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-degrees-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): log k = 16.19 - 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 kJ mol-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 1 mu M H2ST in anoxic waters.