Si-29, Al-27 and Na-23 solid-state nuclear magnetic resonance studies of combustion-generated ash

Combustion-generated ash was synthesized, heated and analyzed by solid-stat e nuclear magnetic resonance spectroscopy with magic angle spinning (NMR-MA S). The structural information obtained from this study indicates that the ash contains different magnetic resonance environments for Al, Si and Na af ter combustion. The final ash material contains a major amorphous aluminosi licate glass phase with mullite and corundum crystals. Si-29 NMR spectra sh owed a single peak at - 110.5 ppm (delta) for the initial powder sample due to SiO2. When the sample was heated to 1500 degreesC the peak shifts to lo wer field (delta = -99 ppm) due to the formation of aluminosilicate amorpho us glass containing alkali or alkaline earth metals. The Al-27 NMR spectra of the initial sample gives a single peak at 10.1 ppm due to the AlO6 (octa hedral group) present in the Al2O3. When the sample is heated to 1500 degre esC, a new peak appeared at 50 ppm due to the formation of AlO4 tetrahedral represent the mullite and aluminosilicate compounds. The Na-23 NMR spectra of both the initial and the heated samples (at 1500 degreesC) give rise to a single peak at -14.3 and -28.7 ppm, respectively. The sharp peak at -14. 3 ppm is contributed from the crystalline Na2O and the broad peak around -2 8.7 ppm is due to the Na+ present to compensate SiO- present in the amorpho us aluminosilicate glass material. The presence of low amounts of heavy met als Cr (2800 ppm), Cd (2700 ppm) and Pb (2800 ppm) in the ash generally doe s not show any influence on the Al-27, Si-29 and Na-23 solid-state NMR spec tra. On the other hand, the ash sample containing higher amounts of heavy m etals (Cd = 14210 ppm, Cr = 13120 ppm and Pb = 13140 ppm) showed significan t effect on the Al-27 NMR spectra. The replacement of Al3+ from its octahed ral sites by Cr3+ leads to the decrease in the intensity of the AlO6 octahe dral peak. The presence of Cr3+, Cd2+ and Ph2+ in the aluminosilicate matri x exerts a deshielding effect on Si-29 resonance and thus shifts delta to l ow-field. The results clearly indicate an interaction between the heavy met als and the ash matrix compounds. (C) 2001 Elsevier Science Ltd. All rights reserved.