ALUMINUM SPECIATION IN ALUMINUM-SILICA SOLUTIONS AND POTASSIUM-CHLORIDE EXTRACTS OF ACIDIC SOILS

To differentiate chemical species of Al in soil solutions, a method us ing nuclear magnetic resonance (NMR) was applied to laboratory prepara tions 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: Al-13([AlO 4Al12(OH)(24)(H2O)(12)](7+)), Al-SYM, and Al-NON. The Al-13 and Al-SYM were quantitatively determined by Al-27-NMR at 63 and 0 ppm, respecti vely. The Al-NON was defined as the Al that cannot be determined by Al -27-NMR. The Al-SYM was attributed to electrically symmetric octahedra l Al including monomer land dimer) HyA ions, and the Al-NON was attrib uted to electrically asymmetric Al including HAS and polymer HyA ions. The concentrations of Al-13 decreased with increasing Si/Al molar rat io of the mixed solutions, and Al-13 was not detected at Si/Al greater than or equal to 0.28. With increasing Si/Al molar ratio, the concent ration of Al-SYM increased slightly, while Al-NON increased significan tly. In the KCL extracts from soils, the Al-13 was not detected at all . This result suggests that Al-13 is not formed in these soils, or tha t the persistence of Al-13 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 Al-27-NMR spectra. The chemical shift of resonance p eak of Al-ORG was close to that of the 1:1 complex of AL oxalate. On a verage, 92 and 96% of the KCl-exchangeable Al existed as Al-SYM and Al -SYM plus Al-ORG respectively.