Coupling adsorption-photocatalytic reduction of Cr(VI) by metal-free N-doped carbon.

The efficient removal of toxic hexavalent chromium (Cr(VI)) is crucial for waste water treatment. Herein, we report a new strategy to couple adsorption and in situ photo-reduction of Cr(VI) to Cr (III) using metal-free, N-doped carbon facilely derived from naturally abundant biomass cellulose. Experimental results exhibited the removal rate of Cr(VI) can be significantly enhanced from 43.25 mg/g to 98.25 mg/g after visible light irradiation under acidic conditions. We demonstrated that toxic Cr(VI) ions were firstly adsorbed on N-doped carbon via electrostatic attraction, and then photo-reduced into Cr(III), followed by re-adsorption through chemical complexation. The carbon sp2-hybridized structures and electro-attracting graphic-N groups (N-(C)3) are proposed to be responsible for this photo-reduction effect. This work reveals the efficient removal of heavy metals through the cooperative adsorption and photo-reduction using the materials synthesized from biomass waste.