5.2.1 Factors affecting the mobility and accessibility of elements in terrestrial (soil) environments
Elements can occur in the soil in either the solid phase or the aqueous soil solution. In the solid phase, ions can be bound to organic and inorganic soil components in various ways, including ion exchange and surface complexation, or they can exist in minerals or be co-precipitated with other minerals in the soil. In the soil solution, the elements can exist either as free ions or as complexes
with organic groups, such as amino, carboxyl, and phenolic groups, or inorganic groups, such as carbonate, chloride, hydroxide, nitrate, and sulfate. Ions in solution are generally bioaccessible, and ions in the solid phase of the soil may become accessible if environmental conditions change (National Research Council, 2003).
Because most soil solutions are not saturated with respect to their inorganic components, continuous dissolution from the solid phase tends to occur, and dissolution kinetics determine the bio- accessibility of ions derived from soil minerals. Conversely, sorption of ions, compounds, and complexes limits their bioaccessibility.
Sorbed compounds can occur as surface complexes or as surface precipitates or clusters. Ion exchange occurs mainly at sites where there is a permanent electrical charge (not pH dependent) on clay minerals that have undergone isomorphic substitution. Isomorphic substitution is replacement of ions in the clay mineral lattice with other ions of lower charge. Soils with significant negative charge have a high cation exchange capacity and low cation mobility. Soils high in clay typically have the highest cation exchange capacity. Ion exchange is affected by the speciation of elements as reflected in their oxidation state, as this also affects the net charge on their ions or on other electrically charged derivatives (Table 5).
Table 5. Elemental species that may determine the accessibility of elements in the soil solutiona
Element Aerobic soils
Anaerobic soils Chromium Cr(OH)3 (low to neutral pH) Cr(OH)3
Nickel NiO, NiCO3, Ni(OH)2 NiS
Arsenic Ca3(AsO4)2, Mg3(AsO4)2, As2O5 As2S3
Cadmium Cd(OH)2, CdCO3 CdS
Mercury HgCl2, HgO, Hg(OH)2 HgS
Lead PbO, PbCO3, Pb3(CO3)(OH)2 PbS
a Modified from Hayes & Traina (1998).
Abundant bioaccessible amounts of essential nutrients, such as phosphate and calcium, can decrease plant uptake of non-essential but chemically similar substances, in this case arsenate and cad- mium. More complex interactions are also observed. For example,
CuII toxicity may be related to low abundances of FeII, ZnII, sulfate, and/or molybdate (Adriano, 1986, 1992; Chaney, 1988).
In summary, soil conditions that cause precipitation or sorption of elements reduce their soil mobility and bioaccessibility. The elements that tend to be the most mobile and bioaccessible are those that form weak bonds with organic or inorganic soil components or those that complex with ligands in solution and that are not adsorbed to soil particles.
5.2.2 Factors affecting the mobility and accessibility of elements in sediment environments
Determining the bioaccessibility of elements sorbed to sedi- ments is key to understanding their potential to accumulate in aquatic organisms and to induce toxic effects in them and in the ultimate human consumers. It is clear from the published data that total element concentrations in sediments are poorly related to the bioaccessible fraction (Ruiz et al., 1991; De Vevey et al., 1993;
Allen & Hansen, 1996). A recent document (USEPA, 2005) describes the use of equilibrium partitioning sediment benchmark procedures to derive concentrations of metallic element mixtures in sediment that are not harmful to benthic organisms. The equilibrium partitioning approach is applicable across sediments and designed to allow for the bioaccessibility of chemicals in different sediments in relation to an appropriate biological effects concentration.
A large amount of the total elemental constitution of most sedi- ments is in a residual fraction as part of the natural minerals that make up the sediment particles (USEPA, 2005). These residual elements are not bioaccessible. The remaining elements in sediments are adsorbed to or complexed with various sediment components and may be bioaccessible. In oxidized sediments, cations may be adsorbed to clay particles, iron, manganese, and aluminium oxide coatings on clay particles, or dissolved and particulate organic mat- ter. As the concentration of oxygen in sediment decreases, usually because of microbial degradation of organic matter, oxide coatings begin to dissolve, releasing adsorbed cations. In oxygen-deficient sediments, many cations react with sulfide produced by bacteria and fungi to form insoluble sulfides. Many chemical species may be released from sorbed or complexed phases into sediment pore water in ionic, bioavailable forms following changes in oxidation/
reduction potential. Microbial degradation of organic matter may also release adsorbed species to pore water. Certain bacteria are able to methylate some ionic species, such as those of arsenic, mercury, and lead, to produce organic derivatives that are more bioaccessible than the original inorganic species.
The dominant role of the sediment sulfides in controlling metal cation bioaccessibility seems to be clear (Di Toro et al., 1990, 1991;
Ankley et al., 1991). Sulfides are common in many freshwater and marine sediments and are the predominant form of sulfur in anaerobic sediments (usually as iron(II) sulfide [FeS]). The ability of sulfide and metal cations to form insoluble precipitates with water solubilities below the toxic concentrations of the cations in solution is well established (Di Toro et al., 1990). This accounts for the lack of toxicity from sediments and sediment pore waters, even when high metal concentrations are present (Ankley et al., 1991). Ankley et al. (1991) showed that the solid-phase sediment sulfides that are soluble in weak cold acid, termed acid volatile sulfides, are a key factor in controlling the toxicity of cations of elements such as copper, cadmium, nickel, lead, and zinc. Toxicity due to the cations of these elements is not observed when they are bound to sediment and when, on a molar basis, the concentration of acid volatile sul- fides is greater than the sum of the molar concentrations of metals.
When the ratio of the sum of the simultaneously extracted metallic elements to the concentration of acid volatile sulfides exceeds 1.0 on a molar basis, toxic effects due to cations of the elements may be expressed, if the cations are not complexed by other ligands. Thus, the element to acid volatile sulfides ratio can be used to predict the fraction of the total metal concentration present in sediment that is bioaccessible as cations, and hence provides the basis for risk assess- ment.
Limitations to the metallic element to acid volatile sulfides ratio approach occur when the concentration of acid volatile sulfides is low — for example, in fully oxidized sediments. Most sediments have at least a small zone where the sediments are oxic near the sediment–water interface. The importance of this zone has been demonstrated for copper relative to acid volatile sulfides and accumulation of copper in the midge (Chironomus tentans) (Besser et al., 1996). In these situations, other phases (i.e. iron and
manganese oxides, dissolved organic carbon, and particulate organic carbon) can play an important role in determining the bioaccessi- bility of potentially toxic elements.