Co-transport of chromium(VI) and bentonite colloidal particles in water-saturated porous media: Effect of colloid concentration, sand gradation, and flow velocity
Introduction
Groundwater pollution is often the result of human activities. Any activity that leads to the diffusion of chemicals or wastes into the environment has the potential to contaminate the groundwater. When groundwater is polluted, its cleaning will be difficult and expensive (Burri et al., 2019). Many processes can affect the fate and transport of pollution in the groundwater. Some of these essential processes are transport, absorption, and biological decomposition. Transport occurs when pollutants travel with groundwater. Absorption occurs when pollutants link themselves to soil or colloid particles. Absorption can slow down or sometimes facilitate the movement of pollution in groundwater. Biological degradation occurs when microorganisms use hazardous substances as an energy source (Speight, 2019).
Migration and transport of colloid particles are present in natural porous media as an engineering concern in groundwater pollution (Ghiasi et al., 2020). These colloid particles are often derived from clay minerals, iron oxides, and aluminum, silicate and natural organic substances (Sen and Khilar, 2006). Many researchers have worked on the transport of pollutants, colloid particles and also their interactions. Most studies have concluded that the colloidal particles facilitate the pollutants transport in groundwater (Magee et al., 1991; Roy and Dzombak, 1996; Puls and Powell, 1992; Saiers and Hornberger, 1996; Kersting et al., 1999; Kretzschmar et al., 1999; Sen et al., 2002a, Sen et al., 2002b; Zhuang et al., 2003; De Jonge et al., 2004; Šimůnek et al., 2006; Cheng et al., 2016; Emerson et al., 2016; Benhabib et al., 2017; Ma et al., 2017; Snousy et al., 2018). However, some have stated that in certain conditions, the colloidal particles have a negative impact on the pollutants transport and causing adsorbed pollutant to the particles trap between the grains. (Bekhit et al., 2006; Bekhit et al., 2009; Sen and Khilar, 2006; Kheirabadi et al., 2017; Ghiasi et al., 2020; Peng et al., 2017).
In the previous studies with column experiment, the effect of different parameters and conditions were investigated (such as pH, ion strength, flow velocity, particle size, and colloid concentration) (Ryan and Elimelech, 1996; Roy and Dzombak, 1997; Um and Papelis, 2002; Torkzaban et al., 2015). Roy and Dzombak (1996) empirically found that the reduction of ion resistance could be the cause of colloid mobility. Saiers and Hornberger (1999) experimentally reported that colloid mobility increases with increasing pH and flow velocity. Bradford et al. (2002) investigated the physical factors influencing the transport of colloid in the wet condition. Their results showed that the amount of trapped colloid and colloid output concentration are highly dependent on the soil grain size and colloid size. Bennacer et al. (2017) investigated the coupled effects of ionic strength, particle size, and flow velocity on transport and deposition of suspended particles in saturated porous media. Ge et al. (2018) studied the role of U (VI) concentration, pH and ionic strength on co-transport of U (VI) and colloids and showed. Won et al. (2019) illustrated that large colloidal particles lead to clogging and reduce pollutant transport. He et al. (2019) demonstrated the increasing silver nanoparticles transmission occurred in lower particles diameter sizes. Li et al. (2019) perused the Fe-colloid co-transport under hydrochemical and hydrodynamic condition. They showed that increasing the silicon concentration and humic acid colloids, promote and inhibit Fe transport, respectively. They used three porous media with the same average diameter and different specific area and concluded that the porous media with the highest specific area has the lowest Fe transport.
Despite numerous experiments in this area, the effects of different conditions and parameters on pollutant transport in the presence of colloid particles have been limited only to investigating the effects of ionic strength and pH and there are little studies perused other parameters effect. Therefore, it is essential to investigate the effects of other important parameters such as flow velocity, aggregates size and colloid concentration on pollutant and colloids co-transport, which a few articles studied some of these parameters effect in recent years (Yang et al., 2019a, Yang et al., 2019b).
Heavy metals have high absorbance properties (Morsali et al., 2019) and could be transported to groundwater by soil or colloid particles. Anthropogenic activities such as electroplating, leather tanning, wood preservation can release large amounts of Cr(VI) in liquid and solid waste forms (Patterson, 1985). The Cr(VI) is a significant pollutant in groundwater because of its highly toxic and highly soluble in water and availability of transport to long distances in the groundwater (Somasundaram et al., 2011). So the presence of colloid particles could have a remarkable role in Cr(VI) transmission in groundwater. Thus, the main purpose of this study is to investigate the effect of colloid particles presence on the transport of Cr(VI) and the effect of flow velocity aggregates size and colloid concentration on Cr(VI) and colloids co-transport.
Section snippets
Materials and methods
The chromium (Cr(VI)) transfer in the presence of bentonite colloidal particles experiment is done in a laboratory column. The schematic experiment setup has been demonstrated in Fig. 1. Preparing the experimental setup, materials used and how the experiment was conducted were all based on the standards mentioned in the “Guidelining protocol for soil-column experiments assessing fate and transport of trace organics” (Gibert et al., 2014). and procedure of Bradford et al. (2002) experiment. In
Cr(VI) transport
Cr(VI) transfer experiment was carried out in the saturated porous media without bentonite colloidal particles as a first test. A sampling of this experiment was in 330, 390, 480, 600, 780 and 1020 s after running the test begins. The 330 s and 1020s sampling times are times of colored material first particles arrival to the end of the column and colored material thoroughly mixed at the end of the column respectively. The other sampling times and increasing time intervals have been selected due
Conclusion
Colloidal particles can facilitate or retain pollutant transmission in groundwater and its presence and concentration variation could have a remarkable effect on significant groundwater pollution transmission such as Cr(VI). Also, the physical properties groundwater environment and groundwater velocity are effective in transmitting pollution to groundwater. Therefore, the transport of Cr(VI) in the presence of bentonite colloidal particles was studied in an experimental column filled with sand.
Author statement
B.G. completed this study as part of his PhD thesis under the supervision of M.H·N. and advisery of A.M.M. The paper was written by B.G. while technical support and revisions were provided by M.H.N and A.M.M.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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