Kinetic Study of Hydroxyl and Sulfate Radical-Mediated Oxidation of

Feb 2, 2017 - Advanced oxidation processes (AOPs), such as hydroxyl radical (HO•)- and sulfate radical (SO4•–)-mediated oxidation, are alternati...
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Kinetic Study of Hydroxyl and Sulfate Radical-Mediated Oxidation of Pharmaceuticals in Wastewater Effluents Lushi Lian,† Bo Yao,† Shaodong Hou,‡ Jingyun Fang,‡ Shuwen Yan,† and Weihua Song*,† †

Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, China School of Environmental Science and Engineering, Sun Yet-sen University, Guangzhou 510275, China

‡

S Supporting Information *

ABSTRACT: Advanced oxidation processes (AOPs), such as hydroxyl radical (HO•)- and sulfate radical (SO4•−)-mediated oxidation, are alternatives for the attenuation of pharmaceuticals and personal care products (PPCPs) in wastewater effluents. However, the kinetics of these reactions needs to be investigated. In this study, kinetic models for 15 PPCPs were built to predict the degradation of PPCPs in both HO•- and SO4•‑-mediated oxidation. In the UV/H2O2 process, a simplified kinetic model involving only steady state concentrations of HO• and its biomolecular reaction rate constants is suitable for predicting the removal of PPCPs, indicating the dominant role of HO• in the removal of PPCPs. In the UV/K2S2O8 process, the calculated steady state concentrations of CO3•− and bromine radicals (Br•, Br2•− and BrCl•−) were 600-fold and 1−2 orders of magnitude higher than the concentrations of SO4•−, respectively. The kinetic model, involving both SO4•− and CO3•− as reactive species, was more accurate for predicting the removal of the 9 PPCPs, except for salbutamol and nitroimidazoles. The steric and ionic effects of organic matter toward SO4•− could lead to overestimations of the removal efficiencies of the SO4•−-mediated oxidation of nitroimidazoles in wastewater effluents.

■

for PPCPs.14 Unlike HO•, SO4•− prefers to react with organic compounds through an electron transfer mechanism; therefore, H-abstraction and addition processes have minor contributions to SO4•− oxidation.8 Consequently, SO4•−-mediated AOPs can complement HO•-mediated AOPs with diverse reactivity, product patterns, and energy efficiencies.15−17 Meanwhile the highly reactive oxidants HO• and SO4•− both result in the oxidation of effluent organic matter (EfOM) and inorganic ions presented in the wastewater effluents.18 Since micropollutant PPCPs are found at concentrations 3 to 5 orders of magnitude lower than the concentrations of the remaining matrix components, most of the active species (and therefore the added oxidant) are consumed by the matrix. The occurrence of PPCPs in aquatic systems is seasonally and spatially diverse,19,20 which indicates that the occurrence pattern of PPCPs is not stationary. Moreover, the concentrations of PPCPs that remain in wastewater are quite low, ranging from ng L−1 to μg L−1.21,22 Detecting and monitoring individual PPCPs is laborious and time-consuming and involves costly mass spectrometers. Hence, it is necessary to develop kinetic models to assess removal efficiencies. In general, the simplified kinetic model involves two key parameters: the

INTRODUCTION Pharmaceuticals and personal care products (PPCPs) are a group of important emerging contaminates that have been released from wastewater treatment plants due to their incomplete elimination by existing biological treatment processes.1,2 PPCPs have long-term adverse effects on aquatic ecosystems. Moreover, PPCPs are a concern for downstream drinking water supplies. Hence, advanced techniques are being investigated to attenuate PPCPs from wastewater effluents. Advanced oxidation processes (AOPs) are attractive alternatives, that use highly reactive species (RS), such as hydroxyl radical (HO•), sulfate radical (SO4•−), and chloride radical (Cl•).3 Hydroxyl radical-mediated AOPs are one group of the most commonly used AOPs, such as UV/H2O2, O3/H2O2, and Fe2+/ H2O2.4,5 Numerous studies have demonstrated that HO• reacts with PPCPs through three competing pathways: addition, hydrogen abstraction, and electron abstraction.3,6 Sulfate radical-mediated AOPs have recently gained a substantial amount of scientific attention. A series of methods have been used to generate sulfate radical through the activation of peroxydisulfate (PDS), including UV irradiation,7 heat,8 ozone,9 quinone10 and transition-metal catalysis11 methods. PPCPs can be decomposed by SO4•− due to its high redox potential (2.5−3.1 V),12 which is comparable with that of HO• (1.9−2.7 V).13 In contrast to well-studied HO• reaction rate constants, only a few SO4•− reaction rates have been reported © 2017 American Chemical Society

