Kinetics and Mechanisms of Radiolytic Degradation of Nitrobenzene

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Environ. Sci. Technol. 2007, 41, 1977-1982

Kinetics and Mechanisms of Radiolytic Degradation of Nitrobenzene in Aqueous Solutions SHU-JUAN ZHANG,† HONG JIANG,† M I N - J I E L I , † H A N - Q I N G Y U , * ,† HAO YIN,‡ AND QIAN-RONG LI‡ Department of Chemistry and Structure Research Laboratory, University of Science & Technology of China, Hefei, Anhui 230026, China

Among these methods, ionizing radiation has been proven to be a promising approach for contaminant destruction and an effective technique for elucidating reaction mechanisms and kinetics, because the generation of active species from radiolysis avoids dosing of additional agents to the system (12, 13). Radiolysis of water results in the formation of a suite of species, in which hydroxyl radicals (•OH) and hydrated electrons (eaq- ) are predominant, as shown in eq 1. The values in parentheses are the radiation chemical yields of these species at pH 7.0, viz., G values, which are defined as the number of formed or decomposed molecules per 100 eV of absorbed energy (12, 14).

H2O D> •OH (2.7), eaq- (2.7), H• (0.6), H2 (0.45), H2O2 (0.7) (1)

Steady-state radiolysis experiments were performed to gain insight into the kinetics and mechanisms of nitrobenzene (NB) degradation in aqueous solutions. The degradation of NB under γ-ray irradiation followed pseudo-first-order kinetics, and the pseudo first-order rate constant and the initial G value of NB decomposition were functionally related to both the initial NB concentration and the irradiation dose rate. Under oxidative conditions, complete mineralization of NB was achieved, whereas no total organic carbon reduction was observed under reductive conditions. The radiolytic products of NB under various conditions were identified using FTIR and GC-MS analyses. The mechanisms behind the radiolytic degradation of NB under both oxidative and reductive conditions were proposed schematically in light of the degradation products observed. In addition, calculations based on ab initio molecular orbital and density functional theory provided support for the proposed mechanisms and the preferred pathways among all the possible reactions.

Introduction As an important raw material in the chemical industry, nitrobenzene (NB) is widely present in various wastewaters, constituting a serious threat to the environment. Due to the electron-deficient property of the aromatic ring resulting from the substitution of the nitro group, NB is a biorefractory compound. Conventional biological processes are not efficient for the treatment of NB-laden wastewaters. Therefore, intense efforts have been made to find efficient and costeffective treatment methods. Both oxidative and reductive approaches have been developed for the remediation of NB-rich wastewaters. Direct reduction of NB by elemental iron (Fe0) to aniline was used as an in situ remediation (1, 2). Carbon-catalyzed reduction of NB by hydrazine also led to the formation of aniline (3). The mechanisms behind the reduction of NB to aniline have been elucidated clearly. Advanced oxidation processes based on the generation of highly oxidative hydroxyl radicals either catalytically or noncatalytically, such as photocatalysis (4), Fenton’s reaction (5), ozone-based oxidation (6), sonolysis (7), supercritical oxidation (8), ionizing radiation (9), and various combinations of them (10, 11), have been employed for the complete mineralization of NB. * Corresponding author phone: +86 551 3607592; fax: +86 551 3601592; e-mail: [email protected]. † Department of Chemistry. ‡ Structure Research Laboratory. 10.1021/es062031l CCC: $37.00 Published on Web 02/15/2007

