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).