Spatial distribution, geochemical behaviors and risk assessment of antimony in rivers around the antimony mine of Xikuangshan, Hunan Province, China

Antimony (Sb) is a toxic and carcinogenic heavy metal, and main group V with an atomic weight of 121.75 in periodic table element (Johnston et al. 2020), and its outer-orbital electron configuration is s²p³. Therefore, Sb is considered to be the primary pollutant of both the United States and European Union (Montserrat et al. 2002). Sb usually exists in environments affected by anthropogenic activities. Although people have paid more and more attention to Sb in recent decades, the basic knowledge of the environmental geochemical behavior of Sb in natural aquatic systems is still insufficient (Guo et al. 2018). Sb is not only a pollutant, but also a suspected carcinogen (He et al. 2019). Medical evidence shows that inhalation of Sb can cause damage to the respiratory system, liver and skin (Gebel et al. 1997; Guo et al. 2019). Excessive Sb content inhaled by the human body may also cause acute heart disease (McCallum 2005).

The current reserves of Sb are about 1.8 million tons, and the Sb reserves in southwestern China account for 80% of the global production capacity (Dupont et al. 2016). During the past 100 years or so, mining and smelting activities in the XKS area of Hunan have caused water, soil and sediment pollution by Sb and other metals (He et al. 2012). The principal sources of Sb include mine wastewater and untreated tailings discharge, percolation and runoff (Guo et al. 2018). In nature, Sb exists in the form of antimonite (Sb₂S₃) (Hu et al. 2015). The continuous development and use of Sb has led to uncontrolled release of its compounds into the environment. Therefore, it has been identified that the concentration of Sb in the environmental medium is higher than the background concentration (He et al. 2012).

The background value of dissolved Sb in natural water bodies is generally below 1.00 μg/L, and the Sb content detected in water bodies in many mining areas far exceeds national standards and World Health Organization standards. The Sb content in the upstream and downstream rivers of five abandoned Sb mines in Slovakia was assessed, and the results showed that the Sb content in the upstream and downstream river water samples of different river mining areas is very different, and the downstream content is 8–180 times (Hiller et al. 2012). The median Sb concentration of 1.70 μg/L in the water samples gathered in the upper reaches of the Susuljiu River, and the Sb concentration in the water samples flowing in the mining area is 7,000 μg/L (Cidu et al. 2014). Taking mine wastewater and river water in the XKS area of Hunan as the research object, it was found that the Sb content is 1,300–21,790 μg/L and 37–63 μg/L (He & Yang 1999), and 2–6,384 μg/L (Wang et al. 2011). After collecting the percolation water of the tailings dam in the Sb mining area of XKS it was found that the Sb content ranged from 4,581 to 29,423 μg/L, which exceeded the Chinese drinking water quality standard (Sb:5 μg/L) 5,884.60 times (Zhu et al. 2009). Water-rock interaction has a strong influence on the chemical characteristics of water bodies. For example, the dissolution of carbonate minerals results in an increase in the content of Ca²⁺ and Mg²⁺, and the ion exchange process often increases the content of Na⁺ and K⁺ (Mendoza et al. 2016; Salcedo et al. 2017).

The Sb content of shallow drinkable groundwater at XKS mine area has exceeded China's national drinking water quality guidelines, posing a serious threat to the health of residents in the mine area. The hydrogeochemistry data and water-rock interactions revealed the condition leading to the formation of high Sb groundwater. The chemical composition of low Sb groundwater is mainly influenced by the dissolution of carbonate minerals and the chemical composition of high Sb groundwater is controlled by ion-exchange interaction and dissolution of carbonate minerals and silicate minerals, among which ion-exchange interaction is the most important factor (Hao et al. 2020). The environmental geochemical behavior of Sb in various water bodies and sediments in the XKS area were mainly controlled by the process of oxidation and adsorption/combination with environmental matrix, mainly as Fe/Al (hydr)oxide, and spatial distribution of decreased Sb concentrations in some surface waters resulted from the dilution effect of river water and adsorption of environmental matrix (Guo et al. 2018).

The heavy metal element Sb pollution in the water environment around the mining area is very common. For a large number of studies on the heavy metal elements of different waters, most of the researches have been carried out on the content and chemical form, spatial distribution characteristics, pollution source analysis and formation mechanism, and have achieved fruitful research results (Guo et al. 2018; Hao et al. 2020). However, preliminary research on the source analysis of heavy metal elements in water bodies often stays in qualitative analysis, and quantitative evaluation is rarely possible. The quantitative source analysis of heavy metal elements has more practical application value and promotion significance for identifying pollution sources, analyzing pollution processes and causes of pollution.

