CHARACTERIZATION OF AL(III) SPECIES IN BASIC ALUMINUM-CHLORIDE FLOCCULANTS BY MEANS OF FERRON METHOD AND AL-27 NUCLEAR-MAGNETIC-RESONANCE

Flocculant processes as a treatment step in water and wastewater purification technology are of increasing importance. Partially neutralized aqueous aluminium chloride solutions - the basic aluminium chlorides - are often used as flocculants in water conditioning. The present paper describes the reactions which appear in these solutions by their dilution, the identification of occurring cationic aluminium species, and the relations between the composition of the solutions and their efficiency as flocculants. The solutions were quantitatively analyzed using Al-27 NMR and the ferron method; the latter method offers a simple and inexpensive alternative for identification and quantification of aluminium cations and can facilitate investigations of the Al speciation at concentrations too low for analysis by NMR. The distribution of aluminium cations in basic aluminium chloride solutions changes drastically by the dilution while applied as flocculants because the equilibrium strongly depends on the concentration. The dynamic changes following the dilution of partially neutralized solutions were investigated simply by mixing a solution with water and immediate analysis by the ferron method. It could be shown that rearrangement reactions occur in the system, partially overlapping each other; the oligomeric cations seem to be especially instable. Furthermore, a partial change from octahedral to tetrahedral coordination of the aluminium in the species can be observed. At extremely low aluminium concentrations, as in the case of application of the basic aluminium chloride solutions for flocculation, monomeric and especially transition polymeric and polymeric aluminium cations, respectively appear. The ratio of these cations to each other also depends on the time up to the flocculation. Accordingly, these cations especially the different polymeric aluminium species seem to be important for the efficiency of the basic aluminium chlorides as flocculants in water conditioning.