Kinetics and mechanism of carbamazepine degradation by a modified Fenton-like reaction with ferric-nitrilotriacetate complexes

Journal of Hazardous Materials 252–253 (2013) 155–165

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Kinetics and mechanism of carbamazepine degradation by a modified Fenton-like reaction with ferric-nitrilotriacetate complexes Sheng-Peng Sun, Xia Zeng, Ann T. Lemley ∗ Graduate Field of Environmental Toxicology, Cornell University, FSAD, 273 HEB, Ithaca, NY 14853, United States

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

 NTA-modified Fenton-like reaction leads to rapid degradation of CBZ at pH 7.0.  Kinetics of CBZ degradation and influence of operating conditions were studied.  FeIII (NTA)(OH)2 2− is the most likely • active iron species for OH production.  Potential mechanism of • OH formation and degradation pathways of CBZ are proposed.  NTA is biodegradable; this system has good potential for environmental application.

a r t i c l e

i n f o

Article history: Received 8 January 2013 Received in revised form 20 February 2013 Accepted 24 February 2013 Available online xxx Keywords: Carbamazepine Fenton-like reaction Nitrilotriacetic acid Hydroxyl radical (• OH) Kinetics

a b s t r a c t This study investigated the kinetics and mechanism of carbamazepine (CBZ) degradation over an initial pH range of 5.0–9.0 by a modified Fenton-like reaction using ferric-nitrilotriacetate (FeIII -NTA) complexes. The results indicate that CBZ degradation by FeIII -NTA/H2 O2 can be described by pseudo first-order kinetics and mainly attributed to hydroxyl radical (• OH) attack. Ten intermediates were indentified during the degradation process, including hydroxy-CBZs, 10,11-epoxy-CBZ, quinonid CBZ derivatives, dihydroxy-CBZs, and hydroxy-CBZ-10,11-diols. The steady-state concentration of • OH, ranging from 3.8 × 10−16 to 2.1 × 10−13 M, was strongly dependent on the concentration of FeIII , the initial pH, and H2 O2 :FeIII and NTA:FeIII molar ratios. Optimal conditions of [FeIII ] = 1 × 10−4 M, [H2 O2 :FeIII ] = 155:1 and [NTA:FeIII ] = 3:1 were obtained for the degradation of CBZ at neutral pH (7.0) and ambient temperature (25 ◦ C); the corresponding degradation rate constant of CBZ, kapp , was 0.0419 (±0.002) min−1 . The value of kapp increased with increasing pH from 5.0 to 9.0 due to the strong pH-dependence of FeIII -NTA complexes; FeIII (NTA)(OH)2 2− was the most likely active iron species to activate H2 O2 to produce • OH. The temperature dependence of CBZ degradation by FeIII -NTA/H2 O2 was characterized by an activation energy of 76.16 kJ mol−1 . A potential mechanism for the formation of • OH by FeIII -NTA/H2 O2 and possible degradation pathways of CBZ are proposed. © 2013 Elsevier B.V. All rights reserved.

1. Introduction

∗ Corresponding author. Tel.: +1 607 255 1944; fax: +1 607 255 1093. E-mail address: [email protected] (A.T. Lemley). 0304-3894/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2013.02.045

Low level contamination by pharmaceuticals is an emerging environmental issue due to their extensive use and continuous release into the environment [1]. The presence of small concentrations of pharmaceuticals in the aquatic environment has been

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associated with chronic toxicity, endocrine disruption and the development of pathogen resistance [2]. Moreover, pharmaceuticals in surface water have the potential to contaminate sources of drinking water. Although most pharmaceuticals are not believed to pose significant human health threats at concentrations currently found in water, their presence is still a concern due to their unpredictable ecotoxicological effects and is the foundation for much of the resistance to widespread implementation of potable water reuse [3,4]. Carbamazepine (CBZ) is widely used for the treatment of epilepsy, trigeminal neuralgia and some psychiatric diseases [5]. It is highly persistent and resistant to biological treatment processes and has been frequently detected in effluents from sewage treatment plants (STPs) and in surface waters [6,7]. The removal efficiency of CBZ in conventional STPs is usually < 10% [8,9]. The removal of CBZ by constructed wetlands (24–36% in winter and 48% in summer) and sand filters (< 20%) is not successful [10,11]. The advanced wastewater treatment with membrane bioreactor (MBR) is also not effective (<20%) [12]. Recently, it was reported that fungi, Cunninghamella elegans, Umbelopsis ramanniana, Trametes versicolor, Ganoderma lucidum and Pleurotus ostreatus, can successfully metabolize CBZ [5,13]. However, the major metabolite, 10,11-epoxy-CBZ, has pharmacological activity similar to that of CBZ and is very stable in vitro/vivo [5,13]. MnVII and FeVI can oxidize CBZ rapidly (kCBZ,MnVII = 3 × 102 M−1 s−1 and kCBZ,FeVI = 70 M−1 s−1 ), but they mediate incomplete oxidation of CBZ to organic byproducts [14]. The efficient removal of CBZ is achieved by nanofiltration and carbon nanotube sorption [15,16]. Nevertheless, these methods are non-destructive, costly and/or potentially lead to secondary pollution. Therefore, it is still necessary to develop cost-effective treatment approaches. The hydroxyl radical (• OH) has received increased attention as a potential tool to eliminate so-called emerging micro-pollutants from water [17]. • OH is different from other oxidants due to its high reactivity and low selectivity. The second-order rate constant for CBZ with • OH is ∼8.8 × 109 M−1 s−1 [18]. Fenton/Fenton-like processes, where H2 O2 reacts with FeII /FeIII to generate • OH via radical chain reactions, are effective at treating micro-pollutants. One challenge, however, is to overcome the necessary acidic conditions (optimum pH 2.8–3.0). The use of chelating agents is successful by preventing precipitation of FeIII [19–26] at neutral pH. Ethylenediaminetetraacetic acid (EDTA) is the most commonly used agent and is effective; however, it is non-biodegradable. Nitrilotriacetic acid (NTA), however, is easily biodegradable and could be a safe and environmentally benign replacement for EDTA. The use of • OH in degrading three probe pesticides through decomposition of H2 O2 by FeII -/FeIII -NTA complexes at pH 7.0 was recently reported [27]. It was also reported that the H2 O2 decomposition rate by FeIII -NTA complexes was strongly influenced by the FeIII concentration, the NTA:FeIII molar ratio, the H2 O2 :FeIII molar ratio, and the solution pH [28]. This work, however, was done by the traditional optimization method (i.e., examining a single factor at a time while fixing all other variables at one level), which does not study the effect of interactions between multiple variables. Moreover, this study did not elucidate the mechanism for the formation of • OH. In the present study, we use CBZ as a probe and investigate its degradation kinetics and mechanism in a modified Fenton-like reaction with FeIII -NTA complexes to more completely understand the mechanism for the formation of • OH in these systems. The specific goals are: (i) to evaluate the potential of modified Fenton-like reactions with FeIII -NTA complexes to degrade CBZ in aqueous solution under an initial neutral pH (5.0–9.0); (ii) to investigate the kinetics and establish the optimal range of operating conditions for the efficient degradation of CBZ by employing response surface methodology (RSM); (iii) to identify the role of NTA in this system and to elucidate the mechanism of • OH formation; and (iv)

