The complexity and interconnectedness of circular cities and the circular economy for sustainability

In our study, we are evaluating the dynamic interplay between circular cities and the circular economy, revealing connections shaped by new research insights, using Web of Science database. We have in depth analysed 671 publications from 1990 to mid-2023, employing Leximancer and VOSviewer tools. Our analysis uncovered key themes, including buildings, waste management, value chains, and sustainable development, linking the circular cities concept and circular economy framework. Emerging research trends, such as urban metabolism, urban mining, governance models, the built environment, sustainability benchmarks, and value chain management, signify deliberate shifts towards higher circular development and implementation. These trajectories harmonise with overarching sustainable development goals. Contemporary research emphasises urban mining, circular indicators, adaptive resource reuse


| INTRODUCTION
Implementing circular economy principles in cities requires understanding the underlying cities' patterns and features to foster circularity.A necessity is exposed by cities themselves, amplified with growth and increased energy and resource consumption.The United Nations (UN) introduced that 55% of the global population resides in cities, with the initial trends expecting an increase of up to 68% by 2050 (UN, 2019).Such trends are challenging, especially considering the environmental and societal impacts following the expanding directions.Cities and their urban areas occupy only 2%-3% of the entire earth's surface.They are responsible for up to 80% of global energy consumption, followed by 75% of the current world's natural resource consumption (Dodman et al., 2017).Cities with their activities produce up to 50% of global solid waste and are directly and indirectly responsible for up to 70% of global greenhouse emissions (OECD, 2021).These impacts are expected to increase with the rise of so-called 'megacities' which calls for an immediate reaction from policymakers and researchers to find possible solutions (UN, 2019).
In response to the increasing challenges posed by urbanisation, the United Nations recognised the critical importance of incorporating cities into its Agenda 2030, established in 2015.This led to the formulation of Sustainable Development Goal 11, 'Sustainable cities and communities', which encompasses a range of targets to address various aspects of urban development (Lehner & Blaschke, 2021).These targets, such as inclusive and sustainable urbanisation (Target 11.3), are aligned with the broader areas of sustainable development, including considerations of people, partnership, and prosperity (Tremblay et al., 2020;Vanhuyse et al., 2021).The concept of sustainable cities was thus established, emphasising the integration of social, environmental, and economic dimensions to design and construct urban developments that enhance habitat resilience, self-sufficiency, job creation, and efficient resource utilisation (Crane et al., 2021;Hassan & Lee, 2015;Shi et al., 2022).By adopting such an integrated approach, cities have emerged as vital centres for promoting and advancing other sustainable development goals, such as quality education (Goal 4) and decent work and economic growth (Goal 8) (Deininger et al., 2019;Petit-Boix & Leipold, 2018).Nevertheless, cities are complex systems, demanding advanced sustainable systems thinking to effectively address the challenges they present (Bai et al., 2016;Johnson, 2012;Wijkman et al., 2019).Establishing a model for sustainable cities is inherently intricate with diverse facets encompassing infrastructure, management, and societal dynamics (Bettencourt, 2015;Esmaeilian et al., 2018).
The urbanisation challenges further led to the emergence of several conceptual frameworks, offering diverse city models to facilitate the implementation of sustainable development principles in urban environments.Alongside the integration of smart technologies, green urban areas, and measures to enhance resilience, another crucial concept gaining traction is the circular economy.This model is specifically designed to improve the circularity of material flows within urban areas, promoting efficient resource utilisation and minimising waste generation in a scientifically informed manner.
Circular economy represents an engaging approach that could foster cities to achieve impact reduction and move towards more sustainable solutions.However, circular economy principles implementation in cities faces many challenges, including industrial, financial, and social (Geng et al., 2009).Thus, it is essential to understand circular city implementation pathways and targeted goals.The conceptualization of circular cities, in general, follows the core aspects and circular economy definitions.In practice, cities should promote and demonstrate a transition from a linear to a circular economy, using the advantages of the urban space, city functions and departments, and collaboration with residents, businesses, and the research community (Novak et al., 2021).According to Ellen MacArthur Foundation (EMF) (2019), cities need to address economic, environmental, and social statuses, which can be achieved by implementing innovative urban designs, prolonging usage and maintaining materials and their values, and promoting the regeneration of natural systems.A circular city is described as a city where circular economy concepts, new alliances, and cooperation between different stakeholders are utilised (Langergraber et al., 2020).
However, there is an ongoing discussion about circular city concepts and their essential elements.Marin and De Meulder (2018) exposed that the circular city concept raises uncertainty regarding the details, such as designing the urban landscape for circular economy transition and the inclusion of multidisciplinary teams to integrate various approaches (e.g., political and industrial ecology).Circularity is, in many ways, also a theme of smart cities, as indicated by Prendeville et al. (2018) or Sujata et al. (2016).For example, Aceleanu et al. (2019) implemented circular economy principles of waste management in the smart city model, focusing on energy production, while Andrade and Yoo (2019) identified the circular economy and the Internet of Things (IoT) as a crucial combination to spur smart city development.Circular economy has been recognised as a vital component in attaining sustainability goals in the smart city (Parra-Dominguez et al., 2021).The recent research emphasises a need to examine definition frameworks and concepts of city models towards a circular economy, as cities present a focal point for implementing a circular economy (Lakatos et al., 2021).Exploiting the circular city concept is essential to avoid ambiguity and uncertainty with other concepts (e.g., smart cities or green cities).
