Enhanced electron transport and selective reduction of nitrate to nitrogen by sludge biochar-supported nanoscale zero-valent iron

In the process of nitrate reduction by nano zero-valent iron (nZVI), the preparation of nZVI/sludge biochar (nZVI/SBC) by introducing SBC will help to improve the aggregation and low nitrogen selectivity. Herein, the pyrolysis temperature of SBC was changed, and the excellent reduction capacity with the minimum charge transfer resistance were obtained at 300 °C. In addition, nZVI/SBC prepared with a mass ratio of 1:1 obtained a nitrate removal efficiency of 95.23 % and a nitrogen selectivity of 35.33 %. The corrosion of Fe 0 at the appropriate ratio provided sufficient electrons for the reduction, and the further transformation into Fe 3 O 4 promoted the outward transport of electrons. The electrons were transferred to the solid surface through the SBC, which promoted the accumulation and collision of nitrogen intermediates and induced the production of nitrogen. The nitrate removal efficiency of nZVI/SBC remained above 66 % in the wide pH range (3.0–11.0), which demonstrated practical nitrate remediation capabilities. • nZVI/SBC achieved the nitrate removal efficiency of 95.23 % and the nitrogen selectivity of 35.33 %. • The nZVI/SBC1:1 (SBC pyrolyzed at 300 °C) showed the smallest charge transfer resistance and stronger electron transport. • Hydrogen ion consumption and nitrogen accumulation on the solid surface of nZVI/SBC induced the formation of N 2 . • The SBC dispersed nZVI and acted as an electron shuttle to receive and donate electrons to improve the N 2 selectivity.