Two-step synthesis of coconut shell biochar-based ternary composite to efficiently remove organic pollutants by photocatalytic degradation

Designing environmentally friendly, low cost and efficient photocatalysts is vitally important for degradation of organic pollutants. Herein, a ternary composite-ZnO/ACSC@TiO2, constituted by activated coconut shell derived biochar (ACSC), TiO2 and ZnO, was successfully synthesized by two-step hydrothermal method. It demonstrated that TiO2 could be uniformly wrapped on ACSC surface during first step to form core-shell structures (ACSC@TiO2). It was beneficial not only to enhance adsorption capacity for organic pollutants and absorption ability for light, but also to form C-doped TiO2 with a relatively narrow bandgap to expand light absorption of TiO2 from UV to visible light. Subsequently, ZnO was introduced through second step to generate type-II heterojunctions with ACSC@TiO2, which further reduced bandgap value of the ternary photocatalyst to promote photogenerated carrier generation and efficiently diminished recombination of e--h+ pairs. As expected, the optimal prepared catalyst with 10 wt% of ZnO (10%ZnO/ACSC@TiO2) exhibited excellent adsorptive and photocatalytic abilities for removal of tetracycline (TC) and Rhodamine B (RhB) with different initial concentrations. Particularly, its total removal efficiency for TC and RhB was 97.6% and 99.4%, respectively under 300 W xenon lamp irradiation (25 mg/L of organic pollutants, 1.0 g/L catalyst and natural pH in 60 min). Investigations on catalytic mechanism and degradation pathways proved that 10%ZnO/ACSC@TiO2 could remove RhB and TC by deep degradation. Its enhanced synergy of adsorption and photocatalysis could efficiently accelerate mineralization rates of RhB and TC. This biomass derived biochar-based ternary composite as photocatalyst with optimized energy band structures and microstructures would have good industrial application potential.