environments Review The Way of Macroplastic through the Environment Simone Lechthaler 1,2,* , Kryss Waldschläger 1 , Georg Stauch 2 and Holger Schüttrumpf 1 1 Institute of Hydraulic Engineering and Water Resources Management, RWTH Aachen University, 52062 Aachen, Germany; [email protected] (K.W.); [email protected] (H.S.) 2 Department of Geography, Physical Geography and Geoecology, RWTH Aachen University, 52062 Aachen, Germany; [email protected] * Correspondence: [email protected] Received: 28 August 2020; Accepted: 22 September 2020; Published: 24 September 2020 Abstract: With the focus on microplastic in current research, macroplastic is often not further considered. Thus, this review paper is the first to analyse the entry paths, accumulation zones, and sinks of macroplastic in the aquatic, terrestrial, and atmospheric environment by presenting transport paths and concentrations in the environment as well as related risks. This is done by applying the Source–Pathway–Receptor model on macroplastic in the environment. Based on this model, the life cycle of macroplastic is structurally described, and knowledge gaps are identified. Hence, current research aspects on macroplastic as well as a sound delimitation between macro- and microplastic that can be applied to future research are indicated. The results can be used as basic information for further research and show a qualitative assessment of the impact of macroplastic that ends up in the environment and accumulates there. Furthermore, the applied model allows for the first time a quantitative and structured approach to macroplastic in the environment. Keywords: macroplastic; life cycle; sources; sinks; transport paths; knowledge gaps 1. Introduction Plastics are synthetic macromolecules consisting of compounds of monomers produced synthetically or by natural product conversion [1]. Due to their high economic and technological importance, they have a larger production volume (in m3) worldwide than steel and aluminium [2]. However, the great advantages of the material—its lightness, durability and low-cost production— are leading to an ever-increasing environmental problem: due to large production values, short product life, and unconscious handling of the material, 3% of the produced plastic leaks into the marine environment and accumulates there due to its persistence [3,4]. In order to carry out a sound analysis of this problem, some information is still missing: How is plastic waste distributed in the environment, where are hotspots and sinks in the environment, and which organisms are affected [5–9]? Furthermore, the question of “missing plastic” comes into focus [3,10], as measured concentrations of (micro-) plastic in some environmental areas do not appear to increase [11–13]. This can be explained by an overestimation of the input of macroplastic (MaP) into the ocean as well as an underestimation of plastics that accumulates at the ocean surface, the water column of the oceans, or the removal of plastic from the ocean surface by degradation, fragmentation, and increase in density [14,15]. For this reason, the question of possible sinks within the life cycle is particularly important for further understanding. In this paper, the Source–Pathway–Receptor (SPR) model is used to summarise the current state of knowledge and to reveal further research areas. The model was already applied to microplastics [16]. So this paper is a further step towards a comprehensive consideration of plastics in the environment. The initially unidirectional model can be extended over time to a multidirectional model by adding Environments 2020, 7, 73; doi:10.3390/environments7100073 www.mdpi.com/journal/environments Environments 20202020,, 77,, 73x FOR PEER REVIEW 2 of 30 model by adding new insights on sources, transport routes, and receptors as well as newinterdependencies insights on sources, between transport the different routes, components. and receptors The as wellSPR asmodel interdependencies is built up on betweenthe basis the of disources,fferent pathways components. and receptors The SPR modeland can is thus built be up used on theto assess basis any of sources, environmental pathways pollution and receptors [16,17]. andThe canmethodical thus be usedclassification to assess of any these environmental sections allows pollution a better [16, 17structuring]. The methodical of the processes classification and ofa thesestepwise sections extension. allows The a better SPR model structuring is characterised of the processes by its andsimplicity, a stepwise its flexibility, extension. and The the SPR abili modelty to isrecognise characterised relationships by its simplicity, in complex its flexibility,systems. This and themakes ability it possible to recognise