SORPTION OF CO, NI, CU, CD AND PB ON NA-MONTMORILLONITE: A LABORATORY AND MODELING STUDY

The overall goal of this study is to develop improved methods for quantifying the fate of trace metals in the environment. One important control on the fate and mobility of trace metals is their interaction with soil and sediment constituents, especially clay minerals. Therefore, in this study sorption of Cu, Pb, Ni, Co, Cd on Wyoming Na-Montmorillonite (SWY-2) is investigated as a function of pH, ionic strength and sorbate/sorbent ratio. For each experiment, 0.5 g/L of montmorillonite is added to a mixture of NaNO₃ (0.001 to 0.1M) and metal (5·10^-7 to 5·10^-5 M) and initially equilibrated for 24 hrs under atmospheric conditions. The pH is then titrated from ~3 to 10 with aliquots taken at ~0.3 to 0.5 pH intervals. Each aliquot is further equilibrated for 24 hrs, the pH remeasured, and then the samples are centrifuged, filtered and the supernatant analyzed for metal concentration via ICP-OES. Sorption increases from low to high pH as expected. The pH₅₀ generally decreases in the order Cd≈Co>Ni>Pb≈Cu with pH₅₀ ranging from ~3 for all the metals in 0.001 M NaNO₃ to 8.9 for Cd in 0.01 M NaNO₃. For a given cation, adsorption increases strongly with decreasing ionic strength and decreasing sorbate/sorbent ratio. Optimized stability constants are derived for each adsorption edge using the FITEQL software. A diffuse double layer surface complexation model is used, assuming that metals bind to both a variable charge edge site and a permanent charge face site. Several stoichiometries (e.g bidentate or monodentate complexes on the variable charge site) are tested for each metal. Good fits could be achieved for individual edges, however, median stability constants based on the range of ionic strength and sorbate/sorbent ratios measured for each metal did not produce good fits for the entire suite of adsorption data.