Environ. Sci. Technol. 2000, 34, 2014-2017
Dechlorination of Pentachlorophenol by Zero Valent Iron and Modified Zero Valent Irons YOUNG-HUN KIM† AND E L I Z A B E T H R . C A R R A W A Y * ,† Environmental Engineering Specialty Area, Department of Civil Engineering, Texas A&M University, College Station, Texas 77843-3136
The disappearance of pentachlorophenol (PCP) from aqueous solutions in contact with zero valent metals (ZVMs) may be due to dechlorination reactions or sorption to ZVMrelated surfaces. Previously reported results on PCP and zero valent iron measured only PCP loss from aqueous solutions and attributed this loss to reaction. In this study, the total amount of unreacted PCP, both that in aqueous solution and that sorbed to ZVM-related surfaces, was measured using a modified extraction method. PCP dechlorination was confirmed by following the appearance of tetrachlorophenol isomers. The results indicate that the rate of dechlorination is much slower than previously reported. In our experiments, electrolytic zero valent iron with a surface area of 0.12 m2/g resulted in an observed first-order rate constant ((95% confidence limits) of 3.9 ((0.7) × 10-3 h-1 or a half-life of approximately 7.4 days. Normalized to surface area, the rate constant (kSA) is 3.2 ((0.6) × 10-4 L m-2 h-1. Four amended irons prepared by coating iron with palladium (Pd/Fe), platinum (Pt/Fe), nickel (Ni/Fe), and copper (Cu/Fe) were also used and showed slower removal rates as compared to unamended iron (estimated half-lives of 36-43 days). Slower reaction rates obtained with amended irons as compared to iron have not been previously reported. Overall, this study conclusively demonstrates PCP dechlorination by iron and several bimetallic ZVMs and indicates that it is essential to separate reaction and sorption processes.
Introduction The use of zero valent metals (ZVMs) for the treatment of halogenated hydrocarbons in wastewaters and groundwaters has been the focus of much recent research (1-6). The majority of these studies are concerned with compounds such as chlorinated methanes, ethanes, and ethenes. These compounds represent important environmental contaminants because they are widespread and mobile (7, 8). The characteristics of ZVM treatments (e.g., relatively low installation and operation costs) may also be advantageous for the treatment of contaminants such as pentachlorophenol (PCP), which is moderately water-soluble at neutral pH (9, 10). Sites contaminated with such compounds may act as long-term, low-level sources because they desorb slowly from * Corresponding author phone: (864)646-2189; fax: (864)646-2260; e-mail:
[email protected]. † Present address: Department of Environmental Toxicology, Clemson University, Clemson Institute of Environmental Toxicology, P.O. Box 709, 509 Westinghouse Rd., Pendleton, SC 29670-0709. 2014
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surfaces of soils and aquifer solids or dissolve slowly from nonaqueous phases. Aromatic compounds have not been as easily dechlorinated as trichloroethylene (TCE) and perchloroethylene (PCE). Dechlorination of polychlorinated biphenyls (PCBs) by zero valent iron has been demonstrated at temperatures of about 400 °C in the absence of water (11). At 200 °C or below, little dechlorination of PCBs occurred. However, nanoscale Fe and Pd/Fe dechlorinated PCBs at room temperature with biphenyl detected as a product (12). Relatively rapid PCP degradation was reported by Ravary and Lipczynska-Kochany, whose figures indicate 40-50% reductions in initial PCP concentrations in 3-6 h with acidwashed zero valent iron at room temperature (13). However, apparently only the aqueous phase was sampled, and dechlorination was not confirmed by analysis of products such as lesser chlorinated phenols or chloride. Neurath and co-workers studied the dechlorination of chlorinated phenols and concluded that initial rapid loss of tri-, tetra-, and pentachlorophenol is due to sorption to metal surfaces (14). Rates of dechlorination by iron have been increased by using palladium, a known hydrodechlorination catalyst (1518), as a coating on the zero valent iron surface. Muftikian and co-workers demonstrated rapid degradation of PCE with Pd/Fe (19). Grittini showed that the Pd/Fe bimetallic system can degrade PCBs but did not quantify the degradation (20). A report by Fernando and co-workers proposed that the enhanced reactivity of Pd/Fe may be due to the adsorption of hydrogen (H2), generated by iron corrosion, on palladium (15). The disappearance of chlorinated organic compounds from aqueous solutions contacting ZVMs may be due to dechlorination reactions or sorption to ZVM-related surfaces. Concurrent sorption and reaction have been demonstrated for PCE, TCE, and cis- and trans-1,2-dichloroethene with cast iron metal filings by Burris and Allen-King and co-workers (21-23). They later demonstrated sorption to be primarily due to graphitic inclusions in Fisher 40 mesh cast iron filings and to follow Traube’s rule for sorption of hydrophobic organic solutes (24, 25). In the present study, the importance of considering both reaction and sorption is illustrated for PCP with electrolytic zero valent iron and bimetallic ZVMs.
