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Thermally activated persulfate (TAP) oxidation of antiepileptic drug carbamazepine in water
Chemical Engineering Journal 228 (2013) 765–771
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Thermally activated persulfate (TAP) oxidation of antiepileptic drug carbamazepine in water Jing Deng a, Yisheng Shao a,b,⇑, Naiyun Gao a, Yang Deng c, Shiqing Zhou a, Xuhao Hu a a
State Key Laboratory of Pollution Control Reuse, Tongji University, Shanghai 200092, China China Academy of Urban Planning & Design, Beijing 100037, China c Department of Earth and Environmental Studies, Montclair State University, Montclair, NJ 07043, USA b
h i g h l i g h t s Thermally activated persulfate (TAP) technology can decompose CBZ efficiently. Sulfate radicals play the primary role in TAP oxidation. The best CBZ degradation can be achieved at acidic conditions. Coexisting anions and cations exhibit opposite effect on the CBZ degradation. Six intermediate products are identified using LC–MS/MS.
a r t i c l e
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Article history: Received 21 November 2012 Received in revised form 30 April 2013 Accepted 4 May 2013 Available online 21 May 2013 Keywords: Thermally activated persulfate (TAP) Carbamazepine (CBZ) Oxidation Kinetics Intermediate products
a b s t r a c t Sulfate radicals-based advanced oxidation processes have been applied in water treatment and in situ chemical oxidation. Batch experiments were conducted to investigate the influencing factors including persulfate dosage, initial carbamazepine (CBZ) concentrations, solution pH, coexisting inorganic anions and cations on the decomposition of CBZ using thermally activated persulfate (TAP) technology. The results showed that TAP oxidation was efficient process for the CBZ degradation in water. The generation of sulfate radicals was accounted for the CBZ degradation in TAP system. The CBZ degradation rate constant increased as persulfate dosage increased and decreased as the initial CBZ concentrations increased. The CBZ decomposition rate decreased with the increasing pH and the best degradation occurred at pH 3. The exception was the strong alkaline condition under which a higher CBZ degradation performance was achieved. Coexisting inorganic anions slowed down the CBZ degradation to different degrees and the 2 inhibiting effect abided by the following order: CO2 3 > HCO3 > Cl > SO4 > NO3 . In contrast, coexisting cations could significantly enhance the CBZ degradation, and the promoting effect was in the order of Fe2+ > Cu2+ > Fe3+. In this study, six major intermediate products were generated during the TAP oxidation. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Concern regarding the occurrence of pharmaceuticals and their metabolites, commonly referred to as pharmaceutically active compounds (PhACs) in water environment has been growing. The ever-increasing demand and consumption of pharmaceuticals, combined with an incomplete metabolism in human body, have led to increasing concentrations in wastewaters and associated receiving surface waters [1]. It has been reported that more than 80 PhACs were detected in aquatic environments in the United
⇑ Corresponding author at: State Key Laboratory of Pollution Control Reuse, Tongji University, Shanghai 200092, China. Tel.: +86 21 65982691; fax: +86 21 65986313. E-mail address: [email protected] (Y. Shao). 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.05.044
States and even in drinking water samples [2,3]. Among numerous PhACs, carbamazepine (CBZ) is one of the most widely prescribed and very important drugs for the treatment of epilepsy, trigeminal neuralgia and some psychiatric diseases. The worldwide annual consumption of CBZ is 1014 tons [4]. The effluents in different European wastewater treatment plants (WWTP) uncovered that more than 75% are not able to remove CBZ efficiently and the effluent concentrations are often reduced in the range of 0–10% only [5]. Concentrations of PhACs detected in European surface waters are usually in the range of several ng L1. However, maximum concentrations of CBZ can reach up to several lg L1 [6]. Therefore, the elimination of CBZ from drinking water and wastewater has become a topic of concern. Due to its resistance to the conventional water and wastewater treatment processes, advanced oxidation processes (AOPs) have been recently applied
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to remove CBZ in aqueous solutions. Keen et al. [7] reported that the biodegradation of CBZ can be enhanced after UV/H2O2 advanced oxidation. Vogna et al. [8] also found that UV/H2O2 oxidation treatment caused an effective removal of CBZ and complete CBZ abatement was obtained after 4 min reaction. In addition, other AOPs such as O3/UV/H2O2 [9], UV/TiO2 [10], UV/Fe2+/H2O2 [11] and US/Fe0/H2O2 [12] were also proved to be efficient processes in the degradation of CBZ. All these oxidation techniques mentioned above are generally characterized by the generation of highly reactive hydroxyl radicals (OH), which can rapidly oxidize a broad range of organic pollutants with a high second order rate constant (108–109 M1 s1) [13]. Recently, sulfate radicals-based advanced oxidation processes (SR-AOPs) have gained great interest in water treatment and in situ chemical oxidation (ISCO). Sulfate radicals (SO 4 ) are more selective than hydroxyl radicals for the oxidation of organic pollutants [14]. Furthermore, better mineraliazation could be achieved through the oxidation by sulfate radicals [15]. Due to its relatively high stability, aqueous solubility and low cost, persulfate is commonly used as a source of sulfate radicals in field application. Activation of persulfate can be accomplished by various methods, such as heat, base, transition metals and ultraviolet light. Among these methods, thermally activated persulfate (TAP) is considered as a clean source of sulfate radicals and has already been applied for the removal of numerous organic contaminants [16–19]. In this study, we systematically investigated the oxidative degradation of CBZ by TAP technology in laboratory scale. The present work aimed at providing insight into the technical feasibility of TAP oxidation for CBZ. Firstly, the degradation kinetics and mechanism in TAP system was elucidated. Furthermore, some key operating parameters including temperature, persulfate dosage, initial CBZ concentrations, solution pH, coexisting anions and cations were also evaluated. Finally, the reaction intermediates of CBZ oxidation were identified using LC–MS/MS. From the environmentally-friendly point of view, TAP oxidation could be a promising technology for the treatment of PhACs in water. 2. Materials and methods 2.1. Materials All chemicals were at least analytical grade, expect as noted. CBZ (99.0%) was obtained from J&K Chemical Co. Ltd. (Shanghai, China). Methanol, acetonitrile (HPLC grade) and phenol (P99.5%) were purchased from Sigma–Aldrich Chemical Co. Ltd. (USA). The other reagents including zero-valent iron (Fe0, P98%), sodium persulfate (Na2S2O8, P99.5%), tert-butyl alcohol (TBA, P99.5%), sodium nitrate (NaNO3, P99.0%), sodium sulfate (Na2SO4, P99.0%), sodium chloride (NaCl, P99.5%), sodium carbonate (Na2CO3,P99.8%), sodium bicarbonate (NaHCO3, P99.8%), calcium sulfate (CaSO4, P97.0%), manganese sulfate monohydrate (MnSO4H2O,P99.0%), copper sulfate pentahydrate (CuSO45H2O, P99.0%), ferrous sulfate heptahydrate (FeSO47H2O, P99.0%) and ferric nitrate nonahydrate (Fe(NO3)39H2O, P98.5%) were supplied by Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). All the solutions were prepared in deionized water provided by a MilliQ water system (18 MX tcm, Millipore, USA). 2.2. Experimental procedures The stock solutions of CBZ (80 lM) and sodium persulfate (80 mM) were prepared before the experiments. If necessary, the desirable experimental concentrations of solutions were diluted with deionized water. Batch experiments were carried out in a series of 40-mL brown borosilicate glass bottles equipped with Teflon-
