Henry Mackeown

Urs Von Gunten

Justine Criquet
email: justine.criquet@univ-lille.fr

Iodide sources in the aquatic environment and its fate during oxidative water treatment -A critical review

Keywords: drinking water, oxidation, iodine, disinfection by-products, chlorination, chloramination, I-THM

Iodine is a naturally-occurring halogen in natural waters generally present in concentrations between 0.5 and 100 µg L -1 . During oxidative drinking water treatment, iodine-containing disinfection byproducts (I-DBPs) can be formed. The formation of I-DBPs was mostly associated to taste and odor issues in the produced tap water but has become a potential health problem more recently due to the generally more toxic character of I-DBPs compared to their chlorinated and brominated analogues. This paper is a systematic and critical review on the reactivity of iodide and on the most common intermediate reactive iodine species HOI. The first step of oxidation of I -to HOI is rapid for most oxidants (apparent second-order rate constant, kapp > 10 3 M -1 s -1 at pH 7). The reactivity of hypoiodous acid with inorganic and organic compounds appears to be intermediate between chlorine and bromine. The life times of HOI during oxidative treatment determines the extent of the formation of I-DBPs. Based on this assessment, chloramine, chlorine dioxide and permanganate are of the highest concern when treating iodide-containing waters. The conditions for the formation of iodo-organic compounds are also critically reviewed.

From an evaluation of I-DBPs in more than 650 drinking waters, it can be concluded that one third show low levels of I-THMs (<1 µg L -1 ), and 18% of exhibit concentrations > 10 µg L -1 . The most frequently detected I-THM is CHCl2I followed by CHBrClI. More polar I-DBPs, as iodoacetic acid in particular, have been reviewed as well.

Finally, the transformation of iodide to iodate, a safe iodine-derived end-product, has been proposed to mitigate the formation of I-DBPs in drinking water processes. For this purpose a pre-oxidation step with either ozone or ferrate(VI) to completely oxidize iodide to iodate is an efficient process. Activated carbon has also been shown to be efficient in reducing I-DBPs during drinking water oxidation.

Introduction

Chemical disinfectants are widely used in drinking water to serve two main purposes: inactivation of microorganisms during primary treatment and avoiding their regrowth in distribution systems [START_REF] Sedlak | The Chlorine Dilemma[END_REF]. Chemical oxidants are also used for iron and manganese removal (Allard et al., 2013a), abatement of taste and color [START_REF] Bruchet | Role of oxidants and disinfectants on the removal, masking and generation of tastes and odours[END_REF] and micropollutants [START_REF] Von Gunten | Oxidation Processes in Water Treatment: Are We on Track?[END_REF](von Gunten, , 2003)), improving the coagulation and filtration efficiency, and preventing algal growth in sedimentation basins and filters [START_REF] Spellman | Handbook of Water and Wastewater Treatment Plant Operations[END_REF]. Commonly applied chemical oxidants in water treatment are chlorine, chloramine, ozone, chlorine dioxide and permanganate [START_REF] Von Gunten | Oxidation Processes in Water Treatment: Are We on Track?[END_REF].

Disinfectants mainly react with dissolved organic matter (DOM) to produce undesired disinfection byproducts (DBPs) [START_REF] Sedlak | The Chlorine Dilemma[END_REF]. Since chloroform was first identified [START_REF] Rook | Formation of haloforms during chlorination of natural waters[END_REF][START_REF] Bellar | The Occurrence of Organohalides in Chlorinated Drinking Waters[END_REF]), more than 700 other DBPs have been detected [START_REF] Richardson | Water Analysis: Emerging Contaminants and Current Issues[END_REF].

The majority of identified DBPs in drinking water are halogenated compounds, through a direct incorporation of chlorine into DOM or indirectly by the incorporation of bromine and/or iodine in presence of bromide and/or iodide [START_REF] Hrudey | Disinfection by-products and human health[END_REF]. To minimize exposure to DBPs, several compounds are currently regulated in numerous countries (four trihalomethanes (THMs), five haloacetic acids (HAAs), chlorite and bromate). THMs and HAAs were chosen for regulation as they are generally found at the highest concentrations [START_REF] Hrudey | Disinfection by-products and human health[END_REF]. It was assumed that applying conditions to effectively mitigate formation of regulated THMs and HAAs would also limit the occurrence of many other known or unknown unregulated DBPs [START_REF] Krasner | The formation and control of emerging disinfection by-products of health concern[END_REF][START_REF] Hrudey | Disinfection by-products and human health[END_REF]. However, an application of alternative disinfectants to chlorine to minimize the formation of the regulated DBPs can favor the formation of potentially more toxic emerging DBPs, including iodinated compounds and nitrogenous compounds such as nitrosamines [START_REF] Krasner | The formation and control of emerging disinfection by-products of health concern[END_REF][START_REF] Dong | Formation of Iodinated Disinfection Byproducts (I-DBPs) in Drinking Water: Emerging Concerns and Current Issues[END_REF]. It has been demonstrated for all classes of halogenated DBPs, that iodine-containing species (I-DBPs) are generally the most toxic [START_REF] Gonsioroski | Iodoacetic acid inhibits follicle growth and alters expression of genes that regulate apoptosis, the cell cycle, estrogen receptors, and ovarian steroidogenesis in mouse ovarian follicles[END_REF][START_REF] Wagner | CHO cell cytotoxicity and genotoxicity analyses of disinfection by-products: An updated review[END_REF], followed by bromine-containing and chlorine-containing analogues [START_REF] Krasner | Halogenated DBPs and emerging issues[END_REF].

Iodinated trihalomethanes (I-THMs) were the first studied I-DBPs due to concerns about a strong medicinal taste and odor in drinking water [START_REF] Hansson | Iodoform taste complaints in chloramination[END_REF] and have received more attention subsequently to improve the understanding of their formation (Bichsel and von Gunten, 2000a;[START_REF] Krasner | Occurrence of a New Generation of Disinfection Byproducts[END_REF]Hua and Reckhow, 2007a;[START_REF] Jones | The impact of bromide/iodide concentration and ratio on iodinated trihalomethane formation and speciation[END_REF][START_REF] Allard | Mechanistic Study on the Formation of Cl-/Br-/I-Trihalomethanes during Chlorination/Chloramination Combined with a Theoretical Cytotoxicity Evaluation[END_REF].

After presenting the different iodine sources in the environment, this review paper analyses oxidation pathways of iodide and reactive iodine species by selected oxidants used in drinking water treatment and assesses second-order rate constants of the corresponding reactions. The aqueous iodine chemistry is presented to better understand the fate of iodide and the formation of iodinated disinfection by-products in drinking water. The available second-order rate constants for the reactions of iodine with inorganic and organic compounds are also presented and discussed.

The different factors influencing the formation of I-DBPs are discussed and a modeling is provided to explain the interplay between bromide concentration and pH. An assessment of the occurrence of I-DBPs worldwide and a risk assessment related to I-DBP formation and mitigation strategies complete this review.

Iodine in atmospheric, aqueous and terrestrial natural systems

Iodine is a naturally-occurring halogen in many chemical forms and oxidation states, ranging from -1 to +7 [START_REF] Risher | Toxicological profile for iodine. Toxicological profile for iodine u.s. department of health and human services[END_REF]. Iodide (-1) and iodate (+5) are the predominant species in the aqueous environment [START_REF] Preedy | Comprehensive Handbook of Iodine: Nutritional, Biochemical, Pathological and Therapeutic Aspects[END_REF]. Iodine occurs naturally as the stable isotope ( 127 I) and the radioisotope ( 129 I), additionally, eight other synthetic radioisotopes exist [START_REF] Risher | Toxicological profile for iodine. Toxicological profile for iodine u.s. department of health and human services[END_REF].

The global cycling of iodine is presented in detail by [START_REF] Fuge | Iodine and human health, the role of environmental geochemistry and diet, a review[END_REF] in a review of the role of iodine environmental geochemistry on human health. The following discussion of the iodine cycle will mainly focus on its transport and its role for water resources used for drinking purposes.

The marine environment: The largest compartment of available iodine

Despite only representing about 1% of the total iodine [START_REF] Preedy | Comprehensive Handbook of Iodine: Nutritional, Biochemical, Pathological and Therapeutic Aspects[END_REF], the ocean is the largest compartment of accessible iodine corresponding to an average concentration of between 45 and 60 µg L -1 [START_REF] Risher | Toxicological profile for iodine. Toxicological profile for iodine u.s. department of health and human services[END_REF]. By contrast, chlorine and bromine have an average seawater concentration of 19.4 g L -1 and 67 mg L -1 respectively [START_REF] Millero | The composition of Standard Seawater and the definition of the Reference-Composition Salinity Scale[END_REF]. In the open ocean, more than 95% of the dissolved iodine is present as inorganic iodine (mainly iodide and iodate) [START_REF] Preedy | Comprehensive Handbook of Iodine: Nutritional, Biochemical, Pathological and Therapeutic Aspects[END_REF][START_REF] Fuge | Iodine and human health, the role of environmental geochemistry and diet, a review[END_REF]. Iodate, is the most abundant species, as it is the thermodynamically stable form in oxygenated, alkaline seawater (Fuge, 2013). However surface and shallow shelf waters are enriched in iodide (up to 50%) (Fuge, 2013;[START_REF] Preedy | Comprehensive Handbook of Iodine: Nutritional, Biochemical, Pathological and Therapeutic Aspects[END_REF]. This is a result of the conversion of stable iodate to metastable iodide, through the action of phytoplankton, some macroalgae, and possibly through some abiotic processes such as photolysis (Figure 1) [START_REF] Fuge | Iodine and human health, the role of environmental geochemistry and diet, a review[END_REF]. Once formed, iodide is slowly re-oxidised to iodate (Fuge, 2013), after a first oxidation step of iodide to iodine, mediated by iodide-oxidising bacteria, followed by spontaneous hydrolysis to HOI, which finally disproportionates to iodate (Bichsel and von Gunten, 2000b;[START_REF] Amachi | Microbial Contribution to Global Iodine Cycling: Volatilization, Accumulation, Reduction, Oxidation and Sorption of Iodine[END_REF].

Volatile organo-iodine species such as CH3I, CH2I2 and CH2ClI can be formed through biological activity [START_REF] Amachi | Microbial Contribution to Global Iodine Cycling: Volatilization, Accumulation, Reduction, Oxidation and Sorption of Iodine[END_REF]. From the knowledge based on aqueous chemistry of halogenation of organic matter and considering the formation of HOI as the key reactant in cells, it would be expected that iodoform (CHI3) is the first product formed through the haloform reaction. Subsequently, this may lead to the formation of CH3I by dehalogenation. However, this mechanism has never been confirmed in living organisms. Organoiodine compounds, mainly iodoform, may also be formed by HOI and I2 -produced by ozone (see section 2.2.4.) -reacting with dissolved organic matter at the sea surface (Bichsel and von Gunten, 2000a;[START_REF] Martino | A new source of volatile organoiodine compounds in surface seawater[END_REF].

Atmospheric iodine chemistry

All the volatilized compounds photodissociate rapidly in the atmosphere (Figure 1) -with a lifetime of a few seconds for I2 up to 10 days for CH3I -to generate the iodine atom (Keppler et al. 2003;[START_REF] Sherwen | Iodine's impact on tropospheric oxidants: a global model study in GEOS-Chem[END_REF]. Subsequently, iodine atoms react mainly with ozone in the lower atmosphere to generate iodine monoxide (IO • ) [START_REF] Preedy | Comprehensive Handbook of Iodine: Nutritional, Biochemical, Pathological and Therapeutic Aspects[END_REF]. Iodine and iodinated compounds are then an important ozone-depleting family especially in the global marine upper troposphere [START_REF] Saiz-Lopez | Iodine chemistry in the troposphere and its effect on ozone[END_REF]. Different atmospheric species were characterized, e. g., OIO, I2O2, HOI, IONO2, I2O2, I2O3, I2O4 and I2O5. Polymerisation of oxides could also lead to the production of iodine oxide particles (IOPs) (Figure 1) [START_REF] Preedy | Comprehensive Handbook of Iodine: Nutritional, Biochemical, Pathological and Therapeutic Aspects[END_REF]. Iodine is transferred to the land surface through wet and dry deposition [START_REF] Risher | Toxicological profile for iodine. Toxicological profile for iodine u.s. department of health and human services[END_REF]. The iodine content of rainwater is usually in the range 0.5 to 2.5 µg L - 1 [START_REF] Fuge | Iodine and human health, the role of environmental geochemistry and diet, a review[END_REF]. Rainwater concentrations of iodine are higher near the coast, wherefore, soils in the coastal zone are subjected to a greater input of iodine [START_REF] Fuge | Iodine and human health, the role of environmental geochemistry and diet, a review[END_REF]. However further inland, there is apparently no significant correlation between the iodine content in the soil and the distance from the sea because this also depends on total annual rainfall [START_REF] Johnson | The geochemistry of iodine and its application to environmental strategies for reducing the risks from iodine deficiency disorders (IDD)[END_REF]. Over the sea or along the coast, iodate has been found as the dominant species in several studies [START_REF] Hou | Time Series of 129I and 127I Speciation in Precipitation from Denmark[END_REF][START_REF] Truesdalea | The variation of iodate and total iodine in some UK rainwaters during 1980-1981[END_REF], whereas further inland, organo-iodine can become the most dominant fraction, with lower fractions of iodate than iodide [START_REF] Gilfedder | Iodine and bromine speciation in snow and the effect of orographically induced precipitation[END_REF].

Iodine in soil

In comprehensive worldwide databases of published iodine analysis, iodine concentrations varied from < 0.1 to up to 660 mg kg -1 , with a median at 3 mg kg -1 [START_REF] Johnson | The geochemistry of iodine and its application to environmental strategies for reducing the risks from iodine deficiency disorders (IDD)[END_REF][START_REF] Smyth | Distribution of iodine in soils of Northern Ireland[END_REF].