to identify the major intermediates and propose possible degradation pathways of CBZ. RSM is used as the approach for analyzing and modeling the effects of multiple variables and their responses and optimizing the process [29] because it is capable of evaluating not only a single variable influencing the degradation kinetics but also the interactions between the multiple variables, a better approach for understanding the system. 2. Experimental 2.1. Chemicals CBZ and NTA were obtained from Acros Organics Company (New Jersey, USA). Ferric chloride hexahydrate (FeCl3 •6H2 O), hydrogen peroxide (30%, w/w), formic acid and 2-propanol were obtained from Mallinckrodt Baker, Inc. (Phillipsburg, USA). HPLC grade methanol and pure water were obtained from Fisher Scientific Company (Fair Lawn, USA). All chemicals were analytical grade reagents and were used directly without further purification. MilliQ water (18.2 M cm, 25 ◦ C) was used for the preparation of solutions. 2.2. Kinetics experiments Each experiment was conducted in a 0.125 L glass conical flask, which was placed in a water bath. A magnetic stirrer was used to provide good mixing of the reaction solutions; the temperature was controlled by a HAAKE DC10-K20 refrigerated circulator. Aqueous stock solution of NTA (0.1 M) was prepared in 0.2 M sodium hydroxide, and aqueous stock solution of FeCl3 (0.1 M) was prepared in 0.1 M sulfuric acid. Typically, FeIII -NTA complexes were freshly prepared by mixing an appropriate amount of NTA and FeCl3 stock solution for each experiment. An appropriate amount of FeIII -NTA catalyst was added into 0.1 L CBZ aqueous solution. The solution pH was adjusted to the desired level with 1.0/0.1 M sodium hydroxide or 1.0/0.1 M sulfuric acid, which was measured using an Accumet AB15 pH meter (Fisher Scientific). The reaction was initiated by adding the appropriate amount of hydrogen peroxide, and a 1.0 ml sample was taken out at regular intervals and immediately mixed with 0.1 ml methanol to quench the reaction. 2.3. Experimental design A central composite design (CCD) with four factors and five levels was applied in the present study (Table 1). The independent input variables included the concentration of FeIII (X1 ), the initial pH (X2 ), and the molar ratios of H2 O2 : FeIII (X3 ) and NTA: FeIII (X4 ). The variables Xi were coded as xi according to Eq. (1): xi =

Xi − Xi,0 ıXi

, (i = 1, 2, . . . , 4)

(1)

where Xi is the real value of the independent variable, Xi,0 is the value of Xi at the center point of the investigated area and the ıXi is the step change. The response variable was fitted by a quadratic polynomial equation: Y = b0 +

n  i=1

bi xi +

n  i=1

bii xi 2 +

n n−1  

bij xi xj

(2)

i=1 j=i+1

where Y is the response; b0 is the offset term (constant); bi , bii and bij are the linear, quadratic and interaction coefficients, respectively; xi and xj are the code values of the independent input variables.

S.-P. Sun et al. / Journal of Hazardous Materials 252–253 (2013) 155–165

157

Table 1 Experimental ranges and levels of the variables tested in the CCD. Independent variables

Symbol

[FeIII ]0 (×10−3 M) Initial pH [H2 O2 ]0 : [FeIII ]0 (molar ratio) [NTA]0 : [FeIII ]0 (molar ratio)

X1 X2 X3 X4

Actual values of the coded variable levels −2

−1

0

1

2

0.02 5 5 0

0.06 6 55 1

0.10 7 105 2

0.14 8 155 3

0.18 9 205 4

2.4. Analytical methods

3.2. Kinetics of CBZ degradation by FeIII -NTA/H2 O2

The concentration of CBZ was determined by high-performance liquid chromatography (HPLC, Agilent 1200). The degradation intermediates were identified by an Agilent 1200 HPLC coupled to a 6130 quadrupole mass spectrometer. The details of the chromatographic conditions are provided in the supplementary data. The speciation of CBZ, NTA and FeIII -NTA complexes were modeled by using the simulation software AQUASIM 2.1d [30].