lined screw caps. All the bottles were placed in a constant temperature water bath stirring reactor (DHJF-2005, Zhengzhou Changcheng Science and Industry Trade Co. Ltd. China). The solution pH was adjusted to desirable values by adding 0.2 M H2SO4 or NaOH solutions to each bottle. Once the bottles placed into the water bath stirring reactor, the activated reaction began. At designated time intervals, samples were sacrificed from each bottle and 100 lL methanol was immediately added. After that, the sample vials were cooled at 4 °C in an ice bath for about 5 min to further quench the reaction. All batch tests were performed in triplicate at least to ensure the reproducibility and to estimate experimental errors. 2.3. Analytical methods The concentrations of CBZ were quantified using a reversedphase high performance liquid chromatograph (HPLC) (Waters 2010, USA). CBZ was detected using a UV–Vis detector (Waters 2489) at an excitation wavelength of 286 nm. The column used was a reversed-phase Symmetry C18 column (250 mm 4.6 mm i.d., 5 lm, Waters, USA). The injection volume of sample was 20 lL and the column temperature was maintained at 35 °C. The mobile-phase solvent profile was 40% Milli-Q deionized water and 60% acetonitrile at a constant flow rate of 1 mL min1. Total Organic Carbon (TOC) was monitored for 4 h using a TOC analyzer (TOC-L, Shimadzu) equipped with an ASI-V autosampler. A pH meter (PHS-3G, Leici Corp., China) equipped with a pH electrode was used to monitor pH. For the identification of intermediate products, samples were analyzed using a LC–MS/MS system that included a Waters HPLC (2010, USA) coupled with a triple stage quadrupole mass spectrometer (Thermo Scientific TSQ Quantum Access MAX, USA) with positive-mode electrospray ionization (ESI+). Separation was performed with a C18 HPLC capillary column (100 2.1 mm i.d., 5 lm, Thermo, USA). The flow rate was set to 0.24 mL min1, being eluent A deionized water (containing 0.1% formic acid) and eluent B acetonitrile. The initial conditions of the elution gradient programmed were 90%A:10%B. From 5 to 30 min the eluent B was increased to 70%, held for 10 min and returned to initial conditions in 10 min. The injection volume of sample was set at 20 lL. 3. Results and discussion 3.1. TAP oxidation for CBZ degradation Temperature is a critical parameter in TAP system, which will determine the degree of persulfate activation. Fig. 1 shows the semi-logarithmic graph of [CBZ]/[CBZ]0 under different temperatures as a function of reaction time. As exhibited, the excellent linear fits (R2 > 0.99) of experimental data indicated pseudofirst-order reaction kinetics of the CBZ degradation:
ln
½CBZ ¼ kapp t ½CBZ0
ð1Þ
where kapp (min1) is pseudo-first-order rate constant; [CBZ]0 and [CBZ] (lM) are the molar concentrations of CBZ at time 0 and t, respectively. As shown, the CBZ degradation using TAP oxidation is temperature dependent and the degradation rate significantly increases as the temperature increases. Only 11.74% CBZ is removed at 40 °C over 120 min; as the temperature increases to 70 °C, almost complete CBZ removal is observed over 80 min. When the temperature increases from 40 to 70 °C, the degradation rate constants increase approximately 60 fold (from 0.0010 to 0.0566 min1). This observation is proved by other researchers that more active species
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Fig. 1. CBZ degradation using TAP oxidation under different temperatures. Experimental conditions: [CBZ]0 = 40 lM; [PS]0 = 1 mM; [pH]0 = 5.13; T = 40–70 °C; reaction time = 120 min.
Fig. 2. CBZ degradation using TAP oxidation in the presence of different scavengers. Experimental conditions: [CBZ]0 = 40 lM, [PS]0 = 1 mM, [Phenol] = [EtOH] = [TBA] = 400 mM, T = 60 °C, reaction time = 120 min.
(sulfate or hydroxyl radicals) may yield at higher temperatures resulting in faster target pollutants degradation [18,19]. As can be seen from the inset image of Fig. 1, ln kapp shows a good linear relationship with 1/T, which illustrates that the thermal activated reaction is accordance with the Arrhenius equation (R2 = 0.999). The Arrhenius equation and its linear form can be expressed as follows:
Table 1 Second-order rate constants for reactions of selected scavengers with hydroxyl radicals.
kapp ¼ A expðEa =RTÞ
ð2Þ
ln kapp ¼ ln A Ea =RT 1
Scavengers
Phenol Ethanol (EtOH) tert-butyl alcohol (TBA)
Reaction rate constants (M1 s1) Sulfate radicals
Hydroxyl radicals
8.8 109 (1.6–7.7) 107 (4–9.1) 105
6.6 109 (1.2–2.8) 109 (3.8–7.6) 108
Reference
[20] [21] [21]
ð3Þ 1
where A (kJ tmol ) is the Arrhenius constant; Ea (kJ mol ) is the apparent activation energy, R is the universal gas constant (8.314 103 kJ(mol K)1), T is absolute temperature (K). The Arrhenius constant (A) and activation energy (Ea) for the CBZ degradation by TAP oxidation are 39.4 ± 0.9 and 120.4 ± 2.6 kJ mol1, respectively. Ghauch and his colleagues [16,17] also applied TAP oxidation for the removal of beta-blockers bisoprolol (BIS) and anti-inflammatory drugs ibuprofen (IBU). In this study, the calculated activation energies were 119.8 ± 10.8 and 168.0 ± 9.5 kJ mol1, respectively, which elucidated that TAP oxidation had the similar degradation ability of CBZ and BIS. However, compared to the aforementioned two pharmaceuticals, IBU was more difficult to remove by TAP oxidation. 3.2. Degradation mechanism in TAP system In this study, we access the degradation mechanism and the predominant species which play the major role through adding excessive radical scavengers. Fig. 2 shows the effect of different scavengers including EtOH, phenol and TBA on the CBZ degradation using TAP oxidation. Second-order rate constants for reactions of selected scavengers with hydroxyl radicals and sulfate radicals are listed in Table. 1. As shown, 87.02% CBZ is removed in the absence of any scavenger. When 400 mM EtOH and phenol are added into solutions, only 21.58% and 0.85% CBZ are removed, respectively. EtOH and phenol both exhibit strong inhibiting effect in TAP system. Compared with EtOH, phenol exerts stronger inhibiting effect because of its higher reaction rate with sulfate radicals (several hundred fold greater than that with EtOH). However, 52.58% CBZ removal is observed in the presence of the same amount of TBA. TBA exhibits high reaction rate with hydroxyl radicals and relatively slow reaction rate. Many previous studies used TBA as the scavenger for hydroxyl radicals. Qi et al. [22] reported that the ozonation of 2,4,6-trichloroanisole (TCA) catalyzed by
the raw bauxite was strong inhibited in the presence of TBA. In this study, due to its relatively low reaction rate with sulfate radicals, higher removal of CBZ is achieved in the presence of TBA. On the whole, the slower the reaction rate is, the lower the decrease of CBZ removal is. The removal of CBZ in the presence of EtOH and TBA can be attributed to the oxidation of sulfate radicals. In summary, sulfate radicals may be the dominant species for the CBZ degradation in this study. 3.3. Effect of persulfate dosage The effect of initial persulfate dosage on the CBZ degradation was investigated within the persulfate dosage ranges of 0.2– 4 mM at 60 °C. Fig. 3 plots the semi-logarithmic graph of [CBZ]/ [CBZ]0 under different persulfate dosages as a function of reaction time. As presented, the degradation rates under different persulfate dosages all abide by pseudo-first-order kinetics. As the persulfate dosage increases from 0.2 to 4 mM, the degradation rate constant of CBZ enhances from 0.0034 to 0.0744 min1. This phenomenon was also observed by other researchers when treating 1,1,1-trichloroethane (TCA) and methyl tert-butyl ether (MTBE) contaminants at various persulfate dosage in TAP system [19,23]. Furthermore, as shown in the insert image of Fig. 3, the degradation rate constant exhibits a linear trend as a function of persulfate dosage in this study (kapp = 0.0186 [PS]0 0.0016, R2 = 0.992). This is consistent with Tan et al. [18], who reported that the degradation rate of diuron was positively proportional to persulfate dosage (kapp = 0.690 [PS]0). 3.4. Effect of initial CBZ concentration The effect of initial CBZ concentration on the CBZ removal has been investigated from 10 to 80 lM at 60 °C. Fig. 4 depicts the semi-logarithmic graph of [CBZ]/[CBZ]0 under different CBZ
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Fig. 3. CBZ degradation using TAP oxidation under different persulfate dosages. Experimental conditions: [CBZ]0 = 40 lM, [PS]0 = 0.2–4 mM, T = 60 °C, reaction time = 120 min.
