# Characteristics and Health Risk Assessment of Cd in Soil around a Municipal Solid Waste Incineration Plant in Chengdu

Authors: Chunmei Wei; Zhilian Gong; Jie Luo
Journal: Mathematical Problems in Engineering
 (2022)

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## Abstract

Taking the Chengdu Municipal Solid Waste Incineration (MSWI) Plant in China as object, totally 100 ambient soil samples from 20 sampling points were taken for experiment. This paper analysed the characteristics of Cd in soil. By analyses, the distribution and health risk assessment were taken into consideration. The results demonstrated that the Cd value in soil in the surroundings of the MSWI plant was beyond the acceptable level according to standards (GB15618-2018). The risk assessment in this experiment showed that the carcinogenic and noncarcinogenic risks via inhalation exposure were in the accessible levels.


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## Body

## 1. Introduction

Compared with landfill and composting technology, incineration technology has been paid more and more attention because of its advantages of reduction and resource utilization [1–5]. The number of MSWI plants increased from 220 to 389, the capacity of MSWI increased from 61 million tons to 121 million tons, and the proportion of incineration technology increased from 35% to 50% between 2015 and 2020. At the same time, as the flue gas from waste incineration contains heavy metals, these heavy metals will settle into the soil with the flue gas. The excessive heavy metals in the soil not only damage the ecological environment of the soil but also migrate to the atmosphere, water, and other environmental media and finally endanger the ecology and human health through food chain, breathing, drinking water, and skin contact [6–9].So far, there are lots of reports about the content of heavy metal in the soil around the MSWI plant at home and abroad, but whether the MSWI plant has a significant impact on the content of heavy metal elements in the surrounding soil is still controversial. There are differences in the health risk assessment of heavy metals in the soil around the waste incineration plant [10–13]. Wang Yushan analysed the heavy metal pollution around a MSWI plant in the Pearl River Delta using RI and USEPA [14]. The results showed that Cd content was 0.15 times of background value. Both carcinogenic risk and noncarcinogenic risk were acceptable. Guo Yanhai investigated the pollution status of Cd in the soil around a MSWI plant in Shanghai [15] and evaluated its ecological risk by using RAC. It was found that the Cd risk index was the highest, reaching the medium risk level.This study takes the soil around the MSWI plant as the research objection, Chengdu, China. Cd content and morphology were analysed, respectively, based on the above results, pollution severity of Cd in the soil was evaluated by Nemerow index method, and the health risk assessment was assessed by USEPA. It is hoped that this result can provide reference for environmental authorities.The remaining paper is organized in the following way: Section2 elaborates all the methodologies used in this paper to make results stronger. Section 3 studies the research methods. Section 4 gives the detail about the experimental results and discusses in well-mannered way. Section 5 consists of the conclusion of this paper.


## 2. Materials and Methods

In this area of paper, the technique of collecting sample data for experiment is studied. After this, we deal with samples in different ways to get the desired results.


### 2.1. Study Area and Sample Collection

The land around the MSWI plant in Chengdu is mainly farmland and wasteland, and the terrain is relatively flat. Combined with the local topography, the regional three-dimensional meteorological field was simulated by CALMET model, and then the monitoring points were set accordingly [16]. The distribution of sampling points is shown in Figure 1.Figure 1
The distribution of sampling points.At each sampling point, five equal samples were taken within the range of 5 × 5 m, and 1 kg soil was obtained by quartering method. The sampling depth was 0 ∼ 10 cm. The soil samples were put into polyethylene sealed bags, stored, and transported at −4°C, dried in the laboratory for 48 h, screened, and then stored in clean brown bottles.


### 2.2. Sample Handling

First, impurities were removed from all the samples before screening with 60 mesh nylon. Second, all the samples were digested by microwave digestion in HNO3-HF-HClO4 digestion system. Third, the digested samples will be heated to remove the acid at about 200°C. Fourth, HCl was added to the treated samples and they were heated and there was constant volume after cooling. The procedure for digestion is shown in Table 1.Table 1
Digestion scheme.
Heating temperature (°C)Soaking time (min)Pressure (MPa)120102150202180102Due to the large content of Cu2+ and Pb2+ in the soil, this study used sodium pyrophosphate to eliminate the interference of Cu2+. The interference of Pb2+ was eliminated by adding potassium sulfate and barium chloride solution.