3. Results

The results show that the degradation kinetics of CBZ by FeIII -NTA/H2 O2 can be described by pseudo first-order reaction kinetics with respect to the aqueous concentration of CBZ. The apparent first-order kinetic rate constants, kapp , under given conditions for each optimization experiment, are presented in Table 2. The regression coefficients (R2 ) are greater than 0.95, indicating that the degradation kinetics of CBZ by FeIII NTA/H2 O2 follows pseudo first-order reaction kinetics well. Based on Eq. (3), the steady-state concentration of • OH was calculated (Table 2).

3.1. Degradation of CBZ at neutral pH

kapp = k•OH,CBZ ∗ [• OH]ss

Fig. 1 shows the degradation of CBZ by FeIII -NTA, NTA/H2 O2 , and FeIII -NTA/H2 O2 at neutral pH. No degradation of CBZ by FeIII -NTA or NTA/H2 O2 was observed, and the degradation efficiency of CBZ by FeIII /H2 O2 was only 3.3% after 180 min. Alternatively, significant degradation of CBZ was observed by FeIII NTA/H2 O2 , and the degradation efficiency of CBZ was > 99% after 120 min. Through confirmation by • OH scavenging experiments with 2-propanol (k• OH,2-propanol = 6 × 109 M−1 s−1 ) [31], the degradation of CBZ by FeIII -NTA/H2 O2 can be attributed to attack by • OH. Fig. 2 shows that the degradation of CBZ by FeIII -NTA/H2 O2 is significantly inhibited in the presence of 2-propanol. The inhibition efficiency was 40% and 80% at a molar ratio of 2-propanol:CBZ of 100:1 and 800:1, respectively. FeIII /H2 O2

(3)

is the second-order rate constant of CBZ with • OH

where k• OH,CBZ (8.8 × 109 M−1 s−1 ); and [• OH]ss is the steady-state concentration of • OH in the aqueous solution (M). It can be seen that the steadystate level of • OH is strongly dependent on the concentration of FeIII , initial pH, H2 O2 :FeIII and NTA:FeIII molar ratios, ranging from 3.8 × 10−16 to 2.1 × 10−13 M. 3.3. Optimization of the degradation of CBZ by RSM To better understand the influence of the identified variables on the degradation kinetics of CBZ by FeIII -NTA/H2 O2 , the kapp was used as the response variable of RSM. By fitting the values of kapp in Table 2 to Eq. (2), a quadratic polynomial model was derived as follows:

Y (min−1 ) = 0.02975 + 0.02258x1 + 0.01184x2 + 0.00793x3 + 0.00743x4 + 0.00553x12 +0.00043x22 − 0.00288x32 − 0.00398x42 + 0.00849x1 x2 + 0.00626x1 x3

(4)

+0.00561x1 x4 + 0.0038x2 x3 + 0.00068x2 x4 + 0.00168x3 x4 The analysis of variance (ANOVA) for Eq. (4) is shown in Table 3. The P-value of the model is smaller than 0.05, indicating that the 1.0

C2-propanol/CCBZ (molar ratio) 0 100:1 200:1 400:1 800:1

1.0

0.8

0.8

Fe -NTA NTA + H2O2

0.6

C/C 0

C/C0

III

III

Fe + H2O2

0.4

III

Fe -NTA + H2O2

0.4

0.2 0.0

0.6

0.2

0

20

40

60

80

100

120

140

160

180

Time (min) Fig. 1. Degradation of CBZ under different processes; ([CBZ] = 6.35 × 10−5 M, [FeIII ] = 1 × 10−4 M, [H2 O2 :FeIII ] = 105:1, [NTA:FeIII ] = 2:1, initial pH 7.0 and temperature = 25 ◦ C).

0.0

0

20

40

60

80

100

120

140

160

180

Time (min) Fig. 2. Effect of 2-propanol on the degradation of CBZ by FeIII -NTA/H2 O2 ; ([CBZ] = 6.35 × 10−5 M, [FeIII ] = 1 × 10−4 M, [H2 O2 :FeIII ] = 155:1, [NTA:FeIII ] = 3:1, initial pH 7.0 and temperature = 25 ◦ C).

