A Bibliometric Analysis of Research Trends in Biodegradation of Plastics

The rapid growth in the production and application of plastic globally has resulted in plastic pollution with a negative impact on the environment, especially the marine ecosystem. One main disadvantage in the majority of polymers is disposal after a useful life span. Non-degradable polymers create severe difficulty in plastic waste management that might end up in landfills or wash into the ocean. The biodegradation of plastic waste is one solution to this critical problem of pollution. Hence, there is a need to consider the advancement of research in this subject area, in pursuit of a way out of plastic pollution. Thus, this study was designed to map the biodegradation of plastic-related research from 2000 to 2021. Statistical information on the topic was recovered from the Web of Science Core Collection and analysed using the bibliometrix package in RStudio statistical software, while data visualisation was conducted via VOSviewer. Our evaluation indicated that the amount of research on the biodegradation of plastic increased over the last decade, and the annual growth rate of publication trends was 11.84%. The study revealed that 1131 authors wrote the 290 analysed documents, with a collaboration index of 4.04. Cooper DG (n = 11) was the most relevant author, McGill University (n = 21) was the most active university, and the Journal of Polymers and the Environment (n = 19) the leading journal. The outcome of this study can guide prospective research and offer vital information for improving the management of plastic waste.


Introduction
Plastics are synthetic or semi-synthetic materials made through the polymerisation of organic and/or inorganic compounds [1,2]. Because of their vast domestic and industrial applications, there is a continuous increase in the production and consumption of plastics. As of 2017, 8.3 billion metric tonnes of original plastics have been produced, of which 76% became waste. Only 9% of plastic waste is recycled, 12% incinerated, and 79% ends up in landfills [3,4]. The majority of bio-based and petroleum-based plastics, such as polyethylene, polypropylene, polyvinyl chloride, and polystyrene, are not biodegradable. Consequently, the build-up of non-biodegradable plastics in soil resulted in decreased soil fertility, among other ecological and health challenges [5,6]. Similarly, plastic materials are difficult to manage and often end up in marine environments, beaches and surfaces of the shallow seabed, as well as the abysses. The degradation of large polymers leads to the proliferation of micro-plastic particles, which are poisonous to marine species. The micro-plastic particles aid the accumulation of other contaminants capable of impairing the feeding and growth of aquatic organisms, thus, damaging the robustness of marine species [3,7]. Therefore, researchers and other stakeholders have shown huge interest to surmount the build-up of non-biodegradable plastics in the environment [1,8].
The biodegradation of plastics depends on some factors, such as the composition or molecular weight of the plastics, environmental conditions (e.g., temperature, soil reaction), and the presence of suitable microorganisms [5,9,10]. Because most polymers are

Data Mining
Numerous scientific databases can be mined for information on bibliometric analysis of research production in a specific field [18,20]. WoS is one of the most dependable and all-inclusive databases for bibliometric studies with millions of varying quality and high-impact scientific articles [16]. Thus, WoS core collection was mined for data on bibliometric analysis of research trends on biodegradation of plastics from 2000 to 2021. The search strategy was: TITLE (("Biodegradation" OR "biological degradation" OR "microbial degradation" OR "enzymatic degradation") AND (*plastic* OR "polymer")). A set of documents (n = 456) were retrieved but the search was refined to scientific research articles (n = 374). Afterwards, manual validation to remove those articles that are not within the time range and do not relate with our focus by going through all the abstracts was performed, of which 84 articles were excluded. As such, 290 articles were found suitable for bibliometric analysis in this study. These documents were retrieved from WoS and saved for further processing.

