Characteristics of heavy metal accumulation in five wild plants in Huize Lead-Zinc mining area

In order to screen out the plants used to repair heavy metal pollution in the soil, five plants and surface soil were collected in the Huize lead-zinc mine area, centered on the hyperaccumulator plant Arabis alpina L. var. parviflora Franch, measured the heavy metal content of in shoot and root of plant and surface soil, and analyzed the characteristics of heavy metal accumulation in plants. The results showed that the soil Cd pollution in the Huize lead-zinc mining area was the most serious; among the five plants, the Cd bioconcentration factor(BCF) and translocation factor(TF) of A. alpina were more than 1, and the TF of Pb was more than 1; the TF of Anaphalis margaritacea, Cyananthus inflatus and Arenaria orbiculata to Cu and Zn were more than 1, the TF of Juncus effusus to Cd and Zn were more than 1. These five plants had good tolerance to heavy metals and were of great significance to the remediation and restoration of heavy metal contaminated soil in lead-zinc mining areas.


Introduction
Lead-zinc mine is an important strategic mineral resource in China [1]. Long-term mining and smelting activities of mineral resources have produced a large number of slag piles and abandoned land, resulted in serious heavy metal pollution in the mining area [2]. Heavy metal pollution of soil in mining areas will lead to soil degradation in farmland and crop yield reduction, thereby threatening the quality and safety of soil in mining areas [3]. Yunnan is rich in mineral resources, with lead-zinc deposits ranking first in the country and second in Asia [4]. The Huize lead-zinc mine in Yunnan is one of the representatives of large-scale lead-zinc deposits in my country. There were old slag piles and waste land generated during mining and smelting in the mining area, which made the surrounding farmlands seriously polluted by heavy metals [5]. Heavy metals enter the human body through soil and crops, and accumulate in the human body, seriously endangering human health.
There are many ways to deal with heavy metal pollution in the soil, among which the greenest and most effective is phytoremediation. This article discussesd Arabis alpina L. var. parviflora Franch(A. alpina), Anaphalis margaritacea(A. margaritacea), Cyananthus inflatus(C. inflatus), Arenaria orbiculata(A. orbiculata) and Juncus effusus(J. effusus) plants and their rhizosphere surface soil in the Huize lead-zinc mining area were sampled and investigated, and their heavy metal enrichment characteristics were studied and analyzed to provide a theoretical basis for remediation of heavy metal soil pollution in the mining area.

Overview of the study area
The Huize lead-zinc mining area is located in the northeastern part of Yunnan, in Huize County, Qujing City, Yunnan Province. The terrain is high in the southwest and low in the northeast. It is mainly mountainous, with an average elevation of 2183 m. It has a subtropical monsoon climate. The annual average temperature is 12.6 ℃. The sampling points were Chihong site(CHS), Xiaomaping site(XMP) and Sanduoduo site(SDD) (Figure 1).

Sample collection and processing
The area where the A. alpina southern mustard grows in the mining area was selected, and five plants and surface soil were collected in the area, such as A. alpina, A. margaritacea, C. inflatus, A. orbiculata and J. effusus, which grew naturally and distributed more frequently. Sealsd all samples with polyethylene plastic bags and took them back to the laboratory.
Firstly, the plant samples were rinsed with tap water to clean the soil attached to the surface of the plants, and then the plants were rinsed with deionized water three times, and the plants were divided into shoot and root. Secondly, place the plants in an oven at 105 ℃ for 30 min, and dry them at 75℃ to a constant weight. Finally, they were crushed with a stainless steel grinder. The soil sample is naturally air-dried and passed through a 0.149 mm nylon sieve.

Concentrated
HNO 3 -HClO 4 and concentrated HNO 3 -H 2 O 2 digestion methods were used to determine the content of Pb, Cd, Cu and Zn in soil and plants, and then measured with flame atomic absorption spectrophotometer.

Data processing and statistical analyses
Bioconcentration factor (BCF)=heavy metal content in shoot of plant (mg kg -1 )/heavy metal content in soil (mg kg -1 ) Translocation factor (TF)=heavy metal content in shoot of plant (mg kg -1 )/heavy metal content in root of plant (mg kg -1 ) The data were collected and analyzed by Microsoft Excel. One-way analysis of variance (one-way ANOVA) and significance test(P=0.05) were done by IBM SPSS Statistics 22.

Soil heavy metal content in mining area
Compared with the national soil environmental quality standard, the average content of Pb, Cd, Cu and Zn in the soil was 15.9 times, 38.4 times, 2.7 times and 35.4 times of the standard. Compared with the soil environmental background value in Yunnan Province, the average content of Pb, Cd, Cu and Zn in the soil was 157.1 times, 262.1 times, 2.9 times and 78.9 times of the standard (Table 1). It can be seen that all four types of heavy metals pollute the soil in the mining area, of which Cd pollution is the most serious.

Characteristics of heavy metal content in plants in mining area
The contents of Pb, Cd, Cu and Zn in the five planted objects in the mining area were quite different (   (Table 3). It can be seen that the TF of four heavy metals in A. alpina are all>1, and it had a strong transport capacity.

Discussion
In order to adapt to the severe living environment in places with severe heavy metal pollution, plants have evolved certain defense mechanisms through long-term natural selection [8]. Studies have shown that dominant plants growing in mining areas have a certain tolerance to heavy metals, but each plant has different adaptability and resistance to different heavy metals. Wan et al. investigated the plants in four mines in Hunan and found that Viola principis had a strong ability to accumulate Cd, Pb and As [9]. Li Siliang et al. studied the heavy metal accumulation characteristics of dominant plants naturally grown in four lead-zinc mines in Zhejiang Province and found that Elsholtzia argyi and Sedum plumbizincicola had the ability to accumulate Cd [10].
In this study, Pb, Cd, Cu and Zn in the surface soil collected from the Huize lead-zinc mining area all exceeded the standard. Among them, Cd was the most polluted, which caused heavy metal pollution in the surrounding farmland soil and was not conducive to crop growth. The content of heavy metals in plants was related to the content of heavy metals in the soil where the plants grow, and the ability of plants to transport and absorb heavy metals [6]. A. alpina had a strong ability to accumulate Cd, its BCF and TF were 1.91 and 1.32, and had good transport capacity for Pb, Cu and Zn; the TF of A. margaritacea, C. inflatus and A. orbiculata to Cu and Zn>1, it can be used as Cu and Zn tolerant plants; the TF of J. effusus to Cd and Zn> 1, it can be used as a tolerant plant for Cd and Zn. Therefore, studying the plants that grow naturally in mining areas and screening out plant varieties with heavy metal accumulation and tolerance are of great significance for soil restoration and ecological restoration in mining areas.

Conclusion
The most serious soil heavy metal pollution in Huize lead-zinc mining area was Cd, followed by Pb, Zn and Cu. Among the five plants, A. alpina had a strong ability to accumulate Cd and a good transport ability to Pb, Cu and Zn. A. margaritacea, C. inflatus and A. orbiculata had good transport ability to Cu and Zn. J. effusus had good transport ability to Cd and Zn.