Transport of Surface-Modified Nano Zero-Valent Iron (SM-NZVI) in Saturated Porous Media: Effects of Surface Stabilizer Type, Subsurface Geochemistry, and Contaminant Loading

This study examined the transport behavior of nano zero-valent iron (NZVI) coated with three types of stabilizers (i.e., polyacrylic acid, Tween-20, and starch) in saturated sand-and soil-packed columns under varying geochemical conditions. The cations or ionic strength and humic acid (HA) affected the transport of NZVI in varying degrees for different types of surface-modified NZVI (SM-NZVI). The effects of HA on the transport of SM-NZVI were different in sand-and soil-packed columns. In the sand-packed column, the presence of HA exerted an effect on the particle-particle interaction (i.e., aggregation), resulting in either enhanced or decreased transport of SM-NZVI. However, in the soil-packed column, the HA not only influenced the particle-particle interaction but also exerted an effect on the particle-soil grain interaction (i.e., deposition). Additionally, a significant enhancement in the transport of SM-NZVI in the soil-packed column was observed with increasing particle concentration. Moreover, the adsorption of arsenic on the surface of SM-NZVI exhibited insignificant effect on the transport of SM-NZVI. The release of arsenic from the arsenic-loaded SM-NZVI was detected when subjected to flushing with phosphate-containing groundwater. This fundamental understanding of the subsurface transport of SM-NZVI is of critical importance for the benign use and risk management of SM-NZVI.