THE FORMATION OF POLYNUCLEAR AL13 UNDER SIMULATED NATURAL CONDITIONS

Polynuclear aluminum species can be formed in nature by the dissolution of Al-containing minerals, as a consequence of soil and surface water acidification, followed by neutralization processes. Under simulated natural conditions in the laboratory, i.e., by allowing acidic Al(III) solutions to flow over granulated marble at flow velocities of 3-4 mm/min, large fractions of monomeric Al(III) were transformed to the Al13O4(OH)24(H2O)127+ polymer (thereafter referred to as Al13 or Al-13(7+)), which was measured by Al-27-NMR spectroscopy. Over two-thirds of the monomeric Al(III) was converted to polynuclear Al13 even when gibbsite particles were mixed with the marble granules. The Al13 polymer was also formed in stirred batch experiments containing marble granules and dissolved phthalate or salicylate. The results suggest that Al13 formation is easily possible in nature if the total AI(III) concentration is high enough. In these experiments, Al13 was detected when the total dissolved AI(III) was 1.9 X 10(-4) mol/L. The minimum total Al(III) concentration required to produce Al13, which can be predicted from available thermodynamic data, is on the order of 10(-5) mol/L.