Selective oxidation of tetracyclines by peroxymonosulfate in livestock wastewater: Kinetics and non-radical mechanism

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Selective oxidation of tetracyclines by peroxymonosulfate in livestock wastewater: Kinetics and non-radical mechanism Jiabin Chena,d, Jie Xub, Tongcai Liua, Yajie Qianc, Xuefei Zhoua,d,*, Shaoze Xiaoa, Yalei Zhanga,d,* a

State Key Laboratory of Pollution Control and Resources Reuse, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, PR China School of Environmental Science and Engineering, Suzhou University of Science and Technology, Suzhou 215001, PR China c College of Environmental Science and Engineering, Donghua University, Shanghai, 201620, PR China d Shanghai Institute of Pollution Control and Ecological Security, Shanghai, 200092, PR China b

A R T I C LE I N FO

A B S T R A C T

Editor: Danmeng Shuai

Tetracyclines (TCs) discharged from livestock wastewater have received worldwide concerns owing to their potential threats to the ecosystem and human health. Advanced oxidation processes always exhibit low efficiency to remove TCs in livestock wastewater due to the radical scavenging by water matrices. Herein, we report selective elimination of TCs by peroxymonosulfate (PMS) in livestock wastewater. A kinetic model was developed to describe the rapid degradation of TCs by PMS in the real livestock wastewater. The radical scavenging study and electron paramagnetic resonance (EPR) technique excluded the contribution of radical species (e.g., SO4%−) in the PMS-promoted oxidation of TCs. Theoretical calculations revealed the electrophilic attacks of PMS most likely located on the B-ring of TCs. Transformation product analysis further elucidated that hydroxylation dominated in the PMS-promoted oxidation of TCs, and N-demethylation also significantly contributed to chlorotetracycline (CTC) oxidation by PMS. These results demonstrate a promising strategy to eliminate TCs in livestock wastewater, because PMS shows specific reactivity towards TCs, and thus suffers less interference from the complicated water matrices.

Keywords: Tetracyclines Livestock wastewater Peroxymonosulfate Kinetics Non-radical mechanism

1. Introduction Antibiotics are extensively used to treat diseases and also used for growth promotion in livestock industry (Zhang et al., 2018). In China, antibiotics consumed by animals were estimated to account for appropriately 52 % of total antibiotic use in 2013 (Zhang et al., 2015a). Because antibiotics are poorly absorbed by animals, the majority of them are excreted either unchanged or as metabolites after consumption. Hence, a substantial amount of antibiotics is expected to be released to the environment. Antibiotics have been widely detected in wastewater (Ben et al., 2018), surface water (Li et al., 2018), and groundwater (Ma et al., 2015). Prevalence of antibiotics in the environment may exert selective pressure on the microorganisms and trigger the bacterial resistance, and also pose potential toxic effects on the ecosystem and human health (Chen et al., 2016a). Tetracyclines (TCs), such as tetracycline (TTC), oxytetracycline (OTC), and chlorotetracycline (CTC) (labeled as A to D from right to left, Fig. 1), are the most widely used antibiotics in the livestock farming, and thus have been frequently detected in livestock

wastewater (LW) with concentrations up to hundreds of μg/L (Cheng et al., 2018). The LW is regarded as a major source for TCs contamination in the environment. Although some swine farms are equipped with conventional treatment facilities (e.g., lagoons and anaerobic digester) before LW discharging into the environment, they are designed to remove contaminants like nitrogen and phosphorus, and thus show low efficiency to remove TCs (Chen et al., 2017). Hence, it was imperative to develop techniques to efficiently eliminate TCs from LW. In fact, various strategies have been explored to remove TCs, such as photo-degradation (Tian et al., 2019; Gómez-Pacheco et al., 2012), activated carbon adsorption (Martins et al., 2015), membrane filtration (Srinivasa Raghavan et al., 2018), and oxidative degradation (Wang et al., 2018). TCs are expected to be susceptible to destruction by various oxidants, owing to the presence of multiple electron-rich groups in the structure. For example, TCs exhibit substantial reactivity towards various oxidants, such as Fe(III) (Wang et al., 2016), KMnO4 (Jiang et al., 2020), ClO2 (Wang et al., 2011), O3 (Hopkins and Blaney, 2014), and advanced oxidation processes (AOPs) (Jeong et al., 2010). Sulfate radical (SO4.−)-based AOPs (SR-AOPs) have received

⁎ Corresponding authors at: State Key Laboratory of Pollution Control and Resources Reuse, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, PR China. E-mail addresses: [email protected] (X. Zhou), [email protected] (Y. Zhang).

https://doi.org/10.1016/j.jhazmat.2019.121656 Received 24 September 2019; Received in revised form 30 October 2019; Accepted 9 November 2019 0304-3894/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Jiabin Chen, et al., Journal of Hazardous Materials, https://doi.org/10.1016/j.jhazmat.2019.121656

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Fig. 1. Structures and properties of TCs.

