The rate of hexavalent chromium reduction by a soil fulvic acid (SFA) was measured in aqueous solutions where concentrations of Cr(VI), H+, and SFA were independently varied. Rates of reduction increase strongly with decreasing pH. Typical Cr(VI)-SFA reactions display a nonlinear reduction of Cr(VI) with time that cannot adequately be modeled by either firstorder or second-order rate equations. An empirical rate equation that treats the SFA as a continuum of reactive groups which reduce Cr(VI) a t varying rates adequately describes the effects of solution parameters on the rates of Cr (VI) reduction. The rate equation is R = kr[HCrO~-l[SFAIICr(VI)laPIH+l~ where [Cr(Vl)]o is the initial Cr(VI) concentration. The rate coefficient (kr) varies with the fraction (Xe) of SFA reducing equivalents oxidized and can be written as kXe", where k is (4.27 f 0.68) x lo-' L mol-' s-' and n is -0.94 f 0.04. The parameters p and q are 0.47 f 0.11 and 0.45 f 0.03, respectively. The rate equation is based on experiments where up to 5% of the SFA was oxidized. Experiments conducted at higher [Cr(VI)]o show excellent agreement between experimental and modeled results even when 21% of the SFA is oxidized.

The remediation of chromium-contaminated sites requires knowledge of the processes that control the migration and transformation of chromium. Advection, dispersion, and diffusion are physical processes affecting the rate at which contaminants can migrate in the subsurface. Heterogeneity is an important factor that affects the contribution of each of these mechanisms to the migration of chromium-laden waters. Redox reactions, chemical speciation, adsorption/desorption phenomena, and precipitation/dissolution reactions control the transformation and mobility of chromium. The reduction of CrVI to CrIII can occur in the presence of ferrous iron in solution or in mineral phases, reduced sulfur compounds, or soil organic matter. At neutral to alkaline pH, the CrIII precipitates as amorphous hydroxides or forms complexes with organic matter. CrIII is oxidized by manganese dioxide, a common mineral found in many soils. Solid-phase precipitates of hexavalent chromium such as barium chromate can serve either as sources or sinks for CrVI. Adsorption of CrVI in soils increases with decreasing chromium concentration, making it more difficult to remove the chromium as the concentration decreases during pump-and-treat remediation. Knowledge of these chemical and physical processes is important in developing and selecting effective, cost-efficient remediation designs for chromium-contaminated sites.

The rate of hexavalent chromium reduction by soil humic substances (SHSs) was investigated in aqueous solutions where the temperature, ionic strength, background electrolyte, [Fe(III)], and [Cr(III)] were independently varied. Rate experiments were conducted with an excess of SHS over Cr(VI). An Arrenhius plot for the reduction of Cr(VI) by a soil fulvic acid and a soil humic acid indicates that the activation enthalpies for oxidation of these substances are nearly the same (63 ± 1 and 61 ± 3 kJ mol-1, respectively) and the activation entropies are significantly different (−160 ± 5 and −203 ± 9 J mol-1 K-1, respectively). Rates of reduction are not significantly altered due to changes in either background electrolyte or ionic strength. The presence of Cr(III) slightly inhibits the rate of reduction by soil humic acid, but not that of soil fulvic acid. Ferric iron increases the rate of Cr(VI) reduction, even when only a small amount of Fe(III) is added to the system. Fe may enhance the reduction of Cr(VI) by being alternately reduced by the SHS and then oxidized by the Cr(VI) as part of a redox cycle. The reduction of FeCrO4 + complexes via a parallel reaction pathway may also enhance Cr(VI) reduction in the Cr−Fe−SHS system.

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The remediation of chromium-contaminated sites requires knowledge of the processes that control the migration and transformation of chromium. Advection, dispersion, and diffusion are physical processes affecting the rate at which contaminants can migrate in the subsurface. Heterogeneity is an important factor that affects the contribution of each of these mechanisms to the migration of chromium-laden waters. Redox reactions, chemical speciation, adsorption! desorption phenomena, and precipitation/dissolution reactions control the transformation and mobility of chromium. The reduction of CrvI to Crl' can occur in the presence of ferrous iron in solution or in mineral phases, reduced sulfur compounds, or soil organic matter. At neutral to alkaline pH, the Cr"' precipitates as amorphous hydroxides or forms complexes with organic matter. Cr"'' is oxidized by manganese dioxide, a common mineral found in many soils. Solidphase precipitates of hexavalent chromium such as barium chromate can serve either as sources or sinks for Cr'v'. Adsorption of Crv' in soils increases with decreasing chromium concentration, making it more difficult to remove the chromium as the concentration decreases during pumpand-treat remediation. Knowledge of these chemical and physical processes is important in developing and selecting effective, cost-efficient remediation designs for chromium-contaminated sites.