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

Abstract

AbstractTo 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+), AlSYM, and AlNON. The Al13 and AlSYM were quantitatively determined by 27Al‐NMR at 63 and 0 ppm, respectively. The AlNON was defined as the Al that cannot be determined by 27Al‐NMR. The AlSYM was attributed to electrically symmetric octahedral Al including monomer (and dimer) HyA ions, and the AlNON 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 AlSYM increased slightly, while AlNON 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 (AlORG) was also directly detected in KCl extracts from upper horizons by 27Al‐NMR spectra. The chemical shift of resonance peak of AlORG was close to that of the 1:1 complex of Al oxalate. On average, 92 and 96% of the KCl‐exchangeable Al existed as AlSYM and AlSYM plus AlORG, respectively.