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Journal of Resources and Ecology >

2024 , Vol. 15 >Issue 4: 909 - 917

DOI: https://doi.org/10.5814/j.issn.1674-764x.2024.04.012

Impact of Human Activities on Ecosystem

Soil Heavy Metal Pollution and Bioavailability in Baoshantao Mining Area, China

  • CAO Yuhong , * ,
  • LU Chenhao
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  • School of Ecology and Environment, Anhui Normal University, Wuhu, Anhui 241002, China
* CAO Yuhong, E-mail:

Received date: 2023-08-27

  Accepted date: 2024-01-05

  Online published: 2024-07-25

Supported by

The National Natural Science Foundation of China(42371185)

The Anhui Normal University College Students Innovation and Entrepreneurship Training Program(2022056511)

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Abstract

In areas with a high geological background of heavy metals, some edible plants could pose a serious threat to human health. In order to find effective methods to remove heavy metals or reduce their harm, this study investigated the enrichment conditions of five soil heavy metals, Cd, Pb, Cu, Zn and Cr, in four edible plants in a mining area, Baoshantao, in eastern China that has a high geological background of metals, and two groups of experiments were designed to investigate the effects of passivators on their enrichment. The results showed that the soil heavy metal content in the study area has a certain degree of spatial variability. The five heavy metal element contaminants in the soil are in the order of Cd>Cu>Zn>Pb>Cr. The enrichment coefficients and the transfer coefficients of different edible plants were different for the different heavy metals. The two groups of passivators showed better passivating effects with an increase in passivating agent dosage. The smaller the enrichment coefficient of water spinach, the lower the bioavailability. The results of this study can provide a scientific basis for the restoration of soil heavy metal pollution and the safe use of land in areas with a high geological background of heavy metals.

Key words： edible plant; enrichment coefficient; transfer coefficient; fertilizer passivator; spatial distribution of heavy metals

Cite this article

CAO Yuhong , LU Chenhao . Soil Heavy Metal Pollution and Bioavailability in Baoshantao Mining Area, China[J]. Journal of Resources and Ecology, 2024 , 15(4) : 909 -917 . DOI: 10.5814/j.issn.1674-764x.2024.04.012

