The Applications of Chemical Analyses of Sediments and Soils in ...

appreciably available. In acid soils (pH 4.5 - 6.5) selenium usually occurs as the basicferric selenite which has very low solubility and is almost completely unavailable. Inalkaline soils selenium can be oxidized to the selenate and become water soluble andavailable (National Research Council, 1971). Where alkaline soils are formed fromparent rock containing selenium the plants may contain 0.1 to 10 ppm dry weight. Ifacid soils are formed plants can contain 0.02 to 0.2 ppm. Selenium accumulator plantssuch as Astragalus can contain from 50 to 1,000 ppm compared with 1 to 5 ppm innearby non-accumulators.IRON AND MANGANESEIron and to a lesser extent manganese are normally present in sediments andsoils in major amounts and play important roles with respect to other metals.The activity of Fe 3+ in equilibrium with ferric hydroxide decreases 1,000x foreach unit increase in pH. Hydrolysis products combine to form polymeric complexessuch as Fe 2 (OH) 2 4+ .It has been shown by X-ray diffraction and Mössbauer spectroscopy that themajority of iron in dried surface sediments of a wide range of Canadian Lakes consistsof crypto-crystalline ferric hydroxide (Coey et al, 1974). Iron is also often present aschlorite with illite, and possibly pyrite and ferrous phosphate.Very rapid absorption of heavy metals in sediments can occur on hydrated ferricand manganese oxides (Chen, 1972). The absorptive capacity is greater on freshly17