Elsevier

Water Research

Volume 225, 15 October 2022, 119144
Water Research

Enhanced heavy metal adsorption on microplastics by incorporating flame retardant hexabromocyclododecanes: Mechanisms and potential migration risks.

https://doi.org/10.1016/j.watres.2022.119144Get rights and content

Highlights

  • HBCD inherent in PS MPs greatly promotes the metals’ adsorption.

  • log KMeOH can be used as an indicator parameter for the adsorption affinity.

  • The –O and –Br groups of HBCD/PS MPs are involved in the metals’ adsorption.

  • HBCD enhances the long-range transfer of the metals via MPs.

Abstract

Microplastics (MPs) are known to act as carriers of heavy metals; however, little is known about the intrinsic chemical additives of MPs, such as hexabromocyclododecane (HBCD), in terms of the adsorption behaviors and migration risks of heavy metals on MPs. Here, we reported the potential mechanisms and risks of HBCD inherent in polystyrene (PS) MPs with Cu(II), Ni(II), and Zn(II) adsorption/desorption. A comparison of the adsorption capacity of the metals onto HBCD/PS composites (HBCD/PS) MPs (10.31–20.76 μmol/g), pure MPs (0–3.60 μmol/g), and natural minerals (0.11–13.88 μmol/g) showed that the addition of HBCD significantly promoted the metals adsorption onto the HBCD/PS MPs, and even exceeded that of natural particles. Isotherms and thermodynamic data suggested that the adsorption process of the metals onto the HBCD/PS MPs was spontaneous and endothermic, and that the adsorption was a mainly multi-ion process with an inclined direction. Furthermore, the results of SEM-EDS, FTIR, and XPS analyses, as well as density functional theory well explained that the metals were mainly adsorbed on the –O and –Br groups of the HBCD/PS MPs via electrostatic interactions and surface complexation. More importantly, by comparing the desorption activity with natural river water and seawater, HBCD inherent in MPs can enhance the long-range transfer of metals carried by the HBCD/PS MPs from contamination sources to potential sink like oceans. Thus, the HBCD/PS MPs with high loading of Cu(II), Ni(II), and Zn(II) could be potential secondary sources of these metals in seawater. Overall, these findings revealed the potential risks of flame retardant in MPs associated with metal migration, and advocated that flame retardant-related waste MPs should be included in coastal sustainable development.

Introduction

The COVID-19 pandemic emphasized the indispensable role of plastic products in our daily lives. Due to their excellent properties such as lightweight, flexibility, durability, and chemical stability, plastics are used in personal protective equipment, packaging applications, and commercial products, which saw unprecedented growth during the pandemic (Patrício Silva et al., 2021; Prata et al., 2020; Roberts et al., 2021). However, the deterioration of the environmental situation due to poor management of plastic waste has further reinforced the widespread perception of plastic as a harmful pollutant (Parashar and Hait, 2021). More seriously, these plastics are subjected to physical, chemical, and biological degradation into smaller plastic fibers, known as microplastics (MPs, particle size < 5 mm), that pose a serious environmental pollution problem (Guan et al., 2020; Thompson et al., 2004).

As an abundant and persistent pollutant, MPs not only pose a direct or indirect threat to ecological environments (Ya et al., 2021; Zhang et al., 2022) and organisms (Li et al., 2022b; Liu et al., 2022a), but also tend to adsorb some pollutants and migrate easily, thus causing even more severe ecological and toxicological effects (Ouyang et al., 2022; Pestana et al., 2021; Wang et al., 2021). In fact, most commercial plastics contain inherent chemical additives that are required to meet the process and application requirements for better use in life (Do et al., 2022; Fauser et al., 2022; Yan et al., 2021). These chemical additives may be released into the environment together with MPs (Fauser et al., 2022). Tun et al. (2022) found that about half of the additives (e.g., phthalates and antioxidants) in a study of drainage field soils were sourced from MPs. Cao et al. (2022) revealed that MPs were a potential source of phthalate esters in the water environment and estimated their release to be approximately 57.8 to 16,100 kg of phthalate esters per year. Several recent studies have also demonstrated that the inclusion of chemical additives in MPs could cause greater ecological risks and toxicity to organisms compared to virgin MPs (Jang et al., 2021; Na et al., 2021). However, most previous studies focused on pure MPs acting as carriers of heavy metals, and information on the role of chemical additives in the migration and fate of MPs and pollutants was highly lacking. This knowledge gap has emerged as one of the central uncertainties in our understanding of the role of MPs in the migration and fate of heavy metals.