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Table 2 Experimental design matrix and results of the degradation kinetics of CBZ by a NTA-modified Fenton-like process. Run

Actual values

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Coded values

Pseudo-first-order reaction kinetics

X1 [FeIII ]0 (×10−3 M)

X2 Initial pH

X3 [H2 O2 ]0 : [FeIII ]0 (molar ratio)

X4 [NTA]0 : [FeIII ]0 (molar ratio)

x1

x2

x3

x4

kapp (min−1 )

R2

0.10 0.06 0.06 0.14 0.10 0.10 0.10 0.10 0.06 0.10 0.10 0.14 0.10 0.10 0.10 0.14 0.06 0.06 0.14 0.10 0.14 0.02 0.18 0.06 0.14 0.06 0.14 0.06 0.10 0.14

7 6 6 8 5 7 9 7 8 7 7 8 7 7 7 8 8 8 6 7 6 7 7 6 6 6 6 8 7 8

105 55 155 155 105 105 105 105 55 205 105 55 105 105 5 155 55 155 55 105 155 105 105 55 55 155 155 155 105 55

2 1 1 3 2 2 2 2 1 2 0 1 4 2 2 1 3 3 3 2 3 2 2 3 1 3 1 1 2 3

0 −1 −1 1 0 0 0 0 −1 0 0 1 0 0 0 1 −1 −1 1 0 1 −2 2 −1 1 −1 1 −1 0 1

0 −1 −1 1 −2 0 2 0 1 0 0 1 0 0 0 1 1 1 −1 0 −1 0 0 −1 −1 −1 −1 1 0 1

0 −1 1 1 0 0 0 0 −1 2 0 −1 0 0 −2 1 −1 1 −1 0 1 0 0 −1 −1 1 1 1 0 −1

0 −1 −1 1 0 0 0 0 −1 0 −2 −1 2 0 0 −1 1 1 1 0 1 0 0 1 −1 1 −1 −1 0 1

0.0268 0.0020 0.0045 0.1083 0.0059 0.0287 0.0602 0.0289 0.0062 0.0354 0.0002 0.0375 0.0306 0.0318 0.0042 0.0711 0.0087 0.0147 0.0327 0.0324 0.0499 0.0013 0.1056 0.0050 0.0118 0.0074 0.0233 0.0092 0.0299 0.0565

0.9934 0.9972 0.9938 0.9613 0.9948 0.9908 0.9689 0.9917 0.9681 0.9907 0.9651 0.9816 0.9891 0.9885 0.9995 0.9620 0.9775 0.9872 0.9799 0.9887 0.9917 0.9965 0.9826 0.9984 0.9895 0.9950 0.9882 0.9585 0.9917 0.9731

model is significant. A high correlation coefficient (R2 ) of 0.9802 suggests a good agreement between the experimental values and the model predicted values. The validity of the model was further tested by several additional experiments, and the results are shown in Table 4. The comparison of the model predicted kapp of CBZ versus the experimental values is provided in Fig. S1. The ANOVA revealed a significant interaction effect of the initial pH and the concentration of FeIII on the degradation kinetics of CBZ (F = 37.69; P < 0.0001). Fig. 3(a) shows the kapp of CBZ as a function of the initial pH and the concentration of FeIII at a fixed H2 O2 :FeIII molar ratio of 105:1 and NTA:FeIII molar ratio of 2:1. At a low level

[OH]ss (×10−13 M)

0.5076 0.0379 0.0852 2.0511 0.1117 0.5436 1.1402 0.5473 0.1174 0.6705 0.0038 0.7102 0.5795 0.6023 0.0795 1.3466 0.1648 0.2784 0.6193 0.6136 0.9451 0.0246 2.0000 0.0947 0.2235 0.1402 0.4413 0.1742 0.5663 1.0701

of FeIII (0.02–0.05 mM), increasing the initial pH from 5.0 to 9.0 leads to a slight change in the degradation rate of CBZ; however, the values of kapp are dramatically improved (by 4–7 times) when increasing the initial pH from 5.0 to 9.0 at high levels of FeIII (> 0.1 mM). In addition, increasing the concentration of FeIII from 0.02 to 0.18 mM greatly improves the degradation rate of CBZ over the entire pH range investigated, and the improvement becomes more pronounced at a higher level of pH. A significant interaction effect of the molar ratio of NTA:FeIII and the concentration of FeIII on the degradation kinetics of CBZ was observed (F = 16.48; P = 0.0010). Fig. 3(b) shows the kapp of CBZ

Table 3 Analysis of variance (ANOVA) for the derived RSM model. Source a

Model X1 :FeIII X2 :pH X3 :H2 O2 :FeIII X4 :NTA:FeIII X1 × X2 X1 × X3 X1 × X4 X2 × X3 X2 × X4 X3 × X4 X1 2 X2 2 X3 2 X4 2 Residual Lack of fit Pure error Cor total

Sum of squares

Degree of freedom

Mean square

F-value

P-value

0.022765 0.01224 0.003365 0.001511 0.001326 0.001153 0.000628 0.000504 0.000231 7.29E−06 4.49E−05 0.000839 5.05E−06 0.000228 0.000435 0.000459 0.000437 2.18E−05 0.023224

14 1 1 1 1 1 1 1 1 1 1 1 1 1 1 15 10 5 29

0.001626 0.01224 0.003365 0.001511 0.001326 0.001153 0.000628 0.000504 0.000231 7.29E−06 4.49E−05 0.000839 5.05E−06 0.000228 0.000435 3.06E−05 4.37E−05 4.36E−06

53.17405 400.2678 110.0526 49.39534 43.36524 37.69145 20.52007 16.48147 7.555279 0.238392 1.467956 27.42116 0.165203 7.456864 14.2318

<0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 0.0004 0.0010 0.0149 0.6324 0.2444 0.0001 0.6902 0.0155 0.0018

10.0327

0.0100

a: R2 = 0.9802, adjusted R2 = 0.9618.

