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The role of plastics in the accumulation and release of trace elements in the environment

Published online by Cambridge University Press:  03 August 2023

Hayden Masterton Open the ORCID record for Hayden Masterton [Opens in a new window] ,
Helena Ruffell ,
Sally Gaw  and
Olga Pantos
Hayden Masterton*
Affiliation:
ESR, Christchurch Science Centre, Christchurch, New Zealand
Helena Ruffell
Affiliation:
University of Canterbury, Christchurch, New Zealand
Sally Gaw
Affiliation:
University of Canterbury, Christchurch, New Zealand
Olga Pantos
Affiliation:
ESR, Christchurch Science Centre, Christchurch, New Zealand
*
Corresponding author: Hayden Masterton; Email: hayden.masterton@esr.cri.nz
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Abstract

Plastics are an emerging class of environmental contaminants whose impacts are not yet fully understood. Trace elements, another class of environmental contaminant and commonly associated with plastics, have been widely researched and are known to be toxic to organisms. However, the combined impacts of these two contaminants on the environment remain unclear. Here, we reviewed the current knowledge of the types and concentrations of trace elements associated with plastics, the role of plastics in creating new exposure routes, the processes involved in the release of trace elements from plastics, and the transport of plastics through environmental compartments. Trace elements inherent in plastics, due to addition during manufacture for formation or functional properties, are typically present at higher concentrations than those that are acquired from the environment, and consequently are likely to have greater impacts. Trace elements are continuously released into environmental matrices from plastics but may be released at higher concentrations when exposed to rapid changes in environmental conditions (pH, ionic strength, redox potential, salinity, UV levels). Plastics potentially provide additional exposure routes for organisms to trace elements. For example, exposure to trace elements may occur when organisms ingest plastics, use them for shelter and nest building or as a surface to attach onto. Further research to improve our understanding of this complex contaminant should focus on environmentally relevant studies on trace element release and their effects.

Topics structure

Topic(s)

Impact

Subtopic(s)

Contamination

Keywords

plasticmicroplasticstrace elementsadditivesnon-intentionally added substances (NIAS)
Type
Review
Information
Cambridge Prisms: Plastics , Volume 1 , 2023 , e15
Creative Commons
Creative Common License - CCCreative Common License - BY
This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0), which permits unrestricted re-use, distribution and reproduction, provided the original article is properly cited.
Copyright
© The Author(s), 2023. Published by Cambridge University Press

Impact statement

This systematic review summarises current knowledge on toxic trace elements that are associated with plastics, including inherent trace elements (intentionally or non-intentionally added) as well as those acquired from the environment. It considers how, once in the environment, plastics cycle through ecosystem compartments and assesses the potential impacts of associated trace elements on the organisms they interact with. Mechanisms through which trace elements may be released into the environment or organisms were assessed along with the environmental fate of plastics to determine the impacts of plastic-associated trace elements and identify settings where impacts are likely to be higher. Routes through which organisms may be exposed to trace elements, that would not occur in the absence of plastics, were also identified. Key knowledge gaps were identified, and as plastics are ubiquitous environmental contaminants, further research on the environmental impacts of plastic-associated trace elements is urgently needed.

Introduction

Plastics are an emerging class of environmental pollutants which have been identified in all environmental compartments so far examined (air, biota, soil and water), including remote areas such as Antarctica (Furness, Reference Furness1985; Reed et al., Reference Reed, Clark, Thompson and Hughes2018; Webb et al., Reference Webb, Ruffell, Marsden, Pantos and Gaw2019; Pereira et al., Reference Pereira, Rodríguez, Blasco-Monleon, Porter, Lewis and Pham2020; Aves et al., Reference Aves, Revell, Gaw, Ruffell, Schuddeboom, Wotherspoon, LaRue and McDonald2022). Human reliance on plastic has been driven by its low cost and versatility in a wide range of applications. Plastic is used for packaging, building and construction materials, transport, electronics, medical supplies, and household, leisure and sporting equipment (PlasticsEurope, 2021).

More recently it has been acknowledged that plastics often contain trace elements (TEs) added during the production and manufacture of plastic items, as well as acquiring them from the environment (Turner and Filella, Reference Turner and Filella2021). The potential for plastics to act as vectors of TEs over long distances and increase the exposure of organisms to TEs is of concern. Trace elements are non-degradable persistent contaminants and therefore may bioaccumulate and biomagnify in biota, thus resulting in greater effects on organisms higher up the food chain (Wu et al., Reference Wu, Cobbina, Mao, Xu, Zhang and Yang2016b).

