Subsurface transport of carboxymethyl cellulose (CMC)-stabilized nanoscale zero valent iron (nZVI): Numerical and statistical analysis

Abstract

Subsurface remediation using nanoscale zero valent iron (nZVI) is a promising in-situ technology that can transform certain groundwater contaminants into non-toxic compounds. However, field scale implementation of nZVI technology has faced major challenges due to poor subsurface mobility, limited longevity and well clogging, all leading to a shorter nZVI travel distance. This distance nZVI travels in the subsurface is an important parameter since it influences the amount of contaminants that can be reached and thereby remediated. There are several factors which may affect nZVI travel distance such as groundwater velocity, injection concentration and rate, lag period (duration when nZVI injection is stopped), solution viscosity, and subsurface heterogeneity. Although various studies have been performed to reveal the effect of different factors on nZVI transport in homogeneous domains, few studies have focused on heterogeneous media, which is more representative of field conditions. In this study, a statistical analysis was performed using a two-dimensional numerical model which simulated carboxymethyl cellulose (CMC) stabilized nZVI transport in randomly distributed soil permeability fields of two aquifers to examine the factors that have the greatest impact on nZVI travel distance. Among all possible factors, field scale solution viscosity and injection rate had a statistically significant effect on nZVI travel distance in both the horizontal and vertical directions, as well as, on the attached mass. Additionally, the lag period between injections had a statistically significant effect on the attached mass, but not the travel distance. These results suggest that having a long injection period followed by a short lag phase during field deployment may result in less nZVI attachment. Lastly, aquifer heterogeneity impacted the nZVI spread while the impact of intrinsic groundwater velocity and injection concentration was found not to be statistically significant. Results from this numerical study can aid in field-scale CMC-nZVI injection by identifying key factors for remediation optimization.

Keywords: Colloid transport; Field scale; Groundwater remediation; Nanoscale zero valent iron; Soil heterogeneity; Statistical analysis.