Chemical effects on the deposition of non-Brownian particles

Abstract

In this work, the effects of changes in solution chemistry on the deposition of non-Brownian latex particles in beds of glass bead porous media are examined. Experimental results are presented for the effect of varying [Ca 2+] at pH 7 on the zeta potential and initial deposition of 4 μm diameter polystyrene particles in packed columns of 0.4 mm diameter glass beads. The experimental results show that initial removal efficiencies can be reduced by approximately two orders of magnitude under unfavorable chemical conditions, i.e., in the presence of repulsive electric double layer (EDL) interactions. As conditions become more unfavorable, deposition is gradually decreased. The effects of chemistry on deposition are modelled by coupling surface chemistry, surface interaction force, and particle transport theories. Particle deposition is modelled using trajectory calculations with Happel's sphere-in-cell porous media model. Chemical effects are incorporated by including EDL interaction forces in the trajectory model. The EDL interaction force is calculated from experimentally-derived zeta potentials, assuming that these represent the potential at the onset of the diffuse layer. A comparison of model predictions and experimental results reveals quantitative agreement for favorable chemical conditions. However, the usual failure of deposition models that include EDL interaction effects to correctly predict the efects of unfavorable chemical conditions on deposition is observed. The model predicts an abrupt decrease to zero removal at a critical repulsive force while the results illustrate a gradual decrease in deposition as the conditions become more unfavorable. An analysis of some of the possible explanations for the failure of the ‘chemical model’ is presented. The effects of a stochastic distribution of particle and collector zeta potentials does not account for the discrepancy between theory and experiment. The effects of surface roughness, heterogeneous surface chemistry properties, salvation forces, hydrophobic effects, and dynamic aspects of EDL interactions may explain the observations. Estimates of the importance of some of these effects for the experimental system studied are presented. Using the trajectory model, characteristic times for the interaction of EDLs as a particle is transported towards a collector are calculated. These times can be compared to estimates of characteristic times for the relaxation of diffuse layers, charge transfer in fixed layers, and fluctuations in repulsive interaction energy barriers in examinations of theories to describe the observed effects of chemistry on particle deposition.