Paper 2C-35, in: A.R. Gavaskar and A.S.C. Chen (Eds.), Remediation of Chlorinated and Recalcitrant Compounds—2002. Proceedings of the Third International Conference on Remediation of Chlorinated and Recalcitrant Compounds (Monterey, CA; May 2002). ISBN 1-57477-132-9, published by Battelle Press, Columbus, OH, www.battelle.org/bookstore.
RATE AND PATHWAY CHANGES IN PCE DECHLORINATION BY ZERO-VALENT IRON IN THE PRESENCE OF CATIONIC SURFACTANT Zhaohui Li (
[email protected]), and Cari Willms (University of Wisconsin – Parkside, Kenosha, Wisconsin) Pengfei Zhang and Robert S. Bowman (New Mexico Institute of Mining and Technology, Socorro, New Mexico) ABSTRACT: Chlorinated solvents are major groundwater contaminants at industrial sites, DOE facilities, and military installations. The recent use of zero-valent iron (ZVI) as a permeable barrier to degrade chlorinated solvents has attracted great attention. Batch and column tests were performed to study changes in degradation pathways of perchloroethylene (PCE) by ZVI in the presence of sorbed cationic surfactants. Compared to unmodified ZVI, when ZVI was coated with hexadecyltrimethylammonium the apparent rate constants of PCE degradation and trichloroethylene (TCE) accumulation increased by a factor of five. The fraction of PCE reduced via hydrogenolysis vs. belimination is 0.58-1.00 in the presence of sorbed surfactant admicelles on ZVI, while the fraction is 0.12-0.25 in the absence of surfactant. Although the shift in PCE degradation mechanism from mainly b-elimination to mainly hydrogenolysis generates more TCE in the system, the overall rate of disappearance of PCE and TCE is still faster in the presence of surfactant. INTRODUCTION Zero-valent iron (ZVI) as an inexpensive reductant for groundwater remediation has attracted great attention in the past ten years. Although a wide range of organic contaminants such as chlorinated solvents and nitro aromatics can be reduced by ZVI, the reduction rates for chlorinated aromatics and most of the chlorinated aliphatics are relatively slow, with half-lives on the order of days or more (Gillham and O’Hannesin, 1994; Johnson et al., 1996). Thus, several approaches have been used enhance the reduction rates of chlorinated compounds and other organic contaminants, including coating ZVI with other metals such as Pt, Ni, and Cu (Liang et al., 1997). Sorbed organic molecules on ZVI surfaces also affect the reduction rates of contaminants by ZVI. Perchloroethylene (PCE) reduction rates increased by factors of 319 when ZVI was modified by cationic surfactants (Li, 1998; Alessi and Li, 2001). A slight increase in PCE reduction was achieved when ZVI was modified by a non-ionic surfactant (Loraine, 2001). No enhancement in PCE reduction was found when an anionic surfactant was used (Li, 1998; Loraine, 2001). Reduction of carbon tetrachloride and trichloroethylene (TCE) by ZVI was inhibited by sorbed natural organic matter, but several quinonoid compounds (juglone, lawsone, and anthraquinone disulfonate) increased the reduction rate of carbon tetrachloride by ZVI (Tratnyek et al., 2001). In a column study using a pelletized mixture of ZVI and surfactant-modified zeolite, an increase of 3 in PCE degradation rate constants was achieved (Zhang et al., 2002). To date, however, no column tests have been performed to examine the effects of sorbed surfactants on PCE reduction rates and pathways by ZVI.
