Honeycomb-structured biochar from waste pomelo peel for synergistic adsorptive and photocatalytic removal of Cr(VI)

Abstract Efficient removal of hexavalent chromium (Cr(VI)) as a well-known highly toxic heavy metal is extremely necessary for people’s health and environmental safety. Biochar from waste biomass has attracted widespread attention in pollutant removal due to abundant pores, high surface area and low-cost, especially the presence of persistent free radical (PFRs). Here, waste pomelo peel was utilized to synthesize the activated biochar with the interconnected porous frameworks (APC). The obtained APC has an exponentially high specific surface area of 997.46 m 2 ·g −1 as compared with pristine biochar (PPC) (8.78 m 2 ·g −1 ) due to distinctive three-dimensional (3D) honeycomb-structured framework with interconnected pores. Meanwhile, through such green synthesis with the introduction of no other substances besides KOH, the APC is enriched with the oxygen-containing functional groups of C=O, C-H and O–H, which can firmly grasp Cr(VI) ions and convert them into low toxic Cr(III) ions. The APC exhibits excellent adsorption and photocatalytic removal of Cr(VI). The adsorption capacity of Cr(VI) reaches a maximum of 307.3 mg·g −1 as compared with the PPC (194.2 mg·g −1 ), which obeys pseudo-second-order kinetics and is a spontaneous endothermic process mainly controlled by intraparticle diffusion. Subsequently, the enriched Cr(VI) can be completely reduced under illumination by the existence of PFRs. Compared with PPC, the APC has strong light absorption, low band gap, more reactive oxygen species (ROS) and rapid charge carrier transfer, leading to high photocatalytic activity. Based on the trapping experiments and quantitative analysis of reactive species, the removal mechanism of Cr(VI) over APC is proposed. This work affords an effective approach to construct the honeycomb-like APC for the synergistic adsorptive and photocatalytic removal of Cr(VI), and hierarchical porous frameworks and PFRs are particularly beneficial for the enhancement of Cr(VI) removal, exhibiting potential practical application in wastewater treatment. Graphical Abstract