Hotspots and Trends of Layered Double Hydroxide-based Adsorbents for Polluted Water Treatment: Insights from Bibliometric Analysis

The use of layered double hydroxides (LDHs) as adsorbent for water treatment has been gaining relevance in recent years. In this context, this work aimed to map, through a bibliometric study, the extent of research that deals with the theme. The scientific database used was the Web of Science, and the chronology of the search consideredthe period from 1997 to 2022. The bibliometix R-package and VOSviewer software were used in this study. The searches retrieved a total of 663 documents, from 69 countries, distributed among all continents, which China (328), India (51) and Japan (40) were the most productive countries. Important journals in the environmental area and with high impact factor, such as Chemical Engineering Journal (44), Applied Clay Science (38), Journal of Hazardous Materials (35) and Chemosphere (27) most published in the area. The network of keywords used by the authors indicates that the publications retrieved deal mainly with aspects related to the efficiency of (LDHs) in the removal of different pollutants, the composition, the synthesis route and the association with other materials and/or techniques. The result of this study constitutes an important tool for directing future research on the subject.


Introduction
Rapid population growth, uncontrolled exploitation of natural resources, and the use of a wide variety of chemicals in activities essential to humans such as agriculture, industrial processes, among others, have been causing enormous environmental instability worldwide (Koutavarapu et al. 2021). Among the main impacts generated by human intervention on the natural environment, water pollution has stood out as one of the most serious problems of today's society (Nasrollahzadeh et al. 2021). In addition to limiting the availability of an essential resource for human life (Fan and Fang 2020), the degradation of water quality, interferes with socioeconomic development worldwide, since water plays an important role in several sectors essential to industrial development, functioning as a key element in the production of food and energy, for example (Mao et al. 2015). Moreover, when contaminated, water becomes the primary mode by which potentially toxic chemicals come into contact with the human and animal community, since such substances, when soluble, are more easily transported and distributed through the water cycle (Oller et al. 2011;Yang et al. 2019).
The inadequate management of contaminated water by conventional treatment systems has contributed to the expansion of studies dealing with the development of alternative treatment systems that are able to remove the varied classes of pollutants resistant to conventional treatments and ensure that the treated water is healthy for reuse and/or consumption (Kıdak and Doğan 2018). Among the various technologies that have been evaluated for their efficiency in treating contaminated water, the Adsorption process is most prevalently cited in review papers (Z. Li et al. 2020). This process is preferred over others because it has some important advantages such as relatively simple design, easy operation, high cost-effectiveness, ability to remove pollutants at very low concentrations, low energy consumption, and high availability of raw materials (Fiyadh et al. 2019). Moreover, the adsorption process has a wide applicability, being the most favorable method for removing contaminants of organic and/or inorganic nature (Kim et al. 2022).
The efficiency of the adsorption process depends on the surface characteristics of the adsorbent material and how these interact with the specific pollutants to be adsorbed (Fiyadh et al. 2019). Although it is considered the best adsorbent for cleaning liquid solutions, the widespread use of activated carbon is restricted due to its high cost (da Silva Santos et al. 2021;D. H. S. Santos et al. 2020) (Wang et al. 2010). One ton of such material produced from coconut husk, for example, costs, before shipment, about USD950-1400 (Ahmedna et al. 2000). The high cost of producing and marketing AC, in addition to contributing to the lack of access to commercially treated water for a significant fraction of the population in developing countries, given the low per capita income of these countries, also limits the ability of industries to treat their effluents to the required environmental standards prior to disposal-with some industries operating outside legal limits-thereby exacerbating environmental pollution (Ntuli and Hapazari 2013).
In this regard, the interest in the development of unconventional adsorbent materials that are efficient in removing a variety of pollutants of organic and/or inorganic nature, easy to obtain, low toxicity, good regenerative capacity and, in particular, low cost, has been the main object of studies in this area (Kumar et al. 2019). Recent progress in nanoscience and nanotechnology has helped researchers to design nanoscale materials with the desired structure and functionality (Sohrabi et al. 2021). In this perspective, layered double hydroxides (LDHs) have been receiving more and more attention.
Layered double hydroxides comprise a class of twodimensional anionic clays with a structure similar to the mineral hydrotalcite ([Mg 6 Al 2 (OH) 16 ]CO 3 -4H 2 O) (Mohapatra and Parida 2016). The general formula of LDHs is represented by the equation M 2+ 1-x M3 + x (OH) 2 .An-x/ n.zH 2 O, where M 2+ and M 3+ correspond to di-and trivalent cations, respectively, and Anis an anion. The cations are responsible for the positive charges presented by the lamellae, while the anionic species along with water molecules, present in the interlamellar regiation, makes the material stable (Mishra et al. 2018).
Commonly known as anionic clays or hydrotalcite-like materials, LDHs are easily synthesized using lowcost materials and unsophisticated equipment (Jijoe et al. 2021). These materials exhibit a unique molecular structure with highly controllable chemical composition of the cationic layers and interlamellar region. Thus, LDHs with different element compositions and metal cation ratios can be produced. These materials exhibit differences in their morphology and crystal structure, which allows them to be designed for a specific purpose (Keyikoglu et al. 2022).
The use of LDHsas adsorbent materials in the remediation of environmental problems has been the most commonly reported application in the literature. Although an increasing interest in the use of these materials as adsorbent materials is observed, there is a gap in statistical research trend information on this topic. In this sense, the present work aims, through a bibliometric study, to analyze the scientific guidelines on the suitability of layered double hydroxides (LDHs) as adsorbent agents for contaminants in aqueous media in order to prospect new research directions and/or strategies with potential to boost the use of these materials.
The article is structured as follows: initially, a brief background on bibliometric analysis, source and data selection is presented. Then, the general characteristics of the retrieved research, the areas of study on the topic, the journals that published the most on the subject and also the main areas of research are addressed. Finally, the perspectives and conclusions of the work are presented.

