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Review

Bibliometric Based Analysis of Hydrogels in the Field of Water Treatment

by
Yakun Yuan
1,
Shuhu Xiao
1,*,
Bingfei Yan
1,2,*,
Songtao Shen
3,
Runtao Dou
4 and
Xiaohe Xu
1
1
State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China
2
College of Water Sciences, Beijing Normal University, Beijing 100875, China
3
College of Environment and Resources, Southwest University of Science and Technology, Mianyang 621010, China
4
Weifang Ecological Environment Monitoring Center of Shandong Province, Weifang 261111, China
*
Authors to whom correspondence should be addressed.
Sustainability 2024, 16(18), 8194; https://doi.org/10.3390/su16188194
Submission received: 12 August 2024 / Revised: 5 September 2024 / Accepted: 9 September 2024 / Published: 20 September 2024
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Abstract

:
Hydrogels exhibit distinctive features. These properties make them suitable for applications across various fields, such as environment, energy, and medicine. In this paper, we conducted a comprehensive search on the CNKI and Web of Science databases spanning from 2000 to 2024. Using tools like CiteSpace and VosViewer, we visualized the evolution, composition, hot spots, and trends in hydrogels in the field of water treatment sustainability. The results show that from 2000 to 2024, there has been a gradual increase in the number of publications in this field. China leads in the total number of publications; although, it ranks fourth in average citation rate. Seven out of the top ten research institutions are based in China. Additionally, three journals, including the International Journal of Biological Macromolecules, Chemical Engineering Journal and Carbohydrate Polymers, stand out, with a relatively high number of publications. The identified research hotspots include a range of preparation methods, including the freezing method and cross-linking method. Additionally, other preparation methods and the examination of water retention rate and adsorption isotherm are part of the research focus. The primary emphasis is on studying the adsorption of heavy metals, microplastics, and organic pollutants in dye wastewater. The main adsorption mechanisms investigated are chelation, electrostatic attraction, and functional group interaction. These findings have potential applications in water purification, seawater desalination, and atmospheric condensation. For the authorized patents in hydrogel-related fields, with the continuous improvement in innovation ability and the continuous enhancement of intellectual property protection awareness, the number of authorized patents continues to rise. China has seven of the top ten institutions in the number of patents granted, and the total number of patents granted in China ranks first in the world. Future work should focus on methods for synthesizing new pollutants and other target pollutants and on improving adsorption efficiency. Additionally, the development of cost-effective hydrogel materials with improved treatment efficacy is essential to advance sustainable practices in water treatment.

