CONSIDERATION ON THE OCCURRENCE OF THE AL13 POLYCATION IN NATURAL SOIL SOLUTIONS AND SURFACE WATERS

Abstract

Equilibrium speciation calculations were performed (1) for soil solutions and streamwaters collected in central and eastern France and (2) for simulated waters at 0 and 25°C, to assess the highest concentration of Al13 that could be reached in waters in the absence of complexing ligands other than OH−. A comprehensive and updated set of aqueous Al species, including polymeric hydroxyaluminosilicates (HAS), and their corresponding thermodynamic formation constants, were used. Results suggest that the concentration of the Al13 polycation in natural waters has been largely overestimated in some past studies using equilibrium models to calculate Al speciation, owing to oversimplification (many Al ligands not considered) and the unrecognised temperature dependence of some formation constants. The Al13 concentration in mildly acidic natural waters may not exceed a few μmol l−1 at AlT on the order of 10−4 mol l−1 and should be less than 1 μmol l−1 at AlT=10−5 mol l−1. Monomeric Al–Si species may not significantly interfere with the formation of Al13, but the formation of both HAS polymers (proto-imogolite precursors) and organo-Al complexes have a marked detrimental effect on the Al13 concentration. The maximum concentration of Al13 decreased upon increasing temperature from 0 to 25°C. In contrast, the pH range wherein Al13 may occur increases slightly with temperature and the most acidic pH value above which Al13 may be formed has been underestimated. At T=25°C, the Al13 polycation may be a significant Al species (4 to 5% of AlT) at pH < 4.5 if AlT > 10−4 mol l−1. The results of this study and the use of HAS polymers to calculate Al speciation in moderately natural acidic soil solutions were in better accordance with soil mineralogy. This research suggests strongly that Al13 should be negligible in natural soil and surface waters and may not control either Al3+ activity or Al-trihydroxide formation through polymerisation/depolymerisation steps. Also, from a biological point of view, the toxicity of Al13 to plants and aquatic organisms in natural conditions may be considered to be very low.