Detrimental Sulfate Effects on Formation of Al-13 Tridecameric Polycation in Synthetic Soil Solutions
1995
Kerven, G. L. | Larsen, P. L. | Blamey, F. P. C.
The chemistry of Al in soil solutions and in surface waters is complex and has included evidence of the tridecameric polycation, known as Al₁₃, in an acid forest soil. The Al₁₃ polycation is the dominant species formed when Al solutions are partially neutralized under controlled laboratory conditions, and solution culture studies have shown Al₁₃ to be highly toxic to root growth. Doubt has been raised, however, as to the occurrence of the Al₁₃ 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 Al₁₃ polycation using both ²⁷Al 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 Al₁₃ polycation. However, SO₄ at ≥50 µM 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 Al₁₃ polycation. The addition of similar concentrations of SO₄ after neutralization had no effect on the measured Al₁₃ concentration. The presence of ≥250 µM SO₄ resulted in no detectable Al₁₃ 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 Al₁₃ polycation is unlikely to be formed in measurable quantities.
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