Land Reclamation Using Typical Coal Gasification Slag in Xinjiang: A Full-Cycle Environmental Risk Study

Abstract

A rising quantity of coal gasification slag (CGS) is produced annually. Land reclamation is a valuable method for efficiently utilizing coal gasification slag on a large scale. The ecological influence of CGS during land reclamation has not been widely investigated. This article covers the entire CGS use cycle for land reclamation, which includes generation, storage, and disposal. The environmental risk of using CGS for land reclamation was assessed by combining four environmental risk assessment methods. The results show no environmental risk for coal gasification coarse slag (CGCS) and coal gasification fine slag (CGFS) at the generation and storage stages. However, a concern remains regarding manganese leaching from CGCS during the storage stage. In the disposal phase, no environmental risk is present when up to 15% of CGCS and CGFS are applied to land reclamation projects. However, the environmental risk of disposing of 100% of CGS in a landfill cannot be disregarded. Conversely, the full-cycle use of CGS for land reclamation carries no environmental risk.

1. Introduction

The utilization of coal resources has been promoted by the modern coal chemical industry. China is currently experiencing a surge in coal gasification technology, which is the forerunner of this industry [1]. As a consequence, CGS production is increasing, owing to the extensive endorsement of coal gasification technology. This coal gasification slag can only be disposed of through landfills, which not only occupies a large amount of land but also results in a waste of land resources, increasing the economic burden. Furthermore, it seriously damages the quality of soil and endangers human health [2,3]. Therefore, the correct disposal of CGS is essential.

Land reclamation is an important method for large-scale resource utilization of coal gasification slag. The composition of CGS includes silicon dioxide, calcium, and magnesium, which are abundant in soil. Thus, CGS can potentially replace topsoil as a solution for the shortage of high-quality topsoil [4]. Additionally, CGS contains amorphous aluminosilicates that are easily dissolved in soil and can be absorbed by plants, making it an effective silicon fertilizer [5,6]. Furthermore, the pore structure of CGS can enhance soil texture, mitigate soil salinization, and improve soil water retention capacity [7]. Nevertheless, raw coal-derived CGS is often enriched with Cr, Mn, Ni, As, and other heavy metals. Therefore, potential environmental hazards associated with land reclamation must not be overlooked [8].

Currently, there are limited studies on the environmental risks associated with CGS. Some studies focus on the risks of heavy metals after CGS has been processed into products or modified [9,10,11,12,13]. In terms of the environmental risks of CGS itself, existing research only evaluates the impact of heavy metal migration by measuring the concentration of leached heavy metals [14,15,16]. For instance, measurements taken in the Ningdong region of China reveal that heavy metals leached from CGS result in concentrations that exceed the regulatory limits for groundwater, including Cr, Mn, Ni, Zn, Ba, and Pb [14]. Furthermore, the leaching concentrations of As, Se, Mo, Sb, and Tl in CGS generated through entrained-flow coal gasification technologies exceeded the upper limits for Level III groundwater [16]. In practical land reclamation scenarios, the use of CGS typically requires multiple stages. During each stage, the heavy metals in the coal gasification slag not only pose direct leaching risks to the environment but also endanger land and related practitioners through soil accumulation [17], dust, direct contact, and other modes. Evaluating the environmental risk of CGS for land reclamation solely based on leachate composition is insufficient. It is crucial to comprehend the reclamation process entirely and to establish a full-cycle environmental risk assessment system for coal gasification slag during land reclamation.

In light of the aforementioned issues, this paper identifies the typical pollutant types in CGS based on raw coal analysis, outlines the approach for assessing the environmental risk of coal gasification slag in each stage of land reclamation, establishes the full-cycle environmental risk assessment system of CGS for land reclamation, analyzes the land pollution risk and human health risk of CGS in each process, and provides the risk protection measures.

2. Materials and Methods

2.1. Materials

The chemical company situated in Qitai County, Changji Hui Autonomous Prefecture, Xinjiang Uygur Autonomous Region, supplied the coal gasification slag that was used in this study. Due to the region’s depleted soil, severe salinization, and minimal vegetation cover, there is an acute requirement for land reclamation. Consequently, this investigation examines the environmental risks associated with the use of coal gasification slag for soil reclamation.

2.2. Sampling and Testing Methods

2.2.1. Sample Collection

The firm’s slag pool, filter slagging site, coarse slag yard, fine slag yard, and the farmland near the firm were chosen as sampling locations.

