Occurrence, mobility, and potential risk of uranium in an abandoned stone coal mine of Jiangxi Province, China

doi:10.21203/rs.3.rs-3920524/v1

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Research Article

Occurrence, mobility, and potential risk of uranium in an abandoned stone coal mine of Jiangxi Province, China

https://doi.org/10.21203/rs.3.rs-3920524/v1

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published 02 Sep, 2024

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Metal sulfide oxidation in abandoned exposed stone coal mines leads to the generation of Acid Mine Drainage (AMD), characterized with high uranium concentration, which is a major concern for local public health. This work employs an approach of geochemical analysis and modelling to determined the mode of occurrence of uranium. Additionally, potential environmental risks were evaluated.

The results revealed that the primary source of uranium pollutants in the surrounding environmental media was attributed to the weathering of mine waste. Concentrations of harmful elements (e.g., U, Fe, sulfate) and acidity in water rapidly decreased to background levels with increasing distance from the mine. The geochemical distribution characteristics of sediments and water exhibit notable similarities. The species of uranium underwent a transformation as uranium in mine waste rocks migrated to environmental media. In acidic pit water, uranium primarily existed as uranyl sulfate, gradually transitioning downstream to complexes dominated by hydrophosphate and carbonate. This transition was accompanied by the coprecipitation of significant amounts of uranium with phosphate and iron hydroxides. Results from the geoaccumulation index (I_geo) and risk assessment codes (RAC) indicated that uranium in unweathered coal waste rocks and newly formed pit sediments posed a high environmental risk, with a bioavailable fraction reaching up to 26.44% and 48.0%, respectively. This research holds significant importance in devising remediation and management strategies for abandoned coal mines to mitigate the impact of uranium release and mobility on the surrounding ecological environment.

Stone coal

Occurrence and mobility of uranium

Sequence chemical extraction

Environmental risk

Acid Mine Drainage (AMD)

The weathering of stone coal waste is the main source of uranium pollution.
The mode of occurrence and environmental conditions determine the mobility of uranium.
Newly formed pit poses strong acid generation capacity and high environmental risk.

Stone coal, extensively found in the southern regions of China, is a combustible black shale of early Paleozoic (in some instances, Permian) age. It is characterized by a low calorific value, high coal rank, and elevated ash and sulfur content (Dai et al. 2018). Covering over 1600 km in a parallel arc in southern China (Steiner et al. 2001), stone coal resources are mainly concentrated in provinces such as Hunan, Zhejiang, Hubei, Anhui, Jiangxi, and Guangxi, with a total reserve of 61,876.7 Mt, almost exclusively distributed in the Lower Cambrian (Dai et al. 2018). With large areas and shallow burial, most stone coal is open-pit mined near the surface. With the adjustment of national policies, stone coal mines with low caloric value and serious environmental pollution have been gradually closed, resulting in the emergence of a large number of abandoned mines.

Uranium, the heaviest naturally occurring radioactive element, possesses both chemical toxicity and radioactivity (Chen et al. 2017). It has been established that the presence of organic matter significantly contributes to the enrichment and mineralization of uranium, resulting in the extensive accumulation of uranium within stone coal deposits. According to a study from Qi et al. (2011), stone coal has been identified as an unconventional uranium resource in China. In specific regions, uranium is enriched to the extent of deposits (such as carbon-silica mudstone type) (Wang et al. 2018; Zhang 2000). Moreover, stone coal can be classified as a typical pyrite-type substance and high sulfur coal (Chou 2012), with a total sulfur content ranging from 2–4% (mainly pyrite-type sulfur) (Dai et al. 2018). The oxidation and dissolution of pyrite in stone coal due to weathering contribute to the production of AMD, promoting the release of uranium (Lavergren et al. 2009). The weathering of uranium-enriched stone coal is considered a major contributor to uranium pollution in stone coal mining areas (Peng et al. 2004). Therefore, the mobility of uranium and the resulting environmental contamination caused by the weathering of uranium-enriched stone coal have attracted significant attention.

A study on the environmental impact of the development and utilization of radioactive stone coal mines in the five provinces of Hubei, Hunan, Anhui, Jiangxi, and Zhejiang, which account for over 90% of stone coal reserves, revealed that the average specific activity of ²³⁸U in the soil of mining areas was 370 Bq kg^− 1, eight times higher than the background average (Ye et al. 2004). In a study conducted by Xu et al. (2018) within the stone coal strata of East China, the monitoring of natural radioactivity levels in air, solid, water, and plant media unveiled that the effective dose of γ-radiation exceeded the threshold (5 mSv/a). Additionally, the concentrations of total α and total β in groundwater at specific samples exceeded the environmental standard limits by 10 to 30 times (Xu et al. 2018). Radiological environmental monitoring data obtained by Wei et al. (2021) in a stone coal mining area of Anhui Province also indicated that the mean concentration of total α in pit water was 3.27 Bq L^− 1, exceeding the highest allowable discharge concentration of 1 Bq L^− 1 (Wei et al. 2021). High content of uranium in stone coal mines can seriously threaten ecological and human health, but little is known about how it migrates into the environmental medium.

In summary, existing research has provided valuable insights into the total uranium concentration in stone coal mines and the surrounding environmental media. However, there remains a lack of information regarding the occurrence of uranium and its potential environmental risks. Indeed, understanding the occurrence of elements is fundamental for deciphering its mobilization process and evaluating their environmental impacts, with the quantification of trace elements in various chemical fractions holding greater significance than their total concentration (Zhou et al. 2014).

