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Sb(III) and Sb(V) Sorption onto Al-Rich Phases: Hydrous Al Oxide and the Clay Minerals Kaolinite KGa-1b and Oxidized and Reduced Nontronite NAu-1 Anastasia G. Ilgen*,† and Thomas P. Trainor† †
Department of Chemistry and Biochemistry, University of Alaska Fairbanks, P.O. Box 756160, 900 Yukon Drive, Room 194, Fairbanks, Alaska 99775-6160, United States S Supporting Information *
ABSTRACT: We have studied the immobilization of Sb(III) and Sb(V) by Al-rich phases - hydrous Al oxide (HAO), kaolinite (KGa-1b), and oxidized and reduced nontronite (NAu-1) - using batch experiments to determine the uptake capacity and the kinetics of adsorption and Extended X-ray Absorption Fine Structure (EXAFS) Spectroscopy to characterize the molecular environment of adsorbed Sb. Both Sb(III) and Sb(V) are adsorbed in an inner-sphere mode on the surfaces of the studied substrates. The observed adsorption geometry is mostly bidentate corner-sharing, with some monodentate complexes. The kinetics of adsorption is relatively slow (on the order of days), and equilibrium adsorption isotherms are best fit using the Freundlich model. The oxidation state of the structural Fe within nontronite affects the adsorption capacity: if the clay is reduced, the adsorption capacity of Sb(III) is slightly decreased, while Sb(V) uptake is increased significantly. This may be a result of the presence of dissolved Fe(II) in the reduced nontronite suspensions or associated with the structural rearrangements in nontronite due to reduction. These research findings indicate that Sb can be effectively immobilized by Al-rich phases. The increase in Sb(V) uptake in response to reducing structural Fe in clay can be important in natural settings since Ferich clays commonly go through oxidation−reduction cycles in response to changing redox conditions.
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profiles.1,10 On the other hand, there is experimental evidence that Sb can be mobile, especially in oxic conditions,11,12 and can be transported in soil solution along preferential flow paths.10,13 Similar to other trace metal(loid) species, Sb partitions to mineral phases and a significant amount of Sb is found in association with solid phases in soil and sediment environments.14 These partitioning reactions are likely the predominant factor controlling the extent of Sb mobilization. Under typical surface environmental conditions Sb can exist in two oxidation states: +3 and +5, and the extent to which Sb partitions onto mineral surfaces depends on the oxidation state and composition of the aqueous matrix. Previous studies have observed that reduced forms of antimony from weathering sulfide ore deposits,15 oxidizing smelter residues,16 or spent bullets7 readily oxidize to Sb(V) and partition onto metal-oxide phases present in soil or suspended in water. In river sediments, Sb was found to be predominantly associated with Fe and Al phases.17 Laboratory studies on natural sorbents have shown that Sb(V) is effectively adsorbed by soils rich in amorphous Fe(OH)3 and humic acid.18 Humic acid immobilizes Sb(III) more effectively than
INTRODUCTION Antimony (Sb) has an average abundance of 0.5−2 μg/g in sedimentary rocks, Al(OH)3 >FeOOH.22 Both Sb(III)23 and Sb(V)24 adsorb strongly to Mn dioxide. Antimony(III) can also be removed from solution by synthetic hydroxyapatite.25 Synthetic goethite readily adsorbs Sb(III) in a wide pH range, and Sb(V) at pH
