PHOSPHATE SORPTION BY SYNTHETIC AMORPHOUS ALUMINUM HYDROXIDES - A AL-27 AND P-31 SOLID-STATE MAS NMR-SPECTROSCOPY STUDY

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.