Colloid transport in porous media: Experiments versus theory

The transport of synthetic polystyrene latex microspheres was examined in packed glass beads and quartz sand under unfavorable deposition conditions (where repulsive interaction exists between microspheres and porous media grain surfaces). In glass beads, the concentrations of retained microspheres decreased hyper-exponentially (faster than exponentially) with increasing transport distance. The hyper-exponential retained profiles indicate a decrease of deposition rate coefficients with distance. In quartz sand, retained microsphere concentrations increased first, and then decreased with transport distance, indicating nonmonotonic variation of deposition rate coefficients with transport distance. These observations demonstrate the ubiquity of deviation of retained profiles from theoretical prediction and that the form of deviation is highly sensitive to system conditions.; Deposition rate coefficients increased with increasing flow rate under favorable deposition conditions, consistent with expected trends from filtration theory. In contrast, under unfavorable deposition conditions, deposition rate coefficients decreased with increasing flow rate, suggesting a mitigating effect of hydrodynamic drag on deposition. The re-entrainment rate was negligible under favorable deposition conditions, but was significant under unfavorable deposition conditions and increased with increasing flow rate, further suggesting an influence of hydrodynamic drag under unfavorable conditions.; X-ray microtomography (XMT) was used to examine the importance of grain to grain contacts (not considered in filtration theory) on colloid deposition in porous media (glass beads and quartz sand) under both favorable and unfavorable deposition conditions. Under favorable conditions, the magnitudes of deposition and the profiles of retained microsphere concentrations with transport distance agreed well with filtration theory predictions, even though a significant fraction of deposition occurred at grain to grain contacts. This observation indicates the importance of grain to grain contacts can not be ignored for micron-sized colloids based on the fact that filtration theory predicts very well the deposition of such colloids. Under unfavorable deposition conditions, deposition of the 36-mum microspheres occurred dominantly at grain to grain contacts. Retained microsphere concentrations increased first, then decreased with distance, as observed for the 1.1-mum microspheres in quartz sand. It is proposed that translation and subsequent deposition at grain to grain contacts of surface-associated microspheres produced the nonmonotonic retained profiles.