The purpose of this study was two-fold: (1) to further investigate surfactantcatalyzed PCE reduction by ZVI in flow-through systems; (2) to investigate changes in PCE reduction pathways in the presence of cationic surfactant. MATERIALS AND METHODS Materials. The ZVI was obtained from Fisher Scientific (Catalog No. S71953) with a particle size of less than 0.13 mm (100 mesh) and was used without pretreatment. The surfactant used was hexadecyltrimethylammonium bromide (HDTMA-Br) from Aldrich. PCE and TCE were also from Aldrich. Surfactant Modification. In each of 12 500-mL centrifuge bottles, 40 g of ZVI and 400 mL of 2.5-mmol/L HDTMA-Br solution were mixed on a shaker table at 150 rpm and room temperature for 24 hours. The mixture was then centrifuged and the supernatant removed for analysis of HDTMA. The ZVI was then washed with 400 mL of distilled water and then air-dried. The final HDTMA loading on the ZVI was 8.0 mmol/kg, similar to the HDTMA loading in a batch study performed earlier (Alessi and Li, 2001). Batch Experiments. For batch kinetic studies, 2.5 g of unmodified or surfactantmodified ZVI and 20 mL of PCE (20 mg/L) were put into a 20-mL headspace glass vial and sealed with PTFE-lined septa. The vials were shaken at room temperature and 8 rpm on a tumbling solution mixer for varying amounts of time. 50 mL of supernatant was withdrawn from each vial using a 100-mL gas-tight syringe and injected into an HPLC for PCE and TCE analyses. The pH of each sample was also measured. Triplicate samples were sacrificed at each time interval. Appropriate blanks were included as controls. Transport Experiments. Separate transport experiments were performed under identical conditions using either PCE or TCE input solutions. Duplicate glass chromatography columns (Ace Glass, Vineland, NJ), 30-cm long by 2.5-cm diameter, were filled with raw ZVI or modified ZVI. Column end caps and most of the fittings and tubing were made of Teflon. Each end cap was fitted with a three-way stainless steel valve for influent and effluent sampling. The fluid delivery system consisted of a programmable multi-channel syringe pump (Yale Apparatus, Wantagh, NY) equipped with four 10-mL glass gastight syringes (Hamilton, Reno, NV). The contaminant solution was fed from a 12-L collapsible Teflon bag (Alltech, Deerfield, IL). The columns were first flushed with distilled water in an upward flow mode at a flow rate of 0.25 mL/min for three days until saturation (as determined by a stable column weight) was attained. Detailed column parameters can be seen in Table 1. The input PCE concentration was 22 mg/L while that of TCE was 11 or 6 mg/L. The flow rate was maintained at 2.00 mL/min. Influent and effluent samples (50 mL) were collected using a 100-mL Hamilton syringe to pierce through the Teflon-lined septum in the sampling port of the three-way valves. The samples were immediately injected into an HPLC for analysis of PCE and TCE. The solution pH was measured periodically. All experiments were performed at room temperature (~22°C). Methods of Analysis. PCE and TCE concentrations were determined using an Alltech
HPLC equipped with a 4.6- by 150-mm Regis ODS column and a UV-Vis detector with the wavelength set at 195 nm. The mobile phase was 70% acetonitrile/30% water. At a flow rate of 2 mL/min, the retention times for PCE and TCE were 3.1 and 2.1 min, respectively. The detection limit was 0.05 mg/L and the linear response range was up to 40 mg/L for both PCE and TCE. Calibration curves with linear regression coefficients no less than 0.99 were constructed based on four to six standards. HDTMA was analyzed via an HPLC using the method of Li and Bowman (1997). Solution pH was measured using an Orion 710 pH/ISA meter with an Orion Ross combination pH electrode. Hydraulic conductivity of the iron was measured using a falling head method. TABLE 1. Flow Conditions for Column PCE Degradation Tests. Columns Modified ZVI Unmodified ZVI Solid Weight, g 388.0 390.2 454.9 454.4 Pore Volume, mL 73.7 75.1 71.2 73.3 Porosity 0.48 0.49 0.47 0.48 Bulk Density, g/cm3 2.6 2.6 3.0 3.0 Hydraulic Conductivity, cm/s 1.2´10-3 1.1´10-3 0.7´10-3 0.8´10-3
