Adsorption isotherms and kinetic studies for the removal of toxic reactive dyestuffs from contaminated water using a viologen-based covalent polymer

Abstract

A polyviologen-based adsorbent was prepared via polymerization of a viologen-dialdehyde with a hydroxyl-substituted aryl-dihydrazide in acidified water. The viologen-based covalent polymer aerogel was then generated by freeze–drying. The adsorption of Reactive Blue 19 (RB19) dye onto this viologen-based covalent polymer in aqueous solutions was explored. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier-transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) analysis, and energy-dispersive X-ray (EDX) spectroscopy were employed to study the surface morphology and chemical composition of the cationic polyviologen before and after the adsorption of RB19. The thermal stability of the prepared cationic polyviologen was also explored. The polymer adsorbent displayed the formation of nanoparticles (70–120 nm) with a high surface area (32 m² g⁻¹) upon dispersion in an aqueous environment. The adsorption experiments were conducted as a function of the polymer amount (0.1–0.5 g L⁻¹), contact time (5–30 min), pH_o (3.0–9.0), and initial dye concentration. It was found that a total amount of 0.5 g L⁻¹ of the polymer removes up to 99% of 20 mg L⁻¹ RB19 solution preadjusted to pH_o 3 in 10 min. Under optimum conditions, the adsorption course was best described by the Lagergren pseudo-first-order kinetics and Dubinin–Radushkevich isotherm models. The results of the kinetic and isotherm analyses proposed that physisorption is the main mechanism of adsorption.