Journal of Hazardous Materials 300 (2015) 39–47

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Kinetic and mechanistic investigations of the degradation of sulfamethazine in heat-activated persulfate oxidation process Yan Fan a,1 , Yuefei Ji a,1 , Deyang Kong b , Junhe Lu a,∗ , Quansuo Zhou a a b

College of Resources and Environmental Science, Nanjing Agricultural University, Nanjing 210095, China Nanjing Institute of Environmental Science, Ministry of Environmental Protection of PRC, Nanjing 210042, China

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• Heat-activated persulfate induced sulfamethazine oxidation.

• Aniline moiety in sulfamethazine molecule was reactive site.

• Sulfate radical was identified as the predominant oxidizing species.

• Presence of bicarbonate and chloride enhanced SMZ degradation. rearrangement followed SO2 extrusion occurred during transformation.

• Similes-type

a r t i c l e

i n f o

Article history: Received 5 May 2015 Received in revised form 23 June 2015 Accepted 24 June 2015 Available online 27 June 2015 Keywords: Heat-activated persulfate oxidation Sulfamethazine Sulfate radical Reaction products Transformation mechanisms

a b s t r a c t Sulfamethazine (SMZ) is widely used in livestock feeding and aquaculture as an antibiotic agent and growth promoter. Widespread occurrence of SMZ in surface water, groundwater, soil and sediment has been reported. In this study, degradation of SMZ by heat-activated persulfate (PS) oxidation was investigated in aqueous solution. Experimental results demonstrated that SMZ degradation followed pseudo-first-order reaction kinetics. The pseudo-first-order rate constant (kobs ) was increased markedly with increasing concentration of PS and temperature. Radical scavenging tests revealed that the predominant oxidizing species was SO4 • − with HO• playing a less important role. Aniline moiety in SMZ molecule was confirmed to be the reactive site for SO4 • − attack by comparison with substructural analogs. Nontarget natural water constituents affected SMZ removal significantly, e.g., Cl− and HCO3 − improved the degradation while fulvic acid reduced it. Reaction products were enriched by solid phase extraction (SPE) and analyzed by liquid chromatography-electrospray ionization-triple quadrupole mass spectrometry (LC-ESI–MS/MS). 6 products derived from sulfonamide S N bond cleavage, aniline moiety oxidation and Smiles-type rearrangement were identified, and transformation pathways of SMZ oxidation were proposed. Results reveal that heat-activated PS oxidation could be an efficient approach for remediation of water contaminated by SMZ and related sulfonamides. © 2015 Elsevier B.V. All rights reserved.

1. Introduction

∗ Corresponding author. Tel: +85 25 84395164; fax: +86 25 84395210. E-mail addresses: [email protected] (Y. Ji), [email protected] (J. Lu). 1 The first two authors have contributed equally to this work. http://dx.doi.org/10.1016/j.jhazmat.2015.06.058 0304-3894/© 2015 Elsevier B.V. All rights reserved.

Sulfamethazine (SMZ) is widely used in livestock and aquaculture as an antimicrobial agent and growth promoter [1]. Run-off from farms, leakage form septic tanks, and direct discharge from aquaculture are pathways responsible for the occurrence of SMZ in natural waters [2,3]. Environmental concentrations of SMZ generally range from nanogram per liter to microggram per liter.

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Y. Fan et al. / Journal of Hazardous Materials 300 (2015) 39–47

