
==== Front
Sci Rep
Sci Rep
Scientific Reports
2045-2322
Nature Publishing Group UK London

39223289
71502
10.1038/s41598-024-71502-3
Article
Analysis of heavy metals in sediments with different particle sizes and influencing factors in a mining area in Hunan Province
Que Wenpiao 1
Yi Liwen yilw@hunnu.edu.cn

12
Wu Yueting 1
Li Qiuping 3
1 https://ror.org/053w1zy07 grid.411427.5 0000 0001 0089 3695 College of Geographical Sciences, Hunan Normal University, Changsha, 410081 China
2 grid.411427.5 0000 0001 0089 3695 Hunan Normal University Key Laboratory of Geospatial Big Data, Changsha, 410081 China
3 Nanchang No.10 Middle School, Nanchang, 330000 China
2 9 2024
2 9 2024
2024
14 2031818 10 2023
28 8 2024
© The Author(s) 2024
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ Open Access This article is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License, which permits any non-commercial use, sharing, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if you modified the licensed material. You do not have permission under this licence to share adapted material derived from this article or parts of it. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by-nc-nd/4.0/.
Heavy metals mainly exist on the surface of sediment particles and are transported using particulate matter as carriers. Therefore, the particle size of sediment particles can affect the adsorption, release, and migration of heavy metals. This study aim to investigate the distribution characteristics and chemical fraction of Cd, Pb, and As contents in sediments of different particle sizes using the BCR method, and to determine the key factors affecting the distribution of heavy metals through mineralogical methods such as XRD and EDS. The results revealed that the overall content of various forms of heavy metals increases with the decrease of particle size, mainly presents in fine particles. The mineralogical analysis results indicated that fine particles predominantly contained clay minerals such as chlorite and illite and coarse particles mainly include primary minerals. Due to the mining areas in the middle-upstream, Cd, Pb and As were primarily associated with galena, sphalerite and pyrite, respectively. The distribution of heavy metals is jointly influenced by sediment particle size and sediment material composition.

Keywords

River sediment in mine area
Particle size
Heavy metals
Speciation
Subject terms

Ecology
Environmental sciences
Hydrology
Hunan Innovative Province Construction Chenzhou National Sustainable Development Agenda Innovation Demonstration Zone Special FundsInvestigation project on the causes of geological hazards in Hunan Province in the past decadeissue-copyright-statement© Springer Nature Limited 2024
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pmcIntroduction

Since the Industrial Revolution, the enormous demand for metal resources has driven the development of minerals. In the process of ore development and mining, heavy metals and their compounds are released into the water environment along with wastewater1. However, heavy metals entering rivers cannot be biodegraded, but enter the food chain through biological adsorption and enrichment, posing a threat to biological and human health2–4. Cd is one of the most toxic elements in environmental pollution5,6. Numerous studies have shown that Cd pollution has adverse effects on aquatic organisms and can migrate in water bodies through water flow and food chains, forming cyclic hazards7. The toxicity of Pb is long-term and persistent, and its main hazards to the human body include damage to the kidneys, liver, reproductive system, nervous system, and basic physiological processes of cells8. As poisoning can lead to cell apoptosis, neurological disorders, and may also inhibit DNA repair, causing chromosomal abnormalities9. Therefore, the study of heavy metals in river sediments is particularly important.

Recently, various studies have demonstrated that heavy metals primarily exist on the surface of sediment particles, which act as carriers for their migration and transport. The particle size of sediment particles is an important characteristic that significantly influences the migration, deposition and the distribution of heavy metals10. Research has revealed that the composition of sediment particle sizes holds considerable significance in determining the distribution of heavy metals. By analyzing the material composition, heavy metal content, and forms of occurrence in sediments of different particle sizes, researchers have been able to identify the key factors that govern the distribution of heavy metals. Currently, there have been many studies on the relationship between heavy metals and sediment particle size composition. However, previous studies have mostly focused on the heavy metal content of sediments with different particle sizes. There is relatively little research on the control factors of heavy metal distribution in sediments with different particle sizes, as well as the relationship between the particle size effect of heavy metals and hydraulic transport.