The environmental fate of plastics and their cycling within and between environmental compartments is a key factor to be considered when assessing the impacts of plastic-associated TEs. Sources of plastic entering the environment include wastewater treatment plant (WWTP) discharges, landfill leachate and aerial resuspension, agriculture, fishing, illegal and accidental littering, and the breakdown of large plastic materials through general wear and tear during usage (Wu et al., Reference Wu, Yang and Criddle2016a; Koelmans et al., Reference Koelmans, Mohamed Nor, Hermsen, Kooi, Mintenig and De France2019; Li et al., Reference Li, Wang, Su, Zou, Duan and Zhang2020; Ruffell et al., Reference Ruffell, Pantos, Northcott and Gaw2021). Once released into the environment there is a continuous flow of plastics between air, soil, biota, surface water, groundwater and sediments (Li et al., Reference Li, Wang, Su, Zou, Duan and Zhang2020). Major transport mechanisms include water cycling, deposition, wind erosion, incomplete incineration of plastic waste, wave action and sea turbulence, and interactions with biota (Ryan, Reference Ryan2015; Dris et al., Reference Dris, Gasperi, Mirande, Mandin, Guerrouache, Langlois and Tassin2017; Allen et al., Reference Allen, Allen, Moss, Le Roux, Phoenix and Sonke2020; Prinz and Korez, Reference Prinz, Korez, Jungblut, Liebich and Bode-Dalby2020; Watt et al., Reference Watt, Picard, Maldonado, Abdelwahab, Mielewski, Drzal, Misra and Mohanty2021; Weiss et al., Reference Weiss, Ludwig, Heussner, Canals, Ghiglione, Estournel, Constant and Kerhervé2021; Pantos, Reference Pantos2022). The transport of plastics can be affected by the density and the additives present, as well as the size and morphotype of each individual particle (Hoellein et al., Reference Hoellein, Shogren, Tank, Risteca and Kelly2019; Kumar et al., Reference Kumar, Sharma, Verma, Jha, Singh, Gupta, Chandra and Prasad2021; Shamskhany et al., Reference Shamskhany, Li, Patel and Karimpour2021). These characteristics of individual particles can change depending on environmental conditions such as UV exposure, weathering and degree of biofouling, making it difficult to predict the fate of environmental plastics (Moret-Ferguson et al., Reference Moret-Ferguson, Law, Proskurowski, Murphy, Peacock and Reddy2010; Liço et al., Reference Liço, Boci and Drushku2014; Lagarde et al., Reference Lagarde, Olivier, Zanella, Daniel, Hiard and Caruso2016).

This review examines the current state of knowledge around the interaction between plastic-associated TEs and the environment. The relative environmental hazards of TEs added during manufacture are compared to those of TEs acquired from the environment, and the composite contaminant that the TE-plastic combination forms. Potential exposure routes of organisms to TEs associated with plastics, and the role environmental conditions play in the level of risk are also examined. The knowledge gaps that need addressing to better understand the impacts and risk of plastics, a now ubiquitous class of anthropogenic contaminant, are identified.

Plastic-associated trace elements

Trace elements may either be inherent within plastics, due to their use in polymer manufacturing, used as additives for improving or adding desirable properties, or acquired from the surrounding environment through adsorption due to physicochemical surface properties of plastic (Bridson et al., Reference Bridson, Gaugler, Smith, Northcott and Gaw2021; Turner and Filella, Reference Turner and Filella2021).

Inherent

Many toxic substances are inherent within plastic products, having been deliberately added in order to add or increase some desired properties of the plastic. These are referred to as additives (Bridson et al., Reference Bridson, Gaugler, Smith, Northcott and Gaw2021). Additives (organic or inorganic) are added to the base polymer during production for improving fire, UV and heat resistance; for adding specific or desirable colours; and as fillers for reducing cost or increasing hardness and stiffness (Gradin et al., Reference Gradin, Howgate, Seldén, Brown, Allen and Bevington1989; Sendra et al., Reference Sendra, Pereiro, Figueras and Novoa2021). Plastics may also have other toxic substances incorporated within the polymer matrix during production, such as catalyst residues (e.g., antimony compounds used as a catalyst in the production of polyester) and impurities that are unintentionally added, referred to as non-intentionally added substances (NIAS) (Bridson et al., Reference Bridson, Gaugler, Smith, Northcott and Gaw2021). The amount of TE additives present in the final plastic product as a mass percentage varies between polymer and product types but can range from a few percent to half of the total mass, in the case of inorganic fillers (Hahladakis et al., Reference Hahladakis, Velis, Weber, Iacovidou and Purnell2018). Trace element use, commonly used compounds, and chemical formula are summarised in Table 1. Some inorganic additives have multiple purposes, such as zinc oxide (ZnO, filler and pigment) and antimony trioxide (Sb2O3, flame retardant and pigment) (Turner and Filella, Reference Turner and Filella2021). These TEs are not chemically bound to the polymer matrix and as a result can diffuse throughout the polymer and into the surrounding environment due to concentration gradients (Wilson et al., Reference Wilson, Young, Hudson and Baldwin1982; Mercea et al., Reference Mercea, Losher, Petrasch and Tosa2017; Chen et al., Reference Chen, Chen, Yao, Artigas, Huang and Zhang2019; Mao et al., Reference Mao, Gu, Bai, Dong, Huang, Zhao, Zhuang, Zhang, Yuan and Wang2020). This creates concern regarding the impacts of TEs being released into the surrounding environment.

Table 1. Trace elements used as additives within plastics products, including the most commonly used compounds and chemical formula

Acquired

The ubiquity of both TEs and plastic in the environment allows for interactions potentially resulting in TEs being acquired by plastics through adsorption. Factors determining the adsorption of TEs to plastics include polymer type, extent of weathering, particle size and concentration, salinity, pH, dissolved organic matter, and temperature (Yang et al., Reference Yang, Cang, Sun, Dong, Ata-Ul-Karim and Zhou2019; Wang et al., Reference Wang, Yang, Cheng, Zhang, Zhang, Jiao and Sun2019a; Guo et al., Reference Guo, Liu and Wang2020; Wang et al., Reference Wang, Zhang, Wangjin, Wang, Meng and Chen2020b; Aghilinasrollahabadi et al., Reference Aghilinasrollahabadi, Salehi and Fujiwara2021). The rate at which polymers adsorb TEs generally occurs rapidly, and there is a constant transfer of TEs between the plastics and matrices they are in contact with (Guo et al., Reference Guo, Liu and Wang2020).

