Pierre-François Biard
email: pierre-francois.biard@ensc-rennes.fr

Thom Thi Dang

Jenny Bocanegra

Annabelle Couvert

Intensification of the O3/H2O2 advanced oxidation process using a continuous tubular reactor filled with static mixers: proof of concept

Keywords: Ozone, hydrogen peroxide, peroxone, advanced oxidation process, tubular reactor, static mixers

An innovative implementation of the O 3 /H 2 O 2 advanced oxidation process was proposed to intensify the hydroxyl radical generation. Natural or drinking waters, containing atrazine as a probe compound, were spiked with H 2 O 2 and further continuously mixed to a pre-ozonated solution in a homogeneous tubular reactor filled with static mixers. Hydraulic residence times in the range 10 s -140 s were set at different sampling ports. The experimental results confirmed a very high ozone decomposition rate, concomitant with a high hydroxyl radical exposure (R ct in the range from 10 -7 to 10 -6 ), especially during the initial ozone decomposition phase (between 10 and 20 s). Equimolar initial concentrations of hydrogen peroxide and ozone were optimal to maximize the hydroxyl radical generation and to minimize their relative consumptions. The influence of the water matrix on the ozone decomposition and the hydroxyl radical generation was limited. This study is a proof of concept that using a homogeneous tubular reactor would be more effective than a gas-liquid reactor to apply the peroxone process.

Introduction

Several technologies can be applied for emerging contaminants removal such as nanofiltration, adsorption on activated carbon, ozonation and advanced oxidation processes [START_REF] Bolong | A review of the effects of emerging contaminants in wastewater and options for their removal[END_REF]. Through the generation of very reactive hydroxyl radicals HO° in water, advanced oxidation processes (AOPs) appear particularly appealing [START_REF] Andreozzi | Advanced oxidation processes (AOP) for water purification and recovery[END_REF][START_REF] Gunten | Ozonation of drinking water: Part I. Oxidation kinetics and product formation[END_REF][START_REF] Glaze | Chemistry of water treatment processes involving ozone, hydrogen peroxide and ultraviolet radiation[END_REF][START_REF] Glaze | Advanced oxidation processes. Description of a kinetic model for the oxidation of hazardous materials in aqueous media with ozone and hydrogen peroxide in a semibatch reactor[END_REF]. They lead to the formation of more biodegradable and often less toxic intermediates and can be advantageously implemented upstream from biological treatments [START_REF] Buffle | Ozonation and advanced oxidation of wastewater: effect of O 3 dose, pH, DOM and HOscavengers on ozone decomposition and HO° Generation[END_REF][START_REF] Oller | Combination of Advanced Oxidation Processes and biological treatments for wastewater decontamination-A review[END_REF][START_REF] Bilińska | Comparison between industrial and simulated textile wastewater treatment by AOPs -Biodegradability, toxicity and cost assessment[END_REF]. The peroxone AOP (O 3 /H 2 O 2 ) involves a radical chain mechanism based on the ozone decomposition initiated by the hydroperoxide anion HO 2 - [START_REF] Gunten | Ozonation of drinking water: Part I. Oxidation kinetics and product formation[END_REF][START_REF] Von Sonntag | Chemistry of ozone in water and wastewater treatment: From basic principles to applications[END_REF][START_REF] Beltrán | Ozone reaction kinetics for water and wastewater systems[END_REF][START_REF] Biard | Determination by reactive absorption of the rate constant of the ozone reaction with the hydroperoxide anion[END_REF][START_REF] Beltrán | Theoretical aspects of the kinetics of competitive first reactions of ozone in the O 3 /H 2 O 2 and O 3 /UV oxidation processes[END_REF].

The literature about emerging contaminants removal by the peroxone process is particularly extensive. Except for a few studies implemented in batch homogeneous reactors [START_REF] Chen | Treatment of Volatile Organic Chemicals on the EPA contaminant candidate list using ozonation and the O 3 /H 2 O 2 Advanced Oxidation Process[END_REF][START_REF] Rosenfeldt | Comparison of the efficiency of OH radical formation during ozonation and the advanced oxidation processes O 3 /H 2 O 2 and UV/H 2 O 2[END_REF][START_REF] Acero | Oxidation of Acetamide Herbicides in Natural Waters by Ozone and by the Combination of Ozone/Hydrogen Peroxide: Kinetic Study and Process Modeling[END_REF][START_REF] Acero | MTBE oxidation by conventional ozonation and the combination ozone/hydrogen peroxide : efficiency of the processes and bromate formation[END_REF][START_REF] Acero | Characterization of oxidation processes : ozonation and the AOP O 3 /H 2 O 2[END_REF][START_REF] Katsoyiannis | Efficiency and energy requirements for the transformation of organic micropollutants by ozone, O 3 /H 2 O 2 and UV/H 2 O 2[END_REF], in which a controlled amount of ozone was injected at the initial time in a water spiked with H 2 O 2 , these studies were implemented in gas-liquid reactors operated semi-continuously or continuously [START_REF] Bilińska | Comparison between industrial and simulated textile wastewater treatment by AOPs -Biodegradability, toxicity and cost assessment[END_REF][START_REF] Nélieu | Degradation of Atrazine into Ammeline by Combined Ozone/Hydrogen Peroxide Treatment in Water[END_REF][START_REF] Yasunaga | New approach for optimization of ozonehydrogen peroxide water treatment[END_REF][START_REF] Prado | Comparison of Different Advanced Oxidation Processes Involving Ozone to Eliminate Atrazine[END_REF][START_REF] Benitez | The Effectiveness of Single Oxidants and AOPs in the Degradation of Emerging Contaminants in Waters: A Comparison Study[END_REF][START_REF] Witte | Ozonation and advanced oxidation by the peroxone process of ciprofloxacin in water[END_REF][START_REF] Tizaoui | Landfill leachate treatment with ozone and ozone/hydrogen peroxide systems[END_REF][START_REF] Gago-Ferrero | Ozonation and peroxone oxidation of benzophenone-3 in water: Effect of operational parameters and identification of intermediate products[END_REF]. The use of stirred batch homogeneous reactors, especially by Acero and his coworkers, was motivated by the fact that this configuration allows to control perfectly the initial amount of oxidants and to calculate the hydroxyl radical exposure, often balanced to the corresponding ozone exposure, through the R ct calculation [START_REF] Elovitz | Hydroxyl radical/ozone ratios during ozonation processes. I. The R ct concept[END_REF]. Thus, using the peroxone process, the total hydroxyl radical exposure is not necessarily higher, with R ct values close to that measured in ozonation, but the hydroxyl radical formation is faster, allowing to design more compact reactors with the peroxone process [START_REF] Rosenfeldt | Comparison of the efficiency of OH radical formation during ozonation and the advanced oxidation processes O 3 /H 2 O 2 and UV/H 2 O 2[END_REF].

