Anaëlle Gabet

Hélène Métivier

Christine De Brauer

Gilles Mailhot

Marcello Brigante
email: marcello.brigante@uca.fr

Hydrogen peroxide and persulfate activation using UVA-UVB radiation: Degradation of estrogenic compounds and application in sewage treatment plant waters

Keywords: estrogens, decontamination, wastewaters, hydroxyl radical, sulfate radical, AOPs, photolysis

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Whilst estrogens are known to be hydrophobic and have significantly high partition coefficients (log K ow = 4) [START_REF] Pal | Impacts of emerging organic contaminants on freshwater resources : Review of recent occurrences, sources, fate and effects[END_REF], their presence in surface waters and river sediments has been widely reported around the world [START_REF] Anderson | Endocrine disruption due to estrogens derived from humans predicted to be low in the majority of U.S. surface waters[END_REF][START_REF] Praveena | Occurrence of selected estrogenic compounds and estrogenic activity in surface water and sediment of Langat River (Malaysia)[END_REF][START_REF] Valdés | Screening Concentration of E1, E2 and EE2 in Sewage Effluents and Surface Waters of the "Pampas" Region and the "Río de la Plata" Estuary (Argentina)[END_REF][START_REF] Zuo | Determination of estrogenic steroids and microbial and photochemical degradation of 17α-ethinylestradiol (EE2) in lake surface water, a case study[END_REF]. The most commonly detected estrogens are estrone (E1) and 17β-estradiol (E2), which are naturally excreted by humans and animals, as well as 17α-ethinylestradiol (EE2), a synthetic hormone used in contraceptive pills. It has been reported that these three hormones are the main estrogenic compounds found in domestic sewage treatment plant (STP) effluents [START_REF] Amin | The occurrence, fate, and distribution of natural and synthetic hormones in different types of wastewater treatment plants in Iran[END_REF][START_REF] Desbrow | Identification of estrogenic chemicals in STW effluent. 1. Chemical fractionation and in vitro biological screening[END_REF][START_REF] Huang | Occurrence, removal, and fate of progestogens, androgens, estrogens, and phenols in six sewage treatment plants around Dianchi Lake in China[END_REF]. [START_REF] Laurenson | Ethinyl estradiol and other human pharmaceutical estrogens in the aquatic environment : A review of recent risk assessment data[END_REF] reported that one human being excretes on average 19.00 μg of E1, 7.70 μg of E2 and 0.41 μg of EE2 per day. As a consequence, these hormones tend to reach concentrations up to tens of ng L -1 in domestic STP wastewaters, particularly in urban areas where their impact is significant. The current treatment processes are not sufficient to provide appropriate degradation rates. Therefore, STP effluents which are released into the environment are likely to contain E1, E2 and EE2 at concentrations up to several ng L -1 [START_REF] Amin | The occurrence, fate, and distribution of natural and synthetic hormones in different types of wastewater treatment plants in Iran[END_REF][START_REF] Desbrow | Identification of estrogenic chemicals in STW effluent. 1. Chemical fractionation and in vitro biological screening[END_REF][START_REF] Huang | Occurrence, removal, and fate of progestogens, androgens, estrogens, and phenols in six sewage treatment plants around Dianchi Lake in China[END_REF].

When entering the human or animal body via external sources, these hormones are considered as endocrine disrupting chemicals because their biological activity can interfere with natural hormone activities. It has been reported that these estrogens have a negative impact on river wildlife, particularly downstream from STPs. [START_REF] Purdom | Estrogenic effects of effluents from sewage treatment works[END_REF] have highlighted that the exposure of rainbow trout to EE2 concentrations of approximately 1 ng L -1 resulted in an abnormal production of vitellogenin, a protein normally synthesized only during female gestation. A study carried out on several British rivers has shown that surface water estrogens can cause varying degrees of feminisation in male fish populations [START_REF] Jobling | Widespread Sexual Disruption in Wild Fish[END_REF].

These observations were also reported by [START_REF] Kidd | Collapse of a fish population after exposure to a synthetic estrogen[END_REF] in an in situ study in Canada on fathead minnow. They observed that concentrations around 5 ng L -1 of EE2 impacted the gonadal development of males. Each of these reported effects have led to a disruption in reproduction, and ultimately a decline in the population of wild species within impacted areas.

To avoid the contamination of surface waters by estrogens, the efficacy of various quaternary treatments on domestic STPs is under investigation. There are two categories of treatment, physical and chemical. Physical treatments include the adsorption of micropollutants on activated carbon and membrane processes (microfiltration, nanofiltration, ultrafiltration or reverse osmosis) whereas chemical treatments include advanced oxidation processes (AOPs) such as ozonation and UV based processes [START_REF] Bui | Multicriteria assessment of advanced treatment technologies for micropollutants removal at large-scale applications[END_REF]. The former processes are costly and require specialist treatment facilities. This is a barrier to their use on small and medium domestic STP effluents (< 10 000 inhabitant equivalent). In order to remove estrogen from these effluents, chemical treatments are necessary. [START_REF] Cédat | Are UV photolysis and UV/H2O2 process efficient to treat estrogens in waters? Chemical and biological assessment at pilot scale[END_REF] have highlighted the efficiency of a UVC/H 2 O 2 process to remove the estrogenic activity of estrogen spiked water samples. Numerous recent studies have also shown the ability of sulfate radicals (SO 4 •-), generated through persulfate UVA or UVC photo-activation, to degrade micropollutants in wastewater [START_REF] Hakim | Degradation of theophylline in a UV254/PS system : Matrix effect and application to a factory effluent[END_REF][START_REF] Li | A mechanistic understanding of the degradation of trace organic contaminants by UV/hydrogen peroxide, UV/persulfate and UV/free chlorine for water reuse[END_REF][START_REF] Olmez-Hanci | S 2 O 8 2-/UV-C and H 2 O 2 /UV-C treatment of Bisphenol A: Assessment of toxicity, estrogenic activity, degradation products and results in real water[END_REF][START_REF] Palharim | Comparison between UVAand zero-valent iron-activated persulfate processes for degrading propylparaben[END_REF], including estrogens [START_REF] Angkaew | UV-activated persulfate oxidation of 17β-estradiol : Implications for discharge water remediation[END_REF]. In the majority of cases, the photo-activation in AOPs is carried out with UVC radiation (254 nm). However, they have several disadvantages compared to UVA and UVB radiations: UVC lamps are more expensive and can be hazardous to manipulate. The use of UVA or UVB radiations also aims at more sustainable processes. They are less energyconsuming and could even be replaced by solar light for real-scale application. Lamps are nevertheless easier to use and to control for research purposes than solar light.

In this work we investigated the ability of UVA and UVB activation of H 2 O 2 and S 2 O 8 2processes to degrade three commonly found hormones in wastewaters: 17β-estradiol, estrone and 17α-ethinylestradiol. Preliminary degradation studies were carried out on 17β-estradiol in milli-Q water and few parameters such as oxidant dosage were investigated. Subsequent studies were conducted to establish the efficacy of the technique on STP effluent spiked with the three estrogens. Moreover, estrogenic activity assays were performed during degradation process to follow the harmfulness evolvement during irradiation.

