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.