Oxidation of aqueous sulfides in the presence of manganese dioxide

A comparison of the oxidation potentials indicates that manganese dioxide is thermodynamically capable of oxidizing aqueous solutions cf hydrogen sulfide. A three part study was undertaken to: 1) determine the kinetics of the oxidation both in the presence and absence of oxygen, 2) measure the end products of the reaction, and 3), examine a possible treatment scheme using a fluidized bed of MnC2 particles. The oxichtion reaction was found to be first order with respect to the disappearance of sulfides under both aerobic and anaerobic conditions. The reactions are both pH dependent with the maximum rate occuring at pH 8.. By examining the products of the reaction, polysulfides, with x between 2 and 5) were found to be significant intermediates in both the aerobic and anaerobic situations and their presence is believed to explain the pH dependence. Under anaerobic conditions the quantity of manganous ions produced per sulfide ion oxidized is largely dependent on the end products of the reaction, which in turn depends on the pH. At pH 8 and below, sulfur is the primary end product and the ratio is equal to 1. Above pH 8 thiosulfate is farmed and the ratio is higher. When oxygen is present, the major product at the lower pH values is still elemental sulfur, and at pH 9, thiosulfate and sulfate anions are j jfound. In the presence of oxygen and high pH, the Mn ions are reoxidized, hence MnC2 acts as a catalyst in the reaction. In the fluidized bed experiments, good sulficfe removal was obtained at pH 7, 8, and 9 as long as the ratio was greater than 2. Mn++ ions were also reoxidized when the pH was greater than 8. Elemental sulfur was not formed, probably because the high degree of turbulence prevented nucleation. The principle products were sulfate at low pH and thiosulfate at higher values. Accordingly, a reaction scheme was proposed involving two competing reactions