Article pubs.acs.org/IECR
Degradation of Toluene by a Selective Ferrous Ion Activated Persulfate Oxidation Process Anhua Long,†,‡ Yang Lei,† and Hui Zhang*,† †
Department of Environmental Engineering, Hubei Biomass-Resource Chemistry and Environmental Biotechnology Key Laboratory, Wuhan University, Wuhan, 430079, China ‡ Jiangxi Science and Technology Normal University, Jiangxi, 330013, China S Supporting Information *
ABSTRACT: The Fe2+-activated persulfate (PS) oxidation of toluene was evaluated using single-factor tests and response surface methodology (RSM). The results indicated the degradation efficiency increased as the PS and Fe2+ concentrations increased, while excessive dosages inhibited the degradation efficiency. The pH had an insignificant effect on the degradation of toluene. Based on the RSM optimization approach, the degradation efficiency reached a maximum of 84.4%. The main intermediates were also separated and identified by GC−MS, and a plausible degradation pathway of toluene was proposed. Further studies of surfactant recovery from flushing effluents mixed with toluene and surfactant were explored. Compared with the Fe2+/H2O2 process, a great enhancement of the toluene degradation efficiency and lower sodium dodecyl sulfate (SDS) degradation efficiency were obtained by the Fe2+/PS process. The results demonstrated that the Fe2+/PS selective oxidation process could be used for SDS recovery and for the degradation of contaminants present in soil flushing effluents.
1. INTRODUCTION BTEX (benzene, toluene, ethylbenzene, and xylenes) are generally used as solvents and are the main constituents of petroleum products, particularly gasoline, jet fuels, and kerosene. Due to their semivolatility, low solubility, strong absorption, and various metabolites, BTEX are among the most frequently co-occurring binary mixtures in complete exposure pathways at hazardous waste sites.1 Environmental media, such as air, water, and soil, are often contaminated with these chemicals, which are identified as hazardous air and industrial wastewater pollutants.2 Long-term exposure to BTEX can lead to neurological impairment because they are toxic or potentially toxic to humans.1,2 In addition, the U.S. Environmental Protection Agency (EPA) has established maximum contaminant levels (MCL) of 0.005 mg/L (benzene), 1.0 mg/L (toluene), 0.7 mg/L (ethylbenzene), and 10.0 mg/L (xylenes) in public drinking water systems.3 Accordingly, such pollution prompts the need for investigations of the available technology for the control and remediation of BTEX contaminants. In recent years, the application of activated persulfate (PS) oxidation has emerged as a novel effective technology for the remediation of BTEXcontaminated air, soil, and water.4−7 This technology could generate sulfate radicals (SO4•−) as described in eq 1. SO4•− can react as a strong oxidant for organic compounds in aqueous systems in three different ways: electron transfer, addition, and hydrogen abstraction.8,9 Sodium persulfate (Na2S2O8) is normally selected as an oxidant10 for in situ chemical oxidation (ISCO) because potassium persulfate (K2S2O8) has a low water solubility and ammonium persulfate ((NH4)2S2O8) is relatively volatile.8 As a natural substrate, ferrous ion (Fe2+) has the advantages of being inexpensive and nontoxic, and it has been widely applied as a catalyst to effectively activate PS.11−14 © 2013 American Chemical Society
S2 O82 − + activator (Fe2 +, heat, UV) → SO4•− + (SO4•− or SO4 2 −)
(1)
