1. Introduction
Rapid industrial production has led to large-scale deposition of
metal-contaminated waste into soil and water and emission to the
atmosphere, causing widespread ecosystem damage. As a result, wastewater
treatment is becoming increasingly complex, and the transport of heavy
metal ions (e.g., Pb2+, Cd2+,
Cr2+, Zn2+, and
Cu2+) in groundwater has become the focus of
considerable research interest in environmental geotechnology (Tansel
and Rafiuddin, 2016; Hudcova et al., 2018; Wang et al., 2019).
The presence of mobile solid particles, such as colloids, clay
particles, bacteria and silicon powders (SPs), can provide additional
media for the transport of heavy metal ions, thereby resulting in an
increase or decrease in the transport rate of heavy metal ions (Syngouna
and Chrysikopoulos, 2016; Kamrani et al., 2018). Clearly, a better
understanding of the cotransport of heavy metal ions and SPs is
essential for the remediation of contaminated soils. Karathanasis et al.
(1999) showed that the existence of colloids enhanced metal ion
transport and that Zn2+ was more mobile than
Cu2+. Malkovsky et al. (2015) revealed
colloid-facilitated transport of uranium by groundwater from the ore
zone to the surface of Earth. Recently, Abbar et al. (2019) investigated
the cotransport of heavy metals and kaolinite particles in sand columns
and evaluated the potential role of kaolinite in heavy metal transfer
and the influence of flax geotextiles on the transfer of these
pollutants.
The interactions of heavy metal ions with the soil matrix and their
transport processes in pores can differ substantially (Raikova et al.,
2019; Valsala and Govindarajan, 2019). Wu and Li (1998) reported that
the mobility of several typical heavy metal ions followed the order of
Cu2+>Zn2+>Pb2+>Cd2+ in neutral leachate (pH=7) but
followed the order of
Cd2+>Zn2+>Cu2+>Pb2+ in more acidic leachate. Tang et
al. (2013) indicated that water-soluble organic carbon could compete for
adsorption sites or form soluble complexes with heavy metal ions to
reduce their adsorption onto soil media to facilitate the transport of
heavy metal ions. Kumpiene (2015) investigated the microbial functional
diversity, biochemical activity, heavy metal ion availability and soil
toxicity of Cd2+-, Pb2+-, and
Zn2+-contaminated soils to restore soil ecological
functions. Ma et al. (2016) showed that the rapid transport of soil
colloids facilitated As(V) transport by inhibiting As(V) adsorption onto
sand. Lee et al. (2019) revealed that the outflow concentration of heavy
metal ions, such as Cr2+ and Pb2+,
in the pore solution increases with increasing colloidal particle
concentration, while Cd2+ and Cu2+lagged behind the release of colloidal particles; that is, the physical
and chemical effects in the solution notably impacted heavy metal ion
migration promoted by different suspended particles.
The presence of SPs can facilitate or inhibit heavy metal ion transport,
depending on the interactions between heavy metal ions and SPs and their
deposition mechanism onto the solid matrix of the porous medium,
including influencing factors such as the ionic concentration, solution
pH, double layer repulsive force, van der Waals force and critical salt
concentration, especially the particle size of SPs (Chrysikopoulos and
Katzourakis, 2015; Valsala and Govindarajan, 2019). These factors are
influenced by the physical-chemical changes in the various suspended
matter in seepage flow. For example, positively charged heavy metal ions
can be easily adsorbed onto negatively charged SPs, which will
conversely affect the deposition coefficient and dispersion process of
SPs. Moreover, SPs with a small particle size can facilitate heavy metal
ion transport due to the size exclusion effect (Bennacer et al., 2017;
Russell and Bedrikovetsky, 2018), whereas SPs with a particle size
larger than the pore size of the porous medium can become entrapped due
to pore constrictions, which can therefore inhibit heavy metal ion
transport. Wang et al. (2012) investigated the effect of the particle
size on the retention and transport of nanoparticles in saturated porous
media and examined the applicability of the
Derjaguin-Landau-Verwey-Overbeek (DLVO) theory to describe the
deposition of small nanoparticles on sand surfaces. Alem et al. (2015)
proposed that, regardless of the hydraulic operating conditions, the
median diameter of the particle-size distribution (PSD) of the recovered
particles clearly increased over time due to physical clogging of the
porous media.
The restriction mechanisms on the movement of individual suspensions
(e.g., SPs or heavy metal ions) such as straining and filtration have
been extensively examined previously. Ahfir et al. (2017) investigated
the removal performance of suspended particles with a diameter range of
2−30 µm in porous media based on median diameter variations of the
deposited suspended particles along a column and breakthrough curve
evolution in the effluent. Li et al. (2018) simulated the microscale
changes in colloidal particle transport and deposition in porous media
and replicated the flow field by reconstructing the pore structure, the
main purpose of which focused on the deposition probability and the
spatial distribution of deposited particles. The deposition process is
always inseparable from the detachment mechanism. In this aspect, the
experimental works of Cui et al. (2017) aimed to delineate the
detachment characteristics of deposited quartz powder particles in
porous media, and they revealed that changing the flow direction was
more effective than changing the flow rate through a two-dimensional
sandbox packed with quartz sand. Despite the many studies on the coupled
transport of heavy metal ions and suspended particles in flowing water
(Bradford et al., 2009; Kim and Walker, 2009), the deposition mechanisms
of heavy metal ions together with suspended particles along the
migration distance remain to be elucidated.
In this paper, the deposition characteristics and migration distance of
lead ions (Pb2+) in the presence of SPs were
investigated in porous media in a long one-dimensional laboratory column
experiment at three injection concentrations of Pb2+,
two particle sizes of SPs, and two Darcy velocities. The deposited
Pb2+ and SPs along the column were measured, and the
PSDs and microstructure photos at different migration distances were
obtained after test completion. The test results indicated that the
presence of SPs may promote or inhibit Pb2+ migration,
which is closely related to the concentration of injected
Pb2+, the particle size and concentration of injected
SPs, the seepage velocity, and the change in absolute zeta potential in
the surface charge.