Materials and methods

Chemicals and reagents

17β-estradiol (E2), estrone (E1) and 17α-ethinylestradiol (EE2) were purchased from Sigma-Aldrich, as well as hydrogen peroxide (H 2 O 2 ) (30% in water) and sodium persulfate (Na 2 S 2 O 8 ). Acetonitrile was supplied by Carlo Erba Reagents. Ultrapure water was obtained from a milli-Q system. Wastewaters were collected at the outlet of the treatment from the "3 rivières" urban STP, Clermont-Ferrand, France in September (STPW1) and December (STPW2) 2019. This STP is equipped with a conventional activated sludge process. STP waters were filtered on a paper filter followed by a filtration on a CHROMAFIL® Xtra RC-45/25 syringe filter from Macherey-Nagel. Main physico-chemical parameters after filtration are reported in Table SM1.

Irradiation experiments

UVA and UVB irradiations were carried out in a 150 mL Pyrex reactor, magnetically stirred and kept at room temperature (20°C) by a cooling system. The reactor was placed in a homemade rectangular box equipped on the top with four polychromatic fluorescence tubes (UVA F15W/350BL, Sylvania Blacklight, Germany, or UVB G15T8E, Sanyo Denki, Japan). The UVA (λ max = 352 nm) and UVB (λ max = 308 nm) lamp emission spectra were measured on top of the reactor using an optical fibre and a charge-coupled device spectrophotometer (Ocean Optics USD 2000 + UV-vis), calibrated using a DH-2000-CAL Deuterium Tungsten Halogen reference lamp (Figure SM1). UV-visible spectra of the estrogens, oxidants and STPW2 were carried out with a Cary 300 scan UV-visible spectrophotometer and reported in Figures SM2, SM3 and SM4. H 2 O 2 concentration was followed using p-hydroxyphenylacetic acid (HPAA, purity > 98%) and horseradish peroxidase (POD), according to the spectrofluorimetric quantification method [START_REF] Miller | Hydrogen peroxide measurement in seawater by (phydroxyphenyl)acetic acid dimerization[END_REF] with a Varian Cary Eclipse fluorescence spectrophotometer setting excitation wavelengths at 320 nm and emission maximum at 408 nm. The formation of the dimer of HPAA was correlated with the concentration of H 2 O 2 using standard solutions.

Estrogen stock solutions (1 mM) were prepared in acetonitrile and stored in the dark at 4°C. Solutions of 100 mL containing 5 µM of estrogens and different oxidant precursor concentrations (from 0 to 5 mM) were irradiated under polychromatic wavelengths (UVA or UVB). Such concentration of estrogens do not require the use of pre-concentration techniques before HPLC analysis which avoids a source of errors while using relatively low concentration. 1 mL of solution was withdrawn at fixed interval times for HPLC quantification of estrogen concentrations. However, small volume variations did not impact the irradiation efficacy.

Sample analysis and data processing

Estrogen concentrations were followed using a Waters Acquity Ultra High Performance Liquid Chromatography (UPLC) system equipped with a BEH C18 column (100  2.1 mm, 1.7 µm), coupled to a diode array detector (200-400 nm) and a fluorescence detector (λ ex = 280 nm, λ em = 305 nm). Elution flow rate was 0.6 mL min -1 and eluents were a mixture of milli-Q water and acetonitrile. A gradient raising the acetonitrile percentage from 30% to 70% in 4 minutes and then 1 min constant at 70% was used. Injection volume was 6 µL and column temperature was fixed at 40°C.

Concentration of estrogen during irradiation was fitted by the following first order equation:

where C 0 and C t are respectively the initial concentration and the concentration at time t and k' is the pseudo-first order rate constant.

The error bars associated to the rate data represent 3σ, derived from the scattering of the experimental data around the fit curves (intra-series variability).

Laser flash photolysis

A time resolved spectroscopy was used to determine the second order rate constant of hydroxyl and persulfate radicals with E1, E2 and EE2 but also with the organic and inorganic carbon of the STPW. The second order rate constants allowed to estimate the radicals selectivity between the various species in solution.

All experiments were carried out using the fourth harmonic (λ exc = 266 nm) of a Quanta Ray GCR 130-01 Nd:YAG laser system instrument and an energy of ∼45 mJ/pulse. The experimental setup has been described before [START_REF] Wu | Sulfate Radical Photogeneration Using Fe-EDDS : Influence of Critical Parameters and Naturally Occurring Scavengers[END_REF]. Briefly, for hydroxyl radical reactivity, a competition kinetic method using thiocyanate anion was used and reactivity was determined following the absorption at 450 nm of (SCN) 2

•-transient [START_REF] Huang | UVA-UVB activation of hydrogen peroxide and persulfate for advanced oxidation processes : Efficiency, mechanism and effect of various water constituents[END_REF]. For sulfate radical, the decay (k', s -1 ) of SO 4

•-signal at 370 nm was plotted as a function of quencher concentration (i.e. estrogen or carbon from STPW) concentration. The slope of the linear correlation gives the value of the second order rate constant M -1 s -1 ).

Estrogenic activity removal

The Arxula Yeast Estrogen Screen assay (A-YES) ready-to-use test kit was provided by Newdiagnostics (Germany). This assay allows to quantify the estrogenic activity caused by estrogen-active substances in aqueous samples. The results are obtained in E2 equivalent concentration. It includes the use of genetically modified Arxula adeninivorans yeast cells which contain the gene for human estrogenic receptor. The estrogenic activity of the aqueous solution is correlated to the chromogenic activity of the final solution [START_REF] Hettwer | Validation of Arxula Yeast Estrogen Screen assay for detection of estrogenic activity in water samples : Results of an international interlaboratory study[END_REF].

The assays were carried out in 96-well plates. The calibration standards were analysed in duplicates, in the range of 1 to 80 ng L -1 , along with two blanks. The samples were diluted to fit in the studied range and analysed in triplicates.

Results and discussion

Effects of H 2 O 2 and S 2 O 8 2-under UVA and UVB radiations on the degradation of E2

In Figure 1 . This trend was attributed to the photoactivation of both radical precursors (reactions R1 and R2) leading to the generation of highly oxidative species such as hydroxyl (HO • ) and sulfate (SO 4

•-) radicals in solution. Faster degradation of E2 in the presence of UVB compared to the UVA lamp was predicted considering higher absorption of both oxidant precursors at shorter wavelength irradiations (Figure SM3). . In fact, E2 degradation can be attributed to the higher photolysis yield of radicals in the S 2 O 8 2-system compared to H 2 O 2 .