The sequential chemical extraction method, originally proposed by the European Community Bureau of Reference for heavy metal elements in soils and sediments (Rauret et al. 1989), has been adapted for the quantitative study of trace element occurrence in stone coal (Dong et al. 2018; Lin et al. 2017). This method categorizes element fractions into water-soluble, weak acid-extractable, reducible, oxidizable, and residual fractions. It is widely thought that the sum of water-soluble and weak acid-extractable fractions, known as the bioavailable fraction, is more susceptible to absorption by organisms and transfer through the food chain to humans (Zhang et al. 2021). In contrast, the residual fraction is generally associated with silicate minerals, stable in the natural environment, and virtually immobile. The toxicity and mobility of harmful elements depend not only on their total concentrations but also on their speciation (Sun et al. 2020). Zhang et al. (2021) and Zhou et al. (2014) attempted to characterize and assess the potential environmental risk of harmful elements by using the proportion of the bioavailable fraction (RAC) rather than the total concentration, as total metal concentrations may overestimate the actual level of environmental risk. For instance, weathered soils in Anji, Zhejiang, showed that although the total Pb content (53.82 mg kg^-1) was much higher than that of Cd (1.25 mg kg^-1), the stable residual form of Pb accounted for 76.34%, while the bioavailable fraction of Cd reached 38.22%. Consequently, Cd posed a higher ecological risk in the area, emphasizing the importance of considering speciation in environmental risk assessments (Zhang et al. 2021).

Jiangxi, with one of the most abundant stone coal resources in China (predicted total resource reserve of 6834.03 Mt) (Dai et al. 2018), faces challenges associated with open-pit mining, resulting in substantial abandoned waste rocks (average uranium content reported as 100.64 mg kg^-1) (Wei and Li, 2023a). In the process of weathering, the abandoned waste rocks may release uranium and pollute the local water system and soil. However, there is a lack of understanding of controlling factors for uranium release, migration behavior, and associated environmental risks. The study's objectives include: (1) determining the mode of occurrence of uranium in stone coal mining area; (2) identify the source, release and mobility of uranium; and (3) evaluating the potential risks using I_geo and RAC method. The research results extend our understanding of the source and mobilization mechanism of uranium pollution in abandoned stone coal mine.

2.1 Study area

The research area is situated within Shishi Town and Huanggu Town in Shangrao City, Jiangxi Province, China (Fig. 1). This region typically experiences a subtropical monsoon climate characterized by elevations ranging from 65 to 331 meters. The average temperature is 17.8°C, and the annual precipitation is 1954.0 mm, indicative of intense chemical and biological weathering. The stone coal-bearing strata is the Lower Cambrian Hetang Formation, marked by black thin-layered carbonaceous siliceous rocks, carbonaceous shale, and carbonaceous siliceous shale. The stone coal is situated in the lower part of the Hetang Formation, consisting of three layers with an average total thickness of 20 meters and an average heating value of 5.62 MJ kg^-1 (Guo 1990). The stone coal formed in a slope environment between the Yangtze Platform and the Jiangnan Deep Sea Basin, developing under anoxic and euxinic depositional conditions (Awan et al. 2020; Wang et al. 2017). Since the 1980s, the stone coal in the research area has been progressively exploited, utilizing on-site combustion for lime production (Guo 1990). This exploitation process has led to significant alterations in the topography, resulting in numerous exposed steep rock walls, open-pits, and piles of stone coal waste rocks (Wei and Li, 2023b). These open-pits and abandoned waste rock piles, exposed to the air and acid rain, present potential sources of uranium contamination. There are two main types of mines with significant differences in formation time, including old pits (more than 20 years old) and new pits (about 3 years old).

2.2 Sample collection

In July 2023, a total of 12 stone coal waste rocks were collected in the research area, comprising 2 weathered samples and 10 unweathered samples. Additionally, 26 soil samples were obtained from farmland, along with 9 surface water samples (including pit water, stream water, and river water) and 5 sediment samples. The waste rocks were sourced from representative open-pit stone coal mining areas, with each sampling point collecting 5 sub-samples randomly to form a composite sample. The sample numbers were prefixed with G (unweathered, G1 ~ G10) and GW (weathered, GW1 and GW7), indicating the degree of weathering as per the method proposed by Peng et al. (2004). In the farmland of the watershed on the eastern side of the stone coal mine, soil samples were collected (prefixed with S, S1 ~ S26). At each sampling point, 5 sub-samples were randomly collected from a 10 m × 10 m plot using a zigzag sampling method, with a sampling depth of approximately 20 cm. Water samples were collected from various sources, including the new pit (W1, W6), old pit (W2 ~ W4), stream flowing through the mining area (W7, W9), and Raobei River (W5). Additionally, river water was collected upstream as a control (W8) (Fig. 1). Sediment samples were collected parallel to the stream and river water sampling points, with sample numbers prefixed with DW.

Solid samples, encompassing waste rock, soil, and sediment, were collected using a quadrant method and sealed in plastic bags for transportation to the laboratory. The samples underwent natural air-drying at room temperature, and after the removal of impurities such as debris and gravel, they were crushed and sieved through a 200-mesh for subsequent analysis. Field measurements of water samples, including pH, Eh, and electrical conductivity (EC), were conducted using a portable multi-parameter water quality meter (YSI ProPlus). All water samples were filtered through a 0.45 µm filter, stored in clean polyethylene bottles previously soaked in a 1:1 HNO₃ solution, with one bottle acidified to pH < 2 using HNO₃ for the analysis of major cations and metal elements, and the other bottle kept as the original sample for the analysis of anions. The collected water samples were stored in a refrigerator at 4°C and promptly transported to the laboratory for testing.

