Tungsten, a transition metal element of Group VI in the Periodic Table of elements, occurs as a trace element in natural waters (Koutsospyros et al., 2006). Due to the nontoxicity of tungsten in the form of metal or alloy and its weak migration in the environment, its environmental impacts have not attracted enough attention for a long time. However, over the last decade, it has been demonstrated that the solubility and possible leaching of tungsten is higher than previously determined (Dermatas et al., 2004; Bednar et al., 2009; Sen Tuna and Braida, 2014). Tungsten, in the natural waters and sediments, occurs predominantly as monomeric tungsten oxyanion WO42− (where tungsten is in its highest oxidation state, + 6) (Kletzin and Adams, 1996). Furthermore, recent studies showed that mono-tungstate was mobile in aquatic systems and exhibited severe ecotoxicological impacts (Seiler et al., 2005; Bednar et al., 2009; Clausen and Korte, 2009; Tuna et al., 2012; Johannesson et al., 2013). For example, the environmental geochemical investigation indicated that the abnormal concentration of tungstate in the environment may be closely related to the outbreak of leukemia in young children (Sheppard et al., 2007; Steinberg et al., 2007). In addition, clinical studies have shown that high levels of tungstate in people's urine or blood can trigger seizures, strokes or cardiovascular disease (Marquet et al., 1996; Tyrrell et al., 2013). In view of the above, tungsten is newly regarded as an arisen environmental pollutant by the United States Environmental Protection Agency (EPA, 2008).
China possesses the maximum tungsten resources (1,900,000 metric tons) in the world, which is also been the world’s central tungsten consumer and exporter (USGS, 2018). However, overexploitation and outdated technology in past decades produced a large amount of tungsten slags (Liu et al., 2010). Notably, the cumulative amount of tungsten slag in China reached 1 million tons, what is worse, more than 70 thousand tons of tungsten slag were produced annually (Li et al., 2019a). Due to lack of proper management measures and economic commercial disposal, substantial amounts of tungsten slags were directly exposed to the land surface, or remained uncovered in massive piles, releasing tungsten into the environment by certain geochemical processes, thus contaminating the underground waters and surrounding soils.
For example, tungsten, in soils of Zakamensk known as one of the biggest ore mining centers in the former Soviet Union, was 42–55 times higher than the background area, and the concentrations of heavy metal in the investigated area significantly exceeded the background value as well, creating a severe hazard for the environment (Timofeev et al., 2018). Furthermore, the highest tungsten concentration of samples in soils of central tungsten mines in Ganzhou was 318 mg/kg, approximately 62.4 times greater than the concentrations of Jiangxi background (Zheng et al., 2020). In contrast to most of the research focusing on the concentration analysis and pollutants level assessment of tungsten in the vicinity of tungsten mines, while few studies have been conducted to investigate the leaching characteristics and mechanisms of the tungsten-rich slags and sediments under special conditions.
In this study, two kinds of sequential extraction procedures (SEP) and batch leaching experiments on tungsten-rich slags and sediments were conducted to identify the release characteristics, the pollution and ecological risks of tungsten-rich samples under special conditions. In addition, the total concentrations of tungsten-rich samples were obtained with digestion. Considering that tungsten precipitates as a tungstic acid under conventional digestion conditions, various digestion methods were performed to determine the applicability and accuracy.