Industrial applications of the peroxone process are also implemented in gas-liquid reactors which are often bubble columns. This configuration, which combines the ozone transfer and the chemical reactions in only one equipment, appears natural first since the fast ozone reaction with H 2 O 2 might enhance the ozone transfer [START_REF] Beltrán | Theoretical aspects of the kinetics of competitive first reactions of ozone in the O 3 /H 2 O 2 and O 3 /UV oxidation processes[END_REF]. Few industrial applications, such as the HiPOx or the PRO 3 Mix processes, are based on the use of intensified gas-liquid reactors like static mixers [START_REF] Knol | Conversion of organic micropollutants with limited bromate formation during the Peroxone process in drinking water treatment[END_REF][START_REF] Bowman | HiPOx Advanced Oxidation of TBA and MTBE in Grounwater[END_REF]. A major drawback of gas-liquid reactors to apply the peroxone process is the complex control of the oxidants doses since both the ozone mass-transfer rate and the reaction rate are interdependent. Thus, homogeneous reactors allow potentially to set easily high initial ozone concentrations, contrarily to heterogeneous systems [START_REF] Sunder | Oxidation of tri-and perchloroethene in aqueous solution with ozone and hydrogen peroxide in a tube reactor[END_REF]. In 2006, Buffle and his co-workers developed a continuous quench flow system to study the ozonation of natural and waste waters during the initial phase of the ozone decomposition, often called the instantaneous ozone demand (IOD) phase, which last for around 20 s [START_REF] Buffle | Ozonation and advanced oxidation of wastewater: effect of O 3 dose, pH, DOM and HOscavengers on ozone decomposition and HO° Generation[END_REF][START_REF] Buffle | Phenols and amine induced HO° generation during the initial phase of natural water ozonation[END_REF][START_REF] Buffle | Measurement of the initial phase of ozone decomposition in water and wastewater by means of a continuous quench-flow system: Application to disinfection and pharmaceutical oxidation[END_REF]. This outstanding work emphasized that ozone reacts mainly with natural organic moieties during this initial phase, leading to a very high hydroxyl radical production. R ct values (10 -7 to 10 -6 orders of magnitude), around two to three orders of magnitude higher than during the second phase (10 -9 to 10 -8 orders of magnitude), were measured in both natural and waste waters. Later, Biard and his co-workers measured even high R ct values (10 -4 order of magnitude) after only 30 ms of reaction time using the peroxone process [START_REF] Biard | Intensification of VOC gas-liquid mass transfer in a compact scrubber using the peroxone process: kinetic study and hydroxyl radical tracking[END_REF]. Sunder and Hempel used a tubular reactor filled with static mixers to study the perchlorothylene and trichloroethylene oxidations [START_REF] Sunder | Oxidation of tri-and perchloroethene in aqueous solution with ozone and hydrogen peroxide in a tube reactor[END_REF].

A pre-ozonated demineralized water (with a flow-rate of 200 L h -1 ) was continuously mixed to a contaminated water spiked with H 2 O 2 (with a flow-rate of 20 L h -1 ). High conversion rates (close to 100%) were obtained for a reaction time lower than one min but the potential of this configuration was not justified and assessed by comparison to the traditional design of the peroxone process.

Thus, all these observations suggest that this high and fast hydroxyl radical production during the initial phase would be advantageously used to intensify the peroxone process in a homogeneous reactor. Thus, a bench scale process was designed in this study to demonstrate the feasibility of such a process which requires to achieve separately the ozone transfer and the chemical reaction steps.

Two different water matrices, a drinking water and a natural water, spiked with H 2 O 2 and with atrazine as an efficient hydroxyl radical probe compound [START_REF] Biard | Efficient catalytic ozonation by ruthenium nanoparticles supported on SiO 2 or TiO 2 : Towards the use of a non-woven fiber paper as original support[END_REF], were mixed to a pre-ozonated solution in a continuous homogeneous tubular reactor filled with static mixers to improve the micromixing efficiency. Such a tubular reactor was particularly convenient to control the reaction time through the hydraulic residence time in the range 10-140 s, compatible with the duration of the IOD phase.

Contrarily to the design of Sunder and Hempel [START_REF] Sunder | Oxidation of tri-and perchloroethene in aqueous solution with ozone and hydrogen peroxide in a tube reactor[END_REF], the pre-ozonated solution flow-rate was lower than the treated water flow-rate. Besides, the pre-ozonated solution was prepared with the same water matrix than the treated water.

Material and methods

Experimental set-up description

The experimental set-up was composed of four main parts (Fig. 1). Pictures of the experimental setup are available as supplementary material (Figs S1 andS2). The part # 1 was devoted to the ozone generation (at a concentration of nearly 120 g m 3 under the normal condition for temperature and pressure) and has been already described in a previous article [START_REF] Dang | Assessment of a Stirred-Cell Reactor Operated Semicontinuously for the Kinetic Study of Fast Direct Ozonation Reactions by Reactive Absorption[END_REF]. The saturated pre-ozonated solution was prepared in the Part # 2 through a continuous bubbling in a temperature controlled gasliquid reactor (supplied in Pyrex® by Cloup, France) equipped with a glass porous diffuser at the bottom and a mechanically agitated turbine. This reactor was filled with around 2L of the studied water matrix (but free of atrazine and/or H according to the low reaction rate between H 2 O 2 and atrazine [START_REF] Prado | Comparison of Different Advanced Oxidation Processes Involving Ozone to Eliminate Atrazine[END_REF]. Another centrifugal pump (Iwaki MD-30FX, Japan) was used to feed continuously the Part #4.

Table 1: Summary of the operating conditions.

Variables Operating conditions range Regular operating conditions C HP,0 (ppm) 0 -20 3.5 C Oz,0 (ppm) 1.6 -5 4.5-5 C At,0 (ppm) 0 -4 3.0-3.5

F 1 (L h -1 ) 2 -8 6 F 2 (L h -1 ) 2 -8 2 F T (L h -1 )
0 - 16 8 The pre-ozonated solution (coming from the Part #2 at a flow-rate F 2 ) and the solution to treat (coming from the Part #3 at a flow-rate F 1 ) were continuously mixed (Part #4) in a tubular reactor (6.35 mm of inner diameter) in PTFE®. F 1 and F 2 were measured and controlled by float type flow meters equipped with a valve (Brooks Sho-Rate R6-15-B, USA). The reactor was operated at steady state. The reactive medium was sampled at different hydraulic residence times owing to four sampling ports (made of stainless steel fittings provided by Legris, France) and at the tube outlet.