Circular city concepts are challenging, especially regarding the inclusion of several circular economy and sustainability principles.
The challenge regarding circular economy implementation emerged as the circular economy has been primarily initiated in the industrial sector and the rising need for compatible models (Camilleri, 2020;Williams, 2019a).A shift from business activities to other city aspects, such as urban landscape design, was proposed (Marin & De Meulder, 2018), and the inclusion of bioeconomy, and regenerative design (Horn & Proksch, 2022).Furthermore, the need for tools to provide a basis for circular city implementation was exposed (Girard & Nocca, 2019), as well as evaluation models and frameworks to examine the success of implementing a circular economy at the city level (Gao et al., 2021a;Rodrigues & Franco, 2019).
Understanding the present state of the circular city is indispensable for fostering future sustainable development.Therefore, we have conducted a comprehensive research trends analysis, focusing on the interlinkages of circular city and circular economy concepts.A content analysis of research papers enables conceptual mapping and identifying challenges.This approach provides an opportunity to devise a structured framework, highlighting the advantages of circular cities and circular economy concepts to accelerate sustainable development.Such conceptualization is essential in gaining perspectives on existing challenges that facilitate progressive transformation towards circularity in cities.Consequently, this research can inform and guide experts in circular and sustainable development and serve as a valuable resource for policymakers when addressing the integration of circular economy aspects in urban areas.Consequently, this research can inform and guide experts in circular and sustainable development and serve as a valuable resource for policymakers when addressing the integration of circular economy aspects in urban areas.

| METHODOLOGY
This section represents a methodological approach, including the following steps: data collection, content analysis, and concept mapping, as outlined in Figure 1.

| Data collection and extraction
In conducting our research, we have considered Donthu et al. (2021) direction to select an appropriate scientific database for the collection of research papers.The databases with substantial collections considered for this purpose were Web of Science (WoS), SCOPUS, and Google Scholar, as identified by Cascajares et al. (2021).While some authors have expressed a preference for utilising SCOPUS (Falagas et al., 2008;Mingers & Lipitakis, 2010), and others have opted for a combination of two or more databases (Archambault et al., 2009;Ullah et al., 2023), we decided to focus on one database.This decision, as explained by Donthu et al. (2021), aimed to eliminate the need for data consolidation and minimise potential human errors during the collection phase.
In the preliminary search conducted in Web of Science, Google Scholar and SCOPUS, we observed a considerable overlap in the research papers retrieved.Considering the findings of Cascajares et al. (2021), we chose to focus our search on the Web of Science, which incorporates the Journal Citation Reports (JCR), encompasses highimpact journals and is partially a foundation for determining journal impact factors (Clarivate Analytics, 2022).The database's rigorous content selection standards and daily maintenance practices make it a reliable source for content analysis (Pasko et al., 2021;Rousseau & Rousseau, 2017) and comprehensive usage counts (Pranckutė, 2021).
The first step in our research involved selecting the appropriate time horizon, following the methodology used by Donthu et al. (2021).The search was conducted in the WoS databases, covering the period from 1 January 1990 to 30 June 2023.Specific settings included only research published entirely in English or a dual language format (e.g., Spanish and English).We also restricted the search to scientific and review articles, encompassing articles published in books or as book chapters, as they are conducive to bibliometric and content analysis (Donthu et al., 2021;Mingers & Lipitakis, 2010).Using the keywords 'circular city/cities' and 'circular economy' under these settings, we identified 1807 relevant papers.
To further refine the initial research results, we implemented a two-step screening procedure suggested by Padilla-Rivera et al. (2020).In the first step, we evaluated the research papers from the initial search by carefully examining both their titles and abstracts.A prerequisite for inclusion in this step was that the title and abstract focused on incorporating both circular economy and circular city concepts in the research.Papers failing to meet this criterion were excluded, leaving us with 1327 applicable papers for the second step.management, and material flow analysis), indirectly indicating the contextual importance of circular economy and circular cities. Papers that satisfied at least one of these two factors, preferably both, were deemed suitable for inclusion in the final database.After applying this two-step procedure, we arrived at 671 papers, which were subsequently employed in both bibliometric and content analysis.
Following the guidelines set forth by Donthu et al. (2021), we utilised VOSviewer and Leximancer as the visualisation software for conducting content analysis.For a comprehensive review of the selected papers, we employed VOSviewer and Leximancer software, following the recommendations of van Eck and Waltman (2022) for 'Full Record and Cited References' in VOSviewer and referring to the full PDF documents for Leximancer (Leximancer, 2022), respectively.These tools allowed us to explore the content and patterns within the selected papers for our research.