Experimental Section Chemicals. Ethyl acetate (GC grade, 99.9+%), hexadecane (ACS grade, 99%), palladium chloride (5% by mass in 10% HCl), platinum chloride, nickel chloride, copper(II) chloride, and hydrochloric acid were obtained from Aldrich Chemical Co. (Milwaukee, WI). The zero valent iron used was iron powder (electrolytic, particle size 100 mesh and smaller) obtained from Fisher Scientific. The iron surface area measured by N2 adsorption (Quantasorb Jr. model QSJR3) and 95% confidence limits (quadruplicate measurements) were 0.12 ((0.006) m2/g. All chemicals were used as received. Purified water was generated by a Barnstead Nanopure system and showed a minimum resistivity of 17.5 MΩ‚cm. Bimetals Preparation. Bimetallic ZVMs were prepared by mixing acidic solutions of metals with zero valent iron. The stock solutions were 1.8% Pt, 2.0% Cu, 2.4% Ni, and 5.0% Pd in 10% HCl solution. For the preparation of each bimetallic ZVM reagent, 1.00 mL of the stock solution was diluted to 200 mL with purified water and then added to 100.0 g of acid-washed iron powder (200 mL of 0.1 N HCl/100 g of iron for 10 min). The contents were mixed on a shaker table for 1 h, rinsed with purified water and acetone, and dried in air at room temperature. The bimetals were dark gray in color 10.1021/es991129f CCC: $19.00
2000 American Chemical Society Published on Web 04/12/2000
with no visual evidence of oxide formation. Assuming 100% plating of the added metal onto iron, the mass fractions of Pt, Cu, Ni, and Pd in the iron bimetals produced were 180, 200, 240, and 500 ppm, respectively. Subsequent preparations of bimetals and atomic absorption analysis of the metal solutions before and after exposure to iron showed that 70100% of the metal is removed (26). The Pd/Fe and Ni/Fe bimetals prepared were also subsequently used in TCE dechlorination experiments and showed increases in rates as compared to iron alone (26). Compared to bimetal preparations described in the literature, the four bimetals prepared for these experiments are of relatively low loading and are similar in loading and iron type to bimetals described by Fennelly and Roberts (19, 20, 27-29). Reactor System. EPA VOA amber vials (20 mL, Fisher Scientific) were used as batch reactors. To each prewashed vial, 10.00 ((0.02) mL of water and 1.00 ((0.01) g of ZVM were added. PCP was added as a 10 or 20 µL spike of an ethyl acetate stock solution. Immediately after the PCP addition, the vials were capped with lead foil (3M Co.) and Teflon lined silicone septa and open-top screw caps. The lead-lined septa were shown to minimize losses in previous experiments with TCE and showed no effect on the dechlorination reactions (26). Control vials were prepared identically except for the exclusion of ZVMs. All vials were placed on an orbital shaker at room temperature (23 ( 1 °C) and shaken at 100 rpm. At each sampling time, three reaction vials and two control vials were removed for extraction and analysis of chlorinated phenols. Extraction. Two types of extractions were used in this study. The first method is a typical liquid-liquid extraction using ethyl acetate as a solvent. In these extractions, 5 mL of ethyl acetate were injected through the