Adjusting the Structural Properties of Fly Ash through Systematic Modifications for Enhanced Methylene Blue Removal:...

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 (q_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.