Recoverable CS@UiO-66-NH2@rGO hydrogel beads for efficient capture of polystyrene microplastics with a visible-light-responsive interface

Microplastic pollution has emerged as a growing concern in aquatic environments, highlighting the need for recoverable materials capable of efficiently capturing dispersed particles. In this study, chitosan (CS), UiO-66-NH2, and reduced graphene oxide (rGO) were integrated into CS@UiO-66-NH2@rGO hydrogel beads via bead formation and solidification, followed by in situ solvothermal growth of UiO-66-NH2. The resulting ternary beads exhibited a rough, hierarchical surface morphology together with an N2-accessible Brunauer–Emmett–Teller (BET) surface area of 298.76 m2 g−1; scanning electron microscopy (SEM) observations, rather than the BET surface area alone, indicated an enlarged interfacial region accessible to polystyrene (PS) particles. Under the selected experimental conditions, the beads achieved a PS removal efficiency of 97.6 ± 0.6% and an adsorption capacity of 390.4 mg g−1, approaching equilibrium within approximately 45 min. At the standard test temperature of 303 K, the constrained pseudo-first-order (PFO) model provided a better empirical description than the pseudo-second-order (PSO) model, with corresponding RMSE values of 15.66 and 20.71 mg g−1, respectively. Nevertheless, both models were subject to boundary limitations, and neither consistently outperformed the other across the investigated temperature range. At 303 K, nonlinear Langmuir fitting based on four quantifiable equilibrium data points yielded a maximum adsorption capacity (qm) of 506.24 mg g−1 (R2 = 0.9840). The beads retained a PS removal efficiency of 91.8 ± 1.1% after six adsorption–desorption cycles and maintained removal efficiencies above 82% in PS-spiked natural-water matrices. The incorporation of rGO reduced the apparent band gap to 2.56 eV, suppressed photoluminescence (PL) emission, enhanced the photocurrent response, and decreased the Nyquist semicircle diameter; collectively, these observations are consistent with the formation of a visible-light-responsive composite interface that promotes charge separation and interfacial charge transfer. Mechanistic analyses suggested a multicomponent capture process involving external-surface adhesion, hydrophobic and π-related interactions, dispersion forces, roughness-assisted retention, and physical entrapment, with electrostatic attraction potentially contributing at pH values below the isoelectric point (IEP). Overall, the developed hydrogel beads represent a reusable platform for rapid PS capture coupled with a photoresponsive material interface.