Bibliometric Analysis
This method allows the identification, organization, and evaluation of the constituent elements of a specific study area, and is therefore an important tool for reviews. It is a comprehensive technique related to mathematical and statistical methods to discover the distribution, variation, and quantity of publications in public databases on a given subject (Gallego-Valero et al. 2021).

Data Source
The Web of Science (WoS) database was used to perform the bibliometric analysis. WoS has high visibility in various fields of knowledge, a selection filter for prestigious publications, and is also widely used to conduct bibliometric studies (Jiang et al. 2018).

Data Selection and Processing
The sample of documents analyzed in this study was obtained using the terms ("layered double hydroxides" OR "anionic clays" OR "hydrotalcite-like materials") AND ("adsorption" OR "adsorbent") AND ("treatment OR "remediation") AND (water OR "contaminated water" OR "effluents" OR "waste water") in the topic field of publications. The literature search was performed with all terms that, according to the literature, are commonly used to refer to LDHs in order to to obtain results more in line with reality (Vaccari 1998). From this result, the documents were refined by applying search criteria for chronology considering the date of the first publication retrieved on the subject in the database until March 2022 . No there were restrictions for the fields of knowledge and type of document. The bibliometix R-package and VOSviewer software for used in bibliometric analysis and scientific mapping of retrieved data. The use of software has been fundamental in bibliometric analyses. Among the numerous software available, VOSviewer and Bibliometrix were chosen for the development of this work because they have free, updated versions (after 2019) and a graphical interface. Among these selection criteria there was the exclusion of other important software such as Scimat, Sci2 Tool that could also be used, for example.
The objective of this work is to sketch an overview of the scientific production in this new field of research, evaluating aspects such as authors, publication vehicles, areas of knowledge, related keywords, among others, in order to explore this research in a deeper way, providing a important reference for the scientific community to know and to relate the productions and authors of these areas and subareas. In this way, it was decided to use all types of documents and without restriction or specification of any language in order to gather the most general panorama possible of scientific production.

