Enhanced degradation of carbon tetrachloride by surfactant-modified zero-valent iron

Enhanced degradation of carbon tetrachloride by surfactant-modified zero-valent iron first-author corresp ‡Corresponding Author ‡Corresponding Author †E-mail:guanbaohong@zju.edu.cn †E-mail:guanbaohong@zju.edu.cn guanbaohong@zju.edu.cn Institute of Environmental Engineering, Zhejiang University, Hangzhou 310027, China Institute of Environmental Engineering, Zhejiang University, Hangzhou 310027, China Sorption of carbon tetrachloride (CT) by zero-valent iron (ZVI) is the rate-limiting step in the degradation of CT, so the sorption capacity of ZVI is of great importance. This experiment was aimed at enhancing the sorption of CT by ZVI and the degradation rate of CT by modification of surfactants. This study showed that ZVI modified by cationic surfactants has favorable synergistic effect on the degradation of CT. The CT degradation rate of ZVI modified by cetyl pyridinium bromide (CPB) was higher than that of the unmodified ZVI by 130%, and the CT degradation rate of ZVI modified by cetyl trimethyl ammonium bromide (CTAB) was higher than that of the unmodified ZVI by 81%. This study also showed that the best degradation effect is obtained at the near critical micelle concentrations (CMC) and that high loaded cationic surfactant does not have good synergistic effect on the degradation due to its hydrophilicity and the block in surface reduction sites. Furthermore degradation of CT by ZVI modified by nonionic surfactant has not positive effect on the degradation as the ionic surfactant and the ZVI modified by anionic surfactant has hardly any obvious effects on the degradation. Enhanced degradation Sorption Surfactants Synergistic effect Zero-valent iron (ZVI) INTRODUCTION 1 As one of the primary organic solvents and industrial chemicals, chlorinated hydrocarbons are extensively applied in chemical industry, medicine, pesticide and many other fields. However, the application of chlorine for disinfection of drinking water is giving rise to poisonous chlorinated hydrocarbons. Therefore, chlorinated hydrocarbons become the most common organic pollutant in soils and groundwater (Shoemaker, 1995 1995 ). Many chlorinated hydrocarbons can lead to cancer, monstrosity and mutagenesis. As a result, they are listed as strictly regulated pollutants in many countries. Degradation of chlorinated hydrocarbons in water by wet oxidation requires high energy (Qin et al., 2001 2001 ) and the degradation product—chloride ion has severe causticity in supercritical water oxidation (SCWO). Particularly, as the Lewis acid (electron acceptor), the multi-chlorine pollutants cannot be treated effectively by advanced oxidation process (AOP). Biological treatment is a potential way, but the microorganism growth should be well controlled. In recent years, as a simple, efficient and low-cost treatment method, the degradation of chlorinated hydrocarbons in industrial wastewater and the groundwater by ZVI (zero-valent iron) at room temperature has been noticed by many researches. As an extensively used metal, iron has favorable dechlorination effect on many chlorinated compounds, such as aliphatics and aromatics and pesticides (Lookman et al., 2004 2004 ; Guan and Meng, 2005 2005 ; Hara et al., 2005 2005 ; Sayles et al., 1997 1997 ). In order to accelerate degradation, many researchers tried to modify the ZVI by coating a second catalyst metal such as Pd (Doong and Lai, 2005 2005 ) on the iron surface or increasing the surface area by reducing particle size to enhance the reactivity (Choe et al., 2001 2001 ). Many researches show that the use of surfactant could enhance the concentration of organism in solution in soil and in the sediments (Ko et al., 1998 1998 ; Burris and Antworth, 1992 1992 ). Ko et al.( 1998 1998 ) showed that the use of anionic surfactants enhances the concentration of aromatic compounds on kaolinite particles. The surface concentration depends on the kind and concentration of the surfactant. Cationic surfactant has strong sorption capacity in the surface of solid thus effectively enhancing the degradation rate of chlorinated hydrocarbons (Alessi and Li, 2001 2001 ; Zhang et al., 2002 2002 ). Sayles et al.