1. Introduction

With the rapid development of the economy, heavy metal pollution of water bodies, as well as new and existing issues related to the water environment, such as heavy metal pollution and emerging pollutants, have become increasingly prominent. Water pollution control is not only the need to protect human health and maintain ecological balance but also a necessary work to promote economic development and green transformation and is of great significance to achieve sustainable development. These problems pose a serious threat to the ecological environment and human health. Consequently, the earth’s water resources are experiencing unprecedented impacts [1]. There are various methods for removing pollutants from wastewater. Traditional physical–chemical methods, such as ion exchange, filtration, membrane separation, and adsorption, along with chemical methods like electrochemistry and advanced oxidation, have proven effective. Biological methods also yield good results in pollutant removal. Ion exchange is a reversible chemical process that removes specific contaminants by exchanging ions in solution with charged ions on an exchanger [2], the type of exchanger, dose, contact time, initial ammonia concentration, presence of anions and cations, pH, exchange capacity, and concentration of regenerated solution all affect the ion exchange effect [3]. The ion exchange method has the advantages of small footprint, good stability, simple operation, easy maintenance, reusability, heavy metal removal, and low temperature resistance [4]; however, during the regeneration process, pollutants are transferred from the treated water to the waste sludge, increasing the treatment cost and environmental burden [5]. Membrane separation technology is a complex method of separating solvents from solutes using synthetic polymer or inorganic membranes. The technology uses differential driving forces across membranes to facilitate the transport and separation of specific components in water. Membrane separation is the third generation of water treatment technology, which has the advantages of simplicity, compact design, low energy consumption, minimal pretreatment, and low environmental sensitivity. However, the membrane separation method has some disadvantages, such as high cost and easy plugging of pores [6]. The electrochemical method has strong oxidation capacity, adaptability to harsh environment, simple equipment, no additives, good chemical stability, and long service life [7]. But it faces challenges such as high energy consumption and operating costs. In addition, toxic intermediate by-products may be produced during wastewater treatment [2]. Among these, adsorption stands out as a cost-effective approach in water treatment. Activated carbon is a commonly known adsorbent material for removing pollutants from water streams and drinking water sources, but its widespread use is hindered by high costs [8]. Hydrogels, as adsorbent materials, have attracted significant attention for their application in water environment treatment. Different treatment methods are suitable for targeting specific pollutants. The search for processes and materials to address diverse pollutants is of great significance for the future development of water purification [9].
The term “hydrogel” first appeared in the literature in 1894 [10] and has since found applications in several fields, such as water treatment and biomedicine [11]. Hydrogels are typically defined in two ways: Firstly, it is defined as a crosslinked polymer matrix, a water-soluble material with a three-dimensional structure. Secondly, it is defined as a hydrophilic, three-dimensional polymeric material with the ability to absorb large amounts of water and retain it in the matrix without dissolving [12]. Hydrogel represents a novel type of polymeric material with a three-dimensional reticulated porous structure, characterized by a large specific surface area, high porosity, and surface activity. These properties can be utilized for the treatment of heavy metals, microplastics, organic pollutants, etc., in wastewater [13]. The presence of hydrophilic groups, such as -NH2, -OH, -COOH, etc., in hydrogels enhances their water adsorption properties [14]. To further enhance adsorption capabilities, researchers have demonstrated that incorporating polymers such as chitosan, alginate, cellulose derivatives, etc., into hydrogels can improve their adsorption capacity. Currently, research on hydrogels is primarily confined to laboratory studies involving material preparation, characterization, and understanding pollutant removal mechanisms. No research team has yet conducted a comprehensive statistical analysis of the hydrogel field from a bibliometric perspective.
The preparation methods for hydrogels encompass ion crosslinking, emulsification, electrostatic complexation, and self-assembly [15,16]. The ionic crosslinking method employs sodium alginate in conjunction with bivalent metal ions to generate nanoparticles. An increase in the concentration of both the crosslinking agent and sodium alginate solution results in a larger nanoparticle diameter. When concentrations reach a specific threshold, these nanoparticles can undergo polymerization to yield hydrogels. The emulsification technique involves the dispersion of minuscule droplets within an immiscible liquid under surfactant influence, resulting in either oil-in-water or water-in-oil emulsions [17]. Electrostatic complexation refers to composite materials formed through intermolecular electrostatic interactions involving polycationic electrolytes, such as polyacrylamide and chitosan [18]. Self-assembly reactions entail the cross-linking of two polymers at molecular interfaces or within molecules themselves to create micelles. The hydrogels commonly applied in water/wastewater treatment were mainly classified into three classes, hydrogel beads, hydrogel films, and hydrogel nanocomposites, according to their shape and physicochemical properties [19]. Hydrogels can be classified into natural hydrogels and synthetic hydrogels based on the origin of their polymer networks. Natural hydrogels are derived from naturally occurring polysaccharides, such as starch, chitosan, and cellulose. In contrast, synthetic hydrogels are formulated using raw materials like polyethylene glycol, polyvinyl alcohol, and polymers such as poly (L-lactic acid) and poly (L-lactide) [20].
Bibliometric methods emerged as a field of research in the 1960s and have two primary applications: evaluative and relational. Evaluative bibliometrics aim to assess the impact of scientific work, typically by comparing the relative contributions of two or more individuals or groups. On the other hand, relational bibliometrics reveal relationships in research, such as the perceived structure of a research field, the emergence of new research frontiers, or patterns of national and international collaborative authorship [21]. The quantity of publications is on the rise, and bibliometrics allow for the measurement of numerous publications and citations, as well as the analysis, evaluation, and visualization of these trends [22]. The authors employed CiteSpace and VosViewer to visually represent the relevant literature, utilizing these tools to analyze the number of publications, research influence, and emerging keywords. Through this analysis, the authors explore the research history of hydrogels in the field of water treatment, identify research priorities, and forecast future research directions. The flow chart of bibliometrics is shown in the figure below (see Figure 1).

2. Data Sources and Research Methodology

2.1. Data Sources

The data were sourced from the Web of Science Core Collection (WoS) and CNKI databases. In the WoS database, a search was conducted using subject terms with the time frame set from 1 January 2000 to 1 July 2024. The search formula employed was TS = (“hydrogel” or “aquagel” or “aquogel” or “hydrogels”) and TS = (“water treatment” or “water purify” or “water cleans” or “water remediation” or “water recovery”). This search yielded a total of 1097 papers. The search was further refined by limiting the article type to Article or Review. After a subsequent screening process, 1082 documents were deemed effective. In this screening process, particular attention was given to keywords that appeared more than eight times. A total of 252 keywords were identified from the selected documents. In the CNKI database advanced search, the specified search terms were “hydrogel”, “water treatment”, “water purification”, and “water body”. The document type was specifically limited to academic journals. This search yielded a total of 226 documents.

2.2. Research Methodology

Bibliometrics involve the research and analysis of published scholarly literature or associated data, along with the statistics and description of the relationships between these published documents [23]. The VosViewer bibliometric software, utilized in this study, is well-suited for constructing and visualizing bibliometric maps. These maps can be generated to represent author or journal networks using co-citation data or keywords networks using co-occurrence data. This program allows the construction of comprehensive bibliometric maps in intricate detail. CiteSpace, a free tool, is capable of analyzing, detecting, and visualizing scientific literature. By performing arithmetic operations on research frontier terms, this tool facilitates a dynamic understanding of the essence of the knowledge base. The exact meaning of co-citation clustering can be clearly marked using the concept of research frontier professional terms, significantly simplifying the complexity of visualization [24]. CiteSpace provides a visual, accurate, and easy way to analyze the research history of a discipline and thus predict future research directions, while VOSviewer provides a better visualization [25]. Utilizing both methods in this paper enhances the scientific rigor of the obtained results.

