Zahra Wehbi

Rani Taher

Jalal Faraj

Cathy Castelain

Mahmoud Khaled

for Sustainable Development (EURACA) Hybrid Thermoelectric Generators-Renewable Energy Systems: A Short Review on Recent Developments

Keywords: Renewable Energy, Thermoelectric Generators (TEGs), Hybrid Systems

The development of different renewable energy techniques has shown their ability in limiting the environmental crisis and meeting future needs. In recent years many efforts have been made in order to produce hybrid systems that work on integrating renewable energy systems with thermoelectric generators (TEGs) in order to increase energy efficiency. This review attempts to discuss and summarize different configurations of hybridizing TEGs with different renewable energy sources (solar, fuel cell, biomass) underlying the concepts and approaches used to reach these hybridizations. This review will give necessary information about this type of hybridization and encourage for future research due to its promising results.

Introduction

Recently, reducing environmental problems resulting from high energy demand has captured the global attention for finding some strategies that compensate for the usage of fuel and decrease the level of pollution crisis [START_REF] Huneidi | Energy modeling and photovoltaics integration as a mitigation measure for climate change impacts on energy demand[END_REF]. Accordingly, alternatives such as heat recovery [START_REF] Ni | Thermodynamic and economic analysis of a novel cascade waste heat recovery system for solid oxide fuel cell[END_REF][START_REF] Hu | Analysis of energy, economy, and carbon emission of nested cascade refrigeration cycle with heat recovery system[END_REF], energy management [START_REF] Andrei | Knowledge demands for energy management in manufacturing industry-A systematic literature review[END_REF][START_REF] Si | Configuration optimization and energy management of hybrid energy system for marine using quantum computing[END_REF] and renewable energy [START_REF] Akarsu | Optimization of electricity and hydrogen production with hybrid renewable energy systems[END_REF][START_REF] Hassan | Integrated off-grid hybrid renewable energy system optimization based on economic, environmental, and social indicators for sustainable development[END_REF][START_REF] Kirim | Technical and economic analysis of biogas and solar photovoltaic (PV) hybrid renewable energy system for dairy cattle barns[END_REF] have been adopted by many institutions and researchers all over the world in order to reduce energy costs as well as environmental issues. The utilization of renewable energy technologies such as solar [START_REF] Vassiliades | Building integration of active solar energy systems: A review of geometrical and architectural characteristics[END_REF], wind [START_REF] Kiunke | Key factors influencing onshore wind energy development: A case study from the German North Sea region[END_REF], geothermal [START_REF] Pešić | Geothermal energy in Serbia-Current state, utilization and perspectives[END_REF], fuel cells [START_REF] Vichard | Hybrid fuel cell system degradation modeling methods: A comprehensive review[END_REF] … etc. as clean energy production is significantly increased in many countries. However, renewable energy sources are highly dependent on weather conditions like wind speed or solar irradiance [START_REF] Rahman | Environmental impact of renewable energy source based electrical power plants: Solar, wind, hydroelectric, biomass, geothermal, tidal, ocean, and osmotic[END_REF][START_REF] Xu | Optimal operation of geothermalsolar-wind renewables for community multi-energy supplies[END_REF]. To overcome the drawbacks of a single renewable energy source system and to increase its efficiency, coupling of various energy systems is introduced forming hybrid systems that are more reliable and environmentally friendly [START_REF] Song | Impacts of renewable portfolio standards on multi-market coupling trading of renewable energy in China: A scenario-based system dynamics model[END_REF]. In this context, the hybridization of thermoelectric generators (TEGs) with a conventional power generation system has been extensively introduced [START_REF] Dong | Energy generation and storing electrical energy in an energy hybrid system consisting of solar thermal collector, Stirling engine and thermoelectric generator[END_REF][START_REF] Farhani | Photovoltaic-thermoelectric (PV-TE) hybrid system for thermal energy harvesting in low-power sensors[END_REF].

Thermoelectric generators are solid-state semiconductor devices that can generate electricity due to temperature gradient using Seebeck effect [START_REF] Wen | Performance prediction of the concentrated hybrid power/thermal system incorporating thermoelectric generators[END_REF][START_REF] Tohidi | Thermoelectric Generators: A comprehensive review of characteristics and applications[END_REF]. These devices are characterized by their free gas emissions, simplicity in Fig. 1. Studies performed on different renewable energy systems integrated with TEGs for the past 10 years. maintenance with no moving parts which have the ability to generate power for a long-life span [START_REF] Alegria | Experimental development of a novel thermoelectric generator without moving parts to harness shallow hot dry rock fields[END_REF]. TEGs are widely used in waste heat recovery applications mainly that of photovoltaic cells where excess electricity can be generated and more than 23% efficiency enhancement might be reached [START_REF] Ma | Performance analysis of a new hybrid system composed of a concentrated photovoltaic cell and a two-stage thermoelectric generator[END_REF][START_REF] Ramadhani | Optimal heat recovery using photovoltaic thermal and thermoelectric generator for solid oxide fuel cell-based polygeneration system: Technoeconomic and environmental assessments[END_REF]. From the literature, several researchers have investigated methods in order to enhance the power and performance for renewable energy systems upon integrating these systems with thermoelectric generators (Fig 1). This paper presents a short review based on recent studies that focuses on the applications of different renewable energy systems with thermoelectric generators. The paper is divided as follows: section 2 describes the development and working principle of thermoelectric generators. Section 3 presents the performance of TEGs when integrated with several renewable energy systems and section 4 will be dedicated to conclusions and future perspectives.

Nomenclature

TEG

Thermoelectric generator FC Fuel cell ZT Figure of merit PV Photovoltaic

Thermoelectric Devices and their working principle

Thermoelectric technology is one of many green technologies which is able to convert heat into electricity and vice versa. Seebeck effect which goes back to 1821 and is based on potential generation across a certain load due to temperature difference between two junctions (hot and cold) is the fundamental concept for a thermoelectric generator. Until 1990s, thermoelectric devices were uncommonly used due to their low processing efficiency. However, renewed interest in thermoelectric devices has been introduced into research due to global warming alerts and the need of a sustainable, durable and reliable electric source [START_REF] Ge | Experimental study of thermoelectric generator with different numbers of modules for waste heat recovery[END_REF][START_REF] Lan | Performance comparison of a thermoelectric generator applied in conventional vehicles and extended-range electric vehicles[END_REF].

There are many types of thermoelectric materials that can be classified based on their thermal and electrical conductivities as well as seebeck coefficient such as BaUO3, CoSb3, SnSe, and many others. However, Bismuth Telluride (Bi2Te3) is the most commonly used one due to its high effectiveness at room temperature. The working principle of a thermoelectric device can be either as a power generator or as a thermoelectric cooler. These devices consist of flat arrays of P (charged holes h+) and N (charged e-) types that are connected electrically in series and thermally operating in parallel. As a power generator, the temperature difference causes the movement of both electrons and holes from the hot side to the cold side resulting in a potential difference that will drive current through the device when it is connected to an external load (see Fig 2). The temperature gradient between the two junctions of a thermoelectric device directly affects the output power which can be increased by the increase of this gradient and the efficiency of the device. The efficiency of a thermoelectric generator is given as the ratio of the power supplied to the external load by the heat absorbed at the hot junction [START_REF] Zhang | Integrating material engineering with module design optimization: A new design concept for thermoelectric generator[END_REF][START_REF] Ko | Design of a thermoelectric generator-assisted energy harvesting block considering melting temperature of phase change materials[END_REF][START_REF] Wen | Performance prediction of the concentrated hybrid power/thermal system incorporating thermoelectric generators[END_REF][START_REF] Fernández-Yáñez | Thermal management of thermoelectric generators for waste energy recovery[END_REF].

