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Water Research 171 (2020) 115374
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Water Research
journal homepage: www.elsevier .com/locate/watres
New insight into the substituents affecting the peroxydisulfatenonradical oxidation of sulfonamides in water
Renli Yin a, b, Wanqian Guo b, *, Nanqi Ren b, Lixi Zeng a, Mingshan Zhu a, **
a Guangdong Key Laboratory of Environmental Pollution and Health, School of Environment, Jinan University, Guangzhou, Chinab State Key Laboratory of Urban Water Resource and Environment, School of Environment, Harbin Institute of Technology, Harbin, China
a r t i c l e i n f o
Article history:Received 19 September 2019Received in revised form11 November 2019Accepted 5 December 2019Available online 8 December 2019
Keywords:SulfonamidesSubstituentsNonradical oxidationPersulfateDFT calculationPathways
* Corresponding author.** Corresponding author.
E-mail addresses: [email protected] (W. Gu(M. Zhu).
https://doi.org/10.1016/j.watres.2019.1153740043-1354/© 2019 Published by Elsevier Ltd.
a b s t r a c t
The large consumption and discharge of sulfonamides (SAs) have potentially induced antibiotic resis-tance genes, posing inestimable threats to humans and ecosystems. In the present study, five SAs withdifferent substituents were regarded as target compounds to be degraded using the nonradical domi-nated peroxydisulfate (PDS) activation process by the combination of 1O2 oxidation and direct electrontransfer. The degradation rates, toxicities and pathways of SAs largely varied with their substituents. Forinstance, sulfathiazole with five-membered substituent had the highest degradation rate of 0.19 min�1,which was 3.8 times as the rate of sulfanilamide (0.05 min�1) without substituent. Then the theoreticalcalculation was adopted to further confirm that different substituents on the SAs could influence themolecular orbital distribution and their stability, thus resulting in the different removal rate of SAs.Finally, the products of different SAs were concisely deduced to take insight into the effects of differentsubstituents on SAs degradation pathways. It was demonstrated that the geometrical differences amongvarious SAs caused by the different substituents contributed to the different degradation pathways ofSAs. Representatively, the special Smiles-type rearrangement pathway was occurred in the six-membered SAs instead of in the five-membered SAs, which inversely resulted in the slower degrada-tion rate of six-membered SAs than the five-membered SAs. Thus, the present study provides a valuableinsight into the effects of substituents on the degradation rate and transformation pathways of SAs in thenonradical PDS activation process.
© 2019 Published by Elsevier Ltd.
1. Introduction
Sulfonamides (SAs), introduced in 1937, are the first effectiveantibiotics and the most widely used medicines to treat and pre-vent infections for both humans and animals because of their lowcost, high chemical stability, and broad antibiotic activity (Dorettoet al., 2014). However, SAs cannot be completely metabolized byhumans and animals, and may be excreted into the aquatic and soilenvironment (Nguyen et al., 2019; Sun et al., 2018). Long-termexposure to SAs antibiotics may pose a huge risk on humanhealth owing to the formation and prevalence of sulfonamide-resistant genes (Chen et al., 2019; Oliveira et al., 2019; Yan et al.,2018). For example, the National Center for Toxicological Research
declares that sulfamethazine is carcinogenic (Feng et al., 2015).Further, the sulfonamide resistance genes are reported to be themost abundant resistant phenotype in the Haihe River of China,which may eventually enter humans to interrupt human metabo-lites (Dang et al., 2017).
