Coupled Factors Influencing Concentration Dependent Colloid Transport and Retention in Saturated Porous Media.

The coupled influence of input suspension concentration (C_i), ionic strength (IS) and hydrodynamics on the transport and retention of 1.1 micron carboxyl modified latex colloids in saturated quartz sand (150 micron) was investigated.  Results from batch experiments and interaction energy calculations indicated that unfavorable attachment conditions occurred during these experiments (pH=10).  The percentage of retained colloids in column experiments decreased with C_i at intermediate IS conditions (31 or 56 mM), when the colloids were weakly associated with the solid phase by a shallow secondary energy minima.  These concentration effects were observed to depend on the system hydrodynamics and on transients in C_i, 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, 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 were increasingly non-exponential at lower values of C_i (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 C_i.  Higher values of C_i 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.