Advances in Studies on Heavy Metals in Urban Soil: A Bibliometric Analysis

Abstract

Recent advancements in urban economies and population growth have led to an escalation in urban soil heavy metal pollution, thereby posing significant threats to human life and health. This paper presents a comprehensive bibliometric analysis, detailing the developmental trajectory, research hotspots, and emerging trends in the field of urban soil heavy metal pollution research. Utilizing the CiteSpace and VOSviewer software tools, we conducted an analysis of 1247 articles sourced from the Web of Science Core Collection Database (WoSCC) spanning the period from 2000 to 2022. Our analysis revealed a significant upward trend in the number of publications during the period 2000–2022, a trend expected to persist. Ahmad Kafeel, Khan Zafar Iqbal, and Huang Biao emerged as the leading authors in this domain. The journal, Science of the Total Environment, held the most influence, while China led in the number of publications, with the Chinese Academy of Sciences as the foremost contributor. The research predominantly focused on source apportionment of urban soil heavy metal pollution, pollution risk assessment, and the application of environmental magnetism. Future research priorities include assessing the human health risks from diverse sources of heavy metal pollution, a key aspect of ensuring urban safety and soil health. Our findings delineate the evolutionary trajectory of urban soil heavy metal pollution research over the past two decades and underscore the viability of employing a dynamic bibliometric approach to investigate this research discipline.

1. Introduction

Heavy metals are persistent environmental pollutants; once introduced into the soil, they become challenging to eradicate, significantly altering the soil’s physicochemical properties and resisting biodegradation or thermal degradation. Exceeding certain thresholds, these heavy metals inflict toxicological effects on soil microorganisms, plants, and animals, seriously impairing soil enzyme and microbial activities [1,2,3]. Plants absorb heavy metals through their roots and leaves, a process primarily influenced by the plant species and their inherent defense mechanisms against toxicity. A notable decrease in the photosynthetic activity of plants poisoned by heavy metals leads to reduced biomass production [4]. Additionally, plant physiological and biochemical activities—including respiration, metabolism, transcription, translation, and cell cycle—are affected. The toxicity of different heavy metals to plants is cumulative. For instance, in barley plants, the combined effect of Cu and Cd results in stunted growth of roots and stems [5]. Heavy metals accumulate in large quantities via bioaccumulation in the ecosystem’s food chain [6] and enter the human body through food, air, water, and skin contact—with food being the primary pathway [7]. Regular intake of heavy metals can hinder growth and weaken the immune system [8], posing direct or indirect threats to human health. Furthermore, heavy metals can infiltrate the human body via skin and the air [9]. These metals are inherently mutagenic and carcinogenic [10] and can cause a spectrum of health problems related to growth and development, cardiovascular, respiratory, dermal, reproductive, and immune systems in severe cases [11]. The harmful effects of even low concentrations of heavy metals should be considered in both plant and human contexts. Case in point: widespread chronic Pb poisoning in the ancient Roman Empire; Cd poisoning in Toyama Prefecture, Japan, in 1931; the globally shocking Hg poisoning in Minamata Bay, Kumamoto Prefecture, Japan, in 1956; and severe Cr pollution in California, USA in 1993. These incidents of heavy metal poisoning have led to tragic and even life-threatening consequences for tens of thousands of people. The issue of heavy metal accumulation in soil remains a topic of persistent emphasis and is worthy of continued focus.

Urban soil is a critical component of the urban ecosystem, with its quality significantly impacting residents’ quality of life and health. While urban soils inherit certain fundamental characteristics from their original natural counterparts, their nature differs markedly due to dense populations and frequent human activities. These soils exhibit unpredictable stratification, poor permeability, low water content, and high trace element content [12,13], serving as key repositories and decomposition sites for heavy metals and other pollutants. Coupled with the limited environmental carrying capacity and self-purification ability of urban soils, the rapid expansion of urban areas due to accelerating urbanization and industrialization introduces an influx of heavy metals. This influx poses a significant burden on urban ecosystems and biogeochemical cycles, triggering various environmental issues, such as deterioration of soil functions, alterations in soil structure, and changes in soil properties [14,15].

Heavy metals enter urban soils via two principal pathways: natural sources and anthropogenic disturbances [16]. Natural sources primarily relate to parent material and soil formation processes. However, human activities—such as fossil fuel combustion, waste incineration, traffic emissions, metal smelting, and intensive agricultural practices—exert a more pronounced influence on soil heavy metal content [17,18]. Particularly in agricultural production activities, despite the infrequent use of urban soil, the extensive production and application of mulch exacerbate pollution. Heavy metals are deposited into the soil as toxic gases, dust, and through pedogenetic processes of weathering of parent materials at trace levels. These heavy metals accumulate in the human body via inhalation, ingestion, or skin contact and absorption, significantly threatening the health and well-being of urban residents, especially children [19,20,21]. Although areas like parks and residential zones are not typically used for food crop cultivation, they also serve as significant transmission sites for heavy metals to humans [22,23]. Studying the distribution of heavy metals in urban soil is instrumental in safeguarding human life and health against prolonged exposure to polluted environments.

Urban soil heavy metal pollution is a pressing research focus both domestically and internationally. Key areas of analysis encompass soil heavy metal concentration [24,25,26], pollution sources [27,28,29], spatial distribution [30,31,32], risk assessment [33,34,35], and soil remediation technology [36,37,38]. With the accelerated urbanization and rapid socio-economic development in recent years, the problem of heavy metal pollution in urban soil should be solved urgently.

Tong et al. [39] undertook a comprehensive systematic review of heavy metal content, spatial distribution, pollution levels, and human health risk assessment in the soils of 71 Chinese cities spanning the years 2013 to 2019, based on the online literature data. Their findings advocate for prioritizing the control of Cd and Hg in Chinese urban soils. Wei et al. [40] conducted a decade-spanning review of heavy metal pollution research in several Chinese cities and identified elevated levels of heavy metal pollution in urban road dust compared to urban soils. Barsova et al. [41] conducted a survey of soil heavy metal pollution in various regions of Russia from 2008 to 2017, revealing zinc’s predominance as the most prevalent metal in soils situated alongside highways.

While extensive studies, including bibliometric analyses, have been conducted on urban soil heavy metal pollution both domestically and internationally, they often concentrate on one or two aspects—such as distribution characteristics, evaluation, remediation, or source analysis—lacking a systematic review of the global research status and evolutionary trend in this field. Accordingly, this paper offers a comprehensive review of the research conducted in this field over the past two decades using bibliometric methods, primarily aided by CiteSpace and VOSviewer software. The paper thoroughly analyzes the research structure and quantifies the information data in this field, presenting the development trajectory, research focus, and evolution trend in this field via visual mapping. This study lays the foundation for future research on the relationship between urban soil heavy metal pollution and ecosystem services, soil health, sustainable development, and agricultural productivity.