Received: Revised: Accepted: Published: 2954

November 3, 2016 January 31, 2017 February 2, 2017 February 2, 2017 DOI: 10.1021/acs.est.6b05536 Environ. Sci. Technol. 2017, 51, 2954−2962

Article

Environmental Science & Technology steady state concentrations of the reactive species ([RS]SS) and the bimolecular reaction rate constants of the PPCPs with RS (kPPCPs,RS), therefore the pseudo first order degradation rates of PPCPs (kPPCPs) can be expressed as follows: kPPCPs = kPPCPs,RS × [RS]SS

Table 1. Summary of Water Qualities of Wastewater Effluent units pH DOC CO32− HCO3−

(1)

The above kinetic model does not consider the effects of secondary RS, which are generated from the reaction of the water matrix with the primary RS as follows: RS• + Cl− → RS− + Cl• •

−1

mgC L mgC L−1 mgC L−1

Cl− Br− F− NO3− SO42−

mg mg mg mg mg

L−1 L−1 L−1 L−1 L−1

103.66 0.46 0.74 9.95 7.64

Einstein L−1 s−1 (0.37 mW cm−2) based on iodide-iodate actinometry.25 The reaction system consisted of 20 mL of wastewater (DOC = 4.78 mgC L−1, pH 8.23) spiked with 1.0 μg L−1 for per PPCPs. Radicals were generated by adding 1.0 mM H2O2 or PDS. To eliminate the effects of HO• in the UV/PDS, 10 mM tert-butyl alcohol (TBA) was added into the solutions because the reaction rate of HO• with TBA (kHO• = 6 × 108 M−1 s−113) is approximately 3 orders of magnitude faster than the reaction rate of SO4•− with TBA (kSO4•− = 4.0 × 105 M−1 s−126). [HO•]SS was modeled by using Kintecus V5.75.27 The modeling results indicated that >95% of HO• is scavenged by 10 mM TBA, whereas only 0.04% of SO4•− is scavenged by TBA. After using subjecting samples to different irradiation doses, aliquots were withdrawn, and methanol was immediately added to the samples (≈ 4% v/v) to scavenge any radicals that formed slowly during the storage period. Then, 1.0 mM Na2SO3 was added to the samples prior to UHPLC-online SPEMS analysis to quench the residual H2O2 or PDS. Control experiments were conducted using H2O2/PDS/UV alone for the PPCPs spiked wastewater effluent. Second-Order Reaction Rate Constants and the Steady State Radical Concentrations. The second-order reaction rate constants of PPCPs and EfOM with HO• and SO4•− were determined based on competition kinetics in ultrapure water using para-chlorobenzoic acid (pCBA) as a probe as described by Lutze et al.8 Twenty μM of both pCBA and the target compound (T) were added in the ultrapure water, and 2.5 mM phosphate buffer was used to adjust the pH to 7.0. The second-order reaction rate constants of the target compounds were derived

(2)

RS• + CO32 − → RS− + CO3•−

units 8.23 4.78 0.33 10.61

(3)

•−

These secondary RS (Cl /CO3 ) remain relatively high reactivity and may also react with PPCPs,23,24 leading to faster decay rates than the decay rates predicted by eq 1. Based on the kinetic model, it is promising to evaluate the effectiveness of AOPs for removing specific PPCPs based on the bimolecular reaction rate constants of PPCPs with RS and the steady state concentrations of RS. Therefore, the objective of this study is to examine the feasibility of using eq 1 to predict the removal of PPCPs in the effluents and to explore the roles of secondary RS in the AOPs. In this study, UV/H2O2 and UV/PDS were employed as models for HO•- and SO4•−-mediated oxidation, respectively. The second-order reaction rate constants of 13 PPCPs with SO4•− and CO3•− as well as 8 PPCPs with HO• were determined first. The simplified and secondary radicals involved in the kinetic models based on eq 1 were applied to predict the removal efficiencies of PPCPs in HO•/SO4•−-mediated oxidation.

