H2O2 with nitrilotriacetic acid

Chemical Engineering Journal 244 (2014) 44–49

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Enhanced heterogeneous and homogeneous Fenton-like degradation of carbamazepine by nano-Fe3O4/H2O2 with nitrilotriacetic acid Sheng-Peng Sun a, Xia Zeng a, Chun Li b, Ann T. Lemley a,⇑ a b

Graduate Field of Environmental Toxicology, Cornell University, FSAD, 273 HEB, Ithaca, NY 14853, United States Department of Chemistry, Ithaca College, 355 Ctr for Natural Sciences, Ithaca, NY 14850, 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

 Degradation kinetics of CBZ by nano-

Model of heterogeneous and homogeneous Fenton-like degradation of carbamazepine by nano-Fe3O4/ H2O2 with NTA.

Fe3O4/H2O2 with NTA was investigated.  Enhanced degradation of CBZ was obtained in the presence of NTA.  Use of NTA greatly decreases the usages of H2O2 and nano-Fe3O4.  The mechanism involves both heterogeneous and homogeneous Fenton-like reactions.

Homogeneous Fenton-like reaction

FeII - / FeIII-NTA

Bulk

aq

Degradation products

H2O2

OH aq

aq

NTA

CBZ

Fe3O4 FeII- /

FeIII-NTA

H2O2

OH Surf

Surf

CBZ

Surf

Fe3O4 Degradation products

Heterogeneous Fenton-like reaction

a r t i c l e

i n f o

Article history: Received 15 November 2013 Received in revised form 13 January 2014 Accepted 15 January 2014 Available online 24 January 2014 Keywords: Fenton-like reaction Nano-Fe3O4 Nitrilotriacetic acid Carbamazepine Fe-NTA complexes

H2O + O2

a b s t r a c t The influence of a biodegradable agent, nitrilotriacetic acid (NTA), on the degradation of the model compound carbamazepine (CBZ) by the nano-Fe3O4/H2O2 system was investigated. The results showed that the presence of NTA can provide better performance of the nano-Fe3O4/H2O2 system for CBZ degradation at reduced usage of H2O2 and nano-Fe3O4. The optimal concentration of NTA was determined to be 0.5– 1.0 mM. The first-order rate constant, kapp, for CBZ degradation by 1.0 g L1 of nano-Fe3O4 and 100 mM of H2O2 in the presence of 0.5 mM of NTA at an initial neutral pH was determined to be 4.32  102 min1, which was 80 times larger than that in the absence of NTA. In addition, the kapp value increased with the increase in nano-Fe3O4 concentration from 0.1 to 3.0 g L1, and the H2O2 concentration increased from 5 to 600 mM. The initial pH of CBZ solutions over a range of pH 5.0–9.0 did not affect the kapp value of CBZ (p = 0.08). A decrease in the initial concentration of CBZ from 6.35  102 to 1.06  102 mM resulted in an increase of the kapp value from 4.32  102 to 1.51  101 min1. Moreover, the results indicated that the degradation intermediates were generated by the attack of OH. The degradation mechanism of CBZ included both an enhanced heterogeneous Fenton-like reaction on the Fe3O4 nanoparticle surface and an enhanced homogeneous Fenton-like reaction in the aqueous phase by Fe-NTA complexes. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction In situ chemical oxidation (ISCO) is attracting increased attention for the treatment of toxic and biorefractory contaminants ⇑ Corresponding author. Tel.: +1 (607) 255 1944; fax: +1 (607) 255 1093. E-mail address: [email protected] (A.T. Lemley). 1385-8947/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2014.01.039

in groundwater and soils [1–3]. Previous studies have shown that various organic contaminants in water and soil can be oxidized by H2O2 in the presence of iron minerals such as ferrihydrite, goethite, hematite, pyrite and magnetite [4–9]. The formation of hydroxyl radical (OH) has been proved to be the reactive species for the degradation of contaminants in this system [7–10]. The significant advantage of this reaction over the traditional Fenton’s

