The rates of the reduction of Cr(VI) with Fe(II) were measured in NaCl, NaClO4, and natural seawater as a function of pH (1.5-8.7), temperature (5-40°C) and ionic strength (I = 0.01-2 M). The pseudo first-order rate constant (log k1) showed a parabolic dependence on pH decreasing from 1.5 to 4.5 and increasing from 5.5 to 8.7. The kinetics of the reaction in these two regions of pH also showed different influences of temperature, ionic strength, and reductant concentration. The overall rate expression over the entire pH range is determined. In oxic aqueous systems Cr(VI) compete with O2 in the oxidation of Fe(II) and an extension of the rate law for Cr(VI) reduction with Fe(II) in oxygenated solutions is proposed. The application of this extended rate law to environmental conditions suggests that this reaction influences the distribution of oxidized and reduced species of chromium in oxic and anoxic waters.
ASJC Scopus subject areas
- Geochemistry and Petrology