Bibliometrics and visualization analysis of microplastics research in water from 2011 to 2023

Millions of metric tons of plastic are produced annually. Countless large items of plastic debris accumulate in marine habitats worldwide and may persist for centuries (McCormick et al. 2014). Plastics are classified as macroplastics (>25 mm), mesoplastics (5–25 mm), microplastics (<5 mm), and nanoplastics (0.1 μm) (Thompson et al. 2004). Microplastics are plastic particles, fragments, and fibers with sizes or equivalent dimensions of less than 5 mm. Due to their small size and large quantities, they are easily broken down, transported, and accumulated in the environment, and have the capacity to carry other pollutants. Once microplastics enter environmental media, they pose a significant threat to ecosystems and biological health. Microplastic pollution affects humans, animals, plants, and the broader environment. Research shows that microplastics can enter organisms through air, soil, and water, threatening ecosystem integrity. For example, Prata and Dias-Pereira found that microplastics enter terrestrial and aquatic organisms via soil and water, and accumulate through the food chain, impacting plant growth and animal health. Enyoh et al. reviewed the toxicological effects of microplastics, highlighting the risks to ecological balance and organism health. Studies show that microplastics accumulate across species, causing physiological damage and threatening survival. Bamigboye et al. emphasized that microplastics entering the human body via the food chain could pose long-term health risks, including endocrine disruption and cancer. As a result, controlling microplastic pollution is critical for global environmental protection and public health (Enyoh et al. 2020; Prata & Dias-Pereira 2023; Bamigboye et al. 2024).

Microplastics are extensively present in various environmental media, including water bodies, soils, atmospheres, and even polar glaciers (Kye et al. 2023). In March 2022, the fifth session of the United Nations Environment Assembly endorsed the ‘Draft Resolution on Ending Plastic Pollution.’ The resolution advocates for the creation of an intergovernmental negotiating committee tasked with achieving a globally binding agreement by 2024. This agreement aims to comprehensively address the entire lifecycle of plastic products, spanning from their production and design to recycling and disposal. In March 2023, a research report published in the journal Public Library of Science Omnibus estimated that approximately 17 trillion pieces of plastic, primarily microplastics, are present on the ocean surface, most of which have been discarded in the ocean since 2005. This pollution has escalated to unprecedented levels over the past 15 years (Eriksen et al. 2023).

The issue of microplastic pollution has emerged as a prominent topic in interdisciplinary research, spanning global environmental science, earth science, agricultural science, and marine science. In 2022, Professor Xia Yankai's team at Nanjing Medical University identified various types and quantities of microplastics and dye particles in human blood clots (Wu et al. 2023). A study in June 2023 found that plastic containers used for baby food release significant amounts of particles when heated in a microwave (Kazi et al. 2023). In January 2024, research from Columbia University revealed that 1 L of water from three leading plastic bottled water brands in the United States contained approximately 110,000–370,000 plastic particles, of which 10% were microplastics and 90% were nanoscale plastics (Qian et al. 2024). On 28 June 2024, the 2024 Guangdong-Hong Kong-Macao Marine Environment Protection and Green Development Forum reported that 1,277 marine species have been found to ingest microplastics (Xu 2024).

Researchers from various countries have conducted extensive research on microplastics. These studies involve the environmental impacts of microplastics on ecosystems and biodiversity, including microplastics in water bodies, soil, and atmosphere, as well as their effects on marine animals, terrestrial organisms, and human health; the continuous improvement of technical means related to sampling, extraction, separation, and identification of microplastics; involving tracking the lifecycle of microplastics, research and development of microplastic alternatives, pollution control, regulations and policies, and international cooperation. Research on microplastics has made some progress, but further research and exploration are still needed in terms of a complete scientific understanding of microplastics, environmental impact assessment, and management strategies.