Simulation Model. PCE or TCE breakthrough curves (BTCs) were simulated using the numerical model HYDRUS-1D version 2.0 (Šimůnek et al., 1998). This model used the following set of partial differential equations to describe the one-dimensional transport of solutes (up to four chain members) undergoing sequential first order decay and linear reversible sorption in porous media (Šimůnek et al., 1998): r b K d ,1 r ö ¶ 2C ¶C æ æ ö ¶C (1) m S ,1 ÷÷C1 ç1 + b K d ,1 ÷ 1 = D 21 - v 1 - çç m L ,1 + q ¶x ¶x è q è ø ¶t ø r b K d ,i -1 r b K d ,i ö æ ö ¶ 2Ci ¶C æ r ö ¶C æ m S ,i -1 ÷÷C i - 1 (2) - v i - çç m L ,i + m S ,i ÷÷C i + çç m L ,i -1 + ç1 + b K d , i ÷ i = D 2 q ¶x è q q ¶x ø ¶t è ø è ø
where Ci and Si are the solute concentrations in liquid and on solid phases, t the time, rb the bulk density of ZVI, q the porosity, D the hydrodynamic dispersion coefficient, v the pore water velocity, x the distance, Kd,i the distribution coefficient, mL,i and mS,i the aqueous and solid phase first order decay constants, and i = 2, 3, 4. The overall first order decay constant mT that determines the steady-state breakthrough concentration is defined as: r K æ ö (3) mT = ç m L + b d m S ÷ q è ø A dispersivity (aL = D/v while neglecting molecular diffusion, Šimůnek et al., 1998) value of 1 cm was used in all simulations. Since reduction of organics by ZVI is considered to be a surface reaction (Weber, 1996), mL was set to zero and the values of Kd and mS were estimated by either inverse modeling (for the parent chemical) or forward modeling (for the daughter products) of the experiment data. To determine the fraction of hydrogenolysis during PCE degradation, the Kd and mS values for PCE and TCE were first estimated by inverse modeling of the BTCs from separate PCE and TCE transport experiments. These parameter values were then used
during forward modeling and the input PCE concentration was varied to match the TCE generation data. The fraction of hydrogenolysis was calculated as the ratio of the fitted PCE input concentration to the actual PCE input concentration. RESULTS AND DISCUSSION Batch PCE Degradation Test. Batch PCE degradation and TCE generation results are plotted in Figure 1. Linear correlation between ln (C/C0) and time revealed pseudo firstorder reaction (Figure 2): ln(C / C0 ) = - kt (4) where C is the PCE solution concentration at time t, C0 is the initial PCE concentration, and k is the pseudo first-order rate constant. The rate constants were 0.002 and 0.03 h-1 for unmodified and modified ZVI, respectively. The PCE concentration was lowered by the first sampling time but remained constant for the first 36 hours. This initial decrease in PCE concentration may indicate the sorption of PCE by modified ZVI. After 36 hours, PCE concentrations declined dramatically in modified system while slowly in the unmodified system. As the PCE concentration decreased, the TCE concentration increased up to about 100 hours, when the TCE concentration started to decrease due to the lowered PCE concentration. Using the apparent rate constants obtained from Eq. (4) for PCE and TCE and stepwise hydrogenolysis of PCE to TCE, the TCE concentration with time can be determined from (Sivavec et al., 1996): [TCE ] =
ak PCE ( e -k PCE t - e -kTCE t )[ PCE ]0 kTCE - k PCE
(5)
where kPCE and kTCE stand for apparent rate constants for PCE and TCE degradation, [PCE]0 for initial PCE concentration, and a for the fraction of PCE degrading to TCE via hydrogenolysis. The a values were 0.15 and 0.65 for unmodified and modified ZVI, indicating a change in PCE degradation pathway. Even with a shift in PCE degradation from mainly b-elimination to mainly hydrogenolysis, the overall aqueous contaminant concentration was still lower in the surfactant-modified system at all stage of the experiment. Thus, the unfavorable increase in hydrogenolysis was offset by a much faster contaminant removal rate. 0.0 Unmodified ZVI
-0.5
15
ln (C/C0)
Concentration (mg/L)
20
-1.0
10
Water only
-1.5
5
-2.0
Modified ZVI
-2.5
0 0
50
100 Time (h-1)
150
200
FIGURE 1. PCE degradation by modified (l) and unmodified ZVI (n). Control is shown as u. Open symbols are TCE concentrations generated. Lines are fitted results from Eq. (4) and (5).
