Main-chain donor-acceptor polyhydrazone mediated adsorption of an anionic dye from contaminated water

Abstract A versatile synthetic approach was applied to synthesize a polyviologen (PV.nBr) polymer via step-growth polymerization of a tetraethylene glycol-linked trinapthyl-based dihydrazide with a bipyridinium-based dialdehyde (BIPY·2Br DA) in water under acidic conditions (pH, 2–3). The polymeric structure was constructed from alternating units of π-electron rich dihydroxy-naphthalene and π-electron deficient bipyridinium (BIPY) moieties linked together via covalent acylhydrazone bonds. Ultraviolet-visible (UV–Vis) absorption analysis confirmed the occurrence of both intra- and intermolecular donor-acceptor charge transfer (DA-CT) interactions between the electron donor and acceptor units. The synthesized polymer was subjected to lyophilization in order to produce a sponge-like aerogel with a large surface area and high porosity. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) characterization revealed that the polymer possessed a uniform sheet-like morphology in the solid state following lyophilization. The ability of the novel polymer to adsorb reactive orange 122 (RO122) dye from synthetic wastewater was investigated at different operational conditions and was determined to follow pseudo-first-order kinetics. The results indicated that the novel PV.nBr can efficiently and rapidly remove RO122 under a wide range of solution pH with up to 97% uptake. The novel PV.nBr aerogel adsorbed RO122 by electrostatic attractions between the BIPY dication groups of the polymer and the anionic sulphonate groups of RO122. Adsorption isotherms revealed that the data was best fit to the Dubinin-Radushkevich model, with a calculated mean free energy of adsorption of 2.04 KJ/mol, corresponding to a physisorption dominated process.