ON THE AVERAGE EQUILIBRIUM OH/Al MOLAR RATIO FOR ALUMINUM ADSORBED BY A SYNTHETIC CATION EXCHANGER

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Abstract
Published data concerning the average OH/Al molar ratio (basicity) of Al adsorbed by smectite and by a sulfonic resin were reviewed and found to be in disagreement. The data available for the sulfonic resin were considered further and judged to be ambiguous because of the unaccounted-for possibility of Al hydrolysis during the preparation of Al resin. In order to resolve the conflict among the published data, the basicity of Al adsorbed by an Na resin from solutions of AlCl3 and by a Ca resin from suspensions of acidified, x-ray amorphous Al(OH)3 was investigated. These experiments were not subject to the possible hydrolysis effect that complicated the interpretation of the earlier published data on sulfonic resin. The data presented in this paper show that the average basicity of hydroxy-Al adsorbed by Na resin from a solution of AlCl3 increases as the solution concentration decreases and is always greater than the average basicity of Al in a resin-free solution of the same total Al concentration. This effect would be expected, in particular, when an Al resin is leached to remove Cl- after treatment with aqueous AlCl3. The average basicity of Al adsorbed by Ca resin in the presence of acidified Al(OH)3 was found to be 2.50 ± .06 in the measured pH range of 4.5–5.4, independent of the relative composition of the Ca resin-Al(OH)3 system. The maximum basicity observed in these model experiments (OH/Al ≅ 2.5) agreed with the many available data on the average basicity of hydroxy-Al adsorbed by smectite but disagreed with the existing published data for Al resin which are systematically lower (OH/Al ≅ 2). The reason for the discrepancy is believed to lie with the presence of hydroxy-Al, which was adsorbed on the Al resin during its preparation, and which, if not taken into account, would result in a low estimate of the OH/Al molar ratio.

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