3. Results and Analysis

3.1. Development History

The number of articles reflects the annual development trend, offering insights into the researchers’ attention to hydrogel in the field of water treatment [26]. From 2000 to 2022, this timeframe can be categorized into three stages based on the overall number of published articles (see Figure 2). The period from 2000 to 2012 represents a slow phase of development, characterized by a small overall number of published articles. The years 2013–2016 mark a stage of fluctuating development where the number of published papers increased compared to the previous stage but exhibited unstable growth. Since 2017, there has been a stable development stage. From 2000 to 2022, a total of 1308 articles were published on WoS and CNKI. The number of articles increased from 3 in 2000 to 109 in 2024 (until 1 July), representing a significant leap and indicating growing attention from researchers both domestically and internationally in this field. The average number of articles published on WoS from 2000 to 2024 is 43.28/year. Notably, the number of articles increased significantly after 2013, and the period from 2020 to 2022 alone accounted for 63.12% of the total articles, indicating a recent period of rapid development. In contrast, the number of articles published by CNKI is relatively small, averaging only 9.04 articles from 2000 to 2024. Before 2019, the yearly publication count was modest, but it started to rise in 2019, reaching 16 articles in 2024 (until 1 July). Comparing the number of articles published in the CNKI database and WoS, there appears to be a certain lag in the domestic research in this field. However, the overall upward trend in the number of articles reflects an increasing focus from researchers in this area.

3.2. Composition of the Research Force

3.2.1. Composition of Issuing Countries

Through the analysis of relevant data, articles related to hydrogel in the field of water treatment have been published by a total of 70 countries (see Table 1). China leads in the number of articles, comprising 42.14% of the total, and also has the highest total number of citations at 16,560. Among the top ten countries with the most articles, there is a balanced representation, including six developed and four developing countries. This phenomenon may be due to the fact that developed countries have a more complete scientific research system and higher investment in scientific research, which makes researchers have good scientific research conditions and economic support, and then put more energy into scientific research work. The country with the highest average number of citations is Canada, reaching 60.87 times, while China ranks third, with an average of 36.32 citations per article; this suggests that Canada holds a significant voice and influence in the field of hydrogel water treatment. Chinese researchers have extensively explored various types of composite hydrogels. For instance, Sun et al. [27]. prepared magnetic xylan/polyacrylic nanocomposite hydrogel adsorbents using straw xylan and Fe3O4 nanoparticles, investigating their methylene blue adsorption capacity. Another study by Zhang and collaborators extensively focused the preparation of poly (vinyl alcohol)-xanthan gum hydrogels, exploring their swelling behavior, mechanical and rheological properties, as well as adsorption characteristics [28]. Malaysian scholars have contributed substantially, with their documents cited 313 times, emphasizing the production of new materials with favorable properties through the mixing of two or more polymers. They have highlighted several new types of hydrogels and gelation technologies [29].

3.2.2. Distribution of Issuing Journals

Through the analysis and statistics of related literature in the WoS database, the literature on hydrogel in water treatment from 2000 to 2024 originated from a total of 326 journals. The top ten journals are detailed in Table 2. JCR zoning involves ranking the impact factors of all journals within a specific discipline in descending order for the previous year and then dividing them into four equally proportioned zones, each accounting for 25%, namely Q1, Q2, Q3, and Q4. Notably, only the JCR partition of RSC Advances and Journal of Applied Polymer Science is Q2; the rest are Q1. This indicates a relatively high publishing level in this field. The impact factors of Chemical Engineering Journal, Carbohydrate Polymers, and Journal of Membrane science rank in the top three, with values of 13.3, 10.7, and 8.4, respectively. The most cited article in the Chemical Engineering Journal suggests that forward osmosis desalination relies significantly on the ease and efficiency of solute extraction from desalinated water, and it proposes hydrogel expansion as an innovative regeneration method [30]. The most cited article in Carbohydrate Polymers reviews the properties and preparation steps of chitosan–cellulose blends and nanocellulose-reinforced materials for various applications of chitosan bio-nanocomposites [29]. This highlights that the study of composite hydrogels is currently a prominent direction in research. It is important to read these articles to gain a deeper understanding of the preparation of high-performance hydrogels. For the functionalization of ultrafiltration membrane with polyampholyte hydrogel and graphene oxide to achieve dual antifouling and antibacterial properties, the most frequently cited article in the Journal of Membrane science, developed a ultrafiltration polyether sulfone membrane (PES) with dual antifouling and antibacterial properties. After, the study found that this material’s leaching rate is very low, suitable for long-term use in water treatment [31].
Table 3 lists the ten most frequently cited articles in WoS. For example, in the most frequently cited, Recent advances in regenerated cellulose materials, a variety of regenerated cellulose materials and cellulose matrix composites were summarized [32]. The removal of heavy metal ions (such as Cu2+, Fe2+, and Pb2+) from water by cellulose-based hydrogels and their purification effect in drinking water were analyzed [33]. Synergistic Energy Nanoconfinement and Water Activation in Hydrogels for Efficient Solar Water Desalination refers to a light-absorbing spongy hydrogel (LASH) which studies energy limitations at polymer–nanoparticle interfaces and water activation achieved by polymer–water interactions, and water vaporization achieved by LASH can remove more than 99.9 percent of seawater through solar desalination [34]. In One step fabrication of graphene oxide enhanced magnetic composite gel for highly efficient dye adsorption and adsorption catalysis, Catalysis presents the preparation of GO/poly(vinyl alcohol) (PVA) composite gels (mGO/PVA CGs), which only exhibit convenient magnetic separation capabilities. Moreover, the adsorption capacity of cationic methylene blue (MB) and methyl violet (MV) dyes is significantly enhanced, which is of great significance for the treatment of dye wastewater [35].

