Waste-derived nano-Al₂O₃-loaded pyranopyrazole composite for high-capacity cadmium and methylene blue removal with mechanistic and DFT validation

Abstract

Upcycling industrial aluminum waste into functional adsorbent materials offers a sustainable strategy for addressing complex wastewater contamination by coexisting organic dyes and toxic heavy metals. In this study, a waste-derived nano-Al₂O₃-loaded pyranopyrazole composite is rationally designed to enable high-capacity and simultaneous removal of methylene blue and Cd(II) ions from aqueous systems. The heteroatom-rich pyranopyrazole framework provides multiple organic binding domains, while nano-alumina incorporation introduces additional Al-O active sites and surface hydroxyl groups that collectively enhance interfacial adsorption efficiency. Comprehensive structural, spectroscopic, and microscopic characterization confirms homogeneous alumina anchoring and the formation of a stable organic-inorganic hybrid interface. The resulting composite exhibits markedly improved adsorption performance relative to the pristine pyranopyrazole, achieving maximum experimental uptake capacities of 189 mg g⁻¹ for methylene blue and 343 mg g⁻¹ for Cd(II) under optimized conditions. Adsorption kinetics follow a pseudo-second-order model, indicating surface-controlled chemisorption, while equilibrium data are best described by the Freundlich isotherm, reflecting adsorption on energetically heterogeneous surfaces. Thermodynamic analysis reveals that both adsorption processes are spontaneous and exothermic. Spectroscopic evidence combined with density functional theory calculations demonstrates that adsorption proceeds through synergistic coordination, electrostatic attraction, hydrogen bonding, and π-interactions involving nitrogen- and oxygen-donor sites of the organic framework together with alumina-derived Al-O centers. The composite maintains high removal efficiency over repeated adsorption-desorption cycles and exhibits effective performance in real wastewater matrices. Overall, this work establishes a clear structure-interaction-performance relationship for waste-derived alumina-organic hybrids and highlights their potential as sustainable, reusable, and multifunctional adsorbents for advanced water remediation targeting both organic dyes and heavy-metal contaminants.