Microwave-Induced Combustion of Coal for Further Sulfur Determination by Inductively Coupled Plasma Optical Emission Spectrometry or Ion Chromatography

In this work, a method using microwave-induced combustion (MIC) was optimized in order to obtain a fast, simple, efficient and greener sample preparation method for coal digestion and further determination of sulfur by inductively coupled plasma optical emission spectrometry (ICP OES). Accuracy was evaluated by comparison of the results with those obtained using elemental analysis by UV-fluorescence, microwave-assisted acid digestion with determination by ICP OES and by analysis of certified reference materials of coal. Some parameters that influence MIC method, such as the type and concentration of absorbing solution, the necessity of using a reflux step as well as the cooling time were carefully optimized. No statistical difference was observed for combustion without reflux followed by 5 min of cooling, in comparison to the reference values obtained by elemental analysis. Complete digestion was obtained with this fast and simple method (total digestion program required only 6 min) and using a diluted acid solution for quantitative recovery. The feasibility of digests for sulfur determination as sulfate by ion chromatography (IC) with conductivity detection was also demonstrated. The possibility of determining other elements, commonly monitored in coals, is another advantage of this high-efficiency digestion method (MIC) combined with the multielemental capacity of ICP OES or IC instruments.