Therefore, the research objectives of this study are as follows: (1) to analyze the spatial distribution characteristics of Sb, (2) to conduct a quantitative source analysis of Sb, and (3) to carry out suitability evaluation of irrigation and ecological risk assessment of surrounding water bodies in the Hunan XKS area.

XKS area is located 13 km north of Lengshuijiang City, Hunan Province. Its geographical coordinates are 111°27′30″–111°30′30″ east and 27°30′44′′–27°48′00′′ north (Figure 1). This mine covers an area of 26 km², divided into the southern mining area and northern mining area, known as the ‘antimony capital of the world’ (Wen et al. 2016).

The northeastward Lianxi River flows through the mining area, and the Zijiang River, the second largest river in Hunan Province, flows along the mining area. The climate in the study area is a typical continental subtropical humid monsoon climate. The area is relatively cold in winter and cool in summer due to its extreme terrain, foggy and long frost season. According to meteorological data from the Lengshuijiang Meteorological Bureau from 1949 to 2012, the average annual rainfall is 1,381.6 mm and the average annual temperature in the area is 16.7 °C. The study area has abundant antimony reserves and proven reserves of 2.5 million tons (Hu et al. 2016). The mineral composition of XKS antimony deposit is relatively simple. The ore minerals are mainly antimonite. The Tongjiayuan deposit contains a small amount of antimony oxide, and the gangue ore is mainly Quartz, next is calcite. The XKS antimony mining area is composed of four deposits: the old mine, Tongjiayuan, Feishuiyan and Wuhua, of which all the old mine deposits have been mined; Wuhua deposits are relatively small, and have been classified as private mining; the main mining deposits are Feishuiyan deposit and Tongjiayuan deposit.

A total of 21 water samples on different paths of two water systems (Qingfeng River and Xuanshan River) in the north and south mining areas of the XKS in Hunan were collected between March and April 2020. Among them, the Qingfeng River collected nine water samples, and the Xuanshan River collected 12 water samples (Figure 1).

Before collection, plastic sampling bottles were rinsed with distilled water two or three times and then rinsed with sample water another two or three times. Since the water depth of the sampling point is less than 0.5 m, sampling is performed at 1/2 water depth. For each sample, three bottles of 500 mL and one bottle of 5,000 mL were collected.

All water samples were filtered using a 0.45 μm glass fiber membrane upon collection (Cidu et al. 2014; Hao et al. 2021). To prevent Sb precipitation, the water samples for Sb analysis were acidified with 1:1 (V/V) dilute nitric acid to a pH < 2.0. Precautions for preservation should be carried out in accordance with ‘Technical Regulations on the Preservation and Management of Water Quality Sampling Samples’ (HJ493-2009).

The concentrations of major anions (chloride and sulfate) and major cations (calcium, magnesium, sodium, and potassium) were determined by ion chromatography (Dionex Integrion IC, Thermo Fisher, U.S.). The concentrations of bicarbonate were determined by acid-base titration (Wen et al. 2018). The Sb concentration was measured using an Agilent 7700× inductively coupled mass spectrometer (ICP-MS) with indium as the internal standard (Wen et al. 2016). The values for pH and total dissolved solids (TDS) were obtained in the field using a portable pH meter (HANNA H18424) and a portable conductivity meter (HANNA H1833), respectively (Wen et al. 2016, 2018).

To ensure the precision and accuracy of the analytical results, 10% of the samples submitted for analysis are blind samples. In addition, for each water sample, each chemical analysis was performed in triplicate. A valid data is the average of three tests, and the maximum relative standard deviation of these three tests should be less than 10%. Sb element analysis accuracy is 0.001 mg/L, other ion analysis accuracy is 0.01 mg/L. By adding Sb standard solution to the water samples, the recovery rate of Sb element was determined to be above 95%; by calculating the ion balance error, the analytical accuracy of the obtained ion concentration was further verified. The absolute ion balance errors in these water samples were all lower than 5%. In addition, 20% of the water samples were re-analyzed, and the error between the two analytical results was equal to or less than 10%.