Annual Production Trends
Variations in the number of published research documents in a specific field are an important pointer for the developmental trend [24]. A plot of publications over time with statistical assessment would play a part in comprehending the research condition as well as prospective trends. About 55% of the documents were published between 2015 and 2021- Figure 1a. The highest publication was observed in 2020 with 36 documents, accounting for 12.4% of the total documents. The observed annual growth rate was 11.84%, an indication of a positive research trend in the biodegradation of plastics over time. The growing public attention to the circular economy of plastic pollution and the efficient biodegradation approach may be responsible for the upsurge in publications. Microorganisms with the potential for efficient plastic degradation could result in new prospects for palliating plastic pollution. Globally, the top five productive countries in biodegradation-ofplastics-related research are China with the highest number of documents (n = 92), followed by the USA (n = 82), India (n = 47), Japan (n = 45), and Canada (n = 31). Though China accounts for the largest production of plastic materials globally [25,26], China still makes a significant effort to remove plastic pollution through the implementation of policies and numerous funded projects. Countries with the greatest contribution are shown in Figure 1b nations that ascribe huge significance to scientific research and they publish in journals indexed in WoS. Though the statistic from WoS may not be a true representation of all published scientific articles in a specific field, because some researchers are indifferent about the journals' quality or they are interested in journals with fast publications only without considering visibility. Similarly, mean total citations per year were analysed over the same period and there is variation in the citation pattern. The highest citation was observed in 2017, followed by 2020, whereas the least citations were observed in 2005. Numerous factors can affect the citation of a research document. These include year of publication and open or paid access journal. Older publications are expected to have more citations than new ones [27]. However, papers in open access journals are easily available to other scholars and, thus, more cited than those in paid access. Citations are presumed to indicate the quality of the research, though in some cases, this is not so. The relationship between research quality and citations is a continuous debate in scientometrics [28].

Relevant Institutions and Authors
The most relevant institution in biodegradation-of-plastics-related research in terms of publication between 2000 and 2021 was McGill University, Canada, with 21 publications, followed by Universiti Putra Malaysia (n = 10), N.M. Emanuel Institute of Biochemical Physics, Russian Academy of Sciences (n = 7), Agricultural University of Athens (n =

Relevant Institutions and Authors
The most relevant institution in biodegradation-of-plastics-related research in terms of publication between 2000 and 2021 was McGill University, Canada, with 21 publications, followed by Universiti Putra Malaysia (n = 10), N.M. Emanuel Institute of Biochemical Physics, Russian Academy of Sciences (n = 7), Agricultural University of Athens (n = 6), and China Pharmaceutical University (n = 6). Details of the top 20 relevant institutions are shown in Table 2. Several studies have used bibliometrics to assess relevant institutions and authors in a specific field of interest [29,30]. The data from WoS revealed that 1131 authors wrote the 290 documents used for the analysis, with 3.9 authors per document- Table 1. The productivity and citation impact of authors were analysed as a function of h_index in Rstudio. The top five authors are Cooper DG (n = 11, total citations = 315, h_index =8), Nicell JA (n = 9, total citations = 236, h_index = 7), Briassoulis D (n = 6, total citations = 510, h_index = 6), Degli-Innocenti, F (n = 5, total citations = 191, h_index = 5), and Tosin, M (n = 5, total citations = 189, h_index = 5)- Table 3. Most of the Cooper DG publications are focused on the application of microorganisms, such as Bacillus subtilis, Rhodococcus rhodochrous, Rhodotorula rubra, and aerobic mesophilic microorganisms, consisting of bacteria, fungi, and yeast, for the degradation of different plastic materials [31][32][33][34]. H_index is a true indicator of researchers' contribution and attainment but it is not suitable for assessing multidisciplinary fields [35,36]. Likewise, citations are a deficient method of quantifying an author's impact in a field, since many issues affect the citation of a research publication [37]. In our analysis, although Cooper DG has the highest number of publications (n = 11), he is not among the top five researchers with the highest total citations. They are Wu WM (752 total citations), Yang Y (689 total citations), Yang J (644 total citations), Jiang L (641 total citations), and Briassoilis D (510 total citations). Year of publication is one of those factors influencing paper citations, notwithstanding the publication year (2002) of Cooper DG, Wu WM, Yang Y, Yang J, and Jiang L, whose publication year was 2014, as well as Briassoulis D with a publication year of 2007, attained higher citation than Cooper DG.