Both coal gasification coarse slag (CGCS) and coal gasification fine slag (CGFS) are continuously created at the slag pool and the filter slagging site, respectively. Therefore, at the slag pool and the filter slagging site, the systematic sampling approach was used. The minimum number of samples was set to be 100, and the sample size was determined to be 500 g [18]. Every day, 10 samples of CGCS and CGFS are collected at the slag pool and filter slagging site, respectively, with a sampling interval of 1 h. Thus, 100 samples were obtained at each location over the course of 10 days. The ten samples collected at the same time were combined into one specimen, so we have ten specimens from each sampling location.

The size of the slag yard of CGCS and CGFS is large, so the chessboard sampling approach was selected. The slag yard for CGCS and CGFS was divided into 100 grids, with each compartment collecting CGCS and CGFS.

Soil samples were taken from the farmland. From the fields, 0–20 cm of topsoil was collected with a ring knife. The sampling method used was the same as at the slag site, and 10 specimens were collected.

2.2.2. Selection of Typical Pollutants

According to information provided by the firm, the raw and auxiliary materials in the coal gasification process, such as raw coal, fresh water, wastewater from the Methanol to Olefins (MTO) section, clarified water from the grinding pond, and additives, are the sources from which the pollutants in the coal gasification slag originate. Table S1 in the Supplementary Material shows the source, composition, and potential pollutants of each raw and auxiliary material. Actual analyses of the pollutants in raw and auxiliary materials (Tables S2 and S3) indicate that only fluoride, mercury, arsenic, zinc, and manganese are present in significant amounts (see Tables S4 and S5 for details of the analytical methods).

2.2.3. Sample Determination

In total, 500 samples and 50 specimens were collected from the five sampling locations. Each specimen was separated into three sections to assess the leaching concentration and total amount of the five typical pollutants. In other words, the typical pollutant concentration of each specimen was examined three times, for a total of 150 tests. The average of the three test findings was used as a test of analysis quality. Tables S4 and S5 report the test procedures.

2.3. Environmental Risk Evaluation Methodology

2.3.1. Nemerow Composite Pollution Index Method

The single-factor pollution index can reflect the exceeding multiple and pollution degree of a single pollutant and analyze the key pollutants in the region. The calculation formula is [19]:

$$P_i=\frac{C_i}{S_i}$$

(1)

where $P_i$ is the ith pollutant’s single-factor pollution index value; $C_i$ is the ith pollutant’s concentration in the leachate from coal gasification slag; and $S_i$ is the ith pollutant’s primary standard value in “GB 8978-1996” [20], in mg/L.

The Nemerow composite pollution index can comprehensively evaluate the pollution risk of land caused by the leaching of typical pollutants in CGS. The formula is [19]:

$$P_n=\sqrt{\frac{P_{imax}^2+P_{ave}^2}{2}}$$

(2)

where $P_n$ is the value of the Nemerow composite pollution index for coal gasification slag; $P_{max}$ is the maximum single-factor pollution index value for each pollutant; and $P_{ave}$ is the average single-factor pollution index value for each pollutant.

2.3.2. Potential Ecological Risk Index Method

Based on the Nemerow composite pollution index method, the potential ecological risk index method introduces toxicity parameters, which can indirectly reflect the health risks to humans caused by the leaching of typical pollutants in CGS. The formula is [19]:

$$RI=\sum _{i=1}^n{E}_r^i=T_i\times P_i$$

(3)

where $RI$ is the comprehensive potential ecological risk index for coal gasification slag; $E_r^i$ is the ith pollutant’s potential ecological risk; $T_i$ is the ith pollutant’s toxicity parameter; and the values for Zn, As, Mn, and Hg are 1, 10, 1, and 40 [19,21,22], respectively. Fluoride’s ecological risk index cannot be calculated since there is no reference.

2.3.3. Geo-Accumulation Index Method

The geo-accumulation pollution index approach compares the difference in pollutant content in the soil before and after industrialization to discriminate soil pollution caused by human activities. The risk of land pollution brought on by the buildup of heavy metals when CGS is mixed with the soil can be assessed using this method. The formula is [21]:

$$I_{geo}={\log }_2\frac{C_i}{1.5S_i}$$

(4)

where $I_{geo}$ is the ith pollutant’s geo-accumulation index; $C_i$ is the ith pollutant’s concentration; and $S_i$ is the ith pollutant’s geochemical background value, in mg/kg. The geochemical background values for Zn, As, Mn, Hg, and fluoride are 68.8, 11.2, 688, 20, and 478, respectively [23].

