A kinetic study of trace element leachability from abandoned-mine-polluted soil treated with SS-MSW compost and red mud. Comparison with results from sequential extraction.

ABSTRACT

The effect of adding treated red mud, a by-product of alumina production, to soil polluted by an abandoned mine and characterised by high concentrations of heavy metals, relatively low reaction grade, and low organic carbon content, was investigated. Also studied was addition of both red mud and compost (produced from source-separated municipal solid waste)--the synergistic action of red mud and compost could be exploited to achieve both metal trapping and an increase in organic carbon content. Leaching batch tests were performed on four different systems soil, soil and treated red mud, soil and compost, soil and compost plus treated red mud. Dilute sulfuric acid and EDTA solution (liquid/solid ratio 101) were used in the tests--sulfuric acid to "mimic" acid rain and EDTA in accordance with general methods for estimating "plant-available" metals. Sequential extraction was also applied to the same samples. The use of relatively non-specific extractant reagents in the leaching tests led to a kinetic approach (already proposed in literature), because measurements of trace elements extracted at equilibrium cannot be related to their speciation. Comparison of information obtainable by the kinetic approach to evaluation of data from leaching tests with results from sequential extraction enabled evaluation whether the "kinetic fractionation method", a relatively rapid and simple procedure, furnishes adequate information about the mobility and bioavailability of trace elements. Especially interesting results were obtained for Mn, Zn, and Ni, present in large amounts in the soil studied--their leachability was significantly reduced by addition of red mud and compost, suggesting interesting perspectives in soil-remediation activity.