Geochemistry of mercury in an intertidal flat of the Scheldt estuary

Abstract

Sediments were sampled on the 'Groot Buitenschoor', an intertidal flat located at about 60 km from the Scheldt's river mouth. Hg concentrations ranged from 30 to 1756 ng g-l. The concentrations were strongly correlated with fine grain fraction, organic matter content and sulphide concentrations. Incubation experiments were performed in order to determine the potential methylation rate of Hg as well as biotic and abiotic factors influencing this transformation. About 1 to 2% of the added inorganic Hg is converted into methylmercury. This conversion rate points to the same equilibrium ratio as was observed in natural sediments, indicating an equilibrium between methylation and demethylation reactions in the sediments. Incubation of a sterilised sediment sample significantly decreased the methylation rate, but the methylmercury concentrations observed are still ten times higher than the natural (unspiked) sediment. This result could be due to a chemical (non-enzymatic) methylation of mercury. Sulphate reducing bacteria are the main species responsible for the methylation of Hg at this site. Addition of Na2MoO 4, a specific inhibitor of sulphate reducing bacteria, decreased the methylation rate to the abiotic level (sterilised sediment). High sulphate reduction rates, however, lead to lower methylation rates. Increased formation of sulphides due to microbial sulphate reduction leads to enhanced HgS formation and this reaction competes with the methylation process. HgS is in fact the major product formed by the reaction of sulphate reducing bacteria with Hg species. About 50% of the Hg spiked to the sediments is transformed into HgS during the incubation experiments, and that compound is practically unavailable for methylation in contrast to other bound forms of Hg.

Total mercury and methylmercury in sediments and in the polychaeteNereis diversicolor at Groot Buitenschoor (Scheldt estuary, Belgium)

“…The geochemical behaviour of Hg in estuarine sediments is dominated by the redox conditions prevailing in the sediments and, in particular, closely linked to the S-cycle due to two main reasons: (i) in anoxic conditions Hg will rapidly precipitate as HgS and (ii) sulfate reducing bacteria have been shown to be the prime methylators of Hg in estuarine sediments (Craig and Moreton, 1986;Compeau andBartha, 1985, 1987;Gilmour and Capone, 1987;Berman et al, 1990;Leermakers et al, 1993;Choi and Bartha, 1994). The sulfur cycle in the sediments at Groot Buitenschoor was intensively investigated by Panutrakul (1993).…”

“…High levels of heavy metals such as Pb and Cd Srinetr, 1992) and Hg (Leermakers et al, 1993) have been recorded in sediments in this area, and benthic organisms have significantly decreased in species diversity and biomass densities in the last 30 years (Van Impe, 1985;Ysebaert et al, 1993). Of the remaining species, Nereis diversicolor and Corophium volutator account for up to 95% of the benthic population and have even increased in number in the last decade (Van Impe, 1985).…”

Spatiotemporal variations of internal P-loading and the related mechanisms in the large shallow Lake Chaohu

“…In the present study, the spatial patterns of clay, Al, Fe concentrations and PSI in the sediments were similar to that of TP concentrations in the sediments, and PSI showed significantly positive relationships with Fe, Al concentrations, but not with Ca concentration, suggesting that spatial patterns of TP concentrations in the sediments were affected by different P adsorption ability due to different mineral compositions and grain sizes. Previous studies indicated that organic matter contents in the sediments were higher in the western than in the eastern part of the lake [36] , and there was a positive correlation between clay content and organic matter content in the sediment [42,43] . The elevated clay contents in the surface sediments of the western part of the lake may be caused by increased sorption sites due to the increase in fine grain of the sediment affected by large amounts of organic matter induced by human activities.…”

Chemical and physical transformations of mercury in the ocean: a review

“…A significant role in methylmercury production in sediments and estuaries is played by sulfate-reducing bacteria (Leermakers et al, 1993;Matilainen, 1995;Benoit et al, 2001a;Harmon et al, 2007). Recent studies showed that in sediments mercury could be methylated also by other groups of bacteria, such as Fe-reducers in the genus Geobacter (Mason, 2012).…”

“…However, it was shown that concentrations of dimethylmercury (subpicomolar) can be higher than concentrations of monomethylmercury (less than 0.05 pM) in intermediate and deep waters (Black et al, 2009). The production of dimethylmercury by microorganisms and its liberation to the environment are supposed to be a detoxification mechanism (Leermakers et al, 1993;Hobman et al, 2000). Monperrus et al (2007) observed that mercury methylation in the surface seawater varies seasonally: high methylation rates are observed when water temperatures are high and nanoplankton are present in sufficient amount.…”