Chitosan-multilayered graphene oxide hybrid beads for $\protect \text{Zn}^{2+}$ and metoprolol adsorption

Abstract

Chitosan (CS) hydrogel beads and hybrid beads made of a blending of CS hydrogels and Multilayer Graphene Oxide (MGO) were synthesized. The hybrid beads were prepared by gelation in NaOH solution of a 1 wt% CS acid solution with addition of MGO at either 1.5 wt% or 3 wt% loading rates. Prepared beads were characterized by infrared spectroscopy, thermogravimetric analysis (TGA), scanning electron cryo-microscopy and Brunauer–Emmett–Teller (BET) specific surface area measurements. $\text{Zn}^{2+}$ and Metoprolol (MTP) adsorption kinetics and isotherms were studied on the pristine and hybrid CS hydrogel beads. The adsorption kinetics of $\text{Zn}^{2+}$ and MTP in hybrid beads is limited by the diffusion to the MGO sites depending on their accessibility. While pure CS is not efficient for the MTP adsorption, the Langmuir-type isotherms of the 3 wt% MGO hydrogel beads (dose: 5 mg/100 mL) show 163 mg$\cdot $g$^{-1}$ maximum adsorption uptake. The MTP adsorption kinetics and isotherm suggest a MTP trapping on the MGO anionic sites (carboxylate groups) by electrostatic interactions. The $\text{Zn}^{2+}$ adsorption capacities are the highest for the 3 wt% MGO hydrogel beads (236 mg$\cdot $g$^{-1}$), and only of 40 mg$\cdot $g$^{-1}$ for the pure CS beads. The presence of $\text{Zn}^{2+}$ adsorption sites in the hybrid bead, such as MGO carboxylate groups giving electrostatic interactions, and CS amine groups leading to complexation, provides synergic adsorption effects. The competitive adsorption of $\text{Zn}^{2+}$ with respect to MTP in equimolar mixture was observed on hybrid beads (dose: 200 mg/100 mL) at 2 mmol$\cdot $L$^{-1}$ initial total concentration. At pollutant initial total concentration lower than 1.5 mmol$\cdot $L$^{-1}$, no competition occurs. The regeneration at pH 4 of the hybrid beads toward MTP or $\text{Zn}^{2+}$ adsorption was found to be 35–40% of the initial adsorption uptake for five adsorption/regeneration cycles.

Keywords

• Chitosan
• Multilayer graphene oxide
• Water treatment
• $\text{Zn}^{2+}$
• Metoprolol