For example, SMZ concentration of 15–328 ng L−1 was detected in tropical waters in Mekong Delta, Vietnam [4]. In China, comparable concentration of SMZ was observed in natural waters, e.g., 0.53–89.1 ng L−1 in surface water of Yangtze estuary [5] and 2.05–623.27 ng L−1 in Huangpu River, Shanghai [6]. Since SMZ is used primarily as veterinary medicine, the occurrence of SMZ in natural waters reflects the contamination derived from livestock wastes and aquaculture [4–6]. Natural attenuation processes, such as adsorption, photodegradation and microbial transformation are major pathways responsible for the fate of SMZ in environment [7,8]. For instance, photodegradation and adsorption to sediments were found to be the major pathways controlling SMZ’s persistent in lakes [9]. Adsorption to minerals followed by oxidative transformation may be another important pathway for SMZ attenuation in environment [10]. However, owning to the lower attenuation rate and continuous releasing mode, SMZ and other antibiotics of concern show “pseudo-persistent” characteristics [2]. The widespread occurrence of these antibiotics may induce antibiotic-resistant genes,thus, have sublethal chronic and unpredicted effects on ecosystem and human health [3,11]. Therefore, developing cost-effective and environmental-friendly treatment techniques is required to removal SMZ and related antibiotics and associated risks. Advanced oxidation processes (AOPs) are viable ways to remove sulfonamides like SMZ in water [12]. Traditional AOPs based on the generation of highly oxidative hydroxyl radical (HO• ), such as Fenton (Fe2+ /H2 O2 ), electron-Fenton, UV/H2 O2 , O3 /H2 O2 have been widely applied in wastewater treatment and pollution control [13,14]. More recently, sulfate radical (SO4 •− )-based AOPs gained growing attention. SO4 •− can be produced via activation of persulfate (PS) and peroxymonosulfate (PMS) by ultraviolet light, transit metals, base and sonolysis [15]. SO4 •− has a relatively high standard reduction potential of 2.6 V [15], which is comparable to that of HO• (E0 = 2.7 V) [16]. Second-order rate constants for reactions of SO4 •− with organic compounds range from 106 to 109 M−1 s−1 [17]. SO4 •− is thought to be a more selective radical than HO• . Reactions between SO4 •− and organic compounds occurred mainly through electron transfer mechanism [17]. This characteristic renders SO4 •− being less scavenged by nontarget water constituents (e.g., NOM), therefore increasing its steady-state concentration. SO4 •− -driven AOPs have become an emerging in situ chemical oxidation (ISCO) technique for remediation of contaminated groundwater and soil [15]. Among various activation methods, heat activation is of particular interest. In heat activation, PS is usually utilized as a precursor of SO4 •− , and symmetrical cleavage of the peroxide bond results in the formation of SO4 •− , which further oxidizes the pollutants [15]. Several recent studies documented the effectiveness of heat-activated PS oxidation of herbicides [18,19], industrial chemicals [20,21], and pharmaceuticals [22,23]. Heat-activation possesses several advantages compared with other activation approaches. For instance, heat activation requires no additional chemicals, which minimizes the consumption of PS during its pre-mixing with activators [15]. Owning to its high chemical stability, PS can be delivered through a long distance to contaminant zone in aquifers prior to activation [24]. Heat-activation was also frequently employed to explore the reaction mechanisms between SO4 •− and various contaminants due to its simplicity and higher efficiency [25]. In field application, heat activation was commonly combined with in situ thermal remediation (ISTR) [15]. However, identifying the optimal operational parameters is crucial for successful application of heat-activated PS oxidation process. For example, increasing temperature facilitates PS decomposition and contaminants oxidation, therefore, shortening the remediation time [15]. However, higher temperature increases the operational cost and may also favor radical-to-radical

reactions instead of radical-to-contaminant reactions, thus, lowering the treatment efficiency [15]. In addition, the effects of various natural water constituents which are expectedly to be encountered in treatment are not well-documented and warrant further study in heat-activated PS oxidation. In this work, we attempted to assess the feasibility of employing heat-activated PS to degrade SMZ in aqueous solution. The motivation of this study is to explore a viable method to eliminate SMZ and related sulfonamide antibiotics in waters. Kinetic studies were performed to gain insights into the influence factors including temperature, PS dosage, pH, dissolved natural organic matter (NOM), chloride, and bicarbonate. Solid phase extraction followed by liquid chromatography–tandem mass spectrometry (SPE-LC–MS/MS) was used to identify reaction intermediates and products. Based on the structural elucidation of the intermediates/products, detail mechanisms and transformation pathways for SMZ oxidation by heat-activated PS were proposed. 2. Materials and methods 2.1. Reagents and materials Chemicals, suppliers, and purities are listed in the Supplementary data, Text S1. 2.2. Experimental setup Batch kinetic studies were conducted in 33 mL screw-cap cylindrical glass vials with Teflon septa at predetermined temperature (i.e., 40–60 ◦ C) controlled by a thermostated water bath (Xianou Instrument Manufacture Co., Ltd., Nanjing). The temperature range adopted herein was consistent with previous reports [18–20]. Prior to heat activation, appropriate volume of SMZ stock solution (150 ␮M) and PS stock solution (100 mM) were transferred into the vials to achieve a total 20 mL solution with predetermined molar ratio of SMZ and PS. Control experiments without PS were carried out under identical conditions and showed no loss of target compounds, indicating SMZ and substructural analogs are hydrolysis-resistant and thermally stable. The reactions for aniline (AN), sulfanilic acid (SAA), and 2-amino-4,6-dimethylpyrimidine (ADPD) were performed under identical conditions. Initial pH was adjusted by 0.01 M H2 SO4 or NaOH to desired value. No buffer was used in the present study to avoid potential reactions between these additives and SO4 •− . The effects of HCO3 − , Cl− and SRFA were studied to evaluate the effects of typical natural water constituents on heat-activated PS oxidation of SMZ. Aliquots (0.5 mL) were withdrawn at predetermined time intervals and chilled in an ice bath for 10 min to stop the reaction and kept in a 4 ◦ C refrigerator thereafter until further treatment and analysis. All the experiments were carried out in duplicates, and the data were averaged. The standard deviations were usually within 5–10% unless otherwise stated. 2.3. Analytical methods SMZ was analyzed by a Hitachi L-2000 high performance liquid chromatography (Hitachi, Japan) equipped with an L-2455 diode array detector. Detailed HPLC setup can be found in Table. S1, Supplementary data. Quantification of analytes was based on multipoint standard calibration. Degradation products were concentrated by solid phase extraction (SPE) using Waters Oasis hydrophilic–liphophilic balance (HLB) cartridges. Detailed experimental procedures are given in Text S2, Supplementary data. Reaction products were identified using HPLC with tandem mass spectrometry (HPLC-MS/MS), consisting of an Agilent 1200 series HPLC coupled to a G6410B triple quadrupole mass spectrometer (Agilent Technologies, USA). Separation was accomplished using a