Oxidant precursors photolysis yield under the polychromatic UVA and UVB lights were determined following their degradations in solutions containing respectively H 2 O 2 and S 2 O 8 2as the only species. 1 mL of methanol (hydroxyl and sulfate radicals quencher) was added to the solutions (100 mL) to prevent self-quenching between the radicals and the oxidant precursors (i.e. radical reactivity) and ensure photolysis as the only degradation path. S 2 O 8 2-photolysis constant (

) were determined to be 1.93 ± 0.09 10 -5 s -1 under UVA radiation and 2.72 ± 0.13 10 -5 s -1 under UVB radiation while for H 2 O 2 lower constants ( ) of 1.48 ± 0.07  10 -6 s -1 under UVA radiation and 6.08 ± 0.33  10 -6 s -1 under UVB radiation were measured.

In Figure 2, the correlation between E2 pseudo-first order rate constant and oxidant precursor concentrations is presented. An increase of E2 degradation was observed when the andR4). Hydrogen peroxide plays a role of hydroxyl radical scavenger, enhanced at high concentrations (Table SM2).

Considering the H 2 O 2 initial concentration and the second order rate constants between HO • and E2 (Table 1) ( = 2.9  10 10 M -1 s -1 ) and between HO • and H 2 O 2 ( = 2.7  10 7 M -1 s -1 ) [START_REF] Buxton | Critical Review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals (⋅OH/⋅O-in Aqueous Solution[END_REF], we can estimate that in the presence of H 2 O 2 at 5 mM, about 48% of the hydroxyl radicals react through reaction R3 leading to the strong decrease of the reactivity towards E2 and the formation of less reactive species i.e. HO 2

• /O 2 • -(pKa = 4.88). HO • + H 2 O 2 → H 2 O + HO 2 • (R3) HO • + E2 → E2ox (R4)

Effect of STP effluent on the degradation of E2 in the different systems

In this work, two STP effluents sampled at different times of the year were characterised.

Similar E2 degradation rates were obtained when spiked into STPW1 and STPW2. STPW2

will be considered for the rest of this study. In ) ions are known as possible interfering species during radical based degradation processes [START_REF] Tao | Phenanthrene decomposition in soil washing effluents using UVB activation of hydrogen peroxide and peroxydisulfate[END_REF][START_REF] Zhang | Impact of Chloride Ions on UV/H 2 O 2 and UV/Persulfate Advanced Oxidation Processes[END_REF]. The inhibition effect on E2 degradation was tested for each individual ion to their concentration measured in STPW2 (Table SM1) and no significant impact was observed (Figure SM5). Only in the presence of chloride ions a slight inhibition (< 5%) of E2 degradation can be observed and attributed to the formation of less oxidant species such as dichloride radical ions (Cl 2

•-

) [START_REF] Armstrong | Standard electrode potentials involving radicals in aqueous solution: inorganic radicals (IUPAC Technical Report)[END_REF].

The composition of the organic matter in STPW2 has not been determined. Several studies highlighted that effluent compositions have a large variation range, depending on the influent characteristics but also on the type of treatment process upstream [START_REF] Imai | Characterization of dissolved organic matter in effluents from wastewater treatment plants[END_REF][START_REF] Ma | Characterization of isolated fractions of dissolved organic matter from natural waters and a wastewater effluent[END_REF][START_REF] Yu | Effect of effluent organic matter on the adsorption of perfluorinated compounds onto activated carbon[END_REF][START_REF] Zhang | Characterization of isolated fractions of dissolved organic matter from sewage treatment plant and the related disinfection byproducts formation potential[END_REF]. In this work, the organic matter reactivity has been standardised on the organic carbon reactivity.

Considering that the reactivity of organic carbon with hydroxyl and sulfate radicals is respectively = 2.5  10 8 M C -1 s -1 and = 2.2 10 8 M C -1 s -1 (Table 1), we can estimate that about 82% and 74% of HO • and SO 4 •-are scavenged by organic matter of STPW2 in the H 2 O 2 and S 2 O 8 2-systems respectively. The inhibition effect is not far from the experimental value reported in Table 2 and shows that the organic matter is mainly responsible for slowing down E2 degradation in STPW2. In their study on the degradation of Bisphenol A, [START_REF] Olmez-Hanci | S 2 O 8 2-/UV-C and H 2 O 2 /UV-C treatment of Bisphenol A: Assessment of toxicity, estrogenic activity, degradation products and results in real water[END_REF] have also determined that the natural organic matter from raw freshwaters was the main HO • and SO 4 •-scavengers, and [START_REF] Ghauch | Contribution of persulfate in UV-254 nm activated systems for complete degradation of chloramphenicol antibiotic in water[END_REF] reported that inorganic anions had a minor implication in the degradation inhibition.

However, [START_REF] Ma | Impacts of inorganic anions and natural organic matter on thermally activated persulfate oxidation of BTEX in water[END_REF] observed that carbonates were also significant scavengers, whereas the scavenging effect of chlorides depended on the studied pollutants and their concentration. k' (s -1 ) in milliQ water 1.3  10 -4 1.7  10 -3 9.9  10 -4 8.8

 10 -3 k' (s -1 ) in STPW2 effluent 1.4  10 -5 1.2  10 -4 1.2  10 -4 1.1  10 -3
Inhibition (%) 89 93 88

Comparison between E1, E2 and EE2 degradation under UVB radiation

Figure 3 illustrates E1, E2 and EE2 degradations under UVB radiation of a mixture (5 µM each) in milliQ water and in STPW2. For the same reasons as explained about E2 in section 3.1., E1 and EE2 degradations are faster when using S 2 O 8 2-than in the presence of H 2 O 2 due to the higher sulfate radical formation rate. The study was carried out with 2 mM of oxidant precursors in order to minimize the oxidant precursors quenching effect (see section 3.1.). It was also seen that all the hormones degradations are inhibited in a wastewater matrix.

In all the studied systems, E1 degradation was faster than E2 and EE2 degradations, which are similar. The three hormones are subjected to photolysis and photo-induced degradation with oxidant S 2 O 8 2-or H 2 O 2 . As seen in Table 1, second order reaction rate constants between each hormone and HO • or SO 4

•-have the same order of magnitude. Therefore, E1 faster degradation is explained by its faster photolysis. In milli-Q water only, Figure 4 shows that E1 reached 95% degradation after 4 hours under UVB radiation while E2 and EE2 degradations were around 15%.

This phenomenon also advantaged E1 degradation in STP water. As seen on Figure 3, its degradation was less inhibited than E2 and EE2 degradations by STP water constituents, particularly when using H 2 O 2 (inhibition of 33%). Because hydrogen peroxide has a slower degradation effect compared to persulfate, photolysis is effective in larger proportion. It also allows E1 degradation to be less impacted by the inhibition effect from the STP water scavengers.

