Sulfate Leached from Phosphogypsum Is Transformed in a Hydrogen-Based Membrane Biofilm Reactor

The high level of sulfate in phosphogypsum (PG), a byproduct of phosphoric acid production, offers an option of recovering elemental sulfur (S0). The first step is reducing sulfate to soluble sulfide, which can then be partially oxidized to S0. We evaluated sulfate reduction to soluble sulfide using a hydrogen-based membrane biofilm reactor (H2-MBfR) from PG leachate (PG water). The H2-MBfR was initiated using synthetic sulfate medium prior to switching to PG water, and it achieved sulfate removal of 70-80% and similar to 60% of influent S as soluble sulfide. Upon switching to PG water, sulfate removal flux increased due to higher sulfate surface loading, but soluble sulfide kept declining and precipitates began forming. Venting the fibers to release accumulated CO2 increased the H2 availability and improved flux. Batch operation increased the generation of soluble sulfide, as sulfate was reduced biologically instead of precipitating as CaSO4 (as verified by X-ray diffraction and solubility calculations). Alkalinity analyses quantified the effects of precipitation, mainly CaSO4, on the sulfide reduction performance. While H2-MBfR demonstrated promise for reducing sulfate to sulfide in PG water, its long-term success will require that calcium be minimized to reduce abiotic sulfate removal, while H2 delivery must slightly exceed the H2 demand for biological sulfate reduction to sulfide.