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Aluminum Speciation in Aluminum-Silica Solutions and Potassium Chloride Extracts of Acidic Soils

Div. of Plant Ecology, National Inst. of Agro-Environmental Sciences, Ministry of Agriculture, Forestry and Fisheries, 3-1-1 Kan-nondai, Tsukuba, Ibaraki 305, Japan

Satoru Taniguchi and Katsutoshi Sakurai

Faculty of Agriculture, Kochi Univ., Nankoku, Kochi 783, Japan

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

ABSTRACT

To differentiate chemical species of Al in soil solutions, a method using nuclear magnetic resonance (NMR) was applied to laboratory preparations of hydroxyaluminum (HyA) and hydroxyaluminosilicate (HAS) ions ([OH]/[Al] = 2.0), and 1 M KCl extracts from acidic soils. The Al in a Al-OH-Si(OH)₄ mixture was separated into three fractions: Al₁₃ ([AlO₄Al₁₂(OH)₂₄(H₂O)₁₂]⁷⁺), Al_SYM, and Al_NON. The Al₁₃ and Al_SYM were quantitatively determined by ²⁷Al-NMR at 63 and 0 ppm, respectively. The Al_NON was defined as the Al that cannot be determined by ²⁷Al-NMR. The Al_SYM was attributed to electrically symmetric octahedral Al including monomer (and dimer) HyA ions, and the Al_NON was attributed to electrically asymmetric Al including HAS and polymer HyA ions. The concentrations of Al₁₃ decreased with increasing Si/Al molar ratio of the mixed solutions, and Al₁₃ was not detected at Si/Al 0.28. With increasing Si/Al molar ratio, the concentration of Al_SYM increased slightly, while Al_NON increased significantly. In the KCl extracts from soils, the Al₁₃ was not detected at all. This result suggests that Al₁₃ is not formed in these soils, or that the persistence of Al₁₃ in the soil solution is strongly inhibited by adsorption or precipitation reactions. The presence of organically complexed Al (Al_ORG) was also directly detected in KCl extracts from upper horizons by ²⁷Al-NMR spectra. The chemical shift of resonance peak of Al_ORG was close to that of the 1:1 complex of Al oxalate. On average, 92 and 96% of the KCl-exchangeable Al existed as Al_SYM and Al_SYM plus Al_ORG, respectively.

Received for publication January 29, 1997.

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