Degradation of thiacloprid via unactivated peroxymonosulfate: The overlooked singlet oxygen oxidation

Journal Pre-proofs Degradation of thiacloprid via unactivated peroxymonosulfate: The overlooked singlet oxygen oxidation Tongcai Liu, Danyu Zhang, Kai Yin, Chunping Yang, Shenglian Luo, John C. Crittenden PII: DOI: Reference:

S1385-8947(20)30255-2 https://doi.org/10.1016/j.cej.2020.124264 CEJ 124264

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Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

16 November 2019 19 January 2020 27 January 2020

Please cite this article as: T. Liu, D. Zhang, K. Yin, C. Yang, S. Luo, J.C. Crittenden, Degradation of thiacloprid via unactivated peroxymonosulfate: The overlooked singlet oxygen oxidation, Chemical Engineering Journal (2020), doi: https://doi.org/10.1016/j.cej.2020.124264

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Degradation

of

thiacloprid

via

unactivated

peroxymonosulfate: The overlooked singlet oxygen oxidation Tongcai Liu

a,b,

Danyu Zhang b, Kai Yin

a,b,*,

Chunping Yang a, Shenglian Luo b, and

John C. Crittenden c

a

College of Environmental Science and Engineering, Hunan University, Changsha

410082, P. R. China b

State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University,

Changsha 410082, P. R. China c Brook

Byers Institute for Sustainable Systems and School of Civil and Environmental

Engineering, Georgia Institute of Technology, 828 West Peachtree Street, Atlanta, Georgia 30332, United States

* Corresponding Authors. E-mail: [email protected] (K. Yin).

1

ABSTRACT: Unactivated peroxymonosulfate (PMS) degrading organic contaminants has been reported. Previous studies have focused on direct PMS oxidation, while the role of singlet oxygen (1O2) is often overlooked. Here, we studied the oxidation of thiacloprid (THIA) of the neonicotinoid insecticides by PMS without explicit activation. According to electron spin resonance spectroscopy and quenching experiments, the nonradical mechanism (i.e., direct PMS oxidation and 1O2 oxidation) was responsible for the THIA degradation. There are two pathways to generate 1O2. The pyridine nitrogen of THIA structure can combine with PMS to produce epoxide, which is beneficial to generation of 1O2. However, the contribution of 1O2 from PMS selfdecomposition can be ignored. According to kinetic solvent isotropic effect analysis, the overall THIA degradation was attributed to 36.5% 1O2 oxidation and 63.5% direct PMS oxidation. A structure-activity assessment and density functional theory suggested that thioether sulfur, amidine nitrogen and the cyanoimino groups were the main reaction sites. Product analysis further confirmed the involvement of 1O2 and PMS. Moreover, three major THIA degradation pathways were proposed, including (ⅰ) heterolytic cleavage of peroxo bonds and transfer of oxygen atoms, (ⅱ) electron transfer and (ⅲ) oxidation of cyanoimino groups to generate nitroso/nitro-THIA. In addition, identification byproducts (Except for TP141) had lower ecotoxicity toxicity (using ECOSAR) in fish, daphnid and green algae. Effects of pH and natural water matrices on THIA oxidation via PMS were further evaluated. This study investigated the overlooked 1O2 oxidation reaction to elucidate the mechanism. The new mechanistic 2

knowledge has important implications for other contaminants for their interactions with PMS. Keywords: Peroxymonosulfate; Thiacloprid; Nonradical mechanism; Singlet oxygen.

3

1. Introduction Peroxymonosulfate (HSO5, PMS) is considered to be an environmentally friendly oxidant in several applications (e.g., bleach, disinfectant, soil remediation and water treatment) [1-4]. In general, sulfate radical (E0 (SO4•/SO42-) = 2.5  3.1 V) and/or hydroxyl radical (E0 (•OH/H2O) = 1.9  2.7 V) are considered to be the main active radicals and are produced through PMS activation by ultraviolet (UV) irradiation, heat, ultrasound or transition metal catalysts [5-11]. However, there are some disadvantages to these methods, such as external energy inputs, toxic metal leaching, and intensive chemical consumption [8, 12, 13]. Moreover, the nonselective property of •OH and high reactivity of SO4•− and •OH toward coexisting organic and inorganic substrates (e.g., NOM and Cl−) result in a variety of competitive reactions that have negative affect on the advanced oxidation processes (AOPs) that use them (e.g., increased disinfection byproducts and oxidized chlorine species) [14-18]. PMS is considered as a mild oxidant (1.85 V), and exhibits high reactivity towards various contaminants in water [19-21]. Recently, the direct oxidation of contaminants by PMS based on nonradical pathways has been discovered [3, 19, 22-24]. Previous studies have shown that unactivated PMS can directly oxidize organic pollutants, while ignoring the role of other reactive species (e.g., singlet oxygen (1O2)). As a moderately reactive electrophile,

1O

2

is effective in oxidizing a variety of electron-rich

pharmaceuticals [25], flame retardants [26] and neonicotinoid insecticide [27]. 1O2 is less susceptible to interference from other coexisting organic and inorganic substances in the natural water owing to its higher selectivity compared to •OH and SO4•− [28]. It 4

is well-known that PMS can self-decompose in water to form 1O2, with widely varying rate constants (0.02-0.11 M-1 s-1) [29]. Zhou et al. reported that organic benzoquinone decomposes of PMS, and ketones (because of the presence of a carbonyl moiety (C=O)) have been found to accelerate 1O2 production, which can subsequently oxidize sulfamethoxazole [25, 30]. In the carbon material catalyzed PMS oxidation process, 1O

