Chapter 13 Influence of Phosphate on Adsorption and Surface Precipitation of Lead on Iron Oxide Surfaces

Abstract

Lead and phosphate sorption on goethite-coated and uncoated quartz sand was measured experimentally and modeled within a reaction-based framework. Single sorbate batch experiments and experiments with lead and phosphate present together were conducted. Adsorption of lead and phosphate to goethite-coated sand was dominated by adsorption to the goethite coating. A surface complexation model adapted from models for pure goethite successfully simulated lead and phosphate adsorption to goethite-coated sand over a broad range of pH and total sorbate concentrations; the inclusion of a surface complexation reaction for lead adsorption to the quartz surface was necessary to improve the model fit. Lead sorption on goethite-coated sand was enhanced by the presence of phosphate. The effect was most pronounced at low pH. The enhanced lead uptake was predicted by the combination of the single sorbate surface complexation models. The adsorption of phosphate at low pH decreased the surface charge and potential, which increases the extent of lead adsorption. No ternary surface complexes were needed to model the dual sorbate results. When a reaction for the precipitation of chloropyromorphite (Pb 5 (PO 4 ) 3 Cl) was included, the model predicted precipitation only at the lowest pH and highest phosphate loading studied. Over most experimental conditions, including conditions that were initially supersaturated with respect to chloropyromorphite, the equilibrium model predicted that adsorption was the dominant mechanism of lead sorption; however, the actual mechanisms may be controlled by the relative rates of precipitation and adsorption reactions.