Thermoelectric cooler works opposite to thermoelectric generator using Peltier effect. In this case a direct current passing through the two junctions can produce or absorb heat based on the direction of the current as shown in Fig 3. This type is highly recommended upon controlling the temperature of a certain device or system however it is quite dependent on many parameters such as ambient temperature, thermoelectric properties and the current flow. Due to their ability to convert low amount of heat into electricity, optimizing a thermoelectric material in order to reach the best performance is crucial and highly dependent on the type of the material used. The performance of the TE device is estimated depending on its operating temperature and the figure of merit (ZT). Figure of merit depends on the seebeck effect, the thermal and electrical conductivity. Thus, improving the figure of merit and decreasing the thermal conductivity of a thermoelectric material can significantly enhance its performance. [START_REF] Aswal | Key issues in development of thermoelectric power generators: High figure-of-merit materials and their highly conducting interfaces with metallic interconnects[END_REF][START_REF] Giri | Challenges and strategies to optimize the figure of merit: Keeping eyes on thermoelectric metamaterials[END_REF][START_REF] Saini | An optimum thermoelectric figure of merit using Ge2Se2 monolayer: An ab-initio approach[END_REF][START_REF] Ghosh | Thermoelectric figure of merit enhancement in cement composites with graphene and transition metal oxides[END_REF]

Hybrid thermoelectric devices (TEG) -Solar photovoltaic systems

The solar renewable energy systems are the most widespread technologies due to high solar radiations available. To utilize the harnessed energy in useful applications, solar energy systems in its two versions solar thermal modules and photovoltaic cells (PV) are able to convert the solar radiation directly into thermal and electrical energy respectively. Solar photovoltaic system (PV) are excessively used and integrated into buildings due to their highpower density compared to other renewable energy technologies. In order to increase the conversion efficiency of a PV system, several cooling technologies that take into consideration the surface operating temperature have been utilized. Due to the fact that the rise in a PV surface temperature can drop its efficiency, thermoelectric modules are introduced in order to utilize the waste heat dissipated from the photons below or above the solar cell band-gap energy and convert it to electricity. [START_REF] Yang | Maximum efficiency and parametric optimum selection of a concentrated solar spectrum splitting photovoltaic cell-thermoelectric generator system[END_REF][START_REF] Shittu | Advancements in thermoelectric generators for enhanced hybrid photovoltaic system performance[END_REF][START_REF] Ko | Annual performance evaluation of thermoelectric generator-assisted building-integrated photovoltaic system with phase change material[END_REF] Recently, integrating a PV solar cell with a thermoelectric module is widely performed and studied, where, the overall output power of the hybrid system is the total sum of the power of the PV and TE modules [START_REF] Ebrahimi | Multi-criteria evaluation, and dynamic modeling of combining thermal photovoltaic and thermoelectric generators to extend electricity generation at night[END_REF][START_REF] Zhang | A hybrid system integrating photovoltaic module and thermoelectric devices for power and cooling cogeneration[END_REF]. To date, the hybridization of PV/TE systems can not only increase the efficiency conversion of a solar cell, but also helps in reducing the heating up of these cells and consequently increasing their life span. The methods of developing an optimized hybrid PV/TE system can be merged with other additional components like cooling systems and can be classified into two designs; spectrum splitting PV-TE and a combined PV-TE as shown in Fig. 4. As shown in Fig. 4 (a), when the sunlight is subjected to the two parallel modules (PV and TEG), the PV works to absorb the UV regions and convert it directly to electricity. As for underlying TEG module it is able to absorb the rest of the radiations (IR region) and heats up the hot side of the TEG device [START_REF] Cotfas | Critical factors and parameters for Hybrid Photovoltaic-Thermoelectric Systems; Review[END_REF]. A heat sink can be placed at the bottom side of the combined system to ensure a temperature gradient through the thermoelectric module. In Fig. 4. (b) the solar radiations are divided into two parts; the solar radiation below the cut-off wavelength is absorbed by the PV cell and radiations above this wavelength are reflected to the thermoelectric device (TEG). [START_REF] Mahmoudinezhad | Experimental investigation on spectrum beam splitting photovoltaic-thermoelectric generator under moderate solar concentrations[END_REF] Different approaches have been conducted by researchers for optimizing the PV-TE systems for a better performance. Studies take into consideration PV and TE parameters, additional systems (Heat sinks, cooling mediums or concentrators) in analysing any proposed system. Fig. 5 shows the increase in the overall efficiency in a hybrid PV-TEG system compared to a stand-alone PV system based on the same operating conditions. The efficiency increment shows that the enhancement in the overall efficiency may exceed 30% for a certain design or condition. The optimization and developments of a hybrid PV-TEG system is based on modifications and enhancements regarding materials and operating conditions and designs of both PV cells and TEG modules that can affect the conversion efficiency of the combined system. Table 1 shows the effect of each operating condition on the performance of the system.

TE Material

Depends on the figure of merits ZT.

Optimal ZT can increase the hybrid system overall efficiency.

PV structure

Nano-structure surface of a PV cell and cooling system integration are essential for a better performance. Nanofluid proved their ability to remove access heat more efficiently than water.

TE structure

Decrease in the thermoelectric leg length and surface area can increase the temperature gradient between the hot and cold side and consequently increase energy generation.

Concentration ratio

The overall efficiency varies with the increase of the CR depending on the PV-type used.

Solar irradiance

Its increase can increase the system power gain.

Ambient temperature

Conversion efficiency decrease with ambient temperature increase.

Power generation

The amount of output power of the hybrid system is relatively higher than stand-alone PV and TE modules.

Overall efficiency

Integrating the best way to couple the PV and TE modules for a better thermal resistance optimization based on the system's working conditions.

Cost

The hybrid PV-TE cost is significantly higher than a stand-alone PV. TE contributes the highest part of the total cost. Compared to the amount of energy generated by the TE device, the hybrid system is not economically feasible and needs more improvements.

Life cycle

Hybrid PV-TE increases the life span of the PV module.

Hybrid thermoelectric devices (TEG/TEC) -Fuel Cells

Fuel Cells

Fuel cells are electrochemical systems that use the chemical energy of the fuel in order to produce electricity and heat without any moving parts. These cells are characterized generally by their low emissions and high energy efficiency. A typical fuel cell consists of two electrodes, the anode and the cathode in addition to an electrolyte placed in between with high conductivity. The fuel oxidation (hydrogen) happens at the anode forming electrons flowing through the load circuit and releasing cations H+ from the reactants and flow out through the electrolyte to the cathode (oxygen), where the electrons reduction takes place. [START_REF] Le | Performance degradation in proton-conducting ceramic fuel cell and electrolyzer stacks[END_REF][START_REF] Aubry | Fuel cell diagnosis methods for embedded automotive applications[END_REF][START_REF] Li | Variational-principle-optimized porosity distribution in gas diffusion layer of high-temperature PEM fuel cells[END_REF][START_REF] Westenberger | FC Types: PEMFC/DMFC/AFC/PAFC//MCFC/SOFC[END_REF][START_REF] Zhang | A novel cooperative design with optimized flow field on bipolar plates and hybrid wettability gas diffusion layer for proton exchange membrane unitized regenerative fuel cell[END_REF] The electrochemical reaction occurring in the fuel cell results in water, heat and electricity production. As shown in Generally, all fuel cell types are clean and sustainable energy sources and they are highly advantageous due to fuel flexibility. However, fuel cells can generate a significant amount of waste heat during their operation, so managing the operating conditions such as temperature and efficiency, and reducing losses such as ohmic, concentration losses and many others in a fuel cell are critical approaches in several thermal applications or heat exchangers for enhancing energy production. [START_REF] Fu | Proton membrane fuel cell stack performance prediction through deep learning method[END_REF][START_REF] Raja | Numerical analysis of the influence of flow direction on heat distribution performance of a PEM fuel cell with serpentine flow field deign[END_REF] Hybridization of fuel cell with thermoelectric devices has been actually valid in 2010 where a modelledTEG-PEMFC system for recovering waste heat from the high temperature PEM fuel cell stack was presented. To support the co-design and co-optimization of the system a three-dimensional TEG module in ANSYS FLUENT has been implemented. In addition, an experimental validation was carried out in which the TEGs were installed at the exhaust pipe surface. However, this study didn't make a clear optimization of the performance or the operation of the system. Based on the reviewed state or arts, waste heat recovery, thermal management, efficiency improvements and cogeneration processes are main strategies that are studied within a fuel cell and thermoelectric device hybrid system Fig. 6. Schematic diagram and operating principle of a fuel cell in order to improve its function and increase energy savings. Tables 2 shows some studies based on the hybridization between fuel cells and thermoelectric generator systems. Table 1. Studies on the hybridization between TEG and fuel cells.

Reference

Hybridization

Research approach

[50] FC-TEG A 3-D finite element model developed in COMSOL Multiphysics that simulates both the FC and the TED subsystems. the proposed model can easily simulate the TED as both a thermoelectric generator (TEG) or as a Peltier device for cooling and heating.

[51] FC-TEG This study uses the thermoelectric device to further improve the exergetic and temperature performance of the conventional fuel cell based combined heat and power (FC-CHP) system.