Generally, the molecules of SAs contain a core sulfanilamidestructure and different N-bound substituents of five- or six-membered heterocyclic rings, such as isoxazole, thiazole, pyrimi-dine, etc (Ji et al., 2017). Several studies have investigated theoxidative transformations of SAs and indicated that the toxicity,degradation rates and pathways of SAs strongly depend on theirsubstituents (Batista and Nogueira, 2012; Soriano-Correa et al.,2018). For example, the LD50 (half lethal dose) to mouse of sulfa-thiazole (STZ) and sulfanilamide (SA) was reported to be400 mg kg�1 and 2900 mg kg�1, respectively (data from theChemical Toxicity Database, http://www.drugfuture.com/toxic/),which indicates that the substituents can largely influence thetoxicity and threats of SAs to environment or humans. Additionally,there are some differences in degradation efficiency and
R. Yin et al. / Water Research 171 (2020) 1153742
mechanism between five-membered and six-membered SAs. Forexample, Boreen et al. observed that the photodegradation rates ofSAs with six-membered heterocyclic substituents were markedlylower than the rates of those containing five-membered hetero-cyclic substituents, which indicated that the type of the sub-stituents could cause different reaction rate of SAs (Annel. Boreenet al., 2004, 2005). With respect to the degradation pathways, forfive-membered heterocyclic substituents, the cleavage of the SeNbond on the sulfanilamide group appeared to be the dominantcleavage (Ji et al., 2015; Yin et al., 2017). In contrast, the primaryproducts formed in six-membered SAs degradation were identifiedas sulfur dioxide extrusion and Smiles-type rearrangement prod-ucts (Annel. Boreen et al., 2005; Fan et al., 2015; Ji et al., 2017).These results indicated that the SAs with different substituents haddifferent transformation pathways, which might cause differentinfluences on the aquatic environments. Thus, it is hypothesizedthat the substituents on the core sulfanilamide structure can makedifferences in the reactivity and transformation pathways ofdifferent structured SAs.
Recently, the nonradical activation of persulfate process isconsidered to be a potential process for pollutants removal inaquous environments (Duan et al., 2018a). This is because thenonradical-dominated systems have several advantages in theiradaption to diverse pH circumstances (acidic/neutral/basic), resis-tance to the ubiquitous inorganic ions, selectivity to organic pol-lutants and moderate redox potential (Chen et al., 2018; Lee et al.,2015; Tang et al., 2018). Several studies have demonstrated thatperoxydisulfate (PDS)-based nonradical oxidation process can beefficiently used to degrade many pollutants, including dyes, phe-nols and phamaceuticals in aquatic environments (Duan et al.,2016; Gao et al., 2018; Guan et al., 2019; Yin et al., 2019; Yunet al., 2018; Zhu et al., 2018b). Additionally, the nonradical PDSoxidation processes also showed good potential for SAs degrada-tion (Kang et al. 2016, 2018,bib_Kang_et_al_2018,bib_Kang_et_al_2016; Wang et al., 2019b). However, these studies mainly focusedon the discussion of persulfate activation process and just took theSAs as the normal targets, instead of taking deep insight into thedifferent degradation rate and mechanism induced by the varioussubstituents. Whether the degradation rates and pathways of SAswith different substituents in the nonradical PDS oxidation pro-cesses differs from the traditional radical oxidation process is stillunclear. Thus, it is necessary to deeply analyze the effects of sub-stituents on the SAs degradation in the PDS-based nonradicaloxidation process for the complete removal and transformation ofSAs from wastewater.
To our knowledge, no study has comprehensively take insightinto the effects of substituents on the reactivity, reaction rate andassociated mechanism of SAs in PDS-based nonradical oxidationprocess. Thus the objective of this study is to investigate the effectof the substituents on the degradation performance and pathwaysof SAs by PDS nonradical activation process and take insight intothe reason and mechanism that cause these differences. Anonradical-dominated peroxydisulfate activation by reduced gra-phene oxide (rGO) system (rGO/PDS) was established anddemonstrated. Five representative SAs with different substituents,including five-membered heterocyclic groups (sulfamethoxazole(SMX), STZ) and six-membered heterocyclic groups (sulfamerazine(SMR), sulfamethazine (SMT)) and one free of substituents (SA),were chosen to investigate the effects of different substituents onthe degradation of SAs in the PDS nonradacal oxidation process.Firstly, the differences in the degradation rate of SAs were found todepend on the type of substituents. Then the DFT calculation wasadopted to analyze the differences in charge distribution and thereactive sites of various SAs to further demonstrate the substituentsaffecting the reactivity of SAs. Finally, the degradation pathways of
five SAs were concisely proposed to illustrate the differences in thetransformation pathways caused by various substituents.
2. Materials and methods
2.1. Materials
All the SAs (SA, SMX, STZ, SMR, SMT, over 98% purity), PDS(K2S2O8) and 5,5-dimethyl-1-pyrroline (DMPO) were purchasedfrom Sigma-Aldrich and used as received without further purifi-cation. The structures and the relevant data of chosen SAs arepresented in Table 1. 2,2,6,6-tetramethyl-4-piperidinol (TEMP, 99%)were purchased from the Tokyo Chemical Industry Co., Ltd. Meth-anol, ethanol, tert-butanol (TBA), acetonitrile and formic acid ofHPLC grade were purchased from Tedia and Ficher. Other chemicalsof analytical grade or better were purchased from SinopharmChemical Reagent Co., Ltd.