2.3 Chemical analysis

The major elemental oxide content of stone coal waste rocks powder, obtained after lithium borate-lithium nitrate high-temperature fusion, underwent analysis using X-ray fluorescence spectrometry (XRF, PANalytical PW2424). Total sulfur and total organic carbon (TOC) content in waste rocks and sediment samples were determined using a carbon-sulfur analyzer (LECO CS844). Loss on ignition (LOI) was measured by weighing the samples after calcination at 1000°C in a muffle furnace. The carbonate and bicarbonate content in sediments were determined by titration according to the standard method LY/T1251-1999. After digestion with nitric acid, hydrochloric acid, perchloric acid, and hydrofluoric acid, trace elements in solid samples were quantified using inductively coupled plasma optical emission spectrometry (ICP-OES, Agilent 5110) and inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7900). The pH values of soil and sediment samples were determined according to the method specified in NY/T 1377–2007, using a pH meter (OHAUS STARTER 3100). For water samples, major cations (Na⁺, K⁺, Mg²⁺, Ca²⁺) and trace elements (Fe and U) were analyzed using inductively coupled plasma mass spectrometry (ICP-MS, LabMS 3000). Major anions (F^-, Cl^-, NO₃^-, SO₄^2-) were determined by ion chromatography (IC, Swiss Metrohm 881). Total phosphorus was measured using a UV-visible spectrophotometer (UV, 1800PC).

2.4 Mineral analysis

X-ray powder diffraction (XRD) was utilized to ascertain the mineral composition of both waste rocks and sediment samples. The XRD analysis was conducted using a Japanese Rigaku X-ray diffractometer (Ultima IV) with Cu K-alpha Ni-filtered radiation. The scanning range was set from 5° to 45° (2θ) with a step size of 0.02° and a scanning speed of 2°/min. Qualitative identification of the main minerals and quantitative analysis of identified minerals were performed using MDI Jade software.

2.5 Sequential extraction method (SEM)

In addition to determining the total concentration of the radioactive element U, identifying its bioavailable fraction and potential mobility capability is also of significant importance. To understand the occurrence of uranium, an adapted version of the SEM was employed, following the national standard GB/T 25282 − 2010. Through this method, the water-soluble fraction (L₀), weak acid extractable fraction (L₁), reducible fraction (L₂), oxidizable fraction (L₃), and residual fraction (L₄) content of uranium in solid media such as waste rocks, sediment, and soil in the stone coal mining area were quantitatively determined. The detailed extraction process is outlined in Table S1, and the concentration of U in each extract was measured using ICP-MS.

2.6 Hydrogeochemical modeling and geochemical mapping

The PHREEQC hydrogeochemical software was utilized to simulate and calculate the species of U in the water of the stone coal mining area (using the waterq4f database). ArcGIS 10.2, a geographic information system, was employed to illustrate the topographical features of the study area, as well as the spatial distribution of sampling points. Additionally, the maps displayed the spatial distribution of U content for the soil samples. All maps were created using the WGS 84 geographic coordinate system as the spatial reference.

3.1 Mineral and geochemistry of stone coal waste rocks

Table 1 presents the mineral composition of weathered and unweathered waste rock samples, revealing significant differences between the two. In unweathered waste rock (G), quartz dominated the mineral composition (73.0%), followed by calcite, clay minerals, and small amounts of pyrite (3.5%), dolomite, orthoclase, and plagioclase. The mineral composition of the waste rocks changed significantly after weathering, with a reduction in easily weathered pyrite to 0.8%, and the weathering decomposition of feldspar into clay minerals (Huang et al. 2017), increasing clay mineral content (12.3%).

Table 1

The mineral composition of waste rocks and sediments
Sample	Quartz	Prthoclase	Plagioclase	Calcite	Dolomite	Pyrite	Clay minerals	Ggypsum	Siderite
G^a	73.0	2.9	0.2	9.3	2.5	3.5	8.6	NMI	NMI
GW^a	73.8	0.6	NMI^a	11.2	1.3	0.8	12.3	NMI	NMI
DW6	35.6	1.3	5.0	NMI	0.6	NMI	53.3	3.5	0.7
DW8	42.7	2.9	8.5	NMI	NMI	0.5	44.8	ND	0.6

The unit of all minerals is %. ^aG and GW refer to unweathered waste rock and weathered waste rock, respectively; NMI, no corresponding mineral identified.

