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Chloride-enhanced oxidation of organic contaminants by Cu(II)-catalyzed Fenton-like reaction at neutral pH
Accepted Manuscript Title: Chloride-enhanced oxidation of organic contaminants by Cu(II)-catalyzed Fenton-like reaction at neutral pH Authors: Hongshin Lee, Juhee Seong, Ki-Myeong Lee, Hak-Hyeon Kim, Jaemin Choi, Jae-Hong Kim, Changha Lee PII: DOI: Reference:
S0304-3894(17)30769-0 https://doi.org/10.1016/j.jhazmat.2017.10.020 HAZMAT 18925
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
24-7-2017 8-9-2017 9-10-2017
Please cite this article as: Hongshin Lee, Juhee Seong, Ki-Myeong Lee, Hak-Hyeon Kim, Jaemin Choi, Jae-Hong Kim, Changha Lee, Chloride-enhanced oxidation of organic contaminants by Cu(II)-catalyzed Fenton-like reaction at neutral pH, Journal of Hazardous Materials https://doi.org/10.1016/j.jhazmat.2017.10.020 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Chloride-enhanced oxidation of organic contaminants by Cu(II)-catalyzed Fenton-like reaction at neutral pH
Hongshin Leea, Juhee Seonga, Ki-Myeong Leea, Hak-Hyeon Kima, Jaemin Choia, Jae-Hong Kimb, Changha Leea,*
a
School of Urban and Environmental Engineering, Ulsan National Institute of Science and
Technology (UNIST), 50 UNIST-gil, Ulju-gun, Ulsan 44919, Republic of Korea b
Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut
06511, USA
Submitted to Journal of Hazardous Materials
*Corresponding author: Phone: +82-52-217-2812 Fax: +82-52-217-2809 E-mail: [email protected] 1
Graphical Abstract
Highlights
The Cu(II)-catalyzed Fenton-like reaction was accelerated in the presence of Cl.
The Cu(II)/H2O2/Cl− system was optimized at neutral pH.
Cu(III)-chloro complexes were suggested as major REACTIVE oxidants.
The Cu(II)/H2O2/Cl− system can be a useful approach to treat saline wastewater.
Abstract
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The Cu(II)-catalyzed Fenton-like reaction was found to be significantly accelerated in the presence of chloride ion (i.e., the Cu(II)/H2O2/Cl system), enhancing the oxidative degradation of organic contaminants at neutral pH. The degradation of carbamazepine (a select target contaminant) by the Cu(II)/H2O2 system using 1 M Cu(II) and 10 mM H2O2 was accelerated by 28-fold in the presence of 10,000 mg/L Cl at pH 7. The observed rate of carbamazepine degradation generally increased with increasing doses of Cu(II), H2O2, and Cl, and exhibited an optimal value at around pH 7.5. Various other organic contaminants such as propranolol, phenol, acetaminophen, 4-chlorophenol, benzoic acid, and caffeine were also effectively degraded by the Cu(II)/H2O2/Cl system. Experiments using oxidant probe compounds and electron paramagnetic spectroscopy suggested that cupryl (Cu(III)) species are the major reactive oxidants responsible for the degradation of these organic contaminants. The enhanced kinetics was further confirmed in natural seawater.
Keywords: copper, Fenton-like reaction, chloride ion, hydrogen peroxide, cupryl ion
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1. Introduction For decades, transition metal-catalyzed Fenton or Fenton-like reactions have been widely studied for the removal of organic contaminants in water and wastewater. Iron has been most extensively explored due to it high catalytic activity to convert hydrogen peroxide (H2O2) into reactive oxidants (i.e., the Fenton reaction) [1]. Other transition metals such as copper [2], aluminum [3], cobalt [4], and silver [5] have been also reported to generate reactive oxidants via the Fenton-like reactions. In recent studies, Cu(II)-catalyzed Fenton-like reactions have been shown to oxidize recalcitrant organic contaminants and to inactivate microorganisms at neutral pH [2, 6-10] contrary to traditional Fenton reactions that function only at lower pH values. Cupryl (Cu(III)) species have been suspected as the major reactive oxidants produced during these reactions [8, 11-13]. Although Cu(II)-catalyzed Fenton-like reactions are potential alternatives to the iron-based Fenton reaction, the application of the Cu(II)/H2O2 pair is limited by the slow reaction kinetics of Cu(II) reduction by H2O2 (rate-determining step, RDS) [13]. In order to accelerate the Cu(II) reduction, the irradiation of UV light [9] and the use of a reducing agent such as hydroxylamine [11] have been attempted. In addition, a recent study showed that the addition of bicarbonate ion
4
(HCO3) can be also instrumental because copper-carbonate complexes enhance kinetics for their redox reactions with H2O2 [14]. Similar to HCO3, chloride ion (Cl) is also known to form a complex with copper to enhance redox reactivity with H2O2 [15, 16]. However, little is known about the effect of Cl on the degradation of organic contaminants in the Cu(II)/H2O2 system. The degradation efficiencies of organic contaminants in the Cu(II)/H2O2/Cl system need to be evaluated under varying reaction conditions for facile translation to practical applications. In addition, there is a dearth of information on how the complexation of copper with Cl alters the mechanism of the Fenton-like reaction and consequently affects the nature of resultant reactive oxidants. Such information shall be particularly useful when the copper-catalyzed Fenton-like reactions are applied to treat organic contaminants in saline wastewater, brackish water, or seawater. The objectives of this study are (i) to investigate the effect of Cl on the degradation of organic contaminants by copper-catalyzed Fenton-like reactions and (ii) to identify major reactive oxidants responsible for the contaminant degradation in the Cu(II)/H2O2/Cl system. For these purposes, the degradation kinetics of carbamazepine (a select target contaminant) in the Cu(II)/H2O2/Cl system was examined with varying reaction parameters including pH and concentrations of Cu(II), H2O2, and Cl. The Cu(II)/H2O2/Cl system was further tested for
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degradation of different organic contaminants. In addition, to gain insight into the nature of reactive oxidants, experiments were performed using probe compounds for reactive oxidants and electron paramagnetic resonance (EPR) spectroscopy.
2. Materials and methods 2.1. Reagents All chemicals were of reagent grade and used as received without further purification except for 2,4-dinitrophenyl hydrazine (DNPH). DNPH was recrystallized three times from acetonitrile to remove impurities. Chemicals used in this work include copper(II) sulfate, iron(II) sulfate, iron(III) perchlorate hydrate, H2O2 solution (35 wt.%), Sodium chloride (NaCl), perchloric acid (HClO4), sodium hydroxide (NaOH), 5,5-dimethyl-1-pyrrolidine N-oxide (DMPO), ethylenediaminetetraacetic acid (EDTA), propranolol. acetaminophen, caffeine, benzoic acid, 4hydroxybenzoic acid (4-HBA), phenol, 4-chlorophenol, carbamazepine, coumarin, 7hydroxycoumarin (7-HC), methanol, formaldehyde (HCHO), tert-butanol (all from SigmaAldrich Co.), and acetonitrile (J.T. Baker Co.). All solutions were prepared using 18.2 MΩ·cm Milli-Q water from a Milli-Q ultrapure water-purification system (Millipore Co.). Stock solutions of copper(II) sulfate (1 mM) and organic contaminants (5 M) were prepared and
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stored at 4oC until use. The stock solution of H2O2 (1 M) was prepared freshly prior to the experiment. Natural seawater was collected from the east coast of Busan city, immediately filtered with a 0.45-µm Nylon membrane, and stored at 4°C until use; several water quality parameters of the seawater sample were presented in Table S1 (the supplementary data).
2.2. Batch experiments for oxidative degradation of organic contaminants Experiments were performed in a 100-mL Pyrex flask at room temperature (20 2oC). The reaction solution was prepared by adding Cu(II) (or Fe(II) for some experiments) and the organic contaminant (and Cl−). The solution pH was adjusted using 1 N HClO4 and 1 N NaOH solutions (1 mM phosphate buffer was used for pH 7). The reaction was initiated by adding an aliquot of a freshly prepared stock solution of H2O2. Samples (1 mL) were withdrawn at predetermined times and immediately filtered through a 0.45-µm PTFE syringe filter. The possible reactions of Cu(II) in samples were quenched by adding EDTA (4 mM). All experiments were carried out at least in duplicate, and the average values with standard deviations (error bars) were presented.