3.2.3. Author Composition

According to the statistics of authors who published articles in this field in WoS, there are 4854 authors. Among them, the team led by Zhuang from the Chinese Academy of Sciences has published the most articles (10 articles) (see Table 4). Their research focuses on the removal of antibiotics by graphene hydrogel [36], as well as the removal and adsorption of tetracycline using alginate/graphene hydrogel [37,38]. The articles from Thakur Sourbh’s team have a relatively high citation frequency, reaching 146.43. His team primarily focused on the investigation of hydrogels composed of various materials, including gelatin-based hydrogels [12], sodium-alginate-based hydrogels [39], cellulose-based hydrogels [40], and lignin hydrogels [41]. Additionally, they conducted comprehensive research on the adsorption of methylene blue by sodium-alginate-based hydrogels [42]. Among the top ten authors, eight are researchers in China, indicating a high level of attention and contribution to this field from Chinese researchers.
Figure 3 illustrates the cooperative relationships among authors publishing articles in the field of hydrogel water treatment. The size of each circle corresponds to the author weight. Observing this Figure, it becomes evident that authors such as Zhuang Yuan, Zhang Wei, and Wang Gang have established their own academic groups. However, the connections between these groups are not very close, indicating a lack of collaboration between teams in this field. Notably, Zhang Wei’s team exhibits relatively higher collaboration within the group compared to others.

3.2.4. Analysis of Major Research Institutions

As depicted in Table 5, the top ten research institutions, in terms of the number of publications, represent four countries. The Chinese Academy of Sciences holds the top position with 41 articles, followed by the University of Chinese Academy of Sciences in second place with 16 articles, and Zhejiang University in third place with 14 articles. Tsinghua University and the University of Johannesburg secure the fourth and fifth positions with 13 articles. Notably, five out of the ten institutions are Chinese, signifying the considerable attention that Chinese researchers dedicate to this field. While the Chinese Academy of Sciences leads in both publications and citations, it is noteworthy that its average citation frequency is only moderate. Amirkabir University of Technology claims the highest average citation frequency at 81, followed by Zhejiang University of Technology at 76.36 and the University of Johannesburg at 62.77. Researchers from the Chinese Academy of Sciences have categorized their studies on hydrogels into two main areas: the preparation and performance characterization of novel hydrogels and the exploration of adsorption and removal performance of different hydrogels on heavy metals [43], antibiotics [36], and dye wastewater [44]. Cranfield University’s research on various hydrogel nanocomposites is notably in-depth. One of its most cited articles details advancements in modifying sodium-alginate-based hydrogels for the adsorption and removal of toxic pollutants [39]. Analyzing the collaborative network between institutions, it is evident that the Chinese Academy of Sciences maintains closer connections with other institutions. However, the overall number of research institutions in the field of hydrogel in water treatment is limited, and the collaborative network among them is not as closely interconnected (see Figure 4).

4. Analysis of Research Hotspots and Trends

4.1. Analysis of Research Hotspots

A bibliometric analysis was conducted to identify the application hotspots of hydrogels in the field of water treatment. The retrieval time span was set from 2000 to 2024, focusing on subject words, and the article type was limited to “Article” or “Review”. In total, 1082 papers were retrieved. Keywords that appeared more than eight times were counted, and meaningless words were excluded. Synonyms were merged, resulting in a co-occurrence relationship among 252 keywords after clustering. After excluding search keywords such as “hydrogel”, “aquagel”, “water treatment”, and “water cleans”, prominent keywords emerged, including “adsorption” (390 occurrences), “removal” (258 occurrences), “chitosan” (145 occurrences),“nanoparticles” (132 occurrences),“methylene-blue” (126 occurrences), “graphene oxide” (96 occurrences), and “composite”(93 occurrences). Other notable keywords include “heavy-metals” (70 occurrences), “cellulose” (60 occurrences), and “alginate” (48 occurrences). The recent research on hydrogels has primarily centered around the preparation methods, modes of action, treatment objects, and applications of various hydrogel types. Among the extensively researched hydrogels are alginate hydrogels [45], graphene-based hydrogels [46], chitosan-based hydrogels [47], and cellulose hydrogels [48]. Cluster analysis reveals that the research hotspots in the field of water treatment hydrogels can be categorized into three main groups (see Figure 5).

4.1.1. Preparation and Characterization of Hydrogels

Currently, hydrogel preparation primarily utilizes freezing and cross-linking methods. For instance, researchers have employed graphene in the one-step freeze-drying process to prepare alginate/nano-fibrillated cellulose double-network hydrogels. They subsequently measured the adsorption capacity and compressive strength of crystalline violet [49]. Kono and his team utilized carboxymethylcellulose sodium salt (CMC) and β-cyclodextrin (β-CD) as raw materials to cross-link hydrogels. They suspended these materials in an alkaline medium with ethylene glycol diglycidyl ether (EGDE) as a crosslinking agent, employing a suspension cross-linking method. This process resulted in the preparation of novel hydrogel beads with molecular adsorption capacity [50]. The researchers focused on characterizing hydrogel properties, particularly examining compressive strength, swelling, water retention, adsorption isotherm, and adsorption kinetics. For example, Li et al. synthesized a double-network hydrogel employing chitosan–cellulose composites as a rigid net and polyacrylamide as a flexible net. They investigated the swelling performance, water-locking capacity, and stability of this double-network hydrogel [51]. Wu’s team synthesized porous nanocomposite hydrogels using biocarbon as adsorbent nanoparticles and polyacrylamide hydrogel as the matrix. Their study explored the adsorption isotherm, adsorption kinetics, and adsorption mechanism [52].

