Degradation of UV Filter Ethyl 4-Aminobenzoate (Et-PABA) Using a UV-Activated Persulfate Oxidation Process

applied sciences

Article Degradation of UV Filter Ethyl 4-Aminobenzoate (Et-PABA) Using a UV-Activated Persulfate Oxidation Process

Ruirui Han 1, Qiang Wu 2, Chihao Lin 2, Lingfeng Zhang 2, Zhicai Zhai 2, Ping Sun 2,* , Yingsen Fang 2,*, Jiaqiang Wu 1 and Hui Liu 2,* 1 Nanhu College, Jiaxing University, Jiaxing 314001, China 2 College of Biological, Chemical Sciences and Engineering, Jiaxing University, Jiaxing 314001, China * Correspondence: [email protected] (P.S.); [email protected] (Y.F.); [email protected] (H.L.); Tel.: +86-0573-8364-6045 (P.S.); +86-0573-8364-3695 (Y.F. & H.L.) Received: 24 June 2019; Accepted: 16 July 2019; Published: 18 July 2019

Featured Application: This work provides basic data, application references and theoretical guidance of the UV/PDS process for the removal of PABA-type UV ﬁlters in water.

Abstract: In this paper, the ultraviolet/persulfate (UV/PDS) combined oxidation process was used to remove the ethyl 4-aminobenzoate (Et-PABA), one of the typical 4-aminobenzoic acid (PABA)-type UV filters. The effects of various factors on the removal of Et-PABA using the UV/PDS process were investigated, and the degradation mechanisms of Et-PABA were explored. The results showed that the UV/PDS process can effectively remove 98.7% of Et-PABA within 30 min under the conditions: UV intensity of 0.92 mW cm 2, an initial concentration of Et-PABA of 0.05 mM, and a PDS concentration of · − 2 mM. The removal rate of Et-PABA increased with the increase in PDS dosage within the experimental range, whereas humic acid (HA) had an inhibitory effect on Et-PABA removal. Six intermediates were identified based on HPLC–MS and degradation pathways were then proposed. It can be foreseen that the UV/PDS oxidation process has broad application prospects in water treatment.

Keywords: advanced oxidation processes (AOPs); UV-activated; persulfate; ethyl 4-aminobenzoate (Et-PABA)

1. Introduction In recent years, skin cancer has attracted widespread attention owing to a series of risks related to ultraviolet (UV) radiation, sunburn, and premature skin aging [1]. Thereby UV filters are widely used in personal care products (PCPs) as UV-absorbing (shielding) agents that protect against UVA and UVB radiation and protect human health [2]. Most PCPs such as cosmetics, sunscreens, hair care products, detergents, etc., contain one or more UV filters in everyday products that are closely related to our lives. These organic UV filters are prone to the persistent pollution of the environment due to their large dosage, stable physical and chemical properties, and resistance to degradation. Those UV filters have become a new class of contaminants as they continue to enter the environment daily [3]. Residues of organic UV filters have been detected in various media such as surface water, sediment and aquatic organisms [4–7]. Several studies have shown that UV filters have potential endocrine disrupting effects that adversely affect organisms such as human tissues, mammals, animals, and fish, and cause serious health threats [8–11]. UV filters are relatively stable in nature and are resistant to biodegradation, making it difficult for municipal wastewater treatment plants (MWTPs) to remove them thoroughly [12–14]. Among them, p-aminobenzoic acid (PABA)-type compounds are the earliest and most widely used sunscreen products, thus their exposure characteristics, ecological effects and

Appl. Sci. 2019, 9, 2873; doi:10.3390/app9142873 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 2873 2 of 11 environmental behavior have become research hotspots in environmental fields in recent years [15,16]. They can exist for a long time and are dispersed in the environment due to their relatively high photochemical stability and low biodegradability. Thus, it is urgent to find efficient ways to remove PABA-type UV filters thoroughly. In recent years, advanced oxidation technology based on sulfate radicals (SR-AOPs) has become a new research hotspot, and it has some practical applications at home and abroad [17]. Persulfate (PS), used to produce sulfate radicals (SO4 −), is new type of oxidant used in in-situ chemical oxidation [18]. PS, including peroxymonosulfate (PMS) and peroxydisulfate (PDS), can be activated by ultraviolet light (UV) [19], ultrasound, heat [20], transition metal ions or their oxides [21–28], and nanocarbon [29–31]. Compared with hydroxyl radicals ( OH), SO4 − has higher redox potentials (E0 = 2.60 V) at neutral pH value that can effectively remove organic contaminants in water via an oxidation reaction. UV-based AOPs have great potential due to their high treatment efficiency, non-toxicity and low cost. Many researchers [32,33] used the UV-activated persulfate process to study the removal of refractory organic pollutants. Since UV technology has been widely used in MWTPs, it is advantageous to use UV to activate persulfate. In this case, UV cannot only disinfect and sterilize the water [34], but also activate persulfate to produce strong oxidizing SO4 − to remove organic pollutants, thus it is more practical to study UV-activated persulfate to remove PABA-type UV filters [35]. However, previous studies have been mainly focused on the photocatalytic degradation of PABA [36–39] or ethyl 4-aminobenzoate (Et-PABA) [40], one of the typical PABA-type UV filters. Information regarding the SO4 − degradation process of Et-PABA is limited, which hinders the application of this technique in PABA-type UV filter treatments. In this paper, the UV-activated PDS process was used to remove Et-PABA. The effects of PDS dosage, pH value, humic acid (HA) and other factors on the removal of Et-PABA during the UV/PDS process were investigated. The degradation intermediates of Et-PABA were identified and the possible degradation pathways were proposed. This work provides basic data, application references and theoretical guidance of the UV/PDS process for the removal of PABA-type UV filters in water.

2. Materials and Methods

2.1. Materials and Reagents Et-PABA (99%) was provided by Macklin Biochemical Co., (Shanghai, China). Formic acid, methanol (MeOH), ethanol (EtOH), and tert-butanol (TBA) were HPLC grade and acquired from Sigma-Aldrich. Other reagents, such as peroxydisulfate (PDS), sodium chloride (NaCl), sodium nitrate (NaNO3), sodium sulfate (Na2SO4), sodium hydrogen carbonate (NaHCO3), potassium dihydrogen phosphate (KH2PO4), sodium hydroxide (NaOH), and sulfuric acid (H2SO4) were all analytical reagent (AR) grade, and were acquired from Aladdin (Shanghai, China).

2.2. Experimental Methods A schematic diagram of the laboratory-made ultraviolet light irradiation device is shown in Figure1. The size of the reactor was 30 30 30 cm. The ultraviolet light irradiator was equipped with × × two low-pressure mercury lamps (254 nm, G4T5, Philips, Kossaka, Poland; 4W). The UV lamps were arranged in parallel above a crystallizing dish (90 cm in diameter) and were approximately 10 cm from the surface of the solution, where the intensity of UV irradiation was determined by a digital UV-C meter (0.92 mW cm 2). In order to ensure the stability of the light source, the UV lamps were preheated · − for 30 min. Generally, UV filters have been detected in water with concentrations up to µg/L. In this experiment, 0.05 mM of Et-PABA was chosen for convenience detection by LC and LC-MS [29,30]. 100 mL solution of Et-PABA and 2 mM PDS were added into a crystallizing dish, then magnetically stirred at 800 rpm. A quantity of 800 µL of the solution was sampled at different reaction times, filtered through a 0.45 µm filter, and added into 1.5 mL chromatography vials. The reaction was stopped with 200 µL of MeOH quencher. Appl. Sci. 2019, 9, x 3 of 11 magnetically stirred at 800 rpm. A quantity of 800 μL of the solution was sampled at different reaction Appl.times, Sci. filtered2019, 9, 2873through a 0.45 μm filter, and added into 1.5 mL chromatography vials. The reaction3 of 11 was stopped with 200 μL of MeOH quencher.

