Ozonation Combined with Sonolysis for Degradation and

May 21, 2009 - In this study, the nitroaromatic compound m-nitrotoluene (MNT) was ... At pH 10.0, initial concentration of MNT 400 mg/L, ozone dose 2...
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APPLIED CHEMISTRY Ozonation Combined with Sonolysis for Degradation and Detoxification of m-Nitrotoluene in Aqueous Solution Zhiqiao He, Runye Zhu, Xing Xu, Shuang Song,* and Jianmeng Chen College of Biological and EnVironmental Engineering, Zhejiang UniVersity of Technology, Hangzhou 310032, People’s Republic of China

Min Xia* School of EnVironmental Science and Engineering, Shanghai JiaoTong UniVersity, Shanghai 200240, People’s Republic of China

Sonolytic ozonation (US/O3 oxidation) is an advanced oxidation process that has become a focus of intense investigation. In this study, the nitroaromatic compound m-nitrotoluene (MNT) was chosen as a model substrate for degradation experiments in which pH, initial concentration of MNT, ozone dose, and US density were varied. At pH 10.0, initial concentration of MNT 400 mg/L, ozone dose 2.4 g/h, and US energy density 88 W/L, the efficiency of MNT removal reached 98% after 120 min. Of the initial degradation rate of MNT abatement, 9.8, 4.1, and 1.1 mg/(L min) were observed with US/O3, O3, and US, respectively. The variation of the concentrations of related anions (oxalate, acetate, formate, and nitrate ion) during the reaction process was ascertained by ion chromatography (IC). The major intermediates detected by gas chromatography/mass spectrometry (GC/MS) were 3-nitrobenzaldehyde, 3-nitrobenzoic acid, nitrobenzene, benzene, butene diacid, oxalic acid, and acetic acid. A degradation pathway is proposed on the basis of these findings. The acute toxicity of the reaction solution to zebra fish (Danio rerio) was estimated after US/O3 treatment. According to the decreasing lethal rate during the US/O3 process, it could be deduced that US/O3 is a powerful tool with which to achieve effective reduction of the toxicity of MNT. Introduction Nitroaromatic chemicals are commonly used to synthesize a variety of industrial products, including explosives, pesticides, and precursors for the manufacture of many products, such as dyes, pharmaceuticals, and plastics.1 As a consequence of widespread use, nitroaromatics have become serious environmental contaminants, posing a threat to human health and stimulating considerable research efforts. Nitroaromatics are highly stable and refractory to conventional treatments,2–5 which makes the remediation of wastewater containing nitroaromatics difficult. Problems associated with traditional treatments of industrial wastewater containing nitroaromatics have prompted environmental engineers and scientists to search for innovative solutions. Recently, various chemical treatment processes were proposed as options for the removal of organic contaminants. One promising approach uses advanced oxidation processes (AOPs), which are generally based on the generation of highly reactive species such as hydroxyl radicals ( · OH), and have the advantages of being nonselective, leading to no secondary pollution, and being particularly effective in removing persistent and biorefractory pollutants from water.6,7 One AOP has been in use for over a decade for complete degradation of organic pollutants in environmental decontamination; the combination of ozonation and sonolysis (US/O3) has been applied for the treatment of aromatic compounds.7–10 US/ * Corresponding authors. Tel.: 86-571-88320726; fax: 86-57188320276; e-mail: [email protected] (S.S.). Tel.: 86-21-54748019; fax: 86-21-54741065; e-mail: [email protected] (M.X.).

