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

Abstract The rates of reduction of chromium (VI) to chromium (III) with H 2 S 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)] d t =−k [Cr(VI)][H 2 S] T can be determined from ( SD =0.07 in log k): log k = 16019−1.06 pH − 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 1μM or H 2 S T in anoxic waters.