Ultra-high selective removal of CR and Cr(VI) from aqueous solutions using polyethyleneimine functionalized magnetic hydrochar: Application strategy and mechanisms insight

Herein, a novel polyethyleneimine (PEI) functionalized magnetic hydrochar (MHTCP-5) adsorbent with low cost and easy recovery is unveiled, which exhibits excellent adsorption capacity for CR and Cr(VI) from aqueous solution. And the maximum adsorption capacities are respectively 1019.2 and 287.7 mg/g, exceeding those of most previously reported biochar-based adsorbents. Surprisingly, MHTCP-5 possesses ultra-high selectivity for both CR and Cr(VI) from mixed solutions, which was the first time proposed in this study. In addition, a series of adsorption experiments are conducted under different conditions to investigate the adsorption behavior of MHTCP-5. The results suggest that the adsorption of CR and Cr(VI) onto MHTCP-5 followed Elovich and Freundlich models, and the surfactants have a significant influence on the adsorption capacity due to electrostatic interactions. Moreover, MHTCP-5 exhibits extremely fast adsorption rate for Cr(VI) ions, which can reduce the Cr(VI) concentration below WHO allowable value (0.05 mg/L) for drinking water within only 5 min, and the cycle experiments manifest that MHTCP-5 still exhibits excellent Cr(VI) adsorption capacity after 17 cycles. The adsorption-reduction mechanisms of CR and Cr(VI) onto MHTCP-5 are systematically investigated based on FT-IR and XPS. Additionally, the outstanding removal efficiency of CR and Cr(VI) in simulated wastewater as well as the superior reusability and fixed-bed column adsorption capacity of MHTCP-5 adsorbent indicate that the adsorbent has great potential in the treatment of actual wastewater containing CR and Cr(VI).