Fate and Transport of Nano Zero-Valent Iron (NZVI) in Subsurface Porous Media during Groundwater Remediation

Nanoscale zero-valent iron (NZVI) technology is quickly becoming the most widely used nanotechnology for environmental remediation. NZVI has been successfully applied for degrading chlorinated organics and immobilizing heavy metals in the subsurface. While the use and development of NZVI are understandably heralded as a promising environmental nanotechnology, fundamental questions remain on its long-term effectiveness, fate and health risks in the environment. In this research, systematic experiments were performed to investigate the fate and transport of NZVI particles under conditions relevant to in-situ remediation. The environmental fate of different types of NZVI (bare NZVI and surface-modified NZVI (SM-NZVI)) before and after chemical reactions was investigated with batch experiments. In addition, the colloidal stability and transport behavior of different types of SM-NZVI were examined under subsurface conditions with batch and column experiments.;Aggregation, sedimentation, and Cr/As desorption of three types of NZVI were investigated under various conditions. It was found that the aggregation behavior of the NZVI differed from one another in regard to reaction time and ionic strength, associated with the respective critical size for sedimentation. Sedimentation of NZVI was positively related to the concentrations and average particle sizes. Moreover, desorption of Cr/As from Cr/As loaded NZVI was detected in the presence of typical groundwater ions, as well as natural organic matter. However, in comparison to the insignificant desorption of Cr, the desorption of arsenic was substantial.;The desorption of arsenic from As(V)-loaded NZVI (As-NZVI) was further investigated under varying conditions. The effects of the As(V)/NZVI mass ratio, pH, and aging time on arsenic desorption from As-NZVI by phosphate were investigated. Less arsenic desorption was observed at lower pH or higher As(V)/NZVI mass ratio. Compared with fresh As-NZVI, the amount of phosphate-extractable arsenic significantly decreased in As- NZVI aged for 30 or 60 days.;The effects of humic acid (HA) and Ca2+ on the colloidal stability of NZVI coated with three types of stabilizer (polyacrylic acid (PAA), Tween-20 and starch) were evaluated. Either increased stability or decreased stability was observed for different SM-NZVI in the presence of HA. The presence of Ca2+ exerted a slight influence on the settling of NZVI modified with PAA or Tween-20, in the absence or presence of HA. However, the presence of Ca2+ caused significant aggregation and sedimentation for starch-modified NZVI in the absence of HA; and induced an even higher degree of aggregation and sedimentation in the presence of HA.;The effects of groundwater geochemistry on the transport of different types of SM-NZVI were investigated in saturated sand- and soil-packed columns. The cations or ionic strength in groundwater decreased the transport of SM-NZVI to various degrees for different types of SM-NZVI. The effect of HA on the transport of SM-NZVI differed in sand- and soil-packed columns. Moreover, the adsorption of arsenic on the surface of SM-NZVI exhibits insignificant effect on the transport of SM-NZVI; and the release of arsenic from the arsenic-loaded SM-NZVI was detected when subjected to flushing with phosphate-containing groundwater.