1 Introduction

With the advancement of natural weathering, industrialization, and urbanization, natural and human factors have led to the release of various elements into the environment through the soil-crop system, affecting soil properties, food safety, and public health (Rizzu et al., 2020). Agri-food safety issues caused by the pollution of various elements have received increasing attention from society, among which heavy metal pollution has become a global environmental problem.
Mining areas with a high geological background of heavy metals and mining, smelting, and transportation activities can cause heavy metal soil pollution problems. For example, the risk of heavy metals in reclaimed soils in bauxite mining areas (Cui et al., 2021), heavy metal pollution in farmland soil around the mining area (Chen et al., 2022), heavy metals in soil under different land use patterns in geological high-background and pollution superimposed areas (Wang et al., 2022), and health risks from mining soil (Zhang et al., 2022a) have been examined. The concentrations of heavy metals such as Cd, Pb, Hg, As, Zn, Cu, Ni, and Cr are determined by collecting samples of farmland soil, atmospheric dust, water bodies, suspended solids, and sediments (Song et al., 2021; Liu et al., 2022). The methods for analyzing the sources of heavy metal pollution in site soil mainly include principal component analysis, positive definite matrix factorization, absolute factor score-multiple linear regression, and single and multi-parameter isotope tracers. Combined applications of multiple source analysis and multi-parameter isotope tracers are the development trend of future research on the source analysis of heavy metal pollution in soil (Yu et al., 2021). Some studies have identified the priority sources of heavy metal pollution in typical metallurgical mining areas by combining positive matrix decomposition models with geostatistical analysis (Gu et al., 2022; Zhou et al., 2022). Others have used the Muller’s geoaccumulation index, the potential ecological risk index, and spatial analysis methods to explore the trends of heavy metal attenuation from coal-fired power generation and zinc smelting enterprises from production areas to the surrounding areas (Ta et al., 2021).
To reduce the heavy metal contents in agricultural products, and ensure national economic and social security, it is necessary to restore contaminated soil from the source (Fajardo et al., 2019). Heavy metal soil remediation methods include engineering, passivation, phytoremediation, soil leaching, and others. Passivation remediation refers to adding conditioners to the soil to change the forms of heavy metals that occur in the soil and reduce their bioavailability, thus reducing the contents of heavy metals in soil and agricultural products. Passivation remediation is simple to operate, does not affect agricultural production, and can realize simultaneous remediation and production; therefore, this method is widely used to control heavy metal soil pollution and reduce the risk from agricultural products (Shangguan et al., 2022). Researchers have reviewed the compound pollution of multiple heavy metals and their chemical immobilization in contaminated soil (Cao et al., 2011), while others have investigated the effects of phosphate and lime on the passivation of copper and zinc in single and mixed contaminated red soil and paddy soil (Zhang et al., 2008). Some studies have examined the remediation effect of bamboo biochar on reducing the bioavailability and bioaccumulation of heavy metals in sediment (Zhang et al., 2018), while others have summarized the in situ passivation remediation materials for cadmium-contaminated alkaline agricultural soils (Xie et al., 2018). The mechanisms of passivation repair include physical, chemical, biological, and combined effects. Chemical passivation refers to the application of a certain amount of passivator substances in soil to minimize the bioavailability and mobility of heavy metals through a series of reactions, such as complexation, adsorption, precipitation, ion exchange, and redox reactions. The heavy metal fixation capacity of iron-based nanoparticles for soil and freshwater bioassays has been shown to reduce ecotoxicological effects (Wang et al., 2020). Biochar stabilization of heavy metal-contaminated soil poses challenges (Wang et al., 2020). Although there are many reports on the use of passivating agents to remediate heavy metal-contaminated soil and reduce the contents of heavy metals in agricultural products, few studies have used different passivators on the contents of five heavy metals in water spinach and compared their passivation effects.
By absorbing heavy metals from the soil and transferring them from the roots to the whole body of the plant, the heavy metal content in the plant will increase rapidly. Therefore, soil heavy metals in heavy metal mining areas and their surroundings, such as the high-background area of crop plant growth, vary due to differences in the bioavailability of different heavy metals (Zhang et al., 2010; Sebei et al., 2018), so the plant heavy metal content in the plant body is also different. If the enrichment coefficient of crop plants to a certain soil heavy metal is high, then the content of that metal in the crop plants will be relatively high, and the heavy metals in the plants will also accumulate in the human body through the food chain when people eat the crop plants, causing the accumulation of heavy metals and seriously threatening human health (Wang et al., 2018). Risk assessments of the exposure to potentially toxic elements in rice and bulgur have been conducted (Sofuoglu and Sofuoglu, 2018), and some studies have pointed to health risks from dietary intake of cadmium and lead among kindergarten children in several cities and villages in China (Watanabe et al., 2017).
In view of the above issues, in order to understand heavy metal soil pollution in high-background geological areas, explore methods for alleviating ecological hazards, prevent heavy metals in soil from threatening human health, and contribute to the development of effective measures for soil heavy metal pollution control and residents’ crop food safety and land use, the Baoshantao polymetallic mining area was selected as the sampling location. The bioaccumulation and transfer characteristics of five soil heavy metals were examined in four different edible plants with two different methods for consuming the edible above- and underground parts of the plants. A laboratory pot experiment was carried out to explore the effects of two passivators on the bioavailability of metals in the edible plants and the remediation effect on heavy metal soil pollution in a high geological background area.

