Environmental geochemistry of potentially toxic elements in tailing of the Ahangaran Pb-Zn mine, Hamadan province

Document Type : Research Article

Authors

• Mahrokh Ghanbari Mohazzab ¹
• Giti Forghani Tehrani ²
• Afshin Qishlaqi ³

¹ Msc Student, Department of Environmental Geology and Hydrology, Faculty of Earth Sciences, Shahrood University of Technology, Shahrood, Iran

² Associate Professor, Department of Environmental Geology and Hydrology, Faculty of Earth Sciences, Shahrood University of Technology, Shahrood, Iran

³ Assistant Professor, Department of Environmental Geology and Hydrology, Faculty of Earth Sciences, Shahrood University of Technology, Shahrood, Iran

10.22067/econg.2024.85154.1094

Abstract

The present study aims to investigate the environmental geochemistry of potentially toxic elements (PTEs) in tailing samples of the Ahangaran Pb-Zn mine, located 95 km southeast of Hamadan. For this purpose, physico-chemical properties of 14 tailing samples, mineralogy and the concentration of PTEs were studied. The values of pH, carbonate content, and cation exchange capacity (CEC) ranged between 7.7-9.4, 50.5-64.5 %, and 27.3-35.1 meq/100g, respectively. The main clay mineral detected in the studied samples was montmorillonite; quartz and siderite were the most frequent mineral phases in the samples. The tailing samples, based on the total concentration of potentially toxic elements and the calculation of environmental indices, were enriched in Pb, Zn, Cd, As and Ag, and were of high ecological risk. Human Health Risk Assessment (HHRA) revealed that the hazard quotient of PTEs was much higher for children than adults. The highest values of non-carcinogenic risks via ingestion, dermal contact and inhalation were obtained for As, Mn, and Cd, respectively. The ingestion of As, Cd, and Cr for children, and the ingestion of Cd for adults are associated with probable carcinogenic risks. Based on the obtained results, disposed tailings around the Ahangaran mine can be considered a potential source to pollute the groundwater resources and agricultural soils. Therefore, appropriate environmental management of waste disposal as well as taking remediation actions deem necessary.
 
Introduction
During the last decades, extensive anthropogenic activities around the world enhanced the input of PTEs into different compartments of the environment. Mining, processing, and smelting of sulfidic ores are considered the most important source of PTEs in the environment. The waste materials produced during the ore processing (i.e. tailings) have been classified as threatening hazardous wastes that need specific environmental management and treatment  (Banerjee et al., 2023). 
Ahangaran mine located 95 km SE Hamadan is a large Pb-Zn producer wherein the ore mineral is processed by flotation, and the produced tailings are dumped in tailing ponds around the mine. The present study aims to investigate the concentration and environmental geochemistry of PTEs in mine tailings and to assess the human health risk imposed by PTEs through the standard method of USEPA (1989). 
 
Materials and methods
Sampling, sample preparation and analyses
14 surface composite tailings samples were collected from the tailing ponds (0-30 cm) by a stainless steel trowel. The samples were dried at room temperature. pH, carbonate content, and cation exchange capacity (CEC) of the samples were determined in particles < 0.15 mm. To determine the total concentration of major and trace elements, 1gr of tailings passed through a 230 mesh sieve (63 μm) was digested by a mixture of hot concentrated acids (HF + HClO₄ + HCl + HNO₃) in the open system ( Jeffery and Hutchinson, 1981). The concentrations of major and trace elements in the digested solutions were measured by an ICP-OES instrument.
pH was determined using the standard method of the USEPA test method (USEPA,1989). The carbonate content was measured by titration method, and CEC was determined based on the USEPA standard method (USEPA,1989). X-Ray Diffraction (XRD) method was used to investigate the mineralogy of the samples.
 
