Enhancing Cr (VI) Adsorption of Chestnut Shell Biochar through H₃PO₄ Activation and Nickel Doping

A high-efficiency nickel-doped porous biochar (PCNi₃) has been successfully synthesized from chestnut shell waste via a two-step chemical activation treatment with H₃PO₄. The influences of microstructure, surface morphology, elemental composition, surface functional groups, specific surface area, porosity, pore-size distribution, and chemical properties of the surface state on the removal of Cr (VI) from water were thoroughly investigated by using XRD, FESEM, FTIR, Raman, BET, and XPS testing methods, N₂ adsorption, and XPS testing techniques respectively. The results indicate that the treatment of H₃PO₄ activation and nickel doping can effectively improve microstructure characteristics, thus promoting Cr (VI) adsorption capacity. The effects of initial solution pH, solution concentration, time, and temperature on remediation are revealed. The Cr (VI) uptake experiments imply that the adsorption curves of PCNi₃ fit well with the Freundlich model, the pseudo-second-order kinetic model, and the Elovich model. The adsorption process of PCNi₃ can be regarded as a spontaneous endothermic reaction limited by diffusion among particles and porosity. The adsorption mechanisms of PCNi₃ are ion exchange, complexation, electrostatic adsorption, and coprecipitation with the assistance of surface active sites, porosity, Ni⁰ particles, and Ni₇P₃. With these advantages, PCNi₃ reveals an extraordinary Cr (VI) removal capacity and a strong ability to reduce Cr (VI) to Cr (III).