ADSORPTION OF LEAD ON SINGLE PHASES AND BINARY MIXTURES OF HFO AND KAOLINITE

Metal oxy-hydroxides and phyllosilicate minerals play a significant role in the fate and transport of heavy metals in the environment (Bertsch and Seaman, 1999, Proc Nat Acad Sci 96, 3350). Chemical speciation of metals affects their bioavailability and chemical reactivity (e.g., Stumm and Morgan, 1996, Aquatic Chemistry). Surface complexation models (SCMs) based on equilibrium thermodynamic principles have been successfully used to quantify adsorption of heavy metals on pure solid minerals, including phyllosilicates and oxy-hydroxides. For natural sediments with mixed mineralogy, Davis et. al. (1998, EST 32, 2820) suggested two different SCM approaches, namely, the component additivity and generalized composite models. In this study, adsorption of lead on pure HFO and pure kaolinite and on binary assemblages of HFO and kaolinite have been measured as a function of pH (~2-9), total metal concentration (10^-4 to 10^-6M), ionic strength of the electrolyte (0.1 to 0.001M NaNO3) and kaolinite to HFO ratio. In all experiments, adsorption of Pb increase with increasing pH; adsorption edges are much sharper for pure HFO compared to pure kaolinite. In binary systems, adsorption starts at pH ~2 and 100% adsorption occurs at pH ~4.5 for 1:1; ~5.5 for 5:1 and 10:1; ~6.0 for 50:1, 100:1 and 500:1 and ~6.9 for 5000:1 mixture. Adsorption edges for mixed systems shift towards the pure kaolinite system with increasing kaolinite:HFO, but never reach the pure kaolinite edge. This suggests that even very small quantities of HFO have a strong influence on Pb adsorption. Deviations between measured adsorption and predictions from thermodynamic calculations assuming no mineral-mineral interactions will be used to test the component additivity approach.