2.2. Real water samples

tremendous attentions as efficient and promising technologies for destructing organic contaminants (Zhang et al., 2015b; Luo et al., 2019; Zou et al., 2019). SO4.− possesses higher redox potential (E0 = 2.5–3.1 V) (Chen et al., 2016b), longer life-time (t1/ 2 = 30−40 μs) (Ao et al., 2019), and broader pH application range than HO· (Zhang et al., 2015c). It can be generated from the activation of persulfates (PS), including peroxymonosulfate (PMS) and peroxydisulfate (PDS), with external energy or catalysts (Zhang et al., 2015b). Indeed, PS activation with UV photolysis (Hu et al., 2019), ultrasound (Eslami et al., 2016), heating (Ji et al., 2016), transition metals are effective to generate SO4.− for efficient oxidation of TCs (Guan et al., 2018). However, LW is commonly known to contain high concentrations of suspended solids, organic matters, and nutrients. The complicated water matrices may act as significant sinks for the radicals, e.g., SO4.−, in SR-AOPs. For example, natural organic matters manifest considerable reaction constants with SO4.− (2.35 × 107 M-1 s-1) (Qian et al., 2018), and ammonia nitrogen (NH4+-N) shows substantial reactivity towards SO4.− (1.4 × 107 M-1 s-1) (Zhang et al., 2015d). Hence, the severe scavenging effect of water matrices may remarkably reduce the utilization efficiency of radicals towards target contaminant, i.e., TCs. Recently, non-radical oxidation in PS-based AOPs has attracted increasing concerns, because this approach shows a lower oxidation potential but higher selectivity towards contaminants, thus providing a promising strategy in water treatment (Chen et al., 2019; Duan et al., 2018). PMS was reported to effectively oxidize arsenite (Wang et al., 2014), bromide (Zhou et al., 2018a) and hydrogen sulfide without formation of reactive radicals (Betterton and Hoffmann, 1990). More recently, PMS was also used to oxidize some organic contaminants in water, and the selective reactivity towards contaminants is highly dependent on their structures. For example, our recent study reported selective transformation of β-lactam antibiotics with the thioether sulfur on the six- or five-membered rings as the main reactive sites (Chen et al., 2018). Later, sulfonamides (SAs) are amendable to direct oxidation by PMS at the site of N atom on the benzene ring (Ji et al., 2018). Also, the tertiary and secondary aliphatic N4 amines on piperazine ring of fluoroquinolones were found to be vulnerable to oxidation by PMS (Zhou et al., 2018b). In this work, we also discovered that PMS showed specific high reactivity towards TCs in LW without generation of SO4.−. We are thus motivated to conduct an in-depth study to elucidate the PMS-promoted degradation of TCs in LW. A kinetic model was developed to describe the rapid degradation of TCs by PMS in the real water matrices. Afterwards, theoretical calculation combined with product analysis revealed the electrophilic reaction mechanism between TCs and PMS. Compared to SR-AOPs, this non-radical process is less affected by the versatile water matrices (e.g., Cl-, NH4+, humic acid) in LW owing to the selectivity and moderate redox potentials.

LW was collected from a swine farm located in the southeast region of China. Samples were taken at the wastewater sinks alongside piggeries, and the wastewater was composed of swine urine, swine bathing water and piggery washing water. LW sample were filtered through 0.45-μm glass fiber filters immediately upon the laboratory and then stored at 4 °C before use. Water parameters of LW are shown in Table S1. Surface water (SW) and municipal wastewater (WW) were sampled from a river and a municipal wastewater treatment facility near the swine farm, respectively. Water parameters of SW and WW are shown in Table S2. 2.3. Experimental procedures Batch reactions were conducted in 100-mL glass serum bottles, and the reaction solution was continuously mixed with a magnetic stirring at 22 °C. Phosphate buffer (10 mM) was used to control the solution pH for all the reactions. Typically, TCs were spiked into the LW (pH 7.0) to achieve 50 μM, and then PMS (0.5 mM) was added to initiate the reaction. The sample aliquots were taken at the selected time intervals, and then immediately quenched by excess amount of Na2S2O3. Afterwards, the samples were filtered through 0.45-μm membrane, stored in 2 mL amber vials, and analyzed within 24 h. For comparison, the radical processes, such as PDS/UV and Co(II)/PMS were also investigated to degrade TCs in LW. Co(II) is considered as the most efficient transition metal ion for PMS activation to generate SO4.−, and UV is effective to activate PDS for SO4.− generation (Zhang et al., 2015b). Co(II) was added into LW containing TTC and phosphate buffer (pH 7.0), and then PMS was added to initiate the reaction. For UV/PDS, the solution containing PDS, TTC and phosphate buffer (pH 7.0) was irradiated by UV to start the reaction. The following procedures were identical to that with PMS alone. To investigate the specific reaction between TCs and PMS, various concentrations of PMS (0.5–1.5 mM) were added into TCs solution. For the impact of water matrices, Cl−, NH4+-N, mixed metal ions, humic acid at concentrations equivalent to LW were spiked into the reaction solution of TCs and PMS in DI. Scavengers (MeOH, TBA and FFA) were added to investigate the reactive species contributed to contaminant degradation. All the experiments were conducted in duplicate or more. 2.4. Analytical methods Target TCs were analyzed by a high performance liquid chromatograph (HPLC, 1260, Agilent Technology, USA) equipped with a Zorbax SB-C18 column (4.6 × 250 mm, 5 μm), and a UV detector. Isocratic elution was employed by a mixture of water with 10 mM oxalic acid (A) and pure acetonitrile (B) at 75:25 v/v. The detection wavelength was 275 nm Transformation products were analyzed on a HPLC system (Utimate 3000, Dionex, USA) with a Zorbax SB-C18 column (2.1 × 150 mm, 5 μm), and connected to a triple quadrupole mass spectrometer (TSQ Quantum Ultra EMR, Thermo Fisher Scientific, USA). The detailed conditions are summarized in Text S2. The electron paramagnetic resonance (EPR) spectra were monitored using an EMX-8/2.7 spectrometer (BRUKER, Germany), with the details presented in Text S3. PMS

