Research progress on the transportation of poly- and perfluoroalkyl substances (PFASs) in freshwater based on CiteSpace

Poly- and perfluoroalkyl substances (PFASs) are a family of emerging organic contaminants that exist widely in aquatic environments. PFASs are hydrophobic, oleophobic, and chemically and thermally stable owing to the unique properties of C–F bonds (Li et al. 2023). Since the 1940s, PFASs and their precursors have been widely produced and utilized in various industrial applications and commercial products such as aqueous film-forming foam (AFFF), personal care products, textiles, and materials resistant to greases, dirt, and water (Dasu et al. 2022). Freshwater includes rivers and their branches, creeks, freshwater lakes, reservoirs, ponds, etc. These water bodies play a decisive role in the survival and reproduction of human beings and other organisms as well as economic and social development (Dudgeon et al. 2006; Krivtsov et al. 2022). Due to their dissolubility and stability, the mobilization of PFASs from various origins into freshwater ecosystems occurs frequently, facilitating their accumulation within these aquatic environments.

PFASs have been reported in sediments, water, and biota at ng/g, ng/L, and μg/L levels, respectively (Ng et al. 2022; Suski et al. 2023). Tang et al. analyzed 35 types of legacy and emerging PFASs in the Poyang Lake revealing concentrations ranging from 23 to 1,000 ng/L in water and 0.26–2.9 ng/g dry weight in sediments, with short-chain and emerging PFASs such as perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) (Tang et al. 2022). Industrial facilities are major point sources of the emission of PFASs to freshwater. Xu et al. evaluated PFAS environmental risks in a constructed wetland fed by industrial tailwater, and their findings suggested that with the accumulation and continued importation of PFASs, the risks remained notably substantial (Xu et al. 2022). Humans are widely exposed to PFASs through the consumption of aquatic food and drinking water, and researchers have reported thyroid, liver, and kidney diseases as well as metabolic dysfunction are related to PFAS exposure (Zeng et al. 2019).

A comprehensive understanding of PFAS transport and migration in freshwater systems is essential for assessing ecotoxicological risk and developing remediation strategies, ensuring the long-term viability of water resources, and contributing to the development of water management practices that prioritize sustainability, energy efficiency, and environmental responsibility (Raihan et al. 2023). While previous research has investigated transport behavior and reaction models for specific PFASs such as PFOA and PFOS (Jahn et al. 2023), it is essential to recognize that diverse types of PFASs exhibit distinct physicochemical properties (Wanzek et al. 2023), and the transport behaviors vary across different media environments. A comprehensive summary of research progress on the transportation of PFASs in freshwater environments serves to enhance our understanding of current research frontiers and hotspots that are helpful to guide future research.

Scientometrics, established in 1969, employs quantitative approaches to research developments within scientific fields. Bibliometric analysis, as a comprehensive investigative approach, integrates scientific and statistical methods to conduct quantitative research on given information. Moreover, it enables the identification of the impact and progression of scientific research, advancements in specific fields, and prevailing growth trends (Moodley et al. 2015). CiteSpace software was introduced in 2003 to conduct data visualization analyses and exhibits flexibility by facilitating co-occurrence and co-citation analyses, along with the capability for keywords’ clustering analysis (Chen 2004). In this study, CiteSpace software version 6.2.R6 was used to analyze research publications collected from the Web of Science (WoS) core database in the field of transportation of PFASs in freshwater.

The WoS Core Collection was selected as the data source. WoS is the most comprehensive academic information resource covering a wide range of disciplines, including the most influential core academic journals in each research field. According to Bradford's Law, a few core journals reveal adequate information to reflect achievements and progress in science (Brookes 1969). The searching keywords of WoS were set as follows: TS = (PFAS* OR *fluoroalkyl substances OR perfluorinated compounds) AND TS = (transport OR migration) AND TS = (aquatic OR freshwater OR wastewater OR inlandwater OR river* OR lake OR creek OR sewage OR estuary OR pond* OR reservoir OR stream OR runoff). The data range included all years and as of 2023.11.02, 433 publications had been searched out while meeting abstract, correction, book chapters and proceeding papers were excluded (Chen et al. 2023; Wang et al. 2023). After filtering out duplicate and unrelated publications, 306 papers were obtained. Combining the publications, 43 papers focused on the risks of PFASs in fresh water to humans and freshwater organisms. A total of 40 papers were based on the overall environment, including the aquatic environment, water bodies or the environment.

