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

Nanoparticles (NPs), especially nanoscale zero-valent iron (nZVI) particles, have been extensively used to directly treat contaminated zones in aquifers because of their desirable properties, i.e., high specific surface area and potential mobility. Understanding the transport of nZVI particles, through water-saturated porous media has important implications for many natural and engineered systems. For the first time, we used 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. Experimental observations, including (i) the more rapid migration of the front relative to bulk transport of the injected solution of NPs and (ii) the retention of NPs, with 27% of the iron retained at the conclusion of deionized water flushing, highlight the important controls of complex pore structure on the resulting retardation, attenuation and efflux of NPs. Complementary numerical simulations evaluate sample heterogeneity and its effects on local transport properties. In general, the model considering four regions of distinct porosities shows improved performance, as highlighted by the low overall residual sum of squares (0.041 to 0.138), compared to another model assuming a homogeneous pore structure (0.044 to 0.328). 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.