Transport and retention of clay particles in saturated porous media. Influence of ionic strength and pore velocity

The aim of this work was to experimentally evaluate the role of ionic stren gth and pore velocity on clay suspension transport and retention through a saturated porous medium. A smectite suspension was injected into columns fi lled with a very fine quartz sand. Experiments were carried out at constant pore velocity with increasing ionic strength adjusted with a divalent elec trolyte (CaCl2) and at constant ionic strength (using three chemical condit ions) with decreasing pore velocity. Typical colloid breakthrough curves sh ow two important behaviors: a constant outlet concentration value after a t ransient phase, and a pronounced tailing effect at the end of the injection step. No differences were observed between the mean travel time of a solut e tracer and that of the clay suspension, The classical advection-dispersio n equation coupled with a first-order two-site kinetics model was used to r eproduce the experimental breakthrough curves. The kinetic model consisted in a site with irreversible deposition and a reversible site used to reprod uce the transient phase preceding the plateau of the experimental breakthro ugh curves, The particle fraction kept by the porous medium increases with ionic strength; consequently the kinetic parameters of the numerical model vary with chemistry. The irreversible sorption rate (K-irr: equivalent to a clean-bed filter coefficient) increases with ionic strength and was direct ly determined from experimental data, With increasing ionic strength, the d eposition rate (K-d) for the reversible sorption increases whereas the rele ase rate (K-r) decreases. The kinetic parameters of the reversible site sho w an evolution with pore velocity similar to that observed in kinetics mode l used for modeling solute transport in double porosity media. With decreas ing pore velocities, the retention of clay particles increases but the kine tic deposition coefficient of the irreversible site decreases. Particle dep osition can also be described and reinterpreted in terms of collector effic iency using the concept of the sphere-in-cell model. The collector efficien cy, which adds a correction to the kinetic parameter with the residence tim e, is a more consistent way to represent particle retention. Its value incr eases with increasing ionic strength and decreasing pore velocity. (C) 2001 Elsevier Science B.V. All rights reserved.