General Characteristics of the Retrieved Samples
Through the searches performed in the WoS database, a total of 663 documents were retrieved, of which 89.54% (595 records), the absolute majority, refer to scientific articles, 9.05% (60 records) correspond to review documents, and the less than 2% remaining correspond to the sum of other types of records such as meeting abstracts, book chapters, news, editorial material, among others.
Articles printed in English correspond to 98.6% of the total records, followed by articles published in Chinese, 0.9% of the total. The proportion of the sum of the other three languages with retrieved records, which include French, Polish, and Turkish, is less than 0.5 percent. The histogram presented in Fig. 1a shows the variations in the number of retrieved publications related to the theme between the years 1997 and 2022. The data obtained show, in general, an upward trend. In the first nine years (1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005), academic research in this area was in its early stages, with a publication rate of 1.75 papers per year, a negligible figure. In the subsequent years, 2006 to 2015, a greater number of documents were retrieved, reaching an average of 15.5 publications per year, a rate 8.9 times higher than that observed previously (1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005). However, in more recent periods (2016 to 2022), the publication rate has increased rapidly and substantially, with the average number of publications being about 3.1 times higher than in the previous period evaluated (2006 to 2015). In the year 2022, by the time the data were surveyed (3/21/2022), 10 studies on the topic had already been published. These results suggest that in the coming years, annual publications on this topic will continue to grow.

Relevant Countries in Research on the Topic
The papers published on topic in the period 1997-2022 come from a total of 69 countries. Figure 1b shows the map of countries with results in the research, the darker the color of the country/region, the more papers there are in that country/region. Although most studies in this field come from a relatively small number of countries, such as China, India, USA, Japan, Brazil, Saudi Arabia, Australia, France, South Korea and Canada, it is notably explicit that countries from all continents and with a wide diversity of economic and socio-cultural characteristics produce relevant research in this area of knowledge. Figure 2a presents the percentages of the 10 most productive countries/territories on the topic. China is the most prolific territory, accounting for 47.96% of the total, with 318 retrieved papers. The second place is occupied by India (7.69% of the total, 51 papers), followed by Japan with 40 papers, representing 6.03% of the total. Subsequently we have USA (40 documents, 6.03%), Brazil (28 documents, 4.22%), Saudi Arabia (28 documents, 4.22%), Australia (25 documents, 3.77%), France (25 documents, 3.62%), South Korea (24 documents, 3.62%) and Canada (17 documents, 2.56%).
The worsening problem of water scarcity in China, caused by serious problems of contamination of drinking water and underground sources by arsenic and fluoride, further associated with rapid economic growth and the large number of industrial production activities in the country, resulted in increased investment in research aimed at the use of advanced technology for water treatment, making this nation a power in this area of technology, which justified the consolidation of China as the country that produces the most on the subject (Wu, 2020).
Social network analysis was then applied to analyze the coauthorship relationships among all the producing countries/territories, the results are displayed in Fig. 2b. Each point represents a node in the network, with nodes being equivalent to countries. The larger the node, the larger the number of cooperations performed by these countries. The lines between the nodes indicate the occurrence of cooperation between the countries, and the thickness of the line is proportional to the number of publications retrieved from these cooperations.
Clearly, China, the largest producer of papers on the topic, also has the largest number of collaborations with other countries/territories, given the size of the node in Fig. 2b which also objectively represents the most frequent cooperation networks. The Chinese have produced publications in cooperation with several countries, with their collaborations with the US, India, Saudi Arabia, Canada, Australia, the UK, and Korea standing out as the most intense, a fact denoted by the thickness of the lines of connections. The U.S. presents not very expressive cooperation data, besides interacting with a reduced number of countries/ territories, its main productions are the fruit of an intense cooperation with the Chinese, which, according to Fig. 2b, is the most frequent among all the others, with a total of 23 retrieved documents. Figure 3 presents the annual average of document citations. The 10 most frequent cooperation between countries/regions, obtained by the bibliometrix package (version 3.1.4), are summarized in Table 1.
Overall, it is observed that the overall citation of this field shows a zigzag trend and indicates that more and more scholars are paying attention to this field in recent years. The average of total citations for each article reached 9.52 times per year. The article published by (Liang et al. 2017) is the most cited.  Table 2. Researcher Wang XK appears with the highest number of publications (20, 3.01%) from Chinese Academy of Sciences followed by Wang XX from North China Electric Power University (13, 1.96%). In the sequence the researchers Qian GR (11, 1.65%), Zhang XL (11, 1.65%) and Yu SJ (10, 1.50%) also add significant contributions.
Another important parameter to consider is the number of citations. In this case, Wang XK tops the list with a total number of 1246 citations, Wang XXA was second with the most citations (1099), followed by Hayat T, Yu SJ and Ok YS who had a total of 593, 523 and 497 citations, respectively.