The input of iodine to soil occurs predominantly through transfer from the atmosphere [START_REF] Risher | Toxicological profile for iodine. Toxicological profile for iodine u.s. department of health and human services[END_REF]. The input of iodine to soil also occurs through weathering of recent marine sediments or through the decay of vegetation or animals. Soil iodine is mainly in an organic form with variable fractions of iodate and iodide [START_REF] Fuge | Iodine and human health, the role of environmental geochemistry and diet, a review[END_REF]. After transfer from the atmosphere, dissolved inorganic iodine is rapidly transformed to organic formsthrough abiotic or biotic processes [START_REF] Seki | Laccase-Catalyzed Oxidation of Iodide and Formation of Organically Bound Iodine in Soils[END_REF][START_REF] Yamaguchi | Inorganic iodine incorporation into soil organic matter: evidence from iodine K-edge X-ray absorption near-edge structure[END_REF]. For example, MnO2 has been found to oxidise I -to I2 and then to iodate (IO3 -) or iodinated organic compounds in the presence of humic substances (Gallard et al., 2009). Organic-rich soils have the highest iodine fixation potential, especially peat soils, and humus may constitute the primary reservoir of iodine. Other components such as iron(III)-and/or aluminium(III)-(hydr)oxides and clay minerals also enhance the iodine fixation potential [START_REF] Johnson | The geochemistry of iodine and its application to environmental strategies for reducing the risks from iodine deficiency disorders (IDD)[END_REF][START_REF] Fuge | Iodine and human health, the role of environmental geochemistry and diet, a review[END_REF], due to anion exchange processes on their positively charged surfaces [START_REF] Preedy | Comprehensive Handbook of Iodine: Nutritional, Biochemical, Pathological and Therapeutic Aspects[END_REF].

Iodine in fresh waters

Most natural surface waters have total iodine concentrations between 0.5 and 100 µg L -1 and often below 20 µg L -1 (Table S1). In 807 samples taken in rivers across Europe, iodide concentrations ranged from < 0.01 to 104 µg L -1 with a quite low median concentration (0.33 µg L -1 ) [START_REF] Salminen | Geochemical Atlas of Europe. Part 1 -Background Information, Methodology and Maps[END_REF].

Central continental and mountainous areas in particular tend to have lower concentrations, typically in the range 0.1 to 18 µg L -1 [START_REF] Salminen | Geochemical Atlas of Europe. Part 1 -Background Information, Methodology and Maps[END_REF].

Compared to surface waters, groundwaters are generally more enriched in iodine with concentrations generally below sea concentrations (i.e. < 60 µg L -1 ). Iodine in groundwaters mainly originates from breakdown of iodine-containing organic matter, leaching from marine sediments in slow-moving aquifers, and evapo-concentration in arid areas [START_REF] Fuge | Iodine and human health, the role of environmental geochemistry and diet, a review[END_REF]. The main species of iodine in groundwater is generally iodide with a small proportion of iodate [START_REF] Tang | Geochemistry of iodine-rich groundwater in the Taiyuan Basin of central Shanxi Province, North China[END_REF], but in some cases organic iodine can also be important [START_REF] Zhang | Iodine in groundwater of the North China Plain: Spatial patterns and hydrogeochemical processes of enrichment[END_REF].

Nevertheless, iodine concentrations higher than in sea water were reported in a some drinking waters, with iodine levels in the mg L -1 range [START_REF] Tang | Geochemistry of iodine-rich groundwater in the Taiyuan Basin of central Shanxi Province, North China[END_REF][START_REF] Zhang | Iodine in groundwater of the North China Plain: Spatial patterns and hydrogeochemical processes of enrichment[END_REF]. 72% of 950 water samples in the southwestern Taiyuan Basin, China had iodine concentrations > 150 µg L -1 [START_REF] Tang | Geochemistry of iodine-rich groundwater in the Taiyuan Basin of central Shanxi Province, North China[END_REF].

The iodine content of surface waters is also influenced by anthropogenic activities. Wastewaters from hydraulic fracturing are characterized by high chloride, bromide and iodide concentrations (≤ 56 mg L - 1 ). They may impact drinking water utilities downstream from disposal or accidental spill sites [START_REF] Harkness | Iodide, Bromide, and Ammonium in Hydraulic Fracturing and Oil and Gas Wastewaters: Environmental Implications[END_REF]. Additional iodine may originate from agricultural activities by application of pesticides or herbicides containing methyl iodide or via fertilizers with trace levels of iodine [START_REF] Moran | Sources of iodine and iodine 129 in rivers[END_REF][START_REF] Salminen | Geochemical Atlas of Europe. Part 1 -Background Information, Methodology and Maps[END_REF]. Another source of iodide to surface and groundwaters is the discharge of persistent iodinated pollutants such as iodinated X-ray contrast media (ICM), which are a class of pharmaceuticals used in the medical imaging of soft tissues. Due to poor removal through conventional wastewater treatment -in some cases as low as 10% -ICM concentrations in surface waters can exceed 1 µg L -1 [START_REF] Schulz | Transformation of the X-ray Contrast Medium Iopromide In Soil and Biological Wastewater Treatment[END_REF][START_REF] Duirk | Formation of Toxic Iodinated Disinfection By-Products from Compounds Used in Medical Imaging[END_REF][START_REF] Xu | Distribution and relevance of iodinated X-ray contrast media and iodinated trihalomethanes in an aquatic environment[END_REF] (Table S2).

In some cases, iodine may be added directly to water to make it drinkable and/or to relieve endemic goitre in certain regions [START_REF] Maberly | Effect of Iodination of a village watersupply on goitre and thyroid function[END_REF][START_REF] Vigneri | Iodine supplementation to a public water supply: An efficient method for correcting iodine deficiency, in: Iodine Deficiency in Europe: A Continuing Concern[END_REF][START_REF] Smith | Comparison of Byproduct Formation in Waters Treated with Chlorine and Iodine: Relevance to Point-of-Use Treatment[END_REF]. However, salt iodization for iodine supplementation to iodine-deficient populations remains the most appropriate and widespread measure to relieve endemic goitre [START_REF] Eastman | The Iodine Deficiency Disorders[END_REF][START_REF] Bürgi | The toxicology of iodate: a review of the literature[END_REF]. Iodine-based disinfectants (iodine solutions or iodine resins) have been a popular option for point-of-use drinking water disinfection by campers, the military, and rural consumers in developing countries [START_REF] Goodyer | Safety of Iodine Based Water Sterilization for Travelers[END_REF][START_REF] Smith | Comparison of Byproduct Formation in Waters Treated with Chlorine and Iodine: Relevance to Point-of-Use Treatment[END_REF]. However, use of iodination for full-scale water disinfection has been rare, due to the high cost of elemental iodine [START_REF] Smith | Comparison of Byproduct Formation in Waters Treated with Chlorine and Iodine: Relevance to Point-of-Use Treatment[END_REF]. Therefore, this source will not be a major contribution to iodine in fresh water systems.

Fate and reactivity of iodide and aqueous iodine during drinking water treatment

Specific removal of iodide

Due to the absence of regulatory limits on bromide and iodide -except in Australia: 0.5 mg L -1 for iodide (NHMRC and NRMMC, 2011) -little effort has been made to develop drinking water treatment for bromide and/or iodide removal. However, based on the need to mitigate Br/I-DBP formation the potential benefits in lowering bromide and iodide concentrations have been assessed. Some processes, which are commonly used in drinking water treatment -while not used specifically for halide removal -may also remove bromide and iodide before a disinfection step. It has been shown that membrane processes such as reverse osmosis, nanofiltration, ion exchange membranes, electrodialysis have good to excellent halide removal efficiencies -with reverse osmosis being the most effective [START_REF] Watson | Strategies for the removal of halides from drinking water sources, and their applicability in disinfection by-product minimisation: A critical review[END_REF]. Studies on bromide and/or iodide removal using adsorption techniques (layered double hydroxides, silver impregnated activated carbons and carbon aerogels, ionexchange resins, aluminium coagulation) -some of which are used in drinking water treatmentshowed varied results, as their efficiency was mainly limited by interference from competing anions and DOM present in water resources [START_REF] Sánchez-Polo | Removal of bromide and iodide anions from drinking water by silver-activated carbon aerogels[END_REF][START_REF] Watson | Strategies for the removal of halides from drinking water sources, and their applicability in disinfection by-product minimisation: A critical review[END_REF].

Oxidation of iodide and iodine

During oxidative water treatment, I -is first oxidised to reactive iodine species (usually HOI) which can further react with oxidants and/or organic matter (Figure 2). The specific reactivity of iodide and iodine with the main oxidants used in water treatment is decribed in this section. The reactivity of several other oxidants (Mn(VII), MnO2, Fe(VI) as well as peroxides HSO5 -, S2O8 2-, H2O2 and their activation processes) has been recently reviewed [START_REF] Li | Oxidation of iodide and hypoiodous acid by non-chlorinated water treatment oxidants and formation of iodinated organic compounds: A review[END_REF], wherefore, only kinetic information will be presented here for these oxidants (Table 1). A compilation of all oxidation processes is provided in Table 1 and Figure 2, including those, which are not discussed in detail in this publication.

Overall, the reactivity of oxidants towards iodide increases in the following order: MnO2 < MnO4 -< ClO2 < NH2Cl < HFeVIO4 -< HOCl < O3 < HOBr (Table 1 and Figure 2). The oxidation of I -to HOI is rapid for most oxidants (kapp > 10 3 M -1 s -1 at pH 7), but relatively slow for permanganate and MnO2. For chlorination, chloramination, ozonation and permanganate, HOI is the initial oxidation product in oxidative drinking water treatment. In the case of chlorine dioxide, ferrate, sulfate radicals, hydrogen peroxide and UV treatment, iodine atoms are formed which then can undergo various follow-up transformations [START_REF] Ye | Formation of iodinated disinfection by-products during oxidation of iodide-containing water with potassium permanganate[END_REF][START_REF] Zhang | A comparison of iodinated trihalomethane formation from chlorine, chlorine dioxide and potassium permanganate oxidation processes[END_REF][START_REF] Kralchevska | Ferrates (FeVI, FeV, and FeIV) oxidation of iodide: Formation of triiodide[END_REF].

The further reactivity of iodine with various oxidants is also discussed with second-order rate constants, the main products and half-life times being summarized in Table 1. At neutral pH, the oxidants can be divided into three groups: (i) HOCl, HOBr, NH2Cl, O3 and S2O8 -for which the secondorder rate constants for the reactions with I -are higher than those for HOI; (ii) HSO5 -and MnO4 -, which have similar second-order rate constants for the reactions both with I -and HOI and (iii) HFeO4 -, which is more reactive towards HOI than towards I -. In the later case, electrostatic repulsion has been hypothetized to explain this paradox [START_REF] Shin | Reactions of Ferrate(VI) with Iodide and Hypoiodous Acid: Kinetics, Pathways, and Implications for the Fate of Iodine during Water Treatment[END_REF]. Within the first group, the differences in reactivities are high. The half-life times of reactive iodine species such as HOI during oxidative treatment will determine the extent of the formation of iodinated organic compounds (see Figure 2 and section 5.1).

Chlorine

If iodide-containing waters are treated with chlorine, iodide (I -) is rapidly oxidised to hypoiodous acid (HOI) or hypoiodite (OI -) (Eqs. 1 and 2). The rate of this reaction depends on the pH, i.e. the speciation chlorine with HOCl or ClO -(pKa (HOCl/OCl -) = 7.53 (Martell, 1976)) [START_REF] Nagy | Non-Metal Redox Kinetics: Oxidation of Iodide by Hypochlorous Acid and by Nitrogen Trichloride Measured by the Pulsed-Accelerated-Flow Method[END_REF][START_REF] Gerritsen | Non-metal redox kinetics: hypochlorite and hypochlorous acid reactions with cyanide[END_REF]Bichsel andvon Gunten, 1999, 2000b) (Table 1).

HOCl + I -HOI + Cl -

(1)

OCl -+ I -OI -+ Cl - (2)
These reactions occur via a Cl + transfer and a formation of ICl as a transient species. This intermediate product is unstable and hydrolyses to OI - [START_REF] Kumar | Atom-Transfer Redox Kinetics: General-Acid-Assisted Oxidation of Iodide by Chloramines and Hypochlorite[END_REF]. During disinfection by chlorine, HOCl is the dominant reactive species for the reaction with iodide (k(HOCl + I -) ⩾ 10 6 k(OCl -+ I -)) [START_REF] Deborde | Reactions of chlorine with inorganic and organic compounds during water treatment-Kinetics and mechanisms: A critical review[END_REF]. For typical concentrations of chlorine in drinking water treatment, HOCl/OCl - transforms I -to HOI/OI -within < 1 ms [START_REF] Bichsel | Oxidation of Iodide and Hypoiodous Acid in the Disinfection of Natural Waters[END_REF] (Table 1).

A further oxidation of HOI by chlorine occurs through a combination of second-and third-order reactions [START_REF] Bichsel | Oxidation of Iodide and Hypoiodous Acid in the Disinfection of Natural Waters[END_REF]. There are two hypothetical pathways for HOI oxidation by HOCl: (i) The I-atom in HOI is attacked by the O-atom of HOCl, leading to iodite (IO2 -) formation, which is quickly further oxidised by HOCl to IO3 -; (ii) HOI is attacked by the Cl-atom of HOCl leading to the formation of an intermediate with a iodine-chlorine bond (HOI-ClOH) which then either dissociates back to the initial compounds or further reacts with a second HOCl to form HOI(ClOH)2 which would quickly hydrolyse to IO3 - [START_REF] Bichsel | Oxidation of Iodide and Hypoiodous Acid in the Disinfection of Natural Waters[END_REF]. Eqs. ( 3) and (4) show the stoichiometries of these reactions with the corresponding rate constants in Table 1 [START_REF] Bichsel | Oxidation of Iodide and Hypoiodous Acid in the Disinfection of Natural Waters[END_REF]. In these reactions, the formation of the intermediate species is the rate limiting step resulting in the measured second-order rate constants.

2 HOCl + HOI IO3 -+ 2 Cl -+ 3H + (3) 2 OCl -+ HOI IO3 -+ 2 Cl -+ H + (4)

Eq. ( 4) is the dominant reaction, contributing between 60% (pH 7) and 99.5% (pH 9) to the overall HOI oxidation by chlorine [START_REF] Bichsel | Oxidation of Iodide and Hypoiodous Acid in the Disinfection of Natural Waters[END_REF]. This process is relatively slow, with a half-life time of 1 hour under typical drinking water treatment conditions (Table 1).

In the presence of bromide, HOBr and OBr -(pKa (HOBr/OBr -) = 8.8) formed from the reaction between chlorine and bromide [START_REF] Heeb | Oxidative treatment of bromide-containing waters: Formation of bromine and its reactions with inorganic and organic compounds -A critical review[END_REF] are also able to oxidise I -to HOI or OI -(Table 1) through an analogous reaction pathway. The formation of iodate is accelerated due to the relatively high reactivity of HOBr and OBr -with HOI/OI -(Table 1), which outcompetes the reactions with chlorine [START_REF] Criquet | Iodate and Iodo-Trihalomethane Formation during Chlorination of Iodide-Containing Waters: Role of Bromide[END_REF].

