Passivation effect of Fe⁰ in the removal of Cr (Ⅵ) from groundwater and the optimization of electrochemical depassivation parameters

Zero-valent iron (Fe⁰) is widely used in the remediation of groundwater polluted by Cr(Ⅵ). However, there is a problem that passivation reduces the remediation efficiency. First of all, Fe⁰ was used to remove Cr (Ⅵ) in groundwater, and passivated iron filings with different passivation degrees were prepared. Subsequently, the electrochemical method was used to deactivate the passivated iron filings, and the effects of electrode setting, electrolysis voltage, electrolysis time and electrode distance on the depassivation effect were studied by single factor test and orthogonal test. Meanwhile, the depassivation solution and the iron filings before and after depassivation as well as the precipitate produced during the depassivation process were analyzed. The results showed that the depassivation effect was the best when passivated iron filings were used as anode. The depassivation effect first increased and then decreased with the increase of electrolysis voltage, decreased with the increase of electrode distance, and increased with the increase of electrolysis time. The influence of three factors on the depassivation effect from high to low was as follows: electrolysis time > electrolysis voltage > electrode distance. The results of the X-ray diffractometer, scanning electron microscope and energy dispersive spectroscopy analysis showed that the activity of passivated iron filings could be recovered effectively due to surface precipitation shedding under electrochemical action. The optimum conditions for depassivation of passivated iron filings were as follows: passivated iron filings for anode, electrolysis voltage = 10 V, electrolysis time = 60 min, electrode distance = 2 cm. Under these conditions, the removal rate of Cr (Ⅵ) by iron filings after depassivation could be restored to more than 90% of the original; Cr (Ⅲ) precipitate could not be dissolved, but the concentration of Fe in the solution could be increased during depassivation process. These research results could provide a useful reference for improving the remediation effect and material utilization rate of Cr (Ⅵ) polluted groundwater by Fe⁰.