2.2. Activation process and analysis
The reduced graphene oxide (rGO) was chosen as the activatorfor PDS in nonradical oxidation by the metal-free catalysis (Duanet al., 2018b). The preparation and characterization of rGO weredescripted in Test S1 and Fig. S1. All the degradation experimentswere performed in beakers containing 100 mL solution on mag-netic stirrers. Each runwas performed in duplicate and the averagevalues were adopted to ensure reproducibility. Batch experimentswere carried out at a PDS concentration of 0.6 mM, an rGO dosageof 0.1 g L�1 an initial SAs concentration 40 mM and initial pH 5.0(adjusted by 0.1 M of H2SO4 and NaOH, without buffer). First, therGOwas added into the solution containing the SAs and was stirredfor 30 min to achieve adsorption equilibrium. Then, the activationprocess of SAs degradation was started by adding a certain amountof PDS. At each given interval, the target reaction solution (1.0 mL)was withdrawn, immediately quenched with excess sodium thio-sulfate and filtered into a vial for UPLC analysis according to themethods in our previous study (Yin et al., 2018b). The oxidizedproducts were firstly extracted by the solid phase extraction (SPE)process (Lindsey et al., 2001). Then the target compounds and theirsubstructures were analyzed using a LC-Triple TOF 5600-MS withan electron-spray ionization (ESI) interface. The detailed methodsof UPLC, LC-MS and SPE process are shown in Text S2. The non-radical oxidation process was demonstrated by the scavengerquenching studies, EPR spectra determination and electrochemicalanalysis.
2.3. Computational methods
Theoretical calculations were adopted to reveal the reason fordifferent degradation performance and mechanism of SAs causedby the different substituents in the rGO/PDS system. Geometryoptimization of the SAs was simulated using the density functionaltheory (DFT) method (Gurkan et al., 2012; Yin et al. 2017,2018a,bib_Yin_et_al_2017,bib_Yin_et_al_2018a). The DFT calcula-tions on the energy and NBO charge were carried out in a Gaussian09 software, using the method of B3LYP/6-31 þ G** combined withthe IEFPCM solvent model. The electron density in HOMO, LUMO,their energies and the energy gap between HOMO and LUMO(DE¼ ELUMO � EHOMO) are important stability indices, which can aidin explaining the effect of the different substituents on SAs degra-dation performance (Chen et al., 2019; Wang et al., 2019a). HigherHOMO energy indicates higher electrophilic reaction activities,while lower LUMO energy means higher nucleophilic reactionability. In addition, a molecule with a smaller gap between HOMOand LUMO is more reactive (Luo et al., 2018). Thus the energies of
Table 1Structures of five sulfonamides.
SAs Acronym Molecule Structure Molecule weight (g mol�1)
Sulfanilamide SA C6H8N2O2S 172.21
Sulfamethoxazole SMX C10H11N3O3S 253.28
Sulfatiazole STZ C9H9N3O2S2 255.32
Sulfamerazine SMR C11H12N4O2S 264.30
Sulfamethezine SMT C12H14N4O2S 278.33
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different structured SAs are firstly compared to illustrate theirdifferent structure reactivity. Furthermore, the different chargedistribution of SAs caused by different substituents are adopted topredict the possible active sites of different SAs by the rGO/PDSprocess, which gives theoretical direction in proposing the degra-dation pathways and searching for the different pathways of SAscaused by different substituents. Thus the calculation results canhelp reveal the differences and similarities in the degradationmechanism of five SAs with different substituents.