Many studies have shown that in unpolluted sediments, heavy metals are mainly concentrated in the mineral lattice of particulate matter in the residual state. After being polluted, heavy metals interact with sediment substrates through different binding mechanisms, adsorb to mineral surfaces, or bind to insoluble substances such as carbonate, iron and manganese oxides, organic matter/sulfides, and carbonate11. In order to assess the migration capacity of heavy metals, different chemical continuous extraction methods have been developed to analyze the morphological components of heavy metals. The study of heavy metal chemical fraction is the key to the expression of the geochemical characteristics of heavy metals in soil and sediments, and has important practical significance. The form of heavy metals is affected by the mechanical composition of soil and sediment, pH, organic matter, etc. Studies have found that with the increase of clay content of fine component soil, the content of available heavy metals also increases, and pH affects the form of heavy metals by changing the particle charge, adsorption–desorption and precipitation-dissolution equilibrium12.

Therefore, this study concentrated on a river located in a mining area in Hunan Province and aimed to analyze the distribution characteristics of Cd, Pb and As in sediments with varying particle sizes and study the release mechanism of heavy metals during hydraulic transmission by examining the mineralogy. To accomplish this, researchers divided the sediments into various particle size components and investigated the content, occurrence forms, organic matter, and major elements of heavy metals in each fraction. This study utilized XRD and EDS techniques to examine the mineralogy of sediments with different particle sizes and identified the key factors that impact the distribution and migration of heavy metals under hydraulic transmission. The research contributes to the development of strategies for mitigating internal secondary pollution caused by heavy metals.

Materials and methods

Sample collection

The research area is located in a watershed in the southern part of Hunan Province. The watershed is located in a mountainous area, flowing from south to north, with a total length of 33.0 km and a watershed area of 117.0 km2. The area has abundant metal reserves, including lead, zinc, tin, tungsten, copper and other polymetallic minerals. After inspecting the mining area in the research area, the river was divided into upstream, middle-upstream, middle-downstream and downstream along the water flow direction based on the scale range of the water body. There are two mining areas distributed in the research area. The deposit in the middle-upstream of the mining area is a type of skarn type lead zinc deposit. The mining areas in the middle-downstream are a historical mining site which is far from the main river channel. Currently, mining activities in the middle-downstream have basically stopped.

To ensure representative sampling, sampling points were established in each river section. Surface sediment samples were collected at these points, with on-site recording of latitude, longitude, river conditions, sample characteristics, and surrounding pollution. The sediment sampling took place at locations where the water surface made contact with the river bank and the water flow was comparatively slow. Using shovels, approximately 2 kg of surface sediments (0–15 cm depth) were collected along the river bank. Three samples were collected from each sampling section to account for variability. Subsequently, the sediment samples were mixed thoroughly, placed in plastic bags, and transported back to the laboratory for drying and subsequent data determination. By employing this systematic sampling approach, the researchers aimed to ensure comprehensive and representative coverage of the study area, facilitating reliable data analysis and interpretation. The specific distribution of these sampling points is as follows in Fig. 1.Fig. 1 Distribution of sampling points.

Research method

Five groups of particle size components were divided into 18, 35, 60, 120 and 200 mesh sediments, namely coarse sand (1.00–0.50 mm), medium sand (0.50–0.25 mm), fine sand (0.25–0.125 mm), very fine sand (0.125–0.075 mm) and silty sand (< 0.075 mm). The sediment samples of each particle size were dissolved by hydrochloric acid, nitric acid, hydrofluoric acid and perchloric acid to determine the content of heavy metals, and the ratio of these four acids is 3:9:2:1 (HJ 1315–2023). The BCR sequential extraction method stipulated in the national standard was adopted to extract weak acid extraction state, reducible state, oxidizable state and residual heavy metals from soil. The content of heavy metals was determined by inductively coupled plasma-mass spectrometer (Perkinelmer avio500, SG). Sediment organic matter was determined by potassium dichromate oxidation and external heating method, major elements content was determined by liquid chromatography-atomic fluorescence spectrometry (BAF-2000, CN) and sediment particle size composition was determined by laser particle size analyzer (Mastersizer 2000, UK).

To ensure the accuracy of the analysis results, an average of 1 sample is taken every 10 samples during the analysis process to make a parallel sample. Simultaneously conduct blank experiments and use the standard substance for analysis of sediment composition in water systems (GBW07309) for quality control. The method detection limits of Cd, Pb, and As are 0.03 mg/kg, 1 mg/kg, and 0.2 mg/kg, respectively. The relative standard deviation(RSDs) of the collected three samples of all heavy metals were less than 5%. All of the reagents were of analytical grade, and all of the solutions were prepared by ultra-pure water.

