Bioxidative dissolution of cinnabar by iron-oxidizing bacteria

Bioavailability and methylation of bulk mercury sulfide in paddy soils: New insights into mercury risks in rice paddies

Mercury sulfide (HgS) constitutes the largest Hg reservoir in the lithosphere but has long been considered to be not bioavailable and a minor participant in Hg biogeochemical cycling. Herein, we report that bulk α-HgS can be dissolved and methylated in paddy soils, especially with rice culture. Bulk α-HgS exposure did not affect rice growth compared to the control group but significantly increased methylmercury (MeHg) contents in the rhizospheric soils (e.g., 80.15% and 108.71% higher for bulk α-HgS treatment vs. control at the seedling and maturation stages, respectively). Moreover, bulk α-HgS exposure induced an apparent accumulation of MeHg (50% higher for bulk α-HgS treatment vs. control) and markedly elevated total Hg (THg) in rice grains. The presence of DOM and reduced sulfide in paddy soils was further evidenced to drive the mobilization and dissolution of bulk α-HgS, thereby resulting in THg and MeHg accumulation in rice grains. These findings highlight the bioavailability of HgS in rice paddies and suggest that bulk HgS should be considered when assessing Hg health risks and developing efficient remediation approaches in Hg-contaminated croplands.

Mercury methylation from mercury selenide particles in soils

Selenium-inhibited monomethylmercury (MeHg) production is an attractive strategy for mitigating the risks of MeHg exposure. However, it is poorly understood the methylation potential of mercury selenide (HgSe) particles during their aging in soils and sediments. Net MeHg production in three floodplain soils amended with different geochemical species of mercury selenides, i.e., dissolved inorganic mercury freshly mixed with selenite (Hg(II)+Se(IV)), HgSe nanoparticles (45.2 ± 0.5 nm) and microparticles (> 1 μm) is examined. Among mercury types, the methylation from nanoparticulate HgSe was similar to (0.05 - 0.5 % vs. 0.1 - 0.4 %, yellow brown soil) or 12.9 - 21.0 times lower (0.02 - 0.1 vs. 0.6 - 1.5 %, black soil) than that from Hg(II)+Se(IV); however, net MeHg production from HgSe nanoparticles (0.02 - 0.5 %) was 1.9 - 15.5 times greater than HgSe microparticles (< 0.05 %) in all soils. Furthermore, net MeHg production from nanoparticulate HgSe varied significantly among soil types, attributable to differences in soil organic matter contents (2.4-5.8%) and microbial methylator community among soils. These results address the importance of geochemical intermediates of mercury selenide precipitation reactions and soil properties in MeHg production, and develop Se-based remediation strategy to minimize negative effects of MeHg on environmental and human health.

L-cysteine addition enhances microbial surface oxidation of coal inorganic sulfur: Complexation of cysteine and pyrite, inhibition of jarosite formation, environmental effects

Indigenous microorganisms were used to remove inorganic sulfur from high sulfur fat coal, and effect of L-cysteine on coal surface and biodesulfurization was investigated. It was found that L-cysteine addition enhanced coal biodesulfurization, and the optimal L-cysteine dosage was 1.6 g/L. With the optimal L-cysteine dosage, the Sulfobacillus were the dominant pyrite-oxidizing bacteria. After biodesulfurization for 30 days, the inorganic sulfur in coal decreased from 3.038% to 0.437%. L-cysteine was adsorbed on the coal surface through amino, carboxyl and sulfhydryl groups, and cysteine-Fe complex was formed by the interaction between interfacial -SH group of L-cysteine and pyrite, which was beneficial to sulfur transfer. Meanwhile, L-cysteine addition improved the adsorption of microorganisms on coal surface though reducing the Zeta potential of coal particle. The structural change of coal during the biodesulfurization showed that the pyrite was solubilized by Sulfobacillus to realize the removal of inorganic sulfur from coal, and L-cysteine addition inhibited the jarosite formation through improvement of pyrite bio-oxidation and corresponding pH decrease, which avoided the dissolved sulfur returning back to coal again. Moreover, the coal biodesulfurization with L-cysteine addition also presented obvious environmental benefit.