Introduction 介绍

Increasing evidence confirms that microplastics (MPs) distribute in nearly every ecosystem on the planet, including marine and freshwater environments, soil, atmosphere, and even Arctic areas (Murphy et al. 2016; Auta et al. 2017; Morgana et al. 2018; Chen et al. 2020; Wang et al. 2022c). One estimation shows that the global release of primary MPs from commercial and household activities into the environment is in the order of 3.2 million tons/year (Boucher and Friot 2017). Another estimation shows that up to 430,000 and 300,000 tons MPs per year release into European and North American farmlands, respectively (Nizzetto et al. 2016). MPs can cause various negative impacts on environments and the organisms therein. After ingestion by organisms, MPs can accumulate in their digestive tract (Ma et al. 2020; Wang et al. 2022d), or excrete as fake feces, interfering with their energy flow (Ma et al. 2020). Previous studies have confirmed a series of negative consequences caused by MPs, such as oxidative stress, stunted growth and reduced fecundity in marine organisms, inhibited photosynthesis in phytoplankton in freshwater environments, and damage in liver organs in animals (Huang et al. 2021; Sun et al. 2022b). After entering the soil, MPs can affect soil properties and communities and their functions, inhibit plant nutrition and growth, and then cause serious damage to agroecosystems (Sun et al. 2022b; Wang et al. 2022b, 2022c, 2022d). MPs can also pose a human health risk through the food chain (Avio et al. 2017; Prata et al. 2020; Huang et al. 2021). Notably, MPs are also found in human blood, placenta, and lungs (Amato-Lourenço et al. 2021; Ragusa et al. 2021; Leslie et al. 2022), posing a potential health risk.
越来越多的证据证实，微塑料 (MP) 几乎分布在地球上的每个生态系统中，包括海洋和淡水环境、土壤、大气，甚至北极地区（Murphy 等人， 2016 年；Auta 等人， 2017 年；Morgana 等人， 2018 年；陈等人， 2020 ；王等人， 2022c ）。一项估计显示，全球商业和家庭活动向环境中释放的初级 MP 量约为 320 万吨/年（Boucher 和 Friot， 2017 年）。另一项估计表明，每年有多达 43 万吨和 30 万吨 MP 分别释放到欧洲和北美农田中（Nizzetto 等人， 2016 年）。 MP 会对环境及其中的生物体造成各种负面影响。被生物体摄入后，MP可以在其消化道中积聚（Ma et al. 2020 ；Wang et al. 2022d ），或作为假粪便排出体外，干扰其能量流动（Ma et al. 2020 ）。以往的研究已经证实MPs会造成一系列负面后果，如海洋生物的氧化应激、生长发育迟缓和繁殖力降低、淡水环境中浮游植物光合作用受到抑制、动物肝脏器官受损等（Huang等， 2021 ；Sun）等2022b ）。 MPs进入土壤后会影响土壤性质和群落及其功能，抑制植物营养和生长，进而对农业生态系统造成严重破坏(Sun等， 2022b ；Wang等， 2022b ， 2022c ， 2022d )。国会议员还可以通过食物链对人类健康构成风险（Avio 等人， 2017 年；Prata 等人， 2020 年；Huang 等人，2020 年）。 2021 ）。值得注意的是，MP 也存在于人体血液、胎盘和肺部中（Amato-Lourenço 等人， 2021 年；Ragusa 等人， 2021 年；Leslie 等人， 2022 年），构成潜在的健康风险。

Heavy metals are common environmental pollutants in various ecosystems. Previous studies have found that MPs and heavy metals coexist in marine, freshwater, and soil environments (Turner 2016; Zhou et al. 2019; Khalid et al. 2021). Heavy metals are used as catalysts in plastic production and can release into the environment with the decay of MPs (Hahladakis et al. 2018). MPs with small sizes and large specific surface areas can adsorb metals through surface electrostatic interaction, biofilm, or natural organic matter to form new complexes, thus producing a carrier effect on heavy metals (Cao et al. 2021; Gao et al. 2021). MPs can transport heavy metals into living organisms, leading to joint toxicity (Liu et al. 2021a). Co-contamination of MPs and heavy metals can alter soil microbiota and biological processes involved in C and N cycling (Wang et al. 2022c; Salam et al. 2023; Zhang et al. 2023). Co-exposure to MPs and heavy metals can cause severer oxidative stress responses and higher toxicity in higher plants, particularly crops (Kumar et al. 2022; Wang et al. 2022b; Huang et al. 2023), posing a threat to food safety. In particular, MPs carrying heavy metals can enter the bodies of humans and animals through ingestion, inhalation, and skin contact, causing health problems (Cao et al. 2021).
重金属是各种生态系统中常见的环境污染物。先前的研究发现MPs和重金属在海洋、淡水和土壤环境中共存（Turner 2016 ；Zhou et al. 2019 ；Khalid et al. 2021 ）。重金属在塑料生产中用作催化剂，并会随着 MP 的腐烂而释放到环境中（Hahladakis 等人， 2018 ）。尺寸小、比表面积大的MPs可以通过表面静电相互作用、生物膜或天然有机物吸附金属，形成新的复合物，从而对重金属产生载体效应(Cao等， 2021 ；Gao等， 2021 )。 MP 可以将重金属转运到生物体中，导致关节毒性（Liu et al. 2021a ）。 MP 和重金属的共同污染可以改变土壤微生物群和参与碳氮循环的生物过程（Wang 等人， 2022c ；Salam 等人， 2023 ；Zhang 等人， 2023 ）。 MPs和重金属的共同暴露会导致高等植物，特别是农作物出现更严重的氧化应激反应和更高的毒性（Kumar等人， 2022 ；Wang等人， 2022b ；Huang等人， 2023 ），对食品安全构成威胁。特别是，携带重金属的MP可以通过摄入、吸入和皮肤接触进入人类和动物体内，造成健康问题（Cao et al. 2021 ）。

There have been some excellent reviews on the combined pollution of MPs and heavy metals. Kutralam-Muniasamy et al. (2021) reviewed the detection and analysis methods, pollution status, and migration risks of MPs and heavy metals in the environment. Gao et al. (2021) reviewed the behavior and influencing factors of heavy metal adsorption by MPs. Liu et al. (2021a) reviewed the effects of MPs on the mobility, bioavailability, and toxicity of heavy metals. Several excellent reviews have addressed the interactions of MPs and heavy metals in aquatic or terrestrial environments and their combined effects on living organisms and humans (Naqash et al. 2020; Cao et al. 2021; Khalid et al. 2021; Liu et al. 2021b, 2022; Kumar et al. 2022; Khoshmanesh et al. 2023). However, these reviews only focus on one or several particular environmental directions. Bibliometrics can provide a scientific approach to assess research trends from the growth of literature on a particular topic through a visual approach (Li et al. 2022a; Zeb et al. 2022) and can analyze all relevant countries, institutions, journals, authors, references, and keywords in the collected publications (Chen et al. 2016), which can help to understand the current advances, hotspots, and trends of a specific topic in an intuitive way. To our knowledge, no bibliometric analysis has been conducted on the interaction of MPs and heavy metals at a global scale.
关于 MP 和重金属的综合污染，已有一些精彩的评论。库特拉拉姆-穆尼亚萨米等人。 ( 2021 )对环境中MPs和重金属的检测分析方法、污染现状及迁移风险进行了综述。高等人。 ( 2021 )综述了MPs吸附重金属的行为及其影响因素。刘等人。 ( 2021a ) 回顾了 MP 对重金属迁移率、生物利用度和毒性的影响。几篇优秀的评论讨论了 MP 和重金属在水生或陆地环境中的相互作用及其对生物体和人类的综合影响（Naqash 等人， 2020 年；Cao 等人， 2021 年；Khalid 等人， 2021 年；Liu 等人， 2021b） ， 2022 ； Khoshmanesh等人。然而，这些评论仅关注一个或几个特定的​​环境方向。文献计量学可以提供一种科学方法，通过视觉方法评估特定主题的文献增长的研究趋势（Li et al. 2022a ；Zeb et al. 2022 ），并且可以分析所有相关国家、机构、期刊、作者、参考文献、以及收集的出版物中的关键词（Chen et al. 2016 ），可以帮助直观地了解特定主题的当前进展、热点和趋势。据我们所知，尚未在全球范围内对 MP 与重金属的相互作用进行文献计量分析。