There are three key mechanisms by which TEs have been reported to accumulate in plastics (Figure 1). Firstly, sorption through surface complexation or electrostatic interactions can occur when charged TEs interact with polar or charged regions on plastic surfaces (Zhang et al., Reference Zhang, Pap, Taggart, Boyd, James and Gibb2020a; Cao et al., Reference Cao, Zhao, Ma, Song, Zuo, Li and Deng2021). Charged regions on polymer surfaces arise from the presence of alkene (C=C), carbonyl (C=O) and hydroxyl (-OH) functional groups which can result from environmental weathering (Bandow et al., Reference Bandow, Will, Wachtendorf and Simon2017). Certain plastics (polystyrene [PS], polyethylene terephthalate [PET] and polyvinyl chloride [PVC]) contain polar regions inherent to the polymer chain (Brennecke et al., Reference Brennecke, Duarte, Paiva, Caçador and Canning-Clode2016; Liu et al., Reference Liu, Shi, Wang, Dai, Li, Li, Liu, Chen, Wang and Zhang2021). The presence of charged additives and other contaminants also results in charged regions on polymer surfaces enabling electrostatic interactions with TEs (Holmes et al., Reference Holmes, Turner and Thompson2012; Lin et al., Reference Lin, Li, Hong, Yuan, Sun, He, Xue, Lu, Liu and Yan2022). Liu et al. (Reference Liu, Wu, Chen, Guo, Zhao, Lin, Li, Zhao, Lv and Huang2022) investigated the adsorption of cadmium (Cd), copper (Cu), chromium (Cr) and lead (Pb) to polypropylene (PP), PS and PVC microplastics (MPs) and identified that halogen bonds and π- π interactions also contribute to the adsorption of TE to plastics in addition to electrostatic interactions. Trace elements can also become associated with environmental plastics through sorption to biofilms and hydrous oxides on the surface of the plastics (Ashton et al., Reference Ashton, Holmes and Turner2010; Guan et al., Reference Guan, Qi, Wang, Wang, Wang, Lu and Qu2020).

Figure 1. Interactions of trace elements with surface functional groups and charged regions of plastics in the environment. Adapted from Cao et al. (Reference Cao, Zhao, Ma, Song, Zuo, Li and Deng2021).

Adsorption of TEs is dependent on the polymer type and the TE. For example, adsorption of Cu was much greater for polyamide (PA) and polymethyl methacrylate (PMMA) (323.6 and 41.03 μg/g, respectively) compared to polyethylene (PE), PS, PET and PVC (<10 μg/g) (Yang et al., Reference Yang, Cang, Sun, Dong, Ata-Ul-Karim and Zhou2019). This enhanced adsorption was attributed to the polar surface functional groups of PA and PMMA. In contrast, greater amounts of strontium (Sr) adsorbed onto PP and PS than PA (52.4, 51.4 and 31.8 μg/g, respectively) (Guo et al., Reference Guo, Liu and Wang2020). The extent of adsorption of Cu to UV-aged PA and PMMA was correlated with the change in C=O functional groups (Yang et al., Reference Yang, Cang, Sun, Dong, Ata-Ul-Karim and Zhou2019). Similarly, UV-ageing of PET increased the adsorption capacity for Cu from 51.2 to 178.2 μg/g, as well as Zn from 32.7 to 81.5 μg/g (Wang et al., Reference Wang, Zhang, Wangjin, Wang, Meng and Chen2020b).

In laboratory studies, it has been demonstrated that water chemistry plays a key role in the adsorption of TEs to plastics. Changing pH alters the adsorption of different TEs, with maximum adsorption generally reached at pH 6–10 for the studied TEs, Cd, Cu, Pb and Zn (Gao et al., Reference Gao, Li, Sun, Zhang, Jiang, Cao and Zheng2019; Wang et al., Reference Wang, Yang, Cheng, Zhang, Zhang, Jiao and Sun2019a; Wang et al., Reference Wang, Zhang, Wangjin, Wang, Meng and Chen2020b). At lower pH, adsorption was less due to the presence of H+ ions which may outcompete positively charged TEs for binding sites. Conversely at higher pH, TEs begin to form hydroxyl complexes and precipitate, in some cases reducing adsorption (Wang et al., Reference Wang, Zhang, Wangjin, Wang, Meng and Chen2020b). Changes in salinity and ionic strength can alter the adsorption of metal ions to plastics (Wang et al., Reference Wang, Yang, Cheng, Zhang, Zhang, Jiao and Sun2019a). Adsorption of Cd to high-density polyethylene (HDPE) was reduced from approximately 68 μg/g to 11 μg/g by the addition of 1 mg/L sodium chloride (NaCl) due to Na outcompeting Cd for binding sites on the surface and Cl complexing with Cd (Wang et al., Reference Wang, Yang, Cheng, Zhang, Zhang, Jiao and Sun2019a). This increased ionic strength of high saline conditions also compresses the electrical double layer surrounding plastic particles, lowering repulsive forces and increasing aggregation of plastics, leading to a decrease in surface area and hence adsorption capacity (Alimi et al., Reference Alimi, Farner Budarz, Hernandez and Tufenkji2018). While increased dissolved organic matter (DOM) concentration in solution increased the sorption of Ag to PS MPs, the increased sorption was attributed to greater adsorption of silver (Ag) to DOM bound to PS MPs (Abdolahpur Monikh et al., Reference Abdolahpur Monikh, Vijver, Guo, Zhang, Darbha and Peijnenburg2020).