2 Materials and methods

2.1 Collection and testing of soil and plant samples

The study area is located in the Baoshantao mining district of Fenghuangshan Village, Tongling City, China. More than 30 metals, such as copper, silver, lead and zinc, have been identified there, which is typical of areas with a high heavy metal geological background.
From March to April 2021, 40 soil samples and four local edible plants were collected from Baoshantao: greengrocery (Brassica chinensis, noted as code Q in the figures and tables), water spinach (Ipomoea aquatica, noted as K), radish (Raphanus sativus L., noted as L), and ginger (Zingiber officinale Roscoe, noted as S). Greengrocery and water spinach are eaten as the aboveground stems and leaves, while radish and ginger are root and tuber vegetables, so their underground parts are consumed by people.
Twelve samples were collected for each plant, and the corresponding soil was excavated at the same time.
Different detection methods were used to determine the characteristics and properties of the soils and the heavy metals they contained. According to the methods recommended by “Soil environmental quality—Risk control standard for soil contamination of agricultural land (GB15618- 2018),” Cr, Cu, and Zn were detected with flame atomic absorption spectrometry, while Pb and Cd were determined with graphite furnace atomic absorption spectrophotometry. Soil pH was measured with the potentiometric method. Soil organic matter, nitrogen, phosphorus, and potassium were measured using the potassium dichromate volumetric method, Kjeldahl method, perchloric acid-sulfuric acid method, and flame atomic spectrophotometry, respectively.

2.2 Soil culture experiment

The planting soil used in the soil cultivation test was taken from the research area. Two treatment groups were set up: calcium magnesium phosphate fertilizer for group C and Si-Ca-Mg-K fertilizer for group S. Each treatment group included three different passivator-added gradient treatments. Thus, the experiment included a total of six treatment groups, C1, C2, and C3 and S1, S2, and S3, and a blank was set up for the K0 group. The compositions and added amounts of the soil passivator in the groups are shown in Table 1.
Table 1 Data for the passivators in the pot experiments
Treatment group Passivator Main ingredient Added amount (g g-1 soil)
K0 - - 0
C1 Calcium magnesium phosphate fertilizer (referred to as Ca-Mg-P fertilizer) P2O5 18%, CaO 45%, SiO2 20%,
MgO 12%, other substances 5%		 0.005
C2 0.015
C3 0.030
S1 Silicon calcium magnesium potassium fertilizer (referred to as Si-Ca- Mg-K fertilizer)
 SiO2 25%, CaO 25%, MgO 12%,
S 10%, Sylvite 10%, Small molecular carbon 2%, Alginic acid 5%, Other substances 11%		 0.005
S2 0.015
S3 0.030
The pot experiment began on the morning of April 20, 2021, and watering occurred every evening. The harvest began on the morning of July 19, 2021, and lasted a total of 90 days. Origin 2018 was used to analyze the results and display them in charts.

2.3 Data processing

The spatial distribution map of heavy metals in the soil was completed on the ArcGIS 10.5 platform using ordinary kriging (OK) interpolation. The box plots were completed on the Origin 2021 platform, and SPSS software was used to analyze the potential ecological risks and other data differences.
Biological effectiveness was expressed by enrichment and transfer coefficients. The enrichment coefficient is the ratio of the equilibrium concentration of pollutants in organisms to the concentration of the pollutants in the living environment. This ratio reflects the ability of plants to enrich and absorb a certain element in the soil. The enrichment coefficient was calculated with the following formula:
EC=Cp/Cs
In the formula, EC represents the enrichment coefficient; Cp represents the heavy metal content in the plant; and Cs represents the heavy metal concentration in the root soil.
EC represents the degree of difficulty or ease of elemental migration in the soil-plant system and is an evaluation index that reflects the ability of plants to absorb and transfer heavy metals in their body. The higher the EC, the greater the concentration of heavy metals in the plant. As there is a certain balance between plant biomass and heavy metal enrichment, the critical value of the enrichment coefficient is generally set to 0.5, which can ensure that the selected plants have a specific biomass.
The transfer coefficient is the ratio of the concentration of heavy metals in the aboveground part of the plant to the concentration in the underground part. The transfer coefficient was calculated with the following formula:
TC=Ca/Cb
In the formula, TC represents the transfer coefficient; Ca represents the concentration of heavy metals in the aboveground parts of the plants; and Cb represents the concentration of heavy metals in the underground parts of the plants, which is represented in grams per kilogram. A transfer coefficient greater than 0.5 indicates that a plant can transfer most of the heavy metals to the aboveground parts, which is conducive to the recovery and utilization of heavy metals.

