Pore-scale simulation of solute and colloid transport and direct comparison with column and microfluidics experiments

Abstract

In this study, pore-scale simulation tools to resolve flow and transport using the lattice Boltzmann (LB) method and the random walk particle tracking method (RWPT) were developed to solve advection-diffusion of solute or colloidal particles through porous media. Both LB and RWPT codes were parallelized to enable direct simulations of column and microfluidic experiments, conducted by collaborators, in acceptable computational times. The RWPT code, specifically, can generate tracer concentration profiles at the outlet known as breakthrough curves (BTCs). To directly compare with column experiments, digitalized images of the columns that contains about 10 million fluid voxels were directly used in LB and RWPT simulations. LB simulation was used to obtain the velocity field in the column. Using the average advection velocity from the LB simulation, input parameters for the RWPT simulation were determined to match the Péclet number of the column experiment. The breakthrough curves of the column experiment for non-adsorbing solutes such as iodide (I−) agree with those from the experiments. For transport of solute involved in equilibrium sorption-desorption processes between the solution and the porous medium such as cesium (137Cs+), a probabilistic method was developed. Numerical batch experiments were performed to determine the probabilities that reproduce the partition coefficients measured in the experiments. Without relying on curve fitting methods or empirical correlations, RWPT simulations reproduced retarded breakthrough curves matching the experimental data. To simulate colloid transport experiments conducted in microfluidic porous media models with embedded collectors, which are beads with surface charge that irreversibly adsorbs colloidal particles with finite adsorptive capacities, RWPT was extended to include linear and nonlinear finite adsorptivity models. Flow and transport in bead-based microfluidic porous media analogues with mixed surface charges were simulated. Colloid transport and deposition from simulations were compared to experiments on the level of breakthrough curves. With the inclusion of physics-based electrostatic interaction range and dynamic blocking functions, RWPT simulations captured the kinetics of this complex advection-diffusion-adsorption process and generated breakthrough curves that are in good agreement with the experiments. Lastly, the movement of colloids under the influence of hydrodynamic lubrication that hinders colloid deposition was modeled as an anisotropic random walk. Preliminary numerical simulations, performed on a body centered cubic (BCC) domain, suggested that near-wall hindered diffusion can facilitate transport of reactive colloids.