International Journal of Environmental Research and Public Health

Article Removal of Sulfamethoxazole, Sulfathiazole and Sulfamethazine in their Mixed Solution by UV/H2O2 Process

Guangcan Zhu *, Qi Sun, Chuya Wang, Zhonglian Yang and Qi Xue

School of Energy and Environment, Key Laboratory of Environmental Medicine Engineering of the Ministry of Education, Southeast University, Nanjing 210096, Jiangsu, China; [email protected] (Q.S.); [email protected] (C.W.); [email protected] (Z.Y.); [email protected] (Q.X.) * Correspondence: [email protected]; Tel.: +86-025-837-955-15



Received: 11 April 2019; Accepted: 19 May 2019; Published: 21 May 2019


Abstract: Sulfamethoxazole (SMZ), sulfathiazole (STZ) and sulfamethazine (SMT) are typical sulfonamides, which are widespread in aqueous environments and have aroused great concern in recent years. In this study, the photochemical oxidation of SMZ, STZ and SMT in their mixed solution using UV/H2O2 process was innovatively investigated. The result showed that the sulfonamides could be completely decomposed in the UV/H2O2 system, and each contaminant in the co-existence system fitted the pseudo-first-order kinetic model. The removal of sulfonamides was influenced by the initial concentration of the mixed solution, the intensity of UV light irradiation, the dosage of H2O2 and the initial pH of the solution. The increase of UV light intensity and H2O2 dosage substantially enhanced the decomposition efficiency, while a higher initial concentration of mixed solution heavily suppressed the decomposition rate. The decomposition of SMZ and SMT during the UV/H2O2 process was favorable under neutral and acidic conditions. Moreover, the generated intermediates of SMZ, STZ and SMT during the UV/H2O2 process were identified in depth, and a corresponding degradation pathway was proposed.

Keywords: sulfonamides; mixed solution; UV/H2O2 oxidation; photodegradation; advanced oxidation process

1. Introduction The ubiquitous occurrence of pharmaceutical compounds as well as their conversion products and metabolites have been recognized as a rising environmental issue of global concern because they are widely and increasingly used for human and veterinary therapeutic purposes, and are constantly released into the aqueous environment. Antibiotics are a large family of emerging pseudo-persistent contaminants that pose adverse effects on the society and environment due to their antibacterial properties and biological activity [1]. Even at very low concentration, the antibiotics can still lead to an increase of bacterial resistance against antibiotics, and these resistant genes are usually persistent [1,2]. Sulfonamide is a collection of important synthetic sulfanilamide derivatives and is widely used in human and veterinary medicine [3–5]. It is reported that there are eight common sulfonamides including sulfamethoxazole, sulfamethizole, sulfadiazine, sulfacetamide, sulfisoxazole, sulfanilamide, sulfasalazine and sulfadoxine [6]. In this work, sulfathiazole (STZ), sulfamethoxazole (SMZ) and sulfamethazine (SMT) were chosen as the targets because: (a) given that these molecules are polar, amphoteric, water-soluble substances with light and thermal stability [1,7], they possess high migration ability and can easily and quickly spread in the environment [1,7]; (b) SMZ, SMT and STZ are the most heavily reported drugs in the surface water of China and exhibit a high pollution level [8–10]; and

Int. J. Environ. Res. Public Health 2019, 16, 1797; doi:10.3390/ijerph16101797 www.mdpi.com/journal/ijerph Int. J. Environ. Res. Public Health 2019, 16, 1797 2 of 15

(c) all the sulfonamides are found in the wastewater treatment plant effluent, groundwater, surface water and even drinking water supply [5,10]. The detection of sulfonamides in the treated drinking water and wastewater treatment plant effluent indicates that conventional water and wastewater treatment processes cannot effectively eliminate sulfonamides. For these aforementioned reasons, it is highly desired to develop reliable water treatment methods that can efficiently remove sulfonamides at a trace level, especially in China. In recent years, advanced oxidation processes (AOPs) have attracted more attention as complementary approaches to traditional water treatment or as alternative treatment methods before the discharging of industrial wastewater. Besides, with the generation of highly reactive and nonselective electrophiles such as hydroxyl radicals, AOPs can remove numerous nonbiodegradable organic pollutants and residual organic pollutants at a trace level in a wide range of water compositions [11]. The photolysis, photocatalysis, H2O2-enhanced photolysis, ozonation, electrochemical oxidation and Fenton are frequently studied AOPs [1,6,11–13]. Among these AOPs, the UV/H2O2 process consists of a combination of two well-known processes, direct UV photolysis and peroxidation, that lead to the generation of hydroxyl radicals, which make this process simpler and more effective for the removal of organic contaminants compared to peroxidation or photolysis respectively [14,15]. It is reported that some sulfonamides, such as sulfaquinoxaline sodium [14], sulfamethoxazole [16], sulfamethazine and sulfapyridine [17], can be efficiently removed by the UV/H2O2 process, and the photochemical 1 oxidation of a single antibiotic with a high concentration (>10 mg L− ) was elucidated. However, other contaminations in the water may dramatically affect the removal of the target pollutant by competitively interacting with hydroxyl radicals and photons or with each other among these compounds. The study about the oxidation of antibiotics with a lower concentration in the mixed solution using the UV/H2O2 process is quite limited. In this work, the photochemical oxidation of SMZ, STZ and SMT in their mixed solution using the UV/H2O2 process was innovatively explored. The effects of various parameters including the initial concentration of the mixed solution, UV light intensity, H2O2 dosages and the initial mixed solution pH on the removal of target antibiotics were evaluated. Based on the identification of the generated intermediates and the proposing of the corresponding degradation pathway, the feasibility of using the UV/H2O2 process for the degradation of sulfonamides was demonstrated.

2. Materials and Methods

2.1. Chemicals and Reagents SMZ, STZ and SMT were purchased from Sigma-Aldrich, USA. The purity of all the above standards was >98%. The HPLC grade acetonitrile was purchased from Supelco Co. USA. The formic acid used in the high-performance liquid chromatography was guaranteed reagent. The hydrogen peroxide (H2O2, 30% w/w) was obtained from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). The analytically pure hydrochloric acid (HCl) and sodium hydroxide (NaOH) were used to adjust the pH. The ultrapure water produced by the Milli-Q water purification system was used to prepare all solutions.

2.2. Experimental Procedures The photochemical experiments were carried out in a 2.5 L cylindrical glass sleeve reactor (360 mm height, 100 mm diameter) at room temperature (25 2 C). The reactor was covered with aluminum ± ◦ foil to eliminate the effects of natural light and avoid the exposure of UV radiation. The reactor was operated in batch mode and a magnetic stirrer device was placed at the bottom of the reactor to provide a rapid mixing. A low-pressure mercury UV lamp emitting at 254 nm was inserted in the center of the cylindrical reactor and protected in a quartz sleeve (outer diameter 25 mm, length 305 mm). In all cases, the water sample containing a mixture of the three compounds was added to the reactor. 1 The initial concentration of the mixed solution of 100 µg L− was set. The predetermined concentration Int. J. Environ. Res. Public Health 2019, 16, 1797 3 of 15 of hydrogen peroxide was added to the reactor and immediately subjected to UV irradiation to start the reactions. After initiating the reactions, samples were collected at regular intervals, and the concentrations of antibiotics were measured using a high-performance liquid chromatography (HPLC). The involved H2O2 solution and sulfonamide solution were prepared at the time of use. For comparison, the direct photolysis experiment (no H2O2 was added into the reactor) and control experiment with H2O2 (without UV irradiation) were conducted in the same conditions. A series 1 1 mixture solution with different concentrations of sulfonamides ranging from 50 µg L− to 200 µg L− was used to investigate the influence of initial mixed solution concentration. In order to determine the effect of UV light intensity, the specific intensity was set as 5, 10 and 15 W respectively. Series 1 concentrations of H2O2 (0, 30, 55 and 100 mg L− ) was selected to evaluate the influence of H2O2 concentration. Three initial solution pH values (5.0, 7.0 and 9.0) were used to determine the influence of initial solution pH. The pH of other water samples was not adjusted except for the experiments focusing on the effect of pH on the sulfonamides degradation. All experiments were performed three times with independent replication of data (n = 3).

2.3. Analytical Methods The antibiotics concentration was analyzed using a HPLC (Agilent 1100) equipped with a UV detector and a C18 reverse-phase column (2.1 50 mm, 1.7 µm). The mobile phase was an 80:20 × 1 (v/v) mixture of acidified water (0.1% formic acid) and acetonitrile at a flow rate of 0.2 mL min− . The UV detector wavelength was set as 265 nm. The injection volume and column temperature were 10 µL and 25 ◦C, respectively. The collected water samples during the experiment were directly used for the determination of residual target compounds by HPLC according to the above conditions. The pre-prepared target compound solution of different concentrations was used to create the calibration curve, which was employed to calculate the concentration of residual target compounds. Under 1 this test condition, the detection limit of the selected sulfonamides was 5 µg L− , and the relative standard deviations (RSD) of the three substances were all less than 0.2% to meet the detection requirements. The pH of the solution was measured using a PHSJ-4A pH meter. The UV-Vis spectra of the samples were obtained on a UV-1280 spectrophotometer (Shimadzu, Japan). The identification of the generated intermediates during the degradation of sulfonamides in the UV/H2O2 process was performed using an Agilent 1100 LC equipped with an Agilent 6530 Series Triple Quadrupole mass spectrometer (LC/Q-TOF-MS) system. The compound was ionized in the electrospray ionization (ESI) and operated in positive mode. The selected ion monitoring (SIM) mode and a scan range from 40 m/z to 3200 m/z was used to obtain the spectra of sulfonamides and their intermediates. The injection 1 volume (20 µL) was conducted using an autosampler at a flow rate of 1 mL min− through an XDB-C18 (4.6 cm 50 mm 1.8 µm, Agilent, USA) column. The mobile phase was a mixture of acidified water × × (0.1% formic acid) and acetonitrile and it was followed by a gradient elution mode: (1) maintain the 10% acetonitrile in 0–5 min; (2) acetonitrile proportion gradually increased to 100% within 50 min; (3) 100% acetonitrile holding for 3 min. The gas temperature and source voltage were 400 ◦C and 4.0 KV, respectively. The degradation of sulfonamides was evaluated based on the following equation:

C Ct Sulfonamides degradation rate (%) = 0 − 100% (1) C0 × where C0 and Ct indicate the concentration of sulfonamides at the beginning and at the given reaction time t, respectively. In addition, in order to illustrate the degradation rate of UV/H2O2 process for the removal of sulfonamides quantitatively, the kinetics of sulfonamides degradation was investigated using the following pseudo-first-order reaction kinetic model:

ln Ct = ln C kt (2) 0 − Int. J. Environ. Res. Public Health 2019, 16, 1797 4 of 15

where C0 and Ct represent the concentration of sulfonamides at the beginning and at the given reaction time t, respectively. k is the pseudo-first-order degradation rate constant which is obtained by the slope of the linear regression of the sulfonamides degradation data points.

3. Results and Discussion

Int. J. Environ. Res. Public Health 2019, 16, x 4 of 17 3.1. Comparison of Absorption Spectra of Target Compounds In3.1. order Comparison to investigate of Absorption whether Spectra the of UVTarget light Compounds irradiation centered at 254 nm can degrade SMZ, STZ and SMT,In the order absorption to investigate spectra whether of SMZ, the STZUV light and irradiation SMT were centered measured at 254 with nm the can wavelength degrade SMZ, ranged fromSTZ 200 and to 400 SMT, nm, the and absorption the result spectra is shown of SMZ, in STZ Figure and1 a.SMT The were SMZ measured has a strongwith the absorption wavelength peak aroundranged 265 nm.from However,200 to 400 nm, STZ and has the two result strong is show absorptionn in Figure peaks 1a. The located SMZ at has 258 a strong nm and absorption 284 nm, and thosepeak of SMT around occur 265 at nm. 240 However, nm and 262STZ nm,has two respectively. strong absorption Therefore, peaks all located these threeat 258 antibioticsnm and 284 possess nm, a and those of SMT occur at 240 nm and 262 nm, respectively. Therefore, all these three antibiotics strong absorption of the 254-nm UV light irradiation, indicating that all these three antibiotics could be possess a strong absorption of the 254-nm UV light irradiation, indicating that all these three degraded by the UV lamp at the main wavelength of 254 nm. antibiotics could be degraded by the UV lamp at the main wavelength of 254 nm.