2. Data Sources and Methodology

2.1. Research Method

Bibliometrics emerged in the early 20th century as an interdisciplinary field that combines mathematics, statistics, and metrology to quantitatively analyze research topics. Bibliographer Pritchard first proposed the term “bibliometrics” in 1969 as an alternative to bibliography, marking the development of bibliometrics as an independent discipline. Since then, it has been widely utilized across numerous disciplines, such as medicine [42], management [43,44], engineering and technology [45], environmental science [46,47]. Bibliometrics serves as an effective method for searching, mining, analyzing, and summarizing the underlying patterns in large data sets. Leveraging modern big data and computer technology, bibliometric analysis results can be effectively presented through clear and concise visual knowledge graphs. This analysis enables us to extract detailed information about the number of publications, authors, journals, countries, institutions, references, keywords, and so forth. Simultaneously, visual analysis aids in interpreting data more scientifically and comprehensively, facilitating the exploration of the intrinsic connections between different data sets.

The purpose of this study was addressed using two software applications: CiteSpace (6.1.R6, USA) and VOSviewer (1.6.18, The Netherlands). CiteSpace is a Java-based software for citation visualization and analysis, developed in the context of scientometrics and data visualization [48]. It presents the structure, patterns, and distribution of scientific knowledge through visual analysis, and the visual graphs produced by this method are often referred to as scientific knowledge maps. The software provides three views, cluster, time zone, and timeline. Its document co-citation analysis feature aids in understanding the development process and trends In the study area [49]. The parameters of CiteSpace for this study were set as follows, the period was from January 2000 to December 2022, individual time slices were set to 1 year, and Pathfinder and Pruning Sliced networks were chosen as the clipping method. Research institution, keywords, and document co-citation were visually analyzed, and emerging keywords were detected. VOSviewer, on the other hand, is a software tool for creating maps and visualizing analyses based on network data [50]. It provides three types of visualization, network visualization, overlay visualization, and density visualization, with the core concept being “co-occurrence clustering”. This study utilized VOSviewer to analyze authors, journals, and countries. Many scholars prefer VOSviewer for constructing bibliometric maps due to its relative simplicity and the aesthetic quality of the plots produced [51,52,53]. The combined use of both software applications enables a complementary analysis.

Moreover, in the core author analysis portion of this study, we employed Price’s law. This principle posits that half of the documents written on the same topic originate from a group of highly productive authors. Numerically, the size of this group equals the square root of the total number of all authors. Price’s law has significantly contributed to the organization and analysis of authorship data. It is commonly used in academia to identify core authors, as it asserts that the minimum number of publications from core authors in a field is $m=0.749\sqrt{n_{max}}$, where $n_{max}$ represents the number of documents published by the most prolific authors [54].

2.2. Data Sources and Search Strategy

Web of Science is the world’s most authoritative, widely used, and comprehensive repository of scholarly information covering the largest number of disciplines [55,56]. It has hundreds of millions of citation records and is widely recognized for its high quality for document analysis [57]. To ensure the authority and accuracy of the documents, this study employed the Web of Science Core Collection (WoSCC) as the document search database (last accessed on 10 January 2023). For the sake of enhancing transparency and reproducibility, here we provide detailed information regarding the subdatasets obtained from WoSCC and their respective coverage periods, as outlined in reference [58]: Science Citation Index Expanded (SCIE), from 1900 to present; Social Sciences Citation Index (SSCI), from 1900 to present; Arts & Humanities Citation Index (AHCI), from 1975 to present; Conference Proceedings Citation Index—Science (CPCI-S), from 1990 to present; Conference Proceedings Citation Index—Social Sciences & Humanities (CPCI-SSH), from 1990 to present; Book Citation Index—Science (BKCI-S), from 2005 to present; Book Citation Index—Social Sciences & Humanities (BKCI-SSH), Emerging Sources Citation Index (ESCI), from 2005 to present; from 2005 to present. Current Chemical Reactions (CCR-E), from 1985 to present; Index Chemicus (IC), from 1993 to present. The search strategy adopted was TS = (Soil OR Soils OR Peat OR Humus) AND TS = (Cities OR Towns OR Municipalities) AND TS = (Metals Heavy OR Heavy Metals OR Heavy Metal OR Metal Heavy) AND TS = (Environmental Pollution OR Pollution Environmental OR Soil Pollution OR Pollution Soil), encompassing the period from 2000 to 2022. The search was refined to include articles and reviews and was conducted in English. All obtained documents were manually reviewed based on title and abstract content and exported as plain text files, with record content selected as complete records with cited references. Post de-duplication, a total of 1247 documents (1224 articles, 23 reviews) were retrieved. The specific data collection method used is illustrated in Figure 1.

For a more accurate analysis of national publication volume statistics, data were appropriately modified; publications from England, Scotland, Ireland, and Wales were collated into the UK publication collection, while those from Taiwan Province and Hong Kong Special Administrative Region were incorporated into the China publication collection. Subsequently, using the visualization software VOSviewer (1.6.18, The Netherlands), we quantified the number of publications and the intensity of collaboration for each country. This data was then combined with Gephi (0.9.7) to generate a national collaboration network chord diagram. Notably, VOSviewer is a tool for constructing visual bibliometric maps (https://www.vosviewer.com/, accessed on 13 October 2022), and Gephi is a software used for data visualization in network analysis (https://gephi.org/, accessed on 28 December 2022). Both are readily accessible, open-source freeware that were instrumental in this study.

3. Results and Discussion

3.1. Publication Statistics

The dataset used in this study comprises 1247 documents from 98 countries, contributed by 4925 authors from 1491 institutions, and published in 322 journals. These documents cited 34,842 references from 11,864 journals. In addition, while investigating the trends in publications related to urban soil heavy metal pollution prior to the year 2000, we maintained a consistent and rigorous selection strategy. We thoroughly searched the WoSCC collection for the pre-2000 literature and discovered that relevant publications first appeared in 1991, with the number of publications exceeding five in 1995. Afterward, the annual publication count remained relatively stable at approximately five publications, indicating a lack of a noticeable upward trend. Descriptive statistics of the interannual growth in the number of publications from 2000 to 2022 are depicted in Figure 2. From 2000 to 2005, the number of publications on urban soil heavy metal pollution was relatively low, totaling fewer than 130 papers. From 2006 to 2016, the number of publications rose moderately, averaging 45 publications per year. The upward trend became more pronounced from 2017 to 2022, with a significant increase observed in 2020. Notably, in the last four years (2019–2022), the number of publications consistently exceeded 100, and the combined number of publications in these four years accounted for 43.38% of the total publications. This suggests an increasing scholarly interest in this research area in recent years. Overall, annual publications on urban soil heavy metal pollution rose significantly from a single publication in 2000 to 160 in 2022. The cumulative increase in publications aligns perfectly with a third-order polynomial fit: y = 0.1052x³ − 0.6929x² + 14.915x − 26.33, R² = 0.999, where y represents the annual number of publications during 2000–2022 and x denotes the year.