In addition, cupric oxide (CuO) -one of the major corrosion products of copper pipes -can activate HOCl, thus enhancing its reactivity towards HOI and increasing the rate of formation of iodate compared to the absence of cupric oxide. Iodate can also be further oxidised to periodate (IO4 -) by CuO-activated hypohalous acids [START_REF] Liu | Chlorination of Iodide-Containing Waters in the Presence of CuO: Formation of Periodate[END_REF].

Chloramines

Unlike the reaction between monochloramine and bromide, which mainly leads to the formation of a mixed haloamine NHBrCl [START_REF] Luh | Kinetics of Bromochloramine Formation and Decomposition[END_REF] the product of the reaction of monochloramine with iodide is assumed to be hypoiodous acid (Eq. ( 5)). NH2Cl transforms I -into HOI/OI -within < 15 minutes under typical drinking water treatment conditions [START_REF] Bichsel | Oxidation of Iodide and Hypoiodous Acid in the Disinfection of Natural Waters[END_REF] (Table 1).

NH2Cl + H + + I -+ H2O HOI + Cl -+NH4 + (5)

The second-order rate constant for the reaction of iodide with dichloramine is much lower than with monochloramine (Table 1). This difference is explained by the predominant reaction of NH3Cl + in chloramination processes [START_REF] Heeb | Formation and reactivity of inorganic and organic chloramines and bromamines during oxidative water treatment[END_REF] along with the difference of the pKa of mono-(1.5) and dichloramine (-6.0) [START_REF] Trogolo | Equilibria and Speciation of Chloramines, Bromamines, and Bromochloramines in Water[END_REF]. NCl3 oxidises I -with a higher second-order rate constant than the two other chloramines but no values were determined [START_REF] Kumar | Atom-Transfer Redox Kinetics: General-Acid-Assisted Oxidation of Iodide by Chloramines and Hypochlorite[END_REF] (Table 1). This leads to the formation of an intermediate NCl3I -which further decomposes by a first-order process to NHCl2 and ICl [START_REF] Nagy | Non-Metal Redox Kinetics: Oxidation of Iodide by Hypochlorous Acid and by Nitrogen Trichloride Measured by the Pulsed-Accelerated-Flow Method[END_REF].

A further oxidation of HOI to iodate by monochloramine can be neglected with a low second-order rate constant (Table 1) leading to a long lifetime of HOI and minor iodate formation in chloramination processes [START_REF] Bichsel | Oxidation of Iodide and Hypoiodous Acid in the Disinfection of Natural Waters[END_REF]. [START_REF] Zhu | Modeling the formation of TOCl, TOBr and TOI during chlor(am)ination of drinking water[END_REF] proposed the formation of the mixed haloamine NHClI (k = 2.0 x 10 -3 M -1 s -1 ) to explain the iodine incorporation into DOM in their model, however this species has not been experimentally confirmed.

In bromide-containing waters, bromochloramine (NHBrCl) as well as HOBr, NH2Br and NHBr2 can be formed during chloramination [START_REF] Luh | Kinetics of Bromochloramine Formation and Decomposition[END_REF][START_REF] Trofe | Kinetics of Monochloramine Decomposition in the Presence of Bromide[END_REF]. Small amounts of HOBr may also be formed from hydrolysis of chloramines (by HOCl that can react with Br -to HOBr) [START_REF] Zhai | Formation of Brominated Disinfection Byproducts during Chloramination of Drinking Water: New Polar Species and Overall Kinetics[END_REF] or bromamines [START_REF] Luh | Kinetics of Bromochloramine Formation and Decomposition[END_REF]. Bromamines are typically more reactive than their chloramine analogues, however the reactivity of I -with bromamines in drinking water treatment has not been reported yet [START_REF] Heeb | Formation and reactivity of inorganic and organic chloramines and bromamines during oxidative water treatment[END_REF].

Ozone

Ozone (O3) oxidises I -very rapidly to OI -, which is in equilibrium with HOI (section 4.1). The reaction occurs by an oxygen atom transfer via an intermediate adduct I-OOO - [START_REF] Liu | Kinetics and Mechanisms of Aqueous Ozone Reactions with Bromide, Sulfite, Hydrogen Sulfite, Iodide, and Nitrite Ions[END_REF]von Gunten, 2003). O3 transforms I -to HOI/OI -within < 1 ms during drinking water treatment (Eq.6) [START_REF] Bichsel | Oxidation of Iodide and Hypoiodous Acid in the Disinfection of Natural Waters[END_REF] (Table 1). Due to the fast direct ozone reaction, the oxidation by the secondarily formed hydroxyl radicals ( • OH) can be neglected (Allard et al., 2013b).

I -+ O3 OI -+ O2 (6)
In contrast to aqueous chlorine, both HOI and OI -are quickly further oxidized to iodate by ozone [START_REF] Bichsel | Oxidation of Iodide and Hypoiodous Acid in the Disinfection of Natural Waters[END_REF], Table 1). Oxidation by ozone occurs predominantly via HOI for pH ⩽ 8. At pH > 8, the oxidation of OI -by ozone contributes significantly to the overall oxidation process [START_REF] Bichsel | Oxidation of Iodide and Hypoiodous Acid in the Disinfection of Natural Waters[END_REF]. The second-order rate constants of these reactions (Table 1) lead to half-life times for HOI/OI -of 0.35 or 0.92 s, respectively for pH 9 and 6 and 1 mgO3 L -1 . The overall stoichiometry of the reaction shows a consumption of 3 moles of ozone to oxidise iodide to iodate (Eq. ( 7)) [START_REF] Bichsel | Oxidation of Iodide and Hypoiodous Acid in the Disinfection of Natural Waters[END_REF].

3 O3 + I -IO3 -+ 3 O2 (7)

Chlorine dioxide

The chlorine dioxide (ClO2) reaction with iodide is complex. Until recently, most of the studies were performed either at low pH and/or with a high iodide concentration. ClO2 has been shown to form Iatoms (I . ) and finally I2 via electron-transfer reactions (Eq. ( 8); k(ClO2/I -) = 1.87 x 10 3 M -1 s -1 [START_REF] Fabian | The Kinetics and Mechanism of the Chlorine Dioxide -Iodide Ion Reaction[END_REF]. A further redox reaction between ClO2 -and I -is possible (Eq. ( 9)) [START_REF] Kern | Iodine Catalysis in the Chlorite-Iodide Reaction[END_REF], however, in slightly acidic to neutral solutions, the corresponding second-order rate constant is several orders of magnitude lower and this reaction can be neglected under typical drinking water treatment conditions [START_REF] Fabian | The Kinetics and Mechanism of the Chlorine Dioxide -Iodide Ion Reaction[END_REF] (Table 1).

2 ClO2 + 2 I -2 ClO2 -+ I2 (8)

ClO2 -+ 4 I -+ 4 H + 2 I2 + Cl -+ 2 H2O (9)

A significant portion of ClO2 degrades to free available chlorine upon reaction with DOM (about 20-25% under drinking water treatment conditions) which can oxidize I -to HOI and eventually to iodate.

Similarly to chlorination, bromide could enhance the formation of iodate through the formation of HOBr during this process [START_REF] Rougé | In Situ Formation of Free Chlorine During ClO2 Treatment: Implications on the Formation of Disinfection Byproducts[END_REF][START_REF] Terhalle | Chlorine Dioxide-Pollutant Transformation and Formation of Hypochlorous Acid as a Secondary Oxidant[END_REF]. It has been reported that the presence of bromide enhances the formation of DBPs and shifts the speciation to more brominated species [START_REF] Yang | Formation of disinfection byproducts upon chlorine dioxide preoxidation followed by chlorination or chloramination of natural organic matter[END_REF] including brominated I-THMs [START_REF] Guo | Factors affecting the formation of iodo-trihalomethanes during oxidation with chlorine dioxide[END_REF], despite the absence of reactivity between bromide and ClO2 [START_REF] Hoigné | Kinetics of reactions of chlorine dioxide (OClO) in water-I. Rate constants for inorganic and organic compounds[END_REF].

Ultraviolet irradiation, advanced oxidation and other processes

Iodide can absorb UV light to form an excited state I -aq* which may relax back to the ground state or form an iodine atom and a solvated electron in close proximity (Eq. ( 10)). These radicals may combine once more in the cage or diffuse into the bulk (Eq. ( 10)) [START_REF] Jortner | On the photochemistry of aqueous solutions of chloride, bromide, and iodide ions[END_REF].

I -+ hν (I -*) -> (I • + e -) (10)
In the absence of electron scavengers, the back reaction predominates [START_REF] Rahn | Potassium Iodide as a Chemical Actinometer for 254 nm Radiation: Use of lodate as an Electron Scavenger[END_REF]. Enhancement of photoxidation of As(III) has been found in the presence of iodide through the formation of iodine atoms [START_REF] Yeo | Iodide-Mediated Photooxidation of Arsenite under 254 nm Irradiation[END_REF].

The reaction of hydroxyl radicals (formed during advanced oxidation) with iodide is diffusion limited (Table 1) and forms the hydroxyl iodide radical anion (HOI -• ) which decomposes to I • and HO -or further reacts with iodide to a I2 -• radical [START_REF] Yamaguchi | Hemibonding of Hydroxyl Radical and Halide Anion in Aqueous Solution[END_REF]. The further reaction of • OH with HOI is important in atmospheric chemistry with a second-order rate constants of k= 4.7 10 8 to 3 x 10 9 M -1 s -1 [START_REF] Hammaecher | A theoretical study of the Habstraction reactions from HOI by moist air radiolytic products (H, OH, and O (3P)) and iodine atoms ( 2P3/2)[END_REF][START_REF] Riffault | Kinetic and mechanistic study of the reactions of OH with IBr and HOI[END_REF]. It is in the same order as the reactivity of • OH with HOBr (2 x 10 9 M -1 s -1 ; [START_REF] Buxton | Critical Review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals (⋅OH/⋅O-) in Aqueous Solution[END_REF]). In presence of hydrogen peroxide (H2O2) (present in AOPs such as UV/H2O2 or O3/H2O2), HOI is quickly reduced to I -(kH2O2/HOI = 9.8 × 10 3 M -1 s -1 ), which generally limits the formation of I-DBPs [START_REF] Shin | Kinetics of the reaction between hydrogen peroxide and aqueous iodine: Implications for technical and natural aquatic systems[END_REF][START_REF] Shin | Reactions of Ferrate(VI) with Iodide and Hypoiodous Acid: Kinetics, Pathways, and Implications for the Fate of Iodine during Water Treatment[END_REF][START_REF] Valent | Simulations of the Iodine Interphase Transport Effect on the Oscillating Bray-Liebhafsky Reaction[END_REF]. This may have consequences for post-chlorination/-chloramination after the UV/H2O2 process, because iodide will persist and may lead to the formation of I-DBPs [START_REF] Shin | Reactions of Ferrate(VI) with Iodide and Hypoiodous Acid: Kinetics, Pathways, and Implications for the Fate of Iodine during Water Treatment[END_REF].

While persulfate (S2O8 2-) and peroxymonosulfate (HSO5 -) can oxidise iodide directly to I2 or HOI [START_REF] Li | Oxidation of iodide and hypoiodous acid by non-chlorinated water treatment oxidants and formation of iodinated organic compounds: A review[END_REF], the sulfate radical (SO4 -• ) formed from their activation has been suggested to lead to the iodine atom. [START_REF] Wang | Transformation of iodide and formation of iodinated by-products in heat activated persulfate oxidation process[END_REF] hypothesized that iodide follows the same transformation pathway as bromide when oxidised by sulfate radicals to form radical iodine species, which undergo a self reaction to free iodine (I2). The second-order rate constant of the oxidation of iodide by sulfate radicals (Eq. ( 26))

is not yet known (Table 1).

Iron (III) has been reported to oxidise iodide, however, the mechanism has only been studied for high levels of iodide leading to the formation of I3 - [START_REF] Vrkljan | Kinetics and Mechanism of Iodide Oxidation by Iron(III): A Clock Reaction Approach[END_REF]. Nevertheless, the formation of I-DBPs has been highlighted during FeCl3-based coagulation [START_REF] Ding | Coagulation of Iodide-Containing Resorcinol Solution or Natural Waters with Ferric Chloride Can Produce Iodinated Coagulation Byproducts[END_REF].

Fate of other iodine sources

Iodinated X-ray contrast media (ICM) are very recalcitrant compounds, wherefore, their degradation has gained some attention recently. ICM can be degraded by direct photolysis (254 nm), whereby I -is released without formation of HOI or IO3 - [START_REF] Allard | Photodecomposition of iodinated contrast media and subsequent formation of toxic iodinated moieties during final disinfection with chlorinated oxidants[END_REF]. Irradiation at 254 nm was shown to be quite effective in degrading diatrizoic acid, iotalamic acid, iohexol, iopromide and iopamidol (quantum yields 0.034 -0.071 mol×Einstein -1 ) [START_REF] Tian | Photodegradation kinetics of iopamidol by UV irradiation and enhanced formation of iodinated disinfection by-products in sequential oxidation processes[END_REF][START_REF] Allard | Photodecomposition of iodinated contrast media and subsequent formation of toxic iodinated moieties during final disinfection with chlorinated oxidants[END_REF].

Chlorine poorly reacts with ICM, with iopamidol having the highest reactivity (k(HOCl + iopamidol) =

1.5 x 10 -2 -1.66 x 10 -3 M -1 s -1 and k(OCl -+ iopamidol) = 0.45 -0.94 M -1 s -1 [START_REF] Wendel | Transformation of Iopamidol during Chlorination[END_REF]Tian et al. 2017a), releasing iodine at slightly alkaline pH [START_REF] Duirk | Formation of Toxic Iodinated Disinfection By-Products from Compounds Used in Medical Imaging[END_REF][START_REF] Wendel | Transformation of Iopamidol during Chlorination[END_REF].

