Factors Governing Metal Availability and Transformation

Metals can be present in solid or aqueous (including pore water) phases. In the aqueous phase, metals can exist as free cations, which is generally the most toxic form, or as complexes with either inorganic elements or organic compounds. 

Toxic metals associated with wetland soils are present in various forms dissolved, adsorbed, bound to carbonates, to Fe and Mn oxides, to sulfides, and insoluble organic matter forms, and within the crystalline structure of primary minerals (Shannon and White, 1991). The amount of organic matter and clay minerals, the soil acidity (pH), and the sediment oxidation-reduction status (Eh) of soils are very important physicochemical properties influencing the mobility of toxic metals. 

FIGURE 12.1 Schematic of the process governing heavy metal transport in wetland environments. 

Humus is an amorphous, hydrophillic, acidic, partly aromatic, generally dark colored, and structurally complex material, resulting from the microbial degradation of plant detritus. Humus can be further classified as follows (a) humic acids a fraction that is soluble in alkali but precipitates on acidification of the solution, (b) fulvic acids a fraction that remains in solution after the extraction is acidified, and (c) humin a fraction that cannot be extracted by either alkali or acid (see Chapter 5 for details). 

The reactions of metals with organics are particularly important in wetland soils containing soluble metals. Organics in soils can form stable complexes with the reduced soluble form of some metals such that the metal is maintained in a water-soluble form for several hours or days in an oxidizing environment (see Chapter 10 for additional details). 

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