The reduction of selenium(IV) by hydrogen sulfide in aqueous solutions

The rates for the reduction of Se(IV) by sulfides were measured in NaCl solutions as a function of pH (2-10), temperature (10-40 °C) and ionic strength (I = 0.01-1 M). The pseudo first-order rate constant (log k1) showed a complex dependence on pH with values decreasing from pH 2 to a minimum at pH 4.5, then increasing from pH 5.5 to a maximum near pH 8 and decreasing again at pH higher than 8. The values of the overall kinetic constant (k) calculated from the values of k1/[H2S]T can be determined from equations: logk=-0.15pH-1355.5/T+0.44I0.5+7.74 for the pH range 2-4.5 (σ = ±0.16), logk=0.28pH-1090.9/T+0.60I0.5+4.68 for the pH range 5.5-7.6 (σ = ±0.10) and logk=-0.50pH-1572.1/T+0.45I0.5+12.67 for the pH range 7.7-10 (σ = ±0.05), from 10 to 40 °C and from 0.01 to 1 M ionic strength. The effect of pH and ionic strength on the reaction suggests that the reactions in natural waters are due to the following interactions: H2SeO3+H2S↔products HSeO3-+H2S↔products HSeO3-+HS-↔products while under strong alkaline conditions (pH > 9) a fourth contribution by SeO32-+HS-↔products is also possible. The overall rate expression over the entire pH range investigated can be determined from (H2A = H2SeO3; HA = HSeO3−; A = SeO32−) k=(kH2S-H2A[H+]3+kH2S-HAKHA[H+]2+kHS-HAKHAK1s[H+]+kHS-AKHAKAK1s)/{([H+]2+KHA[H+]+KAKHA)([H+]+K1s)} where kH2S-H2A=2409±566M-1min-1, kH2S-HA=464±233M-1min-1, kHS-HA = 4190 ± 573 M−1 min−1 and kHS-A = 650 ± 382 M−1 min−1 and KHA, KA and K1s are the dissociation constants of selenous acid and hydrogen sulfide. Kinetic runs in natural freshwater samples and natural seawater have confirmed results obtained in simple NaCl solutions suggesting that this reaction may play an important role under anoxic and hypoxic conditions. Sulfide concentrations under these conditions may range from micromolar to millimolar level and Se(IV) half times are in the order of a few hours to minutes, respectively.