Keywords

1 Introduction

Plastics have been widely used in various fields and applications due to especially their unique properties of low-cost, durability, lightness, hygiene, and corrosion resistance. The global production of plastic products exceeds 3.48 × 108 tons per year, resulting in a considerable amount of plastic waste (Li et al. 2020). Inevitably, plastic waste enters the aquatic environment, and it is estimated that more than 2.5 × 105 tons of plastics are floating on the global ocean surface (Eriksen et al. 2014). Large plastic debris can be easily removed from the environment, whereas plastic debris with small size is prone to be ignored and difficult to remove from environmental matrices.

Microplastics, commonly defined plastic particles less than 5 mm, receive increasing public attention and research interests all over the world (Zhang et al. 2021). Microplastics are derived from primary microplastics and secondary microplastics (Hamidian et al. 2021). The former was originally made of small-sized polymers for special purposes such as pharmaceuticals and personal care products. The latter is mainly created by fragmentation and degradation of large particles of plastics. Microplastics have been found in a wide range of ecosystems, including aquatic systems (oceans, rivers, and lakes), soils and sediments, and atmospheric air (Wang et al. 2021a). Initially, the majority of research was focused on the maritime environment. As a result of the interest in the sources and transfer channels of microplastics, more attention is being directed to additional environmental compartments.

Microplastics induce significant public attention owing to the persistence and ubiquity in water environment and the threats to ecosystems. Because of their small size, microplastics can be ingested by organisms and aquatic creatures, potentially accumulating in the food chain (Ribeiro et al. 2019). The negative consequences of microplastics involve physical injury to the gastrointestinal tract of organisms, and toxic impacts resulting from polymers and additives in the microplastics (Barboza et al. 2020). Additionally, microplastics can be carriers to concentrate and transfer pollutants. Therefore, the interaction between microplastics and pollutants and the removal of microplastics have attracted much attention (Jiang et al. 2022a, b; Bian et al. 2022). With increasing awareness of the worldwide distribution and potential risk of microplastics, more study is being conducted on identifying the origins and transfer pathways, disclosing the threats to ecosystems, attempting to regulate microplastics discharges, and removing microplastics from the environment.

Bibliometric analysis is an important method to reveal the past research evolution of microplastics, providing a better understanding of the emerging research areas. Mathematical techniques are employed to examine the published documents based on macro-perspective. The bibliometric method is related to informetrics and scien-tometrics, which provide fundamental theory and methodology for bibliometric analysis (Hood and Wilson 2001). A bibliometric analysis of the literature on a certain issue provides vital information for the topic and research progress (Ertz and Leblanc-Proulx 2018). Currently, numerous review papers (Andrady 2011; Cole et al. 2011) and bibliometric studies (Pauna et al. 2019; Palmas et al. 2021) on microplastics have been carried out.

Publication performance on microplastics and the main focuses and their development trends were studied by using the Science Citation Index Expanded (SCI-EXPANDED). This study addresses the data analysis of microplastics documents in terms of different criteria, identifying the occurrence and types of microplastics, emerging interests of the research fields, the research gap in the current state, and future perspective.

2 Materials and Methods

This work conducts a data-driven bibliometric study based on a literature review, aiming to explore indicators for further research on microplastics. The literature data were obtained from the SCI-EXPANDED database in Web of Science, which is one of the most important databases for scientific research. The database of the Web of Science is listed in Supporting Material (Text S1). The journal impact factor in 2020 was based on Journal Citation Reports in 2020. The published literature was collected after June 30, 2021. It was pointed out that the SCI-EXPANDED was useful to search published literature but not employed for bibliometric studies (Ho 2020a). Therefore, it is essential to have a data treatment but have data directly from the database of SCI-EXPANDED for bibliometric studies. It was reported that “front page” containing paper title, abstract, and author keywords could be used as a filter for bibliometric studies (Ho 2020b; Fu et al. 2012). KeyWords Plus supplied supplementary search items extracted from paper titles cited by authors and amplified author-keyword and title-word indexing (Garfield 1990). The documents searched by KeyWords Plus were not closely relevant to the target topic (Fu and Ho 2015). The search keywords “microplastics,” “microplastic,” “micro-plastics,” and “micro-plastics” were searched by topic in the database. It resulted in 4972 documents from 1991 to 2020, which may be relevant to the topic. 4026 documents were used as microplastics publications for a bibliometric study because no search keywords were found on the ‘front page’ of 946 documents. The document records were downloaded and manually coded for analysis using Microsoft Excel 2016 (Li and Ho 2008; Ho 2021).

