Adsorption of metal ions on graphene sheet for applications in environmental sensing and wastewater treatment

Abstract

Experimental and computational studies showed that metal ion binding to graphene nanosheets involves a new mechanism of the interaction, ion-graphene charge transfer. Surprisingly, highly acidic= ions showed the highest affinity and ion-induced dipole appears to play a very important role in this interaction. Aqueous suspensions of graphene nanosheets (not graphene oxide or reduced graphene) were prepared by bovine serum albumin-assisted exfoliation (biographene) and metal ion binding carried out by optical spectroscopy, zeta potential analysis and Raman data. In DFT computational studies, the interactions of metal ions with graphene sheets of 20 Å × 12 Å (containing 46 carbon atoms) were examined. Interactions of the alkali and alkali earth metals were examined by M026x theory with 6–31 g basis set and d3 dispersion, while those of the transition metal systems have been examined by B3LYP theory with LanL2DZ basis set. The basis set superimposition errors were avoided using counterpoise calculations. The computed complexation energies (∆EAdsorption) were compared with the experimental values. These studies showed strong correlation of binding energies with the metal ion wlwctronegativity and charge density wkwxr, involving charge transfer between the metal ion and graphene. These surprising results imply a significant role for ion and ion-induced dipole interactions as a possible guiding factor in predicting metal ion binding affinities. A new hypothesis that charge transfer from the graphene π-cloud to the metal ion contributes to thπe binding free energy is proposed. Graphene is a strong candidate, therefore, for detecting and adsorbing metal ions from wastewater streams for sensing and environmental applications.