The Morphology of Colloid Deposition onto Rough Surface under Unfavorable Conditions.

This study examines the morphology of colloid deposition onto rough surfaces under unfavorable conditions via theoretical analysis and laboratory column experiments. Saturated column experiments were conducted with bacteria- and virus-sized latex particles and different collectors (sand and glass beads) at different ionic strengths and flow rates. A three-step procedure was used for colloid retention and release, following dissection of the columns to determine the retained colloids. Results show that (i) the main retention processes of the large colloid include primary- and secondary-minimum depositions and straining/wedging, whereas for the small colloid are primary- and secondary-minimum depositions and retention in immobile zones; (ii) colloids deposited in secondary minima mainly reside aside the asperities (up gradient of them in terms of flow direction) and these deposited colloids can be released by reduction of solution ionic strength but are relatively insusceptible to hydrodynamics; (iii) colloids deposited in primary minima mainly reside atop the asperities and the deposited colloids cannot be released by changing solution ionic strength but are greatly influenced by flow velocities; (iv) The collision efficiency is not only a function of solution ionic strength and flow velocity, but also influenced by surface roughness via changing the interaction energies between colloids and collectors and hydrodynamic and adhesion torques. Our study indicates that it is critical to combining mechanical (e.g., increase flow rate) and chemical (e.g., decrease solution ionic strength) methods for efficient particle removal, which is required in many industrial and environmental cleaning processes (e.g., cleaning of semiconductor surfaces and remediation of contaminated soil).