POORLY ORDERED HYDROUS FE OXIDES, COLLOIDAL DISPERSION AND SOIL AGGREGATION .1. EFFECT OF HUMIC MACROMOLECULES ON SURFACE AND COLLOIDAL PROPERTIES OF FE(III) POLYCATIONS

Model humic macromolecules, obtained by auto-oxidation of the catechol-glycine system, were adsorbed on poorly ordered hydrous iron oxides, Fe(OH)2.8. The pH was nearly constant at c. 6 in order to stimulate natural soil and river environments. When the amount of organic carbon adsorbed on 1 g of poorly ordered ferrihydrite increased from 0 to 70 mg, the ligand OH exchange was very small, but the zero point of charge of suspensions progressively shifted from 7.3 to 5.6, and net surface charge, which was initially positive, progressively became negative, suggesting an adsorption mechanism with positive charge neutralization. Consequently, colloidal stability changed from rapid to slow flocculation. The ferrihydrite-haematite exothermic reaction partially shifted from 400-degrees-C to higher temperatures as a function of adsorbed organic matter, and ferrihydrite solubility progressively increased by formation of soluble iron-organic complexes. These results were explained more satisfactorily by the concept of colloidal organo-mineral association than by organic coatings on surfaces.