0
50
100 Time (h-1)
150
200
FIGURE 2. PCE degradation by modified (l) and unmodified ZVI (n). Control is shown as u.
25 20 15 10 5 0 0
10
20 30 40 Pore Volume
50
60
FIGURE 3. PCE BTCs from HDTMAmodified ZVI (u,n) and unmodified ZVI (l,s) columns. Open symbols are input PCE concentrations. Lines are simulated results.
TCE Concentration (mg/L)
PCE Concentration (mg/L)
PCE Column Test. PCE BTCs from ZVI and HDTMA-ZVI columns can be seen in Figure 3. The steady-state effluent PCE concentrations were about 15 mg/L and 4 mg/L for unmodified and modified columns, respectively, resulting in a reduction by a factor of four in the effluent PCE concentration (Figure 3). The fitted parameters for PCE column test are listed in Tables 2. A five-fold increase in Kd value and a four-fold increase in mT value were achieved after surfactant modification (Table 2). Since the mS remained the same after modification, the increase in mT was due to the increased Kd, indicating that the sorption of PCE on HDTMA-modified ZVI was on reactive sites. The steady-state TCE concentrations produced from reduction of PCE were 0.5 mg/L for the unmodified columns and 7.5 mg/L for modified columns (Figure 4). The fraction of PCE undergoing hydrogenolysis was 0.12-0.25 for the unmodified columns and 0.58-1.00 for the modified columns. These values were similar to the ones obtained from batch tests (this study; Alessi and Li, 2001), and again showed a shift in PCE degradation pathways. Similar to the batch study, the sum of PCE and TCE concentrations in the effluent of the surfactantmodified ZVI was lower than that in the effluent of the raw ZVI (Table 2). 8 6 4 2 0 0
10
20
30 40 Pore Volume
50
60
FIGURE 4. TCE BTCs from HDTMA-modified ZVI (u,n) and unmodified ZVI (l,s) columns, due to stepwise reduction of PCE. Lines are simulated results.
TABLE 2. Transport Parameters Determined by Simulating the Experimental Breakthrough Data at an Initial PCE Concentration of 22 mg/L. Parameter SM1 SM2 UM1 UM2 -1 mS (hr ) 0.06±0.02 0.06±0.02 0.07±0.03 0.06±0.02 -1 mT (hr ) 2.6±0.4 2.7±0.7 0.7±0.2 0.6±0.2 Kd (L×kg-1) 8.5±1.1 9.2±1.5 1.7±0.4 1.6±0.3 Steady-State PCE 4 4 15 15 Concentration (mg/L) Steady-State TCE 7.6 7.5 0.6 0.5 Concentration (mg/L) Fraction of 0.61, 1.00§ 0.58, 0.93§ 0.12, 0.18§ 0.12, 0.25§ Hydrogenolysis pH 7.7 8.8 §Commas separate results from different TCE Kd and mS values (11 mg/L input and 6 mg/L input, discussed below).
14 12 10 8 6 4 2 0 0
10
20
30 40 Pore Volume
50
60
FIGURE 5. TCE breakthrough curves from HDTMA-modified ZVI (u,n) and unmodified ZVI (l,s) columns. Open symbols are for input PCE concentrations. Lines are simulated results.
TCE Concentration (mg/L)
TCE Concentration (mg/L)
TCE Column Test. With an input TCE concentration of 11 mg/L, the effluent TCE concentrations were 4-5 mg/L and 8 mg/L, or 60% and 30% reduction, for unmodified and modified ZVI, respectively (Figure 5). As the input TCE concentration was reduced to 6 mg/L, the effluent TCE concentrations were 0.6 and 1.6 mg/L, or 90% and 70% reduction, for unmodified and modified ZVI, respectively (Figure 6). The fitted parameters for TCE degradation are listed in Tables 3. Compared to an increased PCE reduction by surfactant-modified ZVI, the TCE reduction was inhibited in the presence of cationic surfactant. The Kd value for unmodified and modified ZVI remained the same while a three-fold decrease in mS resulted in an overall decrease in mT (Table 3). The results suggest that surface hydrophobicity plays an important role in contaminant reduction by ZVI. The HDTMA-modified ZVI is more hydrophobic than unmodified ZVI. PCE, with a lower aqueous solubility and higher octanol-water partition coefficient than TCE, partitions more effectively onto the surfactant-modified surface. The higher Kd of PCE on the modified ZVI surface resulted in enhanced PCE reduction. 8 7 6 5 4 3 2 1 0 0
10
20 30 40 Pore Volume
50
60
FIGURE 6. TCE breakthrough curves from HDTMA-modified ZVI (u,n) and unmodified ZVI (l,s) columns. Open symbols are for input PCE concentrations. Lines are simulated results.