Respectively four and seven helical static mixers of 15 cm length (Koflo, Japan) were regularly inserted in the first section (1 meter length) and in the following sections (2 m length) of the tubular reactor as schematized in Fig. 1. The residence time was deduced from the tube length and diameter, from the total liquid flow-rate (F T = F 1 + F 2 ) and from the static mixers void fraction ( = 0.709). Most experiments have been performed with an empty tube superficial velocity of 0.067 m s -1 (F T = 8 L h -1 , Table 1) corresponding to a Reynolds number of 435 (calculating with the empty tube superficial velocity). Such a low Re number is sufficient using helical static mixers to drive to a turbulent regime and a plug flow according to Fang and Lee [START_REF] Fang | Micromixing efficiency in static mixer[END_REF]. In that case, the residence time at the different sampling points was in the range 18 -137 s. The initial concentrations (Table 1), calculated just after the mixture, at the T-junction, were deduced from F 1 and F 2 and from the concentrations measured at the Parts #2 and #3. Usually, the relative removal efficiency of any species i ( i ) and the relative consumptions (C HP /C Oz , C HP /C At and C Oz /C At ) were evaluated at different reaction times according to respectively Eqs. 1 and 2: C i is the concentration of any species i (expressed in mol L -1 ). The subscripts HP, Oz and At stand respectively for hydrogen peroxide, ozone and atrazine. The subscript 0 corresponds to the initial time t 0 .

0 0 , , i i i i C C C    Eq. 1.

Analytical methods

Ozone, H 2 O 2 and atrazine concentrations were quantified at each sampling point and at the tubular reactor outlet. To avoid the evolution of the solute concentrations between the sampling and the analysis, all the samples were introduced gravitationally quickly in amber glass vials containing adapted amounts of quenching reagents (glycine or sodium thiosulfate as explained later). The time between the sampling and the quenching was estimated to less than 1 s. Thus, the ozone concentration was quantified by the indigo colorimetric method which provides no interaction with H 2 O 2 and the atrazine [START_REF] Bader | Determination of ozone in water by the Indigo method[END_REF]. A Helios UV-Vis spectrophotometer (Shimadzu, Japan) was used for the indigo absorbance measurement, performed a few minutes after the sampling. The H 2 O 2 concentration was quantified by the iodometric method in which iodide is catalytically oxidized in iodine at acidic pH. An excess of glycine in powder was previously introduced in the sampling vial to selectively quench O 3 . Indeed, glycine reacts quickly with O 3 (k = 3.4×10 3 L mol -1 s -1 at pH = 8 according to Pryor et al. [START_REF] Pryor | Kinetics of ozonation. 2. Amino acids and model compounds in water and comparisons to rates in nonpolar solvents[END_REF]) but slowly with H 2 O 2 [START_REF] Berger | Ozone and hydroxyl radicals induced oxidation of glycine[END_REF], allowing the selective quantification of H 2 O 2 .

The H 2 O 2 concentration was uninfluenced by the glycine addition. The iodine titration by sodium thiosulfate (Titrisol, Merck) was carried out a few minutes after the sampling. An excess of sodium thiosulfate in powder was used to quench the oxidants [START_REF] Glaze | Advanced oxidation processes. Description of a kinetic model for the oxidation of hazardous materials in aqueous media with ozone and hydrogen peroxide in a semibatch reactor[END_REF][START_REF] Takenaka | Rapid reaction of sulfide with hydrogen peroxide and formation of different final products by freezing compared to those in solution[END_REF][START_REF] Lee | Oxidative transformation of micropollutants during municipal wastewater treatment: Comparison of kinetic aspects of selective (chlorine, chlorine dioxide, ferrateVI, and ozone) and non-selective oxidants (hydroxyl radical)[END_REF] prior to the atrazine quantification using an ultra performance liquid chromatograph (UPLC Acquity H-Class) coupled to a tandem mass spectrometer (Quattro Premier triple quadripole) provided by Waters Corporation (USA). The pH was measured using a pH meter 315i (WTW, Germany) and a pH probe SI Analytics (N1043A, Germany).

Waters' characteristics

Two water matrices were studied: (i) the drinking water from the city of Rennes in France (pH = 8.05, alkalinity = 1.7 mmol of HCO 3 -L -1 , total organic carbon (TOC) = 2.39 ppm) and (ii) a raw ground water sampled in a well (pH = 7.13, alkalinity = 0.74 mmol of HCO 3 -L -1 , total organic carbon (TOC) = 4.06 ppm). This water was filtrated between 17 and 30 µm (Filters 11788722 from Fisher Scientific, USA).

These two water matrices were synthetically polluted by known amounts of atrazine, previously dissolved in a stock solution (50 ppm) in an ultrasound bath.

Results and discussion

Atrazine removal by ozonation

Atrazine oxidation by single ozonation (Fig. 2) has been investigated first for two different initial atrazine concentrations (≈ 3 ppm and ≈ 0.3 ppm) targeting ratios C Oz,0 /C At,0 equal to 7 and 70 (C Oz,0 ≈ 10 -4 mol L -1 ). The corresponding concentration time-course plots are available as supplementary material (Fig. S4 and Fig. S6). The atrazine removal efficiency ( At ) increased from 17% (10%) to 68% (52%) for reaction times of respectively 18 and 137 s for C At,0 = 3 ppm (0.3 ppm). In the meantime, the ozone removal efficiency ( Oz ) increased from 35% (42%) to 66% (71%) for reaction times of respectively 18 and 137 s for C At,0 = 3 ppm (0.3 ppm). The better atrazine consumption at 3.0 ppm can be justified by less significant competitive effects.

