Deep eutectic solvent assisted facile and efficient synthesis of nitrogen-doped magnetic biochar for hexavalent chromium elimination: Mechanism and performance insights

Dual-modification of biochar through magnetization and nitrogen-doping can greatly enhance the removal performance for Cr(VI). Deep eutectic solvent (DES), as a class of novel green solvent, has gained much attention in the research field of material science. In this study, a DES produced by complexing of FeCl3 and urea was adopted for the first time to assist the facile and efficient preparation of a nitrogen-doped magnetic biochar (NMBC) by using peanut shell (PS) as raw stock. The results indicated that FeCl3/urea-based DES could act as fine iron and nitrogen sources in the formation of NMBC. More importantly, the DES catalyzed the pyrolysis process of PS and promoted the pore development on biochar. The synthesized NMBC showed satisfactory removal efficiency for Cr(VI), with the adsorption process well depicted by Langmuir isotherm and pseudo-second-order kinetic models. The results revealed that iron species, nitrogen-and oxygen-containing groups in NMBC made great contributions to Cr(VI) capture. The comprehensive mechanism included pore-filling, electrostatic attraction, cation bridging, ion-exchange, reduction and surface complexation. The present study provides a new insight to guide the design of functional biochar for Cr(VI) decontamination from water.