[52] FC-TEG The hybrid system is superior to the stand-alone MCFC system as the bottoming TEG can effectively increase the maximum power density

[53]

MCFC-TEG A new MCFC-based hybrid systemto harvest the waste heat from MCFC for additional cooling production.the power density and efficiency of the proposed system is effectively increased by 3.4% and 4.0% comparing with the stand-alone MCFC.

[54] FC-TEG A thermoelectric device is proposed to recover the waste heat from a PAFC. Maximum power density and the corresponding efficiency are increased by over 2.0%.

[55] FC-TEG The equivalent maximum power density of the AFC can be increased by up to 23%. The effects of the operating current density, operating temperature, heat conductivity, and integrated parameter on the performance of the hybrid system are revealed.

[56] TEG-FC The system presents a thermal management system for a fuel cell who exploits the bi-directional characteristics of the TE device to achieve both temperature control and the possibility for energy harvesting when active control is not required.

Hybrid Thermoelectric devices -Biomass Energy systems

The chemical energy that results from the burning biomass is considered as a promising and reliable alternative for heat production which can be directly converted to different energy forms (electricity, heating, agricultural uses, etc). Biofuel significantly competes coal, petroleum fuel or nuclear fuel as a recycled energy source that can help in preventing or reducing some environmental problems. Biomass energy systems differs from other renewable energy systems when it comes to their high dependency on weather conditions and changes like solar or wind energy. However, biomass power plants encounter many challenges like costs, capacity or location. Studies on biomass energy systems are based on their design to energy conversion system about their configuration size in addition to their hybridizations with other renewable energy system. Table 3 shows recent studies on the hybridization of biomass systems and TEGs. Table 3. Recent studies on hybrid systems between biomass and TEGs.

References Integration of TEG with biomass energy systems

Research approach System performance [57]

The TEG module is placed between two stainless steel plates. At the bottom of the plate a heat gun supplies the heat flow. The top plate is cooled by water.

Testing the feasibility of a biomass power plant and a TE device for the best integration.

The performance of the TEG is enhanced with a better contact between its surfaces and the cooling and heating structures. A better compression rate leads to a higher power output and surface heat uniformity.

[58]

The TEG system is placed on the surface of a catalytic reactor, which is used for cleaning which serve as a heat source of hot junction on the TEG.

Examine the wasted heat that is recovered from a biomass gasifier.

The maximum and minimum conversion efficiencies of the TEG ranges between 10.9% and 2.8% The performance of the TEG shows a maximum power output of 6.1Wand a power density of 193.1 W/m 2 .

[59]

The hot side of the TEG device is inserted into the flue gas duct, whereas a fresh air duct is placed at the cold side.

The electric energy amount that could be recovered by a TEG module when integrated to a biomass fired thermal oil heater.

Analysing different thermoelectric materials it has been found that the best material for the temperature range 300-573 K is the p-type (Bi,Sb)2Te3 and n-type hot forged Bi2Te3.the maximum electrical power of the module is 4.4 W.

[60]

Adding the TE WHRSs at different locations of a system consisting of a gas turbine, ORC and domestic water heater.

Integrating a thermoelectric WHRS in a biomass power generation system for improving its performance.

The total output heat from the stack enter the WHRS the efficiency will improve by 0.35%, where it will increase by placing the WHRS at the condenser of the ORC. The operating parameters of the system significantly affects its performance.

Conclusions

Renewable energy systems and technologies have been developed in order to meet the worldwide energy demands. Introducing thermoelectric generators into renewable energy systems has been widely used in order to increase their efficiencies since they are highly relying on ambient conditions. Thermoelectric generators can be integrated with renewable energy sources like solar, biomass systems in order to implement a hybridized system with high efficiency. Further studies can be performed on the hybridization between thermoelectric generators and geothermal energy systems.

Fig. 2 .

 2 Fig. 2. Structure of a TEG based on Seebeck effect.

Fig. 3 .

 3 Fig. 3. Schematic for TE cooling or heating based on Peltier effect

Fig. 4 .Fig. 5 .

 45 Fig. 4. Schematic of the hybrid PV-TEG: (a) combined (PV-TEG) system; (b) spectrum splitting (PV-TEG) system

  Fig 6 the basic reactions taking place in a fuel cell model are as follows: At the anode: H2 → 2H++2e-At the cathode: 1/2O2 + 2H+ + 2e-→ H2O Overall reaction: H2+ 1/2 O2 → H2O + Heat +Electricity Five different categories of fuel cells are classified in the literature depending on the type of the electrolyte used: Alkaline fuel cell (AFC), Phosphoric acid fuel cell (AFC), Proton exchange membrane Fuel cell (PEMFC), Molten carbonate fuel cell (MCFC), Solid oxide fuel cell (SOFC) and direct fuel cell that uses liquid fuel such as methanol, ethanol, etc.


Yuan Gao

Wenxia Fan

Zhong Zhang

Yang Zhou

Zhu Zeng

Kai Yan

Jun Ma

Khalil Hanna

Transformation mechanisms of iopamidol by iron/sulfite systems: involvement of multiple reactive species and efficiency in real water

Keywords: Iodinated X-ray contrast media, sulfur radicals, Fe(IV), organic carbon

Although the ability of iron/sulfite system for decontamination purposes has been investigated, the complex reactive species generated and the underlying transformation mechanisms remain elusive. Here, we have comprehensively examined the transformation of iopamidol (IPM), a representative of iodinated X-ray contrast media, by iron catalyzed sulfite oxidation process under different water chemistry conditions.

Multiple reactive intermediates including Fe(IV), SO4 •-, and SO5 •-were identified by conducting a series of experiments. Eight transformation products were detected by mass spectrometry analysis, and correlation with the nature of involved reactive species has been made. Further, the transformation pathway including amide hydrolysis, deiodination, amino and hydroxyl groups oxidation was proposed. Interestingly, these transformation products could be removed through adsorption to iron precipitates formed via pH adjustment. Combining Fourier transform infrared spectroscopy and Xray photoelectron spectroscopy, we revealed an effective way to reduce the amount of transformation products in the treated solutions. Since the iron/sulfite process appears to be less sensitive to natural organic matter, it exhibited very good efficiency for IPM removal in real water samples, even with a high organic carbon loading. These findings may have strong implications in the development of novel oxidation process based on the sulfite/iron systems for wastewater treatment.

Introduction

Iodinated X-ray contrast media (ICM), used for medical imaging of internal organs, blood vessels and soft tissues, are the most widely administered intravascular pharmaceuticals with the worldwide consumption of 3.5 × 10 6 kg per year [START_REF] Duirk | Formation of Toxic Iodinated Disinfection By-Products from Compounds Used in Medical Imaging[END_REF]. This class of pharmaceuticals are specifically designed to be metabolically stable and almost completely excreted from the body via urine or faeces (Pé rez et al., 2006).

Due to their high solubility and poor biodegradability, conventional wastewater treatments are not effective in abating ICM, resulting in their occurrence in surface water and even drinking water ranging from ng L -1 to μg L -1 [START_REF] Kormos | Occurrence of Iodinated X-ray Contrast Media and Their Biotransformation Products in the Urban Water Cycle[END_REF][START_REF] Putschew | Occurrence of triiodinated X-ray contrast agents in the aquatic environment[END_REF][START_REF] Ternes | Occurrence and Behavior of X-ray Contrast Media in Sewage Facilities and the Aquatic Environment[END_REF][START_REF] Xu | Distribution and relevance of iodinated Xray contrast media and iodinated trihalomethanes in an aquatic environment[END_REF]. Iopamidol (IPM), a representative of ICM, is the most frequently detected at concentration level up to μg L -1 . Because of their high environmental concentrations and potential to leak through wastewater treatment plants and even reach drinking water, ICM has become chemicals of emerging concern to the public. Although ICM is believed not to be harmful, any subtle effects of mixtures, their metabolites, or other micropollutants are currently unknown and may pose ecological or human health issues [START_REF] Ens | Development, validation, and application of a novel LC-MS/MS trace analysis method for the simultaneous quantification of seven iodinated X-ray contrast media and three artificial sweeteners in surface, ground, and drinking water[END_REF].

