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

Abstract

Environmental contextLead(II) is a toxic metal pollutant with many anthropogenic sources. We show that lead(II) is bound more strongly to soil surfaces than previously understood. This knowledge may lead to better models for lead(II) dissolution from the soils, which will improve risk assessments for this metal. AbstractLead(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 Pb2+ (i) to 2-line ferrihydrite, and (ii) to a mor layer. The results showed that ferrihydrite has heterogeneous Pb2+ binding. Use of a surface complexation model indicated that ~1 % of the surface sites adsorbed Pb2+ more strongly than the remaining 99 %. Although only one surface complexation reaction was used (a bidentate complex of the composition (≡FeOH)2Pb+), three classes of sites with different affinity for Pb2+ were needed to simulate Pb2+ binding correctly over all Pb/Fe ratios analysed. For the mor layer, Pb2+ 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).