Optimization of sulfate leaching from Phosphogypsum for efficient bioreduction in a batch bioreactor using a sulfate-reducing microbial consortium

Abstract

Phosphogypsum (PG) is generated annually from phosphoric acid production. This by-product contains 50% of sulfate (SO42−), which can be recycled biologically. Biorecovery of elemental sulfur (S0) involves firstly converting SO42− into biogenic sulfide using sulfate-reducing bacteria under anaerobic conditions, then oxidizing the sulfide produced into S0 using sulfur-oxidizing bacteria. This study focused on selecting and evaluating natural microbial consortia from SO42− rich biotopes for their application in the initial first step of S0 biorecovery. The PG sample was first comprehensively characterized using ICP-MS/OES, XRD, FTIR, and TGA analyses. Subsequently, SO42− leaching optimization was performed through a design of experiments approach. The analysis revealed that CaSO4 was the major component of the PG (78%), and the optimal parameters for leaching of SO42− from PG were at a NaOH concentration of 7.4% and PG concentration of 158 g.L−1. Furthermore, the MYC-consortium isolated from a hydrothermal niche demonstrated the highest reduction capacity among the studied consortia. It also exhibited the highest performance under SO42− concentrations up to 3.5 g.L−1, salinity levels up to 30 g.L−1, and in the presence of 60 ppm of Zn(II), 3 ppm of Cd(II), and 6 ppm of Pb(II). Moreover, this consortium showed a high capacity for reducing SO42− in the bioreactor (90% after 6 weeks) when utilizing SO42− from the PG-leached solution, as attested by the chemical analyses of the precipitate formed in the bioreactor. This research offers a comprehensive insight into the potential of MYC-consortium to convert leached SO42− from PG into sulfide, which is the most critical step in the S0-biorecovery.