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

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 (R ² > 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.