FigureFigure 1. UV 1. UV spectrum spectrum scans scans of of sulfonamides sulfonamides ((a), degradation of of SMZ SMZ (b), (b STZ), STZ (c) and (c)and SMT SMT (d) in (d ) in differentdifferent oxidation oxidation treatments, treatments, and degradationand degradation rate constantsrate constants for sulfonamides for sulfonamides degradation degradation at di atfferent different oxidation treatment (e). Reaction conditions: initial concentration of the mixed solution 100 1 oxidation treatment (e). Reaction conditions: initial concentration of the mixed solution 100 µg L− μg L−1 (including SMZ 100 μg L−1, STZ 100 μg L−1 and SMT 100 μg L−1), UV light intensity 5 W, H2O2 (including SMZ 100 µg L 1, STZ 100 µg L 1 and SMT 100 µg L 1), UV light intensity 5 W, H O dosage −1 − − − 2 2 dosage1 55 mg L . 55 mg L− .

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3.2. Decomposition of Sulfonamides in the Mixed System Figure1b–d shows the degradation of sulfonamides in the mixed system under di fferent conditions. 1 When only 55 mg L− of H2O2 was added into the reaction system, the degradation of sulfonamides was negligible. When only sole UV irradiation was performed, the degradation of sulfonamides was quite limited, which might be attributed to the high molar absorption coefficient of sulfonamides 1 at 254 nm [14]. The degradation rate constant of SMZ, STZ and SMT was found to be 0.155 min− , 1 1 0.044 min− and 0.016 min− , respectively, when there was only sole UV irradiation in the system (Figure1e). However, when H 2O2 and UV irradiation were performed together, the degradation rate of sulfonamides was substantially enhanced compared to that of sole UV irradiation. At this 1 1 time, the degradation rate constant of SMZ, STZ and SMT increased to 0.276 min− , 0.178 min− 1 and 0.130 min− , respectively (Figure1e). According to previous studies, the oxidation of organic compounds through UV/H2O2 process can be divided into three stages: direct photolysis, H2O2 oxidation and indirect photolysis [11,18]. As is shown in Figure1b–d, the H 2O2 oxidation did not affect the transformation of sulfonamides, while the decomposition of sulfonamides occurred under UV radiation or UV/H2O2 oxidation, suggesting that both direct photolysis and indirect photolysis contributed to the decomposition of sulfonamides. Similar phenomena were reported, for instance, the removal of sulfaquinoxaline sodium [14] and pharmaceutically active compounds (PhACs) [19] through the UV/H2O2 oxidation process. This result indicates that the UV/H2O2 process was favorable for the degradation of sulfonamides in the mixed solution.

3.3. Effect of Initial Concentration of the Mixed Solution on Degradation Efficiency The initial concentration of the mixed reaction solution is an important parameter because the specific concentration in environment water system and treated wastewater are quite different. 1 The experiments were carried out at UV light intensity of 5 W and the H2O2 dosage of 55 mg L− . Figure2 shows the e ffect of initial concentration of the mixed solution on degradation efficiency in the UV/H2O2 system. The degradation efficiency of SMZ decreased with the increase of the initial concentration of mixed solution in the range of test concentrations (Figure2a). The SMZ degradation 1 rate was 99.9% for the 50 µg L− mixed solution after 20 min of reaction, which decreased to 97.4% and 1 1 76.6% when the mixed solution concentration was increased to 100 µg L− and 200 µg L− , respectively. The degradation pattern fitted the pseudo-first-order kinetic model, and the kinetic constant decreased 1 1 1 from 0.352 min− to 0.197 min− and 0.076 min− respectively, when the initial concentration of the 1 1 1 mixed solution increased from 50 µg L− to 100 µg L− and 200 µg L− . Similar results were obtained when it comes to STZ degradation (Figure2b) and SMT degradation (Figure2c). In addition, it is worth noting that a nearly complete STZ, SMZ and SMT removal was achieved at the lowest concentration of 1 50 µg L− . This result is meaningful because the specific concentrations in most ambient waters will not be more than the tested concentration in this range. The reduced reaction rate at a higher mixed solution concentration was attributed to competition between various sulfanilamide molecules and/or their intermediates formed during the oxidation reaction process [20]. At a higher concentration, the permeation of photons in the solution was reduced, which resulted in a lower concentration of hydroxyl radical. In addition, the sulfonamides and their converted products compete with the generated hydroxyl radicals in UV/H2O2 system [20–22]. Namely, the efficiency of sulfonamides removal was suppressed when the initial mixed solution concentration was enhanced. Int. J. Environ. Res. Public Health 2019, 16, 1797 6 of 15 Int. J. Environ. Res. Public Health 2019, 16, x 6 of 17

Figure 2. EffectEffect of initial concentration of of the the mixed mixed solution solution on on the the removal removal of SMZ ( a),), STZ STZ ( (b)) and and SMT ( (cc)) and and degradation degradation rate rate constants constants for for sulfonamides sulfonamides degradation degradation at at different different initial initial concentration concentration −1 1 of the mixed solution ( d). Reaction Reaction conditions: conditions: UV UV light light intensity intensity 5 5 W, W, H H22OO22 dosagedosage 55 55 mg mg L L−. .

3.4. Effect Effect of of UV UV Light Light Inte Intensitynsity on on Degradation Degradation Efficiency Efficiency The effect effect of of UV UV light light intensity intensity on on the the removal removal of sulfonamides of sulfonamides is shown is shown in Figure in Figure 3. The3. µ 1 experimentsThe experiments were werecarried carried out at outinitial at initialconcentration concentration of mixed of solution mixed solution of 100 μ ofg L 100−1 andg the L− Hand2O2 1 dosagethe H2O of2 dosage 55 mg of L 55−1. mgIn Lall− .investigated In all investigated reaction reaction times, times, the theremoval removal efficiency efficiency of of sulfonamides sulfonamides substantially enhanced when the intensity of the UV light was enlarged. When When the the UV UV light light intensity intensity was set as 5 W, thethe degradationdegradation rate rate of of SMZ SMZ was was 17.1% 17.1% and and 99.2% 99.2% for for the the reaction reaction time time of of 1 min1 min and and 23 23min, min, respectively. respectively. And And the the complete complete removal removal of SMZ of SMZ occurred occurred when when the reactionthe reaction time time was morewas more than than25 min. 25 Whenmin. When the UV the light UV intensity light intensity was enlarged was enlarged to 10 W and to 1510 W,W aand considerable 15 W, a enhancementconsiderable enhancementof the SMZ removal of the SMZ was achieved.removal was A complete achieved. removal A complete of SMZ removal was achievedof SMZ was at the achieved reaction at time the reactionof 12 min time and of 8min 12 min for the and UV 8 lightmin intensityfor the UV of 10light W andintensity 15 W, respectively.of 10 W and Meanwhile, 15 W, respectively. the SMZ 1 1 Meanwhile,degradation the rate SMZ constants degradation greatly increasedrate constants from 0.197greatly min increased− to 0.845 from min 0.197− with min the−1 increaseto 0.845 ofmin UV−1 withlight intensitythe increase from of 5 UV W tolight 15 W.intensity A similar from trend 5 W was to observed15 W. A insimilar the degradation trend was ofobserved STZ and in SMT. the degradationIn the UV/H 2ofO 2STZprocess, and SMT. the hydroxyl In the UV/H radical2O2 wasprocess, formed the throughhydroxyl the radical following was formed equation through [23]: the following equation [23]: H O + hv 2 OH (3) 2 2 → · HO ℎ𝑣→2∙OH (3) The photolysis rate of H2O2 directly depended on the incident power. With a low UV light intensity, The photolysis rate of H2O2 directly depended on the incident power. With a low UV light the photolysis of H2O2 was limited. With a higher UV light intensity, more hydroxyl radical was intensity, the photolysis of H2O2 was limited. With a higher UV light intensity, more hydroxyl radical generated through the photodissociation of H2O2, and thus their removal rate was enhanced [22,23]. was generated through the photodissociation of H2O2, and thus their removal rate was enhanced [22,23].

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FigureFigure 3. 3. EffectEffect of of UV UV light light intensity intensity on on the the removal removal of of SMZ SMZ ( (aa),), STZ STZ ( (bb)) and and SMT SMT ( (cc)) and and degradation degradation raterate constants constants for for sulfonamides sulfonamides degradation degradation at at different different UV UV light light intensity ( (dd).). Reaction Reaction conditions: conditions: 1 1 initialinitial concentration concentration of of the the mixed mixed solution solution 100 100 μµg L −1, ,HH2O2O2 2dosagedosage 55 55 mg mg L− L1.− .

3.5. Effect of H2O2 Dosages on Degradation Efficiency 3.5. Effect of H2O2 Dosages on Degradation Efficiency Figure4a–c depicts the changes of sulfonamides degradation e fficiency in a UV-irradiated reactor Figure 4a–c depicts the changes of sulfonamides degradation efficiency in a UV-irradiated 1 with different H2O2 dosages (0, 30, 55 and 100 mg L− ). In addition, the initial concentration of the reactor with different H2O2 dosages (0, 30, 55 and 100 mg L−1). In addition, the initial concentration of mixed solution and the UV light intensity was set as 100 µg L 1 and 5 W, respectively. The degradation the mixed solution and the UV light intensity was set as −100 μg L−1 and 5 W, respectively. The 1 efficiency of sulfonamides enhanced with the increasing of the H2O2 concentration from 0 mg L− to degradation efficiency of sulfonamides enhanced with the increasing of the H2O2 concentration from 1 100 mg L− . In the absence of H2O2, the photolysis of sulfonamides gave rather moderate results and 0 mg L−1 to 100 mg L−1. In the absence of H2O2, the photolysis of sulfonamides gave rather moderate led to a slow degradation of sulfonamides. When H2O2 was not added to the system, the removal results and led to a slow degradation of sulfonamides. When H2O2 was not added to the system, the efficiency of STZ, SMZ and SMT was 54.7%, 93.3% and 29.3% at the time of 20 min, respectively. The removal efficiency of STZ, SMZ and SMT was 54.7%, 93.3% and 29.3% at the time of 20 min, addition of 30 mg L 1 to 100 mg L 1 increased the STZ, SMZ and SMT degradation from 73.1%, 96.5% respectively. The addition− of 30 mg− L−1 to 100 mg L−1 increased the STZ, SMZ and SMT degradation and 72.1% to 94.9%, 100.0% and 94.7% at a reaction time of 20 min, respectively. The degradation from 73.1%, 96.5% and 72.1% to 94.9%, 100.0% and 94.7% at a reaction time of 20 min, respectively. of sulfonamides at different H2O2 dosage fitted the pseudo-first-order kinetic models based on the The degradation of sulfonamides at different H2O2 dosage fitted the pseudo-first-order kinetic Equation (2) and this model had a good fitness (R2 > 0.94) with the degradation of sulfonamides. When models based on the Equation (2) and this model had a good fitness (R2 > 0.94) with the degradation the H O dosage was 0, 30, 55, and 100 mg L 1, the degradation rate constant of SMZ, STZ and SMT 2 2 − −1 of sulfonamides. When the H2O2 dosage1 was 0, 30, 55, and 100 mg L , the degradation1 rate constant was 0.139, 0.171, 0.197, and 0.266 min− , 0.039, 0.073, 0.137, and 0.181 min− , and 0.014, 0.063, 0.101, of SMZ, STZ and1 SMT was 0.139, 0.171, 0.197, and 0.266 min−1, 0.039, 0.073, 0.137, and 0.181 min−1, and 0.158 min− , respectively. It could be seen that the degradation rate constants of STZ, SMZ and −1 andSMT 0.014, increased 0.063, by 0.101, 357.2%, and 91.6% 0.158 and min 1043.5%,, respectively. respectively, It could when be the seen H2 Othat2 concentration the degradation increased rate 1 1 constantsfrom 0 mg of L −STZ,to 100SMZ mg and L− SMT. The increased H2O2 could by absorb 357.2%, the 91.6% photon and energy 1043.5%, from respectively, UV irradiation when and the the −1 −1 Hhemolytic2O2 concentration splitting ofincreased the O–O from bond 0 inmg H L2O 2toresulted 100 mgin L the. The generation H2O2 could of activated absorb the hydroxyl photon radicalsenergy fromand atomic UV irradiation oxygen [ 12and,20 the]. The hemolytic enhancement splitting of theof the H2 OO-O2 concentration bond in H2O would2 resulted lead in to the the generation increase of ofenergy activated harvesting hydroxyl and radicals hydroxyl and radical atomic generation, oxygen [12,20]. and thus The the enhancement oxidative destruction of the H2O of2 concentration sulfonamides wouldwas promoted. lead to the increase of energy harvesting and hydroxyl radical generation, and thus the oxidativeIn addition, destruction the of reaction sulfonamides rate constant was promoted. as a function of H2O2 dosage is shown in Figure4d. The increaseIn addition, of the the H 2reactionO2 dosage rate resulted constant in as the a enhancedfunction of reaction H2O2 dosage rate constant is shown for thein Figure degradation 4d. The of increaseSTZ, SMZ of andthe SMT,H2O2 alongdosage with resulted a robust in the linear enhanced relationship reaction between rate constant H2O2 dosage for the and degradation reaction rate of STZ, SMZ and SMT, along with a robust linear relationship between H2O2 dosage and reaction rate

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2 constantconstant (R(R2 > 0.96).0.96). This This result result indicates indicates that the H 2OO22 essentiallyessentially had had the the same same effect effect on the reactions of these compoundscompounds inin thethe rangerange ofof testedtested concentration.concentration.

