Saturday, 15 July 2006

Adsorption of Humic Acid on Minerals as Examined by FTIR, NMR, and Elemental Analysis.

Saikat Ghosh, Seunghun Kang, Prasanta C. Bhowmik, and Baoshan Xing. Dept of Plant, Soil, and Insect Sciences, Univ of Massachusetts, Stockbridge Hall, Amherst, MA 01003

Mineral-humic complexes are commonly distributed in the natural environments and play a significant role in regulating the transport and retention behavior of hydrophobic organic contaminants. This study examined the characteristics of the mineral-HA complexes as well as the structural composition of humic acids (HA) upon sequential sorption on kaolinite, montmorillonite, and goethite surfaces. Solid state 13C NMR and diffuse reflectance infrared Fourier transformed (DRIFT) spectroscopy techniques were employed to determine the fractionation behavior of HA. Elemental analysis data clearly indicated increasing polarity of the unadsobed HA fractions after coating. DRIFT spectra of the clay-HA complexes demonstrated higher sorption of aliphatic fractions. DRIFT spectra of both kaolinite-HA and montmorillonite-HA complexes showed small peaks at 2926 and 2856 cm-1 region inferring the C-H stretching of CH3 and CH2 groups. The DRIFT spectra of the goethite-HA complexes were dominated by the presence of strong absorption bands of carboxylic moiety. Signature of the phenolic fractions was also observed on goethite surface. DRIFT results of the left over HA fractions after coating on minerals are also in compliance with the mineral-HA complex data. NMR results showed higher sorption of aliphatic moieties on clay minerals while goethite preferentially sorbed carboxylic and phenolic fractions. The above observations distinctly points out that mineral surface property dictate the fractionation behavior of HA.

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