S.-P. Sun et al. / Chemical Engineering Journal 244 (2014) 44–49

reagent is that it occurs at neutral pH or nearly neutral pH and with less generation of iron-containing sludge. Magnetite and pyrite are the most effective catalysts as compared to the other iron oxides due to the FeII in their structure, which enhances the generation rate of OH [5,10]. The added advantage in the system is that magnetite can be easily separated by magnetic separation after the reaction. However, one major disadvantage in this process is that a large amount of H2O2 is required in order to achieve desirable degradation rates and efficiencies [7,9,11]. This is due to the fact that the decomposition of H2O2 by iron minerals goes through both a radical pathway to generate OH and also through a non-radical pathway by which the H2O2 is directly decomposed to H2O and O2 at neutral pH [10,11]. Chelating agents have recently been used in such systems to enhance the degradation of contaminants. Work in the literature reported the degradation of pentachlorophenol (PCP) by a Fenton-like reaction with micro-sized magnetite (micro-Fe3O4) in the presence of chelating agents with rates in the following order: oxalate > ethylenediaminetetraacetic acid (EDTA) > carboxymethyl-b-cyclodextrin (CMCD) > citric acid > tartaric acid > succinic acid > none chelating agent [12]. The degradation of bisphenol A (BPA) by a Fenton-like reaction with nano-sized BiFeO3 (nanoBiFeO3) in the presence of chelating agents had the order: EDTA > nitrilotriacetic acid (NTA) > glycine > formic acid > tartaric acid > none chelating agent [13]. The positive impact of the chelating agents in these systems is obvious; however, understanding of the mechanism is still inconsistent. One explanation is that the dissolved iron from iron oxides by the chelating agents leads to an enhancement of the homogeneous Fenton-like reaction in the aqueous solution [12]. Another explanation by Wang et al. reported that the improvement of the OH formation on the nano-BiFeO3 surface by the chelating agents is responsible for the enhanced degradation of BPA, i.e. via an enhanced heterogeneous Fenton-like reaction but not related to the dissolved iron from nano-BiFeO3 [13]. Although EDTA has been successfully used in the enhanced homogeneous and heterogeneous Fenton-like reaction at neutral and near neutral pH conditions [1,12–19], it is nonbiodegradable in the environment. Thus, residual metal-EDTA complexes could lead to adverse health and environmental effects [20]. As a promising substitute for EDTA, NTA is a readily biodegradable agent [20]. Our previous study reported the efficient degradation of the emerging contaminant carbamazepine (CBZ) by a modified Fenton-like reaction with FeIII-NTA complexes at an initial pH range of 7.0–9.0 [21]. In addition, the modified Fenton-like reaction with FeIII-NTA complexes has also been proved to be effective for the degradation of pesticides such as 2,4-dichlorophenoxyacetic acid (2,4-D), atrazine, fenuron and parachlorobenzoic acid at pH of 6.0–7.0 [22,23]. FeIII ðNTAÞðOHÞ2 complexes have been demon2 strated to be the most likely active iron species in catalyzing H2O2 to generate OH, and/or have a lower OH scavenging activity compared to the other species of FeIII-NTA complexes [21]. Moreover, we also observed the first successful use of the heterogeneous Fenton-like reaction with nano-Fe3O4 for the effective degradation of CBZ at neutral pH [7]. However, the effect of NTA on the degradation of contaminants by the nano-Fe3O4/H2O2 system at neutral pH has not been reported. In the present study, we use CBZ as a model contaminant to explore this system. The objectives of the study are to (i) assess how the presence of NTA influences the performance of the nano-Fe3O4/H2O2 system for CBZ degradation at neutral pH; (ii) evaluate the effect of NTA concentration, Fe3O4 concentration, H2O2 concentration, the initial pH and the initial concentration of CBZ on the degradation kinetics of CBZ; and (iii) clarify the degradation mechanism of CBZ by the nano-Fe3O4/ H2O2 system in the presence of NTA, i.e., homogeneous and/or heterogeneous Fenton-like reactions.

