Mercury-contaminated wastewater from industries such as chlor-alkali and battery industries, is a severe threat to aquatic ecosystems and must be treated before discharge. Theoretically, the sulfidogenic process is an ideal biotechnology for the low-cost treatment of mercury-contaminated wastewater because biogenic sulfide can precipitate inorganic mercury (Hg²⁺) to form a safe by-product, namely-insoluble mercuric sulfide (HgS_(s), K_sp= 10^-36 at 20 °C). However, most sulfate-reducing bacteria (SRB) can methylate Hg²⁺ to form neurotoxic methylmercury (MeHg), which accumulates in aquatic food webs and is thus a severe health hazards to humans. Hence, conventional sulfidogenic processes driven by SRB are not suitable for the treatment of mercury-contaminated wastewater.

The novel finding...[ Read more ]

Mercury-contaminated wastewater from industries such as chlor-alkali and battery industries, is a severe threat to aquatic ecosystems and must be treated before discharge. Theoretically, the sulfidogenic process is an ideal biotechnology for the low-cost treatment of mercury-contaminated wastewater because biogenic sulfide can precipitate inorganic mercury (Hg²⁺) to form a safe by-product, namely-insoluble mercuric sulfide (HgS_(s), K_sp= 10^-36 at 20 °C). However, most sulfate-reducing bacteria (SRB) can methylate Hg²⁺ to form neurotoxic methylmercury (MeHg), which accumulates in aquatic food webs and is thus a severe health hazards to humans. Hence, conventional sulfidogenic processes driven by SRB are not suitable for the treatment of mercury-contaminated wastewater.

The novel finding of the present study is that long-term feeding of elemental sulfur (S⁰) alters the microbial community structure and eliminates the mercury methylation potential in SRB-abundant activated sludge. During a 6-month cultivation period of elemental sulfur-reducing bacteria (S⁰RB) from SRB-abundant seeding sludge, the methylation capability of the S⁰RB-enriching sludge gradually diminished to a negligible level. When exposed to 5 mg/L of Hg²⁺, no MeHg was produced (less than detection limit, 0.01 μg/L) in the S⁰RB-enriched sludge, whereas 1.49 μg/L of MeHg accumulated in the SRB-enriched sludge. Similar results were obtained when this experiment was reproduced with a different seeding sludge.

The sulfide production rates of the S⁰RB-enriched sludge and SRB-enriched sludge were 15.3 mg S/g volatile suspended solids (VSS)-h and 6.3 mg S/g VSS-h, respectively, which indicates that long-term S⁰ feeding promoted sulfidogenic activity and thus enhanced Hg²⁺ precipitation. Moreover, S⁰ feeding altered the microbial community, suppressed the growth of SRB, and enriched S⁰RB, such as Sulfurospirillum and Pseudomonas.

Geobacter, a known mercury-methylating genus, was the dominant genus in the S⁰RB-enriched sludge, and its relative abundance increased with the cultivation time. This finding conflicts with no MeHg accumulation in the S⁰RB-enriched sludge, implied that the methylation ability of Geobacter might be hindered or the MeHg might be degraded by other genera in the S⁰ reduction system. The mechanism study proved that MeHg degradation by S⁰RB and the inhibition effect of polysulfide on mercury methylation contributed to no MeHg accumulation in the S⁰RB-enriched sludge.

We developed a biological S⁰ reduction process driven by S⁰RB for mercury-contaminated wastewater and investigated its long-term performance in mercury removal and MeHg accumulation for 326 days. Receiving mercury-contaminated wastewater containing 0-50 mg Hg²⁺/L, 99.4%±1.4% of the influent Hg²⁺ was removed by biogenic sulfide. MeHg was undetectable in the bioreactor, indicating that the biological S⁰ reduction process using S⁰RB can efficiently treat mercury-contaminated wastewater, with a high Hg²⁺ removal efficiency and no MeHg accumulation.