Effect of Soil Fulvic and Humic Acids on Pb Binding to the Goethite/Solution Interface: Ligand Charge Distribution Modeling and Speciation Distribution of Pb

The effect of adsorbed soil fulvic (JGFA) and humic acid (JGHA) on Pb binding to goethite was studied with the ligand charge distribution (LCD) model and X-ray absorption fine structure (XAFS) spectroscopy analysis. In the LCD model, the adsorbed small JGFA particles were evenly located in the Stern layer, but the large JGHA particles were distributed over the Stern layer and the diffuse layer, which mainly depended on the JGHA diameter and concentrations. Specific interactions of humic substances (HS) with goethite were modeled by inner-sphere complexes between the −FeOH₂^0.5+ of goethite and the −COO^– of HS and by Pb bridges between surface sites and COO^– groups of HS. At low Pb levels, nearly 100% of Pb was bound as Pb bridges for both JGFA and JGHA. At high Pb levels and low HS loading, Pb–goethite almost dominated over the entire studied pH range, but at high HS loading, the primary species was goethite–HS–Pb at acidic pH and goethite–Pb at alkaline pH. Compared with JGFA, there was a constant contribution of Pb bridges of about 10% for JGHA. The linear combination fit of EXAFS, using Pb–HS and Pb–goethite as references, indicated that with increased HS loading more Pb was bound to adsorbed HS and less to goethite, which supported the LCD calculations