SULFIDE AND METHANE FORMATION IN SOILS AND SEDIMENTS

Abstract

ABSTRACT In suspensions of Crowley soil, and under conditions of controlled pH and redox potential, sulfide formation took place in the pH range 5.5 to 8.5 and in the redox potential range −175 to −350 mV. At each pH tested, the rate of sulfide formation increased with a decrease of the redox potential. The order of reaction of sulfide formation was found to be ill defined. The optimum pH for both sulfide and methane formation was 6.7. Empirical data were in agreement with thermodynamically predicted values. Nitrate reduction, manganous and ferrous ion, and sulfide and methane formation occurred sequentially according to thermodynamic principles. However, some overlap did occur in the later stages of the formation of manganous, ferrous, and sulfide ions, possibly due to delayed dissolution of manganese and iron compounds. A small amount of methane was formed at redox potentials as high as −120 mV. Nitrate and sulfate or their reduction products repressed methane formation. The effect of nitrate appeared to be twofold: first, it delayed methane formation until the reduction of nitrate was complete and the redox potential was lowered sufficiently for further anaerobic reactions to proceed. Secondly, it exerted a toxic effect on methane formation. The major effect of sulfate appeared to be toxic, although a slight increase of the redox potential did occur at high sulfate concentrations.