Adsorption of Cr (VI) ion from aqueous solution on acrylamide – grafted starch (Coccinia abyssinicca) – PVA/PVP/chitosan/graphene oxide blended hydrogel: isotherms, kinetics, and thermodynamics studies

Abstract

ABSTRACT Adsorption-based research toward biodegradable polymers has received much attention in recent years due to environmental concerns. Polysaccharides in this domain are interesting starting materials for the preparation of novel adsorbents. In this work, novel type of biopolymer-based hybrid hydrogel (HH) was designed for removal of Cr (VI) ion from aqueous solution. Polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), acrylamide-grafted native starch (Coccinia abyssinica) (ST-g-AAm), chitosan (CS), and graphene oxide (GO) were used to prepare PVA/PVP/ST-g-AAm/CS/GO HH. Physiochemical properties of freeze-dried hydrogel were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, thermogravimetric analysis, and swelling tests. Adsorptions of Cr (VI) ion onto the hydrogel as functions of initial Cr (VI) ion concentration, pH, time, hydrogel dose, and temperature have been studied. The adsorption data agree with Langmuir isotherm at 25°C and follow pseudo-second-order kinetic model at pH of 2. The maximum adsorption capacity of the hydrogel was 93 mg g−1. The obtained negative standard Gibb’s free energy (∆G° = – 1.120 kJ mol−1) and negative enthalpy (∆H° = – 2.360 kJ mol−1) reveal the spontaneity and exothermic nature of Cr (VI) ion adsorption. Moreover, the adsorption thermodynamics shows enthalpically favoring host–guest complexion along with decrease in entropy.