| Contents Notes |
Laboratory studies were conducted to examine the effects of pH and oxidation-reduction status (redox potential) on soil levels of trace and toxic metals in selected chemical forms. Studies were also conducted to see how metal availability to plants was affected by soil redox conditions. The elements studied included Cu, Zn, Cd, Pb, Cr, Ni, and As. Chemical availability and plant uptake studies under controlled pH and redox potential indicated that various organic phases predominate in retaining Cu under reducing conditions (i.e., low redox potential). As redox potential is increased, however, a very marked transformation occurs so that potentially available Cu becomes strongly associated with the reducible (hydrous iron oxide) phase under well oxidized conditions. Soluble, exchangeable, and chelate-extractable Cu also increased with increasing redox potential. Results with rice were mixed, but increasing oxidation conditions tended to increase Cu in corn. Dissolved and exchangeable Zn increased with decreasing soil pH and increased to a lesser degree with increasing redox potential. A very large increase in plant Zn levels occurred with increasing redox potential. Compared with most other trace and toxic metals studied, a much larger proportion of the total Cd extracted was recovered in the most readily available chemical forms (dissolved and exchangeable). Cd appears to be less strongly associated (immobilized) with high-molecular-weight organics and is weakly bound to hydrous iron oxides. This factor contributes to the potential for ready accumulation of Cd by plants grown in Cd-contaminated soils. In most of the experimental combinations of sludge-soil mixtures and pH used in the laboratory microcosms, plant Cd concentrations increased greatly as the redox potential increased. Except for exchangeable lead being greater at pH 5.0 than at higher pH levels, changing pH or redox potential had little effect on Pb levels in other chemical forms. Unlike results for Cd and Zn, oxidation status had little, if any, effect on plant levels of Cr, Ni, and As. Predominant chemical forms in the sludge-amended soils and oxidation effects on levels in these chemical forms are discussed in the report. This research demonstrates the important effects of soil redox potential in regulating the chemical mobility and availability to plants of Zn and Cd -- two key contaminants in sludge materials that may be applied to soils. Where alternatives exist for disposal of contaminated sludge, wet or poorly oxidized soils would be more effective for immobilizing Zn and Cd and would thus result in lower accumulations of these metals in crop plants or native plant populations on uncultivated land. |
