Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Okada, K. Articles by MacKenzie, K. J. D. Search for Related Content GeoRef GeoRef Citation

REACTION OF PHOSPHATE COMPOUNDS WITH A HIGH-SILICA ALLOPHANE

¹ Department of Metallurgy and Ceramics Science, Graduate School of Science and Engineering, Tokyo Institute of Technology, O-okayama, Meguro, Tokyo 152-8552, Japan
² School of Chemical and Physical Sciences, Victoria University of Wellington, P.O. Box 600 Wellington, New Zealand

^* E-mail address of corresponding author: kokada{at}ceram.titech.ac.jp

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 (G) 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 phosphate-loaded 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 Q⁰ 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.

Key Words: Allophane • Phosphate-loading • Sorption Properties • Structure Change