Enhanced Cr(VI) sequestration and longevity of milli-sized zero-valent iron facilitated by micro-electric field: Column study

Surface passivation caused by Fe-Cr precipitates limits the application of milli-sized zero-valent iron (mFe 0 ) in filter tanks for Cr(VI) removal. A mFe 0 -based column was established using sand as dispersing media, coupled with a micro-electric field (MEF, 1.2 mA/cm 2 ), to accelerate iron corrosion and enhance Cr(VI) sequestration. A control column without MEF was employed for comparison. Surface analysis verified that the MEF induced mFe 0 particles into three-electrodes with bipolar charges and facilitated electron transfer from iron-core to surrounding O 2 /H 2 O. Ferrous ions from iron corrosion afterwards reduced Cr(VI) and precipitated into secondary minerals (e.g., magnetite, lepidocrocite, and green rust). These minerals effectively sequestered Cr(VI) in their matrices through adsorption-reduction/precipitation on Fe 0 and sand surfaces. Compared to the mFe 0 column, the MEF-mediated mFe 0 (MEF-mFe 0 ) column demonstrated a 2.5-fold increase in kinetic rate constant and a 10-fold decrease in Cr(VI) migration rate, leading to a 9.5-fold augment in Cr(VI) sequestration capacity and a 3.7-fold extension in longevity. Under different hydraulic retention times and influent concentrations, the MEF-mFe 0 column outperformed the mFe 0 column with 6.0 to 9.6-fold higher Cr(VI) sequestration and 3.5 to 5.7-fold longer lifespan. Our study revealed that MEF-assistance represented a promising approach to enhance mFe 0 activation for subsequent Cr(VI) sequestration from water.