Monochloramine does not degrade ICM [START_REF] Duirk | Formation of Toxic Iodinated Disinfection By-Products from Compounds Used in Medical Imaging[END_REF][START_REF] Wendel | Transformation of Iopamidol during Chlorination[END_REF]. During ozonation several ICM (iopamidol, iopromide, iomeprol, diatrizoate) are partially oxidized, mainly by the secondarily formed hydroxyl radicals [START_REF] Ternes | Ozonation: a tool for removal of pharmaceuticals, contrast media and musk fragrances from wastewater?[END_REF]. However, most studies used very high ozone doses (10-16 mg L -1 ) and degradation is not complete under more typical ozonation conditions [START_REF] Huber | Oxidation of Pharmaceuticals during Ozonation of Municipal Wastewater Effluents: A Pilot Study[END_REF][START_REF] Huber | Oxidation of Pharmaceuticals during Ozonation and Advanced Oxidation Processes[END_REF][START_REF] Kovalova | Elimination of Micropollutants during Post-Treatment of Hospital Wastewater with Powdered Activated Carbon, Ozone, and UV[END_REF][START_REF] Magdeburg | Evaluating the efficiency of advanced wastewater treatment: Target analysis of organic contaminants and (geno-)toxicity assessment tell a different story[END_REF][START_REF] Ning | Ozone and chlorine reactions with dissolved organic matter -Assessment of oxidant-reactive moieties by optical measurements and the electron donating capacities[END_REF][START_REF] Ternes | Ozonation: a tool for removal of pharmaceuticals, contrast media and musk fragrances from wastewater?[END_REF]. ICMs such as diatrizoate have also been reported to be degraded by zerovalent iron under aerobic conditions [START_REF] Stieber | Treatment of Pharmaceuticals and Diagnostic Agents Using Zero-Valent Iron -Kinetic Studies and Assessment of Transformation Products Assay[END_REF][START_REF] Zhou | Factors influencing DBPs occurrence in tap water of Jinhua Region in Zhejiang Province, China[END_REF] and by sulfate radicals [START_REF] Hu | Enhanced degradation of iopamidol by peroxymonosulfate catalyzed by two pipe corrosion products (CuO and δ-MnO2)[END_REF][START_REF] Wang | Oxidative degradation of iodinated X-ray contrast media (iomeprol and iohexol) with sulfate radical: An experimental and theoretical study[END_REF].

IO3 -can be phototransformed to I -during UV irradiation at 254 nm (Eqs. (13-15)) (Zhang et al., 2016a), with pseudo-first order kinetics and a quantum yield of 0.0591 mol Einstein -1 (Tian et al., 2017a).

However, phototransformation of iodate will not be important for typical UV disinfection in drinking water. It also has been shown that the reduction of IO3 -to iodide by UV/H2O2 is faster than with UV alone [START_REF] Tian | Photochemical degradation of iodate by UV/H2O2 process: Kinetics, parameters and enhanced formation of iodo-trihalomethanes during chloramination[END_REF]. This is probably due to the efficienct reduction of the intermediate HOI by hydrogen peroxide [START_REF] Shin | Kinetics of the reaction between hydrogen peroxide and aqueous iodine: Implications for technical and natural aquatic systems[END_REF].

Aqueous iodine chemistry

In oxidative water treatment, HOI formed from the oxidation of I -can have various fates. The possible reactions of HOI -illustrated in Figure 2 -are (i) disproportionation, (ii) oxidation to iodate or (iii) reaction with DOM.

Speciation of aqueous iodine

Once formed, HOI is in equilibrium with I2, OI-, I3 -and H2OI + as described by Eqs. (11-13) (Table 2). At low pH, I2 is the dominant species (Figure 3). At high pH, HOI dissociates to OI -with a pKa = 10.4 ± 0.1 (Bichsel and von Gunten, 2000b). A pKa-value of 10.64 has been published by extrapolation to zero ionic strength [START_REF] Schmitz | Inorganic reactions of iodine(+1) in acidic solutions[END_REF]. Therefore, under typical drinking water conditions, HOI is the major species and OI -is less significant.

Figure 3

Table 2: I2 is in equilibrium with I3 -and polyiodide anions, which is controlled by the iodide concentration (Eq.

14 in Table 2 and Figure S1). ICl (Eqs. 15-16, Table 2) has also been reported to be an important iodinating agent in Cl --rich waters [START_REF] Rose | Iodination of Dimethenamid in Chloraminated Water: Active Iodinating Agents and Distinctions between Chlorination, Bromination, and Iodination[END_REF].

Disproportionation of hypoiodous acid

The disproportionation of HOI eventually leads to iodate formation (Eqs. 17-20, Table 2), with reactions ( 17) and ( 18) being rate-determining. The disproportionation of HOI is catalysed by buffers such as phosphate, borate or acetate and dominates the kinetics of HOI disproportionation (Bichsel and von Gunten, 2000b;[START_REF] Opoku-Agyeman | New Method for the Synthesis of Iodohydrins[END_REF]. A value of k = 0.3 M -1 s -1 has been proposed (Table 2) for Eq. ( 17) in absence of buffers (Bichsel and von Gunten, 2000b). The value of Schmitz ( 2004) k= 25 M -1 s -1 has been determined in presence of at least 5 x 10 -2 M of perchloric acid which could be sufficient to catalyse the disproportion reaction. The rate constants range from 50 to 5000 M -2 s -1 for the buffer catalysed reaction (Bichsel and von Gunten, 2000b).

The overall reaction (Eq. ( 20)) remains very slow under typical drinking water conditions (pH 6-8, 0-5 mM carbonate, 1-10 µg L -1 iodine), with an initial half-life of HOI ranging from a few days to years.

Therefore, IO3 -formation by this pathway is of minor importance during drinking water treatment (Bichsel and von Gunten, 2000b). Ferrate and permanganate have been shown to catalyse the disproportionation of HOI (at pH > 9 for ferrate [START_REF] Shin | Kinetics of the reaction between hydrogen peroxide and aqueous iodine: Implications for technical and natural aquatic systems[END_REF] and pH > 8 for permanganate [START_REF] Zhao | Reactions of hypoiodous acid with model compounds and the formation of iodoform in absence/presence of permanganate[END_REF]. Thus, the fate of iodine will depend on different parameters such as the reaction of HOI with inorganic and organic compounds as well as the applied oxidants or reductants (e.g., MnO4 -, FeO4 2- H2O2).

Hypoiodous acid reactivity with inorganic compounds

Table 3 summarizes the published rate constants for reactions of iodine with inorganic compounds.

Unlike HOCl and HOBr, HOI is unreactive toward NH3 [START_REF] Bichsel | Oxidation of Iodide and Hypoiodous Acid in the Disinfection of Natural Waters[END_REF]. HOI/OI -can react with Cl -and Br -to form the intermediates ICl and IBr in a third-order reaction involving H + , which rapidly hydrolize back to HOI (2.4 x 10 6 s -1 and 8 x 10 5 s -1 , respectively) (Troy et al., 1991;[START_REF] Vogt | Iodine Chemistry and its Role in Halogen Activation and Ozone Loss in the Marine Boundary Layer: A Model Study[END_REF][START_REF] Wang | Kinetics of Hydrolysis of Iodine Monochloride Measured by the Pulsed-Accelerated-Flow Method[END_REF].

Table 3 Compared to HOCl or HOBr, the information on the HOI reactivity with inorganic compounds is scarce [START_REF] Deborde | Reactions of chlorine with inorganic and organic compounds during water treatment-Kinetics and mechanisms: A critical review[END_REF][START_REF] Heeb | Oxidative treatment of bromide-containing waters: Formation of bromine and its reactions with inorganic and organic compounds -A critical review[END_REF]. In particular, no rate constants are available for the reactions with metals and metalloids of interest such as iron (Fe(II)) and manganese (Mn(II)),

or with other anions such as sulfite, cyanide, nitrite and thiocyanate. According to the literature, HOI reaction with sulfite proceeds rapidly, forming sulfate and releasing iodide [START_REF] Morgan | Some reactions of inorganic iodine compounds[END_REF]. The second-order rate constants for the reactions of HOI with sulfite and hydrogen sulfite were assumed to be > 10 9 M -1 s -1 [START_REF] Pechtl | Modelling iodide? iodate speciation in atmospheric aerosol: Contributions of inorganic and organic iodine chemistry[END_REF][START_REF] Xu | Kinetics and Mechanism of the Reactions of Pentathionate Ion by Different Halogen-Containing Oxidizing Agents[END_REF].

Hydrogen peroxide has been shown to reduce HOI while it is able to oxidise I -. These reactions constitute one of the first oscillating reaction (Bray-Liebhafsky) described in literature [START_REF] Bray | A periodic reaction in homogeneous solution and its relation to catalysis[END_REF].

Hypoiodous acid reactivity with organic compounds

The two main types of moieties which have been investigated for HOI reactivity are phenols (Table 4) and α-methyl carbonyl compounds (Table 5). These classes of compounds are known constituents of NOM (Natural Organic Matter). NOM which contain α-methyl carbonyl or phenol groups are known precursors of I-DBPs such as THMs, iodinated acids or iodinated phenols [START_REF] Ding | A Picture of Polar Iodinated Disinfection Byproducts in Drinking Water by (UPLC/)ESI-tqMS[END_REF].

Recently, several other types of compounds have been studied including carboxylic acids and flavones [START_REF] Zhao | Reactions of hypoiodous acid with model compounds and the formation of iodoform in absence/presence of permanganate[END_REF].

Phenolic compounds

Free iodine reacts with phenolic compounds via (i) electrophilic aromatic substitution [START_REF] Wang | Transformation of iodide and formation of iodinated by-products in heat activated persulfate oxidation process[END_REF], or via (ii) electron transfer, leading to the formation of benzoquinones [START_REF] Zhao | Reactions of hypoiodous acid with model compounds and the formation of iodoform in absence/presence of permanganate[END_REF]. The apparent second-order rate constants for the reactions of iodine with phenolic compounds (Table 4) are pH-dependent (Bichsel and von Gunten, 2000a). The contribution of each reaction to the overall reactivity is illustrated in Figure 4. Similarly to chlorine and bromine [START_REF] Criquet | Reaction of bromine and chlorine with phenolic compounds and natural organic matter extracts -Electrophilic aromatic substitution and oxidation[END_REF], the reactivity of iodine with phenolic compounds is high and is dominated by the reaction between HOI and the phenolate (Eq. ( 24)) at circum neutral pH (Figure 4) with species-specific rate constants ≤ 10 8 M -1 s -1 (Table 4). The reaction rate is minimal at pH 3 to 5 and increases to a maximum at the mean of the pKa values of phenol/phenolate and HOI/IO -(i.e., usually around pH 10). In this range, the apparent second-order rate constant increases by a factor of 10 per pH unit increase (Bichsel and von Gunten, 2000a). The apparent second-order rate constants also increase for pH below 3.

This was attributed to the reactivity of H2OI + towards phenols (Eq. ( 21)) (Bichsel and von Gunten, 2000a;[START_REF] Rose | Iodination of Dimethenamid in Chloraminated Water: Active Iodinating Agents and Distinctions between Chlorination, Bromination, and Iodination[END_REF]. [START_REF] Rose | Iodination of Dimethenamid in Chloraminated Water: Active Iodinating Agents and Distinctions between Chlorination, Bromination, and Iodination[END_REF] introduced the additional reaction between H2OI + and the phenolate (Eq. ( 22)) in their modeling as it involves both the nucleophile (H2OI + ) and the electrophile (phenolate). This reaction shows a similar contribution to the reaction between HOI and phenol and explains the differences observed between the apparent rate constant calculation from pH 3 to 5 (Figure 4). However, the consideration of this fast reaction for the determination of the other second-order rate constants does not modify substantially the second-order rate constants of reactions 23 and 24 (Table 4) [START_REF] Rose | Iodination of Dimethenamid in Chloraminated Water: Active Iodinating Agents and Distinctions between Chlorination, Bromination, and Iodination[END_REF] nor the maximum pH (Figure 4). A comparison of the species-specific second-order rate constants clearly shows the following trend: kHOI/PhO-> kH2OI+/PhOH > kHOI/PhOH. The higher reactivity of H2OI + compared to HOI towards phenol can be explained by the higher electrophilicity of H2OI + compared to HOI, whereas the different reactivities of HOI towards phenol and phenolate can be explained by the higher nucleophilicity of the phenolate compared to the phenol (Eqs. ( 23) and ( 24)) (Bichsel and von Gunten, 2000a). However, the impact of buffers should also be considered as iodination of phenol is a buffer catalysed process [START_REF] Berliner | Kinetics of the Iodination of Phenol[END_REF]. [START_REF] Zhao | Reactions of hypoiodous acid with model compounds and the formation of iodoform in absence/presence of permanganate[END_REF] showed a significant increase of the apparent second-order rate constants for the reaction of iodine with phenol in the presence of increasing concentrations of phosphate and borate buffers. The reactivity of HOI with phenolic moieties lies between the reactivities of HOBr and HOCl. Its reactivity varies significantly depending on the nature and location of the substituents on the phenol (Figure 5). The QSAR modeling first presented in [START_REF] Lee | Kinetics of the Oxidation of Phenols and Phenolic Endocrine Disruptors during Water Treatment with Ferrate (Fe(VI))[END_REF] from the data of Bichsel and von Gunten (2000a) has been extended here with additional second-order rate constants from Table 4. For cases for which only apparent second-order rate constants were available, the species-specific secondorder rate constant k(HOI/PhO -) was estimated neglecting the other reactions and considering the fraction of the phenolate species from its pKa value. This approximation could be applied for the apparent second-order rate constants available at pH 7 and 8 because, under these conditions, k(HOI/PhO -) represents more than 95 and 99 % of the overall reactivity of iodine towards phenol, respectively (Figure 4). However, it was not possible to determine the species-specific second-order rate constants for resorcinol and phloroglucinol with this approach. The apparent second-order rate constants are only available at pH 8 and the di-deprotonated forms which have much higher reactivity could not be neglected in these cases. Thus, these compounds have been excluded from the regression.

The slope obtained (-6.2) for the species-specific second-order rate constant kHOI/phenolate as a function of the Hammett constant (σ) is lower than in a previous study (-7.0 (Lee et al., 2005)). Nevertheless, the reactivity of HOI is more impacted by substituent moieties than for chlorine or bromine which exibit similar and lower slopes (-3.0 and -3.5, respectively [START_REF] Heeb | Oxidative treatment of bromide-containing waters: Formation of bromine and its reactions with inorganic and organic compounds -A critical review[END_REF])) (Figure 5).