3 Results and analysis

3.1 Spatial distribution of soil heavy metals in the study area

The spatial distribution of heavy metals is shown in Fig. 1, including a digital elevation mode (DEM) map and five kriging maps for the distribution of each heavy metal. The pH in the soil was between 5.09 and 8.03, the K content was between 239.90 mg kg-1 and 2435.79 mg kg-1, the soil moisture content was between 4.76% and 39.9%, and the soil organic matter was between 4.19g kg-1 and 35.08g kg-1. The concentrations of the soil heavy metals Cd, Pb, Cu, Zn, and Cr ranged from 2.88mg kg-1 to 4.23mg kg-1, 75.58 mg kg-1 to 909.22 mg kg-1, 74.71 mg kg-1 to 805.48 mg kg-1, 115.15 mg kg-1 to 988.56 mg kg-1, and 77.48 mg kg-1 to 104.1 mg kg-1, respectively.
Fig. 1 Sampling points and spatial distribution of heavy metals in the study area
Figure 1 shows that altitude was related to the distributions of the various heavy metals, but the degree of that connection differed. Pb, Cu, and Zn are distributed in large patches, while Cd and Cr are distributed in fragmented platelets. The high value of Cd was located in the center of the study area. Except for the southwestern corner, most of the Pb was in the low-value area. The low-value area of Zn was roughly the same as that of Pb, and these two may have come from the same source. The high values of Cu were distributed in the east. The high values of Cr were distributed in the north and the low values were in the south. The soil heavy metal content in the study area has a certain degree of variability and is influenced to some extent by external mines. The contamination levels of the five heavy metal elements in the soil are in the order of Cd>Cu> Zn>Pb>Cr.

3.2 Biological availability of heavy metals in the soil for the four edible plants

The heavy metal concentrations in the four plants and their roots are shown in Fig. 2. The enrichment coefficients of the four plants and their roots for the five different kinds of soil heavy metals are shown in Fig. 3.
Fig. 2 Concentrations of five heavy metals in the different parts of four plants

Note: The names of the four plants are shown in Section 2.1, where a represents the above-ground part; b represents the underground part.

Fig. 3 Enrichment coefficients for the five heavy metals in the different parts of four plants

Note: The names of the four plants are shown in Section 2.1, where a represents the above-ground part; b represents the underground part.

3.2.1 Accumulation and transfer of Cd

The data in Fig. 2 show that the Cd concentrations were 0.16 mg kg-1 and 0.19 mg kg-1 in the above- and underground parts of greengrocery, respectively; with corresponding amounts of 6.20 mg kg-1 and 6.83 mg kg-1 in water spinach, respectively; 0.29 mg kg-1 and 0.50 mg kg-1 in radish, respectively; and 1.41 mg kg-1 and 1.71 mg kg-1 in ginger, respectively. According to the National Standard for Food Safety, Limit of Pollutants in Food (GB2762-2017, China), greengrocery is a leafy vegetable. Its Cd standard limit is 0.2 mg kg-1, and the Cd concentrations in the above- and underground parts of greengrocery in the study area were within the standard limit. Water spinach is a stem vegetable, and radish and ginger are root and tuber vegetables, respectively. The Cd standard limit of 0.1 mg kg-1 for the above- and underground parts of water spinach, radish, and ginger are clearly exceeded by the samples in this study, thus, consuming them is risky. Figure 3 shows that the enrichment coefficients for the Cd soil concentration were 0.09 and 0.11 for the above- and underground parts of greengrocery, respectively; with corresponding coefficients of 3.60 and 3.96 for water spinach, respectively; 0.17 and 0.29 for radish, respectively; and 0.82 and 0.99 for ginger, respectively. The enrichment coefficients of the Cd soil concentration for water spinach and ginger were greater than 0.5, indicating a high enrichment of the heavy metal Cd in the soils of water spinach and ginger. In Table 2, the transfer coefficients of the Cd concentrations in the four plants are in the order of water spinach, ginger, greengro-cery, and then radish. The transfer coefficients were greater than 0.5, indicating that these four plants can transfer most of the heavy metal Cd above-ground, which is conducive to recycling Cd.
Table 2 Transfer coefficients of the four plants to the five heavy metal elements
Serial number Plant code Transfer coefficient
Cd Pb Cu Zn Cr
1 Q 0.82 0.83 0.71 0.73 0.68
2 K 0.91 0.62 0.73 0.63 0.58
3 L 0.59 0.80 0.79 0.79 0.68
4 S 0.83 0.42 0.30 0.73 0.55
In the four edible plants, the enrichment coefficients of Cd from large to small were in the order of water spinach, ginger, radish, and greengrocery; that is, water spinach had the greatest bioavailability of Cd and greengrocery the least. Therefore, there is a risk of high Cd consumption in water spinach that is grown in high geological background areas.