Estrogenic activity removal and effect of hormone mixture under UVB radiation

The aim of the YES assay was to ensure that the estrogen degradation goes along with a decline in the estrogenic activity of the sample, responsible for its harmfulness. The estrogenic activity of a sample was expressed in E2 equivalent concentration. The degradation of a mixture of the three hormones (0.5 µM each) in milli-Q water under UVB radiation and oxidant precursors (0.1 mM) was investigated. Concentrations of hormones were lower than previous experiments in order to get closer to environmental concentrations, however the ratio estrogen/oxidant precursor was similar to keep consistency. The estrogenic activity of the mixture is reported in Figure 5 and compared to the degradation of each estrogen during UVB irradiation. The use of both oxidant precursors shows a concordance between the decrease in E2 and EE2 concentrations and the decline in estrogenic activity. E1 faster degradation seems to have a minor effect on the estrogenic activity of the solution, which is mainly governed by E2 and EE2 concentrations because of their higher estrogenic potencies. Considering that the estrogenic potency of E2 is 1, those of E1 and EE2 are respectively 0.1 and 1.2 in agreement with literature data [START_REF] Murk | Detection of estrogenic potency in wastewater and surface water with three in vitro bioassays[END_REF]. Theoretical estrogenic activity of the solution based on the estrogenic potencies of each estrogen and on their concentrations have the same order of magnitude than experimental values. Therefore, degradation products do not seem to have a significant impact on the total estrogenic activity of the solutions. This could be due to a low estrogenic activity of the by-products or to the fast disappearance of potentially high estrogenic activity compounds.

However, while the estrogen concentration has fallen below 99.9% of the initial concentration after 24 hours when using H 2 O 2 and after 3 hours when using S 2 O 8 2-

, the estrogenic activity remains around 30 nM which represents almost 3% of the initial estrogenic activity. It is unknown whether this is due to the persisting estrogenic activity potentially caused by degradation by-products, to the proximity with the limit of quantification (about 15 nM), or to an experimental contamination. Anyway, after 24 hours of treatment using H 2 O 2 and 3 hours using persulfate, a strong decrease of estrogenic activity, more than 97%, is obtained.

Figure 5. Estrogenic activity assessment of a mixture of E1, E2 and EE2 (0,5 µM each) depending on UVB irradiation time.

H 2 O 2 0,1 mM in A, S 2 O 8 2-0,1 mM in B, pH 6.

Conclusion

In this work, UVA and UVB photoactivation of hydrogen peroxide and persulfate were tested for the degradation of three common estrogens: E2, E1 and EE2. It was seen that UVA and UVB radiations are both efficient to produce hydroxyl and sulfate radicals, although photoinduced degradation is faster under UVB radiation. However, higher efficiency of sulfate radicals formation was observed compared to the hydroxyl radicals under these irradiation wavelengths. Although an increase in the oxidant precursor concentration produced faster degradation, this phenomenon did not follow a linear trend because the radicals are quenched by the oxidant precursors, particularly with H 2 O 2 .

In a mixture, E2, E1 and EE2 are competing to react with the generated radicals because the different reaction constants are similar. E1 was seen to degrade faster because it undergoes higher photolysis than E2 and EE2 under UVB radiation. E2 degradation speed was slowed down by approximately 90% in a STP effluent. Experiments have shown that the dissolved carbon and particularly the organic carbon present in the matrix was the main quencher of the hydroxyl and sulfate radicals. The YES assay was seen to give very enriching data confirming that the studied processes allow to remove efficiently the estrogenic activity responsible for the estrogens harmfulness in the environment. These experiments were carried out on a labscale. However, further experiments on a pilot or real scale are required to fully assess the different processes efficiency on real wastewaters.

Declaration of Competing Interest

All authors declare no conflict of interest.

Figure 1 .

 1 Figure 1. Effect of the oxidant precursor concentration (from 0 to 5 mM) on E2 (5 µM) degradation in milli-Q water at pH 6 under various conditions: UVA/H 2 O 2 (A), UVA/S 2 O 8 2-

  photogenerated sulfate radical can be estimated to only 18%. The calculations for all oxidant

Figure 2 .

 2 Figure 2. E2 pseudo-first order rate constants (s -1 ) depending on the oxidant precursor concentrations at pH 6: H 2 O 2 (A) and S 2 O 8 2-(B), under UVA and UVB radiations. Dashed red lines estimate the curves without the oxidant precursors scavenging effects. The error is ± 3σ, obtained from the scattering of the experimental data.

Figure 3 .

 3 Figure 3. E1, E2 and EE2 pseudo first-order degradation rate constants in a mixture (5 µM each) under UVB radiation. Comparison between the use of H 2 O 2 and S 2 O 8 2-(2 mM) as an oxidant precursor and milli-Q water (pH 6) and STP wastewater (pH 8) as a matrix. Inhibition percentages between milli-Q water and STPW2 are mentioned.

Figure 4 .

 4 Figure 4. E1, E2 and EE2 photolysis under UVB radiation in milli-Q water (pH 6.5).

  , E2 degradations were followed under UVA and UVB radiations in the presence

	of different H 2 O 2 or S 2 O 8	2-concentrations (from 0 to 5 mM). Despite no photolysis observed
	under UVA, about 15% degradation of E2 was observed after 4 hours under UVB. In all
	systems, E2 undergoes faster disappearance in the presence of H 2 O 2 and S 2 O 8 2-

Table 1

 1

	•-with estrogens or STPW2 carbon, at neutral pH. Data are in M -1 s -1 for . Second order rate constant of HO • and SO 4 estrogens and in M C -1 s -1 for STPW2. TOC constants were determined after acidification of STPW2 to pH 4 to remove the
				inorganic carbon.
		E2		EE2	E1	STPW2 TC	STPW2 TOC
	2.91±0.09  10 10	1.81±0.02  10 10	2.85±0.03  10 10	2.8±0.1  10 8	2.5±0.1  10 8
	2.66±0.03  10 9		1.84±0.02  10 9	4.11±0.04  10 9	2.4±0.1  10 8	2.2±0.2  10 8
	On the contrary, S 2 O 8 2-scavenging effect is minor due to the lower reactivity constant
	between SO 4	•-and S 2 O 8	2-(	= 6.1  10 5 M -1 s -1 ) (McElroy & Waygood, 1990)

Table 2

 2

	, E2 pseudo-first order degradation rate

Table 2

 2

	. E2 pseudo-first order degradation rate constants in milli-Q water or STPW2 in the presence of H 2 O 2 or S 2 O 8 2-(2
	mM) under UVA/UVB radiations at neutral pH.
	UVA/H 2 O 2	UVA/S 2 O 8 2-	UVB/H 2 O 2	UVB/S 2 O 8 2-

Acknowledgements

The authors wish to acknowledge the financial support of the "Région Auvergne-Rhône-Alpes" for their financial support through the "Pack Ambition Recherche". This work was performed within the framework of the EUR H2O'Lyon (ANR-17-EURE-0018) of Université de Lyon (UdL), within the program "Investissements d'Avenir" operated by the French National Research Agency (ANR). This work was also supported by the "Federation des Recherches en Environnement" through the CPER "Environnement" founded by the "Région Auvergne," the French government and FEDER from European community. The authors thank Muriel Joly for her involvement and her help in conducting the A-YES assays.