Figure 1. TheThe apparatus apparatus used used for for the the ethyl ethyl 4-am 4-aminobenzoateinobenzoate (Et-PABA) degradation.

2.3. Analysis Analysis Methods The concentration of Et-PABA was determineddetermined by high-performancehigh-performance liquid chromatography (HPLC) equipped with an Zorbax SB-C18 column (4.6 250 mm, 5 µm) (Aglient, Santa Clara, CA, (HPLC) equipped with an Zorbax SB-C18 column (4.6 × 250 mm, 5 μm) (Aglient, Santa Clara, CA, µ USA). The chromatographic conditions were: Injection volume volume 10 10 μL; flow ﬂow rate 1.0 mL/min; mL/min; column temperature 30 °C;◦C; wavelength 282 nm; mobile phase was was methanol methanol and and formic formic acid acid solution solution (0.30%) (0.30%) mixed by a volume ratio of 4:6. The removal eefficienciesﬃciencies can be obtainedobtained byby EquationEquation (1).1.

C − C C0 C r = 0 r = − (1) C0 (1) C0 where, r is the removal efficiency; C0 is the concentration of the original solution; C is the concentration where,of the solution r is the after removal treatment efficiency; using the C oxidation0 is the concentration process; and t ofis thethe reaction original time. solution; C is the concentrationThe samples of the were solution extracted after treatment and enriched using using the oxidation Waters Oasis process; HLB and columns t is the reaction (Waters, time. USA), and thenThe samples were analyzed were extracted by an Qtrap and 5600enriched+ (AB using Sciex, Waters Redwood, Oasis CA, HLB USA) columns MS coupled (Waters, with USA), an and ESI thensource were and analyzed an ACQUITY by an UPLCQtrap 5600+ (Waters, (AB Milford, Sciex, Redwood, MA, USA) CA, [31 USA)]. MS coupled with an ESI source and an ACQUITY UPLC (Waters, Milford, MA,USA) [31]. 3. Results and Discussion 3. Results and Discussion 3.1. Degradation of the Et-PABA Using Different Oxidation Processes 3.1. DegradationThe removal of etheffi cienciesEt-PABA of Using Et-PABA Different using Oxidation UV alone, Processes the PDS alone and the UV/PDS combined process,The respectively,removal efficiencies were evaluated. of Et-PABA As using shown UV in alone, Figure the2A, PDS the alone removal and etheffi UV/PDSciency of combined Et-PABA process,using a singlerespectively, UV process were was evaluated. only 23.7%, As shown because in the Figure main 2A, ultraviolet the removal radiation efficiency wavelength of Et-PABA of 254 usingnm lies a single just within UV process the absorption was only band 23.7%, of because Et-PABA the [32 main]. The ultraviolet UV process radiation may cause wavelength photolysis of 254 of nma part lies of just Et-PABA within becausethe absorption a small band amount of Et-PAB of activeA [32]. radicals The suchUV process as OH may may cause be generated photolysis in of the a waterpart of to Et-PABA indirectly because oxidize a thesmall contaminants amount of active [41]; the radicals degradation such as e●ffiOHciency may ofbe Et-PABA generated using in the the water PDS processto indirectly alone oxidize was only the 21.2%, contaminants because PDS [41]; can the theoretically degradation oxidize efficiency and degradeof Et-PABA parts using of the the organic PDS processcompounds. alone By was contrast, only 21.2%, under because the same PDS reaction can theo conditions,retically oxidize the removal and degrade efficiency parts of Et-PABA of the organic using compounds.the UV/PDS combinedBy contrast, process under increased the same rapidly, reaction and conditions, the degradation the removal efficiency efficiency can reach of 98.7% Et-PABA in 30 usingmin. Thethe synergyUV/PDS valuecombined was calculatedprocess increased according rapi todly, Reference and the [42 degradation]. It was a positive efficiency value can (0.918), reach which98.7% in stands 30 min. for The a synergistic synergy evalueffect thatwas maycalculated greatly according improve the to Reference removal e ffi[42].ciency It was of Et-PABA.a positive SO value4 − (0.918),generated which from stands the PDS foractivated a synergistic by the effect ultraviolet that may light greatly hasa improve higher oxidation–reduction the removal efficiency potential of Et- PABA.of E0 = 2.60SO4●− V, generated and its strong from oxidizing the PDS ability activated can transformby the ultraviolet organic matterlight has into a an higher excited oxidation– state and reductionthus, the organic potential contaminants of E0 = 2.60 V, can and be removedits strong fromoxidizing water ability efficiently can transform via an oxidation organic reaction matter into [43]. anMoreover, excited Figurestate and2B showsthus, the that organic the UV contaminants/PDS process can can be also removed efficiently from remove water PABA,efficiently indicating via an oxidationthe universality reaction of a[43]. UV /Moreover,PDS process Figure for the 2B removal shows ofthat PABA-type the UV/PDS UV filtersprocess in can water. also Particularly, efficiently removethe degradation PABA, indicating efficiencies the of universality Et-PABA and of PABAa UV/PDS using process the UV for process the removal were slightly of PABA-type higher than UV that using the PDS process after a 20 min reaction. This may be due to the fact that if UV radiation is increased or the reaction time is extended, UV alone can also remove contaminants from the water, Appl. Sci. 2019, 9, x 4 of 11 filters in water. Particularly, the degradation efficiencies of Et-PABA and PABA using the UV process were slightly higher than that using the PDS process after a 20 min reaction. This may be due to the fact that if UV radiation is increased or the reaction time is extended, UV alone can also remove contaminants from the water, while PDS does not. Therefore, as time goes on, the individual UV process exhibits a better removal efficiency than the PDS alone. These results were consistent with the results of the previous study [32]. Subsequently, we also studied the removal efficiency of Et-PABA using PDS activated by heat Appl.energy. Sci. 2019As ,shown9, 2873 in Figure 2C, the degradation rate did not increase significantly with the increase4 of 11 in the reaction temperature, mainly due to the relatively stable properties of PDS, thus PDS could not be activated by heat energy at low temperatures (25~45 °C) [44]. Dhaka et al. [32] also found the whileUV/PDS PDS process does not. was Therefore,a time efficient as time process, goes on,compar the individualed to the heat-activated UV process exhibits PDS oxidation a better of removal methyl eparaben.fficiency than the PDS alone. These results were consistent with the results of the previous study [32].