3.2.2 Accumulation and transfer of Pb

In Fig. 2, the Pb concentrations were 167.34 mg kg-1 and 202.20 mg kg-1 in the above- and underground parts of greengrocery, respectively; with corresponding values of 55.97 mg kg-1 and 88.01 mg kg-1 in water spinach, respectively; 167.34 mg kg-1 and 209.18 mg kg-1 in radish, respectively; and 327.71 mg kg-1 and 446.24 mg kg-1 in ginger, respectively. According to the Pb limit index in food specified in GB2762, the Pb standard limits are 0.3 mg kg-1. for greengrocery and water spinach and 0.2 mg kg-1 for radish and ginger. The above- and underground parts of the four edible plants clearly exceed their standard limits, so their consumption has safety risks. The data in Fig. 3 show that the enrichment coefficients for the Pb soil concentration were 0.24 and 0.29 for the above- and underground parts of greengrocery, respectively; with corresponding coefficients of 0.08 and 0.13 for water spinach, respectively; 0.24 and 0.30 for radish, respectively; and 0.27 and 0.64 for ginger, respectively. The enrichment coefficient of the Pb concentration in the underground part of ginger was greater than 0.5, indicating that the edible part of ginger is enriched in lead. In Table 2, the order of the plants for their Pb transfer coefficients from large to small was greengrocery, radish, water spinach, and then ginger. Moreover, the transfer coefficients of Pb other than ginger were greater than 0.5, indicating that the three other plants can transfer most of the Pb aboveground, which is beneficial for recycling this heavy metal.
In the four edible plants, the enrichment coefficients of Pb range from large to small in the order of ginger, radish, greengrocery, and then water spinach. That is, ginger has the greatest bioavailability of Pb and water spinach the smallest. Therefore, ginger grown in high geological background areas has a risk of high Pb contamination.

3.2.3 Accumulation and transfer of Cu

In Fig. 2, the Cu concentrations were 545.68 mg kg-1 and 768.92 mg kg-1 for the above- and underground parts of greengrocery, respectively; with corresponding values of 672.18 mg kg-1 and 922.98 mg kg-1 for water spinach, respectively; 768.92 mg kg-1 and 967.35 mg kg-1 for radish, respectively; and 471.27 mg kg-1 and 793.72 mg kg-1 for ginger, respectively. The data in Fig. 3 show that the enrichment coefficients of the Cu soil concentration were 0.22 and 0.31 for the above- and underground parts of greengrocery, respectively; with corresponding coefficients of 0.27 and 0.37 for water spinach, respectively; 0.31 and 0.39 for radish, respectively; and 0.19 and 0.63 for ginger, respectively. The enrichment coefficient of the Cu concentration for the underground part of ginger was greater than 0.5, indicating that there was a certain amount of enrichment of the edible underground part of ginger for the soil heavy metal Cu. The transfer coefficients of the four plants for the Cu concentrations from large to small are radish, water spinach, greengrocery, and then ginger, and the transfer coefficients of Cu other than ginger were greater than 0.5. This shows that, except for ginger, the other plants can transport most of Cu to the shoot, which is beneficial for removing Cu from the soil.
In the edible parts of the four plants, the enrichment coefficients of Cu ranged from large to small in the order of ginger, radish, water spinach, and then greengrocery. That is, ginger had the greatest bioavailability of Cu and greengrocery the smallest. Therefore, ginger grown in the high geological background areas has a high risk of Cu contamination.

