In situ immobilization of metals in contaminated or naturally metal-rich soils

Abstract

The chemical behaviour of metals is primarily governed by their retention and release reactions of solute with the soil matrix. Liming increased the soil pH, resulting in increased adsorption of Zn, Cu, and Cd in soils, which in consequence decreased the concentration of easily soluble Cd fraction in the soils and the uptake of this metal by wheat (Triticum aestivum) and carrots (Daucus carota). Metal adsorption also depended on the presence of clay and organic matter, and thus the soils having highest amounts of clay (e.g., alum shale) and (or) organic matter (e.g., peat soil) showed the highest adsorption for these metals. Among the materials (Fe and Mn oxides and aluminosilicates) having high binding capacity for metals, the immobilizing capacity of birnessite (MnO2) was higher than that of other materials. Addition of synthetic zeolites significantly reduced the metal uptake by plants. Application of zeolite to a soil resulted in increased dissolved organic carbon (DOC) concentration in the leachate, which in consequence increased the leaching of Cd and Zn. Addition of beringite (an aluminosilicate) to a Zn-contaminated soil resulted in increased shoot length and leaf area of bean (Phaseolus vulgaris) and a significant reduction in Zn concentration in leaves (from 350 to 146 mg kg-1). Cadmium concentration in ryegrass and the concentrations of diethylenetriaminepentaacetic acid (DTPA)- extractable Cd, Cu, Ni, and Zn in the soil decreased significantly (P < 0.05) with increasing amounts of organic matter (peat soil and cow manure) added to soils. These effects were assumed to be related to immobilization of metals due to formation of insoluble metal--organic complexes and increased cation exchange capacity (CEC). An overview of the results showed that the products tested (lime, Fe/Mn-containing compounds, aluminosilicates, and organic matter products) can reduce the solubility and the plant uptake of metals but their immobilizing capacity is limited (sometimes through their side effects). Key words: aluminosilicates, contaminated soils, in situ immobilization, leaching, metal oxides, organic matter, plant uptake.