In this study, VOSviewer, Pajek64, and CiteSpace were used to analyze the literature on the interactions of MPs and heavy metals. The objective of this study was to visually analyze the current hotspots and trends in MPs and heavy metals, as well as the association or collaboration analysis of leading journals, countries, institutions, and authors. Finally, based on the results and knowledge gaps, we recommended several priority directions. Our results can help the researchers to comprehensively recognize the research advances and future perspectives of this field.
在本研究中，使用 VOSviewer、Pajek64 和 CiteSpace 来分析有关 MP 与重金属相互作用的文献。本研究的目的是直观地分析 MP 和重金属的当前热点和趋势，以及领先期刊、国家、机构和作者的关联或合作分析。最后，根据结果和知识差距，我们推荐了几个优先方向。我们的研究结果可以帮助研究人员全面认识该领域的研究进展和未来前景。

Methodology 方法论

Date source and search criteria
日期来源和搜索条件

The data were obtained by searching the Web of Science Core Collection database from 2008 to July 5, 2022, using the following terms: microplastic * OR nanoplastic * OR micro-plastic OR (nano)microplastic * OR (micro)nanoplastic * AND heavy metal * OR Cu OR Pb OR Zn OR Fe OR Cr OR Cd OR Hg OR Ni OR Mn OR Cobalt OR arsenic. A total of 840 search results were obtained without restrictions of document type or data category. Then, we selected the results by reading their titles and abstracts. Finally, the search results, including complete records in plain text format and cited references, were exported for subsequent data analysis.
数据是通过搜索 2008 年至 2022 年 7 月 5 日期间的 Web of Science 核心合集数据库获得的，使用以下术语：微塑料 * OR 纳米塑料 * OR 微塑料 OR (纳米)微塑料 * OR (微)纳米塑料 * AND 重金属* 或铜或铅或锌或铁或铬或镉或汞或镍或锰或钴或砷。在不限制文档类型或数据类别的情况下，总共获得了 840 条搜索结果。然后，我们通过阅读标题和摘要来选择结果。最后，导出搜索结果，包括纯文本格式的完整记录和引用的参考文献，以供后续数据分析。

Scientometric analysis 科学计量分析

To reflect the hotspots and trends of a specific domain in multiple dimensions, VOSviewer (Version 1.6.18), Pajek64 (Portable 5.15b), and CiteSpace (Version 5.8.R3) were used to analyze the document types, years, authors, institutions, countries, journal sources, keywords, and references to form visual network maps. Data were summarized using Microsoft Excel 2016. The importance of node content is represented by the size of nodes and the thickness of lines in the visualization diagram (Padilla et al. 2018). The size of nodes indicates the number or frequency, and the line between nodes indicates the association. A thicker line represents a closer relationship (Gao et al. 2019). Therefore, the research trend can be displayed by analyzing the network visualization graph generated from literature data, and the research prospects can be obtained. Some hotspot articles are also reviewed when we discuss the research progress and knowledge gaps.
为了多维度反映特定领域的热点和趋势，使用VOSviewer（1.6.18版本）、Pajek64（Portable 5.15b）、CiteSpace（5.8.R3版本）对文献类型、年份、作者、机构进行分析、国家、期刊来源、关键词和参考文献，形成可视化网络地图。使用 Microsoft Excel 2016 汇总数据。节点内容的重要性通过可视化图中节点的大小和线条的粗细来表示（Padilla et al. 2018 ）。节点的大小表示数量或频率，节点之间的线表示关联性。线越粗代表关系越密切（Gao et al. 2019 ）。因此，通过分析文献数据生成的网络可视化图可以展示研究趋势，并得出研究前景。在讨论研究进展和知识差距时，我们也会回顾一些热点文章。

Results and discussion 结果与讨论

The quantity and type of publications and the dominant journals
出版物数量、类型及主导期刊

A total of 552 articles on MPs and heavy metals were selected for this study, including original research articles (471 items, accounting for 85% of the total number) and review articles (81 items, 15%) (Fig. 1a). The first publication on MPs and heavy metals was published on June 14, 2014, by Holmes et al. in the journal “Marine Chemistry.” Since 2019, the number of publications has increased significantly (Fig. 1b, Table S1). From 2014 to 2021, the number of papers grew exponentially (R2 = 0.9915). In 2021, the annual number of articles published reached 228, accounting for 41.30% of the total number.
本研究共选取了552篇有关MPs和重金属的文章，其中包括原创研究文章（471篇，占总数的85%）和综述文章（81篇，占15%）（图1a ）。第一份关于 MP 和重金属的出版物由 Holmes 等人于 2014 年 6 月 14 日出版。在《海洋化学》杂志上。自2019年以来，出版物数量显着增加（图1 b，表S 1 ）。从2014年到2021年，论文数量呈指数增长（ R 2 = 0.9915）。 2021年全年发表文章数达到228篇，占总数的41.30%。

Fig. 1 图1
figure 1

The number (a), document type (b), journals (c) and country cooperation (d). The details of country labels are shown in Tables S3 and S4. The size of the node represents the number of publications; the connecting line represents the cooperation between the countries
数量（ a ）、文献类型（ b ）、期刊（ c ）和国家合作（ d ）。国家标签的详细信息如表S 3和S 4所示。节点的大小代表出版物的数量；连接线代表国家之间的合作

Full size image 全尺寸图像

These publications were published by a total of 124 journals. The top 16 journals are shown in Fig. 1c and Table S2. There were close citation relationships among these journals. The journal Science of the Total Environment published the highest number of papers (88 papers, accounting for 15.94%), followed by Journal of Hazardous Materials (61, 11.05%), Environmental Pollution (50, 9.06%), Chemosphere (50, 9.06%), and Marine Pollution Bulletin (44, 7.97%). All five journals received citations more than 1000 times.
这些出版物共有 124 种期刊发表。排名前 16 的期刊如图1c和表 S 2所示。这些期刊之间存在着密切的引用关系。 Science of the Total Environment 期刊发文数量最多（88 篇，占比 15.94%），其次是 Journal of Hazardous Materials（61 篇，占比 11.05%）、Environmental Pollution（50 篇，占比 9.06%）、Chemosphere（50 篇，占比 9.06）。 %) 和海洋污染公报 (44, 7.97%)。所有五种期刊的引用均超过 1000 次。

