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Colloid transport through soil and other porous media under transient flow conditions—A review

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Abstract Understanding colloid transport in porous media under transient‐flow conditions is crucial in understanding contaminant transport in soil or the vadose zone where flow conditions vary constantly. In this article, we provide a review of experimental studies, numerical approaches, and new technologies available to determine the transport of colloids in transient flow. Experiments indicate that soil structure and preferential flow are primary factors. In undisturbed soils with preferential flow pathways, macropores serve as main conduits for colloid transport. In homogeneously packed soil, the soil matrix often serves as filter. At the macroscale, transient flow facilitates colloid transport by frequently disturbing the force balance that retains colloids in the soil as indicated by the offset between colloid breakthrough peaks and discharge peaks. At the pore‐scale and under saturated condition, straining, and attachment at solid–water interfaces are the main mechanisms for colloid retention. Variably saturated conditions add more complexity, such as immobile water zones, film straining, attachment to air–water interfaces, and air–water–solid contact lines. Filter ripening, size exclusion, ionic strength, and hydrophobicity are identified as the most influential factors. Our review indicates that microscale and continuum‐scale models for colloid transport under transient‐flow conditions are rare, compared to the numerous steady‐state models. The few transient flow models that do exist are highly parameterized and suffer from a lack of a priori information of required pore‐scale parameters. However, new techniques are becoming available to measure colloid transport in real‐time and in a nondestructive way that might help to better understand transient flow colloid transport. This article is categorized under: Science of Water > Hydrological Processes Science of Water > Water Quality
Different colloid discharge concentration peak timings compared to water transients. (a) Occurrence of colloid concentration peaks as a result of preferential flow (note the occurrence of the colloid concentration peak before the discharge peak; Reprinted with permission from Majdalani et al., 2007); (b) colloid concentration peaks soon after the onset of irrigation and during drainage (i.e., drainage‐induced secondary peaks; Reprinted with permission from Zhuang et al., 2007); (c) occurrence of colloid concentration peaks concurrent with sharp changes in discharge (the line indicates flow rate and the dots indicate concentrations; Reprinted with permission from El‐Farhan et al., 2000); (d) colloid concentration peaks concurrent with discharge peaks (Reprinted with permission from Auset et al., 2005)
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The 19‐port sampling system at the bottom of an intact/undisturbed soil column to differentiate different flow pathways (a) and the water flux collected by the 19‐port sampling system (b) (Reprinted with permission from Mohanty, Bulicek, et al., 2015)
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Illustration of the novel DNA‐labeled particle tracing technique (Reprinted with permission from McNew et al., 2018)
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Colloids interacting with imbibition (a, b, e) and drainage (c, d, f) fronts on a hydrophilic and a hydrophobic substrate (Reprinted with permission from Lazouskaya et al. (2013), Figures 1 and 2). Two interface positions on the particle (for the two surface tension force directions, that is, φ > h and φ < h) are shown (left and right column). The earlier interface position is represented with the dashed line; only the surface tension force F*σ and position angle φ* are shown for this interface. For the later interface position, the direction and components of the surface tension force and other forces are shown. Force arrows do not represent force magnitudes. Panels e and f show the corresponding torques denoted as TD = FDlD, TA = FAlA, and Tr = Frlr. The torques are analogous for a hydrophobic substrate and are not shown. Point O denotes the point of rotation and all torques are calculated with respect to O. The large arrows indicate flow direction
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Illustration of film straining mechanisms. Illustration of pendular rings and water films, and colloids retained in disconnected pendular rings and thin water films (a) (Reprinted with permission from Wan & Tokunaga, 1997), and illustration of the thin water film straining mechanism (b) (Reprinted with permission from Wan & Tokunaga, 1997)
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The mechanisms of colloid retention at SWIs, release from an immobile/stagnant water zone, and retention at and release from air–water interfaces (AWIs). In the filter ripening process, (a) colloids attached to SWIs can serve as extended retention sites (Reprinted with permission from Keller & Auset, 2007). (b) Colloids are released from an immobile water zone when the wetting front passes and the immobile water zone reconnects to the bulk water flow (Reprinted with permission from Gao et al., 2006). (c) Colloids continuously retained at an air bubble surface (Reprinted with permission from Gao et al., 2006), but (d) colloids can be released from pendular rings when water invades the pore space (Reprinted with permission from Gao et al., 2006)
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