Characterization of top 100 researches on e-waste based on bibliometric analysis

With rapid development of energy, information, and communication technology, e-waste problem has become one of the global issues to be settled urgently. The main features on publication years, journals, countries and institutions, authors, keywords, and content types of the 100 most-cited articles on e-waste had been unfolded in this research. The direction and way forward were illustrated, the trends to date were demonstrated, and the terrain and pathways were evaluated on the research of e-waste. Bibliometric analysis method was applied to analyze various attributes on the 100 most-cited articles which were retrieved from WoSCC on May 25, 2021, by utilizing the software tools Microsoft Excel 2016 and VOS viewer 1.6.9. The publication year and citation number of the 100 articles ranged between 2003 and 2017 and from 83 to 925, respectively. Environmental Science & Technology (n=17) published the maximum articles. Waste Management, Journal of Hazardous Materials, and Environmental Science & Technology were the core journals on e-waste. One hundred twenty-three institutions and 25 countries were involved in publishing the 100 articles. Three hundred seventy authors contributed to the 100 articles in total. A total of 267 keywords occurred in the 100 articles. The keywords “e-waste” and “recycling” held the highest occurrences. The study content of the 100 articles could be classified into four types including the characteristic-and-property type, the environment-and-health type, the management-and-economic type, and the technique-and-processing type. Overall completeness and applicability of the evidence found in this study were verified sufficiently; the potential biases in the review process were also considered. The innovations of the research from the past bibliometric analysis work on e-waste were stated, and the implication for practice and research of this study were explained as well. 2007, 2008, and 2009 were a peak of the researches on e-waste, while the recent years were experiencing a valley. China and its institutions were most influential in this field on e-waste. India was becoming more and more influential on e-waste research in the world. Nigeria was the research center in Africa, and Brazil was the research center in Latin America. Wong Minghung was the most important expert on e-waste. The impact on environment and human being’s health was the hot topic of researches on e-waste; the characteristic and property of e-waste were studied not enough. The researches of technique and processing would be the direction and way forward in the study field on e-waste. The characteristic and property on e-waste would need more attention to be researched. The researchers could develop new pathways based on and beyond the four types of content evaluated in this research.


Introduction
Nowadays, with electric and semiconductor technologies making a tremendous advancement compared with 20 years ago which made the consumer demand for electrical energy and information devices grow at an incredible speed, the global countries and districts are being urgent to resolve the tough problem of e-waste. (Ongondo et al. 2011).
Electrical and electronic equipment (EEE) is usually defined as items or wares which are made with electrical or electronic parts. E-waste is defined as the EEEs which have been abandoned as waste or rubbish without reusing (Wath Responsible Editor: Philippe Garrigues Researchers from different regions and countries have implemented a variety of studies, in the progress of researching e-waste, such as environment conditions at e-waste recycling site (Wang et al. 2012;Zhang et al. 2014;Huang et al. 2011;Ha et al. 2009), the influences on of e-waste human (Zheng et al. 2015;Zheng et al. 2008;Zheng et al. 2013;Chen et al. 2011), the public management on e-waste (Yin et al. 2014;Tanskanen 2013;Afroz et al. 2013;Yu et al. 2010), and the processing and recycling technique of e-waste (Akcil et al. 2015;Zhang and Xu 2016;Cui and Zhang 2008;Mishra et al. 2019;Pant et al. 2012). However, as so far there were just a few studies ( Filippo Corsini et al. 2012;Liming Zhang et al. 2019;Gao et al. 2019aGao et al. , 2019b to study the present situation and demonstrate the direction and way forward of researching e-waste using bibliometric analysis method. Compared with these existing bibliometric analysis work on e-waste, there were some featured innovations and advancements of this research. Firstly, the TS words for researching specially included "waste printed circuit board," "CRT," "LCD" and "spent lithium-ion battery," "informal economy," "informal labor," "livelihoods," and "trade" or "unequal flows." Waste printed circuit boards, CRTs, LCDs, and spent lithium-ion batteries were significant part of e-waste nowadays, which could cover the literatures on e-waste more fully. And the search words informal economy and informal labor could select better the literature on e-waste from India and various places in Africa. The words trade and unequal flows were terms that would show up in assessing North America and European research foci (Grant and Oteng-Ababio 2013;Oteng-Ababio 2012;Grant and Oteng-Ababio 2016;Nnorom and Osibanjo 2008;Caravanos et al. 2011;Lepawsky and Mather 2011). Secondly, the analyzed 100 articles on e-waste were with the highest citations, of much greater value, which guaranteed the results and conclusion more reliable. Most important of all, the study obtained the new findings about the research center in Africa and Latin America, the trends and hot topics to date, the direction and way forward, and the terrain to be more concerned.
Bibliometric analysis method is usually utilized as a statistical and quantitative analysis tool of study publications which could extract measurable data and collect valuable knowledge from a publication (Gao et al. 2019a(Gao et al. , 2019bLu et al. 2019aLu et al. , 2019bShi et al. 2020aShi et al. , 2020bLiu et al. 2020). It has been applied in various study fields to discover the current situation, the key topics, and the tendency in study works, because this method could well evaluate the literature traits of specific research subject and the influence of study publications by assessing countries and institutions, journals, authors, and keywords of specific publication (Gao et al. 2019a(Gao et al. , 2019b.
The research implemented a bibliometric analysis on the 100 most-cited articles of e-waste aiming at analyzing the distribution of publication year, countries and institutions, journals, keywords, authors, numbers of citations, and other features, identifying the collaboration among countries and institutions, illustrating the research process and trends to date, and evaluating the research terrain and pathway so far on ewaste. All the members in our team expect that efforts in studying the 100 highly cited articles can provide some valuable information for enhancing global public knowledge on ewaste, decelerating the speed of e-waste generation, diminishing the global amount of e-waste every year, elevating the management level of e-waste, and advancing the technology of processing and recycling e-waste.

