Modeling the Removal of Hexavalent Chromium from Aqueous Solution Using a Pyridinium-Based Poly(ionic liquid)

Chromium-(VI) is a highly toxic pollutant commonly found in industrial wastewater, requiring effective removal strategies. While various remediation technologies exist, many suffer limitations from high costs, energy demands, or inefficiency. Adsorption has emerged as a particularly promising alternative due to its low cost, operational simplicity, and wide range of applicable materials. Recently, focus has turned toward poly-(ionic liquids) (PILs), a class of versatile hybrid materials that combine the advantageous properties of polymers and ionic liquids, including thermal stability, low volatility, and diverse interaction capabilities with target pollutants. In this work, a pyridinium-based PIL containing BF₄ ^- as the counterion was evaluated for Cr-(VI) removal from aqueous solutions. Adsorption behaviors were analyzed using a set of selected models fitted by nonlinear regression on the original scale. Equilibrium was interpreted using heterogeneous/sigmoidal isotherms (Sips and Krishnamurti), showing good agreement with the experimental data. Kinetics were likewise well captured and generally outperformed traditional baselines (pseudo-first order and pseudo-second order), providing consistent rate and time scale estimates across conditions. These results suggest a complex adsorption mechanism involving both surface heterogeneity and possible multilayer formation. Complementary density functional theory (DFT) calculations supported the experimental findings, revealing that CrO₄ ^2- undergoes chemisorption, while HCrO₄ ^- and Cr₂O₇ ^2-, the dominant species under acidic conditions, are primarily adsorbed through physisorption involving electrostatic and dispersion forces. Overall, this work provides new insights into the adsorption mechanisms of Cr-(VI) on PILs and highlights the potential of these advanced materials for heavy metal remediation in environmental systems.