For one corresponding author, it was used as corresponding author (Ho 2012), while the last one of multiple corresponding authors was used as corresponding author (Ho 2019). As to single-author articles, the author was designated as the first and corresponding author (Ho 2014a). For affiliations, England, Scotland, Northern Ireland, Wales, and the Falkland Islands were assigned to the United Kingdom. Greenland was assigned to Denmark (Tchuifon Tchuifon et al. 2017). Additionally, affiliations from the USSR were assigned to Russia or Ukraine (Wambu et al. 2017). Four citation indicators were defined in this work: (1) C0, (2) Cyear, (3) TC2020, and (4) CPPyear (Ho 2013, 2014b; Wang and Ho 2011). The citation indicators were used for bibliometric analysis.

3 Results and Discussion

3.1 Documents Summary

The documents used for bibliometric analysis were collected for the period 1991–2020. The total number of filtered documents relevant to microplastics is 4026. The citation indicator of CPPyear can be employed to describe the citations per publication more accurately (Ho and Ho 2015). Recently, the author number of each publication was also applied to analyze the types of documents related to specific topic (Monge-Nájera and El Ho 2017). As listed in Table 1, 14 types of documents were involved, and the type of article was the top one with an APP of 5.3, accounting for 84% of 4204 documents. Since some documents could be assigned to different document types, the total percentage was higher than 100% (Usman and Ho 2020). For example, 83 documents were assigned to proceedings papers and articles. The document type of retracted publications had the highest CPP2020 of 124 which is due to the highly cited retracted publication with a TC2020 of 100 or more by Lönnstedt and Eklöv (Lönnstedt and Eklöv 2016). Document type corrections had the highest APP of 6.1. In addition, each microplastics-related article had an average of 5.3 authors. The study reported by Gorsky et al. had a maximum author count of 145 (Gorsky et al. 2019). The 3546 articles were employed for bibliometric study, and this is due to the complete structure of article-type research.

Table 1 Citations and authors according to document type
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3.2 Language of Publications

Many bibliometric studies regard publication languages as one basic content (Wang and Ho 2011). There were nine languages in use, and English accounted for 99% of the 3546 articles. Some other languages were as follows: Russian (11 articles), German (9 articles), Japanese (6), Chinese (5), French (5), Ukrainian (4), Korean (1), and Spanish (1). The CPP2020 of articles in English was 32, remarkably higher than that of non-English articles (2.5). Moreover, the APP of articles written in English was 5.3, higher than that of non-English articles (3.0). It should be noted that most of the journals in the Clarivate Analytics database are published in English.

3.3 The Variation of Publications

Figure 1 presents the variation of TP and CPP2020. A significant increase in the number of articles was observed from 109 in 2015 to 1372 in 2020, indicating that microplastics receive increasing attention in the research field. The increase in the number of articles can be attributed to researchers’ finding a new topic or developing research interests in microplastics. In 2014, with 70 articles, we had the highest CPP2020 of 190. Three of the top ten cited articles were published in 2014, including articles by Eriksen et al. (2014), Cozar et al. (2014), and Van Cauwenberghe and Janssen (2014), which ranked third, fourth, and tenth, respectively. A total of 1650 microplastics articles (47% of 3546 articles) were not cited by published studies in the publication year (C0 = 0) (Ho and Kahn 2014).

Fig. 1
A pareto chart plots the number of articles and citations per publication versus year. Number of articles is plotted as a line that passes through the points (1991, 0), (2015, 100), (2017, 350), (2019, 1300). The data of C P P 2020 are as follows. (1993, 5), (2003, 35), (2011, 162), (2014, 190).

The number of articles and citations per publication by year. CPP2020: number of citations (TC2020) per publication

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3.4 The Subject Category of Web of Science