Results and discussion

Content and distribution of heavy metals in different particle sizes

The content of heavy metals in the upstream sediments is low, while the content of heavy metals in the middle-upstream and middle-downstream is high. The overall content of Pb and As reach the highest value in the middle-upstream, Cd reaches the highest value in the middle-downstream. According to the distribution of heavy metal content in sediments of different particle sizes, as shown in Fig. 2, the contents of heavy metals Cd, Pb and As increase with the decrease of particle size in the upstream, middle-downstream, among which the contents are highest in the < 0.075 mm silty sand and extremely fine sand of 0.125–0.075 mm. The content in 1–0.5 mm coarse sand is low. These sections are relatively far from the mining area and are less affected by the lead–zinc mines of middle-upstream. The distribution of heavy metals in different particle sizes is more affected by the surface effect of fine particles. Many studies have shown that with larger specific surface area, fine particles carry larger negative charge and free energy on their surface13, and contain more active ingredients14. Liu et al. (2018). believed that the enrichment of heavy metal elements on fine particles may be related to the adsorption, precipitation and complexation of clay minerals, iron and manganese oxides, organic matter and sulfide on fine particles15, so the chemisorption amount of heavy metals also increased. However, this law is changed in the middle-upstream, Cd, Pb and As show high content in coarse sand and fine particles ranging from 1 to 0.5 mm. Research shows that not all elements are controlled by particle size, while some elements such as Pb are easily controlled by their material sources16. The middle-upstream are most strongly affected by mining areas, and the particle size effect of heavy metals in sediments is interfered by provenance.Fig. 2 Distribution of heavy metal content in sediments with different particle sizes.

In the middle-upstream of lead–zinc mining areas, a large amount of mineral debris is imported into the river sediments, such as galena, sphalerite, pyrite and other minerals are deposited in the riverbed. Pb can exist in galena in the form of secondary mineral crystal particles, and can also exist in pyrite17, Cd, As are usually associated metals in associated lead–zinc ore bodies18,19. The particle size of many mineral detritus is relatively coarse. For example, the particle size of sphalerite is about 0.5 mm, the particle size of galena can reach 1.2 mm, and the particle size of pyrite can also reach 1 mm20, so the mineral detritus rich in heavy metals are concentrated on the coarse particles. Secondly, studies have shown that coarse particles are easy to form micro-aggregates, and heavy metals will be embedded in micro-aggregates and preferentially adsorbed by coarse particles, which can also lead to enrichment of heavy metals in coarse particles21. In addition, Cd not only occurs more in the middle-upstream of coarse sand, but also in the middle-downstream of 1–0.5 mm coarse sand and < 0.075 mm silty sand, which is also interfered by provenance. Pyrite left over from historical mining in the middle-downstream of lead–zinc mines is an important source22. In general, the content of heavy metals in sediment components with smaller particle size is higher. However, under the influence of mineral sources in mining areas, some heavy metals are enriched in coarse particles. It can be said that the particle size composition and material composition of sediments jointly affect the distribution of heavy metals.

Occurrence and morphology of heavy metals

The occurrence form of heavy metals is an important representation of their bioavailability and toxicity. The occurrence form of heavy metals in sediments is not only related to their own characteristics, but also related to sediment material composition, particle size composition and environmental conditions (pH, Eh, DO, etc.)23,24. According to the percentage content of heavy metal speciation in the main river sediments, as shown in Fig. 3, Cd is usually associated in nature in the form of compounds in zinc ore, copper ore and other ore bodies, and Cd has a strong main polarization ability, easy to be adsorbed by colloidal substances in soil, so the content of Cd effective state is very high, among which the exchangeable and acid soluble state accounts for the largest proportion, the residual state proportion is the smallest25. The proportion of Pb reducible state is the highest. Galan et al. (2003). indicated that Fe and Mn oxides in sediments have strong obligate adsorption capacity for bivalent lead ions26. As occurs mostly in the mineral lattices of primary and secondary minerals, and the proportion of exchangeable and acid soluble state As in each sampling section is less than 1%, which is almost negligible. Among these heavy metals, Pb is mainly reducible state, Cd is mainly extracted from exchangeable and acid soluble state, and As is mainly residual state. The reduced heavy metals are usually released significantly under strong reduction conditions27. In general, the contribution rate to the secondary release of heavy metals is small, and the released heavy metals are even re-adsorbed due to the good adsorption performance of iron and manganese oxides. The heavy metals extracted from exchangeable and acid soluble have very strong migration ability, and the residual heavy metals are relatively stable. The occurrence form of heavy metals is affected by the material composition and particle size composition of sediments. The available heavy metals also maintain a high proportion in the upstream, middle-downstream of the sediments with a high proportion of clay and silt, indicating that there is a strong correlation between the available heavy metals and sediment particle size. In the reach with a high proportion of fine particles, heavy metals have a stronger release risk. This is similar to the results of previous studies28. Although the proportion of clay and silt is also high in the middle-downstream near the lead–zinc mine, due to the influence of mineral debris in the mining area, the proportion of heavy metals is high, and heavy metals exist in the crystal lattice of minerals29. Therefore, under the combined influence of sediment material composition and particle size composition, The content and form of heavy metals in different sediments are different.Fig. 3 The percentage content of heavy metal forms in the sediment of the main river channel.