The order of reactivity observed HOBr > HOI > HOCl is somehow not logical. Based on electronegativity of halogens (Cl: 3.16; Br: 2.96; I: 2.66) the polarisation of the O-X bound increases from chlorine to iodine. Then HOI, based on this aspect, should exhibit the highest reactivity. The steric hindrance due to the bigger size of iodine atom could be affect its reactivity, however, the second-order rate constants of o-, m-and p-bromophenols do not support this hypothesis; o-bromophenol should be more reactive compared to p-bromophenol (Figure 5 and Table 4) if steric hindrance was an important factor. Iodine atom is known to be more polarisable than other halogens, this could be an important aspect in the specific reactivity, modifying the relative strength of O-X bonds, making OH a better leaving group in HOBr than HOI. Likely, this factor overrules the effect of the electronegativity such that HOI has an intermediate reactivity between HOCl and HOBr.

Ketones and carboxylic acids

Similarly to chlorine and bromine, the stoichiometry of the reaction between HOI and carbonyl compounds is 3 to form a triiodomethyl group. At neutral pH, the enolization step is rate-determining and the overall reaction depends only on the carbonyl concentration and the pH and not on the concentration of HOI (Table 5). The triiodocarbonyl compound then hydrolyses with the formation of CHI3 and the corresponding carboxylic acid (RCOOH) (Bichsel and von Gunten, 2000a). The enolization step is catalysed by OH -and H2O, and therefore, the rate of HOI consumption includes two rate constants kH 2 O and kOH-[OH -] which reflects the pH dependence (Bichsel and von Gunten, 2000a).

3-oxopentanedioic acid exhibits a substantial reactivity towards HOI with an apparent second-order rate constant of 4 x 10 3 M -1 s -1 at pH 7 [START_REF] Zhao | Reactions of hypoiodous acid with model compounds and the formation of iodoform in absence/presence of permanganate[END_REF]. This ketone may exhibit a different mechanism without the enol-formation. The reactivity of carboxylic acids is low, the apparent secondorder rate constant for the reaction of iodine with citric acid at pH 8 has been reported as < 1 M -1 s -1 [START_REF] Zhao | Reactions of hypoiodous acid with model compounds and the formation of iodoform in absence/presence of permanganate[END_REF].

Other compounds

Second-order rate constants for the reactions of DOM with HOI have been determined to be in the range of 0.1-0.4 M -1 s -1 on a C-atom concentration basis for DOM in two natural surface waters (Bichsel and von Gunten, 2000a). This reactivity is much lower than for HOBr which has been estimated to be in the range 10 2 -10 4 M -1 s -1 in the first phase and 100 times lower in the slow phase of reaction with DOM [START_REF] Heeb | Oxidative treatment of bromide-containing waters: Formation of bromine and its reactions with inorganic and organic compounds -A critical review[END_REF].

Besides phenols and carbonyl compounds, other organic model compounds (allyl alcohol, glucosamine, glycine, toluene, and oxalate) were tested for their reactivity with HOI. The corresponding second-order rate constants were < 1 M -1 s -1 or even < 0.1 M -1 s -1 (Bichsel and von Gunten, 2000a). Moderately nucleophilic aromatic compounds like dimethenamid exhibit a low reactivity towards HOI (≤ (9.07 ± 1.95) × 10 -4 M -1 s -1 ), wherefore, in this case, the reactivity with H2OI + ((6.53 ± 0.10) × 10 6 M -1 s -1 ) is predominant under typical drinking water conditions [START_REF] Rose | Iodination of Dimethenamid in Chloraminated Water: Active Iodinating Agents and Distinctions between Chlorination, Bromination, and Iodination[END_REF].

Factors influencing the formation of I-DBPs

Role of oxidant type/dose and residence time

Monochloramine unlike chlorine is unable to oxidise HOI to iodate leading to a longer lifetime of HOI and hence a potential increase of I-DBP formation compared to chlorination process. However, as

shown previously, the kinetics of the formation of iodate by chlorine is also quite slow (Table 1). This life-time is reduced in presence of bromide (see section 3.1). Overall, due to the slow oxidation kinetics of HOI by chlorine, chlorine dioxide, permanganate, and chloramine, the formation of I-DBPs can be of concern when treating iodide-containing waters. In contrast, iodate has been shown to be a major sink for iodide during ozonation, treatment with ferrate and peroxymonosulfate (Table 1). Overall, the yield of iodinated DBPs for the main oxidants for typical water treatment conditions generally follows the pattern summarized in Table 6.

In addition to the oxidant type also the oxidant dose needs to be considered. Low free chlorine residual concentrations leads to higher organic I-DBPs concentrations [START_REF] Ioannou | Occurrence and variability of iodinated trihalomethanes concentrations within two drinking-water distribution networks[END_REF]) and higher doses of chlorine reduce the formation of organic I-DBPs [START_REF] Hua | Effect of Bromide and Iodide Ions on the Formation and Speciation of Disinfection Byproducts during Chlorination[END_REF]. Several studies have reported that high residence times in distribution systems lead to higher organic I-DBPs concentrations in chlorinated or chloraminated waters [START_REF] Ioannou | Occurrence and variability of iodinated trihalomethanes concentrations within two drinking-water distribution networks[END_REF][START_REF] Serrano | Seasonal evaluation of the presence of 46 disinfection by-products throughout a drinking water treatment plant[END_REF].

Role of total iodine/iodide and bromide concentrations

Generally, higher iodide concentrations are associated with higher concentrations and numbers of organic I-DBPs during oxidation processes. This has been shown in several occurrence [START_REF] Parsons | Study into the formation of disinfection by-products of chloramination, potential health implications and techniques for minimisation[END_REF][START_REF] Richardson | Occurrence and Mammalian Cell Toxicity of Iodinated Disinfection Byproducts in Drinking Water[END_REF] and laboratory studies (Bichsel and von Gunten, 2000a;[START_REF] Jones | The impact of bromide/iodide concentration and ratio on iodinated trihalomethane formation and speciation[END_REF][START_REF] Kristiana | The formation of halogen-specific TOX from chlorination and chloramination of natural organic matter isolates[END_REF][START_REF] Zhang | A comparison of iodinated trihalomethane formation from chlorine, chlorine dioxide and potassium permanganate oxidation processes[END_REF]. A linear relationship has been established between I-THM formation and iodide content [START_REF] Parsons | Study into the formation of disinfection by-products of chloramination, potential health implications and techniques for minimisation[END_REF] but this could not be generalized [START_REF] Postigo | Formation of iodo-trihalomethanes, iodohaloacetic acids, and haloacetaldehydes during chlorination and chloramination of iodine containing waters in laboratory controlled reactions[END_REF][START_REF] Richardson | Occurrence and Mammalian Cell Toxicity of Iodinated Disinfection Byproducts in Drinking Water[END_REF]. [START_REF] Mackeown | Removal of disinfection by-product precursors by ion exchange resins[END_REF] showed a linear correlation for low iodide concentrations (< 8 µg L -1 ), which was not observed anymore for higher iodide contents. For chlorination at typical chlorine doses of 1-3 mg L -1 , 20% of the iodide was incorporated in organic I-DBPs at pH < 8 in one study [START_REF] Dillon | CHAPTER 38: Risk-Based Approach to the Formation of DBPs of Concern in UK Drinking Water[END_REF]. A similar trend of iodine and bromine incorporation has been observed in other studies, ranging from 10 to 40% of halogen bound to DOM [START_REF] Criquet | Reaction of bromine and chlorine with phenolic compounds and natural organic matter extracts -Electrophilic aromatic substitution and oxidation[END_REF][START_REF] Hua | Effect of Bromide and Iodide Ions on the Formation and Speciation of Disinfection Byproducts during Chlorination[END_REF][START_REF] Zhao | Reactions of hypoiodous acid with model compounds and the formation of iodoform in absence/presence of permanganate[END_REF]. In some cases, even low levels of iodide (≤ 1 µg L -1 ) can yield measurable levels of organic I-DBPs [START_REF] Mackeown | Formation and removal of disinfection by-products in a full-scale drinking water treatment plant[END_REF][START_REF] Richardson | Occurrence and Mammalian Cell Toxicity of Iodinated Disinfection Byproducts in Drinking Water[END_REF].

The formation of I-DBPs was also related to the presence of ICMs in source waters [START_REF] Duirk | Formation of Toxic Iodinated Disinfection By-Products from Compounds Used in Medical Imaging[END_REF][START_REF] Tugulea | The influence of precursors and treatment process on the formation of Iodo-THMs in Canadian drinking water[END_REF]. MacKeown et al. ( 2020) showed a higher iodate formation after chlorination than the initial iodide content in the raw water (70% increase), demonstrating an oxidation of other sources of iodide. While ICMs could be important as anthropogenic micropollutants, their concentrations usually do not explain the formed TOI [START_REF] Mackeown | Formation and removal of disinfection by-products in a full-scale drinking water treatment plant[END_REF]. It seems more likely that this deviation results from an underestimation of the iodide concentration due to iodide complexation by NOM (see section 5.4.2). Therefore, total iodine measurements in addition of iodide may be necessary.

Important amounts of iodide can be added during food preparation by the use of iodinated table salt.

Up to several hundreds of µg L -1 of different I-DBPs were produced during cooking through the reaction with residual chlorine or chloramine from tap water, which increased the toxicity of the samples [START_REF] Cao | Household cooking with seaweed salts and chloraminated tap water produce harmful iodinated disinfection by-products[END_REF]Pan et al., 2016b;[START_REF] Yan | Behaviour of I/Br/Cl-THMs and their projected toxicities under simulated cooking conditions: Effects of heating, table salt and residual chlorine[END_REF].

Increasing the bromide concentrations in waters significantly enhances the rate and extent of iodate formation possibly reducing the yield of organic I-DBPs [START_REF] Criquet | Iodate and Iodo-Trihalomethane Formation during Chlorination of Iodide-Containing Waters: Role of Bromide[END_REF]. It has been shown that HOBr accelerates the oxidation of HOI to iodate during chlorination by a bromide-catalysed process.

The second-order rate constants for the reaction of bromine with iodine (k(HOBr/OI -) = 1.9 x 10 6 M -1 s -1 and k(OBr -/OI -) = 1.8 x 10 3 M -1 s -1 ) are much higher than the corresponding rate constants for chlorine (k(HOCl/OI -) = 8.2 M -1 s -1 and k(OCl -/HOI) = 52 M -1 s -1 , Table 1). The effect of the bromide/iodide ratio on the formation and speciation of I-THMs by chlorine followed by ammonia addition showed that CHCl2I is the main I-THM in the absence of bromide. At a molar bromide:iodide ratio of 10, mixed Cl-/Br-/I-THMs without any clear dominating species were formed. When the molar bromide:iodide ratio was increased to 50, Br-/I-THMs clearly became the major species with CHBr2I being most important [START_REF] Allard | Mechanistic Study on the Formation of Cl-/Br-/I-Trihalomethanes during Chlorination/Chloramination Combined with a Theoretical Cytotoxicity Evaluation[END_REF]. This change in I-THM speciation is due to the formation of HOBr which is a better substituting agent than chlorine [START_REF] Westerhoff | Reactivity of natural organic matter with aqueous chlorine and bromine[END_REF][START_REF] Acero | Kinetics and mechanisms of formation of bromophenols during drinking water chlorination: Assessment of taste and odor development[END_REF].

This may lead to higher concentrations of brominated precursors, which consecutively react with HOI to mixed Br-/I-THMs [START_REF] Allard | Mechanistic Study on the Formation of Cl-/Br-/I-Trihalomethanes during Chlorination/Chloramination Combined with a Theoretical Cytotoxicity Evaluation[END_REF]. In addition, increasing bromide concentrations in chloraminated solutions also lead to a significant decrease in the I-THM formation and changes in species distribution due to the formation of bromine and NHBrCl which may lead to more brominated precursors than NH2Cl (Luh and Marinas, 2012;[START_REF] Jones | The impact of bromide/iodide concentration and ratio on iodinated trihalomethane formation and speciation[END_REF][START_REF] Liu | Formation of iodo-trihalomethanes, iodo-acetic acids, and iodo-acetamides during chloramination of iodide-containing waters: Factors influencing formation and reaction pathways[END_REF].

Role of natural organic matter

Natural organic matter is the major precursor for the generation of organic DBPs during disinfection.

DOM reacts with hypohalous acids (HOX) by electrophilic aromatic substitution leading to the incorporation of halogen atoms into the organic matrix or by redox processes leading to an oxidation of the DOM and the release of the halide ions [START_REF] Criquet | Reaction of bromine and chlorine with phenolic compounds and natural organic matter extracts -Electrophilic aromatic substitution and oxidation[END_REF]. The specific ultraviolet absorbance at 254 nm (SUVA) -with a high SUVA value generally indicating a high aromatic content [START_REF] Reckhow | Chlorination of humic materials: byproduct formation and chemical interpretations[END_REF]) -has been widely used as an indicator of the DBP formation potential of DOM (Hua and Reckhow, 2007b;[START_REF] Jones | The impact of bromide/iodide concentration and ratio on iodinated trihalomethane formation and speciation[END_REF][START_REF] Hua | Correlation between SUVA and DBP formation during chlorination and chloramination of NOM fractions from different sources[END_REF][START_REF] Liu | Formation of iodo-trihalomethanes, iodo-acetic acids, and iodo-acetamides during chloramination of iodide-containing waters: Factors influencing formation and reaction pathways[END_REF]. [START_REF] Liu | Formation of iodo-trihalomethanes, iodo-acetic acids, and iodo-acetamides during chloramination of iodide-containing waters: Factors influencing formation and reaction pathways[END_REF] showed from the chloramination of model compounds that low SUVA substances favored the formation of I-DBPs compared to high SUVA substances. This could be related to the numbers of active sites, low SUVA waters exhibiting a limited number of active sites, while high SUVA DOM could incorporate iodine in many different sites favouring an increase of TOI in contrast to individual low molecular weight DBPs [START_REF] Allard | Mechanistic Study on the Formation of Cl-/Br-/I-Trihalomethanes during Chlorination/Chloramination Combined with a Theoretical Cytotoxicity Evaluation[END_REF]. The same behaviour has been observed with natural waters or NOM extracts in several studies [START_REF] Jones | The impact of bromide/iodide concentration and ratio on iodinated trihalomethane formation and speciation[END_REF][START_REF] Liu | Formation of iodo-trihalomethanes, iodo-acetic acids, and iodo-acetamides during chloramination of iodide-containing waters: Factors influencing formation and reaction pathways[END_REF], but the opposite was found in other studies [START_REF] Wang | A comparison of iodinated trihalomethane formation from iodide and iopamidol in the presence of organic precursors during monochloramination[END_REF]Zhang et al., 2016b). However, when TOI was measured, DOM with high SUVA shows the highest concentrations of unknown I-DBPs formed (Hua and Reckhow, 2007b) or lower iodate formation [START_REF] Criquet | Iodate and Iodo-Trihalomethane Formation during Chlorination of Iodide-Containing Waters: Role of Bromide[END_REF]. Consequently, the aromatic fraction seems to be more reactive with iodine to form TOI. Conversely, low SUVA DOM with aliphatic moieties, already containing smaller molecular weight (MW) precursors, could lead to the formation of measurable DBPs such as I-THMs.