3. Results and discussion
3.1. Performances of SAs degradation in the rGO/PDS process
Batch experiments were conducted to investigate the degrada-tion performance of five SAs (SA, SMX, STZ, SMR, SMT) in the rGO/PDS process. All the five SAs remained stable in the solution by theinactivated PDS (results shown in Fig. S2(a)). The adsorption equi-librium of the SAs on the rGO was reached in 30 min before theaccess of the PDS and the results of adsorption was shown inFig. S2(b). However, in the presence of both rGO and PDS, the fiveSAs were efficiently degraded in 30 min (Fig. 1(a)). For instance,under the same reaction condition ([PDS] ¼ 0.6 mM,[rGO] ¼ 0.1 g L�1, [SAs]0 ¼ 40 mM, pH ¼ 5.0 and time ¼ 30 min), thedegradation efficiencies of the five SAs were all higher than 80%.Therein, the degradation efficiency of sulfathiazole (STZ) was the
Fig. 1. Degradation performances (a), and pseudofirst-order (b) of SAs degradation. ExperimeT ¼ 25 �C.
highest and reached to 99.7%. These results indicated that the rGO/PDS could efficiently removed SAs. To further confirm the oxidationcapacity and application potential of rGO/PDS on SAs degradation,SMT was chosen as a case to study the effects of rGO dosage, PDSconcentration, initial pH and natural inorganic ions in water on thedegradation performance, discussed in Test S3 and shown in Fig. S3.The results showed the rGO/PDS process have strong oxidationability towards pollutants, wide adaptability under different envi-ronmental conditions and free of natural ions inhibition, whichindicated that rGO/PDS process was a stable and efficient tech-nology that potential for the real wastewater treatment. Addi-tionally, to analyze the fate of the SAs transformation in the rGO/PDS system, the mineralization of SAs wastewater was studiedusing TOC removal rate. It can be seen that all the removal rateswere greater than 80%, of which the highest was 90.1% for STZ(Fig. S2(c)). These results indicated that rGO was appropriate forPDS activation, not only showing high performance for SAs degra-dation but also enhancing the SAs wastewater mineralization toachieve safe transformation. Thus next the oxidation mechanism ofthe rGO/PDS process needs to be deeply revealed to analyze thereason for its high performance for SAs degradation.
3.2. Oxidation mechanism of the rGO/PDS process
To identify the oxidation mechanism of the rGO/PDS process,EPR experiments trapped by DMPO and TEMP were conducted to
ntal conditions: rGO dosage ¼ 0.1 g L�1, [PDS]0 ¼ 0.6 mM, [SAs]¼ 40 mM, pH ¼ 5.0, and
R. Yin et al. / Water Research 171 (2020) 1153744
determine the possible oxidizing species. The characteristic signalsof DMPO¡OH and DMPO¡SO4 peaks were observed in the EPRspectrum trapped by DMPO in the rGO/PDS process (Fig. 2(a)),suggesting that the$OH and SO4$
� might be the possible reactiveoxidation species generated in rGO/PDS process. In addition, TEMPwas chosen as the trapping agent for 1O2, and the characteristicsignals of the three-line ESR spectrum with equal intensities(Fig. 2(b)) indicate that 1O2 may be another reactive oxidationspecie in the rGO/PDS process. Thus, the EPR detection resultsindicated that the $OH, SO4$
� and 1O2 might jointly contribute toSAs decomposition by rGO/PDS process.
To determine the respective contribution of the above-mentioned species ($OH, SO4$
� and 1O2) to SAs degradation, SMTwas taken as an example to conduct the quenching tests usingdifferent scavengers (methanol, tert-butanol (TBA) and NaN3) andthe analytical method was referred to in our previous study (Yinet al., 2018b). Fig. 2(c) showed that the addition of methanol orTBA at 0.3 M or 0.6 M had no effect on the SAs degradation. Thedegradation curves with methanol and TBA were almost the samewith the control, which indicated that $OH and SO4$