O3 is more efficient and uses less energy than either ozonation or sonolysis alone. When ozone is used alone under alkaline conditions, it is decomposed and generates very reactive free radicals, such as · OH, which is nonselective and much more powerful than molecular oxidants.11 When sonolysis is combined with the ozonation system, the additional thermal decomposition of O3 results primarily in increased production of the hydroxyl radical.8,9 Furthermore, the ultrasonic irradiation enhances the mass transfer and decomposition of O3, and produces more · OH for more efficient degradation.12,13 Among the nitroaromatic compounds, m-nitrotoluene (MNT) is commonly used in organic synthesis, specifically in the synthesis of dyes, toluidines, nitrobenzoic acids, and explosives. There is growing interest in developing efficient and economically feasible methods to destroy MNT, and these include the UV/H2O2 process, electrochemical reduction, and the reduction of aqueous ammonium sulfide.14–16 To the best of our knowledge, however, little attention has been paid to the US/O3 degradation of MNT. Our previous works have investigated the degradation of p-nitrotoluene (PNT) in aqueous solution by US/ O3.17 In view of the fact that the position of substituent group would affect the level of substrate oxidation,18–20 the reactivities and degradation mechanisms of toluenes are expected to depend on the position of nitro group in the benzene ring. Therefore, this study evaluated the effects of different experimental parameters, such as pH, dosage of ozone, energy density of ultrasound, and initial concentration of MNT, on the efficiency of MNT removal. Variation in the concentrations of related ions and organic intermediates was examined in order to elucidate the mechanism of MNT degradation by US/O3. Unlike earlier

10.1021/ie801566z CCC: $40.75  2009 American Chemical Society Published on Web 05/21/2009

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reports, which were focused on improving the efficiency of this process through the evolution of residual model substrates or total organic carbon (TOC), a new parameter, acute toxicity, was studied here in an attempt to achieve an environmentally safe effluent in response to increased social and political concern about environmental risk. This study was done under optimal conditions, and samples were collected at different time-points to examine the evolution of intermediates and acute toxicity. Experimental Section Materials. m-Nitrotoluene (liquid form, molecular mass 137.14 g/mol) with purity >99.5% was obtained from Shanghai Fenghe Chemical Industry Co. Ltd. and used as received. MNT is a clear yellow liquid with a freezing point of 16.1 °C. It is readily soluble in ethanol, benzene, and diethyl ether. The solubility of MNT in water at 303 K is 500 mg/L, and the TOC content of 400 mg/L MNT in aqueous solution is 199 ( 13 mg/L. Apparatus. As shown earlier, the sonolytic ozonation apparatus consisted of an audible-frequency sonicator and an ozone generator.21 Sonication was done in a cylindrical Pyrex glass reactor (diameter 80 mm, height 120 mm) coupled with an ultrasonic processor (YIY-UL04-B, Shanghai Yiyuan Ultrasonic Equipment Co. Ltd., Shanghai, China) equipped with a 20 kHz converter (diameter 106 mm, height 225 mm) and a pure titanium probe. The tip of the probe was 16 mm in diameter and 164 mm in height and was immersed to about 3 cm in the solution. The reactor was immersed in a water bath to keep the solution at a constant temperature of 30 ( 2 °C. An O3 supply system (CHYF-3A, Hangzhou Rongxin Electronic Equipment Co. Ltd.) was used to produce ozone from dry pure oxygen. The ozone diffuser, placed at the bottom of the reactor, was a cylindrical unit with coarse porosity, and the amount of ozone supplied to the reactor was controlled by varying the oxygen feed to the generator. Surplus ozone was trapped in gas absorption bottles containing 2% KI solution. Procedures. The initial pH of the solution was adjusted to different values (2.0, 4.0, 6.0, 8.0, 10.0, and 11.0) by adding the appropriate amount of phosphate buffer, which was prepared in deionized water by mixing calculated amounts of sodium hydroxide solution and phosphoric acid solution. Initial concentrations of 100, 200, 300, and 400 mg/L MNT were chosen to investigate the influence of the initial concentration on the removal of MNT. We used different doses of ozone (0.6, 1.4, 2.1, and 2.4 g/h) and various levels of US density (44, 88, 132, and 176 W/L) as well. Samples of 5 mL were withdrawn from the reactor at different time points, and identified and quantified by means of high-pressure liquid chromatography (HPLC). Additionally, after 5-fold dilution with deionized water, the diluted samples were analyzed for TOC, organic intermediates, and inorganic ions. Analysis. Quantification of MNT was done by comparison with the retention time of the standard compound on HPLC (1200 series, Agilent, USA). Aliquots of 5 µL were injected into an Eclipse XDBC-18 HPLC column (150 mm × 4.6 mm, 5 µm film thickness) and eluted with a mobile phase of water/ methanol (30:70, v/v). Carboxylic acids and inorganic anions were analyzed with a Dionex model ICS 2000 ion chromatograph, a Dionex IonPac AS19 analytical column (250 mm × 4 mm), a Dionex IonPac AG19 guard column (250 mm × 4 mm), a dual-piston (in series) pump, and a Dionex DS6 conductivity detector. Suppression of the eluent was achieved with a Dionex anion ASRS electrolytic suppressor (4 mm) in the autosuppression external water mode.