Zhongyi Fang

Jie Zhao

Yuan Li

Yu Wang

Tian Qiu

Yanlin Wu
email: wuyanlin@fudan.edu.cn

Wenbo Dong
email: wbdong@fudan.edu.cn

Gilles Mailhot

Improving Fenton-like System with Catechin, an Environmental-friendly Polyphenol: Effects and Mechanism

Keywords: Fenton system, plant polyphenol, catechin, hydroxyl radical, reduction mechanism 1

Fenton-like system had long been modified to promote its adaptability and efficiency by adding extra components like metal chelators or reducers. Nonetheless, chelator such as ethylene diamine tetraacetic acid (EDTA) was harmful in high dosage.

While strong reducers could efficiently promote the generation speed of hydroxyl radicals (HO • ), they could also get oxidized easily during storage time, for which further application was costly. In this study, (+)-catechin (CAT), an environmentalfriendly component, was proved to be an efficient promoter of Fenton-like system on Bisphenol A (BPA) degradation. The modified system achieved 92% of BPA removal in one hour. The mechanism was studied, and the reaction procedure between CAT and Fe(III) together with the degradation pathways of CAT were identified. Fe(III) got chelated first by CAT and quickly reduced to Fe(II) via a single inner-compound electron transfer. The intermediates of CAT could further reduce one and a half more potion of Fe(III) on average. Consequently, massive amount of Fe(II) turned Fenton-like procedure to Fenton reaction and thus promoted the generation of HO • .

Introduction

Since the Fenton reagent was firstly introduced by H.J.H Fenton in 1894 [START_REF] Fenton | LXXIII.-Oxidation of tartaric acid in presence of iron[END_REF], it had been developed to several branches, such as Fenton-like process, photo-Fenton process, and ligand-modified Fenton process to extend its applicability and improve efficiency. Those processes had shown high efficiency of degrading some persistent organic pollutants (POPs) or endocrine disruptor chemicals (EDCs) by producing large amount of HO • based on the reaction illustrated as Eqs (1)-(4) (Fenton-like process: 1-4; Fenton process: 4). In latest cases, the Fenton and Fenton-like processes had already been applied in industrial wastewater treatment in real situation. Bautista et al. [START_REF] Bautista | Application of Fenton oxidation to cosmetic wastewaters treatment[END_REF] utilized traditional Fenton system to treat wastewater of low BOD 5 /COD rate from cosmetic industry. Changotra et al. [START_REF] Changotra | Treatment of real pharmaceutical wastewater using combined approach of Fenton applications and aerobic biological treatment[END_REF] treated real pharmaceutical wastewater with three different Fenton processes combined with aerobic biological treatment.

Although those applications were proved to be successful to some extent, there were still some defects yet to improve. The major limitation of Fenton processes was pH value. To overcome the limitation on pH, chelators such as EDTA, oxalate, citrate and [S,S]-Ethylene diamine-N,N′-disuccinic acid (EDDS) were added to promote the system. Hu et al. [START_REF] Hu | EDTA-Fe(III) Fenton-like oxidation for the degradation of malachite green[END_REF] achieved 92.7% malachite green degradation in neutral pH with EDTA-modified Fenton-like system. Manenti et al. [START_REF] Manenti | Insights into solar photo-Fenton process using iron(III)-organic ligand complexes applied to real textile wastewater treatment[END_REF] tested oxalate, citrate and EDDS as organic ligands and accomplished 69.1% mineralization with modified photo-Fenton system. However, EDTA had low biodegradability [START_REF] Tandy | Extraction of heavy metals from soils using biodegradable chelating agents[END_REF][START_REF] Huang | Development of a new homogenous photo-Fenton process using Fe(III)-EDDS complexes[END_REF][START_REF] Metsärinne | Photodegradation of ethylenediaminetetraacetic acid (EDTA) and ethylenediamine disuccinic acid (EDDS) within natural UV radiation range[END_REF]. Hence, it was crucial to find an efficient chemical compound which i) improved the Fenton process efficiency and ii) was easy to degrade. Plant polyphenols were a group of organic compounds with multiple phenol structures generated in plant metabolism procedure. They were often detected in a vast amount in roots, leaves, fruits, mass-produced food and beverage made from plants. According to previous researches, fruits such as grapes and berries might contain 0.3‰ of polyphenol in fresh weight, especially in tea leaves [START_REF] Zuo | Simultaneous determination of catechins, caffeine and gallic acids in green, Oolong, black and pu-erh teas using HPLC with a photodiode array detector[END_REF]. The concentration of polyphenols could reach up to 30% in dry weight of green tea [START_REF] Graham | Green tea composition, consumption, and polyphenol chemistry[END_REF].

The naturally high productivity showed the potential for exploitation and manufacture of certain polyphenol. Polyphenols were also believed to bring health benefits on cardiovascular protection and anti-oxidation, and they were becoming popular as the additive or health care products [START_REF] Graf | Flavonols, flavones, flavanones, and human health: epidemiological evidence[END_REF][START_REF] Arts | Polyphenols and disease risk in epidemiologic studies[END_REF][START_REF] Pandey | Plant polyphenols as dietary antioxidants in human health and disease[END_REF]. The associated benefits of these compounds were often based on the reducibility of polyphenols [START_REF] Sabaikai | UV light photo-Fenton degradation of polyphenols in oolong tea manufacturing wastewater[END_REF]. From the perspective of chemical structure, the sensitivities toward oxidant of both o-phenolic hydroxyl group and p-phenolic hydroxyl group were accounted for the reducibility of polyphenol which raised the possibility to modify traditional Fenton-like system as a reducer.

Catechin was polyphenol classified under flavonoid category. They were typical flavan-3-ols, characterized by a resorcinol group on its A ring, and a catechol group on its B ring. Due to the two chiral carbons, there were four isomers, including (+)-catechin, (-)-catechin, (-)-epicatechin and (+)-epicatechin. However, (+)-catechin (Scheme 1) was the major catechins present in green tea [START_REF] Rai | Assessment of preclinical effect of (+)-catechin hydrate on sexual function: an in silico and in vivo study[END_REF][START_REF] Spizzirri | Synthesis and characterization of a (+)-catechin and L-(+)-ascorbic acid cocrystal as a new functional ingredient for tea drinks[END_REF] and was chosen as the research target in this study. As stated in previous studies [START_REF] Hynes | The kinetics and mechanisms of the reaction of iron(III) with gallic acid, gallic acid methyl ester and catechin[END_REF], the catechol group on its B ring provides possible chemical reactivity both in chelation and reduction, whereas the process was not clear. Wang et al. [START_REF] Wang | Catechin as a new improving agent for a photo-Fenton-like system at near-neutral pH for the removal of inderal[END_REF] studied the photodegradation of inderal in a UV/Fe(III)/CAT system at near neutral pH. The high degradation efficiencies were obtained and they were strongly dependent on pH, following the order pH 3.0 < 5.0 < 7.0 < 6.0. The reaction between Fe(III) and CAT was considered as chelation. Wu et al. [START_REF] Wu | Enhancement of iron-mediated activation of persulfate using catechin: From generation of reactive species to atenolol degradation in water[END_REF] reported the degradation of atenolol in Fe(III)/CAT /S 2 O 8 2system at acid condition (pH=3.3). The mechanism was concluded as the reduction of Fe(III) by CAT to produce Fe(II) and PS was activated by Fe(II) to produce large amount SO 4