� radicals didnot contribute to the removal of SAs. Thus, the SAs degradation inrGO/PDS process was attributed to the nonradical oxidation of theprocess. Significantly, NaN3 (0.6e12 mM) could largely inhibit SMTdegradation, which suggested 1O2 was important oxidative speciein the rGO/PDS process and confirmed the rGO/PDS process was anonradical-dominated oxidation process. To further verify thenonradical oxidation of the rGO/PDS process, KI was used asanother radical quencher in the rGO/PDS system to exclude thecontribution of the radicals bounded on the rGO surface, because KIcan quickly react with the surface-bound free radicals to quenchthe SAs degradation process (Chen et al., 2016; Feng et al., 2017). Itcan be seen from the results that the KI (5 mM) showed a littleinhibition on the SAs degradation in the rGO/PDS system, whichindicated that the radicals bounded on the rGO surface had limitedcontribution to SAs degradation in the rGO/PDS system. These re-sults further demonstrated that the nonradical oxidation processwas the dominant oxidation pathway of the rGO/PDS process thatcontributes to the SAs degradation. However, NaN3 cannotcompletely quench the oxidation of SMT in the rGO/PDS process,
Fig. 2. Oxidation mechanism of rGO/PDS process. (a): EPR spectra trapped by DMPO; (b): EPspectroscopy; (e): linear sweep voltammetry. Experimental conditions: rGO dosage ¼ 0.1 g
which indicates that there may be another nonradical pathway inthe rGO/PDS activation process benefitting the degradation of SMT.According to the results of recent studies (Yun et al., 2018; Zhuet al., 2018a), it was concluded that the possible nonradicaloxidation pathways of the rGO/PDS activation process included thefollowing three aspects: (1) PDS direct oxidation; (2) singlet oxygen(1O2) oxidation; (3) rGO-mediated direct electron transfer betweenPDS and SAs. Pathway (1) had been excluded and pathway (2) hadbeen demonstrated by the above experimental results. Thus thepathway (3) was proposed to be another nonradical oxidationpathway of the rGO/PDS activation process. The electron-transfernonradical pathway was further identified by electrochemicalmeasurements. A smaller semicircle diameter in electrochemicalimpedance spectroscopy (Fig. 2(d)) demonstrated that the charge-transfer resistance of the rGO/PDS/SMT system was significantlydecreased compared to that of rGO/PDS. Moreover, an obvious in-crease of current on the carbon paper electrode occurred duringlinear sweep voltammetry (Fig. 2(e)) recorded upon the addition ofSMT, suggesting that a current flow formed from SAs to the meta-stable reactive complex between PDS and rGO. These results indi-cated that electron transfer was another important nonradicaloxidation pathway of rGO/PDS activation process. This was alsofound in the research of Zhu and Lee on carbon materials for PDSactivation processes (Yun et al., 2018; Zhu et al., 2018a). Above all,the rGO/PDS process was a nonradical-dominated oxidation pro-cess, combined the 1O2 oxidation and rGO-mediated direct electrontransfer between PDS and SAs, which contributed to the efficientdegradation of SAs. Compared with the traditional radical oxidationprocesses, the nonradical processes have the advantages of highselectivity and moderate reaction conditions, which might result inthe different degradation rates and pathways for various pollutants.
3.3. Differences in the degradation rates of SAs
According to the above results, the five different SAs can beefficiently removed the rGO/PDS process, indicates that theoxidation on the core structure of sulfanilamide plays importantrole in the SAs degradation process for their similar structure. Hhowever, it can be seen there are some evident differences from the
R spectra trapped by TEMP; (c): Effects of scavengers; (d): electrochemical impedanceL�1, [PDS]0 ¼ 0.6 mM, [SAs] ¼ 40 mM, pH ¼ 5.0, and T ¼ 25 �C.
Table 2The HOMO and LUMO Distribution of SAs Calculated at B3LYP/6�31G** Level ofTheory. The Left and Right Sides of SA Molecules are the Aniline and SubstitutedHeterocyclic Rings, Respectively. The Unit of All Molecular Orbital Energies is eV.
HOMO LUMO EHOMO ELUMO DE kobs
SA �6.09 �0.94 5.15 0.05
SMX �6.11 �1.18 4.93 0.10
STZ �6.17 �1.44 4.73 0.19
SMR �6.15 �1.39 4.76 0.09
SMT �6.08 �1.27 4.81 0.09
R. Yin et al. / Water Research 171 (2020) 115374 5
results that differences exist in the SAs degradation efficiency (from80% to 99.7%) and the degradation rates of the five SAs are also notuniform. All these five SAs have very similar structures and containthe same sulfanilamide structure, but have different substituents.Thus it is hypothesized that the different substituents on the SAscan result in varying reactivity of SAs, which would cause differentremoval rate and performance of SAs degradation by the nonradicalrGO/PDS process.
