Transport and Straining of Colloid-sized Materials in Saturated Porous Media: Experimental and Numerical Analysis.

The growing body of research on colloids and colloid-facilitated transport of various contaminants such as radionuclides, pesticides, heavy metals etc. provides strong evidences to prove the potential of colloids to carry highly sorbing contaminants through the subsurface causing groundwater contamination. Colloid attachment and colloid straining have been identified as key retention mechanisms in saturated porous media. This study investigated transport and retention of glass bead colloids (diameter 1-10 μ m) and soil colloids extracted from volcanic ash soils (less than 1 μ m) in saturated Toyoura sand at different colloid concentrations and different flow rates by means of a series of column experiments. Deposition profiles of colloids were also examined. The results showed that glass beads colloids exhibited complete retention (presumably due to straining) at both high and low flow rates. Soil colloids, on the other hand, showed complete retention at a low flow rate but showed significant recovery at the high flow rate. Particle size distribution analysis of influent and effluent colloids revealed the deposition of smaller colloids (due to attachment) as well as larger colloids (due to straining) in the porous media. Deposition profiles of both colloid types were nonmonotonic with multiple peaks. Deposition profiles for the glass bead colloids exhibited a large peak near the column inlet and two smaller secondary peaks in deeper layers. In contrast, the deposition profiles for soil colloids exhibited the highest peak toward the bottom of the column. Particle size distribution analysis of deposited glass bead colloids revealed that the largest colloids were deposited near the column inlet (due to mechanical filtration and straining). Numerical analysis carried out using the HYDRUS 1D code assuming first-order attachment, detachment and straining models revealed that straining was the dominant retention mechanism for both colloid types. The simulated colloid deposition profiles of both colloid types, however, failed to capture the observed multiple peaks in the deposition profiles.