•-. In general, there were three major explanations of the reaction between Fe(III) and CAT respectively, regarding CAT as a ligand [START_REF] Wang | Catechin as a new improving agent for a photo-Fenton-like system at near-neutral pH for the removal of inderal[END_REF], a reducer [START_REF] Wu | Enhancement of iron-mediated activation of persulfate using catechin: From generation of reactive species to atenolol degradation in water[END_REF][START_REF] Grzesik | Antioxidant properties of catechins: Comparison with other antioxidants[END_REF] or both a ligand and reducer at the same time [START_REF] Hynes | The kinetics and mechanisms of the reaction of iron(III) with gallic acid, gallic acid methyl ester and catechin[END_REF], yet none of these theories had been fully confirmed. Based on the chemical reactivity and its environmental-friendly feature as a plant polyphenol, CAT was potentially suitable to modify the conventional Fenton system.

Scheme 1. Structure of (+)-catechin BPA was a kind of EDCs which had estrogenic activity by interacting with estrogen receptors in human body. As reported in previous study, the maximum concentration in wastewater could reach 0.15 mg/L in some Canadian and US municipal wastewater treatment plants [START_REF] Mohapatra | Physico-chemical pre-treatment and biotransformation of wastewater and wastewater sludge--fate of bisphenol A[END_REF], and because it was hard to biodegrade [START_REF] Dorn | Degradation of bisphenol A in natural waters[END_REF][START_REF] Tian | Photocatalytic Degradation of Bisphenol A in Wastewater Using Nanometer TiO 2 Film[END_REF], leftover BPA in effluent could still be detected. In this study, BPA was used as a model pollutant. The effect of CAT on the modified Fenton-like system on BPA degradation was investigated. Mechanism of the reaction between CAT and Fe(III), radical identification, effect of pH value and key influencing factors were carefully studied. The degradation pathway of BPA was also reported. added at the concentration level of this study [START_REF] Sharma | Mechanistic study of photo-oxidation of Bisphenol-A (BPA) with hydrogen peroxide (H2O2) and sodium persulfate (SPS)[END_REF]. Fe(III) were the last one to be mixed into the solution of BPA, CAT and H 2 O 2 , working as a trigger of the degradation procedure and making this step the time zero point. Immediately after each sampling, methanol was added to the samples in order to scavenge radicals and stop the degradation. To prevent photolysis, the experiments were conducted in dark brown glass vials and HPLC vials were sheltered from light using an auto-sampler with a hood. All degradation experiments were conducted at least twice.

Methods

Analysis method

The concentration of BPA was determined by high-performance liquid chromatography (U3000, Dionex, USA) with Diode Array Detector (HPLC-DAD).

The detecting wavelengths were set at 226 nm and 280 nm to preclude the interference from the signal of intermediates. Samples of 10 μL each, pretreated with methanol were injected into a C-18 reversed phase column (150 mm × 4.6 mm × 5 μm, Kromasil, Sweden). The temperature of the column oven was set at 40°C. The mobile phase containing 70% of methanol (phase A) and 30% of water (phase B) was pumped into the system at the flow rate of 1 ml min -1 [START_REF] Cespedes | Integrated procedure for determination of endocrine-disrupting activity in surface waters and sediments by use of the biological technique recombinant yeast assay and chemical analysis by LC-ESI-MS[END_REF]. Fe(II) concentration were spectrophotometrically measured based on the reaction between Fe(II) and ferrozine which formed a complex that was detected by UV-vis spectrophotometer at 562 nm an ultra high performance liquid chromatography (UHPLC) in full scan mode.

Stopped-flow Method for Intermediates Analysis.

Intermediates produced by the reaction between CAT and Fe(III) over the time span of several seconds were identified by UV-vis spectrum equipped with Stopped-flow mixing system (Applied Photophysics, SX20). The system was set to take 1000 scans in each time gap from 350 nm to 750 nm.

Computational method.

The main computational methods were density functional theory (DFT) and time-dependent density functional theory (TDDFT). Gaussian09 program (Frisch et al., 2009) was used to perform both DFT and TDDFT calculations, and the three-parameters hybrid density-functional (B3LYP) was used in all DFT calculations setting to 6-31 + g (d,p) basis. All excited-state geometry optimizations as open shell species were described with an unrestricted approach, while ground-state geometry optimizations as closed-shell species were explained with a restricted method.

Results and discussion

Effect of Fenton-like system modified by CAT on BPA degradation

The effect of CAT-modified Fenton-like system on degradation of BPA was compared with that of conventional Fenton-like system and photo-Fenton system. To rule out the effect attributed to potential reaction, in presence of CAT, between BPA and H 2 O 2 or reaction between BPA and Fe(III), experiments excluding one of that four basic components were also carried out (Fig. 1).

Role of ROS in the degradation process

Reactive oxygen species (ROS) were often considered as the core of traditional , experiments with air-saturated solution were also conducted so that potential impact from DO or indirect contribution of O 2 •-would not be omitted. Air flow was pumped into all the solutions with 20 minutes before the experiment. However, rather than promoting the BPA degradation, the result showed

Role of CAT

As the result above showed, CAT barely changed the way how traditional Fenton-like system worked which still relied on the oxidation by HO • , but it improved the efficiency of BPA removal. Although the mechanism was unclear, it was inferred that Fe(III) might react with CAT. To test the assumption, Fe(II) was determined in the mixture solution of Fe(III) (50 μM) and CAT (6.25-100 μM) (Fig. 4). As the result, more than 80% of Fe(III) were reduced to Fe(II) in an hour, when CAT : Fe(III) was more than 1:2. While the concentration of CAT was less than a quarter of Fe(III), the formed Fe(II) concentrations were varied depending on initial concentration of CAT, but with a constant proportional relationship of roughly 2.5 times of CAT. The degradations of CAT were also followed and the results were shown in Fig. SM2.

When the CAT concentrations were lower than 25 μM, they were quickly 100% consumed at the beginning 3 min of the reaction. Data of the experiment revealed a reduction process between CAT and Fe(III), which was in accordance to some studies on catechol and other polyphenol substances [START_REF] Mentasti | Reactions between iron(III) and catechol (o-dihydroxybenzene). part I. Equilibria and kinetics of complex formation in aqueous acid solution[END_REF]. Nonetheless, previous studies also hinted that catechol or compounds with similar functional group tended to react as a complex agent when Fe(III) was involved [START_REF] Andjelkovic | Iron-chelation properties of phenolic acids bearing catechol and galloyl groups[END_REF][START_REF] Aguiar | Mecanismo e aplicações da reação de fenton assistida por compostos fenólicos redutores de ferro[END_REF]. The reduction procedure was, therefore, deduced to be a set of complicated reaction instead of a single elementary reaction. To gain a deeper insight into the reduction process, Stopped-flow UV-vis spectrum was obtained (as shown in Fig. 5). In the first second, absorbance rose steeply at 390 nm, 450 nm and 685 nm, implying formation of multiple compounds.