A total of 9531 journals were indexed Journal Citation Reports in 2020, and 178 subject categories were involved. The relationship between article number in a specific subject category and publication year provides some information about research trends and the interactions (Ho et al. 2010). Table 2 shows the top ten subject categories. In 2020, the environmental sciences category was the most productive category with 2449 articles (69% of 3546 articles), followed distantly by other categories. This implies that microplastics become emerging pollutants and gain great attention due to potential environmental threats. Compared to the top ten categories, microplastics articles in the multidisciplinary sciences category had the highest CPP2020 (55), and it was followed by the environmental engineering category (CPP2020 of 50). The APP in the environmental engineering category was 5.7, and the metallurgy and metallurgical engineering category had an APP of 3.8. Figure 2 shows the variation trend of the top five subject categories. Microplastics-related studies were reported chiefly in the category of environmental sciences. The category of environmental sciences has been the most popular since 2011. The first microplastics article in the category of environmental sciences was published in 2006 by Ng and Obbard in Marine Pollution Bulletin (Ng and Obbard 2006). Furthermore, the category of multidisciplinary chemistry become popular in recent years, ranking tenth in 2020. The of multidisciplinary materials science category published 217 microplastics-related articles and ranked fourth, but ranked 11th in 2020 since only 27 articles were reported. Journals could be assigned to different subject categories, and hence the total percentage was larger than 100%. For example, Water Research journal was assigned to the environmental engineering category, the environmental sciences category, and the water resources category.

Table 2 The top ten productive Web of Science category
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Fig. 2
A line graph plots the number of articles versus year. The data are as follows. Environmental sciences (2016, 150), (2018, 600). Marine and freshwater biology (2016, 50), (2020, 270). Environmental engineering (2020, 200). Multidisciplinary materials science (2020, 10). Toxicology (2020, 100).

Microplastic articles in the Web of Science categories

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3.5 Analysis Based on Journals

A total of 3546 articles related to microplastic researches were reported in 566 journals. These journals covered 112 subject categories of Web of Science. The top five journals publishing more than 100 microplastics-related articles included: Marine Pollution Bulletin (IF2020 = 5.553) with 573 articles (16% of 3546 articles), Environmental Pollution (IF2020 = 8.071) with 426 articles (12%), Science of the Total Environment (IF2020 = 7.963) with 361 articles (10%), Environmental Science & Technology (IF2020 = 9.028) with 172 articles (4.9%), and Chemosphere (IF2020 = 7.086) with 135 articles (3.8%). All above journals were related to the field of environment, suggesting microplastics gained great growing attention and research interests due to environmental problems. Science with three articles, places first with the highest IF2020 of 47.728, followed by Nature Nanotechnology with one article (IF2020 = 39.213), and Advanced Materials with one article (IF2020 = 30.849). Microplastics-related studies were preferred by top journals, such as Science and Nature Nanotechnology, indicating the importance and popularity of the microplastics topic.

3.6 Analysis Based on Countries

The articles (0.28% of 3546 articles) without affiliation information were excluded from the analysis. The 3536 articles related to microplastic studies were from 107 countries. Among them, a total of 2539 single-country articles were from 55 countries, while 997 articles with international collaborations were from 101 countries. The top 13 productive countries, publishing over 100 articles, are displayed in Table 3. These countries included seven European countries (Germany, UK, Italy, France, Spain, Netherlands, and Portugal), three American countries (USA, Canada, and Brazil), two Asiatic countries (China and Russia), and one Oceanian country (Australia). Additionally, South Africa published 52 articles, ranking first in Africa. The indicators, including TP, IP, CP, FP, RP, and SP were applied to compare publication performance (Hsu and Ho 2014). China had the highest publication indicators, with a TP of 25%, an IP of 6%, a FP of 23%, and a RP of 23%. USA was the most collaborative country, with 230 collaborative articles and a CP of 23%. Russia was poor in collaborative studies due to the most single-author articles with SP of 22%. The variation of published articles for the top five countries is shown in Fig. 3. The annual number of microplastics-related publications was no more than 10 before 2014, primarily reported by Russia. A sharp increase was found in China after 2017. China, the USA, Germany, the UK, and Italy were also the top five countries on the total number of articles in 2020.

Table 3 Top 13 productive countries with TP > 100
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Fig. 3
A line graph plots the number of articles versus the year. The data are as follows. China (2018, 110), (2019, 225), (2020, 440). U S A (75, 2018), (2020, 175). Germany (2018, 60), (2020, 150). U K (2017, 40), (2020, 100). Italy (2017, 10), (2019, 60), (2020, 100). The values are estimated.