2

is also produced due to the rich carbonyl structure (similar to quinonoid groups) on

the edge of the material [9, 31, 32]. Duan et al. proposed that heteroatoms (N) could activate the inert carbon framework by adjusting the charge density of adjacent carbon atoms, thereby reinforcing the interaction between catalyst and PMS and promoting the cleavage of the O−O bond in PMS [31]. Some studies have confirmed that carbon skeletons with sp2 hybridization can even induce nonradical degradation pathways (e.g., 1O

2-mediated

pathway), because the conjugated π system therein was favorable for the

electron transfer [31, 32]. Pyridinic nitrogen, which have lone-pair electrons (similar to ketones), can serve as unpaired stable radicals to capture the electrophilic species of peroxide in the nitrogen-doped carbon material catalyzed PMS oxidation process [31]. However, it is unknown whether organic substance (e.g., containing pyridine nitrogen group) could activate PMS to produce 1O2. Therefore, the role that 1O2 plays in unactivated PMS oxidation is needed to develop an understanding of 1O2 performance and generation mechanism. Neonicotinoid insecticides are the most widely consumed class of insecticides due to their high insecticidal activity, moderate water solubility and longevity in the environment [33, 34]. Thiacloprid (THIA) is a representative neonicotinoid insecticide 5

that has strong toxicity (due to the cyanoimino group and thiazole ring) and it is nonbiodegrable [35]. Various techniques have been explored to degrade THIA, including Fenton processes, photocatalytic degradation, UV/chlorine, and ozonation [33, 36, 37]. THIA is expected to be susceptible to degradation by PMS and 1O2 owing to the presence of electron-rich moieties, such as amidine nitrogen and thioether sulfur. While the neonicotinoid insecticide THIA (containing pyridine nitrogen group) was treated by unactivated PMS, we encountered experimental results that could not be fully explained through direct PMS oxidation. This new discovery motivated us to conduct in-depth research to elucidate the mechanism of PMS treatment of THIA. And until now, the mechanism of unactivated PMS toward THIA has not been investigated. In this work, we focus on the degradation of THIA via unactivated PMS. Specifically, we determined: (i) the process of degrading THIA by unactivated PMS, (ii) the mechanism of

1O

2

generation, byproducts, and pathways of THIA

transformation, (iii) the toxicity of byproducts using Ecological Structure-Activity Relationship Model (ECOSAR), and (iv) the effects of natural water matrices on the degradation of THIA.

2. Materials and methods 2.1. Chemicals In this study, PMS (Oxone, 2KHSO5•KHSO4•K2SO4, ≥ 47% KHSO5 basis), imidacloprid (IMD, 98.5%), thiacloprid (THIA, AR), 6-chloronicotinic acid (6CNA, 99%), thiazolidine (THIZ, 98%), sodium azide (NaN3, AR), 9, 10-diphenylanthracene 6

(DPA, AR) 2,4,6-trimethylphenol (TMP, AR), ethanol (EtOH, AR), Tert-butanol (TBA, AR) sodium thiosulfate (Na2S2O3, AR), sodium chloride (NaCl, AR), sodium bicarbonate (NaHCO3, AR), sodium hydroxide (NaOH, AR) were purchased from Aladdin Chemistry Co. Ltd (Shanghai, China). Deuterium oxide (99.8%, D2O) was purchased from J&K chemicals (Beijing, China). Methanol for high-performance liquid chromatography (HPLC) was purchased from Honeywell Burdick & Jackson (Ulsan, Korea). Humic acid (HA) was obtained from Shanghai Luzhong Chemical Reagent Co., Ltd (Shanghai, China) and used to represent natural organic matter (NOM). Due to the limited solubility, DPA stock solutions were made in acetonitrile: chloroform mixture (1:1, v:v). All the stock solutions were prepared with ultrapure water (18.2 MΩ·cm) purified by a Milli-Q system, stored at 5°C before use. The information of natural water samples is provided in the Supporting Information (SI) Text S1 and Table S1. 2.2. Experimental Procedures Reactions were carried out in 100 mL amber vials containing the target contaminant (THIA) and the oxidant (PMS) in the dark at room temperature (25 ± 1°C). The initial solution pH was adjusted by a 10 mM phosphate buffer to the desired values, and the background water matrix components (i.e., Cl, HCO3, SO42‒ and NOM) were added as necessary. Samples were collected at a scheduled interval and immediately quenched with excess sodium thiosulfate (Na2S2O3), and Na2S2O3 does not affect the reaction. All experiments were conducted at least in triplicate, and the average values were represented. The error bars in all figures represents the standard deviation. 2.3. Analytical Methods 7

Analytical details are provided in Text S2 and Tables S2-S3. 2.4. Calculation of Frontier Electron Densities The Gaussian 09 is often applied for the study of pollutant degradation [38-40]. The frontier electron densities (FEDs), the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) based on the optimized geometry of the organic compounds were calculated using the Gaussian 09 with the density functional theory (DFT) with a basis set of B3LYP/6-31 G (d, p). 2.5. Toxicity Analysis The acute and chronic toxicities for fish, daphnid and green algae were calculated using the Ecological Structure-Activity Relationship Model (ECOSAR) program. The details are provided in Text S4. The classification of chronic and acute toxicity based on the Globally Harmonized System of Classification and Labelling of Chemicals is given in Table S4.