Relevant Journals
The distributions of research scope in a specific topic are best explained in journals and subject categories in bibliometric analysis [38].   The analysis of relevant journals showed that the studies in this field are more interdisciplinary, with a focus on the environment. Plastics are not only an environmental concern but also a hazard to microorganisms, soil, plants, human health, and the food chain, among others [39,40]. The increasing impact of plastics on the environment calls for an inclusive assessment of the ecological risk and effect on human health. Studies on the interaction among plastics, microorganisms, and human health may be a likely development in the biodegradation of plastics.

Bibliographic Coupling Analysis
Bibliographic coupling analysis is a comparison measure that uses citation analysis to establish the relationship between documents based on the number of references shared [41]. This is to affirm the probability that the documents treat similar subjects and generate an information map of the research authors, institutions, and journals to show the collaborative network in biodegradation-of-plastics-related research. In this study, bibliographic coupling of authors, journals, and institutions was carried out using the full counting method in VOSviewer software. For bibliographic coupling analysis, we chose a minimum of three documents and 23 journals to meet the criteria. The bibliographic coupling link strength with other sources was calculated for each journal. The 23 journals were grouped into four clusters of 10, 6, 4, and 3 items in VOSviewer. Environmental Science and Technology has the highest total link strength of 574 with 11 documents and 995 citations, followed by the Science of Total Environment with 10 documents, 440 citations, and 564 total link strength- Figure 2a

Collaboration Network Analysis
Collaboration network analysis can offer useful data for individual scholars, institutions, and countries seeking cooperation partners or groups to expand their field of research and realise the purpose of academic exchange [24]. This analysis reveals the reality of scientific research and academic communication among authors, institutions, and countries at various levels [42]. In this study, co-authorship of authors, institutions, and countries was analysed using the full counting method in VOSviewer. For the author's analysis, the maximum number of authors per document was fixed as 25 and 1151 authors were observed. When the minimum number of documents per author was five and the minimum number of citations of an author was zero, only five authors met the threshold. The minimum number of documents per author was further reduced to three and the

Collaboration Network Analysis
Collaboration network analysis can offer useful data for individual scholars, institutions, and countries seeking cooperation partners or groups to expand their field of research and realise the purpose of academic exchange [24]. This analysis reveals the reality of scientific research and academic communication among authors, institutions, and countries at various levels [42]. In this study, co-authorship of authors, institutions, and countries was analysed using the full counting method in VOSviewer. For the author's analysis, the maximum number of authors per document was fixed as 25 and 1151 authors were observed. When the minimum number of documents per author was five and the minimum number of citations of an author was zero, only five authors met the threshold. The minimum number of documents per author was further reduced to three and the number of authors that meet this requirement increased to 22  For the institution's collaboration analysis, with a maximum of 25 institutions per document, 404 institutions were observed. Only five institutions met the threshold when the minimum number of documents per institute was five and the minimum number of citations was zero. Reducing the minimum number of documents per institute to three increased the number of institutions to 22. Most of the 22 institutions are not linked. The biggest group of connected institutions is three; they are Stanford University, Beihang University, and BGI Shenzhen and the connection is shown in Figure 3b. These analyses show that there is no strong collaboration among authors and institutions in the biodegradation-of-plastics-related research. More effort is needed to promote academic exchange in the biological degradation of plastic.
For the country's co-authorship analysis, 52 countries were observed with the maximum number of countries per document being 25. With a minimum of five documents in a country and the minimum number of citations as zero, 18 countries met the selected criteria. A further reduction in the minimum number of documents of a country to three increased the number of countries that satisfy the requirement to 32. Some of the 32 countries are not connected. The biggest group of connected countries consists of 21 countries. The 21 countries were grouped into five clusters of six, four, four, four, and three items in VOSviewers. Clusters are grouped by the rate of shared co-occurrence terms representing each country. Terms with the same colour indicate they are strongly connected, thereby classified into the same cluster- Figure 3c. The more research publications a country has, the bigger the size of its circle; the larger the scale of the cooperation is, the thicker the connecting line [43]. The line connecting two items is the measurement of the level of cooperation between the two terms and is called link strength. The highest link strength between countries is 9 and found between China and the USA.  For the country's co-authorship analysis, 52 countries were observed with the maximum number of countries per document being 25. With a minimum of five documents in a country and the minimum number of citations as zero, 18 countries met the selected criteria. A further reduction in the minimum number of documents of a country to three increased the number of countries that satisfy the requirement to 32. Some of the 32 countries are not connected. The biggest group of connected countries consists of 21 countries. The 21 countries were grouped into five clusters of six, four, four, four, and three items in VOSviewers. Clusters are grouped by the rate of shared co-occurrence terms representing each country. Terms with the same colour indicate they are strongly connected, thereby classified into the same cluster- Figure 3c. The more research publications a country has, the bigger the size of its circle; the larger the scale of the cooperation is, the thicker the connecting line [43]. The line connecting two items is the measurement of the level of cooperation between the two terms and is called link strength. The highest link strength between countries is 9 and found between China and the USA. The top three counties with the highest link strength are the USA (1906 citations, 24 link strength), China (1428 citations, 16 link strength), and Switzerland (202 citations, 7 link strength). This analysis shows there is no country from Africa among the collaborating countries; hence, much work is needed in this research area in Africa.