Hexabromocyclododecane (HBCD), a typical brominated flame retardant, is extensively applied as insulation material for building materials, decorative textiles, and electronics (Covaci et al., 2006; Fromme et al., 2014; Lee et al., 2019). Generally, HBCD is mixed with polystyrene (PS) plastics through extrusion and high-impact methods (Duan et al., 2016), which produces PS plastics containing HBCD (HBCD/PS) that are frequently detected in the environment (Covaci et al., 2006; Lee et al., 2019; Lin et al., 2021a). For example, HBCD was detected in substantial amounts in PS plastics (up to 14.5 mg/g) collected in the coastal region of South Korea, Hong Kong, Canada, and Japan, implying that HBCD/PS composite contamination was a common and global environmental problem (Jang et al., 2017). Additionally, HBCD has attracted worldwide attention due to its potential persistence, toxicity, and bioaccumulation (Jang et al., 2021; Liu et al., 2018). This has led to the fact that the addition of HBCD to MPs can therefore cause more serious toxic effects on organisms in the environment than virgin MPs (Lackmann et al., 2022). In our previous study, we found that HBCD could significantly promote the enrichment behavior of Pb(II) onto PS MPs (Lin et al., 2021b), which greatly enhanced the ability of MPs to serve as contaminants carriers in the environment. Therefore, it is necessary to further investigate the potential roles of HBCD in the adsorption of other heavy metals onto MPs and their potential migration risks, especially in the land-sea migration process.

Cu(II), Ni(II), and Zn(II) are heavy metals widely used in multiple industries, such as agriculture, the electroplating industry, and the dyestuff industry (Wang et al., 2020; Yildirim et al., 2020). When released into water bodies without proper treatment, these heavy metals can cause damage to human health and ecosystems due to their toxicity and biodegradability. Notably, these metals can be highly toxic to lower and higher organisms when exceeding certain concentrations (Li et al., 2022a; Tang et al., 2021b), and are thus included in the Water Framework Directive (Quevauviller, 2009). Moreover, Cu, Zn, and Ni are concerned heavy metals in estuaries due to their historical and humanistic residues (Hong et al., 2020). In this regard, we selected these three heavy metals as the target pollutants. Overall, the aims of this work are to i) reveal the potential roles and mechanisms of HBCD inherent in PS MPs with Cu(II), Ni(II), and Zn(II) adsorption; ii) investigate the influence of water chemistry (e.g., solution pH, salinity, dissolved organic matter (DOM)) on the adsorption processes; and iii) evaluate the potential migration risks of HBCD/PS MPs associated with the metals desorption in river water and seawater media.

Section snippets

Materials and Chemicals

The PS MPs and HBCD/PS MPs (HBCD: 2% of total weight) were obtained from a local material plant (Lianyungang Qihang Flame Retardant Material Co., Ltd, Lianyungang, China). The detailed production process of the HBCD/PS is shown in Fig. S1. The main difference between the PS MPs and HBCD/PS MPs is that the latter have an additional step in the preparation process that involves the addition of HBCD. Comparison of FTIR and XRD spectra of PS, HBCD, and HBCD/PS demonstrates that HBCD and PS were

Comparison of adsorption capacity

In the control experiments, while the HBCD or treated HBCD only accounted for 2% of the mass fraction of the HBCD/PS MPs, it contributes 41.59–57.68 %, 46.53–56.61 %, and 38.64–54.70 % to the Cu(II), Ni(II), and Zn(II) adsorption by HBCD/PS MPs, respectively (Fig. S3). However, the treated PS only contributes 2.15–7.71 % to the metals adsorption by HBCD/PS MPs. There results imply that HBCD plays an important role in the metals adsorption by HBCD/PS MPs. Furthermore, the adsorption ability of

Conclusions

Herein, the potential roles and risks of HBCD inherent in PS MPs associated with Cu(II), Ni(II), and Zn(II) adsorption/desorption were investigated. The results showed that the addition of HBCD greatly promoted the adsorption of Cu(II), Ni(II), and Zn(II) onto the MPs. The SEM-EDS, FTIR, and XPS analyses as well as density functional theory well explained that the metals were mainly adsorbed on the –O and –Br groups of the HBCD/PS MPs through electrostatic interactions and surface complexation.

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

In memory of our best friend, Dr. John Merefield. We gratefully acknowledge the support of this work by National Natural Science Foundation of China (31870483, 31530008) and National Important Scientific Research Programme of China (2018YFC1406600). We thank TopEdit (www.topeditsci.com) for its linguistic assistance during the preparation of this manuscript. The authors would like to acknowledge the editors and anonymous reviewers whose comments and advice greatly improved the quality of this

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