2. Materials and method 2.1. Chemicals Sources of chemicals are provided in the Supporting Information (SI) Text S1.

2

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in LW. As Fig. 2A shows, rapid degradation of TTC was observed by PMS/Co(II) in DI water; whereas the degradation was remarkably retarded in LW. It was thus suggested that LW matrices likely competed with TTC for the generated SO4.− in PMS/Co(II) system, thus reduced the degradation of TTC. Similarly, PMS decomposition in LW was slower than that in DI water (Fig. 2B). Co(II) was likely to complex with the organic contaminants in LW, thus reducing the availability of Co(II) for PMS activation. On the other hand, other matrices in LW might severely compete with Co(II) towards PMS. The severely inhibitory effect of LW matrices on TTC degradation was also observed when treating TTC by PDS/UV in LW. Noting that PDS underwent slow decomposition along TTC degradation by PDS/UV in DI water, while PDS decomposed rapidly at first and then the decomposition slowed down to a halt in LW (Fig. 2B). The rapid decomposition of PDS in the initial stage could be attributed to the consumption by LW matrices, and the subsequent cease of PDS decomposition indicated the inefficient activation of PDS by UV in LW. LW matrices were complicated, and thus might adsorb photons with 254 nm wavelength. Indeed, the UV254 of LW sample was determined to be 2.29, thus preventing the UV irradiation to penetrate through the bulk LW to activate PDS. Hence, the activated PS systems, e.g., PDS/UV and PMS/Co(II), were not suitable for eliminating TTC in the real LW owing to the scavenging effect of radicals as well as the attenuation of UV radiation by the complicated matrices. Furthermore, we also evaluated TTC degradation by other oxidants, and the results show that TTC was rapidly degraded by ozone or free chlorine in DI water, whereas its degradation was significantly reduced in LW (Figure S2). Hence, LW matrices exerted adverse impact on TTC degradation by ozone or free chlorine. Overall, the PMS-promoted TTC degradation is less affected by LW compared to the activated PS and other common oxidants, manifesting that PMS is a promising technique to remove TCs in LW.

was determined by a spectrophotometric method proposed by Liang et al. (Liang et al. (2008)), as detailed in Text S4. The UV–vis spectra of TCs solution along the reaction were monitored on a spectrophotometer (UV-1600PC, Shanghai Mapada Instruments Co., Ltd., China). 2.5. Calculation of frontier electron densities Frontier electron density (FED) calculations were operated by the Gaussian 09 program on a basis set of B3LYP/6−31 G (d, p). The electron densities of highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) based on the optimized geometry of organic compounds were obtained and visualized. Radical addition always occurs at positions with higher FED2HOMO + FED2LUMO; while electron extraction usually take place at atoms with larger 2FED2HOMO in the electrophilic reactions (Pan et al., 2018). Hence, the values of 2FED2HOMO were used to predict electrophilic reaction sites. 3. Results and discussion 3.1. Comparison of PMS and activated PS on TC degradation in LW The activated PS processes were previously reported to efficiently degrade various organic contaminants due to the generation of highly reactive radicals, e.g., SO4.− (Tsitonaki et al., 2010). Our experiments showed that PMS alone promoted fast degradation of TCs without external activators. As shown in Fig. 2, TTC (50 μM) was completely degraded after 20 min when PMS (500 μM) was present in DI water (pH 7.0), while significant decomposition of PMS was also observed along TTC degradation. It was thus suggested that TCs exhibit high reactivity towards PMS oxidation at neutral condition. Importantly, the PMSpromoted degradation of TTC was slightly affected by the complicated water matrices. Indeed, the degradation trend of TTC by PMS in LW resembled that occurred in DI water, and the degradation rate was only slightly inhibited in LW. In contrast to TTC degradation, the decomposition trend of PMS in LW was quite different from that in DI water. To be specific, PMS decomposed rapidly upon the addition of PMS to LW, and subsequently slowed down along the reaction. In DI water, however, the initial rapid decomposition of PMS was not observed, and the decomposition trend was similar to that in the second stage in LW. This result indicated that PMS was intensively consumed by the water matrices in LW, leading to the slight inhibition of TTC degradation by PMS in LW. Besides TTC, other commonly used TCs, e.g., OTC and CTC, were also susceptible to PMS oxidation in LW (Figure S1). Hence, the PMS-promoted TCs degradation was almost immune to the complicated matrices in LW. We further investigated the degradation of TTC by the activated PS