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159

Fig. 3. 3D response surface graphs of kapp values of CBZ showing the influence of variables: (a) the initial pH and the concentration of FeIII ; (b) the molar ratio of NTA:FeIII and the concentration of FeIII ; (c) the molar ratio of NTA:FeIII and the initial pH; (d) the molar ratio of H2 O2 :FeIII and the concentration of FeIII ; (e) the molar ratio of H2 O2 :FeIII and the initial pH; and (f) the molar ratios of NTA:FeIII and H2 O2 :FeIII (temperature = 25 ◦ C).

as a function of the molar ratio of NTA:FeIII and the concentration of FeIII at a fixed H2 O2 :FeIII molar ratio of 105:1 and initial pH 7.0. It can be seen that increasing the molar ratio of NTA:FeIII leads to improvement of the degradation rate of CBZ, and the improvement becomes more pronounced at a higher level of FeIII . An optimum molar ratio of NTA:FeIII is increased from 1.5:1 to 4:1 by increasing the concentration of FeIII from 0.02 to 0.18 mM. Moreover, the effect of the molar ratio of NTA:FeIII and the initial pH on the degradation kinetics of CBZ shows no interdependence (F = 0.24; P = 0.6324) (Fig. 3(c)). A significant interaction effect of the molar ratio of H2 O2 :FeIII and the concentration of FeIII on the degradation kinetics of CBZ

was also observed (F = 20.52; P = 0.0004). Fig. 3(d) shows the kapp of CBZ as a function of the molar ratio of H2 O2 :FeIII and the concentration of FeIII at a fixed NTA:FeIII molar ratio of 2:1 and initial pH 7.0. It can be seen that increasing the molar ratio of H2 O2 :FeIII leads to a slight change in the degradation rate of CBZ at a low level of FeIII ; however, the values of kapp are greatly improved by increasing the molar ratio of H2 O2 :FeIII at a high level of FeIII . An optimum molar ratio of H2 O2 :FeIII increases from 55:1 to 205:1 with an increase of FeIII concentration from 0.02 to 0.18 mM. Furthermore, a slight interaction between the molar ratio of H2 O2 :FeIII and the initial pH is also observed (F = 7.56; P = 0.0149). An optimum molar ratio of H2 O2 :FeIII increases from

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Table 4 Experimental conditions and results for model validation. Run

1 2 3 4 5 6 7 8 9 10 11 12

Actual values

Coded values

Experimental values

X1 [FeIII ]0 (×10−3 M)

X2 Initial pH

X3 [H2 O2 ]0 : [FeIII ]0 (molar ratio)

X4 [NTA]0 : [FeIII ]0 (molar ratio)

x1

x2

x3

x4

kapp (min−1 )

0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10

5 6 7 8 9 7 7 7 7 7 7 7

155 155 155 155 155 155 155 155 30 55 105 205

3 3 3 3 3 0.5 1 2 2 3 3 3

0 0 0 0 0 0 0 0 0 0 0 0

−2 −1 0 1 2 0 0 0 0 0 0 0

1 1 1 1 1 1 1 1 −1.5 −1 0 2

1 1 1 1 1 −1.5 −1 0 0 1 1 1

0.0090 0.0242 0.0419 0.0550 0.0819 0.0095 0.0199 0.0347 0.0137 0.0195 0.0285 0.0396

105:1 to 205:1 with an increase of the initial pH from 5.0 to 9.0 (Fig. 3(e)). The ANOVA revealed no interaction effect between the molar ratio of NTA:FeIII and the molar ratio of H2 O2 :FeIII (F = 1.47; P = 0.2444). Fig. 3(f) shows the kapp of CBZ as a function of the molar ratio of NTA:FeIII and the molar ratio of H2 O2 :FeIII at a fixed FeIII concentration of 0.1 mM and initial pH 7.0. The results indicate that the optimum molar ratios of NTA:FeIII and H2 O2 :FeIII were 3:1 and 155:1, respectively. The experimental kapp of CBZ was 0.0419 (±0.002) min−1 , which is very close to the predicted value of 0.0399 min−1 under the given conditions.

R2

Model predicted values kapp (min−1 )

0.9862 0.9866 0.9700 0.9839 0.9653 0.9496 0.9818 0.9856 0.9946 0.9931 0.9902 0.9908

0.0090 0.0240 0.0399 0.0567 0.0743 0.0122 0.0217 0.0348 0.0114 0.0207 0.0332 0.0409

orientation of bonds (stereoisomers) or have completely different structures (structural isomers) [32]. Fig. 5 shows CBZ degradation by FeIII -NTA/H2 O2 with the simultaneous formation and evolution of the major intermediates. The intermediates increased significantly during the first 30 min (70% removal of CBZ), and then began to decrease till they completely disappeared at 150 min.

3.4. Temperature-dependent degradation of CBZ by FeIII -NTA/H2 O2 Fig. 4(a) shows that the degradation of CBZ is significantly improved by increasing temperature from 15 to 45 ◦ C. The apparent activation energy (Ea ) for the degradation of CBZ by FeIII -NTA/H2 O2 was calculated according to the Arrhenius equation, Eq. (5). kapp = A exp(−

Ea ) RT

(5)

where A is the pre-exponential factor, Ea is the apparent activation energy (J mol−1 ), R is the ideal gas constant (8.314 J mol−1 K−1 ), and T is the absolute temperature (K). Fig. 4(b) shows the plot of lnkapp as a function of 1/T. A high regression coefficient (R2 = 0.9992) indicates a good linear relationship between lnkapp and 1/T. An Ea value of 76.16 kJ mol−1 was obtained from the slope of the plot (i.e., -Ea /R). The value of A was determined to be 9.04 × 1011 min−1 from the intercept of the plot (i.e., ln A). As a result, the kapp of CBZ as a function of the reaction temperature is characterized as follows: kapp = 9.04 × 1011 exp(−

