Ammonia nitrogen sources and pollution along soil profiles in an in-situ leaching rare earth ore

Highlights

• Ammonia nitrogen (AN) pollution and acidification are serious along deep soil profiles in the WCED-REOs.

• Spatial patterns of AN and pH were first released in the polluted tailing.

• Percentage of different forms of ammonium were first quantified in tailing soils.

• Adsorption and desorption process of ammonia nitrogen were revealed in tailing soils.

Abstract

The ammonium sulphate ((NH₄)₂SO₄) in-situ leaching process is the most widely used extraction technology for weathered crust elution-deposited rare earth ores (WCED-REOs). Highly concentrated (NH₄)₂SO₄, a representative leaching agent, is often used in the leaching process of WCED-REOs. However, this in-situ leaching process causes nitrogen pollution in the soil, surrounding surface and ground water due to the high concentrations of (NH₄)₂SO₄ solutions used as a long term leaching agent. To date, the mechanism behind the variations in ammonia nitrogen (AN) in deep soil profiles is unclear. We conducted vertical and lateral soil sampling and analyzed the collected samples for soil moisture, pH, ammonia forms, and AN contents in soil profiles deeper than 500 cm in an in-situ leaching mining area of Ganzhou, Jiangxi Province, southern China. The results show that primary chemical pollutants in the soil are derived from residual leaching agents with high acidities and concentrations of AN. Twelve years after the mining process was completed, the mean pH values of the tailings in the mining area were 3.90 and 4.87 in its lower reaches. Due to the presence of chemical residues, the AN concentration was 12–40 times higher than that of the raw ore soil before it was mined. The percentages of different ammonium forms in the rare earth tailing soil were 65%, 30%, and 5% for the water-soluble, exchangeable, and fixed ammonium forms, respectively. The results of this study support effective prevention and remediation treatment of environmental problems caused by AN pollution of the soil in WCED-REOs.

Introduction

Rare earth elements (REEs) with specific application values have become important strategic resources for all countries (Golev et al., 2014). With the rapid development of science and technology, the demand for REEs are increasing dramatically (Dutta et al., 2016). They are widely used in traditional industries, such as steel and non-ferrous metal industries, and many other fields, while middle and heavy REEs are specifically associated with cutting-edge materials and high-tech industries (Chen, 2011; Du and Graedel, 2013). Weathered crust elution-deposited rare earth ores (WCED-REOs) are mainly rich in middle and heavy REEs with higher atomic masses, including Dy, Lu, and Y (Chi et al., 2012; Yang et al., 2013), and are widely distributed in Jiangxi, Fujian, and other provinces in southern China. The REE resources contained in these deposits account for 80% of the total global REE reserves (Packey and Kingsnorth, 2016; Nie et al., 2020), and their exploitation restricts the global supply of middle and heavy REEs (Orris and Grauch, 2002; Sonich-Mullin et al., 2012; Zepf, 2013). With continued increases in mining exploration, WCED-REOs have also been discovered in Laos, Thailand, Indonesia, Madagascar, United States, and other countries in recent years (Migaszewski et al., 2019; Nie et al., 2020). The mining of WCED-REOs is popular and effective on scales from 1 to 100 km². The environmental pollution caused by the process involved in mining WCED-REOs has also aroused widespread attention (Orris and Grauch, 2002; Sonich-Mullin et al., 2012; Tian et al., 2010; Zepf, 2013). In the context of the economy and long-term stability of countries such as China and the United States, the environmental costs of gradually increased mining activity are enhanced by one to several times, and even limits the development of global REE mining to a certain extent (Orris and Grauch, 2002; Huang et al., 2011; Chi and Liu, 2019).

The process used to mine WCED-REOs has undergone two large leaps forward: from sodium chloride and (NH₄)₂SO₄ leaching in the 1980s, and from pool, heap, and in-situ leaching in the 1990s (Nie et al., 2020). In-situ leaching via (NH₄)₂SO₄ has been widely employed to mine WCED-REOs. It mainly involves the injection of leaching agents into the ore bodies, expanding cavities in the mountain to collect the rare earth ore fluid in a liquid form, and transporting the fluids to a processing plant for further handling and extraction of wet REEs(Chi et al., 2005). In the process of in-situ leaching, when REEs encounter the more active NH₄⁺, they exchange with NH₄⁺ and are desorbed into a rare earth mother liquid, while the excess leaching NH₄⁺cations, remain in the soil (Tian et al., 2010). During the leaching process, large amounts of (NH₄)₂SO₄ and an altered soil environment (pH, Eh, etc) can activate toxic heavy metals (such as Pb and Zn) in the soil. In addition, heavy metal pollution is serious in the soil surrounding REE mining areas, and its enrichments were also observed in surface water and groundwater (Xu et al., 2015; Pan and Li, 2016).

