The rate of sulfide oxidation by δMnO 2 in seawater

Abstract

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°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[H 2S] T dt = k[H 2S] τ[MnO 2] . The rate constant, k, for seawater (S = 35.8, pH = 8.17) at 25°C was found to be 436 M −1 min −1, or 0.0244 m −2 1 min −1 when [MnO 2] is expressed in surface area (m 2/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 H 2S. As the concentration of HS − increases above pH = 5 the rate of the reaction decreases. The rate of sulfide oxidation by MnO 2 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 Ca 2+ and Mg 2+ caused the rates to decrease, apparently by absorbing on the surface of manganese dioxide. Measurements made in artificial seawater (Na +, Mg 2+, Ca 2+, Cl −, and SO 2− 4) were found to be in good agreement with the measurements in actual seawater. Phosphate was found to inhibit the reaction significantly.