There is a high demand for large amounts of fuels, industrial chemicals, various fertilizers, pesticides, and drugs, which has resulted in the generation of millions of tons of hazardous tailings in many locations around the world. These contaminants have a significant impact on the quality of water, soil, and ecosystems. Mine tailings, animal manures, waste, and sewage, as well as leaks and penetration of petrochemical, oily, and acidic materials into the ground, are the primary causes of soil contamination (Cameselle et al. 2013; Yang et al. 2014; Zhang et al. 2010).
The mining industry is a significant economic activity that contributes significantly to the global economy. Although mining is critical to economic growth, it has significant environmental consequences that cannot be overlooked. The mining industry has a wide range of environmental effects on both small and large scales (Karaca et al. 2019; Ortiz-Soto et al. 2019). Mines produce 10,000 to 600,000 tons of tailings per year, which contain heavy metals, combustible materials, hazardous waste, and contaminating gases in varying concentrations. Zinc (Zn), chromium (Cr), arsenic (As), lead (Pb), cadmium (Cd), and mercury (Hg) are among the hazardous metals discovered at contaminated sites.
Several measures have been implemented in recent decades to reduce the problems caused by mining activities, which has resulted in the development of novel methods for the remediation of contaminated soils, such as bioremediation, phytoremediation, soil cultivation, extraction of contaminant vapors, solidification and stabilization, thermal contamination removal, soil washing, biological remediation, and electrokinetics (Ma et al.2001; Pham and Sillanpää 2020; Zou et al. 2008). It should be noted that these methods are not universal, and the method chosen depends on the location and properties of the contaminated site, the type of contamination, and the contaminant's properties.
Electrokinetic remediation is a promising method for cleaning soils, particularly fine-grained soils contaminated with organic and inorganic materials (Kim et al. 2012; Lee and Kim 2010). In comparison to other soil remediation methods, electrokinetics is a relatively safe, effective, simple, and cost-effective method (Zou et al. 2020; Moghadam et al. 2016; Villen-Guzman et al. 2018). Electrical migration, electrophoresis, diffusion, and electroosmosis are some of the contaminant transport processes that can be used in electrokinetic remediation. When an electric current is applied to the soil, electrolysis reactions or water decomposition will occur at the electrodes. Because of the oxidation phenomenon, electrolysis reactions at the anode generate oxygen gas and hydrogen ions, whereas they generate hydrogen gas and hydroxide ions at the cathode (Saichek and Reddy 2014). The electrolysis reactions at the electrodes are depicted in Equations 1 and 2:
4H2O + 4e− \(\to\)2H2 + 4OH− (2)
Given the electrolysis reactions at the electrodes, it produces an acidic front at the anode and a basic front at the cathode, resulting in a decrease and increase in pH at the anode and cathode, respectively. Ions are also attracted to one another and produce water when they collide.
The pH level in the soil sample changes due to the presence of electroosmotic (EO) flow and ionic migration. The pH of the soil sample rises as it moves from the anode to the cathode. The increased pH level of the soil sample on the cathode side caused by water electrolysis results in heavy metal deposition, which reduces the efficiency of electrokinetics. To overcome this limitation, solutions other than distilled water can be used near the electrodes to reduce the pH, preventing heavy metal deposition and, in some cases, causing their release from the soil. The electrokinetic process is heavily influenced by soil pH and electrolysis reaction (Giannis and Gidarakos 2005). The solutions used must create the proper pH conditions in the soil and interact with the metals in the soil to cause them to be removed (Reddy and Chinthamreddy 2003). In general, the efficiency of organic and inorganic matter removal is affected by chemical processes in the electrodes, soil mechanical properties, soil moisture, pore fluid properties, contaminant compounds, voltage and current intensity applied, electrolyte type, and electrokinetic test conditions (Ouhadi et al.2010).
Soil washing is a method of removing contaminants from soil that uses either a physical or chemical technique. The water used in this process can be pure water or contain additives like acids, bases, surfactants, solvents, or separating agents that aid in the separation of contaminants from soils and sediments. Organic compounds, heavy metals, pesticides, and petroleum products are among the contaminated target groups of this process (Dermont et al. 2008; Urum et al. 2003). Soil washing can be done in situ or ex situ (Dermont et al. 2008). During in-situ soil washing, chemical solutions are injected into or sprayed on the contaminated area to mobilize contaminants. After mixing the extraction solution with the contaminants, the contaminant solutions are collected and discarded using suction filtration methods (Dermont et al. 2008). Lower-permeability oils are deemed unsuitable for in-situ washing, necessitating the excavation of contaminated soil.
According to published research, some researchers have made significant efforts to use soil washing techniques in the electrokinetic method to increase metal removal efficiency. Because the solutions raise the pH of the soil in the presence of an electric field, causing metals to be excreted and dissolved. Giannis et al. demonstrated that soil pH and leaching solutions are the most important factors in dissolving and excreting cadmium metal from soil under electric field, and that this method was capable of removing 85 percent of cadmium from contaminated soil.
The purpose of this study was to look into the effectiveness of electrokinetic zinc removal from mine tailings using various purging and washing solutions. This study included two series of experiments. The soil was saturated with distilled water and various solutions were used as filtration solutions in the first series, while the soil was washed with different acids at a fixed concentration of 0.1 M in the second. During the experiment, the distribution of the heavy metal zinc was also investigated.