Role of Sediment in Mercury Cycling in the Environment



Journal Title

Journal ISSN

Volume Title



Mercury (Hg) contamination in aquatic sediment is a significant concern due to its transformation to methylmercury (MeHg) in anaerobic conditions. MeHg is a neurotoxin that can bioaccumulate and biomagnify in the food web, leading to potential health risks in humans. Hg methylation is a biologically mediated process, and sediment Hg bioavailability for methylating bacteria can be quite variable, wherein high concentrations of Hg in sediment do not necessarily result in elevated rates of methylation. Although dissolved forms of Hg considered the most mobile and bioavailable forms of Hg, quantifying the bioavailable fraction of Hg2+ for methylating bacteria is challenging. This difficulty arises from the influence of various factors, such as sediment characteristics, the presence of organic matter and sulfur species, that can alter Hg speciation and bioavailability. This research pursued three main objectives. Firstly, it aimed to examine the effect of cyclic inundation/drainage conditions, particularly during storm flows, on Hg release and methylation in dynamic environment of riverbanks. The second goal of this research was to develop a standardized approach to experimentally assess biologically available Hg, using an assay that can measure the methylation potential of any sediments. Lastly, the research aimed to identify predictive approaches for Hg methylation and determine the sediment characteristics that most contributed to the observed methylation. The field and laboratory assessments of cyclic storm effect on Hg leaching from sediment into porewater showed that drainage/inundation conditions can lead to a higher release of non-particulate Hg, which is the result of both a higher exchange between the bank and river as well as introducing aerated water into the bank that tends to decrease Hg partitioning into immobile solid phases. The results also demonstrated that MeHg concentration at the bank–water interface is highest under base flow when conditions are more reduced due to the absence of oxic water exchange with the surface water. For the second and third objectives, a jar microcosm experiment was designed resembling in-situ conditions by providing reduced condition when used in an anaerobic chamber and by utilizing sediments with near in-situ density and moisture contents. It successfully identified differences in Hg methylation potential of sediments. Assessment of seven different sediments using the jar microcosm showed that porewater THg measurements using filtered water, serve as a reliable tool for predicting sediment bioavailability, as long as significant levels of colloidal/ particulate carbon are not present. The use of 0.2 μm filtered water however, showed that it can remove a substantial fraction of non-bioavailable colloidally bound Hg and correlate well with sediments methylation potential. The effect of sediment particle size and Hg speciation on Hg methylation was also investigated in two sediments. One sediment had over 67% organic thiol-Hg, while the other sediment contained over 65% metacinnabar (β-HgS). Both sediments were fractionated into four size fractions (<0.5 μm, 0.5-2 μm, 2–45 μm, >45 μm), each fraction was directly exposed with methylating bacteria (Desulfovibrio Desulfuricans ND132). Results revealed that the sediment fractions dominated by Hg-thiol species demonstrated 1.5-4 times higher Hg dissolution compared to the size fractions of the sediment dominated by metacinnabar. However, no correlations were found between surface area and MeHg concentration within the fractions of both sediments. In another study utilizing jar microcosms, the methylation potential of Hg-contaminated undersea pipeline was examined under two scenarios: natural release of Hg from partially corroded pipelines and Hg release from fully corroded equipment (worst-case scenario). The findings revealed that only a small fraction of Hg on the equipment can be methylated. In the worst-case scenario with complete corrosion, less than 0.2%, and in a naturally release scenario less than 0.01% of the Hg present on the equipment, was methylated. This methylation was positively correlated with the dissolved fraction of Hg measured in the exposed sediment with the equipment.



Mobility, Sediment, Methylmercury, Mercury, Bioavailability