2.3. Experiments using oxidant probe compounds and EPR spectroscopy
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Methanol, benzoic acid, and coumarin were used as probe compounds for reactive oxidants such as Cu(III) and hydroxyl radical (OH) and their oxidation products (i.e., HCHO, 4-HBA, and 7-HC, respectively) were analyzed. Excess probe compound was employed to maximize the scavenging of reactive oxidants and minimize the secondary oxidation of products [2, 11]. The EPR spectroscopy with DMPO as a spin-trapping agent was used to detect OH. A solution containing Cu(II), Cl, and DMPO was prepared in a Pyrex flask, and H2O2 was added to initiate the reaction. After 5 min of reaction, samples were withdrawn and the EPR signals of DMPOradical adducts were acquired by a CW/Pulse EPR system (JES-X310, JEOL Co.) under a microwave frequency of 9.414 GHz, a microwave power of 0.998 mW, and a modulation frequency of 100 kHz.
2.4. Analytical methods Organic compounds were analyzed using a HPLC equipped with a UV/vis absorbance detector; the detection wavelengths were 227 nm for benzoic acid, 230 nm for 4-chlorophenol, 241 nm for acetaminophen, 270 nm for caffeine, 270 nm for 4-HBA, 277 nm for phenol, 285 nm for propranolol, 285 nm for carbamazepine, 320 nm for 7-HC, and 350 nm for HCHO. HCHO was analyzed after DNPH derivatization [17]. Separation was performed on a ZORBAX Eclipse
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XDB-C18 column (250 mm 4.6 mm, 5 m) using a binary mobile phase consisting of aqueous phosphoric acid solution (0.1 v/v %) and acetonitrile, at a flow rate of 1 – 1.5 mL/min. The concentration of H2O2 was measured by a UV/vis spectrophotometer (S-3100, Scinco Co.) using a titanium sulfate method [18].
3. Results 3.1. Carbamazepine degradation in the Cu(II)/H2O2 system in the presence of Cl The addition of Cl significantly increased the kinetics of carbamazepine degradation in copper-Fenton-like system; as much as 96% degradation was achieved within 2 h (Fig. 1a). In contrast, the carbamazepine degradation by the Cu(II)/H2O2 in the absence of Cl was minor (approximately 11% degradation in 2 h). The combination of H2O2 and Cl without Cu(II) also did not degrade carbamazepine during the entire reaction time. The decomposition of H2O2 by the Cu(II)/H2O2 system was correspondingly accelerated in the presence of Cl (Fig. 1b). The degradation of carbamazepine by the Cu(II)/H2O2 and Cu(II)/H2O2/Cl systems both followed pseudo first-order kinetics (Fig. S1 in the supplementary data). The observed rate constant for the carbamazepine degradation by the Cu(II)/H2O2/Cl system (k = 2.8 × 10-2 min1) was approximately 28-fold higher than that by the Cu(II)/H2O2 system (k = 1.0 × 10-3 min1).
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3.2. Effects of Cu(II), H2O2, Cl concentrations and solution pH, and degradation of various organic contaminants The degradation rate of carbamazepine, expressed in terms of observed pseudo first-order rate constant k, increased as Cu(II), H2O2, and Cl concentrations increased (Fig. 2a). The increase of the k value gradually plateaued with increasing concentrations of Cu(II) and H2O2, whereas such a trend was not observed at elevated concentrations of Cl. The carbamazepine degradation was also significantly affected by the solution pH (Fig. 2b). The rate of carbamazepine degradation was the greatest at around pH 7.5, which was consistent with the trend previously reported in the Cu(II)/H2O2 system without Cl [9]. The Cu(II)/H2O2/Cl system was also found to be effective in degrading a range of micropollutants commonly found in water and wastewater (Fig. 3). All target contaminants, propranolol. acetaminophen, caffeine, benzoic acid, phenol, 4-chlorophenol and carbamazepine, were degraded by more than 90% in 2 h. The observed degradation rates were in the order of caffeine < benzoic acid < 4-chlorophenol < acetaminophen < carbamazepine < phenol < propranolol (refer to the inset of Fig. 3).
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3.3. Oxidation of probe compounds and EPR spectroscopy A significantly higher amount of HCHO was produced (from the oxidation of methanol) in the Cu(II)/H2O2/Cl system compared to the Cu(II)/H2O2 system, while the HCHO production in the H2O2/Cl system was minor (Fig. S2a). In the Cu(II)/H2O2/Cl system, a significant amount of 4-HBA and 7-HC was also produced as the reaction proceeded, whereas both Cu(II)/H2O2 and H2O2/Cl systems produced a negligible amount of 4-HBA and 7-HC (Figs. S2b and S2c). When the concentrations of HCHO, 4-HBA and 7-HC after 2 h of reaction time were compared, the concentration of HCHO appeared much higher than those of 4-HBA and 7-HC for both the Cu(II)/H2O2 and Cu(II)/H2O2/Cl systems (Fig. 4a). The EPR spectroscopy results confirmed the production of the DMPO-OH spin adduct in the Cu(II)/H2O2/Cl system. The hyperfine splitting pattern was consistent with that of DMPO-OH reported in the literature as well as that observed in Fe(II)/H2O2 system which was employed herein as a control (hyperfine constants of aN = aH = 14.9 ± 0.1 G with an intensity ratio of 1:2:2:1) [19]. It is interesting to note that, despite lower DMPO-OH signal in Cu(II)/H2O2/Cl system than that in the Fe(II)/H2O2 system, the degradation rate of carbamazepine and the yield of HCHO were higher in the Cu(II)/H2O2/Cl system than the Fe(II)/H2O2 system (Fig. 4c).
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3.4. Degradation of carbamazepine in seawater Results in Fig. 5 show the degradation of carbamazepine by the Cu(II)/H2O2 and Fe(II)/H2O2 systems in the natural seawater catchment (refer to Table S1 for the seawater constituents; carbamazepine was added at 5 M). The degradation of carbamazepine by the Cu(II)/H2O2 system was greatly enhanced in seawater compared to the DI water; the observed pseudo firstorder rate constant increased by 88-fold from 1.0 × 10-3 min1 to 8.8 × 10-2 min1 (Cu(II)/H2O2 vs. Cu(II)/H2O2/Seawater in Fig. 5a). The Cu(II)/H2O2/Cl system with Cl at the concentration equivalent to seawater (13,559 mg/L) exhibited the similar kinetics to the Cu(II)/H2O2/Seawater system. In contrast, the traditional Fenton system (Fe(II)/H2O2) exhibited the opposite trend (Fe(II)/H2O2 vs. Fe(II)/H2O2/Seawater in Fig. 5b). The addition of Cl (13,559 mg/L) to the Fe(II)/H2O2 system inhibited the carbamazepine degradation by approximately 15% in 2 h. The inhibitory effect was much greater in seawater, indicating that seawater constituents other than Cl additionally hinder the Fe(II)/H2O2 system.