4.1.2. The Mode of Action of Hydrogels and the Contaminants Treated

Adsorption emerges as the dominant mode of action, occurring 289 times. During the swelling process, hydrogel exhibits the ability to absorb water, and the polar groups within them attract water molecules into the network, causing them to become bound water. The osmotic pressure between the interstitial water within the network and the free water outside facilitates the hydrogel’s absorption of additional water molecules [53]. Hydrogels target various substances, including heavy metal ions, microplastics, dye wastewater, organic pollutants, and more. Adsorption stands out as an effective method for removing heavy metals, with various options for adsorbents, such as activated carbon. However, activated carbon faces challenges, such as low regeneration efficiency and the production of purified water with poor quality [54]. The effectiveness of hydrogel adsorbents in removing metal ions is closely tied to the efficient contact between the adsorbents [55]. Hydrogel exhibits a high selective adsorption capacity for Cu2+, Pb2+, and Fe2+. Some researchers have discovered that chitosan-based hydrogel proves effective in treating nuclear wastewater by removing radioactive metal ions. Through a series of treatments, this hydrogel can bring the levels of radioactive metal elements to meet international standards, thereby mitigating the risk associated with the accumulation of nuclear wastewater to a certain extent [56]. The modified chitosan-based hydrogel can also effectively adsorb the spilled oil in water. The researchers prepared a polymer porous gel and coating using non-toxic and environmentally friendly chitosan, citral, and glutaraldehyde as raw materials. The gel has excellent adsorption capacity, high porosity, low density, and soil degradability and has great application potential in the field of environmental purification of oil pollution [57]. The adsorption of heavy metal ions by hydrogels primarily results from the chelation of these ions with -CO, -OH, and -CO2H groups present in the hydrogel [58]. Polyvinyl alcohol is an environmentally friendly hydrogel material with high water solubility [59]; so, researchers have developed a polyvinyl alcohol hydrogel composite membrane for synchronous wastewater treatment and resource recovery. They enhanced the pollution resistance of the cellulose ester forward permeable membrane by introducing a polyvinyl alcohol hydrogel coating, maintaining electrical neutrality and ensuring effective ammonium retention [60]. Microplastics, characterized by particles with diameters of less than 5 mm, represent an emerging pollutant [61]. Microplastics, characterized by a large specific surface area, can be reduced in size. Their inherent properties, including a large specific surface area, small size, hydrophobicity, and susceptibility to external factors, make them particularly impactful on the environment [62]. Currently, methods for microplastic removal include adsorption, photodegradation, flocculation, among others. Researchers have successfully utilized the hydrophobic nature of developed hydrogels to effectively remove microplastics from water bodies [56]. Dye wastewater poses a significant challenge in industrial settings due to its complex composition and large volume [63]. The mechanism underlying the adsorption of methylene blue dye involves electrostatic attraction (between TiO2 or COO- and cationic dyes) and hydrogen bonding (between -OH and imine dyes) [64]. While various methods exist for treating dye wastewater, adsorption stands out as a cost-effective and efficient option. In the initial studies on hydrogel adsorption for dye wastewater, researchers utilized a range of materials, including cellulose [10], chitosan [65,66], among others. However, these studies revealed the emergence of various problems. Subsequently, researchers shifted their focus to polymer hydrogels, synthesizing hydrogels from different materials to address drawbacks associated with single-material hydrogels. A distinct advantage of polymer hydrogels, setting them apart from other hydrogels, is their ability to undergo reversible volume changes in response to external stimuli [67]. Certain studies have highlighted that the mechanism behind the adsorption of dye effluents by sodium alginate/polyacrylamide composite hydrogels involves the interaction of amino and azo functional groups of the dyes with carboxyl functional groups of the hydrogel nanocomposites, as well as π–π interactions [39]. In a bibliometric study, it was revealed that while hydrogels are widely used to address various objects and wastewater treatment challenges, there is a relatively limited number of studies focusing on antibiotic-like substances.

4.1.3. Application of Hydrogels

Hydrogels in the field of water treatment can be divided into water purification, seawater desalination and atmospheric condensation. In the context of drinking water treatment, the utilization of hydrogels avoids issues associated with the traditional use of disinfection reagents, such as the generation of toxic ions and the development of drug resistance. Moreover, the application of hydrogels enhances the overall efficiency of microbial retention [68]. The significance of hydrogel in water purification becomes evident when comparing its role in treating drinking water and the aforementioned applications. Furthermore, hydrogel proves valuable in seawater desalination, with polyelectrolyte hydrogel finding application in permeable membranes [67]. Notably, some researchers have innovatively developed smart desalination materials by capitalizing on the varying swelling capacity of temperature-sensitive hydrogels. This approach aims to achieve desalination by absorbing substantial amounts of freshwater from seawater through a semi-permeable membrane at low temperatures and releasing seawater into the hydrogel at higher temperatures due to a reduction in swelling capacity [69]. Two primary methods for collecting water from the atmosphere are condensation and adsorption. Among these, the adsorption method is considered more energy-efficient and environmentally friendly, as it consumes less energy. The researchers achieved a further reduction in the enthalpy of water evaporation by constructing a polymer network of hydrogels. This innovative approach allows for the regulation of the water state, minimizing energy losses and ensuring the efficient delivery of evaporated water.

