Enhanced breakage of the aggregates of nanoscale zero-valent iron via ball milling

Aggregates of nanoscale zero-valent iron (nZVI) are commonly encountered for nZVI in aqueous solution, particularly during large-scale nZVI applications where nZVI is often in a highly concentrated slurry, and such aggregates lower nZVI mobility during its in-situ remediation applications. Herein, we report that the ball milling is an effective tool to break the nZVI aggregates and thereby improve the nZVI mobility. Results show that the milling (in just five minutes) can break the aggregates of a few tens of microns to less than one micron, which is one-tenth of the size that is acquired via the breakage using the mechanical mixing and ultrasonication. The milling breakage can also improve the efficacy of the chemical conditioning method that is commonly used for the nanoparticle stabilization and dispersion. The milling breakage is further optimized via a study of the milling operational factors including milling time, bead velocity, bead diameter, and chamber porosity, and an empirical equation is proposed combining the bead collision number during the milling. Mechanistic study shows that the high efficacy of the milling to break the aggregates can be explained by the small eddy created by the high shear rate produced by the close contact of the milling beads and may also relate to the direct mechanical pulverization effect. This study provides a high efficacy physical method to break the nanoparticle aggregates. The method can be used to improve the nZVI mobility performance by milling the nZVI slurry before its injection for in-situ remediation, and the milling may also replace the mechanical mixing during the nZVI stabilization via surface modification.