Aluminum speciation in aluminum-silica solutions and potassium chloride extracts of acidic soils

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.