4.2. Research Trend Analysis

Analyzing emergent words in a specific field allows for a comprehensive understanding of research hotspots and frontiers over different time periods [70]. The emergent word function in CiteSpace was employed to analyze keywords spanning from 2000 to 2024, using a minimum duration of one year. The emergent words were scrutinized based on the time periods exhibiting the most significant influence. The red range on the analysis indicates the time period with the greatest change in frequency, representing the period with the most notable impact [71].
The word “acrylic acid” in the WoS began its appearance in 2003 and concluded in 2016, spanning a duration of 13 years with an emergent intensity of 4.03. This signifies that “acrylic acid” served as a research hotspot from 2003 to 2016. Similar patterns are observed for other keywords. Notably, “chitosan” emerged as a focus during 2007–2009, while “hybrid hydrogel” gained prominence in 2014–2017. The appearance of “nanocomposite hydrogel” in 2022–2024 highlights the evolving research focuses and directions over different time periods. This figure illustrates that researchers’ interests and emphases varied during specific periods. As society and the economy continue to develop, future research endeavors may prioritize enhancing the efficiency of hydrogels in pollutant treatment and exploring novel hydrogel types. These advancements are pivotal for advancements in water purification and treatment (see Figure 6).
CiteSpace was utilized to analyze the prominent terms in 167 academic journal papers on hydrogels in the field of water treatment retrieved from the CNKI database. The analysis reveals a certain lag in Chinese researchers’ exploration of hydrogels compared to their international counterparts. For instance, while international researchers began investigating cellulose-doped hydrogels in 2017, it was not until 2019 that domestic researchers initiated similar studies. In light of this situation, our scientific research community should continuously enhance innovation capabilities and strive for leadership. A notable trend identified in the literature is the use of hydrogel materials to adsorb antibiotics, addressing the removal of new pollutants. This attention to new pollutants aligns with the growing emphasis in China on addressing emerging environmental challenges. The 14th Five-Year Plan for National Economic and Social Development in the People’s Republic of China (PRC) and the Outline of Long-term Goals in 2035 clearly states that “attention should be paid to the treatment of new pollutants”. The treatment of new pollutants is an important decision made by the CPC Central Committee and the State Council [72] in response to the national call. Future researchers may focus on the removal of new pollutants from water using hydrogels (see Figure 7).
Hydrogels are widely used in water environment, and the biotoxicity of hydrogels has been studied to some extent. Researchers exposed earthworms to terpolymer hydrogels (acrylic acid, acrylamide, and 2-acrylamido-2-methyl-1-propane-sulfonic acid) and modified cowhide xyloides and found that the lignin-modified hydrogels produced oxidative stress and acute lethal toxicity in Eisenia fetida [73]. However, not all hydrogel materials are biotoxic. For example, Farid-ul-Haq, M, and their team developed a novel Ph-responsive hydrogel using methylene–bisacrylamide (MBA) as a crosslinking agent through the free-radical polymerization of common mugshell seed mucus/hydrogel (AVH) and acrylic acid (AA) monomer. It was found that the hydrogel did not show any hematological, biochemical, or histopathological changes in rat and rabbit models, so it is a non-toxic hydrogel [74]. By using MTT assay, the toxicity levels of chitosan/polyacrylamide hydrogels were found to be between 0 and 1. The results of a cell culture showed that chitosan/polyacrylamide hydrogels were beneficial to cell adhesion and growth and could be used as substrate for cell mechanics research [75]. Therefore, the toxicity of hydrogels may be related to the addition of ingredients. For the application of hydrogels in the field of water treatment, we should also pay great attention to their biological toxicity to avoid the use of hydrogels to cause damage to other organisms, thus destroying the ecological balance.

5. Relevant Patent Analysis

The IncoPat Technology Innovation information platform from Beijing incoPat Co., Ltd., Beijing, China. was used to analyze the relevant patents of hydrogels in the field of water treatment. The search is TIAB = (hydrogel*) AND TIAB = (“water treatment” OR “water purify” OR “water cleans” OR “water remediation” OR “water recovery”), set for the years 2000 to 2024. A total of 230 items were retrieved and analyzed visually.

5.1. Application Trend Analysis

As can be seen from the figure, the number of patents granted from 2000 to 2015 is relatively small, belonging to the slow development stage; 2016 to 2019 belongs to the period of fluctuating growth, and 2020 to now belongs to the rapid development stage (see Figure 8). Due to the data-screening deadline of 1 July 2024, the relevant data for 2024 is not fully displayed, and it is expected to reach 60 items. Through comparison, it can be found that this change trend is similar to the number of relevant articles. In the early stage, researchers paid relatively little attention to this field, resulting in fewer innovative results. With the progress of technology and the continuous enhancement of people’s awareness of intellectual property protection, the number of patent grants has also been increasing.