An analogue behaviour has been hypothesized for brominated DBPs (Hua and Reckhow, 2007b).

However, these conclusions on I-DBPs are weakened by the fact that iodine incorporation into DOM depends on other factors (e.g. bromide concentration, oxidant exposure, iodate formation) that are not always well or clearly set and comparable in the different studies with NOM or natural waters. It has also to be noted that DBP formation potential tests (DBP-FP) are not appropriate in the case of I-DBPs due to the high extent of conversion of iodide to iodate in the presence of high concentrations of chlorine. For increasing SUVA and/or for high molecular weight fractions, a higher chlorine consumption is expected by direct reaction with the DOM, producing more chlorine-containing sites and leaving less chlorine available for oxidation of HOI to iodate [START_REF] Allard | Mechanistic Study on the Formation of Cl-/Br-/I-Trihalomethanes during Chlorination/Chloramination Combined with a Theoretical Cytotoxicity Evaluation[END_REF]Zhang et al., 2016b). Considering the oxidation state of DOM, its partial pre-oxidation could change the production of DBPs. This has been shown in the case of UV/H2O2 pretreatment before chloramination for which the production of I-THM increased, due to the partial oxidation of the DOM, lowering its aromatic structure and forming small molecular weight DOM fractions [START_REF] Zhang | Formation of iodo-trihalomethanes (I-THMs) during disinfection with chlorine or chloramine: Impact of UV/H2O2 preoxidation[END_REF].

Role of pH

The predominance of main iodine reactive species (i.e. HOI and IO -) depends on pH as well as chlorine and bromine speciation but also chloramine stability. Thus, the formation and speciation of I-DBPs is influenced by pH. The formation rate of iodate during chlorination (or bromination) has an optimal pH at the mean of pKa values of chlorine (or bromine) and iodine reactive species [START_REF] Criquet | Iodate and Iodo-Trihalomethane Formation during Chlorination of Iodide-Containing Waters: Role of Bromide[END_REF],

i.e., 9.0 for chlorine and 9.6 for bromine. Therefore, the optimal pH for iodate formation lies between these two values and depends on the bromide level. This range of pH is also close to the optimal pH for the reactivity of phenolic moieties with HOX, and therefore potentially for the formation of I-DBPs (Figure 6).

It has been shown for chlorination that I-THM formation increases with increasing pHfrom 5.5 to 8.5 , specifically for low MW fractions of DOM (Zhang et al., 2016b).

Several studies reported that during chloramination the formation of I-THMs increases with increasing pH in the pH range 6-8.5 for source waters spiked with either iodide or iopamidol [START_REF] Duirk | Formation of Toxic Iodinated Disinfection By-Products from Compounds Used in Medical Imaging[END_REF] while TOI remained stable [START_REF] Hua | DBP formation during chlorination and chloramination: Effect of reaction time, pH, dosage, and temperature[END_REF]. However, only iodoform has been shown to increase with a pH increase, while mixed Cl-Br-I-DBPs decreased [START_REF] Wang | A comparison of iodinated trihalomethane formation from iodide and iopamidol in the presence of organic precursors during monochloramination[END_REF]J. Zhang et al., 2016b). The stability of monochloramine and the possible role of chlorine formed at lower pH has been hypothetised to explain these results [START_REF] Hua | DBP formation during chlorination and chloramination: Effect of reaction time, pH, dosage, and temperature[END_REF].

Finally, the pH also affects the stability of I-DBPs: similarly to TOCl or TOBr, TOI degradation -which occurs faster than for TOBr and TOCl -increases with both increasing pH and temperature, notably due to the dehalogenation of I-DBPs [START_REF] Rahman | Effect of pH and Temperature on Halogenated DBPs (Master of Science (MS))[END_REF][START_REF] Abusallout | Effect of temperature and pH on dehalogenation of total organic chlorine, bromine and iodine in drinking water[END_REF]. However, half-live times are quite high, t1/2 > 20 days under realistic conditions [START_REF] Rahman | Effect of pH and Temperature on Halogenated DBPs (Master of Science (MS))[END_REF]. Acidic conditions have been shown to also increase the decomposition of certain I-DBPs such as iodophenols (Hu et al., 2018b). S4) were used to take into account the formation of different oxidants (bromine and iodine)

with their pH-dependent speciation, and their reactivities towards phenol, which was chosen as a surrogate for natural organic matter. The second-order rate constants for the reaction of iodine with phenol determined by Bichsel and von Gunten (2000a) were used (Table 1), the reactivity of H2OI + with the phenolate species is negligible for the pH range of the modelling (pH 6.5 -9.0). The speciesspecific second-order rate constants allow to take into account the kinetics of the selected reactions as a function of the pH. For this purpose, DOM was simulated by considering the acid-base speciation of phenolic moieties. Substituted phenol structures are associated with fast reacting DOM moieities controlling the rapid initial HOX consumption phase [START_REF] Westerhoff | Reactivity of natural organic matter with aqueous chlorine and bromine[END_REF]. The slower phase for the reactions between HOI and other DOM moieties has not been taken into account in this modeling.

A concentration of 20 µg L -1 of iodide has been assumed with bromide concentrations of up to 400 µg L -1 corresponding to a bromide:iodide molar ratio of ≤ 32:1. 20% DOM-incorporation of halides has been considered to mimic the behaviour of chlorine, bromine and iodine reactive species with DOM [START_REF] Criquet | Reaction of bromine and chlorine with phenolic compounds and natural organic matter extracts -Electrophilic aromatic substitution and oxidation[END_REF]. The remaining 80% of the reaction of iodine reactive species with the organic moieties results in the release of iodide and the oxidation of the organic substance. Figure 7 shows the modelled relative formation of iodate as a function of the bromide concentration and the pH for a reaction time of 4000 s (Table S5). At low bromide levels, low levels of iodate are formed and most of the HOI formed during chlorination is incorporated into organic moieties. This is due to the orders of magnitude higher reactivity of HOI with phenolate compared to the oxidation of HOI to iodate by chlorine (note that 90% of iodide is converted either to iodate or bound to organic matter for pH > 7.0 (Table S5)). At higher bromide levels, the competition of HOI with HOBr for phenolic moieties leads to the formation of brominated phenolic compounds instead of iodinated compounds.

Therefore, the organic moieties become limiting and the residual HOI is either rapidly oxidised to iodate by the residual HOBr, or when HOBr is depleted, slowly oxidised to iodate by HOCl.

As previously discussed (section 5.4), the optimal pH either for the formation of iodate or reaction with phenolic moieties is > 9. However, the modeling shows that the decrease of pH favors the formation of iodate, independently of the bromide level. For the selected conditions (DOC of 5 mgC/L and a chlorine dose of 15 µM (i.e. about 1 mgCl2/L, Figure 7a), pH ≤ 7.0 and [Br -] ≥ 250 µg/L are needed for a ≥ 50 % conversion of iodide to iodate.

Increasing chlorine doses relative to the phenolic moieties favors the formation of iodate. For example at pH 7, the formation of iodate increases from 6 to 23% of the initial iodide concentration by increasing the chlorine dose from 15 to 50 µM in the absence of bromide, and from 68 to 90% in presence of 400 µg L -1 Br -.

Analysis and Occurrence of iodinated disinfection by-product classes in drinking water

Monitoring of individual I-DBPs

I-THMs were the first I-DBPs identified in drinking waters in 1976 [START_REF] Shackelford | Frequency of organic compounds identified in water[END_REF]. They were studied due to early concerns about medicinal taste and odours in drinking water [START_REF] Hansson | Iodoform taste complaints in chloramination[END_REF] with the lowest odour threshold of 0.03 µg L -1 for iodoform [START_REF] Cancho | Determination of the Odor Threshold Concentrations Of Iodinated Trihalomethanes in Drinking Water[END_REF].

The different classes of organic I-DBP can generally be grouped into two categories: aliphatic and aromatic compounds. To date, the reported aliphatic I-DBPs mainly comprise iodinated trihalomethanes (I-THMs), iodinated aliphatic acids, and iodinated haloacetamides. These compounds have quite divers physical chemical properties and Figure 8 provides an overview of appropriate methods for the detection of different target compounds. A majority of the first I-DBP species reported in drinking water were quantified by GC-MS (Hua and Reckhow, 2007a,b;[START_REF] Krasner | Occurrence of a New Generation of Disinfection Byproducts[END_REF][START_REF] Plewa | Chemical and Biological Characterization of Newly Discovered Iodoacid Drinking Water Disinfection Byproducts[END_REF]. Liquid chromatography has been also used subsequently to analyse haloacetamides (HAcAms) [START_REF] Chu | Trace determination of 13 haloacetamides in drinking water using liquid chromatography triple quadrupole mass spectrometry with atmospheric pressure chemical ionization[END_REF], different acids and iodophenols [START_REF] Ding | A Picture of Polar Iodinated Disinfection Byproducts in Drinking Water by (UPLC/)ESI-tqMS[END_REF]Pan et al., 2016a).

So far, more than 650 drinking waters were analysed for organic I-DBPs in more than 40 studies worldwide (Table S6). , 2018), China [START_REF] Ding | Occurrence, profiling and prioritization of halogenated disinfection by-products in drinking water of China[END_REF], Korea [START_REF] Woo | Determination of six iodotrihalomethanes in drinking water in Korea[END_REF], and Canada [START_REF] Tugulea | The influence of precursors and treatment process on the formation of Iodo-THMs in Canadian drinking water[END_REF] and the largest focusing mainly on chloramination in the US [START_REF] Richardson | Occurrence and Mammalian Cell Toxicity of Iodinated Disinfection Byproducts in Drinking Water[END_REF]. One third of the studies in literature show low levels of I-THMs (<1 µg L -1 , Figure S1 and Table S6), but a substantial number of studies showed larger concentrations of I-THMs, with a third with concentrations between 1 and 5 µg L -1 and 18% of the studies showing concentrations > 10 µg L -1 . The highest concentrations of I-THMs were found in North America and Pakistan, followed by China and UK (Figure S2).

The most frequently quantified I-THMs are CHCl2I followed by CHBrClI (Table S6 and Figure S3) with concentrations typically in the range 2 to 4 µg L -1 . In many laboratory studies on I-THM formation, CHI3 was found to be the major species. However, these studies were often conducted with low Br -/I -ratios and/or very high iodide concentrations, which favors the CHI3 formation. In contrast, CHI3 is the least occurring I-THM in most real water samples. Nevertheless, CHI3 has been recently reported as the main I-THM in several distribution systems (Wei et al., 2013a;[START_REF] Tugulea | Iodo-THM Formation During Chlorination of Source Waters With Naturally Occurring Ammonium and High Sodium Content, in: Disinfection By-Products in Drinking Water[END_REF][START_REF] Tugulea | The influence of precursors and treatment process on the formation of Iodo-THMs in Canadian drinking water[END_REF], usually accompanied by high total I-THM concentrations. These waters had particularly high concentrations of iodide (20 -130 µg L -1 ) and naturally occurring ammonium (0.6 to 3.3 mg L -1 ). The concentration of I-THMs were shown to be higher than the regulated THMs in these particular waters.

A tendency for higher I-THM concentrations during summer than during winter is generally observed [START_REF] Parsons | Study into the formation of disinfection by-products of chloramination, potential health implications and techniques for minimisation[END_REF][START_REF] Tugulea | The influence of precursors and treatment process on the formation of Iodo-THMs in Canadian drinking water[END_REF], which was related to rising iodide levels during summer in the different resources (surface and groundwaters). [START_REF] Weinberg | The Occurrence of Disinfection By-Products (DBPs) of Health Concern in Drinking Water: Results of a Nationwide DBP Occurrence Study[END_REF], also showed some treatment plant-specific seasonal trends with more I-THMs formed in waters with higher temperatures. But the lack of data on iodide concentrations in most source waters makes it impossible to distinguish between possible seasonal effects in temperature and variations in iodide levels.

A slight overall increase of I-THMs, limited compared to regulated THMs, has been observed in distribution systems either using chlorine or chloramines [START_REF] Tugulea | The influence of precursors and treatment process on the formation of Iodo-THMs in Canadian drinking water[END_REF]. A significant increase in the I-DBP concentrations can however be expected in distribution system, if iodide is not fully oxidized and/or already incorporated in the organic matrix during primary oxidation, especially considering a possible catalytic effect of pipe residues [START_REF] Liu | Chlorination of Iodide-Containing Waters in the Presence of CuO: Formation of Periodate[END_REF].

Other iodinated halomethanes have also been detected (although not necessarily quantified) in water:

chloroiodomethane was first reported in drinking water samples in the UK in the mid-1970's, but has not been often monitored since then [START_REF] Rumsby | REVIEW OF THE CURRENT TOXICOLOGICAL AND OCCURRENCE INFORMATION AVAILABLE ON IODINATED DISINFECTION BY-PRODUCTS[END_REF]. It has been reported both in chlorinated waters [START_REF] Khan | Incidence of chlorination byproducts in an institutional drinking water distribution network, Islamabad, Pakistan, using response surface methodology[END_REF] and in laboratory chloraminated source waters [START_REF] Postigo | Characterization of iodinated disinfection by-products in chlorinated and chloraminated waters using Orbitrap based gas chromatography-mass spectrometry[END_REF], Bromoiodomethane and diiodomethane were also identified in chloraminated water [START_REF] Symons | Factors Affecting Disinfection By-Product Formation During Chloramination[END_REF]. The formation of iodinated mono or di-halomethanes is not possible based on the so-called haloform reaction involved in the formation of THMs. In this reaction the formation of a halomethane depends on three consecutive halogen incorporations at a methyl group in α-position of a ketone [START_REF] Frimmel | Formation of Haloforms in Drinking Water[END_REF]. The reactivity of the halogenated intermediate compound is increased by the presence of 1 or 2 halogens. In addition, the carbanion formed (X3C -) after hydrolysis is stabalized by the inductive effect of the halogens and constitutes a good leaving group. Mono-or di-halogenated compounds are typically formed from degradation of THMs or other compounds like HAAs by hydrolysis or biodegradation. Chloroiodomethane, bromoidomethane and diiodomethane have been identified as degradation products of I-THMs in spiked river water [START_REF] Cancho | Determination, synthesis and survey of iodinated trihalomethanes in water treatment processes[END_REF].