45

2. Materials and methods 2.1. Chemicals The nano-Fe3O4 used in this study was obtained from Nanostructured & Amorphous Materials Inc. (Houston, Texas). The average diameter of the nano-Fe3O4 was 30 nm and the specific surface area was 48 ± 2 m2 g1 [24]. CBZ and NTA were obtained from Acros Organics Company (New Jersey, USA). H2O2 (30%, w/w) was 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 was used for the preparation of solutions. 2.2. Batch kinetic experiments The degradation experiments were carried out in 50 ml polypropylene centrifuge tubes at 23 ± 2 °C in the dark. The NTA stock solution (0.1 M of NTA) was prepared in 0.2 M of NaOH. In general, an appropriate dosage of NTA was added to a CBZ aqueous solution. The solution pH was then adjusted to a desired value using 0.1 M H2SO4 and 0.1 M NaOH. After the pH adjustment, 25 ml of the prepared solution was mixed with an appropriate dosage of nano-Fe3O4 in a 30 ml glass bottle, and the mixture was subjected to ultrasonic irradiation for 3 min in an ultrasound bath (100 W, 42 kHz, Branson 2510R-DTH). No significant degradation of CBZ (<3%) was observed during the ultrasonic irradiation. The mixture was then transferred to a 50 ml centrifuge tube and filled to 50 ml with the prepared solution. The suspensions were shaken for 1 h to reach the adsorption equilibrium prior to the addition of H2O2. Samples taken before and after shaking showed that the physical adsorption of CBZ on nano-Fe3O4 was 2–3%. The Fenton-like reaction was started by adding an appropriate dosage of H2O2. At different time intervals, 1.0 ml of suspension samples were collected in 1.5 ml centrifuge tubes and immediately mixed with 0.1 ml of methanol to quench the reaction. The quenched samples were centrifuged three times (5 min each) at 13,400 rpm to separate Fe3O4 nanoparticles, and the supernatant liquid was collected for analysis. The degradation of CBZ in the aqueous phase was performed as follows: After 1 h adsorption in the dark, Fe3O4 nanoparticles were removed from the solution by centrifugation (6000 rpm) and followed by filtration through 0.2 lm filters. The same concentration of H2O2 was then applied to the collected aqueous phase. All experiments were performed at least twice or more. 2.3. Analytical methods The change in CBZ concentration was determined by the HPLC method, and the degradation intermediates were identified by LC–MS. The details have been reported in a previous study [7]. The total dissolved iron was determined by atomic absorption spectrophotometry (Shimadzu AA-6300, Shimadzu Scientific Instruments, Inc.). 3. Results and discussion 3.1. Comparison of CBZ degradation by the nano-Fe3O4/H2O2 system in the absence and presence of NTA Fig. 1 shows a comparison of the degradation of CBZ by the nano-Fe3O4/H2O2 system in the absence and presence of NTA at neutral pH. It can be seen that the presence of NTA dramatically improved the degradation efficiency of CBZ. In the absence of

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S.-P. Sun et al. / Chemical Engineering Journal 244 (2014) 44–49

creased from 6.60  103 to 5.63  102 min1. The kapp values reached a plateau and did not increase further with an NTA concentration from 1.0 to 4.0 mM. This result can be explained by the surface-controlled reactions between NTA and nano-Fe3O4. At a low level of NTA concentration (i.e., <1.0 mM), the surface concentration of NTA on the nano-Fe3O4 particles increased with the increase of NTA concentration. However, the surface concentration of NTA reached maximal values when NTA concentration was >1.0 mM. As a result, no significant changes in the kinetics of CBA degradation were observed by increasing NTA concentration from 1.0 to 3.0 mM, but it is worth noting that the kapp value decreased slightly at 4.0 mM of NTA. This is most probably due to the competitive consumption of OH by the excess NTA at high concentration (kOH,NTA = 108–109 M1 s1) [25]. Fig. 1. CBZ degradation by nano-Fe3O4/H2O2 system in the absence and presence of NTA. Experimental conditions: CBZ = 6.35  102 mM, Fe3O4 = 1.0 g L1, H2O2 = 100 mM, NTA = 0.5 mM, initial pH = 7.0 and temperature = 23 ± 2 °C.

