As a strategic resource, rare earth elements were widely used in various industries, such as clean energy (Judge et al 2017), agricultural (Abdelnour et al 2019), military and other fields (Massari et al 2013), especially, the middle and heavy REEs were closely related to cutting-edge technology products (Dushyantha et al 2020). Ion-absorbed rare earth ores were the main source of medium and heavy REEs, which was widely distributed at seven provinces in southern China, such as Ganzhou area, Jiangxi (Tang et al 2018). Numerous scholars (Bao et al 2008) have discovered that REEs attached to ion absorbed REM were easily desorbed through ion exchange when encountering with positive ions (such as Na+, NH4+ and H+). However, the leaching process of REM using ammonium sulfate in forms of the pool leaching, dump leaching and in-situ leaching (Liu et al 2014) might not only induced the release of associated toxic metals (Liu et al 2020), but also resulted in a series of environmental issues, such as co-pollution of associated toxic metals and REEs, soil fertility degradation, land desertification, headwater pollution and downstream farmland damage (Xie et al 2020, Feng et al 2012, Zhou et al 2015). Furthermore, these pollutants (ammonium nitrogen, associated toxic metals and REEs) remained in REM potentially polluted the surface water and groundwater through surface runoff and leakage, which posed serious health risks to humans (Huang et al 2009, Rao et al 2017). Thus, abandoned rare earth mine disposal and restoration were especially urgent.
Recently, compared with traditional engineering techniques for remediating contaminated soil, phytoremediation was a convenient, cost-effective, non-destructive and promising solution for extracting pollutants from contaminated soil, especially the sites with contamination spread over a large area (Wang et al 2017, Cioica et al 2019, Punia 2019). It was well known that toxic metals and other pollutants in the contaminated soil were extracted by plant accumulation in harvestable shoot parts (phytoextraction) or root uptake and translocation into shoots (phytostabilization) (Pedron et al 2011, Banerjee et al 2016). After phytoextraction, these harvestable plants could be disposed of incinerated to provide thermal energy or recover valuable metals (Sas-Nowosielska et al 2004). However, restoration of ion-absorbed REM was extremely slow and difficult owing to the hostile growing conditions, such as lack of organic matter and toxic metals contamination (Pang et al 2003). Thus, the appropriate plant species selection to resist these unfavorable conditions was essential for rehabilitating abandoned rare earth mines.
Vetiver grass, as a fast-growing perennial C4 grass, possessed tall stem (1-2m) and extensive strong root system (up to 3-4m deep) (Vargas et al 2015). Moreover, vetiver was very popular in India due to its special economic value including its roots could be processed into volatile essential oil and leaves could be used as feeds for cattle, goats and horses (Chen et al 2020). Importantly, vetiver grass tolerated various adverse conditions, such as prolonged drought, extreme temperatures, acidity, alkalinity and high concentrations of toxic metals (Danh et al 2009, Roongtanakiat et al 2008). Thus, among various types of plants, vetiver grass was considered to be one of the most promising plants for mine restoration due to its unique morphological and physiological characteristics (Shu et al 2002). Nevertheless, recent studies of vetiver grass have solely reported on the soil erosion control (Mondal et al 2020), landfill rehabilitation and mine site stabilization, such as iron ore mine, gold mine tailings, coal mines and lead mine tailings (Banerjee et al 2019, Wari et al 2019). There were currently few researches that have been tested to evaluate the extraction capacity of REEs and Non-REEs using Vetiver grass from abandoned ion-absorbed rare earth mines.
Therefore, the objectives of the present study were (1) to determine physical-chemical parameters and toxic metals content of contaminated sites, (2) to explore the feasibility analysis for vegetation restoration of rare earth mine, (3) to investigate toxic metals uptake ability of the roots and shoots of vetiver and (4) to assess the transportation of these toxic metals in the soil-vetiver grass system.