| The attachment, detachment, and deposition of colloids, 0.33-μm in diameter, at a glass surface of a narrow parallel-plate channel were experimentally investigated. Colloids were suspended in synthetic groundwater, similar to a natural groundwater at the Nevada Test Site. Experiments were conducted at pH 8, ionic strengths of 0.003, 0.025, and 0.05 M and flow rates between 0 and 0.16 ml/hr (average velocities between 0 and 2.8 × 10−3 cm/s). A video microscopic setup and image processing routines were developed to in situ enumerate attached, detached, and deposited colloids, and to track previously occupied locations at the surface. The setup utilized dark field microscopy to visualize colloids with sufficient resolution (0.5 μm) and provided a relatively large field of view (330 x 245 μm) to increase statistical accuracy of extracted data. A “moving window” approach was employed to establish connectivity between successive images captured every 90-s.; Within the range of flow rates employed, colloid deposition at 0.003 M was negligible. At 0.025 M, colloid attachment and detachment exhibited two successive regions with time: an initial nonlinear region and a linear region. At 0.05 M, colloid attachment exhibited similar behavior with time, while the detachment process was persistently linear. Colloid deposition fluxes reached a constant = 0 for no-flow condition, >0 for flow and 0.025 M, and <0 for flow and 0.05 M. The final colloid attachment, and detachment fluxes increased linearly with flow rate and almost exponentially with ionic strength.; Analysis of results suggested that colloid attachment and detachment processes were controlled by continuous activation and deactivation of sites at the glass surface. Results were interpreted based on a two-sites model, in which two types of sites were identified: Sites I, formed during the preconditioning of the glass surface with particle-free solution; and Sites II, formed during the experiments. The concentration of Sites I increased linearly with preconditioning times >17 hours, and with ionic strength. Tracking of previously occupied sites indicated that no attachment occurred later at these sites. Analysis suggested that the deactivation of these sites, which was attributed to glass dissolution, was not caused by, but associated with detached colloids. |
