Spectroscopic and chemical evaluation of in situ chemical immobilization for the remediation of heavy metal contaminated soils

Scope and method of study. Six in situ chemical treatments for the immobilization of cadmium (Cd), lead (Pb), and zinc (Zn) in contaminated soils were investigated. Chemical and spectroscopic methods were used to characterize the soils and evaluate the effectiveness of the amendments. The initial objective was to compare the effectiveness of phosphate, organic and/or alkaline amendments in smelter contaminated soils. The second objective was the evaluation of three phosphate amendments of varying solubility in similar soils. Sequential extraction was used to evaluate amendment effectiveness by monitoring changes in metal extractability. The final objective was to identify the formation of new metal containing phases in the soil-amendment systems using spectroscopic techniques including, diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS), solid state nuclear magnetic resonance (NMR), and powder X-ray diffraction (XRD).; Findings and conclusions. In situ chemical immobilization is a viable technique for the remediation of Cd, Pb, and Zn contaminated soils. The effectiveness of the amendments depends on the nature of the amendment, the initial form of the metal contaminate and the soil matrix. The solubility of phosphate amendments did not determine the effectiveness of the amendment. These results indicate that the controlling mechanism for immobilization of metals by phosphate amendments is not dissolution-precipitation, New metal phosphate, carbonate, and hydroxide phases were identified spectroscopically in the soil-amendment mixtures. These data provide evidence for the formation of metal phases through both precipitation and surface reactions. Spectroscopic results indicate that it is not possible to predict whether new phases will be observed.