Impact of hybrid natural deep eutectic solvent and polyacrylamide flocculant systems on the flocculation of highly stable graphene oxide suspensions

Graphene oxide (GO)-based nanomaterials have garnered significant attentions for various industrial applications due to their remarkable structural and physicochemical properties. Consequently, the impact of the increased discharge of GO into the environment also becomes a matter of significant concern due to the toxicity of GO nanoparticles to human health and the ecosystem. Therefore, efficient removal of GO from wastewaters before discharge into water bodies is crucial and remains a critical challenge in wastewater treatment due to the formation of highly stable GO suspensions in aqueous media. Accordingly, this work investigates the flocculation of aqueous GO suspensions using a range of single and hybrid coagulant/flocculant systems comprising a choline chloride-based natural deep eutectic solvent (NADES) coagulant and commercial high-molecular-weight polyacrylamide (PAM) based flocculants with variable surface charge densities. The effect of variations in the coagulant/flocculant charge type, charge density, and dosage on GO deflocculation was examined using residual turbidity, zeta potential, and floc size measurements. Cationic PAMs demonstrated the best performance among single flocculant systems with GO removal efficiency exceeding 97% and compact settleable flocs (D-50 > 20 mu m) at optimum doses in the range of 15-20 mg/L. The high charge density of FO 4800 SH further enhanced the GO removal efficiency by 1.1% and reduced the required optimum dose by 5 mg/L compared to FO 4350 SH. Hybrid flocculant systems revealed that anionic PAMs are the best performing flocculants when utilized with a fixed concentration of NADES coagulant. High GO removal efficiencies exceeding 98%, maximum zeta potential reduction, and the formation of large flocs (D-50 > 20 mu m) were achieved. The low charge density variant AN 923 SH offered better flocculation conditions by reducing the optimum PAM dose required by 5 mg/L compared to AN 956 SH. The NADES/AN 923 SH hybrid system was determined to be the optimal combination for a highly efficient treatment of GO suspensions at a fixed NADES dose of 0.048 M and a low optimum PAM dose of 5 mg/L that resulted in a GO removal efficiency of 98.1%, turbidity of 2.5 NTU, zeta potential of - 3.7 mV, and a mean D-50 floc size of 26.1 mu m.