Stabilization of nanoscale zero-valent iron (nZVI) with modified biochar for Cr(VI) removal from aqueous solution

[Display omitted] •Raw biochar (BC) and three modified BC were used to stabilize nZVI particles.•Cr(VI) removal by nZVI supported on four types of biochars were compared.•Effects of nZVI/BC mass ratio, pH and initial Cr(VI) concentration were examined.•The mechanisms of Cr(VI) removal by nZVI supported on modified biochar were probed. Three types of modified biochar (BC) were produced respectively with acid (HCl) treatment (HCl-BC), base (KOH) treatment (KOH-BC) and oxidation (H2O2) treatment (H2O2-BC) of raw biochar. Both the raw biochar and modified biochars supported zero valent iron nanopartilces (nZVI) (i.e. nZVI@BC, nZVI@HCl-BC, nZVI@KOH-BC and nZVI@H2O2-BC) were synthesized and their capacities for Cr(VI) removal were compared. The results showed that the nZVI@HCl-BC exhibited the best performance and the underlying mechanisms were discussed. The surface elemental distribution maps of the nZVI@HCl-BC after reaction with Cr(VI) showed that Fe, Cr and O elements were deposited on the surface of HCl-BC evenly, indicating that the formed Cr(III)/Fe(III) could settle on the surface of HCl-BC uniformly rather than coated only on the nZVI surface. This reveals that the supporter HCl-BC could also play a role in alleviating the passivation of nZVI. Besides, the effects of mass ratio (nZVI/HCl-BC), pH, and initial Cr(VI) concentration on Cr(VI) removal were examined. At lower mass of HCl-BC, nZVI aggregation cannot be fully inhibited on the surface of HCl-BC, whereas excessive biochar can block the active sites of nZVI. Additionally, it was found that Cr(VI) removal by nZVI@HCl-BC was dependent on both pH and initial Cr(VI) concentration.