According to the heavy metal content map of sediment morphology with different particle sizes, as shown in Fig. 4, the major forms of heavy metals in sediments with different particle sizes do not change. In the upstream, middle-downstream and downstream, the content of heavy metals in various forms generally increases with the decrease of particle sizes, which also leads to the concentration of total heavy metals in fine particles. Among them, reducible Pb, exchangeable and acid soluble Cd, and residual As increase greatly with particle size, which is due to the large proportion of major forms of heavy metals, and the change trend is more obvious. Studies have shown that the specific surface of particulate matter, as well as the contents of carbonate, Fe and Mn oxides, and organic matter may be important reasons for the concentration of heavy metals in fine particle size15. In the middle-upstream of the section near the mining area, the contents of exchangeable and acid soluble state and oxidizable state heavy metals also increase with the decrease of particle size, but the reduced and residual heavy metals in the middle-upstream still maintain a high storage in the coarse sand, which may be influenced by the mineral debris and iron and manganese oxides in the middle-upstream of the coarse particles of lead–zinc ore. Residual state and reducible state heavy metals occur more in the lattices of sphalerite, galena and pyrite mineral clastic and on iron and manganese oxides23. From the above studies, it can be seen that the distribution of various forms of heavy metals is affected by the grain size effect and material composition of the sediments. The reducible state and residual state heavy metals in the middle-upstream near the mining area are mainly concentrated in the coarse sand of 1–0.5 mm, while the other sections of all forms of heavy metals tend to accumulate in the < 0.075 mm silt and the extremely fine sand of 0.125–0.075 mm. The particle size effect of heavy metals will lead to the concentration of toxic heavy metals in fine particles. Study indicates that unsteady content of heavy metals is significantly negatively correlated with particle size, and the toxicity of heavy metals will increase with the decrease of particle size30. At the same time, due to the characteristics of river sediment movement itself, fine particles are more likely to migrate with the current, and unsteady heavy metals are enriched in fine particles, which greatly increases the risk of heavy metal pollution diffusion.Fig. 4 Heavy metal content map of sediment morphology with different particle sizes (Note: D1–D5 represents coarse sand—silty sand sediment components).

Sediment XRD analysis results

As shown in Fig. 5, the XRD spectral diffraction results of sediment particles show strong similarity in mineral composition of sediments with different particle sizes in the same section, but the height of diffraction peaks is different, indicating that the mineral content is different. Primary minerals such as quartz, feldspar and mica appear in each section, and the summit of primary minerals is relatively higher in the sediment components with coarse particle sizes. Because the primary mineral is directly from the parent rock, through physical weathering to form a coarser sand, so in the coarse sand 1–0.5 mm content is higher. However, the diffraction peak of secondary minerals such as chlorite and illite is relatively higher when the silt is < 0.075 mm. Due to biological weathering and chemical weathering, the mineral particles of secondary minerals are relatively fine, and the particle size of secondary minerals is generally less than 0.02 mm31. The upstream are less disturbed by human activities, and the content of primary minerals such as quartz and feldspar is higher, especially in the coarse sand. The middle-upstream and middle-downstream contain relatively more clay minerals such as chlorite and illite, and are more concentrated in silt. The middle-upstream of sediments are most affected by the mining area, and the contents of sulfide metal minerals such as galena (PbS) and pyrite (FeS) are large. These minerals are important occurrence minerals of heavy metals32. The diffraction peak of heavy metal minerals is still high in the coarse sand from 1 to 0.05 mm, indicating that the content of metal minerals in the coarse particles is large. The middle-downstream and downstream sediments also contain small amounts of pyrite and galena, which may have been transported down from the upper reaches by the current. Accordingly, clay minerals in the mineral composition of the sediment content is one of the factors influencing the fine particle adsorption of heavy metal, metal sulfide minerals is the important cause leading to the hierarchical sediment coarse particle enrichment of heavy metals.Fig. 5 Mineral composition of sediments with different particle sizes (Note: a, b, c and d represent the upstream, middle-upstream, middle-downstream and downstream, respectively).

