Research PaperThe mechanistic understanding of potential bioaccessibility of toxic heavy metals in the indigenous zinc smelting slags with multidisciplinary characterization
Graphical Abstract
Introduction
Since the past smelting processes were less effective, a small amount of minerals were extracted from sulfide ores, but large quantities of toxic metals were frequently retained in smelting slags (Wan et al., 2021, Zhao et al., 2021). For Example, Wan et al. (2021) reported that the copper smelting slag contained 0.1% of As, 1.2% of Cu, 0.3% of Pb and 1.8% of Zn. Moreover, the utilization rate of smelting slags produced was still low, resulting in a vast amount of solid wastes to be disposed (Gu et al., 2021, Zhang et al., 2021a). There was an estimation that the annual production of steel slag is about 100 million tons in China, while there was around 22% of utilization rate for these slags (Pan et al., 2015). At present, the land-based disposal of smelting wastes is the most common practice. It was reported that over 80% of copper melting slags were directly stockpiled on slag disposal sites in many countries, without proper treatment and management (Zhang et al., 2021d) (In addition, many researchers have reported that toxic metals in smelting slags might be released with prolonged time (Jarosikova et al., 2017, Liu et al., 2018). Some studies have also indicated that the biogeochemical and physical weathering of smelting slags under an open environment would accelerate the mobilization and migration of potentially harmful elements to the surrounding soils and surface waters (Gaulier et al., 2020; Liu et al., 2021; Xu et al., 2021a). This pollution would endanger the local ecosystem and pose the potential risks to residents living near the industrial sized smelters (Zhang et al., 2021b). Therefore, an in depth investigation on the mineralogy and geochemistry for better understanding the potential environmental impacts of smelting slag dumps was an essential step for their appropriate disposal and resource utilization (Zhang et al., 2021c, Zhou et al., 2021).
In addition, how to scientifically evaluate the health risks related to smelting slags was also of great importance for management decision-making of smelting slag dump sites. Many previous studies have indicated that a close association were found between the bioaccessibility of heavy metals determined using in vitro gastrointestinal assays and the relative bioavailability obtained from in vivo animal models (Li et al, 2015; Louzon et al, 2020; Yin et al, 2021). Furthermore, in vivo methods had various disadvantage such as high cost, time consumption, poor repeatability and ethical controversy, which greatly restricted its wide application in human health risk assessment (Hao et al., 2021, Xiao et al., 2022). Therefore, several in vitro methodologies, such as Physiologically Based Extraction Test (PBET) (Ruby et al, 1996), Solubility Bioaccessibility Research Consortium (SBRC) (Juhasz et al, 2009a), In-vitro Gastrointestinal (IVG) methods (Rodriguez et al., 1999), have been developed as the effective procedures to evaluate the bioaccessible fractions of heavy metal contaminants in various environmental matrices, including vegetables, soils, dusts, sediments and tailings (Fujimori et al., 2018, Khelifi et al., 2021, Toujaguez et al., 2013, Wang et al., 2021, Zheng et al., 2021). However, limited studies have reported that the potential release of heavy metals in the unstable smelting slags was influenced by the main controlling factors, including the mineralogical and morphological characteristics of these smelting slags and their particle sizes, as well as the metal fractionation. Moreover, most studies focused on the mineralogy of smelting wastes for metallurgical recycling, while the detailed characterization of these smelting wastes was rarely performed to understand the key factors controlling the bioaccessibility of heavy metals under the human gastrointestinal system (Ettler et al, 2021).
Therefore, this work built on our previous studies was conducted to address the scientific questions as follows: (i) How were the great differences in the metal bioaccessibility determined using three In Virto assays; (ii) How to identify the main potential controls influencing the metal bioaccessibility; (iii) how to provide a better understanding of the solubility mechanism of heavy metals in the human gastrointestinal system using multidisciplinary methods. The present study results would provide a scientific and significant guidance for the sustainable utilization and environmental risk management of smelting slags.
Section snippets
Sample collection
The studied smelter contaminated site is located at a concentrated region of the indigenous zinc smelting actives with the longsmelting history in northwestern Guizhou Province, southwestern China. Detailed information about the sampling locations and sample preparation was present in the previous studies by (Wang et al., 2021, Xu et al., 2021b). The smelting slag sample was air–dried and accurately weighed at 1kg and sieved through a 10–mesh sieve. Then, four particle–size fractions, including
The metal bioaccessibility assessment
The bioaccessible metal fraction (%) in four particle size fractions of the zinc smelting slags are shown in Fig. 1. A comparison based on three in vitro assays indicated that thehigher bioaccessibility of heavy metals (4.54–91.39%) in GP was determined using SBRC assay, while PBET obviously exhibited their higher bioaccessible fraction (0.42–32.87%) in GIP. The significant difference was probably due to the difference in vitro extraction operations (Zhou et al., 2021b). Among the three In
Conclusions and environmental implications
Based on the present scientific backgrounds, PBET, SBRC and IVG assays were used in this study to compare the differences of bioaccessible metals between four particle sizes of the zinc smelting slags. In addition, multidisciplinary methods such as SEM/EDS, XRD, FTIR and XPS were systematically conducted to present the distinction in morphological characteristics, phase compositions and chemical structures of the zinc smelting slags, as well as the occurrence states of the main elements. The
CRediT authorship contribution statement
Da-Mao Xu: Conceptualization, Methodology, Formal analysis, Investigation, Data curation, Analysis, Interpretation, Writing – original draft, Writing – review & editing, Final approval of the manuscript. Rong-Bing Fu: Funding acquisition, Project administration, Writing – review and editing. All authors have read and agreed to the published version of the manuscript.
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.
Acknowledgements
This research was supported by the National Key Research and Development Program of China (2019YFC1805205). The authors are grateful to the editors and anonymous reviewers for their positive comments and suggestions to significantly improve the original version of this manuscript.
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