Enhanced Cr(Ⅵ) reduction by zero-valent iron and ferroferric oxide wet ball milling: Synergy of electron storage and electron transfer

The electron storage quantities and electron transfer rate of the reductants are the most critical factors to determine the contaminants degradation in aqueous solution. Zero-valent iron (Fe0) has been considered as a potential reducing agent due to its considerable amount of electron storage, and a variety of modification methods have been explored to strengthen the low electron transfer rate of the Fe0 in the past 30 years. In this study, Fe0-Fe3O4-BM was prepared by ball milling with ethylene glycol as a liquid grinding aid for the reduction of Cr(Ⅵ). In the batch experiment with pH = 3.0, C0 = 30 mg·L-1 and solid–liquid ratio = 1 g·L-1, the Cr(Ⅵ) removal rate nearly 100 % for Fe0-Fe3O4-BM was reached, significantly higher than 38.9 % and 5.3 % respectively for Fe0 and Fe3O4, with the Cr(Ⅵ) removal capacity exceeding its theoretial sum from individual milled Fe0 or Fe3O4. The removal process conformed with pseudo-second-order kinetic, implying that electron transport was the principle limiting step. Semiconductor properties of Fe3O4 played a decisive role in the iron composites, reflecting the greater electron transfer rate and lower resistance of Fe0-Fe3O4-BM. This study revealed that the synthesised Fe0-Fe3O4-BM composite showed a splendid synergy between the abundant electron storage of Fe0 and the fast electron transfer of Fe3O4, with micro-nano structure prepared by wet ball milling providing a highly effective reagent with a large-scale production potential for in-situ injection based groundwater remediation.