Short-Term Stability of Immobilized Heavy Metals in Soils: Greenhouse Experiment.

In situ remediation of metals-contaminated soils by using chemical amendments to these sites has been given recently increasing attention. Although the resulting metal compounds are stable under favorable environmental conditions, there is some concern about the dissolution of these compounds in the soil due to variation in some soil properties. A greenhouse experiment was conducted to investigate the behavior of the immobilized metals compounds Cd, Cu, Zn, and Pb, due to some changes brought about by variation in soil pH, and redox conditions of the soil environment, following the cleaning up of the soil by the in situ remediation technique. Surface soil samples (calciorthids) from three metals-contaminated sites (Atitalaquia I, Atitalaquia II and Tezantepec) previously treated with raw (PR-1), acidulated phosphate rock at 25% (PR-25) and 50% (PR-50) solely or combined with composted biosolids) were equilibrated with 0.5 M MgCl2 or 0.5 M NH2 OH – HCl solution at four pH levels. Soil extracts following centrifugation were saved for final pH determination and metals analysis. Results showed a significant degree of stability for Cd, Cu, and Pb for soils Atitalaquia II and Tezantepec, in maize shoots. Conversely, the immobilization of Zn in maize shoots was not very effective with varying pH and redox conditions, indicating Zn immobilization is more sensitive to environmental variations. Similar results were found in roots, however, with a higher degree of stability. Treatment with 50% of acidulated rock phosphate combined with biosolids seemed to be the most effective treatment, in terms of stability of in-situ immobilized heavy metals, overall the applied treatments in the greenhouse experiment. Next step will be to study with greater depth these findings and apply to field experiment.