3.2. Transformation kinetics Before evaluating the transformation kinetics of TTC by PMS in LW, we investigated the effect of PMS concentration on TTC oxidation in DI water at neutral condition. TTC degradation exhibited pseudo-firstorder kinetics at different initial concentrations of PMS (Fig. 3A). The calculated pseudo-first-order constants (kobs-TTC) linearly increased with increasing PMS concentrations. Plot of log(kobs-TTC) versus log([PMS]) showed good linearity with the slope close to unity (Fig. 3B), indicating the first-order relationship between kobs-TTC and PMS concentration. Hence, the reaction between TC with PMS could be expressed by a second-order kinetics. Apparent second-order reaction constant between PMS and TTC (k2,app in Eq. (2)) was then calculated to be 12.39 ± 1.87 M−1 s-1 by dividing kobs-TTC by PMS concentration. The calculated k2,app value was close to those for the reaction between PMS

Fig. 2. Degradation of TTC (A) and PMS (B) in LW and DI water, respectively. [PMS] =500 μM, [TCs] =50 μM, [Co(II)] =40 μM, pH 7.0. Note: solid line: LW, dash line: DI water. 3

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(denoted as TTC collectively), anionic TTC (denoted as TTC−) and dianionic TTC (denoted as TTC2-) species, at the investigated pHs (Fig. 4), and [TTC]t was the sum of all acid-base species of TTC [TTC]t = [TTCH+] + [TTC] + [TTC−] + [TTC2-] = αi [TTC]t

(4)

Where α1, α2, α3, α4 represent the distribution coefficient of TTCH+, TTC, TTC− and TTC2-, respectively. Hence, the pH-dependent oxidation of TTC by PMS could be further described by the species-specific reaction between PMS and TTC, which could be obtained by incorporating Eqs. (3) and (4) into Eq. (2) as shown in Eq. (5).

−

d[TTC ]T = k2,app−TTC∙ [TTC ]T ∙ [PMS]T = dt

m, n

∑ ki,j∙αi∙βj . i, j

[TTC ]T ∙ [PMS]T

(5)

Hence, m, n

k2,app−TTC =

∑ ki,j∙αi∙βj i, j

Fig. 3. Effect of PMS concentration on the degradation of TTC in DI water (A), and log (kobs-TTC) vs log (CPMS) (B). [TTC] =50 μM, pH 7.0. Note: the ratio is PMS/TTC.

ki,j represents the specific second-order rate constant for reaction between species i of TTC and species j of PMS and evaluates the contribution of species of TTC or PMS to the overall k2,app-TTC. The values of ki,j were determined by least-squares regression of the experimental data of k2,app-TTC to Eq. (6), which use the user-defined nonlinear regression equation in the Origin software to minimize the square of the difference between predicted and measured values. Because the mole fractions of TTCH+-SO52− and TTC-SO52− are extremely low (Figure S3), the reaction between SO52− and TTCH+ (or TTC) could be neglected in the overall reactions. Hence, the specific reactions between SO52− and TTCH+ (α1 × β2) and SO52− and TTC (α2 × β2) were not considered when modelling the experimental data with the kinetic equations. As shown in Fig. 4, The experimental data of k2,app-TTC could be well fitted with the kinetic model, and thus the reaction between PMS and TTC could be well explained by the species-specific reactions. The contribution of species-specific reactions to the overall reaction was demonstrated in Figure S4. As Figure S4 shows, the cationic and neutral species of TTC show low reactivity towards PMS oxidation, and their reaction insignificantly contributed to the overall reaction between PMS and TTC. In contrast, the anionic and dianionic TTC species exhibit considerable reactivity towards PMS, and the specific reaction between TTC-/TTC2− and PMS predominantly accounted for PMS reacting with TTC. For SO52−, TTC- exhibited higher reactivity than TTC2−. In comparison with the species-specific reaction between TTCand SO52−, strong electrostatic repulsion exists between TTC2− and SO52−. SO52− could get close to TTC2− more difficultly than TTC-, thus remarkably reducing their interaction and reactivity. In LW, however, some components were likely the potential consumption factors for PMS. Indeed, PMS decomposition in LW was extremely fast after immediate addition of PMS, and then gradually slowed down. Hence, PMS decomposition in LW could be ascribed to reaction with two types of substrates, which were highly and lowly reactive towards PMS, respectively. Based on this assumption, the PMS decay in LW could be described by Eq. (7):

and β-lactam antibiotics (Chen et al., 2018), but was much smaller than those for the reactions between TTC with HClO (Wang et al., 2011), ClO2 (Wang et al., 2011), and O3 (Hopkins and Blaney, 2014).