Contributing countries, institutions, and authors
贡献国家、机构和作者

The authors of the 552 papers come from 841 institutions in 70 countries (Figs. 1 and 2). The partnerships of major contributing countries (top 30) and institutions (top 44) are shown in Table S3, Table S4, and Table S5, respectively. The country with the highest number (267) of published papers is China, accounting for 48.37%, followed by India (47, 8.51%), America (44, 7.97%), UK (34, 6.16%), South Korea (33, 5.98%), Australia (26, 4.71%), and Spain (26, 4.71%). Chinese ACAD SCI has published the most papers (35), accounting for 6.34%, followed by UNIV Chinese ACAD SCI (21, 3.80%), Univ Plymouth (18, 3.26%), Tongji Univ (15, 2.72%), and Hunan Univ (15 papers, 2.72%). As shown in Fig. 1d, the major contributing countries have close cooperation. The publications of the major contributing institutions were published after 2020 (Fig. 2a). The cooperation among the major contributing organizations is limited (Fig. 2c).
552篇论文的作者来自70个国家的841个机构（图1和图2 ）。主要贡献国家（前30名）和机构（前44名）的伙伴关系分别如表S 3 、表S 4和表S 5所示。发表论文数量最多的国家（267 篇）是中国，占 48.37%，其次是印度（47 篇，8.51%）、美国（44 篇，7.97%）、英国（34 篇，6.16%）、韩国（33 篇）。 ， 5.98%）、澳大利亚（26， 4.71%）和西班牙（26， 4.71%）。中国 ACAD SCI 发表论文最多（35 篇，占比 6.34%），其次是 UNIV 中国 ACAD SCI（21 篇，3.80%）、普利茅斯大学（18 篇，3.26%）、同济大学（15 篇，2.72%）、湖南大学大学（15 篇论文，2.72%）。如图1d所示，主要出资国合作密切。主要贡献机构的出版物均在2020年后出版（图2a ）。主要贡献组织之间的合作是有限的（图2c ）。

Fig. 2 图2
figure 2

The overlay (a), density (b), and network (c) of top 44 institutions. The size of the node represents the number of publications; the connecting line represents the cooperation between the institutions
排名前 44 的机构的覆盖 ( a )、密度 ( b ) 和网络 ( c )。节点的大小代表出版物的数量；连接线代表机构之间的合作

Full size image 全尺寸图像

The cooperation map of the authors can reflect the current situation of mutual communication and cooperation in the field. A total of 2623 authors contributed to the 552 papers, and 39 authors published more than 5 articles. The author with the largest number of publications (15 papers) is Andrew Turner, from Plymouth University, followed by Zhengguo Song’s team (10 papers), and Julien Gigault (9 papers) (Table S6). Among these 39 authors, 32 of them had cooperative papers, particularly in the year 2021 (Fig. 3). These authors’ information is shown in Table S6. Figure 3 c shows that there are nine author collaboration clusters, but there is no collaboration among the authors outside of the clusters.
作者的合作图谱能够反映该领域相互交流与合作的现状。共有2623位作者贡献了552篇论文，其中39位作者发表了5篇以上的文章。发表论文数量最多的作者（15篇论文）是来自普利茅斯大学的Andrew Turner，其次是宋正国团队（10篇论文）和Julien Gigault团队（9篇论文）（表S 6 ）。在这39位作者中，有32位有合作论文，特别是在2021年（图3 ）。这些作者的信息如表S 6所示。图3c显示有9个作者协作集群，但集群外的作者之间没有协作。

Fig. 3 图3
figure 3

The overlay (a), density (b), and network (c) of top 39 authors. The size of the node represents the number of publications; the connecting line represents the cooperation between the authors
前 39 位作者的叠加 ( a )、密度 ( b ) 和网络 ( c )。节点的大小代表出版物的数量；连接线代表作者之间的合作

Full size image 全尺寸图像

Annual variation analysis of high co-citation and burst references
高同被引和突发参考文献的年度变化分析

Co-citation analysis provides a tool to quantify and visualize the thematic evolution of a specific research area (Cobo et al. 2011). The co-citation of documents was displayed using VOSviewer. Among the 552 articles, their references with more than 50 citations are listed in Table S7. Burst references can reveal the high attention to a research topic in a certain period. Fig. S1 shows the top 25 references with the strongest citation bursts. The annual variation of some important co-cited references in the field of MPs and heavy metals since 2014 is shown Fig. 4 and Fig. S2. Details of these articles are shown in Table S8. By analyzing these cited references, several research directions of high concern can be summarized below.
同被引分析提供了一种工具来量化和可视化特定研究领域的主题演变（Cobo 等人， 2011 ）。使用VOSviewer显示文献的共引情况。在552篇文章中，被引用次数超过50次的参考文献列于表S 7中。爆发引用可以揭示某一时期某个研究主题的高度关注度。图 S 1显示了引用爆发最强的前 25 篇参考文献。自2014年以来MP和重金属领域一些重要同被引用文献的年度变化如图4和图S 2所示。这些文章的详细信息如表 S 8所示。通过分析这些引用的参考文献，可以总结出几个备受关注的研究方向。

Fig. 4 图4
figure 4

Annual variation of co-cited references (Sankey diagram, evolutionary process, and citation hotspot change). Red marks represent the first citation of a particular type of research. The wider the connection between the two documents in the figure, the closer the evolutionary relationship is
同被引用参考文献的年度变化（桑基图、演化过程和引用热点变化）。红色标记代表特定类型研究的首次引用。图中两个文档之间的联系越广，进化关系越密切

