Upcycling Philippine-generated coal fly ash residue after rare earth element extraction via mechanochemical activation for enhanced methylene blue removal: Kinetic and equilibrium studies

The REE-extracted Philippine-generated coal fly ash (RECFA) residue, obtained after rare earth element (REE) extraction, was upcycled via mechanochemical activation (MA-RECFA) to enhance methylene blue (MB) adsorption in aqueous solutions. Physicochemical characterization, including X-ray Fluorescence (XRF), X-ray Diffraction (XRD), Scanning Electron Microscopy – Energy Dispersive Spectroscopy (SEM-EDS), Fourier Transform Infrared Spectroscopy (FTIR), and Laser Particle Size Analysis (LPSA), confirmed structural and morphological modifications following activation, including partial amorphization, increased surface disorder, and a 36.38% increase in specific surface area. Batch adsorption experiments showed that MA-RECFA achieved a higher maximum adsorption capacity of 24.21 mg g⁻¹ compared to 9.98 mg g⁻¹ for RECFA, representing a 142.6% improvement. Kinetic analysis indicated that the pseudo-second-order model best described the adsorption behavior of both materials, suggesting that adsorption was governed by surface interaction processes and adsorption-site availability. Intraparticle diffusion analysis further revealed a multi-step adsorption mechanism involving both external surface adsorption and internal diffusion. Equilibrium studies showed that the Langmuir model adequately described the adsorption process, indicating predominantly monolayer adsorption, while the Freundlich model suggested increased surface heterogeneity after mechanochemical activation. Overall, the enhanced adsorption performance of MA-RECFA was attributed to the combined effects of structural modification and increased availability of active sites induced by mechanochemical treatment. These results demonstrate the potential of mechanochemical activation as a simple and effective strategy for upcycling coal fly ash residues for dye removal applications.