Characterization of Adsorbed Dicarboxylic Acids onto Minerals.
Seunghun Kang and Baoshan Xing. Dept of Plant, Soil, and Insect Sciences, Univ of Massachusetts, Stockbridge Hall, Amherst, MA 01003
Adsorption of dicarboxylic acids (succinic, glutaric, adipic, and azelaic acid) by kaolinite, montmorillonite, and goethite at different pH conditions was investigated using in situ Attenuated Total Reflectance Fourier Transformed InfraRed (ATR-FTIR) and ex situ Diffuse Reflectance Infrared Fourier Transformed (DRIFT) spectroscopy. The order of sorption capacity of the minerals used was: montmorillonite > goethite > kaolinite. The highest dicarboxylic acid adsorption was observed at pH 4 compared to the sorption at pH 7 and pH 9, which indicates that sorption is highly pH-dependent and related to the surface characteristics of minerals. With in situ ATR-FTIR analysis, succinic acid was observed to form outer-sphere complexes with the mineral surfaces at pH 4, 7, and 9. From DRIFT spectra of the freeze-dried complex samples, we observed the inner-sphere coordination between the carboxyl acid groups and the mineral surfaces at pH 4, 7, and 9. Therefore, we propose that organic acids in aqueous environment prefer to adsorb onto clay minerals by outer-sphere complexation, while inner-sphere complexation is favored under dry conditions, indicating that binding of organic acids onto minerals under dry condition is stronger than that under wet condition. Therefore, we expect different conformations and aggregate structures of sorbed organic acids as influenced by complexation types, which may have significant implications for carbon sequestration and cycling in soils.