Insights on the effects of pH and Fe(II) regeneration during the chromate sequestration by sulfidated zero-valent iron

Recent studies showed that Fe(II) acts as the major electron donor when sulfidated zero-valent iron (S-ZVI) is used for reductive removal of pollutants. However, few studies have focused on the utilization efficiency and regeneration rate of Fe(II). In this work, the dynamic interaction between pH and S-ZVI for Cr(VI) removal and Fe(II) production was investigated at several pH levels. It was found that the maximum utilization efficiency and regeneration rate of Fe(II) was obtained at pH 5, which is consistent with the Cr(VI) removal by S-ZVI. To further study the role of Fe(II) in Cr(VI) reduction, an innovative binary correlation analysis between aqueous Cr(VI) and Fe(II) was applied with the addition of 1,10-phenanthroline as a way to control Fe(II) availability for Cr(VI) reduction. The analysis revealed that the Cr(VI) sequestration was initially controlled by Fe-0/FeS and then by Fe (II), which accounts for the majority of the Cr(VI) sequestration. Additionally, the effects of several operational parameters (e.g., S/Fe ratio, S-ZVI dosage and initial concentration of Cr(VI)) on Cr(VI) removal were also evaluated and the results indicated that the removal of Cr(VI) exhibited a bimodal pattern with increasing S/Fe ratio, but followed a unimodal pattern with increase of S-ZVI dosage and chromium concentration. This study sheds additional insight into the dynamic interaction between pH and Fe(II), which is significant for further improving the efficiency of Fe(II) on contaminants.