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MATERIALS AND METHODS Chemicals. All the PPCPs and isotopic internal standards used in this study were purchased from Sigma-Aldrich or Toronto Research Chemicals (TRC) unless otherwise stated. Individual stock solutions of PPCPs were prepared in DI-H2O at 100 mg L−1. A mixture of all the PPCPs stock solutions was prepared in DI-H2O with a concentration of 1.0 mg L−1 for each component. Individual stock solutions of the internal standard compounds were prepared in methanol at 100 mg L−1. The stock solution containing all the internal standard compounds was prepared in methanol using a concentration of 0.55 mg L−1 for each compound. Effluents. The water investigated in this study was collected from a municipal WWTP located in Jiangsu Province, China. The tested WWTP treats domestic (85%) and industrial wastewater (15%) from the eastern district of the Taicang City using a circulatory activated sludge treatment system. The secondary effluent was first filtered by 0.45 μm membrane, and then was concentrated 5-fold by using a reverse osmosis membrane system for convenient storage, and the concentrates were stored in gastight, plastic, opaque bottles at 4 °C. The RO concentrates were diluted 5-fold when used as working solutions in all studies. The DOC content of the working solutions was acquired using a TOC analyzer (Shimadzu, TOC−CPH/CN). The ions were analyzed using an Ion Chromatograph (Metrohm 883, IC). The parameters of the working effluent are shown in Table 1. UV/PDS and UV/H2O2 Experiment. The photo reactor used for the AOPs was equipped with a low-pressure lamp peaking at 254 nm (Trojan Technologies Lamp, 25 W). The fluency rate of the lamp was measured to be 7.97 × 10−4

( ) against the degradation of pCBA

from eq 4 by plotting ln

T T0

⎛ pCBA ⎞ (ln⎜ pCBA ⎟), which revealed a linear function with a slope that ⎝ 0⎠ could be used to calculate kradical (T). ⎛ pCBA ⎞ ⎛T ⎞ k radical(T ) ⎟ ln⎜ ⎟ = ln⎜⎜ k radical(pCBA) ⎝ pCBA0 ⎟⎠ ⎝ T0 ⎠

(4)

The steady state concentrations of the radicals ([radical]SS) in the effluent were determined by the degradation of pCBA. The initial concentration of pCBA was 0.5 μM, and radicals were generated by adding 1.0 mM H2O2 or PDS. The degradation of pCBA by radicals in the effluent followed pseudo first-order kinetics, and the rate constants, k′, were ⎛ pCBA ⎞ obtained by plotting ln⎜ pCBA ⎟ against time (t). In addition, ⎝ 0⎠ [radical]SS can be calculated from eq 5. k′ = k radical(pCBA) × [radical]SS

(5)

The reported second-order reaction rate constants for the reaction of pCBA with HO• and SO4•− are 5.0 × 109 M−1 s−128 and 3.6 × 108 M−1 s−1,29 respectively. 2955

DOI: 10.1021/acs.est.6b05536 Environ. Sci. Technol. 2017, 51, 2954−2962

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Environmental Science & Technology The concentrations of H2O2 and PDS were 1.0 mM in all the experiments described above. The degradation of pCBA and target PPCPs was determined using an Agilent 1260 HPLC system coupled with a DAD detector and a C18 column (4.6 mm × 75 mm, 1.8 μm, Agilent Zorbax Eclipse Plus). For the mobile phase, various isocratic mixtures of 0.05% trifluoroacetic acid in water and acetonitrile were used. The flow rate was 1.0 mL min−1. All compounds were determined using the maximum UV absorption. Online SPE-LC-MS/MS. An UHPLC-online SPE system (Agilent 1290) coupled with a Triple Quadrupole MS system with an electrospray ionization source (Agilent 6430) was applied to quantify the degradation of PPCPs during AOPs. A C18 column (2.1 mm × 50 mm, 1.8 μm, Agilent ZORBAX SBC18 HD) with a controlled temperature of 30 °C was used to separate the PPCPs in the effluent. The gradient elution table and MS details referred to Table S1 and S2, Supporting Information (SI). An Agilent 1290 infinity flexible cube was employed to achieve online SPE.30 First, a 900 μL sample was withdrawn and delivered to one of the two online SPE columns with 3 mL of H2O by using the flush pump. Then, the valve was changed to the position that coupled the SPE column with the analysis column in which the analytes were eluted from the SPE column in the back-flush mode. Simultaneously, the other SPE column began regeneration with 5 mL acetonitrile and followed with 10 mL H2O. The flow rate was set at 1.5 mL min−1.

