A controlled approach to optimising nanoparticle retention in porous media using retained particle distribution

Nanoparticles are increasingly used in applications that require transport through porous media—including subsurface remediation, enhanced oil recovery, and in situ delivery of reactive agents—yet their in-situ distribution is often difficult to control because retention can occur through multiple coupled mechanisms. Here, a retained particle size distribution (RPSD) framework was introduced that quantifies nanoparticle retention by combining (i) influent/effluent particle size distributions measured over time and (ii) independent pore size distribution data of the porous medium. Using iron oxide nanoparticles dispersed in a partially hydrolysed polyacrylamide (HPAM) polymer solution, sand-pack core flooding and ex-situ stability tests were carried out to evaluate the impacts of temperature (25–50 °C) and brine ionic composition. Increasing temperature shifted the suspension from a single mode (∼27 nm) to a bimodal distribution with a dominant aggregate population (∼122 nm), indicating thermally enhanced aggregation. Increasing salinity (especially with divalent ions) promoted aggregation through electrostatic double-layer compression and changes in polymer/colloid interactions. Mapping RPSD against pore sizes allowed to distinguish attachment to grain surfaces (retained sizes well below pore throats), physical straining or entrapment (sizes comparable to pore throats), and pore blocking (sizes exceeding pore throats), as well as to track their onset with injected pore volumes. The proposed RPSD-based mapping provides a practical, measurement-driven tool to design nanoparticle injections that achieve targeted retention while minimising unintended migration in porous media.