Comparative Nitrate Adsorption Performance of Cobalt and Iron-Based Coordination Polymers Using RSM-Based Process Optimization

This research investigates cobalt-based and iron-based coordination polymers as advanced adsorbents for removing nitrate from water, addressing the increasing demand for effective and customizable treatment materials. Both coordination polymers were synthesized through solvothermal methods using terephthalic acid as the organic linker and were characterized by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), which verified their crystalline, porous structures and uniform metal dispersion; Fourier-transform infrared spectroscopy (FTIR) was used to analyze surface characteristic functional groups of the samples before and after adsorption. Batch adsorption tests combined with response surface methodology (RSM), based on a Box–Behnken (BBD), were employed to optimize key operating conditions, including adsorbent dose (0.1–0.5 g/L), pH (3–7), and temperature (25–45 °C). Under optimal conditions (pH 3–5, 0.3 g/L, 30–35 °C), the Co-coordination polymer achieved a maximum nitrate removal of 54.1% and an adsorption capacity of 212.8 mg/g, while the Fe-coordination polymer reached 30.5% removal with a capacity of 35.0 mg/g. Kinetic studies were well fitted by the pseudo-second-order (PSO) model for the Co-coordination polymer (R2 = 0.992–0.997), indicating chemisorption control, whereas the Fe-coordination polymer exhibited diffusion-driven behavior. The equilibrium data fit the Langmuir model well for both, confirming monolayer adsorption. The findings suggest that the Co-coordination polymer provides superior nitrate removal owing to stronger metal–anion interactions, whereas the Fe-coordination polymer offers more stable but lower adsorption capacity.