 2007 American Chemical Society

Both highly oxidative and reductive conditions can be feasibly established through saturation of irradiated solutions with N2O to convert eaq- to •OH or through addition of sodium formate (HCO2Na) to scavenge •OH and H• and purging the solutions with argon gas (Ar) to eliminate O2 (15). The CO2•radical formed upon scavenging •OH and H• with HCO2will generally react with organic compounds, producing the same species as those formed by reduction with eaq- (16). The convenience to create oxidative or reductive conditions using ionizing radiation makes it an excellent approach for elucidating reaction mechanisms. The radiolytic oxidation and reduction of NB in aqueous solutions have been extensively investigated, including the unambiguous identification of all one-electron redox intermediates and their redox properties and associated kinetics, by using pulse radiolysis and time-resolved transient absorption spectroscopy (17-21). Some radiolytic products of NB aqueous solutions irradiated with γ-rays under deaerated conditions have been identified by reversed-phase high-performance liquid chromatography (HPLC) and multicomponent UVvis spectrometry (9). However, the previous investigations of the radiolysis of NB focused on the understanding of the fundamental chemistry of radical reactions in NB aqueous solutions. Information about the mechanistic details and quantitative correlations in the degradation of NB and its intermediates under steady-state irradiation is still limited. Therefore, the main objectives of the present work were to elucidate the mechanisms behind the radiation-induced degradation of NB and consequently to provide kinetic insight into the utilization of γ-rays for remediation of NB-laden wastewaters. In addition to the experimental approach, ab initio molecular orbital (MO) theory and density functional theory (DFT) were employed to calculate the changes of Gibbs free energies in the radiolytic degradation of NB to provide convincing support for the proposed NB degradation mechanisms.

Experimental Section Materials. NB (C6H5NO2), dichloromethane (CH2Cl2), HCO2Na, and anhydrous magnesium sulfate (MgSO4), all purchased from Shanghai Chemical Reagent Co., were of analytical grade. Methanol was chromatographic purity grade. N2O and Ar gases were of high purity (>99%). Except that CH2Cl2 was redistilled prior to use, other reagents were used directly without further purification. Sample Preparation. Throughout the experiments, all solutions were prepared with water doubly distilled using a quartz distillatory and were contained in 250 mL gastight Pyrex glass vials. A 60Co γ-source with an activity of about 60 kCi (2.22 × 1015 Bq) was used for irradiation. For product VOL. 41, NO. 6, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. G Values of Dominant Radicals from Water Radiolysis and Initial G Values of NB Decomposition condition

dominant radicals and their Gr

Gv

G-NB

oxidative Gr(•OH) ) 5.4, Gr(H•) ) 0.6 5.4 (•OH) 2.2 ( 0.3 reductive Gr(eaq-) ) 2.7, Gr(CO2•-) ) 3.3 3.5 (eaq-) 1.8 ( 0.3

analysis, NB solutions with an initial concentration (C0) of 0.9 mM were irradiated at an irradiation dose rate (Dr) of 55 Gy min-1 for given time intervals. Prior to irradiation, several portions of them were saturated with N2O, whereas several others were saturated with Ar gas after the addition of HCO2Na (0.1 M). For kinetic analysis, aerated NB solutions were irradiated over 5 h with several C0 levels from 0.4 to 4.0 mM at a constant Dr of 55 Gy min-1 or with a fixed C0 of 0.8 mM at Dr from 15 to 105 Gy min-1. All experiments were performed at ambient temperatures around 20 °C. Analytical Approach. NB concentration was determined using an HPLC instrument (HPLC-1100, Agilent Inc.) with a Hypersil-ODS reversed-phase column and detected at 254 nm using a VWD detector. The mobile phase was a mixture of water with 0.1% acetic acid and methanol (40:60) delivered at a flow rate of 1 mL min-1. The total organic carbon (TOC) concentration was measured by using a TOC analyzer (VCPN, Shimadzu Co.). The GC-MS and FTIR analyses were the same as those described in the previous work (15), except that redistilled CH2Cl2 was used as the extraction agent. Computational Approach. Ab initio MO and DFT calculations were performed with the GAUSSIAN 03 program (22). Structural optimization and frequency analysis were performed at the B3LYP/6-31G(d) level of theory. Full frequency analyses were conducted to confirm that the optimized structures were minima or saddle points. Singlepoint energies were calculated at the B3LYP/6-311++G(3df,3pd) level. To simulate the reactions in aqueous solvents, the effects of bulk solvent were taken into account with the continuum reaction field scheme, and they were evaluated with the united atom topological model to build the cavity under the polarized continuum model (PCM) scheme. PCM calculations were performed at the B3LYP/6-311++G(3df,3pd) level. Both the electrostatic and nonelectrostatic contributions were included for the total solvation energies. In the calculation, the energies of solvation of H3O+ (23) and H2O (24) were experimentally measured values. The energy of eaq- is an indefinite value. It is usually regarded as being lower than 100 eV, but cannot be neglected as zero. Only the energies of the products were considered in the calculation. All the adscititious energies were reckoned in the process that could overcome the activation energies of the reactions.

