Redox Interactions of Cr(VI) and Substituted Phenols: Kinetic Investigation.
The kinetics of the reduction of Cr(V1) to Cr(II1) by substituted phenols in aqueous solution were studied over environmentally relevant ranges of reactant concentrations, pH, temperature, and ionic strength. At a fixed pH, the reaction was first order with respect to both the concentration of phenol reductant and the total concentration of monomeric Cr(V1). Reaction rates increased as much as 4 orders of magnitude from pH 5 to pH 1. The apparent reaction order with respect to [H+l varied between 0.2 and 2 over this same pH range, but also depended on the structure of the phenol. Comparison of the reactivities of 14 substituted phenols revealed a strong substituent effect. A t pH 2, rates for the 14 phenols spanned more than 5 orders of magnitude with reactivity generally increasing with the electron-donating character of the substituent (methoxy > methyl > chloro, aldehyde > nitro). Substituent effects are described by a linear correlation of the second-order rate constants ( ~ A ~ o H ) with the phenol half-wave potential (Elp). Representative reaction half-lives fall within the time scales of major environmental transport processes and suggest that the kinetics of these interactions are of significant concern in assessing sites contaminated with chromium and phenolic compounds.