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The occurrence and abundance of MPs in the environment and organisms is of the highest priority. The first widely recognized study on MPs was published in Science in 2004, which reported the occurrence of MPs in marine sediments (Thompson et al. 2004). Thereafter, increasing studies have confirmed the occurrence of MPs in aquatic and terrestrial environments (Andrady 2011; Browne et al. 2011; Horton et al. 2017). In general, microplastic abundance is higher in the areas of intensive human activities, such as ports (Claessens et al. 2011) and agricultural soil (Zhang and Liu 2018). The abundance of MPs is also high in the places where material exchange occurs frequently, such as coastlines (Cole et al. 2011; Pan and Wang 2012), estuaries (Nicolaus et al. 2015), and Subtropical Gyre (Ter Halle et al. 2017). One survey found that MPs were distributed in three dimensions in the environment, and even in coastal areas as deep as 2 m underground (Turra et al. 2014). MPs are also widely detected in sewage treatment plants (Mason et al. 2016). It is found that MPs can only be largely removed in the primary treatment process, and secondary and tertiary wastewater treatment cannot effectively remove MPs (Carr et al. 2016). MPs have also been found in more remote areas, such as the deep ocean (Van Cauwenberghe et al. 2013), remote mountain lakes (Free et al. 2014), and the North Pole (Lusher et al. 2015; Amélineau et al. 2016). Meanwhile, MPs can be ingested by a variety of organisms, including marine (Cole et al. 2011) and freshwater organisms (Eerkes-Medrano et al. 2015). The researchers examined the abundance of MPs in fish (Boerger et al. 2010; Lusher et al. 2013) and mussels (Qu et al. 2018), through consuming which humans may have access to large amounts of MPs in their diets (Van Cauwenberghe and Janssen 2014). In an aqueous environment, the density, size, shape, ageing degree, and abundance of MPs will determine their availability to living organisms (Lima et al. 2014; Botterell et al. 2019).
MP 在环境和生物体中的出现和丰度是重中之重。第一篇得到广泛认可的关于 MP 的研究发表在 2004 年的《科学》杂志上，报告了海洋沉积物中 MP 的存在（Thompson 等， 2004 ）。此后，越来越多的研究证实MP在水生和陆地环境中存在（Andrady 2011 ；Browne et al. 2011 ；Horton et al. 2017 ）。一般来说，人类活动密集的地区微塑料丰度较高，例如港口（Claessens等， 2011 ）和农业土壤（Zhang和Liu， 2018 ）。在物质交换频繁的地方，如海岸线(Cole et al. 2011 ；Pan and Wang 2012 )、河口(Nicolaus et al. 2015)、亚热带环流(Ter Halle et al. 2012 )等物质交换频繁的地方，MP的丰度也很高。 2017 ）。一项调查发现，MP在环境中呈三维分布，甚至在地下2 m深处的沿海地区也有分布（Turra et al. 2014 ）。 MP 在污水处理厂中也广泛检测到（Mason 等， 2016 ）。研究发现，MPs只能在一级处理过程中大量去除，二级和三级废水处理无法有效去除MPs(Carr等， 2016 )。在更偏远的地区也发现了MP，例如深海（Van Cauwenberghe等人， 2013年）、偏远的高山湖泊（Free等人， 2014年）和北极（Lusher等人， 2015年；Amélineau等人，2015年）。 2016 ）。同时，MP 可以被多种生物体摄入，包括海洋生物（Cole 等，2017）。 2011 ）和淡水生物（Eerkes-Medrano 等人， 2015 ）。研究人员检查了鱼类（Boerger 等人， 2010 年；Lusher 等人， 2013 年）和贻贝（Qu 等人， 2018 年）中 MP 的丰度，通过食用这些贻贝，人类可以从饮食中获取大量 MP（Van Cauwenberghe）和詹森2014 ）。在水环境中，MP 的密度、大小、形状、老化程度和丰度将决定它们对生物体的可用性（Lima 等人， 2014 年；Botterell 等人， 2019 年）。

The second hotspot is the adsorption, enrichment, and carrying of heavy metals by MPs. The adsorption of heavy metals by MPs occurs commonly in the environment (Rochman et al. 2014a; Wang et al. 2017). The main mechanism underlying heavy metal adsorption by MPs is electrostatic interaction (Guo et al. 2020). The association of heavy metals with MPs is closely related to the concentration of heavy metals in the environment (Zhou et al. 2019). The ability of MPs to adsorb heavy metals is also affected by pH (Turner and Holmes 2015), temperature (Wang et al. 2020b), surface characteristics of MPs (Avio et al. 2017; Tang et al. 2020), and exposure time (Rochman et al. 2014a). Some studies have found that the aged MPs adsorb much more pollutants than the original MPs (Holmes et al. 2014; Wang et al. 2020b). A field investigation found that MPs can be enriched with dangerous metals, such as Cd and Pb, increasing the risk of metals ingested by organisms (Ashton et al. 2010). MPs can act as carriers of heavy metals and other pollutants, and thus facilitate the transport of pollutants in aquatic environments (Browne et al. 2013; Brennecke et al. 2016; Alimi et al. 2018). MPs effectively increase the intake of Cd, Pb, Br, and Hg by organisms (Turner 2018; Fernández et al. 2020). MPs and related contaminants ingested by low-trophic organisms can be transferred to higher-trophic organisms (Wright et al. 2013).
第二个热点是MPs对重金属的吸附、富集和携带。 MP 对重金属的吸附在环境中普遍存在（Rochman 等人， 2014a ；Wang 等人， 2017 ）。 MPs 吸附重金属的主要机制是静电相互作用（Guo et al. 2020 ）。重金属与MPs的关联与环境中重金属的浓度密切相关(Zhou et al. 2019 )。 MP 吸附重金属的能力还受到 pH 值（Turner 和 Holmes 2015 ）、温度（Wang 等人， 2020b ）、MP 表面特性（Avio 等人， 2017 ；Tang 等人， 2020 ）和暴露时间的影响。 （Rochman 等人， 2014a ）。一些研究发现，老化的 MP 比原来的 MP 吸附的污染物要多得多（Holmes 等， 2014 ；Wang 等， 2020b ）。一项现场调查发现，MP 中可能富含危险金属，例如镉和铅，增加了生物体摄入金属的风险（Ashton 等人， 2010 ）。 MP可以作为重金属和其他污染物的载体，从而促进污染物在水生环境中的传输（Browne等人， 2013 ；Brennecke等人， 2016 ；Alimi等人， 2018 ）。 MP 有效增加生物体对 Cd、Pb、Br 和 Hg 的摄入量（Turner 2018 ；Fernández et al. 2020 ）。低营养级生物体摄入的 MP 和相关污染物可以转移到高营养级生物体中 (Wright et al. 2013 )。

Third, the toxicity of MPs to animals attracts increasing attention. MPs can accumulate in living organisms and cause inflammation, tissue damage, oxidative stress, neurotoxicity (Brandts et al. 2018), endocrine system function, lipid and energy metabolism, and the expression of genes (Rochman et al. 2014b; Lu et al. 2016; Rodriguez-Seijo et al. 2017; Barboza et al. 2020). MPs can cause physical damage and chemical transfer of toxicants when ingested by organisms (Wright et al. 2013; Eerkes-Medrano et al. 2015;). Co-exposure to MPs and heavy metals can adversely affect living organisms, such as inhibited growth and increased metal accumulation (Barboza et al. 2018; Wang et al. 2022d). A previous study found that polystyrene MPs can enhance the accumulation and toxicity of Cd in zebrafish (Lu et al. 2018).
第三，MPs对动物的毒性日益引起人们的关注。 MP可以在生物体中积聚并引起炎症、组织损伤、氧化应激、神经毒性（Brandts等， 2018 ）、内分泌系统功能、脂质和能量代谢以及基因表达（Rochman等， 2014b ；Lu等，2014）。 2016 ；罗德里格斯·塞乔等人，2020 ； MP 被生物体摄入后会造成物理损伤和有毒物质的化学转移（Wright 等人， 2013 年；Eerkes-Medrano 等人， 2015 年；）。同时接触 MP 和重金属会对生物体产生不利影响，例如生长受到抑制和金属积累增加（Barboza 等人， 2018 年；Wang 等人， 2022d ）。之前的一项研究发现，聚苯乙烯 MPs 可以增强斑马鱼体内镉的积累和毒性（Lu et al. 2018 ）。