Figure 1. Relative removal efficiencies of 20 PPCPs from wastewater under UV/H2O2 and UV/PDS treatment and a specific radiation dose of 400 mJ cm−2. [H2O2] or [PDS] = 1 mM; initial concentration of PPCPs [PPCP]0 = 1 μg L−1; [DOC] = 4.78 mgC L−1.

first-order degradation rate for direct photolysis of PPCPs. Both [HO•]SS and [SO4•−]SS were determined to be (1.03 ± 0.01) × 10−13 and (2.21 ± 0.06) × 10−13 M, respectively, when using pCBA as a chemical probe. The bimolecular reaction rate constants of HO•/SO4•− with PPCPs are shown in Table 2. Seven of the HO• reaction rate constants of PPCPs are available from the references;31−34 the other eight reaction rate constants were obtained in this study using the competition method.8,34 In contrast with well-studied HO• rate constants, only two of the SO4•− reaction rate constants were reported in the references.14,35 The remaining 13 rate constants were measured based on the competition method. All of the competition kinetic results are shown in SI Figure S1 and S2. From the [RS]SS and bimolecular reaction rate constants, the pseudo first-order decay rates of PPCPs were calculated using eq 6, and the experimental decay rates were simultaneously observed for various doses of UV irradiation. For HO•mediated oxidation, the calculated degradation rates for the 15 PPCPs based on the biomolecular HO• rate constants and [HO•]SS corresponded well with the experimental degradation rates (within 0.5−2 times the experimental values), as illustrated in Figure 2. This result suggested that the combined use of the HO• reaction rate constants of the individual PPCPs and [HO•]SS may be effective for evaluating the likelihood of the effective removal of PPCPs by HO•−-mediated AOPs. Previous studies, which applied HO• rate constants to predict the removal of PPCPs from reverse osmosis retentate using γ irradiation, have also observed analogous results.32 Unlike the well-fitted HO•-mediated oxidation, the predicted first-order degradation rates of SO4•− oxidation were not correlated with the experiment results, as shown in Figure 2. The experimental decay rates of ractopamine, salbutamol, carbamazepine and propranolol were significantly faster than the predicted ones, implying that this kinetic model underestimated their removal efficiencies but could still be used as a conservative tool for predicting their removal. For ronidazole, ornidazole, tinidazole, dimetridazole, and metronidazole, the experimental decay rates were significantly lower than the predicted decay rates, indicating the simplified kinetic model is not suitable for these compounds. To explain the above conflicting results, the effects of the water matrix on the removal efficiencies must be considered.

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RESULTS AND DISCUSSION Removal Efficiencies of PPCPs in the effluent by UV/ H2O2 and UV/PDS. Fifteen PPCPs were selected in our study due to their widespread occurrence in aquatic environments, including β-adrenergic agonists, β-blockers, nitroimidazoles, psychiatric drugs, lipid regulators and anti-inflammatories. All PPCPs were spiked in the effluent at 1.0 μg L−1 to mimic their actual concentrations and quantified using UHPLC-onlineSPE-MS/MS. The oxidation of PPCPs by UV254 nm radiation or H2O2 or PDS alone in the dark was negligible compared to the HO•/SO4•− oxidation. Upon irradiation of the H2O2 or PDS spiked effluents, the removal efficiencies of the 15 PPCPs ranged from 33% for metronidazole to 66% for ractopamine in UV/H2O2 and from 10% for ronidazole to 99% for ractopamine in UV/PDS, respectively. Seven PPCPs presented higher removal efficiencies via SO4•−-mediated oxidation than by HO•-mediated oxidation, whereas the other PPCPs showed opposite results, except for carbamazepine, which resulted in equal removal efficiencies for both HO• and SO4•−-mediated oxidation. SO4•−-mediated removal efficiencies clearly cover a much broader range than HO•-mediated process, as shown in the boxplot in Figure 1. This result was consistent with the comparison of the removal efficiency of five PPCPs in the UV/ PDS and UV/H2O2 treatments,18 indicating that PPCPs react more selectively with SO4•− than with HO•. Simplified Kinetic Model for Predicting the Degradation of PPCPs. To develop a kinetic model for predicting the degradation rates of PPCPs in AOPs, both the steady state concentrations of RS and the bimolecular radical reaction rate constants must be considered, and eq 6 was used. k′ = kPPCPs,RS × [RS]SS + kd

(6)

Where kPPCPs,RS is the bimolecular reaction rate constant of the reactive species (HO•/SO4•−) with PPCPs, kd is the pseudo 2956

DOI: 10.1021/acs.est.6b05536 Environ. Sci. Technol. 2017, 51, 2954−2962

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Environmental Science & Technology

Table 2. Selected PPCPs, Their Structures, and the Secondary Reaction Rate Constants with Sulfate, Hydroxyl and Carbonate Radicalsa

a

* Denotes the second-order rate determined in this study.

investigated. As shown in eqs 7 to 13, H2O2, HCO3−/CO32−, Br− and EfOM present varied HO• reaction rate constants. HO• + H 2O2 → HO2• + H 2O

2.7 × 107 M−1 s−1

(7)