Comparison of development trends among the top five productive countries

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3.7 Analysis Based on Institutions

Table 4 demonstrates the top ten institutions as characterized by six indicators (Hsu and Ho 2014). Single-institution articles accounted for 33% of 3536 articles, while inter-institutionally collaborative articles accounted for 67%, suggesting that many researchers conducted collaborative studies on microplastics. It is worthwhile that the Chinese Academy of Sciences in China, the Russian Academy of Sciences in Russia, the National Research Council (CNR) in Italy, and the French Research Institute for Exploitation of the Sea (IFREMER) in France are national government institutions, rather than universities. The Chinese Academy of Sciences was ranked first and had the highest publication indicators, with a TP of 4.1%, a CP of 5.9%, a FP of 2.7%, and a RP of 2.5%. University of Chinese Academy of Sciences in China took the second position with TP of 2.4% and CP of 3.6%. Russian Academy of Sciences had the maximum publication indicators with IP of 3.1% and SP of 11%. All these 86 articles collaborated with the Chinese Academy of Sciences in China. However, the university had no institution-specific articles or single-author articles, respectively, and three first-author articles and corresponding-author articles. Chinese Academy of Sciences and the Russian Academy of Sciences was the most productive institutions, probably because they have a number of departments or branches (Li et al. 2009).

Table 4 Top ten productive institutions
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3.8 The Important Articles in 2020

The publication indicator, C2020, could offer supplementary insights into understanding the influence of highly cited articles (Ho 2012). The ranking of 3546 microplastics-related articles differed significantly for sorting by TC2020 or sorting by C2020. Among these publications, 22% articles exhibited C2020 = 0 and 17% articles had TC2020 = 0. In addition, 73% of the top 100 C2020 publications were among the top 100 TC2020 papers. A total of 2508 articles (71% of 3546 articles) contained microplastics-related keywords in the Title. 3296 articles (94% of 3507 articles with abstracts) had search keywords in the Abstract. 2106 articles (74% of 2854 articles with author keywords) had microplastics-related keywords in author keywords. Seven of the top 20 articles on TC2020 had microplastics-related keywords in all Title, Abstract, and author keywords. For example, publications reported by Eriksen et al. (. 2013), Van Cauwenberghe and Janssen (2014), Claessens et al. (2011), Woodall et al. (2014), Van Cauwenberghe et al. (2013), Farrell and Nelson (2013), and Fendall and Sewell (2009) ranked ninth with TC2020 of 589, tenth with TC2020 of 583, 13rd with TC2020 of 537, 15th with TC2020 of 532, 16th with TC2020 of 512, 17th with TC2020 of 511, and 19th with TC2020 of 465, respectively.

Figure 4 manifested the citation variation of the top ten highly cited articles with microplastics-related keywords in the Title or author keywords. A study conducted by Barnes et al. (2009) ranked first on annual citations between 2012 and 2020 in the field of microplastics. An article by Browne et al. (Browne et al. 2011) had a similar trend of increasing citations. Table 5 shows the top ten highly cited papers with microplastics-related keywords in the Title or author keywords. The top 10 publications were reported by 20 institutes derived from 11 countries. The UK reported five of the top ten highly cited publications, followed by the USA (3 articles), Finland (2), and one each by Australia, Belgium, Canada, France, Germany, Ireland, Norway, and Switzerland. It can be deduced that microplastics receive great attention in most developed countries, and this can be ascribed to the massive production and consumption of plastic products and increasing environmental awareness. The University of Plymouth in the UK reported four articles among the top ten highly cited papers, and it was followed by the Algalita Marine Research Foundation in the USA and the University of Exeter in the UK, which published two of the top ten articles, respectively.

Fig. 4
A line graph plots the number of citations versus the year. The data are as follows. Barnes et al (2009, 10), (2018, 250). Browne et al (2018, 215), (2020, 425). Cole et al (2017, 80), (2020, 245). Moore (2015, 60), (2018, 110). Lusher et al (2018, 125), (2020, 180). The values are estimated.

Citations of the top five most frequently cited articles with search keywords in their title or author keywords

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Table 5 The top ten most frequently cited articles with search keywords in their title or author keywords
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Among 3546 microplastics-related articles, four articles ranked in the top ten TC2020 and C2020, indicating the most frequently cited and most impactful articles. The four important articles in 2020, considering high citations and impacts, were discussed as below:

  1. 1.

    Accumulation and fragmentation of plastic debris in global environments (Barnes et al. 2009)

    TC2020 = 1737, rank first and C2020 = 469, rank first.

    In this work, the global plastics production and the accumulation of plastic waste were briefly surveyed. The presence of plastic debris in global environments was discussed in detail, as was the accumulation trend. It was found that the particle size of plastics in the environment decreased and the abundance and worldwide distribution of microplastics increased in the past decades. Many studies on microplastics reported the occurrence and abundance of microplastics in different regions, providing better knowledge of the sources, quantities, and distribution. More valuable and comparable data were still required due to the variation in sampling methodology. In addition, it was pointed out that the environmental consequences of microplastics were still poorly understood.

  2. 2.