Considering their low biodegradability, ICM may also accumulate in the environment. Also, ICM might serve as the iodine source promoting the formation of iodinated byproducts [START_REF] Duirk | Formation of Toxic Iodinated Disinfection By-Products from Compounds Used in Medical Imaging[END_REF][START_REF] Pantelaki | Formation of iodinated THMs during chlorination of water and wastewater in the presence of different iodine sources[END_REF]. For this reason, removal of ICM in water has been the subject of numerous studies [START_REF] Cao | Sulfite enhanced transformation of iopamidol by UV photolysis in the presence of oxygen: Role of oxysulfur radicals[END_REF][START_REF] Chan | Degradation of iopromide by combined UV irradiation and peroxydisulfate[END_REF][START_REF] Jeong | Degradation mechanisms and kinetic studies for the treatment of X-ray contrast media compounds by advanced oxidation/reduction processes[END_REF][START_REF] Mao | Accelerated oxidation of iopamidol by ozone/peroxymonosulfate (O3/PMS) process: Kinetics, mechanism, and simultaneous reduction of iodinated disinfection by-product formation potential[END_REF][START_REF] Ternes | Ozonation: a tool for removal of pharmaceuticals, contrast media and musk fragrances from wastewater?[END_REF][START_REF] Tian | Chlor(am)ination of iopamidol: Kinetics, pathways and disinfection byproducts formation[END_REF][START_REF] Wendel | Transformation of Iopamidol during Chlorination[END_REF][START_REF] Zhao | Degradation of iopamidol by three UV-based oxidation processes: Kinetics, pathways, and formation of iodinated disinfection byproducts[END_REF].

Recently, ozone, chlorine, and chlorine dioxide were used as selective oxidant to remove ICM, which showed very limited efficiency with the maximum rate constant up the order of 10 M -1 s -1 [START_REF] Ternes | Ozonation: a tool for removal of pharmaceuticals, contrast media and musk fragrances from wastewater?[END_REF][START_REF] Tian | Chlor(am)ination of iopamidol: Kinetics, pathways and disinfection byproducts formation[END_REF][START_REF] Wendel | Transformation of Iopamidol during Chlorination[END_REF].

However, hydroxyl (•OH) and sulfate radicals (SO4 •-) generated from UV/H2O2, O3/H2O2 or O3/PMS systems were found to be able to transform ICM compounds with the reaction rate constant of ~10 9 M -1 s -1 [START_REF] Chan | Degradation of iopromide by combined UV irradiation and peroxydisulfate[END_REF][START_REF] Jeong | Degradation mechanisms and kinetic studies for the treatment of X-ray contrast media compounds by advanced oxidation/reduction processes[END_REF][START_REF] Mao | Accelerated oxidation of iopamidol by ozone/peroxymonosulfate (O3/PMS) process: Kinetics, mechanism, and simultaneous reduction of iodinated disinfection by-product formation potential[END_REF]. As compared to OH, SO4 •-has been reported to be less influenced by natural organic matter in real water matrix [START_REF] Gara | Kinetics of the sulfate radical-mediated photo-oxidation of humic substances[END_REF][START_REF] Lutze | Degradation of Chlorotriazine Pesticides by Sulfate Radicals and the Influence of Organic Matter[END_REF]. Thus, sulfate radical based oxidation technologies could be considered as promising for ICM removal in wastewater.

Transition metal catalyzed sulfite is one of the possibilities for SO4 •-generation through chain propagation reactions [START_REF] Brandt | Kinetics and Mechanism of the Iron(III)-catalyzed Autoxidation of Sulfur(IV) Oxides in Aqueous Solution. Evidence for the Redox Cycling of Iron in the Presence of Oxygen and Modeling of the Overall Reaction Mechanism[END_REF][START_REF] Yermakov | Iron-Catalyzed Oxidation of Sulfite: From Established Results to a New Understanding[END_REF].

As compared to persulfates (peroxydisulfate and peroxymonosulfate), commonly employed as SO4 •-precursor, sulfite takes the advantage of lower toxicity and competitive price. For instance, the LD50 (oral, rat) of sulfite is 3560 mg/kg, three times higher than that of peroxydisulfate (802 mg/kg) (Zhou et al., 2018a). Among various transition metals, iron is the most widely used one for environmental application because of its cost effectiveness and environmentally friendly nature. This ironcatalyzed sulfite oxidation process in the atmospheric liquid phase has been extensively investigated due to its implication on the generation of acid rain [START_REF] Brandt | Transition Metal-Catalyzed Oxidation of Sulfur(IV) Oxides. Atmospheric-Relevant Processes and Mechanisms[END_REF]. Recently, the iron/sulfite system has been investigated for water contaminants removal (e.g., dyes, aniline, and carbamazepine) [START_REF] Chen | Decolorization of Orange II in Aqueous Solution by an Fe(II)/sulfite System: Replacement of Persulfate[END_REF][START_REF] Dong | Mechanistic insight into the generation of reactive oxygen species in sulfite activation with Fe(III) for contaminants degradation[END_REF][START_REF] Yu | Efficient oxidation of bisphenol A with oxysulfur radicals generated by iron-catalyzed autoxidation of sulfite at circumneutral pH under UV irradiation[END_REF][START_REF] Yuan | Enhanced oxidation of aniline using Fe(III)-S(IV) system: Role of different oxysulfur radicals[END_REF]. In these studies, multiple reactive intermediates including SO 4 •-, •OH and peroxornonosulfate radical (SO 5 •-) were proposed to be the candidates of potential oxidant. For instance, [START_REF] Chen | Decolorization of Orange II in Aqueous Solution by an Fe(II)/sulfite System: Replacement of Persulfate[END_REF] reported that the efficient decolorization of orange II in the Fe(II)/sulfite system was mainly attributed to the contribution of SO4 •-, while •OH and SO5 •-contribution were considered as minor. Other reports showed that the combination of Fe(II) or Fe(III) and sulfite could generated SO5 •-, SO4 •-or •OH, and then effectively remove organic contaminants [START_REF] Dong | Mechanistic insight into the generation of reactive oxygen species in sulfite activation with Fe(III) for contaminants degradation[END_REF][START_REF] Yuan | Enhanced oxidation of aniline using Fe(III)-S(IV) system: Role of different oxysulfur radicals[END_REF]. Although the good ability of the iron/sulfite system for contamination removal has been investigated, the underlying mechanisms of generation of reactive species and target compound degradation remain elusive. In addition, very little is known about the application of this process for ICM transformation, and much less on the influence of key factors (e.g., pH and water matrix) on the removal performance.

In this work, the performance and mechanism of the iron/sulfite system for the degradation of IPM were investigated for the first time. The primary objectives of the present work were to (i) assess the oxidation efficiency of the iron/sulfite system in IPM transformation; (ii) identify the reactive intermediates responsible for IPM degradation; (iii) elucidate the reaction mechanism based on the identified oxidation products; (iv) evaluate the influence of pH adjustment of the resulting solutions and water matrix on the performance of the iron/sulfite system.

Experimental Section

Materials

All chemicals were of analytical purity or higher and used without further purification unless otherwise stated. IPM (98%) was obtained from Macklin Biochemical Co., Ltd. Sodium sulfite (Na2SO3; AR, ≥97.0%) and ethanol (AR, ≥ 95.0%) were purchased from Chemical Reagent Co. Ltd. Iron nitrate nonahydrate (Fe(NO3)3 • 9H2O, AR), ferrous sulfate heptahydrate (FeSO4•7H2O; AR, ≥ 99.0%), and tert-butyl alcohol (TBA >99.5%) were purchased from Aladdin Biochemical Co., Ltd.

Peroxymonosulfate (PMS, available as Oxone), methyl phenyl sulfoxide (PMSO, ≥ 97%) and methyl phenyl sulfone (PMSO2; GC, ≥ 98%), humic acid (HA) and acetic acid for HPLC were obtained from Sigma-Aldrich. Methanol and acetonitrile of chromatographic grad were obtained from Merck KGaA. All solutions were prepared using deionized (DI) water (18.2 MΩ•cm) from a Millipore purification system. The stock solutions of HA were purified by repeated pH adjustment, filtration, and precipitation. Stock solutions of sulfite and ferrous sulfate were freshly prepared before experiments. Real water samples were taken from the filtered water of secondary effluent of Lijiao Wastewater Treatment Plant (located in Haizhu district of Guangzhou in China; Dissolved organic carbon (DOC) of 5.6 mg L -1 , UV254 absorbance of 0.075 cm -1 , Cl -of 1.8 mM and pH of 7.3) and central lake in Guangzhou (GPS coordinate of 23°03'07.3"N/113°23'21.8"E; DOC of 12.6 mg L -1 , UV254 absorbance of 0.083 cm -1 , Cl -of 1.6 mM and pH of 6.8). After filtering through glass fiber membranes, these water samples were stored at 4 °C.

