Application of a Theoretical Framework for Understanding Sulfur Granule Formation during the Anaerobic Digestion Process

Abstract

This research applies a previously validated bioenergetics model to determine whether the formation of elemental sulfur (S0) from hydrogen sulfide is thermodynamically possible during the anaerobic digestion (AD) process. The investigation was sparked by recent studies that reported that the presence of electronically conductive materials (e.g., magnetite) in the AD process could potentially facilitate elemental sulfur formation from hydrogen sulfide rather than the formation of ferrous sulfide (FeS), which is typically formed during the addition of iron salts (e.g., ferric chloride) into the AD environment for hydrogen sulfide control. The net Gibbs free energy change for the overall reaction resulting from the oxidation of hydrogen sulfide to elemental sulfur (S0) during the microbial conversion of organic matter to methane (CH4) was estimated to be -9.90 kilojoules per mole of electrons transferred (kJ/mol e−). The exothermic characteristics of the overall bioenergetics reaction suggest that there may be a competitive growth advantage for anaerobic microorganisms that can establish themselves within the vicinity or region of electronically conductive materials. Based on the overall energy transfer estimated during S0 and CH4 formation, a theoretical biomass yield (Y) of 0.097-g of volatile suspended solids formed per gram of chemical oxygen demand removed (VSS/gram COD) was obtained. Given that the estimated theoretical Y is well within the range anticipated for facultative bacteria grown under anaerobic conditions, the bioenergetics model appears to be well suited for predicting microbial reaction products from simple thermodynamic considerations. These present modeling results indicate that S0 formation is theoretically possible during methane formation. However, reported experimental evidence suggests that the presence of an electron transfer shuttle is required to facilitate this reaction.