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A gas chromatography/mass spectrometry (GC/MS) (GC, Varian cp 3800; MS, Varian Saturn 2000 mass spectrometer) system equipped with a wall-coated open-tubular (WCOT) fused silica series column (30 m × 0.25 mm) was used to identify the intermediate products of MNT degradation. The column temperature was held at 80 °C for 2 min, then increased at 15 °C/min to 250 °C. The other experimental conditions included the following: EI impact ionization, 70 eV; carrier gas, helium; injection temperature, 270 °C; source temperature, 70 °C. Total organic carbon (TOC) was measured with a TOC-VCPH total organic carbon analyzer. The solution pH was measured with a pHs-25 instrument (Rex Analytical Instrument Co. Ltd., Shanghai, China), and the concentration of ozone in the ozone/ oxygen mixture was determined by iodometry.22 Acute Toxicity Assays. The 48 h acute toxicity assays using zebrafish (Danio rerio) as the model species were done at different stages of MNT degradation to evaluate the variation of toxicity. The overall testing procedures followed the OECD (Organisation for Economic Co-operation and Development) guideline for zebrafish.23 Briefly, each toxicity test was done in triplicate using five fish for each test in a beaker with 1000 mL of solution containing MNT and intermediates for set lengths of time between 15 and 120 min. The sensitivity of zebrafish was first tested according to the ISO (International Organization for Standardization) method.24 Water was filtered through activated carbon before being used to make the solutions. The temperature was maintained at 26 ( 1 °C and a minimum 4 mg/L of dissolved oxygen was supplied by air filtered through activated carbon. The solutions were changed every 12 h to maintain concentrations of pollutants. Mortality was recorded at 12 h intervals for a total of 48 h, and the results are expressed as lethal rates, calculated as the number of immobilized animals divided by total animals. Results and Discussion Comparison of Three Processes (US, O3, and US/O3) for the Degradation and Mineralization of MNT. We investigated the respective roles of the US, O3, and US/O3 schemes by measuring the TOC destruction and the MNT removal rate. Since the US/O3 process involves different reactive species, it is reasonable to compare different runs using the initial degradation rate. Figure 1 and Table 1 show the experimental data obtained when ultrasound, ozonation, or their combination was applied. An increased degradation of MNT was observed in the combined US/O3 process as compared to the individual unit processes, and the initial reaction rate of TOC and MNT reduction were ∼55% and ∼88% higher for the combined system than for the linear combination of ozonation alone and sonolysis alone, respectively, suggesting a synergistic increase when the two schemes operated simultaneously. Three factors may have contributed to the increased efficiency of MNT degradation. First, the mechanical action of US enhances the dissolution of O3 and the generation of additional · OH. Second, owing to the cavitation effect of ultrasound, myriads of tiny air bubbles are formed, enabling most of the O3 to enter the liquid phase or react on the gas/liquid interface. Finally, aeration of ozone might increase the turbulence of the aqueous solution and enable more related substances to migrate from the collapsing cavities into the bulk of the solution. It could be inferred from the above that ultrasound enhances the mass transfer and decomposition of O3, and the increased production of free radicals is responsible for the better performance. Effect of Operating Parameters. Effect of pH. US/O3 oxidation of MNT aqueous solution was done in the reactor at

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Figure 1. Comparison of US, O3, and US/O3 for the removal and mineralization of MNT: pH 10.0; initial concentration of MNT 400 mg/L; temperature 30 °C; US energy density 88 W/L; O3 dose 2.4 g/h. Table 1. Initial Degradation Rate of Various Operation Parameters during MNT Removal

pH

init concn (mg/L)

O3 dose (g/h)

US energy density (W/L)

init degradn rate (mg/(L min))