Four seconds after mixing, absorbance at two wavelengths (450 nm and 685 nm) peaked and diminished gently to a low level while the other one (390 nm) still kept on growing. Since the peaked times at those wavelength were different from each other, it could be confirmed that one terminal product and two intermediates were produced during the reaction. The intermediate with absorbance at 685 nm was consistent with catechol-group complexed Fe(III). The terminal product (named as CatQ 2 ) and the other intermediate (named as CatQ 1 ) both had strong absorbance near 390 nm, which were estimated to possess quinone, semi-quinone or comparable structure [START_REF] Hynes | The kinetics and mechanisms of the reaction of iron(III) with gallic acid, gallic acid methyl ester and catechin[END_REF]. Grounded on the structure of CAT, the hydroxyl group on B ring was the only complexing group but the easiest part to be oxidized to quinone as well. It meant that the complex of The structures were also double checked via computational method. To further confirm the structure of three products, the UV-vis spectrum of them were compared with the computational result of DFT model. The DFT model simulated the possible spectrum based on the input structure, and the results were in accordance with the spectrum obtained by the Stopped-follow UV-vis spectrum instrument (Table 1).

According to the results of ESR, HO • In summary, most of the Fe(III) were complexed with CAT and then reduced by an inner-complex electron transfer to form Fe(II). Since Fe(II) could barely complex with catechol group [START_REF] Mentasti | Reactions between iron(III) and catechol (o-dihydroxybenzene). part I. Equilibria and kinetics of complex formation in aqueous acid solution[END_REF][START_REF] Aguiar | Mecanismo e aplicações da reação de fenton assistida por compostos fenólicos redutores de ferro[END_REF], the complex decomposed quickly and released a huge quantity of Fe(II) to the system, which promoted the generation of HO

Effect of CAT/Fe(III) ratio, pH and common anions

To seek the best condition of the modified system on BPA degradation, several influencing factors were tested or introduced to the system. Ratio impact of CAT and Fe(III) was tested from 1/8:1 to 2:1. Based on the data, two different trends which were divided by inflection point between 1/2:1 and 1/4:1 were noticed (Fig. 6a).

When ratio went down from 2:1 to 1/2:1, BPA got removed much faster and more thoroughly over the experiment. As the ratio went deeper down to 1/4:1 or even lower, BPA removal gradually slowed down, but the similar removal effect was achieved.

The outcome was partially in conformity with the pattern of Fe(II) generation: as the CAT outnumbers Fe (III) in 1/3 or more, some CAT was spared and might compete on HO • that hindered BPA removal [START_REF] Husain | Hydroxyl radical scavenging activity of flavonoids[END_REF]. As for the similarity in removal effect in low CAT/Fe(III) ratio, it showed the efficiency of CAT modification was quite high.

Nonetheless, numerous products were created during the degradation process, which blocked further degradation. In order to get the best degradation result, following experiments applied the ratio of CAT and Fe(III) as 1/6:1. Concerning that Fe(III) formed different hydroxide species under different pH, the initial pH value was tested as another influencing factor. The data illustrated in Fig. 6b showed that the system worked well in both acidified and neutral environment.

There were more than 70% of BPA removal but the degradation of BPA was completely stopped if the pH value was above 8.0. It was known that Fe(III) and Fe(II) would completely precipitate in pH 8.0 or above, and as a result, the system stopped yielding HO • . On the other hand, less BPA was degraded as the pH went down in high acidity solution (pH=3). This phenomenon happened since H + competed with Fe(III) on CAT complexation and changed the distribution of Fe(III) species. Either of the factors might have negative impacts on BPA degradation.

In this study, all chemicals were solved in deionized water or HPLC class organic solvent, while in real water treatment, inlet water possibly contained varieties of anions that may bring unknown influence. To ensure the modified system was also suitable for water treatment accompanied with anions, several common anions were selected to add in the reaction solution (Fig. 6c). In the experiments, all the tested anions including chloride (Cl -), phosphate (PO

Conclusion

The addition of CAT promoted the reduction reaction from Fe(III) to Fe(II) through a five-step process: complexation, reduction, release, second reduction and final reduction, leading to a higher productivity of HO • in the Fenton-like system.

Thus, CAT was proved to be an efficient agent to modify Fenton-like system in neutral pH on BPA degradation and very possibly on removal of many other compounds which was also sensitive towards HO • . Moreover, the result demonstrated that the modified system kept its efficiency not only in neutral pH in deionized water, but also in acidic environment or with common anions. The degradation pathway of BPA had also been studied (shown in Fig. SM3). Considering the high potential for massive production of CAT and other plant polyphenols, together with their negligible risk of ecosystem and human health, results of this study present a promising future of applying plant polyphenols in Fenton-like system to promote its efficiency in real

[ 26 ]

 26 . H 2 O 2 concentration was determined by a dosage method based on the reaction between H 2 O 2 and POPHA under the catalysis of peroxidase [27]. Electron spin resonance (ESR) experiments were carried out on a Bruker spectrometer (JEOL, FA200). 5,5-Dimethyl-1-Pyrroline-N-Oxide (DMPO) was used as a spin trap to detect HO • and O 2 •-in water and ethanol, respectively. The reaction intermediates were analyzed by quadruple time-of-flight (Q-TOF) mass spectrometry (MS) coupled with

Fig. 1 .

 1 Fig. 1. Degradation of BPA in different system. ([BPA] 0 = 2 mg L -1 /8.76 μM, [H 2 O 2 ] 0 = [CAT] 0 = [Fe(III)] 0 = 50 μM, pH=6.5)

Fenton-like system

  on contaminant removal. To fully understand the significant change brought by CAT and explore the role of different ROS in the degradation process, ESR analysis was performed with DMPO as spin trapping reagent. The results were shown in Fig.2. In ultrapure water, ESR displayed signal around 333.5 mT in magnetic field with four peaks in 1:2:2:1 ratio, which were the characteristic pattern for HO •[START_REF] Finkelstein | Spin trapping. Kinetics of the reaction of superoxide and hydroxyl radicals with nitrones[END_REF]. Ethanol was used as solvent in another test to quench HO • , so that patterns from other radicals were not affected by signal overlapping. As illustrated in Fig.2b, the dominant signal was in 1:1:1:1 ratio, accompanied with two minor signals after the second and third peak which was often interpreted as the existence of superoxide anion radicals (O 2 •-)[START_REF] Finkelstein | Spin Trapping of Superoxide[END_REF][START_REF] Zhang | Degradation of p-nitrophenol using a ferrous-tripolyphosphate complex in the presence of oxygen: The key role of superoxide radicals[END_REF]. It was concluded from the ESR analysis that both HO • and O 2•-were produced.

