Consideration on the occurrence of the Al-13 polycation in natural soil solutions and surface waters

Equilibrium speciation calculations were performed (1) for soil solutions a nd streamwaters collected in central and eastern France and (2) for simulat ed waters at 0 and 25 degreesC, to assess the highest concentration of Al-1 3 that could be reached in waters in the absence of complexing ligands othe r than OH-. A comprehensive and updated set of aqueous Al species, includin g polymeric hydroxyaluminosilicates (HAS), and their corresponding thermody namic formation constants, were used. Results suggest that the concentratio n of the Al-13 polycation in natural waters has been largely overestimated in some past studies using equilibrium models to calculate Al speciation, o wing to oversimplification (many Al ligands not considered) and the unrecog nised temperature dependence of some formation constants. The Al-13 concent ration in mildly acidic natural waters may not exceed a few pmol l(-1) at A l-T on the order of 10(-4) mol l(-1) and should be less than 1 mu mol l(-1) at Al-T = 10(-5) mol l(-1). Monomeric Al-Si species may not significantly interfere with the formation of All.?, but the formation of both HAS polyme rs (proto-imogolite precursors) and organo-Al complexes have a marked detri mental effect on the Al-13 concentration. The maximum concentration of Al-1 3 decreased upon increasing temperature from 0 to 25 degreesC. In contrast, the pH range wherein Al-13 may occur increases slightly with temperature a nd the most acidic pH value above which Al-13 may be formed has been undere stimated. At T=25 degreesC, the Al-13 polycation may be: a significant Al s pecies (4 to 5% of Al-T) at pH < 4.5 if Al-T > 10-4 mol l(-1). The results of this study and the use of HAS polymers to calculate Al speciation in mod erately natural acidic soil solutions were in better accordance with soil m ineralogy. This research suggests strongly that Al-13 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 Al-13 to plants an d aquatic organisms in natural conditions may be considered to be very low. (C) 2001 Elsevier Science Ltd. All rights reserved.