Iodinated acids: haloacetic acids (I-HAAs) and others

In chlorinated and chloraminated drinking waters, four iodinated haloacetic acids (I-HAAs) were detected in different studies: (mono)iodoacetic acid (ICH2CO2H), chloroiodoacetic acid (ClICHCO2H), bromoiodoacetic acid (BrICHCO2H), and diiodoacetic acid (I2CHCO2H) [START_REF] Plewa | Chemical and Biological Characterization of Newly Discovered Iodoacid Drinking Water Disinfection Byproducts[END_REF][START_REF] Krasner | Occurrence of a New Generation of Disinfection Byproducts[END_REF][START_REF] Richardson | Occurrence and Mammalian Cell Toxicity of Iodinated Disinfection Byproducts in Drinking Water[END_REF][START_REF] Cuthbertson | GAC to BAC: Does it make chloraminated drinking water safer?[END_REF]. The reported concentrations were in the low ng L -1 to low μg L -1 range with maximum concentrations of 1.7 μg L -1 for iodoacetic acid and approximately 0.5 μg L -1 for chloro-and bromoiodoacetic acid. So far, diiodoacetic acid has only been observed at < 1 ng L -1 in drinking waters. Triiodoacetic acid has been reported in several laboratory studies. However, tri-HAAs generally decompose very fast in drinking water at ambient temperature [START_REF] Zhang | Decomposition of trihaloacetic acids and formation of the corresponding trihalomethanes in drinking water[END_REF], and it is unlikely to detect them in drinking water. The most frequently monitored I-HAA, until now, has been iodoacetic acid (ICH2CO2H) (Table S6). Other acids were identified in chloraminated drinking water in US nationwide DBP occurrence studies [START_REF] Weinberg | The Occurrence of Disinfection By-Products (DBPs) of Health Concern in Drinking Water: Results of a Nationwide DBP Occurrence Study[END_REF][START_REF] Krasner | Occurrence of a New Generation of Disinfection Byproducts[END_REF] with concentrations < 1 µg L -1 for 23 drinking water treatment plants [START_REF] Richardson | Occurrence and Mammalian Cell Toxicity of Iodinated Disinfection Byproducts in Drinking Water[END_REF](Table S6). Another iodoacid, 3,5-diiodosalicylic acid was detected at concentrations in the 0.17-1.48 ng L -1 range in 9 tap waters in China (Hu et al. 2018a).

Other iodinated disinfection by-products

Among the 4 surveys that monitored iodinated haloacetonitriles (I-HANs) in drinking waters (Table S6),

only [START_REF] Ding | Occurrence, profiling and prioritization of halogenated disinfection by-products in drinking water of China[END_REF] detected iodoacetonitrile (0.2-4.2 μg L -1 ) in only 4 out of 60 DWTPs applying chlorine in China. Considering acetamides, sampling of DWTPs showed that 3 out of 7 samples in China contained chloroiodoacetamide (<0.04 -0.21 µg L -1 ) and 3 out of 7 samples contained bromoiodoacetamide (<0.05 -0.15 µg L -1 ) [START_REF] Chu | Trace determination of 13 haloacetamides in drinking water using liquid chromatography triple quadrupole mass spectrometry with atmospheric pressure chemical ionization[END_REF].

Iodobutanal was the first iodinated haloaldehyde (I-HAL) detected in a survey carried out in the US [START_REF] Weinberg | The Occurrence of Disinfection By-Products (DBPs) of Health Concern in Drinking Water: Results of a Nationwide DBP Occurrence Study[END_REF][START_REF] Krasner | Occurrence of a New Generation of Disinfection Byproducts[END_REF]). Subsequently, iodoacetaldehyde was detected at concentrations of 0.62 to 4.5 µg L -1 in 4 US drinking water treatment plants applying chloramination, even for iodide concentrations below the detection limit of 5 µg L -1 in the source waters [START_REF] Jeong | Occurrence and Comparative Toxicity of Haloacetaldehyde Disinfection Byproducts in Drinking Water[END_REF]. While iodoacetaldehyde has not been further monitored in drinking water in other studies, disinfection experiments with river water have shown that its formation is possible during chlorination as well [START_REF] Postigo | Formation of iodo-trihalomethanes, iodohaloacetic acids, and haloacetaldehydes during chlorination and chloramination of iodine containing waters in laboratory controlled reactions[END_REF].

Various types of iodophenols have been quantified in tap water such as iodinated Ihydroxybenzaldehydes, I-hydroxybenzoic acids, and I-phenols [START_REF] Wuilloud | Determination of iodinated phenol species at parts-per-trillion concentration levels in different water samples by solid-phase microextraction/offline GC-ICP-MS[END_REF]Pan et al. 2016;Hu et al. 2018a). Their concentrations were < 1 ng L -1 in 18 Chinese tap waters (Pan et al. 2016;Hu et al. 2018a), except for 2,4-diiodo-6-nitrophenol, with a maximum concentration of 24 ng L -1 (Pan et al. 2016). Additionnally, 4 iodinated dipeptides (3-I-/3,5-di-I-Tyr-Ala and 3-I-/3,5-di-I-Tyr-Gly) have been identified in tap water samples [START_REF] Huang | Formation and Occurrence of Iodinated Tyrosyl Dipeptides in Disinfected Drinking Water[END_REF].

Total (organic) iodine, Iodide, iodate and hypoiodous acid

Total iodine comprises of iodide, iodine, iodate and organic iodine for which inductively coupled plasma -mass spectrometry (ICP-MS) demonstrated superior sensitivity in comparison to other detection techniques (Tables S6-S8). However, considering the fate of iodine in water treatment, speciation is recommended. More than 60 methods for the determination of iodide and/or iodate by ion chromatography with various detection methods with detection limits that are often quite high for iodide, typically around 10 µg L -1 , and even higher for iodate (Table S7). A postcolumn reaction has been shown to lower the detection limits to 1 µg/L for iodate with high selectivity by a post-column reaction forming I3 -which can be detected by a UV/Vis detector [START_REF] Salhi | Simultaneous determination of bromide, bromate and nitrite in low μg l-1 levels by ion chromatography without sample pretreatment[END_REF][START_REF] Weinberg | Post-Ion-Chromatography Derivatization for the Determination of Oxyhalides at Sub-PPB Levels in Drinking Water[END_REF]. Lately, HPLC or IC methods coupled to ICP-MS have been shown to be more sensitive, with detection limits of 0.2 µg L -1 (MacKeown et al., 2020) (Table S8). Other chromatographic techniques (Table S7) are time-consuming due to extensive sample pre-treatment, and suffer from possible interferences [START_REF] Zhang | A Novel Approach for the Simultaneous Determination of Iodide, Iodate and Organo-Iodide for 127I and 129I in Environmental Samples Using Gas Chromatography-Mass Spectrometry[END_REF]) but they could be interesting for complex matrices such as seawater. Iodide specific electrodes were also developed, but they are not very sensitive (≥ 10 µg L -1 ) [START_REF] Criquet | Iodate and Iodo-Trihalomethane Formation during Chlorination of Iodide-Containing Waters: Role of Bromide[END_REF].

Considering HOI, this species has been determined after reaction with phenol as the sum of o and piodophenol (Bichsel and von Gunten, 2000a;[START_REF] Shin | Reactions of Ferrate(VI) with Iodide and Hypoiodous Acid: Kinetics, Pathways, and Implications for the Fate of Iodine during Water Treatment[END_REF]. The DPD colorimetric method is also available, however, it is not specific. A compilation of methods for the total organic halogen including iodine (TOI) measurement has been published previously [START_REF] Chen | Methods for total organic halogen (TOX) analysis in water: Past, present, and future[END_REF][START_REF] Kristiana | The forest or the trees: a critical review on the analysis of total organic halogen (TOX) in drinking waters and its utility as a water quality parameter[END_REF]. In addition to the classical technics, TOI can also be determined by measuring total iodine and total inorganic iodine (iodide + iodate) separately and then subtract inorganic from total iodine [START_REF] Gilfedder | Iodine and bromine speciation in snow and the effect of orographically induced precipitation[END_REF]) (Table S9). However, the uncertainties of both analyses should be taken into account and the impact of potential interferences of the iodide analysis due to complexation by DOM needs more investigation to not overestimate the organic fraction by this approach [START_REF] Mackeown | Formation and removal of disinfection by-products in a full-scale drinking water treatment plant[END_REF].

Research needs

Occurrence studies for real waters are not (well) distributed globally (Figure S2 and Table S6). For example, Australia has some data available for iodide showing very high concentrations in raw waters (up to several hundreds of µg L -1 , [START_REF] Gruchlik | Impact of bromide and iodide during drinking water disinfection and potential treatment processes for their removal or mitigation[END_REF]. This may lead to potentially high I-DBP concentrations in the distributed waters. However, only data from 3 treated waters is reported in literature. In Europe, while not a priori an area rich in iodine, France shows a relatively high proportion of I-DBP occurrence (detection in 30% of the samples, (ANSES, 2018)). The highest concentrations of iodide occur in eastern Germany, The Netherlands and around the Mediterranean Sea [START_REF] Salminen | Geochemical Atlas of Europe. Part 1 -Background Information, Methodology and Maps[END_REF], where no or only little information on I-DBPS is available. In contrast, the absence of either chlorination or chloramination processes in e.g., The Netherlands is highly beneficial to mitigate I-DBPs formation [START_REF] Rosario-Ortiz | How do you like your tap water? : Safe drinking water may not need to contain a residual disinfectant[END_REF].

Overall, it remains difficult to compare occurrence and speciation of I-DBPs for different sites. Even for the most commonly reported I-THMs, many of the early studies did not measure all 6 I-THMs.

Furthermore, some of the early studies used analytical methods with higher quantification limits and/or a lack of standards -especially for other I-DBP classes than I-THMs -making it difficult to obtain occurrence trends. Significant improvement has been made since the early 2000s, in the past 10 years also for more polar I-DBP classes. Nevertheless, many of the parameters which greatly influence I-DBP formation were not systematically measured. These parameters should be monitored in future studies if I-DBP occurrence is to be correlated to water characteristics and treatment processes. Such studies would help to develop and validate prediction models which would be useful for water utilities to estimate formation potentials for I-DBPs and improve the overall risk assessments.

Risk assessment and mitigation

Toxicity of iodinated disinfection by-products and priorization

Although toxicological evidence from in vivo assays and molecular epidemiology studies for I-DBPs is still lacking [START_REF] Cortés | Genotoxicity of disinfection byproducts and disinfected waters: A review of recent literature[END_REF], different cytotoxicity and genototoxicity assays have been performed for various I-DBPs (Tables S10 andS11). Iodoacetic acid is the I-DBP which has been subjected to the highest number of different assays, allowing a comparison of hamster (CHO-AS52 and CHO-K1), mouse (NIH3T3, ovarian follicles), and human (HepG2, Caco-2, and CCD 841 CoN) cell lines (Table S10 andS11). The results show more than 2 orders of magnitude variability for cytotoxicity (Table S10), highlighting the difficulty in comparing different assays.

A compilation of 103 DBPs providing the lowest cytotoxic concentrations, LC50 values (lethal concentration for 50% of a population), and other indices with CHO cells concluded that I-DBPs are more cytotoxic and genotoxic than their brominated and chlorinated analogues [START_REF] Wagner | CHO cell cytotoxicity and genotoxicity analyses of disinfection by-products: An updated review[END_REF]. A more detailed discussion of the toxicity of each class of I-DBPs where the authors used mammalian cell cytotoxicity and genotoxicity data to provide a rank ordering of the relative toxicities is provided elsewhere [START_REF] Dong | Formation of Iodinated Disinfection Byproducts (I-DBPs) in Drinking Water: Emerging Concerns and Current Issues[END_REF].

In contrast, iodate is considered non-toxic because it is quickly reduced in vivo to I -and is therefore the desired sink for iodine during water treatment. In some countries it is even added to table salt as a prevention for goitre [START_REF] Bürgi | The toxicology of iodate: a review of the literature[END_REF].

To prioritize DBP classes, different methodologies have been proposed recently. A metric called "TIC-Tox" [START_REF] Plewa | TIC-Tox: A preliminary discussion on identifying the forcing agents of DBP-mediated toxicity of disinfected water[END_REF] compares measured DBP concentrations -or semi-quantitative GC peak areas -weighted by metrics of toxic potency (cytotoxicity or genotoxicity index value) to determine chemical drivers of overall toxicity in water [START_REF] Allard | Mechanistic Study on the Formation of Cl-/Br-/I-Trihalomethanes during Chlorination/Chloramination Combined with a Theoretical Cytotoxicity Evaluation[END_REF][START_REF] Chuang | Comparison of Toxicity-Weighted Disinfection Byproduct Concentrations in Potable Reuse Waters and Conventional Drinking Waters as a New Approach to Assessing the Quality of Advanced Treatment Train Waters[END_REF][START_REF] Dong | Formation of Iodinated Disinfection Byproducts (I-DBPs) in Drinking Water: Emerging Concerns and Current Issues[END_REF][START_REF] Jeong | Occurrence and Toxicity of Disinfection Byproducts in European Drinking Waters in Relation with the HIWATE Epidemiology Study[END_REF][START_REF] Li | Drinking Water Disinfection Byproducts (DBPs) and Human Health Effects: Multidisciplinary Challenges and Opportunities[END_REF][START_REF] Stalter | Mixture effects of drinking water disinfection by-products: Implications for risk assessment[END_REF]. Based on this methodology, iodoacetaldehyde, iodoacetonitrile as well as iodoacetic acid could be important I-DBPs due to their high toxicities and significant concentrations (up to µg L -1 range). Iodophenols and iodoacetamides have reportedly high toxicites, especially the three iodoacetamides (iodoacetamide, bromoiodoacetamide, chloroiodoacetamide), however, only few occurences studies included these compounds and their concentration were far below the µg L -1 range (section 6.1).