Another hotspot is the effects of MPs in terrestrial ecosystems (de Souza Machado et al. 2018; Zhang et al. 2022b). MPs can enter and accumulate in soils through multiple pathways, producing ecological effects on soil physical, chemical, and biological properties and soil functionality (Wang et al. 2022c; Zhao et al. 2022a). MPs can interfere with the germination of plants (Bosker et al. 2019), and impede plant growth, inhibit photosynthesis, and interfere with nutrient metabolism, causing oxidative damage and genotoxicity in plants (Zhang et al. 2022b). MPs can change soil properties and soil ecosystem functions via mediating plant growth and earthworm’s health (Boots et al. 2019) and soil microbial communities (Huang et al. 2019). As a vector of metals, MPs can increase the exposure of metals to earthworms and improve the bioavailability of metals (Hodson et al. 2017), following which the combined toxicity of MPs and heavy metals in terrestrial ecosystems has become a hotspot attracting wide attention (Khalid et al. 2021).
另一个热点是 MPs 对陆地生态系统的影响（de Souza Machado 等人， 2018 ；Zhang 等人， 2022b ）。 MPs可以通过多种途径进入土壤并在土壤中积累，对土壤物理、化学、生物性质和土壤功能产生生态效应（Wang等， 2022c ；Zhao等， 2022a ）。 MPs会干扰植物的发芽(Bosker et al. 2019 )，并阻碍植物生长，抑制光合作用，干扰养分代谢，造成植物氧化损伤和遗传毒性(Zhang et al. 2022b )。 MP可以通过调节植物生长和蚯蚓的健康​​（Boots et al. 2019 ）和土壤微生物群落（Huang et al. 2019 ）来改变土壤性质和土壤生态系统功能。 MPs作为金属的载体，可以增加蚯蚓对金属的暴露量，提高金属的生物利用度(Hodson et al. 2017 )，随后MPs与重金属在陆地生态系统中的联合毒性问题成为人们广泛关注的热点。哈立德等人， 2021 ）。

Cluster analysis of hot keywords
热门关键词聚类分析

There are a total of 2480 keywords in the 552 papers. We combined some keywords with the same meaning, such as “microplastics (MPs),” “microplastic,” and “microplastic pollution” (Table S9). After that, 167 keywords with more than 5 keyword occurrences were used to draw the knowledge graph of the keyword co-occurrence network (Fig. 5). In order to analyze keyword clustering more clearly, we exported map, network, and VOS files of Fig. 5 from VOSviewer, imported them into Pajek, and rearranged them with “Kamada-Kawai” and “In Y Direction” methods to obtain Figs. 6, S3, and S4, respectively.
552篇论文中共有2480个关键词。我们将一些具有相同含义的关键词组合在一起，例如“微塑料（MP）”、“微塑料”和“微塑料污染”（表S 9 ）。之后，使用167个出现次数超过5次的关键词绘制关键词共现网络的知识图谱（图5 ）。为了更清楚地分析关键词聚类，我们从VOSviewer导出图5的地图、网络和VOS文件，导入Pajek，并使用“Kamada-Kawai”和“Y方向”方法重新排列，得到图5和图5。分别如图6 、S 3和S 4所示。

Fig. 5 图5
figure 5

The overlay (a), density (b), and network (c) of keywords analysis. The size of the node represents the frequency of the keyword; the connecting line represents the total link strength
关键词分析的叠加（ a ）、密度（ b ）和网络（ c ）。节点的大小代表关键词出现的频率；连接线代表总链接强度

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Fig. 6 图6
figure 6

The network of keyword clusters mapped by “Kamada-Kawai” (A) and “In Y Direction” (a), respectively. The size of the node represents the frequency of the keyword; the connecting line represents the total link strength
分别由“Kamada-Kawai”（A）和“Y 方向”（a）映射的关键字簇网络。节点的大小代表关键词出现的频率；连接线代表总链接强度

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Understandably, the keyword microplastics has the highest frequency (364 times). Other hot keywords include “metals” (319 times), “adsorption” (159 times), “marine” (151 times), “pollution” (107 times), “sediments” (84 times), “debris” (81 times), “toxicity” (78 times), “contamination” (70 times), “accumulation” (68 times), “cadmium” (63 times), “resin pellets” (63 times), and “nanoplastics” (60 times) (Table S10).
可以理解的是，关键词“微塑料”的出现频率最高（364次）。其他热门关键词包括“金属”（319次）、“吸附”（159次）、“海洋”（151次）、“污染”（107次）、“沉积物”（84次）、“碎片”（81次） ）、“毒性”（78次）、“污染”（70次）、“蓄积性”（68次）、“镉”（63次）、“树脂颗粒”（63次）、“纳米塑料”（60次） ）（表 S 10 ）。

The Overlay map shows the average occurrence year of each keyword (Fig. 5a and Fig. S3). Some keywords appear earlier (green color), such as marine, debris, resin pellet, “chemicals,” and “pellets.” Some other keywords appear later (yellow color), such as toxicity, “PS-MPs,” “soil,” “removal,” “bioaccumulation,” “copper,” “arsenic,” “zinc,” “biofilm,” “communities,” and “organisms.” In particular, the keywords such as microplastics and metals must have appeared early, but the average time is after 2020, indicating the rapid growth of research in this area in recent years, which is consistent with the results in Fig. 1b. The density map shows a density visualization of keywords and hotspot intensities (Fig. 5b and Fig. S4). The keywords with the highest density (red and yellow) are microplastics, metals, adsorption, marine, pollution, sediments, debris, toxicity, contamination, accumulation, “water,” resin pellets, cadmium, nanoplastics, and PS-MPs. The Network map shows the co-occurrence relationship of keywords (Fig. 5c). The analysis of high-frequency keywords in each keyword cluster can reveal the research hotspots in the field of MPs and heavy metals from 2014 to 2022. These keywords are divided into six clusters (Fig. 6), and different colors indicate that the keywords belong to different clusters. The detailed information on each hot keyword is shown in Table S10.
Overlay图显示了每个关键词的平均出现年份（图5a和图S3 ）。一些关键词出现较早（绿色），例如海洋、碎片、树脂颗粒、“化学品”和“颗粒”。其他一些关键词稍后出现（黄色），例如毒性、“PS-MP”、“土壤”、“去除”、“生物累积”、“铜”、“砷”、“锌”、“生物膜”、“社区” 、”和“有机体”。尤其是微塑料、金属等关键词肯定出现较早，但平均时间在2020年以后，说明近年来该领域的研究增长迅速，与图1b的结果一致。密度图显示了关键词和热点强度的密度可视化（图5b和图S4 ）。密度最高的关键词（红色和黄色）是微塑料、金属、吸附、海洋、污染、沉积物、碎片、毒性、污染、积累、“水”、树脂颗粒、镉、纳米塑料和 PS-MP。网络图显示了关键词的共现关系（图5c ）。对每个关键词簇中的高频关键词进行分析，可以揭示2014年至2022年MPs和重金属领域的研究热点。这些关键词被分为6个簇（图6 ），不同颜色表示关键词所属到不同的集群。每个热门关键词的详细信息如表S 10所示。

