Super-oxidized porous carbons for lanthanide sorption

Super-oxidized porous carbon (SOPC) synthesized starting from spruce cones is demonstrated as an efficient sorbent for two lanthanide elements (Ce and Yb) from their nitrate solutions. Strong oxidation treatment applied to activated carbon with an extremely high BET surface area (∼3400 m² g^-1) resulted in the synthesis of carbon material with an oxidation degree on the same level as graphene oxide (C/O = 2.1), a nanoporous structure with rather narrow pore size distribution and a relatively large BET surface area of ∼1200 m² g^-1. Despite the decrease in surface area, hydrophilic SOPC demonstrates strong enhancement of Ce and Yb uptake reaching the level of 140-170 mg g^-1, about 20-25 times higher compared to the hydrophobic non-oxidized activated carbon precursor under the same conditions. Analysis of advanced synchrotron radiation X-ray spectroscopy data allows us to assign the increase in sorption of metal cations to a high abundance of oxygen functional groups double-bonded to carbon. These groups are mostly located at generalized "pore edges" or vacancy defects in the atomically thin carbon walls of SOPC according to modelling of X-ray Raman scattering (XRS) spectra at the carbon K edge. Surprisingly, effects of nano-confinement are not revealed by the analysis of extended X-ray absorption fine structure (EXAFS) spectra at the Yb L₃ edge, suggesting that Ce and Yb cations are accommodated at the entrance sites of sub-nm-sized pores of SOPC in the hydrated state while maintaining an oxidation state found in the solutions. The SOPC enables the removal of up to 99% of Ln from solutions with a concentration of 10 mg L^-1 of Yb or Ce. Further increases in sorption capacity are likely to be possible if larger portions of the surface area can be preserved after similarly strong oxidation treatment.