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.

A theoretical library of N1s core binding energies of polynitrogen molecules and ions in the gas phase.

Polynitrogen molecules and ions are important building blocks of high energy density compounds (HEDCs). High energy bonds formed at the N sites can be effectively probed by X-ray photoelectron spectroscopy (XPS) at the N K-edge. In this work, with the density functional theory and the ΔKohn-Sham scheme, we simulated the N1s ionic potentials (IPs) of 72 common polynitrogen molecules [tetrazoles, pentazole (N5H), diazines, triazines, tetrazines, furazans, oxazoles and oxadiazoles], ions [pentazolate anion (cyclo-N5-), pentazenium cation (N5+), etc.], and molecular (NH3⋯N5H, H2O⋯N5H) and ionic (NH4+⋯N5-, H3O+⋯N5-) pairs, as well as mononitrogen relatives. These constitute a small theoretical database for absolute N1s IPs with an average accuracy of ca. 0.3 eV. To understand the structure-IP relationship within this family, effects of side substituent and bridging groups, local bonding types (amine or imine N), charge and protonation states, and vibronic coupling were analyzed based on selected systems. This study in the gas phase collects inherent chemical shifts of nitrogen in high-energy NN and NC bonds, which provides an essential reference into XPS interpretations of more complex HEDCs in the solid state. We especially highlight the evident N1s chemical shifts induced by protonation for nitrogen in the five-membered ring (N5H versus cyclo-N5-, ca. 7 eV; NH3⋯N5H versus NH4+⋯N5-, ca. 3 eV; H2O⋯N5H versus H3O+⋯N5-, ca. 2 eV), and suggest XPS as a sensitive tool in determining the hydrogen positions in pentanitrogen-based HEDCs.