| State-of-the-art analysis
The state-of-the-art analysis is based on review methods, using computer assistance to identify publications, authors, and journals.It also enables papers' content interconnectedness and overlapping (Han et al., 2020).The methods rely on statistical and mathematical analyses forming and enabling organisational, classificational, and quantitative publication pattern evaluations (Pritchard, 1969).The main goal is network creation, using citation and content analysis to give insight into a specific topic or theme evolution, supported by visualisation interconnections (Peris et al., 2018).
VOSviewer was used for additional map construction in a more specialised graphical mapping representation (van Eck & Waltman, 2010).VOSviewer uses the 'Visualisation of similarities' method, providing a low-dimensional visualisation in which objects reflect their similarity as accurately as (van Eck & Waltman, 2007).VOSviewer enables the creation of visualisation maps constructed for mapping co-citation and keywords co-occurrence.The benefit of our research is that a VOSviewer option links strengths, which allows insights into the individual nodes' distribution and interconnectedness.

| Content analysis
Leximancer focuses on text-mining and content analysis, with the primary goal of automatically generating topic models based on words or concepts gathered from the collection of publications or text materials (University of Surrey, 2022).Leximancer is a software tool for analysing natural-language text data (Biroscak et al., 2017) and has been employed to interpret and visualise complex data (Campbell et al., 2011).The software is based on algorithms, which are statistical and comprehend nonlinear dynamics, as well as machine learning, creating categories and interconnectedness via relationships (Angus et al., 2013).Leximancer recognises the main concepts and their correlations, using thematic and semantic inquiries using two co-occurrence information, semantic and relational (Smith, 2003), and builds the themes by grouping concepts into clusters (Cretchley, Gallois, et al., 2010).In such analysis, papers are examined for the concepts' presence and frequency, such as words, phrases, and definitions, where a concept list and relationships are visualised via a concept map (Hyndman & Pill, 2017).Therefore, it represents a profound understanding of interconnectedness among circular cities and circular economy and their related themes and allows exploring future research trends.Each theme is centred around the dominant concept, which gives the theme its name.Such distribution enables a visual representation of concepts, their strength, and the formation of provided data (Cretchley, Rooney, & Gallois, 2010).Generating concept maps and networks enable a holistic and comprehensive approach towards significant themes and topics.
Additionally, the circles and their size visually enhance the distribution of themes and concepts.The colour of the circles (themes) represents a heat 'importance' map.The warmer colours (e.g., red and yellow) represent the most significant themes, followed by colder colours, such as green, blue, and purple (Engstrom et al., 2022).Concepts are connected to the theme in which they are positioned, and their distance indicates their relationships (Biesenthal & Wilden, 2014).
Semantically strong concepts are closer and even overlapping, while weak concepts are distanced (Campbell et al., 2011).

| RESULTS
This section represents the results of state-of-the-art analysis, as well as bibliometric and content analyses and research trends conceptualisation.

| Publications, citations and their growth
To statistically visualise the current development trend of circular cities and circular economy is crucial to carry out bibliometric and content analyses.Circular economy publications rapidly increased, most notably in the period between 2019 till June 2023 (Figure 2), with citations that obeyed the same trend.
From Figure 2, we can perceive a substantial publication and citation increase since 2016.Such an acceleration within the research areas is associated with the circular economy popularisation via Ellen MacArthur Foundation and the European Commission's activities, including the Circular economy action plan in December 2015 and its follow-up in March 2020 (European Commission, 2020).Our survey comprehended time until the middle-end of 2023.Thus, the number of publications, according to the existing trend, will increase further.
The growing research interest lies within the industrial and consumption-related needs of companies, cities, and nations trying to preserve resources and materials for further economic development.
With the integration of circular economy principles, circular cities are becoming prevalent topics as potential problem-solver for the upcoming environmental crisis (materials, food, and energy).

| Content analysis
Content analysis has been carried out in the circular city and circular economy research using a two-step approach.First, we employed Leximancer to determine the most prevalent concepts in the publication database and the interconnectedness between circular city and concepts network perspective.Further, the results were analysed using VOSviewer to identify the co-occurrence of several concepts, giving a more detailed view of the concepts' distribution.

| Leximancer content analysis
We have employed Leximancer for modelling concepts and interpreting complex data.Leximancer enables text analysis, building concepts and themes from the selected data (Cretchley, Gallois, et al., 2010).