Bibliometrics is a scientific method for quantitatively analyzing the development and evolution of scientific research in a certain field, offering a better understanding of the research trend and emerging interests (Tan et al. 2024). CiteSpace and VOSviewer are widely used visualization software tools for creating knowledge maps. These tools enable the visual analysis of scientific literature, including publication time, keywords, author affiliations, and more. Through CiteSpace and VOSviewer, researchers can quantitatively express the research status, hotspots, existing problems, and development trends in a specific field (Wang et al. 2023a). This study employs CiteSpace and VOSviewer to analyze articles published in core journals indexed by Web of Science (WOS) from 2011 to 2023. The objective is to provide a comprehensive overview of research on ‘microplastics’ in aquatic environments, offering guidance and data support for researchers in this field and enabling them to quickly grasp the state of current research. Additionally, CiteSpace and VOSviewer facilitate the identification of collaborative relationships among countries, institutions, and researchers, thereby promoting opportunities for learning, communication, and collaboration. Keyword analysis helps clarify research focuses and emerging trends, contributing to the advancement of microplastic research and the development of practical solutions for microplastic governance and control.

The data for this article were sourced from the Web of Science Core Collection using advanced search techniques. The search terms utilized were ‘microplastic*’ and ‘micro plastics*’ (TI = microplastic* and micro plastics*), with the following search formula: ((TI = (‘microplastic*’ OR ‘micro plastic*’ OR nanoplastic* OR nano plastic* OR ‘micro-sized plastic*’)) AND TS = (‘ocean’ OR ‘sea’ OR ‘lake’ OR ‘river’ OR ‘marine’ OR ‘water’)) NOT TS = (‘coast’ OR ‘beach’ OR ‘land’ OR ‘soil’ OR ‘food’ OR ‘sediment’ OR ‘deposit’ OR ‘ground’). The search timeframe encompassed all years from 2011 to 2023, with the document type set as ‘article and review’. The filtered literature was downloaded and saved as a plain text file in the format of ‘Full Record with Cited References’ for literature analysis data samples. After deduplication using CiteSpace software and manual screening, a total of 4,309 English-language articles related to the search terms were collected for analysis. Among these, 4,017 were research papers, and 292 were review articles (as of 31 December 2023).

CiteSpace (version 6.3.R1) and VOSviewer software (version 1.6.20) were used to analyse the document types, years, authors, co-cited authors, countries, institutions, journal sources, co-cited journals, keywords, and co-cited references to form social network maps. Data aggregation and analysis were conducted in Microsoft Excel 2016, and related figures were drawn with Origin 9.0 software.

Scimago Graphica is utilized to create academic cooperation maps between various countries and regions. Circle labels represent national entities and have undergone analysis. The area of each circle, rather than its diameter, is directly proportional to the number of publications in the corresponding country. Data processed by VOSviewer is imported into Scimago Graphica, and the top ten countries with the highest publication counts are selected for visual analysis.

In analyzing major research institutions, circles denote individual institutions, each with a minimum publication threshold of 20. Visual analysis is performed on these 74 institutions, considering their cumulative publication and citation counts, and a network of collaborative relationships among them is illustrated. The minimum number of articles required for an author is set at 10.

Keywords within specific research domains accurately reflect their focal topics, making keyword co-occurrence analysis a valuable method for identifying dominant research themes. Our analysis, conducted using CiteSpace, involves the following steps:

• 1. Extract keywords from the articles and consolidate similar terms; for instance, merge ‘plastics,’ ‘plastic waste,’ ‘nanoplastics,’ ‘microplastics,’ and ‘microplastics’ into ‘microplastics.’

• 2. Construct a keyword co-occurrence network and perform keyword clustering analysis. Then, rerun the software, select ‘Pathfinder’ in cutting mode, and use the log-likelihood ratio algorithm to identify nominal terms from the keywords in the cited literature for cluster naming. Finally, choose the top seven clusters with the highest co-occurrence density.

• 3. Summarize the dominant research topics related to microplastics in aquatic environments.

The keyword with the highest explosive intensity is highlighted in red, signifying a sudden increase in its usage frequency during this period, while blue denotes a relatively low usage phase. In the parameter settings, this study defines the time span from 2011 to 2023, with yearly time slices. Burst detection algorithms are employed to analyze keywords, utilizing cosine as the connection strength. The filtering criteria for the keyword ‘Top’ are set to a threshold of 30, allowing for the selection of the top 30 keywords exhibiting the strongest citation explosion for analysis.

