Equilibrium modelling and kinetic studies on adsorption of cadmium from lake water by a magnetic covalent organic framework

Abstract

Predictive, field-relevant descriptions of cadmium attenuation remain essential for safeguarding freshwater systems. This study evaluates a magnetically retrievable covalent organic framework (COF@Fe3O4) sorbent for Cd2+ removal from lake water, integrating characterization, equilibrium modelling, and kinetics. The composite was characterized by FT-IR, XRD, and SEM, and its pHpzc was determined. Batch tests were conducted over a pH range of 5.0-9.0, varying contact times, and initial Cd2+ concentrations of 20-100 mg L-1. High percent removals were achieved under mildly alkaline conditions, with residual Cd close to the detection limit at pH 9.0 and about 98% removal at the operative dose using simple magnetic handling. Equilibrium behavior was analyzed with parallel nonlinear and linear treatments using consistent error functions. Langmuir provided the best description, yielding qmax = 86.99 mg g-1 with KL = 0.0318 L mg-1 in nonlinear fitting and qmax = 93.67 mg g-1 with KL = 0.0281 L mg-1 from the linear form, indicating monolayer uptake on a finite set of sites. Toth also fit well and reflected mild site-energy heterogeneity. Time-dependent uptake for 100 mg L- 1 Cd2+ was the most consistent with a pseudo-second-order model, and intraparticle diffusion analysis gave a positive intercept, indicating a diffusion contribution that is not the sole rate-limiting step. Reusability and structural stability of the material was verified by magnetic recovery with water-only rinsing, maintaining performance across cycles. Overall, the combined characterization, isotherm, and kinetic evidence together with high removals supported COF@Fe3O4 as a practical, magnetically separable platform for Cd2+ treatment in lake water.