Microwave-assisted synthesis of self-assembled network of Graphene oxide-Polyethylenimine-Polyvinyl alcohol hydrogel beads for removal of cationic and anionic dyes from wastewater

Abstract

A novel hybrid network of graphene oxide-polyethylenimine-polyvinyl alcohol hydrogel (GPPH) beads are synthesised via microwave route. Functionalisation of graphene oxide (GO) with amine and amide moieties are confirmed by XPS and FT-IR spectra. The SEM micrographs and BET substantiate slightly mesoporous morphology of the beads. The HRTEM images along with SAED pattern confirm the presence of GO in GPPH. The adsorbent has 809.82 μmol g−1 phenolic, 906.65 μmol g−1 lactonic and 756.69 μmol g−1 carboxylic groups, which are quantified by Boehm titration. Differential thermogram and X-ray diffractogram ascertain that GPPH has greater thermal stability and lower degree of crystallinity than GO and polvinyl alcohol (PVA). The adsorbent is used to remove a variety of dyes such as azo (congo red, reactive orange, methyl orange, eriochrome black T), triarylmethane (brilliant green, crystal violet, malachite green, fuschin basic), xanthene (fluorescein sodium salt, eosin yellow, rhodamine B), anthraquinone (alizarin red S), indigoid (indigo carmine) and thiazine dyes (methylene blue). These dyes have pH-dependent adsorption at the surface of GPPH. Electrostatic interactions, hydrogen bonding, non-covalent interactions and pore diffusion help in sorption of dyes. The equilibrium adsorption data fit well in Langmuir isotherm with qmax ranging from 68.44 to 862.07 mg g−1 and follows pseudo-second-order kinetics. GPPH is effective in 90%–95% removal of alizarin red S, brilliant green, crystal violet, eosin yellow, eriochrome black T and reactive orange in simulated waste water containing 100 mg L–1 of NO3−, SO42−, PO43−, HCO3− Cl−, Mg2+, Ca2+, K+ and Na+ ions. The adsorbent has good reusability even after 5th regeneration cycle. In a fixed bed column, 77% (reactive orange), 75% (eosin yellow) and 56% (brilliant green) are adsorbed by GPPH. Moreover, the column of GPPH is reusable which shows that the adsorbent has good commercial applicability.