COLLOIDAL REMOVAL DURING TRANSPORT IN VARIABLY-SATURATED HANFORD SEDIMENTS

We investigated the transport of natural colloids through variably-saturated Hanford sediments. Column experiments were conducted under steady-state water flow with effective water saturation ranging from 0.55 to 1.0. Colloid transport was described with a mobile-immobile region model accounting for first-order colloid deposition from the mobile region. Under saturated conditions, no colloids were removed from the liquid phase during transport, while under unsaturated conditions colloids were removed from the mobile water region. Though model simulations indicated that colloid deposition inside the column was irreversible under constant chemical and hydrological conditions, colloids deposited within the column could quantitatively be recovered in the column outflow by re-saturating the column after each unsaturated-flow colloid breakthrough, indicating that colloid deposition inside the column was triggered by film straining in the presence of the liquid-gas interface instead of physical-chemical surface deposition. As colloids were smaller than the calculated thickness of adsorbed thin water films, colloid retention in the system was attributed to the non-uniformity of water film thickness distribution owing to the heterogeneity of Hanford sediments. Colloids were retained only when water film thickness was smaller than their sizes by means of capillary forces. Heterogeneity effects of Hanford sediments on colloidal removal were explored by relating first-order colloid deposition coefficients, which increased with decreasing system saturation, to water film thickness profiles in the system simulated from various probability distribution functions.