10.0 10.0 2.0 4.0 6.0 8.0 10.0 11.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0

400 400 400 400 400 400 400 400 300 200 100 400 400 400 400 400 400

0 2.4 2.4 2.4 2.4 2.4 2.4 2.4 2.4 2.4 2.4 2.1 1.4 0.6 2.4 2.4 2.4

88 0 88 88 88 88 88 88 88 88 88 88 88 88 44 132 176

1.1 ( 0.16 4.1 ( 0.31 3.5 ( 0.49 4.0 ( 0.15 5.7 ( 0.43 8.7 ( 1.1 9.8 ( 1.2 9.1 ( 1.0 9.1 ( 1.2 6.9 ( 0.80 4.1 ( 0.34 6.8 ( 0.25 4.3 ( 0.37 3.1 ( 0.29 7.2 ( 0.46 9.8 ( 0.85 9.9 ( 0.57

various pH values (2.0, 4.0, 6.0, 8.0, 10.0, and 11.0). As shown in Figure 2a and Table 1, the initial degradation rate were 3.5, 4.0, 5.7, 8.7, 9.8, and 9.1 mg/(L min) at the initial pH values of 2.0, 4.0, 6.0, 8.0, 10.0 and 11.0, respectively. It is clear that when the pH was increased from 2.0 to 10.0, the reactivity of the system increased. This trend is because ozone decomposition is accelerated rapidly by an increase in pH, especially in alkaline solutions, resulting in enhanced generation of free radicals.25 Hence, the rate of MNT removal increased; however, there was a decrease when pH values reached 11.0. Two main factors

could be responsible for this drop: first, when the initial pH values increased, the amount of the phosphate species added to the solution in our experiment as pH buffers increased, which could react with · OH and the rate of reaction would be slowest in the pH range 2.8-4.5.26 Second, more free radical scavengers HCO3-/CO32- would exist at high pH values, resulting in a decrease in the concentration of available · OH.27,28 Accordingly, the optimal pH for the removal of MNT in our experiments was 10.0. Effect of Initial Concentration of MNT. The rate of MNT disappearance as a function of initial concentration is given in Figure 2b and Table 1. When the initial concentration increased from 100 to 400 mg/L, it was found that the initial degradation rate underwent a rapid increase and then slow increase, which is attributed to the formation of inadequate amounts of the reactive species. Several studies have reported compatible results29,30 and two reasons might explain our observation: (1) the cavities and · OH radicals in the solution approach a steady state for given operating conditions and consequently the available · OH radicals are insufficient for the removal of MNT at higher concentration; (2) a higher initial concentration corresponds to generation of more intermediate products, which would compete with parent substances for · OH. Effect of Ozone Dose. To evaluate the effect of the ozone dose on MNT degradation, sonolytic ozonation experiments with an initial MNT concentration of 400 mg/L were done at four different ozone doses between 0.6 and 2.4 g/h (Figure 2c). Table 1 depicts the initial degradation rate from 3.1 mg/(L min) at an O3 dose of 0.6 to 9.8 mg/(L min) at an O3 dose of 2.4 g/h. An increasing O3 dose clearly exerted a positive influence on degradation, suggesting that the degradation of MNT depended on the dissolution of ozone, which determined the amount of · OH generated and affected the transfer of ozone into the reaction solution. An enlargement of the gas/liquid interface, derived from the ozone dose increase, would lead to an increase of ozone concentrations in the solution and an increase of the formation of free radicals, which result in an increased rate of MNT degradation.31 Nevertheless, with the increased ozone dose, the degradation rate would approach a maximum, since the ozone in the solution would reach saturation at a certain temperature.32 Therefore, energy consumption and the amount of exhaust O3 gas must be taken into account when determining the optimal O3 dose in further studies. Effect of US Density. Figure 2d and Table 1 present the effect of US energy density on the disappearance of MNT. The initial degradation rates vary between 7.2 and 9.9 mg/(L min), corresponding to an applied US density range of 44-176 W/L. The slight increase demonstrates that increasing the US density does not have a noticeable positive effect on the rate coefficients during degradation. Three factors influence the irradiation of US: power density, frequency, and amplitude of the system.33 The greater US density means greater energy supplied to generate cavitation bubbles at a higher temperature, resulting in greater efficiency of degradation. However, when the US density increases, the cavitation bubbles will become very large, resulting in insufficient collapse of the bubbles and production of an acoustic screen; thus, the amount of dissipated energy is increased. Besides, the degassing rate of O3 will increase, affecting the concentrations of O3 and radicals. The factors mentioned above can explain the weak influence of increasing US density on the disappearance of MNT. Toxicity Tests. The acute toxicity assay of reaction solutions was done with adult zebrafish to evaluate the toxicity profile during the course of US/O3 degradation of MNT.34 The control