## 2.1. Study Area and Sample Collection

The land around the MSWI plant in Chengdu is mainly farmland and wasteland, and the terrain is relatively flat. Combined with the local topography, the regional three-dimensional meteorological field was simulated by CALMET model, and then the monitoring points were set accordingly [16]. The distribution of sampling points is shown in Figure 1.Figure 1
The distribution of sampling points.At each sampling point, five equal samples were taken within the range of 5 × 5 m, and 1 kg soil was obtained by quartering method. The sampling depth was 0 ∼ 10 cm. The soil samples were put into polyethylene sealed bags, stored, and transported at −4°C, dried in the laboratory for 48 h, screened, and then stored in clean brown bottles.


## 2.2. Sample Handling

First, impurities were removed from all the samples before screening with 60 mesh nylon. Second, all the samples were digested by microwave digestion in HNO3-HF-HClO4 digestion system. Third, the digested samples will be heated to remove the acid at about 200°C. Fourth, HCl was added to the treated samples and they were heated and there was constant volume after cooling. The procedure for digestion is shown in Table 1.Table 1
Digestion scheme.
Heating temperature (°C)Soaking time (min)Pressure (MPa)120102150202180102Due to the large content of Cu2+ and Pb2+ in the soil, this study used sodium pyrophosphate to eliminate the interference of Cu2+. The interference of Pb2+ was eliminated by adding potassium sulfate and barium chloride solution.


## 3. Research Method

### 3.1. Assessment of Heavy Metal Pollution in Soil

Nemerow index method includes single factor pollution index method and comprehensive index method. The calculation formula is as follows:(1)Pi=CiSi,(2)PN=P2¯+Pmax22.Here, Pi is known as the pollution index of pollutant in soil; Ci is the measured concentration of pollutant in soil; Si is the evaluation criteria for pollutant; PN is known as the Nemerow composite pollution index; P¯ is the average value of single pollution index; and Pmax is the maximum value of single pollution index. Soil environmental quality risk control standard for soil contamination of agricultural land was adopted as the reference standard (GB 15618-2018), and the evaluation results were graded according to the indexes recommended in the technical specification < soil environmental monitoring >HJ/t166−2004.


### 3.2. Health Risk Assessment Model

According to USEPA, Cd is carcinogenic. However, carcinogens also have noncarcinogenic risks, so the carcinogenic and noncarcinogenic risks of Cd were evaluated. Cd in soil is mainly exposed through three ways: oral exposure, skin exposure, and respiratory exposure [17–19]. In this study, exposure routes，exposure levels, and the dose-response relationship of heavy metal Cd were comprehensively considered to determine the probability of harmful results, the acceptable level, and the uncertainty of evaluation results [20, 21].(1)
Exposure assessment model:(3)ERi=C×Ri×ED×EF×ABSiBW×AT.(2)
Noncarcinogenic risk assessment model:(4)HQi=∑ERiRfD.(3)
Carcinogenic risk assessment model:(5)CRi=ERi×SFi,CR=∑CRi.Here, CRi is the carcinogenic risk of pollutant i, c is the concentration of pollutant i in top soil, SFi is the carcinogenic risk slope of pollutants i; ERi is the exposure of pollutants i (carcinogenic effect), Ri is the daily intake of pollutant i for adults; ED is the adult exposure period, EF is the adult exposure frequency, BW is the BWa adult weight, ABSi is the absorption efficiency factor, AT is the mean time of cancer response, RfD is the chronic reference dose of pollutant i, and HQ is the noncarcinogenic risk coefficient of pollutant i.


## 3.1. Assessment of Heavy Metal Pollution in Soil

Nemerow index method includes single factor pollution index method and comprehensive index method. The calculation formula is as follows:(1)Pi=CiSi,(2)PN=P2¯+Pmax22.Here, Pi is known as the pollution index of pollutant in soil; Ci is the measured concentration of pollutant in soil; Si is the evaluation criteria for pollutant; PN is known as the Nemerow composite pollution index; P¯ is the average value of single pollution index; and Pmax is the maximum value of single pollution index. Soil environmental quality risk control standard for soil contamination of agricultural land was adopted as the reference standard (GB 15618-2018), and the evaluation results were graded according to the indexes recommended in the technical specification < soil environmental monitoring >HJ/t166−2004.


