Enhanced Cr(VI) removal by biochar-loaded zero-valent iron coupled with weak magnetic field

Abstract

The ball-milled zero-valent iron (ZVI) biochar composite (BM-ZVI-BC) with excellent dispersion and oxidation resistance was prepared by the ball milling method and to explore its removal capacity to Cr(VI) under weak magnetic field (WMF). Characterization analysis confirmed that the surface of BM-ZVI-BC was successfully loaded with ZVI. A series of experimental factors (mass ratio of ZVI and BC, ball milling time, dosage, and initial pH) were examined to investigate the effect on removal performance in different reaction systems. As the increase of the mass ratio of ZVI to BC and the extension of milling time, the removal of Cr(VI) firstly increased and then decreased, with the best results at 4:1 and 1.5 h respectively. Moreover, the Cr(VI) removal capacity decreased, as the pH increased (pH = 2–8). The removal capacity of Cr(VI) by BM-ZVI-BC reached 364.26 mg·g−1, which was significantly more than that of the ball-milled biochar (BM-BC) (3.92 mg·g−1) and ball-milled zero-valent iron (BM-ZVI) (7.83 mg·g−1). Biochar as a carrier for ZVI could effectively disperse ZVI in solution, mitigating the agglomeration effect when ZVI was present alone. With WMF, the removal capacity of Cr(VI) by BM-ZVI-BC/WMF was increased by 5.67–126.03% compared to BM-ZVI-BC at pH = 2–8. WMF promoted the release of iron ions and mitigated the effect of the passivation layer. The apparent rate constant (kobs) increased from 0.185 min−1 to 0.221 min−1 after the application of WMF, indicating that WMF accelerated the removal of Cr(VI). Overall, the combination of biochar, WMF, and ZVI has potential in the removal of Cr(VI).