To elucidate the above hypothesis, the reaction kinetics of SAswere further revealed through a pseudo-first-order reaction model,which is shown below.
ln (C0/Ct) ¼ kobs$t (1)
Where kobs is the observed pseudo-first-order rate (min�1); t rep-resents the reaction time; and C0 and Ct stand for the SAs concen-tration at 0 min and t min, respectively. Linear relationships werediscovered with high regression coefficients through kineticmodeling analysis (Fig. 1(b)). The observed rates (kobs) of SA, SMX,STZ, SMR, and SMT by the rGO/PDS process were calculated to be0.05, 0.10, 0.19, 0.09, and 0.09 min�1, respectively. Under the sameconditions, the order of the degradation rate isSTZ > SMX > SMT z SMR > SA. SA, an H atom substituted on thecore sulfanilamide structure, is removedwith the lowest rate, whilethe rates of SAs with substituents (SMX, STZ, SMR and SMT) aremuch faster than that of SA. For instance, the observed rate of STZwas 3.8 times to that of SA, respectively, which indicated the sub-stituents could promote the decomposition of the SAs. In addition,the degradation rates of the six-membered heterocyclic-substituted SAs (SMR and SMT) were lower than that of STZ thatcontaining five-membered heterocyclic substituent, which areconsistent with Boreen’s research on the photodegradation of SAs(Annel. Boreen et al., 2004, 2005). Interestingly, the six-memberedheterocyclic-substituted SAs (SMR and SMT) almost have the samevalue of reaction rate, while the differences of rate value can beclearly found for five-membered heterocyclic-substituented SAs(e.g. SMX and STZ). These results suggested that the substituentshave a large influence on the degradation rate of SAs molecules.Thus it is hypothesized that the substituents on the core sulfanil-amide structure can make differences in the reactivity, reactivesites and pathways of different structured SAs. Therefore, next,theoretical calculations are firstly used to analyze the effects ofdifferent substituents on the reactivity of SAs.
3.4. Effects of the substituents on the reactivity of SAs
The electron density in HOMO and LUMO and the energy gapbetween HOMO and LUMO energy (DE ¼ ELUMO � EHOMO) areimportant stability indexes, which can be used to explain the effectof the different substituents on SAs degradation performance.Table 2 shows the HOMO and LUMO distribution of SAs. It can beseen from the results that there are many differences in HOMO andLUMO distribution among SAs. An uniform electron distributionsacross the aromatic portions of the molecules were observed fromthe HOMOs of SA, SMX, SMR, and SMT. However, for STZ, the HOMOdistribution was transferred to the thiazole ring. Additionally, theLUMO of SA, SMX and STZ also mainly were distributed to the ar-omatic ring while the LUMO distribution of SMR and SMT changedto the substituted heterocyclic rings. These changes in electrondistribution would cause the differences in the reactivity of themolecule and their products distribution. For example, the six-membered SAs (SMR and SMT) showed similar HOMO and LUMOdistribution, which resulted in the comparative degradation in rGO/PDS process. However, the HOMO and LUMO distribution weredifferent in the five-membered SAs (SMX and STZ), which induced
the large difference in the degradation rate between SMX and STZby rGO/PDS process. In addition, according to frontier molecularorbital (FMO) theory, the energy gap between HOMO and LUMO(DE¼ ELUMO � EHOMO) are important stability indices, which can aidin explaining the activity and stability of the molecules. A smallervalue of DE indicates the greater reactivity of the molecule. The DEenergies of five SAs are summarized in Table 2. It can be seen fromthe results that the largest DEwas SAwhile the smallest was STZ, soSTZ had the fastest degradation rate and SA had the slowest. Thecomparative DE values of SMX, SMR and SMT induced similardegradation rates. Generally, compounds with electron donatingsubstituents are more susceptible to be oxidized than those withelectron-withdrawing substituents (Hsu et al., 2018). The S atom onthe substituents can give more electron density to the sulfanil-amide bond, thus resulting in the STZ being more reactive thanSMX. These results indicated that the activity and stability of SAswere relative to the energy gap between their HOMO and LUMOand the degradation rates of the SAs showed an inverse relation-ship with DE. Moreover, the difference in the gap energy among theSAs was attributed to the change in the HOMO and LUMO distri-bution on SAs that was caused by the different substituents on SAs.Thus, it is demonstrated that the substituents can influence theelectron distribution and stability of the SAs molecules, resulting inthe difference in SAs degradation rate of SAs by the rGO/PDSprocess.