Fig. 2 .

 2 Fig. 2. ESR analysis performed in different solvent: (a) ultrapure water (b) ethanol. ([H 2 O 2 ] 0 = [CAT] 0 = [Fe(III)] 0 = 50 μM, pH=6.5)To evaluate the role of radicals in the degradation process, two kinds of scavengers were added to the system simultaneously with Fe(III) and the results of quenching experiments were shown in Fig.3. By adding isopropanol into the solution, HO • were quenched before they could attack target contaminants, and as consequence, the concentration of BPA stopped decreasing in just 10 minutes. While the concentration of isopropanol was increased from 5 mM to 10 mM, the BPA degradation was completely inhibited. Chloroform was added to quench O 2•-(k CHCl3, O2 •-= 3.0×10 10 M -1 s -1[START_REF] Haag | Rate constants for reaction of hydroxyl radicals with several drinking water contaminants[END_REF]), but it did not show much impact on the BPA degradation.Comparing to experiments without any scavengers, just 3.7% of BPA degradation was inhibited at the end of the reaction. The 3.7% constraint on BPA removal was probably the result from the competition between chloroform and BPA in reacting with HO • instead of eliminating the effect of O 2•-, for the reaction rate constant between chloroform and HO • was k CHCl3,HO • = 5.0×10 7 M -1 s -1[START_REF] Haag | Rate constants for reaction of hydroxyl radicals with several drinking water contaminants[END_REF].Although O 2 •-were not involved in direct attack on BPA, they might still contribute to the reaction via reduction with Fe(III) or generating extra H 2 O 2 by the recombination of two HO 2 • /O 2 •-radicals. For dissolved oxygen (DO) was considered to enlarge the production of O 2 •-

Fig. 3 .

 3 Fig. 3. Degradation of BPA with scavengers involved. ([BPA] 0 = 2 mg L -1 /8.76 μM,

Fig. 4 .

 4 Fig. 4. Formation of Fe(II) determined by Ferrozine method. ([Fe(III)] 0 = 50 μM, pH = 5.5 adjusted by acetate buffer)

Fig. 5 .

 5 Fig. 5. Stopped-flow UV-vis spectrum of CAT and Fe(III) solution. ([CAT] 0 =

FeScheme 2 .

 2 Scheme 2. The process of the reaction between CAT and Fe(III)

Fig. 6 .

 6 Fig. 6. The impact of the ratio between CAT and Fe(III) (a), initial pH (b) and common anions (c) on BPA degradation ([BPA] 0 = 2 mg L -1 /8.76 μM, [H 2 O 2 ] 0 = [Fe(III)] 0 = 50 μM, [CAT] 0 = 8.3 μM, pH=6.5)

  the speed of BPA degradation. The effects of Cl -, NO 3 -and SO 4 2-were similar and negligible. However, the reason for the obvious inhibition effect of PO 4 3-was the precipitation of Fe(II) or Fe(III) with PO 4 3-(K sp = 4x10 -27 and K sp = 1x10 -36 , respectively).

and materials 2.1 Chemicals

	H 2 O 2 (≥30%), NaCl (≥99.5%), NaNO 3 (≥99.0%), H 2 SO 4 (~98.0%), NaH 2 PO 4
	(≥99.0%), ascorbic acid (≥99.7%) and p-hydroxyphenylacetic acid (POPHA, 98%)
	were purchased from Sinopharm Chemical Reagent Co., Ltd. Bisphenol A (BPA, 99%)
	and (+)-catechin (CAT, 99%) were obtained from J&K Scientific. Methanol,
	isopropanol, chloroform, acetonitrile, and formic acid all in HPLC grade were also
	supplied by J&K Scientific. Fe(ClO 4 ) 3 •xH 2 O (Fe 10.7% w/w) and catalase
	(2,000~5,000 units/mg) were purchased from Sigma-Aldrich. Peroxidase (≥150
	units/mg) were obtained from Macklin Inc.. Ferrozine (97%) was supplied by Aladdin
	Inc..

2.2 Degradation Experiments.

	The degradation experiments basically involved four solutions: BPA, CAT, H 2 O 2
	and Fe(III) as the target contaminant, modifier, and elementary components of
	Fenton-like system, respectively. Previous researches and pre-experiments both
	confirmed that little decomposition of BPA or CAT would occur when H 2 O 2 was

  2 O 2 system on BPA removal was almost doubled in an hour and the total amount of BPA that was removed remained nearly the same comparing to long-term (300 min) in Fe(III)/H 2 O 2 system experiment (near 90% of BPA degradation). The results proved that CAT can enhance the efficiency of the conventional Fe(III)/H 2 O 2 system on BPA removal.

The control experiment showed that the concentration of BPA decreased 27.6% in Fe(III)/H 2 O 2 system and it had an unstopped decomposition trend till 60 minutes. Further experiments revealed that the degradation would gradually slow down and stopped eventually after 150 minutes of experiments by which nearly 90% of BPA had been removed. In another experiment, BPA was slightly degraded in absence of Fe(III) which was possibly due to the weak oxidation ability of CAT/H 2 O 2 system (as shown in Fig.

SM1

, weak signal of HO • was found). There was almost no degradation of BPA in CAT/Fe(III) system.

When CAT was added, BPA got rapidly degraded to 45.7% in only 20 minutes and the concentration then gradually fell to 41.0% in 60 minutes. The efficiency of CAT/Fe(III)/H

Table 1 .

 1 and O 2 •-were both formed in Fe(III)/CAT/H 2 O 2 system. Moreover, the formation of Fe(III)-CAT complex, CatQ 1 and CatQ 2 were confirmed by stopped-flow UV-vis spectra, UHPLC-QTOF-MS and DFT model calculation. So the Scheme 2 was further confirmed. Comparison between observed peak wavelengths of CAT related compounds and DFT results

  • in presence of H 2 O 2 . On the other side, CAT became extremely reducible as a radical after losing an electron on its catechol group. The radical form of CAT might react with any kind of oxidant including Fe(III), H 2 O 2 , DO and convert to quinone intermediate. Since the

	concentration. It well explained the contradictory on the effect of O 2	•-.
	intermediate was still reactive to oxidant, the flavonol structure was soon transformed
	to the terminal product, flavanone structure. DO might compete for electrons with
	Fe(III) in last two steps, O 2	•-was produced as the consequence, which was also
	reactive to reduce Fe(III). Despite that O 2	•-could be increased by pumping air, it
	would not promote the reduce procedure. Because the total amount of electrons
	available to transfer, regardless of its carrier (O 2 or CAT), was only tied to the CAT

Acknowledgements

This research was funded by the National Natural Science Foundation of China (NSFC 21607116) and National Key R&D Plan (Quantitative Relationship and Regulation Principle between Regional Oxidation Capacity of Atmospheric and Air Quality, 2017YFC0210005).

water treatment.