The extensive use of chromium in metallurgic, leather tanning, electroplating, wood treating and other industries has resulted in numerous sites where aqueous chromium has been released to the subsurface. Chromium can be acutely toxic, carcinogenic, mutagenic and teratogenic. Therefore, it is vital that we understand the processes governing the transport and fate of chromium in the environment. Since both the mobility and toxicity of Cr depend on its oxidation state, redox reactions involving Cr are important in assessing its fate in the environment and its risk to human health.
The rates of hexavalent chromium reduction by a soil fulvic acid (SFA) and several humic acids (SHA) obtained from the International Humic Substances Society were investigated. Experiments were conducted in aqueous solutions where the initial concentrations of Cr(VI), H+, and SHS were independently varied. Additional experiments were conducted where temperature, ionic strength, background electrolyte, Fe(III), and Cr(III) were varied. All of the experiments were conducted with a large excess of soil humic substance [SHS] over [Cr(VI)].
Results clearly demonstrate that Cr(VI) reduction by SHS cannot be modeled by simple first- or second-order rate equations. Empirical equations that treat the SHA as a continuum of reactive functional groups which reduce Cr(VI) at varying rates adequately describe the effects of solution parameters. Reduction of Cr(VI) by SFA follows a rate equation of the form
where q = 0.45±0.03 and kSFA = (4.3±0.7)x10-7 L mol-1 s-1. Xe is the fraction of SHS oxidized. Similarly, the rate equation for the reduction of Cr(VI) by SHA is given by
where k0 = (8.3±1.2)x10-12 s-1 and kSHA = (2.04±0.05)x10-9 L1/2 mol-1/2 s-1. Both of these equations are capable of simulating the Cr(VI) versus time data under conditions that were beyond those for which the equations were defined. Cr(VI) reduction at pH 3 by two other SHAs could also be modeled using the general form of eq. 2.
The number of moles of Cr(VI) that can be reduced per mg of SHS was determined by a Walkley-Black test for each of the humic substances used in this study. These reduction capacities are positively correlated with the H, N, and P contents of the SHS. No significant correlations were found between the reduction capacities and the reported concentrations of structural moieties within the humic substances. The apparent rate coefficients at pH 3 are positively correlated with O and N contents and negatively correlated with the C content of the SHS. These apparent rate coefficients are also positively correlated with ketone/quinone and carboxyl contents.
Activation enthalpies for SHA and SFA are nearly identical (62±2 and 64±4 kJ mol-1, respectively) and the activation entropies are significantly different (-165±6 and -195±13 kJ mol-1 K-1, respectively). The activation enthalpies are 1.1 to 2.3 times larger than those reported for Cr(VI) reduction by simple organic compounds with hydroxyl and sulfhydryl functional groups. The entropies of activation are similar to those reported for Cr(VI) reduction by methylphenols, benzyl alcohol, and glutathione. Rates of reduction are not significantly altered due to changes in either background electrolyte or ionic strength. Cr(III) only slightly inhibits the rate of Cr(VI) reduction by SHA, but not by SFA. Ferric iron increases the rate of Cr(VI) reduction, by being alternately reduced by the SHS and then oxidized by Cr(VI) as part of a redox cycle.
The results suggest that the rate equations are reasonably robust and are applicable under a wide variety of conditions. The general form of the rate equation for soil humic acid is applicable to several humic acids and in lieu of specific experimental data, the rate coefficients reported here can provide order of magnitude estimates of the rate coefficients for other soil humic acids.