Table S6 displays the grade standards for each pollution assessment.

2.3.4. Human Health Risk Assessment

(1)

Exposure calculation and parameter selection

Human health risk was calculated according to the second kind of land use standard in “HJ 25.3-2019” [24]. The calculation formulas are as follows [25]:

$${OISER}_{ca}=\frac{{OSIR}_a\times {ED}_a\times {EF}_a\times {ABS}_O}{{BW}_a\times {AT}_{ca}}\times {10}^{-6}$$

(5)

$${OISER}_{nc}=\frac{{OSIR}_a\times {ED}_a\times {EF}_a\times {ABS}_O}{{BW}_a\times {AT}_{nc}}\times {10}^{-6}$$

(6)

$${DCSER}_{ca}=\frac{{SAE}_a\times {SSAR}_a\times {EF}_a\times {ED}_a\times E_v\times {ABS}_d}{{BW}_a\times {AT}_{ca}}\times {10}^{-6}$$

(7)

$${DCSER}_{nc}=\frac{{SAE}_a\times {SSAR}_a\times {EF}_a\times {ED}_a\times E_v\times {ABS}_d}{{BW}_a\times {AT}_{nc}}\times {10}^{-6}$$

(8)

$${PISER}_{ca}=\frac{{PM}_{10}\times {DAIR}_a\times {ED}_a\times PIAF\times (fspo\times {EFO}_a+fspi\times {EFI}_a)}{{BW}_a\times {AT}_{ca}}\times {10}^{-6}$$

(9)

$${PISER}_{nc}=\frac{{PM}_{10}\times {DAIR}_a\times {ED}_a\times PIAF\times (fspo\times {EFO}_a+fspi\times {EFI}_a)}{{BW}_a\times {AT}_{nc}}\times {10}^{-6}$$

(10)

where ${OISER}_{ca}$ and ${OISER}_{nc}$ are the carcinogenic and non-carcinogenic exposure to oral intake of CGS, respectively; ${DCSER}_{ca}$ and ${DCSER}_{nc}$ are the carcinogenic and non-carcinogenic exposure to skin intake of CGS, respectively; and ${PISER}_{ca}$ and ${PISER}_{nc}$ are the carcinogenic and non-carcinogenic exposure to respiratory intake of coal gasification residue, in mg/kg, respectively.

Other parameters refer to “HJ 25.3-2019” [24] and field measurement data, and the values are shown in Table S7.

(2)

Hazard quotient/carcinogenic risk calculation and parameter selection

For the three exposure pathways of oral, respiratory, and dermal ingestion for the typical pollutants in soil, soil mixed with 15% CGS, and coal gasification slag, hazard quotients (HQs) and carcinogenic risks were estimated. Among the typical pollutants in CGS, only arsenic presents a carcinogenic risk, according to research findings from the International Agency for Research on Cancer (IARC). Therefore, only the total arsenic-carcinogenic risk was taken into account. The calculation formulas are as follows [26]:

$${HQ}_{ois}=\frac{{OISER}_{nc}\times C_{sur}}{{RfD}_o\times SAF}$$

(11)

$${HQ}_{dcs}=\frac{{DCSER}_{nc}\times C_{sur}}{{RfD}_d\times SAF}$$

(12)

$${HQ}_{pis}=\frac{{PISER}_{nc}\times C_{sur}}{{RfD}_i\times SAF}$$

(13)

$${HQ}_n={HQ}_{ois}+{HQ}_{dcs}+{HQ}_{pis}$$

(14)

$${CR}_{ois}={OISER}_{ca}\times C_{sur}\times {SF}_o$$

(15)

$${CR}_{dcs}={DCSER}_{ca}\times C_{sur}\times {SF}_d$$

(16)

$${CR}_{pis}={PISER}_{ca}\times C_{sur}\times {SF}_i$$

(17)

$${CR}_n={CR}_{ois}+{CR}_{dcs}+{CR}_{pis}$$

(18)

where ${HQ}_{ois}$, ${HQ}_{dcs}$, and ${HQ}_{pis}$ represent the hazard quotients of oral intake, skin intake, and respiratory intake of a single heavy metal, respectively; ${HQ}_n$ is the total hazard quotient of a single heavy metal through three exposure pathways; ${CR}_{ois}$, ${CR}_{dcs}$, and ${CR}_{pis}$ represent the carcinogenic risk of oral intake, skin intake, and respiratory intake of a single heavy metal, respectively; and ${CR}_n$ is the total carcinogenic risk of a single heavy metal through three exposure pathways. The above parameters are dimensionless. If the calculated total hazard quotient of a single pollutant exceeds 1 or the total carcinogenic risk exceeds 10⁻⁶, the gasification slag risk is considered unacceptable.

