Lead removal by its precipitation with biogenic sulfide in a membrane biofilm reactor

We evaluated the feasibility of using hydrogen (H₂)-based membrane biofilm reactors (MBfRs) to promote the growth of hydrogenotrophic sulfate-reducing bacteria (SRB) to remove lead (Pb) through its precipitation as lead sulfide (PbS) via biogenic sulfide (HS^-) production. Two MBfRs (R1 and R2) were set-up to treat synthetic water rich in sulfate (SO₄^2-) (585 mg/L) and Pb (50, 100, or 250 mg/L). R1 had one influent that had the Pb and synthetic media mixed together; R2 received the Pb solution and synthetic medium through separate influent lines. Oxygen (O₂) and nitrate (NO₃^-) were secondary electron acceptors in R1 and R2, respectively. R1 and R2 produced enough HS^- (> 73 mg/L) to precipitate Pb, and Pb removal reached >97 %. Chemical equilibrium calculations identified which solids were possible in each stage of operation. Precipitation of Pb with phosphate (PO₄^3-) occurred in the feed solution in R1, but phosphate precipitation was avoided in the R2 influent. The predominant Pb precipitate inside R2 was PbS, which was confirmed by SEM-EDX analysis. The microbial communities of R1 and R2 were dominated by two SRB - Desulfomicrobium and Fusibacter - along with sulfur oxidizer Thiovirga and denitrifier Thauera. Although the presence of electron acceptors other than SO₄^2- enabled other respiratory metabolisms, they did not prevent SO₄^2- reduction to HS^- or the precipitation of PbS.