Soil humic and fulvic acids interaction with clay-mineral surfaces

In natural environments, soil organic matter is intimately associated with mineral constituents of soils, mainly clays, producing a mineral-organic complex whose properties differ substantially from those of the component materials. The surface area of the organic phase attached to the mineral surface and the accessibility of the functional groups or hydrophobic domains implicated in the sorption of cations or organic pollutants have a considerable influence on the behaviour and fate of chemical contaminants in the environment.; An AFM (Atomic Force Microscopy) procedure for investigating the morphology of humic materials deposited on mica was developed. The AFM procedure was used to investigate the surface complexes formed by interaction between well-characterized soil humic fractions (IHSS Standard Soil HA/FA) and a model clay-surface (the muscovite surface) in aqueous solution under various pH, humic concentration and ionic-strength conditions. The degree of coverage of the mineral surface, the time scale and the relative importance of sorption/precipitation processes were studied. For the first time, a method (AFM) that can distinguish between precipitation-sedimentation and true adsorption is now available.; The AFM procedure was then applied to the study of complexes between the same soil humic fractions and a well-ordered kaolinite (KGa-1). The images obtained by AFM were compared with images of the same complex obtained by SEM. The AFM images were very similar to the SEM images, with a higher degree of resolution for the AFM images.; In a complementary experiment, the carbon content and delta 13C in clay/humic complexes, obtained from IHSS Standard Soil HA/FA and Source Clays, was determined for a wide range of humic concentrations.; Humic fractionation during interaction with clay surfaces has been predicted theoretically, but very little direct evidence was available until now. Indications of humic fractionation during the surface interaction were observed in AFM images. delta13C data determined for the complexes of IHSS Standard Soil Humic Acid/Fulvic Acid with four Source Clays clearly demonstrate that a fractionation of the humic material occurs during sorption of the humic material on the clay minerals.