Molecular-scale mechanisms of antimony binding to soil organic matter
Final Report Abstract
Antimony is a potentially toxic metalloid that is increasingly being released into the environment due to its use in a variety of industrial and technical products. Despite increasing Sb inputs to soils where Sb occurs as Sb(V) under oxic conditions, little information is available on sorptive interactions between Sb(V) and heterogeneous soil organic matter. Therefore, we studied the sorption of Sb(V) to particulate organic matter (POM) of a peat soil, examined molecular binding mechanisms by X-ray absorption spectroscopy, and modelled Sb(V) sorption to POM using the Stockholm Humic Model (SHM). Our results showed that sorption of Sb(V) to POM did not cause Sb reduction and decreased from 109 mmol kg-1 at pH 3 to 72.4 mmol kg-1 at pH 5. Although sorption maxima were found at pH 1.8–2.8, up to about 10% of total Sb(V) was still removed from solution at pH 6. An increase in POM concentration and ionic strength as well as the presence of Ca2+ promoted Sb(V) sorption. Sorption kinetics of Sb(V) were slow and generally showed bi-phasic patterns. The slowly sorbing Sb(V) was ascribed to diffusion of Sb(V) into net-negatively charged POM particles. X-ray absorption spectroscopy data were consistent with bidentate Sb(V) complexation by polycarboxylic, hydroxy-carboxylic, and/or polyol ligands. The SHM accurately described Sb(V) sorption data. SHM calculations indicated that Sb(V) sorption to POM may be quantitatively relevant even at pH >7 and impairs the precipitation of Ca-antimonates at acidic pH. Model predictions also showed that Al, Pb, and molybdate can cause substantial desorption of Sb(V) and that Sb(V) complexation by POM in organic soils becomes negligible in presence of >1 wt.% metal oxides. We also explored the potential formation of Sb(V)-metal-POM complexes (“ternary complexes“) at pH 3 and 5 using Fe(III)- and Sr-loaded POM as sorbents for Sb(V). However, X-ray absorption spectroscopy data did not provide evidence of this type of Sb(V) binding. The results of this project imply that although POM has an enormous potential for Sb(V) sequestration over a broad pH range, competing ions and mineral sorbents may strongly decrease Sb(V) binding by POM. This suggests that Sb(V) binding to POM is only relevant in oxic metal-poor soil environments. In addition, our results imply that Sb(V) binding by metalorganic complexes of dissolved or particulate organic matter is insignificant in acidic soils.
Publications
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Antimonate reaction with particulate natural organic matter [Data set]. LUIS
Mikutta C., Christl I., Hockmann K., Niegisch M., Schnee L.S.
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Antimony(V) reaction with particulate natural organic matter: Sorption behavior, binding mechanism, and environmental implications. Geochimica et Cosmochimica Acta, 391, 218-236.
Mikutta, Christian; Christl, Iso; Hockmann, Kerstin; Niegisch, Max & Schnee, Laura S.