Cluster 1 (red) has three core keywords toxicity, accumulation, and fish, indicating that it focuses on the toxicity and accumulation of MPs and heavy metals by organisms (particularly fish). Both MPs and loaded heavy metals can be absorbed by organisms, producing toxic effects. There is a correlation between the metal content in organisms and the metal content adsorbed by MPs isolated from organisms (Zhu et al. 2020), suggesting that MPs can increase the bioaccumulation of heavy metals in living organisms (Yang et al. 2022c). A large number of studies have shown that MPs can aggravate the accumulation of metals in organisms and that co-existing MPs and metals can produce higher toxicity than alone (Lu et al. 2018; Banaee et al. 2019; Wan et al. 2021; Wang et al. 2021b; Luo et al. 2022; Zhang et al. 2022a). For example, polystyrene MPs increased Cd accumulation in zebrafish and co-exposure to Cd and MPs induced oxidative damage and inflammation (Lu et al. 2018). Biofilms can enhance the combined toxicity of MPs and heavy metals (Qi et al. 2021). However, in some cases, due to the adsorption of heavy metals by MPs, the bioavailability of heavy metals is reduced, and the toxicity of heavy metals is delayed (Wen et al. 2018; Wang et al. 2021c).
集群1（红色）具有三个核心关键词毒性、积累和鱼类，表明其重点关注生物体（特别是鱼类）对MP和重金属的毒性和积累。 MP 和负载的重金属都可以被生物体吸收，产生毒性作用。生物体中的金属含量与从生物体中分离出的MPs吸附的金属含量之间存在相关性(Zhu et al. 2020 )，这表明MPs可以增加生物体内重金属的生物富集(Yang et al. 2022c )。大量研究表明，MPs会加剧生物体内金属的积累，MPs与金属共存会产生比单独存在更高的毒性(Lu et al. 2018 ; Banaee et al. 2019 ; Wan et al. 2021 ;王等人， 2021b ；张等人， 2022a ) 。例如，聚苯乙烯MPs增加了斑马鱼体内Cd的积累，并且同时暴露于Cd和MPs会引起氧化损伤和炎症（Lu et al. 2018 ）。生物膜可以增强 MP 和重金属的综合毒性（Qi 等人， 2021 ）。然而，在某些情况下，由于MPs对重金属的吸附，导致重金属的生物利用度降低，延迟了重金属的毒性（Wen等， 2018 ；Wang等， 2021c ）。

Cluster 2 (green) mainly focuses on the adsorption of heavy metals by MPs, including the adsorption mechanisms, kinetics, isotherms, and influencing factors. The adsorption behaviors of heavy metals onto MPs are complex, with common sorption mechanisms such as physical and chemical adsorption, electrostatic force and surface complexation, external and internal diffusion, van der Waals force, π-π interaction, polar interaction, non-covalent interaction, the pseudo-first- or pseudo-second-order kinetics, and the Langmuir or Freundlich models (Gao et al. 2021). The factors influencing the adsorption behaviors of heavy metals by MPs can be divided into categories: (1) the polymer type, size, dose, and surface characteristics of MPs (Gao et al. 2021), (2) the intrinsic properties and concentration of metals (Dong et al. 2019, 2020; Wang et al. 2019; Tang et al. 2021; Li et al. 2022c), and (3) the environmental conditions, such as the solution pH, temperature, salinity, dissolved organic matter, and particulate matter (Gao et al. 2021). Notably, MPs in the environment will undergo ageing, weathering, and degradation, which can cause a series of changes in surface functional groups, polarity, and surface area, and consequently change the adsorption behaviors of heavy metals (Gao et al. 2021). Due to their low density, MPs can move easily in water and thus increase the transport of the leased metals due to the carrier effect (Liu et al. 2021a). Particularly, MPs reduce the adsorption capacity but increase the desorption of heavy metals by soil, leading to increased mobility of these metals (Zhang et al. 2020; Li et al. 2021). Understandably, soil with MPs may have higher bioavailability and toxicity of heavy metals and increased leaching to water bodies.
Cluster 2（绿色）主要研究MPs对重金属的吸附，包括吸附机理、动力学、等温线和影响因素。 MPs对重金属的吸附行为较为复杂，常见的吸附机制有物理和化学吸附、静电力和表面络合、内外扩散、范德华力、π-π相互作用、极性相互作用、非共价相互作用等、伪一级或伪二级动力学，以及 Langmuir 或 Freundlich 模型（Gao 等人， 2021 ）。影响MPs吸附重金属行为的因素可分为几类：（1）MPs的聚合物类型、尺寸、剂量和表面特性（Gao et al. 2021 ），（2）MPs的内在性质和浓度。金属（Dong et al. 2019 , 2020 ; Wang et al. 2019 ; Tang et al. 2021 ; Li et al. 2022c ），以及（3）环境条件，例如溶液 pH 值、温度、盐度、溶解有机物和颗粒物（Gao 等人， 2021 ）。值得注意的是，环境中的MP会发生老化、风化和降解，从而引起表面官能团、极性和表面积的一系列变化，从而改变重金属的吸附行为(Gao等， 2021 )。由于其密度低，MP 可以在水中轻松移动，从而由于载流子效应而增加了租赁金属的传输（Liu 等人， 2021a ）。特别是，MPs降低了吸附能力，但增加了土壤对重金属的解吸，导致这些金属的迁移性增加（Zhang等人， 2020 ；Li等人， 2021 ）。 可以理解的是，含有MP的土壤可能具有更高的重金属生物利用度和毒性，并且增加了对水体的淋滤。

Cluster 3 (blue) focuses on how MPs or heavy metals interact with microorganisms and other contaminants (e.g., antibiotics) in different environments, especially in the soil. MPs can alter the speciation, bioavailability, and toxicity of heavy metals to microorganisms (Wang et al. 2021a; Yang et al. 2022a). The co-occurrence of MPs and heavy metals can modify soil microbial community diversity and structure and their ecosystem functions (Feng et al. 2022; Yin et al. 2022). The ingestion of MPs and heavy metals by organisms increases their intestinal burden and triggers changes in the gut microbial community and functions (Yan et al. 2020; Jiang et al. 2022; Yang et al. 2022b). Biofilms colonizing microplastic surfaces (plastisphere) significantly affect heavy metal adsorption by MPs (Li et al. 2022b). The adsorption of heavy metals by MPs, in turn, affects biofilm formation and ecological functions (Wang et al. 2022a). Microorganisms can influence the fate of MPs (e.g., ageing and degradation) and heavy metals (e.g., transformation and sorption) in the environment, and biofilms have been shown to enhance the transport of metals by MPs and increase their combined toxicity (Qi et al. 2021). Thus, the interactions among MPs, heavy metals, and microorganisms are complex and deserve to be explored.
集群 3（蓝色）重点关注 MP 或重金属如何与不同环境中的微生物和其他污染物（例如抗生素）相互作用，特别是在土壤中。 MP 可以改变重金属对微生物的形态、生物利用度和毒性（Wang 等人， 2021a ；Yang 等人， 2022a ）。 MPs和重金属的共存可以改变土壤微生物群落的多样性和结构及其生态系统功能（Feng等， 2022 ；Yin等， 2022 ）。生物体摄入 MP 和重金属会增加肠道负担，引发肠道微生物群落和功能的变化（Yan 等人， 2020 ；Jiang 等人， 2022 ；Yang 等人， 2022b ）。定殖微塑料表面（塑料球）的生物膜显着影响 MP 的重金属吸附（Li 等人， 2022b ）。 MP 对重金属的吸附反过来会影响生物膜的形成和生态功能（Wang 等人， 2022a ）。微生物可以影响环境中MP的命运（例如老化和降解）和重金属（例如转化和吸附），生物膜已被证明可以增强MP对金属的转运并增加其综合毒性（Qi等人） 2021 ）。因此，MP、重金属和微生物之间的相互作用是复杂的，值得探索。