Figure 4.4. EEffectffect of HH22O2 dosagedosage on on the the removal of SMZ ( a), STZ (b) and SMT ((c)) andand reactionreaction raterate constantconstant asas aa functionfunction ofof HH22OO22 dosagedosage ((dd).). ReactionReaction conditions:conditions: initial concentration of thethe mixedmixed −1 solutionsolution 100100 µμgg LL− ,, UV UV light light intensity intensity 5 5 W. W.

3.6. Effect Effect of Initial Solution pH on DegradationDegradation EEfficiencyfficiency The solution pH is recognizedrecognized as oneone ofof thethe mainmain factorsfactors thatthat aaffectffect thethe photochemicalphotochemical reactionreaction process [14]. The decomposition of sulfonamides during the UV/H O process at pH of 5.0, 7.0, and process [14]. The decomposition of sulfonamides during the UV/H22O2 process at pH of 5.0, 7.0, and 9.0 waswas conductedconducted inin thisthis study.study. TheThe initialinitial concentrationconcentration of thethe mixedmixed solution,solution, UVUV lightlight intensityintensity and H O dosage was set as 100 µg L 1, 5 W and 55 mg L 1, respectively. It is clearly shown in and H2O22 dosage was set as 100 μg L−1, −5 W and 55 mg L−1, respectively.− It is clearly shown in Figure Figure5a-c that5a–c the that degradation the degradation efficiency e ffi ofciency SMZ ofand SMZ SMT and gradually SMT gradually decreased decreased when the when initial the solution initial 1 solutionpH increased. pH increased. The degradation The degradation rate constant rate constantof SMZ and of SMZ SMT anddecreased SMT decreased from 0.316 from min 0.316−1 and min 0.105− 1 1 1 andmin− 0.1051 to 0.145 min −minto−1 0.145 and 0.093 min− minand−1, 0.093 respectively, min− , respectively, when the initial when solution the initial pH solution increased pH from increased 5.0 to from9.0. However, 5.0 to 9.0. as However, opposed asto the opposed degradation to the degradationof SMZ and ofSMT, SMZ the and variation SMT, the of variationinitial solution of initial pH solutionimproved pH the improved degradation the degradation rate of STZ. rateThe ofdegradation STZ. The degradation rate constant rate of STZ constant increased of STZ by increased 29.3% from by 1 1 29.3%0.120 min from−1 0.120to 0.155 min min− to−1 0.155when min the− initialwhen solution the initial pH solution increased pH from increased 5.0 to from 9.0. 5.0The to effect 9.0. The of initial effect of initial solution pH on the degradation rate of antibiotics during the UV/H O process was caused by solution pH on the degradation rate of antibiotics during the UV/H2O2 process2 2 was caused by the thecompetitive competitive processes processes of hydroxyl of hydroxyl radical radical generati generationon and scavenging and scavenging [24]. The [24]. hydroperoxide The hydroperoxide anion anion (HO ) was present as a conjugate base of H O under strong alkaline conditions [22]. When (HO2−) was2 −present as a conjugate base of H2O2 under2 2 strong alkaline conditions [22]. When the pH the pH increased, the proportion of HO anion increased. The HO was more reactive than H O increased, the proportion of HO2− anion2 −increased. The HO2− was more2− reactive than H2O2 and2 the2 and the formation of hydroxyl radicals was enhanced under alkaline conditions [24]. Furthermore, formation of hydroxyl radicals was enhanced under alkaline conditions [24]. Furthermore, H2O2 and H O and hydroxyl radical could react with the HO (Equations (4)–(6)) [14,25] and the reaction rate hydroxyl2 2 radical could react with the HO2− (Equations2− (4)–(6)) [14,25] and the reaction rate of the of the hydroxyl radical with the HO was about 100 times higher than that with H O [24,26]. hydroxyl radical with the HO2− was2 about− 100 times higher than that with H2O2 [24,26].2 2

HHOO+ HOHO− →HHOOO+ O OH+ OH− (4)(4) 2 2 2 → 2 2 OH HO →HOO (5) OH + HO2− H2O + O2− (5) → ∙ OH HO →HO OH (6) OH + HO2− HO2· + OH− (6) This explains why the reaction rate was lower→ under strong alkaline conditions. In addition, the This explains why the reaction rate was lower under strong alkaline conditions. In addition, the rate of self-decomposition of hydrogen peroxide increased with increasing the solution pH that also rate of self-decomposition of hydrogen peroxide increased with increasing the solution pH that also reduced the reaction rates, because the main decomposition product was oxygen and its oxidizing reduced the reaction rates, because the main decomposition product was oxygen and its oxidizing ability was lower than that of hydroxyl radical [14,24,26]. Besides, most sulfonamides contained two ability was lower than that of hydroxyl radical [14,24,26]. Besides, most sulfonamides contained basic functional groups (group of aniline and heterocyclic base) and one acidic functional group two basic functional groups (group of aniline and heterocyclic base) and one acidic functional group (sulfonamide group), their ionization states were controlled by solution pH and acid dissociation constant pKa [21,27]. Thus, these two parameters might affect the degradation rate of sulfonamides. The pKa values of SMZ, STZ and SMT were shown in Table 1. They had a cationic form at pH < pKa1

