Modelling lead(II) sorption to ferrihydrite and soil organic matter

Lead(II) adsorption to soil organic matter and iron (hydr)oxides is strong, and may control the geochemical behaviour of this metal. Here, we report the adsorption of Pb(2+) (i) to 2-line ferrihydrite, and (ii) to a mor layer. The results showed that ferrihydrite has heterogeneous Pb(2+) binding. Use of a surface complexation model indicated that similar to 1% of the surface sites adsorbed Pb(2+) more strongly than the remaining 99 %. Although only one surface complexation reaction was used (a bidentate complex of the composition (equivalent to FeOH)(2)Pb(+)), three classes of sites with different affinity for Pb(2+) were needed to simulate Pb(2+) binding correctly over all Pb/Fe ratios analysed. For the mor layer, Pb(2+) sorption was much stronger than current models for organic complexation suggest. The results could be described by the Stockholm Humic Model when the binding heterogeneity was increased, and when it was assumed that 0.2% of the binding sites were specific for Pb. Use of revised model parameters for nine Vietnamese soils suggest that lead(II) binding was more correctly simulated than before. Thus, underestimation of lead(II) sorption to both (hydr) oxide surfaces and organic matter may explain the failure of previous geochemical modelling attempts for lead(II).