−

d[TTC] = k obs−TTC∙ [TTC] dt

(1)

−

d[TTC] = k2,app−TTC∙ [TTC] ∙ [PMS] dt

(2)

The PMS-promoted degradation of TTC is strongly pH-dependent (Fig. 4). Generally, kobs-TTC increased above pH 3.0 with maximum value observed around pH 9.0, and then sharply reduced when the solution pH further raised. The pH dependency of kobs-TTC could be explained by the different reactivity between PMS and TTC at various pHs. Indeed, the speciation of PMS and TC are highly pH-dependent. For example, PMS possesses two pKa values (pKa1 < 0, pKa2 = 9.4), and thus only HSO5− and SO52- exist at the investigated pHs (Eq. (3)). [PMS] = [HSO5−] + [SO52-] = βj [PMS]t

(6)

(3) −

where β1 and β2 represent the distribution coefficient of HSO5 and SO52-, respectively. TTC has three pKa values (pKa1 = 3.32, pKa2 = 7.78, pKa3 = 9.58), which correspond to the tricarbonyl-amide, phenolic-diketone and dimethylamino groups, respectively. Hence it exhibits four TTC species (Eq. (4)), i.e., TTC (denoted as TTCH+), neutral/zwitterionic TTC

[PMS]t = [PMS]0 ∙ [ωe−k3, sub ∙ t + (1 − ω) e−k 4, sub ∙ t ]

(7)

Where [PMS]0 is the initial concentration of PMS, and [PMS]t is PMS concentration at time t; ω represents the percentage of highly reactive substrates in LW; k3,sub and k4,sub are the pseudo-first-order rate constants for the highly reactive and slowly reactive substrates with PMS, respectively. As Fig. 5A shows, PMS decomposition in LW could be fitted by Eq. 7. The fitting was improved when LW was diluted with DI water for 10 times. We further applied Eq. (7) to model the PMS decomposition in other real water matrices, such as WW, and SW (Figure S5). Results show that PMS decomposition could be well fitted in WW and SW,

Fig. 4. Effect of pH and kinetic modeling for the reaction rate constants of TTC with PMS in DI water. 4

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Fig. 5. Modelling of PMS decomposition (A) and TTC degradation (B). [PMS] =500 μM, [TTC] =50 μM, pH 7.0. Symbol: experimental data; Dash line: fitted with Eq. (7) (A) and Eq. (9) (B); Solid line: fitted with Eq. (11).

The results show that the modified model (Eq. (11)) could well agree with the experimental data of TTC degradation in LW (Fig. 5B), verified the contribution of other components to the degradation of TTC in LW. However, the k5 value was calculated to be 9.9 × 10−4 s-1, lower than the observed degradation rate constant of TTC in LW (3.1 × 10-3 s1 ). This result further confirmed that TTC degradation by PMS in LW was primarily contributed from the specific reactions between TTC and PMS, but not from other factors, such as PMS activated by matrices. Overall, the above evidences strongly suggested PMS exhibits specific high reactivity towards TCs. This reaction process suffers less interference from the water matrices than the radical process, and thus shows high efficiency for eliminating TCs in LW. However, it’s still unclear how TCs react with PMS. We further explore the reaction between TCs and PMS to obtain a fundamental understanding on what characteristics make the reaction specific for TCs.

further verifying the effectiveness of Eq. (7) in describing PMS decomposition in real water matrices. Because the reaction between PMS and TTC followed the secondorder kinetics, and the water matrices could consume PMS in LW, Eq. (7) was incorporated into second-order reaction (Eq. (2)) to describe the degradation of TTC in real LW (Eq. (8)). Eq. 8 was further integrated to generate Eq. (9).

−

d[TTC] = k2,app−TTC∙ [TTC] ∙ [PMS]t dt

(8)

t

⎧ ⎫ −k2, app − TTC ∙ [PMS ]0 ∙ [ωe−k3, sub ∙ t + (1 − ω) e−k 4, sub ∙ t ] dt ⎬ ⎨ 0 ⎭ [TTC]0 ∙e⎩

∫

[TTC] =

(9)

The degradation of TTC in LW was fitted by Eq. (9), and results show that the model underestimated the degradation of TTC in LW, especially in the later stage of the reaction (Fig. 5B). In contrast, this model agreed well with the experimental data of TTC degradation for other water matrices (e.g., WW, SW) and the diluted LW (Figs. 5B and S5B). Noting that TTC contains fused ring systems, and is susceptible to various reactions in abiotic processes (Chen and Huang, 2011), the water components might also interfere with the degradation of TTC in LW, thus rendering TTC degradation deviating from the apparent second-order reaction (Eq. (8)). Hence, other parameters were further incorporated in Eq. (8) to describe the contribution of other components in LW (Eq. (10)), and further integration lead to Eq. (11).