The initial phase (2011–2013) is the emergence of relevant papers on microplastics in water. In this period, there are two papers in 2011, and six papers in both 2012 and 2013. The content and focus of these publications primarily are the identification of microplastic pollution, investigations into the levels of microplastics in aquatic environments, surveys on microplastic content in fish, and the toxic impacts of microplastics on marine organisms.

In the second phase (2014–2017), the number of publications per year is less than 100, with an average annual increase of approximately 20 articles. This growth may be attributed to the heightened attention researchers gave to the environmental research on microplastics following the inclusion of microplastics as one of the ten global environmental issues by the United Nations Environment Programme in 2014.

The third phase (2018–2023) is an explosive growth period, with an annual doubling of publication numbers. In 2016, the Second United Nations Environment Assembly identified microplastic pollution as the second most critical scientific issue in environmental and ecological research (Shah et al. 2023). In 2017, the G20 Summit in Hamburg, Germany, adopted the ‘G20 Marine Waste Action Plan’, elevating microplastic pollution to a global priority. In 2018, global research outputs on microplastics in water surpassed a hundred papers for the first time, reaching 1,114 by 2022, representing a nearly tenfold increase over 5 years. These trends collectively indicate an escalating global concern for research on microplastic pollution in aquatic environments. The growing quantity and quality of research outputs suggest a rising level of expertise and development within this domain can foreshadow a rapid expansion of relevant research outcomes in the future.

A total of 472 journals have published articles related to research on microplastics in aquatic environments. During the initial phase from 2011 to 2013, 14 articles on microplastics in water were published in journals such as Marine Pollution Bulletin (United Kingdom, IF: 5.8), Environmental Science Technology (United States, IF: 12.6), Biology Letters (United Kingdom, IF: 3.3), Current Biology (United States, IF: 9.2), Ecological Indicators (Netherlands, IF: 6.9), PeerJ (United Kingdom, IF: 2.7), Water Air And Soil Pollution (Netherlands, IF: 2.9), among others.

During the slow development period from 2014 to 2017, a total of 199 articles were published in 53 journals, which are classified among the 73 Web of Science categories. As shown in Table 1, the top five journals with the highest publication counts being Marine Pollution Bulletin accounting for 42 articles (21.11%), Environmental Pollution accounting for 30 articles (15.08%), Environmental Science Technology accounting for 15 articles (7.54%), Science of The Total Environment accounting for 9 articles (4.52%), and Analytical Methods accounting for 7 articles (3.52%). Most of these five journals are related to environmental categories, indicating that the topic has received significant research attention from an environmental perspective.

From 2018 to 2023, a total of 4,680 articles are published across 459 journals, falling within 111 Web of Science categories, and the top five categories are Environmental Sciences, Engineering Environmental, Marine Freshwater Biology, Water Resources and Toxicology. Table 2 lists the top five journals with the highest number of publications, which are Science of The Total Environment with 621 articles (15.17%), Marine Pollution Bulletin with 373 articles (9.11%), Environmental Pollution with 341 articles (8.33%), Journal of Hazardous Materials with 261 articles (6.38%), and Chemosphere with 223 articles (5.45%).

The research capabilities and influence of a country/region can be reflected by the number of papers indexed in Science Citation Index (SCI). The 4,309 documents on the studies of microplastics in the water body in this article involved a total of 122 countries and regions around the world. Based on the above timeline division of the study, the initial stage from 2011 to 2013 has a total of 14 articles published, with contributing countries including the United Kingdom, the United States, Canada, Germany, Norway, Australia, Belgium, France, Ireland, Italy, Panama, Portugal, and Switzerland. Among them, the United Kingdom and the United States both published five articles, followed by Canada, Germany, and Norway, which both published two articles, and the remaining countries published one article.

During the third stage (2018–2023), the number of publications from China has rapidly increased to a total of 1,647, accounting for 40.00% of the total publications in this stage, which is more than the combined publication count of the second to fifth ranking countries: the United States (379), Italy (273), Spain (267), and Germany (248). China has gradually become a significant contributor to this field. Several indicators, such as publication count, citation frequency, and H-index, can be comprehensively used to scientifically assess a country's academic influence in the field (Ruan et al. 2022). In terms of the average number of citations per paper, the United Kingdom ranks first with 52 citations per paper, followed by Portugal in second place with 49.95 citations per paper. China ranks third with an average citation frequency of 37.29 citations per paper. However, China's H-index is the highest at 126, indicating that despite the lower average citation rate, China has published highly cited papers in this field. The main countries closely collaborating with China include the United States, the United Kingdom, Germany, and Australia. But collaboration among European Union countries is even closer, as evidenced by institutional analysis. In the future, international research cooperation should be enhanced to address the global challenge of microplastic pollution.