septa, and the vials were shaken for 30 min to extract PCP. In this paper, PCP extracted by this method is referred to as “solventextractable” and is expressed as a concentration based on the aqueous volume (10.00 mL) in the vials. The second type of liquid-liquid extraction is modified by the addition of hydrochloric acid to promote the dissolution of ZVM surfaces and protonation of chlorophenolates, thereby releasing adsorbed PCP and other chlorinated phenols for extraction into ethyl acetate. PCP and tetrachlorinated phenol isomers quantified in these extracts are referred to as “total” and are also expressed as apparent aqueous concentrations. In the modified extractions, ethyl acetate was added to vials as before but was followed by 1 mL of concentrated HCl and replacement of the lead-lined septum with a Teflon-lined septum to avoid dissolution of the lead. The vials were then placed on the shaker table for 10 min. Repeated ethyl acetate extractions without acid were not successful in recovering all the PCP added, even at PCP-ZVM exposure times as short as 4 h. Different amounts of HCl were tested in 4-h samples containing 10 mL of 10 mg/L PCP and 1 g of zero valent iron. Relative percent recoveries of PCP from analyses of triplicate vials with no acid, 1 mL of concentrated HCl, and 2 mL of concentrated HCl were 60.2 ( 7.4, 104.8 ( 4.8, and 107.0 ( 5.2, respectively, as compared to PCP extracted from water with no zero valent iron. A series of PCP standards prepared in water and extracted with and without 1 mL of acid showed identical responses, indicating that the extraction efficiency of PCP from water by ethyl acetate was not a function of pH over the 0-5 range. In the experimental results presented, control vials were always extracted using the same method as the samples. Analysis. From each vial, an aliquot of about 1 mL of the solvent layer was transferred to a GC autosampler vial for analysis. Hexadecane (6.5 mg/L) was added to the ethyl acetate before extractions as an internal standard. An HP G1800A GCD (mass selective detector) with an HP-5 (30m × 0.25 mm i.d.) column was used in analyses of PCP and
FIGURE 1. Loss of solvent-extractable PCP in the presence of iron metals. Error bars indicate 1 SD. Some error bars are smaller than data symbols. The initial [PCP], C0, was 37.5 µM in all cases: (0) control, (9) Fe, (2) Pd/Fe, (×) Pt/Fe, ([) Ni/Fe, (b) Cu/Fe. tetrachlorinated phenol isomers. Splitless mode injection of 1 µL of sample was used. The oven temperature program was 1 min at 50 °C, 20 °C/min to 100 °C, 10 °C/min to 220 °C, and 0.1 min at 220 °C. The helium carrier gas flow rate was 1.0 mL/min, and a mass range of 45-425 m/z was selected. External standards of PCP, 2,3,5,6-tetrachlorophenol (2,3,5,6-TeCP), 2,3,4,5-tetrachlorophenol (2,3,4,5-TeCP), and 2,3,4,6-tetrachlorophenol (2,3,4,6-TeCP) were used to prepare calibration curves. The latter two tetrachlorophenol isomers coeluted. However, their responses to the mass selective detector were within experimental error of each other; therefore, the sum of their concentrations is reported. The calibration curves were linear over the concentration range of interest, and the detection limit was approximately 0.1 mg/L.