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41

Scheme 1. Proposed mechanism for 4-(2-imino-4,6-dimethylpyrimidin-1(2H)-yl) aniline (m/z 215) formation via Similes-type rearrangement followed by SO2 extrusion during heat-activated persulfate oxidation of SMZ.

Waters Symmetry C18 column (3.5 ␮m, 150 mm × 2.1 mm I.D.). The analytical details are presented in the Supplementary data, Text S3. Full scans as well as product ion scans were conducted to determine the quasi-molecular ions and elucidate the structures of major oxidation products. Products m/z 124 and 215 were further quantified using multiple reaction monitoring (MRM) mode with the optimized operating parameters showed in Table S2, Supplementary data. Instrument control, data acquisition and processing were performed using the associated Agilent Mass Hunter Qualitative analysis software (version B.04.00). 3. Results and discussion 3.1. Reaction kinetics 3.1.1. Effects of temperature Increasing the reaction temperature can effectively promote the decomposition of PS, thus facilitating the formation of SO4 •− [15,24]. As a result, degradation rate of SMZ increased with increasing temperature in heat-activated PS system. Correspondingly, the removal rate of SMZ increased from 20% to 70% after 30 min reaction as the temperature increased from 40 to 60 ◦ C. The oxidation of SMZ can be fitted by pseudo-first-order kinetics model described by following equation (Eq. (1)). −

d[SMZ] = kobs [SMZ] dt

(1)

where kobs is the pseudo-first-order rate constant, [SMZ] is the concentration of SMZ at reaction time t. The kobs value can be obtained by linear regression of plot of ln ([SMZ]t /[SMZ]0 ) versus t. As shown in Fig. 1(a), increasing the temperature increased kobs appreciably. The value of kobs increased by 21-fold when the temperature was elevated from 40 to 60 ◦ C. The temperature-dependency of kobs was further analyzed using Arrhenius equation (Eq. (2)). lnkobs = lnA −

Ea RT

(2)

where A is the pre-exponential factor, Ea is the apparent activation energy, R is the universal gas constant (8.314 J mol−1 K−1 ), and T is the absolute temperature. It was demonstrated in Fig. 1(b) that a linear relationship could be developed between ln kobs and 1/T. Activation energy Ea was thus determined to be 126 kJ mol−1 by

linear regression using Eq. (2). This value is comparable to that of other contaminants such as 108 ± 3 kJ mol−1 for trichloroethene [20], 141 kJ mol−1 for atrazine [18], and 166.7 ± 0.8 kJ mol−1 for diuron [19]. 3.1.2. Effects of PS concentration The concentration of PS plays a critical role in heat-activated PS oxidation of contaminants because PS concentration can directly influence the equilibrium concentration of SO4 •− [18–23]. In this study, kobs of SMZ degradation increased noticeably with an increase in PS concentration (Fig. 2(a)). Half-life time of SMZ decreased from 258 to 27 min as PS concentration increased from 0.25 to 4.0 mM. These results clearly demonstrated that increasing the concentration of PS could markedly enhance the degradation of SMZ by heat-activated PS oxidation. Fig. 2(b) shows the variation of kobs as a function of PS concentration. As seen, kobs increased linearly with increasing concentration of PS within the range investigated (i.e., 0.25–4.0 mM). Oxidation of SMZ was a result of reaction by free radicals (e.g., SO4 •− and HO• ). Thus, it indicated that at a certain temperature, the radical yield was constant and proportional to PS concentration. Thus, SMZ oxidation can be described as follows (Eqs. (3) and (4)). [SMZ] = [SMZ]0 e−kobs t