concentrations as a function of reaction time. As shown, the degradation rate is pseudo-first-order with respect to the initial CBZ concentrations. As the initial CBZ concentration increases, the degradation rate constant of CBZ decreases. When the CBZ concentration increases from 10 to 80 lM, the degradation rate constant accordingly decreases from 0.104 to 0.0073 min1. Theoretically, the overall amount of sulfate radicals in TAP system is invariant at the fixed persulfate dosage. At a high CBZ concentration, the part of CBZ to be degraded by sulfate radicals is small relative to the total CBZ. Different from the effect of initial persulfate dosage, the degradation rate constant shows an exponential relationship rather than a linear trend (kapp ¼ 3:14 ½CBZ1:48 , R2 = 0.989). There 0 is no similar conclusion reported using TAP oxidation in previous literatures. Similar finding was reported by Xu et al. [24], in which kapp ¼ 0:0745 ½DMP0:364 was determined for UV/H2O2 oxidation of 0 dimethyl phthalate (DMP) in water.
CBZ degradation in TAP system is pH dependent and acidic condition is more favorable than neutral and alkaline conditions. The best CBZ degradation occurs at pH 3 and its corresponding degradation rate constant is 0.0228 min1. As the pH increases from 5.02 to 8.99, the CBZ degradation rate constant accordingly decreases from 0.0182 to 0.0086 min1. The results were in agreement with the rate constants of methyl tert-butyl ether (MTBE) and diphenylamine degradation by persulfate which also decreased with an increasing pH [19]. However, when the pH reached 11.01, the degradation rate achieves a certain degree of increase. It was reported that the rate constant of persulfate decomposition increased with pH decrease under different temperature [25]. Moreover, more sulfate radicals could be generated through acidcatalyzation in acidic condition, which might accelerate the CBZ degradation [26]. The acid-catalyzation decreases with the pH increase, resulting in the reduction of degradation rate. Lee et al. [27] used microwave-activated persulfate to decompose perfluorooctanoic acid (PFOA) and the maximum PFOA degradation occurred at pH 2.5. In addition, sulfate radicals generated in solutions can lead to radical interconversion reactions to produce hydroxyl radicals [21]. Under alkaline conditions, hydroxyl radicals which exhibit a slightly higher redox potential than sulfate radicals become the dominant radical species in TAP system. However, the CBZ degradation rate decreases because of the generation of a large amount of sulfate ions which may inhibit the reactivity of hydroxyl radicals and sulfate radicals. The inhibiting effect of various anions on 4-chlorophenol degradation by hydroxyl radicals in Fenton’s 2 system was ClO4 NO 3 > SO4 > Cl HPO4 > HCO3 [28]. In addition, when hydroxyl radicals and sulfate radicals coexist in solutions, they can recombine to form HSO 4 and oxygen, which make the CBZ degradation decrease [29]. 3.6. Effect of coexisting inorganic anions
The effect of initial solution pH on the CBZ oxidation has been investigated in the pH range of 3.01–11.01 at 60 °C. Fig. 5 plots the semi-logarithmic graph of [CBZ]/[CBZ]0 under different pH as a function of reaction time. As shown, the CBZ degradation rates at all pH follow pseudo-first-order kinetics over 120 min. The
The effect of inorganic anions has been investigated through adding different inorganic anion at fixed concentration of 10 mM. Fig. 6 depicts the semi-logarithmic graph of [CBZ]/[CBZ]0 under different inorganic anion as a function of reaction time. As shown, different inorganic anion exhibit different inhibiting effect on the CBZ decomposition. The degradation rate constant is 1.756 min1 in the absence of anion; however, when 10 mM anions are added into solutions, the degradation rate constants are 1.59, 1.51, 1.39, 1.05 2 and 0.72 min1 in the presence of NO 3 , SO4 , Cl , HCO3 and 2 CO3 , respectively. The inhibition of inorganic anions on the CBZ 2 degradation is in the order of CO2 3 > HCO3 > Cl > SO4 > NO3 .
Fig. 4. CBZ degradation using TAP oxidation under different initial CBZ concentrations. Experimental conditions: [CBZ]0 = 10–80 lM, [PS]0 = 1 mM, T = 60 °C, reaction time = 120 min.
Fig. 5. CBZ degradation using TAP oxidation under different initial solution pH. Experimental conditions: [CBZ]0 = 40 lM; [PS]0 = 1 mM; [pH]0 = 3.01–11.01; T = 60 °C, reaction time = 120 min.
3.5. Effect of solution pH
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Fig. 6. CBZ degradation using TAP oxidation in the presence of coexisting inorganic anions. Experimental conditions: [CBZ]0 = 40 lM; [PS]0 = 1 mM; [Anions] = 10 mM; T = 60 °C, reaction time = 120 min.
Among these inorganic anions, SO2 4 and NO3 only slightly slow down the CBZ degradation, because they almost do not react with sulfate radicals. It is well known that SO2 4 is the inevitable product of persulfate oxidation. Much higher persulfate concentrations are applied in the field which means large amounts of SO2 4 would generate accordingly. In this case, SO2 effects cannot be ignored 4 which is very valuable information for in situ chemical oxidation application. Compared with SO2 4 and NO3 , Cl can react with sul fate radicals to generate relatively weaker species such as Cl, Cl2 and ClHO [30]. It is noted that the redox potentials of the Cl containing radicals are much lower than 2.6 V of sulfate radicals [14]. Furthermore, the generated Cl2 and Cl could further react with it self to form Cl2 and Cl [31,32]. CO2 3 and HCO3 are the good scavengers for hydroxyl radicals, similar findings were reported everywhere [33]. In this study, CO2 and HCO 3 3 both significantly inhibited the degradation rate of CBZ. When CO2 and HCO 3 3 are added into solutions, it would proceed to react and eventually reach the equilibration of carbonate system. In the solutions with 10 mM initial concentration of CO2 3 (pH = 11.29) or HCO3 (pH = 8.61), the primary compositions 2 at equilibrium is 0.84 mM HCO or 9.78 mM 3 =9:16 mM CO3 2 HCO3 =0:22 mM CO3 , respectively. Similar with hydroxyl radicals, sulfate radicals can also react with CO2 and HCO 3 3 at different 2 rates [19]. Therefore, CO3 and HCO3 performs as a radical scavenger in solutions. CO2 3 exhibits more significant inhibiting effect on 2 CBZ decomposition than HCO 3 , because the reaction rate of CO3 with sulfate radicals is approximately 4 times than that of HCO3 .