Results and Discussion Figure 1 shows the change in solvent-extractable PCP in reaction and control vials. Extraction of the contents of these vials was performed using ethyl acetate only. The controls, which contained no ZVMs, show 100% recovery of the 37.5 µM initial concentration of PCP. The increase in PCP concentration observed for iron at longer times (> 4 h) may be due to pH-related increases in PCP solubility. The ranges of pH values observed with unamended and amended irons were 6.6-8.1 and 6.5-7.0, respectively. These results for PCP removal (i.e., approximately 50% removal in a few hours) are very similar to those obtained by Ravary and LipczynskaKochany for acid-washed iron at 25 °C (13). Their experimental conditions included initial PCP concentrations of 2.7 and 50 µM, electrolytic iron (finer than 100 mesh) at 5 g/20 mL of aqueous solution, no added buffers, and temperatures of 25 and 55 °C. The iron was used in both untreated and acid-washed (0.1 N HCl) forms. On the basis of Figure 1, one might propose that PCP is relatively rapidly dechlorinated by all of the five types of ZVMs investigated and at very nearly the same rates. However, this conclusion is not consistent with previous results showing enhanced reactivity with bimetallic iron (19, 26, 30) and dependence of rates on the type of metal plated on iron (26). In addition, lesser chlorinated daughter compounds were not detected when HCl was omitted from the extraction step. A previous study using FTIR spectroscopy showed that chlorophenolates can be chemisorbed on oxide surfaces via inner-sphere coordination and that the sorbed chlorophenols were not removed from oxide surface by washing with water (31). VOL. 34, NO. 10, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Dechlorination of PCP by iron metal (C0 ) 18.7 µM): ([) PCP, (s) first-order fit, (9) 2,3,5,6-TeCP, (2) 2,3,4,5-TeCP and 2,3,4,6TeCP, (O) mass balance, (0) control. Total concentrations obtained from acid-modified extractions are shown. In Figure 2, the loss of total PCP and the production of total tetrachlorophenol isomers are shown for the case in which zero valent iron was used as the reductant and the chlorophenols were extracted using the acid-modified procedure. Thus, this figure shows changes in concentrations due only to dechlorination reactions. Sorbed chlorophenols have been extracted with high efficiency through protonation of the phenol functional groups and dissolution of metal surfaces by the acid. This is confirmed by the appearance of tetrachlorophenol isomers and by the mass balance calculated from the measured concentrations. Over a period of 25 days, the loss of mass of PCP is fully accounted for by the appearance of the products shown. The loss of PCP due to reaction was fit to a first-order model, and the resulting rate constant (( 95% confidence limits) is 3.9 ((0.7) × 10-3 h-1. Rate constants normalized by iron surface area per aqueous solution volume, based on linear dependence of the reaction rate constant on metal surface area, are commonly reported for ZVM reactions (1, 6, 30, 32-36). Without confirming linear dependence under our experimental conditions, we have calculated a surface area normalized rate constant of 3.2 ((0.6) × 10-4 L m-2 h-1. For comparison, rate constants (L m-2 h-1) for PCE and TCE dechlorination under a range of conditions are 2.1 ((2.7) × 10-3 for PCE (32), 3.9 ((3.6) × 10-4 for TCE (32), and 2.98 ((0.39) × 10-4 for TCE (26). It can be seen that reaction rates of iron with PCP are similar to TCE rates measured in our laboratory, if one assumes linear surface area dependence. The results shown are consistent with losses due to strong sorption rather than dechlorination in Figure 1 and in the results reported by Ravary and Lipczynska-Kochany. Comparison of Figures 1 and 2 indicates that sorption may account for removal of more than 50% of the initial mass of PCP. The importance of considering both sorption and reaction in the study of ZVM reactions is apparent. Previously, the work of Burris, Allen-King, and co-workers showed that sorption could be an important process in ZVM applications (21, 22, 24). While they observed sorption of chlorinated ethenes to be predominantly due to adsorption to graphitic inclusions in cast irons, differences in the iron used in these experiments (electrolytic iron contains very little carbon) and in the chemical behavior of chlorinated phenols as compared to ethenes indicate that sorption due to anion exchange to iron mineral surfaces is a more likely underlying mechanism in the present system (24, 25, 31). Figure 3 shows the results for total concentrations obtained with acid-modified extraction of PCP and its dechlorination products from reductions using Pd/Fe. As in 2016
