Effect of N doping on sludge biochar driving Cr(VI) removal from aqueous solution

N-doped sludge biochar prepared by pyrolysis of municipal sludge impregnated with NH 4 Cl solution and was used to investigate the effects of different factors on Cr(VI) adsorption-reduction. Furthermore, spectroscopic techniques were employed to investigate the structural feature of N-doped sludge biochar and the mechanism of Cr(VI) adsorption-reduction in depth. The results showed that compared with pristine sludge biochar (SBC), the Cr(VI) of removal efficiency and reduction efficiency on NSBC increased by 2.35 and 3.32 times, respectively. The removal of Cr(VI) by biochar was in accordance with the pseudo-second-order kinetic model and Langmuir model, and increasing the reaction temperature promoted the Cr(VI) removal by SBC and NSBC. The maximum adsorption amount of Cr(VI) by SBC and NSBC was 8.76 mg/g and 25.93 mg/g, respectively. The Cr(VI) reduction by NSBC was closely related to pH, and the reduction capacity of Cr(VI) under acidic condition was much higher than that under alkaline condition. Moreover, co-existing cations (Ca 2+, Na + and Mg 2+ ) and co-existing anions (Cl -, NO - 3 and SO 2- 4 ) had no significant effect on Cr(VI) adsorption-reduction by NSBC, while humic acid inhibited the Cr(VI) removal by NSBC. The Cr(VI) removal from aqueous solution by NSBC involved Cr(VI) adsorption-reduction and Cr(III) release-adsorption. During the adsorption-reduction process, the persistent free radicals and N-containing groups on NSBC acted as reduction sites for Cr(VI). This work provided in-depth mechanistic insight for N-doped biochar treating Cr(VI)-containing wastewater.