R. Lookman et al., PHOSPHATE SORPTION BY SYNTHETIC AMORPHOUS ALUMINUM HYDROXIDES - A AL-27 AND P-31 SOLID-STATE MAS NMR-SPECTROSCOPY STUDY, European journal of soil science, 45(1), 1994, pp. 37-44
The sorption of phosphate on amorphous aluminium hydroxides was invest
igated using Al-27 and P-31 solid-state magic-angle spinning nuclear m
agnetic resonance (MAS NMR) spectroscopy, following the effect of diff
erent exposures to soluble phosphate. The spectra obtained were compar
ed with the spectrum of amorphous aluminium phosphate. Aluminium in th
e unreacted hydroxide had a 100% octahedral co-ordination. When dried
at 200-degrees-C and exposed to soluble phosphate, very little (maximu
m 0.1%) amorphous aluminium hydroxide transformed to a tetrahedral co-
ordination (Al bound by oxygen bridges to four P atoms), even after 12
0d. The tetrahedral co-ordination exists in aluminium phosphate gel, a
lthough most of its Al atoms exhibit an octahedral co-ordination. For
the aluminium hydroxide dried at 200-degrees-C, no formation of alumin
ium phosphate in which aluminium is in octahedral co-ordination could
be detected, not even when the aluminium hydroxide was exposed to a ph
osphate solution for 120 d. We concluded that the formation of alumini
um phosphate is restricted to the surface of the hydroxide. Most of th
e phosphate which is bound to the aluminium oxide however may not have
formed a 'bulk solid' aluminium phosphate, but is adsorbed on the int
ernal and external surface of the oxide. The same amorphous aluminium
hydroxide, dried at 70-degrees-C instead of 200-degrees-C, is converte
d much more rapidly to aluminium phosphate when exposed to soluble pho
sphate. We propose a P-induced weathering mechanism to describe P sorp
tion on amorphous aluminium hydroxides at high P concentrations. In ad
dition to NMR, phosphate adsorption experiments conducted on aluminium
hydroxides dried at different temperatures produced evidence that the
porosity of the aluminium hydroxide aggregated particles can also be
a factor controlling the rate of phosphate uptake from solution, if th
e aggregate is stable (is not resuspended) in solution.