As one of the BTEX chemicals, toluene is employed as a model contaminant substance because it is likely to be one of the most common lighter-than-water nonaqueous phase liquid (LNAPL) contaminants and it is often detected in contaminated soil and groundwater as a major petroleum industrial organic waste product.15,16 Toluene has a low solubility and high interfacial tension because it is one of the hydrophobic organic compounds (HOCs).15 Hence, surfactant-based treatment is usually chosen as an effective technology for remediation of toluene-contaminated soil and groundwater,15−18 while the collected wastes, including surfactants and toluene, should be treated before discharge or reuse. A recent review proposed that SO4•− is an electrophilic reagent, and when electron-donating groups are present on an aromatic molecule, the rate of the reaction with SO4•− will increase.8 Moreover, SO4•− is relatively more stable in water than the hydroxyl radical (OH•), and thus may be able to disperse a greater distance in water.19,20 Furthermore, it is generally accepted that SO4•− reacts more selectively via electron transfer than OH•.9,18,21,22 Thus, SO4•− may oxidize toluene with a faster reaction rate than straight chain hydrocarbon surfactants, which do not contain a benzene ring in the flushing effluent. Usually, sodium dodecyl sulfate (SDS) is used as surfactant because it is food grade and is easily biodegradable by soil and/ or aquatic microorganisms.23 Received: Revised: Accepted: Published: 1033
August 11, 2013 December 20, 2013 December 24, 2013 December 24, 2013 dx.doi.org/10.1021/ie402633n | Ind. Eng. Chem. Res. 2014, 53, 1033−1039
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Article
capillary column (30 m length × 0.25 mm i.d. × 0.25 m film thickness) was employed for GC separation. The GC equipment was operated in a temperature programmed mode with an initial temperature of 35 °C held for 2 min and then ramped to 280 °C with a 10 °C/min rate. Helium was used as the carrier gas. Electron impact (EI) mass spectra were scanned from 0 to 550 m/z. The concentration of SDS was determined by the methylene blue method.26 Methylene blue was first added to the supernatant liquid, then the anionic surfactant−cationic dye complex was extracted with chloroform, and the absorption in the CHCl3 phase was measured at λmax = 655 nm using a Rayleigh UV-9100 spectrophotometer (Rayleigh Co., China); from this figure the concentration of SDS was obtained.
The objective of the present study was to optimize toluene degradation in an aqueous solution with the Fe2+/PS process using single-factor tests and response surface methodology (RSM). Three reaction parameters (pH value, Fe2+ concentration, and PS concentration) were selected as single factors, and the optimal conditions were obtained with the RSM optimization approach. Furthermore, Fe2+/PS and Fe2+/H2O2 systems were also applied to evaluate the degradation of toluene present in soil flushing effluents and the recovery of SDS.
2. MATERIALS AND METHODS 2.1. Chemicals and Reagents. All the materials were analytical quality reagents and were used without further purification. Toluene (C 6H 5 CH3 ) was purchased from Shanghai No. 4 Reagent Factory (China). Sodium persulfate (Na2S2O8), hydrogen peroxide (H2O2, 30% w/w), ferrous sulfate (FeSO4·7H2O), and sodium dodecyl sulfate (SDS, CH3(CH2)11SO4Na) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). 2.2. Apparatus and Procedure. Batch Fe2+/PS and Fe2+/ H2O2 experiments were conducted in an undivided oxidation reactor (100 mL conical flask with cover) containing 100 mL of solution. Before each run, a stock solution of toluene was prepared fresh with deionized water/surfactant and the initial concentration (C0) was fixed at 1 mM. Prior to the oxidation process, the initial pH (pH0) of the toluene solution was measured with a Mettler-Toledo FE20 pH meter (MettlerToledo Instruments Co., Ltd., Shanghai, China) and adjusted by sulfuric acid and sodium hydroxide. Average pH values presented in the paper were initial values by varying the pH within the same chamber. Then the PS or H2O2 and Fe2+ solutions were added to the oxidation cell. A magnetic stirrer (Model 78-1, Hangzhou Instrument Motors Factory, China) was applied to agitate the solution and ensure homogeneity throughout the reaction. At different time intervals, aliquots of the mixture were sampled and analyzed. 