Structure and affinity towards Cd2+, Cu2+, Pb2+ of synthetic colloidal amorphous aluminosilicates and their precursors

Amorphous aluminosilicates were synthesised at two different initial reagent concentrations. Depending on the concentration used, the final reaction products were identified as either imogolite or allophane. In millimolar medium (millimolar Al initial concentration and Al/Si = 2), imogolite precursors are synthesised during hydrolysis and condensed by heating to build up imogolite tubes. In decimolar medium, precursors with varying local structures are synthesised and condense upon heating to build up allophanes spheroids aggregates. In each medium, silica gel coprecipitated in proportions less than 10% in mass, which seems to have only a weak influence on aluminosilicate synthesis. The zero point of salt effect of suspensions of imogolite, allophane and their respective precursors were determined, and the affinity of these four products towards Cd2+, Cu2+ and Pb2+ was studied. For imogolite and for both precursors, Cu2+ and Pb2+ adsorption can be described as specific, whereas Cd2+ adsorption is only electrostatic. The three metallic cations display specific adsorption towards allophane suspensions. (C) 1999 Elsevier Science B.V. All rights reserved.