Adsorption of organic matter at mineral/water interfaces: I. ATR-FTIR spectroscopic and quantum chemical study of oxalate adsorbed at boehmite/water and corundum/water interfaces

The types and structures of adsorption complexes formed by oxalate at boehmite (gamma-AlOOH)/water and corundum (alpha-Al2O3)/water interfaces were determined using in situ attenuated total reflectance fourier transform infrared (ATR-FTIR) spectroscopy and quantum chemical simulation methods. At pH 5.1, at least four different oxalate species were found at or near the boehmite/water interface for oxalate surface coverages (Gamma(ox)) ranging from 0.25 to 16.44 mumol/m(2). At relatively low coverages (Gamma(ox) < 2.47), strongly adsorbed inner-sphere oxalate species (IR peaks at 1286, 1418, 1700, and 1720 cm(-1)) replace weakly adsorbed carbonate species, and a small proportion of oxalate anions are adsorbed in an outer-sphere mode (IR peaks at 1314 and 1591 cm(-1)). IR peaks indicative of inner-sphere adsorbed oxalate are also observed for oxalate at the corundum/water interface at Gamma(ox) = 1.4 mumol/m(2). With increasing oxalate concentration (Gamma(ox) > 2.47 mumol/m(2)), the boehmite surface binding sites for inner-sphere adsorbed oxalate become saturated, and excess oxalate ions are present dominantly as aqueous species (IR peaks at 1309 and 1571 cm(-1)). In addition to these adsorption processes, oxalate-promoted dissolution of boehmite following inner-sphere oxalate adsorption becomes increasingly pronounced with increasing Gamma(ox) and results in an aqueous Al(III)-oxalate species, as indicated by shifted IR peaks (1286 --> 1297 cm(-1) and 1418 --> 1408 cm(-1)). At pH 2.5, no outer-sphere adsorbed oxalate or aqueous oxalate species were observed. The similarity of adsorbed oxalate spectral features at pH 2.5 and 5.1 implies that the adsorption mechanism of aqueous HOx(-) species involves loss of protons from this species during the ligand-exchange reaction. As a consequence, adsorbed inner-sphere oxalate and aqueous Al(III)-oxalate complexes formed at pH 2.5 have coordination geometries very similar to those formed at pH 5.1. The coordination geometry of inner-sphere adsorbed oxalate species was also predicted using quantum chemical geometry optimization and IR vibrational frequency calculations. Geometry-optimized Al8O12 and Al14O22 clusters with the reactive surface Al site coordinated by three oxygens were used as model substrates for corundum and boehmite surfaces. Among the models considered, calculated IR frequencies based on a bidentate side-on structure with a 5-membered ring agree best with the observed frequencies for boehmite/oxalate/water samples at Gamma(ox) = 0.25 to 16.44 mumol/m(2) and pH 2.5 and 5.1, and for a corundum/oxalate/water sample at Gamma(ox) = 1.4 mumol/m(2) and pH 5.1. Based on these results, we suggest that oxalate bonding on boehmite and corundum surfaces results in 5-coordinated rather than 4- or 6-coordinated Al surface sites. Copyright (C) 2004 Elsevier Ltd