The trends of publications and the sum of total citations per year (SOTC) are shown in Figure 1. The studies related to the transportation of PFASs in freshwater started late with the first publication in 2005. This study delineated perfluorinated chemical (PFC) concentrations in surface water, contending that the primary source is non-atmospheric in Lake Michigan, and the most probable source was identified as discharges from wastewater treatment (Simcik & Dorweiler 2005). The number of works grew at a relatively low rate during 2005–2014. Researchers paid attention to occurrences and distribution of PFASs in aquatic environments or adsorption behaviors and mechanisms (Du et al. 2014) of PFASs in various absorbents in this stage. The regulatory framework for assessing PFASs in water was established by the United States Environmental Protection Agency (U.S. EPA). Specifically, the analytical methodology designated as Method 537.1, as promulgated by the U.S. EPA, was implemented for the comprehensive detection of 26 distinct PFAS compounds in water samples (Sharma et al. 2016). However, knowledge gaps remained in the transport and removal mechanisms of PFASs.

The publication rate of literature accelerated during 2015–2023. Differing from the embryonic stage, in this period, researchers have focused on the transformation and toxicity of PFASs in water environments. More detection standards for PFASs in water environments have been introduced successively, and these standards have broadened the detecting ranges of types of PFASs and samples (Lipps et al. 2021). As the study developed in-depth, remediation technologies of PFASs (Kucharzyk et al. 2017), as well as the discussion on risks to human health and aquatic ecosystems (Sims et al. 2022), were increasingly becoming prominent.

The national collaboration, institutional and co-authorship networks were employed to assess the influential academic groups and collaborative dynamics related to the transportation of PFASs in freshwater. This analysis aimed to elucidate the interconnected relationships among research groups in the specified field. The cooperation network of the country/region, global institutes and authors are shown in Figure 2, and the details of the network analysis are listed in Tables 1–3.

Our investigation revealed that 42 distinct countries or regions undertook research endeavors concerning the transportation of PFASs in freshwater environments (Figure 2(a) and Table 1). Notably, China (120), the United States (97), Canada (39), Germany (20), and Sweden (19) emerged as the predominant contributors, constituting the top quintile of nations in this domain. The number of publications associated with transportation of PFASs in freshwater demonstrates a substantial asymmetry, with China and the United States displaying significantly elevated levels compared with their counterparts. This pattern indicated the heightened interest and engagement of Chinese and American researchers, underscoring an escalating awareness of PFAS-related challenges within both nations. The United States occupied a preeminent central position within this research domain, possessing a betweenness centrality value of 0.58. Subsequent in rank are China (0.35), Netherlands (0.27), Sweden (0.18), and England (0.15). These metrics signify the pivotal roles played by these countries or regions in facilitating international collaborative initiatives within the research milieu of transportation of PFASs in freshwater.

In total, 205 institutions researched in the field of transportation of PFASs in freshwater (Figure 2(b) and Table 2). Our analysis revealed the pivotal role of Chinese research institutions in the study of transportation of PFASs in freshwater. Specifically, the Chinese Academy of Sciences (Chinese Acad Sci) emerged as a key contributor, demonstrating significance through both a higher publication count (54) and a substantial betweenness centrality of 0.36. Despite a relatively recent initiation of research activities in transportation of PFASs in freshwater by the Chinese Academy of Sciences in 2010, its rapid development propelled it to a leading position in the field. Moreover, the collaborative landscape among Chinese institutions appeared robust. Notably, the Academy of Sciences engages in collaborative research with institutions such as the University of Chinese Academy of Sciences, the Research Center for Eco-environmental Sciences, Hohai University, and the Guangzhou Institute of Geochemistry. These collaborative efforts underscored the institution's active involvement and leadership in fostering cooperative research initiatives within the broader scientific community. Moreover, as the largest research institution in Germany, the Helmholtz Association held a high betweenness centrality of 0.11, and it mainly contributed to the field of transportation of PFASs in freshwater between 2010 and 2015. Environment & Climate Change Canada and US EPA though had a relatively small number of publications, holding high betweenness centrality of 0.09 and 0.08, respectively.

