Bio-inspired surface-functionalization of graphene oxide for the adsorption of organic dyes and heavy metal ions with a superhigh capacity

By utilizing the synergistic effect of poly-dopamine (PD) with functional groups and graphene oxide (GO) with a high surface area, a series of sub-nano thick PD layer coated GO (PD/GO) composites were fabricated by a well-controlled self-polymerization of dopamine via catechol chemistry and used for effectively decontaminating wastewater. The obtained PD/GO could selectively adsorb the dyes containing an Eschenmoser structure and showed an extremely high adsorption capacity up to 2.1 g g(-1), which represents the highest value among dye adsorptions reported so far. The adsorption mechanism was investigated by FTIR analysis, solution pH effect, and some control experiments. It was concluded that the adsorption process was based on the Eschenmoser salt assisted 1,4-Michael addition reaction between the ortho position of the catechol phenolic hydroxyl group of PD and Eschenmoser groups in the dyes. The adsorption isotherms were explored according to the Langmuir and Freundlich models respectively, and matched well with the Langmuir model. The thermodynamic parameters (Delta H, Delta G, Delta S, and E) were also calculated, which suggested an exothermic and spontaneous adsorption process. In addition, PD/GO exhibited an improved adsorption capacity for heavy metal ions (53.6 mg g(-1) for Pb2+, 24.4 mg g(-1) for Cu2+, 33.3 mg g(-1) for Cd2+, and 15.2 mg g(-1) for Hg2+, respectively) than pure PD and GO. Our results indicate the effectiveness of the synergistic effect of individual components on designing new functional composites with high performance.