3.2. Core Authors

The analysis of core author statistics can provide us with a more nuanced understanding of the leading scholars and principal research strengths in this field. According to Price’s law, the value of m can be calculated using VOSviewer software. For this dataset, m ≈ 2.70 was obtained. Thus, authors with three or more publications were designated as core authors in the field. This analysis identified 171 core authors who contributed a total of 683 publications, accounting for 54.77% of the total number of publications. This meets the criterion proposed by Price, which stipulates that half of the publications (50%) should be produced by the core authors. Consequently, there is reason to believe that a relatively stable group of collaborating authors has been established in the field of urban soil heavy metal pollution research.

Table 1. Statistics of top 10 core authors.

Rank	Author	Publications	Citations	C/P	H-Index	Current Affiliation	Country
1	Ahmad, Kafeel	12	138	11.5	5	Jamia Millia Islamia	India
2	Khan, Zafar Iqbal	12	138	11.5	5	University of Sargodha	Pakistan
3	Huang, Biao	12	272	22.67	6	Institute of Soil Science	China
4	Yang, Yong	11	125	11.36	11	Huazhong Agricultural University	China
5	Christakos, George	9	385	42.78	8	California State University System	USA
6	Zhou, Shenglu	9	249	27.67	9	Nanjing University	China
7	Zhao, Yongcun	8	333	41.63	5	University of Chinese Academy of Sciences	China
8	Li, Yan	7	294	42	10	Zhejiang University	China
9	Shi, Zhou	7	305	43.57	6	Zhejiang University	China
10	Wu, Shaohua	7	166	23.71	7	Zhejiang University of Finance and Economics	China

Key: C/P = citations/publications, H-index = Hirsch index.

presents the top ten core authors in the field of urban soil heavy metal pollution research, detailing each author’s number of publications, citations, citations per paper, h-index, current affiliation, and nationality. As illustrated in the table, Ahmad Kafeel, Khan Zafar Iqbal, and Huang Biao are the most prolific authors, each having published 12 papers from 2000 to 2022. Notably, Ahmad Kafeel and Khan Zafar Iqbal have collaborated extensively on the potential risk assessment of heavy metals in soil-food crops [59,60,61]. Yang Yong holds the highest h-index at 11, which indicates that he has long been engaged in research in the field of urban soil heavy metal pollution. He is especially good at using various models to study the spatial and temporal distribution and source identification of heavy metals in regional soils [62,63]. Li Yan has the second highest h-index, and also has a high number of citations. Shi Zhou, who has a similarly high citation rate per paper, works at Zhejiang University. They have collaborated closely on regional soil heavy metal pollution assessment and source apportionment [64], as well as on the identification and evaluation of heavy metal pollution in the soil-crop system [65,66]. Their outstanding contributions extend to analyzing the status quo, spatial distribution characteristics, health risks, and potential driving factors of heavy metal pollution in local Chinese soil [67]. Most collaborative relationships among Chinese scholars occur within the same institution. Lastly, the nationalities of the top ten core authors by publication volume suggest that most researchers in the field of urban soil heavy metal pollution are from Asian countries, including China, Pakistan, and India. With Chinese scholars leading the group, this indirectly indicates China’s significant contributions to the research and development in this field during the specified research period.

3.3. High-Yielding Journals

As Dzikowski suggests, the influence of a journal can be gauged by the number of documents it publishes and the citations these documents receive. The larger these numbers, the greater the journal’s impact [68]. In this study, we identified journals that published at least eight papers on urban soil heavy metal pollution research as core journals in this field. Our use of VOSviewer software revealed 36 journals that fit this criterion (Figure 3). Each node in the figure represents a journal, with larger nodes indicating a higher total link strength. Lines between nodes denote citation relationships between journals, with thicker lines suggesting higher co-citation intensity. The journals Science of the Total Environment, Environmental Science and Pollution Research, and Ecotoxicology and Environmental Safety ranked as the top three for total link strength. Despite its fewer publications, Ecotoxicology and Environmental Safety has the highest number of citations per paper. This journal’s status as an open-source publication and the scholarly interest in the impact of urban soil heavy metal pollution on ecological safety and human health likely contribute to its high citation rate.

This study also provides a summary and analysis of the top ten critical journals in the field based on the number of publications. We detailed the number of publications, citations, citations per paper, the five-year impact factor (IF), and the country of the journal’s affiliation (

Table 2. Statistics of top 10 journals with high productivity.

Rank	Journal	Publications	Citations	C/P	IF	Country
1	Science of the Total Environment	66	5152	78.06	10.237	Netherlands
2	Environmental Science and Pollution Research	62	1621	26.15	5.053	Germany
3	Environmental Monitoring and Assessment	58	1815	31.29	3.42	Netherlands
4	International Journal of Environmental Research and Public Health	47	1064	22.64	4.799	Switzerland
5	Environmental Earth Sciences	43	851	19.79	3.152	Germany
6	Fresenius Environmental Bulletin	39	213	5.46	0.583	Germany
7	Environmental Geochemistry and Health	35	1060	30.29	4.932	Netherlands
8	Environmental Pollution	32	2246	70.19	10.366	United Kingdom
9	Chemosphere	25	2196	87.84	8.52	United Kingdom
10	Polish Journal of Environmental Studies	25	225	9	1.845	Poland

Key: C/P = citations/publications, IF = impact factor.

). The 1247 publications under consideration were published across 322 journals. However, the total number of publications in the top ten journals accounted for 432, constituting one-third of the total number of publications. We found that over the past two decades, most of the papers in this field were published in leading environmental science journals, with a few appearing in medical, agricultural, and forestry science journals. The top five journals in terms of the number of publications were, Science of the Total Environment (66 publications), Environmental Science and Pollution Research (62 publications), Environmental Monitoring and Assessment (58 publications), International Journal of Environmental Research and Public Health (47 publications), and Environmental Earth Sciences (43 publications). This distribution suggests that these five journals have a strong interest in the study of heavy metal pollution in urban soils. According to the citations per paper index, Chemosphere ranked highest (87.84 citations per publication), followed by Science of the Total Environment (78.06 citations per publication) and Environmental Pollution (70.19 citations per publication). This indicates that these three journals command significant attention in this field. With IFs above 8, these three journals exert a strong influence in the research field.

