Sorption of copper and cadmium from the water soluble fraction of an acid mine waste by two calcareous soils
Our previous findings suggested different Cu and Cd retention mechanisms for each of two calcareous soils. Solubility data indicated that the Skumpah soil (30% CaCO3) reacted with the water extract of an acid mine waste to form tenorite (CuO). Batch desorption of the waste-extract equilibrated Kidman soil (0.2% CaCO3) with CaCl2 indicated that cation exchange was the metal retention mechanism. This study tested these hypotheses of metal retention mechanisms in soil columns.
Scanning electron microscopy, back scattering electron microscopy, and x-ray diffraction (XRD) were used to investigate the distribution and forms of Cu in the Skumpah soil that had been leached with waste extracts. Leaching with the mine waste reduced the intensity of calcite and dolomite peaks in diffractograms of soil samples from the upper 8 cm of the columns. Micrographs and Ca and Cu dot maps showed that Ca and Cu were associated with distinct and different particles. This segregation of Ca and Cu, together with the failure to identify tenorite, or any other Cu mineral by XRD, were indications that Cu was retained by a specific adsorption reaction.
Copper and Cd retention in the Kidman soil was investigated via a transport model in which cation exchange was assumed to be the mechanism of metal interaction with the solid phase. Model predictions were compared (1) to metal breakthrough curves for the acid leachate, and (2) to metal release curves obtained by leaching the columns with CaCl2 after metal breakthrough. The model failed to predict Cd breakthrough portions of the curves satisfactorily but was in agreement with the CaCl2 leaching curves for all metals. Results of the modeling study indicated that the composition of the leaching solution and exchange phase determined the degree of uncertainty that could be tolerated in the values of the selectivity coefficients. Further, it appeared that exchange reactions controlled metal retention and transport at acid pH.
Dudley, L. M., J. E. McLean, R. C. Sims, and J. J. Jurinak. 1988. Sorption of copper and cadmium from the water soluble fraction of an acid mine waste by two calcareous soils. Soil Sci. 145:207-214.