Ms. Elovitz et W. Fish, REDOX INTERACTIONS OF CR(VI) AND SUBSTITUTED PHENOLS - KINETIC INVESTIGATION, Environmental science & technology, 28(12), 1994, pp. 2161-2169
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.