The concepts and themes are constructed by extracting semantic and relational information from keywords found within the preselected documents (Gapp et al., 2013).The initial setting within Leximancer  was the preselected database of 671 publications identified to include and relate to both the circular economy and the circular city.The first phase included concept generation, achieved by associating terms related to the meaning of individual words.The semantic importance of a concept node is determined by the presence, word frequency, phrases, and co-occurrences related to it, which leads to its generation (Gapp et al., 2013).We have merged the concepts with the same semantic meaning, for example, city and cities.However, concepts presented in a singular phrase were transformed into compounds, for example, 'circular' and 'economy' into 'circular economy', as suggested by Engstrom et al. (2022).Thus, we have obtained 57 unique concepts for the concept map.Following the methodology from Engstrom et al. (2022), we have initiated several 'recluster' processes and received a concept map with four significant themes (see Figure 3), which were the circular economy, waste, building and value chain.The formation of clusters in the map correlates with the concepts within them.
As can be perceived from the obtained concept map (see Figure 3), the dominant theme is the circular economy, comprising concepts such as 'circular city', 'urban metabolism', 'planning', 'growth', and 'population' focusing on the circular city intentions.
Other comprehended concepts are 'environmental', 'resources', 'strategies', and 'industrial', indicating the importance of a strategic approach to managing resources using circular economy principles.
Concepts 'sustainable development', 'society', 'cultural heritage', and Our interest was to identify the interconnected of the circular city (Figure 4 and Table 2) and its relevance and likelihood of other concepts.The likelihood supplements the count statistics and provide an evaluation of probability.
As seen from   (European Commission, 2015).However, it is essential to mention that the circular idea emerged much earlier when Pearce and Turner (1990) distinguished between a (circular) natural system and a (linear) economic system.
We have also performed a content analysis to identify aspects that intertwine the circular city and the circular economy concept.We undertook the content analysis since it provides insight into the development and evolution of academic literature for the chosen scientific area.Understanding the development is especially important due to the increased publication rates on both concepts in the last 5 years.
An increase in publication rates can quickly dilute the current standing of research and hide potential research gaps, which could push towards new research topics.To our understanding, no similar study was detected considering the content analysis of the two concepts.
Thus our study enables a unique perspective on the complex intertwinement between circular city and circular economy concepts.
We conducted the content analysis via two different software, with Leximancer analysis being paper text-oriented and VOSviewer abstract text-oriented.For this reason, we focused primarily on results from Leximancer, complimenting them with findings from VOSviewer to enrich the research network gained by the analysis.
The 'circular city' was heavily imbued under the circular economy in both Leximancer and VOSviewer analysis results, indicating a close relatedness between the two concepts.While the circular economy's primary focus was the industry sector (Camilleri, 2020;Wuyts et al., 2022), it quickly evolved to include cities (EMF, 2019), as indicated in our results.The next impactful concept closely related to the 'circular city' and circular economy was 'urban metabolism'.While implementing a circular economy in the industry is challenging, it is much less complex than in cities. Cities are a complex system comprising several sublayers that need considering, such as resource flows (Williams, 2023), water consumption (Hong & Park, 2023), energy (Martinez et al., 2021), and socio-economic attributes (Galychyn et al., 2022).Understanding the underlying flows within city areas is crucial to implement a circular economy (Williams, 2023).We also detected a close relatedness of both concepts towards 'sustainable development'.While both concepts incorporate sustainability (Girard & Nocca, 2019), we discerned that the primary focus in cities lies within the social dimension.The results coincide with Williams (2019b) and Ríos et al. (2022), that for cities to transform into circular ones, including society dwelling in urban areas, is paramount for any meaningful progress in the future.
Waste management was the second most focused area for circular economy and circular cities.While from the economic perspective, considering the whole life cycle is the goal (Wuyts et al., 2022), the results indicate that the focus lies within the final phases instead.Particularly for cities, we identified the inclusion of focusing on 'water', 'energy', 'food' and 'material flows' and their 'treatment'.The results coincide with research by Dodman et al. (2017) and OECD (2021), who identified cities as a sink of natural resources.Considering cities as potential resource stocks (Ghisellini et al., 2022), we can establish an effective mechanism to harness the resources and return them to the loop (Haberl et al., 2021), as identified by concepts of 'recycling', 'reuse', 'recovery', and 'treatment'.Further evolution of only treating resources within circular cities is implementing urban manufacturing (Hausleitner et al., 2022), identified by concepts of 'manufacture' and 'materials'.By encouraging optimised usage of recovered local resources, cities could diminish their reliance on resource inflows and push towards higher circularity development.
The following research area, specifically towards circular cities, was the built environment.The built environment constitutes the physical properties of a city (Circular city funding guide, n.d.), as identified by concepts of 'infrastructure', 'building', 'land', and 'construction'.
We perceived a significant interest towards improving construction technologies that will enhance resource flows and reduce resource consumption (Foster & Saleh, 2021;Joensuu et al., 2020).Other indices from 'recycling', 'reuse', and 'recovery' indicate a push towards reintroducing construction materials into the construction segment (Bonoli et al., 2021) in line with the circular economy.A recovery approach could increase city flows, increase energy efficiency and decrease resource losses due to final waste disposition (Velenturf & Purnell, 2017).Lastly, 'project', 'infrastructure', and 'land' concepts indicate that cities implement project-oriented planning to prepare for future circular development (Williams, 2023).With many cities spatially constrained, it is paramount to optimise existing land usage within the city's perimeters (Williams, 2023), which benefits the socio-economics and well-being of city inhabitants (Galychyn et al., 2022).
