Using graphene nano-flakes as electrodes to remove ferric ions by capacitive deionization

In this work, the graphene nano-flakes (GNFs), with superior specific surface area and conductivity which are beneficial to electrosorption, were fabricated as electrodes for capacitive deionization devices. GNFs were prepared by a modified Hummers' method with hydrazine for reduction and GNF electrodes were employed in electrosorption of ferric chloride (FeCl₃). The morphology, structure, porous and electrochemical properties were characterized by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, N₂ adsorption at 77 K and electrochemical potentiostat analyser, respectively. Various working parameters were investigated and it was found that the optimum flow rate and electrical voltage were 25 ml/min and 2.0 V, respectively. It was also found that the electrosorptive capacity of GNFs was 0.5 mg/g for an initial FeCl₃ concentration of 20 mg/l. The maximum equilibrium electrosorption capacity and rate constant for FeCl₃ at 2.0 V were 0.88 mg/g and 0.27 min^-1. These data were validated by both Langmuir isotherm and persudo-first-order adsorption, respectively. Finally, the selective ion electrosorption experiments were repeated in solution with different ions, and it was observed that the electrosorption capacities of cations on the GNFs followed the order of Fe³⁺ > Ca²⁺ > Mg²⁺ > Na⁺.