Effect of metals on the rate of the oxidation of H 2 S in seawater

Recent measurements on the oxidation of H 2 S in the Black Sea yielded rates that were ten times faster than expected. To determine if this increase was due to trace metals, measurements have been made on the oxidation of H 2 S in seawater with added transition metals. Below a concentration level of 100 nM, none of the metals except Fe 2+ affect the rate of oxidation. At higher concentrations the rates increase for all the metals except Zn 2+ . The increase in the rates followed the order. The increase in the rates below sulfide precipitation appears to be related to the formation of MHS + ion pairs that have a higher rate of oxidation than HS − . Only Fe 2+ and Mn 2+ have concentrations high enough to be effective in influencing the rate of oxidation of H 2 S in the waters of anoxic basins and sediments. The estimated rates of oxidation of H 2 S in the Black Sea using the laboratory measurements were found to be in reasonable agreement with the measured values. Since CuHS + has a higher rate of oxidation than HS − , it cannot be used to explain the stability of H 2 S in oxic seawaters. A more likely choice is ZnHS + .