Environ. Sci. Technol. 2001, 35, 2244-2251
Abiotic Transformation of Perchloroethylene in Homogeneous Dithionite Solution and in Suspensions of Dithionite-Treated Clay Minerals V A L E N T I N E A . N Z E N G U N G , * ,† R E Y N A M . C A S T I L L O , †,‡ W I L L P . G A T E S , §,| A N D G A R Y L . M I L L S § Department of Geology, University of Georgia, Athens, Georgia 30602, and Savannah River Ecology Laboratory (SREL), University of Georgia, Aiken, South Carolina 29802
The reductive dechlorination of perchloroethylene (PCE) in homogeneous solutions of dithionite and at the surfaces of dithionite-citrate-bicarbonate (DCB) treated ferruginous smectite and Na-montmorillonite was studied. Transformation products of PCE identified in dosed dithionite-treated samples included TCE, DCE, 1,1,2-trichloroethane (TCA), 1,1dichloroethane (DCA), chloroacetylene, acetylene, ethene, and ethane. The decomposition of dithionite to sulfate yielded both protons and electrons necessary for hydrodechlorination (hydrogenolysis) of PCE. Dithionite treatment of the Fe-poor Na-montmorillonite enhanced reductive dechlorination of PCE relative to dithionite-treated Fe-rich ferruginous smectite, within the range of 11.5137.8 mM dithionite. For the same dithionite concentration, the kinetics of the heterogeneous reactions of PCE was generally faster than that of the homogeneous reaction, and higher concentrations of TCE were measured in the heterogeneous reactions. Interestingly, increases in the mass of the clay minerals used, the Fe2+ content in the clay mineral structure, or the dithionite concentration used did not necessarily enhance the abiotic transformation of PCE, as would otherwise be predicted. The most efficient reductive dechlorination of PCE was observed with 0.5% clay (m/v) treated with 34.5 mM dithionite buffered at pH 8.5. The solid-state transfer of electrons to surfaces and edges, rather than the redox capacity, limited the dechlorination of PCE by reduced ferruginous smectite and/or suspensions containing a higher clay mass. The greater reactivity of dithionite-reduced montmorillonite than similarly treated ferruginous smectite is attributed to (i) the well-documented layer collapse and aggregation of chemically reduced clays that increases with the clay’s iron content, (ii) the location of solid-phase Fe2+ in the reduced clay mineral and whether it is accessible or inaccessible for reaction with PCE at the mineral edges and surfaces where the reactions are thought to occur, and (iii) the greater swellability of montmorillonite versus ferruginous smectite. The faster dechlorination rate of PCE observed with dithionitereduced Fe-poor montmorillonite than similarly reduced ironrich ferruginous smectite suggests that the use of dithionite barriers for in-situ treatment of chlorinated solvent plumes should not be limited to aquifers with Fe-rich sediments. 2244
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 11, 2001
Introduction Many studies conducted with anoxic sediments, soils, aquifer materials, or pure mineral phases have shown that the reduction of oxidized organic pollutants may occur by purely abiotic chemical reactions (1-4). Possible chemical reductants within these environments include reduced forms of inorganic iron and sulfur, such as iron-bearing clay minerals, hydrogen sulfide, iron sulfides, and natural organic matter. Kriegman-King and Reinhard (3) investigated the abiotic transformation of carbon tetrachloride (CTC) by ferrous iron (Fe2+) in pyrite and micaceous minerals in the presence of hydrogen sulfide (HS-). At 25 °C, reactions half-lives (t1/2) were roughly 2600, 160, and 50 d for the homogeneous sulfide (1 mM HS-) solution, vermiculite (114 m2/L), and biotite (55.8 m2/L) systems, respectively. The very long half-life for the homogeneous transformation reaction suggests that heterogeneous reactions were catalyzed by mineral surfaces. The authors determined that the sulfur concentration and bulk ferrous iron content in the minerals limited the rate of reductive transformation of CTC. Clay minerals offer a tremendous surface area for surface-catalyzed reactions. Clay minerals are also ubiquitous in soils and aquifer sediments. The composition of clay minerals varies widely with respect to metal cations (e.g., Si4+, Al3+, Mg2+, Fe3+, Fe2+, and Mn2+) in their crystal structures, but nearly all contain some structural iron. Iron is considered the most significant of all constituent ions because the in situ variation of its oxidation state causes profound changes in many fundamental clay mineral properties (5). Iron-bearing minerals in soils and sediments (e.g., iron oxyhydroxides and iron-bearing layer silicates) generally have high surface areas and are capable of surface reactions with organics solutes (6, 7). Of the different types of clay minerals, smectites are among the most common and abundant group. Smectites consist of anisotropic particles consisting of crystallites of aluminosilicate layers and are recognized for their higher swellability, surface area, and cation-exchange capacity (6). The reduction of octahedral Fe3+ affects many surface-sensitive properties, including properties of the smectites that are conventionally considered as static in nature. This is because the smectitic layers have a strong tendency to aggregate into quasicrystallites during reduction of structural Fe3+ (8), thus decreasing the fraction of the surface area available for reaction (9-12). The results of many previous studies have indicated that biotic and abiotic reduction of octahedral Fe3+ change surface charge density (13), swelling pressure (10), cation-exchange and fixation capacity (11, 13), specific surface area (9), color (14, 15), structural order (12, 16), and hydraulic conductivity (17) of the reduced clays. There are several reviews on the effects of structural Fe2+ and Fe3+ on the physicochemical properties of clays in the published literature (5, 18, 19). Biological and chemical approaches can be used to create subsurface reactive barriers for passive remediation of chlorinated organic solvents. Compared to natural (biological) mechanisms of reducing octahedral Fe3+ in clay minerals to Fe2+ to create in situ reducing conditions in aquifer sediments, the kinetics of chemical (or abiotic) processes * Corresponding author phone: (706)542-2699; fax: (706)542-2425; e-mail:
[email protected]. † University of Georgia, Athens. ‡ Current address: IT Corporation, Buffalo, NY. § University of Georgia, Aiken. | Current address: CSIRO Land and Water, Glen Osmond, SA 5064, Australia. 10.1021/es001578b CCC: $20.00
2001 American Chemical Society Published on Web 04/27/2001
TABLE 1. Structural Formulas, Cation-Exchange Capacities, Iron Contents, and Theoretical Surface Areas of Smectites Used in This Study smectitea
unit-cell formula
CEC (mequiv 100 g-1)
Fe2+ in clay (mmol g-1)
total Fe (mmol g-1)
surface area (m2 g-1)
SWa-1 SWy-2
Na0.95[Si7.40Al0.60][Al1.10Fe2.62Mg0.25]O20(OH)4 Na0.73[Si7.66Al0.34][Al3.07Fe0.44Mg0.56]O20(OH)4
94 78
0.023 0.018
3.211 0.587
704 747
a
SWa-1, ferruginous smectite; SWy-2, Na-montmorillonite.
