Iron and sulfur biogeochemical processes involved in mercury mobility and speciation in an artificial aquifer
Although many data are available on the Hg cycle in terrestrial surface aquatic environments, little is known on its behaviour in subsurface and deep aquifers where environmental conditions (anoxia, water saturation) may be favourable to Hg methylation. Bioavailability of Hg, a prerequisite for its methylation by sulphate or iron reducing bacteria, is mainly controlled by physico-chemical conditions and the strong affinity of Hg for organic matter or iron (oxy)hydroxides. This work presents an original experimental setup combining geochemical and microbiological approaches in order to imitate reactions observed in aquifers (i.e., Hg biosorption, solubilisation versus sequestration, speciation, dual effect of iron and sulphate). Two columns were filled in the lower half with sterile sand and in the upper half with a sterile mixture of sand and iron oxides, initially enriched with Hg(II). The water flow was ascendant. Five septa set regularly along the columns enabled water sampling from the different layers of the column without perturbing water flow or in-situ experimental conditions. After an abiotic rinsing period, the system was inoculated with a bacterial consortium and physico-chemical and microbial parameters were monitored in time and space. The inflowing groundwater was supplemented with sulphate (370 g.L-1 MgSO42-) and lactate (830 mg.L-1 sodium lactate) to encourage sulphate-reducing bacteria in the first column (A) and with molybdate (0.40 mmol.L-1) to inhibit sulphate reduction and glucose (10 g.L-1) to favour iron-reducing bacteria in the second column (B). At the end of the experiment (130 days), microbial methylation potentials were evaluated using stable isotope-spiked incubations, diversity using Denaturing Gel Gradient Electrophoresis (DGGE) followed by band sequencing and Fluorescent in situ hybridisation (FISH). Raman spectrometry was carried out to identify neo-mineral formation and selective extractions provided information on Hg distribution between the solid carrier phases. In column A microbial activity assessed by substrate consumption began after 15 days and induced a progressive decrease in the sulphate concentrations at the column outlet and the visual observation of black FeS precipitates (identified as partially oxidized makinawite). When sulphate reduction peaked after 120 days, iron was leached from the column as well as monomethylmercury (MMHg) and Hg. Bacterial diversity did not vary significantly either in time or space, sulphate reducing bacteria were observed with FISH and a sequenced DNA band bore resemblance to Desulfotomaculum reducens. In column B bacterial activity directly induced iron reduction (measured in the outlet as Fe(II)) as well as Hg leaching and MMHg formation. Shewanella sp. was observed with FISH. In both columns methylation potentials averaged 1% and selective extractions showed a shift of Hg from the ferrihydrite towards the organic matrix and newly formed FeS. These results contribute to understanding the interactions between bacteria and geochemical mechanisms controlling Hg fate and behaviour in saturated aquifers.