The presence of additives within the polymer may also play a role in the adsorption of TEs. For example, adding the flame retardant hexabromocyclododecane to virgin PS increased the adsorption of Cu, nickel (Ni) and Zn (Lin et al., Reference Lin, Li, Hong, Yuan, Sun, He, Xue, Lu, Liu and Yan2022). The enhanced adsorption was attributed to the polar bromine groups within the flame retardant. No other studies considering the effects of additives on TE adsorption were able to be found.

The ambient temperature may determine the extent of adsorption of TEs to plastics as TE adsorption is endothermic (Liu et al., Reference Liu, Shi, Wang, Dai, Li, Li, Liu, Chen, Wang and Zhang2021). In a laboratory study, increasing the temperature from 288 K to 318 K increased the adsorption of Zn from 74.8 to 153.7 μg/g and Cu from 119.4 to 268.4 μg/g onto PET (Wang et al., Reference Wang, Zhang, Wangjin, Wang, Meng and Chen2020b). Similarly, increasing the temperature was also reported to increase the adsorption of Pb and aluminium (Al) onto PET, PA and ethylene vinyl acetate (EVA), albeit to a lesser extent (Öz et al., Reference Öz, Kadizade and Yurtsever2019). In agreement with the laboratory studies, a marine field-based study off the coast of China related the higher concentration of Cd and arsenic (As) adsorbed at one of the three sites to the higher temperatures (Gao et al., Reference Gao, Li, Sun, Zhang, Jiang, Cao and Zheng2019).

The study by Gao et al. (Reference Gao, Li, Sun, Zhang, Jiang, Cao and Zheng2019) is currently the only published field study measuring the accumulation of TEs onto plastics. Peak concentrations for As, Cd, Cr, Cu, manganese (Mn), Pb and Zn were 0.037, 0.023, 0.084, 0.223, 31.3, 0.441 and 0.014 μg/g, respectively, over 9 months. Chromium and Pb had the greatest adsorption to both plastics, peaking at 3 months and plateauing thereafter. The concentration of TEs adsorbed changed with respect to changing concentration in the surrounding water, showing a dynamic equilibrium between the two. Exceptions were Cu and Mn accumulation, having no correlation to the surrounding water concentrations.

Trace element concentrations of environmental plastics

The concentrations of TEs associated with environmental plastics for a range of environmental compartments from multiple studies are summarised in Table 2. Factors identified as influencing the sorption of TEs to environmental plastics include polymer type, population density, nearby land uses and ambient TE concentrations (Ashton et al., Reference Ashton, Holmes and Turner2010; Yang et al., Reference Yang, Cang, Sun, Dong, Ata-Ul-Karim and Zhou2019; Carbery et al., Reference Carbery, MacFarlane, O’Connor, Afrose, Taylor and Palanisami2020). Colour was reported multiple times to be the source of high concentrations of many TEs (Cd, Cr, Cu, molybdenum [Mo], Pb, Sb and selenium [Se]) due to the presence of TE-based pigments inherent in the polymer (Filella and Turner, Reference Filella and Turner2018; Fernandes et al., Reference Fernandes, Farzaneh and Bendell2020; Catrouillet et al., Reference Catrouillet, Davranche, Khatib, Fauny, Wahl and Gigault2021). A key finding is that the concentrations of TEs acquired from the environment are, in most cases, orders of magnitude lower than those that are inherently present. For example, barium (Ba), Cd, Cr, mercury (Hg), Pb, Sb and Zn are common additives to plastics. The maximum concentrations (μg/g) for acquired and inherent TEs, respectively, were Ba (59 and 143,000), Cd (11 and 6,760), Cr (14.2 and 77,100), Hg (0.1 and 810), Pb (151 and 23,500), Sb (0.5 and 27,100) and Zn (288 and 26,700). The orders of magnitude differences between adsorbed and inherent TE concentrations (Table 2) highlight that intentionally added TEs present a greater threat to the environment. Concentrations of TEs adsorbed on environmental plastics are lower than laboratory-based studies, indicating the adsorption capacities of plastics demonstrated in the laboratory are not environmentally relevant.

Table 2. Comparison of trace elements acquired from the environment and total (acquired and inherent) for plastics collected from different environments

Release of plastic-associated trace elements

Trace elements can be released from plastics due to changes to either plastic properties (weathering and fragmentation) or environmental conditions (UV exposure, temperature, pH, salinity, and ionic strength). The extent of desorption will depend on the source of the TE (acquired vs inherent) as well as the environmental setting. As the concentrations of some TEs (Cd, Cr, Hg, Mn, Pb, Sb, Se and Zn) inherent within plastics can be elevated, these TEs will diffuse from the plastic into the surrounding environment due to the concentration gradient (Bridson et al., Reference Bridson, Gaugler, Smith, Northcott and Gaw2021). For example, inherent Cu, Mn, Ni, Pb and Zn were released from virgin PVC incubated in alkaline paddy soils (Meng et al., Reference Meng, Xu, Liu, Li, Sy, Zhou and Yan2021) and Sb is leached from PET bottles into bottled water (Westerhoff et al., Reference Westerhoff, Prapaipong, Shock and Hillaireau2008).

The extent of environmental weathering will determine the proportion of inherent TEs released, as weathering exposes more of the interior of the plastic. For example, the release of a Cd-containing pigment was greater from acrylonitrile butadiene styrene (ABS) particles that were mechanically abraded (0.64 μg/mL) compared to new particles (0.112 μg/mL) (Fowles, Reference Fowles1977). Correspondingly more adsorbed Zn was released from aged low-density polyethylene (LDPE) (71.9%) compared to unaged LDPE (10.8%) into an artificial stormwater solution (Aghilinasrollahabadi et al., Reference Aghilinasrollahabadi, Salehi and Fujiwara2021).