3.2.4 Accumulation and transfer of Zn

In Fig. 2, the Zn concentrations were 236.42 mg kg-1 and 322.39 mg kg-1 in the above- and underground parts of greengrocery, respectively; with corresponding values of 161.19 mg kg-1 and 258.32 mg kg-1 in water spinach, respectively; 870.44 mg kg-1 and 1106.86 mg kg-1 in radish, respectively; and 494.33 mg kg-1 and 677.01 mg kg-1 in ginger respectively. In Fig. 3, the enrichment coefficients of the Zn soil concentration were 0.22 and 0.30 for the above- and underground parts of greengrocery, respectively; with corresponding coefficients of 0.15 and 0.24 for water spinach, respectively; 0.81 and 1.03 for radish, respectively; and 0.46 and 0.63 for ginger, respectively. The enrichment coefficients of the above- and underground parts of radish and the underground part of ginger were greater than 0.5, while the enrichment coefficient of the underground part of radish was greater than 1. This indicates that the underground part of ginger and both parts of radish have considerable enrichment of the soil heavy metal Zn, of which the edible part of the underground part of the radish has a large enrichment of Zn. In Table 2, the order of the plants for the transfer coefficients to Zn from large to small was radish, greengrocery, ginger, and then water spinach. The transfer coefficients of the four plants for Zn are greater than 0.5, indicating that these plants can transfer most of the heavy metal Zn to the shoot, which is beneficial for recovering and utilizing Zn and removing it from the soil.
In the edible parts of the plants, the enrichment coefficients of Zn in order from large to small are radish, ginger, greengrocery, and then water spinach. That is, radish has the greatest bioavailability of Zn and water spinach the smallest. Therefore, for radish plants grown in high geological background areas, there is a risk of high Zn contamination.

3.2.5 Accumulation and transfer of Cr

In Fig. 2, the Cr concentrations were 18.88 mg kg-1 and 27.60 mg kg-1 in the above- and underground parts of greengrocery, respectively; with corresponding values of 49.38 mg kg-1 and 86.09 mg kg-1 in water spinach, respectively; 36.31 mg kg-1 and 53.74 mg kg-1 in radish, respectively; and 23.24 mg kg-1 and 42.12 mg kg-1 in ginger, respectively. According to GB2762, the Cr limit for the four vegetables is 0.5 mg kg-1, and the above- and underground parts of the four edible plants clearly exceed the standard limit. Therefore, consumption has safety risks in all cases. The data in Fig. 3 show that the enrichment coefficients of Cr were 0.13 and 0.19 for the above- and underground parts of greengrocery, respectively; with corresponding coefficients of 0.34 and 0.59 for water spinach, respectively; 0.25 and 0.37 for radish, respectively; and 0.16 and 0.29 for ginger, respectively. Moreover, the enrichment coefficient of Cr in the underground part of water spinach was greater than 0.5. In Table 2, the transfer coefficients of the four plants for Cr in order from large to small are greengrocery, radish, water spinach, and then ginger. The transfer coefficients of the four plants for Cr are all greater than 0.5, indicating that these four plants can transport most of the heavy metal Cr to the aboveground parts, which is conducive to recycling Cr.
The order of the plants for the enrichment coefficients of Cr from large to small was radish, water spinach, ginger, and green vegetable. That is, radish has the greatest bioavailability of Cr and green vegetable the smallest. Therefore, there is a risk of high Cr consumption in radish plants grown in high geological background areas.

