Efficient Removal of Bromothymol Blue Dye Using a Nickel-Based Coordination Polymer: Kinetic and Isotherm Insights for Sustainable Wastewater Treatment

A nickel-based coordination polymer, [Ni(H₂btec)(bipy)]·H₂O, was synthesized from 1,2,4,5-benzenetetracarboxylic acid (H₄btec) and 4,4′-bipyridine (bipy) ligands and subsequently characterized. Its adsorption performance toward bromothymol blue (BTB) dye in aqueous solution was examined under different operating conditions. The material recorded a maximum adsorption capacity of 76.56 mg/g at pH 8.0 after 30 min, showing that alkaline conditions improve the deprotonation of active sites and enhance electrostatic attraction between BTB anions and the nickel centers. Kinetic evaluation indicated that the pseudo-second-order (PSO) model best described the adsorption process (R² = 0.7595), with a rate constant of 0.0135 and an equilibrium capacity of 41.7 mg/g, suggesting chemisorption as the dominant mechanism. Equilibrium studies showed an excellent fit to the Freundlich isotherm (R² = 0.9947), confirming multilayer adsorption on a heterogeneous surface, while the Langmuir model estimated a monolayer capacity of 92.59 mg/g. FT-IR spectra before and after adsorption revealed notable band shifts and new peaks, indicating strong interactions between BTB molecules and surface functional groups, mainly through hydrogen bonding and electrostatic attraction. Overall, the findings demonstrate that [Ni(H₂btec)(bipy)]·H₂O is a stable and efficient adsorbent with strong potential for sustainable removal of BTB and related pollutants from wastewater under mildly alkaline conditions.