3. Results and discussion 3.1. Radical vs Nonradical Reaction Figure 1a shows the time course of THIA degradation (3 μM) in the presence of PMS (1 mM) at pH 7.0. Significant degradation of THIA was achieved in the presence of PMS without activation, and it was stable in the absence of PMS. Generally, •OH and SO4• are the reactive species in activated PMS oxidation processes. To verify whether •OH and SO4• were involved in the reaction, the corresponding scavengers, tert-butanol (TBA) and ethanol (EtOH), respectively, were individually added to the 8

reaction solution [41]. However, the degradation of THIA was not affected in the presence of 100 mM TBA or EtOH, indicating that the degradation of THIA could not be attributed to reaction with •OH and SO4•. Previous studies have reported that superoxide (O2•) could be generated in oxidation processes of persulfate (PDS) [42]. Hence, we added superoxide dismutase (SOD) which converts O2• to H2O2 and this makes it an effective scavenger [43]. As shown in Figure 1a, the presence of SOD showed no effect on the degradation of THIA. Therefore, the role of O2• in THIA degradation can be ruled out as well. As reported in a previous study, 1O2 can be generated when PMS is selfdecomposed in aqueous solutions [19], its second-order rate constant for reaction with THIA is (3.9 ± 1)  107 M-1 s-1 [27]. To verify the participation of 1O2 during the unactivated PMS degradation process, sodium azide (NaN3) was applied, given the relatively high rate constant (2  109 M-1 s-1 ) toward 1O2 (Table S5) [43]. After NaN3 was added to the solution, the degradation of THIA was significantly inhibited. However, as shown in Figure 1b, PMS was decomposed within 30 min when it mixed with NaN3, this phenomenon is consistent with the previous study [19]. This indicated that the inhibition of THIA degradation was due to the decomposition of PMS; consequently, NaN3 cannot be used as quencher to determine the role of 1O2 in the system. To investigate the involvement of 1O2, electron spin-resonance (ESR) technology was applied (Figures S1- S2) [44]. ESR spectra were obtained with TEMP as a trap for 1O2 and DMPO as a trap for SO4• and •OH [19]. Signals corresponding to 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO), which is a TEMPO−1O2 adduct, are 9

shown in Figure S1a. Meanwhile, when PMS was mixed with THIA, the signal TEMPO increased within the first 2 min and decreased after 10 min. The above results indicate that THIA increase the decomposition of PMS to produce 1O2, and 1O2 was the reactive species responsible for THIA degradation. No signals for either trapped DMPO•OH or DMPOSO4• were observed in the PMS system (Figure S1b), which further confirmed that SO4• and •OH were not responsible for THIA oxidation. Many previous studies have reported that contaminants can be directly oxidized by PMS [19, 22, 23, 45]. Therefore, we proposed that the THIA degradation process was attributed to the nonradical pathway involving 1O2 oxidation and direct PMS oxidation. 3.2. Contribution of direct PMS oxidation and 1O2 oxidation To calculate the contribution of direct PMS oxidation and 1O2 oxidation, the kinetic solvent isotope effect (KSIE) was investigated by changing from H2O to a solvent of deuterium oxide (D2O). Many studies have confirmed the reliability of this method [19, 46]. The KSIE is commonly used to verify 1O2 process in aqueous solutions because the solvent-dependent 1O2 quenching rate constant is 16 times slower for D2O than it is for H2O, and the quenching rate constants of 1O2 by H2O (kH2O) and D2O (kD2O) are 2.5  105 and 1.6  104 s-1, respectively [19, 47]. Reactions in H2O and 50% D2O were compared at [D+] = [H+] = 1 × 10−7 M (i.e., pH = 7.0 in water, pHmeasured = 6.8 in 50% D2O), 1 mM [PMS]0, and 3 μΜ [THIA]0. The pKaD value of PMS in D2O solution is 9.76 ± 0.07 compared to 9.4 in H2O [19]. Therefore, PMS exists mainly in undissociated form in D2O at pHmeasured = 6.8. And the reaction rate constant of THIA with 1O2 in a solution containing D2O at a molar fraction of x (x = 50%, kobs, H2O, D2O 10

mix)

can be computed according to the following equation (eq 1) [48]. 𝑘𝐻2𝑂.𝑘𝑜𝑏𝑠,𝐻2𝑂

𝑘𝑜𝑏𝑠,𝐻2𝑂,𝐷2𝑂 𝑚𝑖𝑥 = 𝑥𝐻2𝑂.𝑘𝐻2𝑂 + 𝑥𝐷2𝑂.𝑘𝐷2𝑂 where kobs,

H2O, D2O mix

and kobs,

H2O

(1)

represent the corresponding reaction rate

constants in a solution containing D2O at a molar fraction of x and 100% H2O, respectively, and kH2O and kD2O represents the quenching rate constants of 1O2 by H2O and D2O, respectively. As shown in Figure 2a, when 50% H2O was replaced by D2O, the degradation of THIA was higher than for 100% H2O. Because less quenching by water (D2O) and therefore a higher concentration of 1O2 and a higher rate, which suggested that 1O2 was formed and involved in the degradation of THIA. According to the experimental results, the pseudo first order rate constants of THIA degradation in 100% H2O and 50% D2O were 0.0170 and 0.0212 min-1, respectively. However, according to eq 1, if the degradation of THIA is only due to the effect of 1O2, the THIA degradation rate in 50% D2O should be 0.0320 min-1, suggesting other process (PMS direct oxidation) was responsible for the degradation of THIA. Therefore, we can calculate the first-order reaction rate constant of THIA with 1O2 to be 6.2 × 10‒3 min-1 in H2O solution. It would be much more reasonable to propose that direct PMS oxidation also had contribution to THIA degradation after KISE analysis indicating that 1O

