Phytoremediation Potential of Four Native Plants in Soils Contaminated with Lead in a Mining Area

Metal pollution in soils is an issue of global concern, and lead (Pb) pollution is considered to be the most serious type. The arid and semi-arid areas of Northwest China are rich in Pb ore resources. In this study, four native crops [wormwood (Artemisia capillaris), dandelion (Taraxacum mongolicum), alfalfa (Medicago sativa), and plantain (Plantago asiatica L.)] that grow naturally around tailings slag in a mining area in Northwest China were selected to screen their ecological restoration impacts on Pb-contaminated soil. In pot experiments, four different metal lead pollution gradients (0, 2, 3, and 5‰ w/w) were set, and crop growth indexes such as plant height, root length, and biomass, together with the changes of soil Pb content in different tissues and organs before and after planting were analyzed. The results showed the difference in the amount of accumulated Pb in relation to the level of Pb content in the soil. It was found that wormwood and plantain have great potential as remediation plants for soil metal lead pollution in the arid and semi-arid lead bearing mining areas of Northwest China.


Introduction
Soil metal pollution is a global environmental problem [1]. Lead (Pb) pollution is considered to be the most serious form of metal pollution and the main cause of environmental pollution [2]. Pb is a toxic metal that does great harm to the human body. It is also the second-most harmful metal of all elements, after arsenic [3]. A National Communique on Soil Pollution Survey Bulletin issued in April 2014 found serious Pb pollution in the soil around non-ferrous metal mining areas. Areas exceeding the standard rate of pollution points reached 1.5% [4]. Shaanxi Province is located in the inland hinterland of Northwest China. It is an arid and semi-arid area. The Qinling Mountain area is rich in mineral resources, of which Pb mineral reserves rank fourth of 12 western provinces. Industrial, mining and smelting activities cause Pb pollution to spread rapidly, and soil Pb pollution in the areas surrounding industries and mines is becoming increasingly prominent.
There are many types of remediation techniques. Immobilization techniques, soil washing, and phytoremediation are all commonly used methods of contaminated soil treatment [5]. Phytoremediation technology is widely used for the remediation and treatment of metal pollution in industrial and mining areas [3]. Compared with other remediation methods, phytoremediation technology has significant advantages such as stabilizing surface soil structure, improving soil nutritional conditions and reducing the content of metal pollutants [6]. Pollution remediation and ecological restoration can be carried out in a mining area at the same time. Phytoremediation of metal lead pollution was carried out early in United States and Germany, mainly focusing on screening for Pb super enriched plants. There are also several research studies focusing on the cumulative effects of plants on metal Pb, as well as the interaction between plant growth metabolism and pollution stress. More than 500 types of Pb super enriched plants are found worldwide, mainly Thlaspi arvense, Pueraria, and Psychotria. Passiflora and arbuscular bacteria have been used in pot and field experiments on Pb and other polluted soil bioremediation, with good results [7]. Different plant tissues and organs have different cumulative effects on Pb. Thlaspi rotundifolium (L.) can absorb up to 8500 mg/kg of Pb in stem dry weight [8]. The Pb content of Brassica juncea (L.) stems can reach 1.5% in a nutrient solution culture [9]. The accumulation of Pb in marigold and bidens leaves was found to be high after evaluating the Pb content in different tissues of various plants around a lead-zinc smelter. Sorghum halepense and Panax japonicus can store Pb in roots without affecting growth, showing their potential to stabilize heavy metals [10,11].
In addition, there are complex interactions between plant growth and metabolism, and lead pollution. After studying the absorption and transfer of Pb by plants, it was found that they can effectively avoid the toxic effects caused by Pb accumulation and improve their tolerance by relying on mechanisms such as isolated efflux, induced chelation, and enzymatic reaction [12]. Plants can tolerate Pb by regulating photosynthetic pigment, malondialdehyde content and antioxidant enzyme level [11]. After studying the Pb tolerance and repair potential of Moso bamboo seedlings, it was found that plant growth was inhibited to a certain extent, but more inclusions formed in the root system, and tolerance and biomass were not significantly affected [13].
Wormwood, alfalfa, dandelion, and plantain are common species in arid and semiarid mining areas in Northwest China. They are resistant to drought, cold and barren conditions, and can be used as medicines. Compared with introduced species, they have higher survival rates and biomass. This study focused on the following goals: (1) to explore the effects of Pb-contaminated soil on the growth characteristics of different plants, (2) to clarify the redistribution relationship of Pb between soil and plants, and (3) to identify Pb enrichment in different plant tissues and organs. Plants with good repair potential were selected after final comparison.