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FIGURE 3. Dechlorination of PCP by Pd/Fe (C0 ) 18.7 µM): ([) PCP, (9) 2,3,5,6-TeCP, (2) 2,3,4,5-TeCP and 2,3,4,6-TeCP, (O) mass balance, (0) control. Total concentrations obtained from acid-modified extractions are shown. Figure 2, the appearance of tetrachlorophenols accounts for nearly all of the disappearance of PCP. In general, very close agreement between the controls and the mass balance is achieved. However, the extent of removal of PCP over 25 days by Pd/Fe is much lower than for unmodified iron (approximately 40% vs 90%). Although uncertainties in measured concentrations at small extents of reaction preclude conclusive quantitative treatment, the rate of loss of PCP does not appear to be well described by first-order kinetics, as evidenced by the steeper slope of the curve up to 3 days reaction time as compared to the more gradual loss of PCP at longer reaction times. The other bimetals investigated (Ni/Fe, Cu/Fe, and Pt/Fe) showed similar reaction rates and nonexponential PCP loss. Initial rates of PCP loss estimated from data over 0-3 days ranged from 0.04 to 0.05 µM/h for the bimetals (Cu/Fe > Ni/Fe > Pt/Fe > Pd/Fe). In contrast, the initial rate of PCP loss for iron was 0.09 µM/h over the same time period. Thus, rates of PCP dechlorination by the bimetallic ZVMs were lower than that by iron metal over the full time course of these experiments. Recently, other researchers have studied dechlorination reactions of several chlorinated ethane, ethenes, and methanes using bimetals (16, 19, 27, 29, 30). In all cases, the presence of the metal coating greatly enhanced reaction rates as compared to uncoated iron. We also observed increases in the rate of TCE dechlorination using the Pd/Fe and Ni/Fe bimetals prepared for this study (26). A gradual decrease in the rate of TCE reduction by Ni/Fe in a column reactor was observed by Sivavec et al. (28). After 3 months, the dechlorination rate was similar to untreated iron. In the PCP dechlorination investigated here, the reactivity of bimetals is lower than that of untreated iron. Possible explanations for the differences in reaction rates between iron and bimetallic ZVMs may involve competitive sorption of chlorinated phenols and reactive hydrogen on iron and catalytic surfaces as well as effects of sorption on corrosion. Additionally, the pH-dependent behavior of PCP and lesser chlorinated phenols and its effects on sorption must be considered. The immediate implications of this study for ZVM applications are clear, even though the chlorinated phenol sorption and reaction processes occurring on ZVMs cannot be thoroughly explored on the basis of Figures 1-3 alone. Studies in which only the aqueous phase is sampled to show removal of the contaminant of interest may lead to greatly overestimated reaction rates and the design of insufficient barrier walls or above ground treatment facilities. Independent verification of reaction through mass balance (carbon, chloride, or both) must be obtained. Overall, we
have found that iron metal dechlorinates PCP and TCE at similar rates, under the experimental conditions described. Pd/Fe, as compared to iron, exhibits a retardation in the rate of dechlorination of PCP and a greatly enhanced rate of dechlorination of TCE. Thus, the reactivity of bimetallic ZVMs is strongly dependent on specific properties of the compound to be dechlorinated. The production of tetrachlorinated phenols with both iron and amended irons indicates that practical application of ZVMs to pentachlorophenol reduction may be limited by the reactivity of toxic intermediates.
Acknowledgments The support of DuPont through the Young Professor Award Program and helpful comments from two anonymous reviewers are gratefully acknowledged.
Literature Cited (1) Matheson, L. J.; Tratnyek, P. G. Environ. Sci. Technol. 1994, 28, 2045-2053. (2) Arnold, W. A.; Roberts, A. L. Environ. Sci. Technol. 1998, 32, 3017-3025. (3) O’Hannesin, S. F.; Gillham, R. W. Ground Water 1998, 36, 164170. (4) Orth, W. S.; Gillham, R. W. Environ. Sci. Technol. 1996, 30, 6671. (5) Sayles, G. D.; You, G.; Wang, M.; Kupferle, M. J. Environ. Sci. Technol. 1997, 31, 3448-3454. (6) Warren, K. D.; Arnold, R. D.; Bishop, T. L.; Lindholm, L. C.; Betterton, E. A. J. Hazard. Mater. 1995, 41, 217-227. (7) Ramamoorthy, S.; Ramamoorthy, S. Chlorinated Organic Compounds in the Environment; Lewis Publishers: New York, 1997. (8) Sawyer, C. N.; McCarty, P. L.; Parkin, G. F. Chemistry for Environmental Engineering, 4th ed.; McGraw-Hill: New York, 1994. (9) Crosby, D. G. Pure Appl. Chem. 1981, 1052-1080. (10) Arcand, Y.; Hawari, J.; Guiot, S. R. Water Res. 1995, 29, 131-136. (11) Chuang, F.; Larson, R. A.; Wessman, M. S. Environ. Sci. Technol. 1995, 29, 2460-2463. (12) Wang, C.-B.; Zhang, W.-X. Environ. Sci. Technol. 1997, 31, 21542156. (13) Ravary, C.; Lipczynska-Kochany, E. 209th Natl. Meet., Am. Chem. Soc., Div. Environ. Chem. 1995, 35, 738-740 (Abstr). (14) Neurath, S. A.; Ferguson, W. J.; Dean, S. B.; Foose, D.; Agrawal, A. 213th Natl. Meet., Am. Chem. Soc., Div. Environ. Chem. 1997, 37, 159-161 (Abstr).