(A) Et-PABA (B) 100 100 PABA UV 80 PDS 80 UV UV/PDS PDS 60 60 UV/PDS

40 40

20 20 Removal efficiency (%) Removal efficiency (%) efficiency Removal 0 0 0 5 10 15 20 25 30 0 5 10 15 20 25 30 Time (min) Time (min)

20 Removal Removal efficiency (%)

0 0 5 10 15 20 25 30

Time (min) Figure 2. Removal efficiencies of (A) Et-PABA and (B) PABA using different oxidation processes; Figure 2. Removal efficiencies of (A) Et-PABA and (B) PABA using different oxidation processes; (C) (C) removal efficiency of Et-PABA using persulfate (PDS) activated by heat energy. Conditions: removal efficiency of Et-PABA using persulfate (PDS) activated by heat energy. Conditions: [Et- [Et-PABA] = [PABA]= 0.05 mM, [PDS] = 2 mM, and without pH adjustment. PABA] = [PABA]= 0.05 mM, [PDS] = 2 mM, and without pH adjustment. Subsequently, we also studied the removal efficiency of Et-PABA using PDS activated by heat 3.2. Effects of Various Factors on the Removal of Et-PABA energy. As shown in Figure2C, the degradation rate did not increase significantly with the increase in the reactionThe concentration temperature, of mainlyoxidant due dosage to the directly relatively affe stablects the properties amounts of of free PDS, radicals thus PDS produced; could not thus, be activatedit is necessary by heat to study energy the at loweffect temperatures of PDS dosage (25~45 on the◦C) degradation [44]. Dhaka etof al.Et-PABA. [32] also As found shown the in UV Figure/PDS process3A, the wasdegradation a time effi ratecient of process,Et-PABA compared increased to as the the heat-activated concentrations PDS of PDS oxidation increased of methyl from 1.0 paraben. to 4.0 mM within 15 min, which indicated that the removal of pollutants increased rapidly with the increase 3.2.of oxidant Effects ofdosage Various within Factors a oncertain the Removal range. The of Et-PABA main reason may be that as the concentration of PDS increases,The concentration the production of oxidantof SO4●− dosage and ●OH directly free affradicalsects the also amounts increases, of free which radicals promotes produced; the thus,degradation it is necessary efficiency to studyof Et-PABA the eff inect a ofshort PDS time dosage [45,46]. on The the degradationremaining concentration of Et-PABA. of As PDS shown (when in Figureadding3 A,2.0 the mM degradation PDS) was determined rate of Et-PABA using increased the iodometric as the concentrationstitration method. of PDSThe remaining increased fromrate was 1.0 to 4.0 mM within 15 min, which indicated that the removal of pollutants increased rapidly with the increase of oxidant dosage within a certain range. The main reason may be that as the concentration of PDS increases, the production of SO4 − and OH free radicals also increases, which promotes the degradation efficiency of Et-PABA in a short time [45,46]. The remaining concentration of PDS (when adding 2.0 mM PDS) was determined using the iodometric titration method. The remaining rate was about 27%, indicating that increasing the oxidant dosage is effective but not economical. Appl. Sci. 2019, 9, x 2873 55 of of 11 about 27%, indicating that increasing the oxidant dosage is effective but not economical. Theoretically, 23.523.5 MolMol ofof PDS PDS is is required required to to completely completely mineralize mineralize 1 1 Mol Mol of of Et-PABA. Et-PABA. Thus, Thus, 2 mM2 mM of PDSof PDS (40:1) (40:1) was was chosen chosen in thisin this experiment, experiment, considering considering the the actual actual excess excess consumption consumption of oxidants.of oxidants.

(A) PDS (B) 100 100

1.0 mM 80 Concrol 80 pH=3 2.0 mM pH=5 4.0 mM 60 60 pH=7

pH=9 pH=11 40 40

20 20

Removal efficiency (%) Removal efficiency (%) efficiency Removal 0 0 0 5 10 15 20 25 30 0 5 10 15 20 25 30 Time (min) Time (min) Figure 3. Effects of PDS dosage (A) and pH (B) on the removal efficiency of Et-PABA using UV/PDS. Figure 3. Effects of PDS dosage (A) and pH (B) on the removal efficiency of Et-PABA using UV/PDS. Conditions: [Et-PABA] = 0.05 mM, [PDS] = 2.0 mM, and without pH adjustment. Conditions: [Et-PABA] = 0.05 mM, [PDS] = 2.0 mM, and without pH adjustment. The acidity may affect the existence of free radicals in the UV/PDS process. Thus, the effect of pH onThe the acidity Et-PABA may removal affect the was existence examined of infree this radicals experiment. in the TheUV/PDS initial process. pH values Thus of, the solutioneffect of pH on the Et-PABA removal was examined in this experiment. The initial pH values of the solution were adjusted from 3 to 11 with 0.1 M H2SO4 or 0.1 M NaOH, respectively, to investigate the impact of pHwere on adjusted the removal from e3ffi tociency. 11 with As 0.1 Figure M H23SOB shows,4 or 0.1 M the NaOH removal, respectively e fficiency, to of investigate Et-PABA was the slightlyimpact betterof pH on under the removal acidic conditions, efficiency. while As Figure it was 3B slightly shows, worsethe removal under effic theiency alkaline of Et conditions-PABA wa ins slightly a short timebetter of under less thanacidic 20 conditions min. It may, while be dueit wa tos slightly the existence worse ofunder a large the amountalkaline ofconditions H+ in the in reaction a short time of less than 20 min. It may be due to the existence of a large amount of H+ in the reaction system system which reacts with PDS to produce more SO4 −, the predominate radicals in the UV/PDS system. which reacts with PDS to produce more SO4●−, the predominate radicals in the UV/PDS system. SO4●− SO4 − has a higher redox potential under acidic conditions, thereby accelerating the reaction rate [47]. Lianghas a higher and coworkers redox potential found under that acid-catalyzed acidic conditions decomposition, thereby accelerating of persulfate the reaction anion under rate [ acidic47]. Liang pH and coworkers found that acid-catalyzed decomposition of persulfate anion under acidic pH gives a gives a higher concentration of SO4 − [48]. The slight decrease in the removal efficiency at pH 11 ●− mayhigher be concentration due to the fewer of SO radicals4 [48]. The generated slight decrease as a result in ofthe the removal alkaline efficiency condition. at pH However, 11 may be as due the reactionto the fewer progressed, radicals the generated final removal as a e resultfficiency of wasthe greateralkaline than condition 97%, indicating. However, that as a the wide reaction range ofprogressed, pH value the was final applicable removal for efficiency the Et-PABA was greater degradation than 97% using, indicating the UV/PDS that process.a wide range From of these pH results,value was the applicable UV/PDS process for the isEt expected-PABA degradation to be a prospective using the treatment UV/PDS process in the remediation. From these of results, emerging the contaminantsUV/PDS process in wastewater. is expected to be a prospective treatment in the remediation of emerging contaminaIn then casets in ofwastewater scavenger. reactions between the radicals generated during the UV/PDS process In the case of scavenger reactions between the radicals generated during the2 UV/PDS process and the large number of anions in the natural water body such as Cl−, NO3−, SO4 −, HCO3−,H2PO4−, − − 2− − − itand is ofthe great large significance number of toanions investigate in the thenatural effects water of anions body insuch water as C onl , theNO degradation3 , SO4 , HCO of3 Et-PABA, H2PO4 , init theis of UV great/PDS significance process [49 to]. investigate To compare the the effect effectss of of anions different in water anions on on the the degradation degradation of ofEt- Et-PABA,PABA in sodiumthe UV/PDS chloride, process sodium [49]. nitrate, To compare sodium the sulfate, effects sodium of different hydrogen anions carbonate, on the degradation and sodium of dihydrogen Et-PABA, phosphatesodium chloride, were added sodium to the nitrate, reaction sodium system at sulfate, a concentration sodium of hydrogen 5 mM [31 carbonate], respectively., and As sodium shown dihydrogen phosphate were added to the reaction system at a concentration of 5 mM [31], in Figure4A, Cl − showed no inhibition effect on the decomposition, which may be due to the fact respectively. As shown in Figure 4A, Cl− showed no inhibition effect on the decomposition, which that Cl− can react with sulfate radicals and produce relatively weak free radicals such as Cl , Cl2− may be due to the fact that Cl- can react with sulfate radicals and produce relatively weak free radicals and ClHO [32]. HCO3− anions were thought to be scavengers of hydroxyl radicals that can react ● −● ● − withsuch sulfateas Cl , Cl radicals2 and toCl produceHO [32]. bicarbonate HCO3 anions radicals were inthought the solution. to be scavengers Previous studiesof hydroxyl have radicals shown that can react with sulfate radicals to produce bicarbonate radicals in the solution. Previous studies that Cl− and HCO3− at small concentrations can promote the degradation of organic compounds by have shown that Cl− and HCO3− at small concentrations can promote2 the degradation of organic the UV/PDS process [50]. The other ions, such as NO3− and SO4 −, had no significant effect on the − 2− removalcompounds effi ciencyby the ofUV/P Et-PABA.DS process Although [50]. T mosthe other of the ions common, such as ions NO3 in and the SO water4 , had matrix no maysignificant affect theeffect degradation on the removal rate of efficiency Et-PABA, of the Et- removalPABA. Although efficiency most of Et-PABA of the common using the ions UV /inPDS theprocess water matrix in the presencemay affect of the anions degradation was still impressiverate of Et-PABA when, thethe reactionremoval time efficiency increased of Et to-PABA morethan using 20 the min. UV/PDS process in the presence of anions was still impressive when the reaction time increased to more than 20 min. Appl. Sci. 2019, 9, x 2873 6 of 11

