Interaction of monomeric and polymeric species of metal ions with clay surfaces. IV. Mixed system of aluminium(III) and iron(III)

Abstract

Polymerization of aluminium(III) and iron(III), in mixed solutions of the nitrates, by dialysis against distilled water resulted in polycations having higher Fe/(Fe+Al) mole ratios than those of the starting solutions. A maximum of 25 mole per cent aluminium(III) entered probably into the structure of the po1y[Fe(III)-OH] cation. Ferric aquo-ions polymerized quickly and slower polymerization of aluminium(III) led to the exclusion of the monomeric aluminium(III) during hydrolysis. A more complete polymerization of both aluminium(III) and iron(III) in mixed solutions was initiated by adding sodium hydroxide up to a OH/(Fe+Al) mole ratio of 2.5 and then removing the monomers by dialysis. The resultant polycations had Fe/(Fe+Al) mole ratios close to the original solutions. The pH values and positive charge of these products were higher than any obtained with either pure aluminiurn(III) or iron(III) systems, suggesting that these are copolycations of aluminium(III) and iron(III) rather than a mixture of separate aluminium(III) and iron(III) species. The infrared spectra of these copolycations were different from the pure end members, viz. po1y[Fe(III)-OH] and poly[Al(III)-OH] cations prepared under similar conditions, confirming that they were copolymers rather than a physical mixture of individual polymers. The copolycations coagulated by adding an indifferent electrolyte, sodium nitrate, were amorphous to X-rays. Electron micrographs showed that the morphology of particles in these solutions of copolycations became regular hexagons as the aluminium(III) content increased. The pure aluminium(III) sample showed regular hexagonal particles. In general the particle size decreased with increasing aluminium(III) content. The results obtained for critical coagulation concentration and maximum adsorption for sodium-kaolinite were consistent with the earlier reports for pure aluminium(III) or iron(III) system. Both were related to the positive charge on the copolycations. Maximum adsorption of copolycations on sodium-kaolinite caused charge reversal. The pzc of the copolycation treated kaolinites were between 6.40 and 8.65, the PZC increasing with a1uminium(III) content. The significance of the possible existence of copolycations of aluminium(III) and iron(III) in soil systems is discussed.