NTA, only 6% of CBZ was degraded after 120 min of reaction time by 1.0 g L1 of nano-Fe3O4 and 100 mM of H2O2. However, the degradation efficiency of CBZ was >99% in the presence of 0.5 mM of NTA under the same conditions. It was found that the degradation kinetics of CBZ in both experiments followed pseudo first-order reaction kinetics well, with R2 values >0.99. The pseudo first-order rate constant, kapp, for the degradation of CBZ by 1.0 g L1 of Fe3O4 and 100 mM of H2O2 in the presence of 0.5 mM of NTA was determined to be 4.32  102 min1, which was 80 times larger than that in the absence of NTA (kapp = 5.25  104 min1). In addition, this value is 14 times larger than that of CBZ degradation by 1.84 g L1 of nano-Fe3O4 and 600 mM of H2O2 in the absence of NTA (kapp = 3.03  103 min1) that was demonstrated in previous work [7]. Therefore, the results clearly demonstrate that the presence of NTA can provide better performance of the nano-Fe3O4/ H2O2 system for CBZ degradation at reduced concentrations of nano-Fe3O4 and H2O2.

3.2. Effect of NTA concentration

0.10 80 0.08 60

0.06

40 20 0 0.0

Kinetic rate constant kapp

Reaction time 10 min 30 min 60 min 120 min

0.04 0.02

0.12

0.00 0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

NTA concentration (mM) Fig. 2. Effect of NTA concentration on the degradation efficiency and kinetics of CBZ by enhanced Fenton-like reaction of nano-Fe3O4/H2O2 with NTA. Experimental conditions: CBZ = 6.35  102 mM, Fe3O4 = 1.0 g L1, H2O2 = 100 mM, initial pH = 7.0 and temperature = 23 ± 2 °C.

100 0.10 80 0.08 60

0.06 Reaction time 10 min 30 min 60 min 120 min

40 20

0.04 0.02

Kinetic rate constant kapp

0 0.0

Pseudo-first-order kinetic rate -1 constant, kapp (min )

100

Fig. 3 shows the effect of nano-Fe3O4 concentration on the degradation of CBZ by the nano-Fe3O4/H2O2 system in the presence of NTA. It can be seen that the degradation of CBZ is strongly dependent on the nano-Fe3O4 concentration. An increase in the nanoFe3O4 concentration from 0.1 to 3.0 g L1 has a positive effect on the degradation of CBZ. The degradation efficiency of CBZ at 120 min reaction time increased significantly from 64.7% to 99.6% with increasing nano-Fe3O4 concentration from 0.1 to 1.0 g L1; 100% degradation efficiency was achieved when the nano-Fe3O4 concentration was P 1.5 g L1. In addition, the kapp value continually increased from 8.50  103 to 7.43  102 min1 with the increase of nano-Fe3O4 concentration from 0.1 to 3.0 g L1. The increase in the generation rate of OH was responsible for the larger kapp values of CBZ at the higher concentration of nanoFe3O4. At a given concentration of H2O2, the generation rate of  OH is generally proportional to the nano-Fe3O4 concentration, especially at low levels (i.e., Fe3O4 < 1.5 g L1). It can be seen from Fig. 3 that the kapp values increased linearly with the increase of nano-Fe3O4 concentration from 0.1 to 1.5 g L1 (R2 = 0.99). However, the increase in the kapp value with an increase in nanoFe3O4 concentration became slower at high levels of nano-Fe3O4 concentration (Fe3O4 > 1.5 g L1). This effect is best explained by the scavenging effects on OH of higher nano-Fe3O4 concentration which offsets the increase in OH generated at the higher concentration of nano-Fe3O4. In addition, the agglomeration of nanoFe3O4 particles at high concentration would increase under these conditions, decreasing the accessible surface active sites.