(A) Anions (B) HA 100 100 Control 80 80 Cl- NO - 3 60 60 2- SO4 HCO - 40 3 40 - Control H2PO4 5 mg/L 20 20 10 mg/L Removal efficiency (%) Removal efficienty (%) Removal 20 mg/L 0 0 0 5 10 15 20 25 30 0 5 10 15 20 25 30 Time (min) Time (min) Figure 4. Effects of different anions (A) and humic acid (HA) concentrations (B) on the removal Figure 4. Effects of different anions (A) and humic acid (HA) concentrations (B) on the removal efficiency of Et-PABA. Conditions: [Et-PABA] = 0.05 mM, [anion] = 5 mM, [HA] = 5–20 mg/L, [PDS] = efficiency of Et-PABA. Conditions: [Et-PABA] = 0.05 mM, [anion] = 5 mM, [HA] = 5-20 mg/L, [PDS] = 2.0 mM, and without pH adjustment. 2.0 mM, and without pH adjustment. Humic acid (HA) is one of the main natural organic matter (NOM) in natural water. Thus, Humic acid (HA) is one of the main natural organic matter (NOM) in natural water. Thus, it’s it’s typically used as a representative of NOM to investigate the NOM effect on the UV/PDS oxidation typically used as a representative of NOM to investigate the NOM effect on the UV/PDS oxidation process. Thus, the effects of HA at different concentrations on the removal of Et-PABA were process. Thus, the effects of HA at different concentrations on the removal of Et-PABA were investigated as shown in Figure4B. The degradation rate of Et-PABA decreased with the increase in investigated as shown in Figure 4B. The degradation rate of Et-PABA decreased with the increase in the HA concentration. The removal efficiencies at 30 min were 97.4%, 92.3%, and 66.8%, respectively, the HA concentration. The removal efficiencies at 30 min were 97.4%, 92.3%, and 66.8%, respectively, suggesting that HA had a significant inhibitory effect on the removal of Et-PABA. HA molecules suggesting that HA had a significant inhibitory effect on the removal of Et-PABA. HA molecules contain a large amount of photosensitive substances such as aromatic carboxyl groups, hydroxyl contain a large amount of photosensitive substances such as aromatic carboxyl groups, hydroxyl groups, amine groups, which can directly absorb ultraviolet light, competitively inhibit the absorption groups, amine groups, which can directly absorb ultraviolet light, competitively inhibit the of light energy by the PDS [51], and reduce the yield of free radicals in the solution. At the same absorption of light energy by the PDS [51], and reduce the yield of free radicals in the solution. At the time, the active group of HA will also compete with Et-PABA for active radicals such as SO4 −, thus same time, the active group of HA will also compete with Et-PABA for active radicals such as SO4●−, reducing the removal efficiency of Et-PABA [52,53]. thus reducing the removal efficiency of Et-PABA [52,53]. In order to study the degradation of Et-PABA in real water bodies, two water samples from In order to study the degradation of Et-PABA in real water bodies, two water samples from different sources (river water and tap water from Jiaxing College) were collected and filtered through a different sources (river water and tap water from Jiaxing College) were collected and filtered through 0.45 µm filter. In order to compare with the Et-PABA degradation in pure water, the same concentration a 0.45 μm filter. In order to compare with the Et-PABA degradation in pure water, the same of Et-PABA (0.05 mM) was added to the two real water samples, respectively. The effect of real water concentration of Et-PABA (0.05 mM) was added to the two real water samples, respectively. The on the degradation of Et-PABA using the UV/PS process is shown in Figure5. The degradation rate of effect of real water on the degradation of Et-PABA using the UV/PS process is shown in Figure 5. The Et-PABA in the tap water reached as high as 96.5% within 10 min, which was essentially the same degradation rate of Et-PABA in the tap water reached as high as 96.5% within 10 min, which was as that in pure water. The degradation rate of Et-PABA in surface water decreased, but the removal essentially the same as that in pure water. The degradation rate of Et-PABA in surface water efficiency also reached 94.3% after a reaction time of 20 min. This result may be attributed to the decreased, but the removal efficiency also reached 94.3% after a reaction time of 20 min. This result inhibition of NOM in the real water body and the interaction of other water components, as discussed may be attributed to the inhibition of NOM in the real water body and the interaction of other water above. These results suggest that the UV/PDS process has good adaptability regarding the Et-PABA components, as discussed above. These results suggest that the UV/PDS process has good removal of real water samples. Chen et al. [54] also studied the degradation of 2,4-dichlorophenol in adaptability regarding the Et-PABA removal of real water samples. Chen et al. [54] also studied the the real water using the UV/PDS process. They determined the water quality of several real water degradation of 2,4-dichlorophenol in the real water using the UV/PDS process. They determined the bodies. The concentrations of Cl−, HCO3−, and NO3− in different water samples were 42.5–63.0 mg/L, water quality of several real water bodies. The concentrations of Cl−, HCO3−, and NO3− in different 128.0–136.6 mg/L, and 0.005–0.070 mg/L, respectively. The results also suggested the UV/PDS process water samples were 42.5–63.0 mg/L, 128.0–136.6 mg/L, and 0.005–0.070 mg/L, respectively. The can be further extended to practical applications. results also suggested the UV/PDS process can be further extended to practical applications. 3.3. Degradation Pathways Figure6 presents the possible transformation pathways of Et-PABA based on the six observed TPs. On one hand, SO4 − and OH can directly attack the carbon on the benzene ring, resulting in hydroxylation of Et-PABA. Through this degradation pathway, TP1 was produced. This product was further hydroxylated to produce a product of TP2. On the other hand, TP3 was a product of Et-PABA formed after attacks by ROS caused the C–O bond between the carbonyl group and the ethoxy group to be broken. Under further actions of ROS, due to the oxidation or removal of the amino group, TP3 Appl. Sci. 2019, 9, x 7 of 11

