Chemical species of Al reacting with soil humic acids

Abstract

The formation and presence of aluminum tridecamer (Al 13, [AlO 4Al 12(OH) 24(H 2O) 12] 7+) in solution was strongly inhibited by soil humic acids (HA), as shown by liquid-state 27Al-nuclear magnetic resonance (NMR) analyses. It was also observed that Al 13 coprecipitated with HA in the form of Al 13–HA complexes, as shown by solid-state magic angle spinning (MAS) 27Al-NMR analyses. Tetrahedral Al in the Al 13–HA complexes was gradually converted to octahedral Al, indicating that Al 13 in Al 13–HA complexes was decomposed into Al–HA complexes with reaction time. When HA was present in the acidic Al solution before the partial neutralization, the precipitates formed after the partial neutralization did not contain any tetrahedral Al, indicating the absence of Al 13 and that HA is a strong inhibitor of Al 13 formation. These results indicate that Al 13 is not formed from Al complexed with soil HA after partial neutralization. If Al 13 was added to soils, it would preferentially form precipitates of non-phytotoxic Al 13–HA complex in the presence of excess HA (i.e. a COOH/Al 13 molar ratio of more than 9) and gradually be converted to an octahedral Al–HA complex. Therefore, Al 13 is not a major plant-growth inhibitor in humus-rich acidic soils.