4.3 Particle-size distributions of the SPs with the flowing water
For SPs with smaller particle sizes (e.g.,D 50<10 µm), the preferential flow caused by the size exclusion effect can accelerate the movement of heavy metal ions (e.g., Pb2+) along with that of SPs (Bai et al., 2017; Katzourakis and Chrysikopoulos, 2019). However, for SPs with larger particle sizes (e.g., D 50=13.4 and 24.7 µm), the migration process will experience notable deposition due to the pore blocking effect of the porous medium, and eventually, heavy metal ion migration will be inhibited rather than promoted. The microstructure photos after the transport tests (Figs. 9−11) reveal that the SPs with larger particle sizes are first deposited within a relatively short distance from the injection surface of the sand column (e.g., x =1 cm), while the SP deposition amount gradually decreases with increasing distance. At a certain point, the deposition amount rapidly decreases (e.g., x =30, 60 cm), which is consistent with the measured deposition concentrations along the migration distance of the sand column (Fig. 7).
The so-called straining effect is sensitive to the ratio of the solid particle diameter D 50 to the porous medium grain diameter d g (Ahfir et al., 2017; Bai et al., 2017). In the experiments,D 50/d g=0.0089−0.0165 (i.e., D 50=13.4 and 24.7 µm, andd s=1.5 mm), and as a result, the straining effect becomes dominant relative to the preferential flow (Xu et al., 2006; Johnson et al., 2010; Alem et al., 2015). Large-sized SPs can lead to pore narrowing (see Figs. 9(a), 10(a) and 11(a)), which enhances the SP deposition probability by the straining effect, especially at a low velocity (e.g., v =0.087 cm/s; Fig. 9(a)). Comparing Figs. 9 and 10 reveals that with increasing seepage velocity (e.g.,v =0.087→0.260 cm/s), the SP deposition amounts at the same distance decrease and advance to locations farther away from the injection end. That is, at a low velocity, the porous medium captures most of the injected larger-sized SPs at the entrance. At a high velocity, the water flow carries the particles deeper into the porous medium, producing more gradual changes in the deposition profile. Certainly, at the same seepage velocity, the migration distance of larger-sized SPs will be readily limited to a narrow range (Fig. 11;D 50=24.7 µm).
Fig. 2 shows the PSD of the two selected SPs (i.e.,D 50=13.4 and 24.7 µm) in the different sections of the sand column after transport test completion. Fig. 2 shows that with increasing distance from the injection end, the particle size of the deposited SPs gradually decreases, exhibiting a clear particle-separation characteristic due to the flowing water. For example, for the injected SPs of D 50=13.4 µm comprising a wide range of d =1−60 µm, the median diameter isD 50=26.4, 17.6, 11.3, and 9.5 µm when x =1, 30, 60, and 89 cm, respectively, while for the injected SPs ofD 50=24.7 µm with a range of d =1−120 µm, the median diameter is D 50=42.7, 17.8, 15.0, and 11.5 µm, respectively. Clearly, at the injection end (e.g., x =1 cm), the median diameter of the deposited SPs will be even larger than that of the injected SPs, which will gradually decrease with increasing migration distance, and finally, the median diameter will be smaller than that of the injected SPs near the outflow (e.g., x =89 cm).