100

80 Control 60 Tap water Surface water Appl. Sci. 2019, 9, 2873 40 7 of 11

20 was converted into TP4 and TP6,Removalefficiency (%) respectively. Similarly, hydroxylation of TP4 produced TP5. Finally, Appl.these Sci. intermediates 2019, 9, x of Et-PABA can0 be further degraded to lower organic acids. 7 of 11 0 5 10 15 20 25 30 Time (min) 100 Figure 5. The degradation of Et-PABA in real water using the UV/PDS process. Conditions: [Et-PABA] = 0.05 mM, [PDS] = 2.0 mM, and80 without pH adjustment. Control 60 Tap water 3.3. Degradation Pathways Surface water Figure 6 presents the possible40 transformation pathways of Et-PABA based on the six observed TPs. On one hand, SO4●− and ●OH can directly attack the carbon on the benzene ring, resulting in 20 hydroxylation of Et-PABA. ThroughRemovalefficiency (%) this degradation pathway, TP1 was produced. This product was further hydroxylated to produce0 a product of TP2. On the other hand, TP3 was a product of Et-PABA formed after attacks by ROS caused0 the 5C–O bond 10 between 15 20 the 25carbonyl 30 group and the ethoxy group to be broken. Under further actions of ROS, due Timeto the (min) oxidation or removal of the amino group, TP3 was convertedFigure 5. TheThe into degradation degradation TP4 and TP6,of of Et-PABA Et-PABA respectively. in real real water water Similarly, using using the thehydroxylation UV/PDS UV/PDS process. of TP4 Conditions: Conditions: produced [Et-PABA] [Et-PABA] TP5. Finally, these= intermediates0.050.05 mM, mM, [PDS] [PDS] of == 2.0Et-PABA2.0 mM, mM, and and can without without be fu rtherpH pH adjustment. adjustment. degraded to lower organic acids.

3.3. Degradation Pathways Figure 6 presents the possible transformation pathways of Et-PABA based on the six observed TPs. On one hand, SO4●− and ●OH can directly attack the carbon on the benzene ring, resulting in hydroxylation of Et-PABA. Through this degradation pathway, TP1 was produced. This product was further hydroxylated to produce a product of TP2. On the other hand, TP3 was a product of Et-PABA formed after attacks by ROS caused the C–O bond between the carbonyl group and the ethoxy group to be broken. Under further actions of ROS, due to the oxidation or removal of the amino group, TP3 was converted into TP4 and TP6, respectively. Similarly, hydroxylation of TP4 produced TP5. Finally, these intermediates of Et-PABA can be further degraded to lower organic acids.

Figure 6. Proposed reaction pathways for the oxidationoxidation of Et-PABA usingusing thethe UVUV/PDS/PDS process.process.

4. Conclusions This study explored the removal efficiency, mechanism, and a novel advanced oxidation route for Et-PABA in a water environment using the UV/PDS combined process. The results showed that the combined treatment could significantly improve the degradation of Et-PABA compared to the single processes. The removal efficiency of Et-PABA after the UV/PDS combined process can reach 98.7% in 30 min. The removal rate of Et-PABA increased rapidly upon increasing the dosage of oxidant.

Figure 6. Proposed reaction pathways for the oxidation of Et-PABA using the UV/PDS process. Appl. Sci. 2019, 9, 2873 8 of 11

The presence of anions had no obvious eﬀect on the removal, while HA suppressed the removal of Et-PABA. Moreover, the UV/PDS process had good adaptability to Et-PABA removal of real water samples. Finally, a degradation path of Et-PABA was proposed based on the six detected TPs during the oxidation process. Overall, the UV/PDS process can be used as a promising method for the removal of PABA-type UV ﬁlters in water.