The most important characteristics for measuring water quality and judging whether the rivers in the area can be used for farmland irrigation are as follows: salinity or total salt concentration, sodium absorption ratio (SAR) or Sodium Percentage (Na%) and Kellys index (KI).

Table 1 shows the classification statistics of SAR, Na% and KI irrigation suitability in surface water.

In the formula, RI is the comprehensive ecological risk index, is the potential ecological hazard index of the i-th heavy metal element;is the toxicity response coefficient of the i-th heavy metal element, and is the pollution coefficient of the i-th heavy metal element; is the measured value of the concentration of heavy metal elements in the river; is generally taken as the background value of the river in the study area. Among them, the toxicity response coefficient Sb = 7.

Table 2 shows the potential ecological risk assessment indicators and classification of heavy metal elements.

The 21 samples are divided into two categories. The first is defined as mining non-affected areas at sampling points upstream of the mining area, and the other is defined as mining affected areas at the downstream sampling points affected by mining activities. Table 3 shows the statistical table of the content of various elements in the rivers in the XKS area of Hunan Province. The average pH values of mining non-affected areas and mining-affected sampling points are 7.63 (7.53–7.82) and 7.67 (6.57–8.08) respectively, and the overall water environment is neutral or weakly alkaline. Compared with the ‘Sanitary Standards for Drinking Water’ (GB 5749-2006) (6.5 ≤ pH ≤ 8.5), the pH value of all water samples meets the national standard, and the results of pH are consistent with those of Zhu Jing (Zhu et al. 2009) and the research results of Xu (Xu et al. 2020) are comparable. The average TDS of mining non-affected areas and mining-affected sampling points are 280.33 mg/L (163.00–470.00 mg/L) and 332.50 mg/L (173.00–562.00 mg/L), which are in line with the ‘Sanitary Standard for Drinking Water’ (GB 5749-2006) (TDS ≤ 1,000 mg/L), TDS in all water samples meets national standards, and the results obtained by TDS are lower than the previous results (Zhu et al. 2009). Table 4 compares the results of this study with those of previous studies.

The cation content of the sampling points in the non-affected area of the mining industry is in the order of Ca²⁺ > Na⁺ + K⁺ > Mg²⁺, and the average content is 93.37 mg/L, 14.56 mg/L and 11.43 mg/L, and the content ranges are 59.30–145.00 respectively. mg/L, 8.86–19.93 mg/L and 3.60–26.20 mg/L. The anion content in descending order is SO₄²⁻ > HCO₃⁻ > NO₃⁻ > Cl⁻, and the average content is 168.77 mg/L, 167.33 mg/L, 5.93 mg/L and 3.23 mg/L, and their content ranges are 45.50–402.00 mg/L, 137.00–197.00 mg/L, 2.00–11.60 mg/L and 2.00–4.20 mg/L. The cation content of the sampling points in the mining-affected area is in the order of Ca²⁺ > Na⁺ + K⁺ > Mg²⁺, the average content is 114.20 mg/L, 40.95 mg/L and 11.53 mg/L, and the content ranges are 62.60–184.00 mg/L, 12.84–87.79 mg/L and 4.00–26.20 mg/L. The anion content in descending order is SO₄²⁻ > HCO₃⁻ > Cl⁻ > NO₃⁻, and the average content is 270.99 mg/L, 158.17 mg/L, 10.20 mg/L and 9.18 mg/L, respectively, and their content ranges from 60.20–544.00 mg/L, 31.00–203.00 mg/L, 2.20–30.30 mg/L and 2.20–22.10 mg/L.

A Piper diagram can describe the chemical characteristics of water samples and water-rock interactions (Piper 1984). Figure 2 is a Piper diagram of river water chemistry in XKS, Hunan. In the figure, mining non-affected areas and mining affected areas are clearly distinguished: it can be seen from the distribution of the diamond chart that the rivers in the mining non-affected area are mostly distributed in the left middle part of the diamond, and the water chemical type is mainly SO₄-HCO₃-Ca; the rivers in the mining affected area are mostly distributed in the upper part of the diamond, and the water chemical type is SO₄-Ca type mainly.

