Wet Milling of Zerovalent Iron in Sulfide Solution: Preserving and Securing the Metallic Iron

Herein, a critical problem regarding the milling of zerovalent iron (ZVI) to produce fine ZVI particles is solved. During milling, ZVI is prone to be rapidly oxidized by oxygen, water, and other potential oxidants in the milling environment, which results in significant metallic iron (Fe(0)) loss and raises safety concerns. In this work, a sulfide solution (using Na2S) is introduced as the milling solvent; as benchmarks, ZVI milling under a pure aqueous solution (ZVI(W)) and organic solvents (pure ethanol, ZVI(E)) is also investigated. Produced ZVIs are characterized with six different techniques to assess their Fe(0) content, safety, reactivity, and reductive capacity. The obtained results show that the sulfide solution is effective in minimizing Fe(0) oxidation during milling; the Fe(0) content (∼63%, w/w) of ZVI milled in sulfide solution (ZVI(S)) is more than 10 times that of ZVI(W) after 5 h of milling. ZVI(S) is comparable to ZVI(E) in their Fe(0) contents, reductive reactivity, and capacity, but ZVI(S) is more stable and safer in use; an optimal Na2S concentration of 10 g Na2S/L is recommended. A mechanistic study shows that Fe(0) preservation in a sulfide solution results from a combined effect of the alkaline pH of the solution and the coating of iron sulfides on ZVI. This exploratory study demonstrates an original milling solution for a highly scalable, economical, and safe scheme to produce ultrafine and reactive ZVI with a high Fe(0) content for environmental applications.