5.2. Analysis of Institution

A review of the top ten institutions ranked by patent count revealed that seven are Chinese universities (Figure 9). This indicates a continuous enhancement in the innovative capabilities of these institutions, alongside a robust awareness of intellectual property protection. An analysis of the patents from these entities demonstrates that most composite hydrogel materials focus on the removal of heavy metals, emerging pollutants, and other substances, as well as their application in solar evaporators.

5.3. Analysis of Country

An analysis of countries granted patents shows that China has been granted a total of 170 patents related to hydrogels, accounting for 74% of the total number of countries granted worldwide (Figure 10). The United States and South Korea follow with 17 and 16 patents, respectively. This trend indicates that relevant authorities in China are continuously enhancing patent laws and regulations, addressing existing gaps, and fostering a research environment conducive to innovation [76].

6. Conclusions

(1) Between 2000 and 2024, a total of 1308 papers addressing hydrogel applications in water treatment were published in the WoS and CNKI databases, indicating a consistent upward trend in publication numbers. Notably, China contributed the largest share, with 42.14% of the total publications. Among the top ten journals, only two are classified as Q2, while the remaining journals hold a Q1 classification. The top three journals, ranked by impact factor, are International Journal of Biological Macromolecules, Chemical Engineering Journal, and Carbohydrate Polymers, exhibiting impact factors of 7.7, 13.3, and 10.7, respectively. Among the authors, Zhuang leads with the highest number of publications, totaling 10, and a cumulative citation frequency of 622. The articles by Thakur Sourbh is the most frequently cited, with a remarkable citation frequency of 1025.
(2) The research hotspots of hydrogel in the field of water treatment are primarily categorized into three domains. Firstly, the preparation and performance characterization of hydrogel exhibit variations among different types, with emphasis on factors such as adsorption kinetics and adsorption isotherms. Secondly, the focus lies on understanding the mode of action of hydrogel its interaction with various pollutants. This includes the adsorption of heavy metal ions, microplastics, dye wastewater, and organic pollutants. The adsorption mechanism primarily involves chelating effects, electrostatic attraction, hydrogen bonding, and other interactions between the functional groups in the hydrogel and the pollutants. Thirdly, the application of hydrogel in water treatment includes water purification, seawater desalination, and atmospheric condensation. An analysis of the proprietary advantages associated with hydrogels reveals that the number of patent grants has entered a phase of rapid growth since 2020. Notably, seven out of the top ten institutions globally in terms of patents granted are based in China. Furthermore, China’s total number of licenses constitutes 74% of the global total, indicating a significant enhancement in its awareness regarding intellectual property protection and a continuous improvement in its innovation capabilities.
(3) Analyzing the current research trend in hydrogel in the field of water treatment suggests a future emphasis on continual enhancement of synthesis methods. The focus should be on developing hydrogel materials that are not only cost-effective but also possess broader applicability and superior treatment performance. Additionally, there is a need to pay attention to improving the reusability of the material, harnessing its full potential, thereby better controlling treatment costs and further promoting sustainable development in the field of water treatment. In recent years, people’s attention to artificial intelligence and machine learning has been increasing, so in the future, researchers may use machine learning methods to calculate the preparation conditions and optimization conditions of hydrogels, simulate the application of different groups of hydrogels in the water environment under different conditions, and continuously optimize the removal effect of hydrogel materials on pollutants. It can also provide researchers with new ideas and methods.

