Removal of chromium (VI) by a self-regenerating and metal free g-C3N4/graphene hydrogel system via the synergy of adsorption and photo-catalysis under visible light

We propose g-C3N4 nano-sheets and a graphene three-dimensional (3D) gel system composed of two metal free nano-sheets of g-C3N4 and graphene via the stacking effect of two-dimensional (2D) nano-sheets and the π-π conjugated at low temperature. This material has self-regeneration and containing only C and N, which confirms the ability to efficiently remove chromium (VI) (Cr(VI)) via the synergistic effect of adsorption and photo-catalysis. The graphene sheets offer surface adsorption in the composite, which can lead to the adsorption of Cr (VI) with high adsorption capacity and fast speed. The g-C3N4 nano-sheets have high photocatalytic activity in situ for composite to absorb the high toxicity of Cr (VI). This was rapidly reduced to Cr (III). The adsorption and photocatalytic degradation synergized to achieve self-regeneration. The results showed that 80% Cr (VI) (30 mg/L) can be adsorbed by composite in 30 min, and the removal of Cr (VI) reached 100% in 120 min under visible light irradiation via adsorption and photo-catalysis. The low concentrations (2 mg/L) of Cr (VI) can be achieved via a degradation rate of almost 100% that was maintained for more than 26 h. At the same time, there was a large quantity of hydrogel in the pore space network and micron level structure in the composite. This offers 99% recycling and reuse via a stainless steel filter (pore size 38 μm) without the need to rely on a complex separation system. This solves the difficult problems of powder catalyst recovery.