The core of the author cooperation analysis network is represented in Figure 2(c) and Table 3. Researchers had close cooperation in this field of transportation of PFASs in freshwater. The corporation network was divided into two parts. The right part is larger, and it included Wang and Wang. The team group was led by Wang and they had close collaborations with each other in the field of sources, multimedia distributions, and health risk assessments of PFASs in China. The left team group was led by Ahrens, Lutz and included Lam and Ebinghaus. Their studies focused on the PFASs’ fate, effects, and removal technologies in water environments. Giesy and Zhao had collaborations with both team groups.

Although the authors from China started late in the field, they showed great ability to publish. Among all authors, Wang published most papers, at 13. After reviewing all his papers, we found he had close collaboration with Lu, Yonglong whose research team unveiled the occurrence and transportation of perfluoroalkyl acids (PFAA) in the coastal rivers of South Bohai, where fluoropolymer facilities were concentrated (Wang et al. 2014). The research career of Ahrens, Lutz in the topic related to transportation of PFASs in freshwater was the longest among nine authors. He started his research in the field with studying the distribution of PFCs in fish livers from alpine lakes (Ahrens et al. 2010). In recent years, his team has focused on the transportation in water and the pollution to drinking water by sampling in different media (Nguyen et al. 2022).

Using co-citation clustering analysis, we identify whose publications were highly cited and valued related to the field of transportation of PFASs in freshwater (Figure 3). A larger number of clusters suggest a more diverse and fragmented landscape of research within the topic. A fewer clusters imply a more focused and concentrated area of research. Lindstrom and Houtz were in the strongest cluster ‘#0 groundwater’, and Higgins was in the cluster ‘#1 perfluorooctane sulfonate’. The number of representative scholars in cluster ‘#2 pfoa’ was the most with Wang, Buck, Xiao, Giesy, and Taniyasu. Meanwhile, Prevedouros and Houde were in ‘#2 perfluorooctane sulfonic acid’, and Ahrens was included in ‘#8 pfass’. The rest of the clusters received varying degrees of attention, none of them had representative researchers.

The keywords’ bursting analysis is shown in Figure 4. The keyword ‘surfactants’ demonstrated more considerable prominence during the extended timeframe spanning from 2008 to 2017. Due to their high surface activity, thermal stability, and chemical stability, PFASs were used as surfactants and widely applied in producing non-stick pan coating, water-resistant clothing, and greaseproof food packaging, etc. (Kucharzyk et al. 2017). Owing to the robust C–F bonds they possess and the presence of hydrophilic functional groups, PFASs exhibit a propensity to readily dissolve and persist for a long time in aquatic environments. Thus, PFASs were widely identified across diverse environmental matrices of freshwater. Zhou et al. detected PFASs in water, sediment, and biological samples collected near a fluorochemical industrial base (Zhou et al. 2013). Concerning the persistent presence of PFASs in freshwater arising from surfactant production over an extended period, the European Chemicals Agency (ECHA) has put forth a proposal for the prohibition of PFASs (Wollin et al. 2023).

In the initial period, researchers mainly focused on the wide presence and transport behaviors of PFASs and their precursors such as fluorotelomer alcohols in the environment (Wang et al. 2015). The occurrence of PFASs in human bodies was mainly determined by measuring the concentration in human blood. The burstiness of ‘human blood’ in 2006–2010 indicated that researchers realized bioconcentration of PFASs in the human body, while the specific mechanisms of bioconcentration and toxicity of PFASs were not clear at the time (Fujii et al. 2007). PFASs contain a hydrophilic functional head commonly carboxylic or sulfonic acid. Hence, PFASs commonly include two types: perfluorocarboxylic acids (PFCA) and perfluorosulfonic acids (PFSA). The keyword ‘atmospheric chemistry’ demonstrated strong strength of 4 between 2008 and 2013. Cai et al. investigated spatial distribution and the composition profiles of PFASs in Antarctica and concluded that atmospheric deposition was the main pollution source in lakes (Cai et al. 2012).

