Interaction of monomeric and polymeric species of metal ions with clay surfaces. III. Aluminium(III) and chromium(III)

Abstract

Hydrolysis and polymerization in aluminium nitrate and chromic nitrate solutions with different metal/OH ratios were studied by gel filtration chromatography and ultrafiltration techniques. The characteristics of poly[Al(III)-OH] cations separated by ultrafiltration depended on the molecular weight which was controlled by the OH/Al ratio of the original solution. When the OH/Al ratio was <2.0, the polycations had molecular weight < 50 000, high positive charge and were stable. When the ratio was > 2.0 , the polycations had molecular weight > 100 000, low positive charge and rapidly condensed to gibbsite. Polymerization in chromic nitrate solutions was slow and the separated poly[Cr(III)-OH] cations had low molecular weight (<20000), high positive charge and were stable. The interaction of monomeric and polymeric species of aluminium(III) and chromium(III) with the surfaces of sodium-kaolinite, sodium-bentonite and sodium-illite was studied using hydrolysed nitrate solutions of aluminium and chromium, and separated polycations. The results obtained were consistent with the earlier observations of the iron(III) system, and the behaviour could be explained on the basis of positive charge and probable structure of polycations. The adsorption of aluminium(III) and chromium(III) on clay surfaces from hydrolysed metal ion solutions was related to OH/Al or OH/Cr ratios and pH. The adsorption of polycations was low and related to their positive charge. Maximum adsorption resulted in charge reversal on clays. Addition of the hydrolysed nitrate solutions to clays also caused charge reversal and reduction of negative charge. The adsorption from a mixture of monomers and polymers on bentonite surfaces reduced negative charge markedly due to interlayer formation, while the adsorption of separated polycations in low amounts neutralized the charge on surface hydroxyls and led to the flocculation of clays. The anomalous behaviour of the poly[Al(III)-OH] cations of molecular weight >100 000 could be explained if they consisted of planar sheets, formed from coalesced rings of aluminium octahedra, which crystallized into gibbsite.