Microwave saturation of EPR spectra of oxidised coal

Abstract

Microwave saturation of multi-component EPR spectra of oxidized lignite Mequinenza (Spain) with a carbon content of 65.1 wt % and with a high sulphur content of 10.3 wt % was studied. The coal was oxidized with nitric acid (NHO3), peroxyacetic acid (PAA), and in O2/Na2CO3 system. Three different groups of paramagnetic centres exist in the coal samples analyzed. The EPR spectrum of the demineralised coal was a superposition of broad Gauss (δBpp = 0.75 mT), broad Lorentz 1 (δBpp = 0.42 mT) and narrow Lorentz 3 lines (δBpp = 0.08 mT). The three EPR components with linewidths: 0.58-0.77 mT (Gauss line), 0.30-0.39 mT (Lorentz 1 line) and 0.05-0.06 mT (Lorentz 3 line) were recorded for the oxidized coal. The g-values were obtained for the samples studied in the ranges 2.0043-2.0046 (Gauss lines), 2.0035-2.0038 (Lorentz 1 lines) and 2.0032-2.0034 (Lorentz 3 lines). The broad Gauss and Lorentz 1 lines saturate at low microwave powers. The narrow Lorentz 3 lines of demineralised coal were not saturated at microwave power from the range considered. After the coal oxidation with HNO3, PAA and in O2/Na2CO3 system, the microwave saturation of the narrow Lorentz 3 lines was also observed, which indicated a degradation of the multi-ring aromatic structures upon oxidation. © Versita Warsaw and Springer-Verlag Berlin Heidelberg 2006.