Investigation of the speciation of aluminium in drinking waters by sorption on a strong anionic-exchange resin AG1X8

The sorption properties of the strong anion exchange resin AG1X8 towards aluminium(III) were investigated in view of the study of the speciation of this metal ion in drinking waters. This was done by determining the side reaction coeffcients of the metal ion in the sample solution in the presence of the resin. It was found that aluminium(III) is sorbed on AG1X8 by the formation of Al(OH)3 and Al( OH)4-. The exchange coeffcients were determined at different conditions and the intrinsic complexation constants were evaluated on the basis of the Gibbs-Donnan model. From the exchange coeffcients the ratio of metal ion sorbed on the resin to the free metal ion in solution, K* was evaluated. It is a measure of the sorbing properties of a resin towards a metal ion at a given pH. AG1X8 was used for the determination of total aluminium(III) concentration indrinking water samples by the resin titration (RT) method at pH 7.5 and 6.2. In the first case the total concentration obtained by RT was equal to that determined directly, showing that all aluminium is either quantitatively sorbed or in equilibrium with AG1X8. The side reaction coeffcient, a M(I), was evaluated to be around 10^12 in the considered drinking water samples at this pH. Not any aluminium(III) was sorbed on AG1X8 from the considered drinking water samples at pH = 6.2, suggesting that aluminium is present here as species with a M(I)>10^7. These results were compared with those obtained using Chelex 100, which is able to sorb aluminium(III) also at pH 6.2. More information about the speciation of aluminium can be obtained by comparing the results from two different sorbing resins.