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

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 q_max = 86.99 mg g^-1 with K_L = 0.0318 L mg^-1 in nonlinear fitting and q_max = 93.67 mg g^-1 with K_L = 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.