Results and Discussion Contribution of the Initial Reactive Species to NB Degradation. The initial G values of NB decomposition (G-NB) under various conditions were calculated from the initial slopes of NB concentration versus irradiation dose curves, which represented the utilization efficiency of radicals from water radiolysis in the degradation of NB. Both G values of dominant radicals from water radiolysis (Gr) and G-NB were calculated and are summarized in Table 1. With saturation of N2O, the yield of •OH was nearly doubled, attributed to the conversion of eaq-. According to the principles of competitive reaction kinetics, as indicated in eq 2, where -NB(‚OH) (%) represents the percent

kOHGOH -NB(•OH) (%) ) kOHGOH + kHGH

with NB, respectively, and GOH and GH are the yields of •OH and H•, respectively, taking the rate constants of the reactions for •OH and H• with NB into account (16, 25), •OH made a contribution of approximately 97% to NB degradation under oxidative conditions. In the presence of HCO2Na of 0.1 M and with Ar as the background gas, almost all of the •OH radicals and H• were converted into CO2•-. As mentioned above, the role of CO2•in the degradation of NB was the same as that of eaq-, but with a 3.9 times lower rate constant (26, 27). For conciseness and ease of comparison, the yield of CO2•- was converted into a yield in the form of eaq- by taking the ratio of its rate constant with NB to that of eaq- with NB into account. The total radical yield thus obtained was denoted as a virtual yield (Gv) in the form of a radical species under given conditions. Since eaq- and •OH attacked NB with the same rate constant (16, 27), the ratios of G-NB to Gv should be identical under either oxidative or reductive conditions. The ratio of G-NB to Gv was around 0.5 under the reductive conditions, indicating that half of the radicals were exploited for NB decomposition under the conditions applied. Another half vanished through two possible pathways: in the reactions with degradation intermediates of NB or in the reactions of radical recombination (12). The ratio of G-NB to Gv under the oxidative conditions was 0.4, slightly lower than those under the reductive conditions, suggesting that the utilization efficiency of radicals from water radiolysis was not exactly proportional to their yields. Kinetics of Degradation and Mineralization. The degradation kinetics of NB was studied in aerated aqueous solutions following the disappearance of NB. During 5 h of irradiation the NB concentration (C) decayed exponentially with the irradiation time (t). The profiles of ln C versus t showed a good linearity, demonstrating that the degradation of NB followed pseudo-first-order kinetics. From the slopes of ln C versus t, the pseudo-first-order degradation rate constants (k1) were estimated. Typically, the regression coefficients were greater than 0.980 for the seven data points determined during the 5 h irradiation experiments. The mineralization of NB was measured by the reduction of the TOC concentration as a function of the irradiation time. No TOC reduction was observed when NB aqueous solution was irradiated under the reductive conditions, suggesting that the reductive conditions were not applicable for the complete removal of NB. Under the oxidative conditions, the TOC concentration reduced linearly and could be fitted by zeroth-order kinetics, with a pseudo-zeroth-order rate constant of 3.1 × 10-5 M h-1 at a Dr of 55 Gy min-1. Dependence of G-NB and k1 on Dr or C0. The G-NB and k1 values determined are illustrated as a function of C0 or Dr in Figure 1. Linear regression of G-NB versus C0 gave a high correlation coefficient (>0.95), suggesting that G-NB was positively proportional to C0. In other words, the utilization efficiency of radicals was enhanced by an increase of C0. On the other hand, G-NB decreased exponentially with increasing Dr, implying that a high Dr was not favorable for the efficient radical utilization. The results above are in agreement with general trends of reactions known in radiation chemistry and can be interpreted using the principles of competitive reaction kinetics. In the steady-state radiolysis of NB aqueous solutions, the predominant reactions are as follows:

radical + radical f steady molecules

(3)

radical + NB f degradation intermediates

(4)

(2)

radical + degradation intermediates f degradation products (5)

contribution of •OH to NB degradation, kOH and kH are the second-order rate constants of the reactions for •OH and H•

G-NB is an index of the utilization efficiency of radicals in the degradation of NB. A higher C0 means a higher contribution

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FIGURE 1. (a) Profiles of G-NB and k1 as a function of C0 at a Dr of 55 Gy min-1. (b) Profiles of G-NB and k1 as a function of Dr with a C0 of 0.8 mM. Key: dots, experimental data; solid curve, linear regression; dashed curve, first-order exponential decay regression.