$C_{sur}$ is the concentration of typical pollutants in soil, soil mixed with CGS, or CGS, in mg/kg. ${RfD}_o$, ${RfD}_o$, and ${RfD}_i$ are the reference doses of oral, skin, and respiratory intake of pollutants, respectively. ${SF}_o$, ${SF}_d$, and ${SF}_i$ are the carcinogenic slope factors of oral, skin, and respiratory ingestion pollutants, respectively. $SAF$ is the reference dose distribution coefficient exposed to soil, which is dimensionless. The reference dose and carcinogenic slope factor of different pollutants refer to “HJ 25.3-2019” [24] and other literature [27,28], and the values are shown in Table S8.

2.4. Establishment of an Evaluation System

The use of CGS for land reclamation needs to take into account three processes: generation, storage, and disposal. Because of the short storage time at the generation and storage stages, the environmental risk is mostly generated by the leaching and movement of typical pollutants [29]. The Nemerow composite pollution index method is used to assess water contamination [30]. Because the procedure of finding and evaluating typical pollutants in water bodies is comparable to that of leached pollutants, this method can be used to assess the pollution risk to land caused by pollutant leaching. The potential ecological risk index method considers not only the pollutant concentration but also biological toxicity and includes a toxicity parameter [31,32]. As a result, it can be used to assess the health risk to humans caused by pollutant leaks. The CGCS and CGFS generation areas are the slag pool and the filter slagging site, respectively. Pollutant leaching concentrations at these sites can be used in generation phase estimates. Pollutant leaching concentrations in the slag yard can be used in storage phase estimates. Because of the long storage time at the disposal stages, the accumulation of heavy metals is the primary source of environmental risk during the disposal stage. The geo-accumulation index method is appropriate for assessing heavy metal accumulation in soil [31,33]. As a result, this method can be used to assess the pollution risk to land caused by heavy metal accumulation. Human health risk assessments are commonly utilized to evaluate potential physical hazards to practitioners from long-term exposure to heavy metal pollution [34,35]. As a result, this method can be used to assess the health risk to humans caused by heavy metal accumulation. At this stage, the total amount of pollutant is chosen for calculation.

At the disposal stage, the environmental risks of three different land uses and three different personnel kinds through various CGS disposal methods were covered in this study’s discussion. When CGS is disposed of in a landfill, the landfill soil is entirely composed of CGCS and CGFS. Long-term receptors exposed during the working day include landfill personnel. They are thought to be exposed to pollutants through the following routes: accidental ingestion of gasification slag, skin contact with CGS, and inhalation of volatiles and dust. When coal gasification slag is ecologically disposed of, the reclaimed soil contains 15% CGCS and CGFS. Early tests indicate that soils with a CGS of 15% exhibit superior capacity for retaining water and fertiliser. Long-term receptors exposed during the working day include reclamation personnel. They are thought to be exposed to pollutants through the following routes: accidental ingestion of gasification slag, skin contact with CGS, and inhalation of volatiles and dust. In addition, farmed soil without CGS was used as the control. Long-term receptors exposed during the working day include farming personnel. They are also thought to be exposed to pollutants through the following routes: accidental ingestion of gasification slag, skin contact with CGS, and inhalation of volatiles and dust.

Figure 1 depicts the full-cycle environmental risk evaluation method of coal gasification slag used in land reclamation.

3. Results and Discussion

3.1. The Descriptive Statistics of Typical Pollutants

Low leaching concentration, a wide range in leaching concentration, and a high total amount are characteristics of the pollutants in CGS.

Table 1. Leaching concentrations of typical pollutants (mg/L).