13

HO• + HCO3− → CO3•− + H 2O

8.6 × 106 M−1 s−1 (8)

13

HO• + CO32 − → CO3•− + HO−

3.9 × 108 M−1 s−1 (9)

13

HO• + Br − → HO− + BrOH•−

1.1 × 1010 M−1 s−1 (10)

36,37

Figure 2. Relationship between the experimental pseudo first-order rate and the calculated pseudo first-order rate of 15 PPCPs based on eq 6 in the UV/H2O2 and UV/PDS AOPs. [HO•]SS = (1.03 ± 0.01) × 10−13 M; [SO4•−]SS = (2.21 ± 0.06) × 10−13 M; initial concentration of PPCPs [PPCP]0 = 1.0 μg L−1; [DOC] = 4.78 mgC L−1.

HO• + EfOM → product

3.3 × 104 L mg C−1 s−1

(11)

18

HO• + Cl− → HO− + ClOH•−

Effects of the Water Matrix on the Radical Distributions in both AOPs. Considering the influences of the water matrix on the AOPs, the reactions of HO•/SO4•− with inorganic ions, EfOM, and oxidants (H2O2 or PDS) were

4.3 × 109 M−1 s−1 (12)

37

ClOH•− → HO• + Cl− 2957

6.1 × 109 M−1 s−1

(13)

DOI: 10.1021/acs.est.6b05536 Environ. Sci. Technol. 2017, 51, 2954−2962

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Environmental Science & Technology 37

SO4•− + S2O82 − → SO4 2 − + S2 O8•−

By multiplying the individual concentrations (Table 1) with the bimolecular reaction rate constants, the contributions of the inorganic ions and matrix for quenching HO• were summarized in Figure 3a. The calculation details of the fraction of the water

6.5 × 105 M−1 s−1 (14)

43

SO4•− + HCO3− → HSO4 − + CO3•− 2.6 × 106 M−1 s−1

(15)

44

SO4•− + CO32 − → SO4 2 − + CO3•−

6.1 × 106 M−1 s−1 (16)

45

SO4•− + Cl− → SO4 2 − + Cl•

3.0 × 108 M−1 s−1

(17)

SO4 2 − + Cl• → SO4•− + Cl−

2.1 × 108 M−1 s−1

(18)

SO4•− + Br − → SO4 2 − + Br •

3.5 × 109 M−1 s−1

(19)

43

43

46

SO4•− + NO3− → SO4 2 − + NO3•

2.1 × 106 M−1 s−1 (20)

12

SO4•− + EfOM → product

9.4 × 103 L mg C−1 s−1 (21)

18

The effects of the water matrix on SO4•− are obviously different from the effects of the water matrix on HO•. Chloride was the most effective scavenger for SO4•−, with a fraction of 91.6%. Thus, it is rational to speculate that the amount of secondary Cl• formed in the SO4•−-mediated AOP was significantly higher than that formed in the UV/H2O2 process. To further investigate the effects of the water matrix on the dispersion of radicals, the steady state concentrations of the secondary radicals formed during the two treatments were modeled by using Kintecus V5.75.27 Briefly, the Kintecus V5.75 model contains 179 reactions in the UV/H2O2 and UV/PDS process (SI Table S3), including the reactions of PPCPs and organic matter with HO• and SO4•−, which refer to the models previously applied to estimate the inorganic radicals in the RO brines18 and seawater.38 As shown in Table 3, the modeling results demonstrated that the [CO3•−]SS ((5.80 ± 0.09) × 10−11 M) in the SO4•−-mediated oxidation process was only 1 order of magnitude higher than the [CO3•−]SS ((2.79 ± 0.01)

Figure 3. Contributions of the water matrix (EfOM and inorganic ions) to scavenging HO• (a) and SO4•− (b) in the UV/H2O2 and UV/ PDS AOPs, respectively. The dissolved organic carbon concentration in the wastewater was 4.78 mgC L−1.