Author Contributions: Conceptualization—Z.Z. and P.S.; Formal analysis—R.H. and Y.F.; Investigation—Q.W., C.L., and L.Z.; Supervision—H.L.; Validation—R.H. and J.W.; Writing of original draft—P.S.; Writing of review & editing—H.L. Funding: This research was funded by the National Natural Science Foundation of China (21607058); the Zhejiang Provincial Department of Education General Research Project (Y201840526); the Student Research Innovation Team Funding Project of Zhejiang (2019R417027); the Student Research Training Program of Jiaxing University (2019); and the Key Project of Nanhu college of Jiaxing University (2018). Acknowledgments: We wish to thank anonymous reviewers for their comments, which improved the paper greatly. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. De Vries, E.; Arnold, M.; Altsitsiadis, E.; Trakatelli, M.; Hinrichs, B.; Stockfleth, E.; Coebergh, J. Potential impact of interventions resulting in reduced exposure to ultraviolet (UV) radiation (UVA and UVB) on skin cancer incidence in four European countries, 2010–2050. Brit. J. Dermatol. 2012, 167, 53–62. [CrossRef] [PubMed] 2. Liao, C.Y.; Kannan, K. Widespread occurrence of benzophenone-type UV light filters in personal care products from China and the United States: An assessment of human exposure. Environ. Sci. Technol. 2014, 48, 4103–4109. [CrossRef][PubMed] 3. Krause, M.; Klit, A.; Jensen, M.B.; Soeborg, T.; Frederiksen, H.; Schlumpf, M.; Lichtensteiger, W.; Skakkebaek, N.E.; Drzewiecki, K.T. Sunscreens: Are they beneficial for health? An overview of endocrine disrupting properties of UV-filters. Int. J. Androl. 2012, 35, 424–436. [CrossRef][PubMed] 4. He, K.; Hain, E.; Timm, A.; Tarnowski, M.; Blaney, L. Occurrence of antibiotics, estrogenic hormones, and UV-filters in water, sediment, and oyster tissue from the Chesapeake Bay. Sci. Total Environ. 2019, 650, 3101–3109. [CrossRef][PubMed] 5. Mitchelmore, C.L.; He, K.; Gonsior, M.; Hain, E.; Heyes, A.; Clark, C.; Younger, R.; Schmitt-Kopplin, P.; Feerick, A.; Conway,A.; et al. Occurrence and distribution of UV-filters and other anthropogenic contaminants in coastal surface water, sediment, and coral tissue from Hawaii. Sci. Total Environ. 2019, 670, 398–410. [CrossRef][PubMed] 6. Molins, D.D.; Munoz, R.; Nogueira, S.; Alonso, M.B.; Torres, J.P.; Malm, O.; Ziolli, R.L.; Hauser-Davis, R.A.; Eljarrat, E.; Barcelo, D.; et al. Occurrence of organic UV filters and metabolites in lebranche mullet (Mugil liza) from Brazil. Sci. Total Environ. 2018, 618, 451–459. [CrossRef][PubMed] 7. Apel, C.; Joerss, H.; Ebinghaus, R. Environmental occurrence and hazard of organic UV stabilizers and UV filters in the sediment of European North and Baltic Seas. Chemosphere 2018, 212, 254–261. [CrossRef] 8. Ao, J.; Yuan, T.; Gu, J.; Ma, Y.; Shen, Z.; Tian, Y.; Shi, R.; Zhou, W.; Zhang, J. Organic UV filters in indoor dust and human urine: A study of characteristics, sources, associations and human exposure. Sci. Total Environ. 2018, 640, 1157–1164. [CrossRef] 9. Liu, H.; Sun, P.; Liu, H.; Yang, S.; Wang, L.; Wang, Z. Acute toxicity of benzophenone-type UV filters for Photobacterium phosphoreum and Daphnia magna: QSAR analysis, interspecies relationship and integrated assessment. Chemosphere 2015, 135, 182–188. [CrossRef] 10. Liu, H.; Sun, P.; Liu, H.; Yang, S.; Wang, L.; Wang, Z. Hepatic oxidative stress biomarker responses in freshwater fish Carassius auratus exposed to four commonly benzophenone UV filters. Ecotoxicol. Environ. Saf. 2015, 119, 116–122. [CrossRef] 11. Li, A.; Law, J.; Chow, C.; Huang, Y.; Li, K.; Leung, K. Joint effects of multiple UV filters on zebrafish embryo development. Environ. Sci. Technol. 2018, 52, 9460–9467. [CrossRef][PubMed] 12. Sun, X.; Wei, D.; Liu, W.; Geng, J.; Liu, J.; Du, Y. Formation of novel disinfection by-products chlorinated benzoquinone, phenyl benzoquinones and polycyclic aromatic hydrocarbons during chlorination treatment Appl. Sci. 2019, 9, 2873 9 of 11

on UV filter 2,4-dihydroxybenzophenone in swimming pool water. J. Hazard. Mater. 2019, 367, 725–733. [CrossRef][PubMed] 13. O’Malley, E.; O’Brien, J.W.; Tscharke, B.; Thomas, K.V.; Mueller, J.F. Per capita loads of organic UV filters in Australian wastewater influent. Sci. Total Environ. 2019, 662, 134–140. [CrossRef][PubMed] 14. Liu, H.; Sun, P.; He, Q.; Feng, M.; Liu, H.; Yang, S.; Wang, L.; Wang, Z. Ozonation of the UV filter benzophenone-4 in aquatic environments: Intermediates and pathways. Chemosphere 2016, 149, 76–83. [CrossRef][PubMed] 15. Binni, M.; Lu, G.; Liu, J.; Yan, Z.; Yang, H.; Pan, T. Bioconcentration and multi-biomarkers of organic UV filters (BM-DBM and OD-PABA) in crucian carp. Ecotoxicol. Environ. Saf. 2017, 141, 178–187. 16. Suh, H.S.; Lim, S.K.; Kim, M.K.; Kim, M.H.; Baek, S.H. Risk assessment of ethylhexyl dimethyl PABA in sunscreen cosmetic products. Toxicol. Lett. 2017, 280, 105–106. [CrossRef] 17. Guerra-Rodríguez, S.; Rodríguez, E.; Singh, D.N.; Rodríguez-Chueca, J. Assessment of sulfate radical-based advanced oxidation processes for water and wastewater treatment: A Review. Water 2018, 10, 1828. [CrossRef] 18. Liu, H.; Bruton, T.A.; Doyle, F.M.; Sedlak, D.L. In situ chemical oxidation of contaminated groundwater by persulfate: Decomposition by Fe (III)-and Mn (IV)-containing oxides and aquifer materials. Environ. Sci. Technol. 2014, 48, 10330–10336. [CrossRef] 19. Ye, J.; Zhou, P.; Chen, Y.; Ou, H.; Liu, J.; Li, C.; Li, Q. Degradation of 1H-benzotriazole using ultraviolet activating persulfate: Mechanisms, products and toxicological analysis. Chem. Eng. J. 2018, 334, 1493–1501. [CrossRef] 20. Ji, Y.; Shi, Y.; Dong, W.; Wen, X.; Jiang, M.; Lu, J. Thermo-activated persulfate oxidation system for tetracycline antibiotics degradation in aqueous solution. Chem. Eng. J. 2016, 298, 225–233. [CrossRef] 21. Xu, H.; Wang, D.; Ma, J.; Zhang, T.; Lu, X.; Chen, Z. A superior active and stable spinel sulfide for catalytic peroxymonosulfate oxidation of bisphenol S. Appl. Catal. B Environ. 2018, 238, 557–567. [CrossRef] 22. Alexopoulou, C.; Petala, A.; Frontistis, Z.; Drivas, C.; Kennou, S.; Kondarides, D.I.; Mantzavinos, D. Copper phosphide and persulfate salt: A novel catalytic system for the degradation of aqueous phase micro-contaminants. Appl. Catal. B Environ. 2019, 244, 178–187. [CrossRef] 23. Deng, J.; Ya, C.; Ge, Y.; Cheng, Y.; Chen, Y.; Xu, M.; Wang, H. Activation of peroxymonosulfate by metal (Fe,

Mn, Cu and Ni) doping ordered mesoporous Co3O4 for the degradation of enroﬂoxacin. RSC Adv. 2018, 8, 2338–2349. [CrossRef] 24. Li, R.; Cai, M.; Xie, Z.; Zhang, Q.; Zeng, Y.; Liu, H.; Liu, G.; Lv, W. Construction of heterostructured

CuFe2O4/g-C3N4 nanocomposite as an eﬃcient visible light photocatalyst with peroxydisulfate for the organic oxidation. Appl. Catal. B Environ. 2019, 244, 974–982. [CrossRef] 25. Duan, X.; Su, C.; Miao, J.; Zhong, Y.; Shao, Z.; Wang, S.; Sun, H. Insights into perovskite-catalyzed peroxymonosulfate activation: Maneuverable cobalt sites for promoted evolution of sulfate radicals. Appl. Catal. B Environ. 2018, 220, 626–634. [CrossRef] 26. Ahn, Y.Y.; Bae, H.; Kim, H.I.; Kim, S.H.; Kim, J.H.; Lee, S.G.; Lee, J. Surface-loaded metal nanoparticles for peroxymonosulfate activation: Eﬃciency and mechanism reconnaissance. Appl. Catal. B Environ. 2019, 241, 561–569. [CrossRef] 27. Zhou, Y.; Wang, X.; Zhu, C.; Dionysiou, D.D.; Zhao, G.; Fang, G.; Zhou, D. New insight into the mechanism of peroxymonosulfate activation by sulfur-containing minerals: Role of sulfur conversion in sulfate radical generation. Water Res. 2018, 142, 208–216. [CrossRef][PubMed]

