Adjusting the Structural Properties of Fly Ash through Systematic Modifications for Enhanced Methylene Blue Removal: An Experimental and Computational Investigation

Coal fly ash (FA) is generated in vast quantities, yet remains underutilized. Here, we systematically tuned its surface chemistry and porosity through sequential water washing, HCl digestion (2, 4, and 6 M), and calcination (500, 700, and 900 °C). Water washing followed by acid treatments at higher molarities (4, 6 M) and subsequent calcination at 700 °C proved most effective. The optimized sample (PFA-6M-C700) exhibited a Langmuir maximum adsorption capacity (<i>q</i>_max) of 11.0 mg/g for methylene blue (MB). Furthermore, under identical conditions, PFA-6M-C700 achieved an equilibrium uptake approximately nine times higher than that of as-received FA. This enhancement was attributed to selective leaching of surface carbon, surface enrichment of siliceous phases, removal of Cl-containing residuals, a marked increase in surface area and pore size, and a decreased point of zero charge (PZC) upon modification. In contrast, 2 M HCl was insufficient to enhance porosity and calcination at 900 °C induced sintering. Adsorption followed pseudo-second-order kinetics for all modified samples and fitted the Langmuir model for PFA-6M-C700, which retained 82% of its initial capacity after eight successive uses. Density functional theory calculations revealed that efficient MB binding requires SiO₂ surfaces having a balanced ensemble of lattice oxygen, surface -OH groups, and exposed Al centers, a configuration confirmed in the high-performing adsorbents according to characterization results. Beyond highlighting FA as an adsorbent, this work establishes a systematic modification strategy that can be readily transferred to other fields such as catalysis, construction materials, and additional high-value technologies, opening new economic and environmental opportunities.