Environ. Sci. Technol. 2000, 34, 3229-3234
In Situ Stabilization of Metal-Contaminated Groundwater by Hydrous Ferric Oxide: An Experimental and Modeling Investigation TODD A. MARTIN* AND J. HOUSTON KEMPTON Exponent, 4940 Pearl East Circle, Suite 300, Boulder, Colorado 80301
A potential method is investigated for remediation of metal-contaminated groundwater by in-situ emplacement of an adsorptive coating on the aquifer matrix. The coating is emplaced by sequentially injecting solutes that react as they mix in the aquifer to form a sparingly soluble solid with a high metal-adsorption capacity. Dissolved metals are removed passively as groundwater flows through the treated aquifer. The potential effectiveness of this method was demonstrated by sequentially injecting first ferrous sulfate and then oxygen solutions into a column of unconsolidated sand, producing a coating of hydrous ferric oxide (HFO) as the unretarded oxygen reacted with Fe(II) bound to the sand. The HFO delayed the breakthrough of Cr(VI) and As(V) by 8 and 30 pore volumes, respectively, relative to the unamended material. Attenuation of solutes by the unamended sand was reaction-rate limited, but coupled transport/equilibrium geochemical modeling matched well with the increased metal attenuation by the coating. Potential advantages of this method include the following: (1) coatings are emplaced preferentially in highconductivity zones, reducing problems caused by aquifer heterogeneity; (2) surface disturbance is minimal; (3) regeneration of the coating is straightforward; (4) no hazardous material is generated; and (5) existing geochemical models can help extrapolate to larger scales.
Introduction Remediation of contaminated groundwater remains one of the most intractable problems of environmental restoration. Contaminants typically enter groundwater at concentrations that are thousands or even millions of times above riskbased action levels and then disperse as they are carried through aquifers in flowing groundwater. The typical result is a large volume of groundwater with contaminant concentrations that exceed regulatory standards. Remediation is complicated by a combination of physical limits, such as incomplete delineation of source areas and plume extent, restricted access to the subsurface, differential solute transport due to subsurface heterogeneity, and contaminant diffusion out of low-permeability zones. Chemical phenomena, such as slow dissolution of contaminant sources or slow desorption from the aquifer matrix, further limit the success of remedial efforts. * Corresponding author phone: (303) 444-7270; fax: (303) 4447528; e-mail:
[email protected]. 10.1021/es980861w CCC: $19.00 Published on Web 06/23/2000
2000 American Chemical Society
The magnitude of the groundwater contamination problem is immense: 300 000-400 000 sites nationwide contain contaminated soil and/or groundwater (1). Of the Superfund sites for which Records of Decision have been issued, 75% contained some form of metals contamination (2). Federal expenditures on cleanup of contaminated sites are expected to be between $234 and $389 billion over the next 75 years, with combined expenditures for cleanup of all private and public U.S. sites projected to be between $500 billion and $1 trillion (3). The high cost of restoring these sites is related, in part, to the difficulty of remediating groundwater using existing technologies. There is thus a clear need for alternative technologies to clean up contaminated groundwater. In this paper, we investigate a potential new in-situ method for remediating metal-contaminated groundwater by coating contaminated aquifers with an adsorptive substrate, which then passively removes metals from solution.
Available Groundwater Remediation Technologies Although available techniques for cleanup of metalscontaminated groundwater can be highly effective, all have cost or practicability considerations that limit their utility for specific applications. For example, pump-and-treat technologysflushing water from an aquifer until contaminant concentrations drop below target cleanup levelssis effective at containing groundwater plumes. However, posttreatment audits of field-scale systems have demonstrated that this technology has high short- and long-term operating costs (4, 5), and a recent evaluation of 77 pump-and-treat systems found that regulatory standards had been achieved at only about 10% of the sites evaluated (1). The failure of pumpand-treat remediation to meet cleanup goals can be attributed to ineffective containment of point sources, or to the slow release of contaminants from the aquifer itself by desorption, diffusion out of dead-end pore spaces, or diffusion from macro-scale low-conductivity zones. In-situ technologies have the potential to provide passive or semipassive treatment with minimal long-term operational requirements and site disturbance but often suffer from chemical or physical limitations. Electrokinetic separation techniques, which induce the migration and recovery of ions in groundwater using electrical currents (3), have limited effectiveness due to production of H+, which has high ionic mobility relative to most metals (6). Permeable reactive subsurface walls, often used with impermeable walls to focus the flow of contaminated groundwater, are becoming an accepted technology (1, 7) and have been used to achieve in-situ immobilization of U, Mo, Cr(VI), Sr, Tc, and Ni using various adsorption, reduction, and precipitation reactions (8-13). While these technologies operate passively after installation, few of the reactive wall materials currently used in practice (14) can be cost-effectively regenerated in place, and thus they may require periodic excavation and renewal. Furthermore, reactive subsurface walls are less practical where plumes are large, groundwater is deep, or surface features complicate excavation. Newly developed techniques for containing contamination through in-situ aquifer permeability reduction, such as grouting with supersaturated gypsum solutions (15) or microbially produced polysaccharides (16), seem promising for controlling point sources but are less practical for dispersed plumes. In-situ geochemical stabilization techniques, of which the method presented in this paper is one, reduce contaminant mobility by decreasing its solubility (3). In-situ metal sulfide precipitation has been achieved by direct injection of calcium polysulfide (17) and by passive sulfide leaching from a peat VOL. 34, NO. 15, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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and gypsum substrate (18), although permanent stability presumably would require continued exclusion of oxygen to avoid oxidation and remobilization of metals. Laboratory tests indicate that injection of zeolites, immobilized organic chelates, and microbes can reduce metal solubility in situ (6). Ferrous iron injection has been shown to be a potentially effective approach to immobilizing Cr(VI) by inducing its reduction to less soluble Cr(III) (19). Laboratory tests have demonstrated that in-situ emplacement of HFO in aquiferss apparently by injecting an acidic Fe(III) solution (6) or by simple oxygen injection alone (20)sproduces an adsorptive substrate with minimal decrease in aquifer permeability (3).