28. Lin, C.; Shi, D.; Wu, Z.; Zhang, L.; Zhai, Z.; Fang, Y.; Sun, P.; Han, R.; Wu, J.; Liu, H. CoMn2O4 catalyst prepared using the sol-gel method for the activation of peroxymonosulfate and degradation of UV filter 2-phenylbenzimidazole-5-sulfonic acid (PBSA). Nanomaterials 2019, 9, 774. [CrossRef][PubMed] 29. Liu, H.; Sun, P.; Feng, M.; Liu, H.; Yang, S.; Wang, L.; Wang, Z. Nitrogen and sulfur co-doped CNT-COOH as an efficient metal-free catalyst for the degradation of UV filter BP-4 based on sulfate radicals. Appl. Catal. B Environ. 2016, 187, 1–10. [CrossRef] 30. Sun, P.; Liu, H.; Zhai, Z.; Zhang, X.; Fang, Y.; Tan, J.; Wu, J. Degradation of UV filter BP-1 with nitrogen-doped industrial graphene as a metal-free catalyst of peroxymonosulfate activation. Chem. Eng. J. 2019, 356, 262–271. [CrossRef] Appl. Sci. 2019, 9, x 10 of 11

Appl. Sci. 2019, 9, 2873 10 of 11 30. Sun, P.; Liu, H.; Zhai, Z.; Zhang, X.; Fang, Y.; Tan, J.; Wu, J. Degradation of UV filter BP-1 with nitrogen- doped industrial graphene as a metal-free catalyst of peroxymonosulfate activation. Chem. Eng. J. 2019, 356, 31. Sun,262–271. P.; Liu, H.; Feng, M.; Guo, L.; Zhai, Z.; Fang, Y.; Zhang, X.; Sharma, V.K. Nitrogen-sulfur co-doped 31. industrialSun, P.; Liu, graphene H.; Feng, as M.; an Guo, efficient L.; Zhai, peroxymonosulfate Z.; Fang, Y.; Zhang, activator: X.; Sharma, Singlet V.K. oxygen-dominated Nitrogen-sulfur co-doped catalytic degradationindustrial graphene of organic as contaminants. an efficient peroxymonosulfAppl. Catal. B Environ.ate activator:2019, 251 Singlet, 335–345. oxygen-dominated [CrossRef] catalytic 32. Dhaka,degradation S.; Kumar, of organic R.; contaminants. Khan, M.A.; Paeng,Appl. Catal. K.-J.; B Kurade,Environ. M.B.;2019, 251 Kim,, 335–345. S.-J.; Jeon, B.-H. Aqueous phase 32. degradationDhaka, S.; Kumar, of methyl R.; parabenKhan, M.A.; using Paeng, UV-activated K.-J.; Kurade, persulfate M.B.; method. Kim, S.-J.;Chem. Jeon, Eng. B.-H. J. 2017Aqueous, 321, 11–19.phase [degradationCrossRef] of methyl paraben using UV-activated persulfate method. Chem. Eng. J. 2017, 321, 11–19. 33. Verma, S.; Nakamura, S.; Sillanpȁȁ, M. ApplicationApplication of UV-C LEDLED activatedactivated PMSPMS forfor thethe degradationdegradation of anatoxin-a. Chem. Eng. Eng. J J.. 20162016,, 284284,, 122–129. 122–129. [CrossRef] 34. Moreno-AndrMoreno-Andrés,és, J.; Rios Quintero, R.; Acevedo-Merino, A.; Nebot, E. Disinfection performance using a UVUV/persulfate/persulfate system: Effects Effects derived from didifferentfferent aqueousaqueous matrices.matrices. Photochem.Photochem. Photobiol. Sci Sci.. 20192019,, 18,, 878–883. [CrossRef][PubMed] 35. Xue, Y.;Y.; Dong, Dong, W.; W.; Wang, Wang, X.; Bi,X.; W.; Bi, Zhai, W.; P.;Zhai, Li, H.;P.; Nie, Li, M.H.; Degradation Nie, M. Degradation of sunscreen of agent sunscreen p-aminobenzoic agent p- acidaminobenzoic using a combination acid using a system combination of UV system irradiation, of UV persulphate irradiation,and persulphate iron (II). andEnviron. iron (II). Sci. Environ. Pollut. Res.Sci. 2016Pollut., 23 Res., 4561–4568. 2016, 23, 4561–4568. [CrossRef][ PubMed] 36. Tsoumachidou, S.;S.; Velegraki, T.;T.; Poulios,Poulios, I.I. TiOTiO22 photocatalytic degradation of UV filter filter para-aminobenzoic acid under artificialartificial and solar illumination. J. Chem. Technol. Technol. Biotechnol. Biotechnol. 20162016,, 9191,, 1773 1773–1781.−1781. [CrossRef] 37. Soto-Vazquez, L.; Cotto, M.; Duconge, J.; Morant, C.; Marquez,Marquez, F. SynthesisSynthesis andand photocatalyticphotocatalytic activity of TiOTiO22 nanowiresnanowires in in the the degradation degradation of p-aminobenzoicof p-aminobenzoic acid: acid: A comparative A comparative study withstudy a commercialwith a commercial catalyst. J.catalyst. Environ. J. Environ. Manag. 2016 Manag., 167 2016, 23–28., 167 [,CrossRef 23−28. ][PubMed] 38. Zhou, L.; Ji, Ji, Y.; Y.; Zeng, Zeng, C.; C.; Zhang, Zhang, Y.; Y.; Wang, Wang, Z.; Z.; Yang Yang,, X. X. Aquatic Aquatic photodegradati photodegradationon of ofsunscreen sunscreen agent agent p- p-aminobenzoicaminobenzoic acid acid in inthe the presen presencece of ofdissolved dissolved organic organic matter. matter. WaterWater Res. Res. 20132013, 47, 47, 153, 153–162.−162. [CrossRef] 39. Mao, L.;L.; Meng,Meng, C.; C.; Zeng, Zeng, C.; C.; Ji, Ji, Y.; Y.; Yang, Yang, X.; Gao,X.; Gao, S. The S. The effect effect of nitrate, of nitrate, bicarbonate bicarbonate and natural and natural organic organic matter onmatter the degradationon the degradation of sunscreen of sunscreen agent p-aminobenzoic agent p-aminobenzoic acid by simulatedacid by simulated solar irradiation. solar irradiation.Sci. Total Sci. Environ. Total 2011Environ., 409 2011, 5376–5381., 409, 5376–5381. [CrossRef ] 40. Li, A.J.; Sang, Z.Y.; Chow, C.H.; Law, J.C.F.; Guo,Guo, Y.; Leung,Leung, K.S.Y.K.S.Y. EnvironmentalEnvironmental behaviorbehavior ofof 12 UV filtersfilters and photocatalytic profileprofile of ethyl-4-aminobenzoate. J. Hazard. Mater Mater.. 2017,, 337, 115 115–125.−125. [CrossRef] 41. Mc Kay,Kay, G.; G.; Dong, Dong, M.; M.; Kleinman, Kleinman, J.L.; J.L.; Mezyk, Mezyk, S.P.; S.P.; Rosario-Ortiz, Rosario-Ortiz, F.L. Temperature F.L. Temperature dependence dependence of the reaction of the betweenreaction between the hydroxyl the hydroxyl radical andradical organic and organic matter. matter.Environ. Environ. Sci. Technol. Sci. Technol2011,. 452011, 6932–6937., 45, 6932–6937. [CrossRef ] 42. [Dewil,PubMed R.;] Mantzavinos, D.; Poulios, I.; Rodrigo, M.A. New perspectives for advanced oxidation processes. 42. Dewil,J. Environ. R.; Mantzavinos,Manag. 2017, 195 D.;, Poulios,93–99. I.; Rodrigo, M.A. New perspectives for advanced oxidation processes. 43. J.Monteagudo, Environ. Manag. J.M.;2017 Duran,, 195 ,A.; 93–99. Gonzalez, [CrossRef R.; ][Exposito,PubMed ]A.J. In situ chemicaloxidation of carbamazepine