Additionally, countries and institutions can offer more detailed insights about research on the topic, including key institutions in the field and their collaborations. The academic ranks of institutions indicate their relative positions and levels of achievement concerning international scholarly influence. Analysis of the institution's publication count exported from Web of Science shows that there are 3,616 institutions globally conducting research on microplastics in aquatic environments, with 74 institutions having a publication count exceeding 20. Among all institutions, the Chinese Academy of Sciences in China has the highest number of publications (257 articles, 5.93% of the total), followed by the University of Chinese Academy of Sciences (132 articles, 3.05%), East China Normal University (118 articles, 2.72%), the Centre National De La Recherche Scientifique CNRS in France (116 articles, 2.68%), Ministry of Agriculture Rural Affairs in China (82 articles, 1.89%), the Helmholtz Association in Germany (80 articles, 1.85%), and Nanjing University in China (69 articles, 1.59%).

Keywords are the key points of specific research, providing insights into research trends and hotspots of specific fields. Initially, from 2011 to 2013, the literature emphasized keywords such as ‘chemicals’, ‘pollution’, ‘plastic,’ and ‘plastic fragments.’ At this stage, researchers are directing their attention toward the issue of microplastics in aquatic environments, initiating investigations into the pollution levels of microplastics within bodies of water (Li et al. 2021).

Within the ‘Quantifications’ cluster, key terms include ‘quantification’ (9.84), ‘sea’ (7.37), ‘wastewater’ (7.37), ‘identification’ (7.17), and ‘accumulation’ (5.31). The keyword ‘quantifications’ pertains to the study of quantitative methods for microplastics, while ‘identification’ encompasses techniques for detecting and identifying microplastics. This cluster predominantly centers on quantifying the levels of pollution and accumulation of microplastics in aquatic environments. The ‘biofouling’ cluster encompasses keywords such as ‘biodegradation’ (7.39), ‘biofouling’ (7.39), ‘Pacific Ocean’ (7.39), ‘transport’ (6.94), ‘polyethylene’ (5.05), and ‘freshwater’ (3.91). This cluster primarily explores the transportation and biodegradation of microplastics, particularly polyethylene, in aquatic environments (Oberbeckmann et al. 2021; Wang et al. 2023b).

In the ‘neurotoxicity’ cluster, key terms comprise ‘neurotoxicity’ (7.31), ‘predatory performance, (5.48), ‘magna’ (5.48), ‘quantum dots’ (5.48), and ‘vector-effect’ (5.48). This cluster investigates the neurotoxic effects of microplastics and assesses methodologies for evaluating their toxicity (Chen et al. 2017; Zhang et al. 2017; Anbumani & Kakkar 2018). The ‘depuration’ cluster highlights the top five keywords: ‘particles depuration’ (10.11), ‘photo-oxidation’ (10.11), ‘photo-degradation’ (6.45), ‘Ultraviolet (UV) stabilizers’ (5.04), and ‘bacterial biofilms’ (5.04). This cluster delves into purification methods for microplastics in aquatic environments, covering photocatalytic, photo-oxidation, and microbial degradation techniques (Tofa et al. 2019; Pipkin et al. 2021; Shi et al. 2022; Ding et al. 2023).

Within the ‘numerical modeling’ cluster, the top five keywords include ‘numerical modeling’ (10.44), ‘personal care products’ (6.77), ‘sand’ (6.77), ‘extraction techniques’ (6.77), and ‘sea surface’ (6.77). This cluster is predominantly dedicated to employing numerical models for investigating aspects concerning microplastic pollution (Hardesty et al. 2017; Yang et al. 2021). Within the ‘plastic litter’ cluster, the top five keywords are ‘plastic litter’ (11.42), ‘gas chromatography’ (7.72), ‘polystyrene microplastics’ (7.72), ‘functionalized fullerenes’ (5.69), and ‘River Thames’ (5.69). Plastic waste serves as a significant source of microplastics in the environment, and this cluster examines it from diverse perspectives.