3.3 Effects of passivators on plant accumulation of heavy metals from the soil

The data above showed that in an area with a high soil heavy metal geological background, the bioavailability of Cd was highest for water spinach, while the soil Cd pollution in the study area was the most serious. Therefore, water spinach was selected as the plant in the laboratory soil culture experiment. This experiment lasted 90 days. After 90 days, the pH of the soil ranged from 6.68 to 7.38, the Cd concentrations ranged from 1.58 to 4.24 mg kg-1, the Pb concentrations ranged from 616.545 to 858.55 mg kg-1, the Cu concentrations ranged from 2411.86 to 2899.10 mg kg-1, the Zn concentrations ranged from 1035.98 to 1527.7 mg kg-1, and the Cr concentrations ranged from 136.59 to 166.73 mg kg-1. The average soil values for Pb and Zn in group C were higher than those in group S, while the average values of Cd, Cu, and Cr were higher than those in group S. This indicated that the effects of the Ca-Mg-P fertilizer in passivating cadmium, copper, and chromium were poor, while the Si-Ca-Mg-K fertilizer was less effective at passivating lead and zinc. Compared with the control group (K0), the soil pH of the Ca-Mg-P fertilizer treatment groups C1, C2, and C3 increased by 1.65%, 3.29%, and 10.48%, respectively, and the soil pH of the S1, S2, and S3 groups treated with Si-Ca-Mg-K fertilizer increased by 1.95%, 4.34%, and 6.29%, respectively. Compared with the two inorganic passivators, the passivation effects of the Si-Ca-Mg-K fertilizer on Cd, Cu, and Cr were higher than those of the Ca-Mg-P fertilizer passivator. In contrast, the passivation effects of the Ca-Mg-P fertilizer on Pb and Zn were slightly better than those of the Si-Ca-Mg-K fertilizer passivation agent. Overall, the two kinds of passivation agent had effects. The changes in the heavy metals in water spinach are shown in Fig. 4. The heavy metal enrichment coefficient data for water spinach are shown in Fig. 5.
Fig. 4 Heavy metal contents in water spinach after 90 days of pot cultivation

Note: The group and treatment codes shown in Table 1, where “a” represents the above-ground part and “b” represents the underground part.

Fig. 5 Enrichment coefficients of heavy metals in water spinach after 90 days of pot cultivation

Note: The group and treatment codes are shown in Table 1, where “a” represents the above-ground part and “b” represents the underground part.

This experiment clearly showed that the soil pH increased significantly in both treatment groups, and the increase in soil pH was more obvious with the increase in the amount of passivator. The heavy metal contents in soil after the treatments with two passivators showed that both had effects. In comparison, for Cd, Cu, and Zn in soil, the passivation effect of group S was clearly better than that of group C, while for Pb and Cr in soil, the passivation effect of group C was clearly better than that of group S.
The data in Fig. 4 and Fig. 5 show that the heavy metal contents of water spinach in the control group and the two treatment groups were higher in the underground part than in the aboveground part. Compared with the control group, the enrichment coefficients of Cd in the edible part of water spinach in both treatment groups were significantly reduced, and the decrease was clearer with an increase in the passivator dosage. These results indicated that the application of either of these two kinds of passivators had a great positive effect on the safe planting of water spinach in metal mining areas and surrounding areas with high soil heavy metal geological backgrounds, so they could reduce the harm to human health from soil Cd.
The enrichment coefficients of water spinach for the five heavy metals changed more clearly with the change in the passivator dosage. Compared with group C, the dosage of group S had less of an effect on the bioavailability of heavy metals in water spinach, and the effect of passivation tended to be stable when it reached a certain dose.

