Antimony(V) complexing with O-bearing organic ligands in aqueous solution: an X-ray absorption fine structure spectroscopy and potentiometric study

摘要

The stability and structure of aqueous complexes formed by pentavalent antimony (Sb~v) with simple organic ligands (acetic, adipic, oxalic, citric acids, catechol and xylitol) having O-functional groups (carboxyl, alcoholic hydroxyl, aliphatic and aromatic hydroxyl) typical of natural organic matter (NOM), were determined at 25 deg C from potentiometric and X-ray absorption fine structure spectroscopy (XAFS) measurements. In organic-free aqueous solutions, spectroscopic data are consistent with the dominant formation of Sb~v hydroxide species, Sb(OH)_5 and Sb(OH)_6~-, at acid and near-neutral to basic pH, respectively. Potentiometric measurements demonstrate negligible complexing with mono-functional organic ligands (acetic) or those having non-adjacent carboxylic groups (adipic). In contrast, in the presence of poly-functional carboxylic, hydroxyl carboxylic acids and aliphatic and phenolic hydroxyl, Sb~v forms stable 1:1 or 1:3 complexes in coordination 6 with the studied organic ligands, over a wide pH range pertinent to natural waters (3 <= pH <= 9). The XAFS measurements show that in these species the central Sb~v atom has an octahedral geometry with 6 oxygen atoms from hydroxyl moieties and adjacent functional groups (O = C-OH and/or C-OH) of the ligand, forming bidendate chelate cycles. Stability constants for Sb~v-oxalate complexes generated from potentiometric experiments were used to model Sb~v complexing with di-carboxylic functional groups of natural humic acids. Our predictions show that in an aqueous solution of pH between 1 and 4 containing 1 mu g/1 of Sb and 5 mg/1 of dissolved organic carbon (DOC), up to 15 percent of total dissolved Sb may be bound to aqueous organic matter via di-carboxylic groups.