Table 2 shows that the average content of Sb in the mining non-affected area of the XKS area of Hunan Province is 0.03 mg/L, which content range is 0.01–0.04 mg/L; the average content of Sb in the mining affected area is 2.09 mg/L, where the content range is 0.05–5.36 mg/L. Among them, a comparison of the average value of sampling points in the mining affected area of the north mining area in the XKS area of Hunan Province and the upstream sampling points of the mining area XKSS-DB13 and XKSS-DB14 shows that the Sb exceeds the standard rate of 100%, and the maximum exceeded multiple is 141.40; the impact of mining in the southern mining area, comparing the sampling point of the area with the sampling point XKSS-DB05 in the upstream of the mining area, it is found that the heavy metal element Sb exceeded the standard rate of 100%, and the maximum exceeded multiple is 411.31. Compared with the ‘Environmental Quality Standard for Surface Water’ (GB 3838-2002) (Sb:5.00 × 10⁻³ mg/L), the Sb content in all water samples exceeds the standard, with a maximum exceeding multiple of 1,072 times. The level of Sb in all sampling points in this study is lower than its previous study (Mengchang et al. 2016), but similar to the results of Wang (Wang et al. 2011).

Figure 1 shows that the concentration of Sb in the north mining area increased gradually with the influence of mining activities, then decreased gradually with the dilution of rivers, and reached the maximum near Dongxia Village; the concentration of Sb in the southern mining area increased with the influence of mining activities, and reached the maximum value near Shengli Village.

According to Figure 3(a), the data of each sampling point are distributed below the horizontal line with the molar ratio of Sb and SO₄²⁻ of 0.25 and 0.50. The concentration of SO₄²⁻ in all water samples is much higher, indicating that the SO₄²⁻ in these water samples are multi-sourced. Figure 3(b)–3(g) shows that Sb has a strong correlation with Na⁺ + K⁺, Ca²⁺, Mg²⁺, Cl⁻ and NO₃⁻, but a weak correlation with HCO₃⁻.

There are three sources of ions in natural water: weathering of soil and rocks, atmospheric precipitation and evaporation concentration. Figure 4 is often used to judge which action affects river hydrochemistry. Figure 4 shows that the sampling points of the rivers in the XKS area in Hunan are all in the middle part of the hydrochemical control mechanism diagram, which shows that the ion components in the rivers in the XKS area in Hunan are mainly produced by the weathering process of the rock; and part of the mining-affected area samples have a tendency to spread along the region dominated by evaporation, indicating that the chemical characteristics of the river water in the XKS area are formed under the combined effects of both rock weathering and evaporative concentration, and the effect of weathering on the increase of Sb content is more obvious.

Figure 5(a) uses Ca²⁺/Na⁺ as the ordinate and Mg²⁺/Na⁺ as the abscissa to show the mixing of three types of water: carbonate dissolution, silicate dissolution and evaporite dissolution (Mukherjee and Fryar 2008). Figure 5(a) shows that the sampling point in the mining-affected area is closer to the silicate dissolution area, indicating that the rivers after mining in this area are mainly formed by the dissolution of silicate and contain higher levels of silicate minerals. The rivers in the mining area mainly undergo ion exchange. When there is no reaction, the ratio of Na⁺/(Na⁺ + Cl⁻) is 0.50. The sampling points of the rivers in the XKS area in Figure 5(b) are all distributed in the area where Na⁺/(Na⁺ + Cl⁻) > 0.50. The rivers in the mining area mainly carried out ion exchange.

In summary, the rivers in the mining-affected area of XKS mainly have ion exchange, and the rivers in the non-affected area of mining also involve mineral dissolution.

PMF was used for source apportionment. Due to mining activities, agricultural production and other activities in the XKS area in Hunan, a large amount of pollutants eventually flow into the river, with the result that the same pollution source may emit multiple pollution elements, and the same pollution elements in the Qingfeng River and Xuanshan River of XKS may also be discharged by multiple different pollution sources. It is discharged by many different pollution sources. Therefore, in order to better analyze the heavy metal pollution sources of the rivers in Hunan XKS, this study uses PMF to statistically analyze the samples and calculates the contribution rate of each pollution source. Control and governance provide guidance.

