Efficient hexavalent chromium removal by nano-cerium oxide functionalized biochar: Insight into the role of reduction

Abstract

The strong toxicity of hexavalent chromium (Cr(VI)) have spurred great interests in developing an effective technology to remove it from water. In this study, a cost-effective and recyclable composite nano-cerium oxide functionalized peanut shell biochar (BC-Ce) was synthetized via impregnation-precipitation-pyrolysis process and employed for simultaneous adsorption and reduction of Cr(VI) in water. The BC-Ce possessed better Cr(VI) adsorption potential, and its maximum Cr(VI) adsorption capacity calculated by Langmuir achieved 47.83 mg g−1. Simultaneously, the as-prepared composite displayed obvious reduction behavior for Cr(VI) and partially adsorbed Cr(VI) was reduced to less toxic Cr(III). The effects of composite dosage (0.2–4 g L−1), initial Cr(VI) concentration (20–200 mg L−1) and pH (3−9) on Cr(VI) removal were also investigated. Characterization results indicated that the Cr(VI) reduction induced by BC-Ce was mainly attributed to the comprehensive effect of oxygen-containing functional group (OFGs), carbon-centered persistent free radicals (PFRs), oxygen vacancies and graphitic structure in BC-Ce. The CeO2 supported on the surface of BC facilitated the generation of more OFGs (phenolic-OH and C-O), PFRs and oxygen vacancies, thus provided abundant sites for chemical reduction of Cr(VI). Moreover, the graphitic structure in BC-Ce promoted the transfer of electrons from graphite carbon to Cr(VI). This study provided a new insight into the removal mechanisms of Cr(VI) in water by BC-Ce. Except for the adsorption, the reduction behavior during Cr(VI) removal process by BC-based materials should not be overlooked.