-1 , F 1 = 6 L h -1 , F 2 = 2 L h -1 ).
To compare the weight of the atrazine oxidation by radicals to the weight of the atrazine oxidation by molecular ozone, the same experiments have been repeated with 0.001 M of tert-butanol at the reactor inlet to quench the radical chain without interaction with molecular ozone [START_REF] Buffle | Measurement of the initial phase of ozone decomposition in water and wastewater by means of a continuous quench-flow system: Application to disinfection and pharmaceutical oxidation[END_REF]. In that case, both the atrazine and the ozone consumptions were slow down (Fig. 2), confirming undoubtedly (i)

t-BuOH (Atz 0,3 ppm) t-BuOH (Atz 3 ppm) 0,3 ppm 3 ppm (a) (b) C At,0 = 0.3 ppm C At,0 = 3.0 ppm C At,0 = 0.3 ppm C At,0 = 3.0 ppm With tert-but.
the predominance of the atrazine oxidation by radicals and (ii) the influence of the radicals generated on the ozone consumption rate. On the one hand,  Oz with tert-butanol does not depend on C at,0 and was not significantly different than the one measured without atrazine and H 2 O 2 (section 3.3, Fig. 3 (a)), showing that the ozone decomposition initiation was controlled in that case by reactions with inorganic compounds and with ubiquitous moieties of the natural organic matter, in agreement with the experimental observations of Buffle and Von Gunten (2006) during the instantaneous ozone demand (IOD) phase. It is noteworthy that the reaction of O 3 with the hydroxide anion (at a pH around 8) was too slow to contribute significantly to the ozone decomposition rate [START_REF] Buffle | Ozonation and advanced oxidation of wastewater: effect of O 3 dose, pH, DOM and HOscavengers on ozone decomposition and HO° Generation[END_REF]. Without tertbutanol, the ozone decomposition was sped up by the reactions with radicals involved in the radical chain. On the other hand, contrarily to the results gathered without radical scavenger (tert-butanol)  At does not depend on C At,0 which was characteristics of a pseudo-first order kinetics in excess of ozone, showing that the atrazine consumption in that case was only due to the bimolecular reaction with O 3 .

To quantify the radical production, the ratio of the hydroxyl radical exposure to the ozone exposure (R ct ) was calculated according to Eq. 3 [START_REF] Biard | Efficient catalytic ozonation by ruthenium nanoparticles supported on SiO 2 or TiO 2 : Towards the use of a non-woven fiber paper as original support[END_REF]:

                          3 0 0 1 O t Oz At At HO ct k dt C C C k R , ln Eq. 3.
With k HO°, the second-order reaction constant between HO° and atrazine (3×10 9 L mol -1 s -1 [START_REF] Acero | Degradation kinetics of atrazine and its degradation products with ozone and OH radicals : a predictive tool for drinking water treatment[END_REF]) and k O 3 , the second-order reaction constant between O 3 and atrazine (6.3 L mol -1 s -1 at neutral and basic pH, [START_REF] Beltrán | Advanced oxidation of atrazine in water. I: Ozonation[END_REF]). The R ct values were around 10 -12 using tert-butanol and confirmed an efficient radical scavenging. Without tert-butanol, an average R ct value at 137 s, equal to 5.5×10 -8 (4.0×10 -8 ) for 3 ppm (0.3 ppm) of atrazine, consistent with the order of magnitude found in the literature [START_REF] Yong | Incorporation of initiation, promotion and inhibition in the R ct concept and its application in determining the initiation and inhibition capacities of natural water in ozonation[END_REF][START_REF] Elovitz | Hydroxyl radical/ozone ratios during ozonation processes. II. The effect of temperature, pH, alkalinity, and DOM properties[END_REF], have been measured. The R ct calculated after only 18 s of reaction time had the same orders of magnitude.

Regarding the quick ozone consumption after 18 s, it stressed a fast radical generation, taking into account advantageously the first second of the IOD phase [START_REF] Buffle | Ozonation and advanced oxidation of wastewater: effect of O 3 dose, pH, DOM and HOscavengers on ozone decomposition and HO° Generation[END_REF][START_REF] Buffle | Phenols and amine induced HO° generation during the initial phase of natural water ozonation[END_REF][START_REF] Buffle | Measurement of the initial phase of ozone decomposition in water and wastewater by means of a continuous quench-flow system: Application to disinfection and pharmaceutical oxidation[END_REF].

Ozone decomposition and H2O2 consumption without atrazine

-1 , F 1 = 6 L h -1 , F 2 = 2 L h -1 ).
Different initial H 2 O 2 concentrations (in the range 0-9.50 ppm/0-2.79 10 -4 mol L -1 ) were applied to assess the influence of the ratio C HP,0 /C Oz,0 (in the range 0-3.08 mol mol -1 ) on the O 3 and H 2 O 2 removal efficiencies without atrazine (with C Oz,0 ≈ 10 -4 mol L -1 ). The corresponding concentration time-course plots are presented as supplementary material (Fig. S3).

For C HP,0 ≤ C Oz,0 ,  Oz increased with the reaction time and was complete after 137 s in presence of H 2 O 2 . For C HP,0 > C Oz,0 , the ozone concentration cannot be quantified even at the first sampling points (reaction time of 18 s) owing to a fast ozone decomposition rate (Fig. 3 (a)). This large ozone consumption rate cannot be attributed solely to the initiation reaction with the hydroperoxide anion and involves significant parallel reactions with radicals formed after the mixing of the two solutions, with inorganic compounds and with ubiquitous moieties of the natural organic matter [START_REF] Buffle | Phenols and amine induced HO° generation during the initial phase of natural water ozonation[END_REF]. The lower ozone decomposition rates involved in the study of Sunder and Hempel [START_REF] Sunder | Oxidation of tri-and perchloroethene in aqueous solution with ozone and hydrogen peroxide in a tube reactor[END_REF] In the meantime, in agreement with a corresponding fast ozone consumption,  HP increased sharply before the first sampling point and was in the range 23%-60% after 18 s of reaction time (Fig. S5).

Then,  HP increasing was narrower after the first sampling point and the almost complete ozone disappearance. This slight increasing might be due to reactions with both residual radicals or with byproducts. The literature usually supports a consumption of one mol of H 2 O 2 for two mol of O 3 using the peroxone process (C HP /C Oz = 0.5) whereas the initiation reaction involved the consumption of one mol of HO 2 -for one mol of O 3 [START_REF] Biard | Determination by reactive absorption of the rate constant of the ozone reaction with the hydroperoxide anion[END_REF][START_REF] Acero | Characterization of oxidation processes : ozonation and the AOP O 3 /H 2 O 2[END_REF][START_REF] Merényi | Reaction of Ozone with Hydrogen Peroxide (Peroxone Process): A Revision of Current Mechanistic Concepts Based on Thermokinetic and Quantum-Chemical Considerations[END_REF][START_REF] Paillard | Optimal conditions for applying an ozone-hydrogen peroxide oxidizing system[END_REF]. Thus, C HP /C Oz was most of time in the range 0.5-1 (Fig

Atrazine removal using the peroxone process

The corresponding concentration time-course plots are presented as supplementary material (Figs S4-S6). Both the ozone and H 2 O 2 concentration time-courses were poorly affected by the atrazine addition compared to the reference experiments without atrazine (section 3.3) achieved with the same oxidants concentrations. However, the combination of O 3 and H 2 O 2 enhanced the atrazine oxidation with a major extent, even at low H 2 O 2 inputs (Fig. 4). Indeed, the atrazine removal efficiency was in the range 80-90% (60-100%) after only 18 s at C At,0 = 3.0 ppm (C At,0 = 0.3 ppm).

