Antimony(III) complexing with O-bearing organic ligands in aqueous solution: An X-ray absorption fine structure spectroscopy and solubility study Marie Tella * , Gleb S. Pokrovski Experimental Geochemistry and Biogeochemistry Group, Laboratoire des Me ´canismes et Transferts en Ge ´ologie, LMTG, Universite ´ de Toulouse, UPS, CNRS, IRD, OMP, 14 Av. E. Belin, F-31400 Toulouse, France Received 6 May 2008; accepted in revised form 3 October 2008; available online 22 October 2008 Abstract The stability and structure of aqueous complexes formed by trivalent antimony (Sb III ) with carboxylic acids (acetic, adipic, malonic, lactic, oxalic, tartaric, and citric acid), phenols (catechol), and amino acids (glycine) having O- and N-functional groups (carboxyl, alcoholic hydroxyl, phenolic hydroxyl and amine) typical of natural organic matter, were determined at 20 and 60 °C from solubility and X-ray absorption fine structure (XAFS) spectroscopy measurements. In organic-free aque- ous solutions and in the presence of acetic, adipic, malonic acids and glycine, both spectroscopic and solubility data are consistent with the dominant formation of Sb III hydroxide species, SbðOHÞ nþ 3n ; SbðOHÞ 0 3 and SbðOHÞ  4 , at strongly acid, acid-to-neutral and basic pH, respectively, demonstrating negligible complexing with mono-functional organic ligands (acetic) or those having non adjacent carboxylic groups (adipic, malonic). In contrast, in the presence of poly-functional carboxylic and hydroxy-carboxylic acids and catechol, Sb III forms stable 1:1 and 1:2 complexes with the studied organic ligands over a wide pH range typical of natural waters (3 < pH < 9). XAFS spectroscopy measurements show that in these species the central Sb III atom has a distorted pseudo-trigonal pyramidal geometry composed of the lone pair of 5s 2 electrons of Sb and four oxy- gen atoms from two adjacent functional groups of the ligand (O@C–OH and/or CAOH), forming a five-membered bidendate chelate cycle. Stability constants for these species, generated from Sb 2 O 3 (rhomb.) solubility experiments, were used to model Sb complexing with natural humic acids possessing the same functional groups as those investigated in this study. Our pre- dictions show that in an aqueous solution of pH between 2 and 10, containing 1 lg/L of Sb and 5 mg/L of dissolved organic carbon (DOC), up to 35% of total dissolved Sb binds to aqueous organic matter via carboxylic and hydroxy-carboxylic groups. This amount of complexed Sb for typical natural DOC concentrations is in agreement with that estimated from dial- ysis experiments performed with commercial humic acid in our work and those available in the literature for a range of stan- dardized IHSS humic acids. Our results imply that a significant part of Sb is likely to be bound with humic acids via hydroxy- carboxylic moieties, in the form of bidendate complexes. However, following the strong chemical affinity of Sb III for reduced sulfur, some undefined fraction of Sb III might also be bound to the minor thiol-bearing moieties of humic acids; further stud- ies are required to check this hypothesis. Ó 2008 Elsevier Ltd. All rights reserved. 1. INTRODUCTION Antimony is of significant environmental concern be- cause of its high toxicity (Gurnani et al., 1994) and the growing contamination of waters and soils in certain re- gions from mining, industry, and fire arms (e.g., Scheinost et al., 2006). Knowledge of the effect of major inorganic and organic ligands on the Sb chemical speciation in aqua- tic systems is a first step to predict the antimony behavior in natural waters and polluted sites (e.g., Casiot et al., 2007), its interactions with mineral surfaces (e.g., Belzile et al., 0016-7037/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2008.10.014 * Corresponding author. Fax: +33 5 61 33 25 60. E-mail address: tella@lmtg.obs-mip.fr (M. Tella). www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 73 (2009) 268–290