Abstract
The adsorption of humified clover extract by imogolite was studied to understand the clay-humus interaction in volcanic ash soils. The adsorption occurred apparently following a Langmuir type adsorption curve. Imogolite and allophane with the SiO2/Al2O3 ratio less than 1.5 were both effective adsorbents for humified material and show preference for high-molecular-weight components. The adsorption from concentrated solutions (> 0.1 g of humified material per litre) was reversible to some extent, while the humified material adsorbed from dilute solutions was not readily displaced by repeated washing with water. The adsorption increased with the lowering of pH and was inhibited by addition of sodium citrate and oxalate, while no similar inhibitive action was noted for NaCl. An inhibition of proton uptake in the extent from 25 to 30 per cent due to clayhumus interaction was inferred from the buffer curves determined with an imogolite-humus complex and the respective components.
The result was interpreted in terms of simultaneous operation of two reactions; the first is the “ligand exchange” reaction in which the carboxyl groups of humus molecules penetrate the co-ordination shells of Al atoms on the clay surface, and the second is the surface-segment interaction between the humus molecules and the structure units of imogolite.