(3)

kobs =

(4)



ki ˛i [PS]

where ␣i is the yield of certain radical i (e.g., SO4 •− ) generated by heat-activated PS decomposition, and ki is the second-order-rate constant for SMZ reaction with certain radical i. 3.1.3. Effects of pH pH plays a complex role in heat-activated PS oxidation of contaminants [15,18–23]. At low pH, PS can undergo acid-catalyzed decomposition which depletes PS through non-radical pathways without producing SO4 •− [26]. At high pH, SO4 •− can be transformed to HO• by reaction with hydroxyl ion (Eq. (5)) [15]. SO4 •− + OH− → SO4 2− + HO•

(5) •−

HO•

Mechanisms for reactions of SO4 and with organic compounds could be quite different since SO4 •− reacts with organic compounds primarily through electron transfer, H-atom

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Y. Fan et al. / Journal of Hazardous Materials 300 (2015) 39–47

Fig. 1. (a) Effects of temperature on SMZ degradation by heat-activated persulfate oxidation; (b) plot of ln kobs versus T−1 for Ea calculation via Arrhenius equation. Experimental conditions: [SMZ]0 = 30 ␮M, [PS]0 = 2.0 mM, T = 40–60 ◦ C, pH 7.0, V = 20 mL, and reaction time = 360 min. Error bars represent 95% confidence intervals of replicates (n = 2). ln kobs in (b) was the mean value of duplicates.

Fig. 2. (a) Effects of persulfate concentration on SMZ degradation by heat-activated persulfate oxidation; (b) plot of kobs versus persulfate concentration. Experimental conditions: [SMZ]0 = 30 ␮M, [PS]0 = 0.25–4.0 mM, T = 50 ◦ C, pH 7.0, V = 20 mL, and reaction time = 120 min. Error bars represent 95% confidence intervals of replicates (n = 2), and kobs in (b) was the mean value of duplicates.

abstraction and addition-elimination [17,27], whereas HO• reacts preferentially via addition to C C double bonds and abstracting H from C-H, N-H, or O-H bonds [13]. Therefore, pH affects the reaction by altering the predominant oxidizing species in solution, and thus, the mechanisms of pollutants degradation. Furthermore, pH can change the speciation of target pollutants. For SMZ, when pH < pKa1 (2.79), protonation occurs on the amine group, which blocks the lone pair electron delocalizing from N atom to the aromatic ring and reduces its reactivity to electrophilic radicals (e.g., SO4 •− and HO• ). When pH > pKa2 (7.45), deprotonation of NH of sulfonamide group could reduce it electron-withdrawing effect on aniline moiety and enhance its oxidation by reactive radicals [28]. The effects of pH on heat-activated PS oxidation of SMZ are presented in Fig. 3. The kobs followed the order of pH 9 > pH 7 > pH 3 > pH 5 ≈ pH 10 > pH 11. The removal rate of SMZ is 77% and 86% at pH 7 and 9, respectively, which was plausibly attributed to the less electron-withdrawing effect of deprotonation of sulfonamide nitrogen on aniline moiety as discussed above. Similar enhanced oxidation of sulfonamides at weak basic condition has previously been reported in ozonation [28]. However, the extremely slow degradation in pH 10 and 11 was somewhat unexpected, and might be related to the different reaction mechanisms involving HO• oxidation since SO4 •− was mostly transformed to HO• [29,30]. The low degradation rate at pH 3 and 5 was possibly due to the low

Fig. 3. Variation of pseudo-first-order rate constant as a function of initial solution pH for SMZ degradation by heat-activated persulfate oxidation. Experimental conditions: pH 3.0–11.0, [SMZ]0 = 30 ␮M, [PS]0 = 2.0 mM, T = 50 ◦ C, V = 20 mL, and reaction time = 120 min. Error bars represent 95% confidence intervals of replicates (n = 2).