3.4. High Output Countries and Institutes

To identify which countries have made the most significant contributions to urban soil heavy metal pollution research, we analyzed data from 98 countries. Using VOSviewer, we visualized the top 30 countries based on the number of papers published (Figure 4a). In this visualization, larger nodes represent countries with more papers, and connecting lines represent the strength of collaboration between countries. The thicker the connecting line, the more collaborative papers have been published by the two countries. We imported the collaboration network analysis data from VOSviewer into Gephi for the top 30 countries, creating corresponding country chord diagrams (Figure 4b). Different color areas represent the number of publications from different countries, with the thickness of connecting lines indicating the strength of the link between two countries. These combined plots reveal a marked disparity in the number of publications among countries in this field. A few Asian countries, including China, Iran, and Pakistan, have published the most and are therefore the main contributors to urban soil heavy metal pollution research, with China leading the way. The two countries with the closest collaboration are China and the United States, followed by China and Iran.

Further analysis reveals the countries with high productivity in this field.

Table 3. Statistics of the top 10 productive countries.

Rank	Country	Publications	Citations	C/P	Continent
1	China	555	22,825	41.13	Asia
2	Iran	70	1454	20.77	Asia
3	Turkey	66	714	10.82	Asia and Europe
4	USA	57	1768	31.02	North America
5	Russia	52	515	9.9	Europe
6	Pakistan	49	1002	20.45	Asia
7	India	46	871	18.93	Asia
8	Poland	44	1084	24.64	Europe
9	Saudi Arabia	33	338	10.24	Asia
10	Egypt	31	471	15.19	Africa

Key: C/P = citations/publications.

displays the top ten high-productivity countries, detailing their number of published papers, citation count, citations per paper, and geographical continent. China has an abundance of research achievements in this field, leading in the number of publications, citation count, and average citation count per paper, with a total of 555 publications, 22,825 citations, and approximately 41 citations per paper. This prominence is likely due to the high priority the Chinese government places on addressing heavy metal pollution in soil. The government has successively enacted, introduced, and revised a series of standards and regulations to continuously tighten soil heavy metal emission limits. For instance, the “Soil Pollution Prevention and Control Law” was enacted by the National People’s Congress in 2019, and in 2022, the Ministry of Ecology and Environment issued “Opinions on Further Strengthening the Prevention and Control of Heavy Metal pollution”. These measures underscore China’s attention to soil heavy metal pollution. Iran ranks second, with a total of 70 papers and 1454 citations, averaging about 21 citations per paper. Additionally, the average citation count per paper in the United States is noteworthy, with 57 papers receiving 1768 citations, resulting in more than 30 citations per paper. Judging by the geographic distribution of significant countries, Asian and European countries have a strong collaborative presence.

Research institutions play a crucial role in contributing to the body of research in a field.

Table 4. Statistics of top 15 high productivity institutions.

Rank	Institution	Country	Publications	H-Index
1	Chinese Academy of Sciences	China	140	75
2	Zhejiang University	China	37	36
3	University of Chinese Academy of Sciences	China	33	41
4	Nanjing University	China	24	25
5	China University of Geosciences	China	21	29
6	Huazhong Agricultural University	China	15	13
7	China University of Mining and Technology	China	15	13
8	Henan University	China	11	8
9	Beijing Normal University	China	11	36
10	Chinese Research Academy of Environmental Sciences	China	11	22
11	Islamic Azad University	Iran	10	17
12	Nanjing Forestry University	China	8	7
13	Ministry of Agriculture and Rural Affairs	China	8	19
14	University of Belgrade	Serbia	8	17
15	China Agricultural University	China	8	8

lists the top 15 research institutions that have published the most papers on urban soil heavy metal pollution from 2000 to 2022. As can be seen from the table, these institutions include colleges, universities, and research institutes. The Chinese Academy of Sciences (with 140 publications), Zhejiang University (with 37 publications), and the University of Chinese Academy of Sciences (with 33 publications) are ranked in the top three. The Chinese Academy of Sciences accounts for the largest proportion, at 38.89%. The majority of these research institutions are located in China, which aligns with the data on high-productivity countries, indicating that Chinese research institutions’ investment in, and output of, research on urban soil heavy metal pollution surpass those of other countries. Iran and Serbia follow China. Judging by the h-index of various research institutions, the Chinese Academy of Sciences far surpasses other institutions, suggesting that its published documents are more influential and authoritative.

Inter-institutional collaborations provide insights into the intensity of international research in the field. CiteSpace was employed to generate a collaboration network map of various research institutions (Figure 5). In the figure, the circle nodes represent different research institutions; the larger the circle node, the more papers produced by that research institution. The connecting lines signify collaborative links, with more connecting lines indicating closer collaboration between research institutions. The purple outer circle represents nodes with high intermediary centrality. It is evident that the Chinese Academy of Sciences not only has the highest number of publications but also holds a high intermediary centrality, playing an indispensable bridging role in collaborations between different research institutions. The figure contains 277 nodes, with 167 connecting lines and a network density of 0.0044, indicating relatively limited collaboration and communication among institutions.

3.5. Highly Cited Document Network Analysis

3.5.1. Co-Citation Analysis of Document

Co-citation analysis is a characteristic feature of CiteSpace software. Two documents are said to have a co-citation relationship if they are jointly cited by one or more subsequent documents. The frequency of co-citation underpins the strength of the connection between the two papers, indicating the similarity of their academic backgrounds. As depicted in Figure 6a, the nodes represent the cited documents. The larger the node, the more frequently the document is cited. The lines between the nodes represent co-citation associations, with thicker lines indicating stronger associations. The figure shows that Yang, Q.Q. (2018) [69] is the document with the highest citation frequency, exerting a substantial influence on this study. This paper provides a systematic review of soil heavy metal concentrations in China’s urban industrial and agricultural areas. Based on the data collected, it comprehensively assesses the ecological and health risks of soil heavy metals nationwide. It was found that the pollution and associated risks of three metals—As, Cd, and Pb—were particularly severe. Industrial areas exhibited higher pollution risk than agricultural areas [69]. This review summarizes prior studies and provides guidance for subsequent research. In addition, several nodes with high intermediary centrality, indicated by purple outer circles, are present in the figure. By analyzing the intermediary centrality of key nodes, critical authoritative documents in this research area can be identified.

Table 5. Statistics of the top 10 documents with betweenness centrality.

