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Division S-2—Soil Chemistry

Disappearance of Aluminum Tridecamer from Hydroxyaluminum Solution in the Presence of Humic Acid

^a National Institute for Agro-Environmental Sciences, 3-1-3 Kan-nondai, Tsukuba, Ibaraki 305-8604, Japan
^b Graduate School of Agriculture and Life Science, The Univ. of Tokyo, Yayoi, Bunkyo, Tokyo 113-8657, Japan

^* Corresponding author (hiradate{at}affrc.go.jp)

We investigated the influences of humic acid on the removal of Al tridecamer (Al₁₃) from a hydroxyaluminum (HyA) solution at various humic acid/Al ratios. The Al species contained in the solution were analyzed by using a liquid-state ²⁷Al-NMR and an atomic absorption spectrometer and fractionated into three Al species: (i) Al₁₃, (ii) Al monomer and dimer (Al_SYM), and (iii) other undefined species including aggregated/precipitated Al (Al_NON). By the addition of humic acid to the HyA solution, the concentration of Al₁₃ was rapidly decreased within 0.007 d (10 min). The decrease in Al₁₃ and the increase in Al_NON were more pronounced at a higher humic acid/Al ratio. When the molar ratio of humic acid carboxylic groups to Al exceeded 0.8, Al₁₃ was undetected from solution within 0.007 d. The formation of Al₁₃–humic acid complexes and the aggregation/precipitation of those complexes were a predominant mechanism in removing aqueous Al₁₃ at the early stage of the reaction. Approximately 10 mol of carboxylic groups in humic acid were required to remove 1 mol of Al₁₃ from the HyA solution. Aqueous Al₁₃ had greater preference in precipitating with humic acid than Al_SYM. After 5 to 570 d of aging, the concentration of Al₁₃ and Al_NON also decreased and increased, respectively, both in the presence and absence of humic acid. In conclusion, aqueous Al₁₃ would not exist in soil solution under a high humic acid condition.

Abbreviations: Al₁₃, aluminum tridecamer • Al_NON, undefined species other than Al₁₃ or Al_SYM including aggregated or precipitated Al that cannot be detected by NMR • Al_SYM, Al monomer and dimer • HyA, hydroxyaluminum • NMR, nuclear magnetic resonance

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