76.16 × 103 ) RT

(6)

3.5. Identification and evolution of the degradation intermediates Ten intermediates were identified based on the molecular ion measurement (Fig. S2). They are classified into four groups including molecular ions at: (i) m/z = 250 (three intermediates); (ii) m/z = 252 (three intermediates); (iii) m/z = 268 (three intermediates); and (iv) m/z = 286 (one intermediate). The intermediates in each group have the same mass and empirical formula; however, their molecular structures are different because of their different retention times. This is a common phenomenon during radical attack irrespective of the active species used and leads to the formation of isobaric compounds that differ in their three-dimensional

Fig. 4. The effect of temperature on the degradation kinetics of CBZ by FeIII NTA/H2 O2 ; ([CBZ] = 6.35 × 10−5 M; [FeIII ] = 1 × 10−4 M, [H2 O2 :FeIII ] = 155:1, [NTA: FeIII ] = 3:1, and initial pH 7.0).

S.-P. Sun et al. / Journal of Hazardous Materials 252–253 (2013) 155–165

C/C0 (CBZ)

0.8 0.6 0.4 0.2 0.0

0

20

40

60

80

100

Time (min)

120

140

6x10

4

5x10

4

4x10

4

3x10

4

2x10

4

1x10

4

a CBZ species fraction

CBZ + [M+H] m/z =253 (total) + [M+H] m/z =251 (total) + [M+H] m/z =269 (total) + [M+H] m/z =287 (total)

Total Area

1.0

1.0 0.8 0.6

Cationic CBZ Neutral CBZ Anionic CBZ

0.4 0.2

0 160

0.0

NTA species fraction

b

4. Discussion 4.1. Role of NTA NTA itself does not react with CBZ or catalyze H2 O2 to generate active radicals. There is negligible degradation of CBZ by the traditional Fenton-like reaction with FeIII /H2 O2 because of the formation of ferric hydroxide complexes (e.g. Fe(OH)3 ) at pH > 5.0. The Fenton-like reactivity of Fe(OH)3 is too weak to induce the radical chain reactions. On the contrary, successful degradation of CBZ was obtained in the presence of NTA with a molar ratio of NTA:FeIII from 1:1 to 4:1. NTA complexes strongly with FeIII with a stability constant of log K = 15.9 [33]. The presence of NTA is essential to prevent formation of Fe(OH)3 at neutral pH. FeIII -NTA complexes are the dominant iron species in FeIII -NTA/H2 O, and the rapid degradation of CBZ clearly showed that FeIII -NTA complexes are able to activate H2 O2 to generate • OH. A critical molar ratio of NTA:FeIII of at least 1.5:1 is required to obtain efficient degradation of CBZ. Although a 1:1 stoichiometry of NTA to FeIII is capable of complexing all the FeIII , because both complexed and uncomplexed NTA are degraded by the attack of • OH [28] a critical ratio of NTA:FeIII > 1:1 is required. The optimum molar ratio of NTA:FeIII increased from 1.5:1 to 4:1 with increasing FeIII concentration from 0.02 to 0.18 mM since more • OH was generated at higher concentrations of FeIII , and correspondingly more NTA and FeIII -NTA complexes were consumed.

0

4

6

8

10

12

14

10

12

14

1.0 0.8 0.6

Neutral NTA NTA 2NTA 3NTA

0.4 0.2 0.0

0

2

4

6

8

pH Fig. 6. Speciation of CBZ and NTA as a function of solution pH.

4.2. pH-dependent speciation of FeIII -NTA complexes The maximum kapp of CBZ was obtained at an initial pH 9.0 rather than acidic pH. The pKa values of CBZ are pKa,1 = 2.3 (protonation of the nitrogen atom in the seven-member ring of CBZ) and pKa,2 = 13.9 (deprotonation of the -NH2 group) [15,34,35]; therefore, neutral CBZ is the predominant species at pH 5.0–9.0 (∼100%) (Fig. 6(a)), and a change in the initial pH did not change the fraction of CBZ species. Additionally, the pKa values of NTA are pKa,1 = 1.5, pKa,2 = 2.52 and pKa,3 = 9.59 [33]. More than 99% of uncomplexed NTA, if any, is present in solution as NTA2− at pH 5.0–7.7 (Fig. 6(b)). The kapp of CBZ increased with increasing initial pH from 5.0 to

0.09

1.0

III

Fe (NTA)(H2O)2(H)

0.08

III

2

pH

Fig. 5. The time profile of CBZ degradation and peak area of major intermediates detected by LC-ESI/APCI-MS; ([CBZ] = 6.35 × 10−5 M; [FeIII ] = 1 × 10−4 M, [H2 O2 :FeIII ] = 155:1, [NTA:FeIII ] = 3:1, initial pH 7.0 and temperature = 25 ◦ C).

