Pore waters, sediment speciation

Benoit JM, Gilmour CC, Mason RP, Heyes A (1999) Sulfide controls on mercury speciation and bioavailability to methylating bacteria in sediment pore waters. Environ Sci Technol 33 951-957... 

The first consideration was the speciation and distribution of the metal in the sediment and water. Benthic organisms are exposed to surface water, pore water and sediment via the epidermis and/or the alimentary tract. Common binding sites for the metals in the sediment are iron and manganese oxides, clays, silica often with a coating of organic carbon that usually accounts for ca. 2% w/w. In a reducing environment contaminant metals will be precipitated as their sulfides. There is not necessarily a direct relationship between bioavailability and bioaccumulation, as digestion affects the availability and transport of the metals in animals, in ways that differ from those in plants. 

Although the abundance of silver in the Earth s crust is comparatively low (0.07 pgg-1), it is considered an environmental contaminant and is toxic at the nanomolar level. As an environmental pollutant it is derived from mining and smelting wastes and, because of its use in the electrical and photographic industries, there are considerable discharges into the aquatic environment. Consequently, there have been studies on the geochemistry and structure of silver-sulfur compounds [31]. Silver, either bound to large molecules or adsorbed on to particles, is found in the colloidal phase in freshwater. In anoxic sediments Ag(I) can bind to amorphous FeS, but dissolved silver compounds are not uncommon. A more detailed study of silver speciation in wastewater effluent, surface and pore waters concluded that 33-35% was colloidal and ca. 15-20% was in the dissolved phases [32]. 

P 9.7 Assessing the Speciation of a PCB-Congener in a Sediment-Pore Water... 

The Michael addition mechanism, whereby sulfur nucleophiles react with organic molecules containing activated unsaturated bonds, is probably a major pathway for organosulfur formation in marine sediments. In reducing sediments, where environmental factors can result in incomplete oxidation of sulfide (e.g. intertidal sediments), bisulfide (HS ) as well as polysulfide ions (S 2 ) are probably the major sulnir nucleophiles. Kinetic studies of reactions of these nucleophiles with simple molecules containing activated unsaturated bonds (acrylic acid, acrylonitrile) indicate that polysulfide ions are more reactive than bisulfide. These results are in agreement with some previous studies (30) as well as frontier molecular orbital considerations. Studies on pH variation indicate that the speciation of reactants influences reaction rates. In seawater medium, which resembles pore water constitution, acrylic acid reacts with HS at a lower rate relative to acrylonitrile because of the reduced electrophilicity of the acrylate ion at seawater pH. 

Figure 9 Dissolved mercury speciation in sediment pore waters as a function of sulfide concentration. Note that the most bioavailable form, HgS°, is the dominant chemical form at log S <, (—4.7) (source Benoit et al., 1999a).

Benoit J. M., Mason R. P., and Gilmour C. C. (1999b) Estimation of mercury-sulfide speciation in sediment pore waters using octanol-water partitioning and implications for availabihty to methylating bacteria. Environ. Toxicol. Chem. 18, 2138-2141. 

Studies conducted on Lake Biwa in Japan (38,39) revealed the presence of methylarsenic (III) species, possibly (CH3)As(OH)2 and (CH3)2AsOH, at low concentrations. Speciation and concenttation of arsenic varied with the season, particularly in the euphotic southern basin of the lake where DMA(V) could be the dominant species. Similar compounds had been seen in sediment pore water from Yellowknife, Canada (41). 

In aquatic sediments or soils, there are also a range of trace elements species ranging from ions exchanged to particles, to those bound to organic matter or in various inorganic forms (e.g., oxides, carbonates, sulfides) or as more inert crystalline mineral phases. As in waters, speciation studies in soils and sediments are generally undertaken to better understand the bioavailability of toxic substances and to investigate transport pathways to and from other parts of the ecosystem. Sediment and soil pore waters (soil solutions) are of particular interest because they are in equilibrium with the solid phase and are the medium for contaminant uptake by plants and many other biota. The techniques used for speciation analysis in these aqueous samples differ little from those for waters. 

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