Table S2 summarizes the chemical composition of the waste rocks. The most abundant chemical component was SiO₂, with an average content of 66.09%, followed by CaO, Al₂O₃, MgO, Fe₂O₃, K₂O, and S. The chemical analysis results were consistent with the mineral quantitative analysis conducted using XRD, where Si, Al, Mg were primarily associated with quartz and clay minerals, Ca was mainly controlled by carbonate minerals (i.e., calcite), K was related to feldspar minerals in the waste rock, and Fe and S were closely linked to sulfide minerals (such as pyrite). Compared to unweathered waste rock, the Fe and S content significantly decreased in weathered waste rock, possibly influenced by the oxidation and dissolution of pyrite. The waste rocks demonstrated a high organic matter content, with TOC reaching up to 19.8% and an average of 11.46%. The results of the LOI also supported the abundant organic matter in the waste rock, with an average LOI of 21.09%. Although the organic matter in coal waste rocks was mainly composed of stable organic compounds such as non-degradable n-alkanes and isoprenoids (Zhao et al. 2023), the content of organic matter in weathered waste rock was notably reduced due to long-term weathering (for example, at site G7, TOC decreased from 19.8–9.4% under weathering). The uranium content in the waste rocks ranged from 3.6 to 193 mg kg^− 1, with an average of 84.63 mg kg^− 1, approximately 35 times the world coal background value (2.4 mg kg^− 1) (Ketris and Yudovich 2009), indicating a high degree of enrichment. Although there were only 12 samples, the U content in the waste rocks varied, typically with significantly lower U content in weathered waste rocks (Table S2), indicating the release and mobility of U during weathering processes (Perkins and Mason 2015).

3.2 Hydrochemical characteristics of surface water

Table 2 shows the field measurement parameters and chemistry analysis results for surface water in the study area. The pH of the new pit water (W1 and W6) was 2.64 and 2.88, respectively, indicating strong acidity. These waters contained high concentrations of sulfate (3002 ~ 5832 mg L^− 1), metals (Fe, U), and total P, classifying as AMD (Santofimia et al. 2022). Near the new pits, the total sulfur content of waste rocks (G1 and G10) was notably elevated at 2.17% and 4.10% (Table S2), primarily dominated by pyrite. This constituted a key factor triggering the generation of AMD, as pyrite is prone to oxidation under weathering conditions, leading to the formation of AMD (Acharyaa and Kharelb 2020). Due to the longer formation time, the acid genreation capacity in the old pits have become limited. After dilution by rainwater and neutralization by surrounding carbonate rocks, the pH approached neutrality (ranging from 6.93 to 7.38), with significantly lower concentrations of sulfate, Fe, U, and total P. As shows in Fig. 1C, acidic pit water (W6) flowed downstream, forming Stream 1 which was reddish-brown and noticeable in the field, but with a lower flow rate. Stream 1 flowed north and converged into Stream 2 which was another stream passing through the mining area, with a flow rate approximately 20 times that of Stream 1. W7 was collected at the confluence point. Stream 2 flowed east and eventually affluxed into Raobei River. Due to the strong influence of AMD (W6), downstream W7 also showed significantly higher EC (2136 µs cm^− 1), U (0.404 mg L^− 1), and lower pH (5.63), especially sulfate and Fe concentrations, which were about two orders of magnitude higher than those in natural river water. With increasing distance from the mine and the mixture of other unpolluted streams, the chemical composition concentrations gradually approached normal background levels (e.g., W5). Although, like Stream 1, Stream 2 also passed through the coal mining area, it was mainly supplied by the upstream old pit water. Compared to the former, Stream 2 exhibited lower acidity, EC, SO₄²⁻, and metal concentrations but remained significantly higher than natural river water. The maximum U concentration occurred in W6, where the surrounding waste rocks also had a high U content (G10, 173 mg kg^− 1), with a total sulfur content as high as 4.10%, indicating an active acid-producing process (Du et al. 2017).

Table 2

Hydrochemical characteristics in collected surface water samples
	Guideline value	W1	W2	W3	W4	W5	W6	W7	W8	W9
pH	6 ~ 9^a	2.64	7.38	6.93	7.05	7.38	2.88	5.63	7.77	6.66
Eh	/	550	247	229	270	213	474	251	256	206
EC	/	3830	1538	1306	1464	291	5590	2136	291	1878
Na⁺	/	0.292	53.9	2.57	3.74	2.32	0.626	47.2	1.79	50.3
Mg²⁺	/	1.11	19.4	22.5	28.6	2.23	144	25.8	2.00	22.2
K⁺	/	0.733	7.43	4.05	3.27	1.26	0.425	68.6	1.10	7.21
Ca²⁺	/	21.5	19.8	17.2	23.3	1.90	47.8	28.9	1.55	28.6
SO₄²⁻	250^a	3002	853	689	980	9.30	5832	1276	11.1	1077
HCO₃⁻	/	bdl^c	50	35	47	51	bdl	47	55	48
Cl⁻	250^a	85.18	46.61	60.53	27.81	50.07	20.84	123.3	51.40	72.11
NO₃⁻	10^a	0.913	2.28	0.535	0.937	3.56	bdl	1.40	4.11	2.28
F⁻	1^a	2.10	7.18	0.314	0.973	0.248	16.54	5.98	0.250	5.36
Fe	0.3^a	193	0.385	0.228	0.288	0.188	400	18.5	0.116	0.505
total P	0.2^a	2.26	0.010	0.007	0.013	0.029	6.54	0.422	0.030	0.002
U	0.03^b	0.390	0.039	0.019	0.046	0.128×10^− 2	8.69	0.404	0.125× 10^− 2	0.050

The unit of pH, Eh, and EC is dimensionless, mV, and µs cm^− 1, respectively. Other parameters are mg L^− 1. ^aGuideline values are referenced from the Surface Water Environmental Quality Standards (GB3838-2002); ^bThe guideline value for uranium is derived from the Guidelines for Drinking-Water Quality (Fourth Edition) (WHO 2011). ^cbdl refers to below detectable limit. Data exceeding the specified guideline values are indicated in bold italic.

