Mechanisms of Humus Adsorption on Vermiculite

To further explore rules of the translocation and transformation of soluble organic matter on clay minerals, adsorption behaviors of fulvic acid and humic acid on vermiculite were studied with the aid of multiple analytical techniques including high performance size exclusion chromatography, liquid-state ¹H nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Results show that as various functional groups in humus display different affinities to vermiculite, fractionation of fulvic acid (FA) and humic acid (HA) occur during the adsorption process. Fractions large in molecule size are preferentially adsorbed, whereas fractions lower in molecular size are left in liquid. Elemental analysis shows that HA is more hydrophobic than FA . Compared to FA, HA is larger in molecular weight, thus creating certain steric hindrance effect at iron-adsorbing sites on the surface of vermiculite. When humus is high in concentration (> 20 mg •L^-1), the steric hindrance effect of HA and and cation-π bonding between aromatic groups and iron cations facilitate vermiculite adsorption of FA that is smaller in molecular size. After vermiculite adsorption of HA and FA, pH in the solution increases, indicating that ligand exchange occurs and FA/HA-vermiculite complexes form. Furthermore, the rise of pH of the solution or the fall of the ionic strength enhance electrostatic repulsion between vermiculite and humus, thus lowering vermiculite adsorption of FA and HA. The findings of this study will improve our understanding of the roles of mineral interfacial properties, the effects of FA/HA on adsorption of humus on clay minerals.