Study of aluminium speciation in freshwaters by sorption on a chelating resin

concentration in samples of unknown composition after
separation on a chelating resin was applied to the
determination of aluminium(iii) concentration in real
freshwater samples, using Chelex 100 as a sorbing resin.
The method, called chelating resin titration, was
previously tested on synthetic samples, and allows also the
evaluation of the side reaction coefficient [aM(I)] of the
metal ion in the presence of the resin, which is useful for
the study of metal speciation. To obtain this information,
the sorption equilibria of aluminium(iii) on Chelex 100
under the experimental conditions must be exactly known.
These equilibria were investigated according to a
previously reported method, based on the determination
of the sorption curves of aluminium on Chelex 100 from
solutions of known composition, sometimes containing
chelating substances. It was found that the sorption of
aluminium on Chelex 100 takes place by formation of
three species, AlL, AlL(OH) and Al(OH)3. The
corresponding exchange coefficients were determined.
Despite the fact that Chelex 100 sorbs aluminium(iii)
strongly, it was found that the sorption from fresh waters
was not quantitative, which was ascribed to the strong
complexation of aluminium in the samples considered.
The values of aM(I) of aluminium in some freshwater
samples at natural pH and acidified up to pH 3 were
determined. They were found to be much higher than
those ascribable to hydrolysis or to inorganic and organic
ligands present in natural waters, being for instance 1012.1
at pH 7.48 and 107.5 at pH 4.88. A similar determination
carried out in a 1023 mol kg21 sodium chloride solution
containing a known amount of aluminium at pH Å 8 gave
log aM(I) = 14, completely ascribable to hydrolysis,
showing that an accurate evaluation can be obtained by
the proposed method.