Electrolyte induced rheological modulation of graphene oxide suspensions and its applications in adsorption

In this study, we report the microstructure, rheology and adsorption characteristics of aqueous suspensions of Graphene Oxide (GO) at a volume fraction (ϕ <inf>GO</inf> ) = 0.018, which can be transformed into gels by cation induced charge shielding and cross-linking between GO nanosheets. GO nanosheets of average thickness ∼1.5 nm and a lateral dimension of ∼750 nm are synthesized by Hummer's process. At ϕ <inf>GO</inf> = 0.018, cations of varying size and valence are systematically introduced with electrolytes NH <inf>4</inf> Cl, LiCl, NaCl, KCl, MgCl <inf>2</inf> and FeCl <inf>3</inf> at concentrations ranging from 10 ^-5 –10 ^-1 M to investigate their effect on the rheology of GO suspensions. Our results suggest that depending on the electrolyte concentration, size and the valence of the cation: low viscosity suspensions, fragile gels and solid-like GO-electrolyte gels are formed. The storage modulus (G') of all GO-electrolyte gels increases with the increase in electrolyte concentration and G' follows the order GO-FeCl <inf>3</inf> > GO-MgCl <inf>2</inf> > GO-KCl > GO-NaCl > GO-LiCl > GO-NH <inf>4</inf> Cl. FESEM analysis shows that lyophilized GO-electrolyte gels with 10 ^-1 M electrolytes have a porous morphology resulting from the aggregation of GO nanosheets. The GO-electrolyte gels are shown to adsorb high quantities of oils, with GO-FeCl <inf>3</inf> gels showing a higher adsorption capacity. The GO-NaCl and GO-FeCl <inf>3</inf> lyophilized gels are also shown to adsorb methylene blue dye and follow the pseudo-second-order kinetics of adsorption. Along with higher oil and dye adsorption efficiency, GO-electrolyte gels are easy to recollect after the adsorption, thus avoiding the potential toxicity for bio-organisms in water caused by GO nanosheets.