4 Discussion

People living in high soil heavy metal geological background areas eat foods with high heavy metal contents for a long time, which directly threatens their health. Effective technologies are urgently needed to remediate soil heavy metal pollution. Based on the results of the experiments in this study, phytoremediation alone and phytoremediation using inorganic passivating agents are both feasible methods.
From the soil remediation perspective, different plants can be planted according to the spatial distribution of the heavy metals. For example, water spinach can be grown in places with a high Cd content, and then the heavy metals will be transferred out through harvesting to reduce the background heavy metal content in the soil. From the perspective of reducing the risk of food toxicity, deactivator fertilizers can be applied to vegetable fields to change the soil pH, passivate heavy metals, lock their activity, and make it difficult for the crops to absorb and transfer them. Therefore, the optimal scheme for soil remediation and reducing local food toxicity can be formulated according to the spatial distribution of the heavy metals.
In addition, the two compound fertilizers used in this study contain Ca, Mg, Si, S, K, P, small molecular carbon, alginic acid, and other components, so they can not only supplement soil nutrients and increase their fertility but also passivate the toxic heavy metal elements through adsorption, exchange, precipitation, complexation, etc. The action mechanism of passivating agents in soil remediation also involves a series of physical and chemical reactions, such as π bonds, ion exchange, redox, and others, which can effectively reduce the activity of ions, thereby reducing the environmental risk of heavy metals in soil. At the same time, edible plant intercropping and compound passivating agents can effectively promote the remediation effect of polluted soil (Zhang et al., 2022b). The remediation of farmland soil containing Cd and As is based on a plant rotation pattern with compound passivator application. Commonly used passivation agents include lime, hydroxyapatite, metal oxides, biochar, organic fertilizer, sepiolite, zeolite, bentonite, and newly developed passivation materials, which can be used in combination to improve passivation efficiency (Li et al., 2020; Li et al., 2022), and soil can be repaired by using inorganic passivation to intervene in the regulatory mechanism of microbial sensitivity (Guo et al., 2006; Su et al., 2022).

5 Conclusions

On the whole, the effects of the accumulation and transfer of five soil heavy metals in typical mining areas and their surrounding areas with a high geological background of soil heavy metals were examined in four edible plants. This study not only provides data support for the phytoremediation of soil heavy metal pollution but also provides references for the safety of residents and the safe use of land in areas with a high background of soil heavy metals in terms of food safety and human health. At the same time, soil Cd pollution was the most serious in the comprehensive study area, and the bioavailability of Cd to water spinach was the strongest among the four edible plants. A pot experiment using water spinach as the experimental plant, and two commonly used and easily accessible passivators preliminarily explored a method for reducing soil heavy metal pollution while considering edible quality. The optimal scheme for soil remediation and reduction of local food toxicity can be formulated according to the spatial distribution of the heavy metals. The conclusions of this research can be summarized in five main points.
(1) The soil heavy metal content in the study area is somewhat variable and is influenced to some extent by external mines. The degree of contamination by the five heavy metal elements in the soil is highest for Cd, followed by Cu, then Zn and Pb, and the lowest is Cr.
(2) All the heavy metal contents in the above- and underground parts of the four edible plants in the study area exceeded the standard limits for heavy metal contents stipulated in the National Food Safety Standard Limits of Contaminants in Food (GB2762-2017). Therefore, the health of the residents in the study area is threatened.
(3) Among these four plants, the enrichment coefficients for all five heavy metals were close to or less than 1, except for water spinach, which had Cd enrichment coefficients of 3.60 and 3.96 for the above- and underground parts, respectively. Therefore, the Cd in the soil of this mining area poses the greatest threat to human health.
(4) The bioavailability in water spinach after passivation was significantly lower in both groups, and the bioavailability in the edible part of water spinach was significantly lower than that of the inedible part of water spinach. At the same time, passivation more effectively reduced the Cd transfer coefficient between the above- and underground parts of water spinach, so it is better for reducing the toxicity of edible water spinach.
(5) According to the spatial distributions of the heavy metals, combined with the plant enrichment coefficients and the characteristics of the fertilizer passivators, an optimized plan for soil remediation and the reduction of local food toxicity can be formulated. By collecting enriched plants and using various passivation methods according to local conditions, this approach can reduce heavy metal contamination in mine soils.

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