4. Discussion 4.1. Role of Cl in copper-catalyzed Fenton-like reactions
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It is well known that Cu(II)/H2O2 system produces reactive oxidants by activating H2O2 through the catalytic redox cycle involving the Cu(II)/Cu(I) couple. In the first step, Cu(II) is reduced to Cu(I) by the reaction with H2O2, producing superoxide radical anion (O2) (reaction 1). O2 can be further engaged in redox reactions with Cu(II) and Cu(I) (reactions not shown). Subsequently, Cu(I) reacts with H2O2 to produce reactive oxidants (OH and Cu(III) via one- and two-electron transfer mechanisms, respectively) that are capable of degrading organic contaminants (reaction 2). The reaction 1 is very slow and limits the overall kinetics for the production of reactive oxidants (i.e., RDS). Cu(II) + H2O2 → Cu(I) + H2O2
→
Cu(I) + O2 + H+ (k1 = < 1 M1 s1 at pH 7 [20])
(1)
Cu(II) + OH + OH or Cu(III) + 2OH
(k2 = 4 × 105 M1 s1 at pH 68 [15])
(2)
The addition of Cl changes the kinetics of the above redox reactions by forming copperchloro complexes (reactions 3 and 4) (refer to Figs S3S5 in the supplementary data for the speciation of Cu(I) and Cu(II) in the presence of Cl). The increase in the Cl concentration promotes the Cu(II) reduction by H2O2 by H2O2 because the redox potentials of Cu(II)/Cu(I)chloro complexes (e.g., CuCl+/CuCl0 and CuCl20/CuCl) are higher than that of Cu2+/Cu+ ion couple [15]. In the presence of 10,000 mg/L Cl (0.28 M), the k3 and k4 values were calculated as
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~70 M1 s1 at pH 7.17 and ~170 M1 s1 at pH 68, respectively (reactions 3 and 4); those values were obtained by extra(intra)polating the data from Moffett and Zika (1987) [15]. Since the bottleneck due to the slow rate of reaction 1 is eliminated, the overall catalytic cycle of Cu(II)/Cu(I) is activated, resulting in the enhancement of organic contaminant degradation and H2O2 decomposition (Figs. 1a and 1b). Cu(II)-Cln + H2O2 → Cu(I)-Cln + O2 + H+ (k3 = ~70 M1 s1 at pH 7.17, [Cl]0 = 0.28 M [15]) Cu(I)-Cln + H2O2
(3)
→ Cu(II)-Cln + OH + OH or Cu(III)-Cln + 2OH
(k4 = ~170 M1 s1 at pH 68, [Cl]0 = 0.28 M [15])
(4)
4.2. Factors affecting the organic contaminant degradation by the Cu(II)/H2O2/Cl system Factors including initial concentrations of Cu(II), H2O2, and Cl, and solution pH affect the degradation rate of carbamazepine in the Cu(II)/H2O2/Cl system (Figs. 2a and 2b). The increase in the carbamazepine degradation rate (k) with increasing concentrations of Cu(II) and H2O2 (the reagents for the Fenton-like reactions) shown in Fig. 2a is likely to be simply related to the increased rate of reactive oxidant production. The plateauing of the k value at high concentrations of Cu(II) and H2O2 may be attributed to the scavenging of reactive oxidants by
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Cu(I) and H2O2. The increase of the carbamazepine degradation rate with increasing the Cl concentration is due to the increasing formation of aforementioned Cu(II)-chloro complexes such as CuCl+, CuCl2, and CuCl3 (Fig. S5b in the supplementary data). The k value increased almost linearly with respect to the Cl concentration (Fig. 2a), showing consistent trends with the formation of Cu(II)-chloro complexes (Fig. S5b). This also suggests that the scavenging effect of reactive oxidants by Cl is minor. The degradation of carbamazepine was most efficient at pH 7.5 in the Cu(II)/H2O2/Cl system (Fig. 2b). This trend, also observed in Cu(II)/H2O2 system, is the result of a balance between two factors with opposite pH effects; i.e., as pH increases, more reactive oxidants are produced, but reactivity of some of these oxidants decreases [9]. First, the catalytic decomposition of H2O2 into reactive oxidants accelerates with increasing pH (regardless of the presence of Cl), since the rate of Cu(II) reduction by H2O2 (reactions 1 and 3) increases with increasing pH. The Cu(II) reduction by H2O2 (reactions 1 and 3) proceeds via the deprotonated H2O2 (HO2−, pKa of H2O2 = 11.75) through its complexation with Cu(II) and the subsequent ligand-to-metal charge transfer. Meanwhile, at higher pH, the Fenton-like reaction produces oxidants of less reactivity, most likely Cu(III)-hydroxo complexes [2, 11], that are less reactive toward carbamazepine degradation.
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4.3. Nature of reactive oxidants produced by the Cu(II)/H2O2/Cl system There has been a controversy over the identity of reactive oxidants produced during the copper-catalyzed Fenton-like reactions (i.e., OH vs. Cu(III) from reaction 2) [21-23]. However, recent studies have provided more convincing pieces of evidence that Cu(III), present in different forms (Cu(III)-oxo and –hydroxo complexes, or complexed with potentially other (in)organic ligands), is a dominant oxidant rather than OH [2, 8-11]. Experimental results in this study also provide the evidence to rule out OH as the major oxidant, which resultingly supports the production of Cu(III) species, most likely Cu(III)-chloro complexes (or Cu(III)-chloro-hydroxo complexes), as major reactive oxidants in the Cu(II)/H2O2/Cl− system. First, the results obtained with oxidant probe compounds are not well explained by OH. The formation of hydroxylation products (4-HBA and 7-HC) from the oxidation of benzoic acid and coumarin was negligible compared to the HCHO formation from methanol oxidation (Fig. 4a). If OH had been produced, the yields of 4-HBA and 7-HC relative to HCHO should have reached approximately 23% and 29%, respectively; the maximum yields of HCHO, 4-HBA, and 7-HC by the reactions of methanol, benzoic acid, and coumarin with OH are known to be 100%, 23% and 29%, respectively [24-26].
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Second, the EPR data were not sufficient to support the production of OH as the major reactive oxidant. The Cu(II)/H2O2/Cl− system indeed exhibited the DMPO-OH signal. However, the signal intensity was not high enough to prove the prevalence of OH; note that the Fe(II)/H2O2 system (a reference OH-producing system) produced less reactive oxidants (Fig. 4c), but exhibited a higher DMPO-OH signal than the Cu(II)/H2O2/Cl− system (Fig. 4b). In addition, the DMPO-OH signal in the Cu(II)/H2O2/Cl− system was not affected by the addition of tertbutanol, whereas the signal in the Fe(II)/H2O2 system significantly decreased in the presence of tert-butanol (Fig. S6 in the supplementary data). This observation indicates that the oxidants responsible for the DMPO-OH signal in the two systems are different, and suggests that the oxidant from the Cu(II)/H2O2/Cl− system may not be OH. The DMPO-OH spin adduct can be formed through pathways that do not involve OH [27]. However, additional studies are needed to elucidate the mechanism by which Cu(III) species (likely Cu(III)-chloro complexes in this study) can generate the DMPO-OH signal. It is possible that dichloride radical anion (Cl2−) is generated by the oxidation of Cl− by reactive oxidants (Cu(III)-chloro complexes). However, the inhibitory effect of methanol on the degradation of benzoic acid indicates that Cl2− does not prevail in the Cu(II)/H2O2/Cl− system (Fig. S7 in the supplementary data). If Cl2− were the oxidant responsible for the benzoic acid
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degradation, the inhibitory effect of methanol should be less based on the reported rate constants (k(Cl2− + benzoate) = 2.0 106 M1s1 [27], k(Cl2− + methanol) = 3.5 103 M1s1 [28]).
4.4. Comparison with the iron-catalyzed Fenton (-like) reactions The effect of Cl− was remarkably different between the Fe(II)/H2O2 system and the Cu(II)/H2O2 system (Figs. 5a and 5b). The degradation of carbamazepine by the Fe(II)/H2O2 system was inhibited in the presence of Cl− (consistent with the observations in previous studies [29, 30]), and the inhibition was even greater in natural seawater (Fig. 5b). The negative effect of Cl− on the Fe(II)/H2O2 system was explained by (i) the formation of Fe(II)- and Fe(III)-chloro complexes which have similar or lower reactivity toward H2O2, and (ii) the scavenging of the reactive oxidant (OH) by Cl− and the subsequent conversion into less reactive Cl2− [31]. The greater inhibition of carbamazepine degradation in seawater may be due to the organic substances that serve as iron-chelators and radical scavengers. In contrast, the Cu(II)/H2O2 system in seawater exhibited a similar degradation rate of carbamazepine to the Cu(II)/H2O2/Cl− system at the equivalent Cl concentration, suggesting that the (in)organic constituents in seawater do not exert any inhibitory effects.
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5. Conclusions The Cu(II)-catalyzed Fenton-like reaction enhanced by Cl− was demonstrated to effectively degrade organic contaminants at neutral pH at a relatively low Cu(II) dose. The concentration of Cu(II) mainly used in this study (1 M) is much lower than drinking water standard values (2 mg/L = 31.4 M for WHO [32], 1.3 mg/L = 20.5 M for the United States [33], and 1 mg/L = 15.7 M for Korea [34]). The enhanced kinetics is attributed to the formation of Cu(II)-chloro complexes that are more reactive with H2O2 than uncomplexed Cu(II), promoting the catalytic redox cycling of Cu(II)/Cu(I) for the decomposition of H2O2 into reactive oxidants. Cu(III) species (likely in the form of Cu(III)-chloro complexes) are suggested to be the major reactive oxidant produced in this Cu(II)/H2O2/Cl− system. These oxidants are believed to be as reactive as OH,
and capable of degrading a wide spectrum of organic contaminants. The Cu(II)/H2O2/Cl−
system can be a useful approach to treat organic contaminants in saline wastewater, brackish water, and seawater (e.g., wastewater from agro-food, petroleum and leather industries, aquaculture effluents, effluents from hydro fracking, etc.). The optimized activity at neutral pH and the enhancement by Cl− are great advantages of the copper-catalyzed Fenton-like reaction over the traditional Fenton reaction based on iron.
