Evaluating and Refining Alkalinity Calculations Due to Sulfide and Bicarbonate Accessed by Titration in Anaerobic Sulfate-Reducing Bioreactors

An anaerobic down-flow fixed-structured bed reactor (DFSBR) treating synthetic acid mine drainage (AMD) presented significant drops in alkalinity (from 2.45 to 1.30 g CaCO 3 L −1 ) simultaneously with sulfide (HS − ) precipitation induced by iron addition. To clarify this observation, the theoretical speciation of sulfide in aqueous solution was used to estimate the contribution of HS − for the global alkalinity of the system. Standard solutions of bicarbonate (HCO 3 − ) and sulfide (HS − ) were used to validate the proposed calculations for sulfide and bicarbonate alkalinities. The extent of buffer capacity of sulfide alkalinity under an overload of VFA was also evaluated. An overall assessment of the data obtained from the DFSBR showed that a concentration of total dissolved sulfide (TDS) close to 430 mg L −1 at pH = 8.48 (424 mg HS − L −1 ) provided an effluent alkalinity of 642 mg CaCO 3 L −1 exclusively due to HS − . Therefore, the refined calculations, presented and discussed in this paper, can be considered suitable to differentiate the fractions of alkalinity due to HS − and HCO 3 − obtained by titration. This novel approach provides a better characterization of the buffer capacity in anaerobic systems, mainly in order to avoid an overestimation of the HCO 3 − produced by the sulfate-reducing bacteria (SRB) metabolism and to predict the potential alkalinity loss by sulfide removal.