43. Monteagudo,solutions using J.M.; persulfate Duran, simultaneously A.; Gonzalez, R.;activated Exposito, by A.J.heat In energy, situ chemicaloxidation UV light, Fe2+ions, of and carbamazepine H2O2. Appl. 2+ solutionsCatal. B Environ using persulfate. 2015, 176 simultaneously, 120–129. activated by heat energy, UV light, Fe ions, and H2O2. Appl. Catal. 44. BJi, Environ.Y.; Dong,2015 C.;, Kong,176, 120–129. D.; Lu, J.; [CrossRef Zhou, Q.] Heat-activated persulfate oxidation of atrazine: Implications for 44. Ji,remediation Y.; Dong, C.;of groundwater Kong, D.; Lu, co J.;ntaminated Zhou, Q. Heat-activated by herbicides. persulfate Chem. Eng. oxidation J. 2015, 263 of, atrazine: 45–54. Implications for 45. remediationZhang, R.; Sun, of groundwater P.; Boyer, T.H.; contaminated Zhao, L.; Huang, by herbicides. C. DegradationChem. Eng. of J.pharmaceuticals2015, 263, 45–54. and [CrossRef metabolite] in 45. Zhang,synthetic R.; human Sun, P.;urine Boyer, by UV, T.H.; UV/H Zhao,2O L.;2, and Huang, UV/PDS. C. Degradation Environ. Sci. ofTechnol. pharmaceuticals 2015, 49, 3056–3066. and metabolite in 46. syntheticLiu, Y.; He, human X.; urineFu, Y.; by Dionysiou, UV, UV/H2 OD.D.2, and Kinetics UV/PDS. andEnviron. mechanism Sci. Technol. investigation2015, 49 ,on 3056–3066. the destruction [CrossRef of] 46. Liu,oxytetracycline Y.; He, X.; by Fu, UV-254 Y.; Dionysiou, nm activation D.D. of Kinetics persulfate. and J. mechanismHazard. Mater investigation. 2016, 305, 229–239. on the destruction of 47. oxytetracyclineRomero, A.; Santos, by UV-254 A.; Vicente, nm activation F.; Gonzalez, of persulfate. C. Diuron J.abatement Hazard. Mater.using activated2016, 305 ,persulphate: 229–239. [CrossRef Effect of] [pH,PubMed Fe (II)] and oxidant dosage. Chem. Eng. J. 2010, 162, 257–265. 47.48. Romero,Liang, C.; A.; Wang, Santos, Z.; A.; Bruell, Vicente, C.J. F.;In Gonzalez,fluence of C.pH Diuron on persulfate abatement oxidat usingion activated of TCE at persulphate: ambient temperatures. Eﬀect of pH, FeChemosphere (II) and oxidant 2007, 66 dosage., 106–113.Chem. Eng. J. 2010, 162, 257–265. [CrossRef] 48.49. Liang,Martins, C.; R.C.; Wang, Gmurek, Z.; Bruell, M.; C.J.Rossi, Inﬂuence A.F.; Corceiro, of pH on V.; persulfate Costa, oxidationR.; Quinta-Ferreira, of TCE at ambientM.E.; Ledakowicz, temperatures. S.; ChemosphereQuinta-Ferreira,2007 R.M., 66, 106–113.Application [CrossRef of Fenton][PubMed oxidation] to reduce the toxicity of mixed parabens. Water Sci. 49. Martins,Technol. 2016 R.C.;, 74 Gmurek,, 1867–1875. M.; Rossi, A.F.; Corceiro, V.; Costa, R.; Quinta-Ferreira, M.E.; Ledakowicz, S.; Quinta-Ferreira, R.M. Application of Fenton oxidation to reduce the toxicity of mixed parabens. Water Sci. 50. Tan, C.; Gao, N.; Deng, Y.; Zhang, Y.; Sui, M.; Deng, J.; Zhou, S. Degradation of antipyrine by UV, UV/H2O2 Technol.and UV/PS.2016 J., 74Hazard., 1867–1875. Mater. [2013CrossRef, 260,][ 1008–1016.PubMed] 50.51. Tan,Khan, C.; J.A.; Gao, He, N.; X.; Deng, Khan, Y.; H.M.; Zhang, Shah, Y.; Sui, N.S.; M.; Dionysiou, Deng, J.; Zhou, D.D. S.Oxidative Degradation degradation of antipyrine of atrazine by UV, in UV aqueous/H2O2 and UV/PS. J. Hazard. Mater. 2013, 260, 1008–1016. [CrossRef][PubMed] solution by UV/H2O2/Fe2+, UV/S2O82-/Fe2+ and UV/HSO5/Fe2+ processes: A comparative study. Chem. Eng. J. 51. Khan,2013, 218 J.A.;, 376–383. He, X.; Khan, H.M.; Shah, N.S.; Dionysiou, D.D. Oxidative degradation of atrazine in 2+ 2 2+ 2+ aqueous solution by UV/H2O2/Fe , UV/S2O8 −/Fe and UV/HSO5/Fe processes: A comparative study. Chem. Eng. J. 2013, 218, 376–383. [CrossRef] Appl. Sci. 2019, 9, 2873 11 of 11

52. Nie, M.; Yang, Y.; Zhang, Z.; Yan, C.; Wang, X.; Li, H.; Dong, W. Degradation of chloramphenicol by thermally activated persulfate in aqueous solution. Chem. Eng. J. 2014, 246, 373–382. [CrossRef] 53. Luo, C.; Ma, J.; Jiang, J.; Liu, Y.; Song, Y.; Yang, Y.; Guan, Y.; Wu, D. Simulation and comparative study on 2 the oxidation kinetics of atrazine by UV/H2O2, UV/HSO5− and UV/S2O8 −. Water Res. 2015, 80, 99–108. [CrossRef][PubMed] 54. Chen, J.; Gao, N.; Yang, J.; Wang, C.; Gu, Z.; Jiang, C. Study on the characteristics of 2,4-dichlorophenol in water degraded by UV/PS. Chin. Environ. Sci. 2017, 37, 2145–2149.

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