matrix that reacts with HO•/SO4•− can be found in the SI. EfOM (59.1%) and Br− (23.7%) were two primary contributors for scavenging HO• under the experimental conditions used in this study, even though Br− was present at very low concentrations. Previous studies have also reported that trace amounts of Br− play an important role in scavenging HO•.18,38 The reaction of Br− with HO• can form BrOH•− (eq 9), which can further react with H+, Cl−, and Br− to form other reactive halogen radicals (especially Br•, Br2•− and ClBr•−).36,39 HCO3− and CO32− scavenged 6.9% of HO• to yield CO3•− through eqs 8 and 9. Chloride was not considered as a radical scavenger in the HO•-mediated AOP, despite its high concentration in the wastewater. This result occurred because the reaction of Cl− and HO• is reversible, as shown in eqs 12 and 13, and the reaction rate of the forward reaction (k = 4.3 × 109 M−1 s−1)13 is even slightly lower than that of the back forward reaction (k = 6.1 × 109 M−1 s−1).37 Considering both reactions, the reported observed reaction rate for the reaction of Cl− with HO• is 103 M−1 s−1 at pH 7.0 and increases by a factor of 10 with every decrease in pH unit.40 Since the pH of effluent used in this study was 8.2, the reaction of Cl− with HO• was neglected. Moreover, chloride ions are a less important scavenger in UV/H2O2 processes.41,42 A similar approach was proposed for SO4•−-mediated oxidation, which is summarized in eqs 14 to 21.

Table 3. Modeled Steady-State Concentrations of Inorganic Radicals in the Effluent

2958

[radical]ss (M)

UV/H2O2

[HO•] [SO4•−] [Cl•] [Cl2•−] [Br•] [Br2•−] [BrCl•−] [CO3•−] [NO3•]

(2.72 ± 0.03) × 10−13 (4.98 (7.88 (5.07 (7.11 (4.40 (2.79 -

± ± ± ± ± ±

0.11) 0.01) 0.16) 0.42) 0.11) 0.01)

× × × × × ×

UV/PDS

10−19 10−17 10−16 10−15 10−15 10−12

(1.30 (9.72 (5.79 (9.14 (8.01 (6.07 (2.98 (5.80 (1.41

± ± ± ± ± ± ± ± ±

0.22) 0.05) 0.01) 0.07) 0.80) 0.83) 0.28) 0.09) 0.01)

× × × × × × × × ×

10−15 10−14 10−15 10−13 10−13 10−12 10−12 10−11 10−16

DOI: 10.1021/acs.est.6b05536 Environ. Sci. Technol. 2017, 51, 2954−2962

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Environmental Science & Technology × 10−12 M) in the UV/H2O2; and the concentration of halogen radicals was at least 4 orders of magnitude different. In the UV/ H2O2 process, the concentration of CO3•− (2.79 × 10−12 M) modestly exceed that of HO• (2.72 × 10−13 M), and the concentration of halogen radicals was at least 40-fold lower than of the concentration of HO•. Briefly, SO4•− reacts with Cl− to form Cl• as a primary product and then Cl• can react further with Cl− to form Cl2•−, with a reaction rate of 8.5 × 109 M−1 s−1.47 Next, Cl• and Cl2•− can react with Br− to produce ClBr•−.36,48 The [Cl2•−]SS was 10-fold higher than that of SO4•−. In addition, because Cl2•− is a strong oxidant with a redox potential of 2.0 V,23 Cl2•− could partially contribute to the removal of PPCPs during SO4•− mediate oxidation. The concentrations of bromine radicals (Br•, Br2•−, and BrCl•−) are 1−2 orders of magnitude higher than that of SO4•−, and the redox potential of Br2•− is 1.63 V,49 indicating the promising participation of bromine radicals in the removal of PPCPs. The average formation reaction rate of BrCl•− (3.04 × 109 M−1 s−1) is similar as that of Br2•− (4.52 × 109 M−1 s−1), however, the average scavenge reaction rate of BrCl•− (1.45 × 109 M−1 s−1) is approximately 6-fold higher than that of Br2•− (5.13 × 108 M−1 s−1). Furthermore, BrCl•− is primarily formed from the reaction of Cl• and Br− with the reaction rate of 1.2 × 1010 M−1 s−1. Therefore, the steady-state concentration of Br2•− was 3-fold higher than that of BrCl•− despite of the low concentration of bromide (5.8 μM) as shown in Table 3. The highest concentration of inorganic radicals in the UV/PDS treatment is CO3•−, which was modeled to be 600-fold greater than that of SO4•−. Although HCO3−/CO32− played a minor role in quenching SO4•− (0.8%), Cl• and Cl2•− can react with HCO3−/CO32− to yield CO3•−, with a reaction rate of 107−108 M−1 s−1, as shown in eq 22-25.36 Therefore, the effects of CO3•− on the removal of PPCPs should be considered in the following kinetic model. Cl• + CO3− → Cl− + CO3•−

5.0 × 108 M−1 s−1

k′ = kPPCP,HO • /SO4 •− × [HO• /SO4•−]SS + kPPCP,CO3 •− × [CO3•−]SS + kd

The predicted results based on eq 26 are shown in Figure 4. In the HO•−mediated AOPs, the influence of CO3•− on the k′

