Effective removal of hexavalent chromium from aqueous solution by ZnCl2 modified biochar: Effects and response sequence of the functional groups

In view of the high toxicity and carcinogenicity of the Cr(VI), it is necessary to take some effective technologies to remove the Cr(VI) from the contaminated water, and the investigation into interface reaction mechanism in the process is particularly important for the technology optimization. In this study, the ZnCl2 modified sea buckthorn stones biochar (Z-SSBC) was successfully prepared and used for removing Cr(VI) from aqueous solution, and the SEM, BET, FT-IR analyses demonstrated that the biochar possessed rough surface, large specific surface area (1031 m2/g), mesoporous structure and abundant functional groups. The maximum removal efficiency (98%) of Cr(VI) was obtained at initial pH 2 and 30 ℃, and the media pH had significant impact on the adsorption of Cr(VI) onto the Z-SSBC. The analyses of adsorption kinetics and isotherms demonstrated that the process of Cr(VI) removal by the Z-SSBC included the chemisorption and some degree of intraparticle diffusion, and the Z-SSBC exhibited the fine reusability in five adsorption–desorption cycles. The two-dimensional correlation spectroscopy (2D COS) analysis revealed the sequence of the main functional groups participating in the reaction process (–OH>C=O>–COOH>–CH3). Furthermore, as the electron-donors, the –OH and C=O groups transform Cr(VI) anions into Cr(III), and the reduced Cr(III) ions were immobilized on the surface of the biochar. These results revealed that the prepared adsorbent had a great potential in the application of Cr(VI) removal.