3.5. Prediction of active sites on the SAs
Then the possible reaction sites of five different SAs are pre-dicted to reveal the effects of substituents on the reactive sites ofdifferent structured SAs. Theoretical calculations were employed topredict the possible active sites of SAs in the rGO/PDS process togive directions for proposing the accurate degradation pathways ofSAs. The rGO/PDS process is a nonradical oxidation process domi-nated by the combination of 1O2 oxidation and electron transfer,and it has been known that 1O2 would selectively react towardelectron-donating groups on SAs and the electron transfer processwas also excited at the electron-rich sites of SAs (Gao et al., 2018).Thus, the electron-rich sites on SAs would be the possible activesites attacked during the rGO/PDS process and charge distributionwas adopted as the descriptor. The optimized molecules and
R. Yin et al. / Water Research 171 (2020) 1153746
calculated charge distribution of five SAs are shown in Fig. 3 andTable S1. The 8N, 10N, 9O and 11O atoms were the similar electron-rich sites on SAs, which might be the sites susceptible to attack inthe rGO/PDS process. However, the charge values of the 8N and 10Natoms on the SAs with substituents were much larger than that onthe SA, which might induce the degradation rate of SA is muchlower than that SAs with substituents. Additionally, some nitrogenor carbon atoms on the substituted rings were also the electron-rich sites. The electron-rich sites were distributed on thesubstituted rings of the SAs, which indicated that more reactivesites were existed and might also induce the SAs with substituentto degrade faster than the SA without substituent. Moreover, thesereactive sites on the substituent ring were also not the same, whichmight result in the different degradation rates and pathways of SAs.This finding further demonstrated that the substituents on SAscould attract electrons and influence the charge distribution andstability of SAs, thus resulting in the different degradation rate ofdifferent SAs.
3.6. Effects of the substituents on the degradation pathways of SAs
Finally, the oxidized products were concisely determined byusing the LC-TOF-MS (ESI, both positive and negative mode) andthe degradation pathways of five SAs were proposed detailedly toillustrate the differences in the transformation pathways caused byvarious substituents. Structural assignments of intermediates wereperformed by total product ion scans, according to the accurate m/zvalues and corresponding MS spectra. The detailed data and pro-posed structures of the products and their MS spectra are shown inTables S2eS6 and Figs. S4eS8. According to the proposed structureof the possible oxidized products, the possible degradation path-ways of SAs were deduced in Fig. 4 and Fig. S9. It can be seen fromthe results that the same degradation pathways of the five SAswerethe oxidation of 8N and 10N, which contributed to the highdegradation rate of SAs in the rGO/PDS process. However, the fiveSAs have some particular degradation pathways caused by the ef-fects of the substituents. Compared to SAwithout substituent, otherSAs with substituents (SMX, STZ, SMR, and SMT) have anotherpathway through the oxidation of substituted heterocyclic rings,
Fig. 3. Charge distribution of five SAs ((a): SA, (b): SMX, (c): STZ, (d): SMR, (e): SMT; (Red: e31 þ G (d, p), opt ¼ calcall, freq, scrf¼(IEFPCM, solvent ¼ water), pop ¼ NBO. (For interpretaversion of this article.)