Heavy Metal	Sampling Location	Standard Value	Leaching Concentration	Coefficient of Variation %
Species Total mercury	slag pool	0.05	0.0007 ± 0.0005 c	71.43
	coarse slag yard		0.00060 ± 00003 c	5.00
	filter slagging site		0.0137 ± 0.0114 b	83.21
	fine slag yard		0.0206 ± 0.0008 a	3.88
Total arsenic	slag pool	0.5	0.0062 ± 0.0046 b	74.19
	coarse slag yard		0.0008 ± 0.0000 b	0.00
	filter slagging site		0.0297 ± 0.0281 a	94.61
	fine slag yard		0.0063 ± 0.0000 b	0.00
Total zinc	slag pool	2	0.0000 ± 0.0000 c	-
	coarse slag yard		0.3165 ± 0.0144 a	4.55
	filter slagging site		0.0064 ± 0.0170 c	265.63
	fine slag yard		0.2943 ± 0.0140 b	4.76
Fluoride	slag pool	10	0.5750 ± 0.3504 b	60.94
	coarse slag yard		0.2840 ± 0.0070 c	2.46
	filter slagging site		0.9030 ± 0.3698 a	40.95
	fine slag yard		0.1100 ± 0.0047 c	4.27
Total manganese	slag pool	2	0.0353 ± 0.0487 c	137.96
	coarse slag yard		2.4490 ± 0.1480 a	6.04
	filter slagging site		0.0380 ± 0.0562 c	147.89
	fine slag yard		0.4945 ± 0.0126 b	2.55

Values with superscript letters a, b and c are significantly different across rows (p ≤ 0.05).

displays the pollutant leaching concentrations. Except for the manganese element of the CGCS in the slag yard, which exceeds the first-level standard value in “GB 8978-1996” [20], the concentration of typical pollutants of CGS at other sampling locations is below the first-level standard value. During the storage stage, CGCS may pose environmental risks as manganese may leach into the surroundings. At the slag pool and filter slagging site, the coefficients of variation for each pollutant are notably higher than those at the slag yard. This discrepancy may be due to the considerable variability in the quality of the CGS produced at different times. In the slag yard, CGS generated at each sampling time was mixed well. Except for mercury and arsenic in CGCS, the leaching concentrations of other pollutants vary significantly between sampling locations, suggesting that it is justifiable to distinguish the generation and storage phases of CGS based on sampling locations.

Table 2. Content of typical pollutants (mg/kg).

Category	Total Mercury	Growth Rate %	Total Arsenic	Growth Rate %	Total Zinc	Growth Rate %	Fluoride	Growth Rate %	Total Manganese	Growth Rate %
Soil	1.08	-	7.32	-	42.34	-	35.74	-	578.25	-
15% CGCS	2.01	86	8.22	12	49.48	17	37.42	5	808.66	40
15% CGFS	2.02	87	8.04	10	59.66	41	41.12	15	676.16	17

presents the total amount of typical pollutants before and after the soil was mixed with CGS. The application of CGCS resulted in an 86% rise in total mercury, a 14% increase in total arsenic, a 17% increase in total zinc, a 5% increase in total fluoride, and a 40% increase in total manganese within the soil. More emphasis should be placed on mercury and manganese pollutants. Total concentrations of mercury, arsenic, zinc, fluoride, and manganese in soil underwent an increase of 87%, 17%, 41%, 15%, and 17%, respectively, following the application of CGFS. This data highlights the necessity for more extensive research on mercury pollution.

It is essential to note that, despite the concentration of typical pollutants from leaching being substantially lower than the relevant standard, the direct mixing of CGS in soil will result in a considerable increase in the total amount of typical pollutants. Therefore, it is highly recommended to give more consideration to the environmental risks associated with adding coal gasification slag to the soil.

3.2. Environmental Risk Evaluation of Coal Gasification Slag Generation and Storage

Coal gasification slag presents no risk of land pollution during its generation and storage stages, according to the Nemerow composite pollution index method (Figure 2). All single-factor indices of each pollutant in CGCS and CGFS at both the slag pool and the filter slagging site were below 1. The Nemerow composite pollution index for CGCS and CGFS was 0.043 and 0.069, respectively. These findings illustrate that CGS does not endanger land during the generation stage. A comparison of the risk values of CGCS and CGFS during gasification slag generation indicates that CGFS has a higher risk value. This is due to CGFS having a longer residence time in the gasifier, leading to a longer contact time with heavy metal gases that are volatilized at high temperatures. Therefore, CGCS has a higher concentration of typical pollutants adsorbed [14]. At the slag yard, the single-factor index of manganese in the CGCS was 1.225, and all other pollutants had single-factor indices less than 1. Manganese leaching may pose a pollution risk to the land. However, the composite pollution indices of CGCS and CGFS Nemerow are 0.889 and 0.186, respectively. This shows that while the possibility of manganese leaching needs to be taken into account, CGS as a whole does not pose a risk of land pollution during the storage stage. The values for the hazard of typical pollutants, such as mercury, arsenic, and fluoride, excepting manganese and zinc, decreased during the stockpiling phase when compared to the generation phase. This is due to the rapid leaching of mercury, arsenic, and fluoride, with the majority being leached during the generation phase. Zinc is more susceptible to adsorption on the surface of the gasification slag and leaches in large quantities after a prolonged period of time [36]. CGS is abundant in CaO and alkaline silica-aluminate, which has an alkaline pH at generation [37]. Due to the alkaline environment, manganese leaching is hindered [38], resulting in a longer stockpiling period before the start of leaching.

