Modeling the transport and retention of nanoparticles in a partially saturated pore

Nanoparticles enter the subsurface through various sources such as land application of wastewater, landfill leachates, and reuse of treated wastewater for groundwater recharge. Vadose zone acts as a barrier to protect the groundwater by retaining a portion of the infiltrated nanoparticles at solid-water (SWI), and air-water interfaces (AWI), and air-water-solid contact region. Hence, it is important to understand the movement of nanoparticles in the vadose zone to assess the groundwater contamination potential. A mathematical model is developed to simulate the transport and retention of nanoparticles in a single partially-saturated pore in the soil by accounting for particle deposition at SWI, AWI, and contact region. The transport in the pore is modeled using the advection-diffusion equation and the mass exchange with the SWI, AWI, and contact region are modeled as first-ordered reactions that depend on the interaction energy of particles with the interfaces. Contact region is found to play the dominant role in particle retention than SWI and AWI. Pore-scale results indicate that pore size, half corner angle, particle size, contact angle of the particle with AWI and flow velocity influenced the retention the most. The pore-scale results from this study will be further used to upscale particle transport to the continuum scale using pore-network modeling.