Functional Ionic Liquid-Capped Graphene Quantum Dots for Chromium Removal from Chromium Contaminated Water

New functional ionic liquid (IL) capped graphene quantum dot (GQD) was synthesized as an adsorbent for the removal of toxic heavy metal chromium­(VI) ion (Cr6+). The physicochemical properties of the adsorbent (IL-GQD) were investigated by Fourier transform infrared, X-ray diffraction, atomic force microscopy, Raman, scanning electron microscopy, energy dispersive X-rays, and transmission electron microscopy analyses. The adsorption parameters, namely equilibrium time, solution pH, competing co-ions, dosage, and initial chromium concentration, were optimized for maximum Cr6+ adsorption. The maximum adsorption capacity reached 934.62 mg/g at 40 min in neutral pH; this is much better than most of the other adsorbents reported earlier. In addition, the effect of pH in solution was investigated in the range of 3.0–12.0. The result showed that the lower pH value was found to favor the adsorption. The adsorption kinetics and isotherms fitted well with Langmuir isotherm model and pseudo-second-order kinetic model. The thermodynamic studies indicated that the chromium adsorption process followed a spontaneous and endothermic model. This new functionalization of ionic liquid moieties into graphene quantum dot provides excellent results for the removal of toxic Cr6+. This can be utilized for field applications to reduce the chromium concentration to below the tolerance limit (>0.05 mg L–1).