There is no risk to human health posed by CGS during its generation and storage stages, according to research using the potential ecological risk index method (Figure 3). The potential ecological risk index value of each pollutant in CGCS is highest for total mercury, followed by total arsenic, total manganese, and total zinc, in descending order, at the slag pool and the filter slagging site. The comprehensive potential ecological risk index is 0.707, which is less than 140 and represents a potential low danger that can be ignored. In the context of the CGFS, each pollutant’s potential ecological risk index average ranks in the order of total arsenic, total mercury, total manganese, and total zinc. The comprehensive potential ecological risk index is also minimal at 1.164, which implies that the CGS does not pose a risk to human health during the generation stage. In descending order, the potential ecological risk indices for each pollutant at the slag yard in the CGCS and CGFS are total manganese, total mercury, total zinc, and total arsenic. The CGCS and CGFS have comprehensive potential ecological risk indexes of 1.841 and 1.345, respectively, which are both less than 140. Consequently, it is implied that the gasification slag presents no health risk to humans at the storage stage.

Concluding, although manganese in CGCS may pose a land pollution risk during storage, there is no net environmental risk from coal gasification slag during its generation and storage stages.

3.3. Environmental Risk Evaluation of Coal Gasification Slag Disposal

The study employed the geo-accumulation index method and revealed that CGS at 15% does not pose a pollution risk to the land during disposal (see Figure 4). The geo-accumulation indexes of the different substances in farmland soil were in descending order: total manganese (−0.836), total arsenic (−1.198), total zinc (−1.285), fluoride (−2.654), and total mercury (−4.803). The risk rating for all pollutants was classified as “non-pollution”. The analysis indicates that the collected soil from farmland contains no heavy metals and is of excellent quality. Nevertheless, the geo-accumulation index for pollutants in the reclaimed soil increases in comparison with the geo-accumulation index for pollutants in the agricultural soil, but it is still below 0. This shows that 15% of the CGS increases the possibility of damage to the soil, but it is still below the limit value. Upon disposal, CGS exhibits excellent physical and chemical properties, resulting in improved soil quality and no land pollution risk [39,40]. When CGS is used for landfills, the geo-accumulation index of manganese in coarse slag landfill soil is 0.433, and the geo-accumulation index of zinc in fine slag landfill soil is 0.795, both resulting in a slight pollution risk. The geo-accumulation of all other pollutants was below zero. This implies that, at this stage, the disposal of CGS in landfills can cause soil pollution.

Figure 5 shows the carcinogenic and non-carcinogenic risks posed by typical pollutants in three categories for three exposure routes. The data show that during the disposal phase, 15% of CGS poses no risk to humans. From a job function perspective, the carcinogenic and non-carcinogenic risks to farming personnel are in the acceptable range. Compared to agricultural workers, the carcinogenic and non-carcinogenic risks for reclamation workers are higher but remain within the limits of 10⁻⁶ and 1, respectively. This shows that the soil containing 15% CGS for land reclamation does not have a negative impact on human health during the disposal process. Landfill maintenance personnel have a higher risk of both cancer and non-cancer compared to reclamation and agricultural workers. As the amount of CGS mixed increases, so does the risk to human health from heavy metal accumulation. In addition, the non-carcinogenic risk posed by the accumulation of manganese is greater than 1, posing a non-carcinogenic risk to those who maintain landfills. From an exposure pathway perspective, arsenic’s carcinogenic risk in CGCS and CGFS was rated as ${CR}_{ois}$ > ${CR}_{dcs}$ ≈ ${CR}_{pis}$ under various exposure paths. Depending on the categories of workers, the order of non-carcinogenic risk for each typical pollutant varied by exposure pathway. However, in general, total arsenic, total mercury, total zinc, and fluoride in CGS have the highest non-carcinogenic risk by oral intake, and total manganese has the highest non-carcinogenic risk by respiratory intake, which has the potential to endanger workers’ health. The above results show that oral intake is the main way for the three categories of people to ingest the typical pollutants in CGS, but respiratory intake is the main cause of human health hazards.

