Sulfate Reduction in the Hydrogen-Based Membrane Biofilm Reactor Receiving Calcium Reduced Phosphogypsum Water

Phosphogypsum (PG), a byproduct of phosphate mining, contains sulfate that can be leached and converted to elemental sulfur, thus offering a sustainable opportunity to recover sulfur (S) as a step toward a circular economy. Calcium, at ~15 mM in PG leachate, creates inorganic precipitation that interferes with biological sulfate reduction, the first step of S recovery. Here, we evaluated the effectiveness of using cation-exchange to lower the calcium concentration in water-leached PG (PG water) delivered to a H₂-based membrane biofilm reactor (H₂-MBfR) employed to reduce sulfate to sulfide. A high sulfate flux (1 gS/m²-day) and 65% sulfate reduction were achieved despite a high pH (10) resulting from base production during sulfate reduction. However, soluble sulfide was only 20% of the reduced S, possibly due to precipitation of sulfide, iron, and phosphate, and alkalinity analysis revealed possible formation of polysulfides. Shallow metagenomics of the biofilm documented that Desulfomicrobium was the dominant sulfate-reducing bacterium, while Thauera, a mixotroph capable of sulfate reduction and sulfide oxidation, also was an important genus. The metagenomics also revealed the presence of methanogens and acetogens that competed for H₂ and CO₂. Although calcium removal from PG water improved sulfate reduction and reduced inorganic precipitation in the H₂-MBfR, soluble sulfide generation must be improved by supplying sufficient CO₂ to moderate pH increase due to sulfate reduction and by controlling the H₂-delivery capacity to limit methanogens and acetogens.