Author Contributions

Y.Y.: Conceptualization, data curation, formal analysis, writing—original draft, writing—review and editing. S.X.: conceptualization, project administration, resources, writing—review and editing. B.Y.: conceptualization, project administration, writing—review and editing. S.S.: conceptualization, funding acquisition, project administration. R.D.: conceptualization; resources. X.X.: conceptualization, writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Sichuan Province International Science and Technology Innovation Cooperation/Hong Kong, Macao and Taiwan science and Technology Innovation cooperation project, grant number 2020YFH0120.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Flowchart of this study.
Figure 1. Flowchart of this study.
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Figure 2. Changes in the number of publications on hydrogel in water treatment from 2000 to 2024.
Figure 2. Changes in the number of publications on hydrogel in water treatment from 2000 to 2024.
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Figure 3. Author-assisted relationship network of hydrogel in water treatment.
Figure 3. Author-assisted relationship network of hydrogel in water treatment.
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Figure 4. Collaborative network between institutions in the field of hydrogel research in water treatment.
Figure 4. Collaborative network between institutions in the field of hydrogel research in water treatment.
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Figure 5. Distribution of research hotspots in the field of hydrogel in water treatment.
Figure 5. Distribution of research hotspots in the field of hydrogel in water treatment.
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Figure 6. Emergent keyword mapping of the WoS.
Figure 6. Emergent keyword mapping of the WoS.
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Figure 7. Emergent keyword mapping of the CNKI.
Figure 7. Emergent keyword mapping of the CNKI.
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Figure 8. Number of patents granted from 2000 to 2024.
Figure 8. Number of patents granted from 2000 to 2024.
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Figure 9. The world’s top ten institutions with the number of related patents.
Figure 9. The world’s top ten institutions with the number of related patents.
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Figure 10. The world’s top ten countries with the number of related patents.
Figure 10. The world’s top ten countries with the number of related patents.
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Table 1. Top ten countries with hydrogel publications in water treatment in WoS (2000–2022).
Table 1. Top ten countries with hydrogel publications in water treatment in WoS (2000–2022).
CountryNumber of PublicationsTotal Citation FrequencyAverage Citation Frequency
China4561656036.32
India123312525.41
USA113390234.53
Iran79325741.23
Egypt62114918.53
South Korea42129730.88
Saudi Arabia4070917.73
Australia38223458.79
Canada31188760.87
Italy3162520.16
Table 2. Top ten Journals of Hydrogels in the Field of Water Treatment in WoS (2000–2002).
Table 2. Top ten Journals of Hydrogels in the Field of Water Treatment in WoS (2000–2002).
JournalNumber of Publications 2024 Impact FactorJCR Partition
International Journal of Biological Macromolecules437.7Q1
Chemical Engineering Journal3613.3Q1
Carbohydrate Polymers3110.7Q1
RSC Advances303.9Q2
Gels255Q1
Polymers254.7Q1
Journal of Applied Polymer Science242.7Q2
Separation and Purification Technology208.1Q1
Journal of Membrane science198.4Q1
Desalination188.3Q1
Table 3. Top ten papers with the highest total citations for hydrogels in water treatment in WoS (2000–2022).
Table 3. Top ten papers with the highest total citations for hydrogels in water treatment in WoS (2000–2022).
TitleFirst AuthorJournalThe Year of PublicationTotal Citation Frequency
Recent advances in regenerated cellulose materials.Wang SProgress in Polymer Science2016747
pH-Responsive polymers: synthesis, properties and applications.Dai SSoft Matter2008547
Graphene-based macroscopic assemblies and architectures: an emerging material system.Cong HPChemical Society Reviews2014396
Synergistic Energy Nanoconfinement and Water Activation in Hydrogels for Efficient Solar Water Desalination.Guo YHACS Nano2019357
A review on chitosan-cellulose blends and nanocellulose reinforced chitosan biocomposites: Properties and their applications.Khalil HPSACarbohydrate Polymers2016353
Recent advancements in forward osmosis desalination: A review.Akther NChemical Engineering Journal2015349
An introduction to zwitterionic polymer behavior and applications in solution and at surfaces.Blackman LDChemical Society Reviews2019308
Recent progress in sodium alginate based sustainable hydrogels for environmental applications.Thakur SJournal of Cleaner Production2018299
Development of a sodium alginate-based organic/inorganic superabsorbent composite hydrogel for adsorption of methylene blue.Thakur SCarbohydrate Polymers2016295
One-step fabrication of graphene oxide enhanced magnetic composite gel for highly efficient dye adsorption and catalysis.Cheng ZHACS Sustainable Chemistry and Engineering2015292
Table 4. Top ten authors of hydrogel in water treatment with publications in WoS (2000–2024).
Table 4. Top ten authors of hydrogel in water treatment with publications in WoS (2000–2024).
AuthorNumber of Publications Total Citation FrequencyAverage Citation Frequency
Zhuang Yuan1062262.20
Jiao Tifeng959265.78
Zhang Leixin959265.78
Shi Baoyou950956.56
Zhou Jingxin858973.63
Thakur Vijay Kumar7809115.57
Thakur Sourbh71025146.43
Dai Sheng736752.43
Zhao Changsheng736752.43
Yang Jing712818.29
Table 5. Top ten research organizations in the field of hydrogel in water treatment in WoS based on number of publications.
Table 5. Top ten research organizations in the field of hydrogel in water treatment in WoS based on number of publications.
Research OrganizationNumber of PublicationsTotal Citation FrequencyAverage Citation Frequency
Chinese Academy of Sciences46212846.26
Sichuan University2674928.81
University of Chinese Academy of Sciences1783349
Harbin institute of technology1758034.12
Zhejiang University14106976.36
Tsinghua University 1483159.36
University of Johannesburg1481662.77
Nanjing Forestry University1342032.31
Alexandria University1229624.67
Amirkabir University of Technology1189181
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MDPI and ACS Style

Yuan, Y.; Xiao, S.; Yan, B.; Shen, S.; Dou, R.; Xu, X. Bibliometric Based Analysis of Hydrogels in the Field of Water Treatment. Sustainability 2024, 16, 8194. https://doi.org/10.3390/su16188194

AMA Style

Yuan Y, Xiao S, Yan B, Shen S, Dou R, Xu X. Bibliometric Based Analysis of Hydrogels in the Field of Water Treatment. Sustainability. 2024; 16(18):8194. https://doi.org/10.3390/su16188194

Chicago/Turabian Style

Yuan, Yakun, Shuhu Xiao, Bingfei Yan, Songtao Shen, Runtao Dou, and Xiaohe Xu. 2024. "Bibliometric Based Analysis of Hydrogels in the Field of Water Treatment" Sustainability 16, no. 18: 8194. https://doi.org/10.3390/su16188194

APA Style

Yuan, Y., Xiao, S., Yan, B., Shen, S., Dou, R., & Xu, X. (2024). Bibliometric Based Analysis of Hydrogels in the Field of Water Treatment. Sustainability, 16(18), 8194. https://doi.org/10.3390/su16188194

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