Solvent-free production of nanoscale zero-valent iron (nZVI) with precision milling

Despite its effectiveness in degrading a wide array of environmental contaminants and superior physical characteristics for subsurface delivery, large-scale applications of nanoscale zero-valent iron (nZVI) for environmental remediation have been inhibited by the high costs associated with its conventional production technologies. In this contribution, an environmentally-benign and cost-effective production method of nZVI is demonstrated using a precision milling system. Unlike conventional methods such as chemical synthesis and vapor phase condensation, which typically involve toxic chemicals, sophisticated equipment and extensive labor, the precision milling method relies solely on the mechanical impact forces generated by stainless steel beads in a high-speed rotary chamber to break down the micro iron particles. The system uses no toxic solvents, is completely scalable to large-scale manufacturing. Scanning electron microscope (SEM) and BET surface area analysis independently verify that, after 8 hours of milling, the feed micro iron was effectively reduced to particles with sizes below 50 nm. The surface chemistry and crystal composition of the milled iron were characterized with high-resolution X-ray photoelectron spectroscopy (HR-XPS) and X-ray diffraction (XRD). Reactivity of the milled nZVI was probed through reactions with water and seven model chlorinated aliphatic compounds. The results demonstrate the milled nZVI (8 hour) is more reactive for contaminant degradation than the nZVI synthesized by the widely-adopted borohydride reduction method. The ball milling method thus stands as a promising green process for large-scale nZVI production and enhances the prospect of the nZVI technology for large-scale environmental remediation.