The mercury-contaminated wastewater from the chloralkali and battery industries seriously threatens aquatic ecosystems and must be treated before discharge. Theoretically, sulfidogenic process is an ideal bio-technology for low-cost treatment of Hg-contaminated wastewater because the biogenic sulfide can precipitate Hg(II) to form a safe byproduct – insoluble HgS (K_sp= 10^-36 at 20 °C). However, most sulfate-reducing bacteria (SRB) can methylate Hg(II) to form neurotoxic methylmercury (MeHg) that accumulates in aquatic food webs, resulting in severe health hazards for human beings. Thus, conventional sulfidogenic processes driven by SRB are not suitable for mercury-contaminated wastewater treatment.

In this study, we observed a novel finding that the long-term feeding of elemental sulfur for the cultivation of a sulfate-reducing bacteria (SRB)-abundant seeding sludge, eliminated the mercury methylation potential but kept the sulfidogenic activities remained, with the enrichment of sulfur-reducing bacteria (S⁰RB) in the final microbial community. During the 6-month cultivation of S⁰RB from the SRB-abundant seeding sludge, the methylation capability of the S⁰RB-enriching sludge gradually diminished to a negligible level. When exposed to 5 mg/L Hg²⁺, no MeHg was produced (<limit of detection, 0.01 μg/L) in the S⁰RB-enriched sludge that cultivated for 6 months, while 1.49 μg/L MeHg accumulated in the SRB-enriched sludge. This finding was reproduced with a replicated cultivation work with a different seeding sludge, and the similar results were obtained.

The sulfide production rate of the S⁰RB-enriched sludge and SRB-enriched sludge were 15.3 and 6.3 mgS/gVSS/h, respectively, showing that the long-term S⁰ feeding promoted the sulfidogenic activity. However, S⁰ feeding altered the microbial community, suppressed the growth of SRB and enriched S⁰RB, such as Geobacter, Sulfurospirillum and Pseudomonas.

Interestingly, Geobacter that was identified as a known mercury-methylating genus, was the dominant genus in the S⁰RB-enriched sludge, and its relative abundance increased with the cultivation time. This is an unexpectedly finding, implying that the methylation ability of Geobacter could be hindered in the sulfur reduction system.

Finally, we developed a biological sulfur reduction process driven by S⁰RB for mercury-contaminated wastewater, and investigated its long-term performance on mercury removal and MeHg accumulation. Receiving Hg-contaminated wastewater containing 0 – 50 mg Hg(II)/L for 326 days, S⁰RB in the sulfur reduction bioreactor removed 99.4%±1.4% of the influent Hg(II) by biogenic sulfide. MeHg was always found to be undetectable in the bioreactor, showing that the cultivated S⁰RB could be used for mercury-contaminated wastewater treatment without MeHg concern.