    Accumulation of microplastic on shorelines worldwide: sources and sinks (Browne et al. 2011)

    TC2020 = 1281, rank second and C2020 = 423, rank second.

    Browne et al. (2011) reported a worldwide study on the sources and transfer pathways of microplastics. It was found that fibers from washing clothes were an important source of microplastics. A large amount of microplastic fibers were identified in marine environments, and most of them originated from sewage effluent because of the washing of clothes. This study offered novel insights into the sources, abundance, sinks, and pathways of microplastic into the environment. Subsequently, more research interests are paid on microplastics in the freshwater environment.

  3. 3.

    Microplastic ingestion by zooplankton (Cole et al. 2013)

    TC2020 = 789, rank sixth and C2020 = 249, rank fifth.

    Intake of microplastics by various marine biota has been widely reported by researchers, such as mussels, fish, and seabirds. Cole et al. conducted research on microplastic ingestion by zooplankton due to their important ecological role in marine food webs (Cole et al. 2013). Bioimaging techniques were employed to examine the microplastics in zooplankton in different stages, such as ingestion, egestion, and adherence. Ingestion of microplastics by zooplankton in the ocean was verified, and negative impacts included reduced function and health, transferring pollutants to predators, and the ingesting of fecal pellets. This study not only provides insights into the knowledge of microplastic contamination in aquatic environments but also induces significant attention to the problems of microplastic pollution.

  4. 4.

    Occurrence of microplastics in the gastrointestinal tract of pelagic and demersal fish from the English Channel (Lusher et al. 2013)

    TC2020 = 676, rank eighth and C2020 = 185, rank eighth.

    In this work, the abundance of microplastics in natural environments was investigated through the fish samples from the English Channel. It was reported that the studied pelagic species and demersal species had ingested microplastics. Polyamide (36%) and rayon (58%) were the most common types of ingested plastics. The potential consequences of ingesting microplastics were not studied. The widespread occurrence of microplastics and their ingestion by fish suggest that revealing the potential risks of microplastics in the marine environment is imperative.

3.9 Research Focuses and Their Trends

To better understand the research topic, the keywords in microplastics-related publications were examined. A total of 3257 articles (92% of 3546 articles from 1991 to 2020) published in the active period from 2013 to 2020 were further analyzed for research focuses and their trends. The words in article Title, Abstracts, author keywords, and KeyWords Plus were explored, and microplastics-related articles were ranked based on the study period, which was exhibited in Supplementary Material A, B, and C. The top 20 author keywords commonly mentioned in articles were listed in Table 6. Besides the keywords, including microplastic, microplastics, micro-plastic, and micro-plastics, plastic pollution was the most commonly employed author keyword in 2013–2020 (in 139 articles; 5.1%), followed by pollution (129; 4.8%), marine debris (110; 4.1%), and sediment (109; 4.0%). Based on the results of keywords, it can be deduced that microplastics gain great attention due to environmental problems. The potential pollution from microplastics becomes the key concern. Reports on microplastics date back to the 1970s, and less attention was received until the beginning of the twenty-first century (Shim et al. 2018). Motivated by the report of Thompson et al. (2004), renewed interest over the last decade has made microplastics an emerging research area with an emphasis on environmental pollution. Numerous studies have been reported on microplastics in marine environment, and this is in agreement with the keywords of marine debris. Three keywords “marine debris”, “marine litter”, and “marine pollution” suggest that great attention is paid to microplastics in the marine environment and the potential environmental pollution. The sources, fate, and potential impacts of microplastics are extensively investigated in marine environments (Auta et al. 2017). Subsequently, researchers expanded the focus to freshwater and terrestrial environments since an estimated 80% of microplastics in the marine environment derive from land (Rochman 2018). This meets the top keywords of “sediment,” “freshwater,” and “surface water.” “Freshwater” and “surface water” are used as author keywords since the period of 2015–2016, and the rank of them has increased constantly, especially for “surface water”, implying that microplastics in freshwater environment gain increasing attention in the past several years.

Table 6 Top 20 author keywords in publications related to microplastics
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With the increasing occurrence and abundance of microplastics, the potential threats to marine life gain more interest. Microplastics are of special concern due to their accessibility to many organisms and their potential for physical and toxicological injury (Habibi et al. 2022). Microplastics are of special concern due to the accessible size to many organisms with potential physical and toxicological injury (Habibi et al. 2022). The keywords “ingestion,” “fish,” and “oxidative stress” reveal great interest in the consequences of microplastics on aquatic organisms and animals. The emerging keyword “oxidative stress” since 2017 and its elevated rank suggest in-depth research on the potential threat of microplastics. “Nanoplastics” as an emerging keyword since 2015, and nanoplastics receive special interests owing to the nano-specific features, such as larger surface area, more accessible size for organisms, and difficulty in detection (Koelmans et al. 2015). “Adsorption” and “sorption” refer to the interaction of toxic chemicals with microplastics (Wang et al. 2018), and this is a hot topic relating to the toxicity of microplastics in environments.