UV exposure will also alter the proportion of inherent TEs released, especially for photosensitive compounds such as cadmium sulfide (CdS) and cadmium selenide (CdSe) (Halpin and Carroll, Reference Halpin and Carroll1974), two commonly used pigments. UV irradiation of ABS increased Cd release from 0.332–2.26 to 5.6–17.6 μg/mL (Fowles, Reference Fowles1977). Similarly, 6 hours of UV exposure resulted in four to five times greater release of Sb from PET bottles compared to bottles with no UV exposure (Westerhoff et al., Reference Westerhoff, Prapaipong, Shock and Hillaireau2008). This enhanced release of Sb was attributed to the oxidation of the catalyst residue in PET, Sb2O3 (oxidation state Sb(III)), likely to Sb(OH)6− (oxidation state Sb(V)), thus increasing its solubility (Hu et al., Reference Hu, Kong and He2014). Increased temperature can also enhance the release of TEs. For example, higher concentrations of Sb were released from PET at 80 °C (7.8–9.7 ppb) compared to 22 °C (0.5–0.64 ppb) (Westerhoff et al., Reference Westerhoff, Prapaipong, Shock and Hillaireau2008) as well as greater release of Cd at 37.5 °C (0.332–0.64 μg/mL) compared to 19 °C (0.124–0.22 μg/mL) (Fowles, Reference Fowles1977).

In addition to solution pH having a key role in the adsorption of TEs, it also determines the extent of release of TEs from plastics. At low pH values, H+ ions exchange with TEs already adsorbed to the plastic, and at high pH, TEs can form insoluble hydroxides. Similar competition between ions for binding sites is observed with increased salinity, mainly increased NaCl concentrations (Holmes et al., Reference Holmes, Turner and Thompson2014; Wang et al., Reference Wang, Yang, Cheng, Zhang, Zhang, Jiao and Sun2019a). The smaller hydrated ion radius of Na+ results in the exchange with many TEs adsorbed to plastics, such as Cd (Nightingale, Reference Nightingale1959; Wang et al., Reference Wang, Yang, Cheng, Zhang, Zhang, Jiao and Sun2019a). The formation of Cl complexes with inherent TEs can also result in the release of TEs in saline conditions.

Ingestion of plastics by organisms can result in a greater and more rapid release of TEs and release TEs that may otherwise not be released due to extreme gut conditions such as lower pH and increased surface area resulting from chewing or grinding. The elevated concentrations of inherent TEs may also result in acute exposures (Jones and Turner, Reference Jones and Turner2010; Holmes et al., Reference Holmes, Thompson and Turner2020; Smith and Turner, Reference Smith and Turner2020). Simulated gastric fluids of marine seabirds have been demonstrated to release significant proportions of TEs from <1–78% of Cd, cobalt (Co), Cr, iron (Fe), Mn, Pb and Sb (Turner and Lau, Reference Turner and Lau2016; Shaw and Turner, Reference Shaw and Turner2019; Holmes et al., Reference Holmes, Thompson and Turner2020; Smith and Turner, Reference Smith and Turner2020; Turner et al., Reference Turner, Holmes, Thompson and Fisher2020). These studies had incubation times from 120 to 220 hours; however, a large proportion of each TE was released rapidly followed by a gradual release to steady-state. Release of Pb was the most rapid, reaching a steady-state within the first time point (0.25 hours) (Holmes et al., Reference Holmes, Thompson and Turner2020). Similarly, Cd (0.009–0.53%), Cu (14–19%) and Zn (14–16%) were released from plastics into simulated gastric fluids of marine invertebrates over a 5- to 6-hour period (Jones and Turner, Reference Jones and Turner2010; Martin and Turner, Reference Martin and Turner2019). Again, a rapid release of TEs occurred, with steady-state reached within 30 minutes in some cases, with the exclusion of the continual release of Cu over the 5-hour period (Jones and Turner, Reference Jones and Turner2010). In addition to organisms ingesting plastic and associated TEs in a single region, they can also be transported long distances by organisms. For example, shearwaters (family Procellariidae) can travel over 1,700 km to forage for food for their chicks, and frequently consume plastic (Skira, Reference Skira1986). This foraging behaviour can result in the translocation of plastics and associated TEs between significantly different ambient environmental contamination levels circumventing gradual loss across gradients. Consequently, this may result in food contaminated with high TE loading relative to the local environment being fed to their young in addition to uptake into the foraging adult. Further research on the bioavailability of plastic-associated TEs to organisms is needed. The majority of studies to date have used simple gastric simulants. The more complex nature of digestive tracts also needs to be considered including short-term release due to regurgitation, release into other parts of the digestive tract and more studies on the complex nature of seabird guts, including high lipid contents (Smith and Turner, Reference Smith and Turner2020).