Data source and search strategy
The research implemented an overall retrieval in SCI-E of WoSCC on May 25, 2021. The retrieval was accomplished within the day to eliminate deviation led by the daily updates of the database. Retrieval method was as shown below: TS=("electrical waste" OR "electronic waste" OR "e-waste" OR "waste electrical and electronic equipment" OR "used electronics" OR "WEEE" OR "waste printed circuit board" OR "CRT" OR "LCD" OR "spent lithium-ion battery" OR "informal economy" OR "informal labor" OR "livelihoods" OR "trade" or "unequal flows"). In order to pick up the literatures on e-waste from India and various places in Africa and assess north America and European research foci better, the TS included specially the words such as "informal economy," "informal labor," "livelihoods," "trade," and "unequal flows," which would make the research more international in scope and methods.
The screening procedure was as follows. The first step was that the research team set the sequence of the results based on the "citation frequency" from high to low without time and language restrictions, but with the publication types restricted to "article" and "review." Secondly, two independent reviewers filtered out the 100 most-cited results most related to e-waste. In order to reach agreement, both of them discussed disagreements carefully. To pay more attention on the e-waste problem in the Africa region, the two independent reviewers substituted the last two results with two articles which focused the West Africa around the Accra, Ghana, ewaste hub and also had the high citations in SCI-E. To extract the relevant literature traits of the 100 articles such as title, publication years, journals, institutions and countries, authors, citations, and keywords, the research team exported the citation report and the marked-results spreadsheet of those articles from SCI-E database and took advantage of them fully.

Bibliometric analysis
The relevant traits of publication year, journals, and type of study content were extracted by utilizing Microsoft Excel 2016 (Shi et al. 2020a(Shi et al. , 2020b. It was also applied to generate evidence graphs. The evidence of relevant traits could be presented well by evidence graphing which included the bubble graph and the histogram in this research. Two types of bubble graphs were generated to demonstrate (1) the distribution of articles and citations by year (X-axis: publication year; Y-axis: the total quantity of citations; bubble size: the quantity of articles in the specific year; number label: the anterior: the total number of citations; the rear: the average number of citations.) and (2) the average quantity of citations of articles sorted by types of study content (X-axis: different types of study content; Y-axis: the average quantity of citations; bubble size: the quantity of articles of specific type). The histogram was used to demonstrate the distribution of different types of study content (X-axis: types of study contents; the anterior Y-axis: the quantity of articles; the rear Y-axis: the percentage of citations).
The network maps of countries and authors and the density map of keywords were generated by utilizing VOS viewer 1.6.9 (Leiden University, Leiden, Netherlands). The data in the citation report and the marked-results spreadsheet were normalized before importing in VOS viewer to generate maps. For instance, the normalization of keywords was that various expressions of the same keyword were requisite to employ a uniform expression to eliminate deviation, which was completed artificially by the authors. With respect to the national information identified by the tool, Northern Ireland, unified Wales, England, and Scotland were classified into the UK, and Hong Kong, Macao, and Taiwan were classified into China. The explanation of maps generated by VOS viewer should take into consideration three properties: colors, size, and distance. In the maps, the nodes were interpreted as specific items which stand for countries, authors, or keywords, and the size of them was interpreted as the quantity of articles or occurrences, and the number of the links between them was interpreted as the affinity of their relationship.
Based on the data of the World Bank, the research sorted countries into developing or developed countries. 2019 Journal Citation Reports provided the newest IFs of the related journals.