The last research area was for value chains and 'supply chains'.
Both software analyses intertwined 'supply chains' with 'management', 'market', and 'performance' concepts, indicating that managing flows is essential for a circular economy within cities. 'Supply chains' and broader supply chain networks (Do gan et al., 2023) are important in sustaining necessary materials (e.g., food, energy, and water) for cities to function (UN Habitat, 2022).Value chains are further 'supply chain' improvements, including business activities involving production and providing services.In line with the circular economy, the main focus lies in establishing a reverse supply chain and loop creation, establishing a network between cities, industry, and waste management (Montag, 2023).The 'management', 'assessment', 'indicator', and 'information' concepts indicate research on establishing statistically backed methods for managing value chains (Gao et al., 2021b;Wang et al., 2020).Approach by statistical models enables validated resource flows, increases communications down the supply chains and optimises circular methods within the city perimeters (Montag, 2023).
Another aspect we have focused on in the content analysis was the emergence of specific research topics.Emerging topics represented potential barriers that needed further addressing to implement the circular economy in cities.Thus, the topics interesting for our research appeared most recently in our results.By closer inspection, we identified four topics: urban mining, circular indicators, adaptive reuse, and regeneration.Urban mining relates to increasing the city's self-reliance by utilising existing resources already stored within them (Ghisellini et al., 2022).The emerging topic suggests that cities are looking for additional ways to optimise their resource consumption, increase flow values and improve beneficial social and environmental improvements (Kakkos et al., 2020).would enable the establishment of validated statistical data, which could help cities in finding areas in need of development to achieve circularity (Henrysson et al., 2022).Adaptive reuse is the 'youngest' emerging topic due to the evolution from a previously pragmatic approach to a conceptual one (Lanz & Pendlebury, 2022).The theory behind adaptive reuse is the repurposing of existing structures used within cities (Foster & Saleh, 2021;Gravagnuolo et al., 2021).Adaptive reuse indicates that cities are considering employing higher R-strategies (Potting et al., 2017).Instead of demolishing and recycling infrastructures, cities are opting to prolong their usage and increase their economic value.The last emerging topic was regeneration, which hinted towards restoring natural systems within and on the outskirts of the city's perimeter (Awad & Jung, 2022;Balletto et al., 2022).The built environment, specifically infrastructure, significantly degrades the land, water, air, and environmental health (Domenech & Borrion, 2022).We can conclude from the indices for regeneration that cities are assisting with the degraded land recovery.By employing strategies that promote regeneration, cities improve environmental health, improve natural resources production (e.g., food and water resources) and maintain high health conditions for society living within the city's perimeter (Domenech & Borrion, 2022;Sala Benites et al., 2022).
An integral aspect of our research is identifying prevailing trends within the circular economy and circular cities, including emergent topics and potential research gaps.Evaluating the present developments in these research areas assumes paramount significance in shaping the course of future investigations (Foroudi et al., 2020).
Leveraging the results obtained using Leximancer and VOSviewer software, we have constructed a comprehensive network that illustrates the interconnectedness and similarity of concepts.By grouping these concept networks with exploring emerging topics, we have discerned five prevailing research trends that interweave aspects from both the circular economy and circular cities areas.
Urban metabolism, in conjunction with urban mining, enables an in-depth examination of material flow dynamics within cities and the potential extraction of valuable resources from existing urban stocks.
Embracing urban metabolism is critical for realising circular and ultimately sustainable cities, as it facilitates the systematic inflow and outflow management for various resources, including materials, energy, water, and human populations, within urban environments and their peripheries (Ulgiati & Zucaro, 2019).Given the increased urbanisation, population influx from rural regions, and heightened material consumption trends (Kennedy et al., 2008;Zhang et al., 2018), gaining insights into future urban development is crucial.A noteworthy emergent subject within urban metabolism is the concept of urban mining, which emphasises augmenting self-sufficiency levels in urban areas.Essentially, cities embody a repository of valuable resources, either currently stored or transformed into waste for prospective reclamation (Esmaeilian et al., 2018;Koop et al., 2017).Studies exploring urban mining potential reveal that implementing circular economy principles within cities can support material circulation within these urban areas (Xavier et al., 2023).Through the adoption of such strategies, urban areas can enhance energy utilisation efficiency (Koop et al., 2017), reduce material losses (Esmaeilian et al., 2018), and contribute to employment generation, social upliftment, and environmental restoration (Izdebska & Knieling, 2020;Ulgiati & Zucaro, 2019), all of which fundamentally align with the primary objectives of fostering sustainable cities (Froes & Lasthein, 2020).
