Development of a simple and robust microwave-assisted decomposition method for the determination of rare earth elements in coal fly ash by ICP-OES
A simple and reliable two-step method for the total decomposition of coal fly ash (CFA) materials for the quantitative determination of rare earth elements (REEs) by inductively coupled plasma optical emission spectrometry (ICP-OES) has been developed. In the first step, coal fly ash samples were heated at ∼200 °C on a hotplate for about an hour in the presence of 2 mL of sulfuric acid followed by the addition of 0.5 mL of HF. After completion of the hotplate treatment (HT) process, the sample mixture was further subjected to microwave-assisted decomposition (MWD) after adding 8 mL of 20% HCl (v/v) for its total decomposition. After suitable dilution, the final sample digests were analysed for REEs by ICP-OES (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y and Sc). Critical experimental parameters related to the first step of hotplate decomposition and the second step of MWD processes were optimized for achieving quantitative recovery of REEs (>95%) through the analysis of a standard reference material (SRM) CFA NIST-1633b. The optimized two-step approach was subsequently applied to two Indian coal fly ash samples collected from different thermal power stations. These investigations revealed that the proposed two-step method in conjunction with ICP-OES can be utilized for the quantitative determination of REEs in CFA materials on a regular basis.