This study examines the deposition/release mechanisms involved in colloid retention under unfavorable conditions through theoretical analysis and laboratory column experiments. A Maxwell approach was utilized to estimate the coupled effects of primary- and secondary-minimum deposition. Theoretical analysis indicates that the secondary energy minimum plays a dominant role in colloid deposition even for nanosized particles (e.g., 20 nm) and primary-minimum deposition rarely happens for large colloids (e.g., 1000 nm) when diffusion is the dominant process. Polystyrene latex particles (30 and 1156 nm) and clean sand were used to conduct three-step column experiments at different solution ionic strengths, a constant pH of 10, and a flow rate of 0.0012 cm/s. Experimental results confirm that small colloids can also be deposited in secondary minima. Additional column experiments involving flow interruption further indicates that the colloids deposited in the secondary energy well can be spontaneously released to bulk solution when the secondary energy minimum is comparable to the average Brownian kinetic energy. Experimental collision efficiencies are in good agreement with Maxwell model predictions but different from the theoretical values calculated by the interfacial force boundary layer approximation. We propose a priori analytical approach to estimate collision efficiencies accounting for both primary- and secondary-minimum deposition and suggest that the reversibility of colloid (e.g., viruses and bacteria) deposition must be considered in transport models for accurate predictions of their travel time in the subsurface environments.

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In this study we conducted experiments with saturated columns packed with sand and glass beads to quantitatively examine surface roughness effect on deposition and release of micro‐ and nano‐sized colloids at different solution ionic strengths. Experimental results showed more colloid retentions in both primary and secondary energy minima in sand than in glass bead columns, especially at high solution ionic strengths (e.g., >0.01 M). This observation cannot be explained by the classic Derjaguin–Landau–Verwey–Overbeek (DLVO) theory, which assumes sphere‐smooth surface configuration. We modified the Derjaguin approximation approach and calculated interaction energies, which indicate that the sharp asperities on sand surfaces can facilitate colloid deposition in primary minima by reducing the energy barrier. In addition, the increased attachment in secondary minima in sand columns can be attributed to the presence of the valleys on sand surfaces where colloids associated at secondary minima can be shielded from hydrodynamic shear. Additional theoretical analysis verified that large valleys can locally increase the energy barrier as well as the secondary‐minimum depth, and hence, are favorable for colloid deposition in secondary minima. Whereas the reduction effects of surface roughness on energy barrier has been extensively addressed in the literature, our modified DLVO analysis and experimental results demonstrate that the effect of this mechanism is only effective at high ionic strength for large colloids (e.g., >0.01 M for the 1156 nm colloid in this study). We provide experimental evidence and theoretical demonstration that surface roughness also plays an important role in colloid deposition at secondary minima under unfavorable conditions. Our study provides a more complete understanding of the effect of surface roughness on colloid deposition.

[1] This study presents an a priori model for predicting attachment efficiency of colloid deposition in porous media under unfavorable conditions. The model takes into account coupled effects of diffusion and hydrodynamic forces on colloid attachment efficiency that result from both primary and secondary minimum deposition. Effect of diffusion was quantified using the Maxwell approach, and influence of hydrodynamic drag was determined by comparing adhesive and hydrodynamic torques that act on the attached colloids. Main findings from this study include (1) the attachment efficiency does not change with increase of flow velocity until it reaches a critical value at which the attachment efficiency decreases as flow velocity further increases and (2) the attachment efficiency increases with increasing collector diameter when the condition that adhesive torque is greater than hydrodynamic torque is not guaranteed over the entire collector surface. Whereas the classic filtration theory only takes into account the effects of system hydrodynamics on single collector contact efficiency, we show additionally their effects on attachment efficiency. Furthermore, results of this study imply that in addition to the important role that collector size plays in the straining process, which has been the focus of many recent studies, it also influences the attachment process thus must be considered when describing colloid retention and transport in porous media under unfavorable attachment conditions.