Copper phytotoxicity in a contaminated soil:: Remediation tests with adsorptive materials

A long-contaminated mineral soil containing in excess of 3000 mg kg(-1) total Cu was amended with several materials having metal-adsorptive properties, including humus and iron, aluminum, and manganese oxide/hydroxide, in an attempt to reduce its phytotoxicity. The total soluble (measured by ICP) and labile (measured by ASV) concentrations of Cu were reduced by amendment with 5% (by weight) noncrystalline alumina and 10% (by weight) ferrihydrite, although humus and crystalline (low surface area) iron oxide were ineffective in reducing soluble or labile Cu. None of the soil amendments was able to correct the phytotoxicity indicated by bioassays with maize seedlings. Although manganese oxide (birnessite) lowered the free Cu2+ activity in the soil, phytotoxicity persisted due to an increase in dissolved organic carbon (DOC) and both soluble and labile Cu. Toxicity of Cu to maize shoots appeared to be dependent on both labile Cu as well as free ionic Cu2+. The high surface area (noncrystalline) iron and aluminum hydroxides reduced the solubility of Mo, As, Cd, Pb, and Cr as well as Cu attributable in part to their effectiveness in removing DOC from solution. Other metal solubilities, notably those of Ni and Zn, were not decreased by these hydroxide amendments. In contrast, the manganese oxide, by increasing DOC in the sail, raised the solubility of Pb, Cd, As, and several other metals. Thus, while reactive (high specific surface area) oxides did in the short-term reduce soluble concentrations of specific metals in this contaminated soil, the effects were metal-specific and sensitive to any pH change caused by the amendment.