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(sulfonamide group), their ionization states were controlled by solution pH and acid dissociation constant pKa [21,27]. Thus, these two parameters might affect the degradation rate of sulfonamides. Int. J. Environ. Res. Public Health 2019, 16, x 9 of 17 Int. J. Environ.Int. J. Environ. Res. Public Res. Public Health Health 2019, 162019, x , 16, x 9 of 179 of 17 The pKa valuesInt.Int. J. Environ. J.Int. Environ. of J. Environ.SMZ, Res. Res. Public PublicRes. STZ Health Public Health and 2019 Health 2019 SMT, 16 ,2019 ,16 x ,were x, 16, x shown in Table1. They had a cationic form at pH9 of9 of17< 9 17 pKaof 171 and an anionicand an form anionic at form pH > atpKa pH > pKa[16,221 [16,21].]. These These di ffdifferenterent ionic ionic statesstates of of sulfonamides sulfonamides might might have havea a and anand anionic an anionic form form at pH at 2> pH pKa > 2pKa [16,21].2 [16,21]. These These different different ionic ionic states stat ofes sulfonamides of sulfonamides might might have have a a andconsiderableand anand an anionic anionic an anionic impact form form atonform atpH their pH at> pKapHreactivity> pKa 2> [16,21].pKa2 [16,21]. 2and [16,21]. These light These Theseabsorbingdifferent different different ionic properties. ionic stationic states esstat ofPrevious ofsulfonamideses sulfonamides of sulfonamides study has might might shown might have have that ahave a a considerableconsiderable impactconsiderable on impact their impact on reactivity their on their reactivity andreactivity lightand lightand absorbing lightabsorbing absorbing properties. properties. properties. Previous Previous Previous study study study has has shown has shown shown that that that considerablemolarconsiderableconsiderable absorption impact impact coefficient impact on on their their on reactivity oftheir reactivity SMZ reactivity in and different and light and light absorbing protlight absorbingonation absorbing properties. properties. states properties. followed Previous Previous Previous the study order study studyhas of has protonationshown hasshown shown that that > that molar absorptionmolarmolar absorption coe absorptionfficient coefficient of coefficient SMZ of in SMZ diof ffSMZ inerent different in different protonation prot onationprotonation states states states followed followed followed the order orderthe order of ofprotonation of protonation protonation > >> molarneutralmolar molarabsorption absorption> deprotonation absorption coefficient coefficient coefficient at wavelengthof ofSMZ SMZ of in SMZ indifferent belowdifferent in different 300prot prot nmonation protonation [8].onation From states states this statesfollowed followed perspective, followed the the order orderthe it canorderof ofprotonation also protonation of explainprotonation > > > neutralneutral > deprotonation > deprotonation at wavelength at wavelength below below 300 nm300 [8].nm From [8]. From this perspective,this perspective, it can it alsocan alsoexplain explain neutral >neutralwhydeprotonationneutral SMZneutral > deprotonation> wasdeprotonation > deprotonationmore at wavelengthresistant at atwavelength wavelength atto wavelengthUV below degradation below below 300 below 300 300 nm atnm a300nm [ higher8[8]. ].nm[8]. From [8]. FrompH. Fromthis this this perspective, perspective,thisperspective, perspective, it canit itcan it canalso canalso explain also also explain explainexplain why whySMZ SMZ was morewas more resistant resistant to UV to degradation UV degradation at a higherat a higher pH. pH. why SMZwhy waswhy SMZInwhy moreSMZ order wasSMZ was resistant moreto was more intuitively resistantmore resistant to resistant UV toinvestigate degradation toUV UV to degradation UVdegradation degradationthe ateffect at a at highera higheraof athigher pHa higher pH. pH.on pH. the pH. light absorption properties of In orderIn order to intuitively to intuitively investigate investigate the effectthe effect of pH of onpH theon lightthe lightabsorption absorption properties properties of of In ordersulfonamides, toIn In intuitivelyorder orderIn orderto the tointuitively UV investigateintuitivelyto spectraintuitively investigate ofinvestigate the selected investigate effect thesulfonamid ofthe pHeffect theeffect on effectofthe es of pHat lightpH ofdifferent on pHon absorptionthe onthe pHlight thelight conditions absorptionlight propertiesabsorption absorption were properties ofpropertiesdetermined. sulfonamides,properties of of of sulfonamides,sulfonamides, the UV the spectraUV spectra of selected of selected sulfonamid sulfonamides at esdifferent at different pH conditions pH conditions were were determined. determined. sulfonamides,Itsulfonamides, cansulfonamides, be clearly the the seenUV UVthe spectrafrom UVspectra spectraFigure of ofselected selected 5d–fof selected thatsulfonamid sulfonamid the sulfonamid changees esat inatdifferentes differentpH at differenthad pH apH considerableconditions pHconditions conditions were effectwere determined.were determined.on thedetermined. UV the UV spectraIt canIt of becan selectedclearly be clearly seen sulfonamides seenfrom from Figure Figure 5d–f at di 5d–fthatfferent thatthe pH changethe conditionschange in pH in hadpH were ahad considerable determined.a considerable effect Iteffect on can the on be UVthe clearly UV Itabsorption Itcan can Itbe canbe clearly clearlyspectrabe clearly seen seen of fromtheseen from STZ, Figurefrom Figure SMZ Figure 5d–f and5d–f that 5d–fSMT. that the that Thethe change thechangeabsorption change in inpH pH peaksin had pHhad a of had considerablea theconsiderable a STZ,considerable SMZ effect effectand effecton SMT on the theaton UV pH theUV UV seen from Figureabsorptionabsorption5d–f spectra that spectra theof the change of STZ, the STZ, SMZ in pHSMZ and had SMT.anda SMT. considerableThe absorptionThe absorption epeaksffect peaks of on the theof STZ, the UV STZ, SMZ absorption SMZ and SMTand SMT spectraat pH at pH of absorption= absorption2 wereabsorption substantially spectra spectra spectra of ofthe weaker the ofSTZ, STZ,the SMZthan STZ, SMZ thoseand SMZ and SMT. ofand SMT. pH TheSMT. = The 6 absorptionand The absorption 11. absorption When peaks peaks the peaks ofpH ofthe increased, the ofSTZ, STZ,the SMZ STZ, SMZ the and SMZ UV and SMT absorption and SMT at SMT atpH pH at pH the STZ, SMZ= 2 were and= 2 were substantially SMT. substantially The absorptionweaker weaker than than peaksthose those of of pH theof = pH 6 STZ,and = 6 11.and SMZ When 11. andWhen the SMT pH the increased, pH at pHincreased,= the2 were UV the absorption UV substantially absorption =of 2= were 2SMZ were= 2 substantially wereand substantially STZsubstantially exhibited weaker weaker weaker athan slightthan those than those blue-shift, ofthose ofpH pH of= 6 =pHresulting and 6 and = 11.6 and 11. When inWhen 11. an When the increase the pH pHthe increased, increased,pH of increased,their the molar the UV UVthe absorptionadsorption UVabsorption absorption of SMZof SMZ and andSTZ STZexhibited exhibited a slight a slight blue-shift, blue-shift, resulting resulting in an in increase an increase of their of their molar molar adsorption adsorption weaker thanofcoefficient ofSMZ thoseSMZof andSMZ and ofat STZ and 254 pHSTZ exhibitednm.STZ =exhibited6 Macroscopically, andexhibited a 11. slighta slight Whena slightblue-shift, blue-shift, the theblue-shift, UV pH resulting absorptionresulting increased, resulting in in anof an theinorganicincrease increasean UV increase compounds absorptionof oftheir theirof molartheir molar at of molar 254adsorption SMZadsorption nm adsorption andwas STZ coefficientcoefficient at 254 at nm.254 nm.Macroscopically, Macroscopically, the UVthe absorptionUV absorption of organic of organic compounds compounds at 254 at nm254 wasnm was exhibitedcoefficientenhanced. acoefficient slightcoefficient blue-shift, atThe at254 254 absorptionat nm. 254nm. Macroscopically, resultingnm. Macroscopically, peak Macroscopically, of in SMT an the increasegradually the UV theUV absorption UVabsorption ofincr theirabsorptioneased of molar with oforganic organicof the adsorptionorganic increasecompounds compounds compounds of coepH. at ffi atIn254cient 254 general,at nm 254nm atwas nm 254wasin was nm. enhanced.enhanced. The absorptionThe absorption peak peak of SMT of SMT gradually gradually increased increased with withthe increase the increase of pH. of InpH. general, In general, in in Macroscopically,enhanced.theenhanced. wavelengthenhanced. the The The UV absorption rangeThe absorption absorption absorption of 190–600 peak peak of ofpeak nm, of organicSMT SMT theof graduallySMT light gradually compounds gradually absorption incr increased increased of at easedorganicwith 254 with nmthewith thematter increase was theincrease was enhanced.increase of mostly ofpH. pH. of In relatedpH. In Thegeneral, general, In absorption general,to thein in in the wavelengththe wavelength range range of 190–600 of 190–600 nm, thenm, light the lightabsorption absorption of organic of organic matter matter was mostlywas mostly related related to the to the thedelocalizedthe wavelength wavelengththe wavelength π electron range range rangeof system of190–600 190–600 of 190–600present nm, nm, the innm, the lightthe lightthe structure. absorption light absorption absorption A changeof oforganic organic of in organic pH matter matter would matter was was result mostly was mostly in mostly relateda reductionrelated related to tothe orthe to the peak of SMTdelocalized graduallydelocalized π electron π increased electron system system with present present the in increasethe in structure. the structure. of pH.A change A In change general, in pH in would pH in would the result wavelength result in a reductionin a reduction range or or of delocalizedevendelocalized delocalizedan elimination π electronπ electron π electron of system systemthe systemπ present electron present present in delocalization inthe the instructure. structure.the structure. Aeffect, Achange change A which change in inpH wouldpH inwould wouldpH resultwould result result in resultin a ina change reductiona inreduction a reduction of theor or or 190–600 nm,even theeven an light elimination an elimination absorption of the of of πthe electron organic π electron delocalization matter delocalization was effect, mostly effect, which related which would towould result the delocalizedresult in a inchange a change πofelectron the of the evenabsorptioneven aneven an elimination eliminationan characteristics elimination of ofthe the of andπ theπelectron photelectron π electronolysis delocalization delocalization properties delocalization ofeffect, effect,an organiceffect, which which which substance.would would would result result Namely, resultin ina changea insulfonamides change a change of ofthe the of the absorptionabsorption characteristics characteristics and photand photolysisolysis properties properties of an of organic an organic substance. substance. Namely, Namely, sulfonamides sulfonamides system presentabsorptionpossessedabsorptionabsorption in the thecharacteristics characteristics structure.delocalized characteristics and Aπ and electrons change phot and photolysis photolysis inand pHolysisproperties were properties would properties subjected of result ofan an toorganicof organic thean in aorganicpH reductionsubstance. change.substance. substance. Therefore, Namely, or Namely, even Namely, sulfonamidesin an sulfonamides the elimination sulfonamides absence of possessedpossessed the delocalized the delocalized π electrons π electrons and wereand were subjected subjected to the to pH the change. pH change. Therefore, Therefore, in the in absence the absence the π electronpossessedofpossessed otherpossessed delocalization organic the the delocalized delocalizedthe substrates, delocalized eff πect, electronsπthe electrons which πvariation electrons and wouldand ofwere and werepH subjected resultwerecaused subjected subjected inchanges to a tothe change the topH in pHthe thechange. ofchange.pH light the change. Therefore,absorption absorption Therefore, Therefore, in characteristics inthecharacteristics the inabsence absencethe absence of otherof other organic organic substrates, substrates, the variation the variation of pH of caused pH caused changes changes in the in light the lightabsorption absorption characteristics characteristics and photolysisofof ofother sulfonamides, otherof properties organicother organic organic substrates, andsubstrates, of substrates,thus an organic affectedthe the variation variationthe substance.their variation ofUV ofpH degradationpH ofcaused Namely, pHcaused caused changes changes performance. sulfonamides changes in inthe the inlight lightthe absorption possessedlight absorption absorption characteristics the characteristics delocalizedcharacteristics π of sulfonamides,of sulfonamides, and thusand thusaffected affected their their UV degradation UV degradation performance. performance. electronsof and ofsulfonamides, sulfonamides, wereof sulfonamides, subjected and and thus and thus to affected thethus affected pH affected their change. their UV their UV degradationTherefore, UVdegradation degradation performance. in performance. the performance. absence of other organic substrates, Table 1. Chemical structures and relevant data for selected sulfonamides. the variation of pH causedTable changesTable 1. Chemical 1. Chemical in thestructures lightstructures and absorption relevant and relevant data characteristics fordata selected for selected sulfonamides. ofsulfonamides. sulfonamides, and thus TableTable 1.Table Chemical1. Chemical 1. Chemical structures structures structures and and relevant andrelevant relevant data data for data for selected selected for selected sulfonamides. sulfonamides. sulfonamides. Compound affected theirCompound UVCompound degradation Sulfamethoxazole performance. Sulfathiazole Sulfamethazine CompoundCompoundNameCompound SulfamethoxazoleSulfamethoxazole SulfathiazoleSulfathiazole SulfamethazineSulfamethazine NameName SulfamethoxazoleSulfamethoxazoleSulfamethoxazole SulfathiazoleSulfathiazoleSulfathiazole SulfamethazineSulfamethazineSulfamethazine AcronymNameName Name SMZ STZ SMT AcronymAcronymTable 1. Chemical structuresSMZ SMZ and relevant data STZ for STZ selected sulfonamides. SMT SMT AcronymMolecularAcronymAcronym SMZSMZ SMZ STZ STZ STZ SMT SMT SMT MolecularMolecular C10H11N3O3S C9H9N3O2S2 C12H14N4O2S MolecularformulaMolecularMolecular C10H11CN10H3O113SN 3O3S C9H9NC93HO29SN2 3O2S2 C12H14CN12H4O142SN 4O2S Compoundformula Nameformula SulfamethoxazoleC10CH1011HNC11310NOH3OS11 N3S3 O3S C Sulfathiazole9HC99HN9C3NO9H32OS92N2 S32O 2S2 C12CH12 Sulfamethazine14HNC14412NOH42OS14 N2S4 O2S Molecularformulaformulaformula MolecularMolecular AcronymMolecularweightMolecularMolecular 253.3 SMZ 255.3 STZ 278.3 SMT weightweight 253.3 253.3 255.3 255.3 278.3 278.3 Molecular formulaweight(g/mol)weightweight C10H253.311253.3N3 O253.33 SC 9255.3H255.39N 3255.3O 2S2 278.3C278.312 H278.3 14N4 O2S Molecular weight(g/mol)(g/mol) (g/mol)Pk(g/mol)a1 (g/mol) 1.6253.3 ± 0.2 2.2 255.3 ± 0.1 2.6 ± 278.30.2 (g/mol) Pka1 Pka1 1.6 ± 0.2 1.6 ± 0.2 2.2 ± 0.1 2.2 ± 0.1 2.6 ± 0.2 2.6 ± 0.2 PkPkPka1a2 a1 Pka1 5.7 1.6 1.6± ± 0.2 0.2 ± 1.6 0.2 ± 0.2 2.2 7.22.2 ± 0.1 ± 2.2 0.40.1 ± 0.1 2.6 2.6 ± 0.2 8± 2.6 ±0.2 1 ± 0.2 Pka1 Pka2 Pka2 1.6 5.7 ±0.2 0.2 5.7 ± 0.2 7.2 2.2 ± 0.4 7.20.1 ± 0.4 8 ± 2.61 8 ± 10.2 PkPka2 a2 Pka2 5.7 5.7 ± 0.2 ± 5.7 0.2 ± 0.2 7.2 7.2 ± 0.4 ± 7.2 0.4 ± 0.4 8 ± 8 1 ± 1 8 ± 1 Pk 5.7 0.2 7.2 0.4 8 1 CH3 a2 O N O O N CH CH ±OH O O ± S ±H CH3 3 Molecular N N O O S O ON CHN3 3 CH3 NH2 SO NO H O HN O O NH2 S N S NH2 O O HS ON H MolecularMolecular H H N H ON O O HSO S S H N N N MolecularstructureMolecularMolecular NH2 NH2 S N S NO O NH2 NH2 S N S NN NH2 NH2 SHN S HN N NH S N CH NH OS N H NHNH NH S SNONS N Molecular structurestructureNH 2 2 NH2 OS N S N 3 NH2 2 NH2 S N H SNN N 2 2 2 N N CH structure O CH CH OH H NO NH N O O 3 structurestructurestructure O 3 3 O O O O O ON N CHN CH O O O CHCH3 3 CH3 CH3 3 CH3 3 CH3 CH O H NCH3 CH3 O OS S + O O N CHNC3H3 CH3 O O O S NH3 O O HSONHN + NH + SS N S NH + NH + SHNHSN HN N O O O NH+3 + 3+ S N +3 + 3+ O N N N N NHNH3 NH3 S ONS HN SNSNH NNHNH3 NH3 S SN NS N + O HO H O HON O O 3 3 O N N CH NHNH + NH + S SNHNHSN HNO ON O + OH H NO NH N O N 3 +3 + 3+ NH3 O OS N O O O ON CHN CH NHNH3 NH S SN NS N + 3 3 3 3 ProtonatedProtonatedH N Protonated form(SMTCHCH3) 3 CH3 O O O CHCH3 3 CH3 ProtonatedProtonatedProtonatedO + + O O CHCH CH + ProtonatedProtonated form(SMT form(SMT)+ ) O + 3 3 3 + + Protonated form(SMT+ ) + + + Protonatedform(STZform(STZ form(STZ+ )+ +) ) ProtonatedProtonated form(SMT form(SMT) C)H ProtonatedProtonatedProtonated form(SMZ form(SMZ form(SMZ+) )+ +) form(STZform(STZform(STZ) ) ) ProtonatedProtonatedProtonated form(SMZ form(SMZ form(SMZ) ) ) O NCH3 CH3 O OS S OH CHCH3 CH O O ON O O S O N N3 3 OH H NHO N O O S S S NH2 O O HSONHN O ON N O N NH2 NH2 S N S N H N H N NHNH2 NH S SNHNHS HNO O O NHNH2 NH S SN N S NH NH NH2 SHN S N ChemicalChemicalChemical NH2 2 S N 2 2 2 HH NN NNHNH2 NH S SN ONS N N ChemicalChemicalChemical NH 2 2 NH2 S N S N OOH H NO NH N 2 2 2 Chemical speciation O O O CHCH3 CH3 O O O O O N N CH speciationspeciationspeciation O O O CHCH3 CH Non-protonatedNon-protonated O O ON N N speciationspeciationspeciation 3 3 3 Non-protonated CH3 CH3 Non-protonatedNon-protonatedNon-protonated form(SMZ) form(SMZ) form(SMZ) Non-protonated Non-protonatedNon-protonated form(STZ) Non-protonatedCH CH3 3 CH3 Non-protonatedNon-protonatedNon-protonated form(SMZ) form(SMZ) form(SMZ) form(STZ)form(STZ) Non-protonatedNon-protonated form(SMT) form(SMT) form(STZ)form(STZ)form(STZ) Non-protonatedNon-protonatedNon-protonatedform(SMT) form(SMT) form(SMT) form(SMT) O O ON N N O S O O- O ON O N O O OS S CH CH NH S N - N - O O NH O OS N-O S 3 3 NH2 2 NH2 S -N S- NO- NH 2 NH S N- SS N- S O O N CHNC3HC3 CHH3 NHNH2 2 NH2 S SN NS N NH2 2 S- N- N - O O OO N O O O CHCH CHNH2 2 NH2 S ON SNN N - N - NN 3 3 3 O O NH2 NH2 S -N S- N- - O O O CHCH3 3 CH3 O O NO N -NNHNHNH2 NH S SN SNSNN DeprotonatedDeprotonatedDeprotonated form(SMZ form(SMZ form(SMZ-) -) -) DeprotonatedDeprotonatedDeprotonated form(STZ ) 2 2 2 O O N N DeprotonatedDeprotonatedDeprotonated form(SMZ form(SMZ form(SMZ-) -) -) DeprotonatedDeprotonatedDeprotonated O O OON N NCHN CH - - 3 3 form(STZform(STZ) ) CHCHC3 CHH 3 form(STZform(STZform(STZ-) -) -) DeprotonatedDeprotonated form(SMT form(SMT-)3 -) DeprotonatedDeprotonatedDeprotonatedDeprotonated form(SMT form(SMT form(SMT form(SMT-) -)- ) -) ReferenceReferenceReference [1616,,21 21][16, 21 77 ][ 7 27 27 ] ReferenceReferenceReferenceReference 16,16, 16,21 21 2116, 21 7 7 7 27 27 27 27

