DETRIMENTAL SULFATE EFFECTS ON FORMATION OF AL-13 TRIDECAMERIC POLYCATION IN SYNTHETIC SOIL SOLUTIONS

The chemistry of Al in soil solutions and in surface waters is complex and has included evidence of the tridecameric polycation, known as Al-13, in an acid forest soil. The Al-13 polycation is the dominant species formed when Al solutions are partially neutralized under controlled laboratory conditions, and solution culture studies have shown Al-13 to be highly toxic to root growth. Doubt has been raised, however, as to the occurrence of the Al-13 polycation in natural systems. The objective of this study was to examine the effects of solution composition, comparable with soil solutions extracted from acid soils, on the formation of the Al-13 polycation using both Al-27 nuclear magnetic resonance (NMR) and spectrophotometric methods. The presence of Ca, Mg, K, Cl, and NO3 during partial neutralization had no effect on the formation of the Al-13 polycation. However, SO4 at greater than or equal to 50 mu M during partial neutralization to a OH/Al ratio of R = 2.0 precipitated the major proportion of Al and interfered with the formation of the Al-13 polycation. The addition of similar concentrations of SO4 after neutralization had no effect on the measured Al-13 concentration. The presence of greater than or equal to 250 mu M SO4 resulted in no detectable Al-13 polycation present after the solution had been neutralized to R = 2.5. We suggest that in view of the ubiquitous nature of the SO4 anion in soil solutions and surface waters, the Al-13 polycation is unlikely to be formed in measurable quantities.