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Acknowledgements This research was supported by Korea Ministry of Environment (2017000140005) and the National Research Foundation of Korea (NRF) Grant (NRF-2017R1A2B3006827 and NRF2015R1A5A7037825).
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[18] G.M. Eisenberg, Colorimetric determination of H2O2, Ind. Eng. Chem. Anal., 15 (1943) 327-328. [19] G.S. Timmins, K.J. Liu, E.J.H. Bechara, Y. Kotake, H.M. Swartz, Trapping of free radicals with direct in vivo EPR detection: a comparison of 5,5-dimethyl-1-pyrroline-N-oxide and 5diethoxyphosphoryl-5-methyl-1-pyrroline-N-oxide as spin traps for •OH and SO4•−, Free Radical Biol. Med., 27 (1999) 329-333. [20] J.F. Perez-Benito, Copper(II)-catalyzed decomposition of hydrogen peroxide: Catalyst activation by halide ions, Monatsh. Chem., 132 (2001) 1477-1492. [21] M.K. Eberhardt, G. Ramirez, E. Ayala, Does the reaction of copper(I) with hydrogen peroxide give hydroxyl radicals? A study of aromatic hydroxylation, J. Org. Chem., 54 (1989) 5922-5926. [22] G.R.A. Johnson, N.B. Nazhat, R.A. Saadallanazhat, Reaction of the aquocopper(I) ion with hydrogen-peroxide - Evidence against hydroxyl free-radical formation, J. Chem. Soc. Chem. Comm., (1985) 407-408. [23] G.R.A. Johnson, N.B. Nazhat, R.A. Saadallanazhat, Reaction of the aquacopper(I) ion with hydrogen peroxide - Evidence for a Cu(III) (Cupryl) intermediate, J. Chem. Soc .Farad. T. I, 84 (1988) 501-510.
23
[24] K.D. Asmus, H. Moeckel, A. Henglein, Pulse radiolytic study of the site of hydroxyl radical attack on aliphatic alcohols in aqueous solution, J. Phys. Chem., 77 (1973) 1218-1221. [25] G.W. Klein, K. Bhatia, V. Madhavan, R.H. Schuler, Reaction of hydroxyl radicals with benzoic acid. Isomer distribution in the radical intermediates, J. Phys. Chem., 79 (1975) 1767-1774. [26] G. Louit, S. Foley, J. Cabillic, H. Coffigny, F. Taran, A. Valleix, J.P. Renault, S. Pin, The reaction of coumarin with the OH radical revisited: hydroxylation product analysis determined by fluorescence and chromatography, Radiat. Phys. Chem., 72 (2005) 119-124. [27] S. Goldstein, D. Meyerstein, G. Czapski, The Fenton reagents. Free Rad. Biol. Med. 15 (1993) 435-445. [28] P. Neta, R.E. Huie, A.B. Ross, Rate constants for reactions of inorganic radicals in aqueous solution, J. Phys. Chem. Ref. Data, 17 (1988) 1027-1284. [29] M.-C. Lu, Y.-F. Chang, I.M. Chen, Y.-Y. Huang, Effect of chloride ions on the oxidation of aniline by Fenton's reagent, J. Environ. Manage., 75 (2005) 177-182. [30] J. Kiwi, A. Lopez, V. Nadtochenko, Mechanism and kinetics of the OH-radical intervention during Fenton oxidation in the presence of a significant amount of radical scavenger (Cl -), Environ. Sci. Technol., 34 (2000) 2162-2168.
24
[31] J. De Laat, T.G. Le, Effects of chloride ions on the iron(III)-catalyzed decomposition of hydrogen peroxide and on the efficiency of the Fenton-like oxidation process, Appl. Catal. B-Environ., 66 (2006) 137-146.
[32] WHO (World Health Organization), Guidelines for drinking-water quality. 4th, ed., WHO, Geneva, 2011. [33] U. S. Environmental Protection Agency, National primary drinking water regulations, United States. http://water.epa.gov/drink, 2009. [34] The Ministry of Environment, Drinking water quality standard and test, Republic of Korea. http://www.me.go.kr, 2014.
25
Figure captions
Fig. 1. (a) Degradation of carbamazepine and (b) decomposition of H2O2 in the Cu(II)/H2O2, H2O2/Cl, and Cu(II)/H2O2/Cl systems ([Carbamazepine]0 = 5 M, [Cu(II)]0 = 1 M, [H2O2]0 = 10 mM, [Cl]0 = 10000 mg/L, pH 7).
Fig. 2. Effects of (a) Cu(II), H2O2, and Cl concentrations, and (b) solution pH on the degradation of carbamazepine by the Cu(II)/H2O2/Cl system ([Carbamazepine]0 = 5 M for (a) and (b), [Cu(II)]0 = 1 M for varying [H2O2] and [Cl] in (a), and (b), [H2O2]0 = 10 mM for varying [Cu(II)] and [Cl] in (a), and (b), [Cl]0 = 10000 mg/L for varying [Cu(II)] and [H2O2] in (a), and (b), pH 7 for (a)).
Fig. 3. Degradation of various organic contaminants by the Cu(II)/H2O2/Cl system ([Propranolol]0 = [Acetaminophen]0 = [Caffeine]0 = [Benzoic acid]0 = [Phenol]0 = [4-chlorophenol]0 = [Carbamazepine]0 = 5 M, [Cu(II)]0 = 1 M, [H2O2]0 = 10 mM, pH 7, the inset represents pseudo first-order rate constants.).
Fig. 4. (a) Production of HCHO, 4-HBA, and 7-HC from oxidation of probe compounds by the Cu(II)/H2O2 and Cu(II)/H2O2/Cl- systems ([Methanol]0 = 200 mM, [Benzoic acid]0 = 10 mM, [Coumarin]0 = 1 mM, [Cu(II)]0 = 1 M, [Cl]0 = 10000 mg/L, [H2O2]0 = 10 mM, pH 7, reaction time = 2 h), (b) EPR spectra obtained by spin trapping with DMPO in different systems ([Cu(II)]0 =
26
[Fe(II)]0 = 10 M, [Cl]0 = 10000 mg/L, [H2O2]0 = 10 mM, [DMPO]0 = 1 mM, pH 3 for Fe(II)/H2O2, pH 7 for Cu(II)/H2O2, H2O2/Cl, and Cu(II)/H2O2/Cl, reaction time = 0.5 min), (c) Degradation of carbamazepine and production of HCHO by those systems employed in the EPR analysis. ([Carbamazepine]0 = 5 M, [Methanol]0 = 200 mM, [Cu(II)]0 = [Fe(II)]0 = 10 M, [Cl]0 = 10000 mg/L, [H2O2]0 = 10 mM, pH 3 for Fe(II)/H2O2, pH 7 for Cu(II)/H2O2 and Cu(II)/H2O2/Cl, reaction time = 30 min for HCHO production experiments).
Fig. 5. Effects of Cl and seawater medium on degradation of carbamazepine by the (a) Cu(II)/H2O2 and (b) Fe(II)/H2O2 systems ([Carbamazepine]0 = 5 M, [Cu(II)]0 = 1 M, [H2O2]0 = 10 mM, pH 8 for (a), pH 3 for (b)).
27
(b) 1.0
0.8
0.6 Cu(II)/H2O 2
0.4
H2O 2/Cl-
[H2O2]/[H2O2]0
[Carbamazepine]/[Carbamazepine]0
(a) 1.0
Cu(II)/H2O 2/Cl-
0.9 0.6 0.4
Cu(II)/H2O 2 H2O 2/Cl-
0.2 0.2
Cu(II)/H2O 2/Cl-
0.0
0.0 0
30
60
90
0
120
30
60
90
Reaction time (min)
Reaction time (min)
Fig. 1
28
120
(a) 0.1
(b) Cu(II) H2O2
0.06
k (min-1)
Cl-
k (min-1)
0.01
0.04
0.02 0.001
[Cu(II)]0 (
)
[H2O2]0 (mM) -
0.1
1
10
100 0.00
1
10
100
3
[Cl ]0 (mg/L) 10
100
1000
4
5
6
7 pH
10000
Fig. 2
29
8
9
10
11
C/C0
Caffeine
Benzoic acid
4-chlorophenol
0.04
0.6
Acetaminophen
k (min-1 )
0.06
Carbamazepine
0.08
0.8
Phenol
0.10
Propranolol
1.0
0.02
0.00
0.4
0.2
0.0 0
30
60
90
Reaction time (min)
Fig. 3
30
120
Propranolol Phenol Carbamazepine Acetaminophen 4-chlorophenol Benzoic acid Caffeine
(c)
(b)
HCHO (from Methanol) 4-HBA (from Benzoic acid) 7-HC (from Coumarin acid)
Cu(II)/H2O2 0.20
0.09
HCHO k
80
Cu(II)/H2O2/Cl-
HCHO (mM)
120
0.06
Cl- /H2O2
Intensity (a. u.)