According to the surface water environmental quality standards in China and the WHO drinking water quality standards, the factors exceeding the limits in the study area's surface water included pH, SO₄²⁻, F⁻, Fe, total phosphorus, and U (Table 2). The samples exceeding the standards were mainly pit water and streams passing through the mining area. After flowing into the Raobei River, the river water's (W5) various factors were below the standard limits. The U concentration dropped to 0.128 × 10^− 2 mg L^− 1, which was close to the U concentration in the unpolluted upstream river (W8, 0.125 × 10^− 2 mg L^− 1).

3.3 The mineralogical and geochemical characteristics of sediments

XRD analysis of sediments indicated the absence of carbonate minerals and pyrite (Table 1). However, secondary minerals such as siderite and gypsum were detected. The presence of siderite suggested a high oxidation degree of pyrite, concomitant with the occurrence of AMD (Concas et al. 2006). The appearance of gypsum indicated substantial sulfates in Stream 1 (W6), with a sulfate concentration of 5832 mg L^− 1. The pH and partial chemical element analysis results for five sediment samples are presented in Fig. 2. The pH variation pattern of the sediment was generally consistent with that of the stream and river water. Sediments from the new pit revealed acidity, gradually transitioning to neutrality away from the mining area. The total sulfur content in the sediment correlated positively with acidity, with the highest value observed in Stream 1 (W6). XRD analysis results indicated that sulfur was primarily present in the form of sulfate minerals, such as gypsum (3.5%), suggesting the precipitation of sulfates at this sampling point. The distribution of Fe and Mn content in the sediment showed differences, with the highest Fe content occurring in DW6 and DW7 (both exceeding 14%). Conversely, the Mn content at these points was minimal, indicating the predominant precipitation of Fe-bearing minerals in Stream 1 and Stream 2 rather than Mn. The U content decreased progressively from Stream 1 and Stream 2 to the downstream Raobei River. Sediments in DW6 and DW9, impacted by acidic mine drainage, contained U concentrations approximately four times higher than those in sediments from unpolluted water bodies. The distribution characteristics of total phosphorus in sediments closely resembled those of U, indicating a positive correlation between the two.

3.4 pH and uranium content of soils in the surrounding farmland of stone coal mine

Table 3 presents the statistical results of soil pH and uranium content in the surrounding farmland of the mine. Influenced by the acidic drainage from the mine, the soil pH exhibited slight acidity, with a mean value of 5.25, comparable to the reported soil pH (5.59) in the distribution area of black shale in Anji, Zhejiang Province (Zhang et al. 2021). Overall, the soil pH in the study area was relatively lower than the national soil background values. The mean, minimum, maximum, standard deviation and coefficient of variation of U content in the soil samples are also listed in Table 3, with their spatial distribution illustrated in Fig. 3. In comparison to the national background values (n = 3382, 2.5 mg kg^− 1, referring to Wang et al. 2016), and the regional average in the investigation of natural radiation levels in Shangrao (n = 43, converted to approximately 3.79 mg kg^− 1 from 55.3 Bq kg^− 1, referring to Li et al. 1993), the average soil U content in the study area exceeded the former two by an order of magnitude. The highest measured value (S₂, 191.0 mg kg^− 1) was about 50 times the background level in the Shangrao region. Similar soil U enrichment has been reported in another major stone coal area in Xiushui, Jiangxi Province (Xu et al. 2018).

Table 3 Statistical results of pH and uranium content of soil samples (n=26)

Parameter	Soil					Background values (Wang et al. 2016)
Parameter	Ave	Min	Max	SD	CV	Background values (Wang et al. 2016)
pH	5.25	2.85	7.02	1.39	0.26	8.0
U (mg kg^-1)	39.35	2.27	191.0	37.65	0.96	2.5

Abbreviations: Ave, Arithmetic mean; Min, Minimum value. Max, Maximum value. SD, Standard deviation. CV, Coefficient of variation.

As depicted in Fig. 3, uranium enrichment was observed in the surrounding farmland soils of the mine. At 57% of the sampling points, uranium concentrations surpassed national background values by tenfold, necessitating considerable attention. The spatial distribution of uranium content revealed that in the proximity of open-pit mining areas, such as Ji Yang, He Cun and Shi Shi, soil uranium content was significantly higher compared to non-mined regions. This implied that stone coal, once exposed to the surface through anthropogenic extraction, under the influence of AMD, facilitated the release and mobility of uranium (Parviainen and Loukola-Ruskeeniemi 2019; Perkins and Mason 2015). The lower uranium content in weathered waste rocks compared to unweathered waste rocks, as demonstrated in Table S2, further supported this inference. The sampling point (S2) with the highest uranium content was situated in an open-pit mining area, representing residual soil formed by the weathering of coal seams as the parent rock. Influenced by the uranium enrichment in its parent rock (G2, 162 mg kg^-1), uranium is similarly enriched in the residual soil after weathering (Zhang et al. 2021). Therefore, the natural weathering of stone coal represented a significant source of excessive uranium accumulation in farmland soils within the study area. Additionally, the distribution of uranium content in sediments clearly indicated that uranium pollutants from the mining area were spread along the stream through dissolution and transportation (Concas et al. 2006), resulting in widespread soil uranium contamination.