Results
A total of 148,934 articles were acquired by the primary search which were then ranked by citation frequency. We distinguished 100 studies which are the most relevant to the study theme of e-waste, also were all published in English language. Citation frequency of all the 100 articles ranges from the lowest 83 to the highest 925. The total quantity of citations was 21,728, and the average quantity of citations of the 100 articles was 21,728. The 100 most-cited articles have been presented in Appendix 1.

The distribution of articles and citations by year
The 100 most-cited articles were published between the year of 2003 and 2017. Figure 1 below mainly represents the article and citation distribution by publication year in which X-axis is denoted as the publication year, Y-axis is denoted as the total quantity of citations, and the bubble size is denoted as the quantity of articles in the specific year. The anterior number of the number label beside the bubble is the total quantity of citations, and the rear is the average quantity of citations in the specific publication year.
As seen easily in the Fig. 1, it was in the publication year of 2007 that the total quantity of citations was the highest (n=3435) and in the publication year of 2017 that the total quantity of citations was the lowest (n=502). With respect to the average quantity of citations, the biggest was 325.7 in 2003, and the smallest was 149.8 in 2010, respectively. Across the 15 publication years from 2003 to 2017, the largest number of articles was issued in 2007 which was up to 15 articles represented by the biggest bubbles, followed by 2009 with 14 articles. The least number of articles was issued in 2017 which was 3 articles represented by the smallest bubbles.
The trends of researches so far on e-waste could be clearly recognized from Fig. 1. In detail, the total quantity of citations and articles of the years 2007, 2008, and 2009 was all much higher than other years, which could adequately indicate that 3 years was a peak of the research on e-waste in the past 20 years. However, the total quantity of citations and articles of the recent 5 years in the 100 most-cited articles were very low, which could mean the research on e-waste were experiencing a valley.

Journals
The top 100 cited articles were entirely issued by 29 journals which are all owned by three countries, the USA, the England, and Netherlands. All journals were represented in the Table 1. Among the 29 journals, eleven journals were managed by the USA, accounting for up to 38%, and the other eleven journals were managed by England, followed by Netherlands with 7 journals, accounting for 24%.
The impact factors of all published journals ranged from the highest 26.625 to the lowest 1.192. The Materials Science & Engineering R-Reports held the highest impact factor (IF =26.625) but only issued one article, and the Annals of Global Health with the lowest impact factor (IF = 1.192) also only published just one article. The IFs of three journals were higher than 10, which were the Materials Science & Engineering R-Reports, the Lancet Global Health, and the Renewable & Sustainable Energy Reviews, and their numbers of articles were all just 1, and their ratio of total citations reached 4.77%. However, it was with a much lower impact factor that the three journals all contributed more than or equal to ten articles, which were the Environmental Science & Technology, the Waste Management, and the Journal of Hazardous Materials. Except for the three journals, the number of articles of other journals was all less than 10.
The citation numbers of these journals ranged from the highest 3341 to the lowest 83 in which three journals were more than 2000: to be specific, the Journal of Hazardous Materials with 3341, the Environmental Science & Technology with 3185, the Waste Management with 2833, respectively. The three journals' ratio of total citations was up to about 43.1%, although they had a relatively low impact factor.
Based on the analysis of the article and citation number of these journals, we could conclude that the article and citation number of the three journals were both top 3 among studied 100 articles, which reflected their substantial position in the ewaste field. Consequently, the three journals, Waste

Management, Journal of Hazardous Materials, and
Environmental Science & Technology could be considered to be the core journals in the study field of e-waste. By analyzing the impact factor of these journals, we could also conclude that the article and citation number of one journal do not have an absolute relationship with its impact factor.