Experimental procedure

Batch experiments were conducted in open beakers with 100 mL capacity under magnetic stirring. To initiate the reactions, predetermined amounts of iron ion (50 μM) and sulfite (300 μM) were simultaneously added into stirring solutions containing IPM and/or constituents (e.g., Cl -and humic acid) of interest at desirable concentrations. At specific time intervals, reaction solution was sampled and quenched by excess methanol for the following measurement of IPM or the analysis of transformation products.

Experiments in real waters were conducted following the similar procedure as that in synthetic solutions except that a relatively low concentration of IPM (500 nM) was used to simulate environmental levels.

Unless otherwise stated, the experiments in this study were conducted at controlled temperature (25 ± 1 °C) in ambient atmosphere. The solution pH was adjusted with HClO4 and NaOH. To ensure reproducibility, the experiments were conducted in triplicates with results presented as the average data and the relative standard deviations were also presented.

Analytical Methods

IPM was quantified by using a Waters Alliance e2695 HPLC. The separation was achieved by a Waters XBridge C18 column (4.6×250 mm, 5μm particle size) and the mobile phase consisted of 0.1% acetic acid and methanol with a ratio of 90:10 (v/v) at a flow rate of 0.5 mL/min. The wavelength for IPM was set at 242 nm. PMSO and PMSO2 were analyzed by HPLC/UV at 230 and 215 nm, with the eluent of 0.1% acetic acid and acetonitrile at a flow rate of 1 mL/min. A Sciex QTrap 5500 MS with an ESI source was used for products identification and quantification. The MS instrumental parameters were provided in Text S1. Electron paramagnetic resonance (EPR) analysis using DMPO as a spin-trapping agent was performed on a Bruker A200 spectrometer, and the detailed parameters were shown in Text S2. Fourier transform infrared spectroscopy (FTIR) analysis was conducted on a Shimadzu IRTracer-100 and the samples was prepared by vacuum filtration via 0.45 μm filters. X-ray photoelectron spectrometer (XPS) was conducted on a Thermo Fisher Escalab 250Xi and the samples was prepared by vacuum filtration via 0.45 μm filters followed by vacuum drying. The DOC content of HA stock solution was determined using Shimadzu TOC-L. Solution pH was measured with Leici PHS-3C pH-meter (Shanghai INESA Scientific Instrument Co.Ltd). Dissolved oxygen was detected using Hach HQ40d Portable Meters. Absorbance was measured by Hach DR6000 Spectrophotometer.

Results and Discussion

Degradation of IPM by iron/sulfite

Figure 1 comparatively showed the degradation of IPM in the Fe/sulfite as well as Fe(II)/persulfates (PDS or PMS). The combination of Fe(III) (50 μM) and sulfite (300 μM) led to an efficient abatement of IPM (5 μM) at pHini 4, where ~85% removal of IPM was achieved. Interestingly, Fe(II)/sulfite exhibited a similar reactivity toward IPM removal. Comparatively, Fe(II)/PMS and Fe(II)/PDS systems only achieved ~40% removal under the same conditions. Similar findings were obtained in the case of diatrizoate, a relatively refractory ICM. The removal percentage of diatrizoate in the iron/sulfite system (~50%) was at least more than twice that in the Fe(II)/PMS and Fe(II)/PDS systems (<25%) (Fig. 1). These findings indicated the higher oxidation efficiency of iron/sulfite system in ICM abatement as compared to commonly used iron-based Fenton-like AOPs. Further, the effect of pH ini on the performance of Fe(III)/sulfite system was examined.

It was found that efficiency of Fe(III)/sulfite system increased first as pHini increased from 3 to 4, and then decreased when pHini further increased and became negligible at higher pH values (7 and 9) (Fig. S1a). The pH variation was also measured and the solution pH at the end of reaction (pHfinal) was shown in Figure S1b. To evaluate the reaction kinetics at different pHini, the initial reaction rates kini over the first reaction time (within 1 min) was determined. The k ini value first increased from 0.80 to 1.23 min -1 as pH ini increased from 3 to 4, and then decreased to 0.08 min -1 as pH further increased to pHini 9. This could be probably due to the combined effect of pH on the distribution of sulfite species and iron speciation in solution [START_REF] Guo | Novel Photo-Sulfite System: Toward Simultaneous Transformations of Inorganic and Organic Pollutants[END_REF]. Acidic pH may facilitate the formation of sulfur dioxide, thus decreasing the effective concentration of HSO3 -. At higher pH, iron precipitation takes place, thereby decreasing the dissolved iron concentration available for sulfite activation.

Identification of reactive radical species

Preliminary experiments showed that negligible degradation of IPM was observed in the case of Fe(III) or sulfite alone (Fig. S2). Therefore, Fe(III)-induced activation of sulfite is likely to occur for the generation of reactive species. A radical mechanism has been well reported for the complex reaction of Fe(III) catalyzed sulfite auto-oxidation (eqs. 1-12) [START_REF] Brandt | Kinetics and Mechanism of the Iron(III)-catalyzed Autoxidation of Sulfur(IV) Oxides in Aqueous Solution. Evidence for the Redox Cycling of Iron in the Presence of Oxygen and Modeling of the Overall Reaction Mechanism[END_REF]. The fast formation of Fe(III)-sulfito complex (KFe(III)sulfito = 600) and its subsequent decomposition to Fe(II) and SO3 •-is considered as the core of the reactions (eqs. 1-2), which participate to the generation of SO3 •-as the initiation step of catalytic sulfite oxidation. In the present work, the degradation of IPM was completely inhibited with the addition of EDTA and pyrophosphate, stronger chelators for iron (e.g., KFe(III)-EDTA = 10 24.23 ) (Fig. S3). Then, the generated SO3 •-may combine quickly with oxygen (O2) leading to the formation of SO5 •-(eq. 3). This reactive species may oxidize either sulfite or Fe(II) resulting in the production of sulfurcontaining intermediates including SO4 •-, SO3 •-and HSO5 -(peroxymonosulfate ion) (eqs. 4-6). These intermediates could further induce several redox processes such as oxidizing sulfite or Fe(II) (eqs. 7-12). This was consistent with the evolution of sulfite (continuous degradation) and Fe(II) (initial increase concentration and then decrease) observed during the reaction in the present study (Fig. S4). In addition, •OH might be generated from the reaction of SO4 •-and H2O [START_REF] Neta | Rate Constants for Reactions of Inorganic Radicals in Aqueous Solution[END_REF].

Fe 3+ +HSO 3 -K=600 ↔ FeSO 3 + +H + (1) FeSO 3 + →Fe 2+ +SO 3 - (2) SO 3 -+O 2 →SO 5 - (3) SO 5 -+HSO 3 -→SO 4 -+SO 4 2-+H + (4) SO 5 -+HSO 3 -→SO 3 -+HSO 5 - (5) SO 5 -+Fe 2+ →Fe 3+ +HSO 5 - (6) HSO 5 -+HSO 3 -→SO 3 -+SO 4 -+H 2 O (7) HSO 5 -+HSO 3 -→+2SO 4 2-+2H 2 O (8) HSO 5 -+Fe 2+ →Fe 3+ +SO 4 -+OH - (9) SO 4 -+HSO 3 -→SO 3 -+SO 4 2-+H + (10) SO 4 -+Fe 2+ →Fe 3+ +SO 4 2- (11) SO 4 -+H 2 O→•OH+SO 4 2-+H + (12)
In the case of Fe(II), a rapid formation of FeSO 3 + is possible (eqs. 13-14) [START_REF] Chen | Decolorization of Orange II in Aqueous Solution by an Fe(II)/sulfite System: Replacement of Persulfate[END_REF][START_REF] Guo | Novel Photo-Sulfite System: Toward Simultaneous Transformations of Inorganic and Organic Pollutants[END_REF], which would in turn follow the same reaction chain (eqs.

2-12). Therefore, the same reactive species should be involved in Fe(II)/sulfite and Fe(III)/sulfite systems, thereby explaining their similar performance in IPM degradation as observed in Figure 1a.