Figure 4. Relationship between the experimental pseudo first-order reaction rate and the calculated pseudo first-order reaction rate of 15 PPCPs based on eq 26 in the UV/H2O2 and UV/PDS AOPs. [HO•]SS = (1.03 ± 0.01) × 10−13 M; [SO4•−]SS = (2.21 ± 0.06) × 10−13 M; [CO3•−]SS = (2.79 ± 0.01) × 10−12 M in the UV/H2O2; [CO3•−]SS = (5.80 ± 0.09) × 10−11 M in the UV/PDS; initial concentration of PPCPs [PPCP]0 = 1.0 μg L−1; [DOC] = 4.78 mgC L−1.

values of the PPCPs is not significant. As shown in Table 2, the bimolecular reaction rate constants for the reactions of CO3•− with PPCPs are 2 to 4 orders of magnitude lower than the rate constants of the reactions of HO• with PPCPs. Despite the 10fold higher CO3•− concentrations than HO• concentrations, both of bimolecular reaction rate constants of CO3•− and [CO3•−]SS indicated that the influence of CO3•− on the removal of PPCPs is negligible in HO•-mediated oxidation under the conditions considered in this experiment. For the SO4•−-mediated AOPs, the accuracy of the modified kinetic model was significantly improved. As shown in Figure 4, most of the calculated k′ values were within 0.5−2 of the experimental factors, except for the calculated k′ values for nitroimidazoles and salbutamol. The experimental pseudo firstorder reaction rates of salbutamol are still higher than the calculated rates, indicating the participation of other radicals, such as halogen radicals, in its degradation. However, the group of nitroimidazoles presented significant lower experimental decay rates than the decay rates calculated for SO4•−-mediated oxidation. To explain this unexpected result, we hypothesized that nitroimidazoles might associate with EfOM, which would result in a significantly lower SO4•− reaction capacity than the free style nitroimidazoles due to the steric and ionic effects of EfOM toward SO4•−. Herein, the association of PPCPs with EfOM is also expressed based on their sorption behaviors. Consequently, the sorption ratios of all the tested PPCPs on EfOM were determined by using equilibrium dialysis (for additional details see the SI). The results demonstrated that only 7 PPCPs presented detectable sorption ratios, including propranolol, salbutamol and nitroimidazoles. However, propranolol and salbutamol showed a higher experimental decay rates than calculated ones, this might

(22)

Cl• + HCO3− → Cl− + H+ + CO3•− 2.2 × 108 M−1 s−1 Cl 2•− + CO3− → 2Cl− + CO3•−

(23)

1.6 × 108 M−1 s−1 (24)

Cl 2•− + HCO3− → 2Cl− + H+ + CO3•− 8.0 × 107 M−1 s−1

(26)

(25)

The Involvement of Carbonate Radicals in the Kinetic Model for Predicting the Degradation of PPCPs. Because the method for measuring the bimolecular reaction rate constants of halogen radicals with PPCPs was not available, the kinetic models involving secondary radicals only considered the carbonate radical. The reported redox potential of CO3•− at pH 8.4 is 1.63 V,45 indicating its moderate oxidation capability. Thus, CO3•− may partly participate in the removal of PPCPs in the treatments. The bimolecular reaction rate constants of CO3•− with 13 PPCPs were determined in this study (SI Figure S3) and are listed in Table 2. The pseudo first-order rate constants should be modified to be the summation of the reaction rate constants for the reactions of PPCPs with SO4•− or HO• and CO3•−, as shown in eq 26. 2959

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ENVIRONMENTAL SIGNIFICANCE For HO•-mediated oxidation, EfOM is the main scavenger of HO• followed by Br−. CO3•− and HO• are two predominant reactive species. Compare to the fast HO• rate constants, the slow rate constants of the reactions of CO3•− with PPCP result in its negligible effect on the removal of PPCPs in UV/H2O2 treatment. A simplified kinetic model only involving bimolecular reaction rate constants and steady state HO• concentrations is suitable for predicting the removal of PPCPs during UV/H2O2 treatments, indicating that HO• plays a dominant role in the removal of PPCPs. In the UV/ PDS treatment, the chloride ion was considered as the dominant scavenger of SO4•−. The steady state concentrations of the secondary radicals were higher than that in UV/H2O2 treatment, indicating the more significant influence of inorganic ions on SO4•−. [CO3•−]SS and [Br•]SS are several orders of magnitude higher than [SO4•−]SS, especially [CO3•−]SS, which is 600-fold higher than [SO4•−]SS. CO3•− should be considered in the kinetic model in the UV/PDS treatment to improve the prediction ability of the model. However, the sorption of PPCPs on EfOM is an area of concern. Steric effects and electronic repulsion would lead to the attenuation of the reaction rate of SO4•− with the PPCPs absorbed on the EfOM; therefore, the sorption-reduced effects must be considered for the removal of PPCPs in SO4•−-mediate oxidation.

