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2633485 
Journal Article 
Stability and structure of pentavalent antimony complexes with aqueous organic ligands 
Tella, M; Pokrovski, GS 
2012 
No 
Chemical Geology
ISSN: 0009-2541 
292 
57-68 
WOS:000300339000006 
Despite the rapidly growing concern about antimony pollution of waters and soils, the effect of organic matter on the behavior of this toxic trace element is poorly understood because of a lack of data on Sb-v-organic ligand interactions in aqueous solution. We used in situ potentiometry and X-ray absorption spectroscopy (XAS) to measure in aqueous solution at ambient conditions the stability and structure of aqueous complexes formed by pentavalent antimony (Sb-v) with low molecular weight organic ligands, such as carboxylic acids (acetic, adipic, malonic, lactic, oxalic, citric and salicylic), phenols (catechol), polyols (xylitol and mannitol), and amines (glycine), which have O- and N-functional groups typical of natural organic matter. Potentiometric titrations from pH 2 to 10 demonstrate negligible Sb-v complexing with amine and carboxylic acids with single functional group (acetic acid) or non-adjacent functional groups (adipic acid). In contrast, Sb-v forms stable complexes with poly-carboxylic, hydroxy-carboxylic acids, and with aliphatic and aromatic polyol ligands in the pH range typical of natural waters. XAS measurements show that in these species the Sb-v atom has a distorted octahedral geometry composed of 6 oxygen atoms forming a five- or six-membered bidendate cycle. Stability constants of Sb-v-organic complexes, generated for the first time in this study, were used to model Sb-v binding with natural humic acids containing the same functional groups as those used in this work. Our predictions of Sb-v binding with natural humic acids indicate that in an aqueous organic-rich solution of 1 mu g L-1 Sb and 20 mg L-1 dissolved organic carbon (DOC) up to 40% of total Sb binds to aqueous organic matter via carboxyl and hydroxy-carboxyl functional groups at pH <= 4, whereas at neutral-to-basic pH this amount does not exceed 5%. These estimations are in agreement with direct dialysis measurements conducted with a purified commercial humic acid. The low affinity of Sb-v to organic matter at near-neutral pH contrasts with that of Sb-III whose organic complexes may account up to 80% of total Sb in DOC-rich waters. The large differences in Sb-III versus Sb-v binding to organic matter may be used for tracing in organic-rich sediments and waters the two main Sb oxidation states, which have different toxicities for aquatic organisms. (C) 2011 Elsevier BM. All rights reserved. 
Sb; Humic; Complexation; XAS; Potentiometry; Dialysis