KOH-activated porous biochar with high specific surface area for adsorptive removal of chromium (VI) and naphthalene from water: Affecting factors, mechanisms and reusability exploration

Highlights: • The porous PBC_KOH was successfully prepared by two-step KOH-activated pyrolysis. • The PBC_KOH has a high S_BET of 2183.80 m²/g with excellent adsorption performance. • The maximum uptake of Cr(VI) and NAP by PBC_KOH were 116.97 mg/g and 450.43 mg/g. • Chemical interactions (e.g. complexation and reduction) enhanced the Cr(VI) binding. • Pore filling and π-π stacking interactions promoted the NAP adsorption onto PBC_KOH. Herein, a high-performance porous biochar described as PBC_KOH was successfully synthesized by two-step pyrolysis of corn straw with chemical activation of KOH, and was employed for the elimination of Cr(VI) and naphthalene (NAP) from water. Benefiting from KOH activation, the PBC_KOH was found to possess huge specific surface area of 2183.80 m²/g and many well-developed micropores with average particle size of 2.75 nm and main pore diameters distribution from 1 to 2 nm. The PBC_KOH presented an excellent adsorption performance with a theoretical monolayer uptake of 116.97 mg/g for Cr(VI) and a heterogeneous adsorption capacity of 450.43 mg/g for NAP. The uptake equilibrium was attained within about 120 min for Cr(VI), while about 180 min for NAP following avrami fractional-order model, revealing the existence of multiple kinetics during the adsorption. The thermodynamic results showed that the uptake of both Cr(VI) and NAP occurred spontaneously (-ΔG°), while in an endothermic nature for Cr(VI) (+ΔH°) and an exothermic characteristic for NAP (-ΔH°) with different randomness. Furthermore, the PBC_KOH was believed to enhance the Cr(VI) adsorption mainly through the combination of electrostatic attraction, complexation, ion exchange and reduction action, while achieving the high NAP uptake by pore filling and π-π stacking interactions.