Cluster 4 (yellow) focuses on investigating and assessing the abundance and risks of microplastic pollution or heavy metals in the aquatic environment. MPs widely distribute in both marine and terrestrial ecosystems, and their surfaces are capable of adsorbing and enriching metals (Li et al. 2020; Patterson et al. 2020). There is a positive correlation between the amount of metal enriched on the surface of MPs and the abundance of metal surrounding them (Zhu et al. 2020). Both MPs and heavy metals, such as Cr, Cd, and As, have been found to contaminate water and sediments (Mohsen et al. 2019; Jahromi et al. 2021; Sun et al. 2022a). Heavy metals and MPs have also been detected in aquatic animals, such as bivalves, oysters, sea cucumbers, and fish (Mohsen et al. 2019; Zhu et al. 2020; Jahromi et al. 2021; Vieira et al. 2021; Sun et al. 2022a). Since many of these animals are consumed as sea foods by humans, their ingestion of MPs and associated heavy metals will bring enlarged health risks for consumers.
集群 4（黄色）重点调查和评估水生环境中微塑料污染或重金属的丰度和风险。 MPs广泛分布在海洋和陆地生态系统中，其表面具有吸附和富集金属的能力（Li et al. 2020 ；Patterson et al. 2020 ）。 MPs表面富集的金属量与其周围金属的丰度呈正相关（Zhu et al. 2020 ）。 MP 和 Cr、Cd 和 As 等重金属均被发现会污染水和沉积物（Mohsen 等人， 2019 年；Jahromi 等人， 2021 年；Sun 等人， 2022a ）。在双壳类、牡蛎、海参和鱼类等水生动物中也检测到了重金属和 MP（Mohsen 等人， 2019 年；Zhu 等人， 2020 年；Jahromi 等人， 2021 年；Vieira 等人， 2021 年；Sun 等人）等2022a ）。由于许多这些动物被人类作为海鲜食用，它们摄入的MP和相关重金属将给消费者带来更大的健康风险。

The core keywords in cluster 5 (purple) are pollutants such as persistent organic pollutants and polycyclic aromatic-hydrocarbons, indicating that it is concerned with the interrelationship of MPs or heavy metals with other pollutants in the environment. In many cases, MPs or heavy metals do not singly exist in the environment but co-exist with other pollutants, such as organochlorine pesticides and polycyclic aromatic hydrocarbons (Fred-Ahmadu et al. 2022). These pollutants are even ingested together by living organisms (Borges-Ramírez et al. 2021; Hu et al. 2022; Xiang et al. 2022). Similar to heavy metals, organic pollutants can adsorb and interact on the surface of MPs. For example, the presence of other pollutants alters the adsorption behavior of heavy metals by MPs (Yu et al. 2020; Zhao et al. 2022b). Co-exposure of MPs and heavy metals with a variety of other contaminants can lead to complex toxicity to organisms (Menéndez-Pedriza and Jaumot 2020; Xiang et al. 2022), making it difficult to investigate the interactions of MPs and heavy metals. It is also challenging to assess the biological toxicity of multiple pollutants.
第5组（紫色）的核心关键词是持久性有机污染物和多环芳烃等污染物，表明它与MP或重金属与环境中其他污染物的相互关系有关。在许多情况下，MP或重金属并不单独存在于环境中，而是与其他污染物共存，例如有机氯农药和多环芳烃（Fred-Ahmadu等人， 2022 ）。这些污染物甚至被生物体一起摄入（Borges-Ramírez 等人， 2021 ；Hu 等人， 2022 ；Xiang 等人， 2022 ）。与重金属类似，有机污染物可以在MPs表面吸附并相互作用。例如，其他污染物的存在会改变 MP 对重金属的吸附行为（Yu 等人， 2020 ；Zhao 等人， 2022b ）。 MP 和重金属与多种其他污染物的共同暴露可能导致对生物体产生复杂的毒性（Menéndez-Pedriza 和 Jaumot 2020 ；Xiang 等人2022 ），使得研究 MP 和重金属的相互作用变得困难。评估多种污染物的生物毒性也具有挑战性。

Cluster 6 (cyan) focuses on the identification and characterization of MPs (and heavy metals) in the environment. Understanding the abundance of MPs in the environment and the concentration of metals on the surface of MPs is a prerequisite for further study of their interactions (Mohsen et al. 2019; Kutralam-Muniasamy et al. 2021). MPs and nanoplastics are considered a type of new pollutants for which analytical methods still need to be developed. The separation, identification, and classification of MPs (i.e., polymer type, particle size, shape, color) are difficult due to their small size and the fact that they can aggregate or interact with other environmental media (Bitencourt et al. 2020; Tirkey and Upadhyay 2021). Although analytical methods for heavy metals are relatively mature (Inobeme et al. 2023), it is difficult to separate smaller MPs (e.g., nanoplastics) from environmental samples such as soils, causing challenges in the identification and quantification of the associated metals. The future development of high-throughput and standard methods to identify and accurately characterize MPs and the associated heavy metals would therefore be a ground-breaking initiative (Kutralam-Muniasamy et al. 2021).
集群 6（青色）侧重于环境中 MP（和重金属）的识别和表征。了解环境中 MP 的丰度以及 MP 表面金属的浓度是进一步研究其相互作用的先决条件（Mohsen 等人， 2019 年；Kutralam-Muniasamy 等人， 2021 年）。 MP 和纳米塑料被认为是一种新污染物，其分析方法仍需开发。 MP 的分离、识别和分类（即聚合物类型、粒径、形状、颜色）很困难，因为它们尺寸小，而且它们可以聚集或与其他环境介质相互作用（Bitencourt 等人， 2020 ；Tirkey）和 Upadhyay 2021 ）。尽管重金属的分析方法相对成熟（Inobeme et al. 2023 ），但很难从土壤等环境样品中分离出较小的MP（例如纳米塑料），这给相关金属的识别和定量带来了挑战。因此，未来开发高通量和标准方法来识别和准确表征 MP 及相关重金属将是一项突破性举措（Kutralam-Muniasamy 等人， 2021 ）。

Timeline view of keywords co-occurrence network and burst keywords analysis
关键词共现网络时间线视图及突发关键词分析

The keyword co-occurrence network represents the static scene, but cannot display the dynamic changes in the study area. The timeline view and burst keywords can illustrate the evolution of keywords. The size of the node indicates the frequency of keyword occurrences. The colored lines connecting two nodes represent their co-occurrence relationship (Wang et al. 2020a).
关键词共现网络代表静态场景，但无法显示研究区域的动态变化。时间轴视图和突发关键词可以说明关键词的演变。节点的大小表示关键字出现的频率。连接两个节点的彩色线代表它们的共现关系（Wang et al. 2020a ）。

Fig. S5 shows the evolution of keywords from 2014 to 2022. The adsorption behaviors of metallic pollutants onto MPs are the most frequent topic, accompanied by the keywords like pollution and accumulation. In addition to the appearance of individual metal elements, keywords such as cadmium, bioaccumulation, and toxicity are also mentioned during this period. Over time, the keywords such as risk, ecosystem, and “food web” appeared, indicating that the research areas continued to expand.
图S 5显示了2014年至2022年关键词的演变。金属污染物在MP上的吸附行为是最常见的主题，同时伴随着污染和积累等关键词。除了个别金属元素的出现外，镉、生物富集性、毒性等关键词也被提及。随着时间的推移，风险、生态系统、“食物网”等关键词出现，表明研究领域不断扩大。