The keyword ‘water’ showed a strong strength of 4.76 between 2012 and 2017. Adsorption is the major transport process of contaminants in freshwater environments. Adsorption is not only efficient for the removal of PFASs from water but also influences the distribution of PFASs at interfaces and their fate in aquatic environments (Du et al. 2014). Adsorption mechanisms contain electrostatic interaction, hydrophobic interaction, ligand and ion exchange, and hydrogen bond, while electrostatic interaction and hydrophobic interaction have the greatest influence on the transport process. Due to hydrophobic C–F bonds, negatively charged PFASs can adsorb onto hydrophobic media surfaces (Deng et al. 2012), indicating that hydrophobic interaction could impede the transportation of PFASs, particularly for long-chain PFASs with more C–F bonds. Due to hydrophilic functional head of PFASs, anionic PFASs are the majority in the adsorption process. Electrostatic attraction, electrostatic repulsion, and divalent metal cation bridging effect can affect adsorptions between anionic PFASs and charged media surfaces. PFASs with longer chains are less affected by electrostatic interaction, owing to their longer molecular stretching distance (Li et al. 2023).

With the basic adsorption mechanism of PFASs being clearer, researchers began to focus on topics in ‘surface waters’ and ‘waste water’. As for ‘waste water’, it is mainly associated with WWTPs as a point source of PFASs and its removal technologies (Banks et al. 2020). With the development of mathematical simulation, the experimental methods on physicochemical properties of PFASs are essential to reduce uncertainties of transport, transform, and fate simulation (Liu et al. 2019). Owing to the extensive data accumulated through long-term PFASs monitoring, researchers have been able to investigate the temporal trends of PFASs in freshwater transportation (Shi et al. 2021). In recent years, research related to ‘risk assessment’, ‘perfluorinated alkyl acids’, ‘great lakes’, and ‘river basin’ have been bursting. Aquatic biota are widely exposed to PFASs in freshwater, and human consumption of aquatic food leads to the bioaccumulation of PFASs in the body. Thus, exposure risks of PFASs to environments and human health need to be urgently assessed (Kurwadkar et al. 2022). The Great Lakes is of great importance as this large freshwater system provides drinking water and aquatic food to millions of people. Researchers highlighted PFASs source apportionment (Lin et al. 2021), accumulation, and toxicity (Bartlett et al. 2021) to aquatic organisms in the Great Lakes. The river basin contains almost all types of freshwater bodies. Nevertheless, most studies on PFASs in environments focused on a single type of environment medium or a combination of two. There is a scarcity of research that explores PFASs across various environment media simultaneously (Liu et al. 2022).

The timeline analysis of keywords in different clusters is shown in Figure 5. The cluster ‘#2 bioconcentration’ was highlighted at the beginning. Researchers detected the bioconcentration of PFASs in various aquatic organisms (Conder et al. 2008). Later in 2006–2008, ‘#0 atmospheric chemistry’, ‘#1 fate’, ‘#5 pfas’, and ‘#10 pfaas’ were emphasized. In this period, owing to knowledge gaps in PFASs’ fate in freshwater, researchers focused on the environmental factors affecting PFASs’ fate rather than fate itself (Johnson et al. 2007). Due to their biotoxicity, environmental persistence, and bioaccumulation (Li et al. 2023), it is essential to reduce the use of PFASs and evaluate their transportation and transformation in freshwater environment matrices (Podder et al. 2021). Additionally, as a prevalent type of PFASs, PFAA gradually attracted considerable attention. ‘#0 atmospheric chemistry’ was related to the global distribution of PFASs. Pfotenhauer et al. revealed that wet deposition was an important supplement with PFASs in freshwater (Pfotenhauer et al. 2022). The cluster ‘#7 alpine lake’ was a hotspot along with ‘#1 fate’ in 2010. The occurrence of PFASs in remote areas such as alpine lakes indicated the importance of atmosphere transportation (Taniyasu et al. 2013).

Noteworthily, ‘#2 bioconcentration’ has been highlighted again since 2012. Differing from the previous period, researchers made significant contributions to deepening our understanding of the toxicity of PFASs in organisms (Zhang & Lerner 2012). Meanwhile, ‘#3 air-water interface’ and ‘#4 spatial distribution’ were hotspots at this period. The transfer processes between phases comprise a key factor influencing the transportation and fate of PFASs. Meanwhile, considering the molecular characteristics of PFASs, it is compulsory to also consider adsorption at the air-water interface as a potential retention mechanism influencing spatial distribution in environmental systems (Brusseau 2019). The cluster ‘#6 fluorinated hydrocarbons’ was emphasized in 2017. As a precursor of PFASs, fluorinated hydrocarbons (HFC) can form PFASs through atmospheric degradation, and PFASs were detected in the Arctic ice cap (Hartz et al. 2023).