Degradation efficiency of CBZ (%)

0.12

Pseudo-first-order kinetic rate -1 constant, kapp (min )

Degradation efficiency of CBZ (%)

Fig. 2 shows the degradation of CBZ by the nano-Fe3O4/H2O2 system in the presence of various concentrations of NTA. It can be seen that the increase in NTA concentration from 0.025 to 1.0 mM has a positive effect on the degradation of CBZ. The degradation efficiency of CBZ at 120 min reaction time increased significantly from 55.9% to 100% with the increase of NTA concentration from 0.025 to 1.0 mM, and the kapp value correspondingly in-

3.3. Effect of nano-Fe3O4 concentration

0.00 0.5

1.0

1.5

2.0

2.5

3.0

-1

Fe3O4 concentration (g L ) Fig. 3. Effect of nano-Fe3O4 concentration on the degradation efficiency and kinetics of CBZ by enhanced Fenton-like reaction of nano-Fe3O4/H2O2 with NTA. Experimental conditions: CBZ = 6.35  102 mM, H2O2 = 100 mM, NTA = 0.5 mM, initial pH = 7.0 and temperature = 23 ± 2 °C.

47

3.4. Effect of H2O2 concentration

3.5. Effect of initial pH

0.10 80 0.08 60

0.06

40 20

Reaction time 10 min 30 min 60 min 120 min

Kinetic rate constant kapp

0 4

5

6

7

8

9

0.04 0.02 0.00 10

pH Fig. 5. Effect of initial pH on the degradation efficiency and kinetics of CBZ by enhanced Fenton-like reaction of nano-Fe3O4/H2O2 with NTA. Experimental conditions: CBZ = 6.35  102 mM, Fe3O4 = 1.0 g L1, H2O2 = 100 mM, NTA = 0.5 mM and temperature = 23 ± 2 °C.

changes in the initial pH from 5.0 to 9.0 had little effect on the degradation of CBZ either on the nano-Fe3O4 surface (heterogeneous degradation) or in the aqueous solution (homogeneous degradation). Moreover, it was observed that the final pH for the degradation of CBZ with an initial neutral pH decreased to pH 6.2 after the reaction. This is due to the formation of carboxylic acids during the degradation of CBZ. 3.6. Effect of initial concentration of CBZ Fig. 6 presents the degradation kinetics of CBZ by the nanoFe3O4/H2O2 system in the presence of NTA at various initial concentrations of CBZ. It can be seen that efficient degradation of CBZ was achieved over the entire range tested. The kapp value of CBZ decreased from 1.51  101 to 4.32  102 min1 with the increase of the initial concentration of CBZ from 1.06  102 to 6.35  102 mM. The small kapp values obtained at the high concentration of CBZ can be explained by the fact that more OH is required for the degradation of CBZ and its degradation intermediates at high CBZ concentrations, while the yield of OH would be similar under the same concentrations of nano-Fe3O4, H2O2 and NTA. It is noteworthy that the effect of initial concentration of contaminant on its degradation rates is not only dependent on the specific experimental conditions, but also strongly dependent on the physicochemical properties of the contaminant. Our previous study demonstrated that the degradation rate of p-nitrophenol (p-NP) by nano-Fe3O4/H2O2 system in the absence of NTA

0.12 100 0.10 80 0.08 60

0.06

40 20

Kinetic rate constant kapp

Reaction time 10 min 30 min 60 min 120 min

0

0.04 0.02

Pseudo-first-order kinetic rate -1 constant, kapp (min )