3.5 Sequential chemical extraction and hydrogeochemical simulation results

Utilizing the SEM, the mode of occurrence of uranium in eight representative samples of waste rocks, soils, and sediments was determined. The specific test results are presented in Fig. 4. Dai et al. (2021) summarized the occurrence of uranium in coal, suggesting that in stone coal, uranium was primarily adsorbed to organic matter in the form of uranyl ions, similar to G1, where uranium was predominantly present in an oxidizable fraction (Fig. 4). However, some studies also noted that during the process of sequential chemical extraction, due to the encapsulation of fine minerals by organic matter, uranium present in these stable fine minerals was mistakenly categorized as organically bound, leading to an overestimation of the contribution of organically bound uranium (Dai et al. 2020; Spears, 2013). In this study, uranium in waste rocks was also predominantly present in the residual fraction (up to 85.08%), where it was bound within mineral crystalline structures—a highly stable fraction less likely to be released from rocks and unavailable for biological utilization (Zhou et al. 2014; Zhang et al. 2021). Weathered waste rocks (GW1, GW7) exhibited uranium concentrations significantly lower than unweathered waste rocks (Table S2), but the proportion of the residual fraction was relatively higher, indicating uranium release during weathering (Perkins and Mason, 2015). While uranium in waste rocks primarily existed in relatively stable residual or oxidizable fractions, mobile components (L₀ and L₁) also account for a certain proportion (4.15% ~ 26.44%). Moreover, considering the high total uranium content (Table S2), the absolute amount of uranium potentially released upon exposure to the surface should not be disregarded.

Sediments collected in the new pit (e.g., DW6) contained uranium concentrations significantly higher than downstream river sediments (Fig. 2). Sequential chemical extraction results indicated that uranium in the sediment primarily existed in bioavailable fractions (L₀+L₁, 48%). This fraction of uranium demonstrated strong potential mobility and high environmental risks (Zhou et al. 2014).

The significant presence of clay minerals (53.3%) in DW6, in contrast to the geochemical properties of the stone coal waste rock, implied that these sediments may not have originated from the weathering of the stone coal, but rather have been deposited through external transportation processes. It is worth noting that clay minerals possess a strong capacity for uranium adsorption (Song et al. 2007), and it is plausible that the uranium enrichment observed in the sediments was primarily attributed to its adsorption onto clay minerals, which can be readily reactivated.

The distribution of uranium in different fractions of soils showed the characteristics of oxidizable fraction>reducible fraction≈residual fraction>weak acid-extractable fraction>water-soluble fraction, with the oxidizable state (L₃) accounting for over 50% (Fig. 4). This could be attributed to the enrichment of organic matter in the soil (34.9~103 g kg^-1, referring to Yang et al. 2022). A certain proportion of uranium in the soil was also present in the reducible fraction (19.02% ~ 22.51%, Fig. 4), indicating that uranium was also associated with iron-manganese (hydro)oxide minerals.

It has been reported that the toxicity of harmful trace elements in water is closely related to their chemical species (Liao et al. 2018). Before the remediation and removal of harmful trace elements from AMD, it is essential to consider their chemical fraction. The PHREEQC hydrogeochemical program is widely used to simulate and calculate the chemical species of ions in water-rock interaction processes, including coal mine waste rock (Kříbek et al. 2018; Namaghi and Li 2016; Qureshi et al. 2016; Santofimia et al. 2022). In this study, the PHREEQC software was used to determine the chemical species of uranium in AMD, streams, and Raobei river water (Fig. 5b), where uranium predominantly existed as hexavalent (VI) complexes. Fig. 5a illustrates a positive correlation between the total amount of uranium and acidity. The AMD with low pH contained a significantly higher uranium total concentration. The distribution of uranium species is as follows: UO₂SO₄> UO₂(SO₄)₂^2-> UO₂(HPO₄)₂^2-> UO₂²⁺> UO₂F⁺. Santofimia et al. (2022) observed a similar predominance of uranium in the species of UO₂SO₄ (67% ~ 71%) in acidic waters produced by black shale in the Iberian Massif mountains of Spain. The pH of the acidic pit water demonstrated an upward trend upon flowing into the stream, leading to a substantial deposition of U, accompanied by a significant reduction in the total concentration of U. In nearly neutral streams and river waters, the chemical species of uranium experienced a significant change. Uranium was primarily present in the forms of hydrophosphate and carbonate complexes such as UO₂(HPO₄)₂^2-, UO₂(CO₃)₂^2-and UO₂CO₃, consistent with the simulated results of neutral drainage from a uranium-containing coal waste pile reported by Kříbek et al. (2018).

3.6 The potential environmental risks of uranium

Uranium has been proven to possess developmental toxicity, and inhalation or ingestion of uranium can contribute to internal irradiation. Additionally, substantial exposure to uranium can also result in chemical toxicity. In some cases, the chemical toxicity of soluble uranium compounds may even exceed their radiotoxicity (Domingo 2001). Due to the elevated U content in stone coal (Dai et al. 2018; Yang et al. 2022), the potential environmental impact of uranium during development has attracted widespread attention (Wei et al. 2021; Ye et al. 2004; Zhou 1981). Previous environmental impact studies, although including various media types such as coal, ash, soil, water, air, and biota, predominantly focused on the total concentration of uranium, with little consideration for its chemical fractions. Therefore, to more reasonably evaluate the pollution and potential environmental risks of uranium, this study integrated two widely used approaches for quantitatively assessing the potential environmental risk of solid samples, including the I_geoand RAC (Fu et al. 2019; Ma et al. 2020; Wu et al. 2021; Dong et al. 2021; Zhang et al. 2022). I_geo not only considers the impact of natural geological processes on element background values but also focuses on the influence of human activities on toxic element pollution (Wu et al. 2021). RAC determines the potential environmental risk of potentially toxic metals by determining the proportion of the bioavailable fraction (L₀+L₁). The risk level was determined based on the RAC value, classified as < 1%, 1% ~ 10%, 11% ~ 30%, 31% ~ 50%, and > 50%, representing no risk, low risk, moderate risk, high risk, and very high risk, respectively.