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43

efficiency of PS decomposition to SO4 •− at acidic condition [26]. In addition, the fraction of less reactive protonated SMZ increased with decreased pH, which could also be responsible to the observed low degradation rate of SMZ at pH 3 and 5. 3.2. Predominant oxidizing species at varying pH In order to elucidate the underlying mechanism accounting for the discrepancy of SMZ oxidation rate at different pH in heatactivated PS system, oxidizing species formed in the reaction system was examined. Both SO4 •− and HO• were likely formed and responsible for the elimination of SMZ. Formation of SO4 •− and HO• has previously been confirmed by spectroscopic methods such as electron spin resonance (ESR) [30]. In the present study, alcohols with different reactivity to SO4 •− and HO• (i.e., EtOH and TBA) were employed as radical probes. The difference in alcohol reactivity toward HO• and SO4 •− can be used to distinguish the dominant radical species in reaction solutions (Eqs. (6)–(9)) [31]. EtOH + HO• → intermidiates, k = (1.2 − 2.8) × 109 M−1 s−1

(6)

7 −1 −1 EtOH + SO•− 4 → intermediates, k = (1.6 − 7.7) × 10 M s

(7)

t-BuOH +

HO•

8

→ intermidiates, k = (3.8 − 7.6) × 10 M

−1 −1

s

(8)

5 −1 −1 t-BuOH + SO•− 4 → intermediates, k = (4 − 9.1) × 10 M s

(9)

As shown in Fig. S1(a) in Supplementary data, at pH 3, the removal rate of SMZ after 120 min reaction declined from 77% to 28% and 18% in the presence of 30 mM and 300 mM EtOH, respectively, as compared to non-EtOH controls, suggesting radicals (SO4 •− and HO• ) played an important role. The addition of 300 mM TBA exhibited weak inhibitory effect, indicating HO• played a less importance role. These results suggested that SO4 •− was the predominant reactive species at pH 3. Similar results were obtained at pH 7 (see Fig. S1(b)). Addition of EtOH at either 30 or 300 mM substantially suppressed SMZ oxidation, while adding 300 mM TBA only decreased the removal rate from 77% to 57%. Fig. S1(c) shows that the inhibitory effects of EtOH and TBA on SMZ oxidation at pH 10 was both pronounced, suggesting the importance of HO• increased. Results presented here suggest that SO4 •− was present dominantly and contributed to the oxidation of SMZ from acidic to neutral pH. At basic pH, the contribution of HO• increased, and may even became the dominant oxidizing species [29–31]. Interestingly, at pH 3 and 10, the addition of 30 mM TBA showed promoting effect. The reason for this finding is not clear and warrants further studies. 3.3. Reactive sites in SMZ molecule In order to identify the reactive sites in SMZ molecule for SO4 •− attack, we compared the oxidation of SMZ with its substructural analogs including aniline (AN), 2-amino-4,6-dimethylpyrimidine (ADPD), and sulfanilic acid (SAA) (Fig. 4). As seen, the kobs followed the order of AN > SMZ > SAA ≈ ADPD. This finding suggests that aniline group in SMZ molecule is the preferred site for SO4 •− attack. The lower reactivity of SAA than AN suggests that attaching an electron-withdrawing sulfonic group at para-position reduces the reactivity of aniline group. Meanwhile, the lower reactivity of ADPD could plausibly be attributed to the electron negative effect of the pyrimidine ring. Given the conspicuously low reactivity of ADPD and SAA relative to SMZ, we speculated that the higher reactivity of SMZ should be resulted from the specific molecular structure of SMZ rather than pyrimidine ring and p-amino-benzenesulfonic moiety itself.

Fig. 4. Comparison of SMZ oxidation with substructural analogs: aniline (AN), sulfanilic acid (SAA), and 2-amino-4,6-dimethylpyrimidine (ADPD). Reaction conditions: [Substance]0 = 30 ␮M, [PS]0 = 2 mM, T = 50 ◦ C, pH 7.0, and V = 20 mL. Error bars represents 95% confidence intervals of duplicates.