Int. J. Environ. Res. Public Health 2019, 16, 1797 10 of 15 Int. J. Environ. Res. Public Health 2019, 16, x 10 of 17

Figure 5. Degradation ofof SMZSMZ (a(a),), STZ STZ ( b(b)) and and SMT SMT (c )(c in) in di ffdifferenterent pH pH and and UV UV spectra spectra of SMZ of SMZ (d), STZ(d), STZ(e) and (e) SMTand SMT (f) under (f) under different different pH conditions. pH conditions. Reaction Reaction conditions: conditions: initial initial concentration concentration of the mixedof the 1 −1 1 −1 mixedsolution solution 100 µg 100 L− μ, UVg L light, UV intensity light intensity 5 W, H 52 OW,2 dosageH2O2 dosage 55 mg 55 L −mg. L .

3.7. Identification Identification of of Sulfonami Sulfonamidesdes Degradation Degradation Intermediates The intermediate intermediate products products of of sulfonamides sulfonamides dec decompositionomposition were were analyzed analyzed using using LC/Q-TOF-MS. LC/Q-TOF-MS. The structure of transformation products and their MS fragmentation data in the UV/H O process The structure of transformation products and their MS fragmentation data in the UV/H2O22 process2 of SMZ,of SMZ, SMT SMT and and STZ STZ were were shown shown in Tables in Tables 2–4.2 –SMZ,4. SMZ, SMT SMT and andSTZ STZhad hada retention a retention time timeof 4.184 of 4.184 min, + 3.984min, 3.984min and min 3.864 and 3.864min, respecti min, respectively,vely, and they and all they yielded all yielded a [M+H] a [M+ at+ H]m/z at254, m /279z 254, and 279 256. and Based 256. onBased the onstudy the of study García-Galán of García-Gal et al.án [17], et al. the [17 sulfonamides], the sulfonamides tend to tend lose to the lose sulfonyl the sulfonyl group group under under light irradiation.light irradiation. So the So compound the compound S1 at m/z S1 at 190.0182 m/z 190.0182 was mainly was mainly a product a product of direct of direct degradation degradation by UV. by TheUV. Thedihydroxy dihydroxy compound compound was was identified identified at m/z at m /28z 288.06618.0661 (S3), (S3), which which was was caused caused by by the the hydroxyl hydroxyl radicalradical attacking attacking the the double bond on the isoxazole ring. The The formation formation of of this compound could be explained based onon thethe generation generation of of a a tertiary tertiary carbon-centered carbon-centered radical radical proposed proposed by Huby Hu et al. et [ 28al.]. [28]. The Thecompound compound S2 yielded S2 yielded an m an/z m/z ratio ra oftio 133.0609, of 133.0609, which which corresponded corresponded to the tocleavage the cleavage of S-N of S-N of S3. of TheS3. compounds S4 yielded an m/z of 99.0562, for which the best-fit formula was C H N O, which was The compounds S4 yielded an m/z of 99.0562, for which the best-fit formula was4 6 C42H6N2O, which recognized as a by-product of TiO photocatalysis [9]. The reason for this compound might be described was recognized as a by-product 2of TiO2 photocatalysis [9]. The reason for this compound might be describedas follows: as a follows: five-membered a five-membered ring contained ring acontai doublened bond a double and a bond nitrogen and atom, a nitrogen and the atom, atoms and in the atoms in the five-membered rings were conjugated, which resulting in a weak bond attached to the nitrogen and sulfur bond. The compound S5 at m/z 216.0437 was formed by the opening of the