In short, numerous scholars have extensively researched microplastics in aquatic environments from various perspectives. These investigations encompass the evaluation of pollution levels, migration patterns, conversion processes, detection methodologies, toxicological impacts, purification techniques, and more (Barrows et al. 2017; Li et al. 2024a).

In the third stage, as shown in Figure 5(b) and Table 4, the keyword clustering analysis uncovers four primary clusters: ‘marine litter,’ ‘adsorption,’ ‘oxidative stress,’ and ‘sludge.’ Within the ‘marine litter’ cluster, the top five keywords are ‘abundance marine litter’ (65.36), ‘surface water’ (61.16), ‘Mediterranean Sea’ (60.68), ‘adsorption’ (50.07), and ‘marine debris’ (43.62). This cluster highlights that the ocean bears the brunt of microplastic pollution, primarily as a result of the accumulation of plastic waste from water bodies that eventually find their way into the ocean. Microplastics can act as attachment points for microorganisms, thus altering the composition of microbial communities in marine sediments and disturbing the ecological functions of existing microorganisms (Ivleva et al. 2017). Within the ‘adsorption’ cluster, the top five keywords are ‘water adsorption’ (146.32), ‘sorption’ (114.53), ‘heavy metals’ (38.68), ‘mechanism’ (35.56), and ‘desorption’ (34.71). This cluster explores the adsorption of harmful substances, particularly heavy metals, by microplastics in water, along with the synergistic risks posed by such interactions (Lionetto & Corcione 2021; Guerrini et al. 2022; Kulik et al. 2023; Liu et al. 2023).

Within the ‘oxidative stress’ cluster, key keywords comprise ‘responses oxidative stress’ (147.33), ‘nanoplastics’ (56.09), ‘gene expression’ (55.67), ‘polystyrene microplastics’ (46.93), and ‘gut microbiota’ (46.66). This cluster elucidates the toxic mechanisms of microplastics on marine organisms (Adam et al. 2021; Cordova et al. 2024; Jia et al. 2024). Within the ‘sludge’ cluster, key keywords consist of ‘rapid sand filtration sludge’ (63.06), ‘wastewater treatment plant’ (58.36), ‘wastewater’ (53.58), ‘sewage sludge’ (48.69), and ‘wastewater treatment’ (46.34). This cluster centers on the presence of microplastics in sewage treatment plant sludge, driven by the rising volume of microplastic waste originating from everyday consumables.

In comparison to the second stage, the research on microplastics in aquatic environments during the third stage is more specific, with a primary focus on the following areas: exploring microplastic pollution in oceans (George et al. 2024), investigating the synergistic toxic effects of microplastics and heavy metals (Banaee et al. 2019; Goh et al. 2022), studying the adverse impacts of microplastics on organisms (Mahamud et al. 2022), and delving into microplastics found in sludge from urban sewage treatment facilities (Long et al. 2019).

The keyword ‘marine debris’ demonstrates a burst strength of 12.75, while ‘Mediterranean Sea,’ ‘Pacific Ocean,’ ‘Great Lakes,’ and ‘ocean’ exhibit burst strengths of 7.23, 4.46, 3.67, and 13.04, respectively. These numbers highlight significant findings of microplastics in various bodies of water, including the Mediterranean (de la Fuente et al. 2021) and Pacific oceans (Kukulka et al. 2016). Furthermore, the keywords ‘polycyclic aromatic hydrocarbons,’ ‘persistent organic pollutants,’ and ‘toxic chemicals’ display burst strengths of 4.85, 27.93, and 3.86, respectively, signifying extensive literature discussing the synergistic toxic interactions between microplastics and organic pollutants, heavy metals, and other harmful substances in aquatic environments (Oliveira et al. 2013; Wei et al. 2019; Hamidian et al. 2021; Nguyen et al. 2022).

