Reduction of chromium (VI) with hydrogen sulfide in NaCl media

The rates of reduction of chromium (VI) to chromium (III) with H₂S in NaCl solutions have been measured as a function of temperature (15-50°C). pH (7.5-10.5) and ionic strength (0-2 M). The overall rate constant (k, M^-1 min^-1) in: d[Cr(VI)] dt=-k[Cr(VI)][H₂S]_T can be determined from (SD=0.07 in log k): logk = 16019-1.06pH - 2300.9 T where the ionic strength term has been overlooked because it does not significantly affect the kinetics. The energy of activation is 43.6 ± 1.5 kJmol^-1. The rates of reduction in real seawater samples appeared to be 2-3 times faster than in NaCl at the same ionic strength. Based on these kinetic measurements, half times of Cr(VI) range from few hours (4-6 h) to many days (160-250 d), respectively, at 1 mM or 1μM or H₂S_T in anoxic waters.