Int. J. Environ. Res. Public Health 2019, 16, 1797 11 of 15

five-memberedInt. J. Environ. rings Res. were Public conjugated, Health 2019, 16, whichx resulting in a weak bond attached to the nitrogen11 of 17 and sulfur bond.Int. The J. Environ. compound Res. Public S5Health at 2019 m/,z 16 216.0437, x was formed by the opening of the isoxazole11 ring of 17 and isoxazoleInt. J. Environ. ring Res. and Public the Health loss 2019 of ,one 16, x carbonyl group. The m/z of the S6 ion was higher than that11 of of the 17 the loss of oneInt. J. carbonyl Environ. Res. group.Public Health The 2019 m, 16/,z x of the S6 ion was higher than that of the parent compound,11 of 17 parentInt.isoxazole J. Environ. compound, ring Res. andPublic theindicating Health loss 2019 of, one16that, x carbonyl hydroxyl group. radical The might m/z ofcreate the S6 a ionSMZ was molecule higher thanalong that with11 of of the an17 indicating thatInt. J. hydroxyl Environ. Res. radicalPublic Health might 2019, 16 create, x a SMZ molecule along with an intermediate. The11 of S7 17 had intermediate.parentisoxazole compound, ring The and S7 theindicating had loss the of sameone that carbonyl m/zhydroxyl as thegroup. radi S6, cal butThe might them/z retention ofcreate the S6 a times ionSMZ was ofmolecule thesehigher two thanalong compounds that with of thean isoxazole ring and the loss of one carbonyl group. The m/z of the S6 ion was higher than that of the the same m/wereisoxazoleintermediate.parentz as thequite compound, S6,ring different. but Theand the S7 theindicating Onehad retentionloss hadthe of one sameathat retention timescarbonyl hydroxylm/z ofas time thesethegroup. radi ofS6, 4.049twocal Thebut might the compoundsm/zmin retention of andcreate the the S6 a other times ion wereSMZ was was ofmolecule quite higherthese 2.882 di two min,than ffalongerent. compounds thatindicating with Oneof thean had parentisoxazole compound, ring and theindicating loss of onethat carbonyl hydroxyl group. radical The might m/z ofcreate the S6 a ionSMZ was molecule higher thanalong that with of thean a retention timethatparentwereintermediate. these ofquite compound, 4.049 two different. Thecompounds min S7 indicating and Onehad the had thewere other sameathat retentionnot hydroxylm/z wasthe same as 2.882time the compoundradi ofS6, min, 4.049cal but might indicating themin. Based retention andcreate onthethat previousa other timesSMZ these was ofmolecule reports, these two2.882 compoundstwo min,italong is compounds possible indicating with anto were intermediate.parent compound, The S7 indicating had the same that m/zhydroxyl as the radi S6, calbut might the retention create a times SMZ ofmolecule these two along compounds with an not the sameformintermediate.thatwere compound. these quitetwo two isobaricdifferent. The compounds Based S7 compounds Onehad on hadthe were previous samea retentionnotby m/ztheapplyingreports, sameas time the compound oftwoS6, it4.049 but isfragmentation possible themin. Based retention and on tothe previousformvoltages othertimes twowasof reports,tothese 2.882 isobaricobtain two min,it is compounds“in-source” possible compoundsindicating to wereintermediate. quite different. The S7 Onehad hadthe samea retention m/z as time the ofS6, 4.049 but themin retention and the othertimes was of these 2.882 two min, compounds indicating fragmentationwereformthat these quitetwo two isobaricdifferent. compounds[13]. Thecompounds One S7 had werepresented a retentionnotby theapplying an same iontime frag compound oftwo ment4.049 fragmentation atmin. Basedm/z and 156.0116, onthe previous voltagesother which was reports,to 2.882corresponded obtain min, it is “in-source” possibleindicating to the to by applyingthatwere two these quite fragmentation two different. compounds One voltages had were a retentionnot to the obtain same time “in-source”compound of 4.049 min. Based fragmentationand onthe previous other was reports, [13 2.882]. The min,it is S7possible indicating presented to cleavagethatfragmentationform these two of two isobaricthe compounds [13].sulfonamide Thecompounds S7 werepresented bond. notby This theapplying an ionsame ion fragment fragcompound twoment fragmentationtogether at. Basedm/z with156.0116, on thepreviousvoltages ion which fragments reports,to corresponded obtain itof is m/z“in-source” possible 108.045 to the to an ion fragmentformthat these attwo m two/isobaricz 156.0116, compounds compounds which were not correspondedby theapplying same compound two to fragmentation the.cleavage Based on previousvoltages of the sulfonamide reports,to obtain it is“in-source” possible bond. to This andformcleavagefragmentation 92.050 two of were isobaricthe [13].sulfonamide also Thecompounds existent S7 presented bond. in theby This SMZapplying an ion spectrumion fragment frag twoment andfragmentation together atmost m/z of with156.0116, the theconversionvoltages ion which fragments to products, correspondedobtain of m/z“in-source” indicating 108.045 to the ion fragmentfragmentationform together two isobaric with [13]. the Thecompounds ion S7 fragmentspresented by applying an of ion m/ zfrag 108.045twoment fragmentation at and m/z 92.050156.0116, voltages were which also to corresponded obtain existent “in-source” in theto the SMZ thatfragmentationandcleavage the92.050 degradation of werethe [13]. sulfonamide also The ofexistent SMZ S7 presented bond. wasin the owing This SMZ an ion to ionspectrum fragmentthe frag attackment and together of at most the m/z hydroxyl ofwith156.0116, the the conversion radicalsion which fragments oncorrespondedproducts, isoxazole of m/z indicating ring.108.045 to the In cleavagefragmentation of the [13].sulfonamide The S7 presented bond. This an ion ion fragment fragment together at m/z with156.0116, the ion which fragments corresponded of m/z 108.045 to the spectrum andthecleavagethatand mostS6 92.050the compound, degradation of werethe the sulfonamide conversionalso the ofexistent ion SMZ fragment bond. wasin products, the owing This SMZ at ionm/z to indicatingspectrum fragment the156 attack disappeared and together thatof most the the hydroxyl ofandwith degradationthe an the conversion ion radicalsion fragment fragments of onproducts,SMZ isoxazole at of m/z wasm/z indicating 172.0065 ring.108.045 owing In to andcleavage 92.050 of were the sulfonamide also existent bond. in the This SMZ ion spectrum fragment and together most of with the theconversion ion fragments products, of m/z indicating 108.045 the attack ofappeared,andthethat the S6 92.050the hydroxyl compound, degradation suggesting were also radicals the ofexistentthat ion SMZ the fragment on wasinhydroxyl isoxazole the owing SMZ at radicalm/z tospectrum ring. the156 attacked attack disappeared In and the of themost the S6 benzene hydroxyl compound, ofand the an conversionring. ion radicals fragment the onproducts, ion isoxazole at fragment m/z indicating 172.0065 ring. at In m /z thatand the92.050 degradation were also ofexistent SMZ wasin the owing SMZ to spectrum the attack and of most the hydroxyl of the conversion radicals onproducts, isoxazole indicating ring. In thatappeared,the S6theFrom compound, degradation suggestingthe aforementioned the of that ionSMZ the fragment was hydroxyl discussion, owing at radicalm/z to the the 156 same attackedattack disappeared degradation of the the benzene hydroxyl and intermediates an ring. ionradicals fragment onas isoxazoleSMZ at m/z were 172.0065 ring. found In 156 disappearedthethat S6 the and compound, degradation an ion fragmentthe of ion SMZ fragment was at m owing/z at 172.0065 m/z to the156 attack disappeared appeared, of the hydroxyl suggestingand an ion radicals fragment that on the isoxazole at hydroxyl m/z 172.0065 ring. radical In duringtheappeared, S6From compound,the suggestingthedegradation aforementioned the that ion of theSMT.fragment hydroxyl discussion, The atdegradation radicalm/z the 156 sameattacked disappeared intermediates degradation the benzene and ofintermediates an SMT ring. ion infragment the asUV/H SMZ at 2m/zO were2 oxidation 172.0065 found attacked theappeared,the benzene S6 compound, suggesting ring. the that ion the fragment hydroxyl at radicalm/z 156 attacked disappeared the benzene and an ring. ion fragment at m/z 172.0065 processappeared,duringFrom theincluded: suggesting thedegradation aforementioned (1) S-N that of bond the SMT. hydroxyl cleadiscussion, Thevage degradation radicalin SMT,the sameattacked producing intermediates degradation the S1benzene (m/z ofintermediates 140.0824) SMTring. in the and asUV/H S2 SMZ (m/z2O were2 124.0878),oxidation found From theappeared, aforementionedFrom suggestingthe aforementioned that discussion, the hydroxyl discussion, the radical same the same attacked degradation degradation the benzene intermediates intermediates ring. as as SMZ SMZ were were found found (2)processduring theFrom loss theincluded: the ofdegradation sulfonyl aforementioned (1) S-N group of bond SMT. in discussion,cleaSMT, Thevage generatingdegradation in theSMT, same theproducing intermediates compounddegradation S1 (m/z S3 intermediatesof (m/z 140.0824)SMT 215.1301), in the and asUV/H S2(3)SMZ (m/zhydroxylation2O were2 124.0878),oxidation found during the degradationduringFrom the thedegradation aforementioned of SMT. of The SMT. degradation discussion, The degradation the intermediates same intermediates degradation of ofintermediates SMT SMT in in thethe as UVUV/H SMZ/H2O Owere2 oxidationoxidation found ofduring(2)process the the benzene loss theincluded: ofdegradation sulfonyl ring (1) in S-N SMT,group of bond SMT. producing in cleaSMT, Thevage generatingdegradation the in compoundSMT, theproducing intermediates compound S4 (m/z S1 295.0875),(m/z S3 of (m/z 140.0824)SMT 215.1301), (4)in thethe and additionUV/H S2(3) (m/z hydroxylation22O 2reaction 124.0878),2oxidation of processduring theincluded: degradation (1) S-N of bond SMT. clea Thevage degradation in SMT, producing intermediates S1 (m/z of 140.0824)SMT in the and UV/H S2 (m/z2O2 124.0878),oxidation process included:doubleprocessof(2) the the benzene lossbond (1)included: S-Nof at sulfonyl ringthe bond (1) six-member in S-N SMT,group cleavage bond producing in clea heterocyclicSMT, invage SMT,generating the in SMT,compound ring, producing producingthegenerating compound S4S1 (m/z S1 (mthe (m/z295.0875),/ compoundS3z 140.0824) (m/z 140.0824) 215.1301), (4) S1 the and(m/z addition S2(3) S2140.0824) (m/z hydroxylation (m reaction /124.0878),z 124.0878),and S7of (2)process the loss included: of sulfonyl (1) S-N group bond in cleaSMT,vage generating in SMT, theproducing compound S1 (m/z S3 (m/z 140.0824) 215.1301), and S2(3) (m/zhydroxylation 124.0878), (2) the loss of(m/z(2)doubleof sulfonyl thethe 313.0537). benzeneloss bond of groupat sulfonyl ringInthe addition six-member in in groupSMT, SMT, to producing inthe generating heterocyclicSMT, degradation generating the compound thering, prod compound thegeneratingucts compound S4 similar (m/z the S3 to295.0875), S3compound (mSMZ, (m/z/z 215.1301),the 215.1301), (4) degradation S1 the (m/z addition (3) (3)140.0824) hydroxylation hydroxylation process reaction and also S7of of(2) the the benzene loss of sulfonyl ring in SMT,group producing in SMT, generating the compound the compound S4 (m/z 295.0875), S3 (m/z 215.1301), (4) the addition (3) hydroxylation reaction of foundof(m/zdouble the 313.0537). benzenethe bond oxidation at ringInthe addition six-member inproduct SMT, to producingof the the heterocyclic degradation amine the group compound ring, prod at generating thuctse benzeneS4 similar (m/z the 295.0875),ringto compound SMZ, (S5, them/z (4) degradation S1 293.0718).the (m/z addition 140.0824) Similarly, process reaction and also the S7of of the benzenedoubleof the ring benzene bond in SMT,at ringthe six-member producingin SMT, producing heterocyclic the compound the compound ring, generating S4 (m S4 /(m/zz 295.0875),the 295.0875), compound (4) (4) S1 thethe (m/z addition 140.0824) reaction reaction and S7of of degradationdoublefound(m/z 313.0537). the bond oxidation productsat theIn addition six-member product of STZ to of themainly theheterocyclic degradation amine included group ring, prod that generatinge th uctsS-Ne benzene similar bond the cleavage ringto compound SMZ, (S5, them/zproduct degradationS1 293.0718). (m/z (S1, 140.0824) m/z Similarly, process 101.0179), and also the S7 double bond(m/zdouble at 313.0537). the bond six-member at Inthe addition six-member heterocyclic to the heterocyclic degradation ring, ring, generatingprod generatingucts similar the the to compoundcompound SMZ, the degradation S1 S1(m/z (m 140.0824)/z 140.0824)process and also S7 and sulfonyl(m/zdegradationfound 313.0537). the removed oxidation products In additionproduct product of STZ (S2,to of the mainly m/zthe degradation amine226.0662), included group carbon-carbonprod thate thucts S-Ne benzene similar bond doub cleavagetoring SMZ,le (S5,bond them/zproduct addition degradation 293.0718). (S1, product m/z Similarly, process 101.0179), (S4, m/zalso the S7 (m/z 313.0537).found(m/z 313.0537). the In oxidation addition In addition product to the to of degradationthe the degradation amine group products prod at thuctse benzene similar similar ringto toSMZ, (S5, SMZ, them/z thedegradation 293.0718). degradation Similarly, process process also the 290.0274),foundsulfonyldegradation the removed oxidationhydroxylation products product product of productSTZ (S2, of mainly m/zthe (S5, amine 226.0662), m/zincluded group 272.0176) carbon-carbon that eth S-Nande benzene bondthe aminodoub cleavageringle (S5,group bond m/zproduct additionoxidation 293.0718). (S1, product productm/z Similarly, 101.0179), (S4, on m/z the degradationfound the oxidation products product of STZ of mainly the amine included group thate th S-Ne benzene bond cleavage ring (S5, m/zproduct 293.0718). (S1, m/z Similarly, 101.0179), the also found thebenzenedegradation290.0274),sulfonyl oxidation ringremoved hydroxylation products(S7, product m/z product 270.0017).of ofproductSTZ (S2, the mainly m/z amine (S5, 226.0662), m/zincluded group 272.0176) carbon-carbon atthe the S-N and benzene bondthe doubamino cleavage ringle groupbond (S5, product additionoxidation m/z 293.0718).(S1, product productm/z 101.0179), (S4, Similarly,on m/zthe sulfonyldegradation removed products product of STZ (S2, mainlym/z 226.0662), included carbon-carbon the S-N bond doub cleavagele bond product addition (S1, product m/z 101.0179), (S4, m/z the degradationsulfonylbenzene290.0274), products removedring hydroxylation (S7, of m/zproduct STZ 270.0017). mainlyproduct (S2, m/z (S5, included 226.0662), m/z 272.0176) thecarbon-carbon S-N and bond the doubamino cleavagele groupbond product addition oxidation (S1, product product m/z (S4, 101.0179), on m/z the 290.0274),sulfonyl removed hydroxylation product product (S2, m/z (S5, 226.0662), m/z 272.0176) carbon-carbon and the aminodouble group bond additionoxidation product product (S4, on m/zthe 290.0274),benzeneTable ring hydroxylation2. The (S7, structure m/z 270.0017). productof transformation (S5, m/z products 272.0176) an andd their the MS amino fragmentation group oxidation data in the product UV/H2O on2 the sulfonyl removedbenzene290.0274), product ring hydroxylation (S7, m/z (S2, 270.0017). m /productz 226.0662), (S5, m/z carbon-carbon 272.0176) and the double amino bond group addition oxidation productproduct on (S4, the m /z benzeneoxidationTable ring 2. The (S7,process structure m/z of 270.0017). SMZ. of transformation products and their MS fragmentation data in the UV/H2O2 290.0274), hydroxylationbenzene ring (S7, product m/z 270.0017). (S5, m /z 272.0176) and the amino group oxidation product on the oxidationTable 2. The process structure of SMZ. of transformation products and their MS fragmentation data in the UV/H2O2 benzene ring (S7,Table m/ z2. 270.0017).The structure of transformation products and their MS fragmentation data in the UV/H2O2 Tableoxidation 2.Retention The process structure Time of SMZ. of transformation Molecular Weightproducts and theirCharacteristic MS fragmentation data in the UV/H2O2 NameoxidationTable 2. The process structure of SMZ. of transformation products and their MS fragmentation Moleculardata in the Structure UV/H2O2 oxidationRetention process(min) Timeof SMZ. Molecular(MW) Weight CharacteristicIons Nameoxidation process of SMZ. Molecular Structure Table 2. The structureRetention of(min) transformation Time Molecular products(MW) Weight and theirCharacteristic MSIons fragmentation data in the UV/H2O2 Name Retention Time Molecular Weight Characteristic Molecular Structure Name Retention(min) Time Molecular(MW) Weight 190.0182,172.0045,CharacteristicIons Molecular Structure oxidationName processS1 ofRetention SMZ.(min)1.666 Time Molecular(MW) 189 Weight CharacteristicIons Molecular Structure Name (min) (MW) 190.0182,172.0045,122.0243,109.0524Ions Molecular Structure S1 (min)1.666 (MW) 189 Ions Name Retention Time (min) Molecular Weight (MW) Characteristic190.0182,172.0045,122.0243,109.0524 Ions Molecular Structure S1 1.666 189 190.0182,172.0045, S1 1.666 189 190.0182,172.0045, S1 1.666 189 190.0182,172.0045,122.0243,109.0524 S2 1.713 132 190.0182,172.0045,122.0243,109.0524 133.0609,72.0449 S1S1 1.6661.666 189 189 122.0243,109.0524 122.0243,109.0524122.0243,109.0524 S2 1.713 132 133.0609,72.0449 S2 1.713 132 133.0609,72.0449 S2 1.713 132 133.0609,72.0449 288.0661,156.0119, S2S2 1.7131.713 132 132 133.0609,72.0449 133.0609,72.0449 S3S2 2.6151.713 287 132 133.0609,72.0449 288.0661,156.0119,108.0452,92.0504 S3 2.615 287 288.0661,156.0119, 288.0661,156.0119,108.0452,92.0504 S3 2.615 287 288.0661,156.0119,288.0661,156.0119, S3S3 2.6152.615 287 287 288.0661,156.0119,108.0452,92.0504 S3 2.615 287 108.0452,92.0504108.0452,92.0504 S4S3 2.8432.615 287 98 108.0452,92.0504 99.0562,72.0462 108.0452,92.0504 S4 2.843 98 99.0562,72.0462 S4S4 2.8432.843 98 98 99.0562,72.0462 99.0562,72.0462 S4 2.843 98 216.0437,156.0113, 99.0562,72.0462 S4 2.843 98 99.0562,72.0462 S4 2.843 98 216.0437,156.0113, 99.0562,72.0462 S5 2.785 215 108.0444,92.0507, 216.0437,156.0113, 216.0437,156.0113, S5S5 2.7852.785 215 215 108.0444,92.0507,216.0437,156.0113,108.0444,92.0507, 216.0437,156.0113,65.0395 S5 2.785 215 216.0437,156.0113,108.0444,92.0507,65.0395 S5 2.785 215 108.0444,92.0507,65.0395 S5 2.785 215 270.0556,172.0065,108.0444,92.0507, Int. S5J. Environ. Res. Public2.785 Health 2019, 16, x 215 270.0556,172.0065,108.0444,92.0507,65.0395 12 of 17 S6Int. S6J. Environ. 4.049 Res. Public4.049 Health 2019, 16, x 269 269 65.0395 12 of 17 124.0398,109.0528270.0556,172.0065,124.0398,109.052865.0395 S6 4.049 269 65.0395 270.0552,156.0116,270.0556,172.0065,124.0398,109.0528 S6 4.049 269 270.0556,172.0065,270.0552,156.0116, S6S7 4.0492.882 269 270.0552,156.0116,270.0556,172.0065, S7S7S6 2.8822.8824.049 269 269 270.0556,172.0065,124.0398,109.0528108.0446,92.0504 S6 4.049 269 108.0446,92.0504124.0398,109.0528108.0446,92.0504 124.0398,109.0528 124.0398,109.0528 254.0608,156.0120 254.0608,156.0120,254.0608,156.0120 S8 S8 4.1844.184 253 253 S8 4.184 253 108.0451,92.0505108.0451,92.0505 108.0451,92.0505

