Ammonia nitrogen sources and pollution along soil profiles in an in-situ leaching rare earth ore☆
Graphical abstract
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.
Section snippets
Site description
Sampling was conducted at a WCED-REO and in-situ leaching mining area (24°48′N, 114°51′E, 250–410 m asl) located in Ganzhou-Longnan, which is in southern Jiangxi Province, covering an area of approximately 34.7 km2 (as shown in Supplementary Material 1a, 1b). The mean annual precipitation in this area is 1587 mm and the frequency of acid rain (pH < 5.0) is high (Chen et al., 2014), as shown in Supplementary Material 2. The soil at the sampling site is mainly composed of clay minerals, quartz
Soil pH in the soil profiles
The pH values of the different soil profiles are shown in Fig. 1. The pH of the soil was low in both the tailing soil and the raw ore soil. The raw ore soil in the REE mining area was still weakly acidic without mining disturbances, i.e., the pH was in the range of 5.4–6.0. The surface layer was the most acidic, and the acidity gradually weakened with increasing depth until the values became stable. The pH range of the in-situ leaching tailing soil profiles (P2, P3, and P4) was 3.5–4.7, among
NH4+-N reaction in the in-situ leaching process
In the in-situ leaching process, an ion exchange reaction occurs between the clay minerals and the leaching agents. The cations (NH4+) in the leaching agents are adsorbed and the REE ions are desorbed from the soil. The leaching process is a fast ion-exchange reaction, where the chemical reaction with the ammonium salts (reaction agent) can be expressed as (Tian et al., 2010):where m is a polymer, s
Conclusions
- 1)
The mining process used in WCED-REOs seriously affects the soil acidity of the tailings and the lower reaches of the mine. The soil pH increases slowly but continuously. The specific acidity of the ore soil was pH raw ore = 5.73 > pH lower reaches = 4.87 > pH mine tailing = 3.90.
- 2)
The proportions of the three AN form in the WCED-REOs are different from those of general agricultural soil or polluted soil. The percentages of the different AN form in WCED-REOs were 60% water-soluble ammonium, 35%
Credit author statement
Qiuying, Zhang and Futian Ren: Conceptualization, Methodology, Software, Formal analysis, Investigation, Data curation, Writing Original Draft, Writing-Review & Editing; Conceptualization, Formal analysis, Investigation, Supervision, project administration, funding acquisition; Fadong Li: Methodology, Formal analysis, Supervision, Writing-Review & Editing; Guang Yang: Methodology, Formal analysis, Supervision, Writing-Review & Editing; Guoliang Chen: Investigation, Data curation; Jianqi Wang:
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.
Acknowledgements
The authors gratefully acknowledge the Financial of National Key Research and Development Project of China (grant numbers 2018YFC1801801 and 2016YFD0800301).
References (55)
Global rare earth resources and scenarios of future rare earth industry
J. Rare Earths
(2011)- et al.
Uncovering the end uses of the rare earth elements
Sci. Total Environ.
(2013) - et al.
Global demand for rare earth resources and strategies for green mining
Environ. Res.
(2016) - et al.
Throughfall reduction diminished the enhancing effect of N addition on soil N leaching loss in an old, temperate forest
Environ. Pollut.
(2020) - et al.
Rare earths supply chains: current status, constraints and opportunities
Resour. Pol.
(2014) - et al.
Silicon improves the growth of cucumber under excess nitrate stress by enhancing nitrogen assimilation and chlorophyll synthesis[J]
Plant Physiol. Biochem.
(2020) - et al.
Process optimization of rare earth and aluminum leaching from weathered crust elution-deposited rare earth ore with compound ammonium salts
J. Rare Earths
(2016) - et al.
Soil physiochemical properties and landscape patterns control trace metal contamination at the urban-rural interface in southern China
Environ. Pollut.
(2019) - et al.
Soil acidification of the soil profile across Chengdu Plain of China from the 1980s to 2010s
Sci. Total Environ.
(2020) - et al.
Distribution characteristics of nitrides in soil of south China ion-adsorption rare earth ore mining area[J]
Chinese Rare Earths
(2015)
Extreme enrichment of arsenic and rare earth elements in acid mine drainage: case study of Wiśniówka mining area (south-central Poland)
Environ. Pollut.
Recovery of rare earth elements adsorbed on clay minerals: II. Leaching with ammonium sulfate
Hydrometallurgy
Research progress on leaching technology and theory of weathered crust elution-deposited RE ore
Hydrometallurgy
The impact of unregulated ionic clay rare earth mining in China
Resour. Pol.
Behavior of leaching and precipitation of weathering crust ion-absorbed type by magnetic field
J. Rare Earths
Acidification of Earth: an assessment across mechanisms and scales
Appl. Geochem.
Leach of the weathering crust elution-deposited rare earth ore for low environmental pollution with a combination of (NH4)2SO4 and EDTA
Chemosphere
Mitigation of soil acidification through changes in soil mineralogy due to long-term fertilization in southern China
Catena
Leaching process of rare earths from weathered crust elution-deposited rare earth ore
Trans. Nonferrous Metals Soc. China
Process optimization on leaching of a lean weathered crust elution-deposited rare earth ores
Int. J. Miner. Process.
Cooperation between partial-nitrification, complete ammonia oxidation (comammox), and anaerobic ammonia oxidation (anammox) in sludge digestion liquid for nitrogen removal
Environ. Pollut.
Evaluating the fractionation of ion-adsorption rare earths for in-situ leaching and metallogenic mechanism
J. Rare Earths
China’s ion-adsorption rare earth resources, mining consequences and preservation
Environmental Development
Whole soil acidification and base cation reduction across subtropical China
Geoderma
Seasonal variations in nitrogen mineralization under three land use types in a grassland landscape
Acta Oecol.
Characteristics and prediction model of Jiangxi acid rain
Environ. Sci. Technol.
Swelling of clay minerals during the leaching process of weathered crust elution-deposited rare earth ores by magnesium salts
Powder Technol.
Cited by (57)
Treatment of ammonium nitrogen wastewater using MC-FeCo catalyzed ozonation combined with biological denitrification
2024, Journal of Industrial and Engineering ChemistryCan we redevelop ammonia nitrogen contaminated sites without remediation? The key role of subsurface pH in human health risk assessment
2024, Journal of Hazardous MaterialsNovel process for organic wastewater treatment using aerobic composting technology: Shifting from pollutant removal towards resource recovery
2024, Science of the Total EnvironmentBioseparation of rare earth elements and high value-added biomaterials applications
2024, Bioorganic Chemistry