Deposition and reentrainment of colloids in porous media: Effects of natural organic matter and solution chemistry

The transport of colloidal particles in aquatic porous media is of great environmental interest for its public health implications and its technological applications. Solution chemistry and the adsorption of natural organic matter (NOM) on particles and collectors to a large extent dominate the stability of colloids in these systems.; In this study, the impacts of a variety of NaCl concentrations, pHs, the presence of Ca2+, and the adsorption of NOM on colloidal deposition and reentrainment have been evaluated. The experimental protocol included the characterization of the surface properties of sulfate latex colloids and soda lime glass collectors with and without NOM (mobility measurements, NOM adsorption, NOM adsorbed layer thickness), and deposition/reentrainment experiments conducted using the packed bed technique.; Solution chemistry affected adsorption of NOM and electrophoretic mobilities, both for particles and collectors. In all cases, the adsorption of NOM resulted in more negative electrophoretic mobility. Particle deposition increased with increasing ionic strength, decreasing pH, and in the presence of Ca 2+. Reentrainment was complete at low ionic strength, and decreased to a low value after the ionic strength was increased above a certain threshold. Reentrainment decreased when the solution pH was lowered, and when CaCl 2 was the added salt. Under most chemical conditions, larger attachment efficiencies and smaller fractions of particles recovered were observed in the presence of SRHA. The impact of SRHA on particle stability was more significant at high ionic strength, low pH, and in the presence of calcium. Calculations with a model based on the deposition in the secondary minimum (Maxwell kinetic theory) provided a better fitting of the experimental results than a model based on the primary minimum (Interaction Force Boundary Layer). The secondary minimum model was also consistent with the reentrainment of particles observed under most conditions.; Based on experimental results and model calculations, it is hypothesized that at low ionic strength reversible deposition in the shallow secondary minimum is the principal attachment mechanism (independent of the presence of SRHA), while for higher ionic strengths deposition may be regarded as a combination of capture in the primary minimum and capture in the secondary minimum.