Fabrication of tannic acid-incorporated polyvinylpyrrolidone/polyvinyl alcohol composite hydrogel and its application as an adsorbent for copper ion removal.

An effective tannic acid-incorporated polyvinylpyrrolidone/polyvinyl alcohol composite hydrogel with high-potential sorption capacity was developed for the removal of copper from aqueous solution. The composite hydrogel exhibited pH-dependent swelling, in which swelling and shrinking occurred reversibly with adjustment of the pH of the medium. At pH 4, the maximal adsorption capacity for copper at 30 °C was 297.0 mg g, and the adsorbent dose was 4 g L. The adsorption kinetics were best fitted with a pseudo-second order kinetic model. The adsorption behavior was well predicted by the Freundlish isotherm. The thermodynamics parameters indicated a spontaneous and exothermic reaction with an increase in the entropy of the system. The chemical changes in the film structure before and after adsorption treatment were characterized by Scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS). The FTIR, XPS and XAS results confirmed that Cu bound to the oxygens in the -OH, C = O and N-(C = O)- functional groups on the T-HD. XAS analysis revealed the chemical composition and molecular geometry of the adsorbed copper ions. The single-solute adsorption and coadsorption mechanisms, which provide insight into cobalt-copper, nickel-copper, or nickel-cobalt-copper complex solutions, were investigated. The composite hydrogel exhibited excellent regeneration ability in EDTA solution. Notably, the adsorbent retained an adsorption efficiency exceeding 87% even after five regeneration cycles. On the basis of both adsorbent characteristics and adsorption performance, it was determined that the composite hydrogel has the potential to be used as a platform for developing materials to treat wastewater containing high levels of metal contaminants such as those from the electroplating industry.