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transition metals for activation of persulfate and also found that Mn2+ was an ineffective activator for persulfate. As for Cu2+, it can activate persulfate to generate more sulfate radicals [34]. Although the rate of this reaction is slow and requires a relatively high energy, the process may be accelerated in high temperature oxidation system. The other reason may be the generation of Cu3+ in TAP system, which is unstable and may act as an oxidant for CBZ decomposition [35]. The recurred Cu2+ in this process can continue to activate persulfate. Compared with Cu2+, Fe2+ is more efficient activator for persulfate to generate radicals in TAP system. In this work, once 1 mM Fe2+ is added into the solution, more than 90% CBZ are removed within 10 min. Because once the Fe2+ is attached with persulfate, a considerable amount of powerful sulfate radicals could be yielded transiently. After experiencing a rapid reaction stage, a slower decomposition stage which is not obvious in this study appears because of the accumulation of Fe3+ and the poor recovery of Fe2+. However, adding Fe3+ into TAP oxidation system can speed up the recovery of Fe2+, which can enhance the degradation rate of CBZ. Of note, several previous publications have reported that Fe2+ can react with sulfate radicals and consequently inhibited the degradation of target pollutants [36]. However, this phenomenon only happened when Fe2+ dosage was too high, excessive Fe2+ can react with sulfate radicals. This phenomenon is not observed in this study for the reason that Fe2+ dosage used is below the inhibiting point. 3.8. Identification of the intermediate products The structure for the degradation products of CBZ were proposed based on the full scan mode obtained from LC–MS/MS analysis and the information available from previous studies [12,37,38]. Figs. 8 and 9 show the total ion chromatogram and the proposed structures for the degradation products, respectively. As shown, TAP oxidation of CBZ produces 6 major intermediate products in this study. At retention time of 19.71 min, CBZ is eluted. Due to the addition of hydroxyl or carbonyl groups on the CBZ structure, more polar compounds are generated after TAP oxidation which leads to all the intermediate emerge at lesser retention time. Except for the generation of various hydroxylated intermediates (I, II), many corresponding isomers are also produced in this process, because sulfate radicals can attack the different carbon atoms of the benzene and the azepine rings. For example, intermediate I has its trans and cis stereoisomers. Intermediate III (10,11-10,11-epoxycarbamazepine), which is most common degradation product of CBZ identified in previous
3.7. Effect of coexisting inorganic cations The effect of inorganic cations has been investigated through adding different inorganic cations at fixed concentration of 1 mM. Fig. 7 presents the semi-logarithmic graphs of [CBZ]/ [CBZ]0 under different inorganic cations as a function of reaction time. As shown, the common cations in surface water and groundwater, such as Ca2+ and Mn2+ are found to have no significant impact on the decomposition of CBZ. However, other cations including Fe2+, Fe3+ and Cu2+ can greatly accelerate the degradation rate of CBZ. Compared with the blank experiments (no cations addition), the degradation rate increases about 4.47, 4.65 and 10.34 folds with addition of 1 mM Fe3+, Cu2+ and Fe2+, respectively. The promotion of inorganic cations on CBZ degradation abide by the order of Fe2+ > Cu2+ > Fe3+ > Mn2+ Ca2+. As a transition metal, the influence of Mn2+ is negligible in TAP oxidation. Anipsitakis and his colleagues [21] investigated nine
Fig. 7. CBZ degradation using TAP oxidation in the presence of coexisting inorganic cations. Experimental conditions: [CBZ]0 = 40 lM; [PS]0 = 1 mM; [Cations] = 1 mM; T = 60 °C, reaction time = 120 min.
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Fig. 8. Total ion chromatogram after background subtracted obtained after 90 min reaction in TAP system. Experimental conditions: [CBZ]0 = 40 lM; [PS]0 = 1 mM; [pH]0 = 5.18; T = 60 °C.
Fig. 9. Proposed structures for the degradation products of CBZ identified using LC–MS/MS.
literatures can be formed through the dehydration of intermediate I. Intermediate IV can be produced through the cleavage reaction of the azepine rings of intermediate III [38]. Intermediate V and VI are identified as quinine derivatives and acridone, which were also found during the photodegradation of CBZ [37]. Matta et al. [39] also identified intermediates III and V in Co2+/peroxymonosulfate system. Of note, another common intermediate product acridine cannot detect in this study. This may be attributed to the low persulfate dosage and the corresponding mineralization is only (5.7 ± 1.3)% after 4 h reaction.
4. Conclusions The kinetics and mechanism of carbamazepine (CBZ) degradation using thermally activated persulfate (TAP) oxidation were investigated in this paper. Affecting factors such as persulfate dosage, initial CBZ concentration, solution pH, coexisting inorganic anions and cations were also evaluated. The CBZ decomposition was greatly affected by temperature, persulfate dosage, initial CBZ con centration and pH. Adding CO2 3 and HCO3 in solutions can greatly slow down CBZ degradation in TAP system. On the contrary, Fe2+,
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Cu2+ and Fe3+ can significantly speed up CBZ degradation rate. Six major intermediate products were identified in this study. It was noted that TAP oxidation was an efficient technology for CBZ removal. Acknowledgements This work was supported by the National Major Project of Science & Technology Ministry of China (2012ZX07403-001), the research and development Project of Ministry of Housing and Urban-Rural Development (2009-K7-4) and the National Natural Science Fund Projects (51178321). In addition, the time and insightful recommendations of the reviewers are highly appreciated. References [1] A.R.D. Verliefde, S.G.J. Heijman, E.R. Cornelissen, G. Amy, B. Van der Bruggen, J.C. van Dijk, Influence of electrostatic interactions on the rejection with NF and assessment of the removal efficiency during NF/GAC treatment of pharmaceutically active compounds in surface water, Water Res. 41 (2007) 3227–3240. [2] D.W. Kolpin, E.T. Furlong, M.T. Meyer, E.M. Threman, S.D. Zaugg, L.B. Barber, H.T. Buxton, Pharmaceuticals, hormones, and other organic wastewater contaminants in US streams, 1999–2000: a national reconnaissance, Environ. Sci. Technol. 36 (2002) 1202–1211. [3] C.A. Kinney, E.T. Furlong, S.L. Werner, J.D. Cahill, Presence and distribution of wastewater-derived pharmaceuticals in soil irrigated with reclaimed water, Environ. Toxicol. Chem. 25 (2006) 317–326. [4] Y. Zhang, S.U. Geiben, C. Gal, Carbamazepine and diclofenac: removal in wastewater treatment plants and occurrence in water bodies, Chemosphere 73 (2008) 1151–1161. [5] P. Braeutigam, M. Franke, R.J. Schneider, A. Lehmann, A. Stolle, B. Ondruschka, Degradation of carbamazepine in environmentally relevant concentrations in water by Hydrodynamic-Acoustic-Cavitation (HAC), Water Res. 46 (2012) 2469–2477. [6] L. Puijker, M. Mons, Pharmaceuticals and personal care products in the water cycle – international review, Kiwa Report KWR 04.013, Kiwa N.V. Water Research, Nieuwegein, The Netherlands. [7] O.S. Keen, S. Baik, K.G. Linden, D.S. Aga, N.G. Love, Enhanced biodegradation of carbamazepine after UV/H2O2 advanced oxidation, Environ. Sci. Technol. 46 (2012) 6222–6227. [8] D. Vogna, R. Marotta, R. Andreozzi, A. Napolitano, M. d’Ischia, Kinetic and chemical assessment of the UV/H2O2 treatment of antiepileptic drug carbamazepine, Chemosphere 54 (2004) 497–505. [9] I. Jong-Kwon, C. Il-Hyoung, K. Seung-Kyu, Z. Kyung-Duk, Optimization of carbamazepine removal in O3/UV/H2O2 system using a response surface methodology with central composite design, Desalination 285 (2012) 306– 314. [10] T.E. Doll, F.H. Frimmel, Removal of selected persistent organic pollutants by heterogeneous photocatalysis in water, Catal. Today 101 (2005) 195–202. [11] N. Klamerth, S. Malato, M.I. Maldonado, A. Aguera, A.R. Fernandez-Alba, Application of photo-fenton as a tertairy treatment of emerging contaminants in municipal wastewater, Envion. Sci. Technol. 44 (2010) 1792–1798. [12] A. Ghauch, H. Baydoun, P. Dermesropian, Degradation of aqueous carbamazepine in ultrasonic/Fe0/H2O2 systems, Chem. Eng. J. 172 (2011) 18– 27. [13] F.L. Rosario-Ortiz, E.C. Wert, S.A. Snyder, Evaluation of UV/H2O2 treatment for the oxidation of pharmaceuticals in wastewater, Water Res. 44 (2010) 1440– 1448. [14] G.P. Anipsitakis, D.D. Dionysiou, M.A. Gonzalez, Cobalt mediated activation of peroxymonosulfate and sulfate radical attack on phenolic compounds: implications of chloride ions, Environ. Sci. Technol. 40 (2006) 1000–1007. [15] J. Criquet et al., Degradation of acetic acid with sulfate radical generated by persulfate ions photolysis, Chemosphere 77 (2009) 194–200.
