Enhanced removal of Cd2+ from water by AHP-pretreated biochar: Adsorption performance and mechanism

The sesame straw-derived biochar was successfully prepared via alkaline hydrogen peroxide (AHP) pretreatment in this study. Systematic experimental characterizations, 15 relevant batch and column adsorption models, combined with density functional theory (DFT) calculation were used to investigate the performances and micro-mechanisms of Cd2+ adsorption onto biochar. We found AHP-pretreatment could greatly improve the adsorption performance of biochar for Cd2+. The maximum Cd2+ adsorption capacity of AHP-pretreated biochar (87.13 mg g-1) was much larger than that of unpretreated biochar. Cd2+ adsorption was mainly dominated by the chemisorption of the homogeneous surface monolayer. The hydroxyl and carboxyl groups on the surface of biochar provided preferential adsorption sites, and liquid film diffusion and intra-particle diffusion were two dominant rate-controlling steps. Our results showed that ion exchange, co-precipitation, surface complexation, and Cd2+-π interaction were the dominant adsorption mechanisms. Especially, DFT calculations well-identified that lone-pair electrons during complexation and π electrons during coordination were provided by oxygen-containing functional groups and aromatic rings, respectively. The experimental breakthrough curves fitted better with the theoretical value of the BJP model, compared to Thomas, Yoon-Nelson, and EXY models. Overall, our study provides a promising method for Cd2+ removal from wastewater and resource utilization of agricultural wastes.