Impacts of plastics and trace elements

Understanding the potential impacts of plastic-associated TEs is critical for determining the threat plastic pollution pose to ecosystems. There is however a significant knowledge gap in this area. Studies examining the impacts of plastic and TEs have, to date, focused on the determination of the interactions between plastics and TEs through co-exposure experiments (plastics and TEs added together as separate contaminants) rather than using plastics with bound TEs (either acquired or inherent). Such experiments do not accurately represent real-life exposure conditions resulting from plastic pollution. No synergistic negative effects were reported for the majority of these studies (Oliveira et al., Reference Oliveira, Barboza, Branco, Figueiredo, Carvalho and Guilhermino2018; Fu et al., Reference Fu, Zhang, Fan, Qi, Wang and Peng2019; Wang et al., Reference Wang, Ding, Xiong, Zhu, Li, Jia, Zhu and Xue2019b; Lian et al., Reference Lian, Wu, Zeb, Zheng, Ma, Peng, Tang and Liu2020; Sıkdokur et al., Reference Sıkdokur, Belivermiş, Sezer, Pekmez, Bulan and Kılıç2020; Zhang et al., Reference Zhang, Wang, Chen, Yang and Wu2020b; Cheng et al., Reference Cheng, Feng, Duan, Duan, Zhao, Wang, Gong and Wang2021; Dong et al., Reference Dong, Gao, Qiu and Song2021; Yang et al., Reference Yang, Xu and Yu2022). However, some did see significant reduction in plant root mass, decreased growth and increased TE accumulation in tissues, bioavailability and mortality rate (Lu et al., Reference Lu, Qiao, An and Zhang2018; Abbasi et al., Reference Abbasi, Moore, Keshavarzi, Hopke, Naidu, Rahman, Oleszczuk and Karimi2020; Dong et al., Reference Dong, Gao, Song and Qiu2020; Tunali et al., Reference Tunali, Uzoefuna, Tunali and Yenigun2020; Zhou et al., Reference Zhou, Liu and Wang2020; Wang et al., Reference Wang, Zhang, Zhang, Zhang and Sun2020a; Li et al., Reference Li, Liu, Xu, Wang and Yu2021). The key difference between the majority of studies reporting impacts and those that reported no effect was the choice of polymer. Most studies reporting reduced or no adverse toxicological effects used polymers with active surface functional groups (PS, PET and PVC). On mixing, the TEs may have adsorbed to the plastics, potentially reducing their bioavailability and consequently the toxicological effect of the TEs themselves (Dong et al., Reference Dong, Gao, Qiu and Song2021). This is not environmentally relevant as the surface functional groups of plastics will become fully saturated soon after entering the environment (Guo et al., Reference Guo, Liu and Wang2020). Therefore, the presence of plastics would not decrease the exposure of TEs to organisms as suggested in the cited studies as the majority of TEs will likely remain waterborne. In contrast, toxic effects of TEs were more frequently reported for studies using PE, which has a low level of surface functional groups. In addition, the above studies also do not take into account TEs that are possibly inherent within the virgin plastic used which could be causing effects as well.

There is a significant lack of data on the impacts of plastic-associated TEs on organisms. To date, only two studies on the impacts due to plastic-associated TEs have been published (Wang et al., Reference Wang, Dong, Wang, Ren, Qin and Wang2020c). Polyethylene MPs with sorbed Cd were more toxic to water fleas (Moina monogolica) in comparison to virgin PE-MPs. Exposure of virgin PVC to zebrafish (Danio rerio) resulted in increased metallothionen levels, a metal-binding protein, due to the release of inherent Pb (Boyle et al., Reference Boyle, Catarino, Clark and Henry2020). The paucity of data on the toxicity of plastic-associated TEs is a critical data gap as inherent TEs can be present at elevated concentrations.

The environmental impacts of TEs adsorbed to plastics are likely to be lower than for inherent TEs. When considering the impacts of adsorbed TEs on the environment it could be assumed that they have comparable impacts to TEs sorbed to natural organic matter as natural organic matter and plastics have similar environmental cycling and fate. The higher concentrations of inherent TEs are of greater environmental concern. These TEs are released over time and in some cases rapidly. Environmental conditions that can change rapidly leading to greater release include pH, ionic strength, redox potential, UV exposure and salinity. Settings where these rapid changes can happen include plastic release from sediment to water, transport from freshwater to saltwater, rapid biofilm removal, discharges from WWTPs and landfills, and ingestion. Ingestion should also be highlighted as a significant source of high TE exposure as organisms may incorrectly select plastic as food based on colour (Okamoto et al., Reference Okamoto, Nomura, Horie and Okamura2022) and high inherent TE concentrations are frequently due to their use as pigments. Conversely, environments where these conditions are more stable, such as sediments and groundwater, will slow down the release of TEs, resulting in lesser impacts.

New routes by which organisms are exposed to plastic-associated TEs are of further concern. This includes the above ingestion pathway, but also from direct chemical transfer to organisms without the protection of an exoskeleton that uses environmental plastics for shelter, for nest building or as a surface to live on (Reynolds et al., Reference Reynolds, Ibáñez-Álamo, Sumasgutner and Mainwaring2019). Even if the TE exposure is low relative to other pathways, it is an additional exposure route that would not occur in the absence of plastic and the TEs that they contain.

More environmentally relevant studies are required to address the significant knowledge gaps that exist. This includes the departure from the use of virgin plastics with no adsorbed TEs and unknown inherent TEs, to the use of plastics containing known levels of inherent or acquired TEs under ecologically relevant conditions.

Conclusions

Trace elements that are deliberately added or inherent within plastics are present at much higher concentrations than those acquired in the environment and may therefore have a greater impact on organisms and the environment due to continuous release over time. Factors that determine the extent of TE release include the source of TEs, properties of the associated plastic, the extent of weathering and environmental conditions such as pH and redox potential. Key knowledge gaps identified by this review include the significance of the release of inherent TEs from plastics into environmental matrices over long periods of time, how rapid changes in conditions may cause localised TE hotspots, determining the bioavailable portion of released TEs into marine organisms, determining if plastics provide previously unrecognised exposure pathways to organisms, including direct chemical transfer to organisms in contact with plastics and associated TEs, and lastly determining the impacts of inherent TEs and environmentally acquired TEs towards organisms. As plastics are now considered to be ubiquitous environmental contaminants, further research on the environmental impacts of plastic-associated TEs is urgently needed.