to consider relationships the path in complexof MaP systems.through all This areas makes of the it possible aquatic, to terrestrial, consider theand path atmospheric of MaP through environment. all areas In of comparison the aquatic, to terrestrial, existing andreviews atmospheric or research environment. articles of plastic In comparison waste and to MaP existing in the reviews environment or research (marine articles environments: of plastic waste[18,19] and; freshwater MaP in the environments: environment (marine[20–22]; environments:terrestrial environments: [18,19]; freshwater [23] or environments: regarding ecological [20–22]; terrestrialeffects [24,25] environments:, this one looks [23] at or the regarding whole life ecological cycle and eff ectsall environmental [24,25], this one areas looks by at determining the whole life all cyclepossible and sources, all environmental pathways, areasand receptors by determining (sinks). all possible sources, pathways, and receptors (sinks). DefinitionDefinition Macroplastic. Macroplastic. WhileWhile the the size size of microplastic of microplastic is defined is defined as particles as with particles a diameter with a< diameter5 mm [26]<, MaP5 mm is commonly [26], MaP defined is commonly in distinction defined to in microplastics distinction to asmicroplastics items with a diameter as items ≥ with 5 mm a diameter 5 mm [6,27–29]. However, both size classifications are not internationally standardised. [6,27–29].≥ However, both size classifications are not internationally standardised. Regarding MaP, Regardingother definitions MaP, otherare also definitions published. are For also example, published. BarnesFor et example, al. (2009) Barnes [30] defined et al. (2009)macro [-30debris] defined with macro-debrisa diameter > with20 mm, a diameter the European> 20 mm, Commission the European (2013) Commission [31] defined (2013) it [31as ]items defined > 25 it asmm,items while> 25 other mm, whilestudies other define studies items define > 5 cm items as MaP> 5 [32] cm asor MaPsuggested [32] or to suggested define MaP to defineas items MaP > 1 ascm items [33] >(Hartmann1 cm [33] (Hartmannet al., 2019). et Furthermor al., 2019).e, Furthermore, the term mesoplastic the term mesoplasticis also used iswith also a used size withclassification a size classification of >5–25 mm, of >according5–25 mm, to according which MaP to whichis defined MaP as is a defined fraction as > 25 a fraction mm [34]>. Due25 mm to the [34 ].large Due number to the large of publications number of publicationson microplastic on microplastic(>4400 papers (> 4400in Web papers of Science in Web ofin ScienceAugust in2020, August keyword: 2020, keyword: microplastic*microplastic*), the size), the size definition of MaP 5 mm is preferable and simplifies the distinction with regard to already definition of MaP ≥ 5 mm ≥is preferable and simplifies the distinction with regard to already present presentresearch research work on work microplastics. on microplastics. In addition to the unstandardised size definitions, definitions, the unclarified unclarified terminology is also a problem, problem, as numerous terms are used synonymously beyond the the term term MaP. MaP. The terms “macro litter” [35] [35],, “anthropogenic“anthropogenic litter”litter” [[36]36],, “plastic“plastic litter”litter” [[37]37],, “marine“marine litter”litter” [[38]38],, “marine“marine plastic”plastic” [[39]39] and “plastic debris” [[40]40] areare mostmost frequentlyfrequently used used alongside alongside MaP, MaP, and and the the first first publication publication using using “macroplastic” “macroplastic” in itsin its headline headline was was only only published published in in 2012 2012 [41 [41]]. With. With this this variety variety of of terms, terms, a a clearclear focusfocus onon thethe problem with MaPMaP inin thethe environmentenvironment isis didifficultfficult toto implementimplement withinwithin research.research. Based on aa comprehensivecomprehensive literature review,review, thisthis studystudy suggests to set a newnew standardstandard with the size limit ofof particlesparticles with a diameter 5 mm and the classification with the term “macroplastic”. diameter ≥ 5 mm and the classification with the term “macroplastic”. Bibliographic Collection.Collection. The literature for this review was firstfirst compiled via searches on the WebWeb ofof ScienceScience onon 88 AugustAugust 20202020 (regarding(regarding keywords,keywords, seesee Figure1 1);); then, then, thethe suitablesuitable referencesreferences inin the papers were included. Additionally, Additionally, literature waswas sought via other sourcessources such as Researchgate or the homepages of didifferentfferent journals.journals. Of the papers found,found,