LEAD RETENTION AND COMPLEXATION IN A MAGNESIUM SMECTITE (HECTORITE)

The retention and complexation of Pb2+ in the interlayer of a smectite clay(hectorite) was investigated. Based on XRD results, the Ca-Pb exc hanged clays most likely exhibit random interstratification. A random alternation of one-water-layer (similar to 12.5 Angstrom) and two-wate r-layer (similar to 15.5 Angstrom) hydrates as well as an irrational s et of basal reflections are observed. At a specified humidity, the bas al spacings and IR intensity of the water-bending mode decrease as the lead content in the clays increases. The Ca-Pb exchange isotherm clea rly shows increasing selectivity of the clay for Pb2+ over Ca2+ with i ncreasing Ca2+ concentrations in the clay. The thermodynamic equilibri um constant is 5.18 and the standard free energy change is -4.08 kJ/mo l at 298 K. The higher adsorption specificity for Pb2+ over Ca2+ could be partly due to the difference in ionic potentials and hydration ene rgies between Pb2+ and Ca2+. However, in the solid retrieved from the exchange solution, an infrared spectroscopy study has shown the format ion of a lead hydroxycarbonate complex. The role eventually played by this complex in enhancing the selectivity is unknown. (C) 1995 Academi c Press, Inc.