A Niobium Coordination Polymer as an Efficient Sorbent for Caffeine Detection in Surface Water

Herein, it is reported the synthesis of a niobium-based metal-organic framework (MOF), [Nb-(Bez-(COO)₂)] _n , for the extraction of caffeine from surface waters. The material was synthesized and characterized by Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), and Brunauer-Emmett-Teller (BET) analysis, which confirmed the coordination between the ligand (1,4-benzenodicarboxylic, (Bez-(COO)₂)) and niobium (Nb) with a morphology composed of hexagonal rods, high crystallinity, and a surface area of 94.7 m² g^-1. The extraction process was optimized using a response surface methodology, evaluating three factors: (i) mass of the MOF (100-500 mg), (ii) solution pH (5.0-9.0), and (iii) temperature (25-45 °C). The optimal conditions for caffeine extraction were determined as 10 mg of material, pH 9.0, and temperature of 25 °C. Adsorption studies showed that the Freundlich isotherm model provided the best fit (R ² = 0.9498), suggesting adsorption on a heterogeneous surface. Kinetic studies showed that the intraparticle diffusion model better described the adsorption process (R ² = 0.9554), highlighting physisorption by intraparticle diffusion as the predominant mechanism. Thermodynamic parameters revealed spontaneous and exothermic adsorption, with ΔG values between -5.052 and -4.668 kJ mol^-1. The developed analytical method showed a linear range from 1.0 to 20 μg mL^-1, with good linearity (R ² = 0.9978), a limit of detection and quantification of 0.54 μg mL^-1, and 1.78 μg mL^-1, respectively. Accuracy was confirmed by recovery of 95.6 ± 1.1% at 4.5 μg mL^-1. Moreover, the [Nb-(Bez-(COO)₂)] _n material demonstrated high reusability, maintaining its extraction efficiency after five consecutive adsorption-desorption cycles. These results confirm the robustness, efficiency, and sustainability of [Nb-(Bez-(COO)₂)] _n for environmental monitoring and remediation applications.