Effect of graphene oxide surface modification on the elimination of Co(II) from aqueous solutions

Graphene oxide (GO) was applied as adsorbent to efficiently eliminate the heavy metal ions from wastewater because of its large amount of functional surface groups and high surface areas. However, the contribution of different functional surface groups on the high preconcentration of heavy metal ions to GO is still unclear. Herein the GO was applied as adsorbent for the efficient preconcentration of Co(II) and the effect of functional surface groups on Co(II) removal was studied by batch sorption experiments. The results showed that the maximum sorption capacities of Co(II) on GO, NGO, MGO and NMGO were 43.6, 27.3, 23.2 and 14.6 mg/g, respectively. The elimination of Co(II) was significantly influenced by ionic strength and pH, indicated that the interaction mechanism was primarily controlled by strong surface complexation. The DFT calculation further evidenced that the oxygen-containing surface groups could form shorter bond distance with Co(II) ions than the nitrogen-containing functional groups, and the calculated adsorption energies (E_ad) for GO–COOH–Co (44.05 kcal/mol) and GO–OH–Co (16.44 kcal/mol) were much higher than that for NGO–Co (6.33 kcal/mol). The density of states (DOS) and charge density differences of combined complexes further illustrated the strong surface complexes were formed between Co(II) and the oxygen-containing surface groups of GO surfaces. This work highlighted that the oxygen-containing surface groups played a vital role in the formation of surface complexes with Co(II) on GO, which is crucial for the preparation and application of GO in the radioactive pollution cleanup.