−

d[TTC] = k2,app−TTC∙ [TTC] ∙ [PMS]t + k5 ∙ [TTC] dt

3.3. Radical vs non-radical reaction Generally, both SO4%− and HO% were always considered as the main oxidant species in the activated PS processes. We further investigated whether radical species involved in the PMS-promoted oxidation of TTC. Radical quenchers (e.g., MeOH and TBA) were added to DI water containing PMS and TTC. MeOH reacts with HO% and SO4%− at a high and comparable rate constants (9.7 × 108 M-1 s-1, 3.2 × 106 M-1 s-1 for SO4%− and HO%, respectively), while TBA reacts with HO% at the rate constants ((3.8–7.6) × 108 M-1 s-1) (Liu et al., 2018) much faster than SO4%− ((4–9.1) × 105 M-1 s-1) (Qian et al., 2018). As shown in Fig. 6A and B, the degradation of TTC by PMS was unaffected after addition of 10 mM alcohols, i.e., MeOH and TBA. When the concentrations of alcohols increased to 1000 mM, TTC degradation still kept consistent with that in the absence of alcohols. This result indicated that the PMSpromoted oxidation of TTC was not contributed from the reactive radicals, such as SO4%− and HO%, but was most likely originated from the

(10)

t

⎧ ⎫ t k5 dt −k2, app − TTC ∙ [PMS ]0 ∙ [ωe−k3, sub ∙ t + (1 − ω) e−k 4, sub ∙ t ] dt + 0 ⎨ ⎬ 0 [TTC]0 ∙e⎩ ⎭

∫

[TTC] =

∫

(11) Where k5 represents rate constants of TTC degradation contributed from other components in LW.

Fig. 6. Effect of MeOH (A) and TBA (B) on PMS promoted degradation of TTC, and EPR spectra of PMS/TTC (C). [PMS] =1000 μM, [TTC] =50 μM, pH = 7.0. 5

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3.4.2. Theoretical prediction of reactive sites on TCs As an oxidant, PMS is expected to attack the electron-rich moiety of TCs. The resonance structures of TCs, i.e., phenolic-diketone (BCD-ring) and tricarbonylamide (A-ring) render the electrophilic attack of PMS at various sites on TCs. Indeed, the phenolic group, double bonds and amine groups in TCs with abundant electrons were regarded as the reactive sites towards different oxidants, such as ClO2 (Wang et al., 2011), O3 (Hopkins and Blaney, 2014), MnO4− (Jiang et al., 2020), SO4.- (Guan et al., 2018) and HO· (Wang et al., 2018). The UV–vis spectra verified the BCD-ring involved in the PMS-promoted degradation of TCs. The resonance structure of phenolic-diketone renders the electrophilic attack of PMS at various sites on the BCD-ring. Indeed, PMS was reported to convert monohydric phenols to dihydric phenols, resembling the classical “Elbs peroxydisulfate oxidation” (Kennedy and Stock, 1960; Baker and Brown, 1948). Hence, the phenolic group on the D-ring of TTC seems to undergo electrophilic attack by PMS, generating the quinol and catechol derivatives. Moreover, PMS was able to convert cyclohexene to cyclohexanediol, and the terminal or internal double bond can be hydroxylated by the electrophilic PMS (Kennedy and Stock, 1960). Double bonds on the B-ring also seems to be the reactive site for PMS oxidation. Theoretical calculations have been frequently used to predict the possible reaction mechanism or potential reaction sites of contaminants (Qu et al., 2015). To ascertain the reactive sites on TCs towards PMS, frontier electron densities (FEDs) calculations were conducted to investigate the oxidative degradation behavior of TCs based on the frontier molecular orbital theory. Generally, the frontier orbital theory elaborated that the electrophilic reactions most likely occur in the electron rich (HOMO) regions, while the nucleophilic reactions most likely take place at the moiety of LUMO regions. The PMS-promoted oxidation of TCs was verified to proceed via non-radical process, and thus PMS, as an electrophile, most likely attacks at the sites with high 2FED2HOMO values. To determine the preferential sites in the electrophilic reactions, the FEDs of TCs were calculated and demonstrated in Fig. 8 and Figures S12-S14. As shown in Fig. 8, the HOMOs of TCs are mainly located at the B-ring, suggesting the positions on the B-ring are potential sites for oxidation by PMS. This theoretical calculation revisited the previous assumption that the phenolic group on the D-ring of TTC acts as sites for electrophilic attack. Since the structural difference of TTC and CTC only lies in the D-ring (i.e., chlorine present on the Dring of CTC, but absent in TTC), the presence of chlorine, an electron abstracting group, can reduce the electron density on the D-ring, unfavorable for the electrophilic attack on the D-ring of CTC by PMS. However, the faster degradation of CTC than TTC observed in this work excluded the D-ring as the reactive sites in the electrophilic reaction (Figure S1). The 2FED2HOMO for each atom in TCs were calculated and shown in Figures S12-S14. Results show that the high 2FED2HOMO values are obtained at carbon atoms on the B-ring, indicating that these sites are potentially electrophilic attacked by PMS. To further verify this reaction mechanism, we investigated the transformation products of TCs by PMS oxidation.