Fe 2+ +HSO 3 -↔FeHSO 3 (13) 4FeHSO 3 +O 2 →4FeSO 3 + +2H 2 O (14)

Role of SO4 •-and •OH in IPM degradation

According to eqs. 1-12, SO4 •-and •OH were usually considered as the primary reactive species responsible for contaminant degradation in the iron ions/sulfite system [START_REF] Chen | Decolorization of Orange II in Aqueous Solution by an Fe(II)/sulfite System: Replacement of Persulfate[END_REF][START_REF] Dong | Mechanistic insight into the generation of reactive oxygen species in sulfite activation with Fe(III) for contaminants degradation[END_REF]. To gain further insights into the role of SO4 and •OH with the rate constants of 1.6-7.7 × 10 8 M -1 s -1 and 1.2-2.8 × 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/⋅Oin Aqueous Solution[END_REF][START_REF] Neta | Rate Constants for Reactions of Inorganic Radicals in Aqueous Solution[END_REF]. TBA could effectively scavenge •OH with the rate 3.8-7.6 × 10 8 M -1 s -1 , which was about 3 orders of magnitude higher than the rate constant for the reaction of TBA and SO4 •-(4.0-9.1 × 10 5 M -1 s -1 ) [START_REF] Buxton | Critical Review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals (⋅OH/⋅Oin Aqueous Solution[END_REF][START_REF] Neta | Rate Constants for Reactions of Inorganic Radicals in Aqueous Solution[END_REF]. The rate constants for the reaction of IPM with •OH and SO4 •-are 3.4 × 10 9 M -1 s -1 and 1.6 × 10 9 M -1 s -1 as reported in literature [START_REF] Jeong | Degradation mechanisms and kinetic studies for the treatment of X-ray contrast media compounds by advanced oxidation/reduction processes[END_REF][START_REF] Mao | Accelerated oxidation of iopamidol by ozone/peroxymonosulfate (O3/PMS) process: Kinetics, mechanism, and simultaneous reduction of iodinated disinfection by-product formation potential[END_REF].

Theoretically, the addition of 5 mM TBA or 50 mM ethanol could almost completely suppress the degradation of IPM (5 μM) by

•OH or SO4 •-( k OH • -TBA [TBA] > 100k OH • -IPM [IPM] , k SO 4 --ethanol [ethanol] > 100k SO 4 --IPM [IPM] ; details in Text S3). As
shown in Figure 2a, 5 mM TBA slightly affected the degradation of IPM, excluding the involvement of •OH. This might be due to the slow formation rate of SO4 •-from the reaction of •OH with water (<6 M -1 s -1 , eq. 12). This finding was also confirmed by the EPR analysis where characteristic spectrum of the hydroxyl radical was not observed in the Fe(III)/sulfite system (Fig. 2b). Comparatively, ethanol considerably suppressed the degradation of IPM, suggesting the possible involvement of SO4 •-.

Although the signal of DMPO/SO 4 •-was not observed, the characteristic spectrum of EPR analysis for the DMPO/SO 3 •-adduct (α N = 14.7 G, α H = 15.9 G) was detected (Fig. 2b) [START_REF] Mottley | Sulfate anion free radical formation by the peroxidation of (Bi)sulfite and its reaction with hydroxyl radical scavengers[END_REF]. The absence of DMPO/SO 4 •-signal might be due to the high concentration of DMPO, which could completely trap SO 3 •-and thus inhibited SO 4

•-generation (eqs 3-9) [START_REF] Yuan | A simple Cr(VI)-S(IV)-O2 system for rapid and simultaneous reduction of Cr(VI) and oxidative degradation of organic pollutants[END_REF]. Interestingly, ~15% IPM removal was still achieved in the presence of 50 mM ethanol, suggesting that other reactive species could be involved in the Fe(III)/sulfite system.

Role of Fe(IV) in IPM degradation

In previous studies, the reaction of HSO5 -(i.e., monoanion form of PMS) and Fe(II) depicted as eq. 9 was usually considered as the main source for SO4 •-in the Fe(III)/sulfite system [START_REF] Chen | Decolorization of Orange II in Aqueous Solution by an Fe(II)/sulfite System: Replacement of Persulfate[END_REF][START_REF] Dong | Mechanistic insight into the generation of reactive oxygen species in sulfite activation with Fe(III) for contaminants degradation[END_REF]. However, Fe(IV) has been reported to be the primary oxidant in the Fe(II)/PMS, generated from Fe(II) and HSO5 -, in a recent work (Wang et al. 2018) (eq. 15). This Fe(IV) generation has been proposed based on conversion reaction of methyl phenyl sulfoxide (PMSO) to methyl phenyl sulfone (PMSO2) via oxygen electron transfer. Comparatively, PMSO2 was not generated in the case of free radicals [START_REF] Wang | Is Sulfate Radical Really Generated from Peroxydisulfate Activated by Iron(II) for Environmental Decontamination?[END_REF]. Thus, it is quite plausible that Fe(IV) was involved in the Fe(III)/sulfite system generated from eq. 15. In order to verify this possibility, the degradation of methyl phenyl sulfoxide (PMSO) in the Fe(III)/sulfite system was examined. As expected, the Fe(IV) is known to serve as an oxidant with considerable selectivity. For instance, the rate constants for the reaction of Fe(IV) with phenol and dimethyl sulfoxide (DMSO)

were reported to be k phenol = 4 × 10 3 M -1 s -1 and k DMSO = 1.26 × 10 5 M -1 s -1 , respectively [START_REF] Jacobsen | Reactions of the ferryl ion with some compounds found in cloud water[END_REF]Má rtire et al., 2002). To evaluate the oxidizing ability of Fe(IV) toward IPM, the degradation of IPM in the Fe(II)/PMS and Fe(II)/HOCl systems was examined, wherein Fe(IV) has been reported to be the primary oxidant (Fig. 3 and Fig. S5). Significant IPM degradation was observed suggesting that Fe(IV) could contribute to IPM removal in the Fe(III)/sulfite system.

Interestingly, 50 mM ethanol was found to completely inhibit the degradation of IPM in the Fe(II)/PMS system (Fig. 3), underscoring the role of ethanol as an effective scavenger for Fe(IV). This was also consistent with previous work on the inhibitory effect of ethanol on the Fe(II)/ozone and Fe(II)/HOCl systems, where Fe(IV) served as the main reactive intermediates [START_REF] Liang | Fe2+/HClO Reaction Produces FeIVO2+: An Enhanced Advanced Oxidation Process[END_REF][START_REF] Pestovsky | Reactivity of Aqueous Fe(IV) in Hydride and Hydrogen Atom Transfer Reactions[END_REF].

However, partial inhibition was observed in the Fe(III)/sulfite system upon addition of 50 mM ethanol (Fig. 2a), suggesting that other reactive intermediates, in addition to SO4 •-and Fe(IV), may exist in the Fe(III)/sulfite system.

Role of HSO5 -, SO3 •-and SO5 •-in IPM degradation

Recently, HSO5 -(i.e., monoanion form of PMS) was found to be a mild oxidant towards organic compounds such as estrogens and antibiotics with the rate constants ranging from 0.5-30 M -1 s -1 [START_REF] Zhou | Oxidation of fluoroquinolone antibiotics by peroxymonosulfate without activation: Kinetics, products, and antibacterial deactivation[END_REF][START_REF] Zhou | Oxidation of steroid estrogens by peroxymonosulfate (PMS) and effect of bromide and chloride ions: Kinetics, products, and modeling[END_REF]. Here, the degradation of IPM by HSO5 - was found negligible (Fig. 4a), ruling out the contribution of HSO5 -to IPM degradation in the iron/sulfite system. SO3 •-with a redox potential of E(SO3 •-/HSO3 -) = 0.84 V (vs. NHE) could act as a weak oxidant or a strong reductant (Neta and Huie, 1985a).

