Impact of Colloid Size on its Transport in Porous and Fractured Media

The physicochemical factors affecting the transport of colloids in the subsurface are of foremost interest from both scientific and applicable points of view. One of the most important parameters is the colloid size. The transport of three sizes of latex microspheres, 0.02, 0.2 and1 micrometer was explored in: (1) homogenous silica sand following three different levels of treatment; (2) unsaturated homogeneous sand;(3) unsaturated heterogeneous sand, with three levels of water content; and (4) two natural discrete fractures crossing chalk cores, with equivalent hydraulic apertures of 180 and 350 micrometer. In all case it was found that maximum recovery was obtained for the 0.2 micrometer microspheres, slightly lower recovery for the 1 micrometer colloids, and much lower recovery for the 0.02 micrometer colloids. Retention of the 0.02 micrometern colloids was more sensitive to the level of sand cleaning than that of the larger colloids. In the natural chalk fractures, recovery of the 0.2 and 1 um colloids ranged between 79-99percent while for the 0.02 micrometer colloids recovery was around 75percent. The small colloids retained within the fractures could not be remobilized even under relatively fast flow rates of the dispersive solution, therefore it was concluded that they irreversibly penetrate the complex porous media of the surrounding chalk matrix (average pore size 0.15 micrometer). A comparison between transport through fractures of dense clay particles and buoyant microspheres, within the same size range, reveals that in addition to size, colloid density also plays an important role. Interestingly, despite the different mechanisms that play the dominant role for colloid retention depending on the media collector properties, the common observation is maximum recovery of middle-size colloids (0.2 micrometer) less recovery for larger colloids, and minimum recovery for the smallest colloids.