Structure Characterization of Carboxylic Acid/Mineral Complexes.
Seunghun Kang and Baoshan Xing. Univ of Massachusetts, Dept of Plant, Soil, and Insect Sciences, 24B Stockbridge Hall, Amherst, MA 01003
Storage carbon in soils can be strongly influenced by interactions of carbon compounds with soil minerals. The objective of this study was to determine physicochemical properties of carboxylic acid complexes with minerals at different pH conditions using Fourier transformed infrared (FTIR) with attenuated total reflectance (ATR) and diffuse reflectance (DRIFT) accessory, nuclear magnetic spectroscopy (NMR), atomic force microscopy (AFM), and dynamic light scattering (DLS). Oxalic, benzoic, and phthalic acids were employed for an analogue of natural organic matter in environment, and goethite, kaolinite, and montmorillonite were used as model sorbents. Overall, sorption capacity of the acids was followed: montmorillonite > kaolinite > goethite. Acid adsorption was the highest at pH 4 compared with that at pH 7 and pH 9, which indicates that the sorption is highly pH-dependent and related with the surfaces of clay minerals. With in situ analysis with ATR-FTIR, azelaic acid formed the outer-sphere complexation with the surface of the minerals at all pHs. Meanwhile according to DRIFT spectra with freeze-dried complex sample, we observed the inner-sphere coordination between the carboxyl acid group and the mineral surface at all pHs. Therefore we suggest that organic acid in aqueous environment prefers to adsorb onto clay mineral by outer-sphere complexation, but inner-sphere complexation is involved after drying. Binding of organic acids onto minerals by outer- or inner-sphere coordination may render them resistant to biological or chemical attack, resulting in carbon stabilization in soils.