2

alone cannot account for overall removal of THIA. We can conclude that the

contribution of 1O2 oxidation is approximately 36.5%, while the contribution of PMS direct oxidation is 63.5% in the reaction solution. 3.3. Proposed the pathways of singlet oxygen generation Pathway I: PMS is capable of self-decomposition in water to produce 1O2, but the 11

process is slow [29]. We calculated the observed second-order rate constant for PMS (1 mM) decomposition (kdecomp, PMS) is the value of 5.89 × 10-4 M-1 s-1 in phosphate buffered to pH 7.0 in the absence of THIA, as shown in Figure S3. In this case, the rate at which PMS self-decomposition to generate 1O2 is given by eq 2, and the result of R1O2, form was calculated to be 5.89 × 10-4 μM s-1. 𝑅1𝑂2,

𝑓𝑜𝑟𝑚

=

𝑑[1𝑂2]𝑡 𝑑𝑡

= 𝑘𝑑𝑒𝑐𝑜𝑚𝑝,𝑃𝑀𝑆[𝑃𝑀𝑆]2

(2)

The fraction of 1O2 reacting with THIA is calculated by eq 3: 𝑓1𝑂2,𝑇𝐻𝐼𝐴 =

𝑘1𝑂2,𝑇𝐻𝐼𝐴 [𝑇𝐻𝐼𝐴] 𝑘1𝑂2,𝐻2𝑂 + 𝑘1𝑂2,𝑇𝐻𝐼𝐴 [𝑇𝐻𝐼𝐴]

𝑇𝐻𝐼𝐴 + 1𝑂2→𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠

(3) (4)

where k1O2, H2O is the rate constant for physical quenching of 1O2 by water (2.5  105 s1),

and k1O2, THIA = (3.9 ± 1)  107 M-1 s-1 is the rate constant for THIA reacting with 1O2.

According to the equations, 1.65 × 10-3 μM of 1O2 reacted with THIA in 100 min (eq 3), while 2.463 μM of THIA was removed, corresponding to 0.067% of the THIA reacting through the 1O2 pathway. Therefore, the degradation of THIA by 1O2 produced by PMS self-decomposition is negligible. However, we calculated that the contribution of 1O2 oxidation for THIA degradation was approximately 36.5%, so there should be other ways to produce 1O2. Pathway II: PMS can act as an electrophile or nucleophile by peroxyl group, which depending on the nature of the reactants [24]. The lone-pair electrons of pyridinic nitrogen in the THIA structure, which are similar to ketonic C=O groups, can serve as unpaired stable radicals to capture the electrophilic species of peroxide [31]. It has been reported that the pyridine N has a higher electronegativity and a smaller covalent radius 12

to facilitate electron transfer from the neighboring C, resulting in a decrease in the electron cloud density in other parts of the ring [49]. Hence, the pyridine ring is an electron-deficient system with an electrophilic nature. According to DFT calculations, the LUMO characteristic of a compound’s electrophilicity is located at the pyridine ring (Figure S4a) and the PMS molecule with a peroxy bond possessing nucleophilic properties (Figure S4b) [50]. Electron transfer is expected from the electron-rich nitrogen atoms to PMS molecules via the carbon−N(−)−C(+)−O(−)−peroxide bridge to epoxide. This has been reported for nucleophilic attack of PMS on sp2 carbon bonded to a negatively charged atom and leading to oxidation, such as the attack of PMS on the C=N group of the imidazole side chain of histidine [24]. As shown in Scheme 1, we proposed that PMS bonds with positively charged carbon atoms adjacent to pyridinic N to form an intermediate of epoxide. To demonstrate the formation of an epoxide between deficient carbon atom and the peroxide bond, we performed a fluorescence quenching experiment to verify it. Figure S5 confirmed that the production of epoxide in the reactions. The fluorescence intensity weakens with the increase of the reaction time, indicating that the epoxide formed is unstable and easily decomposed, which indicates that the decomposition of the epoxide may produce 1O2. The unstable intermediate subsequently gains electron from another PMS molecule to produce a peroxymonosulfate anion radical (SO5•−) and the epoxide is broken to form SO3•− radical (eq 5, 6), thereby restores the pyridine ring to create the redox cycle [4, 31]. ESR analysis can be used to detect SO3•− and DMPO/SO3•− adduct can be observed [51]. However, the signal of DMPO/SO3•− adduct was not detected (Figure S1a). The 13