Results and discussion
The physico-chemical properties of tailing samples
pH of the tailing samples varies between 7.7 and 9.4. The neutral to alkaline nature of the samples may enhance the bioavailability of metalloids such as As and Sb. The average amount of carbonate content in the samples is 55.9%, which is in accordance with the alkaline pH of the studied samples. Based on the Metson classification (Metson, 1956), the samples are classified as high CEC. XRD analysis of the samples shows that the predominant clay mineral in the studied samples is montmorillonite, which explains the high CEC values of the studied samples.
 
Concentrations of major and trace elements in tailing samples
The average concentration of elements (mg/kg) in the studied samples decreases in the following order:
Fe > Mn > Pb > Al > Zn > Cu > Sb > Ni > Th > As > Co > V > Cr > Cd > Ag > Sc
The average concentrations of PTEs in the tailings samples are much higher than their respective values in the mean crust composition; therefore, the tailings of the Ahangaran mine may enhance the concentration of PTEs in soils, cultivated crops, and groundwater of the adjacent area. In order to assess the level of contamination, the geoaccumulation index (I_geo) was calculated as follows:
 I_geo= log ₂ [(C_n)⁄1.5(B_n)]
where C_n and B_n are the target element’s concentration in the study sample and the reference material (mean crust composition), respectively. According to the classification of the geoaccumulation index, the studied samples are very highly polluted in Pb, Sb and Ag. 
The enrichment factor (EF) is calculated using the normalized ratio of target element (x) in the study sample to the respective value in the reference (background) material:
EF =
RE refers to the concentration of a normalizing element (Sc). The mean crust composition was considered the background sample. On the basis of the obtained results, the studied samples are characterized as extremely high enriched in Pb, Sb, Ag, Mn, Zn, Cd, As, very high enriched in Th and Cu, significantly enriched in Fe, Co, Mo, and moderately enriched in Ni, Cr, and V. 
Hakanson (1980) defined the Potential Ecological Risk Index (PERI) as follows:
where Ei and Ti are the ecological risk index and biological toxicity factor, respectively. The obtained PERI values of all tailing samples are much higher than 600; thus the ecological risk imposed by tailings is very high and suitable measures should be taken to reduce the pollution level in the study area.
To assess the human health risk imposed by high levels of PTEs in the tailing samples, the average daily dose (ADD, mg/kg/day) of each element through ingestion, inhalation, and dermal contact routes as was calculated:
ADD_inh =                                                                                                      
The Hazard Quotient (HQ) of each pathway was obtained to assess the non-carcinogenic risk of each target element (USEPA, 1989): 
Where RfD_i is the reference dose (mg/kg/day). While HQ≤1 indicates that there is no non- carcinogenic risk, HQ> 1 shows that non-carcinogenic risks are probable through a certain exposure route. Hazard index (HI) is also an indicator of probable non-carcinogenic health risks:
n refers to the number of the studied elements. If the value of HI is ≤1, there is no adverse health effects. HI> 1 indicates likely adverse health effects. Human health risk assessment shows that the HQ values of target elements is higher in children than in adults. The highest non-carcinogenic risk for digestion, inhalation and dermal contact is related to As, Mn, and Cd, respectively. The ingestion of As, Cd, and Cr for children and ingestion of Cd for adults is associated with probable carcinogenic risks.
 
Conclusion
Based on the total concentration of PTEs in the samples and the values of geochemical indices, the studied tailings are highly polluted with Pb, Sb, Mn, Zn, Cu, As, Cd, and Ag. Human health risk assessment shows that ingestion is the most important route for element’s exposure, and children are at higher risks. The highest values of non-carcinogenic risks through ingestion, inhalation and dermal contact pathways were obtained for As, Mn, and Cd, respectively. The ingestion of As, Cd, and Cr for children, and the ingestion of Cd for adults are associated with probable carcinogenic risks.
  
Acknowledgment
The authors gratefully acknowledge the financial support of the Research Office of the Shahrood University of Technology.

Keywords

• Pollution
• Tailing
• Ahangaran Mine
• Sanandaj-Sirjan Zone

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