TABLE 3. Transport Parameters Determined by Simulating the Experimental Breakthrough Data at an Initial TCE Concentration of 11 mg/L and 6 mg/L. Input Parameter SM1 SM2 UM1 UM2 -1 mS (hr ) 0.05±0.01 0.06±0.01 0.16±0.03 0.19±0.04 mT (hr-1) 0.49±0.06 0.54±0.08 1.41±0.02 1.63±0.02 -1 11 mg/L Kd (L×kg ) 1.7±0.1 1.8±0.1 1.4±0.2 1.4±0.2 Steady-State 8 8 5 4 Conc. (mg/L) mS (hr-1) 0.24±0.07 0.22±0.05 0.4±0.2 0.6±0.3 mT (hr-1) 2.2±0.2 2.2±0.2 3.9±0.5 4.9±0.4 -1 6 mg/L Kd (L×kg ) 1.7±0.4 1.9±0.3 1.4±0.6 1.3±0.7 Steady-State 1.5 1.6 0.7 0.4 Conc. (mg/L) pH 6.5 7.5
The effect of surfactants on contaminant reduction by ZVI is poorly understood. Sayles et al. (1997) found that when Triton X-114 (a nonionic surfactant) micelles were present in solution the reduction of DDT (1,1,1-trichloro-2,2-bis(p-chlorophenyl) ethane) by ZVI increased by almost a factor of two, due to increased solubility and enhanced mass transfer of the hydrophobic compound. In contrast, the presence of hydroxypropylb-cyclodextrin in solution lowered the PCE degradation rate due to the partitioning of PCE molecules into the hydrophobic interiors of the cyclodextrin, limiting PCE availability (Bizzigotti et al., 1997). Loraine (2001) observed a PCE reduction rate increase of 39% in batch studies when the ZVI (Fisher Scientific, 0.4 mm) was mixed with 1.5 g/L non-ionic surfactant Triton X-100. Compared to results with nonionic surfactants, the enhancement of PCE reduction by cationic surfactants sorbed on ZVI is much more pronounced. Arnold and Roberts (2000) indicated that adsorption of substrates to reactive sites at the surface, reaction at the surface, and desorption of the products all played important roles in reduction by ZVI, while the rate-limiting step can be any of these processes. Weber (1996) suggested that reduction of organic compounds by ZVI is a surface reaction that requires close contact between the compounds and the iron. For such a reaction, the rate is proportional to the surface concentration of the organic compound (Folger, 1999). Arnold and Roberts (2001) found the reactivity of chlorinated ethenes followed DCE > TCE > PCE, while most other studies (Johnson et al., 1996) found the opposite trend. A partial explanation for the opposing trends is enhanced sorption of highly substituted chloroethenes by impurities (carbon, in particular) on the ZVI surface, resulting in a faster apparent disappearance rate (Arnold and Roberts, 2000). The present study shows that the PCE sorbed by HDTMA was on reactive sites and resulted in enhanced dechlorination. The PCE reduction enhancement by HDTMA-treated ZVI is the same or greater than that by SMZ/ZVI pellets (Zhang et al., 2002). The observed enhancement of PCE reduction by ZVI in the presence of HDTMA may be attributed to (1) increased sorption of PCE directly onto iron surfaces by ironbound HDTMA, accelerating the surface reduction of PCE (Li, 1998; Alessi and Li, 2001), and/or (2) a catalytic effect of the cationic surfactant (Alessi and Li, 2001). In a study of pathways and kinetics of chlorinated ethylene degradation by ZVI, it was found that 87 % of initial PCE underwent b-elimination while hydrogenolysis to TCE accounted for only 10 % of the input PCE (Arnolds and Roberts, 2000). The TCE production (0.6 mg/L of TCE, corresponding to 3% of the initial