The behavior of dissolution/passivation and the transformation of passive films during electrocoagulation: Influences of initial pH, Cr(VI) concentration, and alternating pulsed current

• Initial pH, Cr(VI) and APC could affect the behavior of dissolution/passivation in Fe-EC.
• A dissolution/passivation region was constructed with different initial pH-Cr(VI).
• The film was rich in Fe and Cr at high Cr(VI), whereas with lots of Fe but negligible of Cr at low Cr(VI).
• The film was non-protective at long TAPC, but became more stable and protective at short TAPC.
• Behavior of dissolution/passivation and passive film transformation in Fe-EC was elucidated.

The passivation behavior of an iron anode for electrocoagulation (EC) was first investigated using response surface methodology (RSM). Tested initial pH range, Cr(VI) concentration and alternating pulsed current (APC) were 4.0 to 8.0, 52 to 520 mg L−1 and 10 to 590 s, respectively. The distance between electrodes was 25 mm, and K2SO4 (1 g L−1) was used as the supporting electrolyte in a 2.5 L EC reactor. Results confirmed that initial pH, Cr(VI) concentration, and APC significantly influence the extent of passivation. Then, based on the interaction effect on passivation behavior between initial pH and Cr(VI) in RSM, a pH-Cr(VI)-dissolution/passivation diagram was constructed with galvanostatic measurements. The diagram showed an optimal dissolution region for EC operation. This optimum was characterized by a reasonable final pH for extended precipitation and little passivation. Results of the cyclic voltammetry and X-ray photoelectron spectroscopy revealed a significant difference in the composition and stability of oxide films in the region with more pronounced passivation. Interestingly, the APC had both positive and negative effect on the passivation behavior. Long period of APC (TAPC = 590 s) produced a non-protective film, which favored the Fe0 dissolution. However, a more stable and protective passive film with a uniform structure of Fe and Cr oxides was formed by short TAPC (10 s). Based on the above results, this study elucidated the behavior of dissolution/passivation and the transformation of passive films during the Fe-EC process for Cr(VI) treatment.