How soil releases zinc in areas mined for rare earth elements

Jim Crocker
16th December, 2025

How soil releases zinc in areas mined for rare earth elements

Schematic of the mining area overview and the on-site sampling process.

Image adapted from: Guo et al. / CC BY (Source)

Key Findings

  • In southern China rare earth mining areas, leaching processes using acidic solutions release zinc (Zn) from soil, posing environmental risks
  • Magnesium sulfate (MgSO4) initially released the most Zn into the leachate, but aluminum sulfate (Al2(SO4)3) left a higher amount of Zn remaining in the soil
  • A 3% concentration of aluminum sulfate (Al2(SO4)3) was most effective at transforming zinc in the soil, indicating a greater potential for long-term environmental risk due to increased residual Zn
Ion-type rare earth ores are a critical component in modern technology, but their extraction can lead to significant environmental problems, particularly heavy metal contamination of surrounding soils. A key concern is zinc (Zn), which is often found in high concentrations in these mining areas, and understanding how it’s released during the leaching process—where acidic solutions are used to dissolve and extract the rare earth elements—is crucial for developing safer mining practices. Researchers from Jiangxi University of Science and Technology and the University of Tehran[1] have recently investigated this issue, focusing on how different leaching agents affect zinc’s behavior in soil. The core problem is that while the acidic solutions are effective at extracting rare earths, they also activate and release other metals present in the soil, potentially causing pollution. Previous studies have highlighted the risk of heavy metal release when using new leaching agents[2]. For example, research on weathered crust elution-deposited rare earth ore showed that certain leaching agents, like those containing chloride ions, could leach significant amounts of lead, and that the leaching process isn't simply a constant release but occurs in stages. This study builds upon that knowledge by specifically examining zinc and the impact of different agents commonly used in ion-type rare earth mining. The study involved simulating the leaching process in a controlled laboratory setting, using three common leaching agents: ammonium sulfate ((NH4)2SO4), magnesium sulfate (MgSO4), and aluminum sulfate (Al2(SO4)3). Crucially, they also tested varying concentrations of aluminum sulfate (1%, 3%, 5%, and 7%) to see how the amount of the agent used affects zinc release. The researchers then monitored the zinc concentration in the leachate (the liquid that drains through the soil) over time and analyzed the remaining zinc in the soil to understand how much was being released and what form it was in. The results revealed significant differences in how each leaching agent behaved. Magnesium sulfate (MgSO4) resulted in the highest peak zinc concentration in the leachate, meaning it initially released the most zinc. However, using aluminum sulfate (Al2(SO4)3) led to the highest amount of residual zinc remaining in the soil, reaching levels considered to be a high environmental risk. This is a critical finding, as it suggests that while MgSO4 might have a more immediate impact, Al2(SO4)3 poses a longer-term threat due to the zinc that remains in the soil. Further analysis showed that the concentration of aluminum sulfate also played a key role. Higher concentrations (5% and 7%) were much more effective at releasing zinc from the soil than lower concentrations (1% and 3%). The 3% concentration of Al2(SO4)3 was found to be particularly effective at transforming zinc in the soil, indicating it’s a critical concentration level to consider. The researchers also investigated how each agent was releasing the zinc. Ammonium sulfate ((NH4)2SO4) appeared to work primarily through ion exchange, where ammonium ions displace zinc ions in the soil. Aluminum sulfate (Al2(SO4)3) worked by creating a strongly acidic environment that dissolves the mineral structures holding the zinc, effectively breaking them down. Magnesium sulfate (MgSO4) released zinc through a combination of ion exchange and by altering the structure of the soil itself, making it easier for zinc to move. To assess the overall risk, the researchers used an environmental risk assessment index called the RAC (Risk Assessment Code)[3]. As expected, the highest RAC values were found after leaching with aluminum sulfate (Al2(SO4)3), confirming its greatest potential for environmental damage. This aligns with earlier work that emphasized the importance of considering not just the total metal concentration, but also the stability of metals in compost and soil, using indices like the IR and MRI to evaluate mobility and toxicity[3]. This study provides crucial insights into the complex processes governing zinc release during rare earth mining. It expands on previous research by identifying the specific mechanisms by which different leaching agents activate and release zinc, and highlights the importance of carefully controlling the concentration of aluminum sulfate. The findings offer a theoretical foundation for optimizing “green mining” practices, helping to minimize heavy metal pollution and develop effective remediation strategies. Further research building on this could focus on methods to stabilize zinc in the soil after leaching, preventing its long-term release and mitigating environmental risks, and in conjunction with findings from studies on ion-adsorption rare earth tailings[4], could help provide a more comprehensive understanding of heavy metal release in mining areas.

AgricultureEnvironmentPlant Science

References

Main Study

1) Study on the release pattern of Zn in soil of ionic rare earth mining areas under different leaching conditions

Published 15th December, 2025

https://doi.org/10.1371/journal.pone.0338566

Related Studies

2) Study on Pb release by several new lixiviants in weathered crust elution-deposited rare earth ore leaching process: Behavior and mechanism.

https://doi.org/10.1016/j.ecoenv.2019.110138

3) The usability of the IR, RAC and MRI indices of heavy metal distribution to assess the environmental quality of sewage sludge composts.

https://doi.org/10.1016/j.wasman.2014.04.005

4) Leaching characteristics and environmental impact of heavy metals in tailings under rainfall conditions: A case study of an ion-adsorption rare earth mining area.

https://doi.org/10.1016/j.ecoenv.2024.116642


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