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Figure 2. Effect of process variables on the removal of MNT. (a) Effect of pH: initial concentration of MNT 400 mg/L; temperature 30 °C; US energy density 88 W/L; O3 dose 2.4 g/h. (b) Effect of initial MNT concentration: pH 10.0; temperature 30 °C; US energy density 88 W/L; O3 dose 2.4 g/h. (c) Effect of ozone dose: pH 10.0; initial concentration of MNT 400 mg/L; temperature 30 °C; US energy density 88 W/L. (d) Effect of US energy density: pH 10.0; initial concentration of MNT 400 mg/L; temperature 30 °C; O3 dose 2.4 g/h. Table 2. Lethal Rates of Zebrafish in Diluted Reaction Solutions Withdrawn at Different Time Points: pH 10.0; Initial Concentration of MNT 400 mg/L; Temperature 30 °C; US Energy Density 88 W/L; O3 Dose 2.4 g/h sample time (min) 0

15

60

120

dilution (fold)

lethal rate (%)

16 × 8× 5.3 × 3.2 × 8× 5.3 × 4× 3.2 × 2.6 × 5.3 × 4× 3.2 × 2.6 × 4× 3.2 × 2.6 ×

0(0 53.4 ( 11.6 80 ( 20 100 ( 0 6.6 ( 11.6 13.4 ( 11.6 20 ( 0 46.6 ( 11.6 93.4 ( 11.6 0(0 6.6 ( 11.6 20 ( 0 73.4 ( 11.6 0(0 6.6 ( 11.6 46.6 ( 11.6

solution, which contained no MNT, displayed no toxicity after US/O3 treatment for 120 min. Table 2 illustrates the lethal rates in diluted reaction solutions removed at different time points. The lethal rates were reduced when the reaction time was prolonged, indicating a decreasing aquatic toxicity. The obvious toxicity decrease in oxidized solution points to the formation of some intermediate products, which might be less toxic than the parent compound. Table 2 shows that the rate of toxicity decrease in the first 60 min of the reaction was more rapid than the following decrease. For example, after 60 min of reaction time the lethal rate of the 3.2-fold dilution was decreased from ∼100% to ∼20%, and in the next 60 min, it decreased to ∼6.6%. Combined with the data detected by GC/MS, most of the MNT was degraded after reaction for 60 min, and the

benzene rings of MNT molecules were cleaved, yielding further degradation products, including butene diacid, oxalic acid, and acetic acid, which have a lower toxicity effect than aromatic compounds (e.g., nitrobenzene). With increased reaction time, the reaction byproduct would be expected to be mineralized eventually to carbon dioxide and water. Thus, the reaction solution could presumably become nontoxic after a sufficient length of time. Variation in the Concentration of Related Ions and a Possible Degradation Pathway. After US/O3 treatment, the products remaining in solution were analyzed with IC and the results are presented in Figure 3. After reaction for 120 min, at least 60% of the total N in the initial MNT molecule was converted into NO3-. However NO2- was not detected by the IC analysis. The nitrogen content of the aqueous solution was less than that of the initial nitro group in MNT, likely because some of the nitrogen was evaporated from solution by the formation of N2, NO, or NO2 at different pH values,35,36 and some might have existed in persistent undetected byproduct. As for the contribution of organic acid to TOC, oxalic acid possessed ∼33% of the residual TOC, suggesting that oxalic acid is a relatively stable organic substance, which needs further research to identify an efficient degradation process. Other researchers have obtained results that agree with ours.37 In addition, acetic acid and formic acid accounted for ∼14% and ∼16% of the residual TOC, respectively. The rest of the residual TOC likely consisted of other undetected byproduct. To investigate the degradation mechanism of MNT, intermediate compounds were identified by GC/MS (Table 3). In Figure 4, we propose a possible reaction mechanism according to the GC/MS and IC data. In the process of oxidation of MNT by O3/US, the initial oxidation species attack can occur at the