Sediment EDS analysis results

According to EDS analysis, as shown in Fig. 6, the coarse sand is mainly composed of Si and O elements, mainly quartz sand grains. Silt, on the other hand, is very broken and dispersed in shape, with high porosity and larger surface area. It is a soft aggregate composed of more fine particles, with more flocculation colloid adsorbed on the surface, which may be organic complex. From the atomic percentage of energy spectrum, the atomic percentage of As in the silt at the upstream, middle-downstream and downstream is greater than that of the coarse sand. However, the content of heavy metals in the coarse sand in the middle-upstream is relatively high. This indicates a high content of heavy metals carried by the substances in its coarse sand. Combined with XRD and EDS scan line analysis of sediments, it can be seen that Pb mainly occur in galena, and Cd is obviously shown in the densely distributed area of Zn, which may mainly occur in sphalerite19. The distribution of As and Fe is close to each other. Combined with the characteristics of heavy metals, As may occur mainly in pyrite, partly in galena and sphalerite33. The occurrence minerals of heavy metals are closely related to the release capacity of heavy metals. Caille (2003). proposed that differences in the oxidation rates of sulfur-containing metal minerals would lead to different release rates of heavy metals34. Caetano (2003). suggested that heavy metals co-precipitated or adsorbed with iron and manganese sulfide could be oxidized rapidly after aeration in sediments35. However, heavy metals combined with more stable CuS and pyrite are unlikely to be oxidized in a short period due to their slow oxidation kinetics, among which the oxidation rate of galena and sphalerite is higher than pyrite36. Therefore, the grain size effect of sediments is not the only factor affecting the adsorption and release of heavy metals. The content of clay minerals in the sediment mineral composition and the oxidation rate of metal minerals are also important factors affecting the dynamic behavior of heavy metals.Fig. 6 EDS of sediments with different particle sizes (Note: The a to h shows EDS plots of coarse sand and silt in the upstream, middle-upstream, middle-downstream and downstream).

Distribution rules and influencing factors

In addition to the specific surface and surface energy differences of particulate matter, the material composition of particulate matter is also an important factor that particle size controls the distribution of heavy metals. Tab. 1 shows the contents of organic matter and major elements in sediments with different particle sizes. It is found that the contents of organic matter and major elements in each component of sediments in the middle-upstream are the highest, while the contents in the downstream are the lowest. The content of organic matter in each section ranges from 0.2 to 0.35, and the content of organic matter in different particle sizes increases with the decrease of particle size, and the content of organic matter is the highest in silt < 0.075. The complexation reaction between organic matter and heavy metal has an important effect on the fixation of oxidized heavy metal37, and the charge in organic matter and soil colloid also increases the adsorption of heavy metal ions. It has been confirmed that carbonate particles and clay minerals can selectively adsorb organic compounds, thus increasing the organic matter content of sediment particles, and clay minerals are mainly concentrated in fine-grained sediments38. Al2O3, Fe2O3 and MnO2 basically increased with the decrease of particle size to varying degrees, especially in the silt < 0.075 mm. The other two major elements, MgO and CaO, do not change obviously with grain size. However, Fe2O3 and MnO2 in 1–0.5 mm coarse sand in the middle-upstream of the section are still very high because of the high content of iron and manganese mineral debris in its coarse particles. Therefore, the above results demonstrate that particle size plays a pivotal role in controlling the distribution of heavy metals. This control is achieved by influencing the contents of organic matter, as well as minerals such as Fe, Mn, and Al, and the specific surface area of sediments.Table 1 Main elements and organic matter content of sediments with different darticle sizes.