Experiments were conducted in anaerobic condition to examine the reactivity of SO3 •- toward IPM. Less than 10% of IPM was degraded in the absence of O 2 , where SO 3 •- served as the predominant oxidant (Fig. 4b), underscoring the minor role of SO3 •-in IPM degradation. However, it should be noted that SO3 •-could quickly react with O2

with the second-order rate constant up to 2.5 × 10 9 M -1 s -1 as described in eq. 3 [START_REF] Fronaeus | Iron-Manganese Redox Processes and Synergism in the Mechanism for Manganese-Catalyzed Autoxidation of Hydrogen Sulfite[END_REF]. The evolution of dissolved O2 during the reaction was also monitored (Fig. 4c). The initial decrease of O2 further confirmed the participation of O2 (eq. 3) in the and the relatively high concentration of dissolved O2 (≥155 µM under atmospheric pressure), SO3 •-would be quickly reacted with O2 rather than with IPM. SO5 •-, a relatively stronger oxidant, has been reported to react effectively with several organics such as aromatic amines and hydroxyphenols [START_REF] Huie | Chemical behavior of sulfur trioxide(1-) (SO3-) and sulfur pentoxide(1-) (SO5-) radicals in aqueous solutions[END_REF]Neta and Huie, 1985b). For instance, the rate constants for the reaction of Trolox with SO5 •-was reported to be 1.2 × 10 7 M -1 s -1 , more than 10 times the rate constant for the SO3 •-reaction (~10 6 M -1 s -1 ). Therefore, SO5 •-might also participate in IPM oxidation in the Fe(III)/sulfite system. In contrast to SO4 •-and Fe(IV), SO5 •-is relatively inert toward ethanol with a low rate constant, ~10 3 M -1 s -1 [START_REF] Neta | Rate Constants for Reactions of Inorganic Radicals in Aqueous Solution[END_REF]. Thus, the IPM degradation (~15%) observed in excess ethanol (Fig. 2a) could be resulted from the contribution of SO5 •-. Diphenylamine (DPA) with a relatively higher rate constant with SO5 •-is usually used as scavenger to terminate the contribution of SO5 •- (Neta and Huie, 1985b;[START_REF] Yuan | Enhanced oxidation of aniline using Fe(III)-S(IV) system: Role of different oxysulfur radicals[END_REF]. Here, the effect of DPA on IPM degradation in the Fe(III)/sulfite system was examined. In contrast to the partial inhibitory effect of ethanol, IPM was completely suppressed in the presence of 50 µM DPA (Fig. 4d), further suggesting the role of SO5 •-in IPM degradation in the iron/sulfite system.

Taken together, these findings suggest that multiple reactive species including Fe(IV), SO4 •-, and SO5 •-could contribute to the IPM transformation in the iron/sulfite system.

Transformation mechanism

ESI-MS at precursor ion scan (PIS) mode was used to selectively detect polar halogenated compounds. Given three iodine atoms in IPM, this approach setting precursor ion at m/z 127 was used to detect the primary transformation products of IPM in the Fe(III)/sulfite system. (-324) were observed in its product ion scan spectra, corresponding to the successive loss of two side chain B (-CO-NH-CH-CH2OH-CHOH-). This finding indicated carboxamide side chains were intact in this product and it might be generated from the cleavage of C-N bond in the amide moiety of acylamino side chain A. Following similar procedure, other TPs were identified (Fig. S6-S11) and their information including retention time, molecular mass, chemical formula and proposed molecular structure were shown in Table 1.

TIC of -Prec (127.00): Exp 1, from Sample 5 (Samp-Pre-3) of 202001007 IPC Fe+sulfite-samp-Pre.wiff (Turbo Spray) Max. 6.2e7 cps. To further understand the transformation pathway, the transformation products of IPM by SO 4 •-, SO 5 •-, and Fe(IV) were comparatively detected. Figure S12 exhibited the HPLC/ESI-MS chromatograms of IPM after the treatment of Heat/PDS, which has been acknowledged as the common approach for SO4 •-production [START_REF] Johnson | Persulfate Persistence under Thermal Activation Conditions[END_REF][START_REF] Lee | Persulfate-Based Advanced Oxidation: Critical Assessment of Opportunities and Roadblocks[END_REF]. The production of TP 2-6 was observed, indicating the oxidation role of SO4 •-. To explore the TPs of IPM by SO5 •-, experiments were conducted in the presence of 50 mM ethanol, where the contribution of SO4 •-and Fe(IV) could be terminated and SO5 •-would act as the sole oxidant. TP 1-5 were generated indicated the SO 5 •-role in the formation of these products (Fig. S13). Further, the transformation products of IPM in the Fe(II)/PMS system were also detected to distinguish the products generated from Fe(IV) in the Fe(III)/sulfite system (Fig. S14). As expected, the generation of TP 7 and 8 other than TP 1-6 were observed in the case of Fe(IV), further indicating the participation of Fe(IV) in IPM degradation during Fe(III)/sulfite process.

In addition, the difference in the production formation in the Heat/PDS system (acknowledged SO4 •-system) and Fe(II)/PMS system again indicated the involvement of Fe(IV) in the Fe(II)/PMS system, supporting the formation of Fe(IV) from the reaction of Fe(II) and HSO5 -(eq 15).

The time course profiles of products identified in the Fe(III)/sulfite were also monitored by HPLC/ESI MS at multiple reaction monitoring (MRM) mode. As could be seen in Figure S15, the formation of TP 1 (MW 667) and TP 3 (MW 731) was followed by their decay over time. The other products were gradually accumulated and reached plateau during the reaction time, which could be generated from the oxidation of TP 1 and TP 3 in the Fe(III)/sulfite system. Here, due to the lack of commercial standards, we cannot determine the absolute concentration of each TP, and only peak areas were used for data interpretation.

Based on these identified products, the transformation mechanism for IPM oxidation by the Fe(III)/sulfite system was tentatively proposed as shown in Figure 6.

IPM

Removal of TPs from solutions via pH adjustment

Considering the hydrolysis of iron ion in neutral and alkaline conditions, the formation of iron precipitates through pH adjustment increasing up to neutral 7 and their removal capability towards transformation products were investigated. Interestingly, TP 3, TP 4, TP 7, TP 8 was not detected after pH adjustment to 7. The concentration of iodide before and after pH adjustment was also compared. An efficient iodide removal (~80%)

was also observed, where the iodide in solutions decreased from 1.8 μΜ to 0.3 μΜ after pH adjustment (Fig. S16). These findings indicated that the transformation products of IPM as well as the iodide released from IPM during iron/sulfite process might be effectively removed through adsorption and/or coprecipitation with iron species. To further confirm this, FTIR and XPS were conducted to characterize the precipitates obtained in the Fe(III)/sulfite system after the pH adjustment. In the control group without IPM, the FTIR band recorded at 3400 and the peak at 1640 cm -1 were ascribed to the OH stretching vibration of water molecules (Fig. S17). The peak around 1300 cm -1 could be ascribed to the antisymmetric stretching of NO3 -(resulting from the iron salt used, Fe(NO3)3). The peak near 800 cm -1 could be related to the OH bending modes in FeOOH [START_REF] Amini | Ultra-small and highly dispersive iron oxide hydroxide as an efficient catalyst for oxidation reactions: a Swiss-army-knife catalyst[END_REF]. In the presence of IPM, the new peaks around 2900 cm -1 were ascribed to the alkane C-H. Also, the band around 1050 might be ascribed to C-OH stretching vibration or =CH [START_REF] Saptari | Fourier transform spectroscopy instrumentation engineering[END_REF]. The variation observed in the FTIR spectra with vs without IPM addition might be resulted from the alkane and alcohol functional groups or aromatic ring present in the transformation products. XPS analysis also indicated that Fe III -precipitates were indeed generated in the Fe(III)/sulfite system via pH adjustment (Fig. S18). The peaks around 711 and 725 eV could be attributed to Fe 2p3/2 and Fe 2p1/2 [START_REF] Richardson | Sorption and binding of nanocrystalline gold by Merino wool fibres-An XPS study[END_REF][START_REF] Yang | Efficient Degradation of Organoarsenic by UV/Chlorine Treatment: Kinetics, Mechanism, Enhanced Arsenic Removal, and Cytotoxicity[END_REF].

No difference was observed in presence or absence of IPM, possible due to the low relative amount of organic products as compared to precipitates. Collectively, these results showed that adsorption of iodide-containing transformation products and iodide to iron precipitates could be considered as a way to reduce the to reduce the amount of transformation products in the treated solutions.

Effects of water matrix

Figure 7a showed the effect of water matrix including Cl -and humic acid (surrogate of NOM) on the degradation of IPM in the Fe(III)/sulfite system. The presence of Cl - suppressed the degradation of IPM to a certain extent. With the concentration of Cl - increasing from 0 to 3 mg/L, the percentage of IPM degradation decreased from 85% to 50%. Comparatively, the effect of Cl -on heat/PDS (for SO4 •-generation) and Fe(II)/PMS systems was investigated. An inhibitory effect of Cl -was observed in the scenario of heat/PDS (Fig. S19). In previous studies, it has been documented that Cl - could act as a radical scavenger for SO4 •-(eqs 16-17) with the formation of chlorine radicals (Cl  and Cl 2 -) [START_REF] Neta | Rate Constants for Reactions of Inorganic Radicals in Aqueous Solution[END_REF].

