Ecotoxicological effects of micronized car tire wear particles and their heavy metals on the earthworm (Eisenia fetida) in soil
Graphical abstract
Introduction
Microplastic pollution has emerged as the major global environmental issue (Alimba and Faggio, 2019; Rillig et al., 2019). MPs are generally defined as small plastic pieces less than 5 mm in diameter (Thompson et al., 2004; Arthur et al., 2009; Cole et al., 2011). Compared with larger plastic pieces, MPs are more easily ingested by organisms, increasing the potential for exposure and subsequent effects (Farrell and Nelson, 2013). As MPs have become ubiquitous in the environment, they are being increasingly ingested to organisms, including mammals, birds, fish and aquatic invertebrates (Besseling et al., 2013; Lee et al., 2013; Lusher et al., 2015; Reynolds and Ryan, 2018; Tanaka and Takada, 2016). Although many studies have described the occurrence and adverse effects of MPs in the aquatic environment, less is known about their fate and impact in the terrestrial environment (de Souza Machado et al., 2018; Nizzetto et al., 2016). This is despite that plastics accumulate in soil through multiple sources, including discarded tires and TMPs from roadways, as well as wind-blown debris from landfills, sewage sludge, peeling paint, and agricultural plastic sheeting (Nizzetto et al., 2016; Sundt et al., 2014; Lassen et al., 2015; Steinmetz et al., 2016; Li et al., 2020a).
The automotive industry accounts for about 10% of plastics produced, with the majority of it in tires (Horton et al., 2017). Vehicle tires are composed of elastomers such as styrene-butadiene rubber or butadiene rubber. As the tires wear they generate MPs that can become dispersed in the environment in different ways (de Souza Machado et al., 2018). For example, smaller particles can become airborne and travel some distance depending on their size and weather conditions. Particles can also be transported to nearby soil, as well as streams, rivers, lakes and oceans via runoff. The global per capita tire wear contribution to the environment is estimated at about 0.81 kg year−1 per capita, about 6.1 million tonnes (~1.8% of total plastic production) (Kole et al., 2017), with the total tire wear from >1 billion vehicles estimated to be 3,369,698,000 kg/year, enough to fill the 31 of the largest container ships in the world (Reynolds and Ryan, 2018). Thus, automobile tires are one of the most important sources of MPs in the environment (Nizzetto et al., 2016; Hurley and Nizzetto, 2018; Horton et al., 2017).
Tires have evolved over the years to improve performance and extend their lifespan. Tires are a complex mixture of materials, including rubber, fillers, metals, reinforcing agents, processing aids, accelerators and retarders, adhesives and activators, with each component having different properties, functions, and environmental impacts (Horton et al., 2017; Bläsing and Amelung, 2017; Dris et al., 2016). TMPs vary size depending on numerous factors, but the majority seem to be between in the 350 and 50 μm, though undoubtedly there is a significant portion that are smaller (Vogelsang et al., 2018; Cadle and Williams, 1978).
Earthworms are representative animals of the Annelid Oligochaeta, which is made up of many types of aquatic and terrestrial worms with bare body surfaces and no cuticles. Earthworms can form permanent vertical burrows in the soil, which affect soil structure, water and nutrient supply, as well as microbes and soil animal communities (Lee, 1985). Thus, they influence soil dynamics, i.e. mixing of inorganic and organic parts, nutrient cycle cycles, decomposition of organic matter, aeration and water infiltration and/or drainage in the soil and sedimentary layers. They are also the main food of terrestrial vertebrates and birds, accounting for 80% of the total biomass of soil animals. In addition, they can use sensitive receptors to quickly respond to chemical substances in the soil and can transfer most of the absorbed or ingested heavy metals and polycyclic aromatic hydrocarbons to organisms at higher levels in the food chain through bioaccumulation (van Gestel et al., 2011). Further, earthworms are key indicator organisms for ecotoxicological tests that are commonly used in the European Union, the Organization for Economic Co-operation and Development (OECD), the International Organization for Standardization, and the Food and Agriculture Organization of the United Nations, among others (Piola et al., 2013; Santadino et al., 2014).