An indexing method based on 3 criteria (concentration level, reported frequency and toxicity) was also recently developed [START_REF] Mian | Prioritization of unregulated disinfection by-products in drinking water distribution systems for human health risk mitigation: A critical review[END_REF] to prioritize unregulated DBPs in three categories (i.e., high, medium, and low priorities). The I-DBP group was categorized as low priority compared to other DBP classes, but, only iodoacetic acid and iodoform were considered. An earlier study [START_REF] Hebert | Innovative method for prioritizing emerging disinfection by-products (DBPs) in drinking water on the basis of their potential impact on public health[END_REF] gave scores to DBPs based on 4 criteria (toxicity, occurrence, US-EPA and/or IARC classification, and regulation). This procedure allowed the identification and ranking of three different groups of emerging DBPs: (i) moderate occurrence and high toxicity; (ii) moderate to elevated occurrence and relevant toxicity and (iii) low occurrence and unknown or little toxicity. From this study, dichloroidomethane and iodoacetonitrile were ranked in group (i) with compounds such nitrosamines or trichlorophenol, which are considered the most critical compounds. I-THMs as well as I-AAs and I-HAcAMs were categorized in group (ii) mainly due to their toxicity.

There are some I-DBPs with no toxicity data as of yet, and more occurrence data is needed on I-DBPs in real drinking water systems to improve priorization of I-DBPs. Such priorization can provide useful information to develop control strategies to minimize human health risks resulting from the exposure of consumers to I-DBPs as compared to other DBPs, including the establishment of new drinking water standards.

Mitigation strategies

Prevention of formation

Precursor abatement prior to disinfection is a common strategy for DBP minimisation, in that it nonspecifically minimises all DBPs, both regulated and emerging [START_REF] Watson | Strategies for the removal of halides from drinking water sources, and their applicability in disinfection by-product minimisation: A critical review[END_REF]. As DOM is the major precursor for the generation of organic DBPs, DOM removal prior to disinfection is an effective option. However, when focusing on I-DBPs, there may be also a need for iodide removal if more impaired water resources are used for drinking water production.

Alternatively, the optimization of disinfection through the transformation of iodide to iodate, a safe iodine-derived end-product, has been proposed as a viable option. A pre-oxidation step with either ozone or ferrate(VI) before post-chlorination or especially post-chloramination to oxidize iodide to iodate is an efficient process to mitigate the formation of I-DBPs (Allard et al., 2013b, Zhang et al., 2016b, Wang et al., 2018[START_REF] Tian | Photochemical degradation of iodate by UV/H2O2 process: Kinetics, parameters and enhanced formation of iodo-trihalomethanes during chloramination[END_REF]. Iodide oxidation by ozone is fast enough to avoid excessive formation of bromate, a regulated DBP linked to ozonation of bromide-containing waters (Allard et al., 2013b).

Prechloration before chloramination also leads to a partial iodate formation, whereby reducing the subsequent formation of I-DBPs [START_REF] Bichsel | Oxidation of Iodide and Hypoiodous Acid in the Disinfection of Natural Waters[END_REF]. Therefore, if chloramine is used as a disinfectant, it is recommended to increase free chlorine contact time before ammonia addition to reduce iodo-DBP formation [START_REF] Richardson | Occurrence and Mammalian Cell Toxicity of Iodinated Disinfection Byproducts in Drinking Water[END_REF][START_REF] Jones | I-THM Formation and Speciation: Preformed Monochloramine versus Prechlorination Followed by Ammonia Addition[END_REF][START_REF] Criquet | Iodate and Iodo-Trihalomethane Formation during Chlorination of Iodide-Containing Waters: Role of Bromide[END_REF][START_REF] Allard | Mechanistic Study on the Formation of Cl-/Br-/I-Trihalomethanes during Chlorination/Chloramination Combined with a Theoretical Cytotoxicity Evaluation[END_REF]. However, the formation of I-DBPs, and especially their speciation, in the prechlorination/post-chloramination process depends strongly on the concentration of bromide [START_REF] Allard | Mechanistic Study on the Formation of Cl-/Br-/I-Trihalomethanes during Chlorination/Chloramination Combined with a Theoretical Cytotoxicity Evaluation[END_REF]. It was demonstrated that pre-chlorination minimized iodoform formation, however, short pre-chlorination contact times form higher I-THM concentrations compared to pre-formed NH2Cl due to a large increase in the formation of mixed Cl-Br-I-THM species [START_REF] Jones | I-THM Formation and Speciation: Preformed Monochloramine versus Prechlorination Followed by Ammonia Addition[END_REF][START_REF] Criquet | Iodate and Iodo-Trihalomethane Formation during Chlorination of Iodide-Containing Waters: Role of Bromide[END_REF][START_REF] Allard | Mechanistic Study on the Formation of Cl-/Br-/I-Trihalomethanes during Chlorination/Chloramination Combined with a Theoretical Cytotoxicity Evaluation[END_REF]Zhang et al., 2016b).

Removal of I-DBPs after their formation

Activated carbon adsorption after pre-or intermediate chlorination or pre-chloramination in full-scale drinking water treatment plants has been shown to be beneficial to mitigate DBP concentrations in drinking waters [START_REF] Cuthbertson | Does GAC with Chlorination Produce Safer Drinking Water? From DBPs and TOX to Calculated Toxicity[END_REF][START_REF] Cuthbertson | GAC to BAC: Does it make chloraminated drinking water safer?[END_REF][START_REF] Mackeown | Formation and removal of disinfection by-products in a full-scale drinking water treatment plant[END_REF]. I-THMs showed particurlarly high removal efficiency in full-scale water treatment plants [START_REF] Cancho | Determination, synthesis and survey of iodinated trihalomethanes in water treatment processes[END_REF][START_REF] Mackeown | Formation and removal of disinfection by-products in a full-scale drinking water treatment plant[END_REF]. I-THMs were removed by 77 to 99 % (average of 3 campaigns for each compound), the efficiency increasing with the molecular weight and hydrophobicity of THMs. Iodoacetic acid had a removal efficiency > 90% in GAC filtration (around 20 000 bed volumes). The removal of these compounds was much more efficient than for the THM4 on the same filters (from 15 to 58%) [START_REF] Mackeown | Formation and removal of disinfection by-products in a full-scale drinking water treatment plant[END_REF]. [START_REF] Cuthbertson | Does GAC with Chlorination Produce Safer Drinking Water? From DBPs and TOX to Calculated Toxicity[END_REF]2020) also demonstrated an overall decrease of TOI by (biological) activated carbon filtration and calculated geno-and cyto-toxicities. However, the assumptions were only based on a theoretical assessment of DBP related toxicities. Moreover, the assessment of different parameters such as GAC types, run time, age of filters and contact times would be needed to fully evaluate the potential of such a process.

Conclusion

The increasing demand on water resources will inevitably require increasing use of impaired water sources, containing higher concentrations of iodide/iodine especially in coastal areas. Reactive iodine species can be formed during oxidative water treatment of iodide-containing waters with various oxidants and they react with inorganic and organic compounds. HOI is generally the major reactive species for the reaction with dissolved organic matter. Iodine reactive species are similar to chlorine and bromine in terms of speciation, mechanism of reaction or binding to DOM. The few available second-order rate constants for the reactions of HOI with inorganic and organic compounds do not allow to draw final conclusions in terms of its reactivity, which however generally appears to be intermediate between chlorine and bromine. The largest difference consists of the further oxidation of HOI to iodate, which is a safe end-product for iodine. The formation of iodinated disinfection byproducts is favored when the oxidant is not able to effectively oxidize HOI to iodate (mainly monochloramine). The incorporation of iodine depends strongly on the oxidant type, I -, Br I-DBPs are known to have generally higher toxicities than their chlorinated and brominated analogues.

Iodoacetic acid and iodoacetamides exibit particularly strong cytotoxicity and iodoacetic acid is still the most genotoxic DBP identified to-date and is shown to impair the endocrine system.

Processes which favor the formation of iodate are most efficient in I-DBP mitigation. In particular controlled ozonation allows to form iodate without formation of bromate and a pre-chlorination contact time before chloramination has been demonstrated to be efficient to control I-DBP formation. Figure 2: Scheme of the main reactions involving iodine-containing species in various oxidative processes. The indicated times are half-life times for oxidant/reductant concentrations of 10 µM (see Table 1). 4). Dashed lines considering reactions 21-24 using rate constants from (Rose and Roberts, 2019) (Table 4). pKa(H2OI+/HOI) = -0.3 for both modelings. The contributions of each reaction are also shown as a function of the pH; apparent second-order rate constants calculated by multiplying the species specific second-order rate constant by the proportion of species for different pH. S3). Correlation log(kHOI/PhO-) = 6.5-6.2 ∑σ, R2 = 0.77. log(kHOBr/PhO-) = 7.8-3.5 ∑σ, R2 = 0.80 [START_REF] Heeb | Oxidative treatment of bromide-containing waters: Formation of bromine and its reactions with inorganic and organic compounds -A critical review[END_REF]; log(kHOCl/PhO-) = 4.15-3.0 ∑σ, R2 = 0.95 [START_REF] Gallard | Chlorination of natural organic matter: kinetics of chlorination and of THM formation[END_REF]. i Calculated using pKa (HOCl/OCl -) = 7.53 (Martell 1976); ii Calculated using pKa (HOBr/OBr -) = 8.8 [START_REF] Haag | Ozonation of bromide-containing waters: kinetics of formation of hypobromous acid and bromate[END_REF]); iii catalytic process not included [START_REF] Kern | Iodine Catalysis in the Chlorite-Iodide Reaction[END_REF]; iv Calculated using pKa (HFe VI O4 -/Fe VI O4 2-) = 7.23 [START_REF] Sharma | Oxidation of inorganic contaminants by ferrates (VI, V, and IV)-kinetics and mechanisms: A review[END_REF]); v Calculated using kapp = 7.0 M -1 s -1 [START_REF] Zhao | Kinetic and Mechanistic Aspects of the Reactions of Iodide and Hypoiodous Acid with Permanganate: Oxidation and Disproportionation[END_REF]); vi Calculated using kapp = 6.9 M -1 s -1 [START_REF] Zhao | Kinetic and Mechanistic Aspects of the Reactions of Iodide and Hypoiodous Acid with Permanganate: Oxidation and Disproportionation[END_REF]); vii kapp = 71 M -2 s -1 determined from [START_REF] Allard | Oxidation of iodide and iodine on birnessite (δ-MnO2) in the pH range 4-8 -ScienceDirect[END_REF]); viii Calculated using pKa (HSO5 -/SO5 2-) = 9.3; ix Calculated using pKa (H2O2/HO2 -) = 11.6

Figure 1

 1 Figure 1

Figure 4

 4 Figure 4

Figure 5

 5 Figure 5

Figure 6 5. 5 .

 65 Figure 6 5.5. Kinetic modeling to assess the effect of water quality parameters on I-DBP formation during

Figure 7

 7 Figure 7

  Figure 8 6.1.1. Iodinated trihalomethanes (I-THMs)

  -and DOM concentrations, DOM type, solution pH as well as on the ammonium concentration. Thus, disinfection practices intending to reduce THM4 levels may ultimately increase the formation of I-DBPs in the treated waters, especially in the case of chloramination. Enhanced detection methods will allow for detection of lower concentrations of I-DBPs in the sub ng L -1 range. Occurrence studies mainly detected I-THMs as well as several iodoacids. However, in recent years, more studies were carried out on nitrogenous I-DBPs as well as other polar I-DBPs. One of the main analytical difficulty comprises the quantification of a large number of I-DBPs from different classes with different physical-chemical properties and at low levels.

Figure captions : Figure 1 :

 :1 Figure captions :

Figure 3 :

 3 Figure 3: pH-dependent speciation of iodine (for a concentration of 1 µM I-). Calculations based on equilibria in Table2.

Figure 4 :

 4 Figure4: Apparent second order rate constants for the reactions of iodine with phenol as a function of pH. Solid lines considering reactions 21 and 23-24 using rate constants from(Bichsel and von Gunten, 2000a) (Table4). Dashed lines considering reactions 21-24 using rate constants from (Rose and Roberts, 2019) (Table4). pKa(H2OI+/HOI) = -0.3 for both modelings. The contributions of each reaction are also shown as a function of the pH; apparent second-order rate constants calculated by multiplying the species specific second-order rate constant by the proportion of species for different pH.

Figure 5 :

 5 Figure 5: Correlation between the second-order rate constants (log (k)) for the reaction of HOI with dissociated phenols and the Hammett constant (sigma, σ) (Data in TableS3). Correlation log(kHOI/PhO-) = 6.5-6.2 ∑σ, R2 = 0.77. log(kHOBr/PhO-) = 7.8-3.5 ∑σ, R2 = 0.80[START_REF] Heeb | Oxidative treatment of bromide-containing waters: Formation of bromine and its reactions with inorganic and organic compounds -A critical review[END_REF]; log(kHOCl/PhO-) = 4.15-3.0 ∑σ, R2 = 0.95[START_REF] Gallard | Chlorination of natural organic matter: kinetics of chlorination and of THM formation[END_REF].

Figure 6 .Figure 7 :

 67 Figure 6. Shematic representation of the influence of pH on I-DBP and iodate formation during chlorination and chloramination and I-DBP dehalogenation.

Figure 8 :

 8 Figure 8: Main classes of I-DBPs analysed in previous studies as a function of carbon and nitrogen atom contents and the corresponding analytical methods.

Table 1

 1

	Figure 2

Table 4

 4

Table 5

 5

Table 6

 6

Table 1 :

 1 Compilation of rate constants and half-life times for the oxidation of iodide and hypoiodous acid by various oxidants under typical drinking water treatment conditions (pH 7.0 and with 1980 an oxidant concentration of 10 μM).

	1981
	I -

Acknowledgments

This study was supported by Interreg 2 Seas program via DOC2C's project -contract No 2S01-013. The authors also thank the Region "Hauts-de-France" through the CPER Climibio for support. Dr. Aurélien Moncomble is thanked for fruitful discussions.

 (M -1 s -1) kHOI (M -1 s -1 ) kapp at pH 7 (M -1 s -1 ) References acetaldehyde (9 ± 1) x 10 -8 (9.0 ± 0.4) x 10 -1 5.1 x 10 -6 (Bichsel and von Gunten, 2000a) acetate < 2 x 10 -10 < 3 x 10 -6 < 1 x 10 -8 acetone (1.3 ± 0.4) x 10 -8 (2.5 ± 0.3) x 10 -1 7.