non-radical process. EPR technique was further used to evaluate whether radical species, i.e., SO4%− and HO%, involved in the PMS-promoted oxidation of TTC with DMPO as radical trap. As shown in Fig. 6C, signals corresponding to DMPO adducts of SO4%− and HO% were not observed in the spectrum of PMS/TC solution, which, together with the scavenging results, essentially ruled out the participation of SO4%− and HO% in the PMS-promoted oxidation of TTC. It is noted that PMS undergo slowly and spontaneously decomposition in the solution, the process of which generates singlet oxygen (1O2) (Yang et al., 2018). Typical 1O2 scavengers, e.g., FFA, NaN3 and L-histidine were subsequently added in PMS/TTC solution, and the results showed that 1O2 did not contribute to TTC degradation by PMS (Text S5, Figures S6-S7). Overall, the PMS-promoted degradation of TTC proceeded via non-radical oxidation process and dominated by PMS direct oxidation. To further evaluate whether LW matrices could activate PMS to generate radicals, MeOH was added in the reaction solution and results showed that TTC degradation and PMS decomposition were not affected after the addition of MeOH (Figure S8), indicating TTC degradation was not contributed from the radical species, but from nonradical oxidation, e.g., the direct oxidation by PMS. Furthermore, we tested the degradation of anisole (a frequently used probe for SO4.− and HO·) by PMS in LW (Figure S9). The negligible degradation of anisole with or without MeOH also verified that the radicals were not generated in LW.

3.4. Reaction pathway and mechanism 3.4.1. UV–vis spectra UV-vis absorbance spectrum of TTC solution was measured along the reaction with PMS. TTC exhibits two distinct absorbance bands at 250−300 nm and 340−380 nm, respectively, which were attributed to the resonance groups, i.e., phenolic-diketone (BCD-ring) and tricarbonylamide (A-ring). Generally, the band at 250−300 nm was contributed from both A- and BCD- rings, while 340−380 nm absorbance band was merely contributed from the BCD-ring (Chen and Huang, 2009). As shown in Fig. 7, the UV–vis spectra of TTC solution changed along the degradation by PMS with the two characteristic bands decreased as the reaction proceeded. Moreover, the two bands shifted to lower wavelength after degradation, indicating the BCD-ring involved in the reaction. UV–vis spectra of OTC and TTC were also monitored during the PMS-promoted degradation (Figures S10 and S11), and the variation of the two distinct absorbance peaks at 250−300 nm and 340−380 nm resembled that in the PMS-promoted degradation of TTC, indicating the similar mechanisms in the PMS-promoted degradation of TCs.

3.4.3. Reaction pathway Transformation products of TCs were further analyzed by LCeMS/ MS. The parent compound TTC (m/z 445) possesses two primary mass fragments of m/z 427 and m/z 410, which represent the loss of a NH3 (−17 m/z), and both NH3 and H2O (−35 m/z) from TTC molecular ions (Figure S15). The loss of NH3 and H2O were reported to relate with the amino group at C2 and hydroxyl group at C6, respectively (Chen and Huang, 2010). Analysis of TTC solution with PMS oxidation indicated the generation of eight products with molecular ions of m/z 422, 433, 438, 461a, 461b, 461c, 477a and 477b (Figures S16-S23). Most of these products exhibit similar fragmentation patterns as the parent TTC, e.g., the major fragments of -17 and −35 m/z, suggesting they possess the common tetracyclic rings as in TTC. They are short-written as M-23, M12, M-7, M + 16a, M + 16b, M + 16c, M + 32a, and M + 32b (M

Fig. 7. UV–vis spectra of TTC during the reaction with PMS. 6

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Fig. 8. HOMOs of various TCs.