SO 4 -+Cl -→Cl  +SO 4 2- (16) Cl  +Cl -→Cl 2 -+SO 4 2- ( 17
)
The rate constant for the reaction of Cl2 •-with IPM was estimated to be 2.5 × 10 6 M -1 s - 1 , three orders of magnitude lower than that for the reaction of SO4 •-with IPM [START_REF] Zhao | Degradation of iopamidol by three UV-based oxidation processes: Kinetics, pathways, and formation of iodinated disinfection byproducts[END_REF]. The inhibitory effect of Cl -on heat/PDS system further suggested the relatively lower reactively of chlorine radicals towards IPM as compared to SO4 •-. In contrast, no difference in IPM degradation was observed in the Fe(II)/PMS system with or without Cl -(Fig. S20). This distinct difference in the impact of Cl -on Fe(II)/PMS vs heat/PDS system might be another indication for that Fe(IV) was the primary oxidant in the Fe(II)/PMS system, otherwise a inhibitory effect should be observed. In turn, this findings indicated that Cl -has slight effect on Fe(IV) oxidation toward IPM. So, the inhibitory effect of Cl -on the iron/sulfite system involving multiple reactive intermediates might be explained by that Cl -captured SO4 •-and chlorine radicals was less reactive toward IPM as compared to SO4 •-. Based on above findings, we could speculate that the effect of Cl -on the iron/sulfite system was likely related to the reactivity of chlorine radicals towards contaminants.

As shown in Figure 7b, the degradation of IPM was slightly affected by humic acid (HA), where the percentage of IPM degradation stayed at ~80% when humic acid amount increased from 0.3 to 3 mg C/L. This finding indicated that the oxidation of IPM by reactive intermediates involved in the Fe(III)/sulfite system are slightly affected by HA. Further experiments were conducted to evaluate the performance of Fe(III)/sulfite system in two real water matrixes. As shown in Figure 7c, ~65% of IPM removal were achieved in both filtered secondary effluent and lake water samples. Since these water samples have similar chloride content but differ in organic carbon content (DOC 5.6 mg L -1 , Cl -1.8 mM for secondary effluent, whereas DOC 12.6 mg L -1 , Cl -1.6 mM for central lake), these results corroborate the above-mentioned findings, i.e. the efficiency of iron/sulfite system is less sensitive to natural organic matter, compared to chloride ions. Collectively, these results suggest that the Fe(III)/sulfite system is highly selective and effective for IPM removal, and could be applied in real waters containing high organic carbon content.

Conclusion

In this work, the performance of iron/sulfite system in IPM transformation as well as the mechanism were examined for the first time. Effective abatement of IPM was achieved by the iron/sulfite system, and the removal percentage was at least more than twice as compared to Fe(II)/PMS and Fe(II)/PDS systems. Multiple reactive species including Fe(IV), SO4 •-, and SO5 •-contributed to the degradation of IPM in the iron/sulfite system. The transformation pathways including amide hydrolysis, deiodination, amino and hydroxyl groups oxidation were proposed for the degradation of IPM based on the identification of a total of eight transformation products. In addition, iron precipitates generated via pH adjustment could effectively remove transformation products as well as the iodide in the resulting solutions. In contrast to Cl -, humic acid had a negligible impact on the IPM removal in the Fe(III)/sulfite system.

As a result, the iron/sulfite process exhibited a good efficiency for IPM removal in real water samples, even with a high organic carbon loading. Considering the low cost and good performance of the iron ions/sulfite system, this advanced oxidation process may be considered as a promising technology and an alternative for Fenton-like AOPs (Fe(II)/PMS and Fe(II)/PDS) in remediation of contaminated wastewater. In addition, this novel technology might be used as a possible pre-oxidation of IPM, in order to reduce the potential formation of iodinated by-products originated from ICM.

Fig. 1

 1 Fig. 1 Degradation of IPM (a) or diatrizoate (b) in the iron/sulfite and iron/persulfates systems. Experimental conditions: [IPM] = 5 μΜ, [Fe(II)] = [Fe(III)] = 50 μΜ, [sulfite] = [persulfates] (i.e., [PDS] or [PMS]) = 300 μΜ, and pHini = 4.

Fig. 2

 2 Fig. 2 Effect of alcohol scavengers on IPM degradation in the Fe(III)/sulfite system as well as the EPR spectra obtained from the Fe(III)/sulfite/DMPO system (red line) and the simulation of DMPO-SO3 •-adduct (blue line). Experimental conditions: [IPM] = 5 μΜ, [Fe(III)] = 50 μΜ, [sulfite] = 300 μΜ, pHini = 4, and [TBA] = 5 mM, [ethanol] = 50 mM for (a),or [DMPO] = 10 mM for (b).

  phenyl sulfone (PMSO2) (1.1 μM) was observed from the transformation of 2.8 μM PMSO, suggesting the involvement of other reactive intermediates (possibly Fe(IV)) in the Fe(III)/sulfite system.

Fig. 3

 3 Fig. 3 Effect of alcohol scavengers on IPM degradation in the Fe(II)/PMS. Experimental conditions: [IPM] = 5 μΜ, [Fe(II)] = 50 μΜ, [PMS] = 300 μΜ, pHini = 4, and [ethanol] = 50 mM.

  )/sulfite oxidation process, while further increase of O2 level might be due to the redissolution of O 2 under open atmosphere. Given the fast reaction of SO 3 •-with O 2

Fig. 4

 4 Fig. 4 Oxidation of IPM by PMS under open atmosphere (a), by Fe(III)/sulfite system in the absence of oxygen (b), evolution of oxygen during Fe(III)/sulfite system and effect of DPA on the degradation of IPM in the Fe(III)/sulfite system under a open atmosphere. Experimental conditions: [IPM] = 5 μΜ, [Fe(III)] = 50 μΜ, [sulfite] = 300 μΜ, pHini = 4, and [PMS] = 300 μΜ for (a), or [DPA] = 50 μΜ for (d).

Figure 5

 5 showed the HPLC/ESI-MS chromatogram of IPM after the treatment of Fe(III)/sulfite with eight new peaks corresponding to transformation products (TPs). These TPs were further identified based on their molecular mass and MS/MS fragmentations. For instance, TP 1 eluting at 7.21 min had the molecular ion of m/z 667, 110 Da loss as compared to IPM of m/z 777 (as ) with the retention time of 8.22 min. This product was proposed to be a hydroxylated product C 17 H 23 I 2 N 3 O 9 , consistent with the nitrogen rule and its fragments observed in the product ion scan spectra. The fragment ions of 72 (-CO-CHOH-CH2), 263 (-CO-CHOH-CH2-, -CH-CH2OH-CHOH-, -CO-NH-CH-CH2OH-CHOH-) indicated the intact acylamino side chain A and carboxamide side chains B. TP 2 at the retention time of 11.73 min has the molecular ion of m/z 705. Two major products m/z 587 (-117) and m/z 467

TICFig. 5

 5 Fig.5 The HPLC/ESI-MS chromatograms of IPM after the treatment of Fe(III)/sulfite and fragmentation pattern of TP-1 and TP-2. Experimental condition: [IPM] = 5 μΜ, [Fe(III)] = 50 μΜ, [sulfite] = 300 μΜ, and pHini = 4

-

  EPI (665.90) Charge (+0) CE (-25) CES (10) FT (7.08597): Exp 2, 7.194 to 7.348 min from Sample 5 (Samp-Pre-3) of 20..763.71) Charge (+0) CE (-25) CES (10) FT (11.5654): Exp 2, 11.491 to 12.030 min from Sample 4 (Samp-Pre-2) of ..

Fig. 6

 6 Fig. 6 Proposed pathway of IPM in the Fe(III)/sulfite process.

Fig. 7

 7 Fig. 7 Effect of chloride (a) and humic acid (b) on the degradation of IPM by the Fe(III)/sulfite system as well as the degradation of IPM in real water samples (c). Experimental condition: [Fe(III)] = 50 μΜ, [sulfite] = 300 μΜ, pHini = 4, [IPM] = 5 μΜ for (a) and (b), [IPM] = 500 nΜ for (c) to simulate environmental levels.

  •-and •OH. Ethanol could react quickly with both SO4 •-

Acknowledgments

This study was financially supported by the National Natural Science Foundation of China (52000038 and 21906030), the Program for Guangdong Introducing Innovative and Entrepreneurial Teams (2019ZT08L213), the Guangdong Basic and Applied Basic Research Foundation (2019A1515110321 and 2019A1515011947), the Guangzhou Science and Technology Project (202002030093), the Open Project of State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (No.HC202145), the Key Special Project for Introduced Talents Team of Southern Marine Science and Engineering Guangdong Laboratory (GML2019ZD0403). Thanks for the kind help of Dr. Tao Yang, Wei Song, Hang Bai and Zhengheng Yang regarding the XPS、EPR and FTIR analysis