Lead Pollution and Isotope Tracing of Surface Sediments in the Huainan Panji Coal Mining Subsidence Area, Anhui, China

Abstract

In this study, the provenance of anthropogenic lead, a major pollutant of surface sediments, was determined in Huainan Panji coal mining area. The lead concentrations and the pollution degree were investigated by the enrichment factor. Tessier five-step morphology extraction method was used to discuss its potential hazard. By comparing the lead isotope ratios of surface sediments and surrounding potential polluted end-members, the lead isotope ternary mixture model was appropriated to explore the sources and relative contribution fractions. The results showed that: (1) The lead concentrations ranged from 31.44 to 64.07 mg/kg which was mild-moderate pollution. (2) The chemical forms of lead were residue state > iron-manganese oxidation state > exchangeable state > organic state > carbonate state. (3) The anthropogenic lead in surface sediments originated from soil, vehicle exhaust, coal gangue. And the relative contribution fractions were 51.70%, 30.90%, and 17.40%.

The lead has a wide distribution and easy extraction characteristics, which was widely used in manufacturing. Thus, this metal was an ubiquitous contaminant in coastal marine environments (Choi et al. 2007). With the rapid development of economy, lead contamination in riverine environment has become one of the most concerned issues in natural ecosystems (Lintern et al. 2016; Zhang et al. 2009). Volcanic eruptions and forest fires were the two main natural sources of environmental lead. The anthropogenic activity, such as mining activity, smelting and vehicle exhaust has obviously contributed to leading contamination (Li et al. 2012). Due to its persistence, bioaccumulation and non-degradability in environmental media, long-term exposure may cause damage to nerves, hematopoiesis, cardiovascular and endocrine systems through the food chain (Damian and Barbara 2010; Liu et al. 2013).

In the course of coal mining, the ground sank and formed a large area of subsidence (Sun et al. 2010a). The high groundwater level and the surface runoff caused it formed a collapse lake. Apart from natural weathering, anthropogenic activities are the prime source for lead contamination, what also is a contributor of lead in water (Hao et al. 2008). Sediment plays an important role in the migration and transformation of heavy metals. Because the collapse lake is less circulated with the outside world, more than 99% of heavy metal contaminants are enriched in sediments after being discharged into water bodies (Sun et al. 2010b). Chemical fertilizer and pesticide, mine water discharge, vehicle exhaust, leaching of coal gangue and so on, all of which are the main sources of heavy metal pollution (Tang et al. 2018; Kaur and Goyal 2018; Tanabe et al. 2001; Pulles et al. 2012; Roessler et al. 2016). The ecological environment can be affected when the collapse lake is used in fishery farming or farmland irrigation (Fang et al. 2014).

It is essential to identify the pollution sources for environmental pollution management. There are many evaluations of heavy metal pollution in sediments (Varol 2011; Liu et al. 2003), but each method has its limitations. The lead isotope ratios had been shown to be a useful tracer for lead pollution assessment and source determination (Fillion et al. 2014). Lead has four naturally isotopes: 204Pb, 206Pb, 207Pb, 208Pb, and their abundance variations arise from the radioactive decays of 238U, 235U and 232U to 206Pb, 207Pb and 208Pb (Cheng and Hu 2010). Lead ore has different isotope ratios from the surrounding rocks and other ores, because lead isotope in the crust has increased or decreased depending upon half-lives of their parent isotopes, and Pb ores formed in the past. Therefore lead isotope ratios retain the characteristics of the source area of pollution and play a role in geochemical fingerprinting, which can be used for tracers sources (Lin et al. 2015). The lead isotope fingerprints can be used not only to identify sources, but also to quantitatively assess the contribution of various sources. Using lead isotope ratios to identify lead sources of surface sediments in Huainan Panji have not yet been done and are challenged given the potentially numerous continental sources, however, it is necessary to show the capabilities of differentiation of pollutant sources for multi-source environments using isotopic tracers.

Huainan mining area is located in the middle section of the Huaihe river and is rich in coal resources. It is one of the 14 billion tons of coal production base and one of the six major coal-electricity integration bases determined by the state. Panji mining area is the most densely distributed mining area in Huainan minefield. There are four modern mines, Panyi, Paner, Pansan and Panbei mine. There are three large meteorite mountains in the mining area. The coal mining subsidence area is caused by coal mining activities and is greatly affected by human activities, which is of great significance for the study of heavy metal pollution. In the present study, total concentration, chemical forms and isotope ratios of lead were used to (i) evaluate the enrichment and mobility of lead in sediments; (ii) analyse the different chemical forms of bioavailability; (iii) determine the anthropogenic contribution fractions of lead in environment. The results will provide a scientific basis for the prevention and control of heavy metal pollution in the region.