( 1997 1997 ) found that deoxidation of DDT (dichloro-diphenyl-trichloroethane) by ZVI could enhance the degradation rate by 40% in the presence of Triton-114 (a sort of nonionic surfactant). The enhancement of the reaction rate could be attributed to the increase of DDT products dissolubility. The present study is aimed at finding whether ZVI modified by surfactant has synergistic effect on accelerating the dechlorination. The surfactants involved here are cationic surfactant (cetyl trimethyl ammonium bromide, CTAB; cetyl pyridinium bromide, CPB), nonionic surfactant (Triton-100, Tween 80) and anion surfactant (sodium dodecyl benzene sulfonate, SDBS; sodium dodecyl sulfate, SDS). MATERIALS AND METHODS 1 Reagents 2 ZVI (80 mesh) and surfactants (CTAB, CPB, Triton-100, Tween 80, SDBS and SDS) were all obtained from Shanghai Chemical Co. Carbon tetrachloride (CT) and methanol were purchased from Siyou Chemical Co. All reagents were of AC grade. The critical micelle concentration (CMC) was found in the literature to be 0.9 mmol/L for CTAB (Rosen, 1989 1989 ), 0.6 mmol/L for CPB (Mukerjee and Mysels, 1971 1971 ), 0.25 mmol/L for Triton-100 (Partearroyo et al., 1996 1996 ), 0.185 mmol/L for Tween 80 (Kim et al., 2001 2001 ), 8.0 mmol/L for SDS (Liu and Roy, 1995 1995 ), 2.1 mmol/L for SDBS (Bi et al., 1999 1999 ). Surfactant modification 2 In surfactant modifying studies, 10 g iron powder and surfactant CTAB and CPB of different initial concentrations were respectively put into a 50 ml zero headspace conical flask and sealed with Teflon rubber. The pH remained neutral and had no ionic strength. The conical flask was put onto a shaker table kept shaking for 24 h (a period shown to be sufficient to obtain equilibrium) without light and under the conditions of 25 °C and 220 r/min. After modification, the iron powder was filtrated and the concentration of the filtered solution was analyzed by HPLC. The surfactant loaded could be measured from the difference between the initial and equilibrium surfactant (Li et al., 1999 1999 ). CT degradation 2 In CT degradation studies, 2.0 g unmodified or surfactant-modified ZVI and 20 mg/L CT solution were put into 50 ml zero headspace conical flask and sealed with Teflon rubber immediately. The conical flask was put onto a shaker table, and the reaction was conducted without light. The pH remained neutral and the rotating speed was 220 r/min. Methods of analysis 2 Cationic surfactant was analyzed by HPLC with ODS column using an aqueous solution of 5 mmol/L ptoluenesul fonate and methanol (45:55, v/v) as the mobile phase. Aliquots of 25 μl were injected into the HPLC (Li and Bowman, 1997 1997 ). The CT concentration was measured by GC with ECD detector on a 3 m long and 3 mm diameter glass column. Temperatures of injector, column and detector were set at 180, 80 and 180 °C respectively. The 250 μl samples were put into a 100 ml gas-liquid equilibrium bottle with 50 ml headspace. After a water bath at 38 °C and equilibrium for 40 min, 15 μl zero headspace gas samples were taken and analyzed by GC. RESULTS AND DISCUSSION 1 CT degradation with surfactant-modified ZVI 2 CT degradation by unmodified ZVI or ZVI modified by cationic surfactants followed pseudo first-order reduction (Tamara and Butler, 2004 2004 ; Tratnyek et al., 2001 2001 ): C = C 0 e − kt , where C and C 0 are the solute concentrations at time t and t =0, respectively. Table 1 1 lists the apparent CT rate constants for different surfactant treatment. Control experiment containing different concentrations and 20 mg/L CT showed no CT loss during the experiment, indicating that these surfactants alone would not react with CT. Type of surfactant treatment Input surfactant conc. (mmol/L) Apparent rate constant (min −1 ) Only ZVI 0.00 0.0079 CTAB 0.30 0.0120 CTAB 0.90 0.0143 CTAB 2.70 0.0129 CPB 0.20 0.0125 CPB 0.60 0.0182 CPB 1.80 0.0134 Tween 80 0.06 0.0083 Tween 80 0.19 0.0102 Tween 80 0.56 0.0092 Triton-100 0.08 0.0089 Triton-100 0.25 0.0113 Triton-100 0.75 0.0100 SDS 2.70 0.0072 SDS 8.00 0.0069 SDS 24.00 0.0085 SDBS 0.70 0.0069 SDBS 2.10 0.0066 SDBS 6.30 0.0075 The charge of the hydrophilic group has great influence on the nature of the surfactant. The result of degradation of 20 mg/L CT in the presence of unmodified ZVI and ZVI modified by cationic surfactants is presented in Fig. 1 1 . Both Fig. 1 1 and Table 1 1 show that ZVI modified by two kinds of cationic surfactants can effectively accelerate the degradation. The degradation rate of ZVI modified by CPB was higher than that of the unmodified ZVI by 130%, and the degradation