To display hot topics, Fig. S6 shows the 10 keywords with strength greater than 2, as well as the 10 keywords with strength less than 2 but expected to be hot topics in the future. Debris and “litter” are among the hot keywords. Subsequently, the marine environment, the water environment, the type of plastic particles, and the toxicity have become hot topics. In 2021, researchers started to shift their focus from local phenomena to global impacts, as shown by the emergence of ecosystem. The transfer of MPs and heavy metals through the food web is also a current hotspot. MPs and related contaminants can potentially have long-term effects on biological and human health through the food web and dietary exposure (Huang et al. 2021).
为了显示热门话题，图S 6显示了强度大于2的10个关键词，以及强度小于2但预计未来会成为热门话题的10个关键词。碎片和“垃圾”是热门关键词之一。随后，海洋环境、水环境、塑料颗粒的种类、毒性等成为热门话题。 2021年，研究人员开始将注意力从局部现象转向全球影响，生态系统的出现就表明了这一点。 MP和重金属通过食物网的转移也是当前的热点。 MP 和相关污染物可能通过食物网和饮食暴露对生物和人类健康产生长期影响（Huang 等人， 2021 ）。

To conclude, MPs and heavy metals have been extensively studied in aquatic (particularly marine) environments, but the data from studies in soil and atmospheric environments are relatively lacking. The ecotoxicological studies of MPs and heavy metals on food webs and ecosystems are worth exploring.
综上所述，MPs和重金属在水生（特别是海洋）环境中得到了广泛的研究，但土壤和大气环境的研究数据相对缺乏。 MPs和重金属对食物网和生态系统的生态毒理学研究值得探索。

Conclusions and future directions
结论和未来方向

Using a bibliometric analysis based on VOSviewer, Pajek64, and CiteSpace, the background of knowledge, research performance, and the latest knowledge structure on MPs and heavy metals over the last 9 years were presented and reviewed. A total of 552 articles have been published in 124 journals, such as Science of the Total Environment, Journal of Hazardous Materials, Environmental Pollution, Chemosphere, and Marine Pollution Bulletin. There are 39 authors having more than 5 articles, and Andrew Turner, from Plymouth University, published the largest number of publications (15 papers). A total of 70 countries have published articles related to this field, with China making the largest contribution. The leading institutions and authors have close collaborations. The analysis of the total highly cited literature shows that the hotspots are shifting from marine to terrestrial ecosystems, with focus on the exploration of toxicity mechanisms. Hot keyword analysis shows that the research on MPs and heavy metals has focused on their toxicity and bioaccumulation, the adsorption and desorption behaviors, the environmental pollution and risk assessment, and their detection and characterization.
利用基于VOSviewer、Pajek64和CiteSpace的文献计量分析，对过去9年MP和重金属的知识背景、研究表现和最新知识结构进行了介绍和回顾。在Science of the Total Environment、Journal of Hazardous Materials、Environmental Pollution、Chemosphere、Marine Pollution Bulletin等124种期刊上共发表论文552篇。有39位作者发表了超过5篇文章，来自普利茅斯大学的Andrew Turner发表的出版物数量最多（15篇论文）。共有70个国家发表了与该领域相关的文章，其中中国贡献最大。领先机构和作者有着密切的合作。对高被引文献总数的分析表明，热点正在从海洋生态系统转向陆地生态系统，并重点关注毒性机制的探索。热点关键词分析表明，MPs和重金属的研究主要集中在其毒性与生物富集性、吸附与解吸行为、环境污染与风险评估、检测与表征等方面。

Based on the current bibliometric analysis of the research history and current status of MPs and heavy metals, the following directions for future research should be highlighted.
根据目前对MP和重金属研究历史和现状的文献计量分析，未来研究应重点关注以下方向。

  1. 1.

    Both MPs and heavy metals are persistent in the environment. Considering the spatiotemporal heterogeneity of MPs and heavy metals in the environments, one of the priority directions is to investigate their co-occurrence, source, characteristics, and environmental fate and behaviors, especially in terrestrial ecosystems and soil environments that have not been unveiled sufficiently. MPs with smaller sizes (e.g., nanoplastics) and aged surface generally have a stronger ability to adsorb and enrich heavy metals, which deserve more concern.
    MP 和重金属都在环境中持久存在。考虑到环境中MPs和重金属的时空异质性，优先研究的方向之一是研究它们的共现、来源、特征以及环境命运和行为，特别是在尚未充分揭示的陆地生态系统和土壤环境中。尺寸较小（如纳米塑料）和表面老化的MP通常具有较强的吸附和富集重金属的能力，值得更多关注。

  2. 2.

    Analytical methods should be developed and standardized for effective extraction and accurate quantification of MPs (particularly nanoplastics) and the associated heavy metals from various environmental samples.
    应开发并标准化分析方法，以便从各种环境样品中有效提取和准确定量 MP（特别是纳米塑料）和相关重金属。

  3. 3.

    Most current studies on the interaction of MPs and heavy metals are focused on aquatic (marine) organisms and ecosystems. However, MPs and heavy metals are both common contaminants in terrestrial ecosystems, particularly agroecosystems, posing threats to food safety and security. The co-contamination effects and toxicity of MPs and co-existing heavy metals on terrestrial crops and soil biota should be addressed in future work. There is a need to gain insight into their toxicological mechanisms on organisms using multi-techniques, such as omics (e.g., genomics, transcriptomics, proteomics, and metabolomics).
    目前大多数关于 MP 和重金属相互作用的研究都集中在水生（海洋）生物和生态系统。然而，MPs和重金属都是陆地生态系统特别是农业生态系统中常见的污染物，对食品安全构成威胁。未来的工作应解决 MP 和共存重金属对陆地作物和土壤生物群的共同污染效应和毒性。需要使用组学（例如基因组学、转录组学、蛋白质组学和代谢组学）等多种技术来深入了解它们对生物体的毒理学机制。

  4. 4.

    The impact of MPs on the bioavailability and bioaccessibility of heavy metals and the ability of MPs and heavy metals to be transported along food webs through trophic levels need to be further investigated. It is expected that the ingestion and bioaccumulation of MPs may release the associated heavy metals from organisms to the food chain, and thus biomagnify across trophic levels, posing uncertain ecological and health risks.
    MPs对重金属生物利用度和生物可及性的影响以及MPs和重金属沿着食物网通过营养级运输的能力需要进一步研究。预计 MP 的摄入和生物富集可能会将相关重金属从生物体释放到食物链中，从而在营养级上进行生物放大，从而带来不确定的生态和健康风险。

  5. 5.

    MPs and heavy metals co-occur in the atmosphere and foods and drinking water, thus entering human bodies through inhalation and ingestion (Al Osman et al. 2019; Pironti et al. 2021). Although the presence of MPs in human tissues and their health risk have been reported, the combined toxicity mechanisms and health risks of MPs and heavy metals have not been well elucidated.
    MP 和重金属同时存在于大气、食物和饮用水中，从而通过吸入和摄入进入人体（Al Osman 等人， 2019 年；Pironti 等人， 2021 年）。尽管MPs在人体组织中的存在及其健康风险已有报道，但MPs和重金属的综合毒性机制和健康风险尚未得到很好的阐明。

  6. 6.

    Finally, sustainable strategies are needed to reduce pollution from MPs and heavy metals, such as the use of policy, legislative and regulatory, and environmental interventions in promoting the reduction, reuse, and recycling (i.e., 3Rs) of plastic and metallic wastes, and the development of biodegradable plastics to replace non-degradable polymers.
    最后，需要采取可持续战略来减少MP和重金属的污染，例如利用政策、立法和监管以及环境干预措施来促进塑料和金属废物的减少、再利用和回收（即3R），以及开发可生物降解塑料以取代不可降解聚合物。