The I_geo results for uranium of solid samples, including waste rocks, soil, and sediment, are presented in Fig. 6a. The I_geovalues ranged from -0.72 to 5.74, with average values of 3.88 for waste rocks, 2.99 for soil, and 2.47 for sediment, respectively. According to the I_geo values of waste rocks samples, 8% of the sampling points were uncontaminated (I_geo = 0), while most other sampling points were mostly heavily or severely polluted. The highest soil I_geo value was located at S2 (5.67, weathered residual soil of coal waste), and most soil I_geo values fell within the range of 2 to 4, indicating moderate to strong pollution. The I_geo results for sediments showed that heavily polluted samples were primarily concentrated in the streams passing through the mining area. These results indicated that abandoned mine represented a significant source of uranium pollution, and long-term weathering has resulted in some degree of contamination in the surrounding environmental media.

The RAC results for the eight solid media samples are illustrated in Fig. 6b. The sampling point with the highest RAC value is the sediment from the new pit (DW6, RAC = 48%). In comparison to DW6, DW5 was mainly associated with Fe-Mn hydroxides, organic matter, and residual forms, resulting in a low potential risk. The soil in the mining area exhibited a high organic matter content, and most of the uranium was associated with organic matter, resulting in RAC values below 10, indicating a low-risk level for the ecosystem.

The evaluations based on the I_geo and RAC indicated a certain degree of contamination in waste rocks, soil, and sediments. Particularly, in some sampling points, notably in the waste rocks and sediments from the new mining pit, a considerable amount of bioavailable uranium was observed, suggesting a potential risk of release and mobility.

The mode of occurrence of elements and external environmental factors are the two crucial factor for predicting the mobility behavior during the development and utilization of coal resources, as well as the leaching of coal waste during weathering (Finkelman 1995). Leaching tests on stone coal waste rocks from Hubei Province, China, have shown that the leaching of metal elements (V, Cr, As, Cd, etc.) was not significantly correlated with the total content but was closely related to the content in the water-soluble and exchangeable fractions (Lin et al. 2017). A correlation analysis of U and TOC content was conducted on 55 samples of stone coal-bearing black shale from Enshi, Xiushui, and the research area (Wang et al. 2017; Xue et al. 2018). The results indicated a positive correlation between U and organic matter components (Fig. 7a). In addition to organic matter, the enrichment of uranium in coal was closely related to the phosphorus element (Fig. 7b). Wang et al. (2014) inferred, through electron probe analysis, that the uranium minerals in the black shale of Xiuwu Basin were mainly phosphocalcite and phosphorus-containing uranium minerals. Scanning electron microscopy in-situ analysis of Permian uranium-rich black shale in Enshi, Hubei, also found that uranium was associated with phosphorus minerals (such as xenotime and rhabdophane) (Wang et al. 2017). However, in this study, sequential chemical extraction data of waste rocks indicated that U was mainly present in the residual and organic-bound fractions (Fig. 4). It can be inferred that in addition to being associated with organic matter by hexavalent complexes, a portion of uranium in coal exists in the tetravalent form in uranium minerals or stable minerals. This is evidenced by the presence of some outliers in Fig. 7a (highlighted with black circles). Based on the spatial distribution characteristics of uranium in environmental media such as water, sediment, and soil, the enrichment of uranium is likely to originate mainly from the weathering and mobility of mine waste rock. Two pieces of evidences support this: 1) a significant decrease of uranium content in weathered waste rocks samples (Table S2); 2) the absence of other uranium-polluting industrial activity around the stone coal mine.

The abundant presence of organic matter and sulfide minerals (Tables 2 and S2) makes coal and waste rocks susceptible to chemical weathering effects (Parviainen and Loukola-Ruskeeniemi 2019). When coal is exposed to the surface, sulfide minerals are prone to oxidation and dissolution, generating AMD with high concentrations of Fe, sulfate, and low pH, accompanied by the release and mobilization of uranium (Saria et al. 2006). A comparison of the sequential chemical extraction results for unweathered and weathered waste rocks revealed that the most significant reduction in the uranium are oxidizable, reducible, and weak acid-extractable fractions. The release of uranium primarily occurred under oxidative and strongly acidic conditions, transforming uranium associated with organic matter and Fe-Mn (hydro) oxide minerals into soluble uranyl sulfate complexes. Similar uranium leaching behavior was observed by Yu et al. (2014) through laboratory oxidation experiments and SEM. As AMD flowed into Stream 2, the pH of the water increased (pH=5.63), leading to significant precipitation and accumulation of uranium in the sediment (Fig. 5a). Simultaneously, there was a notable change in the species of uranium, shifting from sulfate complexes to hydrophosphate and carbonate complexes. Kříbek et al. (2018) summarized four ways in which polluted water transfers pollutants to sediment, including adsorption on mineral surfaces, organic matter adsorption complexes, coprecipitation with iron-manganese hydroxides and phosphates, carbonates, forming independent minerals . As shown in Fig. 2, the uranium content in the sediment was positively correlated with total phosphorus and iron, indicating coprecipitation of uranium with phosphate and iron hydroxides.

