A rapid microwave-assisted acid extraction method based on the use of diluted HNO₃-H₂O₂ followed by ICP-MS analysis for simultaneous determination of trace elements in coal samples

Abstract

Before coal processing such as pyrolysis, liquefaction, gasification and combustion, it is very crucial to monitor the trace element concentration levels as that determines the coal quality. Therefore, the current study describes the development of microwave-assisted acid extraction (MW-AAE) method for extraction of 15 trace elements in coal samples prior to their determination using inductively coupled plasma-mass spectrometry. Diluted HNO₃-H₂O₂ was used in order to reduce reagents amount used, eliminate matrix interferences caused by concentrated acids and to decrease waste produced in analytical laboratories. The optimisation of the proposed extraction method was carried out by using a full factorial design (2⁴) involving four factors; that is, temperature, extraction time, HNO₃ and H₂O₂ concentrations. The optimum conditions for the MW-AAE procedure were found to be 200°C, 5 min, 5 mol L⁻¹ and 2 mol L⁻¹ for temperature, extraction time, HNO₃ and H₂O₂ concentrations, respectively. Under optimum conditions, the accuracy of the MW-AAE method was examined by analysing three coal certified reference materials (SARM 18, 19 and 20) and recoveries of 80–115% were achieved for V, Mn, Co, Ni, Cu, Zn, Ga, Ge, As, Sr, Zr, Cs, Ba, Pb and U, except for Ti (10–25%) and Hf (27–70%). In addition, the precision of the proposed method, expressed in terms of relative standard deviation (SD) (n = 15), was within the accepted range (≤3.5%). The method detection limits of 0.001–0.57 µg g⁻¹ for all trace elements under the investigation were similar to the literature reported work, except for Ti (4.00 µg g⁻¹).

Keywords:

• microwave-assisted acid extraction
• trace elements
• diluted HNO₃-H₂O₂
• inductively coupled plasma-mass spectrometry

Acknowledgements

The authors would like to acknowledge South African National Research Foundation (NRF) Scarce Skills for financial support and the University of Johannesburg, Auckland Park Campus (Analytical Facility and Chemistry department), for providing ICP-MS and some other laboratory facilities.