Products conc. (
160 0.15
0.03 0.10 0.00
Fe(II)/H2O2
0.05
40
-0.03 332
0 Cu(II)/H2O2
-
Cu(II)/H2O2/Cl
333
334
335
336
Magnetic Field (G)
Fig. 4
31
337
338
0.00 Cu(II)/H2O2
Cu(II)/H2O2/Cl-
Fe(II)/H2O2
k (min-1)
(a) 200
Fe(II)/H2O2
Cu(II)/H2O2 Cu(II)/H2O2/Cl
Fe(II)/H2O2/Cl-
Cu(II)/H2O2/Seawater
Fe(II)/H2O2/Seawater
-
kCu(II)/H 2O2/Seawater = 0.0881
6
0.6
-ln(C/C0)
[Carbamazepine]/[Carbamazepine]0
0.8
0.4
[Carbamazepine]/[Carbamazepine]0
(b) 1.0
(a) 1.0
R2 = 0.9921
4 kCu(II)/H 2O2/Cl- = 0.0854 R2 = 0.9748 2 kCu(II)/H 2O2 = 0.0010 R2 = 0.9921 0
0.2
0
30 60 90 Reaction time (min)
120
0.8
0.6
0.4
0.2
0.0
0.0 0
30
60
90
0
120
30
60
90
Reaction time (min)
Reaction time (min)
Fig. 5
32
120
S0304-3894(17)30769-0 https://doi.org/10.1016/j.jhazmat.2017.10.020 HAZMAT 18925
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
24-7-2017 8-9-2017 9-10-2017
Please cite this article as: Hongshin Lee, Juhee Seong, Ki-Myeong Lee, Hak-Hyeon Kim, Jaemin Choi, Jae-Hong Kim, Changha Lee, Chloride-enhanced oxidation of organic contaminants by Cu(II)-catalyzed Fenton-like reaction at neutral pH, Journal of Hazardous Materials https://doi.org/10.1016/j.jhazmat.2017.10.020 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Chloride-enhanced oxidation of organic contaminants by Cu(II)-catalyzed Fenton-like reaction at neutral pH
Hongshin Leea, Juhee Seonga, Ki-Myeong Leea, Hak-Hyeon Kima, Jaemin Choia, Jae-Hong Kimb, Changha Leea,*
a
School of Urban and Environmental Engineering, Ulsan National Institute of Science and
Technology (UNIST), 50 UNIST-gil, Ulju-gun, Ulsan 44919, Republic of Korea b
Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut
06511, USA
Submitted to Journal of Hazardous Materials
*Corresponding author: Phone: +82-52-217-2812 Fax: +82-52-217-2809 E-mail: [email protected] 1
Graphical Abstract
Highlights
The Cu(II)-catalyzed Fenton-like reaction was accelerated in the presence of Cl.
The Cu(II)/H2O2/Cl− system was optimized at neutral pH.
Cu(III)-chloro complexes were suggested as major REACTIVE oxidants.
The Cu(II)/H2O2/Cl− system can be a useful approach to treat saline wastewater.
Abstract
2
The Cu(II)-catalyzed Fenton-like reaction was found to be significantly accelerated in the presence of chloride ion (i.e., the Cu(II)/H2O2/Cl system), enhancing the oxidative degradation of organic contaminants at neutral pH. The degradation of carbamazepine (a select target contaminant) by the Cu(II)/H2O2 system using 1 M Cu(II) and 10 mM H2O2 was accelerated by 28-fold in the presence of 10,000 mg/L Cl at pH 7. The observed rate of carbamazepine degradation generally increased with increasing doses of Cu(II), H2O2, and Cl, and exhibited an optimal value at around pH 7.5. Various other organic contaminants such as propranolol, phenol, acetaminophen, 4-chlorophenol, benzoic acid, and caffeine were also effectively degraded by the Cu(II)/H2O2/Cl system. Experiments using oxidant probe compounds and electron paramagnetic spectroscopy suggested that cupryl (Cu(III)) species are the major reactive oxidants responsible for the degradation of these organic contaminants. The enhanced kinetics was further confirmed in natural seawater.
Keywords: copper, Fenton-like reaction, chloride ion, hydrogen peroxide, cupryl ion
3
1. Introduction For decades, transition metal-catalyzed Fenton or Fenton-like reactions have been widely studied for the removal of organic contaminants in water and wastewater. Iron has been most extensively explored due to it high catalytic activity to convert hydrogen peroxide (H2O2) into reactive oxidants (i.e., the Fenton reaction) [1]. Other transition metals such as copper [2], aluminum [3], cobalt [4], and silver [5] have been also reported to generate reactive oxidants via the Fenton-like reactions. In recent studies, Cu(II)-catalyzed Fenton-like reactions have been shown to oxidize recalcitrant organic contaminants and to inactivate microorganisms at neutral pH [2, 6-10] contrary to traditional Fenton reactions that function only at lower pH values. Cupryl (Cu(III)) species have been suspected as the major reactive oxidants produced during these reactions [8, 11-13]. Although Cu(II)-catalyzed Fenton-like reactions are potential alternatives to the iron-based Fenton reaction, the application of the Cu(II)/H2O2 pair is limited by the slow reaction kinetics of Cu(II) reduction by H2O2 (rate-determining step, RDS) [13]. In order to accelerate the Cu(II) reduction, the irradiation of UV light [9] and the use of a reducing agent such as hydroxylamine [11] have been attempted. In addition, a recent study showed that the addition of bicarbonate ion
4
(HCO3) can be also instrumental because copper-carbonate complexes enhance kinetics for their redox reactions with H2O2 [14]. Similar to HCO3, chloride ion (Cl) is also known to form a complex with copper to enhance redox reactivity with H2O2 [15, 16]. However, little is known about the effect of Cl on the degradation of organic contaminants in the Cu(II)/H2O2 system. The degradation efficiencies of organic contaminants in the Cu(II)/H2O2/Cl system need to be evaluated under varying reaction conditions for facile translation to practical applications. In addition, there is a dearth of information on how the complexation of copper with Cl alters the mechanism of the Fenton-like reaction and consequently affects the nature of resultant reactive oxidants. Such information shall be particularly useful when the copper-catalyzed Fenton-like reactions are applied to treat organic contaminants in saline wastewater, brackish water, or seawater. The objectives of this study are (i) to investigate the effect of Cl on the degradation of organic contaminants by copper-catalyzed Fenton-like reactions and (ii) to identify major reactive oxidants responsible for the contaminant degradation in the Cu(II)/H2O2/Cl system. For these purposes, the degradation kinetics of carbamazepine (a select target contaminant) in the Cu(II)/H2O2/Cl system was examined with varying reaction parameters including pH and concentrations of Cu(II), H2O2, and Cl. The Cu(II)/H2O2/Cl system was further tested for
5
degradation of different organic contaminants. In addition, to gain insight into the nature of reactive oxidants, experiments were performed using probe compounds for reactive oxidants and electron paramagnetic resonance (EPR) spectroscopy.