Int. J. Environ. Res. Public Health 2019, 16, x 13 of 17 Int. J. Environ. Res. Public Health 2019, 16, x 13 of 17 Int. J. Environ. Res. Public Health 2019, 16, x 13 of 17 Int. J. Environ. Res. Public Health 2019, 16, x 13 of 17 Table 3. The structure of transformation products and their MS fragmentation data in the UV/H2O2 Int. J. Environ.Table 3. Res. The Public structure Health 2019of transformation, 16, x products and their MS fragmentation data in the UV/H213O2 of 17 Tableoxidation 3. The process structure of SMT. of transformation products and their MS fragmentation data in the UV/H2O2 Int. J. Environ.Tableoxidation 3. Res. The process Public structure Health of SMT. 2019of transformation , 16, x products and their MS fragmentation data in the UV/H213O2 of 17 Int. J. Environ.oxidation Res. process Public Health of SMT. 2019 , 16, x 13 of 17 Int. J. Environ.oxidation Res. process Public Health of SMT. 2019 , 16, x 13 of 17 Int. J. Environ.Table Retention3. Res. The Public structure TimeHealth 2019of transformationMolecular, 16, x Weight products and their MS fragmentation data in the UV/H2O132 of 17 Int.Name J. Environ.Table Retention3. Res. The Public structure TimeHealth of2019 transformationMolecular, 16, x Weight products anCharacteristicd their MS fragmentation Ions data Molecular in the UV/H Structure2O132 of 17 Int.Name J. Environ.Tableoxidation Retention3. Res. The process (min)Public structure Time Health of SMT. of2019 transformation Molecular, 16, x (MW) Weight products anCharacteristicd their MS fragmentation Ions data Molecular in the UV/H Structure2O132 of 17 Int. J. Environ. Res.Name PublicTableoxidation HealthRetention3. The process(min) 2019structure Time , of16 SMT., 1797of transformation Molecular(MW) Weight products anCharacteristicd their MS fragmentation Ions data Molecular in the UV/H Structure2O2 12 of 15 NameoxidationTable 3. The process(min) structure of SMT. of transformation (MW) products anCharacteristicd their MS fragmentation Ions data Molecular in the UV/H Structure2O2 oxidationTable 3. The process(min) structure of SMT. of transformation (MW) products and their MS fragmentation data in the UV/H2O2 oxidationTableRetention 3. The process structure Time of SMT. of transformation Molecular Weight products and their MS fragmentation data in the UV/H2O2 Nameoxidation Retention process Time of SMT. Molecular Weight Characteristic140.0824,123.0558, Ions Molecular Structure NameS1oxidation Retention process(min)2.033 Time of SMT. Molecular(MW) 139 Weight Characteristic140.0824,123.0558, Ions Molecular Structure Table 3. TheNameS1 structure Retention(min)2.033 of transformation Time Molecular(MW) products 139 Weight and theirCharacteristic140.0824,123.0558, MS fragmentation Ions data Molecular in the UVStructure/H O NameS1 Retention(min)2.033 Time Molecular(MW) 139 Weight Characteristic140.0824,123.0558,99.0563,82.0302 Ions Molecular Structure2 2 NameS1 Retention(min)2.033 Time Molecular(MW) 139 Weight Characteristic99.0563,82.0302 Ions Molecular Structure oxidationName process Retention of SMT.(min) Time Molecular(MW) Weight Characteristic140.0824,123.0558,99.0563,82.0302 Ions Molecular Structure Name (min) (MW) Characteristic99.0563,82.0302 Ions Molecular Structure S1 (min)2.033 (MW) 139 140.0824,123.0558, Name RetentionS1 Time (min)2.033 Molecular Weight 139 (MW) Characteristic140.0824,123.0558,99.0563,82.0302 Ions Molecular Structure S1S2 2.0332.160 139 123 124.0878,107.0610,67.0314140.0824,123.0558,99.0563,82.0302 S1S2 2.0332.160 139 123 124.0878,107.0610,67.0314140.0824,123.0558,99.0563,82.0302 S2 2.160 123 124.0878,107.0610,67.031499.0563,82.0302 S1S2 2.0332.160 139 123 124.0878,107.0610,67.0314140.0824,123.0558,140.0824,123.0558,99.0563,82.0302 S1S1 2.0332.033 139 139 99.0563,82.030299.0563,82.0302 S2 2.160 123 124.0878,107.0610,67.031499.0563,82.0302 S2 2.160 123 124.0878,107.0610,67.0314 S2S3 2.1602.965 123 214 124.0878,107.0610,67.0314215.1301,108.0689,198.1035 S3 2.965 214 215.1301,108.0689,198.1035 S2S3 2.1602.965 123214 215.1301,108.0689,198.1035 124.0878,107.0610,67.0314 S2S2S3 2.1602.1602.965 123 123 214 124.0878,107.0610,67.0314215.1301,108.0689,198.1035 124.0878,107.0610,67.0314 S2 2.160 123 124.0878,107.0610,67.0314

S3 2.965 214 215.1301,108.0689,198.1035 S3 2.965 214 215.1301,108.0689,198.1035295.0875,124.0874, 295.0875,124.0874, S3S4 2.9653.812 214 294 215.1301,108.0689,198.1035 S4 3.812 294 295.0875,124.0874, S3S3S4 2.9652.9653.812 214 214294 215.1301,108.0689,198.1035186.0336,108.0450,172.0067295.0875,124.0874, S3S4 2.9653.812 214 294 215.1301,108.0689,198.1035186.0336,108.0450,172.0067 S3 2.965 214 215.1301,108.0689,198.1035186.0336,108.0450,172.0067 186.0336,108.0450,172.0067295.0875,124.0874, S4 3.812 294 295.0875,124.0874,

S4 3.812 294 186.0336,108.0450,172.0067295.0875,124.0874, S4 3.812 294 295.0875,124.0874, S4S4S5 3.8123.8123.218 294 294 292 186.0336,108.0450,172.0067293.0718,212.0823,184.0876295.0875,124.0874, S4S5 3.8123.218 294 292 186.0336,108.0450,172.0067293.0718,212.0823,184.0876295.0875,124.0874, S5 3.218 292 186.0336,108.0450,172.0067293.0718,212.0823,184.0876295.0875,124.0874, S4S5 3.8123.218 294 292 186.0336,108.0450,172.0067293.0718,212.0823,184.0876 S4 3.812 294 186.0336,108.0450,172.0067 S5 3.218 292 186.0336,108.0450,172.0067293.0718,212.0823,184.0876 279.0934,186.0347,124.0882, S5 3.218 292 293.0718,212.0823,184.0876279.0934,186.0347,124.0882, S5S5S6 3.2183.2183.984 292 292 278 279.0934,186.0347,124.0882,293.0718,212.0823,184.0876 S5S6 3.2183.984 292 278 293.0718,212.0823,184.0876279.0934,186.0347,124.0882,108.0458,92.0512,156.0126 S5S6 3.2183.984 292278 293.0718,212.0823,184.0876108.0458,92.0512,156.0126 S5S6 3.2183.984 292 278 293.0718,212.0823,184.0876108.0458,92.0512,156.0126 S5 3.218 292 279.0934,186.0347,124.0882,293.0718,212.0823,184.0876108.0458,92.0512,156.0126 S6 3.984 278 279.0934,186.0347,124.0882, 279.0934,186.0347,124.0882,313.0537,186.0336, S6S6 3.9843.984 278 278 279.0934,186.0347,124.0882,108.0458,92.0512,156.0126 S6S7 3.9844.313 278 312 108.0458,92.0512,156.0126313.0537,186.0336, S6S7 3.9844.313 278 312 279.0934,186.0347,124.0882,108.0458,92.0512,156.0126313.0537,186.0336, S6S7 3.9844.313 278312 279.0934,186.0347,124.0882,108.0458,92.0512,156.0126313.0537,186.0336,142.0052,124.0864 279.0934,186.0347,124.0882,108.0458,92.0512,156.0126142.0052,124.0864 S6S7 3.9844.313 278 312 108.0458,92.0512,156.0126142.0052,124.0864 S6 3.984 278 108.0458,92.0512,156.0126313.0537,186.0336,313.0537,186.0336,142.0052,124.0864 S7S7 4.3134.313 312 312 108.0458,92.0512,156.0126313.0537,186.0336, 142.0052,124.0864313.0537,186.0336, S7 Table 4. The4.313 structure of transformation 312 products an313.0537,186.0336,142.0052,124.0864d their MS fragmentation data in the UV/H2O2 S7 Table 4. The4.313 structure of transformation 312 products an313.0537,186.0336,142.0052,124.0864d their MS fragmentation data in the UV/H2O2 S7Table oxidation 4. The process4.313 structure of STZ. of transformation 312 products an313.0537,186.0336,142.0052,124.0864d their MS fragmentation data in the UV/H2O2 Tableoxidation 4. The process structure of STZ. of transformation products an142.0052,124.0864d their MS fragmentation data in the UV/H2O2 S7oxidation process4.313 of STZ. 312 313.0537,186.0336,142.0052,124.0864 S7oxidation process4.313 of STZ. 312 Table 4. The structure of transformation products an142.0052,124.0864d their MS fragmentation data in the UV/H2O2 Table 4. The structureRetention of transformation Time Molecular products Weight and their MS fragmentation data in the UV/H2O2 Table 4. The structure of transformation products an142.0052,124.0864d their MS fragmentation data in the UV/H2O2 Nameoxidation Retention process Time of STZ. Molecular Weight Characteristic Ions Molecular Structure oxidationName processTable ofRetention4. The STZ.(min) structure Time of transformationMolecular(MW) Weight products anCharacteristicd their MS fragmentation Ions data Molecular in the UV/H Structure2O2 NameTableoxidation 4.Retention The process(min) structure Time of STZ. of transformationMolecular(MW) Weight products anCharacteristicd their MS fragmentation Ions data Molecular in the UV/H Structure2O2 NameoxidationTable 4. The process(min) structure of STZ. of transformation(MW) products anCharacteristicd their MS fragmentation Ions data Molecular in the UV/H Structure2O2 oxidationTable 4. The process(min) structure of STZ. of transformation(MW) products and their MS fragmentation data in the UV/H2O2 oxidationRetention process Time of STZ. Molecular Weight Name RetentionNameTable Time 4. The (min) structure Molecular of transformation Weight (MW) products an CharacteristicCharacteristicd their MS fragmentation Ions Ions data Molecular in the UV/HStructure 2O2 S1oxidation Retention process(min)1.692 Time of STZ. Molecular(MW) 100 Weight 101.0179,74.0078,58.9978 NameS1oxidation Retention process1.692 Time of STZ. Molecular 100 Weight 101.0179,74.0078,58.9978Characteristic Ions Molecular Structure NameS1 Retention(min)1.692 Time Molecular(MW) 100 Weight 101.0179,74.0078,58.9978Characteristic Ions Molecular Structure S1NameS1 1.692Retention(min)1.692 Time Molecular 100(MW) 100 Weight 101.0179,74.0078,58.9978101.0179,74.0078,58.9978Characteristic Ions Molecular Structure Name Retention(min) Time Molecular(MW) Weight Characteristic Ions Molecular Structure Name Retention(min) Time Molecular(MW) Weight Characteristic Ions Molecular Structure S1 (min)1.692 (MW) 100 101.0179,74.0078,58.9978226.0662,124.0222, Name Characteristic226.0662,124.0222, Ions Molecular Structure S1S2 (min)1.6922.208 (MW) 100 225 101.0179,74.0078,58.9978226.0662,124.0222, S1S2 1.6922.208 100 225 101.0179,74.0078,58.9978226.0662,124.0222,226.0662,124.0222,151.0334 S2S2 2.2082.208 225 225 151.0334 S1S2 1.6922.208 100225 101.0179,74.0078,58.9978151.0334151.0334 S1 1.692 100 101.0179,74.0078,58.9978226.0662,124.0222,151.0334 S1S2 1.6922.208 100225 101.0179,74.0078,58.9978226.0662,124.0222, S2 2.208 225 226.0662,124.0222,151.0334 S3 2.483 172 226.0662,124.0222, 173.0318,93.0582 S3S2S3 2.4832.2082.483 172 225 172 173.0318,93.0582226.0662,124.0222, 173.0318,93.0582151.0334 S3 2.483 172 226.0662,124.0222, 173.0318,93.0582151.0334 S2S3 2.2082.483 225172 173.0318,93.0582151.0334 Int.S2 J. Environ. Res. Public2.208 Health 2019, 16, x 225 226.0662,124.0222,151.0334 14 of 17 Int. S2J. Environ. Res. Public2.208 Health 2019, 16, x 225 14 of 17 Int. S3J. Environ. Res. Public2.483 Health 2019, 16, x 172 290.0274,156.0120,290.0274,156.0120, 173.0318,93.0582151.0334 14 of 17 S4S3S4 2.6482.4832.648 289 172 289 290.0274,156.0120, 173.0318,93.0582151.0334 S4 2.648 289 108.0452,92.0507290.0274,156.0120, S3S4 2.4832.648 172 289 290.0274,156.0120,272.0176,172.0069, 173.0318,93.0582108.0452,92.0507 S3S4S5 2.4832.6483.423 172289 271 272.0176,172.0069, 173.0318,93.0582108.0452,92.0507 290.0274,156.0120,272.0176,172.0069,108.0452,92.0507 S3S5 2.4833.423 172 271 272.0176,172.0069, 173.0318,93.0582108.0452,92.0507124.0400,108.0453 S5S3S4S5 3.4232.4832.6483.423 271 172289271 290.0274,156.0120, 173.0318,93.0582 124.0400,108.0453290.0274,156.0120,124.0400,108.0453 S4 2.648 289 290.0274,156.0120,108.0452,92.0507124.0400,108.0453 S4 2.648 289 290.0274,156.0120,256.0217,156.0120,108.0452,92.0507 S4 2.648 289 290.0274,156.0120,108.0452,92.0507 S6 3.864 255 256.0217,156.0120,290.0274,156.0120,256.0217,156.0120,108.0452,92.0507 S6 S4S6 3.8642.6483.864 255 289 255 256.0217,156.0120,108.0452,92.0507108.0452,92.0506 S4S6 2.6483.864 289255 108.0452,92.0506108.0452,92.0507 108.0452,92.0506 108.0452,92.0507108.0452,92.0506 270.0017,189.0123,270.0017,189.0123, S7 S7 3.9003.900 269 269 270.0017,189.0123, S7 3.900 269 270.0017,189.0123,161.0174161.0174 S7 3.900 269 161.0174 161.0174