Degradation efficiency of CBZ (%)

Fig. 5 shows the degradation of CBZ by nano-Fe3O4/H2O2 system in the presence of NTA at various initial pH values of CBZ aqueous solution. No significant difference in the kapp values of CBC was observed over an initial pH range of 5.0–9.0 (p = 0.08). The result is consistent with that of CBZ degradation by the nano-Fe3O4/H2O2 system in the absence of NTA. An explanation is that the pKa values of CBZ, pKa1 = 2.3 and pKa2 = 13.9, are far from the pH range tested, thus changes in the initial pH have little effect on the adsorption of CBZ on nano-Fe3O4 [7]. In addition, although it has been reported that the degradation of CBZ in aqueous solution by FeIII-NTA complexes/H2O2 at pH 9.0 is more effective than that at pH 5.0 [21], the dissolved iron at basic pH conditions was minimal. As a result, the

0.12 100

Pseudo-first-order kinetic rate -1 constant, kapp (min )

The effect of H2O2 concentration on the degradation of CBZ by the nano-Fe3O4/H2O2 system in the presence of NTA is shown in Fig. 4. It can be seen that the degradation of CBZ is strongly dependent on H2O2 concentration. Control experiments showed that no degradation of CBZ was observed after 24 h reaction time in the absence of H2O2 (data not shown). The degradation efficiency of CBZ at 120 min of reaction time increased rapidly from 69.7% to 99.6% with the increase of H2O2 concentration from 5 to 100 mM, and the corresponding kapp value increased from 9.65  103 to 4.32  102 min1. In addition, it can be seen that the kapp value continually increased to 7.16  102 min1 with increased H2O2 concentration to 600 mM. The increase in the kapp value with the increase of H2O2 concentration was due to more OH generated at the high concentration of H2O2 in the range investigated. However, it can be observed that the increase in the kapp values of CBZ became slower at high levels of H2O2 concentration (i.e., H2O2 > 200 mM). This can be explained by the fact that the surface concentration of H2O2 gradually reached close to its maximum value when the H2O2 concentration was >200 mM. In addition, the scavenging effects of H2O2 on OH would also increase at higher concentration of H2O2 (kOH,H2O2 = 1.2–4.5  107 M1 s1) [15]. The most important finding of this study is that the usage of H2O2 in the nano-Fe3O4/H2O2 system can be greatly decreased by the presence of NTA. A comparison study showed that only 69% of CBZ was degraded after 600 min of reaction time by 600 mM of H2O2 and 1.0 g L1 of nano-Fe3O4 at pH 7.0 in the absence of NTA. In this study, 99.6% of CBZ was degraded after 120 min reaction time by 100 mM of H2O2 and 1.0 g L1 of nano-Fe3O4 at pH 7.0 in the presence of 0.5 mM NTA.

Degradation efficiency of CBZ (%)

S.-P. Sun et al. / Chemical Engineering Journal 244 (2014) 44–49

0.00 0

100

200

300

400

500

600

H2O2 concentration (mM) Fig. 4. Effect of H2O2 concentration on the degradation efficiency and kinetics of CBZ by enhanced Fenton-like reaction of nano-Fe3O4/H2O2 with NTA. Experimental conditions: CBZ = 6.35  102 mM, Fe3O4 = 1.0 g L1, NTA = 0.5 mM, initial pH = 7.0 and temperature = 23 ± 2 °C.

Fig. 6. Degradation efficiency and kinetics (inset) of CBZ by enhanced Fenton-like reaction of nano-Fe3O4/H2O2 with NTA at various initial concentrations of CBZ. Experimental conditions: Fe3O4 = 1.0 g L1, H2O2 = 100 mM, NTA = 0.5 mM, initial pH = 7.0 and temperature = 23 ± 2 °C.