FIGURE 2. FTIR spectra of the extracts from γ-irradiated NB aqueous solutions under (a) oxidative conditions and (b) reductive conditions.

of reaction 4 among reactions 3-5. Therefore, G-NB was proportional to C0. Conversely, an increase of Dr reduced the contribution of reaction 4. Consequently, G-NB decayed with Dr. Contrary to the relationship between G-NB and C0/Dr, k1 decayed exponentially with C0 but was proportional to Dr. According to the steady-state assumption, the formation rate of radicals from water radiolysis is equal to their extinction rate. Their concentrations are thus dependent upon Dr and the effective concentration of the reactants. At a fixed Dr, an increase of C0 reduces the steady-state concentration of radicals. Since the pseudo-first-order degradation rate constant is the product of radical concentrations with the absolute second-order rate constants of reactions between radicals and reactants, an increase of C0 ultimately led to a decrease in k1. On the contrary, at a fixed C0, a higher Dr corresponds to a higher steady-state concentration of radicals. As a result, k1 increased proportionally with Dr. Identification of Degradation Products. The products from NB radiolytic degradation under both oxidative and reductive conditions were identified using FTIR and GCMS analysis. As shown in Figure 2a, the FTIR spectra of the extracts from the γ-irradiated NB aqueous solutions under the oxidative conditions had characteristic absorption bands at 3380 and 1530/1338 cm-1, which are the stretching and bending vibrations of O-H (υOH and δOH) on the aromatic ring. Furthermore, with an increasing irradiation dose, the strength of these three bands increased, indicating the gradual substitution of OH on the benzene ring. Under the reductive conditions, as illustrated in Figure 2b, the characteristic absorption band of NO2 (asymmetric υNO2 at 1525 cm-1, symmetric υNO2 at 1348 cm-1, and υC-NO2 at 1348 cm-1) disappeared with irradiation. Meanwhile, new bands at 1480, 1438, and 926 cm-1 (the characteristic absorption bands of -NdN-) and υN-O on the azoxy group at 1300 and 1275

cm-1 were formed with an irradiation time of up to 3 h and later decayed. The prominent character of the extracts obtained from the NB solutions after 5 h of irradiation under the reductive conditions was the strong absorption bands at 3440, 1600, and 1290 cm-1, representing υN-H, δN-H, and υC-NH2 of the aromatic amine, respectively. The GC chromatograms obtained from GC-MS analysis of the extracts from the γ-irradiated NB aqueous solutions under both oxidative and reductive conditions are shown in Figure 3. Under the oxidative conditions, nothing was detectable after 5 h of irradiation, whereas a strong signal existed even after 14 h of irradiation under the reductive conditions. This indicates that the degradation of NB was more efficient under the oxidative conditions than that under the reductive conditions. The products identified with their GC-MS data are summarized in Table 2. Consistent with the FTIR analysis, the degradation products under the oxidative conditions were phenol (1), isomeric nitrophenols 3, 6, and 8, 4-nitrocatechol (4), 1,3-dinitrobenzene (5), and 2-nitrohydroquinone (7). Nitrosobenzene (1′) and its dimer (7′), aniline (2′), azobenzene (4′), 2-phenylazophenol (5′), and azoxybenzene (6′) were the primary reductive products. Analysis of Degradation Mechanisms. On the basis of the products identified in the present work and those published in the literature, the mechanisms behind the radiolytic degradation of NB under variant conditions were proposed and are illustrated in Schemes 1 and 2. The numbers beneath the compounds are their peak numbers in GC chromatograms. The compounds labeled with names, most of which are in the transitional state, were not detected in the present work, but have been well documented in the literature. The items in braces are needed to equalize the reactions, and those following in parentheses are the relative variables of the Gibbs free energies (∆Gaq). Theoretically, a lower ∆Gaq means a higher probability that the reaction will go on. VOL. 41, NO. 6, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. GC chromatograms of the extracts from γ-irradiated NB aqueous solutions under (a) oxidative conditions and (b) reductive conditions.

TABLE 2. Product Analysis in the Radiolytic Degradation of NB by GC-MS

a

peak no.

