Congo Red Removal From Aqueous Solutions Using Plant‐Extract‐Assisted ZnO Nanoparticles: Kinetic and Isotherm Analyses

ABSTRACT Green‐synthesised zinc oxide nanoparticles (ZnO NPs) were prepared using Equisetum arvense extract and evaluated as nano‐adsorbents for Congo red (CR) removal from aqueous solutions. Their physicochemical, morphological, elemental, and crystalline properties were characterised using dynamic light scattering (DLS), zeta‐potential analysis, Fourier‐transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy‐dispersive X‐ray spectroscopy (EDS), and X‐ray diffraction (XRD). DLS showed an average hydrodynamic size of 365.2 nm and a polydispersity index of 0.341, while the zeta potential of −24.7 mV indicated moderate colloidal stability and a tendency toward aggregation. FTIR indicated plant‐derived surface functional groups and a Zn─O absorption feature at 450–500 cm − 1 . XRD showed hexagonal wurtzite ZnO as the predominant crystalline phase. Batch experiments showed that maximum removal efficiency reached 86.79% after 180 min for 30 mg L − 1 CR using 5 mg ZnO NPs, while equilibrium adsorption capacity increased from 3.11 to 71.11 mg g − 1 with increasing CR concentration. The pseudo‐second‐order kinetic model provided the best fit among the evaluated kinetic models (R 2 = 0.9993). The Freundlich fit (R 2 = 0.9327) was consistent with surface heterogeneity, while the low Dubinin–Radushkevich mean adsorption energy suggested a contribution from physical adsorption.