Chemistry of metal retention by soils Several processes are explained
L. J. Evans
Depnrainent of Land Resource Science University of Guelph Guebh, Ontario, NIGZWI. CMadn
For thousands of years, soil has been
the repository of society’s wastes. The excavation of middens, which has provided archaeologists with valuable information on the daily life of past civilizations, has shown that land disposal has been carried out since antiquity. In recent years, however, the rapid increase in the disposal of municipal and industrial wastes in landfdl sites, and the application bcth of leachates from these landfill sites and of sewage sludge to land, is causing much concern about environmental pollution. Moreover, metals also are being added to soil either directly through agricultural practices or indirectly through the atmospheric precipitation of industrial pollutants and automobile emissions. Many metals form stable complexes with biomoleculea, and their presence in even small amounts can be detrimental to plants and animals. The relative toxicity of metals to plants and animals, however, can vary greatly (see box); metals that form stable complexes with ligands tend to be the most toxic (1, 2). The possible contamination of groundwater supplies by metals leached from landfill sites, therefore, is a major environmental wncern. This concern has led many industrial nations to specify the maximum acceptahle concentrations of metals, such as barium, cad1046 Environ. Sci. Technol.. Vol. 23. No. 9. 1989
mium, chromium, lead, mercury, and silver in drinking water. The soils and sediments formed or deposited on the land surface can, however, act as vast reservoirs for the containment of these metals. Soils are important particularly because they contain surface-active mineral and humic constituents involved in reactions that affectmetal retention. The retention of metals by soil or other surface materials has been a focus of much research in recent yean (4). Various mathematical models have been developed to predict the speciation of metals in natural waters, and mechanistic models have been formulated to quantify the retention of metals by both mineral and organic soil constituents (5, 6). The surfaces of fine-grained soil particles are very active chemically; surface sites are negatively or positively charged or they are electrically neutral. Oppositely charged metallic counterions from solutions in soils are attracted to these charged surfaces. The relative proportions of ions attracted to these various sites depends on the degree of acidity or alkalinity of the soil, on its mineralogical composition, and on its
content of organic matter. The adsorption reactions that occur between metallic ions and the charged surfaces of soil particles may invoke either the formation of relatively weak outer sphere complexes through cation exchange reactions or the formation of strongly bound inner sphere complexes through ligand exchange reactions. The actual nature of the association between the charged surface and the counterion depends on the mechanism of retention of the counterion with the surface; the extent of adsorption depends on either the respective charges on the adsorbing surface and the metallic cation or on the intrinsic formation constants for the complexation reactions. In addition to these adsorption reactions, precipitation of new mineral phases also may occur if the chemical compition of the soil solution becomes supersaturated with respect to insoluble precipitates. Of these secondary minerals, the most important are oxides, hydroxides, oxyhydroxides, and carbonates; sulfides, phosphates, and silicates generally are less important.
Speciation of metals in water Metals in natural waters, such as soil solutions, may exist either as free (uncomplexed) ions or as various complexes with both inorganic and organic ligands (7).n o types of soluble complexes are formed between metals and complexant ligands. Outer sphere complexes, or ion pairs, a relatively weak electrostatic associations formed between a hydrated cation and a complexant ligand, in which one or both of the
001%936W891W23-1046$01.5010 0 1989 American Chemical Society
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