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Thermally activated persulfate (TAP) oxidation of antiepileptic drug carbamazepine in water Jing Deng a, Yisheng Shao a,b,⇑, Naiyun Gao a, Yang Deng c, Shiqing Zhou a, Xuhao Hu a a
State Key Laboratory of Pollution Control Reuse, Tongji University, Shanghai 200092, China China Academy of Urban Planning & Design, Beijing 100037, China c Department of Earth and Environmental Studies, Montclair State University, Montclair, NJ 07043, USA b
h i g h l i g h t s Thermally activated persulfate (TAP) technology can decompose CBZ efficiently. Sulfate radicals play the primary role in TAP oxidation. The best CBZ degradation can be achieved at acidic conditions. Coexisting anions and cations exhibit opposite effect on the CBZ degradation. Six intermediate products are identified using LC–MS/MS.
a r t i c l e
i n f o
Article history: Received 21 November 2012 Received in revised form 30 April 2013 Accepted 4 May 2013 Available online 21 May 2013 Keywords: Thermally activated persulfate (TAP) Carbamazepine (CBZ) Oxidation Kinetics Intermediate products
a b s t r a c t Sulfate radicals-based advanced oxidation processes have been applied in water treatment and in situ chemical oxidation. Batch experiments were conducted to investigate the influencing factors including persulfate dosage, initial carbamazepine (CBZ) concentrations, solution pH, coexisting inorganic anions and cations on the decomposition of CBZ using thermally activated persulfate (TAP) technology. The results showed that TAP oxidation was efficient process for the CBZ degradation in water. The generation of sulfate radicals was accounted for the CBZ degradation in TAP system. The CBZ degradation rate constant increased as persulfate dosage increased and decreased as the initial CBZ concentrations increased. The CBZ decomposition rate decreased with the increasing pH and the best degradation occurred at pH 3. The exception was the strong alkaline condition under which a higher CBZ degradation performance was achieved. Coexisting inorganic anions slowed down the CBZ degradation to different degrees and the 2 inhibiting effect abided by the following order: CO2 3 > HCO3 > Cl > SO4 > NO3 . In contrast, coexisting cations could significantly enhance the CBZ degradation, and the promoting effect was in the order of Fe2+ > Cu2+ > Fe3+. In this study, six major intermediate products were generated during the TAP oxidation. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Concern regarding the occurrence of pharmaceuticals and their metabolites, commonly referred to as pharmaceutically active compounds (PhACs) in water environment has been growing. The ever-increasing demand and consumption of pharmaceuticals, combined with an incomplete metabolism in human body, have led to increasing concentrations in wastewaters and associated receiving surface waters [1]. It has been reported that more than 80 PhACs were detected in aquatic environments in the United
⇑ Corresponding author at: State Key Laboratory of Pollution Control Reuse, Tongji University, Shanghai 200092, China. Tel.: +86 21 65982691; fax: +86 21 65986313. E-mail address: [email protected] (Y. Shao). 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.05.044
States and even in drinking water samples [2,3]. Among numerous PhACs, carbamazepine (CBZ) is one of the most widely prescribed and very important drugs for the treatment of epilepsy, trigeminal neuralgia and some psychiatric diseases. The worldwide annual consumption of CBZ is 1014 tons [4]. The effluents in different European wastewater treatment plants (WWTP) uncovered that more than 75% are not able to remove CBZ efficiently and the effluent concentrations are often reduced in the range of 0–10% only [5]. Concentrations of PhACs detected in European surface waters are usually in the range of several ng L1. However, maximum concentrations of CBZ can reach up to several lg L1 [6]. Therefore, the elimination of CBZ from drinking water and wastewater has become a topic of concern. Due to its resistance to the conventional water and wastewater treatment processes, advanced oxidation processes (AOPs) have been recently applied
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to remove CBZ in aqueous solutions. Keen et al. [7] reported that the biodegradation of CBZ can be enhanced after UV/H2O2 advanced oxidation. Vogna et al. [8] also found that UV/H2O2 oxidation treatment caused an effective removal of CBZ and complete CBZ abatement was obtained after 4 min reaction. In addition, other AOPs such as O3/UV/H2O2 [9], UV/TiO2 [10], UV/Fe2+/H2O2 [11] and US/Fe0/H2O2 [12] were also proved to be efficient processes in the degradation of CBZ. All these oxidation techniques mentioned above are generally characterized by the generation of highly reactive hydroxyl radicals (OH), which can rapidly oxidize a broad range of organic pollutants with a high second order rate constant (108–109 M1 s1) [13]. Recently, sulfate radicals-based advanced oxidation processes (SR-AOPs) have gained great interest in water treatment and in situ chemical oxidation (ISCO). Sulfate radicals (SO 4 ) are more selective than hydroxyl radicals for the oxidation of organic pollutants [14]. Furthermore, better mineraliazation could be achieved through the oxidation by sulfate radicals [15]. Due to its relatively high stability, aqueous solubility and low cost, persulfate is commonly used as a source of sulfate radicals in field application. Activation of persulfate can be accomplished by various methods, such as heat, base, transition metals and ultraviolet light. Among these methods, thermally activated persulfate (TAP) is considered as a clean source of sulfate radicals and has already been applied for the removal of numerous organic contaminants [16–19]. In this study, we systematically investigated the oxidative degradation of CBZ by TAP technology in laboratory scale. The present work aimed at providing insight into the technical feasibility of TAP oxidation for CBZ. Firstly, the degradation kinetics and mechanism in TAP system was elucidated. Furthermore, some key operating parameters including temperature, persulfate dosage, initial CBZ concentrations, solution pH, coexisting anions and cations were also evaluated. Finally, the reaction intermediates of CBZ oxidation were identified using LC–MS/MS. From the environmentally-friendly point of view, TAP oxidation could be a promising technology for the treatment of PhACs in water. 