Abstract
The oxidation of H2S with O2 in Black Sea waters was measured at 25°C. The measurements were made on mixtures of deep and surface waters. The oxidation rates were found to be ten times faster than the rates for surface seawater (Black Sea and Gulf Stream) with added H2S. Since the rates were the same for filtered (0.2 µm) and unfiltered waters, the increase in the rates appears to be abiotic and caused by dissolved solutes. To determine if this increase was due to trace metals, measurements have been made on the oxidation of H2S in seawater with added transition metals. Below a concentration of 100 nM, none of the metals except Fe2+ affect the rate of oxidation. At higher concentrations the rates increase for all the metals except Zn2+. The increase in the rates follow the order
Fe2+>Pb2+>Cu2+>Fe3+>Cd2+>Ni2+>Co2+>Mn2+
The increase in the rates below sulfide precipitation appears to be related to the formation of MHS+ ion pairs that have a higher (PbHS+, CuHS+, CdHS+, MnHS+) or lower (ZnHS+) rate of oxidation than HS-. Only Fe2+ and Mn2+ have concentrations high enough to be effective in influencing the rate of oxidation of H2S in anoxic waters and sediments. The effect of Fe2+ on the rate of oxidation is truly catalytic. The oxidation of Fe(II) with O2 yields dissolved and particulate Fe(III) that can catalyze the oxidation and also oxidize H2S with the formation of Fe(II). The effect of Mn2+ appears to be related to the formation of the MnHS+ ion pair that is more reactive than HS-. The estimated rates of oxidation of H2S in the Black Sea were found to be in reasonable agreement with the measured values. Preliminary measurements of the rates of oxidation of SO32- and S2O32- in oxygenated deep Black Sea waters are also presented. These measurements indicate that trace metals may also accelerate the rates of oxidation of SO32-.