Efficient Cr(vi) removal by expanded graphite synergized with oxalic acid under UV irradiation

Expanded graphite (EG), an easily-obtained carbon material with the potential of transferring electrons, was utilized successfully in the removal of hazardous hexavalent chromium (Cr(vi)) by environment-friendly oxalic acid (Ox) under UV irradiation. EG with a unique worm-like structure was obtained via a facile microwave treatment. The results showed that the EG + Ox + UV system had optimum performance, removing 99.32% of the Cr(vi) (1 mM) within 60 min at pH = 3, and the kinetic rate constant of Cr(vi) elimination was 7.95 mol L^-1 min^-1. Three components are potentially involved in the Cr(vi) elimination mechanism by the EG + Ox + UV system: (1) the direct electron transfer (DET) pathway of the EG-Ox-Cr(vi) through the acceleration effect of EG caused the majority removal of Cr(vi) under UV; (2) ·CO₂ ^- generated from Ox photolysis was used to reduce some Cr(vi); (3) ·CO₂ ^- created from Cr(vi)-Ox complexes in the solution through the photoinduced electron transfer (PET) pathway also reduced a little Cr(vi). Overall, the efficient removal of Cr(vi) by the EG + Ox + UV system provided new ideas for future research on Cr(vi) treatment.