Recently, studies have shown that MPs may have adverse effects on terrestrial ecosystems (Ju et al., 2019; Rodriguez-Seijo et al., 2017; Scheurer and Bigalke, 2018; Zhu et al., 2018). For example, polyethylene MPs in soil were found to significantly reduce the growth rate of earthworms and cause inflammation and congestion of the intestinal wall (Huerta Lwanga et al., 2016; Rodriguez-Seijo et al., 2017). Other studies have also shown that earthworms can transport MPs from the surface soil to deeper layers, leading to increased pollution there (Maaβ et al., 2017; Rillig et al., 2017). Toxicity tests using tire leaching fluids have shown that the tire crumb leachate is toxic to a number of species, including luminescent bacteria, invertebrates, fish, and green algae. (Fishbein, 1991; Birkholz et al., 2003). However, research on the molecular and biochemical reactions of earthworms to TMPs in the soil is quite limited (Rodriguez-Seijo et al., 2018; Deng et al., 2017). This article is the first to study the physical and chemical effects of tire microplastics and heavy metals on earthworms in the soil environment.
The purpose of the current study was to (1) evaluate the biochemical effects of the TMPs on the earthworm Eisenia fetida, (2) quantify changes to the TMPs during the study, and (3) determine the uptake of zinc (Zn), cadmium (Cd), and lead (Pb) from the TMPs into the organisms.
Section snippets
Preparation of artificial soil, micronized car tire wear particles, and test organisms
Artificial soils were prepared to conduct a laboratory-scale exposure experiment. Soils were mixed according to OECD guidelines with slight modification (OECD, 2004; Verdu et al., 2018). The specific components were as follows: 20% kaolinite clay (Aladdin, China, https://www.aladdin-e.com/), 70% quartz sand (Greagent-Reagent, China), and 10% sphagnum peat (Jiffy substrates, purchased from a local gardening store). Analytical reagent CaCO3 was used to adjust the initial pH of the soil. Organic
Changes in functional groups on the surface of TMPs during the experiment
The intensity of key functional groups in the FTIR spectrum of TMPs changed with degradation time (Fig. S1). The experimental results show that slight changes in the IR spectrum with or without earthworms. (Fig. S1 A and B). After 28 days, in different degradation environments, narrow absorption peaks appeared at 2900 cm−1 and 1500 cm−1, corresponding to CH(CH2) stretching vibration and CH(CH3, CH2) stretching vibration (Weilun and Yilong, 2019). With earthworms, the absorption intensity of
Changes of functional groups on the surface of TMPs during the experiment
Based on FTIR analyses, the waste tires used in this experiment were comprised of a mix of styrene-butadiene rubber and butadiene rubber. Styrene-butadiene rubber and butadiene rubber are the two most used synthetic rubbers. Styrene-butadiene rubber is an elastomer obtained by copolymerization of butadiene and styrene. Butadiene rubber is a structured synthetic rubber formed by polymerization of butadiene. The only published degradation study under environmental conditions showed that the
CRediT authorship contribution statements
Yingfei Sheng: Experiments, Writing, Data-processing and editing. Yi Liu: Experiments and Data-processing. Keiwei Wang: Data-processing. Yichun Wu: Supervision. James V. Cizdziel: Writing - Reviewing and Editing. Ying Zhou: Supervision and Editing.
Declaration of competing interest
The authors declared that they have no conflicts of interest to this work.
We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
Acknowledgements
The authors are very grateful for grants from the Zhejiang Provincial Natural Science Foundation of China (No. LY20B070011), the Science and Technology Program of Zhejiang Food and Drug Administration, China (No. 2021002) and the Science and Technology Program of Zhoushan, Zhejiang Provincial, China (No. 2019C31042).
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