Novel coal fly ash–chitosan composite for highly efficient, cost-effective and stable removal of lead and chromium from industrial wastewater†

In the present study, a novel and economical adsorbent was synthesized from a coal fly ash–chitosan composite to remove Pb²⁺ and Cr⁶⁺ from aqueous solutions. The characterization of the adsorbent under optimal conditions revealed that it was mesoporous and rich in different functional groups, which enhanced its adsorption properties. The optimal conditions for the adsorption process were achieved at three levels. At the first level, the optimal conditions for fly ash calcination (300 °C for 2 h), H₃PO₄ concentration (0.4 mol L⁻¹), MFA–CS ratio (3 : 1), and effective morphology (nanopowder) for Pb²⁺ and Cr⁶⁺ removal were achieved. At the second level, response surface methodology achieved adsorption capacities of 339.27 mg g⁻¹ for Pb²⁺ removal and 242.84 mg g⁻¹ for Cr⁶⁺ removal under optimal conditions. The third level involved pH standardization, which further enhanced the adsorption capacities to 352.19 mg g⁻¹ for Pb²⁺ removal and 265.13 mg g⁻¹ for Cr⁶⁺ removal. These results were well fitted by the pseudo-second-order kinetic and Langmuir isotherm models, demonstrating that the adsorption progressed via monolayer chemisorption. Removal efficiencies of 86.78% and 67.09% were obtained for Pb²⁺ and Cr⁶⁺, respectively, during their simultaneous removal. Thermodynamic studies confirmed the spontaneity of the adsorption process. The adsorbent demonstrated reusability, retaining its performance over 15 regeneration cycles. In column studies, maximum adsorption capacities of 255.61 mg g⁻¹ for Pb²⁺ and 42.08 mg g⁻¹ for Cr⁶⁺ were achieved, described well by the Thomas model. This cost-effective adsorbent, driven by ion exchange and surface complexation mechanisms, holds significant promise for wastewater treatment.