Mercury speciation in sediments of a tropical coastal environment

The speciation of mercury in tropical coastal areas, although scarcely studied, has presented an odd behaviour when compared with the results obtained in temperate environments. In this work, we measured the concentrations of mercury species (mercuric mercury, methyl-mercury and dimethyl-mercury) in the sediments of Sepetiba Bay (Brazil) and compared these with geochemical parameters. Twenty-eight sediment samples were extracted in an open microwave system and mercury speciation was carried out by ethylation, cryogenic focusing, gas chromatography and quartz furnace atomic absorption spectrometry. Fraction < 63 mu m, organic carbon and total sulphur contents and redox potential were measured in order to outline the geochemical characteristics of the sediments. While mercuric mercury presented concentrations ranging between 22.65 and 134.61 ng g(-1), methyl-mercury concentrations ranged between < 2.0 and 4.4 ng g(-1). Only a few dimethylmercury concentrations were relatively high (up to 14.6 ng g(-1)). The results of mercuric mercury presented an unexpected positive correlation with sulphur contents showing that mercury is not forming stable sulphide complexes, even under very reducing conditions. This would render mercury available for the formation of methyl- or dimethyl-mercury that would be both fixed in the sediments and volatilised.