City governance and framework are crucial in driving circular development and facilitating the transformation towards sustainability.Effective governance is essential for successfully integrating changes in city development, incorporating new policies, considering diverse agendas, and implementing appropriate frameworks (Hansson et al., 2017).Frameworks, in turn, offer guidance to governmental entities on the systematic introduction and monitoring of potential changes, with the UN SDGs serving as target goals to strive for (Winter, 2018).Ongoing research reveals that governance mechanisms foster circularity and sustainable development (Van Zeijl-Rozema et al., 2008) while identifying and addressing risks encountered during implementation phases (Ullah et al., 2021).Moreover, recent investigations illustrated that governance's influence on urban development and the circular principles incorporation, can help in pursuing sustainable city goals (Papageorgiou et al., 2021;Rasoolimanesh et al., 2019).
The built environment is important for urban settings, as it involves optimising land development to enhance residents' social well-being and health, while reducing energy, water, and resource consumption.This aspect is complexly linked with urban metabolism and sustainable development.It is evident that modifications to infrastructure and supporting activities can significantly improve the current state and foster a more circular approach (Krausmann et al., 2017;Ríos et al., 2022).Recent research revealed that adopting circular practices in urban services, consumer practices, cleaner production, and construction can positively achieve circularity (Jaeger-Erben et al., 2021;Joensuu et al., 2020;Mhatre et al., 2021).
A primary focus of current efforts is enhancing the construction sector to align with circular economy principles.By promoting longevity design, recycling, reuse, and maintenance activities, the construction sector can contribute to improved social welfare, reduced resource consumption, and environmental restoration (Joensuu et al., 2020;Ossio et al., 2023).Such initiatives hold significant promise in driving sustainable urban development and fostering city circularity.
Establishing management and value chains underscores the imperative of effectively managing supply chains and loops, encompassing the stages of manufacturing, usage, and recovery, all within the confines of city perimeters.Sustainable development can be achieved by integrating circular principles into management practices, thereby fostering the transition towards circular cities (Joensuu et al., 2020;Pan et al., 2015).Central to this endeavour are value chains and supply chain management, critical components that optimise the material flows within urban environments and provide the necessary tools for an urban metabolism approach (Dwivedi et al., 2021;Esmaeilian et al., 2018).Research efforts are currently directed at enhancing collaboration throughout supply chains to augment the circularity and value of materials (Dwivedi et al., 2021;Mhatre et al., 2021).This collaborative drive promotes flexibility and cooperation among various stakeholders within supply chains and cities (Farooque et al., 2019;Kazancoglu et al., 2020).Furthermore, incorporating smart technologies has emerged as an essential support mechanism in this pursuit (Chauhan et al., 2022).
The last identified research trend centres around sustainable  et al., 2022;Khajuria et al., 2022).The highest priority in cities is thus integrating urban society to participate in circular and sustainable activities and to empower social welfare.Therefore, accelerating efforts towards circular economy practices and transforming cities into circular entities emerges as a promising approach to simultaneously achieve the outlined sustainable goals and enhance the overall well-being of the global society.

| CONCLUSIONS
The papers analysis revealed an increased publication and citation The remaining research papers underwent a comprehensive content analysis based on two key factors in the second step.The first factor involved the presence of both main keywords, 'circular economy' and 'circular city/cities', in a meaningful contextual manner within the content.The second factor focused on including other significant keywords related to the main concepts (e.g., urban concepts, waste F I G U R E 1 The conceptualization of our research setup. Web of Science database classifies all journals into approximately 250 subject categories based on their research fields, giving researchers insight into research focus areas or themes and exposing chronological timeline and theme evolution.A journal can be included in multiple subject categories, which in turn can present specific results as some publications are counted several times, belonging to multiple subject categories.Our results identified 79 subject categories within the circular cities and circular economy of 671 selected publications.The records were associated with the six most significant subject categories, amounting to n = 1222 (total n = 1585), representing 77.10%.The top 10 subject categories are illustrated in Table 1.As perceived, subject categories of circular cities and circular economy mostly belong to the environmental related research, comprehending journals, such as Environmental Sciences (n = 358), Green Sustainable Science Technology (n = 274), Environmental Studies (n = 174), Engineering Environmental (n = 155), Energy Fuels (n = 63), and Urban Studies (n = 49).

F
I G U R E 2 Publication and citation growth in the research fields of circular cities and circular economy.T A B L E 1 Ten most frequent WoS subject categories for circular economy and circular cities.

