Tuesday, November 14, 2006

Effects of Particle Size and Reaction Kinetics on Colloid Attachment and Detachment in Porous Media.

Yan Jin1, Chongyang Shen2, and Yuanfang Huang2. (1) 157 Townsend Hall, Dept. of Plant & Soil Sciences, University of Delaware, Dept. of Plant & Soil Sciences, Newark, DE 19716-2170, (2) China Agricultural University, Department of Soil and Water Sciences, Beijing, 100094, China

A poor agreement between theoretical and experimental colloidal deposition rates under unfavorable deposition conditions has been widely reported in the literature. In this study, we present a new two-step theoretical framework for the kinetics of deposition of colloids: (1) colloids are transported from bulk solution and captured into the secondary minimum; (2) colloids in the secondary minimum overcome energy barrier and are attached in primary minimum. The rate of first step is obtained by numerically solving a convective-diffusion equation with colloidal and hydrodynamic interactions incorporated and with modified boundary conditions; the rate of second step is estimated by an expression that gives an exponential relationship between the rate and the height of energy barriers. The overall deposition rate depends on the dominant process of the two. Saturated colloid transport experiments, with polystyrene latex particles of various sizes (24 nm and 1000 nm) and through clean Accusand, were conducted at different solution ionic strengths. Detachment experiments were carried out with deionized water following the deposition experiments and then each column was sectioned to obtain the spatial distribution profile of the retained particles. The results indicate that the deposition rates of both particles increase with the increase of solution ionic strengths and that attachment of colloids in the primary minimum is more significant when ionic strength is higher. The theoretical deposition rates are in good agreement with experimental deposition rates.