Country and institution
Twenty-five countries in total took participation in the issue of the 100 articles. Developed countries were the majority of them. However, China contributed maximum articles (n=54) which is a developing country, followed by the USA (n=17), while India, Australia, England, Switzerland, and South Korea were ranked in the from third to seventh place with more or equal to 5 publications. The rest of 18 countries all contributed to less than 5 articles. Regrettably, only two in the top 10 countries belonged to developing countries. Remarkably, among the 25 countries except China and the USA which had the highest GDP in the world, India issued the third highest number of articles that is up to 7 articles, which means India was becoming a more and more influential e-waste research country in the world. Besides, Nigeria and Ghana were only the countries from Africa. Especially, Nigeria contributed up to 3 articles more than or equal to some important developed countries, such as Canada, Germany, Netherlands, Italy, France, and New Zealand. Surprisingly, only Brazil came from Latin America.  Figure 2 represents the collaboration of the countries that issued the 100 articles which showed that China, England, and Canada maintained intimate cooperation with other countries.
Twenty-three institutions in total were found to publish two or more articles; among them, two institutions took participation in the publication of more than ten articles. The highest quantity of publications was possessed by the Chinese academy of sciences, which was up to 23 articles, followed by Hong Kong Baptist University with 15 articles. Table 2 represents the top 10 countries and institutions that published the 100 articles.
It is worth noting that in the top 10 institutions, University of Ibadan came from Nigeria. Abia State University was also from Nigeria. University of Ghana came from Ghana. All the three institutes were located in Africa, which shown that the ewaste problem in Africa were becoming serious and raised more and more concern to local research organization. Nigeria could be considered the research center in Africa which contributed the most articles and possessed the most institutes in African countries. In all the contributed institutes, only University of Sao Paulo came from Brazil, Latin America. Therefore, Brazil could be regarded as the research center in Latin America.

Author
A total of 370 authors were involved in the issue of the 100 articles. And 51 ones contributed two or more articles. The author Wong Minghung took part in publishing thirteen articles and could be considered the most productive. There were five authors of whom the number of contributed articles were more than or equal to five; they were Wong Minghung with 13 articles, Li Jinhui with 8 articles, and Chen She jun, Mai Bixian, and Zeng Xianlai all with 5 articles. From Table 3, it can be found that the two authors (Wong Minghung and Li Jinhui) not only had the top two number of contributed articles in the top cited 100 articles, but also the top two number of citations which were 3299 and 1343, respectively. The number of articles and citations of all other authors were below one thousand. Figure 3 represents the cooperation relationship among authors who contributed two or more articles. There are different color clusters where the nodes are the same color and connected with each other, which means these authors have a close cooperation relationship. It can be easily found that there are mainly four study groups based on mutual cooperation in the study field of e-waste separately represented by the red, green, yellow, and blue clusters in the Figure 3. The red group   Table 3 and Figure 3 that Wong Minghung was the most influential expert in the research field of e-waste who had the most articles and held the most intimate cooperation with researchers all around the world.

Keywords
A total of 267 keywords were collected from the top cited 100 articles which had a total frequency of 400. Among all the keywords, the frequency of occurrence of the keyword "ewaste" was the highest (n = 34), and the keywords "recycling" (n=21) and "electronic waste" (n=10) were with the second and third highest frequency of occurrence. The 17 keywords listed in Table 4 were all with 3 or more occurrences. Part of the Table 4 was about e-waste, electronic waste, waste electrical and electronic equipment, weee, and printed circuit boards; part of it was associated with environment and health, such as recycling and soil; some of it were about chemicals, for instance, heavy metal, metal recovery, pbdes, lead, pahs, pcbs, and polybrominated diphenyl ethers (pbdes); some of it were related to management and district, such as china, extended producer responsibility, and waste management. Figure 4 is a density map of keywords in which the hot spot with red or yellow color stands for high occurrence, and the spot with green color stands for low occurrence. According to this rule, we can easily find that the red spot included the  Fig. 3 Network map of author cooperation relationship who contributed two or more articles keywords e-waste and recycling, which have the highest frequency of occurrence, the ones with yellow or light yellow color consists of 7 keywords, mainly about electronic waste, heavy metal, soil, weee, metal recovery, China, and waste electrical and electronic equipment (weee), which have a relatively high frequency of occurrence, meanwhile the green ones includes pbdes, pahs, pcbs, waste management, extended producer responsibility, lead, polybrominated diphenyl ethers (pbdes), and printed circuit boards, which are of low frequency of occurrence.

