Factors affecting the Transport of Carboxymethyl Cellulose coated Zero Valent Iron Nano-particles in Soil

Abstract
Background and objectives: As versatile materials, zero valent iron nanoparticles (NZVI) have been employed for in-situ decontamination of a wide range of water and soil contaminants, including organic chlorinated compounds, pesticides, inorganic anions and heavy metals. To carry out a successful clean-up plan, it is necessary to deliver the decontaminant agent to the vicinity of pollutant. Therefore a considerable number of studies have investigated the factors which affecting transport behavior of NZVI particles in natural subsurface environments. Most of these studies have been utilized model solutions and well defined homogeneous porous media such as glass beads and pure quartz sand. As bare (unmodified) NZVI particles has been found to be immobile even in homogeneous porous media, different coating agent such as Carboxymethyl cellulose (CMC) has been used for surface modification of NZVI in order to improve their mobility in subsurface environments. So far, no investigation has been conducted on undisturbed soil columns, considering the real properties of the media in which the NPs are transported. So in this study, different soil types covering a wide range of soil properties in terrestrial systems are examined and the main characteristics associated with NZVI mobility in saturated soil media.

Materials and methods: Several parameters (n=29) including physiochemical and hydraulic properties of 20 different soil types and nanoparticle characteristics in soil extract suspension were measured and the transport parameters estimated from breakthrough curves employing a two-site kinetic model of advection-dispersion equation. Principal component analysis (PCA) was then used to explore the significant factors which control CMC-NZVI transport. Multi-linear regression model was investigated between the percentage of transported CMC-NZVI through the soil and the properties of soil and nanoparticles.
Results: Results showed that depending on the soil type, 10.2 to 61.9 percent of introduced CMC-NZVI mass passed through the soil columns which indicates CMC-NZVI particles are mobile in soil medium; However CMC-NZVI particles were significantly retained by soils with higher clay contents and salinity. PCA results showed that 7 selected principal components (PC) described 88.2% of the total variance of the input variables where, Solution chemistry had high loading values in PC1 among the examined parameters. A multi-linear regression model developed between two kinds of input variables including primary variables and factor scores (FSs) as secondary variables, and percentage of transported CMC-NZVI through the soil column, showed that regression model employing FSs as secondary input variables presents a better estimation of CMC-NZVI particles transportability in soil with higher R2 and lower RMSE values.
Conclusion: PCA results indicate the significance of solution chemistry, clay content and hydrodynamic properties of soil in CMC-NZVI transport. Results of this study suggest CMC-NZVI particles are mobile enough to be employed for subsurface remediation when clay content and salinity of soil are not so high. However more investigations are need to explore the efficiency of these materials for removing different pollutants from natural soils and subsurface media.