4.7 Article

Stability and structure of pentavalent antimony complexes with aqueous organic ligands

Journal

CHEMICAL GEOLOGY
Volume 292, Issue -, Pages 57-68

Publisher

ELSEVIER
DOI: 10.1016/j.chemgeo.2011.11.004

Keywords

Sb; Humic; Complexation; XAS; Potentiometry; Dialysis

Funding

  1. French Ministere de l'Enseignement Superieur et de la Recherche

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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.

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