Enhanced Cr(VI) reduction and immobilization by Fe0 coupled with biochar through galvanic interaction

Zero-valent iron (Fe 0 ) is widely packed in filters for Cr(VI) removal, but it suffers from ferrochrome precipitate passivation. In this study, a mixed Fe 0 and biochar (BC) system was established and investigated for promoted Fe 0 corrosion and enhanced Cr(VI) removal through iron‑carbon micro-electrolysis (IC-ME) by batch and column experiments with Fe 0 and BC as controls. Due to electrochemical reaction between Fe 0 and BC, there was an enhancement in iron corrosion and subsequent formation of secondary minerals on Fe 0 surface. These minerals included magnetite, lepidocrocite, and goethite, which facilitated Cr(VI) adsorption and reduction. The Fe 0 -BC system therefore obtained a higher Cr(VI) removal rate of 86.3 % within 6 h than those of the BC (7.9 %) and Fe 0 (12.5 %) systems. Chromium species distribution showed that 76.2 % of removed Cr(VI) was converted to Cr(III). The resultant Cr(III) of 68.4 % was immobilized on Fe 0 and BC surfaces due to precipitation and sequestration. The optimal Fe/BC mass ratio was 2:1 for galvanic reaction and Cr(VI) removal. The removal of Cr(VI) by Fe 0 -BC exhibited a pH-dependency, with acidic conditions favoring higher removal rates. Additionally, the column experiment result revealed that Cr(VI) removal capacity of 4.0 mg Cr(VI)/(g Fe 0 ) and operational lifespan of 1851 PV in the Fe 0 -BC column were 6.7 and 3.9-fold greater than those in the Fe 0 column, further confirming that galvanic reaction could effectively alleviate Fe 0 surface passivation. These results indicated that mixed Fe 0 and BC as an effective medium in filter tanks could form IC-ME to enhance Cr(VI) reduction and immobilization during water treatment.