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
R. Wershaw et al., 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, Colloids and surfaces. A, Physicochemical and engineering aspects, 108(2-3), 1996, pp. 199-211
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.