Fig. 8 summarizes the mobility patterns of uranium from stone coal mine under surface weathering. Firstly, uranium-rich waste rocks containing high organic matter and pyrite were exposed to the surface through human mining activities, leading to the oxidation and dissolution of pyrite, generating AMD. As the formation of stone coal is heterogeneous, varying in different regions or even within the same mining area, the sulfide minerals, associated metal elements, and host rocks differ significantly (Parviainen and Loukola-Ruskeeniemi 2019). In this respect, the content of carbonate minerals in the host rocks of coal layers can affect their acid-producing capacity. Lin et al. (2017) found through laboratory leaching experiments with stone coal waste rocks that waste rocks possesses acid-base neutralization capacity, with the pH stabilizing between 8.3 and 8.5 under different initial pH conditions (ranging from 3 to 9). In this study, we consistently observed acid neutralization phenomena in some coal waste rocks, such as in some water samples from the old pits (W2, W3, and W4), which showed near-neutral acidity with pH values of 7.38, 6.93, and 7.05, respectively (Table 2), indicating that weaker capacity of acid production and lower environmental risk.

Secondly, uranium in coal was partly associated with organic matter in hexavalent form, and another part was present in tetravalent form in stable minerals (such as phosphorus-containing minerals). Under strong acidic and oxidative conditions, uranium was leached by AMD, forming soluble species like UO₂SO₄, UO₂(SO₄)₂^2-, and minor amounts of UO₂(HPO₄)₂^2-, UO₂²⁺, and UO₂F⁺. After AMD flowing into stream, it was diluted, resulting in a gradual increase in pH and then the concentrations of sulfate, iron, phosphorus, and uranium in the stream decreased progressively. Simultaneously, corresponding sediments showed a gradual decline in the concentrations of these elements, with uranium primarily co-precipitating with phosphate and iron hydroxides. Finally, the uranium content in downstream river waters sharply decreased to levels near background concentrations, falling below environmental standard limits. The species of uranium shifted predominantly to hydrophosphate and carbonate complexes. It represented the process of natural attenuation (He et al. 2019).

This study investigated the occurrence, mobility behavior, and potential environmental risk of uranium in waste rocks, soils, sediments, and surface water in an abondoned stone coal mining area of Jiangxi Province, China. The results indicated that the abandoned open-pit mine represented the primary source of uranium pollution. The self-purification and natural attenuation of streams were observed. Both sediments and surface water showed similar geochemical characteristics, with high uranium content samples only occurring near the mine. In contrast, uranium-rich soil was more widely distributed in the downstream watershed of mine.

The mode of occurrence and external environment conditions determine the mobility of uranium.

Uranium in waste rocks was leached by AMD to form soluble uranium sulfate complexes. As AMD flowed into other nearly neutral streams, a significant amount of uranium precipitated with phosphates and iron hydroxides, leading to a transformation in the uranium species, with hydrophosphate and carbonate complexes becoming predominant. There is a substantial amount of uranium present in the bioavailable fraction (L₀+L₁) in unweathered waste rocks and sediments from the new pit, having a relatively high potential environmental risk. Further research should focus on laboratory leaching tests to assess the release rate of uranium. Such tests can provide specific information to understand the release and mobility behavior of uranium under different conditions (Petrovic and Fiket 2022). Due to the heterogeneity of the waste rock, it is important to understand the mode of occurence of uranium, acid-producing capacity, and environmental conditions before developing a mine remediation strategy.

Consent to publish: The participant has consented to the submission of the case report to the journal.

Funding: This research was financially supported by “Mechanism of in situ bio-desulfurization and uranium imobilization of acidic uranium-bearing groundwater” (No.2022 NRE-LH-20), jointly foundation funded by China National Uranium Corporation Limited & State Key Laboratory of Nuclear Resources and Environment of East China University of Technology.

Competing interests: The authors have no competing interests to declare that are relevant to the content of this article.

Ethics approval：This is an observational study. No ethical approval is required.

Author’s contribution statements:

Xinxiang Wei: Writing-original draft preparation.

Naizheng Xu: Writing-Reviewing and Editing.

Jiang Li: Supervision, Validation.

All authors have read and agreed to the published version of the manuscript.

Acknowledgements: This research was financially supported by “Mechanism of in situ bio-desulfurization and uranium imobilization of acidic uranium-bearing groundwater” (No.2022 NRE-LH-20), jointly foundation funded by China National Uranium Corporation Limited & State Key Laboratory of Nuclear Resources and Environment of East China University of Technology.

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Occurrence, mobility, and potential risk of uranium in an abandoned stone coal mine of Jiangxi Province, China

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1 Introduction

2 Materials and methods

2.1 Study area

2.2 Sample collection

2.3 Chemical analysis

2.4 Mineral analysis

2.5 Sequential extraction method (SEM)

2.6 Hydrogeochemical modeling and geochemical mapping

3 Results

3.1 Mineral and geochemistry of stone coal waste rocks

3.2 Hydrochemical characteristics of surface water

3.3 The mineralogical and geochemical characteristics of sediments

3.4 pH and uranium content of soils in the surrounding farmland of stone coal mine

4 Discussion

5 Conclusion

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