Effect of the Lower Boundary Condition and Flotation on Colloid Mobilization in Unsaturated Sandy Sediments
2008 (English)In: Vadose Zone Journal, ISSN 1539-1663, E-ISSN 1539-1663, Vol. 7, no 3, 930-940 p.Article in journal (Refereed) Published
Unsaturated soil or sediment columns pose challenges for colloid mobilization studies because the boundary condition imposed at the column outlet may cause experimental artifacts. Our objective was to study in situ colloid mobilization during transient, unsaturated flow as affected by the boundary condition imposed at the column outflow, and to elucidate the mechanisms of colloid mobilization. We conducted colloid mobilization experiments by infiltrating unsaturated, packed sandy sediment columns under two different bottom boundary conditions: a seepage and a suction control. Different flow rates and ionic strengths were used for the column experiments. The mechanisms of colloid mobilization were investigated theoretically using force calculations (adhesive and interfacial forces), complemented with “flotation” experiments, where colloids in the bulk fluid and at the liquid-gas interface were measured separately. More colloids were mobilized under seepage than under suction-controlled boundary conditions. The shape of the colloid breakthrough curves also differed: for the seepage boundary, the maximum of the colloid concentration occurred right at the beginning of the column outflow, but for the suction-controlled boundary, colloid concentrations in the outflow increased gradually before reaching a maximum. Colloid mobilization increased with flow rate and decreased with ionic strength for both boundary conditions; however, colloids were mobilized even at ionic strength exceeding the critical coagulation concentration (CCC). Flotation experiments showed that colloids were located both in the bulk fluid and at the liquid-gas interface at electrolyte concentrations less than the CCC, but only at the liquid-gas interface when the CCC was exceeded. Theoretical considerations confirmed that interfacial forces at the liquid-gas interface exceeded adhesive forces at all ionic strengths used in our experiments. Both experiments and theory showed that the liquid-gas interface had a dominant effect on colloid mobilization. As the movement and configuration of the liquid-gas interface is controlled by the lower boundary condition in column experiments, so is colloid mobilization.
Place, publisher, year, edition, pages
2008. Vol. 7, no 3, 930-940 p.
Other Environmental Engineering Oceanography, Hydrology, Water Resources
IdentifiersURN: urn:nbn:se:uu:diva-163154DOI: 10.2136/vzj2007.0163OAI: oai:DiVA.org:uu-163154DiVA: diva2:462894