Currently, 0.5%–2% of (NH₄)₂SO₄ leaching agents are commonly injected into an ore body at a volume ratio of 1:3 (leaching agent: ore soil) (Tang et al., 2018). However, in the actual mining process, due to a lack of knowledge regarding the leaching mechanism, the concentration and dosage of (NH₄)₂SO₄ solution have been unintentionally increased to improve the REE extraction efficiency, resulting in an excessive amount of ammonium ions persisting in the soil (Yang et al., 2013). This not only changes the soil properties but also leads to the diffusion of (NH₄)₂SO₄ into the adjacent soil and water system via rainwater or runoff, resulting in an unparalleled risk to the environment (Tian et al., 2020; Guo et al., 2019; He et al., 2016a). For example, residual AN in the soil leads to soil acidification due to a large amount of H⁺ produced by AN hydrolysis reactions (He et al., 2016b), which makes the soil much softer, causes an imbalance in plant N, P, and K, and affects vegetation growth in the mining area (Yang et al., 2019). The residual leaching agents washed or leached into the soil via rainfall and runoff also transport heavy metal ions and REEs that subsequently migrate into the deep soil and can eventually enter the groundwater, causing uncontrollable pollution within the mining area (Feng et al., 2012).

Previous studies of WCED-REOs have mainly focused on the leaching mechanism and how to improve the leaching efficiency (Xu et al., 2018). The problem of AN pollution in mine soil is becoming increasingly serious due to the use of high concentrations of (NH₄)₂SO₄ (Wen, 2017). To prevent and control AN pollution, Qiu et al. (2008) studied the transport kinetics of pollutants and optimized the leaching process to reduce the consumption of (NH₄)₂SO₄. Xiao et al. (2015) showed that MgSO₄, a new type of leaching agent without ammonia, can be used instead of (NH₄)₂SO₄ to extract REEs without AN pollution. Yang et al. (2016) explored the spatial distributions of AN in the soil residues of an REE mining area and still found high concentrations of AN in the surface tailing soil after the mining process. They then used a KCl solution as a washing agent instead of water to solve the AN pollution problem quickly and effectively. However, only a few studies have reported on AN variation, its impact factors, and its distribution characteristics in deep soil profiles in WCED-REOs.

In general, previous studies have principally focused on how to reduce AN pollution caused by WCED-REO mining and the potential environmental risks (Deng et al., 2019a). Although a few reports have inferred serious soil pollution surrounding WCED-REOs, there has been limited research regarding how to accurately measure AN pollution at such contaminated sites. Many studies has also been carried out on the adsorption ability of AN by WCED-REO soil (Deng et al., 2019b) with a primary focus on the amount of apparent adsorption. Yet, an in-depth discussion of the proportions of different ammonium forms is currently lacking. The deep soil layer is an essential medium that connects the surface and bedrock, and is the only pathway for solute transport. Moreover, there is little research on the distribution of residual AN in the deep layers of the ore body, which is the source of continuous groundwater AN pollution in mining areas.

In this study, we selected the areas surrounding representative tailings investigate and analyse soil at depths of greater than 500 cm in which in-situ leaching of WCED-REO occurs. We collected soil samples with and without water washing along several deep soil profiles at different depths. Based on our field investigations and soil analyse, the objectives of this study are: to 1) describe the effects of the leaching process on the mine soil; 2) determine the proportions and distribution mechanisms of three forms of ammonium in different soil layers; and 3) discuss the effectiveness of water washing of the tailing soil to eliminate AN pollution. This study provides a scientific basis for the transfer and transformation mechanism of AN in tailings and highlights the efficient removal of residual AN for mine soil reclamation after the mining of a WCED-REO is complete.