Open peer review

To view the open peer review materials for this article, please visit http://doi.org/10.1017/plc.2023.15.

Data availability statement

Data availability is not applicable to this article as no new data was created or analysed in this study.

Author contribution

Hayden Masterton: Conceptualisation, Writing - Original Draft, Writing - Review & Editing. Helena Ruffell: Writing - Original Draft, Writing - Review & Editing. Sally Gaw: Conceptualisation, Writing - Review & Editing. Olga Pantos: Conceptualisation, Writing - Review & Editing.

Financial support

This work was conducted as part of the Aotearoa Impacts and Mitigation of Microplastics (AIM2) project, in receipt of funds from a New Zealand Ministry of Business, Innovation and Employment (MBIE) Endeavour Research Programme grant (C03X1802).

Competing interest

Conflicts of interest: None.

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Figure 0

Table 1. Trace elements used as additives within plastics products, including the most commonly used compounds and chemical formula

Figure 1

Figure 1. Interactions of trace elements with surface functional groups and charged regions of plastics in the environment. Adapted from Cao et al. (2021).

Figure 2

Table 2. Comparison of trace elements acquired from the environment and total (acquired and inherent) for plastics collected from different environments

Author comment: The role of plastics in the accumulation and release of trace elements in the environment — R0/PR1

Published online by Cambridge University Press:  03 August 2023
DOI: https://doi.org/10.1017/plc.2023.15.pr1 [Opens in a new window]
Hayden Masterton [Opens in a new window] Institute of Environmental Science and Research, New Zealand
Revision round: 0
Role: author

Comments

No accompanying comment.

Review: The role of plastics in the accumulation and release of trace elements in the environment — R0/PR2

Published online by Cambridge University Press:  03 August 2023
DOI: https://doi.org/10.1017/plc.2023.15.pr2 [Opens in a new window]
Reviewer_1
Date of review:  13 February 2023
Revision round: 0
Role: reviewer
Recommendation/decision: reject

Conflict of interest statement

Reviewer declares none.

Comments

General

The review aimed to clarify the role of plastics in the accumulation, transport and release of trace elements in the environment. The authors discuss some interesting topics about the trace elements inherent in plastics and those that are acquired from the environment, and differentiated the concentration and types in plastics. However, the review paper missed important information such as following. Firstly, the discussion on the main text is too superficial. The authors only focused on the phenomena and speculation about the plastics and trace elements, but no summary of the related data and mechanism. Secondly, the framework for the review is very messy. Most time, it is very hard for me to understand what the authors is trying to express. Finally, too many contents unrelated to the topic of the review were involved and discussed. What I would like to see is a synthesis of research data and systematic summary on the interaction mechanism and environmental behavior of plastics and trace elements, and that then predictions behind these phenomena are given.

Specific comments:

1. The title needs to be thoroughly revised. The article did not involve the accumulation of trace elements. Also, the author did not discuss the role of plastics in the transport and release of trace elements in depth, but only their concentration in plastics.

2. Page 5, section “Acquired”: How about the adsorption capacity of plastics to trace elements. Please specify. Also, the relationship between the types of trace elements adsorbed by plastics and the type of the inherent trace elements should be discussed.

3. Page 5, “regarding the comparison about acquired and inherent trace elements”: This comparison lacked the reasonable basis, which should be obtained from the results summarized by the same literature or numerous literatures.

4. Page 6, section “The fate and flow of environmental plastics”: This section has little relevance to the topic of the review. I advised to delete it or thoroughly rearranged.

5. Page 6, Line 14-16: How size and morphology of plastics affect their environmental fate? Please explain.

6. Page 7, Line 6-7: How soil porosity and size and morphology of the plastic affected the transport of plastics to groundwater? The author should provide deep discussions.

7. Page 8, section “Release of plastic-associated trace elements”: The overall discussion is simple. Authors should focus on the following questions: Which trace elements in plastics are more likely to be released into environmental media or into organisms, and the concentration levels of released trace elements? The release process over time? The release mechanism? The relationship between the release of trace elements and the surface property of plastics?

Review: The role of plastics in the accumulation and release of trace elements in the environment — R0/PR3

Published online by Cambridge University Press:  03 August 2023
DOI: https://doi.org/10.1017/plc.2023.15.pr3 [Opens in a new window]
Inês Martins [Opens in a new window] University of the Azores, Portugal
Date of review:  27 April 2023
Revision round: 0
Role: reviewer
Recommendation/decision: accept

Conflict of interest statement

Reviewer declares none.

Comments

The manuscript “The role of plastics in the accumulation, transport and release of trace elements in the environment” is an important contribution for the knowledge of how plastics became an emergent pollutant. The manuscript present a good review of previously published research on plastic environmental threat topic and summarizes the amounts of TE known to be acquired by plastic surface wordwide which can be extremely usefull to the following studies

Recommendation: The role of plastics in the accumulation and release of trace elements in the environment — R0/PR4

Published online by Cambridge University Press:  03 August 2023
DOI: https://doi.org/10.1017/plc.2023.15.pr4 [Opens in a new window]
Fengxiao Zhu Nanjing Normal University, China
Date of review:  29 April 2023
Revision round: 0
Role: Handling Editor
Recommendation/decision: major-revision

Comments

Dear authors,

Manuscript Number: PLC-22-0001

Thank you for submitting your manuscript to Cambridge Prisms: Plastics. We have completed the review of your manuscript with a decision that it needs a major revision. The reviewers' comments are appended below.

