Surface complexation modeling of Pb(II) adsorption on mixtures of hydrous ferric oxide, quartz and kaolinite

Pb adsorption is measured on hydrous ferric oxide (HFO), kaolinite and quartz as a function of pH (∼ 2-10), ionic strength (0.001 to 0.1 M NaNO3) and sorbate concentration (1 to 100 µM Pb on 2 g/L solid). The data are used to parameterize diffuse layer surface complexation models (DLMs) for each solid. Pb adsorption edges on HFO show little dependence on ionic strength and are described well by a DLM with Pb forming a monodentate complex on strong and weak surface sites. In contrast, Pb adsorption edges on quartz are strongly dependent on ionic strength and sorbate/sorbent ratios. A single-site DLM with a monodentate Pb complex describes Pb adsorption on quartz well over relatively restricted solution conditions, but cannot reproduce the observed dependence on ionic strength and Pb loading. As for quartz, Pb adsorption on kaolinite depends strongly on ionic strength and Pb loading. Several model approaches are tested, but none produces a good fit to the data over the full range of measured conditions. The best of the tested DLMs for kaolinite assumes Pb binding to a permanent charge (face) site and formation of a monodentate complex on a variable-charge (edge) site. A simple component additivity approach, assuming no mineral-mineral interactions and using models developed for the three pure sorbents generally produces good predictions of Pb adsorption for mixed solid systems. With the possible exception of the HFO and kaolinite system, mineral-mineral interactions between quartz, kaolinite and HFO appear to be smaller than the uncertainties associated with the pure sorbent models.