Aluminium speciation in natural water by sorption on a complexing resin.

Very stable aluminium complexes may be present in natural waters, which can be detected only using appropriate methods. One of them is the resin titration based on the sorption of aluminium on a strongly sorbing resin, Chelex 100. It was here used to detect strong aluminium complexes, and to characterize them by determining their concentration, and the corresponding stability constant. High and low salinity waters were sampled in different sites in the North of Italy. In all the samples aluminium complexes with high stability constant, up to 10^17.4 (M-1) in the less acidic solution, were detected. The stability constant depends mainly on the solution acidity, increasing with increasing pH up to 7. The concentration of the ligands responsible for the strong complexation is similar to that of aluminium (from 0.5 to 1.5 microM), or somewhat lower in the case of estuarine and sea waters. A small fraction of aluminium (from 0% to 2%) in freshwaters, higher in estuarine and sea waters (14% and 10%, respectively), is present in weakly bound forms which could also be the hydrolysis products. The conditional constants of the strong complexes were determined for the different samples examined. They were found to be slightly lower in the case of the high salinity waters, in which a value of 10^16.1 (M-1) at pH 7.5 was obtained. This is probably due to the higher ionic strength in marine water, which strongly influences the complexation of trivalent metal ions, as seen for example also in the hydrolysis. It could be deduced that similar substances, but at different concentration, would be responsible for the aluminium complexation in the sea and freshwaters here examined. They could be natural organics like fulvic substances, or better some particular complexing sites in this substances with very high affinity for aluminium.