MODELING THE EFFECT OF PHYSICAL AND CHEMICAL HETEROGENEITY OF GRAIN SURFACE ON NANOPARTICLE TRANSPORT IN A SINGLE PORE IN SOIL

Abstract

The physical and chemical heterogeneity of soil grains significantly affects nanoparticle transport. However, no quantitative relationships exist for particle deposition rates accounting for grain-surface heterogeneity. This study quantifies the effect of various physicochemical parameters on the nanoparticle deposition rate coefficients in a single heterogeneous pore in soil. A mathematical model is developed to simulate the transport of nanoparticles through an idealized pore of cylindrical shape with rings of the same height on the wall representing roughness. Chemical heterogeneity is considered by assigning positive and negative charges to the top of the ring elements and the remaining area of the pore wall, respectively. Particle transport is simulated by solving the advection-diffusion equation with first-order sorption at the pore wall. Nanoparticle breakthrough curves obtained from simulations are fitted with a 1D advection-dispersion-sorption equation. The pore-averaged deposition rate coefficients obtained thus are satisfactorily described using a power-law relationship vis-a-vis pore-scale parameters. The Damkohler number for nanoparticle attachment to the pore wall is significantly affected by parameters representing pore and particle radii, flow velocity, surface potentials of nanoparticles, and regions of the pore wall having positive and negative charges, and Hamaker constant. However, the Damkohler number for nanoparticle detachment from the pore wall is predominantly influenced by parameters representing particle size, roughness height, surface potentials of nanoparticles, and regions of the pore wall having positive and negative charges, Hamaker constant, and ionic strength. Chemical heterogeneity plays a dominant role in nanoparticle retention than wall roughness. The above relations can be incorporated into a pore-network model to quantify the effect of grain-surface heterogeneity on nanoparticle deposition at the continuum scale.