MECHANISM OF FORMATION OF HUMUS COATINGS ON MINERAL SURFACES .2. ATTENUATED TOTAL REFLECTANCE SPECTRA OF HYDROPHOBIC AND HYDROPHILIC FRACTIONS OF ORGANIC-ACIDS FROM COMPOST LEACHATE ON ALUMINA

Hydrophobic and hydrophilic fractions were isolated from a compost lea chate. The adsorption isotherms of both fractions on alumina were meas ured by attenuated total reflectance infrared spectroscopy. The shapes of the adsorption isotherms of the two fractions were different. The isotherms for the hydrophilic fraction showed little change in surface excess with increasing solution concentration above 4 mg L(-1). The i sotherms for the hydrophobic fraction, on the other hand, displayed a marked increase in surface excess with increasing solution concentrati on. This increase is evidence for the formation of aggregates (admicel les or hemimicelles) on the alumina surface. Linear dichroism calculat ions indicated that more of the carboxylate groups in the adsorbed hyd rophobic molecules than in the absorbed hydrophilic fraction were free to rotate. The hindered rotation of the carboxylate groups in the ads orbed hydrophilic-fraction molecules probably indicates that these gro ups are bound to surface aluminum ions by a bidentate mechanism in whi ch the two oxygen atoms of a single carboxylate group bind to separate aluminum ions.