3.4. Reaction products and transformation pathways Heat-activated PS oxidation of SMZ yielded a series of intermediates/products which were identified by HPLC–MS/MS. Fig. S2 in Supplementary data shows a ESI(+) MS spectrum of a SPE concentrated reaction sample of SMZ under full scan mode, in which several intermediates/products with different m/z were observed as well as the parent SMZ (m/z 279). Molecular structures of these intermediates/products were tentatively elucidated by interpretation of the molecular ion masses and MS–MS fragmentation patterns (Supplementary data, Figs. S3–S8) as well as comparison with previous studies on SMZ transformation in other oxidative processes [8–10,32,33]. Totally, 6 intermediates/products, namely, 2-amino-4,6-dimethylpyrimidine (ADPD, m/z 124), 4-(2-imino4,6-dimethylpyrimidin-1(2H)-yl) aniline (m/z 215), 4-(2-imino4,6-dimethylpyrimidin-1(2H)-yl) nitrosobenzene (m/z 229), 4-(2imino-4,6-dimethylpyrimidin-1(2H)-yl) hydroxyl nitrosobenzene (m/z 245), N-hydroxyl sulfamethazine (N4 -OH-SMZ, m/z 295), and 4-nitro-sulfamethazine (4-NO2 -SMZ, m/z 309) were identified. Sulfonamide bond cleavage is well-known during oxidation of sulfonamide antibiotics by SO4 •− and other oxidants [33,34]. In this work, the cleavage of sulfonamide bond of SMZ led to the formation of ADPD that was confirmed by comparison with an authentic standard. No formation of sulfanilic acid was detected, which was plausibly due to its higher water solubility (1 g L−1 ) and quick elution in C18 reversed phase column. The time-dependent evolution of ADPD was further quantified by HPLC-MS/MS analysis under MRM mode. As shown in Fig. 5, transformation of SMZ produced ADPD rapidly, and the concentration of ADPD maintained nearly constant over the time course investigated, consistent with the lower reactivity of ADPD (Fig. 4). The maximum concentration of ADPD was 0.06 ␮M, accounting for approximately 0.2% of the initial SMZ (i.e., 30 ␮M). One important product of SMZ was m/z 215, corresponding to 4-(2-imino-4,6-dimethylpyrimidin-1(2H)-yl) aniline. The MS–MS spectrum of m/z 215 (Fig. S4) was identical to that reported by Gao et al. [10], confirming its structural assignment. This product, generated by Smiles-type rearrangement followed by SO2 extrusion, was previously found during photochemical and MnO2 -mediated oxidation of SMZ [8,10]. A recent study on UV/PS oxidation of SMZ also reported the formation of this compound [33]. The aniline moiety in SMZ molecule was SO4 •− -reactive as confirmed by comparison with substructural analogs (Fig. 4), and reaction • between SO4 − and aniline could generate an aniliny radical cation [35]. Formation of aniliny radical cation has been confirmed by quantum chemical modeling calculation and transient absorption

44

Y. Fan et al. / Journal of Hazardous Materials 300 (2015) 39–47 •− 6 −1 −1 HO• + HCO− 3 → CO3 + H2 O,k = 8.5 × 10 M s

(11)

2− 2− •− 6 −1 −1 SO•− 4 + CO3 → SO4 + CO3 , k = 6.1 × 10 M s (at pH ≥ 11)

(12)

− 2− • 6 −1 −1 SO•− 4 + HCO3 → SO4 + HCO3 , k = 1.6 × 10 M s (at pH8.4)

(13)

HCO•3 ↔ H+ + CO•− 3 , pKa = 9.5

(14)

HO•

Fig. 5. Time-dependent evolution of 2-amino-4,6-dimethylpyrimidine (ADPD, m/z 124) and 4-(2-imino-4,6-dimethylpyrimidin-1(2H)-yl) aniline (m/z 215) during heat-activated persulfate oxidation of SMZ. Experimental conditions: [SMZ]0 = 30 ␮M, [PS]0 = 2.0 mM, T = 50 ◦ C, V = 20 mL, and reaction time = 120 min. Note that the concentration of ADPD was amplified by 10-fold in the figure.

spectroscopy studies on single-electron oxidation of sulfonamides [36]. It was reported that the para-position of aniliny radical cation of sulfadiazine could be subjected to intermolecular nucleophilic attack in which a pyrimidine nitrogen acted as a nucleophile [10,36]. This attack could lead to the intermolecular rearrangement (Smiles-type), resulting in SO2 extrusion [10,36]. In the present study, the ␣-carbon adjacent to the sulfonamide group possessed strong positive charge in the resulting SMZ radical cation, which facilitated the nucleophilic attack of pyrimidine nitrogen. A detailed mechanism responsible for m/z 215 formation was proposed in Scheme 1. The concentration of m/z 215 increased markedly after the reaction was initialized, reaching maximum around 40–60 min, and then decreased slowly (Fig. 5). Unfortunately, due to the lack of authentic standard, an accurate quantification of m/z 215 and mass balance calculation was impossible. The aniline moiety within m/z 215 molecule could be further oxidized to nitroso-derivatives (m/z 229), which underwent further reaction leading to the formation of hydroxyl nitroso-derivatives (m/z 245). Another important transformation pathway of SMZ oxidation was the stepwise oxidation of aniline moiety, resulting in N4 OH-SMZ (m/z 295) and 4-NO2 -SMZ (m/z 309), sequentially. This pathway has previously been proposed in Co2+ catalyzed peroxymonosulfate oxidation of sulfamethoxazole [37]. Oxidation of aniline moiety provided further evidence that such functionality was the active site in SMZ molecule for SO4 •− attack. Detail transformation pathways responsible for SMZ oxidation by heatactivated PS are described in Scheme 2.