are usually faster and can be easily managed (20, 21), although significant progress has been made toward optimizing microbial reduction (22, 23). Gan et al. (24) studied the relative effectiveness of several reducing agents in reducing structural iron in smectite, including dithionite, sulfide, thiosulfate, hydrazine, ascorbic acid, hydroquinione, and sodium oxalate. Their results indicated that dithionite was the most effective reducing agent. The advantages of using dithionite over other reducing agents are as follows: (i) it is a very strong reductant, (ii) dithionite and its reaction products are relatively nontoxic (25), and (iii) dithionite treatment barriers are relatively easier to engineer than the biologically created systems that have similar physicochemical properties (9, 10, 13, 22, 23, 26-28). As a result, dithionite is favored and has been recommended as a potential environmentally suitable reducing agent for structural iron in clay- and silt-sized layer silicates to artificially control in situ redox conditions (29). Recently, dithionite-treated Hanford sediments were used to reduce hexavalent to trivalent chromium and completely dechlorinated CTC and trichloroethylene (TCE) to the corresponding alkane and alkene (30, 31). These authors concluded that the oxidized organic contaminants were degraded by dithionite-reduced structural iron of phyllosilicates and surface-bound Fe2+ of aquifer sediment. It was not clear from the latter experiments what optimum dose of dithionite was needed to achieve the greatest reduction of Fe3+ and, subsequently, the oxidized contaminant. The goal of this research was to investigate the dechlorination of PCE in homogeneous dithionite solution and by dithionite-treated layered clay minerals that are ubiquitous in aquifer sediments. The specific objectives were to (i) determine the relative reactivity of dithionite reduced ferruginous smectite (Fe-rich) and sodium montmorillonite (Fepoor) and (ii) determine the optimum concentration of dithionite and solids content for efficient transformation of PCE.
Experimental Materials Chemicals and Solvents. High-performance liquid chromatography (HPLC) grade PCE was obtained from Aldrich Chemical Co. (Milwaukee, WI), and ACS grade trichloroethylene (TCE) was obtained from Fisher Scientific Co. (Fair Lawn, NJ). Both reagents were of >99.9% purity as confirmed by GC/MS and used as received. Sodium dithionite (Na2S2O4) and sodium bicarbonate (NaHCO3) powders were obtained from J. T. Baker (Phillipsburg, NJ). Sodium citrate hydrous crystals (Na3C6H5O7‚2H2O) were obtained from EM Science (Gibbstown, NJ). Citrate-bicarbonate buffers were prepared by the method of Stucki et al. (32). HPLC grade methanol from J. T. Baker (Philipsburg, NJ) was used to prepare stock solutions of PCE and TCE. Hexane used in all extractions was HPLC and ACS (ACS) grades and obtained from Fisher Scientific Co. (Fair Lawn, NJ). Deionized water was used to clean apparatus and in preparation of aqueous solutions. Groundwater, obtained from well no. P27D at the Savannah River Site (Aiken, SC), was used in the degradation experiments. The initial pH of the well water was 5.4, but after equilibration in the laboratory it was pH 6.8. The equilibrated specific conductivity was 33 mS/cm, Eh was 369 mV, total
dissolved solids (TDS) was 35 mg/L, and dissolved oxygen was 1.23 mg/L. Clays. Reference clays, Wyoming montmorillonite (SWy2) and ferruginous smectite (SWa-1), obtained from the Source Clay Minerals Repository of the Clay Minerals Society, Columbia, MO, were used in all studies. The structure, formulas, cation-exchange capacities (CEC), specific surface area, and Fe2+ and total Fe contents of the initial clay materials are listed in Table 1. Neither clay has substantial amounts of tetrahedral Fe, as observed by X-ray absorbance nearedge structure (XANES) spectroscopy (33); thus, they differ predominantly in the amount of octahedral Fe and tetrahedral Al.
Methodology Dithionite Reduction of Clay Suspension. The clay fraction was obtained from the original material by wet sedimentation (34). Specifically, 35 g of ferruginous smectite or montmorillonite was dispersed in 1 L of distilled-deionized water for approximately 12 h to separate the clay fraction (nominally