Materials and Methods

Panji mining subsidence area (32°39′11″–32°56′16″N, 116°39′52″–117°05′50″E), located in the north-central part of Huainan city, Anhui Province. The research area includes Panyi, Paner, Pansan and Panbei. Fifteen sediment cores (Fig. 1) were collected in May 2017 by using a gravity core sediment-sampler of 80 mm inner diameter. The GPS was used to record the locations of sample sites. Along the collapsed lake, Farmland soil samples were collected from the upper 20 cm, four subsamples were collected according to the random dot method, and 1 kg was retained after the quadruple method as a soil sample. Coal gangue samples were collected by GB 475-1996, according to the stacked shape, the sub-samples are distributed at the top, waist and bottom. First removed the surface layer of 20 cm and took four subsamples at the top, waist and bottom next. Four subsamples were evenly mixed into one sample, and stored in quarters to 1 kg. Vehicle exhaust samples were collected in the exhaust pipe of large transport vehicles, used a brush to gently scrape the oil ash on the inner wall of the exhaust pipe and store it in a clean sealed bag.

Fig. 1
figure1

Sampling location of surface sediments in Panji coal mining subsidence area

Sediment samples were dried and ground to 200 mesh in an agate mortar. Sample digestion was performed using the national standard method (HJ 832-2017). Each sediment sample was weighted for 0.1 g in a PTFE digestion tank and wetted with distilled water. HNO3 (6 mL) and HCl (3 mL) and HF (2 mL) were added in proper order, and the mixture was screwed and placed in a microwave digestion apparatus (Discover SP-D 24/48). Microwave digestion was started according to Table 1. The lead concentration was measured by Thermo Fisher, iCAP Q, and the detection limit is 0.005–0.01 ppb. Quality control was achieved by adding GBW 07423 soil composition analysis standard material, and the recovery rate was controlled at 95%–98%.

Table 1 Microwave digestion temperature program

The Tessier five-step extraction method (Tessier et al. 1979) was used to analyze the chemical forms of lead (F1: exchangeable state, F2: carbonate state, F3: iron-manganese oxidation state, F4: organic state, F5: residual state).

The lead isotope testing was completed by laser ablation inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences. Instrument accuracy (Agilent7500a): 7Li > 8 Mcps/ppm < 6 cps, 89Y > 20 Mcps/ppm < 50 cps, 205Tl > 12 Mcps/ppm < 5 cps. Lead isotope standard analysis results: NBS981 206Pb/204Pb = 16.931, 207Pb/204Pb = 15.486, 208Pb/204Pb = 36.687.

In the present study, the enrichment factor (EF) (Chen et al. 2007) were calculated to assess the pollution status. The formula as follows:

$$EF=\frac{{{{({C_{P{\text{b}}}}/{C_{Al}})}_{Sample}}}}{{{{({C_{Pb}}/{C_{Al}})}_{Reference}}}}$$
(1)

where (CPb/CAl)Sample is the Pb to Al ratio in the samples, and (CPb/CAl)Reference is the reference material value of Pb to Al ratio, In this study, Al was used as a reference element (Chen et al. 2007).

The contribution fractions of vehicle exhaust, soil and coal gangue were calculated by using the mixed model formula of ternary pollution (Cheng et al. 2010). The formula as follows:

$${R_s}={f_1}{R_1}+{f_2}{R_2}+{f_3}{R_3}$$
(2)
$${N_s}={f_1}{N_1}+{f_2}{N_2}+{f_3}{N_3}$$
(3)
$$1={f_1}+{f_2}+{f_3}$$
(4)

where the \({R_{\text{s}}}\),\({N_{\text{s}}}\)represents 206Pb/207Pb, 208Pb/206Pb of the sample, the subscripts \({N_1}\), \({N_2}\) and \({N_3}\) represent the three major sources, and \({f_1}\), \({f_2}\), \({f_3}\) are their relative contributions.

Results and Disscussion

Figure 2 shows the lead concentrations and enrichment factor in surface sediments from different collapse lakes in the research area. The lead concentrations of each sampling point showed a wide range of 31.44–64.07 mg/kg, and the mean value was 38.72 mg/kg.

Fig. 2
figure2

Lead contents and EFs of surface sediment in Panji coal mining subsidence area EF < 1 none enrichment; 1 < EF < 2 minimal enrichment; 2 < EF < 5 moderate enrichment

Compared with the background value of soil environment in Anhui province (China Environmental Monitoring Center 1990), each sampling point was exceeded. The lead EFs in surface sediments ranged from 0.95 to 2.26, and the mean value was 1.34. According to the enrichment evaluation criteria (Hu et al. 2011), two samples was in moderate pollution level, and 12 samples were in slight pollution level, and one sample was in no pollution. The lead concentrations in the surface sediments of the Panji coal mining subsidence area had a downward trend with the flow direction of the river, indicating that there was a lead pollution source in the upper reaches of the Nihe river. In the upper reaches of the Nihe river, there were large-scale mining areas and numerous amount of coal gangue and fly ash piled up, which will be released into the environment under natural weathering and leaching. The emission of vehicle exhaust during coal transportation was an important source of pollution for lead.

Chen (2017) found that the heavy metal content in the water body of Panji coal mining subsidence area can meet the requirements of class I water quality in China. However, due to heavy metals precipitate quickly and bind strongly to sediments (Simpson and Spadaro 2016), it was possible for metals to detach from sediments and be released back to the water column, negatively impacting water quality (Simpson and Batley 2010).