•− ,

and SO4 Compared with is less reactive with a second-order-rate constant for reactions with organic compounds ranging from 103 to 108 M−1 s−1 [40,41]. Meanwhile, HCO3 • /CO3 •− is more selective, and shows high reactivity only toward electron-rich compounds. For example, phenolic and aniline compounds have higher reactivity with HCO3 • /CO3 •− [41,42]. In such cases, the importance of HCO3 • /CO3 •− may overweight HO• and/or SO4 •− due to its longer life-time and higher steady-state concentration in aqueous solution [43]. Since SMZ contains aniline moiety, the enhancement in SMZ oxidation rate was likely a result of HCO3 • /CO3 •− reaction, which offset the negative effect of SO4 •− and/or HO• scavenging. Similar enhancement of HCO3 − amendment has also been observed by Hu et al. in photocatalytic oxidation of sulfamethoxazole [44]. Note that, the effect of pH changed by HCO3 − addition could be ruled out since the initial solution pH was maintained at 8.5 for each case. In SO4 •− -based AOPs, the halides could be oxidized by SO4 •− to reactive halogen species (RHS) [38,45]. For instance, reactive chlorine species including both reactive chlorine radicals (e.g., Cl• , Cl2 •− and ClOH•− ) and free chlorine (Cl2 , HOCl and OCl− ) could be produced by a suite of reactions initialized by oxidation of Cl− by SO4 •− . These reactive chlorine species are moderate oxidants which can react with electron-rich compounds such as phenols, leading to the formation of chlorinated compounds (Eqs. (15)–(20)) [46]. In the present study, the presence of Cl− at lower concentration (e.g., 5 and 10 mM) had negligible effect on SMZ oxidation. However, higher concentration of Cl− (e.g., 100 and 200 mM) promoted SMZ oxidation appreciably (Fig. 6(b)). The enhancement in SMZ oxidation with increasing Cl− concentration was likely due to the contribution of reactions involving reactive chlorine species, which compensated the depletion of SO4 •− and/or HO• due to Cl− scavenging. − • 2− •− 2− SO•− 4 + Cl ↔SO4 + Cl , E0 (SO4 /SO4 ) = 2.6V •

Cl + Cl



• •− ↔Cl•− 2 , E0 (Cl /Cl2 )

= 2.41V





Cl2(aq) + H2 O → HOCl + H + Cl , E0 (HOCl/Cl ) = 1.48V

3.5. Effects of natural water constituents

R In the present study, HCO3 − , Cl− and natural organic matter (NOM) were selected to systematically investigate their effects on heat-activated PS oxidation of SMZ because they are ubiquitously present in natural waters and have pronounced impacts on SO4 •− based oxidation processes [38,39]. Fig. 6(a) shows that the presence of HCO3 − exhibited promoting effect on heat-activated PS oxidation of SMZ over the concentration investigated (i.e., from 0 mM to 50 mM). HCO3 − and CO3 2− can quench HO• and SO4 •− as radical scavengers to produce carbonate radical (HCO3 • /CO3 •− ) (Eqs. (10)–(14)) [38,39]. (10)



+ Cl•− 2



→ R- Cl + Cl

R- H + HOCl → R- Cl + H2 O

(15) (16)

•− − − Cl•− 2 + Cl2 → Cl2 + 2Cl , E0 (Cl2 /Cl ) = 1.36V +

•− − 8 −1 −1 HO• + CO2− 3 → CO3 + OH , k = 3.9 × 10 M s

HCO3 • /CO3 •−

(17) (18) (19) (20)

Natural organic matter (NOM) is known as a radical sink because ERMs (electron-rich moieties) within NOM molecular structure can be readily attacked by electrophilic radicals such as SO4 •− and HO• [47,48]. Thus, NOM was found to play a negative role in AOPs in previous studies including SO4 •− -based oxidation processes [18,22]. In this study, SRFA was used as a representative of aquatic NOM and its effect on SMZ oxidation was investigated over the range of 0–10 mg L−1 . Fig. 6(c) depicts the presence of SRFA showed inhibitory effect on SMZ oxidation, consistent with the radical quenching effect of NOM. In addition, the antioxidant characteristic