2. Materials and methods 2.1. Materials All chemicals were at least analytical grade, expect as noted. CBZ (99.0%) was obtained from J&K Chemical Co. Ltd. (Shanghai, China). Methanol, acetonitrile (HPLC grade) and phenol (P99.5%) were purchased from Sigma–Aldrich Chemical Co. Ltd. (USA). The other reagents including zero-valent iron (Fe0, P98%), sodium persulfate (Na2S2O8, P99.5%), tert-butyl alcohol (TBA, P99.5%), sodium nitrate (NaNO3, P99.0%), sodium sulfate (Na2SO4, P99.0%), sodium chloride (NaCl, P99.5%), sodium carbonate (Na2CO3,P99.8%), sodium bicarbonate (NaHCO3, P99.8%), calcium sulfate (CaSO4, P97.0%), manganese sulfate monohydrate (MnSO4H2O,P99.0%), copper sulfate pentahydrate (CuSO45H2O, P99.0%), ferrous sulfate heptahydrate (FeSO47H2O, P99.0%) and ferric nitrate nonahydrate (Fe(NO3)39H2O, P98.5%) were supplied by Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). All the solutions were prepared in deionized water provided by a MilliQ water system (18 MX tcm, Millipore, USA). 2.2. Experimental procedures The stock solutions of CBZ (80 lM) and sodium persulfate (80 mM) were prepared before the experiments. If necessary, the desirable experimental concentrations of solutions were diluted with deionized water. Batch experiments were carried out in a series of 40-mL brown borosilicate glass bottles equipped with Teflon-
lined screw caps. All the bottles were placed in a constant temperature water bath stirring reactor (DHJF-2005, Zhengzhou Changcheng Science and Industry Trade Co. Ltd. China). The solution pH was adjusted to desirable values by adding 0.2 M H2SO4 or NaOH solutions to each bottle. Once the bottles placed into the water bath stirring reactor, the activated reaction began. At designated time intervals, samples were sacrificed from each bottle and 100 lL methanol was immediately added. After that, the sample vials were cooled at 4 °C in an ice bath for about 5 min to further quench the reaction. All batch tests were performed in triplicate at least to ensure the reproducibility and to estimate experimental errors. 2.3. Analytical methods The concentrations of CBZ were quantified using a reversedphase high performance liquid chromatograph (HPLC) (Waters 2010, USA). CBZ was detected using a UV–Vis detector (Waters 2489) at an excitation wavelength of 286 nm. The column used was a reversed-phase Symmetry C18 column (250 mm 4.6 mm i.d., 5 lm, Waters, USA). The injection volume of sample was 20 lL and the column temperature was maintained at 35 °C. The mobile-phase solvent profile was 40% Milli-Q deionized water and 60% acetonitrile at a constant flow rate of 1 mL min1. Total Organic Carbon (TOC) was monitored for 4 h using a TOC analyzer (TOC-L, Shimadzu) equipped with an ASI-V autosampler. A pH meter (PHS-3G, Leici Corp., China) equipped with a pH electrode was used to monitor pH. For the identification of intermediate products, samples were analyzed using a LC–MS/MS system that included a Waters HPLC (2010, USA) coupled with a triple stage quadrupole mass spectrometer (Thermo Scientific TSQ Quantum Access MAX, USA) with positive-mode electrospray ionization (ESI+). Separation was performed with a C18 HPLC capillary column (100 2.1 mm i.d., 5 lm, Thermo, USA). The flow rate was set to 0.24 mL min1, being eluent A deionized water (containing 0.1% formic acid) and eluent B acetonitrile. The initial conditions of the elution gradient programmed were 90%A:10%B. From 5 to 30 min the eluent B was increased to 70%, held for 10 min and returned to initial conditions in 10 min. The injection volume of sample was set at 20 lL. 3. Results and discussion 3.1. TAP oxidation for CBZ degradation Temperature is a critical parameter in TAP system, which will determine the degree of persulfate activation. Fig. 1 shows the semi-logarithmic graph of [CBZ]/[CBZ]0 under different temperatures as a function of reaction time. As exhibited, the excellent linear fits (R2 > 0.99) of experimental data indicated pseudofirst-order reaction kinetics of the CBZ degradation:
ln
½CBZ ¼ kapp t ½CBZ0
ð1Þ
where kapp (min1) is pseudo-first-order rate constant; [CBZ]0 and [CBZ] (lM) are the molar concentrations of CBZ at time 0 and t, respectively. As shown, the CBZ degradation using TAP oxidation is temperature dependent and the degradation rate significantly increases as the temperature increases. Only 11.74% CBZ is removed at 40 °C over 120 min; as the temperature increases to 70 °C, almost complete CBZ removal is observed over 80 min. When the temperature increases from 40 to 70 °C, the degradation rate constants increase approximately 60 fold (from 0.0010 to 0.0566 min1). This observation is proved by other researchers that more active species
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Fig. 1. CBZ degradation using TAP oxidation under different temperatures. Experimental conditions: [CBZ]0 = 40 lM; [PS]0 = 1 mM; [pH]0 = 5.13; T = 40–70 °C; reaction time = 120 min.
Fig. 2. CBZ degradation using TAP oxidation in the presence of different scavengers. Experimental conditions: [CBZ]0 = 40 lM, [PS]0 = 1 mM, [Phenol] = [EtOH] = [TBA] = 400 mM, T = 60 °C, reaction time = 120 min.
(sulfate or hydroxyl radicals) may yield at higher temperatures resulting in faster target pollutants degradation [18,19]. As can be seen from the inset image of Fig. 1, ln kapp shows a good linear relationship with 1/T, which illustrates that the thermal activated reaction is accordance with the Arrhenius equation (R2 = 0.999). The Arrhenius equation and its linear form can be expressed as follows:
Table 1 Second-order rate constants for reactions of selected scavengers with hydroxyl radicals.