If you can fully address all the comments below, I invite you to submit a revised manuscript by May 20, 2023.

Kind regards,

Cambridge Prisms: Plastics

Reviewer comments:

Reviewer #1: The MS provides a nice overview on documented cases of plastic fragmentation mediated by macrofauna across different environments and provide useful guidelines for results reporting in future research efforts. I found the MS well written and structured; thus I recommend its full consideration for possible publication.

The review aimed to clarify the role of plastics in the accumulation, transport and release of trace elements in the environment. The authors discuss some interesting topics about the trace elements inherent in plastics and those that are acquired from the environment, and differentiated the concentration and types in plastics. However, the review paper missed important information such as following. Firstly, the discussion on the main text is too superficial. The authors only focused on the phenomena and speculation about the plastics and trace elements, but no summary of the related data and mechanism. Secondly, the framework for the review is very messy. Most time, it is very hard for me to understand what the authors is trying to express. Finally, too many contents unrelated to the topic of the review were involved and discussed. What I would like to see is a synthesis of research data and systematic summary on the interaction mechanism and environmental behavior of plastics and trace elements, and that then predictions behind these phenomena are given.

Specific comments:

1. The title needs to be thoroughly revised. The article did not involve the accumulation of trace elements. Also, the author did not discuss the role of plastics in the transport and release of trace elements in depth, but only their concentration in plastics.

2. Page 5, section “Acquired”: How about the adsorption capacity of plastics to trace elements. Please specify. Also, the relationship between the types of trace elements adsorbed by plastics and the type of the inherent trace elements should be discussed.

3. Page 5, “regarding the comparison about acquired and inherent trace elements”: This comparison lacked the reasonable basis, which should be obtained from the results summarized by the same literature or numerous literatures.

4. Page 6, section “The fate and flow of environmental plastics”: This section has little relevance to the topic of the review. I advised to delete it or thoroughly rearranged.

5. Page 6, Line 14-16: How size and morphology of plastics affect their environmental fate? Please explain.

6. Page 7, Line 6-7: How soil porosity and size and morphology of the plastic affected the transport of plastics to groundwater? The author should provide deep discussions.

7. Page 8, section “Release of plastic-associated trace elements”: The overall discussion is simple. Authors should focus on the following questions: Which trace elements in plastics are more likely to be released into environmental media or into organisms, and the concentration levels of released trace elements? The release process over time? The release mechanism? The relationship between the release of trace elements and the surface property of plastics?

Reviewer #2: The manuscript “The role of plastics in the accumulation, transport and release of trace elements in the environment” is an important contribution for the knowledge of how plastics became an emergent pollutant. The manuscript present a good review of previously published research on plastic environmental threat topic and summarizes the amounts of TE known to be acquired by plastic surface worldwide which can be extremely useful to the following studies.

Decision: The role of plastics in the accumulation and release of trace elements in the environment — R0/PR5

Published online by Cambridge University Press:  03 August 2023
DOI: https://doi.org/10.1017/plc.2023.15.pr5 [Opens in a new window]
Steve Fletcher [Opens in a new window] School of Environment, Geography and Geosciences, University of Portsmouth, United Kingdom of Great Britain and Northern Ireland
Revision round: 0
Role: Editor in Chief
Recommendation/decision: major-revision

Comments

No accompanying comment.

Author comment: The role of plastics in the accumulation and release of trace elements in the environment — R1/PR6

Published online by Cambridge University Press:  03 August 2023
DOI: https://doi.org/10.1017/plc.2023.15.pr6 [Opens in a new window]
Hayden Masterton [Opens in a new window] Institute of Environmental Science and Research, New Zealand
Revision round: 1
Role: author

Comments

No accompanying comment.

Review: The role of plastics in the accumulation and release of trace elements in the environment — R1/PR7

Published online by Cambridge University Press:  03 August 2023
DOI: https://doi.org/10.1017/plc.2023.15.pr7 [Opens in a new window]
Reviewer_1
Date of review:  30 June 2023
Revision round: 1
Role: reviewer
Recommendation/decision: accept

Conflict of interest statement

Reviewer declares none.

Comments

I thanks the serious revisons and response to my questions. In my view, the manuscirpt was thoroughly improved and can be accepted by this Journal.

Recommendation: The role of plastics in the accumulation and release of trace elements in the environment — R1/PR8

Published online by Cambridge University Press:  03 August 2023
DOI: https://doi.org/10.1017/plc.2023.15.pr8 [Opens in a new window]
Fengxiao Zhu Nanjing Normal University, China
Date of review:  04 July 2023
Revision round: 1
Role: Handling Editor
Recommendation/decision: accept

Comments

Manuscript Number: PLC-22-0001R1

Thank you for submitting your manuscript to Cambridge Prisms: Plastics. I am pleased to inform you that your manuscript is now acceptable for publication.

Kind regards,

Cambridge Prisms: Plastics

Reviewer comments:

Reviewer #1: I thanks the serious revisions and response to my questions. In my view, the manuscript was thoroughly improved and can be accepted by this journal.

Decision: The role of plastics in the accumulation and release of trace elements in the environment — R1/PR9

Published online by Cambridge University Press:  03 August 2023
DOI: https://doi.org/10.1017/plc.2023.15.pr9 [Opens in a new window]
Steve Fletcher [Opens in a new window] School of Environment, Geography and Geosciences, University of Portsmouth, United Kingdom of Great Britain and Northern Ireland
Revision round: 1
Role: Editor in Chief
Recommendation/decision: accept

Comments

No accompanying comment.