In Situ Remediation by Emplacing an Adsorptive Coating The conceptual remedial method presented in this investigation involves injecting a series of soluble components into the aquifer, where they react and precipitate as they mix in situ, coating the aquifer with an insoluble, nontoxic substrate that has a high affinity for trace-metal contaminants. This substrate produces a passive treatment zone in the aquifer, which provides long-term remediation of metalscontaminated groundwater. Inducing in-situ precipitation reactions to form an adsorptive substrate requires that the sequentially injected reactants mix completely within the aquifer. Field-scale tests have demonstrated that dispersion can provide for microscale in-situ mixing of sequentially injected solutes at distance from injection wells (21). However, aquifer dispersivity is generally scale dependentsan artifact of variable solute transport rates in the different strata, rather than true dispersion (22), and standard aquifer dispersivity calculations may significantly overestimate actual subsurface solute mixing. We have thus assumed that substrate precipitation reactions would be more complete if mixing occurred by differential transport rates rather than by physical dispersion. Differential solute transport rates are more easily quantified than microscale dispersion, allowing more control over the location of substrate emplacement. In practice, in-situ mixing is achieved by sequentially injecting soluble reactants, slowest-migrating component first, so that precipitation reactions occur as faster-moving solutes overtake slower ones in the aquifer, coating the aquifer matrix with the adsorptive substrate. Ideal substrates should precipitate from a mixture of reactants that have high solubility, low toxicity, differential retardation in the aquifer, and rapid (or at least accurately known), homogeneous reaction kinetics. The resulting substrate should be chemically stable and should have a large surface area and high affinity for metals, and ideally, accurate thermodynamic adsorption parameters should be available to allow modeling of adsorptive attenuation of dissolved metals. For this bench-scale testing, hydrous ferric oxide (HFO), typically reported as FeOOH or Fe(OH)3, was selected as a substrate that met these criteria. HFO is sparingly soluble at groundwater pH greater than 4-5 s.u. (Ksp ) 10-38), provided that conditions remain sufficiently oxidizing to prevent reduction of Fe3+ back to Fe2+. HFO has a large surface area (∼600 m2/g), a strong affinity for many dissolved metals (23), and fast metal adsorption kinetics (e.g., typically equilibrating in a few hours) (24). The adsorption of metals by HFO is pH dependent, and the effects of pH vary between metals, so groundwater pH is an important consideration in evaluating the efficacy of the use of HFO as an in-situ adsorptive substrate. Parameters for diffuse-layer adsorption to HFO have been compiled for most metals and common anions (23) and incorporated into geochemical models (25), allowing for prediction of metal removal efficiency by the HFO coating. Precipitation of HFO in the test column was achieved by sequentially injecting an anoxic Fe(SO4) solution, followed 3230
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by oxygenated water. The HFO precipitation proceeded in two steps-oxidation of Fe(II) by 1 4
1 2
Fe2+ + H+ + O2 S Fe3+ H2O
(1)
then precipitation of HFO by
Fe3+ + 3H2O S Fe(OH)3(s) + 3H+
(2)
FeSO4 is highly soluble (Ksp ) 100.46) under near-neutral pH conditions; dissolved oxygen concentrations approaching 44 mg/L can be achieved by purging with purified oxygen gas (PO2 ) 1 atm; T ) 20 °C; H ) 104.6 atm) or much higher if added as H2O2. Dissolved oxygen injected into the subsurface can be consumed by reduced minerals or by microbial respiration, restricting the feasibility of reaction 1 under highly reducing conditions. In general, however, oxygen is not significantly retarded in oxic, sandy aquifers, such as that simulated in this study, and greater retardation of Fe2+ can be expected, either by adsorption to mineral surfaces or by chemical precipitation, such as reaction with calcite to form siderite (FeCO3). The homogeneous oxidation reaction (reaction 1) is second order with respect to [OH-] and is essentially complete in a few hours under neutral pH conditions (26). Once Fe3+ is produced, precipitation of HFO (reaction 2) is nearly instantaneous.
Experimental Design Laboratory experiments were conducted in two phases, first to emplace the in-situ HFO coating on aquifer material and second, to measure the resulting increase in metal attenuation produced by the coating. All experimental work was performed at Exponent’s Boulder, Colorado laboratory. All analyses for dissolved metals were conducted by the University of Colorado, Boulder, Department of Geological Sciences. X-ray diffraction (XRD) analyses of the sand were conducted at the Colorado School of Mines, Department of Geological Sciences. Materials. The simulated aquifer matrix was a well-sorted, medium sand, with median grain size of 0.6 mm, >99% finer than 2 mm, and
