Multifaceted evaluation of distribution, occurrence, and leaching features of typical heavy metals in different-sized coal gasification fine slag from Ningdong region, China: A case study

https://doi.org/10.1016/j.scitotenv.2022.154726Get rights and content

Highlights

  • The distribution of typical heavy metals has evident particle size dependence.

  • Typical heavy metals in different-sized CGFS have various environmental risk levels.

  • The leaching behavior under different conditions is comprehensively evaluated.

  • Leaching mechanism is related to the structural properties of different-sized CGFS.

Abstract

The coal gasification fine slag (CGFS) from the entrained-flow coal gasification unit faces the challenge of safe disposal and clean utilization in the Ningdong region, China. This study aims to provide complete and thorough understanding of the distribution features, chemical speciation, environmental impact, and leaching behavior of typical heavy metals (i.e., V, Cr, Mn, Ni, Cu, Zn, Ba, and Pb) in the CGFS with different size fractions. The results show that the distribution of selected heavy metals in the CGFS has evident particle size dependence. Except for Zn, the other heavy metals in different size fractions mainly exist in chemical speciation of residual form with the ratio of 50.11–86.69 wt%. Moreover, it is found that the heavy metals in the different-sized CGFS show different RAC (risk assessment code) environmental risk levels and TCLP (Toxicity Characteristic Leaching Procedure) leaching concentrations. Especially, Zn in SGFS-C and SGFS-D posed a high-risk level to the environment, while the heavy metal elements of Cr, Mn, Ni, Zn, and Ba in other size fractions are classified as a medium environmental risk. In addition, the TCLP test results indicate that the leaching concentration of Cr, Mn, Ni, Zn, Ba, and Pb exceeds the groundwater-related regulatory limit in China. The pH-dependent leaching experiments suggest that Pb shows the amphoteric behavior, while the leaching mode of other heavy metals seems to be the cationic pattern. Furthermore, the leachability of the selected heavy metals in small-size fractions of the CGFS should be given more consideration at both acid and alkaline pH ranges. The leaching kinetic results demonstrate that the most effective mechanism to describe the leaching process of Cr, Ni, Zn, and Pb in different CGFS size fractions is the diffusion-controlled theory, which is supported by the different morphological traits of spherical mineral particles and carbon particles in the CGFS.

Introduction

As the largest coal producer and consumer in the world, coal holds a dominant position in the energy structure of China (He et al., 2020; Wang and Song, 2021). It is an irresistible trend for coal conversion and utilization technology to develop in the direction of high efficiency, lower energy consumption, and less environmental pollution (Wang et al., 2019a). Coal gasification is the core technology of coal clean and efficient utilization which provides a basis for the production of clean gas and liquid fuels as well as chemical synthesis (Minchener, 2005; Xu et al., 2015). The coal gasification industry has grown rapidly with strong support from the Chinese government in promoting low-carbon and sustainable energy use. The Ningdong region, Ningxia Hui Autonomous Region, is one of the “China Energy Golden Triangle Area” and the largest coal gasification industrial base in China (Liang et al., 2019). Entrained-flow gasification technology has been most widely used in gasification plants from the Ningdong region due to the advantages of high carbon conversion rate, feed coal flexibility, and productivity (Kong et al., 2021; Lü et al., 2021). The coal gasification fine slag (CGFS) is a by-product produced from the gasification unit due to the incomplete reaction of the raw coal and gasification agents (oxygen, water vapor, etc.). The CGFS is composed of the unreacted char and the coal ash slag. It is noteworthy that millions of tons of SGFS are produced in the Ningdong region every year, and the total amount continues to increase (Guo et al., 2021a). Currently, CGFS is mainly landfilled or deposited in slag dumps as the final disposal method, which is environmentally unfriendly and unsustainable. The selected elements in this work (V, Cr, Mn, Ni, Cu, Zn, Ba, and Pb) are the commonly seen heavy metals in coal gasification fine slag (CGFS) from the Ningdong region, China. Furtherly, these eight elements have a relatively high concentration in CGFS as well as a potential threat to the ecological environment. In other words, these eight elements are the typical toxic heavy metal elements in the CGFS of the Ningdong region, which is also supported by other researchers (Strugała-Wilczek and Stańczyk, 2016; Wang et al., 2021a; Li et al., 2021a). In addition, the CGFS contains numerous nanometric particles. Therefore, the nanometric particles will unavoidably spread into the air and be inhaled into the lungs by the nearby residents through the human respiratory tract after the CGFS accumulated in the slag field is dried, which may lead to a weakened immune system and possibly raise the risk of respiratory tract infection (Vargas Buonfiglio et al., 2017). Furthermore, the heavy metals in CGFS may also pose a risk to human health through the food chain (Hoang et al., 2021). It's worth noting that some of the heavy metals may pollute the precious local soil or groundwater if the CGFS is not disposed properly, which brings the large challenge to the water-scarce Ningdong region (Xu et al., 2021; Wang et al., 2019b). The negative environmental impact of large amounts of the CGFS must be given increasingly widespread attention.