Centrality	Title	Author	Journal	Year
0.26	Source apportionment and health risk assessment of heavy metals in soil for a township in Jiangsu Province, China	Jiang, Y.X.	Chemosphere	2017
0.23	Assessment of heavy metals pollution in urban topsoil from Changchun City, China	Yang, Z.P.	Journal of Geochemical Exploration	2011
0.19	Heavy metals assessment in urban soil around industrial clusters in Ghaziabad, India: Probabilistic health risk approach	Chabukdhara, M.	Ecotoxicology and Environmental Safety	2013
0.16	Pollution features and health risk of soil heavy metals in China	Chen, H.Y.	Science of the Total Environment	2015
0.14	Source identification and health risk assessment of metals in urban soils around the Tanggu chemical industrial district, Tianjin, China	Zhao, L.	Science of the Total Environment	2014
0.14	Heavy metals in urban soils: a case study from the city of Palermo (Sicily), Italy	Manta, D.S.	Science of the Total Environment	2002
0.13	A review of heavy metal pollutions in urban soils, urban road dusts and agricultural soils from China	Wei, B.G.	Microchemical Journal	2010
0.13	Trace metal pollution in urban soils of China	Luo, X.S.	Science of the Total Environment	2012
0.13	Heavy metal pollution in street dust and roadside soil along the major national road in Kavala’s region, Greece	Christoforidis, A.	Geoderma	2009
0.11	Identification of trace element sources and associated risk assessment in vegetable soils of the urbanerural transitional area of Hangzhou, China	Chen, T.	Environmental Pollution	2008

presents detailed information on the top ten documents ranked by intermediary centrality index. The node with the highest intermediary centrality is Jiang, Y.X. (2017), the first document to estimate the hazards to human health from soil heavy metal sources [70]. This document has drawn researchers’ attention to human health risks and provided new ideas for subsequent research.

3.5.2. Document Co-Citation Cluster Analysis

The clustering function of CiteSpace for document co-citation analysis can aid us in extracting common themes from similar documents and discovering emerging trends in urban soil heavy metal pollution research. Clustering labels were extracted using keywords and the LLR algorithm, and the eight most significant clusters were selected to generate a cluster label view (Figure 6b). The clustering profile module value Q = 0.747 (>0.3) and the average silhouette value S = 0.9129 (>0.7) indicate that the clustering effect is significant. In the figure, clusters nearer to yellow at the top are newer, suggesting that the research frontiers they present are relatively recent. The developmental trajectory of the study initially focused on heavy metal pollution in individual cities, primarily within clusters #1 Mexico and #5 Xuzhou, often in conjunction with soil magnetic characteristics. In surface soils, there is a significant correlation between magnetization and heavy metal content in highly contaminated samples [71]. Saturation isothermal residual magnetization intensity (SIRM) is a rapid, non-destructive method for detecting heavy metal pollution [72]. Subsequently, scholars recognized that differences in heavy metal pollution were not only manifest across different cities but were also significant within different functional areas of the same cities. Hence, research began to focus on heavy metal pollution in different functional areas within cities, with geostatistical analysis becoming more widespread during this period [73,74,75]. Geographic information system (GIS) plays a crucial role in determining the spatial distribution of soil heavy metals, with spatial interpolation commonly used to ascertain the spatial distribution of heavy metals [28,73]. The primary clusters within this period include #2 geostatistical analysis and #6 industrial cities. Finally, due to accelerated urbanization, increasing land area, and a broad variety of functional areas, heavy metal pollution became increasingly severe. Researchers began to focus on the evaluation of heavy metal pollution levels and the assessment of potential risks, shifting from ecological health risks to human health hazards. This primarily includes cluster #0 source resolution, #3 ecological risk assessment, #4 ultramafic rocks, and #7 health hazards. This shows that source apportionment and health hazards are emerging themes in urban soil heavy metal pollution research. The number of documents addressing health hazards arising from heavy metal sources is expected to increase further.

3.6. Profiling of Keywords in the Paper

Keywords underscore the core concepts of a document. To gain a clearer understanding of the keywords in this research area, we extracted the top 10 keywords with the highest frequencies, as shown in

Table 6. Statistics of top 10 keywords.

Rank	Frequency	Centrality	Keyword
1	763	0.04	heavy metals
2	737	0.05	pollution
3	342	0.04	risk assessment
4	332	0.01	urban soil
5	312	0.08	city
6	227	0.04	spatial distribution
7	223	0.07	Pb
8	193	0.07	area
9	188	0.08	agricultural soil
10	183	0.08	sediment

. The terms “heavy metals” (763 times), “pollution” (737 times), “risk assessment” (342 times), “urban soil” (332 times), and “city” (312 times) represent the key concepts in this research area. A keyword co-occurrence network can help us identify the research hotspots in the field. A total of 315 keywords from the 1247 papers were mapped by occurrence frequency using CiteSpace (Figure 7). The frequency of keywords highlights the research hotspots in the field. In the graph, the larger the font and the corresponding node of a keyword, the more it represents a research hotspot. The line between nodes represents the strength of the association between the keywords, with thicker lines indicating that two keywords appear in the same paper more frequently.

The three keywords with purple outer circles, namely “dust”, “plant”, and “cd”, serve as key nodes with betweenness centrality over 0.1, acting as crucial hubs connecting different domains. The concentrations of heavy metals in soil are closely related to those in dust. Heavy metals in dust can accumulate in the human body through direct inhalation, ingestion, or skin contact [76,77]. Hand-to-mouth exposure is the primary route for non-carcinogenic risk [27]. When dust falls, airborne heavy metals are deposited into the soil, and similarly, heavy metals in the soil can become airborne through dust and particulate matter. Consequently, studies of urban soil heavy metals often concurrently investigate street dust. Some studies even suggest that dust exhibits a higher degree of heavy metal pollution than urban soil [27]. Recently, the entry of increasing heavy metals into the urban environment through industrial dust and fuel combustion has drawn widespread attention from researchers [40,69]. Cadmium (Cd), one of the most severe soil heavy metal contaminants, is the primary contaminant in urban industrial parks, mining areas, and sewage irrigation areas. Researchers have given significant attention to Cd due to its interference with normal human physiological functions, propensity for chronic or acute toxicity, and classification as a human carcinogen by the International Agency for Cancer [78]. Some studies suggest that the risk of soil Cd exposure is higher in urban populations than in rural ones, emphasizing Cd as a priority for controlling soil heavy metal pollution in mining areas [6]. Heavy metals have toxic effects on plants in soil, causing changes in various physiological characteristics such as plant height, primary root length, and leaf area. However, plants can serve as an effective tool for remediating heavy metal pollution in soil. Phytoremediation, a form of bioremediation, removes soil pollutants through plant extraction, plant fixation, and plant volatilization [79,80], offering a more economical and environmentally friendly solution than traditional physical and chemical remediation techniques. Enhancing bioremediation efficiency is possible by combining plants with plant growth-promoting rhizobacteria (PGPR) [81]. For instance, the bacteria-plant system is found to remediate soils contaminated by Cd and Cr, significantly improving plant tolerance to heavy metals under symbiotic conditions [82]. Nevertheless, phytoremediation techniques are currently limited by the scarcity of suitable plant species, low biomass, and extended growth cycles. Thus, phytoremediation of heavy metals in urban soils presents a significant challenge that requires strategic and effective designs.