impact of dissolved oxygen (DO) can provide another evidence for the generation of SO3•−, because SO3•− rapidly reacts with DO to form SO5•− at a diffusion-controlled rate (2.5×109 M-1 s-1) (eq 5) [52]. As shown in Figure S6, it was observed that the degradation of THIA was inhibited, when N2 was bubbled into the solution and excess DMPO (10 mM) was added. This phenomenon is owing to DO and DMPO trapped the formed SO3•− and terminated the subsequent radical propagation reactions (Scheme 1). The signals arising from SO3•− was not observed in the ESR spectra, which might be due to DO rapidly trapped the formed SO3•− and DMPO/SO3•− adduct is unstable. Afterward, due to the high reaction rate constant (2.0  108 M-1 s-1), two molecules of SO5•− are prone to self-reaction, generating S2O82‒ and 1O2 (eq 7) [50]. As shown in Figure S7 (b), with the reaction proceeds, the presence of PDS (S2O82-) is detected in the system, which further proves that eq 6-eq 7 is valid. This reaction process has also been proven by ESR, when PMS was mixed with THIA, the signal TEMPO adduct increased within the first 2 min, as shown in Figure S1a. As a result, electron transfer between THIA-epoxide and PMS is another way to generate 1O2. In fact, in order to explain the effectiveness of scheme 1, we did some experiments to verify. For example, we tested the formation of S2O82‒ (Figure S7 (b), Text S2); proved the generation of epoxide by fluorescence quenching experiments (Figure S5); and confirmed the role of 1O

2

by the specific reaction experiments of DPA and TMP with 1O2 (Figure S8). These

experiments further confirm that Scheme 1 is valid. Note that, we know that SO5•− is relatively inert toward alcohols [10], which is why neither EtOH nor TBA could effectively scavenge SO5•− radical in Fig. 1. No contribution of SO3•‒/SO5•‒ radicals 14

were observed in the THIA degradation, which might be due to its rapid conversion to 1O

2

or low oxidation potential (1.1 V) [10, 51]. 𝑆𝑂•3 ‒ + 𝑂2→𝑆𝑂•5 ‒

(5)

𝐻𝑆𝑂5― →𝑆𝑂•5 ‒ + 𝐻 + + 𝑒 ― 𝑆𝑂•5 ‒ + 𝑆𝑂•5 ‒ →𝑆2𝑂28 ― + 1𝑂2

(6) (7)

3.4. Determination of Active Sites To determine the reactive sites with respect to the oxidation process (direct PMS and 1O2), PMS-induced degradation was further studied for some similar structures, including imidacloprid (IMD), 6-chloronicotinic acid (6CNA) and thiazolidine (THIZ) (Figure S9). According to the experimental results (Figure 2b), THIZ and IMD can also be degraded by PMS, while 6CNA did not react with PMS. By simultaneously analyzing the above results, we can conclude that the thiazole ring and the imidazole ring might act as active sites for the reaction. As shown in Figure S10, the highest occupied molecular orbital (HOMO), which is characteristic of compound nucleophilicity, is located at the five-membered thiazole ring and the cyanoimino group of THIZ and IMD. Meanwhile, the thioether sulfur on the thiazole ring is known to be susceptible to electrophilic attack by various oxidants (i.e., chlorine dioxide, ferrate and ozone), and PMS is well known to be very reactive toward sulfides and thiohydride, and was used in chemistry as a synthetic route to sulfones and sulfoxides [53, 54]. Therefore, thioether sulfur was vulnerable to direct PMS oxidation, while amidine nitrogen on the thiazole ring and the cyanoimino groups were vulnerable to 1O2 oxidation. 15

In order to explain the difference between the reaction rates of four compounds (THIA, IMD, 6CNA, THIZ) with unactivated PMS, DFT calculations were performed to assess their nucleophilicity. The nucleophilicity with the HOMO energy was used to describe the nucleophilicity N index [55], described as eq 8: N = EHOMO(Nu) (eV) ‒ EHOMO(TCE) (eV)

(8)

where tetracyanoethylene (TCE) was taken as a reference because it has the lowest HOMO energy in a series of molecules, giving a positive nucleophilicity value for various organic compounds (Tables S6-S10). Accordingly, the calculated N index (Table S6) was consistent with the experimental results (Figure 2b), which further verified that the observed rate constants follow the order of kobs,THIZ (0.034 min-1) > kobs,IMD (0.018 min-1) ≈ kobs,THIA (0.017 min-1) > kobs,6CNA (Figure S11). 3.5. Identification of Transformation Products and Reaction Pathways The transformation products (TPs) of THIA by PMS were measured using HPLCESI-tqMS. A total of nine TPs were identified from the THIA degradation, and the structures of the TPs were tentatively proposed by the corresponding MS/MS spectrum. Detailed information of all TPs is summarized in Figures S12-S19. Direct PMS oxidation (Oxygen atom transfer): According to the experimental results, direct PMS oxidation might proceed through two-electron transfer, which involves the heterolytic breakage of the peroxide bond on PMS and an oxygen atom transfer from PMS to the contaminants [22]. A plausible mechanism for the oxidation of THIA by direct PMS oxidation is likely the attack of thioether sulfur by peroxide to generate an intermediate, which was further decomposed to sulfoxide byproducts 16