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Fig. 7. CBZ degradation in the aqueous phase versus in the nano-Fe3O4 suspensions. Experimental conditions: CBZ = 6.35  102 mM, Fe3O4 = 1.0 g L1, H2O2 = 100 mM, NTA = 0.5/1.0 mM, initial pH = 7.0 and temperature = 23 ± 2 °C.

Fig. 8. Dissolved iron concentration in the nano-Fe3O4/H2O2 system in the presence of NTA. Experimental conditions: CBZ = 6.35  102 mM, nano-Fe3O4 = 1.0 g L1, H2O2 = 100 mM, NTA = 0.5/1.0 mM, initial pH = 7.0 and temperature = 23 ± 2 °C.

increased linearly with the increase of p-NP initial concentration from 0.11 to 0.32 mM [11]. This is because p-NP degradation was strongly influenced by the solution pH, which depended on its pKa value. A larger initial concentration of p-NP led to a faster decrease of the solution pH because of the production of more carboxylic acids, which favored the degradation of p-NP. Although the solution pH also decreased during the degradation of CBZ, unlike p-NP the change in the solution pH had negligible effect on the degradation of CBZ (see Section 3.5).

an NTA enhanced homogeneous Fenton-like reaction in the aqueous phase. The results were significantly different from the degradation of CBZ by the nano-Fe3O4/H2O2 system in the absence of NTA, where the degradation of CBZ in the aqueous phase was negligible and the degradation rate of CBZ in the nano-Fe3O4 suspensions was also slow. The mechanism of the enhanced degradation of CBZ by the nano-Fe3O4/H2O2 system in the presence of NTA is proposed in Scheme 1 showing that the presence of NTA affects both the surface and solution reactions. First, NTA is adsorbed on the surface of nano-Fe3O4 particles to form FeII- and FeIII-NTA surface complexes, noted as „FeII- and „FeIII-NTA, via a ligand-exchange reaction. The „FeII- and „FeIII-NTA complexes can react with the adsorbed H2O2 (noted as H2O2,surf) to induce a Fenton-like reaction on the nano-Fe3O4 surface via a heterogeneous Fenton-like reaction. In addition, the „FeII- and „FeIII-NTA complexes can be released from the surface into the aqueous phase, noted as FeIIand FeIII-NTA aqueous complexes. A previous study has reported that the release of metal ions from minerals can be enhanced by chelating agents because the metal-oxygen bonds on the surface of minerals are weakened upon formation of complexes [26]. In order to verify the presence of iron in the aqueous phase of the reaction, samples were taken and analyzed, and results shown in Fig. 8 confirm its presence. After 1 h pre-adsorption, the dissolved iron concentration was determined to be 0.44 and 0.51 mg L1 Fe in the presence of 0.5 and 1.0 mM of NTA, respectively, and increased to 0.73 and 0.80 mg L1 after 2 h of reaction time. Thus the FeII- and

3.7. Heterogeneous and/or homogeneous Fenton-like reaction? As shown in Fig. 7, it can be seen that some degradation of CBZ occurs in an aqueous phase resulting from filtration of nano-Fe3O4 particles from a portion of a Fenton suspension before the reaction commences. The kapp values of CBZ in the filtered phase were determined to be 6.40  103 and 1.00  102 min1 in the presence of 0.5 and 1.0 mM of NTA, respectively. The kapp values of CBZ in the remaining nano-Fe3O4 suspensions were 4.32  102 and 5.62  102 min1 in the presence of 0.5 and 1.0 mM of NTA, respectively. Based on these rate constants, it can be calculated that the contribution rates of the heterogeneous and homogeneous Fenton-like reactions for CBZ degradation are 80–85% and 15–20%, respectively. These results clearly demonstrate that the mechanism of CBZ degradation includes both an NTA enhanced heterogeneous Fenton-like reaction on the Fe3O4 nanoparticle surface and

Homogeneous Fenton-like reaction

FeII - / FeIII-NTA

Bulk

aq

Degradation products

H2O2

OH aq

aq

NTA

CBZ

Fe3O4 FeII- /

FeIII-NTA

H2O2

OH Surf

Surf

CBZ

Surf

Fe3O4 Degradation products

Heterogeneous Fenton-like reaction

H2O + O2

Scheme 1. Mechanism of the enhanced degradation of CBZ by the nano-Fe3O4/H2O2 system in the presence of NTA.

