Coupled factors influencing concentration-dependent colloid transport and retention in saturated porous media

The coupled influence of input suspension concentration (Ci), ionic strength (IS), and hydrodynamics on the transport and retention of 1.1 microm carboxyl-modified latex colloids in saturated quartz sand (150 microm) under unfavorable attachment conditions (pH 10) was investigated. The percentage of retained colloids in column experiments decreased with Ci at intermediate IS conditions (31 or 56 mM) when colloids were weakly associated with the solid phase by a shallow secondary energy minima. In contrast, the effects of Ci on colloid retention were absent when IS was too low (6 mM) or too high (106 mM). The concentration effects under intermediate IS conditions were dependent on the system hydrodynamics, magnitude of Ci, and injection order of Ci, but they were largely independent of the input colloid mass. These observations were explained in part by time- and concentration-dependent filling of retention sites. Only a small fraction of the solid surface area was found to contribute to retention when IS was 31 mM, and micromodel observations indicated that colloid retention was enhanced in lower velocity regions of the pore space that occurred near grain-grain contacts. Consequently, retention profiles for IS = 31 mM conditions were increasingly nonexponential at lower values of Ci (during filling), whereas the observed concentration effect was largely eliminated as retention locations became filled. In addition, micromodel observations indicated that liquid and solid phase mass transfer of colloids to retention locations was influenced by Ci under intermediate IS conditions. Higher values of Ci are expected to produce less relative mass transfer to retention locations due to increased numbers of collisions that knock weakly associated colloids off the solid phase. Hence, the concentration effects were found to be largely independent of input colloid mass during filling of retention sites.