## 3.2. Health Risk Assessment Model

According to USEPA, Cd is carcinogenic. However, carcinogens also have noncarcinogenic risks, so the carcinogenic and noncarcinogenic risks of Cd were evaluated. Cd in soil is mainly exposed through three ways: oral exposure, skin exposure, and respiratory exposure [17–19]. In this study, exposure routes，exposure levels, and the dose-response relationship of heavy metal Cd were comprehensively considered to determine the probability of harmful results, the acceptable level, and the uncertainty of evaluation results [20, 21].(1)
Exposure assessment model:(3)ERi=C×Ri×ED×EF×ABSiBW×AT.(2)
Noncarcinogenic risk assessment model:(4)HQi=∑ERiRfD.(3)
Carcinogenic risk assessment model:(5)CRi=ERi×SFi,CR=∑CRi.Here, CRi is the carcinogenic risk of pollutant i, c is the concentration of pollutant i in top soil, SFi is the carcinogenic risk slope of pollutants i; ERi is the exposure of pollutants i (carcinogenic effect), Ri is the daily intake of pollutant i for adults; ED is the adult exposure period, EF is the adult exposure frequency, BW is the BWa adult weight, ABSi is the absorption efficiency factor, AT is the mean time of cancer response, RfD is the chronic reference dose of pollutant i, and HQ is the noncarcinogenic risk coefficient of pollutant i.


## 4. Results and Discussion

### 4.1. Total Amount and Pollution Assessment of Cd in Soil

The result is represented in Figure2. The pH of soil sample is 7.7(7.5∼8.1). The content of Cd in the soil around MSWI plant is about 0.61∼2.72 mg/kg, which is higher than the risk screening value (pH > 7.5) in the national standard (GB15618-2018). Therefore, it was necessary to strengthen. The regional soil environmental monitoring and agricultural products collaborate monitoring. Compared with other research reports, the content of Cd was higher than that of the soils around the MSWI plants in Spain, India, Shenzhen, and Guangzhou [22–25].Figure 2
Total cadmium content of soil samples.According to the index method, the study area is heavily polluted, and most of them are at high concentration level. Combined with the location analysis of the sampling point, the point was located in the annual dominant downwind direction and the secondary dominant downwind direction.In principle, it should be forbidden to grow edible agricultural products when the concentration of Cd is near the risk control value. The concentration of Cd in the soil decreased with the increase of the distance from the waste incineration plant, which was consistent with the distribution characteristics of point source pollutants. The results showed that the flue gas sinking was the main way to enrich heavy metals and affect the ecological environment.


### 4.2. Speciation Analysis of Cd

It obtained the sources, existing forms, and bioavailability by analyzing the chemical forms of heavy metals. The speciation of Cd was analysed by Tessier continuous extraction procedure, which includes exchangeable, carbonate bound, iron manganese oxide bound, organic bound, and residual. It can be seen that the speciation of Cd roughly met the relationship of exchangeable state > iron manganese oxide bound state > residual state > carbonate bound state > organic bound state, as shown in Figure3. Exchangeable was bioavailable and easily absorbed by vegetation. Therefore, it was easy to be introduced into the food chain, accumulate in organisms, and eventually affect the ecology and human health.Figure 3
Speciation analysis of Cd.The carcinogenic and noncarcinogenic soil exposure values were calculated using the soil metal exposure model. The results are shown in Tables2 and 3.Table 2
Soil exposure to carcinogenic heavy metal Cd in soil (mg (kg d)−1).
Exposure routeIntake exposureInhalation exposureSkin contactAdult4.17 × 10−74.95 × 10−92.39 × 10−9Table 3
Noncarcinogenic soil exposure to heavy metal Cd in soil (mg (kg d)−1).
Exposure routeIntake exposureInhalation exposureSkin contactAdult1.21 × 10−61.43 × 10−86.88 × 10−9


### 4.3. Health Risk Assessment of Cd

The carcinogenic and noncarcinogenic soil exposure values were calculated using the soil metal exposure model. The results are shown in Tables2 and 3.According to the data in Tables2 and 3, the intake exposure > inhalation exposure > skin contact exposure. Therefore, oral intake is the main exposure route for adults. It can reflect the influence degree and main way of surrounding people. Therefore, the food produced around the incineration plant should be the main way for the waste incineration plant to affect human health, which also meant that the control of crops should be considered in the prevention and control work around the waste incineration plant.According to the calculation, the carcinogenic risk value of inhaling soil particles has not exceeded the national standard. Carcinogenic health effect was considered as no threshold effect. As long as exposed, even if the exposure dose was small, long-term exposure also has carcinogenic risk.