The cluster analysis timeline map generated by CiteSpace reveals the temporal emergence of key terms within this field of research, thereby delineating the overall knowledge structure of the discipline and its dynamic progression. As demonstrated in the timeline mapping (Figure 8), research on urban soil heavy metal pollution can be segmented into three distinct phases. The initial phase, spanning 2000 to 2006, was characterized by the high frequency of keywords such as Cu, Cd, Zn, urban, heavy metal pollution, agricultural soil, dust, and plants. The research primarily concentrated on the varying concentrations and types of heavy metals present in urban soil. The second phase, from 2007 to 2016, was characterized by keywords such as spatial distribution, enrichment factors, impact, source apportionment, human health risk, and emission. The research during this period predominantly focused on identifying the sources of urban soil heavy metal pollution and conducting risk assessments. The third phase, from 2017 to 2022, was characterized by keywords such as pollution characteristics, models, traffic, mining areas, and urban environment. The research in this phase centered on understanding the characteristics of soil heavy metal pollution in representative urban areas and assessing the deleterious effects on human health resulting from various pollution sources.

Figure 8 presents the ten keyword clusters related to urban soil heavy metal pollution. The modularity (Q) and silhouette (S) values are 0.3862 and 0.7141, respectively, suggesting a significant clustering effect and robust cluster results. The interconnections between nodes represent the convergences and intersections among the knowledge bases of various research directions. The lighter the node color, the more recent the keyword has emerged. Upon combining cluster information with the atlas, ten keyword clusters emerge within urban soil heavy metal pollution research, which include “source apportionment”, “urban soil”, “environmental magnetism”, “pollution assessment”, “education district”, “sewage irrigation”, “vegetables”, “atmospheric sedimentation”, “heavy metal pollution”, and “magnetic susceptibility”. These clusters embody the current focal points of research in this field. Furthermore, each cluster displays considerable overlap, with heavy metal pollution serving as the central term, thereby forming keyword clusters with high relevance and expandability. To render the research more comprehensive, this paper further consolidates and analyzes these ten significant clusters based on a thorough examination of the principal keywords of the major clusters. Accordingly, research on urban soil heavy metal pollution can be condensed into three primary domains, source apportionment of urban soil heavy metal pollution, pollution risk assessment, and the application of environmental magnetism.

3.6.1. Urban Soil Heavy Metal Pollution Source Apportionment

The theme “source apportionment of heavy metal pollution in urban soil” encompasses the following five clusters, #0 source apportionment, #1 urban soil, #5 wastewater irrigation, #7 atmospheric deposition, and #8 heavy metal pollution. The primary keywords incorporate urban soil, source apportionment, spatial distribution, heavy metal pollution, and wastewater irrigation. In the wake of rapid industrialization and urbanization, the inhabitants of densely populated cities are inevitably exposed to urban surface soil, either directly or indirectly. The escalating issue of urban soil heavy metal pollution poses a severe threat to human health. Hence, understanding the sources of soil heavy metal pollution and implementing reasonable, practical, targeted remediation measures are crucial to urban safety management and soil health preservation. Early research has indicated that the levels of heavy metal pollution in the soils of developed and industrial cities are exceedingly high [79]. For instance, typical coal-mining cities experience severe heavy metal pollution due to coal extraction activities [83]. Moreover, the severity of heavy metal pollution and associated risks in industrial areas far exceed those in agricultural regions [69]. Cadmium (Cd) and lead (Pb) pollution is notably more severe in areas of heavy traffic within city centers [84].

Source apportionment involves identifying various sources of pollution and quantifying their respective contributions. Numerous models and methods are currently available for the source apportionment of soil heavy metal pollution, including geographic information systems (GIS), multivariate statistical methods, isotope tracing methods, the positive matrix factorization (PMF) model, the multivariate receptor (UNMIX) model, and the absolute principal component score-multiple linear regression (APCS-MLR), among others. PMF and multivariate statistical methods, such as principal component analysis (PCA), correlation analysis, and cluster analysis (CA), are frequently employed for source apportionment [83,85,86,87,88]. Each source apportionment method comes with its own advantages, disadvantages, and application limitations, which often necessitates the combined use of multiple methods. Numerous studies have also focused on improving traditional source apportionment models. For instance, the modified PCA-MLRD model, based on PCA and multivariate linear regression of distance (MLRD), can not only identify specific sources but also quantify the impact of each source’s emissions [89]. The robust absolute principal component score-robust geographically weighted regression (RAPCS-RGWR), a new receptor model proposed based on the traditional APCS-MLR model, provides estimated source contributions that are closer to actual soil heavy metal concentrations than traditional models [90]. Eigenvector-based spatial distribution principal component analysis (SD-PCA) models, which assess soil pollution in spatial dimensions, simplify the process of tracing and identifying potential heavy metal sources [91]. GIS effectively explains spatial variability and is commonly employed to visualize the spatial distribution of heavy metals [75,92]. Geostatistical methods such as inverse distance weighting (IDW), ordinary kriging (OK), and ordinary co-kriging (OCK) are all frequently used for spatial interpolation.

Extensive research indicates that anthropogenic factors primarily drive environmental heavy metal pollution. These factors include traffic emissions (comprising tire wear particles, automobile exhaust particles, street surface particles, etc.), domestic emissions (such as waste incineration, sewage irrigation, etc.), industrial emissions (originating from power plants, chemical plants, metallurgical industries, etc.), and atmospheric deposition. Among these, atmospheric deposition represents a principal pathway for the introduction of heavy metal elements into soil, as minute heavy metal particles can infiltrate the soil through dust fall and regular deposition. The presence of heavy metal elements on the surface of atmospheric particles alters the original elemental ratio in the soil and the concentration required to maintain the ecological balance of metal elements. This alteration results from the deposition effect, leading to the formation of new heavy metal oxides, disrupting the food chain, and thereby breaking the ecological balance [93]. Various sources of soil heavy metals pose different threats to human health [60]. Utilizing geostatistics in combination with land use surveys, the health hazards of heavy metal sources in urban soils have been estimated for the first time, waste incineration and textile/dye industries (28.3%), natural pollution sources (45.4%), traffic emissions (5.3%), and the electroplating industry and livestock farming (21.0%). These four sources account for 23.5%, 32.7%, 7.4%, and 36.4% of the total health risk, respectively. Wastewater irrigation of farmland, coupled with intensive agricultural practices such as the excessive use of pesticides and nutrient-based fertilizers, has become a common practice. This practice has led to the accumulation of heavy metals in cultivated land [94]. Studies have found that lead (Pb) toxicity notably increases in soils irrigated with sewage water and is transferred to the edible parts of food crops [95]. The first report on heavy metal transformation in soil-grape systems in China reveals that wastewater irrigation leads to severe soil heavy metal pollution in vineyards, especially with cadmium (Cd) and zinc (Zn). The consumption of grapes grown in sewage-irrigated areas poses potential health risks for consumers [96]. In many countries, the irrigation of agricultural land with untreated or substandard industrial effluents is the primary mode through which heavy metals enter the soil. This process results in surface source pollution, posing severe environmental and health issues to inhabitants. Additionally, polyvinyl chloride (PVC) products, tires, colored pigments, and furnace ashes are potential sources of heavy metals in urban soils.

