Transport of Fe-Based Nanoparticles in Porous Media Facilitated by Xanthan Gum: Non-Monotonic Relation Between Transport Efficiency and Flow Velocity

The transport of Fe-based nanoparticles (Fe-NPs) in porous media is of vital importance for application of Fe-NPs in groundwater remediation, yet their low mobility remains an open question. Here, we conducted column and microfluidic transport experiments combined with rheology experiments and model simulations to investigate the effect of xanthan gum (XG) on the transport of two types of Fe-NPs (nanoparticles of Fe₃O₄ (nFe₃O₄) and zero-valent iron (nZVI)) in quartz sand at different input concentrations and flow velocities. We observed that the rheological modification of water by XG significantly enhanced the transport of both Fe-NPs, and the transport of nZVI was better than that of nFe₃O₄. With the increase of input concentration of Fe-NPs, the transport of nZVI slightly declined, whereas the transport of nFe₃O₄ initially increased and then decreased. The different responses of transport of nFe₃O₄ and nZVI to input concentrations were attributed to the unique shear-thinning rheological properties of XG suspensions with each type of Fe-NPs. We observed a novel non-monotonic relation between transport efficiency and flow velocity, where the transport of both Fe-NPs initially weakened and then enhanced as pore-water velocity rose within a certain range. We demonstrated that the formation of a non-flowing layer of XG on the surface of the porous medium was identified as the mechanism responsible for the non-monotonic transport behavior. These findings provide new insights into transport behavior of Fe-NPs in porous media under the rheological remediation of XG, and have practical implications for application of Fe-NPs in groundwater remediation.