In the Wall Street report, oil, gas, coal, forest and timber, gold and silver, copper, uranium, lead, crude iron, and phosphate, all of which are important global sources, are considered wealth indicators. With this approach, Russia, the United States, Saudi Arabia, and Canada rank first to fourth in the world, and Iran ranks fifth in the world with $ 27.3 trillion in natural underground resources. In the foreign trade sector, Iran in the mining and mineral industries sector with exports of $ 9.2 billion in 2020, has a share of 21% of the total value of the country's exports. Unfortunately, in recent years, due to economic factors, little attention has been paid to the environment and the environmental impact of mining activities has been overshadowed. In this case, it is natural that a very large volume of tailings is produced in different parts of Iran, which will have many environmental effects. The major portion of this wastes is tailings which are the result of metallurgical extraction (Çoruh et al., 2013).
Tailings are usually a by-product of the process of extracting valuable elements in mining. They are different types and usually produced by crushing, grinding, sieving, and chemical processes on the mined ore. After performing these steps, two substances are formed: a valuable concentrate that contains the desired element and a discarded substance that contains different types of pollutants, particles, and chemicals called tailing (Hudson, 2001, Younger and Wolkersdorfer, 2004, Lottermoser, 2010, Zhang et al., 2019). The fast growth of industrial development has created an ever-growing amount of harmful mine tailings. Currently, estimated that the amount of tailings is generated several million tons per year while this volume is exponentially increasing due to high demand and low-grade deposits (Hudson-Edwards and Dold, 2015). Mine tailings are mostly rich in poisonous trace metals, such as As, Pb, Cu, Ni, Mn, Cd, Cr, and etc (Mendez and Maier, 2007, Yang et al., 2013, Uugwanga and Kgabi, 2020). Over the past few decades, pollution of the environment, water, and soil resources have intensified due to mining activities (Al-Jabri et al., 2006).
The existence of heavy-trace metals, especially lead, due to mining activities in the environment, has caused a great deal of concern owing to its toxicity to human life (Moors and Dijkema, 2006, Montanaro et al., 2001, Nurcholis et al., 2017). Leaching of the poisonous metals unavoidably will pose a hazard to the environment and public health worldwide (Ye et al., 2015). Tailings among mine wastes act as the most an important source of environmental contamination (Roussel et al., 2000). Lead and zinc mine tailings have great levels of heavy metals, like Zn, Pb, and low concentration of macronutrients, negligible physical structure, and plenty of salinity (Khan et al., 2019). Solubility and mobility are two important processes in the leaching of heavy metals such as lead from wastes and their entry into water and soil sources. Pollution of soil and water resources to lead causes many diseases for humans and plants. Some scientists reported that the entry of lead into the human food chain can cause various diseases such as the most sensitive brain, abdominal pain symptoms, constipation, headaches, irritability (Schreck et al., 2012), memory problems, inability to have children, and tingling in the hands and feet (Jaishankar et al., 2014).
Heavy metals (Pb, Zn, Cd) concentrations in lead-zinc mine tailings are naturally greater than the earth's crust (Wani et al., 2015). The maximum permissible concentration of lead in drinking water is 10 µg L−1 (Organization, 2008) while the average lead concentration in tailings dam seepage water is more than 500 µg L−1 (Lottermoser, 2010). For example, in some running waters from metal mine sites such as tailings pond water Jumna tin mill; mine site drainage water, Montalbion silver mine and tailings dam seepage water, Mary Kathleen uranium mine, lead concentrations were 607, 36 and 450 µg L−1, respectively. Tailings contain abundant sulfide minerals and these sulfides are a mixture of sulfide with various metals (zinc, lead, copper). This can lead to acid mine drainage (AMD) and damage to the environment. The behavior and mobility of Pb in lead-zinc mine tailings depend on the physicochemical structure and the properties of tailings (Falagán et al., 2017)
The main factor in determining the rate and amount of lead released into the environment by leaching is the concentration and distribution of lead in tailing particles (Ma et al., 2019). Also, the influence of mine tailings on the environment is mainly in relation to the possible concentration of heavy metals in the contacted water with tailings to the total concentration of heavy metals in the tailings. The concentration of heavy metals in a leachate solution affected by the physical-chemical properties of the tailings, such as pH, particle size, water to tailing ratio, heavy metal fractionation, mineralogy, chemical composition, and contact time of the particles with water (Karaca et al., 2016). These factors are the most important key factors affecting the bioavailability of heavy metals and are used to evaluate the toxicity of mineral tailings in various environmental scenarios (Grathwohl and van der Sloot, 2007, Grathwohl and Susset, 2009, Guyonnet, 2010). Leaching and erosion are some of the processes that lead to the transfer of tailing-related contaminants to water and soil resources. Leaching of contaminants from tailings directly affects groundwater sources, especially when the pollutants (like Pb) in the tailing particles are easily transported by acidic rain or any other flowing water and discharged to surface and groundwater sources (Uugwanga and Kgabi, 2020).
A huge amount of Zn-Pb mine tailings are produced in Iran, which has very high concentrations of lead. Environmental impacts of Zn-Pb mine tailings have not been well considered and the mechanism of lead release from these tailings is poorly studied and still unclear. Hence, it is very important to realize the leaching characteristics of lead for better controlling of Zn-Pb mine tailings. However, in a few studies, the effect of liquid to solid ratio, osculation time, pH, and particle size on the process of lead release from tailings has been published. Therefore, in this paper, lead release behavior from tailings and mechanisms affecting this process was tested. For this purpose, batch leaching tests were carried out to investigate the liquid-solid ratio, pH, contact time, and particle size effect on the release of lead from mine tailings. The toxicity level of tailings was investigated by synthetic precipitation leaching procedure (SPLP), toxicity characteristic leaching procedure (TCLP), field leach test (FLT), and leachate extraction procedure (LEP) and the sequential extraction procedure was also applied to evaluate the mobility behavior of lead in tailings.