Except for ratios C HP,0 /C Oz,0 lower than 1, the atrazine concentration did not vary significantly after the first sampling point (18 s), in agreement with a complete ozone decomposition (section 3.2), emphasizing that the tubular reactor is more beneficial during the first seconds of reaction time corresponding to the IOD phase. Besides, the H 2 O 2 addition leads to R ct values (10 -7 order of magnitude), calculated between 0 and 18 s, around one order of magnitude higher than without H 2 O 2 (10 -8 ) with the same operating conditions (Table 3). They were also around one or two orders of magnitude higher than in traditional ozone-based advanced oxidation processes with different water qualities such as the catalytic ozonation or the peroxone process applied in homogeneous stirred batch reactors, in which the R ct was not significantly different than with the ozonation process but with a fastest ozone decomposition [START_REF] Rosenfeldt | Comparison of the efficiency of OH radical formation during ozonation and the advanced oxidation processes O 3 /H 2 O 2 and UV/H 2 O 2[END_REF][START_REF] Acero | MTBE oxidation by conventional ozonation and the combination ozone/hydrogen peroxide : efficiency of the processes and bromate formation[END_REF][START_REF] Acero | Characterization of oxidation processes : ozonation and the AOP O 3 /H 2 O 2[END_REF][START_REF] Elovitz | Hydroxyl radical/ozone ratios during ozonation processes. I. The R ct concept[END_REF][START_REF] Biard | Efficient catalytic ozonation by ruthenium nanoparticles supported on SiO 2 or TiO 2 : Towards the use of a non-woven fiber paper as original support[END_REF][START_REF] Elovitz | Hydroxyl radical/ozone ratios during ozonation processes. II. The effect of temperature, pH, alkalinity, and DOM properties[END_REF][START_REF] Guzman-Perez | Kinetics of catalytic ozonation of atrazine in the presence of activated carbon[END_REF]. Using a homogeneous tubular reactor filled with static mixers to improve the micromixing efficiency, both the amount of HO° radicals produced and their generation rates were higher, i.e. more radicals were formed within a shorter time at identical initial oxidants inputs. It is noteworthy that such high R ct values imply that atrazine reacted only with HO° radicals and not directly with molecular ozone [START_REF] Elovitz | Hydroxyl radical/ozone ratios during ozonation processes. I. The R ct concept[END_REF].

On the one hand, both the R ct and the ozone consumption, quantified through the ratio C Oz /C At , were poorly influenced by the initial hydrogen peroxide concentration when C HP,0 /C Oz,0 was roughly higher than 1 (Table 3) for which both the atrazine and ozone consumptions were almost total. On the other hand, the hydrogen peroxide consumption, quantified through the ratio C HP /C At , tends to increase with C HP,0 /C Oz,0 , excepted for a local minimum obtained for a molar ratio C HP,0 /C Oz,0 around 1. Therefore, to maximize the radical production and to limit the hydrogen peroxide consumption, a molar ratio C HP,0 /C Oz roughly equal to one was optimal.

The R ct values obtained for a low initial atrazine concentration of 0.3 ppm (Table 3) were close to those obtained using 3.0 ppm for C Oz,0 /C At,0 lower or equal to 0.67. For higher C Oz,0 /C At,0 ratios, atrazine was not detected at the sampling point and the R ct was not quantified justifying the selection of both a low and a high atrazine concentrations for the assessment of the proposed process. Indeed, even if a high atrazine concentration can slightly affect the R ct determination and the ozone chemistry, R ct order of magnitude should be close to the one obtained at lower and more realistic atrazine concentrations. Since the ozone and hydrogen peroxide consumptions are mainly related to their self-reaction and to the radical chain, C HP /C At and C Oz /C At values were very high for C At,0 = 0.3 ppm.

Influence of the initial oxidants concentration on the atrazine removal

The influence of the oxidants concentration with an optimal ratio C HP,0 /C Oz,0 ratio of nearly 1 and at a constant atrazine initial concentration (C At,0 = 3.0 ppm) was assessed targeting a ratio C Oz,0 /C At,0 in the range 2.25-6.79. Only the results at 18 s of reaction time are presented but the corresponding concentration time-course plots are presented as supplementary material (Fig. S7 and Fig. S8). The R ct increased slightly with the ratio C Oz,0 /C At,0 (Table 4), proving that atrazine does not inhibit the radical chain [START_REF] Yong | Incorporation of initiation, promotion and inhibition in the R ct concept and its application in determining the initiation and inhibition capacities of natural water in ozonation[END_REF]. The R ct increasing with the ratio C Oz,0 /C At,0 might be justified by a lower competition between O 3 (which is a promoter) and atrazine regarding hydroxyl radicals. Thus, high initial oxidants concentrations respecting a ratio C HP,0 /C Oz,0 close to one would be effective to enhance the radical production.

The atrazine degradation by HO° radicals leads to the formation of many by-products which does not allow to predict the overall reaction stoichiometry [START_REF] Nélieu | Degradation of Atrazine into Ammeline by Combined Ozone/Hydrogen Peroxide Treatment in Water[END_REF][START_REF] Acero | Degradation kinetics of atrazine and its degradation products with ozone and OH radicals : a predictive tool for drinking water treatment[END_REF]. However, the order of magnitude of C Oz /C At is reasonable. For example, Acero et al. measured C Oz /C At in the range from 8.5 (for an atrazine yield of 29%) to 25 (for a total atrazine consumption) using the peroxone process in a batch homogeneous reactor and initial concentrations close to those used in this study [START_REF] Acero | Degradation kinetics of atrazine and its degradation products with ozone and OH radicals : a predictive tool for drinking water treatment[END_REF].

Influence of the reaction time on the atrazine oxidation

The sections 3.2 and 3.3 clearly highlighted that the process is more beneficial before the first sampling point, i.e. for a reaction time lower than roughly 10 to 20 seconds, corresponding to the IOD phase. To assess the influence of the reaction time at this first sampling point, the flow-rates F 1 and F 2 were adjusted to target reaction times in the range 9-35 s (F T was in the range 4-16 L h -1 ). On the one hand,  At (around 83%) is not significantly affected by the reaction time whereas  Oz increased with the reaction time (from 84 to more than 99%) according to the Table 5. On the other hand,  HP decreased surprisingly with the reaction time (i.e. increased with the liquid superficial velocity and flow-rate) although the initial oxidants and atrazine concentrations were unmodified.

