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

Author
item Bradford, Scott
item KIM, HYUNJUNG - University Of California
item HAZNEDAROGLU, BERAT - University Of California
item TORKZABAN, SAEED - Berkeley National Laboratory
item WALKER, SHARON - University Of California

Submitted to: Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/23/2009
Publication Date: 9/15/2009
Citation: Bradford, S.A., Kim, H.N., Haznedaroglu, B.Z., Torkzaban, S., Walker, S.L. 2009. Coupled Factors Influencing Concentration Dependent Colloid Transport and Retention in Saturated Porous Media. Environmental Science and Technology. 43(18)6996-7002.

Interpretive Summary: Our understanding of and ability to predict the fate of colloids, such as pathogenic microorganisms, and colloid-associated contaminants in soils and aquifers is currently limited due to the complex coupling of many factors. The objective of this work was to investigate the coupled influence of colloid concentration with solution chemistry and water velocity on colloid retention mechanisms in sand. Specific solution chemistry conditions were identified when concentration dependent colloid transport is expected. Results from this work also have important implications for quantifying the evolution of colloid retention in soil over time, for determining the potential importance of transients in suspension concentration on colloid fate, and for predicting long-term colloid transport. This information will be of interest to scientists and engineers concerned to predicting the fate of colloids and microorganisms in the environment.

Technical Abstract: The coupled influence of input suspension concentration (Ci), ionic strength (IS) and hydrodynamics on the transport and retention of 1.1 'm carboxyl modified latex colloids in saturated quartz sand (150 'm) 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 Ci 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 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, 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 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. 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.