kapp ¼ A expðEa =RTÞ
ð2Þ
ln kapp ¼ ln A Ea =RT 1
Scavengers
Phenol Ethanol (EtOH) tert-butyl alcohol (TBA)
Reaction rate constants (M1 s1) Sulfate radicals
Hydroxyl radicals
8.8 109 (1.6–7.7) 107 (4–9.1) 105
6.6 109 (1.2–2.8) 109 (3.8–7.6) 108
Reference
[20] [21] [21]
ð3Þ 1
where A (kJ tmol ) is the Arrhenius constant; Ea (kJ mol ) is the apparent activation energy, R is the universal gas constant (8.314 103 kJ(mol K)1), T is absolute temperature (K). The Arrhenius constant (A) and activation energy (Ea) for the CBZ degradation by TAP oxidation are 39.4 ± 0.9 and 120.4 ± 2.6 kJ mol1, respectively. Ghauch and his colleagues [16,17] also applied TAP oxidation for the removal of beta-blockers bisoprolol (BIS) and anti-inflammatory drugs ibuprofen (IBU). In this study, the calculated activation energies were 119.8 ± 10.8 and 168.0 ± 9.5 kJ mol1, respectively, which elucidated that TAP oxidation had the similar degradation ability of CBZ and BIS. However, compared to the aforementioned two pharmaceuticals, IBU was more difficult to remove by TAP oxidation. 3.2. Degradation mechanism in TAP system In this study, we access the degradation mechanism and the predominant species which play the major role through adding excessive radical scavengers. Fig. 2 shows the effect of different scavengers including EtOH, phenol and TBA on the CBZ degradation using TAP oxidation. Second-order rate constants for reactions of selected scavengers with hydroxyl radicals and sulfate radicals are listed in Table. 1. As shown, 87.02% CBZ is removed in the absence of any scavenger. When 400 mM EtOH and phenol are added into solutions, only 21.58% and 0.85% CBZ are removed, respectively. EtOH and phenol both exhibit strong inhibiting effect in TAP system. Compared with EtOH, phenol exerts stronger inhibiting effect because of its higher reaction rate with sulfate radicals (several hundred fold greater than that with EtOH). However, 52.58% CBZ removal is observed in the presence of the same amount of TBA. TBA exhibits high reaction rate with hydroxyl radicals and relatively slow reaction rate. Many previous studies used TBA as the scavenger for hydroxyl radicals. Qi et al. [22] reported that the ozonation of 2,4,6-trichloroanisole (TCA) catalyzed by
the raw bauxite was strong inhibited in the presence of TBA. In this study, due to its relatively low reaction rate with sulfate radicals, higher removal of CBZ is achieved in the presence of TBA. On the whole, the slower the reaction rate is, the lower the decrease of CBZ removal is. The removal of CBZ in the presence of EtOH and TBA can be attributed to the oxidation of sulfate radicals. In summary, sulfate radicals may be the dominant species for the CBZ degradation in this study. 3.3. Effect of persulfate dosage The effect of initial persulfate dosage on the CBZ degradation was investigated within the persulfate dosage ranges of 0.2– 4 mM at 60 °C. Fig. 3 plots the semi-logarithmic graph of [CBZ]/ [CBZ]0 under different persulfate dosages as a function of reaction time. As presented, the degradation rates under different persulfate dosages all abide by pseudo-first-order kinetics. As the persulfate dosage increases from 0.2 to 4 mM, the degradation rate constant of CBZ enhances from 0.0034 to 0.0744 min1. This phenomenon was also observed by other researchers when treating 1,1,1-trichloroethane (TCA) and methyl tert-butyl ether (MTBE) contaminants at various persulfate dosage in TAP system [19,23]. Furthermore, as shown in the insert image of Fig. 3, the degradation rate constant exhibits a linear trend as a function of persulfate dosage in this study (kapp = 0.0186 [PS]0 0.0016, R2 = 0.992). This is consistent with Tan et al. [18], who reported that the degradation rate of diuron was positively proportional to persulfate dosage (kapp = 0.690 [PS]0). 3.4. Effect of initial CBZ concentration The effect of initial CBZ concentration on the CBZ removal has been investigated from 10 to 80 lM at 60 °C. Fig. 4 depicts the semi-logarithmic graph of [CBZ]/[CBZ]0 under different CBZ
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Fig. 3. CBZ degradation using TAP oxidation under different persulfate dosages. Experimental conditions: [CBZ]0 = 40 lM, [PS]0 = 0.2–4 mM, T = 60 °C, reaction time = 120 min.
concentrations as a function of reaction time. As shown, the degradation rate is pseudo-first-order with respect to the initial CBZ concentrations. As the initial CBZ concentration increases, the degradation rate constant of CBZ decreases. When the CBZ concentration increases from 10 to 80 lM, the degradation rate constant accordingly decreases from 0.104 to 0.0073 min1. Theoretically, the overall amount of sulfate radicals in TAP system is invariant at the fixed persulfate dosage. At a high CBZ concentration, the part of CBZ to be degraded by sulfate radicals is small relative to the total CBZ. Different from the effect of initial persulfate dosage, the degradation rate constant shows an exponential relationship rather than a linear trend (kapp ¼ 3:14 ½CBZ1:48 , R2 = 0.989). There 0 is no similar conclusion reported using TAP oxidation in previous literatures. Similar finding was reported by Xu et al. [24], in which kapp ¼ 0:0745 ½DMP0:364 was determined for UV/H2O2 oxidation of 0 dimethyl phthalate (DMP) in water.
CBZ degradation in TAP system is pH dependent and acidic condition is more favorable than neutral and alkaline conditions. The best CBZ degradation occurs at pH 3 and its corresponding degradation rate constant is 0.0228 min1. As the pH increases from 5.02 to 8.99, the CBZ degradation rate constant accordingly decreases from 0.0182 to 0.0086 min1. The results were in agreement with the rate constants of methyl tert-butyl ether (MTBE) and diphenylamine degradation by persulfate which also decreased with an increasing pH [19]. However, when the pH reached 11.01, the degradation rate achieves a certain degree of increase. It was reported that the rate constant of persulfate decomposition increased with pH decrease under different temperature [25]. Moreover, more sulfate radicals could be generated through acidcatalyzation in acidic condition, which might accelerate the CBZ degradation [26]. The acid-catalyzation decreases with the pH increase, resulting in the reduction of degradation rate. Lee et al. [27] used microwave-activated persulfate to decompose perfluorooctanoic acid (PFOA) and the maximum PFOA degradation occurred at pH 2.5. In addition, sulfate radicals generated in solutions can lead to radical interconversion reactions to produce hydroxyl radicals [21]. Under alkaline conditions, hydroxyl radicals which exhibit a slightly higher redox potential than sulfate radicals become the dominant radical species in TAP system. However, the CBZ degradation rate decreases because of the generation of a large amount of sulfate ions which may inhibit the reactivity of hydroxyl radicals and sulfate radicals. The inhibiting effect of various anions on 4-chlorophenol degradation by hydroxyl radicals in Fenton’s 2 system was ClO4 NO 3 > SO4 > Cl HPO4 > HCO3 [28]. In addition, when hydroxyl radicals and sulfate radicals coexist in solutions, they can recombine to form HSO 4 and oxygen, which make the CBZ degradation decrease [29]. 3.6. Effect of coexisting inorganic anions
The effect of initial solution pH on the CBZ oxidation has been investigated in the pH range of 3.01–11.01 at 60 °C. Fig. 5 plots the semi-logarithmic graph of [CBZ]/[CBZ]0 under different pH as a function of reaction time. As shown, the CBZ degradation rates at all pH follow pseudo-first-order kinetics over 120 min. The
The effect of inorganic anions has been investigated through adding different inorganic anion at fixed concentration of 10 mM. Fig. 6 depicts the semi-logarithmic graph of [CBZ]/[CBZ]0 under different inorganic anion as a function of reaction time. As shown, different inorganic anion exhibit different inhibiting effect on the CBZ decomposition. The degradation rate constant is 1.756 min1 in the absence of anion; however, when 10 mM anions are added into solutions, the degradation rate constants are 1.59, 1.51, 1.39, 1.05 2 and 0.72 min1 in the presence of NO 3 , SO4 , Cl , HCO3 and 2 CO3 , respectively. The inhibition of inorganic anions on the CBZ 2 degradation is in the order of CO2 3 > HCO3 > Cl > SO4 > NO3 .