This H 2 O 2 consumption increasing with the liquid superficial velocity suggests that the micromixing efficiency might limit the overall kinetics. Indeed, even if helical static mixers are recognized for their high mixing efficiency, the micromixing time would be in the range 1-50 ms [START_REF] Fang | Micromixing efficiency in static mixer[END_REF]. However, with with and without hydrogen peroxide in natural, waste and drinking waters. Nonetheless, the quite low Reynolds numbers (calculated considering an empty tube), in the range 218-871, were sufficient using helical static mixers to induce a turbulent flow [START_REF] Fang | Micromixing efficiency in static mixer[END_REF]. Thus, the almost constant atrazine yield might be justified by the fact that the lower reaction times were counterbalanced by a more efficient radical production. Indeed, the R ct increased when the reaction time decreased, in agreement with the experimental observations of Buffle et al. [START_REF] Buffle | Measurement of the initial phase of ozone decomposition in water and wastewater by means of a continuous quench-flow system: Application to disinfection and pharmaceutical oxidation[END_REF]. Using static mixers, this phenomenon might be reinforced by the micromixing efficiency improvement at higher liquid flow-rate. Thus, outstanding R ct values higher than 10 -6 were obtained.

This set of experiments confirmed the benefits of a low reaction time, using a high superficial velocity, to improve the mixing efficiency and the radical production and to limit the ozone consumption. For the development of an industrial process based on this concept, a shorter tubular reactor length and a higher superficial velocity, combined with a recirculation loop would be a feasible option to achieve a very high degradation rate in a compact process and with a limited oxidants consumption.

Influence of the water matrix

The optimal conditions of the section 3.3 (C Oz,0 /C HP,0 around 1 with C Oz,0 ≈ 10 -4 mol -1 ,

F 1 = 6 L h -1 , F 2 = 2
L h -1 ) determined using drinking water were applied to a ground water (well water) with an initial atrazine concentration of 3.0 ppm and without atrazine. The corresponding concentration timecourse plots are presented as supplementary material (Fig. S9 and Fig. S10).

Without atrazine, both the hydrogen peroxide and ozone consumptions were slowed down in the ground water compared to the drinking water (Table 6). On the one hand, the alkalinity of the ground water is nearly 2.5 times lower. Hydrogenocarbonate and carbonate anions are considered as the main radical scavengers in natural water but they also play the role of promoters in the peroxone process [START_REF] Acero | Influence of carbonate on the ozone/hydrogen peroxide based Advanced Oxidation Process for drinking water treatment[END_REF]. On the other hand, the ground water has a pH one unity lower than the drinking water.

Thus, an initiation rate significantly lower is expected since the H 2 O 2 conjugated base (HO 2 -) is involved as the main initiator of the ozone decomposition. Consequently, the lower O 3 and H 2 O 2 consumptions for the ground water might be mainly attributed to its lower pH.

R ct and  At after 18 s of reaction time were respectively around two times and 16% lower using well water compared to the drinking water (Table 7). Nonetheless, even using a raw water, which has never been previously ozonated (a high ozone demand should be expected) and with a high organic content, the radical generation potential remained important. The ozone consumption was even lower. Thus, this set of experiments shows that the influence of the water matrix on the HO° generation remains limited. The pH seems to be the most influential parameter through its influence on the H 2 O 2 /HO 2 -balance but it should be confirmed with other water matrices, especially waste waters.

Discussion -comparison with a traditional gas-liquid reactor

Using a traditional semi-batch gas-liquid reactor, with a high initial hydrogen peroxide concentration of 0.025 mol L -1 (around 10 -4 mol L -1 in this study) and an atrazine concentration of 6.95 10 -5 mol L -1

(corresponding to a concentration of round 15 ppm) in a synthetic water matrix, [START_REF] Prado | Comparison of Different Advanced Oxidation Processes Involving Ozone to Eliminate Atrazine[END_REF] determined an atrazine half-life of nearly 10 min at pH 7 [START_REF] Prado | Comparison of Different Advanced Oxidation Processes Involving Ozone to Eliminate Atrazine[END_REF]. In this study, using a ground water with a similar pH, the atrazine half-life was lower than 10 s. Since the experimental conditions of [START_REF] Prado | Comparison of Different Advanced Oxidation Processes Involving Ozone to Eliminate Atrazine[END_REF] were not necessarily optimized, with a very high initial H 2 O 2 concentration, the gas-liquid reactor implemented for the ozone transfer (Part #2) was used to undertake two semi-batch reference experiments with the drinking water matrix with an initial atrazine concentration of around 4 ppm. The ozone gas concentration was set at 20 g Nm -3 to target a dissolved ozone concentration at the equilibrium of nearly 10 -4 mol L -1 (such as in the part 3.3) with a flow-rate of 80 NL h -1 . With a low H 2 O 2 initial concentration of 4 ppm corresponding to nearly 10 -4 mol L -1 , the time necessary to obtain an atrazine removal efficiency of 90% was around 130 s and H 2 O 2 was completely consumed after 140 s.

These results confirm the higher potential of a homogenous tubular reactor compared to a traditional gas-liquid reactor. Using a gas-liquid reactor, the dissolved ozone concentration was negligible as long as H 2 O 2 was present in solution. It shows that the initiation reaction between O 3 and HO 2 -was fast in the liquid film surrounding the gas-liquid interface with a Hatta number higher than 5 [START_REF] Beltrán | Theoretical aspects of the kinetics of competitive first reactions of ozone in the O 3 /H 2 O 2 and O 3 /UV oxidation processes[END_REF]. Thus, hydroxyl radicals would be generated in the liquid film. Since, these radicals are very reactive and do not diffuse in solution, most of them should react in the liquid film to the detriment of the liquid bulk in which the pollutants concentrations is maximal. Thus, the pollutants oxidation by hydroxyl radicals might be limited by their transport from the liquid bulk to the interface. The use of a homogeneous reactor enables the generation of the radical species directly in the liquid bulk with no mass-transfer and transport limitations. Besides, the process operation would be simplified compared to a traditional gas-liquid reactor due to the fact that both the hydrogen peroxide and ozone concentrations can be controlled independently whereas the dissolved ozone and H 2 O 2 concentrations in a gas-liquid contactor are interdependent.

