The classical colloid filtration theory (CFT) is the most commonly used approach for predicting particle transport in saturated porous media. Despite widespread use in modeling and experimental efforts, a growing body of experimental evidence suggests that the deposition behavior of microbial particles (e.g., bacteria and viruses) is inconsistent with CFT.; The objective of this research was to investigate the fundamental mechanisms that give rise to the observed deviation from CFT. Well-controlled laboratory-scale column deposition experiments were conducted using spherical glass beads as model collectors. To examine the validity of CFT, the effluent particle concentration and the profile of retained particles were systematically measured over a broad range of physicochemical conditions. Experiments were conducted using uniform polystyrene latex microspheres, a pathogenic protozoa (Cryptosporidium parvum), and two mutants of a well-characterized Escherichia coli (E. coli) K12 strain.; Deposition studies conducted with different-sized latex microspheres under various solution conditions (e.g., in the presence of anionic surfactant or at high pH), revealed that the observed deviation from CFT was directly related to variations in Derjaguin-Landau-Verwey-Overbeek (DLVO) interactions. Specifically, it was shown that breakdown of CFT in the presence of repulsive colloidal interactions can be attributed to (i) the occurrence of a secondary minimum in the DLVO energy profile, and (ii) charge heterogeneities on particle and collector surfaces. A dual deposition mode (DDM) model which considers the heterogeneity in particle-collector interactions demonstrated excellent agreement with measured particle concentrations and fractions of eluted particles.; Comparison of Cryptosporidium parvum oocyst deposition with that of similarly-sized microspheres indicated that the mechanisms controlling the observed deviation from CFT were the same in both systems. Elution experiments further suggested that specific interactions between oocyst-wall proteins and the glass beads could inhibit oocyst release upon perturbation in solution chemistry. Finally, evaluation of the deposition behavior of two E. coli mutants showed that non-DLVO-type interactions attributed to the presence of cell surface biomolecules further contribute to the observed deviation from CFT.; These findings have important implications for predictions of (bio)colloid transport because the two key mechanisms controlling the deviation from CFT are common to chemical-colloidal interactions in natural and engineered aquatic systems.
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