'
community' indicate the societal related concepts in the circular economy.Concepts 'policy', 'indicators', and 'practices' imply recent research activities, needs and measurements regarding the circular city and economy.Waste is the second most dominant theme, closely following the circular economy.Within the waste theme, we have identified five distinct clusters, the most noticeable including 'materials', 'manufacture', and 'technologies', indicating the inclusion of new technologies (e.g., 4.0 and 5.0 industry) in improving the manufacturing of the product.'Municipal', 'solid waste', 'recycling', 'treatment', and 'wastewater' form the second cluster, hinting towards reprocessing waste resources and their reintroduction into a loop in line with circular economy principles.'Management', 'assessment', and 'information' concepts indicate a need to establish management systems to control material flows and establishing a flow map for future optimisation.The cluster consisting of concepts of 'construction', 'consumption', 'quality', and 'efficiency' indicates the push towards improving circularity attributes of infrastructure.The fifth cluster comprises concepts of 'emissions', 'water', 'energy', 'flows', and 'air', exposing development towards a nexus approach and slow transition towards urban metabolism.The transition towards urban metabolism will improve the understanding of the interaction between society and the environment, providing vital information on processes comprising the urban areas.The third identified theme was the building theme, in which we have identified two significant clusters.The cluster comprising F I G U R E 3 The conceptual map of major themes, as obtained with the Leximancer software.concepts of 'green', 'infrastructure', 'land', 'climate', and 'project' indicates a project-oriented approach to planning and transforming urban areas by including improvements towards climate change.The cluster of 'building', 'energy', 'flow', 'reuse', 'resource', and 'efficiency' concepts suggests incorporating new energy-saving technologies and building concepts in the infrastructure segment, intending to lower resource consumption.The last identified theme was the value chain theme, where we identified two distinctive clusters.The cluster comprising concepts of 'market', 'network', 'performance', 'supply chain', and 'industry sector' indicates a push towards establishing supply chain networks within the industry market, increasing interdependency and connectivity between organisations and information flows.Concepts 'management', 'assessment', and 'information' form the second cluster, covering a managerial approach towards assessing information flows within value chains with assessment methods, with the goal of obtaining validated statistical databases.The overlap of themes provides additional information about research themes.The overlap between theme building and waste includes 'energy', 'flows', 'construction', and 'materials' concepts, suggesting a focus towards managing infrastructure as a resource stock.The notion of infrastructure or cities being stocks of resources is further evident from the overlap between the circular economy, building, and waste theme.The need to manage existing resources within cities is visible from the overlap of waste and value chain themes, especially concepts of 'management', 'assessment' and 'information', indicating that new methods, management approaches, and digitalisation are needed.Improving management approaches in line with the circular economy model F I G U R E 4 A network-based visual representation of the interconnectedness of circular city concepts.T A B L E 2 Circular city concept's likelihood.city's transformation towards a circular one, optimise resource flows and reduce the city's environmental footprint.
3.3.2| VOSviewer analysisFollowing the thematic distribution created in Leximancer software, we have conducted the co-occurrence of keywords within the VOSviewer software.In VOSviewer, we have identified a total of n = 3049 keywords.These were aligned with constraints and connectivity, compiled into a network of n = 139 keywords (Figure5), forming seven thematic clusters and their features.The most frequent thematic clusters are circular economy, sustainable development, and management, followed by construction, economy, industrial symbiosis, and framework.The circular economy is at the centre of a network with six other clusters.We can perceive that the most important keywords in the circular economy cluster are circular cities, barriers, and resource management, which focus on finding solutions for implementing the circular approach in cities.The sustainability cluster, which includes city, waste, urban, governance, and strategies, indicates that circular cities actively implement sustainable development principles.The system, energy, waste, and LCA are the most prominent keywords constituting the management cluster, identifying a need for establishing F I G U R E 5 The visualisation of keyword co-occurrences, as obtained with the VOSviewer software.F I G U R E 6 A time-related network of keyword co-occurrences, as obtained with the VOSviewer software.F I G U R E 7 Research trends obtained from Leximancer and VOSviewer content analysis.systematic management and measurement of waste flows within cities.The construction cluster comprises keywords built environment, urban metabolism, urban mining, material flows and recycling, indicating that the infrastructure is important in establishing the city's circular economy principles.Clusters economy, industrial symbiosis, and framework comprise keywords such as performance, business, efficiency, transition, material flows, and supply chains, indicating a need to optimise current economic models to be more circular and increase their interactions with cities.We have carried out further research to understand the development of circular city and circular economy over time (Figure 6).This was achieved by utilising the VOSviewer function of chronologically analysing and organising keywords.We have considered the average publication year and calculated by mean subtraction and division by the standard deviation to relay the time-scale distribution.Clusters with cold colours (dark blue) represented older keywords, while warmer colours (yellow) represented newer words.Based on cluster distribution, we can perceive that earlier research related to industrial symbiosis, consumption and emissions.The research continued transitioning towards establishing systems networks, energy, waste management, and LCA studies, followed by management, circular city, barriers, framework and governancerelated topics.We can perceive the construction cluster as an emerging research area, including urban mining, material stocks, recycling, and concrete.Other emerging circular city-related topics are circularity indicator, urban agriculture, water, regeneration, supply chain management, adaptive reuse, and industry 4.0.Considering the results by both Leximancer and VOSviewer, we discerned current research trends related to the circular city, presented in Figure 7.As can be perceived from Figure 7, we cross