Co-Occurrence of Author Keywords
Co-occurrence analysis can be used to identify current topics and directions to observe and follow up the advances in scientific research and programs [44,45]. We analysed the co-occurrence of the author's keywords via VOSviewer, shown in Figure 4. The size of a keyword indicates the number of publications in which it occurs, and the distance between two keywords gives a rough estimate of the relationship of the keywords. Using the full counting method, 779 keywords were observed. With the minimum number of occurrences of a keyword fixed at five, only 28 keywords meet the threshold. When we reduced the minimum number of occurrences of a keyword to three, 65 keywords met the requirement. The topmost keywords are: Biodegradation (125 occurrences, 106 link strength), Enzymatic degradation (12 occurrences, 6 link strength), Plastics (11 occurrences, 18 link strength), Polyethylene (11 occurrences, 17 link strength), Biodegradable (11 occurrences, 15 link strength), Composting (10 occurrences, 18 link strength), Degradation (10 occurrences, 6 link strength), Biodegradability (9 occurrences, 9 link strength), Enzymes (8 occurrences, 10 link strength), and Microorganisms (7 occurrences, 10 link strength). These keywords are a reflection of the research focus on the subject over two decades among scholars, and various researchers have utilised keywords to ascertain research directions in a specific field [17,18,20].

Co-Occurrence of Author Keywords
Co-occurrence analysis can be used to identify current topics and directions to observe and follow up the advances in scientific research and programs [44,45]. We analysed the co-occurrence of the author's keywords via VOSviewer, shown in Figure 4. The size of a keyword indicates the number of publications in which it occurs, and the distance between two keywords gives a rough estimate of the relationship of the keywords. Using the full counting method, 779 keywords were observed. With the minimum number of occurrences of a keyword fixed at five, only 28 keywords meet the threshold. When we reduced the minimum number of occurrences of a keyword to three, 65 keywords met the requirement. The topmost keywords are: Biodegradation (125 occurrences, 106 link strength), Enzymatic degradation (12 occurrences, 6 link strength), Plastics (11 occurrences, 18 link strength), Polyethylene (11 occurrences, 17 link strength), Biodegradable (11 occurrences, 15 link strength), Composting (10 occurrences, 18 link strength), Degradation (10 occurrences, 6 link strength), Biodegradability (9 occurrences, 9 link strength), Enzymes (8 occurrences, 10 link strength), and Microorganisms (7 occurrences, 10 link strength). These keywords are a reflection of the research focus on the subject over two decades among scholars, and various researchers have utilised keywords to ascertain research directions in a specific field [17,18,20]. The 65 keywords were divided into eleven clusters of 11,8,8,8,7,6,5,5,3,3, and 1 item in VOSviewer. Each cluster is represented with a unique colour to show the relatedness of the items in the cluster [46]. The keywords in identical clusters normally exhibit stronger links. The node size indicates the number of occurrences of a keyword and the attached lines show the existing connection. Cluster 1 centres on different plastic additives as well as their degradation and is identified by keywords, such as "2-ethylhexanol", "phthalate", "dehp", "plasticizer", "polyesters", and "plastic biodegradation", among others. Different methods of degradation of plastic materials are categorised in Cluster 2, The 65 keywords were divided into eleven clusters of 11,8,8,8,7,6,5,5,3,3, and 1 item in VOSviewer. Each cluster is represented with a unique colour to show the relatedness of the items in the cluster [46]. The keywords in identical clusters normally exhibit stronger links. The node size indicates the number of occurrences of a keyword and the attached lines show the existing connection. Cluster 1 centres on different plastic additives as well as their degradation and is identified by keywords, such as "2-ethylhexanol", "phthalate", "dehp", "plasticizer", "polyesters", and "plastic biodegradation", among others. Different methods of degradation of plastic materials are categorised in Cluster 2, which were identified by keywords, such as "enzymatic degradation", "hydrolytic