Study Area Description
Testing was conducted from 2019 to 2020 in the greenhouse of the Key Laboratory of Degraded and Unused Land Consolidation Engineering, the Ministry of Natural Resources. Fuping County. Where the tests took place, there exists a warm temperate semi-arid climate area with a significant continental monsoon climate. The annual average temperature is 13.3 • C, the annual average ground temperature is 15.6 • C, the annual average sunshine duration is 2352.3 h, and annual average precipitation is 513.5 mm. Total solar radiation is 550 × 10 7 J/m 2 , the annual average wind speed is 2.4 m/s, and the frost-free period throughout the year is 223 day.

Test Soil
The test soil was collected from 0-20 cm topsoil in the Tongguan mining area, Shaanxi Province (110 • 21 40 E, 34 • 30 16 N). The sundries in soil were first removed, soil samples were air dried, and run through a 5.0 mm nylon sieve before pot tests. The basic physical and chemical characteristics of the test soil are shown in Table 1.

Experimental Design
According to the soil environmental quality standard for soil pollution risk control of agricultural lands (GB 15618-2018), the control value of soil Pb pollution risk in agricultural lands (pH > 7.5) is 1000 mg/kg. Mild to moderate pollution is 2~3 times the standard value, and 5 times or more indicates severe pollution. Contaminated soil was prepared by spraying it with a Pb(NO 3 ) 2 solution. The test treatments were set as CK, T1, T2, and T3. The corresponding target Pb contents for polluted soil were 0‰, 2‰, 3‰, and 5‰ (w/w), respectively. After preparation was completed and maintained for 1 month, contaminated soil Pb contents were measured as actual content (w/w). Base fertilizers such as urea, potassium dihydrogen phosphate, and potassium sulfate powder were applied to the stabilized gradient polluted soil. The nitrogen, phosphorus, and potassium contents in the test soil were 225 mg/kg, 65 mg/kg, and 227 mg/kg, respectively. Two and a half kg of contaminated soil were weighed into each pot. Artemisia capillaries, dandelion, alfalfa, and plantain seeds, which were full, uniform and moth free, were selected and represented by letters A, T, M, and P respectively. After the seeds were germinated at room temperature, they were directly sown in each pot with medium spacing. After emergence, the number of seedlings in each pot was kept the same. Using a weighing method, the water content in the basin was kept to about 65% of the field capacity. Three repetitions were set for each treatment, and the plants were harvested after 90 days. Test design and measured Pb content in polluted soil are shown in Table 2. (1) Treatment of soil samples. The soil samples were air-dried, crushed, and ground with wooden rods, sieved (<0.149 mm), and stored until measurement [14] of Pb concentrations following the method outlined in GB/T 17141-1997. (2) Determination of soil samples. Soil samples were weighed, and weights were recorded (with an accuracy of one ten thousandth). Microwave digestion was used for pretreatment according to the USEPA method [15]. After mineralization, soil extracts were filtered (0.45 µm PTFE), diluted, and analyzed. Total Pb concentrations in the soil extracts were determined using an AAS Zeenit 700P atomic absorption spectrometer. The accuracy of the analytical procedure adopted for atomic absorption spectrometer analysis was checked by running standard solutions every 20 samples.