3.6.2. Pollution Risk Assessment

The theme “pollution risk assessment” encompasses the following three clusters, #3 pollution assessment, #4 educational zone, and #6 vegetables. The primary keywords include pollution assessment, agricultural soil, organic matter, educational zone, vegetables, crops, and human health risk. Given that soil heavy metals can readily be transferred to humans via a contaminated food chain, it is crucial to assess their pollution levels and ecological risks. Soil heavy metal concentrations are typically used to assess human health risks, such as chronic or non-carcinogenic hazards, including hazard indicators HIexP (ingestion, inhalation, dermal and carcinogenicity), and CR (cancer risk). It has been found that the HIexP values for lead (Pb) and cadmium (Cd) exceed permissible limits for children, and CR caused by the carcinogenic metals cobalt (Co), chromium (Cr), and Cd are within safe ranges. However, CR is higher in adults compared to children [97]. In terms of non-carcinogenic risks, children are at a greater health risk than adults, which is closely associated with their behavioral activities and physiological characteristics [98]. Playgrounds and educational zones, frequently used by large numbers of young children and their guides, are highly susceptible to surrounding metal pollution events due to the concentration, distribution, and effectiveness of soil heavy metals [99]. The spatial variability of pollution sources results in different concentrations of soil heavy metals in different functional areas of cities. The order of reaching the ecological hazard level varies from one functional area to another. Heavy metal pollution in agricultural soils, which is related to food security and human health, has become a global issue. An analysis of heavy metals in soil, rice, and vegetable samples near industrial mining areas revealed that there are significant differences in the accumulation levels of heavy metals in leafy and non-leafy vegetables, and that food intake and soil intake are two significant pathways for the average daily intake of heavy metals by adults [100]. Heavy metal-organic contaminant complexes, which include polycyclic aromatic hydrocarbons (PAHs), antibiotics, pesticides, and others, are prevalent and complex in soils. Heavy metals and PAHs are toxic chemicals that can cause chronic diseases and carcinogenesis [101]. The same adsorption mechanism to soil organic matter and clay results in a synergistic or antagonistic effect of heavy metals and PAHs in urban soils [102,103]. This synergistic effect generates more severe environmental risks in areas with high population density [104]. Heavy metals are seen as a potential factor in the evolution and development of antibiotic resistance. Some studies have found a positive correlation between heavy metal concentrations and antibiotic concentrations in urban soils, which may be related to the adsorption of metals on antibiotics [105].

Risk assessment of soil heavy metal pollution plays a crucial role in shaping urban pollution management and remediation strategies. The United States and the United Kingdom, as pioneers in risk assessment, have relatively comprehensive frameworks for the health risk assessment of soil pollution. In comparison, Asian countries have initiated research in this area relatively late, and most of their soil heavy metal risk assessment methods employ the index method. Commonly used assessment metrics for heavy metal pollution status include the enrichment factor (EF), geological accumulation index (Igeo), pollution index (PI), and potential ecological hazard index (RI) [39,106,107]. In recent years, the application of algorithmic models in this research field has grown. For instance, the entropy-based Topsis (EMBT) model, which improves the ability to accurately analyze pollution sources by focusing on their locations, can effectively calculate the pollution level of sampling points. The accuracy of this model has been validated through interpolation in geostatistical analysis [108].

3.6.3. Applications of Environmental Magnetism

The theme “applications of environmental magnetism” encompasses two clusters: #2 environmental magnetism and #9 magnetic susceptibility. The primary keywords include environmental magnetism, accumulation, elements, toxicity, among others. Traditionally, soil heavy metal pollution has been monitored by collecting soil samples for chemical analysis to determine heavy metal concentrations. However, this process can be resource-intensive in terms of both time and finances. With advancements in science, technology, and modern magnetism, a new technique for monitoring soil heavy metal pollution has emerged in recent years. Soil magnetism, which provides information on the degree, source, extent, and history of heavy metal pollution, has become an excellent alternative to traditional geochemical methods. All materials in nature exhibit specific magnetic characteristics, and magnetic susceptibility is the most critical indicator of these characteristics. Contaminated soil significantly differs from uncontaminated soil in terms of magnetic susceptibility characteristics. Researchers have found a significant negative correlation between soil heavy metal concentration, magnetic susceptibility, SIRM, and distance from urban centers [26]. This correlation can serve as an effective indicator for monitoring soil heavy metal pollution. The magnetic susceptibility of soils around industrial centers and areas with heavy traffic is higher than that of natural soils in non-urban areas [109]. Soil metal content is often used to evaluate the degree of anthropogenic influence in urban environments [37,38]. The magnetic susceptibility signal detected in some urban top soils originates from ferromagnetic components generated by anthropogenic activities that contain some heavy metals in their molecules. The magnetic characteristics of urban topsoil are dominated by low-coercivity magnetite. Heavy metals are highly enriched in the magnetic fraction of urban topsoil. Their geochemical characteristics are significantly different from those of agricultural topsoil, further indicating that the additional magnetic minerals accumulated in urban topsoil are not inherited from soil-forming parent material and are not formed by soil-forming processes, but are the result of human activities [110]. Certain heavy metals, such as copper (Cu), iron (Fe), lead (Pb), zinc (Zn), cadmium (Cd), etc., have a strong affinity for ferrous materials and increase soil magnetization [37]. The spatial distribution of magnetic concentration and heavy metal content in agricultural soils matches the spatial pattern of parameters in river sediments [111]. In recent years, environmental magnetic methods based on magnetization rates and rock magnetic parameters have been developed as suitable alternative techniques for characterizing and quantifying the extent of soil heavy metal pollution, for identifying the types of various industrial pollution sources [112,113,114,115], and as alternative indicators for determining the spatial distribution of soil heavy metal pollution and anthropogenic emission sources [100,101,106,109,110,116], with an increasing number of applications in various study areas [101,102,107,110,111,117].