degradation", and "biocomposite", among others. Cluster 3 identifies plastic biodegradation material keywords, namely "biodegradable materials", "biodegradation in soil", "biofilm", "composting", and "chitosan", among others. Microbial degradation of plastic is an environmentally benign method of managing plastic pollution [47]. Most research on the biodegradation of plastic is focused on the terrestrial environment, with several microorganisms, such as "bacteria", "fungi", and "microbes", keywords identified in Clus-ter 6 [48][49][50]. There are limited reports of research on marine plastic sources degrading microorganisms [7,51]. Cluster 7 focused on various forms of plastic accumulation in the environment and their biodegradation. The keywords in this cluster include "microplastics", "plastic waste", "polystyrene", "polycaprolactone", and "biodegradation". Previous studies have shown that microplastics are a carrier of toxic substances and other contaminants, which when consumed by organisms, such as fishes and seabirds, could affect their agility, feeding pattern, and reproductive system [52][53][54]. In addition, microplastics have been detected in foods, sea salts, and bottled water, including human stool samples and human placenta [55][56][57][58]. This suggests that microplastics can cause an imbalance in the intestinal microbiota, such as intestinal dysfunction and metabolic disorder [59,60]. Hence, studies on microbial activities on microplastics and their associated impact on human health are among the vital research topics needed.

Co-Citation Analyses
This is meant to determine the relationship of items based on how often they are cited together. Co-citation analysis provides important information on a topic in a specific field of research from the bulk of cited sources, authors, and references, that can assist in the evaluation of the most significant publications on a specific subject [24]. In this study, co-citations of cited references, cited sources, and cited authors were analysed- Figure 5. Similarly, for the co-citation of authors, 6716 authors were identified. With the minimum number of citations of an author as 10, 72 authors meet the criteria. Some of the 72

Limitations
This study presents the mapping of biodegradation-of-plastic-related research between 2000 and 2021. Nevertheless, the analysis might not be all inclusive of research publications on the topic, seeing as we centred on publications indexed in the Web of Science, excluding publications indexed in PubMed and Scopus, among other scientific databases. Similarly, it is likely that we did not use up all the possible keywords connected to the biodegradation of plastics within the specified time, and that might have created a bias in our analysis. Lastly, this study solely visualised the research trends on the topic but did not analyse the substance of every publication to determine the scientific quality or otherwise of the paper.

Conclusions
This study presented a comprehensive scientific mapping of global research on the biodegradation of plastic-related research from 2000 to 2021. About 290 publications were recovered from the Web of Science, written by 1131 authors. The Peoples' Republic of China and the USA were the most prominent countries, with the highest publications and citations. In addition, the most relevant journals are the Journal of Polymers and the Environment and Polymer Degradation and Stability on the subject. We observed a low collaboration network among authors, with the strongest collaboration network between China and the USA, and no African country on the collaboration network analysis. This study suggests that more studies should be focused on the biodegradation-of-plastic-related research that elucidates the concurrent impact of microbial communities and microplastics on both the environment and human health. We believe this study would assist scholars interested in this field to identify potential collaborators, as well as provide valuable information for the effective planning and management of plastic contaminants, especially in the coastal and marine environment.

Conflicts of Interest:
The authors have no relevant financial or non-financial interest to disclose. Furthermore, the funders had no role in the design of the study; in the collection, analysis, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.