Removal and reduction of Cr(Ⅵ) in simulated wastewater using magnetic biochar prepared by co-pyrolysis of nano-zero-valent iron and sewage sludge

Abstract

Magnetic biochar was prepared by one-step pyrolysis using sewage sludge and nano-zero-valent iron particles as raw materials, and applied to remove Cr(VI) in simulated wastewater. When the initial pH of the solution was 2, the initial concentration of Cr(Ⅵ) was 50 mg L−1, the dosage of biochar was 4 g L−1, the reaction time was 24 h, and the reaction temperature was 298 K, the removal capacities of Cr(Ⅵ) and CrTotal were 11.56 mg g−1 and 9.840 mg g−1. Moreover, the removals of Cr(VI) were always greater than CrTotal under all controlled conditions, indicating that Cr(VI) may be preferentially reduced into Cr(III), and then removed by adsorption. The removals of Cr(VI) and CrTotal had the best correlation with the Langmuir model and Freundlich model, and they were followed the pseudo second-order dynamics model much better. The Cr(VI) and CrTotal removals were spontaneous, endothermic, and chemisorption-based processes. The characterization of biochar before and after adsorption and component analysis of the solution after adsorption showed the electron donors for Cr(VI) reduction should be Fe0, Fe2+ and organics in biochar, and Cr(III) was very likely removed by the generation of CrOOH. The rate-controlling steps for Cr(VI) and CrTotal removals may individually be liquid-film/intra-particle diffusion and liquid-film diffusion/chemical reaction.