High-resolution characterization of nanoparticle transport in heterogeneous porous media via low-field nuclear magnetic resonance

Understanding the transport of nanoparticles (NPs), including nanoscale zero-valent iron (nZVI) particles, through water-saturated porous media has important implications for many natural and engineered systems. For the first time, we use spin-echo single point imaging (SE-SPI) of low-field Nuclear Magnetic Resonance (LF-NMR) to monitor nanoparticle transport through a heterogeneous porous medium. The ability of this method to provide information of nano- to micro-scale pore structure and to monitor transient processes is verified by a transport experiment using modified nZVI particles. Early breakthrough of the NP front and large amount of residual NPs (27% of the iron was retained in the core) were observed in the experiment. These observations were due to the presence of preferential flow paths and dead-end pores as demonstrated by micro X-ray Computed Tomography (μCT) imaging. Spatial and temporal data provided by this method enabled numerical simulations to quantify the impacts of spatial heterogeneity in pore structure on NP transport in terms of transport parameters. The simulation that captures the spatial heterogeneity by explicitly considering four regions of distinct porosities outperforms the model assuming a homogeneous pore structure. The maximum of residual sum of squares reduced from 0.328 to 0.138 when considering the impact of pore structure. The transport parameters vary significantly among different regions. For example, in the lowest porosity region of this sandstone, colloid dispersion (D) is 0.10 cm²/min instead of 0.20 cm²/min in the most connected region, while the attachment (k_ac) increased from 0.007 to 0.025 min⁻¹. Overall, SE-SPI imaging is shown to be an important tool in refining transport processes of NPs in heterogeneous porous media with application to constrain complex natural systems.