Treatment and Determination of Plant Samples
(1) Treatment of plant samples. After harvesting, plant height, root length, and biomass were measured under different treatment conditions. Forty-eight samples of selected plants were further assigned to two sections (aboveground and underground organs) and thoroughly washed with tap water and deionized water. After weighing and recording the fresh weights, plant samples were placed in an oven at 105°Cfor approximately 15 min. Then, the temperature was adjusted to 75°C until a constant plant weight was reached, which was taken as the dry weight (g) [4]. (2) Determining the Pb concentrations of plant samples. Samples were milled using an agate mortar and sieved through a 0.25 mm nylon sieve. Weighed samples of approximately 0.1000 g were placed in a polytetrafluoroethylene digestion tank along with 65-68% nitric acid and 30% hydrogen peroxide for digestion. Plant samples were then acid-digested according to the USEPA method [14]. An AAS Zeenit 700P atomic absorption spectrometer was used to determine Pb concentrations in the aboveground and underground organs. Only analytical grade reagents were used for testing, with strict quality control [16].

Method for Evaluation of Coefficient
The plant translocation coefficient (TF, %) [17] was calculated following Equation (1) [18]: The soil Pb removal rate (η, %) was calculated following Equation (2): Bioaccumulation factor (BCF, %) is an index used to measure metal enrichment capacity [19]. BCF was calculated following Equation (3) [20]: In the formula, C S and C r are the concentration (mg kg −1 ) of Pb in the stems and root, respectively. C so is the initial concentration (mg kg −1 ) of Pb in soil, C so is the Pb concentration (mg kg −1 ) in soil after restoration. C pt is the Pb concentration (mg kg −1 ) in harvested tissues.

Data Processing
ANOVA testing and LSD multiple comparisons were performed at the 95% significance level using SPSS 22 (IBM SPSS Statistics, Version 22). One-way ANOVA was used to reveal the effects of different concentrations of metal lead stress on the growth and lead absorption and accumulation of four native plants. Least Significance Difference (LSD) was used to test the significance of each index across different treatment groups (p < 0.05). All figures in this paper were created using ORIGIN 8.5 (OriginLab Corp., Northampton, MA, USA).

Effects of Soil Pb Pollution Level on Plant Growth
Normal plant growth is a prerequisite for restoration [21]. Plant species have different tolerance thresholds of soil Pb. If the soil Pb content is higher than the threshold, plants will have different degrees of toxic reactions [22]. In this study, pot experiments were used to clarify the tolerances of four native plants to soil Pb pollution. Under different Pb pollution levels, the growth status of the four plants was monitored, characterized by plant height, root length, biomass, and other indicators.

Plant Height
The effects of stress under different Pb pollution levels on plant height are shown in Figure 1. It can be seen from Figure 1 that different species responded differently to Pb pollution stress, in terms of plant height. Within a certain range, as pollution level improved, some plant heights increased, but when pollution level exceeded that range the plant height decreased. Under the stress of different Pb pollution levels, there was no significant difference in plant heights between plantain and the control treatment (p > 0.05). As Pb pollution level increased compared with the control, the average plant height decreased in varying degrees, relative decreases ranging from 1.45% to 32.61%. When soil Pb was at severe pollution level, wormwood and alfalfa plant heights were not significant compared with the control treatment (p > 0.05). When the soil Pb pollution level was mild to moderate, wormwood plant heights increased as pollution level increased (p < 0.05). When the soil Pb level was below the light pollution threshold, alfalfa plant heights increased with soil Pb concentration (p < 0.05). USA).

Effects of Soil Pb Pollution Level on Plant Growth
Normal plant growth is a prerequisite for restoration [21]. Plant ent tolerance thresholds of soil Pb. If the soil Pb content is higher than t will have different degrees of toxic reactions [22]. In this study, pot exp to clarify the tolerances of four native plants to soil Pb pollution. Und lution levels, the growth status of the four plants was monitored, ch height, root length, biomass, and other indicators.