Furthermore, a continuous tubular reactor appears more beneficial than stirred homogenous reactors used in other studies focused on the peroxone process [START_REF] Rosenfeldt | Comparison of the efficiency of OH radical formation during ozonation and the advanced oxidation processes O 3 /H 2 O 2 and UV/H 2 O 2[END_REF][START_REF] Acero | Oxidation of Acetamide Herbicides in Natural Waters by Ozone and by the Combination of Ozone/Hydrogen Peroxide: Kinetic Study and Process Modeling[END_REF][START_REF] Acero | MTBE oxidation by conventional ozonation and the combination ozone/hydrogen peroxide : efficiency of the processes and bromate formation[END_REF][START_REF] Acero | Characterization of oxidation processes : ozonation and the AOP O 3 /H 2 O 2[END_REF][START_REF] Huber | Oxidation of Pharmaceuticals during Ozonation and Advanced Oxidation Processes[END_REF], with R ct values around 1 or 2 orders of magnitude higher. Two reasons can be proposed. Firstly, a low hydraulic residence time, corresponding to the initial ozone decomposition phase in which the radical generation is enhanced, can be set with a tubular reactor. To achieve a desired oxidation level, a recirculation loop would be advantageously implemented between the tubular reactor outlet and the treated water feed. Secondly, the use of on-line mixers with a high micromixing efficiency allows to reduce the mixing time to an order of magnitude comparable to those corresponding to the initiation reaction (section 3.5).

Prototype proposal based on this proof of concept

This process might be applied both batch wise and continuously to industrial and urban waster waters (Fig. 5). In this study, the ozone transfer, performed in a gas-liquid agitated vessel, was not optimized yet. An intensified gas-liquid contactor such as a hydro-ejector or static mixers, in which a high ozone partial pressure can be applied to reach a high dissolved ozone concentration, should be used. Furthermore, the low hydraulic residence time in this kind of contactor would allow to limit the ozone consumption during the transfer step. The make-up water used to prepare the pre-ozonated solution could be derived from the water to treat if its ozone demand is not too high, or a fraction of the waste water plant outlet can be recirculated. To maximize the ozone transfer efficiency, a recirculation loop on the ozone flux would be beneficial. Thus, the waste water to treat would be continuously spiked with a desired amount of H 2 O 2 pumped with a metering pump from a commercial solution to respect an optimal ratio C HP,0 /C Oz,0 around one. Control loops based on the ozone and H 2 O 2 dissolved concentrations measurement must be implemented. Then, both the preozonated solution and the treated water would be pumped in a short tubular reactor to achieve a reaction time of a few seconds. A recirculation loop would allow to control the pollution level at the process exit. The next step of the process development will be to implement a lab-scale prototype based on these observations, trying to optimize the ozone transfer, the H 2 O 2 injection, the control of initial H 2 O 2 and O 3 concentrations and the reaction step.

Conclusions

The feasibility of using a homogeneous tubular reactor filled with static mixers to intensify the radical generation using the O 3 /H 2 O 2 advanced oxidation process was evaluated. A pre-ozonated solution was continuously mixed to the polluted water spiked with hydrogen peroxide and atrazine. The results emphasized a very high radical generation, with R ct values with orders of magnitude in the range from 10 -7 to 10 -6 , particularly during the first tens of seconds of reaction time, concomitant with a high ozone decomposition rate corresponding to the IOD phase. An optimal ratio of the initial oxidants concentration C HP,0 /C Oz,0 around one, which correspond to the stoichiometry of the initiation reaction between O 3 and HO 2 -, was optimal to enhance the radical generation and to limit the hydrogen peroxide consumption, whereas the ozone consumption was almost complete.

In addition of an intensified hydroxyl radical generation, this configuration allowed advantageously to carefully control the initial O 3 and H 2 O 2 concentrations and the reaction time through the hydraulic residence time. An increasing of the liquid superficial velocity within the static mixers to increase the turbulences was beneficial to improve the micromixing efficiency, the ozone decomposition rate and the R ct value. Both a drinking and a ground waters were studied. The influence of the water matrix on the ozone consumption and on the radical generation was limited but this should be confirmed with other water matrices, especially waste waters with a high ozone demand. A lab scale prototype including a recirculation loop to obtain a desired level of mineralization was proposed and will be tested in the future.

Glossary

AOP: advanced oxidation process C i : concentration of any species i (in mol L -1 or ppm depending on the situation)

  2 O 2 ) prior to each experiment. The pre-ozonated solution was continuously pumped by a centrifugal pump (Iwaki MD-30FX, Japan) to feed both a dissolved ozone analyzer (ATI Q45H/64 probe supplied by Analytical Instruments) at a flow-rate of 30 L h -1 (bypass line) and the Part #4 of the process. The flow-rate of the by-pass line was measured and controlled by a float-type flow-meter Brooks Sho-Rate R6-15-B (USA).

Figure 1 :

 1 Figure 1: Process flow diagram of the experimental set-up.

Figure 2 :

 2 Figure 2: Evolution of  At (a) and  Oz (b) with the reaction time without H 2 O 2 for C At,0 = 0.30 ppm and 3.0 ppm (drinking water, C Oz,0 ≈ 10 -4 mol L -1 , F 1 = 6 L h -1 , F 2 = 2 L h -1 ).

Figure 3 :

 3 Figure 3 : Evolution of  Oz (a) and of C HP /C Oz (b) with the reaction time without atrazine for different ratios C HP,0 /C Oz,0 in the range 0-3.08 (drinking water, C Oz,0 ≈ 10 -4 mol L -1 , F 1 = 6 L h -1 , F 2 = 2 L

Without H 2 O 2 ,

 22  Oz without atrazine was lower at any time than the ones measured with atrazine (whatever the atrazine concentration) according to Fig.2(b) and Fig.3(a). It indicates that atrazine should play the role of a promoter of the ozone decomposition since it poorly affects the R ct but enhance the ozone decomposition[START_REF] Yong | Incorporation of initiation, promotion and inhibition in the R ct concept and its application in determining the initiation and inhibition capacities of natural water in ozonation[END_REF]. Nonetheless, the choice of an initial atrazine concentration of 3.0 ppm in addition of experiments at 0.3 ppm was necessary since at 0.3 ppm, atrazine could not be quantified while C HP,0 ≥ C Oz,0 (part 3.3).

