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

Surface complexation models (SCMs) have been developed to quantify metal binding to oxide and other mineral surfaces using equilibrium thermodynamic principles. SCMs have been shown to accurately describe well-controlled laboratory investigations of heavy metal (e.g., Pb, Zn, Cd, Cu, Co) adsorption on a variety of pure mineral phases (e.g., goethite, silica, corundum) (Dzombak and Morel, 1990, Hydrous Ferric Oxide). Recently, Davis et. al. (1998, ES&T) suggested two different SCM approaches to characterize adsorption in natural systems, namely, the component additivity and generalized composite models. In this study, Pb adsorption was measured on single minerals (HFO, silica and kaolinite) and binary solid systems comprised mixtures of HFO + silica and HFO + kaolinite, as a function of pH, total Pb concentration (10-4, 10-5 and 10-6M with 2g/L of solid) and ionic strength of the electrolyte (0.001-0.1M NaNO3). Adsorption of Pb increases with increasing pH in all systems. Sharp adsorption edges with little ionic strength dependence observed for the pure HFO system are in agreement with model predictions using existing SCM stability constants from Dzombak and Morel (1990), and suggest that Pb forms strong inner-sphere complexes on HFO. Pb adsorption edges for the pure silica system are considerably shallower than on HFO; the pH at which 50% of the lead adsorbs (pH50%) is ~6, in contrast to the pH50% of ~3 for HFO and ~5 for kaolinite. Pb adsorption edges for mixtures of HFO + silica or HFO + kaolinite are virtually indistinguishable from pure HFO system, suggesting that most of the adsorption occurs on the HFO. Derivation of SCM stability constants for Pb adsorption on the pure silica and kaolinite systems is in progress; results will allow a quantitative test of the component additivity model.

Dzombak, D.A., and Morel, F.M., (1990) Surface complexation modeling, Hydrous Iron Oxide, Wiley.

Davis, J.A., Coston, J.A., Kent, D.B., and Fuller C.C., (1998) Environ. Sci. Technol. 32, 2820-2828.