which promoted the degradation rate of the SAs with substituents.More importantly, the six-membered SMT and SMR were proposedto decompose through a special SO2 extrusion pathway that did notfind in the pathways of five-membered SMX and STZ and the SAfree of substituent. Taking SMT as an example, the special pathwaywas initiated by an anilinyl radical cation of SMT via an electrontransfer mechanism (Fan et al., 2015). Thereafter, the aromaticcarbon adjacent to the SeN bond in the radical cation possessed astrong positive charge and could be subjected to intermolecularnucleophilic attack, while one of the pyrimidine nitrogen served asa nucleophile. Such an attack led to the intermolecular rearrange-ment (Smiles-type), thus generating an SO2 extrusion product. Thetransformation mechanism of the Smiles-type rearrangement ofSMT is shown in Fig. 4. The oxidized products, generated via theoxidation of SMT (a representative SA with six-membered hetero-cyclic ring) and SMX (a representative SA with five-memberedheterocyclic ring) were compared. Representatively, P-214 withm/z 199.0979 and chromatographic retention time at 1.720 minwere proposed as the product generated from the SO2 extrusion inthe six-membered SMTmolecule (Table S6 and Fig. 4), respectively.It is interesting to note that SO2 extrusionwas not observed in SMXwith the five-membered ring. This is mainly attributed to thegeometric differences of SAs caused by different substituents. It canbe seen that the SO2 extrusion product mainly occurred on theaniline ring C holding the S and the N on the substituents. In thefive-membered ring, the distances of the N geometrically awayfrom the aniline ring C holding the Swere 4.09 in SMX, respectively,which were longer than the six-membered ring of the distance inSMT (3.38) between the N of the pyrimidine ring and the anilinering C holding the S. Additionally, the SO2NeC]N angle in SMX isabout �133.1�, whereas it is 26.1� in SMT as measured in theoptimized structures (see Fig. 3). Thus the further distance andlarger angle of the five-membered SAs would cause too large bar-rier for the formation of the radical cation of SAs to initiate theSmiles-type rearrangement pathway (Feng et al., 2019). It should benoted that the geometrical differences among various SAs areattributed to the different substituents on SAs. Thus, it is demon-strated that the substituents on SAs can affect the degradationpathways of SAs. Additionally, it was reported that the
lectron-rich atom; green: electron-deficient atom)). Computational methods: B3LYP/6-tion of the references to colour in this figure legend, the reader is referred to the Web
Fig. 4. Representative degradation pathways of five-membered SMX (a), six-membered SMT (b), and the transformation mechanism of the Smiles-type rearrangement of SMT (c).
R. Yin et al. / Water Research 171 (2020) 115374 7
Fig. 5. Proposed mechanism of the effects of substituents on the degradation of five SAs.
R. Yin et al. / Water Research 171 (2020) 1153748
eNHeSO2�R oxidation in SAs is faster than the SO2 extrusion, thusthe degradation rate of the SAs with the pathway of SO2 extrusionmight be slower than that without SO2 extrusion (Feng et al., 2019).It is inversely explained the reason why the six-membered SAs hasslower degradation rates than five-membered SAs, which isbecause the six-membered SAs is removed through a slower SO2extrusion pathway than the five-membered SAs. Above all, thedifferent substituents on SAs played important roles in both theremoval rates and the transformation pathways of SAs by the rGO/PDS process. Simultaneously, the different transformation path-ways might also inversely influence the removal rate of SAs. Theschematic illustration of the mechanism in the effects of sub-stituents on SAs degradation in the rGO/PDS process is proposed inFig. 5.
4. Conclusions
In summary, this is the first study toward a comprehensiveunderstanding of the effects of different substituents on thedegradation rate and pathways of five SAs by PDS nonradicaloxidation process. Firstly, the differences in the degradation rate ofSAs are found to depend on the type of substituents. The resultsindicated that the existence of the substituents on SAs could pro-mote and influence the degradation rate of SAs. Then the DFTcalculation is adopted to analyze the differences in charge distri-bution and the reactive sites of various SAs to further demonstratethe substituents affecting the reactivity of SAs. As confirmed bytheoretical calculation, the changes in the HOMO and LUMO dis-tribution of SAs by different substituents caused the differences inthe reactivity of molecules and degradation rate of the reaction.Finally, the degradation pathways of five SAs are concisely pro-posed to illustrate the differences in the transformation pathways
caused by various substituents. The geometrical differences amongvarious SAs caused by different substituents contributed to thedifferent transformation pathways of SAs. The six-membered SAswere degraded through a special SO2 extrusion pathway, whichinversely resulted in the slower degradation rate of six-memberedSAs than the five-membered SAs. This finding may help predict thereactivity and rates of other SAs with different types of sub-structures. These analyses from the point view of the effects ofsubstituents on the reactivity and pathways of SAs in this study canpossibly provide direction for the degradation rates and pathwaysof SAs in other oxidation processes.
Declaration of competing interest
The authors declare that they have no known competingfinancial interests or personal relationships that could haveappeared to influence the work reported in this paper.
Acknowledgments
The present study was financially supported by the Projectfunded by China Postdoctoral Science Foundation (No. 55350333)and the National Natural Science Foundation of China (No.51678188 and 21577142). The authors are also grateful to Prof. EddyY. Zeng from the Jinan University for his profound discussions.
Appendix A. Supplementary data
Supplementary data to this article can be found online athttps://doi.org/10.1016/j.watres.2019.115374.
R. Yin et al. / Water Research 171 (2020) 115374 9
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