Y. Fan et al. / Journal of Hazardous Materials 300 (2015) 39–47

45

Scheme 2. Proposed transformation pathways for SMZ degradation in heat-activated persulfate process.

of NOM may also inhibited the oxidative transformation of sulfonamides by reductively re-transferring the radical intermediates to parent compound as observed in photochemical degradation of sulfonamides and aniline moiety-bearing compounds [49,50]. 3.6. Environmental implications The antimicrobial potency of sulfonamides is derived from its antagonistic competition with p-aminobenzoic acid (PABA) for dihydropteroate synthase enzyme inside bacteria, thus inhibiting its growth and reproduction [1]. In this work, heat-activated PS oxidation of SMZ produced a series of intermediates/products with modified molecular structure of sulfanilamide. Thus, antimicrobial potency of these intermediates/products was expected to be reduced. A similar scenario was also likely to occur when other

sulfonamides were treated by SO4 •− -based AOPs. Nevertheless, additional ecotoxicology experiments are needed to confirm this hypothesis. Of the identified intermediates/products, 4-(2-imino-4,6dimethylpyrimidin-1(2H)-yl) aniline (m/z 215) is of concern because this compound has frequently been observed in both natural attenuation and advanced oxidation processes [8,10,33]. Since ecotoxicological effect of this compound is largely unknown, further studies measuring the occurrence and assessing environmental risk of such compound in natural environment is desirable. Particularly, the formation of nitrobenzene derivatives (m/z 309) should be taken into consideration because nitroaromatic compounds are generally known to be toxic and/or mutagenic [51]. In addition, we could not rule out the plausibility that some unidentified intermediates/products of SMZ may possess higher

46

Y. Fan et al. / Journal of Hazardous Materials 300 (2015) 39–47

Fig. 6. Effects of (a) HCO3 − (pH 8.5), (b) Cl− (pH 7.0), and (c) SRFA (pH 7.0) on SMZ degradation by heat-activated persulfate oxidation. Experimental conditions: [SMZ]0 = 30 ␮M, [PS]0 = 2.0 mM, T = 50 ◦ C, pH 7.0, V = 20 mL, and reaction time = 120 min. Error bars represent 95% confidence intervals of replicates (n = 2).

toxicity than parent compound as evidenced by the formation of p-benzoquinone in electrochemical treatment of antibiotic sulfachloropyridazine [52].

ing the ecotoxicological effects of SMZ transformation products are highly desirable. Acknowledgements

4. Conclusions In this study, heat-activated PS oxidation of SMZ in aqueous solution was systematically investigated. It was shown that increasing the temperature and PS dosage significantly enhanced the removal rate of SMZ. SMZ oxidation followed pseudo-firstorder reaction kinetics. SMZ could be degraded efficiently at pH ranged from 7.0 to 9.0, but higher and lower pH showed inhibitory effects. SO4 •− was identified as the predominant oxidizing species responsible for SMX degradation by radical scavengers. Cl− and HCO3 − greatly enhanced the degradation rate of SMZ, however, SRFA showed an inhibitory effect. Six intermediates/products were identified by HPLC-ESI–MS/MS. It was demonstrated that sulfonamide S N bond cleavage, aniline moiety oxidation and Smiles-type rearrangement were the primary SMZ degradation pathways in heat-activated PS system. These identified intermediates/products with modified molecular structure were expected to have less antimicrobial potency than their parent compound. We concluded that heat-activated PS was an efficient approach for eliminating SMZ in waters, but practical application of this technology under environmentally realistic condition should be investigated further because naturally occurring constituents may have deep impacts on SMZ degradation. Further studies assess-

This work was supported by China Postdoctoral Research Funds (2015M570454), Jiangsu Planned Projects for Postdoctoral Research Funds (1402013A), Fundamental Research Funds for the Central Universities (KYZ201407), and the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institute. The content of the paper does not necessarily represent the views of the funding agencies. We gratefully acknowledge two anonymous reviewers for their valuable comments and constructive suggestions. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jhazmat.2015.06. 058 References [1] C. Walsh, Antibiotics: Action, Origins, Resistance, ASM Press, Washington, DC, 2003. [2] K. Kümmerer, Antibiotics in the aquatic environment-a review – Part I, Chemosphere 75 (2009) 417–434.

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Kinetic and mechanistic investigations of the degradation of sulfamethazine in heat-activated persulfate oxidation process.

Sulfamethazine (SMZ) is widely used in livestock feeding and aquaculture as an antibiotic agent and growth promoter. Widespread occurrence of SMZ in s...
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