Effect of alkaline leaching of phosphogypsum on sulfate reduction activity and bacterial community composition using different sources of anaerobic microbial inoculum

Phosphogypsum (PG), a by-product of the phosphate industry, is high in sulfate, (SO₄²⁻), which makes it an excellent substrate for sulfate-reducing bacteria (SRB) to produce hydrogen sulfide. This work aimed to optimize SO₄²⁻ leaching from PG to achieve a high biological reduction of SO₄²⁻ and generate high sulfide concentrations for subsequent use in the biological recovery of elemental sulfur. Five SRB consortia were isolated and enriched from: IS (Industrial sludges), MS (Marine sediments), WC (Winogradsky column), SNV (petroleum industry sediments) and PG (stored Phosphogypsum). The five consortia showed reduction activity when using PG leachate (with water) as source of SO₄²⁻ and lactate, acetate, or glucose as the electron donor. The highest reduction rate (81.5 %) was registered using lactate and the IS consortium (81.5 %) followed by MS (79 %) and PG (71 %). To enhance the concentration of leached SO₄²⁻ from PG for future utilization with the isolated consortia, PG was treated with NaOH solutions (2 % and 5 %). SO₄²⁻ release of 97 % was achieved with a 5 % concentration and the resulting leachate was further diluted to target a SO₄²⁻ concentration of 12.4 g·L⁻¹ for utilization with the isolated consortia. Compared to water leachate, a significantly higher reduction rate was registered (2 g·L⁻¹ of SO₄²) using the IS consortium, demonstrating limited inhibition effect of sulfide⁻ concentration on SRB functionalities. Moreover, metagenomic analysis of the consortia revealed that using PG as a source of SO₄²⁻ increased the abundance of Deltaproteobacteria, including known SRB like Desulfovibrio, Desulfomicrobium, and Desulfosporosinus, as well as novel SRB genera (Cupidesulfovibrio, Desulfocurvus, Desulfococcus) that showed, for the first time, significant potential as novel sulfate-reducers using PG as a SO₄²⁻ source.