Is it possible to efficiently and sustainably remove microplastics from sediments using froth flotation?
Graphical abstract
Introduction
Microplastics (MPs) contamination has gained public attention in recent years and has become a global environmental issue [1]. There are multiple sources of MPs contamination. Due to improper waste management, waste plastics undergo aging, physical collisions, and degradation, inducing the fragment of plastics [2]. Besides, the microbeads in cosmetics and the fibers produced by laundry washing are directly discharged into the municipal pipe network [3]. With the large specific surface area, MPs may adsorb heavy metals, persistent organic pollutants, and biofilms, thereby aggravating the ecological risks [4]. Researchers have provided the evidence of MPs contamination in marine invertebrates and zooplankton [5], [6]. Further evidence of MPs in the guts of fish and marsh birds confirmed the invasion of MPs into food chain [7], [8]. Subsequently, MPs can enter the human body through the digestive tract, respiratory system and skin, causing oxidative stress, cytotoxicity, and chromosomal translocation [9]. Generally, the “final sedimentation tank” of the migration of MPs are the sediments in various environments of sea and river [10]. For sea sediments, the average abundance of MPs in the Black Sea sediments has reached 106.7 items/kg [11], and the MPs accumulation in coastal sediments of the East China Sea has been increasing since 1960 s [12]. The MPs in sediments of freshwater environments can reach up to 3 × 104 items/kg [13]. The plastic fibers found in bivalves in the Persian Gulf are similar to MPs in surrounding sediments [14]. Recent works have focused on MPs distribution and migration in sediments, whereas the development of MPs removal is still in the preliminary stage.
Many removal technologies were employed in the removal of MPs from the water, such as filtration [15], advanced oxidation process (AOPs) [16], adsorption [17], coagulation [18], magnetic extraction [19], dissolved air flotation (DAF) [20], and biodegradation [21]. However, the presence of sediment particles undoubtedly increases the difficulty of MPs removal. Firstly, the MPs concentration in sediments is low, and most removal technologies might not be applicable due to low efficiency. Besides, the interference of sediment particles might block the removal, such as membrane fouling [22] and unexpected adsorption with sediment [21]. The biological removal of microplastics is still relatively inefficient [23]. Only few approaches were applied to remove MPs in sediments with high efficiency. As a common separation method in waste management, density separation can separate light MPs and heavy sediments in a brine solution. Nevertheless, heavy liquids consist of highly concentrated solutions containing halogens, which are dangerous to humans and the environment [24]. Electrostatic separation technique has been used in mineral beneficiation, fine particle separation, tea production, and waste plastic separation [25]. Solid matrices like dirt and silt are commonly charged because of their conduction qualities, whereas MPs are normally non-conductive. However, the energy consumption of the separator should be focused on [23]. Based on the oleophilic attraction between MPs and oil, castor oil was used to remove MPs from different environmental matrices [26]. Nevertheless, the oil separation could not efficiently extract rubber and fluoropolymers, and the oil cost might be expensive. Therefore, limited techniques are currently employed to remove MPs from sediments, and a sustainable and efficient removal method for MPs removal from sediments is an urgent need.
Froth flotation is a separation method in mineral processing [27], and our previous work aims at separating waste plastic by froth flotation and wettability regulation [28], [29]. Accordingly, we speculate that the adherence of air bubbles on hydrophobic MPs surfaces contributes to the MPs floating of MPs while hydrophilic sediment particles submerge. Despite the fact that froth flotation has a bright future, the classical froth flotation might not realize the complete removal of MPs from sediment. Imhof et al. extracted only 55% MPs from the sediments by flotation, and they pointed out that the density separation was more effective [30]. The sediments settle under gravity and cover the air stone, which may hinder the supply of air bubbles and decrease the removal efficiency (Figure S1). The improvement of flotation equipment should be considered. Besides, do MPs in sediment remain hydrophobic? Generally, MPs were implicitly assumed hydrophobic, however, the sediment environment may induce hydrophilization on the surface of MPs [31]. Interfacial interactions between sediment and MPs may account for the limitations of flotation removal. After the clarifying hydrophilization mechanism, we should further restore the hydrophobicity of MPs to improve the removal efficiency, and the addition of collector might be helpful. Finally, the application potential of froth flotation should be further evaluated from the aspects of efficiency and sustainability [32].
In this study, we established a novel flotation system that could achieve continuous MPs removal from sediments. Sediment particles and MPs might collide, rub and stick, enhancing the hydrophilicity of MPs and inhibiting removal [33]. Therefore, we investigated the effects of MPs properties and environmental factors on flotation removal. Meanwhile, we optimized operation conditions, including flotation time and frother dosage. The wettability mechanism of MPs was revealed through surface characterizations and interaction energies calculated by Extended Derjaguin Landau Verwey Overbeek (EDLVO) theory. To restore the hydrophobicity of MPs, we used environmentally friendly sodium oleate as a collector based on the hydrophobic attraction [34]. Subsequently, we proposed an efficient flotation process for MPs removal. To further demonstrate the application potential and sustainability of proposed process, we compared the environment categories of froth flotation removal with density separation and electrostatic separation using life cycle assessment (LCA). The costs of separation methods were evaluated. The findings of this study will strengthen the understanding of the interaction between sediments particles and MPs, and provide a new feasible strategy for MPs removal from sediments.
Section snippets
Preparation of MPs and sediments
Polyvinyl chloride (PVC) and acrylonitrile–butadienestyrene (ABS) were common MPs in sediment environments [35]. Post-consumer PVC and ABS were collected from a local e-waste dismantling plant (Miluo, China). MPs derived from the crushing of waste plastics were screened into different sizes (less than0.074 mm, 0.074–0.125 mm, 0.125–0.5 mm, 0.5–1 mm, 1–2 mm, 2–3 mm, 3–4 mm, 4–5 mm) [36]. MPs derived from waste plastics undergo more aging and abrasion than commercial resin particles. We collected
The effects of MPs properties on MPs removal
Flotation removal was conducted after the contact of MPs with sediments. The hydrophilization effect depended on MPs type, size, and concentration. In sediments, ABS MPs were more vulnerable to these factors and had a lower removal efficiency in various situations than PVC MPs (Fig. 2). In fact, in the absence of sediment, flotation removed over 98.5% of MPs in control experiments. The MPs removal ratio did not change with MPs concentration (Fig. 2a). During the flotation process, MPs tended to
Goal and scope definition
The purpose of this chapter was to analyze and compare froth flotation with other technologies from environmental and economic perspectives. We considered three representative methods for MPs removal from sediment: froth flotation, density separation, and electrostatic separation. The flowsheet of MPs technologies was shown in Fig. 9. Froth flotation did not need to dry the sediment, and the sediment was mixed with fresh water and added with minor surfactant, followed by the flotation
Conclusions
Inspired by the surface hydrophobicity of MPs, froth flotation was evaluated for MPs removal from sediments. It was surprising that wetted MPs in sediments caused poor removal efficiencies. After 24 h incubation time, ABS MPs and the large MPs particles were more sensitive to the hydrophilization effect of sediments. In addition, the sticky lake sediment with 20% moisture might cause a decline of PVC and ABS removal efficiencies to be 54.8% and 48.2%, respectively. When exposed to an acid
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 work is supported by the National Natural Science Foundation of China (21878343). Thanks for the help of Dr. Jiacheng Dai in Fudan University. We sincerely thank the reviewers and editors for their help.
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