2. Materials and methods 2.1. Reagents All chemicals were of reagent grade and used as received without further purification except for 2,4-dinitrophenyl hydrazine (DNPH). DNPH was recrystallized three times from acetonitrile to remove impurities. Chemicals used in this work include copper(II) sulfate, iron(II) sulfate, iron(III) perchlorate hydrate, H2O2 solution (35 wt.%), Sodium chloride (NaCl), perchloric acid (HClO4), sodium hydroxide (NaOH), 5,5-dimethyl-1-pyrrolidine N-oxide (DMPO), ethylenediaminetetraacetic acid (EDTA), propranolol. acetaminophen, caffeine, benzoic acid, 4hydroxybenzoic acid (4-HBA), phenol, 4-chlorophenol, carbamazepine, coumarin, 7hydroxycoumarin (7-HC), methanol, formaldehyde (HCHO), tert-butanol (all from SigmaAldrich Co.), and acetonitrile (J.T. Baker Co.). All solutions were prepared using 18.2 MΩ·cm Milli-Q water from a Milli-Q ultrapure water-purification system (Millipore Co.). Stock solutions of copper(II) sulfate (1 mM) and organic contaminants (5 M) were prepared and
6
stored at 4oC until use. The stock solution of H2O2 (1 M) was prepared freshly prior to the experiment. Natural seawater was collected from the east coast of Busan city, immediately filtered with a 0.45-µm Nylon membrane, and stored at 4°C until use; several water quality parameters of the seawater sample were presented in Table S1 (the supplementary data).
2.2. Batch experiments for oxidative degradation of organic contaminants Experiments were performed in a 100-mL Pyrex flask at room temperature (20 2oC). The reaction solution was prepared by adding Cu(II) (or Fe(II) for some experiments) and the organic contaminant (and Cl−). The solution pH was adjusted using 1 N HClO4 and 1 N NaOH solutions (1 mM phosphate buffer was used for pH 7). The reaction was initiated by adding an aliquot of a freshly prepared stock solution of H2O2. Samples (1 mL) were withdrawn at predetermined times and immediately filtered through a 0.45-µm PTFE syringe filter. The possible reactions of Cu(II) in samples were quenched by adding EDTA (4 mM). All experiments were carried out at least in duplicate, and the average values with standard deviations (error bars) were presented.
2.3. Experiments using oxidant probe compounds and EPR spectroscopy
7
Methanol, benzoic acid, and coumarin were used as probe compounds for reactive oxidants such as Cu(III) and hydroxyl radical (OH) and their oxidation products (i.e., HCHO, 4-HBA, and 7-HC, respectively) were analyzed. Excess probe compound was employed to maximize the scavenging of reactive oxidants and minimize the secondary oxidation of products [2, 11]. The EPR spectroscopy with DMPO as a spin-trapping agent was used to detect OH. A solution containing Cu(II), Cl, and DMPO was prepared in a Pyrex flask, and H2O2 was added to initiate the reaction. After 5 min of reaction, samples were withdrawn and the EPR signals of DMPOradical adducts were acquired by a CW/Pulse EPR system (JES-X310, JEOL Co.) under a microwave frequency of 9.414 GHz, a microwave power of 0.998 mW, and a modulation frequency of 100 kHz.
2.4. Analytical methods Organic compounds were analyzed using a HPLC equipped with a UV/vis absorbance detector; the detection wavelengths were 227 nm for benzoic acid, 230 nm for 4-chlorophenol, 241 nm for acetaminophen, 270 nm for caffeine, 270 nm for 4-HBA, 277 nm for phenol, 285 nm for propranolol, 285 nm for carbamazepine, 320 nm for 7-HC, and 350 nm for HCHO. HCHO was analyzed after DNPH derivatization [17]. Separation was performed on a ZORBAX Eclipse
8
XDB-C18 column (250 mm 4.6 mm, 5 m) using a binary mobile phase consisting of aqueous phosphoric acid solution (0.1 v/v %) and acetonitrile, at a flow rate of 1 – 1.5 mL/min. The concentration of H2O2 was measured by a UV/vis spectrophotometer (S-3100, Scinco Co.) using a titanium sulfate method [18].
3. Results 3.1. Carbamazepine degradation in the Cu(II)/H2O2 system in the presence of Cl The addition of Cl significantly increased the kinetics of carbamazepine degradation in copper-Fenton-like system; as much as 96% degradation was achieved within 2 h (Fig. 1a). In contrast, the carbamazepine degradation by the Cu(II)/H2O2 in the absence of Cl was minor (approximately 11% degradation in 2 h). The combination of H2O2 and Cl without Cu(II) also did not degrade carbamazepine during the entire reaction time. The decomposition of H2O2 by the Cu(II)/H2O2 system was correspondingly accelerated in the presence of Cl (Fig. 1b). The degradation of carbamazepine by the Cu(II)/H2O2 and Cu(II)/H2O2/Cl systems both followed pseudo first-order kinetics (Fig. S1 in the supplementary data). The observed rate constant for the carbamazepine degradation by the Cu(II)/H2O2/Cl system (k = 2.8 × 10-2 min1) was approximately 28-fold higher than that by the Cu(II)/H2O2 system (k = 1.0 × 10-3 min1).
9
3.2. Effects of Cu(II), H2O2, Cl concentrations and solution pH, and degradation of various organic contaminants The degradation rate of carbamazepine, expressed in terms of observed pseudo first-order rate constant k, increased as Cu(II), H2O2, and Cl concentrations increased (Fig. 2a). The increase of the k value gradually plateaued with increasing concentrations of Cu(II) and H2O2, whereas such a trend was not observed at elevated concentrations of Cl. The carbamazepine degradation was also significantly affected by the solution pH (Fig. 2b). The rate of carbamazepine degradation was the greatest at around pH 7.5, which was consistent with the trend previously reported in the Cu(II)/H2O2 system without Cl [9]. The Cu(II)/H2O2/Cl system was also found to be effective in degrading a range of micropollutants commonly found in water and wastewater (Fig. 3). All target contaminants, propranolol. acetaminophen, caffeine, benzoic acid, phenol, 4-chlorophenol and carbamazepine, were degraded by more than 90% in 2 h. The observed degradation rates were in the order of caffeine < benzoic acid < 4-chlorophenol < acetaminophen < carbamazepine < phenol < propranolol (refer to the inset of Fig. 3).
10
3.3. Oxidation of probe compounds and EPR spectroscopy A significantly higher amount of HCHO was produced (from the oxidation of methanol) in the Cu(II)/H2O2/Cl system compared to the Cu(II)/H2O2 system, while the HCHO production in the H2O2/Cl system was minor (Fig. S2a). In the Cu(II)/H2O2/Cl system, a significant amount of 4-HBA and 7-HC was also produced as the reaction proceeded, whereas both Cu(II)/H2O2 and H2O2/Cl systems produced a negligible amount of 4-HBA and 7-HC (Figs. S2b and S2c). When the concentrations of HCHO, 4-HBA and 7-HC after 2 h of reaction time were compared, the concentration of HCHO appeared much higher than those of 4-HBA and 7-HC for both the Cu(II)/H2O2 and Cu(II)/H2O2/Cl systems (Fig. 4a). The EPR spectroscopy results confirmed the production of the DMPO-OH spin adduct in the Cu(II)/H2O2/Cl system. The hyperfine splitting pattern was consistent with that of DMPO-OH reported in the literature as well as that observed in Fe(II)/H2O2 system which was employed herein as a control (hyperfine constants of aN = aH = 14.9 ± 0.1 G with an intensity ratio of 1:2:2:1) [19]. It is interesting to note that, despite lower DMPO-OH signal in Cu(II)/H2O2/Cl system than that in the Fe(II)/H2O2 system, the degradation rate of carbamazepine and the yield of HCHO were higher in the Cu(II)/H2O2/Cl system than the Fe(II)/H2O2 system (Fig. 4c).
11
3.4. Degradation of carbamazepine in seawater Results in Fig. 5 show the degradation of carbamazepine by the Cu(II)/H2O2 and Fe(II)/H2O2 systems in the natural seawater catchment (refer to Table S1 for the seawater constituents; carbamazepine was added at 5 M). The degradation of carbamazepine by the Cu(II)/H2O2 system was greatly enhanced in seawater compared to the DI water; the observed pseudo firstorder rate constant increased by 88-fold from 1.0 × 10-3 min1 to 8.8 × 10-2 min1 (Cu(II)/H2O2 vs. Cu(II)/H2O2/Seawater in Fig. 5a). The Cu(II)/H2O2/Cl system with Cl at the concentration equivalent to seawater (13,559 mg/L) exhibited the similar kinetics to the Cu(II)/H2O2/Seawater system. In contrast, the traditional Fenton system (Fe(II)/H2O2) exhibited the opposite trend (Fe(II)/H2O2 vs. Fe(II)/H2O2/Seawater in Fig. 5b). The addition of Cl (13,559 mg/L) to the Fe(II)/H2O2 system inhibited the carbamazepine degradation by approximately 15% in 2 h. The inhibitory effect was much greater in seawater, indicating that seawater constituents other than Cl additionally hinder the Fe(II)/H2O2 system.