0.8

III

0.07 0.05 0.04

0.4

0.03 0.02

0.2

-1

0.06

0.6

+

III

Fe (NTA)(H2O)2

kapp (min )

Fe (NTA) species fraction

161

-

Fe (NTA)(H2O)(OH) 2III Fe (NTA)(OH)2 III

Fe (NTA)(OH)3

3-

III

kapp at NTA/Fe = 1:1 III

kapp at NTA/Fe = 2:1 III

kapp at NTA/Fe = 3:1 III

kapp at NTA/Fe = 4:1

0.01 0.0

2

3

4

5

6

7

8

9

10

11

12

0.00

pH Fig. 7. Effect of pH on the fractions of FeIII (NTA)(H2 O)2 , FeIII (NTA)(H2 O)(OH)- , FeIII (NTA)(OH)2 2− and FeIII (NTA)(OH)3 3− and the kapp of CBZ by FeIII -NTA/H2 O2 ; ([CBZ] = 6.35 × 10−5 M; [FeIII ] = 1 × 10−4 M, [H2 O2 :FeIII ] = 155:1, [NTA:FeIII ] = 1:1 ∼ 4:1 and temperature = 25 ◦ C).

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Fig. 8. Proposed possible degradation pathways of CBZ by FeIII -NTA/H2 O2 .

7.0. Thus, it can be concluded that the increase of the kapp of CBZ is not due to changes in speciation of uncomplexed NTA, and, as a result, the influence of the initial pH on the degradation of CBZ by FeIII -NTA/H2 O2 is most likely attributable to the pH-dependent speciation of FeIII -NTA complexes. The following 1:1 complexes: FeIII (NTA)(H2 O)2 , FeIII (NTA)(H2 O)(OH)− , FeIII (NTA)(OH)2 2− and FeIII (NTA)(OH)3 3− are the predominant species of FeIII -NTA complexes depending on the solution pH [33,36]. Although some 2:1 complex (FeIII (NTA)2 3− species) is probably formed at a molar ratio of NTA:FeIII > 1:1, the

increase of the kapp of CBZ resulting from increasing the initial pH from 5.0 to 7.0 is not likely due to changes in the FeIII (NTA)2 3− species fraction with pH because, as shown in a reported study, the FeIII (NTA)2 3− species is mainly present at pH 4.0–8.0, and its fraction is not significantly changed at pH 5.0–7.0 [33]. Additionally, the fraction of the FeIII (NTA)2 3− species is smaller than that of FeIII (NTA)(H2 O)(OH)− at a low concentration of FeIII (< 1.0 mM) even when the molar ratio of NTA:FeIII is 3:1 [36]. The proton- and concentration-dependent equilibrium between the 1:1 complexes can be established rapidly according to the following equations

S.-P. Sun et al. / Journal of Hazardous Materials 252–253 (2013) 155–165

163

Fig. 9. Mechanism for the formation of • OH by FeIII -NTA/H2 O2 .

4.3. Role of H2 O2 :FeIII

[33,36]: FeIII (NTA)(H2 O)2 (H)+ ↔ FeIII (NTA)(H2 O)2 + H+ logK0 = -1.03 (7)

FeIII (NTA)(H2 O)2 ↔ FeIII (NTA)(H2 O)(OH)− + H+ logK1 = -4.36 (8)

FeIII (NTA)(H2 O)(OH)− ↔ FeIII (NTA)(OH)2 2− + H+ logK2 = -7.58 (9)

An optimum molar ratio of H2 O2 :FeIII was observed at a fixed level of FeIII . The degradation rate of CBZ did not further improve when the ratio of H2 O2 :FeIII was increased above the optimum value. This can be attributed to the increased scavenging of • OH by excessive H2 O2 . Additionally, the results above clearly indicate that the optimum molar ratio of H2 O2 :FeIII increased from 55:1 to 205:1 when increasing the concentration of FeIII from 0.02 to 0.18 mM. This is because the rate of • OH generation was significantly improved at a high level of FeIII , leading to a larger consumption of H2 O2 via Eq. (11). • OH

+ H2 O2 → •OOH + H2 Ok = 2.7 × 107 M−1 s−1

(11)

4.4. Degradation intermediates and possible pathways

FeIII (NTA)(OH)2 2− ↔ FeIII (NTA)(OH)3 3− + H+ logK3 = -10.72 (10) FeIII (NTA)(H2 O)2 has a monomeric structure coordinated with H2 O; FeIII (NTA)(H2 O)(OH)− has a dimeric structure with ␮-hydroxyl-bridges to FeIII , formed by the dehydration and deprotonation of FeIII (NTA)(H2 O)2 ; FeIII (NTA)(OH)2 2− has a dimeric structure with ␮-oxo-bridges to FeIII , formed by the deprotonation of FeIII (NTA)(H2 O)(OH)− [37]. FeIII (NTA)(OH)3 3− is formed at high pH. The fractions of FeIII (NTA)(H2 O)2 , FeIII (NTA)(H2 O)(OH)− , FeIII (NTA)(OH)2 2− and FeIII (NTA)(OH)3 3− as a function of pH were modeled based on the literature equilibrium constants of Eqs. (7)–(10) (The FeIII (NTA)2 3− species was not included in the present model), and the results are shown in Fig. 7. At pH 5.0–9.0: (i) the fraction of FeIII (NTA)(H2 O)2 decreases significantly with the increase of pH, becoming extremely low at pH > 7.0; (ii) the fraction of FeIII (NTA)(H2 O)(OH)− reaches a maximum at pH 6.0, and then decreases gradually with further increase of pH; (iii) the fraction of FeIII (NTA)(OH)2 2− increases continually with increasing pH from 5.0 to 9.0. Most importantly, changes in the fraction of FeIII (NTA)(OH)2 2− showed a trend similar to the kapp of CBZ found in the present work. These results suggest that FeIII (NTA)(OH)2 2− is likely to be more active in catalyzing H2 O2 to generate • OH and/or has a lower • OH scavenging activity than the other species of FeIII NTA complexes.