(TP268A, TP268B) (Figure 3). 1O

2

oxidation (Electron transfer and cyanoimino oxidation): 1O2 is a selective

oxidizing species and shows a high reactivity toward electron-rich moieties (i.e., amines and phenols); hence, the reaction of 1O2 with contaminants proceed via electron transfer [43]. Figure 3 shows the electron transfer pathways leading to the formation of the radical cation of THIA after reaction with 1O2. Further elimination of H+ leads to an αaminoalkyl radical in the methylene bridge, which subsequently reacts with 1O2 to form TP141, TP127 and TP266. T266 combines with H+ in H2O to form hydroxylation products, and then dehydrates to give a byproduct of TP250. Meanwhile, the generation of nitroso-THIA (TP256) and nitro-THIA (TP272) were mainly due to the oxidation of the cyanoimino group in THIA. This can result in the internal recombination of the nitrothiazolidine imine moiety and the loss of molecular nitrous oxide and produce TP228 [56]. Among the degradation intermediates, in order to verify the presence of carbon-center radicals, DIPPMPO was commonly used as a trapping agent to capture the carbon-center radicals for ESR analysis (Text S2). Figure S20 shows the ESR spectra of DIPPMPO adducts in the PMS/THIA system, the presence of these peaks can be attributed to the formation of carbon-center radicals (Environ. Sci. Technol. 2010, 44, 6815–6821) [56]. To interpret the initial reactions of THIA oxidation by unactivated PMS, Gaussian 09 was used to calculate the following: (i) point charges, (ii) FEDs of the THIA molecule and (iii) the Wiberg bond orders in the skeleton of the THIA molecule (Figure 4 and Table S11). As shown in Figure 4a, C24 (0.474), C15 (0.340) and S22 (0.211) 17

possess higher positive charge values compared with the other atoms. Therefore, the oxidation reactions preferentially occurred at the C24, C15 and S22 atoms, resulting in the formation of TP256, TP272, TP228 and TP268A/B byproducts. The large 2FED2HOMO value that appears at N23 (0.4169), N25 (0.3489), and N21 (0.3639) (Figure 4b), which indicated that the highly substituted thiazole ring was easily oxidized. The isosurface of the HOMO orbital (Figure S10) also showed that the electron density is well distributed on these sites, making them more vulnerable for electron extraction. As a result, the cyanoimino group was oxidized by 1O2 to generate TP256, TP272 and TP228. The Wiberg bond order is a key indicator of bond strength. For chemical bonds of the same type, a small bond order always implies a low bond energy and favorable cleavage [57]. As shown in Figure 4d, the –C11−N21− bond has the smallest bond order (1.0239). Thus, the cleavage of the –C11−N21− bond is also reasonable and consistent with the observation of the products TP141 and TP127. The proposed reaction mechanisms were consistent with the conclusions drawn from experiments showing that the thioether sulfur, amidine nitrogen and cyanoimino group were the primary reactive sites. 3.6. Ecotoxicity Evaluation Determining the ecotoxicity as oxidation occurs is important because the goal of oxidation is to reduce toxicity. We used EPA’s QSAR-based ECOSAR program to predict the ecotoxicity of THIA and its byproducts. The predictions of the ECOSAR program are based on the chemical structures and the basic characteristics of substance. The EC50 values are the concentrations where 50% of specific effect, and LC50 values 18

are the concentrations where ½ of the biota die. The Chronic Value (ChV) represents the chronic toxicity and is defined as the geometric mean of the no-observed-effect concentration and the lowest-observed-effect concentration [39, 58]. The toxicities for fish, daphnid and green algae were calculated, and the results are shown in Figure 5 and Table S12. Green algae and daphnid were more sensitive to THIA and its TPs than fish. Except for the chronic toxicity of byproduct TP141 to daphnid, which is higher than THIA, the acute toxicity and chronic toxicity of the remaining TPs were lower than those of the parent compound THIA. Although the low mineralization for THIA, approximately 13.6, 29.3, and 46.4% (Figure S22), after 60, 100, and 180 min of PMS treatment, respectively. These toxicological test results indicate that the selective oxidation of THIA via PMS without activation would produce byproducts that are less toxic than the parent compound. 3.7. Effects of Real Water Matrix on THIA Degradation Natural water matrices contain different concentrations of inorganic anions, predominately HCO3−, Cl− and SO42−, for neutral to weakly alkaline pH. Also, natural organic matter (NOM) is a constituent in natural waters. Therefore, we examined the effects of pH, these inorganic anions and NOM on THIA oxidation. The effect of pH on THIA degradation by PMS was examined over a wide pH range from 5.0 to 11.0. As shown in Figure S23 (a), as the pH varied from 5.0 to 9.0, no significant difference of kobs for the reaction was observed, but an obvious increase of kobs was observed when the pH increased from 9.0 to 11.0. The pK2 of PMS is approximately 9.4, the increasing OH− concentration accelerates PMS deprotonation to 19

generate SO52−, and the self-decomposition of PMS to produce 1O2 was accelerated with the increase of SO52− [19]. This may be the reason for the increased rate of THIA degradation under alkaline conditions (9.0-11.0). Inorganic anions and NOM are ubiquitous in surface water and groundwater and can exhibit negative effects on the radical-based oxidation of organics [15]. As shown in Figure 6a, no effect on the degradation of THIA by PMS was observed in the presence of different concentrations of HCO3− or SO42‒. Meanwhile, a minor inhibition of THIA oxidation in the presence of HA in concentrations in the range of 5-10 mg/L was observed. This result may be due to the competition reaction of HA and THIA with PMS and the scavenging effect of HA on 1O2 [3]. However, as the concentration of Cl− increased, the degradation of THIA showed a slight promotion. The promotion effect is due to the formation of HOCl and other active chlorine species (i.e., Cl2/ClO−) that are generated from the reaction of Cl− with PMS (2.06  10-3 M-1 s-1) [23, 59]. The degradation of THIA by PMS in different natural water matrices (Table S1) was also investigated (Figure 6b, Figure S23 (b)).We found there was a slight effect in the kobs values in the natural water samples (Surface Water1 (SW1), Surface Water2 (SW2) and Ground Water (GW)) by PMS compared to the ultrapure water (UW) sample. Therefore, compared to the conventional PMS activation and the production of SO4•− and •OH, the PMS-induced degradation also exhibits a specific reactivity and suffers less interference from the natural water matrices.

