It is important to study the formation conditions and the stability of the tridecameric Al polymer (Keggin-type Al-13 polycation, [AlO4Al12(OH)(24)(H2O)(12)](7+), known as Al-13) due to its strong toxicity to living organisms of a soil environment. In order to examine the pH range where toxic Al-13 can exist in aqueous solution, Al-27 NMR spectra for sample solutions containing Al3+ ions with various pH (pH 3.5-6.1) were measured. The results show that the peak due to Al-13 (peak due to 4-coordinated Al around 63 ppm) appeared at pH 3.6-5.7 and the peak intensity was relatively high at pH 4.1-4.8, suggesting that Al-13 can be formed at pH 3.6-5.7, while it can exist dominantly at pH 4.1-4.8. It was also found that Al-13 can stably adsorb onto a chelate resin, Chelex 100, by weak electrostatic interaction. The Chelex 100, with iminodiacetate groups, served as a model compound for surfaces of microbes covered with carboxyl groups and for surfaces of soil particles covered with humic substances having many carboxyl groups. Additionally, decomposition of Al-13 did not occur even after adsorption, and its pH stability range was wide compared to that in aqueous solution. (C) 2009 Elsevier Inc. All rights reserved.