Quinoline-functionalized graphene oxide for enhanced cadmium removal: synthesis, characterization, and mechanistic insights

Abstract

Cadmium contamination in aquatic systems poses serious environmental and human health risks, driving the need for efficient, selective, and regenerable adsorbents. Herein, graphene oxide (GO) was functionalized with 8-hydroxyquinoline (GQ) and 8-hydroxyquinoline-5-sulfonic acid (GQS) via carbodiimide-mediated coupling to introduce strong chelating N,O- and N,O/S-donor sites for enhanced Cd(II) removal. Comprehensive characterization using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS), Brunauer–Emmett–Teller (BET), dynamic light scattering (DLS), and zeta-potential measurements confirmed successful ligand grafting, surface functionalization, and a controlled reduction in surface area. Batch adsorption experiments revealed optimal Cd(II) uptake at pH 6 and a sorbent dose of 2.0 g L⁻¹, with rapid adsorption kinetics well described by the pseudo-second-order model (R² > 0.998). Equilibrium data fitted the Langmuir and Sips isotherms, yielding maximum adsorption capacities of 39.9, 32.9, and 26.6 mg g⁻¹ for GQ, GQS, and GO, respectively, reflecting the increasing density and accessibility of chelation sites. Thermodynamic analysis (ΔH° < 0, ΔG° < 0) indicated a spontaneous, exothermic adsorption process governed primarily by chelation-driven inner-sphere complexation and interfacial dehydration. The sorbents exhibited excellent regenerability, achieving 93.6% Cd(II) desorption using 1.0 M HCl. Importantly, treatment of a real industrial raffinate resulted in 94.2% Cd(II) removal with minimal co-ion interference, confirming high selectivity under realistic conditions. Overall, this study demonstrates that quinoline-functionalized graphene oxide provides a simple, tunable, and reusable platform for high-performance cadmium remediation in complex aqueous matrices.