3.7. Keyword Bursting Detection

CiteSpace offers numerous functionalities. For instance, its keyword burst detection can identify research hotspots and trending topics in different periods. This helps us understand the sudden surge of research hotspots in the field of urban soil heavy metal pollution. With the burst detection algorithm provided by CiteSpace, we can not only detect high-frequency words and analyze the discipline’s research frontiers and evolution trends according to frequency changes, but also visualize the duration and intensity of scholars’ attention to specific topics. The analysis results, as depicted in Figure 9, reveal that the three keywords with the highest burst intensity value from 2000 to 2022 are source resolution (18.92), Pb (lead) (14.33), and Hong Kong (13.27). These three keywords occupy a central position in this research area, with source resolution being a core content of urban soil heavy metal pollution research. Hong Kong is a popular area of study, and Pb is a consistently important keyword in heavy metal pollution research. Urban soil heavy metal pollution research in Hong Kong drew significant attention from 2002 to 2014, aligning with the publication period of a 2001 study by the Hong Kong Polytechnic University. This study found that urban park ground dust in Hong Kong contains numerous heavy metals, with Cd (cadmium) and Pb accumulation being most severe compared to suburban parks [118]. The study of Cd pollution lasted from 2001 to 2012, reflecting the importance researchers place on human health. Keywords with high burst intensity values during 2000–2006 included Cd, Pb, Zn (zinc), Cu (copper), Hong Kong, and soil pollution, indicating a focus on different heavy metal pollution in urban soils. From 2007 to 2016, the keywords with high burst intensity values included model, China, wastewater, accumulation index, and environment, indicating a shift from studying urban soil heavy metal concentrations to pollution assessment and degrees of pollution under different agricultural cultivation patterns. In the period of 2017–2022, keywords with high burst intensity values included source identification, human health risk, and source apportionment, indicating that scholars more frequently combined source apportionment of urban soil heavy metal pollution with human health risk. Notably, the burst of the keywords source identification, source apportionment, ecological risk assessment, human health risk, and pollution characteristics continued until 2022. This suggests an increasing research intensity around these keywords and indicates that they will remain core and hot topics in this research field in the future.

3.8. Limitations of This Study

This study has several limitations due to both subjective and objective factors. First, the bibliometric analysis software has stringent specifications and standards for data analysis. To ensure the quality and integrity of the data collected, this study selected only journal documents from the WoSCC database, excluding other databases such as Scopus and EI Compendex. This means the data analysis may not be fully comprehensive. Furthermore, quantitative analysis and interpretation of data necessitate a deep and comprehensive understanding of the field. While this study is based on an extensive body of literature and maintains effective communication with authoritative figures who have long been engaged in this field, some subjectivity is inevitably introduced in the chart creation and analysis content.

4. Conclusions and Outlook

Amid rapid industrialization and urbanization, the issue of urban soil heavy metal pollution has increasingly come into focus. Leveraging the document corpus on urban soil heavy metal pollution in the WoSCC database, this study employed bibliometric and text analysis methodologies. With the assistance of visualization software such as CiteSpace and VOSviewer, we reviewed and organized the developmental lineage and academic achievements in this field over the past two decades, and validated the applicability of Price’s law in this context. Specifically, we undertook an in-depth discussion and analysis of the volume of publications, key authors, highly productive countries and institutions, notable journals in the field, and keyword clustering in this research area, leading to the following conclusions.

The body of research on urban soil heavy metal pollution has generally grown from 2000 to 2022, with a particularly significant increase in the total number of publications post-2016. This indicates an escalating interest in urban soil heavy metal pollution among international scholars. The volume of publications by core authors met half the criteria proposed by Price, suggesting a relatively stable group of author collaborations. Ahmad Kafeel and Khan Zafar Iqbal, along with Huang Biao, shared the top spot for the number of publications. Most of the high-output journals were among the top-ranked in environmental science, with the Science of the Total Environment (66 publications), Environmental Science and Pollution Research (62 publications), and Environmental Monitoring and Assessment (58 publications) being the top three journals in terms of publication count. Asian countries, particularly China, which accounted for 44.51% of global publications, made significant contributions. China thus emerged as the most influential country in the field of urban soil heavy metal pollution research. In terms of research institutions, the Chinese Academy of Sciences held the top spot in both the number of publications and h-index. Document co-citation mapping and cluster mapping revealed that the most frequently cited paper was authored by Yang, Q.Q. (2018) [69], while the paper by Jiang, Y.X. (2017) [70] had the highest centrality (0.26). Temporal development vein clustering labels suggest that future research will focus on health hazards. Keyword co-occurrence analysis, cluster analysis, timeline analysis, and burst detection analysis indicated that urban soil, heavy metal pollution, risk assessment, and Pb (lead) were the primary research keywords. The health hazards of soil heavy metals were repeatedly cited as a significant human crisis. The research primarily centered around three themes, source apportionment of urban soil heavy metal pollution, pollution risk assessment, and the application of environmental magnetism, with source apportionment ranking first in burst intensity value.

This study has some limitations that are important to acknowledge. Firstly, it’s worth noting that bibliometric analysis software requires high-quality data for accurate results. Even widely used databases like WoSCC can introduce errors in citations due to factors such as metadata quality from publishers, discrepancies in original papers, or inaccuracies during data processing [119]. While these errors are usually infrequent, they can have a more significant impact when dealing with smaller sample sizes [120]. Secondly, our study focused exclusively on the journal literature within the core collection of the Web of Science database, excluding other databases like Scopus. This choice may result in incomplete data analysis as it does not cover all available research sources. Lastly, it is important to highlight that quantitative data analysis and interpretation require a deep and comprehensive understanding of the subject matter. Subjectivity can play a role, underlining the significance of researchers having a strong domain knowledge to ensure the reliability of their analysis and interpretation.

In the future, research on urban soil heavy metal pollution will remain relevant and dynamic due to its significant importance. Subsequent studies should prioritize better assessment of health risks by utilizing source analysis of heavy metals in urban soil. It is crucial to enhance research methods, broaden the scope of investigation, and develop comprehensive research models to refine research ideas. Furthermore, the risk assessment of heavy metal pollution in urban soil should include spatial correlation analysis between different heavy metals and consider the synergistic interactions resulting from mixed metals. Human risk assessment should also account for the physical conditions of different groups, such as the elderly, pregnant women, and other sensitive individuals. Lastly, although significant efforts have been made to remediate soil heavy metal pollution in mining areas and agricultural soils, there remains a lack of economically viable and effective treatment methods. Therefore, more attention should be directed towards soil remediation and sustainable development in densely populated urban areas. With the rapid development of urbanization, it is increasingly necessary for the government to adopt a comprehensive approach to alleviate and prevent heavy metal pollution in urban soil. Nature-based solutions (NBS) play a pivotal role in urban soil pollution remediation and can be included in discussions of urban soil pollution management.