Super-oxidized "activated graphene" as 3D analogue of defect graphene oxide: Oxidation degree vs U(VI) sorption

Porous carbons are not favorable for sorption of heavy metals and radionuclides due to absence of suitable binding sites. In this study we explored the limits for surface oxidation of "activated graphene" (AG), porous carbon material with the specific surface area of similar to 2700 m(2)/g produced by activation of reduced graphene oxide (GO). Set of "Super-Oxidized Activated Graphene" (SOAG) materials with high abundance of carboxylic groups on the surface were produced using "soft" oxidation. High degree of oxidation comparable to standard GO (C/ O=2.3) was achieved while keeping 3D porous structure with specific surface area of similar to 700-800 m(2)/. The decrease in surface area is related to the oxidation-driven collapse of mesopores while micropores showed higher stability. The increase in the oxidation degree of SOAG is found to result in progressively higher sorption of U (VI), mostly related to the increase in abundance of carboxylic groups. The SOAG demonstrated extraordinarily high sorption of U(VI) with the maximal capacity up to 5400 mu mol/g, that is 8.4 - fold increase compared to non-oxidized precursor AG, similar to 50 -fold increase compared to standard graphene oxide and twice higher than extremely defect-rich graphene oxide. The trends revealed here show a way to further increase sorption if similar oxidation degree is achieved with smaller sacrifice of surface area.