M-23 products were odd, quite different from other products and parent compound with even mass, M-7 and M-23 products were thus supposed to contain odd number of nitrogen atoms based on the nitrogen rule. Hence, the deamination likely happened during the oxidation of TTC, most likely at C4 sites. Compared to the N-demethylation and deamination pathways, the PMS-promoted oxidation of TTC primarily proceeded via the hydroxylation process. In the PS activation process with SO4%− generation, SO4%− showed high reactivity towards TTC to generate diverse products, and both N-demethylation and hydroxylation significantly contributed to TTC oxidation by SO4%- (Ji et al., 2016). Compared to the SO4%−, PMS exhibited high selectivity toward TTC oxidation via electrophilic attack at the sites with high HOMOs. Direct oxidation of PMS with contaminants was reported to form a so-called precursor complex at first (Ji et al., 2018), and then proceed via two electron transfer, involving heterolytic cleavage of the PMS peroxide bond and oxygen transfer from PMS to nucleophiles (Chen et al., 2018). Hence the PMS-promoted TTC oxidation was supposed to initiate via a transition state complex between PMS peroxide bond and the carbon at the B-ring of TTC, followed by the heterolytic cleavage of the PMS peroxide bond and electron transfer, generating the hydroxylated TTC products. Transformation product pattern of OTC by PMS quite resemble that observed in TTC, that is, M-23, M-12, M-7, M + 16, and M + 32 products were observed along the reaction, and M + 16 products were the predominant degradation products (Figure S25). It was thus suggested that the hydroxylation process also primarily dominated in the PMSpromoted OTC degradation. Noting that −OH group is already present at C5 position, hydroxylation of OTC cannot occur at C5. The M + 16 products were likely generated via hydroxylation at C4a and C11a positions. Moreover, the presence of −OH group on C5 was likely to form hydrogen bond with the neighboring −OH group at C6, and thus reduce the possibility of −OH at C6 attacking carbonyl group at C11. Such isomerization of OTC might destruct the phenolic-diketone structure, and thus affect the likelihood of electrophilic attack of PMS on the BCD-rings, hence the degradation of OTC was faster than that of TTC (Figure S1). The PMS-promoted CTC oxidation also generated the primary M + 16 and M + 32 products (Figure S26), suggesting hydroxylation dominated in the CTC oxidation. The evolution trend of these hydroxylation products was quite different from those of TTC and CTC. For example, the primary M + 16 products initially increased and subsequently decreased after 120 s, indicating M + 16 as the intermediate products of the reaction. Indeed, three M-12 products were generated, and their intensity continuously increased along the CTC degradation. The above evidences suggest that M + 16 products were most likely further transformed to M-12 products. This reaction was likely to proceed via the N-demethylation reaction at tricarbonylamide, similar to that happened in TTC. Hence, besides hydroxylation process, N-demethylation was the other important pathway for the PMS-promoted oxidation of CTC. Overall, the PMS-promoted oxidation of TCs proceeded via the electrophilic attack at the B-ring of TCs, and the hydroxylation was the dominant reaction mechanism. For CTC, N-demethylation was also significant for the hydroxylation products.

Fig. 9. Evolution of TTC products by PMS oxidation.

represents the molecular weight of TTC), respectively, indicating the net mass loss or gain of the product from the parent TTC. The evolution of products indicated that M + 16 and M + 32 products were the primary products of TTC by PMS oxidation (Fig. 9). The generation of three M + 16 products indicated the hydroxylation involved in TTC oxidation by PMS, and the hydroxylation probably located at the B-ring of TTC. Indeed, the theoretical calculation indicated the B-ring as the potential electrophilic sites for PMS with HOMOs at C5 much higher than other atoms. Hence, hydroxylation most likely occurred at C5, generating M + 16b with the highest intensities among all the products. Noting that the structure difference between OTC (MW: 460 = 444 + 16) and TTC locates at the hydroxyl group at C5, which is only present on OTC, it seems reasonable to assume that the primary product M + 16b was OTC. We subsequently analyzed the authentic OTC chemical with the analytical methods for the TCs products, and found that M + 16b products in the PMS-promoted oxidation of TTC did not match, but quite close to OTC in LC retention time (10.1 min vs 9.56 min). On the other hand, the mass fragmentation of M + 16b quite resembled those of OTC (Figure S24). These evidences suggested that M + 16b was likely the stereoisomeric OTC. The isomerization products of OTC (epi-OTC) have been reported in abiotic conditions with low pH level (Chen and Huang, 2011). The generation of M + 32 products indicated that M + 16 products could further undergo hydroxylation by PMS electrophilic attack (Fig. 10). Moreover, the presence of M + 32 further indicated that the reaction stoichiometry between PMS and antibiotics was higher than 1:1, which was consistent with the decomposition of PMS in Fig. 2. Besides the hydroxylated products, M-7, M-12 and M-23 products were also observed during the PMS-promoted degradation of TTC with much lower intensity than the hydroxylated products. M-12 was likely generated from N-demethylation reaction at C4 of the M + 16 products, because 46C on tricarbonylamide possesses high HOMOs (Figure S12), and was supposed as the potential reactive sites towards PMS oxidation based on the theoretical calculation. It is noted that the mass of M-7 and 7

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Fig. 10. Proposed transformation pathway of TTC by PMS oxidation.

4. Conclusions

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This study demonstrated selective oxidation of TCs by PMS in LW. Unlike the radical oxidation processes, direct oxidation dominated in the TCs degradation by PMS alone, the rapid oxidation of TCs was only slightly affected by the water matrices. A new kinetic model was developed to describe the rapid degradation of TCs in LW. Theoretical calculations revealed the HOMOs of TCs primarily located at B ring, and thus suggesting B ring are potential electrophilic attack sites by PMS. Hydroxylation was further verified as the dominant reaction mechanism for TCs oxidation by PMS. For CTC, N-demethylation also significantly contributed to the PMS-promoted oxidation. PMS shows specific and high reactivity towards TCs, and is less affected by the water matrices than the radical processes. Hence, PMS maintains remarkable treatment efficiency for TCs removal in LW. Declaration of Competing Interests None. Acknowledgements We sincerely thank the National Key R&D Program of China (2018YFD1100500), and National Natural Science Foundation of China (51625804, 51878431). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jhazmat.2019.121656. References Ao, X.W., Sun, W.J., Li, S.M., Yang, C., Li, C., Lu, Z.D., 2019. Degradation of tetracycline by medium pressure UV-activated peroxymonosulfate process: influencing factors,

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