be resulted from the reaction with secondary radicals, like reactive halogen species, with these two PPCPs.50−52 As shown in Table 3, the steady state concentration of Br• is 10-fold higher than that of SO4•−. The redox potential of Br• is 1.96 V,52 indicating its promising participation on the removal of these PPCPs. However, the contribution of halogen radicals could not be quantified due to lack of the method for measuring the bimolecular reaction rate constants. Nitroimidazoles had the relatively greater ability to sorption on the EfOM among all the tested PPCPs, and the highest sorption ratio of 34.1% was observed for ornidazole (Table 4). Table 4. pKa and Sorption Ratios of PPCPs name atenolol metoprolol propranolol metronidazole dimetridazole tinidazole ornidazole ronidazole venlafaxine carbamazepine gemfibrozil ibuprofen naproxen ractopamine salbutamol

pKa1 9.43 9.43 9.50 2.58 2.81 2.30 2.72 1.32 9.26 13.94 4.75 4.41 4.84 9.62 9.23

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

pKa2 0.3 0.1 0.3 0.3 0.3 0.3 0.4 0.3 0.3 0.2 0.4 0.1 0.3 0.1 0.1

13.88 13.89 13.85 14.44

± ± ± ±

sorption ratios (%) 0.2 0.2 0.2 0.1

13.29 ± 0.2 12.99 ± 0.5 14.84 ± 0.2

9.99 ± 0.31 9.97 ± 0.26

Article

4.7 2.4 7.9 27.5 34.1 32.8

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.6b05536. Additional information as noted in the text. Three texts describing the fraction of wastewater matrix reacting with HO• and SO4•− in the UV/H2O2 and UV/PDS AOPs, determination of the sorption rates of PPCPs in the wastewater, and bimolecular reaction rate constants for carbonate radicals with PPCPs. Three figures showing the competition kinetic results of HO•, SO4•− and CO3•−. Four tables showing the gradient elution, MS details for the PPCPs detected by Agilent 6430 triplet quadrupole MS, principle reactions of the wastewater matrix with HO• or SO4•− in the UV/H2O2 or UV/PDS AOPs in the kinetic model, and parameters applied in the calculation of the formation of HO• or SO4•− in the photolysis of H2O2or PDS (PDF)

3.6

Micropollutants have been reported to absorb to DOM, resulting in the alteration of their reaction, via various sorption mechanism.53−56 Nitroimidazoles were present in neutral forms in the effluent (pH 8.2) because their pKa values ranged from 1.82 to 2.81 (Table 4). This indicated that the electrostatic interactions would not be the predominant sorption mechanism. The interaction between π electrons and the hydrogen bonds could be the predominant sorption mechanism. Previous studies have also reported that nitroimidazoles can be adsorbed to activated carbon by dispersive interactions.57 The SUVA of the effluent was 3.77 L mg−1 m−1, which suggested a relatively high aromatic content. Thus, the nitroimidazoles could be absorbed onto EfOM through dispersive interactions between the π electrons in the aromatic rings of the nitroimidazoles and EfOM. EfOM is negatively charged at neutral pH because of the deprotonation of carboxylic and phenolic moieties. Electronic repulsion from negatively charged EfOM would prevent reactions between the absorbed nitroimidazoles and SO4•−/CO3•−, resulting in a lower decay rate than that observed in the modeling results. However, this charge interference would not be anticipated in HO• oxidation processes. Consequently, these results show that the bimolecular reaction rate constants of SO4•− themselves cannot predict the removal of PPCPs in the SO4•−-mediated oxidation process; thus, other secondary radicals, such as halogen radicals and carbonate radicals, should be considered. Meanwhile the steric and ionic effects of dissolved organic matters could lead to overestimate the removal efficiencies toward to SO4•− radicals when PPCPs significantly absorbed on the OM.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-15821951698; e-mail: [email protected]. ORCID

Weihua Song: 0000-0001-7633-7919 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge partial support from the Natural Science Foundation of China (21422702, 21377030, 21677039 and 21607026), the Ministry of Science and Technology of China (2012YQ220113-4) and the China Postdoctoral Science Foundation (2016M590321). W.S. also acknowledges support from the Professor of Special Appointment (Eastern Scholar) at the Shanghai Institution of Higher Learning. 2960

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