S.-P. Sun et al. / Chemical Engineering Journal 244 (2014) 44–49

FeIII-NTA aqueous complexes played an important role in degrading CBZ in the aqueous phase via a homogeneous Fenton-like reaction in contrast to results for homogeneous reactions in the absence of NTA where a negligible amount of dissolved iron (<0.1 mg L1 of Fe) was detected. Moreover, bubble formation from the nano-Fe3O4 was clearly observed. This is due to H2O2,surf decomposition by the nano-Fe3O4 through a non-radical pathway to generate oxygen and H2O. In addition, hydroxyl radical chain reactions would also lead to the generation of oxygen. The degradation intermediates were determined by LC–MS and include hydroxy-CBZs and 10,11-dihydro-10,11-epoxycarbamazepine (10,11-epoxy-CBZ) ([M + H] m/z 253); dihydroxy-CBZs ([M + H] m/z 269); 10,11-dihydro-10,11-dihydroxycarbamazepine (CBZ-10,11-diols) ([M + H] m/z 271); hydroxyl-CBZ-10,11-diols ([M + H] m/z 287); and compounds ([M + H] m/z 251) that probably formed from dihydroxy-CBZ radical and/or quinonoid-CBZ derivatives by an intramolecular reaction with a H2O loss. The results further indicate that the degradation of CBZ was caused by attack of OH. The intermediates are consistent with that shown in the literature of CBZ degradation by the nano-Fe3O4/H2O2 system in the absence of NTA, as well as that by a modified homogenous Fentonlike reaction with FeIII-NTA complexes [7,21]. 4. Conclusion Batch experiments were conducted to investigate the influence of NTA on the degradation kinetics of CBZ by the nano-Fe3O4/H2O2 system. It was found that: (1) CBZ degradation by the nano-Fe3O4/H2O2 system was significantly enhanced by the presence of NTA with an optimal NTA concentration of 0.5–1.0 mM. Additionally, the presence of NTA can greatly decrease the use of H2O2 and nano-Fe3O4 in the nano-Fe3O4/H2O2 system. (2) The kapp values of CBZ were strongly dependent on nanoFe3O4 concentration, H2O2 concentration and the initial concentration of CBZ, but less dependent on the initial pH from 5.0 to 9.0. A kapp value of 4.32  102 min1 was obtained under optimal conditions of 1.0 g L1 nano-Fe3O4, 100 mM H2O2, 0.5 mM NTA and initial pH 7.0 at room temperature (23 ± 2 °C) which is 80 times larger than that in the absence of NTA and under otherwise identical conditions. (3) The presence of NTA greatly enhanced both the heterogeneous Fenton-like reaction on the Fe3O4 nanoparticle surface and the homogeneous Fenton-like reaction in the aqueous phase by the formation of FeII- and FeIII-NTA complexes which were responsible for the enhanced degradation of CBZ. (4) Magnetite is an environmentally benign material for the decontamination of polluted waters and soils which has been used widely to treat environmental contaminants. Since NTA is readily biodegradable, and therefore would not cause secondary pollution, the NTA-assisted nanoFe3O4/H2O2 process offers some benefits that the traditional Fenton’s reagent and the modified Fenton’s reagent cannot offer. For example, it works effectively under neutral pH conditions, using less H2O2 than the modified Fenton’s reagent. Therefore, the NTA-assisted nano-Fe3O4/H2O2 process can potentially be used for In Situ Chemical Oxidation (ISCO).

Acknowledgments The study was funded in part by the College of Human Ecology, Cornell University and in part by the Cornell University

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