Plant Height
The effects of stress under different Pb pollution levels on plant Figure 1. It can be seen from Figure 1 that different species respond pollution stress, in terms of plant height. Within a certain range, as proved, some plant heights increased, but when pollution level exce plant height decreased. Under the stress of different Pb pollution leve nificant difference in plant heights between plantain and the control As Pb pollution level increased compared with the control, the avera creased in varying degrees, relative decreases ranging from 1.45% to Pb was at severe pollution level, wormwood and alfalfa plant heights compared with the control treatment (p > 0.05). When the soil Pb poll to moderate, wormwood plant heights increased as pollution level When the soil Pb level was below the light pollution threshold, alfa creased with soil Pb concentration (p < 0.05).

Root Length
Root systems are where plants come in direct contact with soil P tem is damaged, it will indirectly affect the growth and developme plants, causing dwarfism [23]. Pb inhibits the growth of plant roots a

Root Length
Root systems are where plants come in direct contact with soil Pb. Once the root system is damaged, it will indirectly affect the growth and development of aboveground plants, causing dwarfism [23]. Pb inhibits the growth of plant roots and the development of lateral roots to a certain extent [24,25]. At the same time, root biomass, root surface area, and root volume decline. Some research has also calculated the ratio of plant root length to control root length under different treatment conditions to reflect plant tolerance to heavy metals.
The effects of different Pb pollution levels on plant root length are shown in Figure 2. It can be seen from Figure 2 that plant types responded differently to Pb pollution stress in terms of root length. Most plant root growth was inhibited by soil Pb pollution. In general, alfalfa root lengths were 36% to 113% longer than the control treatment lengths at all pollution levels, whereas other plants had root lengths 1.78% to 37.5% shorter than the control treatment lengths at all pollution levels. Plantain root lengths under different pollution levels were significantly longer than the control (p < 0.05). When the soil target Pb content was greater than 2‰, root length remained relatively stable and did not change significantly as pollution level increased. When soil Pb was at the light pollution level, wormwood, dandelion, and alfalfa root lengths were significantly longer than the control (p < 0.05). In this range, wormwood and dandelion root lengths gradually decreased as pollution level increased, whereas alfalfa root lengths increased. When soil Pb pollution was mild to moderate, wormwood and dandelion root lengths increased with pollution level (p < 0.05). The increase of pollution level to a certain extent promoted root length increases in wormwood and dandelion. In general, alfalfa root lengths were 36% to 113% longer than the contr at all pollution levels, whereas other plants had root lengths 1.78% to the control treatment lengths at all pollution levels. Plantain root len pollution levels were significantly longer than the control (p < 0.05). Pb content was greater than 2‰, root length remained relatively stable significantly as pollution level increased. When soil Pb was at the l wormwood, dandelion, and alfalfa root lengths were significantly lon (p < 0.05). In this range, wormwood and dandelion root lengths gra pollution level increased, whereas alfalfa root lengths increased. Wh was mild to moderate, wormwood and dandelion root lengths incre level (p < 0.05). The increase of pollution level to a certain extent pr increases in wormwood and dandelion.

Total Biomass
High concentrations of Pb inhibit plant growth and even directly [26,27]. Therefore, biomass is an important indicator of Pb toxicity to fects of stress at different Pb pollution levels on biomass are shown in and plantain biomass under different pollution levels were significa control (p < 0.05). Alfalfa biomass under different pollution levels was pared with the control (p > 0.05). Wormwood biomass was significan control only at the moderate to severe pollution level (p < 0.05).
Metal Pb pollution has different effects on plants, which also lea growth indexes with the pollution level [28]. Plantain biomass increa

Total Biomass
High concentrations of Pb inhibit plant growth and even directly lead to plant death [26,27]. Therefore, biomass is an important indicator of Pb toxicity to plants [11]. The effects of stress at different Pb pollution levels on biomass are shown in Figure 3. Dandelion and plantain biomass under different pollution levels were significantly higher than the control (p < 0.05). Alfalfa biomass under different pollution levels was not significant compared with the control (p > 0.05). Wormwood biomass was significant compared with the control only at the moderate to severe pollution level (p < 0.05). and 2021, 10, x FOR PEER REVIEW