Based on the identification results of intermediates in the degradation process of SMZ, SMT and Based on theBased identification on the identification results ofresults intermediates of intermedia intes the in degradationthe degradation process process of SMZ, SMZ, SMT SMT and and STZ,Based the possible on the identificationdegradation resultspathways of intermedia of sulfonamidestes in the through degradation the UV/H process2O2 ofprocess SMZ, SMTcould and be STZ, the possibleSTZ, the degradation possible degradation pathways pathways of sulfonamides of sulfonamides through through the the UV UV/H/H 2OO2 process could could be be STZ,elucidated the possible as shown degradation in Figure 6.pathways The contaminan of sulfonamidests present throughin the aqueous the UV/H solutions22O2 process might could produce be elucidated as shown in Figure 6. The contaminants present in the aqueous solutions might produce elucidated aselucidatedmore shown toxic inasconversion Figureshown 6in .products Figure The contaminants 6. th Thean the contaminan parent present compound.ts present in the Somein the aqueous researchers aqueous solutions solutions evaluated might the potentialproduce produce more toxic conversion products than the parent compound. Some researchers evaluated the potential more toxic conversionmoreenvironmental toxic conversion products impact products thanof sulfonamides the th parentan the parentand compound. its compound. byproducts Some Some [29–31]. researchers researchers They foevaluated evaluatedund that thetoxicity the potential potential was environmental impact of sulfonamides and its byproducts [29–31]. They found that toxicity was environmentalenvironmentalhigher impact at the ofbeginning impact sulfonamides of of sulfonamidesthe reaction and itsand and byproducts the its to byxicityproducts reduced [29 –[29–31].31 gradually]. They They with foundfound the thatfurther toxicity toxicity progress was was higher at the beginning of the reaction and the toxicity reduced gradually with the further progress higherof the reaction. at the beginning This showed of the that reaction the intermediates and the toxicity of sulfonamides reduced gradually were degraded with the intofurther the lessprogress toxic higher at theof beginning the reaction. of This the showed reaction that and the the intermediates toxicity reduced of sulfonamides gradually were with degraded the further into the progress less toxic of ofproducts the reaction. as the This reaction showed thatprogresses. the intermediates Therefore, of sulfonamidesthe environmental were degraded impact intoof sulfonamides the less toxic products as the reaction progresses. Therefore, the environmental impact of sulfonamides productsdegradation as intermediatesthe reaction wasprogresses. less than thatTherefor beforee, degradation.the environmental impact of sulfonamides degradation intermediates was less than that before degradation. degradation intermediates was less than that before degradation. O O + R-NH H2N OS OH 2 + R-NH2 H2N S OH + R-NH H2N SO OH 2 O O O O H2N OS NH2 + R H N S NH H2N H2N SO NH2 + R H N 2 2 + R H2N O O 2 O O H2N OS NH R H2N S NH R H2N SO NH R O O H O O HO HN OS NH R HO HN S NH R HO N SO NH R O O

H2N NH R O H2N NH R O H2N NH R O O O N OS NH R H2N OS NH R O N S NH R H2N S NH R H2NHO SO NH R O N SO NH R HO O O HO O Figure 6. Proposed Odegradation pathways of sulfonamides during the UV/H2O2 oxidation process (R Figure 6. Proposed degradation pathways of sulfonamides during the UV/H2O2 oxidation process (R Figurerepresents 6. Proposed a heterocyclic degradation ring). pathways of sulfonamides during the UV/H2O2 oxidation process (R represents a heterocyclic ring). represents a heterocyclic ring). 4. Conclusions 4. Conclusions 4. ConclusionsTo sum up, the decomposition of sulfonamides with a low concentration in their mixed solution To sum up, the decomposition of sulfonamides with a low concentration in their mixed solution throughTo sum the UV/Hup, the2O decomposition2 process was investigated of sulfonamides in depth. with Thea low degradation concentration of each in their contaminant mixed solution in the through the UV/H2O2 process was investigated in depth. The degradation of each contaminant in the throughcoexistence the systemUV/H2O fitted2 process well waswith investigated the first-order in depth.kinetic The model. degradation The degradation of each contaminant efficiency of inSMZ, the coexistence system fitted well with the first-order kinetic model. The degradation efficiency of SMZ, coexistenceSTZ and SMT system in their fitted mixed well with solution the first-order could be kineticaffected model. by multiple The degradation parameters efficiency including of initialSMZ, STZ and SMT in their mixed solution could be affected by multiple parameters including initial STZconcentration and SMT of in mixed their solution,mixed solution UV light could intensity, be affected H2O2 dosage by multiple and initial parameters solution includingpH. An increase initial concentration of mixed solution, UV light intensity, H2O2 dosage and initial solution pH. An increase concentrationin UV light intensity of mixed and solution, H2O2 dosage UV light could intensity, improve H2O the2 dosage degradation and initial of organicsolution pollutants, pH. An increase while in UV light intensity and H2O2 dosage could improve the degradation of organic pollutants, while inthe UV increase light intensity of the initial and H concentration2O2 dosage could of thimprovee mixed the solution degradation would of suppress organic pollutants,the degradation while the increase of the initial concentration of the mixed solution would suppress the degradation theefficiency. increase The of degradationthe initial concentration of SMZ and SMTof th evia mixed the UV/H solution2O2 processwould suppressgradually the decreased degradation with efficiency. The degradation of SMZ and SMT via the UV/H2O2 process gradually decreased with efficiency.initial pH, Thehowever, degradation the degradation of SMZ and efficiency SMT via of the STZ UV/H increased2O2 process with graduallythe increase decreased of the initial with initial pH, however, the degradation efficiency of STZ increased with the increase of the initial initialsolution’s pH, pH.however, The intermediates the degradation of SMZ,efficiency SMT of and STZ STZ increased in the withUV/H the2O2 increaseprocess ofwere the further initial solution’s pH. The intermediates of SMZ, SMT and STZ in the UV/H2O2 process were further solution’s pH. The intermediates of SMZ, SMT and STZ in the UV/H2O2 process were further

Int. J. Environ. Res. Public Health 2019, 16, x 14 of 17

272.0176,172.0069, S5 3.423 271 124.0400,108.0453

256.0217,156.0120, S6 3.864 255 108.0452,92.0506

270.0017,189.0123, S7 3.900 269 161.0174

Based on the identification results of intermediates in the degradation process of SMZ, SMT and STZ, the possible degradation pathways of sulfonamides through the UV/H2O2 process could be elucidated as shown in Figure 6. The contaminants present in the aqueous solutions might produce moreInt. J. Environ. toxic conversion Res. Public Health products2019, 16 th, 1797an the parent compound. Some researchers evaluated the potential 13 of 15 environmental impact of sulfonamides and its byproducts [29–31]. They found that toxicity was higher at the beginning of the reaction and the toxicity reduced gradually with the further progress ofthe the reaction. reaction. This This showed showed that that the the intermediates intermediates of of sulfonamides sulfonamides werewere degradeddegraded intointo the less toxic products asas the the reaction reaction progresses. progresses. Therefore, Therefor the environmentale, the environmental impact of impact sulfonamides of sulfonamides degradation degradationintermediates intermediates was less than was that less before than degradation. that before degradation.

O + R-NH H2N S OH 2 O O H2N S NH2 + R H2N O O

H2N S NH R O O H HO N S NH R O

H2N NH R O O O N S NH R H2N S NH R HO O O

Figure 6. ProposedProposed degradation degradation pathways pathways of of sulfonamides sulfonamides during during the the UV/H UV/2HO22O oxidation2 oxidation process process (R represents(R represents a heterocyclic a heterocyclic ring). ring).

4. Conclusions Conclusions To sum up, the decomposition of sulfonamides with a low concentration in their mixed solution through the UV/H O process was investigated in depth. The degradation of each contaminant in through the UV/H22O2 2process was investigated in depth. The degradation of each contaminant in the coexistencethe coexistence system system fitted fitted well wellwith withthe first-order the first-order kinetic kinetic model. model. The degradation The degradation efficiency efficiency of SMZ, of STZSMZ, and STZ SMT and SMTin their in theirmixed mixed solution solution could could be beaffected affected by by multiple multiple parameters parameters including including initial concentration of mixed solution, UV light intensity, H O dosage and initial solution pH. An increase concentration of mixed solution, UV light intensity, H22O2 dosage and initial solution pH. An increase in UV light intensity and H O dosage could improve the degradation of organic pollutants, while the in UV light intensity and H2 2O22 dosage could improve the degradation of organic pollutants, while theincrease increase of the of initial the initial concentration concentration of the mixedof the solutionmixed solution would suppress would suppress the degradation the degradation efficiency. The degradation of SMZ and SMT via the UV/H O process gradually decreased with initial pH, efficiency. The degradation of SMZ and SMT via2 the2 UV/H2O2 process gradually decreased with initialhowever, pH, the however, degradation the degradation efficiency of efficiency STZ increased of STZ with increased the increase with ofthe the increase initial solution’sof the initial pH. The intermediates of SMZ, SMT and STZ in the UV/H O process were further identified, and the solution’s pH. The intermediates of SMZ, SMT and 2STZ2 in the UV/H2O2 process were further possible transformation pathways of sulfonamides in the UV/H2O2 process were proposed in this work. The toxicity of sulfonamides was high at the beginning, and the toxicity was gradually reduced as the reaction progresses. This work exhibits the feasibility of the UV/H2O2 process for the degradation of organic pollutants in aqueous environment, which provides new insights for the selection of oxidants in water treatment processes.

Author Contributions: Conceptualization, G.Z.; Methodology, G.Z. and Q.X.; Formal analysis, Q.S.; Writing—original draft preparation, Q.S.; writing—review and editing, Z.Y. and C.W.; Supervision, G.Z.; Project administration, G.Z. and Q.X. Funding: This research was funded by the National Natural Science Foundation of China (No. 51578132) and Major Science and Technology Program for Water Pollution Control and Treatment (2014ZX07405002). Conflicts of Interest: The authors declare no conflict of interest. Int. J. Environ. Res. Public Health 2019, 16, 1797 14 of 15

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