2.3. Analytical Methods. Direct aqueous injection gas chromatography/flame ionization detection (DAI-GC/FID) was applied to investigate the concentration of toluene in the aqueous/surfactant solutions.24,25 The water samples were quantified using a gas chromatograph (GC-14C, Shimadzu) with the following conditions. GC conditions: oven programmed to 150 °C, constant temperature (12 min), N2 (99.999% purity) carrier gas, and SE54 (30 m × 0.32 mm × 0.50 μm) column. FID conditions: temperature set to 250 °C, H2 flow of 60 mL/min, air flow of 400 mL/min, N2 makeup gas, and makeup flow of 20 mL/min. Injector conditions: injection model with split inlets (split ratio = 20:1), injection volume of 1 μL, and 250 °C injector temperature. The total organic carbon (TOC) values were determined by an Analytikjena multi N/C 3100 analyzer equipped with hightemperature combustion and a nondispersive infrared absorption detector. The intermediate products during the Fe2+/PS reaction in the aqueous solutions were detected by gas chromatography− mass spectrometry (GC−MS, VARIAN 450-GC/320-MS). Samples for GC−MS analysis were prepared as follows: 100 mL of reaction solution was divided into five parts, and each part was repeatedly extracted with 5 mL of carbon disulfide. The extracts were then analyzed by GC−MS. A VF-5 MS
3. RESULTS AND DISCUSSION 3.1. Single-Factor Tests. The toluene degradation efficiency depends on several main factors, such as pH, concentration of Fe2+, and concentration of PS in the Fe2+/ PS process. Some preliminary runs were performed to achieve a suitable single factor. Then the optimal initial pH and dosages of PS and Fe2+ were investigated. The initial pH values investigated in this study were 4, 7, and 10 with a Fe2+ concentration of 5 mM and PS concentration of 20 mM. As seen in Figure 1a, the results showed that the toluene degradation efficiency decreased a little as the pH value rose from 4 to 7, and there was no obvious difference between pH 7 and 10. The decreased toluene degradation in the neutral and alkaline media might result from the precipitation of Fe3+. In addition, soluble Fe2+ may decrease due to the formation of Fe2+ complexes when the pH value of the aqueous solution is higher than 4. On the other hand, the Fe3+ oxyhydroxides, such as FeOH2+, Fe2(OH)24+, Fe(OH)2+, Fe(OH)30, and Fe(OH)4−, have low activation efficiencies for persulfate for SO4•− generation.27 SO4•− predominates under acidic conditions, while OH• predominates under basic conditions.20,21,28 When SO4•− was generated by the ferrous iron mediated decomposition of PS via eq 1, it could react with H2O or OH− to produce OH• as shown in eqs 2 and 3.28 Moreover, the solution pH decreased with H+ generation or OH− consumption based on eqs 2 and 3 in the Fe2+/PS process. Furthermore, H+ is further released by the dissociation of HSO4− (eq 4), which was formed through the reaction of OH• and SO4•− (eq 5).29 Therefore, the pH value of the solution would drop with the progress of the reaction and finally tend to remain around 2.5. This may be the reason for the similar degradation efficiency observed at different initial pH values, as indicated in Figure 1a. This indicated that the Fe2+/PS process could be successfully applied in a wide range of initial pH values. SO4•− + H 2O → SO4 2 − + OH• + H+
(2)
SO4•− + OH− → SO4 2 − + OH•
(3)
HSO4 − → SO4 2 − + H+
(4)
SO4•− + OH• → HSO4 − + 1/2O2
(5)
The effect of Fe2+ concentration on the degradation of toluene was investigated by conducting experiments at 0, 2, 5, and 10 mM when the PS concentration was 20 mM and the initial pH value was around 7.0 (background value). As shown in Figure 1b, the toluene degradation efficiency increased as the 1034
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significant as the PS concentration increased, and more PS was consumed via eq 7.11,12,28 SO4•− + S2O82 − → SO4 2 − + S2 O8•−
(7)