In recent years, the studies regarding CGFS have focused primarily on the characterization of physic-chemical properties (Miao et al., 2020a; Miao et al., 2019; Guo et al., 2020a; Wu et al., 2007), effective dehydration technology (Guo et al., 2020b; Guo et al., 2021b; Zhao et al., 2021), and resource utilization approaches (Guo et al., 2021a; Guo et al., 2021c; Wu et al., 2020; Guo et al., 2021d; Guo et al., 2022). However, only a limited number of studies have addressed the potential risk of heavy metals in the CGFS. Thereinto, Tang et al. (Tang et al., 2017) found that the leaching concentration of Be, Mo, and Ba in solid coal gasification wastes from the Ningdong region all exceed the limitation of the Chinese Groundwater Quality Standard. The leachate concentration of Sb and Tl exceeds the primary drinking water quality standards in the United States. In addition, Wang et al. (Wang et al., 2021b) conducted the column leaching experiment according to the M1314 method specified by the U.S. Environmental Protection Agency to estimate the potential risks of hazardous trace elements in solid coal gasification wastes from Northwest China. They found that the leaching rate of As, Se, Mo, and Sb in the wastes is relatively high, which would require consideration of the potential environmental risks of these elements under the CGFS. In the study of Du et al. (Du et al., 2020), the leaching behavior of heavy metals (Ni, Cd, and As) in the fluidized bed CGFS after acid treatment (HAc and HCl) was evaluated. The results showed that the leachability of Ni and Cd in CGFS increased with the increase of HAc and HCl concentrations, while the leaching behavior of As is more complicated. In addition, the acidic treatment of CGFS increases the potential for the release of heavy metals (Ni, Cd, and As) into the environment. Recently, Jiang et al. (Jiang et al., 2021) used a four-step sequential extraction method to examine the chemical speciation of heavy metals in coal gasification slag. They found that both the concentration and the mobile fraction of the selected heavy metals in fine slag are higher than those of coarse slag. More importantly, the graded utilization of the CGFS has become widely recognized as an important approach to the efficient use of the CGFS (Guo et al., 2022; Liu et al., 2021). To be noted, the feed coal has undergone a series of chemical reactions in the gasifier including the gasification of carbon particles and the melting and transformation of minerals in the gasification process. Different-sized CGFS has various properties due to the particular reaction processes in the gasifier, which may lead to the varied occurrence modes of typical heavy metals in the different-sized CGFS (Miao et al., 2019; Guo et al., 2020a; Pan et al., 2016; Lanzerstorfer, 2018). However, the distribution modes and environmental risks of typical heavy metals in different size fractions of CGFS remain unclear. The distribution modes and chemical forms of heavy metals are carefully believed to be important factors to assess the toxic response and environmental risks of heavy metals in solid waste, which should be put into more research (Zhao et al., 2018a).

