Enhanced ferrate(VI) oxidation of micropollutants in water by carbonaceous materials: Elucidating surface functionality

Metal-free carbonaceous materials have increasingly attracted worldwide attention owing to their unprecedented properties in catalytic elimination of environmental pollutants in water. Nevertheless, the surface functionality-dependent catalytic efficacy and activation mechanism for ferrate(VI) ((FeO42-)-O-VI), an emerging green water-treatment agent, have remained largely unknown. Here we have examined if different carbonaceous materials (i.e., hydrochar, graphene oxide (GO), reduced graphene oxide, graphite, and fullerene) can activate (FeO42-)-O-VI and enhance its effectiveness for rapid oxidation of a wide range of micropollutants (i.e., carbamazepine (CBZ), diclofenac, sulfamethoxazole, sulfadimethoxine, trimethoprim, flumequine, atenolol, and caffeine) under mild alkaline conditions. For example, the addition of hydrochar or GO into the reaction system, at pH 9.0, achieved remarkable enhancement of degradation of CBZ as compared to insignificant effect observed for three other carbonaceous materials. The magnitude of enhancement varied with the moieties of micropollutants. The Fourier-transform infrared spectroscopy (FTIR) technique in conjunction with the chemical probe (i.e., methyl phenyl sulfoxide) tests demonstrated chemical interactions of surface C=O groups of hydrochar with (FeO42-)-O-VI, of which high-valent iron-oxo intermediates (i.e., Fe-IV/Fe-V) were generated as the oxidizing species for enhanced remediation. Additionally, this enhancement was seen in five successive catalytic runs, indicating high recyclability of hydrochar to oxidize micropollutants by (FeO42-)-O-VI. These findings suggest that heterogeneous carbonaceous activation of (FeO42-)-O-VI holds promise for advancing water remediation under mild alkaline reaction conditions.