rate of ZVI modified by CTAB was higher than that of the unmodified ZVI by 81%. In surfactant-iron system, a CT degradation model consists of aqueous phase, surfactant layer on the iron surface, reactive sites on the iron surface, and non-reactive sites on the iron surface. For dechlorination to occur, CT is first sorbed onto the iron surface; then the sorbed CT gives rise to a dechlorination reaction. The cationic modified ZVI can increase sorption of CT onto iron surface, resulting in an elevated CT surface concentration on reactive sites of iron. Therefore both the electron transfer for CT and the reduction of sorbed CT are promoted. The efficiency of ZVI modified by nonionic surfactants is similar for the ZVI modified by cationic surfactants (Fig. 2 2 ). The reduction rate on CT of ZVI modified by Tween 80 was 29% higher than the maximum unmodified ZVI and degradation rate on CT of ZVI modified by Triton-100 was 43% higher than that of the unmodified ZVI. Table 2 2 lists the surfactant loading on modified ZVI. With increasing of surfactant concentration in solution, the surfactant sorbed onto the ZVI surface increases. However the degradation rate of CT does not necessarily increase with increasing of surfactant capacity (Tables 1 1 and 2 2 ). The iron-water interface bears negative electricity at high pH values (Tratnyek et al., 2001 2001 ), therefore, as Fig. 3 3 shows, under the condition of lower concentration of cationic surfactant, the iron surface is sorbed by patchy monolayer and the hydrophobic group points to solution so as to sorb the CT in water. The higher concentration of cationic surfactant forms a patchy bilayer on the iron surface which leads to a hydrophilic group and thus higher concentration unfavorable for sorption of CT. Furthermore, too many surfactants on the iron block the reduction sites, resulting in decreasing the reduction rate of CT decrease with the increasing of the modified surfactant concentration. Type of surfactant treatment Input surfactant conc. (mmol/L) Surfactant loading (mmol/kg) Only ZVI 0.0 0.0 CTAB 0.3 0.4 CTAB 0.9 1.0 CTAB 2.7 5.9 CPB 0.2 0.4 CPB 0.6 0.7 CPB 1.8 2.8 Fig. 2 2 shows the reduction of CT by nonionic surfactants modified ZVI. In the case of low concentration of nonionic surfactant, the iron surface is sorbed by patchy monolayer with the modified ZVI having weaker reduction effect on CT when compared to cationic surfactants. In contrast to cationic surfactants modified ZVI, Fig. 4 4 shows that in the case of low concentration of anionic surfactants, the iron surface is sorbed by monolayer and the hydrophilic group points to solution unfavorable for sorption of CT. While in the case of a higher concentration, the anionic surfactant forms a bilayer on the iron surface which leads to a hydrophobic group point to solution. The process is opposite to that in the case when cationic surfactant is used. Initial rapid decrease in CT solution concentration could be found in Figs. 1 1 , 2 2 and 4 4 . As noted by Burris et al.( 1995 1995 ). ZVI can sorb PCE (perchloroethylene), probably due to the presence of carbon in the iron metal. So the initial rapid decrease may be resulted from this effect. The enhanced sorption of CT due to presence of cationic surfactant thus appears to be responsible for the enhanced CT degradation. Influence of pH 2 The reduction of CT is greatly influenced by the change of pH in the solution. At lower pH, the reduction rate constant is twice that of the constant at higher pH. The result is shown in Fig. 5 5 and accords with the ZVI corrosion at lower solution pH and passivation at higher solution pH (Chen et al., 2001 2001 ). The use of phosphate-based buffer solution restrains the reaction, because iron phosphate precipitation results in ZVI coating. CONCLUSION 1 This research showed that the use of cationic surfactants results in the increasing of CT concentration on ZVI and then higher reduction rate. 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Enhanced perchloroethylene reduction in column systems using surfactant-modified zeolite/zero-valent iron pellets 10.1631/jzus.2006.B0702 Project (No. 20030352) supported the Science and Technology Program of Zhejiang Province, China 2006-01-19 2006-03-21 2006-08-15 2022-01-05 Copyright © 2006, Journal of Zhejiang University Science 2006 Journal of Zhejiang University-SCIENCE B (Biomedicine and Biotechnology) 09 07