4. Discussion 4.1. Role of Cl in copper-catalyzed Fenton-like reactions
12
It is well known that Cu(II)/H2O2 system produces reactive oxidants by activating H2O2 through the catalytic redox cycle involving the Cu(II)/Cu(I) couple. In the first step, Cu(II) is reduced to Cu(I) by the reaction with H2O2, producing superoxide radical anion (O2) (reaction 1). O2 can be further engaged in redox reactions with Cu(II) and Cu(I) (reactions not shown). Subsequently, Cu(I) reacts with H2O2 to produce reactive oxidants (OH and Cu(III) via one- and two-electron transfer mechanisms, respectively) that are capable of degrading organic contaminants (reaction 2). The reaction 1 is very slow and limits the overall kinetics for the production of reactive oxidants (i.e., RDS). Cu(II) + H2O2 → Cu(I) + H2O2
→
Cu(I) + O2 + H+ (k1 = < 1 M1 s1 at pH 7 [20])
(1)
Cu(II) + OH + OH or Cu(III) + 2OH
(k2 = 4 × 105 M1 s1 at pH 68 [15])
(2)
The addition of Cl changes the kinetics of the above redox reactions by forming copperchloro complexes (reactions 3 and 4) (refer to Figs S3S5 in the supplementary data for the speciation of Cu(I) and Cu(II) in the presence of Cl). The increase in the Cl concentration promotes the Cu(II) reduction by H2O2 by H2O2 because the redox potentials of Cu(II)/Cu(I)chloro complexes (e.g., CuCl+/CuCl0 and CuCl20/CuCl) are higher than that of Cu2+/Cu+ ion couple [15]. In the presence of 10,000 mg/L Cl (0.28 M), the k3 and k4 values were calculated as
13
~70 M1 s1 at pH 7.17 and ~170 M1 s1 at pH 68, respectively (reactions 3 and 4); those values were obtained by extra(intra)polating the data from Moffett and Zika (1987) [15]. Since the bottleneck due to the slow rate of reaction 1 is eliminated, the overall catalytic cycle of Cu(II)/Cu(I) is activated, resulting in the enhancement of organic contaminant degradation and H2O2 decomposition (Figs. 1a and 1b). Cu(II)-Cln + H2O2 → Cu(I)-Cln + O2 + H+ (k3 = ~70 M1 s1 at pH 7.17, [Cl]0 = 0.28 M [15]) Cu(I)-Cln + H2O2
(3)
→ Cu(II)-Cln + OH + OH or Cu(III)-Cln + 2OH
(k4 = ~170 M1 s1 at pH 68, [Cl]0 = 0.28 M [15])
(4)
4.2. Factors affecting the organic contaminant degradation by the Cu(II)/H2O2/Cl system Factors including initial concentrations of Cu(II), H2O2, and Cl, and solution pH affect the degradation rate of carbamazepine in the Cu(II)/H2O2/Cl system (Figs. 2a and 2b). The increase in the carbamazepine degradation rate (k) with increasing concentrations of Cu(II) and H2O2 (the reagents for the Fenton-like reactions) shown in Fig. 2a is likely to be simply related to the increased rate of reactive oxidant production. The plateauing of the k value at high concentrations of Cu(II) and H2O2 may be attributed to the scavenging of reactive oxidants by
14
Cu(I) and H2O2. The increase of the carbamazepine degradation rate with increasing the Cl concentration is due to the increasing formation of aforementioned Cu(II)-chloro complexes such as CuCl+, CuCl2, and CuCl3 (Fig. S5b in the supplementary data). The k value increased almost linearly with respect to the Cl concentration (Fig. 2a), showing consistent trends with the formation of Cu(II)-chloro complexes (Fig. S5b). This also suggests that the scavenging effect of reactive oxidants by Cl is minor. The degradation of carbamazepine was most efficient at pH 7.5 in the Cu(II)/H2O2/Cl system (Fig. 2b). This trend, also observed in Cu(II)/H2O2 system, is the result of a balance between two factors with opposite pH effects; i.e., as pH increases, more reactive oxidants are produced, but reactivity of some of these oxidants decreases [9]. First, the catalytic decomposition of H2O2 into reactive oxidants accelerates with increasing pH (regardless of the presence of Cl), since the rate of Cu(II) reduction by H2O2 (reactions 1 and 3) increases with increasing pH. The Cu(II) reduction by H2O2 (reactions 1 and 3) proceeds via the deprotonated H2O2 (HO2−, pKa of H2O2 = 11.75) through its complexation with Cu(II) and the subsequent ligand-to-metal charge transfer. Meanwhile, at higher pH, the Fenton-like reaction produces oxidants of less reactivity, most likely Cu(III)-hydroxo complexes [2, 11], that are less reactive toward carbamazepine degradation.
15
4.3. Nature of reactive oxidants produced by the Cu(II)/H2O2/Cl system There has been a controversy over the identity of reactive oxidants produced during the copper-catalyzed Fenton-like reactions (i.e., OH vs. Cu(III) from reaction 2) [21-23]. However, recent studies have provided more convincing pieces of evidence that Cu(III), present in different forms (Cu(III)-oxo and –hydroxo complexes, or complexed with potentially other (in)organic ligands), is a dominant oxidant rather than OH [2, 8-11]. Experimental results in this study also provide the evidence to rule out OH as the major oxidant, which resultingly supports the production of Cu(III) species, most likely Cu(III)-chloro complexes (or Cu(III)-chloro-hydroxo complexes), as major reactive oxidants in the Cu(II)/H2O2/Cl− system. First, the results obtained with oxidant probe compounds are not well explained by OH. The formation of hydroxylation products (4-HBA and 7-HC) from the oxidation of benzoic acid and coumarin was negligible compared to the HCHO formation from methanol oxidation (Fig. 4a). If OH had been produced, the yields of 4-HBA and 7-HC relative to HCHO should have reached approximately 23% and 29%, respectively; the maximum yields of HCHO, 4-HBA, and 7-HC by the reactions of methanol, benzoic acid, and coumarin with OH are known to be 100%, 23% and 29%, respectively [24-26].
16
Second, the EPR data were not sufficient to support the production of OH as the major reactive oxidant. The Cu(II)/H2O2/Cl− system indeed exhibited the DMPO-OH signal. However, the signal intensity was not high enough to prove the prevalence of OH; note that the Fe(II)/H2O2 system (a reference OH-producing system) produced less reactive oxidants (Fig. 4c), but exhibited a higher DMPO-OH signal than the Cu(II)/H2O2/Cl− system (Fig. 4b). In addition, the DMPO-OH signal in the Cu(II)/H2O2/Cl− system was not affected by the addition of tertbutanol, whereas the signal in the Fe(II)/H2O2 system significantly decreased in the presence of tert-butanol (Fig. S6 in the supplementary data). This observation indicates that the oxidants responsible for the DMPO-OH signal in the two systems are different, and suggests that the oxidant from the Cu(II)/H2O2/Cl− system may not be OH. The DMPO-OH spin adduct can be formed through pathways that do not involve OH [27]. However, additional studies are needed to elucidate the mechanism by which Cu(III) species (likely Cu(III)-chloro complexes in this study) can generate the DMPO-OH signal. It is possible that dichloride radical anion (Cl2−) is generated by the oxidation of Cl− by reactive oxidants (Cu(III)-chloro complexes). However, the inhibitory effect of methanol on the degradation of benzoic acid indicates that Cl2− does not prevail in the Cu(II)/H2O2/Cl− system (Fig. S7 in the supplementary data). If Cl2− were the oxidant responsible for the benzoic acid
17
degradation, the inhibitory effect of methanol should be less based on the reported rate constants (k(Cl2− + benzoate) = 2.0 106 M1s1 [27], k(Cl2− + methanol) = 3.5 103 M1s1 [28]).