The intermediates at m/z = 252 were the principal initial degradation products of CBZ. They can be directly attributed to the attack of • OH resulting in the hydroxylation of either the double bond or the aromatic rings of CBZ (+ m/z 16), a major pathway for • OH-induced degradation of organic molecules. Additionally, an alternative pathway is the attack of • OH on the central seven member ring of CBZ, through a non-detected intermediate, to form 10,11-epoxy-CBZ [38]. The intermediates at m/z = 250 were the secondary predominant degradation products. The mechanism might involve an intramolecular reaction, with H2 O loss arising from a non-detected transient dihydroxy-CBZ radical and/or quinonidCBZ. Similar intermediates were detected in the photodegradation of CBZ with FeIII [39], photocatalytic degradation of CBZ by TiO2 [38], and CBZ degradation by ultrasonic/Fe0 /H2 O2 [40]. The intermediates at m/z = 268 were dihydroxy-CBZ species, which were generated from the reaction between • OH and hydroxy-CBZ. Furthermore, the intermediate at m/z = 286 might be generated from rapid hydroxylation of an undetected compound, 10,11-,dihydro10,11-dihydroxycarbamazepine (CBZ-10,11-diol), through attack by • OH. All intermediates disappeared completely after treatment, implying that these major intermediates were further degraded by • OH. In addition, the pH of the solution decreased from 7.0 to 3.9–4.1 after the complete degradation of CBZ due to the formation of short chain organic acids such as formic and oxalic acid.

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Fig. 8 represents the possible degradation pathways of CBZ by FeIII NTA/H2 O2 . 4.5. Mechanism for • OH formation by FeIII -NTA/H2 O2 The results above provide evidence that the • OH generated by FeIII -NTA/H2 O2 played a major role in CBZ degradation. In addition, FeIII (NTA)(OH)2 2− was the most likely active iron species to catalyze H2 O2 to produce • OH. • OH chain paths have been considered a general mechanism for the decomposition of H2 O2 by a variety of iron complexes [41]. However, the mechanism for the formation of • OH by FeIII -NTA/H2 O2 is still not established. Previous study reported that a complex (i.e., FeIII (EDTA)O2 3− ) formed from FeIII -EDTA and H2 O2 at pH 8.2– 10 might play an important role in catalyzing both H2 O2 decomposition and organic substrates oxidation [41]. Recently, the formation of intermediates (most likely peroxocomplexes of FeIII -NTA) from the reaction of FeIII -NTA and H2 O2 at neutral pH was reported [28]. On the basis of previous studies and this study, a potential mechanism for the formation of • OH by FeIII -NTA/H2 O2 is proposed (Fig. 9). The initiation step is: FeIII (NTA)(OH)2 2− initially reacts with H2 O2 to form a complex, named as FeIII (NTA)(OH)2 2− -H2 O2 . FeIII (NTA)(OH)2 2− -H2 O2 may undergo a ground-state one electron transfer from ligand to metal, resulting in generation of FeII -NTA complexes and peroxide radical (•OOH)/superoxide radical (O2 •− ). The propagation step is: the resulting FeII -NTA complexes can react intermediately with H2 O2 to produce • OH and be oxidized to FeIII (NTA)(OH)2 2− . • OH reacts nonselectively with H2 O2 , yielding •OOH/O2 •− . •OOH/O2 •− is capable of both reducing FeIII (NTA)(OH)2 2− to FeII -NTA complexes and oxidizing FeII -NTA complexes to FeIII (NTA)(OH)2 2− , or undergoing self-reaction to form H2 O2 . The reduction of FeIII (NTA)(OH)2 2− to FeII -NTA complexes by •OOH/O2 •− is considered to be much faster than that by H2 O2 . The termination step is: • OH reacts with the organic substances in the solution at diffusion-limited rates, including CBZ and its degradation products, complexed and uncomplexed NTA. Moreover, • OH can also react with •OOH/O2 •− and/or undergo recombination to terminate the chain. 5. Conclusion This study provides insight into the kinetics and mechanism of CBZ degradation by a modified Fenton-like reaction with FeIII NTA complexes. It can be concluded that FeIII -NTA complexes are able to activate H2 O2 to generate • OH at neutral pH, with FeIII (NTA)(OH)2 2− the most likely active iron species. Based on the results with CBZ, the addition of NTA to the traditional Fentonlike process with FeIII /H2 O2 provided efficient degradation of CBZ by FeIII -NTA/H2 O2 at an initial pH range of 7.0–9.0. Since NTA is biodegradable, this system has good potential for environmental applications. The optimal range of operating conditions obtained by employing experimental design methodology can provide a better understanding of the influence of the identified variables on the degradation kinetics of CBZ by FeIII -NTA/H2 O2 , and this will allow the system to be tested under realistic conditions. The major degradation intermediates were identified as hydroxy-CBZs, 10,11-epoxy-CBZ, quinonid CBZ derivatives, dihydroxy-CBZs and hydroxy-CBZ-10,11-diols, and these intermediates can be further degraded by • OH attack and disappear completely after treatment, an important aspect of treating toxic materials. The potential mechanism proposed for the formation of • OH by FeIII -NTA/H2 O2 and for the possible degradation pathways of CBZ provided better understanding of the degradation mechanism in this system which will be useful in future applications.

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