Nano nickel embedded in N-doped CNTs-supported porous biochar for adsorption-reduction of hexavalent chromium

Highlights: • Nickel embedded in N-doped CNTs-biochar was synthesized from sugarcane bagasse. • Ni@N-K-C composites has a high BET specific surface area of 604.62 m²/g. • Activation treatment can improve the doping efficiency of Ni and N atom in biochar. • Activation treatment can improve the yield and length of CNTs growth on biochar. • Cr(VI) removal capacity was enhanced more than 5 times after KOH activation. Nitrogen-doped carbon coated transition metal hybrids for the removal of hazardous hexavalent chromium (Cr(VI)) has attracted increasing attention in wastewater treatment recently. In this study, three-dimensional nano-nickel particles embedded in N-doped carbon nanotubes supported on porous biochar (Ni@N-K-C) were synthesized by a two-stage strategy of KOH activation followed by annealing. The effect of KOH activation treatment on the doping process and Cr(VI) removal properties were investigated. The results indicate that KOH activation can improve the pore parameters and promote subsequent doping of Ni and N and the growth of carbon nanotubes (CNTs). After KOH pretreatment, the specific surface area of Ni@N-K-C increased significantly to 604.62 m²/g. The improved pore structure accelerates the mass diffusion of Cr(VI) ions and provides an available surface for the adsorption and reduction of Cr(VI). Therefore, the Ni@N-K-C obtained at 900 °C showed a high removal capacity for Cr(VI) (824.4 mg/g) and a stronger ability to reduce to Cr(III).