Changes in the oxidation state of trace metals can occur depending on the redox condition of the environment. Redox reactions are thus important in influencing the chemical speciation of a number of metals and metalloids, notably Hg, As, Se, Cr, Pu, Co, Pb, Ni, and Cu (Oscarson et al., 1981; Bartlett and James, 1993; Alloway, 1995; Myneni et al., 1997; Huang, 2000; James and Bartlett, 2000; Adriano, 2001; Sparks, 2003). Redox reactions also exert a great influence in the transformation and reactivity of Fe and Mn oxides in soils that have an enormous capacity to adsorb metals and metalloids (Huang and Germida, 2002). Furthermore, reduction of sulfate to sulfide in an anerobic environment also affects the transformation, solubility, and bioavailability of these pollutants through the formation of highly insoluble metal sulfides.
Masschelyn and Patrick (1994) have summarized the critical redox potentials for the transformation of some metal contaminants in soil environments. There has been little information on how changes in soil redox potential in the rhizo-sphere could affect the transformation of metal and metalloid contaminants. The generation of biomolecules through root exudation and microbial metabolism in the rhizosphere influences the redox potential. From a thermodynamic point of view, complexation of ligands with metals both on solid and solute phases has a dramatic effect on the redox potentials (Stumm and Morgan, 1996). For exemplification, Fe(II) and Fe(III) are used to illustrate the consequences of complexation on the redox potentials because (1) more data are available with this redox pair than with others, and (2) the transformations of Fe are especially important in the redox cycling of electrons in natural environments. The Fe(III)/Fe(II) redox couple can be adjusted with appropriate ligands to any redox potential within the entire range of the stability of water. As illustrated in Figure 1.9, the redox potential at pH 7, EoH (pH 7), decreases in the presence of most complexing lig-ands, especially chelates with oxygen donor atoms, such as citrate, EDTA, and salicylate, because these ligands form stronger complexes with Fe(III) than with Fe(II). Phenanthroline, which stabilizes Fe(II) more than Fe(III), is an exception. But Fe(II) complexes are usually stronger reductants than Fe2+. The range of redox potentials for heme derivatives given on the right in Figure 1.9 illustrates the possibilities involved in bioinorganic systems. The principles exemplified here are applicable to other redox systems. Furthermore, when metals are com-plexed with ligands, the kinetics of their oxidation are substantially retarded. This complexation effect is clearly illustrated in the kinetics of Fe(II) oxidation as influenced by a series of organic ligands (Table 1.3). Therefore, the consequences of complexation on the redox potentials of soils and related environments and the impact on the transformations of metals and metalloids warrant in-depth research.
Although complexation with most complexing ligands, such as biomolecules in the rhizosphere, should decrease the redox potentials, the creation of an oxidized zone adjacent to the plant root in wetland soils has been identified as one process affecting the chemistry of Zn, Cu, and As in soils. In wetland soils, steep gradients in redox potentials develop around plant roots. This process is reflected in the precipitation of FeOOH (iron plaque) on the roots (Otte et al., 1989; Kirk and Bajita, 1995). Compared with the surrounding soil, these Fe-rich plaques on the roots of the saltmarsh plant Aster tripolium are enriched in Zn and Cu (Otte et al., 1989). Zinc also accumulates in the rhizosphere (Oryza sativa L.), which is the result of the formation of a zone of oxidation of Fe2+ to Fe3+ adjacent to the roots (Kirk and Bajita, 1995). Zinc concentration is higher in red roots (with iron plaque) than in white roots; a positive effect of the Fe concentration on the root surface, up to a certain level, on Zn uptake into the xylem fluid has been demonstrated (Otte et al., 1989). Above this level of Fe coating, Zn uptake by the plant is reduced, which is attributed to complete coating of the root surface by
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