The chemical forms of lead exhibited similar distribution patterns, mainly in the forms of F5 (residual state) > F3 (iron–manganese oxidation state) > F1 (exchangeable state) > F4 (organic state) > F2 (carbonate state) (Fig. 3.). The average percentage of residual state was 43.93%. This part of heavy metals was contained in the lattice of primary minerals and secondary silicate minerals, which properties were stable and it was not easy to undergo migration and transformation.

Fig. 3
figure3

Lead forms of sediments in Panji coal mining subsidence area

The exchangeable and carbonate states were sensitive to environmental changes, especially pH, and were readily released under neutral or acidic conditions. The iron–manganese oxidation state meant that heavy metals were encapsulated by iron and manganese oxides or were a part of hydroxide precipitation, which was easily released when the redox potential decreased or the water was anoxic, which causeed secondary pollution to the water body. Organic state referred to the combination of heavy metals with active groups or sulfur ions of organic matter, which was released in strong oxidizing environment. The above four forms were collectively called extractable states and can be used by organisms (Wang et al. 2016). The average percentage of extractable state in the study area was 56.07%, which was greater than the residual state. Most of the residual heavy metals existed in the lattice of primary and secondary silicate minerals, which were stable and difficult to diffuse to the surrounding environment.

Studies have shown that, the extractable state content will increase significantly after the sediments were artificially contaminated by heavy metal elements, which can be judged that the heavy metals in the sediments were affected by human activities (Liu et al. 2007).

Because of lead isotope ratios did not take the place of fractionation in secondary environment, their isotopic ratios were mainly restricted by the initial lead concentrations and the reaction of radioactive elements in the source area. This “fingerprint” characteristic can be well applied to the analysis of environmental pollution sources (Walraven et al. 2014). Different isotopic ratios for 208Pb/204Pb, 207Pb/204Pb, 206Pb/204Pb, 208Pb/206Pb, 206Pb/207Pb in sediments were summarized in Table 2. The natural 206Pb/207 Pb is higher (> 1.20) and the anthropogenic 206Pb/207Pb is lower (0.96 ~ 1.20) (Sun et al. 2017). The 206Pb/207Pb of the surface sediment in the study area was 1.1836 (Table 2), which was < 1.2, indicating that it was affected by the artificial lead source.

Table 2 Lead isotopic composition of sediment and potential sources

The lead isotope ratios of surface sediment and end-members are shown in Fig. 4. The lead isotope ratios of the surface sediments in the study area were basically consistent with those of the Yangtze river and Nan Jing sediments, indicating that the three sources were similar. The lead isotope ratios of surface sediments in the study area were within the range of vehicle exhaust, soil and coal gangue, and the correlation coefficient was 0.8969. Therefore, the lead in the sediments was derived from three sources of vehicle exhaust, soil, and coal gangue.

Fig. 4
figure4

Comparison between lead isotope ratios of the surface sediment and the potential sources

The main sources of lead in the surface sediments of the study area were from soil, the contribution rate was 51.70%; the second was vehicle exhaust emission, the contribution rate was 30.90%; the lowest contribution rate was coal gangue, the contribution rate was 17.40% (Table 3). As a medium of heavy metal pollution, soil was seriously affected by coal mining activities. The relative contribution from soil was 51.70% and the soil lead pollution may result from coal mining activities.

Table 3 Contribution fractions of main sources to lead of the surface sediment

The concentration and chemical forms and isotope ratios were carried out to investigate the level of pollution and source analysis. The conclusions are as follows:

  1. 1.

    The concentration of each sampling point ranged from 31.44 to 64.07 mg/kg. Combined with EF, the lead in the sediments of the collapse lake in Huainan Panji mining area was mild-moderate pollution.

  2. 2.

    The percentage of chemical forms were residual state > iron–manganese oxidation state > exchangeable state > organic state > carbonate state. The extractable state (F1 + F2 + F3 + F4) in the samples was 56.07%, which was greater than the residual state.

  3. 3.

    The lead in the sediments were derived from three sources of soil, vehicle exhaust, and coal gangue, the contribution fractions were 51.70%, 30.90%, 17.40% respectively. Coal mining activities had an important impact on lead pollution in the study area.

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Acknowledgements

This work was financially supported by the Natural Science Foundation of China (Grant Nos. 41373108; 41702176), Science and Technology Development Project of Anhui Traffic and Navigation Engineering (Grant No. JTHW-2017K1). We acknowledge editors and reviewers for polishing the language of the paper and for in-depth discussion.

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Correspondence to Liugen Zheng.

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Zheng, L., Liu, X., Tang, Q. et al. Lead Pollution and Isotope Tracing of Surface Sediments in the Huainan Panji Coal Mining Subsidence Area, Anhui, China. Bull Environ Contam Toxicol 103, 10–15 (2019). https://doi.org/10.1007/s00128-019-02558-5

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Keywords

  • Coal-mining subsidence area
  • Surface sediment
  • Lead
  • Isotope ratio