Pb Distributions and Concentrations in Various Plant Organs
The accumulation of Pb in plant tissues and organs under different le pollution is shown in Figure 4.  Metal Pb pollution has different effects on plants, which also leads to differences in growth indexes with the pollution level [28]. Plantain biomass increased significantly as pollution level increased (p < 0.05). When the pollution level was further increased to severe pollution, biomass decreased significantly. When soil Pb pollution was mild to moderate, wormwood and alfalfa biomass decreased significantly (p < 0.05), and dandelion increased as the pollution level increased (p < 0.05). This was consistent with some studies that showed that plant biomass did not decrease significantly under low concentration lead stress [29].

Pb Distributions and Concentrations in Various Plant Organs
The accumulation of Pb in plant tissues and organs under different levels of soil Pb pollution is shown in Figure 4.

Pb Distributions and Concentrations in Various Plant Organs
The accumulation of Pb in plant tissues and organs under different levels of soil Pb pollution is shown in Figure 4.   Under different treatment conditions, Pb contents in plant tissues and organs reached significant levels compared with the control treatment (p < 0.05). This showed that soil Pb pollution had different effects on the Pb content of four native plants in the northwest mining area. Different plants had different absorption and accumulation characteristics of Pb in polluted soil and increases of Pb content across plant tissues and organs were significantly different as soil Pb content increased. When the target Pb content of polluted soil was less than 5‰, the Pb content of wormwood increased as the soil Pb content increased (p < 0.05). The Pb contents in stems and roots were the highest in T3 (up to 720 mg/kg and 1768 mg/kg, respectively), which were 48.3 and 491 times higher than the control treatment stems and roots. When the target Pb content of polluted soil was less than 3‰, the alfalfa Pb increased with increase in soil Pb content (p < 0.05). The Pb contents in stems and roots of T2 plants were the highest (up to 330 mg/kg and 876 mg/kg, respectively), which were 39.3 and 78.9 times higher than the control treatment stems and roots. The maximum accumulation of Pb in the aboveground and underground parts of some plants occurred in the treatment with different target Pb content in polluted soil. For example, when the target Pb content of polluted soil was less than 5‰, Pb contents of dandelion roots and plantain stems increased with soil Pb content (p < 0.05). The maximum accumulation of Pb was 2284 mg/kg and 383 mg/kg, which were 439.2 and 47.9 times higher than the control treatment. When the target Pb content of polluted soil was less than 3‰, the Pb contents of dandelion stems and plantain roots increased with soil Pb content (p < 0.05). The maximum accumulations of Pb were 596 mg/kg and 3617 mg/kg, or 161.1 and 267.9 times higher than the control treatment. Comparing the Pb content in different tissues and organs of four plants overall, the root contents were generally higher than in the stems. This is consistent with the conclusions of most reports. After Pb enters the soil, it mainly exists in the divalent form of sulfide and oxide. Insoluble sediment formed is usually difficult to move and is not conducive to plant absorption [30] but can still be enriched. This is primarily because Pb mainly exists in the form of precipitation such as lead phosphate and lead carbonate in the root system, and it is difficult to transport Pb absorbed by the root system to the aboveground plant parts [31]. The contaminated soil in this experiment was neutral weak alkaline soil, which further increased the difficulty of lead ion activation.
Translocation coefficients of the four plant species to Pb were calculated according to Equation (2), and the results are shown in Table 3. The absorption, distribution, and accumulation characteristics of Pb in plant stems and roots were also different. The TF reflects metal Pb transport and distribution in plants [32]. TF is also one of the parameters to assess the plant translocation capabilities on metal Pb effectively. According to the Pb TF in the soil plant system (Table 3), TF values for all four plant species were less than 1. This indicates a greater accumulation effect of metal Pb at the roots. This phenomenon is not only directly related to the bioavailability of metal Pb, but also affected by various factors, such as soil physicochemical properties, plant species, and environmental conditions. Under the same soil Pb concentration, the distributions of Pb transfer from root to stems were as follows. Wormwood and dandelion TF values were higher than alfalfa and plantain. The four native plants growing in the natural mining area are able to transport Pb. Especially in dandelion, when the soil Pb mass concentration was beneath the moderate pollution level, TF was greater than 0.499, which can transport a considerable amount of Pb to the stems. Harvesting and cutting may be used to remove Pb from the contaminated soil, which can remediate Pb-contaminated soil in the mining area. Under different soil Pb concentrations, the translocation coefficients of wormwood, alfalfa, and plantain were less than the control treatment. The results demonstrated high resistance to Pb transfer from roots to the stems. In particular, the plantain TF decreased to a more stable state when the pollution level increased to the mild level. It shows that the plant can fix Pb in the roots. This is an effective method to prevent and control the transfer of Pb to the aboveground part of plants and reduce its toxic effects.

