A novel biological sulfur reduction process for mercury-contaminated wastewater treatment.

The sulfidogenic process driven by sulfate-reducing bacteria (SRB) is not suitable for mercury-contaminated wastewater treatment due to the highly toxic methyl-mercury (MeHg) produced by SRB. In our previous study, we observed in short-term batch tests that sulfur-reducing bacteria (S0RB) could remove mercury ions without MeHg production. Thus, the aim of this study is to develop a biological sulfur reduction process driven by S0RB for mercury-contaminated wastewater, and investigate its long-term performance on mercury removal and MeHg accumulation. Receiving mercury-contaminated wastewater containing 0-50 mg Hg(II)/L for 326 days, S0RB in the sulfur-reducing bioreactor showed high tolerance with mercury toxicity, and removed 99.4% ±â€¯1.4% of the influent Hg(II) by biogenic sulfide. MeHg was always found to be undetectable in the bioreactor, even though the sulfidogenic bacteria were exposed to high levels of Hg(II) in long-term trials. The result of qPCR analysis further revealed that the mercury-methylation functional gene (hgcA) concentration in the bioreactor sludge was found to be extremely lower than in the SRB-enriched sludge, Geobacter sulfurreducens PCA and Desulfomicrobium baculatum DSM 4028, implying that there was no or few mercury methylators in the bioreactor. In short, the biological sulfur reduction process using S0RB can efficiently treat mercury-contaminated wastewater, with high Hg(II) removal efficiency and no MeHg accumulation.

Arsenite removal without thioarsenite formation in a sulfidogenic system driven by sulfur reducing bacteria under acidic conditions.

Sulfidogenic process using sulfate-reducing bacteria (SRB) has been used to remove arsenite from the arsenic-contaminated waters through the precipitation of arsenite with sulfide. However, excessive sulfide production and significant pH increase induced by sulfate reduction result in the formation of the mobile thioarsenite by-products and the inefficiency and instability of arsenite removal, especially when the arsenite level fluctuates. In this study, we proposed a novel sulfidogenic process driven by sulfur reducing bacteria (S0RB) for the arsenite removal under acidic conditions. In a long term experiment, efficient sulfide production (0.42 ±â€¯0.2 kg S/m3-d) was achieved without changing the acidic condition (pH around 4.3) in a sulfur reduction bio-reactor. With the acidic sulfide-containing effluents from the bio-reactor, over 99% of arsenite (10 mg As/L) in the arsenic-contaminated water was precipitated without the formation of soluble thioarsenite by-products, even in the presence of excessive sulfide. Maintaining the acidic condition (pH around 4.3) of the sulfide-containing effluent was essential to ensure the efficient arsenite precipitation and minimize the formation of thioarsenite by-products when the arsenite to sulfide molar ratios ranged from 0.1 to 0.46. An acid-tolerant S0RB, Desulfurella, was found to be responsible for the efficient dissimilatory sulfur reduction under acidic conditions without changing the solution pH significantly. The microbial sulfur reduction may proceed through the direct electron transfer between the S0RB and sulfur particles, and also through the indirect electron transport mediated by electron carriers. The findings of this study demonstrate that the proposed sulfidogenic process driven by S0RB working under acidic conditions can be a promising alternative to the SRB-based process for arsenite removal from the arsenic-contaminated waters.