Size and charge fractionation of aqueous aluminium in dilute acidic waters: effects of changes in pH and temperature

Abstract

In natural waters, aluminium occurs in different physico-chemical forms, depending on pH, temperature and the presence of inorganic and organic ligands. Conventional methods for fractionation of Al species do not fully succeed in separating monomeric, i.e., low relative molecular mass (M_r) Al species, from polymeric colloidal Al species. In the present work, hollow-fibre ultrafiltration and acid digestion steps are introduced prior to the ordinary Barnes–Driscoll procedure. By this method, a colloidal Al fraction is separated from the particulate fraction, i.e., the fraction that is able to precipitate by the force of gravity. Both the monomeric and colloidal Al fractions are characterized according to M_r and the chemical properties such as hydroxyquinoline–isobutyl methyl ketone-extractability and Amberlite IR-120 cation exchangeability. Changes in pH and/or temperature were found to be critical for the analytical results of Al. By combining size and charge fractionation techniques, increased information concerning Al species in dilute freshwaters was obtained. One important investigation is the presence of high M_r Al species in the operationally defined monomeric Al fractions.