Remediation Effects of Plants on Soil Pb
The effects of plant growth metal Pb removal from soil are shown in Figure 5, where can be seen that crop types, Pb pollution degree of original soil, and other factors reduced the Pb content in soil. In general, the four plants reduced soil Pb content. Under different soil Pb pollution levels, 90 days after planting the order of soil Pb removal rate was different. When soil Pb was at the mild pollution level but below moderate to severe pollution, Pb removal was in the order of wormwood, plantain, alfalfa, and dandelion. When Pb was at the mild to moderate pollution level, the removal rate was in the order of dandelion, alfalfa, plantain, and wormwood. The average removal rates of soil Pb by different plants were ranked. Removal rates by wormwood and plantain were higher than alfalfa and dandelion. The removal rates were 26.25%, 19.72%, 17.71%, and 13.01%, respectively. With the continuous improvement of soil Pb pollution, the removal rate of soil Pb by plantain decreased from 24.03% to 12.82%. There was no significant relationship between the removal rate of soil Pb by wormwood and pollution degree. However, removal rate reached a maximum of 31.89% when soil Pb was seriously polluted. The maximum removal rates of dandelion and alfalfa on soil Pb appeared when soil Pb was at the mild to moderate pollution level.
Land 2021, 10, x FOR PEER REVIEW 9 of 12 soil Pb pollution levels, 90 days after planting the order of soil Pb removal rate was different. When soil Pb was at the mild pollution level but below moderate to severe pollution, Pb removal was in the order of wormwood, plantain, alfalfa, and dandelion. When Pb was at the mild to moderate pollution level, the removal rate was in the order of dandelion, alfalfa, plantain, and wormwood. The average removal rates of soil Pb by different plants were ranked. Removal rates by wormwood and plantain were higher than alfalfa and dandelion. The removal rates were 26.25%, 19.72%, 17.71%, and 13.01%, respectively. With the continuous improvement of soil Pb pollution, the removal rate of soil Pb by plantain decreased from 24.03% to 12.82%. There was no significant relationship between the removal rate of soil Pb by wormwood and pollution degree. However, removal rate reached a maximum of 31.89% when soil Pb was seriously polluted. The maximum removal rates of dandelion and alfalfa on soil Pb appeared when soil Pb was at the mild to moderate pollution level. The influence of soil Pb pollution level on Pb enrichment coefficient in the soil plant system is shown in Table 4. BCF is an important parameter for characterizing phytoremediation ability and categorizing as hyper-accumulators and accumulator. Similarly, the soil Pb pollution level, soil physicochemical properties, and plant species have an impact on BCF. Plant BCF responses to Pb pollution stress are different, and it was common for BCF in the underground part to be higher than in the aboveground part. There was no significant difference between dandelion aboveground BCF, wormwood underground BCF, and the control treatment when soil Pb was at the light pollution level or below. Under other Pb pollution levels, however, there were significant differences between plant BCF and the control treatment (p < 0.05). BCF values of wormwood, alfalfa, and plantain decreased as Pb pollution increased. The BCF in wormwood aboveground parts reached a maximum under the condition of no pollution. Only BCF in the plaintain underground parts was greater than one under mild to moderate Pb pollution, and the enrichment coefficients of other plant organs were less than one. Under different soil Pb contents, dandelion underground BCF gradually increased, soil Pb pollution increased (p < 0.05), and the maximum value was 0.474 when soil Pb pollution was not higher than severe. In a certain range, plantain root system BCF gradually increased as soil Pb pollu- The influence of soil Pb pollution level on Pb enrichment coefficient in the soil plant system is shown in Table 4. BCF is an important parameter for characterizing phytoremediation ability and categorizing as hyper-accumulators and accumulator. Similarly, the soil Pb pollution level, soil physicochemical properties, and plant species have an impact on BCF. Plant BCF responses to Pb pollution stress are different, and it was common for BCF in the underground part to be higher than in the aboveground part. There was no significant difference between dandelion aboveground BCF, wormwood underground BCF, and the control treatment when soil Pb was at the light pollution level or below. Under other Pb pollution levels, however, there were significant differences between plant BCF and the control treatment (p < 0.05). BCF values of wormwood, alfalfa, and plantain decreased as Pb pollution increased. The BCF in wormwood aboveground parts reached a maximum under the condition of no pollution. Only BCF in the plaintain underground parts was greater than one under mild to moderate Pb pollution, and the enrichment coefficients of other plant organs were less than one. Under different soil Pb contents, dandelion underground BCF gradually increased, soil Pb pollution increased (p < 0.05), and the maximum value was 0.474 when soil Pb pollution was not higher than severe. In a certain range, plantain root system BCF gradually increased as soil Pb pollution increased. When the soil Pb pollution degree was moderate or less, it reached a maximum value of 1.430. With the further increase of soil Pb content, BCF in plantain underground parts decreased. This showed that soil Pb content within a certain range could promote the enrichment of metal Pb in plantain roots. It further proved that root system is the main tissue and organ for plants to directly absorb and transfer nutrient elements and Pb [31]. In order to safely dispose of the contaminated biomass, we need to dig out the roots of the plants when harvesting. Composting, compacting, and ashing of biomass can be employed as volume reduction approaches to reuse [5,33]. The pot experiments achieved results in the condition of alkaline soil conditions. It is important to explore the resistance mechanism of metal Pb in different nature soils, determining certain physiological parameters of the plants. It is crucial for future research to carry out field experiments on contaminated sites and dispose of Pb-containing biomass.