(15) Cheng, I. F.; Fernando, Q.; Korte, N. Environ. Sci. Technol. 1997, 31, 1074-1078. (16) Li, T.; Farrell, J. Environ. Sci. Technol. 2000, 34, 173-179. (17) Kovenklioglu, S.; Cao, Z.; Shah, D.; Farrauto, R. J.; Balko, E. N. AIChE J. 1992, 38, 1003-1012. (18) Lowry, G. V.; Reinhard, M. Environ. Sci. Technol. 1999, 33, 19051910. (19) Muftkian, R.; Fernando, Q.; Korte, N. Water Res. 1995, 29, 24342439. (20) Grittini, C.; Malcomson, M.; Fernando, Q.; Korte, N. Environ. Sci. Technol. 1995, 29, 2898-2900. (21) Burris, D. R.; Campbell, T. J.; Manoranjan, V. S. Environ. Sci. Technol. 1995, 29, 2850-2855. (22) Allen-King, R. M.; Halket, R. M.; Burris, D. R. Environ. Toxicol. Chem. 1997, 16, 424-429. (23) Campbell, T. J.; Burris, D. R.; Roberts, A. L.; Wells, J. R. Environ. Toxicol. Chem. 1997, 16, 625-630. (24) Burris, D. R.; Allen-King, R. M.; Manoranjan, V. S.; Campbell, T. J.; Loraine, G. A.; Deng, B. J. Environ. Eng. 1998, 124, 10121019. (25) Deng, B.; Campbell, T. J.; Burris, D. R. Environ. Sci. Technol. 1997, 31, 1185-1190. (26) Kim, Y.-H. Ph.D. Dissertation, Texas A&M University, College Station, TX, 1999. (27) Fennelly, J. P.; Roberts, A. L. Environ. Sci. Technol. 1998, 32, 1980-1988. (28) Sivavec, T. M.; Mackenzie, P. D.; Horney, D. P. 213th Natl. Meet., Am. Chem. Soc., Div. Environ. Chem. 1997, 37, 83-85 (Abstr). (29) Li, W.; Klabunde, K. J. Croat. Chem. Acta 1998, 71, 863-872. (30) Wan, C.; Chen, Y. H.; Wei, R. Environ. Toxicol. Chem. 1999, 18, 1091-1096. (31) Kung, K. S.; McBride, M. B. Environ. Sci. Technol. 1991, 25, 702-709. (32) Johnson, T. L.; Scherer, M. M.; Tratnyek, P. G. Environ. Sci. Technol. 1996, 30, 2634-2640. (33) Scherer, M. M.; Balko, B. A.; Gallagher, D. A.; Tratnyek, P. G. Environ. Sci. Technol. 1998, 32, 3026-3033. (34) Su, C.; Puls, R. W. Environ. Sci. Technol. 1999, 33, 163-168. (35) Deng, B.; Burris, D. R.; Campbell, T. J. Environ. Sci. Technol. 1999, 33, 2651-2656. (36) Hung, H.; Hoffmann, M. R. Environ. Sci. Technol. 1998, 32, 30113016.
Received for review October 4, 1999. Revised manuscript received February 11, 2000. Accepted February 14, 2000. ES991129F
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