Types of study content
Through reading every article carefully, the top cited 100 articles could be classified into four types according to the study content. The four types of the study contents contained characteristic and property, environment and health, management and economic, technique, and processing. The four types were much different in its number of articles. Specifically, as seen in the Figure 5, there were 4 articles about the characteristic-and-property type, 41 articles related to the environment-and-health type, 29 articles associated with the management-and-economic type, and 26 articles concerning the technique-and-processing type. Obviously, the environment-and-health type held the highest quantity of articles and the characteristic-and-property type with the lowest quantity of articles. The two other types of study content were very close in quantity of articles. The total quantity of citations and the percentage of citations of each type rank as same as its quantity of articles, which was also demonstrated in the figure.
However, the average quantity of citations of each type has different features from the former two indexes. Actually, as seen in the Figure 6, the characteristic-and-property type possessed the lowest average quantity of citations with 166, and the technique-and-processing type owned the highest average quantity of citations with 255.1. The average quantity of  According to the quantity of articles of each type, the impact of e-waste on environment and human being's health could be seen as the hot topic of researches on e-waste. On the contrary, the characteristic and property of e-waste were studied much infrequently. Compared with the environmentand-health type, the quantity of articles of technique-andprocessing type was not enough.
However, the technique-and-processing type with the highest average quantity of citations was the most influential, most valuable to other researchers. Consequently, researches of technique and processing on e-waste would have a great potential to become the direction and way forward in the study field on e-waste. Meanwhile the characteristic and property on e-waste require more concern to promote the integrated development of researches on e-waste, because the quantity of articles and average citations of characteristic and property on ewaste in the 100 top cited articles were both the least.
Actually, researchers in the study field could not be restricted to just the four pathways of researches on e-waste. Instead, the researchers could develop new pathways based on and beyond the four ones evaluated in this research to extend the research terrain on e-waste.

Summary of the main results
This research synthetically analyzed the 100 most-cited articles on e-waste in terms of publication years, journals, countries and institutions, authors, keywords, and study contents. The main study results are as follows: (1)  (3) One hundred twenty-three institutions and 25 countries in total took part in publishing the 100 articles. China and its institutions were most influential in this field with biggest number of article and citations. India was becoming a more and more influential country on e-waste research in the world. Nigeria was the research center in Africa. Brazil was the research center in Latin America. (4) Three hundred seventy authors contributed to the 100 articles in total, and Wong Minghung was the most important expert in this field. (5) A total of 267 keywords were acquired in the 100 articles, and the keywords "e-waste" and "recycling" held  Figure 6 The average quantity of citations of each study content type. Xaxis: different types of study content; Y-axis: the average quantity of citations; bubble size: the quantity of articles of specific type the highest occurrences. (6) Through analyzing its study content comprehensively, the study content of the 100 articles could be classified into four types including the characteristic-and-property type, the environment-and-health type, the management-and-economic type, and the techniqueand-processing type. The impact on environment and human being's health was the hot topic of researches on e-waste; the characteristic and property of e-waste were studied not enough. The researches of technique and processing would be the direction and way forward in the study field on e-waste. The characteristic and property on e-waste need to be paid more attention. The researchers could discover new pathways based on and beyond the four types of study content evaluated in this research.
Completeness and applicability of the evidence The evidence mainly described above found by the study in this paper are of great completeness and applicability. The completeness of the evidence could be mainly justified by data resource, search strategy, and the method. The SCI-E of the WoSCC from which the studied 100 articles were selected is a global citation database in which more than 9200 of the most influential journals across 178 scientific disciplines are indexed now. Because the database is publisherindependent and has the robust evaluation and curation of its data, it is most worth being trusted.
The search strategies were specially designed to guarantee that the retrieved articles were the most professional, valuable, and influential. The cooperation and devotion of all the members in the study team ensured the analyzed material and the attained evidences were relevant and accurate. The software tools applied in this study also made a significant contribution to the integrity of the study.
Known as an useful and effective method, the method of bibliometric analysis was utilized in various study fields, such as computer and information science (Bonilla Claudio and Merigo Jose 2015;Cancino et al. 2017;Heradi et al. 2016;Bornmann and Mutz 2015), library science ( (Liu and Liao 2017), green and sustainable science (Ruhanen et al. 2015), manufacturing and engineering (Fahimnia et al. 2015), and achieved plenty of significant study results, which provided an excellent guide and reference for this study.
The applicability of the evidence can be expected in many ways. The materials and evidences in this study can enhance the awareness (Afroz et al. 2013;Borthakur and Govind 2017) of people, organizations, and nations all around the world on management and recycling of e-waste and provide grounds for officers in formulating policies and laws on e-waste (Pariatamby and Victor 2013;Daum et al. 2017;Gok Gulden et al. 2017).
The evidences are especially meaningful for the researches on e-waste in the future. The succeeding researches can seek the causes hidden under the evidences, for instance, find an overall explanation to the question "Why the most articles were published in the year of 2007?", "Why China and India could be the center of researching on e-waste in Asia, Nigeria could be the center of Africa and Brazil could be the center of Latin America?", and so on (Davis et al. 2019;Laser 2016;Breivik Breivik et al. 2016). The researchers can strengthen the studies on the characteristic and properties of e-waste based on the result that the characteristic-and-property type of study contents possessed the least number of articles and the lowest percentage of citations and discover the new study pathways on e-waste except for the four ones found in this study (Gao et al. 2019a(Gao et al. , 2019bFilippo Corsini et al. 2012).
According to the core journals found in this study, the researchers could find the newest and valuable studies on ewaste more quickly and issue their own study results in proper journal (Solarino 2012;San-Miguel et al. 2011;Black and Davies Sally 1999). In addition, based on the core countries, institutions, and experts found in this study, the researchers in the study field of e-waste could develop and explore elaboration more efficiently with global partners (Buse and Gwin 1998;Osterblom and Bodin 2012;Wagner Caroline et al. 2015).
The evidences are also useful for the development of electrical and electronic industries. The workers in those industries can learn advanced processing technologies and managing approaches of recycling e-waste by reading the articles of the core journals; the entrepreneurs can formulate proper strategies and directions of their companies to comply policies or legislations by keeping familiar with the study status and trends of the core countries, institutions, and experts (Davis and Garb 2015;Li et al. 2015;Wang et al. 2020).
In summary, the evidences found in this study possess an overall and high completeness and applicability.