It's noteworthy that ‘#8 numerical modeling’ appeared in 2009 and 2014. Numerical models were widely used to simulate transportation of PFASs in freshwater since relatively higher costs in terms of both labor and time (Wang et al. 2016). These numerical models were largely process-based, utilizing established theories for their formulation. Recently, integrated models have been set up to simulate PFASs’ fate and transportation in a large basin (Raschke et al. 2022). Integrated models are efficient for PFASs’ transport due to their accuracy and capacity to couple environment metrices together. Additionally, ‘#9 water treatment’ also appeared in 2009 and 2014, suggesting that wastewater and drinking water treatments are essential to remove PFASs from water. Currently, the main removal and degradation technologies include electrochemical degradation, enzymatic transformation, advanced oxidation, and photocatalytic degradation (Araujo et al. 2022).

This research has several limitations: (1) Single data source. This research only set the WoS core Collection as the database. Not all literature works in the field of transportation of PFASs in freshwater are included in WoS, meaning that we could not make a comprehensive analysis of all relevant literature. (2) Insufficient performance of CiteSpace. The CiteSpace software cannot induct synonyms, resulting in ‘PFAA’ and ‘perfluorinated alkyl acids’ occurring in keywords bursting analysis while they exhibit semantic equivalence. Meanwhile, CiteSpace could not determine orders of authors so that we could not distinguish their interrelationships. (3) Due to the scope of the study, specific transportation equations were not discussed in this research.

One significant challenge is the co-occurrence of PFASs with other contaminants, specifically microplastics (MPs). An in-depth exploration of how the presence of MPs may interact with PFASs is needed, elucidating the mechanisms governing their interaction and the potential combined toxicity (Dai et al. 2022). Moreover, hydrological factors, such as water pulse change, ecological water supplementation, and occurrences of intense rainfall and floods, have been recognized as influential determinants in the distribution and transportation of contaminants in freshwater ecosystems. An expanded research focus is warranted to comprehensively investigate the impact of hydrological factors on PFASs’ transport, distribution, and associated ecological implications.

Integrated large-scale modeling is a trend for exploring transportation of PFASs. This involves the incorporation of diverse factors, such as hydrological, environmental, and geographical parameters into a unified model, which contribute to a more holistic understanding of transportation of PFASs in freshwater environments (Raschke et al. 2022). Meanwhile, future investigations should delve into developing and implementing sustainable strategies for mitigating PFASs in freshwater, acknowledging the imperatives of energy efficiency and environmental responsibility (Xu et al. 2022).

This research conducted a bibliometric analysis related to the transportation of PFASs in freshwater. We analyzed publications growth trends and cooperations among countries/regions, institutions, and authors. It is noteworthy that the progress of research topics and trends in this field was emphasized in this study. The growth of publications went through two stages: the ‘embryonic’ stage (2005–2014) and the ‘developing’ stage (2015–2023). In terms of cooperations analysis, although Chinese scholars started research related to the topic late, China has played a leading role since then. According to keywords’ analysis of burstiness and timeline, we traced the development of this research field. Moreover, the current research exhibits a shift from the transportation of specific PFASs in single or dual media to a broader spectrum of the transportation of PFASs across multiple media. Current research has unveiled an expanded understanding of the mechanisms and properties governing the transportation of PFASs in freshwater, surpassing previous knowledge. Joint efforts are recommended in the future to delve into the transportation of PFASs in multimedia and facilitate technological innovation while persistently mitigating the sources of PFASs.

This work was supported by the Key Project of National Natural Science Foundation of China [42330705], the National Science Foundation for Young Scientists of China [52300190], the Fundamental Research Funds for the Central Universities [2022NTST08], the special fund of State Key Joint Laboratory of Environment Simulation and Pollution Control [23Z02ESPCW], and the Beijing Natural Science Foundation [8212028].

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.