Stability and structure of pentavalent antimony complexes with aqueous organic ligands

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 SbV-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 (SbV) 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 SbV complexing with amine and carboxylic acids with single functional group (acetic acid) or non-adjacent functional groups (adipic acid). In contrast, SbV 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 SbV atom has a distorted octahedral geometry composed of 6 oxygen atoms forming a five- or six-membered bidendate cycle. Stability constants of SbV-organic complexes, generated for the first time in this study, were used to model SbV binding with natural humic acids containing the same functional groups as those used in this work. Our predictions of SbV binding with natural humic acids indicate that in an aqueous organic-rich solution of 1 μg 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 SbV to organic matter at near-neutral pH contrasts with that of SbIII whose organic complexes may account up to 80% of total Sb in DOC-rich waters. The large differences in SbIII versus SbV 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.

► We studied the aqueous complexation of SbV with simple organic ligands. ► Stability and structure were determinate combining potentiometry and XAS. ► SbV organic complexation in an aqueous organic-rich solution was modeled. ► At pH≤4, 40% of SbV binds to organic matter via hydroxy-carboxyl functional groups. ► In contrast, at neutral-to-basic pH this amount does not exceed 5%.