Sampling section	Particle size(mm)	OM	Al2O3 (g/kg)	Fe2O3 (g/kg)	MnO2 (g/kg)	CaO (g/kg)	
Upstream	1–0.5	0.22	76.33	40.07	0.95	16.37	
0.5–0.25	0.21	82.05	42.34	1.55	15.71	
0.25–0.125	0.20	83.49	42.71	1.64	11.72	
0.125–0.075	0.23	90.15	44.72	2.78	14.46	
 < 0.075	0.24	120.26	53.96	3.62	17.96	
Middle-upstream	1–0.5	0.26	177.42	142.63	17.61	96.87	
0.5–0.25	0.28	153.18	130.48	10.53	86.12	
0.25–0.125	0.30	176.60	122.09	13.52	79.48	
0.125–0.075	0.31	197.42	133.86	15.10	82.99	
 < 0.075	0.35	233.66	145.39	18.03	52.17	
Middle-downstream	1–0.5	0.24	123.17	50.92	4.40	27.75	
0.5–0.25	0.26	120.13	59.39	4.19	27.96	
0.25–0.125	0.28	122.22	73.11	5.91	52.76	
0.125–0.075	0.30	133.93	78.60	7.35	72.26	
 < 0.075	0.31	165.47	89.43	7.69	76.99	
Downstream	1–0.5	0.26	83.89	44.29	6.43	20.76	
0.5–0.25	0.26	85.72	44.13	7.06	21.58	
0.25–0.125	0.27	86.73	45.47	7.92	28.24	
0.125–0.075	0.29	85.71	59.47	8.52	18.76	
 < 0.075	0.28	90.19	64.23	9.93	15.70	

Particle size distribution is a basic property of natural water particle assemblage, which affects physical transport and biochemical processes in rivers39. The particle size of sediment migrates along with the transport of water flow, and the particle size is the key factor affecting its migration ability. Studies have shown that the mechanical composition of river sediment is different, which will cause the difference in its effect on heavy metal carriers40.

The grain size distribution characteristics of river sediments are affected by the thickness of terrigenous particulate matter and sedimentary environment41, as shown in Fig. 7. Sand (63–2000 μm) at the upstream source occupies the highest proportion, and the suspension capacity of sediment in the upstream reaches is weak due to the influence of gravity, resulting in a low concentration of suspended matter in water. The proportion of clay (< 3.9 μm) and silt (3.9–63 μm) in the rest reaches is relatively high, and the proportion of clay and silt in the middle-upstream and middle-downstream reaches 84% and 76%.Fig. 7 Percentage of sediment particle size composition.

The correlation analysis of heavy metals with clay, silty sand and sand, as shown in Fig. 8, shows that the proportion of clay is positively correlated with the content of heavy metals; Pb and As are significantly correlated with it. There is also a positive correlation between heavy metal content and silty sand, and the correlation between Cd and silty sand is stronger than clay. And sand is negatively correlated with heavy metal content, indicating that the ability of heavy metal occurrence in sand is weaker than that in clay and silty sand. Many studies have shown that due to the larger specific surface area, greater surface energy, and high content of organic matter and clay minerals, fine particles have a stronger ability to adsorb heavy metals42, while the higher content of SiO2 in sand has a "dilution" effect on heavy metals.Fig. 8 Correlation between physical and chemical properties of sediment and heavy metals.

Conclusion

Based on the analysis of sediment material composition, heavy metal content, and the occurrence of heavy metals in different particle size components, the study has identified key factors that control the distribution of heavy metals. The distribution of heavy metal content is influenced by both sediment particle size composition and material composition. The overall content of various forms of heavy metals increases with the decrease of particle size, mainly presents in fine particles. Primary minerals such as quartz, feldspar, and mica are more concentrated in coarse sand, while clay minerals such as chlorite and illite are more concentrated in fine sand. Furthermore, Pb is mainly present in galena, Cd is mainly present in sphalerite, and As is mainly present in pyrite in the sediments of the middle-upstream most strongly affected by the mining area. The distribution of heavy metals is jointly influenced by sediment particle size and sediment material composition such as iron, manganese, aluminum oxides, organic matter, clay minerals.

Author contributions

CRediT authorship contribution statement W.Q.: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Resources, Software, Visualization, Experimentation, Manuscript. L.Y.: Funding acquisition, Project administration, Supervision, Writing—review & editing. Y.W.: Methodology, Software. Writing—review & editing. Q.L.: Software, Validation. All authors have read and agreed to the published version of the manuscript.

Funding

Funding was provided by Hunan Innovative Province Construction Chenzhou National Sustainable Development Agenda Innovation Demonstration Zone Special Funds and Investigation project on the causes of geological hazards in Hunan Province in the past decade.

Data availability

All data generated or analysed during this study are included in this published article and its supplementary information files. The datasets used and analysed during the current study available from the corresponding author on reasonable request.

Competing interests

The authors declare no competing interests.

Publisher's note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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