REDOX INTERACTIONS OF CR(VI) AND SUBSTITUTED PHENOLS - KINETIC INVESTIGATION

The kinetics of the reduction of Cr(VI) to Cr(III) by substituted phen ols in aqueous solution were studied over environmentally relevant ran ges of reactant concentrations, pH, temperature, and ionic strength. A t a fixed pH, the reaction was first order with respect to both the co ncentration of phenol reductant and the total concentration of monomer ic Cr(VI). Reaction rates increased as much as 4 orders of magnitude f rom pH 5 to pH 1. The apparent reaction order with respect to [H+] var ied between 0.2 and 2 over this same pH range, but also depended on th e structure of the phenol. Comparison of the reactivities of 14 substi tuted phenols revealed a strong substituent effect. At pH 2, rates for the 14 phenols spanned more than 5 orders of magnitude with reactivit y generally increasing with the electron-donating character of the sub stituent (methoxy > methyl > chloro, aldehyde > nitro). Substituent ef fects are described by a linear correlation of the second-order rate c onstants (k(ArOH)) with the phenol half-wave potential (E(1/2)). Repre sentative reaction half-lives fall within the time scales of major env ironmental transport processes and suggest that the kinetics of these interactions are of significant concern in assessing sites contaminate d with chromium and phenolic compounds.