Venkata Balarama Krishna Mullapudi
Coal fly ash (CFA), a by-product generated during coal combustion in thermal power plants, represents a valuable secondary source of critical elements (CEs) including lithium (Li), cobalt (Co), gallium (Ga), germanium (Ge), titanium (Ti), and vanadium (V). In this study, a highly efficient ultrasound-assisted extraction (UAE) procedure comprised three sequential steps for the quantitative recovery of the selected CEs from CFA. In the first step, the mineral phases present in the CFA samples were activated ultrasonically using a mixture of ammonium bifluoride (ABF, NH4HF2) and dilute HNO3. The second step involved the evaporation of the silicon matrix together with excess HF. In the final step, the target elements were extracted through ultrasound-assisted leaching with 3%(v/v) HNO3, followed by their determination using inductively coupled plasma-atomic emission spectrometry (ICP-AES). The major experimental parameters-including the sample-to-ABF ratio, sonication bath temperature, sonication time, and extractant concentration-were systematically optimized to maximize the extraction efficiency of Li, Co, Ga, Ge, Ti, and V. The optimized procedure exhibited excellent analytical performance, with recoveries ranging from 95 to 104%, low detection limits (0.18 to 0.76 μg g-1), and good precision (RSD < 6%). The raw CFA sample and its solid residue obtained after completion of the sample preparation procedure were characterized using X-ray diffraction (XRD) and energy-dispersive spectroscopy (EDS). The accuracy and reliability of the developed method were validated through the analysis of the standard reference material (SRM) NIST CFA-1633b. Compared with conventional methods, the developed UAE approach provides significant benefits, including shorter extraction times, reduced reagent usage, and enhanced sample throughput. The successful application of the method to Indian CFA samples further confirms its applicability for routine analysis and its potential utility in evaluating coal fly ash as a viable resource for critical element recovery.