Figure 4 :

 4 Figure 4: Evolution of  At with the reaction time for various ratio C HP,0 /C Oz,0 for C At,0 = 3.0 ppm (a) and 0.30 ppm (b) (drinking water, C Oz,0 ≈ 10 -4 mol L -1 , F 1 = 6 L h -1 , F 2 = 2 L h -1 ).

  comparable reaction time orders of magnitude, Biard et al. (2011), Buffle and von Gunten (2006) and Buffle et al. (2006a,b) observed significant amounts of ozone and parachlorobenzoic acid consumed,

Figure 5 :

 5 Figure 5: Industrial process proposal

F 1 :F 2 :

 12 flow-rate of the waste water at the tubular reactor inlet (L h -1 ) flow-rate of the pre-ozonated solution at the tubular reactor inlet (L h -1 ) F T : total liquid flow-rate in the tubular reactor (L h -1 ) IOD: instantaneous ozone demand k i : reaction rate constant between the atrazine and a species i ( L mol -1 s -1 ) R ct : ratio of the hydroxyl radical exposure to the ozone exposure Re: Reynolds number t : reaction time (s)Greek letters: i : removal efficiency of any species i C i /C i ': ratio of the amount of the species i consumed over the amount of the species i' consumed at a reaction time t

  3.b) when H 2 O 2 was introduced in excess (C HP,0 /C Oz,0 ≥ 1). However, for C HP,0 /C Oz,0 < 1,C HP /C Oz was lower than 0.5. Thus, low inputs of H 2 O 2 are beneficial to enhance the ozone decomposition but with a limited H 2 O 2 consumption. Addition of atrazine in the part 3.3 will allow to confirm if this enhanced ozone decomposition is concomitant with an intensified radical generation.

Table 3 : Values of C Oz /C At , C HP /C At , C HP /C Oz and of the R ct for a reaction time of 18 s (drinking water, C Oz,0 ≈ 10 -4 mol L -1 , F 1 = 6 L h -1 , F 2 = 2 L h -1 ) C HP,0 /C Oz,0 (mol mol -1 ) C Oz /C At (mol mol -1 ) C HP /C At (mol mol -1 ) C HP /C Oz (mol mol -1 ) 10 7 ×R ct

 3

	90% 100%	(a)			18	137	90% 100%
		80%							80%
		70%							70%
	 At	50% 60%						(b)  At	60% 50%
		40%							40%
		30%							30%
		20%							20%
		10%							10%
		0%							0%
			0.22	0.67	1.16	1.47	2.65		0.29	0.71	1.06	1.46	2.47
				C HP,0 /C Oz,0 (mol mol -1 )				C HP,0 /C Oz,0 (mol mol -1 )
		C At,0 (ppm)		3.0	0.30	3.0		0.30	3.0	0.30	3.0 0.30
		0			14.0	278	-		-	-	-	0.42 0.25
		0.22		4.54	90	0.93		24	0.21	0.29	2.12 2.51
		0.67		6.38	62	3.69		28	0.58	0.45	7.23 8.14
		1.16		7.02	68	2.37		38	0.34	0.56	9.63 UD a
		1.47		6.85	74	6.16		50	0.90	0.67	8.74 UD
		2.65		6.82	80	8.92		48	1.31	0.60	9.32 UD
	a : UD = undetermined

Table 4 : Values of  At ,  Oz ,  HP , C Oz /C At , C HP /C At and of the R ct for a reaction time of 18 s and for C At,0 ≈ 3.0 ppm (drinking water, C Oz,0 /C HP,0 ≈ 1.1, F 1 = 6 L h -1 , F 2 = 2 L h -1 ). C Oz,0 /C At,0 (mol mol -1 )  At (%)  Oz (%)  HP (%) C Oz /C At (mol mol -1 ) C HP /C At (mol mol -1 ) 10 7 ×R ct

 4

	2.25	45.1	84.1	39.5	4.20	2.15	5.87
	2.94	61.4	90.9	29.3	4.35	1.62	7.73
	3.68	64.7	94.7	28.7	5.38	1.78	6.94
	4.89	79.5	95.6	16.4	5.88	1.11	8.58
	6.08	88.2	96.8	29.9	6.67	2.34	9.14
	6.79	90.4	93.5	27.1	7.02	2.37	9.63

Table 5 : Values of  At ,  Oz ,  HP and of the R ct for a reaction time of 18 s and for C At,0 = 2.0 ppm (drinking water, F 1 = F 2 , C Oz,0 ≈ 6.55×10 -5 mol L -1 , C HP,0 = 8.08×10 -5 mol L -1 , C At,0 = 9.08×10 -6 mol L -1 ).

 5

t (s) F 1 = F 2 (L h -1 )  At (%)  Oz (%) 

HP (%) 10 7 ×R ct

	35.2	2	82.2	99.6	38.1	4.96
	23.4	3	83.3	99.5	36.6	7.69
	14.1	5	83.9	94.5	41.2	12.5
	11.7	6	83.5	92.4	41.2	14.5
	10.0	7	83.1	89.3	42.8	16.3
	8.8	8	82.8	83.8	44.3	17.5

Table 6 : Comparison in drinking and ground waters of the values of  Oz ,  HP and C HP /C Oz for reaction times of 18 s and for 137 s without atrazine (C Oz,0 ≈ 10 -4 mol L -1 , C Oz,0 /C HP,0 ≈ 1.16, F 1 = 6 L h - 1 , F 2 = 2 L h -1 ).

 6 C HP /C Oz (mol mol -1 )  HP (%)  Oz (%) C HP /C Oz (mol mol -1 )  HP (%)  Oz (%)

		Drinking water			Ground water
	t (s) 18	0.65	58.3	93.6	0.32	25.50	80.3
	137	0.68	64.3	> 99	0.49	49.21	> 99

Table 7 : Comparison in drinking and ground waters of the values of  At ,  Oz ,  HP , C Oz /C At , C HP /C At and of the R ct for a reaction time of 18 s ( C At,0 ≈ 3.0 ppm, C Oz,0 ≈ 10 -4 mol L -1 , C Oz,0 /C HP,0 ≈ 1.16, F 1 = 6 L h -1 , F 2 = 2 L h -1 ). Water nature C HP /C At (mol mol -1 ) C Oz /C At  HP (%)  Oz (%)  At (%) 10 7 ×R ct

 7

	Drinking water	2.37	7.02	27.13	93.52	90.38	9.02
	Ground water	2.05	5.79	22.85	65.77	76.48	4.17

Acknowledgments

The authors wish to thank the Vietnam Ministry of Education and Training and Campus France for their financial supports (Grant N° 793312F & 793315L). We gratefully acknowledge Pierre Largillière for his help for the process design.

Highlights

 A homogeneous tubular reactor was used to apply the O 3 /H 2 O 2 process  Static mixers allowed to improve the micromixing efficiency within the reactor  Outstanding R ct in the range from 10 -7 to 10 -6 were measured  The first seconds of the ozone decomposition phase were the most beneficial  Equimolar initial hydrogen peroxide and ozone concentrations were optimal