Ball Milling-Enabled mechanochemical reduction of Cr(VI) using sponge iron over an extensive pH range

With the assistance of mechanical ball milling (BM), the partially oxidized micro-sponge iron (micro-Fe0@Fe2O3) substantially increased by 42–79 % eradication of Cr(VI) across a broad pH range, in contrast to pristine micro-Fe0@Fe2O3, which has a negligible effect on Cr(VI) reduction as pH varies. Mechanical force applied during BM eliminates the surface oxide layer, exposing the Fe0 core to Cr(VI) and leading to sustained reduction of Cr(VI). The efficiency of micro-Fe0@Fe2O3 is strongly influenced by dissolved oxygen (DO): higher DO results in the generation of stable reductant Fe(II) under acidic conditions at the beginning stage of ball milling, while DO consumption of intermediate reductant Fe(II) hinders the reduction of Cr(VI) under alkaline conditions due to the ball milling action significantly increases the pH. Parameters of BM operation have a significant impact on the reduction of Cr(VI), with rotation speed dominating the removal of Cr(VI). The higher rotation speed of 550 rpm and lower DO of 3.2 mg/L would be valid in the following application. Adsorption, reduction of Cr(VI) to Cr(III), and subsequent precipitation of Fe0.33Cr0.67(OH)3 contribute to the mechanism for Cr(VI) removal by ball milling. The residual solid after the reaction can be disposed as a non-hazardous waste.