Conclusions
A series of indoor simulation experiments were carried out to explore the tolerance thresholds of four common plants to soil Pb in China's northwest lead bearing area. The redistribution of Pb between soil and plants and Pb enrichment in different plant parts were also studied. Based on the experimental results, suitable crops for Pb-contaminated soils with different gradients were selected. When the soil Pb content was less than 2‰, it was suitable to plant wormwood and plantain, and the soil Pb removal rate was greater than 24%. The accumulations of Pb in plantain and dandelion roots were 1.75‰ and 0.72‰, respectively. When soil Pb content was less than 3‰, it was suitable to plant dandelion, alfalfa, plantain, wormwood, and other crops, and the soil Pb removal rate was greater than 20%. The accumulations of Pb in plantain and wormwood roots were 3.62‰ and 1.40‰, respectively. When soil Pb content was less than 5‰, the removal rates of soil Pb by plantain, dandelion, and wormwood were greater than 30%. The accumulations of Pb in roots ranged from 1.76‰-3.11‰. The accumulation of Pb by plants increased as Pb content in the initial soil increased. Under the same treatment conditions, the accumulation of metal Pb in plantain roots was 3.11‰, and that of stems of wormwood was 0.72‰. The results of TF and BCF demonstrated high resistance to Pb transfer from roots to the stems. Compared with other plants, wormwood and plantain have substantial remediation potential and can be used as remediation plants for soil metal Pb pollution in arid and the semi-arid lead mining areas of Northwest China. Funding: This research was funded by Technology Innovation Center for Land Engineering and Human Settlements, Shaanxi Land Engineering Construction Group Co., Ltd. and Xi'an Jiaotong University (2021WHZ0094).