Efficient Hexavalent Chromium Removal by Nano-Cerium Oxide Functionalized Biochar:Insight the Role of Reduction

Abstract

The strong toxicity of hexavalent chromium (Cr(VI)) have spurred great interests in developing effective technologies to remove it from environment. In this study, a cost-effective and recyclable adsorbent nano-cerium (Ce) oxide functionalized peanut shell biochar (BC-Ce) was synthetized by impregnation-precipitation-pyrolysis process and employed in simultaneous adsorption and reduction of Cr(VI) in water. Compared with the original biochar, the as-prepared composite material exhibited better Cr(VI) adsorption ability. Its maximum Cr(Ⅵ) adsorption capacity calculated by Langmuir achieved 47.83 mg·g-1, which was much greater than that of original biochar (9.24 mg·g-1). The BC-based materials displayed obvious reduction behavior for Cr(Ⅵ) and partially adsorbed Cr(VI) was reduced to less toxic Cr(III). When 4 g·L-1 BC-based materials were used to treat 50 mg·L-1 Cr(VI) solution, the Cr(Ⅲ)t/Cr(Ⅵ)0 for BC-Ce reached to 18.10% after adsorption, meanwhile that for BC was only 3.66%. Characterization results indicated that the better 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 mechanisms of Cr(VI) removal from water by BC-Ce, namely, except for adsorption, the reduction behavior should not be overlooked in Cr(Ⅵ) removal by BC-based materials.

Keywords: Biochar, Cerium oxide, Cr(VI), oxygen-containing functional group (OFGs), persistent free radicals (PFRs)