4.4. Comparison with the iron-catalyzed Fenton (-like) reactions The effect of Cl− was remarkably different between the Fe(II)/H2O2 system and the Cu(II)/H2O2 system (Figs. 5a and 5b). The degradation of carbamazepine by the Fe(II)/H2O2 system was inhibited in the presence of Cl− (consistent with the observations in previous studies [29, 30]), and the inhibition was even greater in natural seawater (Fig. 5b). The negative effect of Cl− on the Fe(II)/H2O2 system was explained by (i) the formation of Fe(II)- and Fe(III)-chloro complexes which have similar or lower reactivity toward H2O2, and (ii) the scavenging of the reactive oxidant (OH) by Cl− and the subsequent conversion into less reactive Cl2− [31]. The greater inhibition of carbamazepine degradation in seawater may be due to the organic substances that serve as iron-chelators and radical scavengers. In contrast, the Cu(II)/H2O2 system in seawater exhibited a similar degradation rate of carbamazepine to the Cu(II)/H2O2/Cl− system at the equivalent Cl concentration, suggesting that the (in)organic constituents in seawater do not exert any inhibitory effects.
18
5. Conclusions The Cu(II)-catalyzed Fenton-like reaction enhanced by Cl− was demonstrated to effectively degrade organic contaminants at neutral pH at a relatively low Cu(II) dose. The concentration of Cu(II) mainly used in this study (1 M) is much lower than drinking water standard values (2 mg/L = 31.4 M for WHO [32], 1.3 mg/L = 20.5 M for the United States [33], and 1 mg/L = 15.7 M for Korea [34]). The enhanced kinetics is attributed to the formation of Cu(II)-chloro complexes that are more reactive with H2O2 than uncomplexed Cu(II), promoting the catalytic redox cycling of Cu(II)/Cu(I) for the decomposition of H2O2 into reactive oxidants. Cu(III) species (likely in the form of Cu(III)-chloro complexes) are suggested to be the major reactive oxidant produced in this Cu(II)/H2O2/Cl− system. These oxidants are believed to be as reactive as OH,
and capable of degrading a wide spectrum of organic contaminants. The Cu(II)/H2O2/Cl−
system can be a useful approach to treat organic contaminants in saline wastewater, brackish water, and seawater (e.g., wastewater from agro-food, petroleum and leather industries, aquaculture effluents, effluents from hydro fracking, etc.). The optimized activity at neutral pH and the enhancement by Cl− are great advantages of the copper-catalyzed Fenton-like reaction over the traditional Fenton reaction based on iron.
19
Acknowledgements This research was supported by Korea Ministry of Environment (2017000140005) and the National Research Foundation of Korea (NRF) Grant (NRF-2017R1A2B3006827 and NRF2015R1A5A7037825).
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Figure captions
Fig. 1. (a) Degradation of carbamazepine and (b) decomposition of H2O2 in the Cu(II)/H2O2, H2O2/Cl, and Cu(II)/H2O2/Cl systems ([Carbamazepine]0 = 5 M, [Cu(II)]0 = 1 M, [H2O2]0 = 10 mM, [Cl]0 = 10000 mg/L, pH 7).
Fig. 2. Effects of (a) Cu(II), H2O2, and Cl concentrations, and (b) solution pH on the degradation of carbamazepine by the Cu(II)/H2O2/Cl system ([Carbamazepine]0 = 5 M for (a) and (b), [Cu(II)]0 = 1 M for varying [H2O2] and [Cl] in (a), and (b), [H2O2]0 = 10 mM for varying [Cu(II)] and [Cl] in (a), and (b), [Cl]0 = 10000 mg/L for varying [Cu(II)] and [H2O2] in (a), and (b), pH 7 for (a)).
Fig. 3. Degradation of various organic contaminants by the Cu(II)/H2O2/Cl system ([Propranolol]0 = [Acetaminophen]0 = [Caffeine]0 = [Benzoic acid]0 = [Phenol]0 = [4-chlorophenol]0 = [Carbamazepine]0 = 5 M, [Cu(II)]0 = 1 M, [H2O2]0 = 10 mM, pH 7, the inset represents pseudo first-order rate constants.).
Fig. 4. (a) Production of HCHO, 4-HBA, and 7-HC from oxidation of probe compounds by the Cu(II)/H2O2 and Cu(II)/H2O2/Cl- systems ([Methanol]0 = 200 mM, [Benzoic acid]0 = 10 mM, [Coumarin]0 = 1 mM, [Cu(II)]0 = 1 M, [Cl]0 = 10000 mg/L, [H2O2]0 = 10 mM, pH 7, reaction time = 2 h), (b) EPR spectra obtained by spin trapping with DMPO in different systems ([Cu(II)]0 =
26
[Fe(II)]0 = 10 M, [Cl]0 = 10000 mg/L, [H2O2]0 = 10 mM, [DMPO]0 = 1 mM, pH 3 for Fe(II)/H2O2, pH 7 for Cu(II)/H2O2, H2O2/Cl, and Cu(II)/H2O2/Cl, reaction time = 0.5 min), (c) Degradation of carbamazepine and production of HCHO by those systems employed in the EPR analysis. ([Carbamazepine]0 = 5 M, [Methanol]0 = 200 mM, [Cu(II)]0 = [Fe(II)]0 = 10 M, [Cl]0 = 10000 mg/L, [H2O2]0 = 10 mM, pH 3 for Fe(II)/H2O2, pH 7 for Cu(II)/H2O2 and Cu(II)/H2O2/Cl, reaction time = 30 min for HCHO production experiments).
Fig. 5. Effects of Cl and seawater medium on degradation of carbamazepine by the (a) Cu(II)/H2O2 and (b) Fe(II)/H2O2 systems ([Carbamazepine]0 = 5 M, [Cu(II)]0 = 1 M, [H2O2]0 = 10 mM, pH 8 for (a), pH 3 for (b)).
27
(b) 1.0
0.8
0.6 Cu(II)/H2O 2
0.4
H2O 2/Cl-
[H2O2]/[H2O2]0
[Carbamazepine]/[Carbamazepine]0
(a) 1.0
Cu(II)/H2O 2/Cl-
0.9 0.6 0.4
Cu(II)/H2O 2 H2O 2/Cl-
0.2 0.2
Cu(II)/H2O 2/Cl-
0.0
0.0 0
30
60
90
0
120
30
60
90
Reaction time (min)
Reaction time (min)
Fig. 1
28
120
(a) 0.1
(b) Cu(II) H2O2
0.06
k (min-1)
Cl-
k (min-1)
0.01
0.04
0.02 0.001
[Cu(II)]0 (
)
[H2O2]0 (mM) -
0.1
1
10
100 0.00
1
10
100
3
[Cl ]0 (mg/L) 10
100
1000
4
5
6
7 pH
10000
Fig. 2
29
8
9
10
11
C/C0
Caffeine
Benzoic acid
4-chlorophenol
0.04
0.6
Acetaminophen
k (min-1 )
0.06
Carbamazepine
0.08
0.8
Phenol
0.10
Propranolol
1.0
0.02
0.00
0.4
0.2
0.0 0
30
60
90
Reaction time (min)
Fig. 3
30
120
Propranolol Phenol Carbamazepine Acetaminophen 4-chlorophenol Benzoic acid Caffeine
(c)
(b)
HCHO (from Methanol) 4-HBA (from Benzoic acid) 7-HC (from Coumarin acid)
Cu(II)/H2O2 0.20
0.09
HCHO k
80
Cu(II)/H2O2/Cl-
HCHO (mM)
120
0.06
Cl- /H2O2
Intensity (a. u.)
Products conc. (
160 0.15
0.03 0.10 0.00
Fe(II)/H2O2
0.05
40
-0.03 332
0 Cu(II)/H2O2
-
Cu(II)/H2O2/Cl
333
334
335
336
Magnetic Field (G)
Fig. 4
31
337
338
0.00 Cu(II)/H2O2
Cu(II)/H2O2/Cl-
Fe(II)/H2O2
k (min-1)
(a) 200
Fe(II)/H2O2
Cu(II)/H2O2 Cu(II)/H2O2/Cl
Fe(II)/H2O2/Cl-
Cu(II)/H2O2/Seawater
Fe(II)/H2O2/Seawater
-
kCu(II)/H 2O2/Seawater = 0.0881
6
0.6
-ln(C/C0)
[Carbamazepine]/[Carbamazepine]0
0.8
0.4
[Carbamazepine]/[Carbamazepine]0
(b) 1.0
(a) 1.0
R2 = 0.9921
4 kCu(II)/H 2O2/Cl- = 0.0854 R2 = 0.9748 2 kCu(II)/H 2O2 = 0.0010 R2 = 0.9921 0
0.2
0
30 60 90 Reaction time (min)
120
0.8
0.6
0.4
0.2
0.0
0.0 0
30
60
90
0
120
30
60
90
Reaction time (min)
Reaction time (min)
Fig. 5
32
120