SIGNIFICANCE OF STRAINING IN COLLOID DEPOSITION: EVIDENCE AND IMPLICATIONS

Filtration theory has been developed to characterize colloid attachment when deposition is controlled by chemical interactions between colloids and grain surfaces. Over the past decade considerable research suggests that colloid deposition is frequently not consistent with filtration theory predictions. Filtration theory does not include the potential influence of pore structure, grain-grain junctions, and surface roughness on straining deposition. This work highlights recent experimental evidence that indicates that straining can play an important role in colloid deposition under unfavorable attachment conditions, and may explain many of the reported limitations of filtration theory. This conclusion is based upon micromodel observations, measured colloid size distributions before and after passage through porous media, liberation of deposited colloids when the direction of water flow was reversed, and comparison of effluent concentration curves and spatial distribution data for variously sized sands and colloids. Methods to quantify and mathematically model attachment and straining processes will be reviewed, including the complimentary nature of size exclusion and straining. Implications for colloid straining on transport in saturated systems, the vadose zone, and heterogeneous formations will also discussed.