Mobility of mercury in contaminated marine sediments: Biogeochemical pathways

The availability of dissolved inorganic mercury (DHg) in sediment pore water is a crucial step for the mechanisms of methylmercury (MeHg) generation in the aquatic system. The geochemical form of Hg in sediments and the redox-controlled microbial reactions taking place during early diagenesis regulate the pool of DHg and, consequently, the bioavailability of Hg for methylation. Here, we report new evidence on the biogeochemical mechanisms controlling the pool of DHg in sediment pore water from two box-cores collected from Augusta Bay (Sicily), a marine coastal zone heavily contaminated by chlor-alkali discharges. The content of the total Hg (THg) in the studied sediments (4.13 ÷ 22.2 mg Kg− 1) is largely present as an “immobile” phase, while the labile fractions account for minor percentages (< 2%). Despite the predominance of Hg “immobile” forms, depth profiles of the pore water suggest relevant DHg production (up to 226 ng L− 1) mainly in the deeper levels of the investigated sedimentary column. Specifically, most of the THg appears to be partially mobilized in the “Fe-Mn reduction” zone, as clearly suggested by significant correlations between DHg and dissolved Fe-Mn concentrations in the pore water. The irregular vertical distribution of SO42 −, with evident enrichment in the Fe-Mn reduction zone, could also indicate mechanisms of sulphate generation by sulphide oxidation. Specific microbial populations identified in the sediments appear dominated by chemolithoautotrophic sulphur oxidizing bacteria (SOB, genera Sulfurovum and Thioalkalispira) which could drive the microbial oxidation of sulphides and support, with effects of Mn-Fe oxide reduction, processes of Hg mobilization.