Fig. 4. CBZ degradation using TAP oxidation under different initial CBZ concentrations. Experimental conditions: [CBZ]0 = 10–80 lM, [PS]0 = 1 mM, T = 60 °C, reaction time = 120 min.
Fig. 5. CBZ degradation using TAP oxidation under different initial solution pH. Experimental conditions: [CBZ]0 = 40 lM; [PS]0 = 1 mM; [pH]0 = 3.01–11.01; T = 60 °C, reaction time = 120 min.
3.5. Effect of solution pH
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Fig. 6. CBZ degradation using TAP oxidation in the presence of coexisting inorganic anions. Experimental conditions: [CBZ]0 = 40 lM; [PS]0 = 1 mM; [Anions] = 10 mM; T = 60 °C, reaction time = 120 min.
Among these inorganic anions, SO2 4 and NO3 only slightly slow down the CBZ degradation, because they almost do not react with sulfate radicals. It is well known that SO2 4 is the inevitable product of persulfate oxidation. Much higher persulfate concentrations are applied in the field which means large amounts of SO2 4 would generate accordingly. In this case, SO2 effects cannot be ignored 4 which is very valuable information for in situ chemical oxidation application. Compared with SO2 4 and NO3 , Cl can react with sul fate radicals to generate relatively weaker species such as Cl, Cl2 and ClHO [30]. It is noted that the redox potentials of the Cl containing radicals are much lower than 2.6 V of sulfate radicals [14]. Furthermore, the generated Cl2 and Cl could further react with it self to form Cl2 and Cl [31,32]. CO2 3 and HCO3 are the good scavengers for hydroxyl radicals, similar findings were reported everywhere [33]. In this study, CO2 and HCO 3 3 both significantly inhibited the degradation rate of CBZ. When CO2 and HCO 3 3 are added into solutions, it would proceed to react and eventually reach the equilibration of carbonate system. In the solutions with 10 mM initial concentration of CO2 3 (pH = 11.29) or HCO3 (pH = 8.61), the primary compositions 2 at equilibrium is 0.84 mM HCO or 9.78 mM 3 =9:16 mM CO3 2 HCO3 =0:22 mM CO3 , respectively. Similar with hydroxyl radicals, sulfate radicals can also react with CO2 and HCO 3 3 at different 2 rates [19]. Therefore, CO3 and HCO3 performs as a radical scavenger in solutions. CO2 3 exhibits more significant inhibiting effect on 2 CBZ decomposition than HCO 3 , because the reaction rate of CO3 with sulfate radicals is approximately 4 times than that of HCO3 .
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transition metals for activation of persulfate and also found that Mn2+ was an ineffective activator for persulfate. As for Cu2+, it can activate persulfate to generate more sulfate radicals [34]. Although the rate of this reaction is slow and requires a relatively high energy, the process may be accelerated in high temperature oxidation system. The other reason may be the generation of Cu3+ in TAP system, which is unstable and may act as an oxidant for CBZ decomposition [35]. The recurred Cu2+ in this process can continue to activate persulfate. Compared with Cu2+, Fe2+ is more efficient activator for persulfate to generate radicals in TAP system. In this work, once 1 mM Fe2+ is added into the solution, more than 90% CBZ are removed within 10 min. Because once the Fe2+ is attached with persulfate, a considerable amount of powerful sulfate radicals could be yielded transiently. After experiencing a rapid reaction stage, a slower decomposition stage which is not obvious in this study appears because of the accumulation of Fe3+ and the poor recovery of Fe2+. However, adding Fe3+ into TAP oxidation system can speed up the recovery of Fe2+, which can enhance the degradation rate of CBZ. Of note, several previous publications have reported that Fe2+ can react with sulfate radicals and consequently inhibited the degradation of target pollutants [36]. However, this phenomenon only happened when Fe2+ dosage was too high, excessive Fe2+ can react with sulfate radicals. This phenomenon is not observed in this study for the reason that Fe2+ dosage used is below the inhibiting point. 3.8. Identification of the intermediate products The structure for the degradation products of CBZ were proposed based on the full scan mode obtained from LC–MS/MS analysis and the information available from previous studies [12,37,38]. Figs. 8 and 9 show the total ion chromatogram and the proposed structures for the degradation products, respectively. As shown, TAP oxidation of CBZ produces 6 major intermediate products in this study. At retention time of 19.71 min, CBZ is eluted. Due to the addition of hydroxyl or carbonyl groups on the CBZ structure, more polar compounds are generated after TAP oxidation which leads to all the intermediate emerge at lesser retention time. Except for the generation of various hydroxylated intermediates (I, II), many corresponding isomers are also produced in this process, because sulfate radicals can attack the different carbon atoms of the benzene and the azepine rings. For example, intermediate I has its trans and cis stereoisomers. Intermediate III (10,11-10,11-epoxycarbamazepine), which is most common degradation product of CBZ identified in previous
3.7. Effect of coexisting inorganic cations The effect of inorganic cations has been investigated through adding different inorganic cations at fixed concentration of 1 mM. Fig. 7 presents the semi-logarithmic graphs of [CBZ]/ [CBZ]0 under different inorganic cations as a function of reaction time. As shown, the common cations in surface water and groundwater, such as Ca2+ and Mn2+ are found to have no significant impact on the decomposition of CBZ. However, other cations including Fe2+, Fe3+ and Cu2+ can greatly accelerate the degradation rate of CBZ. Compared with the blank experiments (no cations addition), the degradation rate increases about 4.47, 4.65 and 10.34 folds with addition of 1 mM Fe3+, Cu2+ and Fe2+, respectively. The promotion of inorganic cations on CBZ degradation abide by the order of Fe2+ > Cu2+ > Fe3+ > Mn2+ Ca2+. As a transition metal, the influence of Mn2+ is negligible in TAP oxidation. Anipsitakis and his colleagues [21] investigated nine
Fig. 7. CBZ degradation using TAP oxidation in the presence of coexisting inorganic cations. Experimental conditions: [CBZ]0 = 40 lM; [PS]0 = 1 mM; [Cations] = 1 mM; T = 60 °C, reaction time = 120 min.
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Fig. 8. Total ion chromatogram after background subtracted obtained after 90 min reaction in TAP system. Experimental conditions: [CBZ]0 = 40 lM; [PS]0 = 1 mM; [pH]0 = 5.18; T = 60 °C.
Fig. 9. Proposed structures for the degradation products of CBZ identified using LC–MS/MS.
literatures can be formed through the dehydration of intermediate I. Intermediate IV can be produced through the cleavage reaction of the azepine rings of intermediate III [38]. Intermediate V and VI are identified as quinine derivatives and acridone, which were also found during the photodegradation of CBZ [37]. Matta et al. [39] also identified intermediates III and V in Co2+/peroxymonosulfate system. Of note, another common intermediate product acridine cannot detect in this study. This may be attributed to the low persulfate dosage and the corresponding mineralization is only (5.7 ± 1.3)% after 4 h reaction.
4. Conclusions The kinetics and mechanism of carbamazepine (CBZ) degradation using thermally activated persulfate (TAP) oxidation were investigated in this paper. Affecting factors such as persulfate dosage, initial CBZ concentration, solution pH, coexisting inorganic anions and cations were also evaluated. The CBZ decomposition was greatly affected by temperature, persulfate dosage, initial CBZ con centration and pH. Adding CO2 3 and HCO3 in solutions can greatly slow down CBZ degradation in TAP system. On the contrary, Fe2+,
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