engleski

MICROWAVE AND ULTRASOUND DIGESTION OF COAL ASH

Determination of trace elements in coal ash samples has always been an interesting area of environmental research. A good number of instrumental techniques, atomic absorption spectrometry (AAS), inductively coupled plasma atomic emission spectrometry, X ray fluorescence, and neutron activation analysis have been proposed for multi-elemental analysis of coal ash. The most analytical techniques normally require that coal ashes sample be dissolved in an acidic solution through decomposition of the organic and mineral matrices. Broadly, three dissolution/digestion approaches are categorized as: (1) the open wet ashing ; (2) the dry ashing dissolution ; (3) the closed vessel dissolution assisted by microwave irradiation. Compared to the former two methods, microwave-assisted digestion (MAD) provides a rapid and efficient method, and it also possesses an advantage of reducing volatilization loss and contamination. Another useful sample preparation method for solid samples is ultrasound-assisted digestion (UAD). The main benefits of ultrasound-assisted sample pre-treatment over other methods are the speed of digestion, high sample treatment capacity and low reagent usage. UAD is safer than MAD, because pressure and temperature are substantially lower in ultrasound technique. However, it is only few methodologies developed, applied and validated for real-world application in the determination of the elemental composition of coal ashes. This paper reports the development and optimization of a MAD and UAD procedure for digestion of coal ash samples followed by AAS for the determination of 8 elements (Si, Al, Fe, Na, K, Ca, Mg, Mn). The validity of determination was assessed by using two certified reference materials (GSJ Geochemical Reference Samples) which were also treated in the same way. Several critical parameters of the MAD and UAD procedure (digestion time and temperature, volume of solvent, power of microwave oven and ultrasound bath, single- or two-step methods) were studied for the optimization of the digestion protocol. Especially consideration was on the necessity of adding HF in the acid mixture for both digestion techniques, because coal ash contains an appreciable amount of inorganic matters, which mainly consists of aluminosilicate matrix. In general, results indicating that both digestion methods give almost equal element’s concentrations.

microwave digestion; ultrasound digestion; coal ash

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