Mechanochemical Synthesis of Manganese-Modified Microscale Zerovalent Iron for Efficient Cr(VI) Removal: Performance and Mechanism

Metal doping for improving the reactivity of zerovalent iron (ZVI) has been well studied, while Mn(II)-modified microscale ZVI (Mn-mZVI) has not yet been explored. Herein, ball-milled Mn-mZVI was fabricated and used for Cr(VI) removal. Characterization analysis showed that the structure, composition, and charge of mZVI changed after the Mn(II) modification. The comparative test showed that Mn-mZVI could remove 100% of Cr(VI) within 10 min, whereas mZVI removed negligible Cr(VI) within 60 min. The zeta-potential and electrochemical evidence verified that the enhanced electrostatic attraction and electron-transfer ability contributed to the superior Cr(VI) removal performance of Mn-mZVI. Moreover, the solution pH increase caused the decline of Cr(VI) removal, and the presence of NO₃^– inhibited Cr(VI) removal, whereas other coexisting ions showed little influence on the Cr(VI) removal performance of Mn-mZVI. Chemical and material characterization analyses revealed that Cr(VI) reduction by Mn-mZVI was the combined action of Fe(0) and generated Fe(II). In addition, the reusability of Mn-mZVI was not ideal due to the surface passivation and loss of Mn(II), but the reactivity could be reactivated by ball-milling the reacted Mn-mZVI again with Mn(II). Overall, this work provides a new mentality for mZVI modification and is important to develop promising mZVI-based materials for Cr(VI) pollution control.