Coal ash amorphized via microwave route as a potential adsorbent towards pollutant removal in water

Coal ash was amorphized using a novel, rapid, and simple method, microwave-assisted alkaline fusion, at 1000 W for 3 min, employing solid NaOH as the amorphizing agent. The microwave-amorphized ash (MWAA) was used as an adsorbent to remove crystal violet (CV) dye from an aqueous medium. MWAA was prepared and characterized, and the optimal adsorption conditions were investigated using a central composite rotatable design (CCRD), evaluating the effects of pH and adsorbent dosage. A significant improvement in the pore properties of the amorphized material was observed compared to the raw sample, including an increase in surface area (from 1.23 to 66.28 m² g⁻¹) and pore volume (from 0.0045 to 0.1934 cm³ g⁻¹). The optimal adsorption conditions were pH 9 and an adsorbent dosage of 0.7 g L⁻¹. The maximum adsorption capacity was 142 mg g⁻¹. CV adsorption was found to be spontaneous, exothermic, and favorable. The amorphized coal ash showed good performance after four cycles of reuse. Thus, the amorphization route proposed in this work produces a promising adsorbent from bottom coal ash for wastewater treatment.