Mechanistic model for the cotransport of colloids and radionuclides in saturated porous systems based on the interaction forces between colloids

Abstract

The presence of colloids in groundwater can enhance radionuclide transport by adsorbing them. In general, colloid transport dominates the process of cotransport. Ripening and site-blocking are two common behaviors in the colloid transport process that are essentially affected by intercolloid forces. In this work, a finite difference model was constructed to describe the cotransport behaviors of radionuclides and colloids in saturated porous media by focusing on the forces between colloids. The proposed model was formulated based on the assumption that the presence of nuclides did not influence colloid transport and ignored the decay of radionuclides in a short period. The one-dimensional advection-dispersion transport of nuclides was coupled with the transport of nuclides in the form of pseudo-colloids. The adsorption and desorption processes of nuclides on media, as well as on mobile or filtered colloids, were considered. Moreover, ripening in the presence of attractive forces between colloids and site-blocking in the presence of repulsive forces between colloids were investigated. The distribution ratios of nuclides in the free solute and in colloidal states during cotransport were reported, and the ripening and site-blocking mechanisms were unified through the force field between colloids. The model was verified by comparing the calculated results with the experimental data. In addition, a parameter sensitivity study was carried out to explore the influence of each parameter on the cotransport process. The model proposed in this paper could be helpful to predict the transport and retention risks of nuclides in porous media containing many colloids.