Potential biases in the review process
In order to eliminate the deviation caused by the study, we have made much efforts in many ways. However, there still exist some potential biases in the review process.
The top 100 cited articles retrieved from only SCI-E database may not thoroughly cover all the most valuable studies on e-waste. The search strategies and screening criteria could miss some publications related to e-waste. In terms of citation, this study may not take some other situations into consideration.
Actually, those biases mentioned above do not affect the accuracy and reliability of the final overall results. The evidence found by this study are fundamentally of high completeness and applicability.

Implication for practice and research
This study could provide an instructive significance for other practice and research. When people conduct practices, such as development, engineering, and design, they had better search for the reliable evidences, which could help their activities be more scientific, operative, and effective. As for research, the scholars and academics could utilize the bibliometric analysis or study better method to discover more valuable evidences.

Conclusion
To conclude, the trends of researches to date on e-waste have found that the three years, 2007, 2008, and 2009, were a peak, while the recent years were experiencing a valley. The study found that the journal the Waste Management, Journal of Hazardous Materials, and Environmental Science & Technology were the core journals on e-waste. China and its institutions were most influential in this field. India was becoming more and more influential on e-waste research in the world. Nigeria was the research center in Africa. Brazil was the research center in Latin America. The expert Wong Minghung was the most important expert in this field. "Ewaste" and "recycling" occurred most frequently in the articles on e-waste.
The impact on environment and human being's health was the hot topic of researches on e-waste; the characteristic and property of e-waste were studied inadequately. The researches of technique and processing would be the direction and way forward on e-waste of recycling and management in the future. The characteristic and property on e-waste need to be more concerned. The researchers could develop new pathways based on and beyond the four types of study content evaluated in this research.
Abbreviations WoSCC, Web of Science Core Collection; SCI-E, Science Citation Index-Expanded; IF, Impact factor Supplementary Information The online version contains supplementary material available at https://doi.org/10.1007/s11356-021-15147-z. Acknowledgements The authors give sincere gratitude to the editor and reviewer.
Author contribution Xianghong Chen accomplished the comprehensive research. Jingting Wei conducted the retrieval. Liwei Chen and Bo Xie used the software tool to analyze the data. Xianghong Chen wrote the paper. Ming Liu proposed ideas and structure of the paper. All authors approved the final manuscript.
Data Availability The datasets generated and/or analyzed during the current study are available in the SCI-E of the WoSCC repository (https:// clarivate.com/webofsciencegroup/solutions/webofscience-scie/).

Ethical approval Not applicable
Informed consent No informed consent is required as all analyses were based on previously published studies.

Consent for publication Not applicable
Competing interests The authors declare no competing interests.