4. Conclusions 20

Activated PMS-based AOPs produce radical pathways for target compound destruction. Although this pathway is attractive in many ways, there are some drawbacks, including the external energy required or metal releases from activating catalysts. In real water matrices, the nonselective property and high reactivity of SO4•− and •OH toward organic and inorganic coexisting substrates will result in a variety of competitive reactions that negatively affect the removal efficiency of target pollutants. For instance, (bi)carbonate scavenges SO4•− and •OH to produce less reactive carbonate radicals, and halogen ions, which are prevalent in water, are easily oxidized to active halogen species, which not only reduce the degradation rate of contaminants but also form highly halogenated byproducts. Hence, in this study, the degradation of target contaminant THIA using unactivated PMS was proposed. We demonstrated that THIA is amendable to PMS direct oxidation and 1O2 oxidation via a nonradical pathway without external energy and activators. We found that the pyridine nitrogen structure of THIA can combine with PMS to produce epoxide, which is beneficial to generation of 1O2. Meanwhile, according to KISE analysis, the overall THIA degradation can ascribe to 36.5% 1O2 oxidation and 63.5% direct PMS oxidation. Based on the byproducts identification combined with density functional theory (DFT) calculation, the possible degradation pathways for THAI were proposed in unactivated PMS process. PMS and 1O2 are milder but more selective active species than SO4•− and •OH. The background substances (HA, HCO3‒, Cl‒ and SO42‒) showed a negligible impact on THIA degradation. This negligible impact improves the utilization efficiency of the active species. Therefore, unactivated PMS exhibits a specific reactivity toward THIA 21

that is virtually unaffected by background constituents in water matrices, providing a promising approach to eliminate THIA and reduce its toxicity in water treatment. Furthermore, this study provides a new strategy for the removal of contaminants, which is the role of 1O2 from THIA catalyzed decomposition of PMS.

Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by the National Natural Science Foundation of China (51778218) and the China Postdoctoral Science Foundation (2019M652759). The authors appreciate the support from the Brook Byers Institute for Sustainable Systems, the Hightower Chair and the Georgia Research Alliance at the Georgia Institute of Technology. The views and ideas expressed herein are solely those of the authors and do not represent the ideas of the funding agencies in any form. Additionally, we thank ChemWorx for English editing.

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27

Figure Captions Figure 1. (a) Effect of various scavengers on the degradation of THIA by PMS, and (b) PMS decomposition in solutions. Experimental conditions: [THIA]0 = 3 μM, [PMS]0 = 1 mM, [TBA]0 = 100 mM, [EtOH]0 = 100 mM, [NaN3]0 = 5 mM, [SOD]0 = 1 kU, pH = 7.0 buffer. Figure 2. (a) Degradation of THIA by PMS with 100% H2O, and 50% H2O + 50% D2O, and (b) comparison of degradation rates between THIA and structurally related chemicals (IMD, 6CNA, THIZ) by PMS. Experimental conditions: [Substance]0 = 3 μM, [PMS]0 = 1 mM, pHmeasured = 6.8 in 50% D2O, pH = 7.0 buffer. Figure 3. Proposed structures of byproducts and degradation pathways for THIA oxidation by PMS, and the proposed transients in brackets were not detected. Figure 4. Point charge, FEDs, and Wiberg bond orders calculated by the Gaussian 09 with the B3LYP/6-31(d, p) basis set. (a) Total charges of THIA, (b) 2FED2HOMO values of THIA, (c) FED2HOMO plus FED2LUMO values of THIA, and (d) Wiberg bond orders in the skeleton of the THIA molecule. The arrows imply possible reaction sites. Figure 5. Evolution of acute and chronic toxicities for THIA and its transformation products. Figure 6. (a) Effect of HA (0, 5, 10 mg/L), HCO3– (0, 5, 10 mM), Cl (0, 5, 10 mM) and SO42 (0, 5, 10 mM) on the degradation kinetics of THIA by PMS. (b) Degradation of THIA in UW and natural water samples (SW1, SW2 and GW) by PMS. Experimental conditions: [THIA]0 = 3 μM, [PMS]0 = 1 mM. Scheme 1. Proposed the mechanism of singlet oxygen generation. 28

PMS decomposition,C/C0

(a) 1.0

C/C0

0.8 0.6 0.4 0.2 0.0

Scavenger free 100 mM TBA 100 mM EtOH

5 mM NaN3

1 kU SOD

PMS free

0

20

40

60

80

100

(b) 1.0

0.8 scavenger free 100mM TBA 100mM EtOH 5mM NaN3

0.6 0.4

1 kU SOD

0.2 0.0

0

20

40

60

80

100

Time (min)

Time (min)

Figure 1. (a) Effect of various scavengers on the degradation of THIA by PMS, and (b) PMS decomposition in solutions. Experimental conditions: [THIA]0 = 3 μM, [PMS]0 = 1 mM, [TBA]0 = 100 mM, [EtOH]0 = 100 mM, [NaN3]0 = 5 mM, [SOD]0 = 1 kU, pH = 7.0 buffer.