The keywords “polyethylene” and “polystyrene” are related to the types of microplastics. The existence of different microplastics has been reported in various environments. The major polymer types of microplastics involve (1) polyethylene, (2) polystyrene, (3) polypropylene, (4) polyethylene terephthalate, (5) polyester, (6) polyvinyl chloride, and (7) polyamide. Fig. S1 shows the development of research trends of the eight polymers and microplastics. Polyethylene is the most-frequently-mentioned microplastics followed by polystyrene and polypropylene, and this agrees well with the composition of microplastics in real environment. The number of articles mentioning polyethylene, polystyrene, and polypropylene increased sharply after 2016.

Microplastics in the marine environment are mainly derived from the transport of terrestrial microplastics in the waterbody. Wastewater treat plant is a vital source of microplastics in the waterbody, and atmosphere is also a migration pathway for microplastics (Jiang et al. 2022c). The previous research on the microplastics abundance focused on the natural environments, including marine, freshwater, soil, and atmosphere environments. However, with the microplastics transportation in the food chain, current research about the abundance of microplastics in human blood, feces, and fetuses attracts more attention. The relationship between health and microplastics has been emphasized. The threat of microplastics could be divided into the direct and indirect hazards to ecosystems. In addition to the adsorption of heavy metals, metalloids, and organic pollutants on microplastics surfaces (Wang et al. 2021b), researchers are more interested in the combined toxicity of microplastics and pollutants and the migration of microplastics into organisms. Besides, the focus of research has gradually shifted from microplastic toxicity to aquatic organisms and mice to the effects of microplastics on human health (Prata et al. 2020).

Research on microplastics includes several topics: (1) the occurrence and distribution of microplastics in the marine environment and global regions; (2) the ingestion and potential threat of microplastics to marine life and the ecological environment; (3) the sources and transfer of microplastics from human habits; (4) the interactions between microplastics and other toxic substances for identifying the potential threat of microplastics; and (5) the occurrence and abundance of microplastics in soil, sediments, and the atmosphere. It should be pointed out that the identification of microplastics is highly dependent on the sampling methodology. Research data in the initial stage may be misleading due to inappropriate sampling. With significant advances in sampling technologies, more valuable and comparable data can be reported (Barnes et al. 2009). With better knowledge and an in-depth understanding of the wide existence and penitential threats of microplastics, the control of microplastics discharge and the removal of microplastics from the environment are becoming imperative (Wang et al. 2022; Jiang et al. 2022d).

4 Conclusion

  1. 1.

    A total of 4026 documents of microplastics-related studies were collected; 14 document types were involved, and articles accounting for 84% were used for bibliometric analysis. English was the most widely used language among nine languages.

  2. 2.

    The number of articles increased sharply since 2015, indicating that microplastics received increasing attention in the research field. This can be ascribed to finding a new topic or research interests about microplastics. Web of Science category of environmental sciences was the leading category (69% of 3546 articles). This implies that microplastics become emerging pollutants and gain great attention due to potential environmental threats. Microplastics-related research was published in 566 journals, and the top three most productive journals included Marine Pollution Bulletin, Environmental Pollution, and Science of the Total Environment.

  3. 3.

    Of the 3536 articles on microplastics from 107 different countries, articles from single country accounted for 72%, while the percentage of articles with international collaborations was 28%. China was the most productive country, followed by USA, Germany, UK, and Italy. In additional, single-institution and inter-institutionally collaborative articles were 33% and 67% of these microplastics-related papers. The top 3 institutes were from China, including Chinese Academy of Sciences, University of Chinese Academy of Sciences, and East China Normal University.

  4. 4.

    The most impactful articles in 2020 were discussed. With the increasing occurrence and abundance of microplastics, the potential threats to marine life gain more interest. The major polymer types of microplastics involve polyethylene, polystyrene, polypropylene, polyethylene terephthalate, polyester, polyvinyl chloride, and polyamide. The research focus and perspectives were briefly summarized. This work provides insights into a better understanding of microplastics-related research.