Atmosphere matters: The protection of wet ball milling instead of dry ball milling accelerates the electron transfer effect of sulfurized micron iron/biochar to Cr (VI).

• Na 2 S 2 O 4 , biochar, and micron iron were reunited by wet and dry ball milling. • Wet ball milling promoted faster and complete Cr(VI) removal (100 % in 80 min) • The composite of wet ball milling presented excellent anti-ageing property. • Higher electron selectivity was obtained (59.15% for wet and 46.72% for dry) • Replacing oxides with sulfides facilitated high iron cycling for Cr(VI) removal. Obstacles such as aggregation and low activity of micron zero-valent iron in pollutant treatment could be overcome by sulfidation and biochar support. Ball milling is a promising method for the preparation of iron-based composites but the milling atmosphere is critical. To maximize the reactivity and identify the effects of ball mill atmosphere, ball milling technology under wet and dry atmosphere was applied to prepare composites from mZVI, Na 2 S 2 O 4 , and pine wood biochar (BC) for Cr(VI) removal. The results showed that S-mZVI/BC-wet was highly dispersible with more FeS 2 coverage and thus more efficient on Cr (VI) removal than S-mZVI/BC-dry (99.9 % removal in 80 min vs. 87.9 % removal in 24 h) at the optimum Fe: C mass ratio (1:1), S: Fe molar ratio (1:10), and reaction conditions. Cr(VI) was adsorbed by a spontaneous, endothermic mechanism that combined physisorption and chemisorption. The reduction process was vulcanization efficiency and iron cycle dependent. Reduction outperformed adsorption on the Cr(VI) removal and high reduction rateswere electron-selectively related (59.15 % for S-mZVI/BC-wet and 46.72 % for S-mZVI/BC-dry), implying S-mZVI/BC-wet was more capable. S-mZVI/BC-wet presented excellent anti-ageing property and slightly inferior regeneration performance. In conclusion, the composite prepared by wet ball milling have excellent performance in rapid and complete removal of Cr(VI), and S-mZVI/BC-wet has superior potential in acidic wastewater treatment. [ABSTRACT FROM AUTHOR]