Submitted to: Clays and Clay Minerals
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 7, 1996
Publication Date: N/A
Interpretive Summary: Prediction of the transport of potentially toxic anions requires that we account for the adsorption by soil minerals. Predicting adsorption in natural systems is made complex by anion competition between some but not all ions. Understanding these interactions requires knowledge of the specific species adsorbed as well as the type of bonding. The presence of carbonate species has been reported to reduce B adsorption on oxides. Thi study provides the first in situ infrared spectroscopic information on carbonate speciation at the mineral-water interface. In this study we determined that adsorption of bicarbonate and carbonate onto oxides produces inner-sphere complexes (no water of hydration between the species and the surface). Using FTIR spectroscopy we determined that both bicarbonate and carbonate species compete for surface sites on poorly crystallized Fe oxides and only carbonate ions compete for surface sites on npoorly crystallized Al oxdes.
Technical Abstract: Surface adsorption mechanisms of dissolved inorganic carbon species on soil minerals are not well understood. The interaction of bicarbonate and carbonate ions with x-ray amorphous Al and Fe oxides, gibbsite and goethite was examined by in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. Inner-sphere complexation of bicarbonate and carbonate was supported by a lowering in the anion symmetry due to the interaction with Al and Fe oxides surfaces. Only complexed monodentate carbonate was identified in am-Al(OH)3/aqueous solution at pH 4.1 - 7.8 when the solid was reacted with either NaHCO3 or Na2CO3 solutions. Am-Al(OH)3 was transformed to a crystalline sodium aluminum hydroxy carbonate, dawsonite [NaAl(CO3)(OH)2], and bayerite after reacting with 1.0 M Na2CO3 for 24 h. Gibbsite adsorbed much less carbonate than am-Al(OH)3 and gave weak IR absorption for adsorbed carbonate. Both surface complexed bicarbonate and carbonate species were found in the interfacial region of am-Fe(OH)3 suspensions and their relative distribution depended on solution pH. Only monodentate carbonate was found in the interfacial region of goethite in 1.0 M NaHCO3. A ligand exchange reaction was proposed to describe the interaction of bicarbonate and carbonate with the surface functional groups of Al and Fe oxides.