Precipitationdissolution Reactions Of Metals And Metalloids

Dissolution or precipitation reactions are generally slower than reactions among dissolved species, but it is quite difficult to generalize about rates of precipitation and dissolution (Stumm and Morgan, 1996). There is a lack of data concerning many geochemically important solid-solution reactions. Furthermore, there is not only a continuum between surface complexation (adsorption) and precipitation, but there is also obviously a continuum between heterogeneous nucleation and surface precipitation.

For many of the more abundant elements, such as Al, Fe, and Mn, precipitation of mineral forms is common and may greatly influence or even control their solubility. For most trace elements, direct precipitation from solution through homogeneous nucleation appears to be less likely than adsorption-desorption, by virtue of the low concentration of these metals and metalloids in soil solutions in well-aerated dryland soils. When soils become heavily polluted, metal solubility may reach a level to satisfy the solubility product to cause precipitation. Precipitation may also occur in the immediate vicinity of the phosphate fertilizer zone, where the concentration of heavy metals and metalloids present as impurities may be sufficiently high. Precipitation of trace metals as sulfides may have a significant role in metal transformation in reduced environments where the solution sulfide concentration is sufficiently high to satisfy the solubility product constants of metal sulfides (Robert and Berthelin, 1986).

In aerobic soils, although precipitation of trace metals through homogeneous nucleation is not likely, heterogeneous nucleation may play a significant role in metal transformation because of the presence of mineral, organic, and microbial surfaces that catalyze the nucleation set of crystallization (Huang and Germida, 2002). The energy barrier to nucleation is reduced or removed by surfaces. This is especially true in cases where there are crystallographic similarities between the surface and the precipitation phase. This catalytic process reduces the extent of supersaturation necessary for precipitation to occur. However, precipitation reactions are often slower than adsorption-desorption reactions in soil environments. Figure 1.11 illustrates the growth process from surface complex to surface nucleus to surface precipitate for Cr3+ sorbed by poorly crystalline goethite at pH 4 (Manceau et al., 1992).

Besides physicochemical reactions, metals have easy access to bacterial surfaces through diffusion. Metal sorption and precipitation on bacterial surfaces are interfacial effects. Surface metal concentrations frequently exceed the stoichiom-etry expected per reactive chemical sites within cell walls (Beveridge, 1989; McLean et al., 2002). The sorption of metals can be so great that precipitates can be formed, and distinct minerals are eventually formed through microbial biomineralization (i.e., the formation of minerals by microbes).

In the rhizosphere, activities of free metal ions may be decreased through uptake by plants and microbes. Metal contaminants are complexed substantially with biomolecules in the rhizosphere due to higher concentrations of complexing

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