## 4.1. Total Amount and Pollution Assessment of Cd in Soil

The result is represented in Figure2. The pH of soil sample is 7.7(7.5∼8.1). The content of Cd in the soil around MSWI plant is about 0.61∼2.72 mg/kg, which is higher than the risk screening value (pH > 7.5) in the national standard (GB15618-2018). Therefore, it was necessary to strengthen. The regional soil environmental monitoring and agricultural products collaborate monitoring. Compared with other research reports, the content of Cd was higher than that of the soils around the MSWI plants in Spain, India, Shenzhen, and Guangzhou [22–25].Figure 2
Total cadmium content of soil samples.According to the index method, the study area is heavily polluted, and most of them are at high concentration level. Combined with the location analysis of the sampling point, the point was located in the annual dominant downwind direction and the secondary dominant downwind direction.In principle, it should be forbidden to grow edible agricultural products when the concentration of Cd is near the risk control value. The concentration of Cd in the soil decreased with the increase of the distance from the waste incineration plant, which was consistent with the distribution characteristics of point source pollutants. The results showed that the flue gas sinking was the main way to enrich heavy metals and affect the ecological environment.


## 4.2. Speciation Analysis of Cd

It obtained the sources, existing forms, and bioavailability by analyzing the chemical forms of heavy metals. The speciation of Cd was analysed by Tessier continuous extraction procedure, which includes exchangeable, carbonate bound, iron manganese oxide bound, organic bound, and residual. It can be seen that the speciation of Cd roughly met the relationship of exchangeable state > iron manganese oxide bound state > residual state > carbonate bound state > organic bound state, as shown in Figure3. Exchangeable was bioavailable and easily absorbed by vegetation. Therefore, it was easy to be introduced into the food chain, accumulate in organisms, and eventually affect the ecology and human health.Figure 3
Speciation analysis of Cd.The carcinogenic and noncarcinogenic soil exposure values were calculated using the soil metal exposure model. The results are shown in Tables2 and 3.Table 2
Soil exposure to carcinogenic heavy metal Cd in soil (mg (kg d)−1).
Exposure routeIntake exposureInhalation exposureSkin contactAdult4.17 × 10−74.95 × 10−92.39 × 10−9Table 3
Noncarcinogenic soil exposure to heavy metal Cd in soil (mg (kg d)−1).
Exposure routeIntake exposureInhalation exposureSkin contactAdult1.21 × 10−61.43 × 10−86.88 × 10−9


## 4.3. Health Risk Assessment of Cd

The carcinogenic and noncarcinogenic soil exposure values were calculated using the soil metal exposure model. The results are shown in Tables2 and 3.According to the data in Tables2 and 3, the intake exposure > inhalation exposure > skin contact exposure. Therefore, oral intake is the main exposure route for adults. It can reflect the influence degree and main way of surrounding people. Therefore, the food produced around the incineration plant should be the main way for the waste incineration plant to affect human health, which also meant that the control of crops should be considered in the prevention and control work around the waste incineration plant.According to the calculation, the carcinogenic risk value of inhaling soil particles has not exceeded the national standard. Carcinogenic health effect was considered as no threshold effect. As long as exposed, even if the exposure dose was small, long-term exposure also has carcinogenic risk.


## 5. Conclusions

It is found that the Cd pollution in the soil around the MSWI plant was serious, which was mainly related to the Cd enrichment and refractory. Through the speciation analysis of Cd, it can be concluded that the exchangeable form accounts for the largest proportion, so it was easy to accumulate in the biological chain of plants, which would cause serious impact on the ecological environment. Therefore, the prevention and control of Cd pollution in soil should focus on the exchangeable form. According to the calculation of exposure, the main way of adult exposure was oral intake, and the harm of Cd to human health was mainly manifested in food. Therefore, the soil around the waste incineration plant should not be cultivated as farmland, so as to reduce the amount of food, prevent Cd from entering the ecological food chain, and plant more vegetation that can enrich and control the migration of Cd, so as to reduce the soil pollution. The impact of Cd on ecology and human health is minimized.



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*Source: 5265632-2022-04-14.xml*