The sequential chemical extraction procedure proposed by the European Community Bureau of Reference (BCR) proves to be an essential and practical approach to investigate the occurrence modes of heavy metals in waste, which can be applied to reveal the chemical forms of heavy metals in CGFS (Zhao et al., 2018b). More than that, the Toxicity Characteristic Leaching Procedure (TCLP) formulated by the U.S. Environmental Protection Agency (EPA) is widely adopted to simulate the mobility and leachability of heavy metals in solid waste landfills (Sharma et al., 2021; Zang et al., 2019). However, TCLP can only provide limited information on the leaching of heavy metal due to its single test conditions (Zhao et al., 2020). Changing the pH of the extraction solution has a powerful influence on the leaching behavior of solid waste (Du et al., 2020; Kosson et al., 2014). The Leaching Environmental Assessment Framework (LEAF method 1313) from U.S. EPA provides the procedures to further evaluate the effect of pH value of leaching solution on the leaching ability of heavy metals in CGFS under a broad range of environmental conditions (Kosson et al., 2014; Thorneloe et al., 2010). Furthermore, leaching kinetics are essential to understand the release and dissolution mechanism of the heavy metals in different-sized CGFS, which could provide the necessary basis for the ecological risk control of heavy metals in the CGFS (Alghanmi et al., 2015; Dai et al., 2018). Considering that the increasing amount of CGFS from the Ningdong region is facing the challenge of clean utilization or disposal, it is necessary to carry out a multifaceted evaluation of the occurrence, leaching, and potential environmental risk of the typical heavy metals in different-sized CGFS.

The present study aims to reveal the occurrence, leaching, and potential environmental risk of typical heavy metals in different-sized CGFS through the BCR sequential chemical extraction, the TCLP experimental method, the pH-dependent leaching experiment, and the kinetic analysis, which provides possible guidance on the green disposal and utilization of the CGFS.

Section snippets

Sample collection

The representative CGFS samples used in this study were collected from the typical entrained-flow gasification unit (Gaskombimat Schwarze Pumpe pulverized coal gasification, GSP) in China Energy Group Shenhua Ningxia Coal Industry Group Co., Ltd., which is the largest coal gasification company in Ningdong, China. The gasification temperature of the gasifier is monitored at approximately 1450 °C with a gasification pressure of around 5.0 MPa. The CGFS produced by the incomplete gasification of

Distribution and occurrence patterns of typical heavy metals in the CGFS

As shown in Fig. 2, the concentration of V, Cr, Ni, Cu, Zn, and Pb in the four size fractions of the CGFS ranges from 41.18 to 408.04 μg/g. In addition, the concentration of Mn and Ba varies between 1020.81 and 2440.72 μg/g, which is significantly higher than other elements. A similar outcome was also observed in the study by Jiang et al. (Jiang et al., 2021). With the exception of Cu and Mn, the total concentration of other heavy metals increases with the reduction of the size fractions of the

Conclusions

The distribution of selected typical heavy metals in the CGFS from the Ningdong region has an evident particle size dependence, and the majority of the typical heavy metals have a tendency to concentrate in small size fractions. Zn, Pb, and Ba in different size fractions of the CGFS have a considerable ratio in F1, F2, and F3 fractions, while V, Cr, Ni, Cu, and Mn were found in large quantities in the F4 fraction (60.55–76.39 wt%).

The typical heavy metals in the different-sized CGFS showed

CRediT authorship contribution statement

Yang Guo: Conceptualization, Investigation, Methodology, Writing – original draft. Yixin Zhang: Methodology, Writing – review & editing, Funding acquisition. Xu Zhao: Data curation, Investigation. Jie Xu: Investigation, Resources. Guofeng Qiu: Visualization, Software. Wenke Jia: Investigation. Jianjun Wu: Conceptualization, Funding acquisition, Project administration. Fanhui Guo: Conceptualization, Methodology, Writing – review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the National Natural Science Foundation of China [51974311]; the National Key Research and Development Program of China [2019YFC1904302]; the Natural Science Foundation of Shandong Province [ZR2020KE044]; Yulin University and the Dalian National Laboratory for Clean Energy [YLU-DNL Fund 2021005]. We appreciate the anonymous reviewers for their careful work and valuable suggestions that have helped improve this paper substantially.

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