Herein, preliminary single-factorial experiments identified the ranges for the three main factors. The toluene degradation efficiency was investigated by varying the following variables: pH value (4, 7, or 10), Fe2+ concentration (1, 5, or 9 mM), and PS concentration (5, 20, or 35 mM), as listed in Table SM-1 in the Supporting Information. 3.2. Response Surface Evaluation. The full factorial analysis with three factors in three levels as well as the toluene degradation efficiency results for each run are listed in Table SM-2 in the Supporting Information. The toluene degradation efficiency was developed based on the main effects plot and the interaction plots plotted from Table SM-2 (Supporting Information). The main effects plot indicated that the effects of the pH value and PS concentration on the toluene degradation efficiency were slightly negative (Supporting Information, Figure SM-1),30−32 in keeping with the singlefactor results. However, the Fe2+ concentration had a positive effect because the high (+1) Fe2+ concentration was not in excess when the PS concentration was set at high level (+1). This means that a high level (+1) of the pH value and the PS concentration would lead to a lower toluene degradation efficiency when the pH value was basic and PS was in excess. Optimization of the pretreatment conditions was achieved by employing a Box−Behnken design (BBD) and the polynomial equation describing the toluene degradation efficiency in 30 min as a simultaneous function of pH value (X1), Fe2+ concentration (X2), and PS concentration (X3), as shown in eq 8: Y = 77.73 − 0.27X1 + 19.51X 2 − 0.51X3 + 0.57X1X 2 + 1.53X1X3 + 9.05X 2X3 − 4.72X12 − 16.09X 2 2 Figure 1. Effect of different factors. (a) Degradation of toluene under different solution initial pH values. C0 = 1 mM; [Fe2+] = 5 mM; [PS] = 20 mM. (b) Effect of Fe2+ concentration on the degradation of toluene. C0 = 1 mM; [PS] = 20 mM; pH 7.0. (c) Effect of PS concentration on the degradation of toluene. C0 = 1 mM; [Fe2+] = 5 mM; pH 7.0.
− 5.39X32
The effects of two-factor interactions were conducted with one factor fixed at a high (+1) or low (−1) level while the other was investigated. 33−35 Figure SM-2 in the Supporting Information shows interaction plots that elucidate whether there was an existing interaction between the factors. The interaction effect on toluene degradation efficiency between the Fe2+ concentration and the PS concentration was significant due to the curves crossing, as shown in Figure SM-2 (Supporting Information). That is, the change of one variable would affect the other one. The interaction effect between the pH value and Fe2+ concentration or PS concentration was insignificant. This was also confirmed with the high probability value ((Prob > F) > 0.1) through analysis of variance (ANOVA). The ANOVA analysis is shown in Table SM-3 in the Supporting Information to justify the significance and adequacy of the model. The value of (Prob > F) was used to determine the significance of each model term. Corresponding model terms will be more significant if the value of (Prob > F) becomes smaller while a value over 0.1 implies insignificance.36,37 According to the confidence level selected ((Prob > F) > 0.1), the insignificant model terms (X1, X3, X1X2, X1X3) were removed from the RSM models; then eq 8 could be changed into eq 9 and the ultimate RSM models, in terms of
Fe2+ concentration increased from 0 to 5 mM. The ferrous ion can activate PS to produce SO4•−. Increasing the ferrous ion dosage corresponds to a higher available Fe2+ concentration. However, the toluene degradation efficiency decreased when the Fe 2+ concentration increased to 10 mM. It was hypothesized that either the destruction of SO4•− in the presence of excess Fe2+ or the rapid conversion of all Fe2+ to Fe3+ limited the ultimate oxidizing capability via eq 6.13,14,19 SO4•− + Fe 2 + → Fe3 + + SO4 2 −
(8)
(6)
The effect of PS concentration on the toluene degradation efficiency was evaluated by conducting experiments at 0, 1, 10, and 20 mM. The Fe2+ concentration was 5 mM, and the initial pH value was the background pH value. When the PS concentration increased from 0 to 20 mM, the degradation efficiency increased, as observed in Figure 1c. PS is a source of SO4•− in the system, and more reactive radicals are generated to degrade toluene at higher PS concentrations. On the other hand, the side reaction between PS and SO4•− became more 1035
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the coded factors, were determined to calculate the toluene degradation efficiency. Y = 77.73 + 19.51X 2 + 9.05X 2X3 − 4.72X12 − 16.09X 2 2 − 5.39X32
(9)
The very low (Prob > F) values (