(a)

1.0

0.8 kobs,H2O,D2O = 0.0212 min-1

C/C0

-ln(C/C0)

2.0 1.5

(b) 1.0

100% H2O 50% H2O + 50% D2O

R2 = 0.9919 kobs,H2O = 0.0170 min-1

0.5

0.6 0.4 THIA IMD 6CNA THIZ

0.2

R2 = 0.9974

0.0

0.0 0

20

40

60

80

100

0

Time (min)

20

40

60

80

100

Time (min)

Figure 2. (a) Degradation of THIA by PMS with 100% H2O, and 50% H2O + 50% D2O, and (b) comparison of degradation rates between THIA and structurally related chemicals (IMD, 6CNA, THIZ) by PMS. Experimental conditions: [Substance]0 = 3 μM, [PMS]0 = 1 mM, pHmeasured = 6.8 in 50% D2O, pH = 7.0 buffer. 29

Figure 3. Proposed structures of byproducts and degradation pathways for THIA oxidation by PMS, and the proposed transients in brackets were not detected.

(a) Point charge

(b) 2FED2HOMO 0.3489

0.4169

0.0747 0.0361

0.0073 0.0024

0.0049 0.0059

0.0123 0.0176 0.3639

0.0631

0.0056 0.0115 (d) The Wiberg bond orders

(c) FED2HOMO + FED2LUMO

0.0201 0.0372

0.3712 0.0883 0.0356

1.5242

0.1089

0.0669 0.2228 0.0877 0.2873

1.6758 0.0205 1.0475 1.0570

0.0058 0.0212

30

1.1568

0.2088

0.0375

1.0319

0.1747

0.0056 0.0082

1.4079

1.6175 1.5556

Figure 4. Point charge, FEDs, and Wiberg bond orders calculated by the Gaussian 09 with the B3LYP/6-31(d, p) basis set. (a) Total charges of THIA, (b) 2FED2HOMO values of THIA, (c) FED2HOMO plus FED2LUMO values of THIA, and (d) Wiberg bond orders in the skeleton of the THIA molecule. The arrows imply possible reaction sites.

Daphnid

Fish

1 THIA

TP141 TP268A TP268B TP250 TP272

TP256

102

TP268A TP268B TP268A

TP250

TP127 TP141

102

TP228

TP227

TP256

10

TP272 TP228

TP141 TP127

TP272 TP228 TP256 TP127

100

TP266

10

Very toxic

Toxic

THIA

TP266

TP141 TP127

10

2

Harmful

TP268A TP268B TP250

101

101

2

TP250

Chronic Toxicity ChV (mg/L)

TP250

100

TP268A TP268B

Chronic Toxicity ChV (mg/L)

Chronic Toxicity ChV (mg/L)

TP268A TP268B TP266

101

TP141

TP228

THIA THIA

TP250

TP256 TP127

TP141

100

TP228

TP268A TP268B

102

TP266

TP266

THIA

101

TP272 TP256

101

TP127

100

THIA

Acute Toxicity EC50 (mg/L)

10

100 Acute Toxicity LC50 (mg/L)

Acute Toxicity LC50 (mg/L)

100

Green algea

TP266

2

Not harmful

Figure 5. Evolution of acute and chronic toxicities for THIA and its transformation products.

(b)

Concentration: 

HA (mg/L)

HCO3 (mM)

Cl (mM)

SO42 (mM)

0.012

0.012

kobs (min-1)

kobs (min-1)

(a) 0.016

0.008 0.004 0.000

0.009 0.006 0.003

0 5 10

0 5 10

0 5 10

0 5 10

0.000

UW

SW1

SW2

GW

Figure 6. (a) Effect of HA (0, 5, 10 mg/L), HCO3– (0, 5, 10 mM), Cl (0, 5, 10 mM) 31

and SO42 (0, 5, 10 mM) on the degradation kinetics of THIA by PMS. (b) Degradation of THIA in UW and natural water samples (SW1, SW2 and GW) by PMS. Experimental conditions: [THIA]0 = 3 μM, [PMS]0 = 1 mM.

O

C

N

Cl

S

SO5•‒ SO5•‒

HSO5‒

e‒

THIA

H

HSO5‒ δ‒

＋

O2

S2O82‒

HSO5‒

＋

1O 2

OH‒

SO3•‒

H2O

Scheme 1. Proposed the mechanism of singlet oxygen generation.

O

-

+ SO52‒ Pathway I

HOOS O O H2 C Cl

HSO5‒ THIA 1O

Pathway II 2

(36.5%) N

N

THIA

S N CN

Nonradical process (63.5%)

Oxygen atom transfer Electron transfer Oxidation of cyanoimino group

PMS

Highlights 

Direct PMS and 1O2 oxidation account for THIA degradation via unactivated PMS.



The contribution of direct PMS and 1O2 oxidation was 63.5 and 36.5%, respectively.



Electron transfer between THIA-epoxide and PMS is the dominant way to produce 1O

2.

32



Oxidation of cyanoimino group, transfer of oxygen atom and electron were proposed.

Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

33