Ultrasonic desorption of mercury from contaminated sediments: influence of the chelating agent DMPS

Among the factors which play a role in mercury behaviour in sediments and aquatic environments, the presence of organic matter or, more specifically, of its dissolved fraction (dissolved organic matter, DOM), is probably the most important. DOM is ubiquitous in water, soil, and sediment environments and frequently controls a number of environmental mechanisms that are relevant for the behavior of mercury, in aquatic ecosystems. These processes include mineral dissolution and precipitation, photochemical oxidation and reduction reactions, and the speciation, solubility, mobility, toxicity and fate of metals. The compounds that constitute DOM also indirectly influence the fate of metals in aqueous systems by controlling ecological transformations serving, for example, as a substrate for microbially mediated reactions. In the case of mercury, DOM can promote the activity of microorganisms which are responsible of methylation. We will show in this section that DOM interacts with mercury, in all its forms (Hg(II), HgS(s), and MeHg), very strongly through different mechanisms. Strong complexation can facilitate the passage of mercury from natural and contaminated soils and sediments into streams and groundwater, increasing water column concentrations of mercury and thus the dissolved fraction of mercury and methyl mercury. Promoting dissolution, DOM influences the partitioning of mercury species between dissolved and solid phases. This is a key factor in the assessment of mercury contaminated sites because partitioning between sediments and water influences mobility, reactivity and bioavailability of mercury species. In this section the most important aspects concerning mercury-DOM interactions will be analysed. Firstly I will report some experimental evidence about the strong correlation between mercury dissolved concentrations and DOM, graphically and in terms of conditional stability constants. I will justify then these results from a chemical point of view, describing the nature and the strength of mercury binding to DOM most reactive sites, the sulphur ones. After having described the mechanism of mercury adsorption onto to DOM, I'll provide a brief description of other minor processes which can influence mercury cycling in the aquatic environment.