THE RATE OF SULFIDE OXIDATION BY DELTA-MNO2 IN SEAWATER

The rate of oxidation of hydrogen sulfide by manganese dioxide in seawater was determined as a function of pH (2.0-9.0), temperature (5-45-degrees-C). and ionic strength (0-4 M). The overall rate constant, k, in seawater at pH = 8.17 was found to be first order with respect to both sulfide and manganese dioxide: -d[H2S]T/dt = k[H2S]T[MnO2]. The rate constant, k, for seawater (S = 35.8, pH = 8.17) at 25-degrees-C was found to be 436 M-1 min-1, or 0.0244 m-2 1 min-1 when [MnO2] is expressed in surface area (m2/L). The energies of activation were found to be 14 +/- 1 KJ mol-1 and 10 +/- 1 KJ mol-1, respectively, for pH = 8.2 and pH = 5.0 in seawater (S = 35). The rate increased from pH 2.0 to a maximum at a pH of about 5.0 and decreased at higher pH. This pH dependence was attributed to formation of a surface complex between >MnO- and H2S. As the concentration of HS- increases above pH = 5 the rate of the reaction decreases. The rate of sulfide oxidation by MnO2 is not strongly dependent on ionic strength. The rates in 0.57 M NaCl were found to be slightly higher than the rates in seawater. Measurements made in solutions of the major sea salts indicated that Ca2+ and Mg2+ caused the rates to decrease, apparently by absorbing on the surface of manganese dioxide. Measurements made in artificial seawater (Na+, Mg2+, Ca2+, Cl-, and SO42-) were found to be in good agreement with the measurements in actual seawater. Phosphate was found to inhibit the reaction significantly.