Reaction of Phosphate Compounds with a High-Silica Allophane

The loading of various phosphates on the surfaces of nanoparticles of allophane (1–2SiO2·Al2O3·5–6H2O) was investigated. The allophane used was a high-silica type with a Si/Al ratio of 0.85. The phosphate-sorption isotherm was measured using (NH4)2HPO4 solution, which showed the highest phosphate sorption of the seven phosphates examined. This sorption isotherm was in better agreement with the Langmuir equation than the Freundlich equation. The resulting maximum sorption capacity was 4.87 mmol/g and the Langmuir constant was 0.0033 L/mmol. The sorption energy (ΔG) calculated from the Langmuir constant was −2.96 kJ/mol. The amount of loaded phosphate varied greatly according to the phosphate used, being greater for orthophosphates than for polyphosphates. The amount of loaded phosphate also depended on the cation present, in the order Ca-Na-NH4-phosphate. The Si/Al ratios of the samples were decreased by orthophosphate loading due to the partial replacement of SiO4 by PO4 tetrahedra, but this effect was offset by the partial dissolution of the allophane by polyphosphate loading.The 29Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectra of all the phosphateloaded samples showed an increase of a peak at −90 ppm (the Q1–Q3 polymerized tetrahedral unit) and the decrease of a peak at −78 ppm peak (the Qo monomeric tetrahedral unit). The 31P MAS NMR spectra showed peaks at ~−10 ppm, assigned to Q2 units corresponding to polymerized tetrahedra which consisted of loaded PO4 together with Si(Al)O4. The structure changes produced in allophane by phosphate loading are discussed in light of these data.