The keywords ‘Mytilus edulis l’, ‘zooplankton’, ‘organisms’, and ‘demersal fish’ with intensities of 22.53, 13.08, 8.63, and 11.03, respectively, indicate that aquatic organisms can ingest microplastics, leading to their entry into the food chain (Van Cauwenberghe et al. 2013; Dawson et al. 2018; Sun et al. 2018). Moreover, the keyword ‘quantification’ with a strength of 15.03 emerged between 2015 and 2018, pointing toward quantitative methodologies for microplastic detection (Li et al. 2022). The keyword ‘adsorption’ with a strength of 10.98 from 2003 to 2015 suggests that the prevalent method for removing microplastics from water is through adsorption (Peng et al. 2023). Furthermore, the presence of the keyword ‘surface waters’ indicates a significant research interest in microplastics in surface water (Ulvi & Aydin 2023).

The analysis above provides insights into the research trends of this topic. Initially, studies predominantly investigated microplastic pollution in water bodies, emphasizing the issue of plastic waste contaminating oceans such as the Mediterranean, Pacific, North Sea, and the Great Lakes (Yu et al. 2024). Subsequently, research delved into the presence of microplastics in aquatic organisms and their biological toxicity. Apart from their intrinsic harmful effects, microplastics can also interact with toxic substances like heavy metals in water, compounding risks to organisms. Notably, since 2015, numerous quantitative methods for detecting microplastics have emerged. Recent research efforts have primarily focused on adsorption as a key method for removing microplastics from water bodies. The adsorption method employs adsorbents such as activated carbon, magnetic nanoparticles, and metal oxides to remove microplastics from water. Its advantages include low cost, high efficiency, and ease of operation. Adsorption is considered one of the most promising technologies for microplastic remediation in wastewater. The mechanisms through which adsorbents capture microplastics involve hydrogen bonding, hydrophobic interactions, electrostatic attraction, and van der Waals forces (Wang et al. 2021; Li et al. 2024b; Verma et al. 2024).

This paper presents global research trends on microplastics in water based on 4,309 documents retrieved from the WOS Core Collection. The analysis results reveal a significant growth in the number of publications and citations since 2018, particularly with both 2022 and 2023 surpassing 1,000 published articles. Initially, the United Kingdom led in publishing articles in this field, but post-2018, China emerged as the country with the highest number of publications. Marine Pollution Bulletin not only released early articles in this research domain but also boasts the largest number of published articles. Core journals, such as Environmental Science Technology, Environmental Pollution, and Science of The Total Environment, are pivotal for microplastics research in water. The Chinese Academy of Sciences holds the highest number of publications among all institutions, while the University of Plymouth maintains extensive collaborations with 73 institutions, ranking first in collaborative efforts.

Keyword analysis reveals that research on microplastics in water encompasses various aspects, including assessing pollution levels, examining migration and transformation, analyzing detection techniques, understanding toxic effects, and investigating purification methods. Initial studies primarily targeted microplastic pollution in water bodies, particularly the presence of plastic waste in oceans like the Mediterranean, Pacific, North Sea, and the Great Lakes. Subsequently, attention turned to the detection and impact of microplastics in aquatic organisms, addressing biological toxicity. Furthermore, microplastics, besides their inherent toxicity, can interact with harmful substances like heavy metals in water, compounding risks to organisms. Since 2015, numerous quantitative detection methods for microplastics have emerged. Recent research has concentrated on adsorption as a primary method for removing microplastics from water bodies, with an additional focus on studying microplastics in surface water.

Despite extensive research on microplastics in water, several unresolved issues require further investigation, including (1) establishing unified standards for the collection, analysis, and evaluation of microplastics, along with reliable and standardized testing techniques; (2) evaluating the comprehensive environmental and health impacts of microplastics and developing a risk assessment system; (3) investigating effective, feasible, and economical technologies for the removal of microplastics from water; and (4) creating environmentally friendly alternatives to plastic products.

This study employs CiteSpace and VOSviewer to analyze articles published in core journals indexed by Web of Science from 2011 to 2023. While this database is widely used and comprehensive, it may not capture all relevant articles, particularly those from local or regional journals lacking international indexing. Furthermore, only articles written in English were included, potentially overlooking significant research published in other languages. Future studies could expand the data sources to mitigate this limitation.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.