RTa

MIb

1 2 3 4 5 6 7 8

2.83 3.80 4.23 5.80 8.04 8.48 9.01 9.19

94 123 139 155 168 139 155 139

Oxidative Degradation 94,c 66, 39 123, 93, 77,c 65, 51 139,c 122, 109, 92, 81, 65 155,c 107, 93, 79, 52 168,c 152, 122, 92, 75, 50 139,c 93, 65 155,c 138, 125, 97, 81, 52 139,c 109, 93, 81, 65

phenol nitrobenzene o-nitrophenol 4-nitrocatechol 1, 3-dinitrobenzene m-nitrophenol 2-nitrohydroquinone p-nitrophenol

1′ 2′ 3′ 4′ 5′ 6′ 7′

2.39 2.86 3.96 10.15 11.96 12.74 14.19

107 93 123 182 198 198 214

Reductive Degradation 107,c 77, 51 93,c 66 123, 93, 77,c 65, 51 182, 105, 77,c 51 198,c 170, 121, 93, 77, 65 198, 169, 141, 105, 91, 77,c 51 196, 121, 107,c 78, 52

nitrosobenzene aniline nitrobenzene azobenzene azoxybenzene 2-phenylazophenol nitrosobenezene dimer

Retention time on the GC chromatogram, min.

main fragments

b

Molecular ion. c Base peak in the MS spectra.

Under the oxidative conditions, as shown in Scheme 1, electrophilic •OH as the predominant species was initially added to the electron-deficient aromatic ring of NB (2) at a rate near the diffusion-controlled limit, forming a nitrosubstituted hydroxycyclohexadienyl radical (a), which has been identified in the literature by using pulse radiolysis and 1980

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UV-vis absorption methods (17-20). This was followed by rearrangement, leading to the subsequent formation of isomeric nitrophenols 3, 6, and 8. The ∆Gaq values of nitrophenols 3, 6, and 8 suggest that isomer 8 would be the dominant one. A further hydroxylation resulted in the formation of 4 and 7. As an alternative, denitration occurred

SCHEME 1. Main Pathways and Relative ∆Gaq in NB Radiolysis under Oxidative Conditions

SCHEME 2. Main Pathways and Relative ∆Gaq in NB Radiolysis under Reductive Conditions

to 14 h of irradiation. Therefore, reductive radiolysis of NB could only be used as a pretreatment method. with the formation of 1 and isomeric diphenols 9, 10, and 11 (28). With the ∆Gaq values, the amounts of 9, 10, and 11 might not be of significant difference. The G value of the nitrate ion reported was of a level similar to those of isomeric nitrophenols (20). Ipso substitution of •OH on NB resulted in the detachment of a nitro radical (NO2•). Combination of NB with NO2• led to the formation of 5. The ∆Gaq of the reaction from 2 to 5 suggests that the formation of 5 was not a facile pathway, which was supported by its lower relative intensity in the GC chromatograms. The products above then underwent further hydroxylation and/or denitration, forming ring-opening products, and were completely mineralized within 7 h under the given conditions. Under the reductive conditions, as shown in Scheme 2, eaq- played a key role in the radiolytic degradation of NB. The initial attack of eaq- on NB led to the formation of a nitrobenzene radical anion (b′) (27). The radiolytic reduction was a concerted two-electron-step process from NB to 1′ and 7′, subsequently to phenylhydroxylamine (8′), and finally to 2′. The combination of the reductive intermediates 1′ and 8′ resulted in the formation of 5′ and 6′. A further reduction of them led to the formation of 4′, which was ultimately transformed to 2′ via 1,2-diphenylhydrazine (9′) (1). In light of the ∆Gaq values, the pathway from 1′ to 8′ and then to 2′ should be the preferential one. On the other hand, considerable amounts of 5′ and 6′ were observed as the intermediate products of irradiation. The evolution of 5′ and 6′ to 4′ is clearly illustrated in Figure 3b. Aniline was stable when exposed to the attack of eaq- (29). As illustrated in Figure 3b, a high level of aniline remained in the NB solution even up

Acknowledgments We thank the Ministry of Science and Technology, China (Grant No. 2004AA649300), and the Anhui Natural Science Foundation (Grant No. 050450302) for partial financial support of this study.

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Received for review August 24, 2006. Revised manuscript received December 15, 2006. Accepted January 16, 2007. ES062031L