In conclusion, during the disposal stage, 15% of the CGS does not pose an environmental risk. In fact, the addition of CGS can improve the physical and chemical properties of the soil and the quality of crops [41]. However, the environmental risk created when landfills are used to dispose of 100% of CGS is not negligible. Environmentally sound disposal of coal gasification slag rather than disposal in landfills is recommended.

3.4. Prevention Measures for the Ecological Risk of Coal Gasification Slag

According to the studies mentioned above, environmental risk is not present during the production stage or the storage stage of coal gasification slag. However, it is important to take into account the manganese leaching risk in CGCS during the storage stage. Studies have demonstrated that an acidic environment makes manganese easier to leach [42]. In order to prevent manganese leaching, lime water can be sprayed over the coarse slag yard to create an alkaline environment.

During the disposal stage, 15% of CGS will not endanger the environment and may even enhance the soil’s physicochemical characteristics. CGS disposal in landfills, however, will pose an environmental risk for the time being. Currently, the majority of CGS disposal techniques involve in-situ landfills [2]. This paper suggests ways to reduce the environmental risk that CGS poses at landfills. The simplest prevention and control measure for the land pollution brought on by the buildup of zinc and manganese is to cover the film before the landfill. In other words, an anti-seepage membrane is installed in the slag yard, and the typical pollutants can be stopped by the membrane and prevented from contaminating the soil. This strategy, however, does not lessen the dissolution of the typical pollutants from the source; rather, it just prevents the polluting of land by restricting the path of the pollution. Additionally, this approach is ineffective in addressing the environmental risk to humans. The stabilization and solidification of CGS (such as cement solidification, melting solidification, carbonation solidification, etc.) can decrease the dissolution of pollutants and solve the environmental risks caused by the accumulation of heavy metals on land and in the human body [43,44]. Adsorbents such as hematite, zeolite, and charcoal can be added to the CGS to fix typical pollutants in an iron–manganese oxide-bound form. Through ion exchange and hydrogen bonding, the adsorbent binds to pollutants in the form of ions, which are then incorporated into oxides in an alkaline environment [45,46,47,48,49]. The incorporation of adsorbents can also lessen the environmental risks to land and humans by lowering the concentration of CGS. In addition, the main exposure route of manganese in CGS to human health is respiratory intake, which can also be solved by wearing protective masks.

4. Conclusions

In this study, the full-cycle environmental risk of using CGS for land reclamation was evaluated from two perspectives: human health risk and land pollution risk. The results show that:

(1)

The Nemerow composite pollution index and comprehensive potential ecological risk index of the CGCS in the slag pool and the CGFS at the filter slagging site are both lower than the standard value of 1, indicating that there is no environmental risk from coal gasification slag at the generation stage.

(2)

At the slag yard, the Nemerow composite pollution index and the comprehensive potential ecological risk index of CGCS and CGFS are also below the standard value. However, the value of the single-factor index of manganese in CGCS exceeds the standard value. This suggests that there is no environmental risk from coal gasification slag during storage. Nevertheless, attention needs to be paid to the leaching risk of manganese in the CGCS.

(3)

When 15% of CGS is used for land reclamation, the geo-accumulation index, carcinogenicity risk, and hazard quotient of each typical pollutant do not exceed the standard. This indicates that the use of CGS for land reclamation poses no environmental risk. When all of the CGS is disposed of in a landfill, the geo-accumulation index of manganese in CGCS and zinc in CGFS surpasses the standard value. Additionally, the hazard quotient of manganese in CGS is greater than the standard value. These findings imply that landfill disposal of CGS will create environmental risks.

(4)

Based on the aforementioned research results, during the storage stage, CGS can be mitigated by spraying alkaline lime water on the coarse slag yard. By installing an anti-seepage membrane in the slag yard, curing and stabilizing the coal gasification slag, and incorporating adsorbents into the CGS, potential environmental hazards caused by the CGS in the landfill can be mitigated.