analysed results gained from Leximancer and VOSviewer.Based on the keyword similarities, we established connections between the established keyword groups (concepts, clusters) and time-related keyword distribution to form five distinct research trends related to the circular city.The identified research trends related to circular cities comprise urban metabolism and urban mining, governance and framework, built environment, sustainable development and management and value chains.4| DISCUSSION Our research has indicated several exciting information and features of the circular city and circular economy research from content perspectives.Results revealed an increased number of publications.Consequently, citations regarding the circular economy and circular city since 2015, after the European Commission published and represented an ambitious Circular Economy Package to promote Europe's transition towards a circular economy that will strengthen global competitiveness, boost sustainable economic growth, and create jobs Focusing on circular indicators implies insufficiently developed monitoring frameworks (Girard & Nocca, 2019; Rinc on-Moreno et al., 2021).With proper frameworks, cities could actively monitor their progress and circular development, slowing down their transformation.Developing circularity indicators growth related to the circular economy and circular cities, indicating a significant interest from the scientific community in the two research topics.The WoS categories indicated a research focus on environmental sciences and green technology research, indicating the importance of sustainability environmental dimension, with economy and social dimensions being less represented.The content analysis revealed the most important concepts within the circular city and circular economy research, which included waste, buildings, construction, management, sustainability, framework, value chains and industrial symbiosis.By cross-analysis of results from the Leximancer and VOSviewer, five research trends were identified and established as the primary focus of current research.The prevailing research trends expose a notable concentration on specific development areas, with a central emphasis on transforming contemporary cities into circular cities and pursuing sustainable objectives outlined by the United Nations.Notably, these identified research trends intertwine and synergize, adopting a systemic perspective to approach cities as intricate entities.Urban metabolism and urban mining clustering, along with effective management and value chains, facilitate a comprehensive analysis of urban areas' material and energy flows while capitalising on reverse flows to enhance value creation and foster self-sufficiency in cities. Governance and framework aspects further underscore the importance of establishing improved circular models for cities, considering the artificial nature of the built environment.This scientific approach is expected to provide valuable insights and strategies for cities to transition towards circularity and advance their sustainability objectives effectively.The integration of circular economy principles and circular cities' concepts demonstrates a concerted effort to address not only Sustainable Development Goal 11, which refers to Sustainable Cities and Communities, but also several other crucial goals such as SDGs 1 (No Poverty), 3 (Good Health and Well-being), 4 (Quality Education), and 5 (Gender Equality).Emphasis is placed on enhancing societal well-being in urban environments by promoting economic prosperity, health improvements, and living standards.This convergence of circularity and sustainable development objectives bears substantial potential for fostering a more resilient and equitable future for humanity.It combines shaping the cities physical infrastructure and transforming societal behaviours and lifestyles towards greater circularity and sustainability.Identifying emerging topics such as circular indicators, regeneration, urban mining, and adaptive reuse highlights the need for further investigation.The research focus should promote circular resource flows within urban areas while comprehensively measuring and defining these flows.Additionally, emphasis should be placed on reimagining strategies related to repurposing the built environment, revitalising degraded land, and enhancing the living conditions of city inhabitants.As evident from these topics, the research is gradually advancing towards higher development plans, aligning with the principles of sustainable development and the circular economy.Notably, there is a noticeable shift in the direction of transforming cities, with a greater emphasis on societal integration and city shaping, moving beyond the traditional focus on economic and environmental aspects, such as infrastructure and climate change.Identifying research trends and emerging topics in the circular city and circular economy field offers valuable insights into these concepts' current stage and evolutionary trajectory from a content perspective.This comprehensive view highlights the complexity and interconnectedness of urban activities, various subsystems, and the underlying city processes.Moreover, it underscores the significance of developing appropriate policy frameworks and practical implementation strategies to effectively address the complexities and challenges related to advancing circularity in cities. Certain limitations have come to the fore during the research, which subsequently identifies promising trends for further inquiry within the studied research areas.The foremost limitation pertains to the exclusive utilisation of the Web of Science database.While this database was deliberately selected, initial perusal of SCOPUS and Google Scholar unveiled papers absent in Web of Science, albeit not all possessing science citation indices.The second constraint involves categorising eligible literature, encompassing only research articles and review papers, excluding conference papers, proceedings, monographs, and so forth.The third constraint revolves around the contextual confines of 'circular economy' and 'circular cities.'Although our investigation has delineated compelling research trends and gaps, particularly concerning SDGs, its extrapolation to broader sustainable development or urban sustainability realms needs to be revised.Forthcoming analyses assessing diverse SDGs, urban sustainability models (e.g., smart, green, and resilient cities), and thematic explorations like urban metabolism, material flows, and value chains hold the potential for uncovering underlying patterns and improvements in the delineated research areas.

Table 2 ,
27% of the texts comprehending the concept 'circular city' also include 'urban'.This result is expected as cir- are all complementary and important for accelerating cities towards circular ones, especially as cities are more local oriented.In addition, other concepts, such as 'strategies', 'national', 'challenges', and 'circular economy', as well as 'land' and 'social' require attention, as all these concepts are complementary and need to be considered when planning or researching circular cities.