In this paper, the mass concentration of pH, Na⁺ + K⁺, Ca²⁺, Mg²⁺, Cl⁻, SO₄²⁻, HCO₃⁻, NO₃⁻ and Sb in the rivers of XKS in Hunan are used as the parameters to run the PMF model (Figure 6). The main contributing elements of factor 1 are Ca²⁺, pH and HCO₃⁻. The pH can change the solubility of a variety of solids, thereby affecting the release of metals (Mengchang et al. 2016). The surface charge of minerals is a pH-dependent factor, which can control their dissolution (Jin et al. 2012). Due to the amphoteric characteristics of Sb, Sb is easily transferred to the water environment when the salt and pH change (Zhu et al. 2010); the main contributing elements of factor 2 are Na⁺ + K⁺, Cl⁻, SO₄²⁻ and Sb. Sulfate is a useful marker for studying whether water pollution comes from natural and man-made sources, and the increase of sulfate concentration in natural water is a sign of water deterioration (Bottrell et al. 2008) The main source of Sb is the liquid discharged from human mining activities or mineral waste leached by rainwater. The content of heavy metal antimony in natural water is usually less than 1.00 μg/L (Filella et al. 2002), which is mainly due to the low solubility of antimony minerals. Most of the antimony minerals cannot be dissolved in water (Ning & Xiao 2007). In the XKS area of Hunan, the average concentration of antimony in the water body reached 1.79 mg/L, and there has been a very obvious trend of increasing Sb concentration, indicating that the antimony in the surface water of the study area mainly comes from mining activities.

According to the PMF model to calculate the overall contribution percentage of each source of heavy metal element Sb (Figure 7), it is known that the main source of heavy metal Sb in the rivers of XKS in Hunan is mining activities accounting for 83.60%, followed by ore leaching accounting for 16.40%.

TDS has a significant impact on soil alkalization and crop growth. When irrigated with high TDS rivers, it will harden the soil and inhibit crop growth. The TDS values of the rivers in the study area are 287.26–968.88 mg/L, and the calculated SAR values are 1.00–6.65. In order to understand the quality of irrigation water more intuitively, a classification map of irrigation water based on TDS and SAR was proposed (Figure 8(a)), showing that 80.95% of river samples in the study area are at low SAR and medium salinity levels, and 14.29% of river samples are at low SAR and high. In terms of salinity, 4.76% of the river samples are at the level of medium SAR and high salinity. At the same time, the Wilcox chart reflecting salinity and sodium percentage (Figure 8(b)) shows whether the river can be used for irrigation. The analysis shows that 80.95% of the river samples in the study area are very suitable for irrigation, and 19.05% of the rivers are suitable for the irrigation category.

The Hakanson ecological risk index method is used to comprehensively evaluate the potential ecological risk of river heavy metal pollution. The calculation results are shown in Table 5. It can be seen from Table 5 that the RI of sampling points in the non- affected area of XKS area industry in Hunan is between 6.63 and 7.18, all showing a slight degree of ecological risk. The RI of the sampling points in the mining-affected area is between 22.11 and 2,886.15, 16.67% of the rivers have a low ecological risk; 16.67% of the rivers have a moderate ecological risk; 22.22% of the rivers have a considerable ecological risk; 27.78% of the rivers have a high ecological risk; 16.67% of the rivers have a very high ecological risk. The sampling point XKSS-DB22 in the mining affected area has the highest ecological risk level.

In this study, spatial distribution and pollution formation mechanism were analyzed to reveal the geochemical behaviors of Sb in contaminated rivers around an antimony mine in Hunan Province, China. The main conclusions obtained from this research are as follows:

• (1)

  The average Sb content of mining non-affected areas in XKS of Hunan Province is 0.03 mg/L, and the average Sb content of mining affected areas is 2.09 mg/L. Compared with the surface water environmental quality standard (GB 3838-2002) (Sb:5.00 × 10⁻³ mg/L), the Sb content in all water samples exceeded the standard, the maximum exceeding multiple is 1,072 times, and the Sb element content reached the maximum near Shengli Village.

• (2)

  The main source of Sb in the rivers of XKS in Hunan is mining activities which account for 83.60%, followed by ore leaching, which accounts for 16.40%.

• (3)

  The SAR, Na% and KI indexes of the rivers in the study area indicate that the rivers in the area are suitable for direct irrigation, but 27.78% of the river water bodies have a strong ecological risk after irrigation, and 16.67% have an extremely strong ecological risk.

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.

This work is supported by the Open Fund of State Key Laboratory of Groundwater Protection and Utilization by Coal Mining (grant number SHJT-17-42.17), the Ecological restoration project in Lengshuijiang Antimony Mine area (grant number LCG2020009), and the Fundamental Research Funds for the Central Universities of China (grant number 3142018009).

All relevant data are included in the paper or its Supplementary Information.