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Figure 3. Variation of the concentration of the related ions with time: pH 10.0; initial concentration of MNT 400 mg/L; temperature 30 °C; US energy density 88 W/L; O3 dose 2.4 g/h. Table 3. Intermediates Identified by GC/MS: pH 10.0; Initial Concentration of MNT 400 mg/L; Temperature 30 °C; US Energy Density 88 W/L; O3 Dose 2.4 g/h Figure 4. Probable degradation pathway of MNT by US/O3: pH 10.0; initial concentration of MNT 400 mg/L; temperature 30 °C; US energy density 88 W/L; O3 dose 2.4 g/h.

methyl group of MNT to yield 3-nitrobenzaldehyde (D1), which can be further oxidized to form 3-nitrobenzoic acid (D2). Due to the poor stability of the carboxyl group on D2, it could be transformed easily into nitrobenzene (D3), which would then yield benzene (D4) with the release of NO3-. Parallel with this degradation, the methyl or nitro group might be lost by vaporphase pyrolysis due to the high temperatures that are achieved during bubble collapse conditions, and form the compounds 3-nitrophenyl radical (S1) or 3-methylphenyl radical (S2), respectively. S1 and S2 could be transformed easily into D3 and toluene (S3), respectively, because of their instability in aqueous solution. By O3 and/or · OH oxidation, S3 could be oxidized to yield benzoic acid (S4), which was further decarboxylated to give D4. Similarly, under the oxidation of free radicals such as · OH, D3 was transformed to D4 along with the generation of NO3-. Following this, the aromatic ring of D4 may open to give smaller organic acid molecules, such as butene diacid (D5), oxalic acid (D6), and acetic acid (D7). Certainly, S4 could be oxidized directly to D5, D6, and D7.

1. The disappearance of MNT in aqueous solution occurred in response to US, O3, or US/O3. Ultrasonic irradiation alone yielded negligible results, whereas US/O3 treatment exhibited a higher removal rate compared with ozonation alone. Degradation rate of US/O3 was affected by pH, initial concentration of MNT, O3 dose, and US energy density. 2. Organic intermediates and related ions were detected by GC/MS and IC, respectively. MNT was converted mainly to 3-nitrobenzaldehyde, 3-nitrobenzoic acid, nitrobenzene, benzene, butene diacid, and inorganic anions (e.g., NO3-). The organic intermediates were then degraded into smaller organic acid molecules. Quantitative analysis showed oxalic acid accounted for the majority of the residual TOC in solution. 3. The lethal rate of reaction solution toward zebrafish decreased with increased length of time of US/O3 treatment, indicating a decreasing acute toxicity. These results imply that US/O3 oxidation is a powerful tool to achieve effective reduction of the toxicity of MNT. 4. Compared with our previous studies,17 the degradation rate of PNT was faster than that of MNT by US/O3 oxidation under analogous conditions. This can be explained, in part, as a result of the inductive effect of substituents. Methyl, as an electron donor, is an activator as well as ortho- and para-directing in electrophilic aromatic substitution, while nitro substituent which belongs to the electron receptor, deactivates the phenyl ring and is meta-directing in the reaction. Thus, the removal of PNT, with a nitro substituent in the para-position, by · OH attack was more favored than that of MNT with a nitro substituent in metaposition of the methyl group on benzene ring. Acknowledgment

Conclusions The conclusions drawn from this study are summarized as follows:

This work was supported by the National Basic Research Program of China (Grant No. 2009CB421603) and the Natural Science Foundation of Zhejiang Province (Grant No. Z5080207).

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ReceiVed for reView October 17, 2008 ReVised manuscript receiVed March 17, 2009 Accepted May 2, 2009 IE801566Z