High and low affinity sites of ferrihydrite for metal ion adsorption: Data and modeling of the alkaline-earth ions Be, Mg, Ca, Sr, Ba, and Ra

摘要: The alkaline-earth metal ion series comprises Be, Mg, Ca, Sr, Ba, and Ra. Calcium (Ca) magnesium (Mg) are the most abundant ions in nature their interaction with mineral surfaces of (hydr)oxides (e.g. ferrihydrite, Fh) affects bioavailability, mobility, geochemical cycling many relevant ions. adsorption Ca2+ Mg2+ to well-characterized freshly precipitated Fh has not been extensively measured yet systems a large variation pH (5–10), ionic strength (0.01–1 M), (0.002–2 μmol m−2). Nor have such data interpreted surface complexation model that regards structure adsorbed complexes state-of-the-art insights into this nanomaterial. primary collected study (M2+/Fe) were scaled consistent manner area derived recently developed probe-ion methodology, before these charge distribution (CD) model, using MO/DFT/B3LYP/6-31+G** optimized hydrated geometries obtain independently CD coefficients. pH-dependent behavior Ca Mg is rather similar. Both cations (M2+) form predominantly bidentate inner-sphere ([tbnd](FeOH)2Δz0MΔz1), possibly as binuclear double corner (2C) complex according EXAFS. This binding mechanism explains relatively high H+/Ca2+ exchange ratio related pH-dependency adsorption. Modeling reveals quantifies site heterogeneity Fh, distinguishing low affinity sites for M2+ evaluated rationalize phenomenon identify possible configurations. increase [tbnd]FeOH-M2+ bond may be due redistribution within specific sets Fe1 polyhedra at underlying solid set common oxygen an insufficient neutralization. According our structural analysis 2.2–2.8 nm particles, [tbnd]FeOH density involved (∼0.3 ± 0.1 nm−2) aggress found by modeling (0.30 0.03 nm−2). Extending literature data, comprising full ions, radius was found, i.e. Be2+