PCE concentration) by unmodified ZVI in this study is in agreement with these results. However, a significant increase in aqueous TCE concentration (7 mg/L, corresponding to 30% of the initial PCE concentration) was observed when HDTMA-modified ZVI was used (Figures 1 and 4). Since the TCE is the product of hydrogenolysis while the product of reductive elimination is dichloroacetylene (Arnolds and Roberts, 2000), the significant TCE accumulation when HDTMA-ZVI was used compared to unmodified ZVI indicates that a relatively larger portion of input PCE underwent hydrogenolysis compared to reductive elimination. Therefore, a mechanism change occurs in the presence of sorbed surfactant. CONCLUSIONS The results from this study show that PCE reduction by ZVI is greatly enhanced in the presence of a sorbed cationic surfactant. The enhancement appears to be due to
greater sorption of more hydrophobic compounds onto the ZVI surface after surfactant modification. The results from this study also show a change in PCE degradation pathway from mainly b-elimination to mainly hydrogenolysis in the presence of cationic surfactant, resulting in greater TCE accumulation. The increased chlorinated ethylene production during PCE or TCE reduction may be a concern, since the less chlorinated ethylenes are more toxic. Nevertheless, the overall contaminant removal rate was still faster with the modified ZVI compared to unmodified ZVI. ACKNOWLEDGMENTS Funding for this research was provided by the Wisconsin Groundwater Program. Jeff Alley helped in sample collection during column experiments. REFERENCES Alessi, D. S., and Z. Li. 2001. “Synergistic Effect of Cationic Surfactants on Perchloroethylene Degradation by Zero Valent Iron.” Environ. Sci.Technol. 35(18): 3713-3717. Arnold, W. A., and A. L. Roberts. 2000. “Pathways and Kinetics of Chlorinated Ethylene and Chlorinated Acetylene Reaction with Fe(0) Particles.” Environ. Sci. Technol. 34(9): 1794-1805. Bizzigotti, G. O., D. A. Reynolds, and B. H. Kueper. 1997. “Enhanced Solubilization and Destruction of Tetrachloroethylene by Hydroxypropyl-b-Cyclodextrin and Iron.” Environ. Sci. Technol., 31 (2): 472-478. Fogler, H. S. 1999. The Elements of Chemical Reaction Engineering. 3rd ed. Prentice Hall, Upper Saddle River, NJ. Gillham, R. W., and S. F. O’Hannesin. 1994. “Enhanced Degradation of Halogenated Aliphatics by Zero-Valent Iron.” Ground Water 32 (6): 958-967. Johnson, T. L., M. M. Scherer, and P. G. Tratnyek. 1996. “Kinetics of halogenated organic compound degradation by iron metal.” Environ. Sci. Technol. 30 (8): 2634-2640. Li, Z.1998. “Degradation of Perchloroethylene by Zero Valent Iron Modified with Cationic Surfactant.” Adv. Environ. Res. 2(2): 244-250. Liang, L., N. Korte, J. D. Goodlaxson, J. Clausen, Q. Fernando, and R. Muftikian. 1997. “Byproduct Formation During the Reduction of TCE by Zero-Valent Iron and Palladized Iron.” Ground Water Monitoring & Remediation 17 (1): 122-127. Loraine, G. A. 2001. “Effects of Alcohols, Anionic and Nonionic Surfactants on the Reduction of PCE and TCE by Zero-Valent Iron.” Water Res. 35(6): 1453-1460. Sayles, G. D., G. You, M. Wang, M. J. Kupferle. 1997. “DDT, DDD, and DDE Dechlorination by Zero-Valent Iron.” Environ. Sci. Technol. 31(12): 3448-3454.
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