Main Areas of Study on the Theme
The results of the bibliometric analysis allow us to distinguish between the different disciplines to which the scientific articles analyzed belong. It should be noted that an article may belong to more than one category; therefore, the results are analyzed in percentages. In all, 25 different areas of knowledge are covered with publications on the topic. Figure 5 shows the top 10 areas of study. Among the various research areas, the most important are Environmental Sciences with 16.63% of the total, followed by Environmental Engineering (14.07%), Physical Chemistry (13.17%), Chemical Engineering (12.18%), Multidisciplinary Materials Sciences (10.12%), Multidisciplinary Chemistry (6.83%), Water Resources (4.03%), Mining (3.7%), Nanoscience Nanotechnology (2.55%) and Green Sustainble Science Technology (2.49%). The item identified as "other" (13.99% of the total) includes a wide and diverse range of knowledge areas such as microbiology, mathematics, geology, microscopy, toxicology, biophysics, among others.

Journals that most Published on the Subject
The total number of retrieved documents is distributed among 217 journals. With 6.63% of the total sample of articles (44 documents) the Chemical Engineering Journal, a comprehensive journal for Environmental Chemical Engineering, was the most productive journal, followed by Applied Clay Science (38), Journal of Hazardous Materials (35) and Chemosphere (27). Together, these four journals published 21.49% of the total articles on this research topic. In addition to those mentioned above, other important journals also published within this theme. The information of the 10 journals that publish the most articles on the research topic and their main indexes are summarized in Table 3.

The Main Fields of Research
The keywords of a scientific document may offer important information about the main ideas and trends of a given theme, and are therefore an extremely important tool in bibliometric analyses. In this paper a total of 1475 keywords were identified, of which the vast majority had only one (1179, 79.7%) or two (210, 14.62%) occurrences, while 69 (4.06%) of the keywords had 5 or more occurrences. Thematic evolution analysis can be used to detect, quantify and visualize specific fields of research, and can visually show the evolution of the theme in recent years. A strategy map shown in Fig. 6 divided into four quadrants shows the degree of connection between the clusters and between the keywords in the cluster.
The second and third quadrants are the most developed, with strong centrality and high impact being the most relevant for the current field. Similar words are found in the first and fourth quadrants, indicating a certain linearity in the use of HDLs.
The strategic diagrams allow visualizing the research field as a set of themes, mapped and classified into four groups, categorized in terms of density and centrality: (I) cluster engine (first quadrant, with high density and strong centrality); (II) highly developed and isolated clusters (second upper left quadrant, with marginal importance for the research field; 'specialized topics'); (III) declining or emerging clusters (third quadrant, with low density and low centrality; 'emerging or disappearing themes'); and (IV) basic and transversal clusters (fourth quadrant, with important but undeveloped themes; &rsqu;transversal and general themes&rsqu;) (Alcaide-Muñoz et al. 2017;Cobo et al. 2012). It is noteworthy that the sphere represents a cluster of words (or theme) and the name of each one of these is related to the most recurrent word and/or theme; the volume of the spheres corresponds to the number of associated articles -the larger the sphere, the greater the number of articles that cited that word as a keyword (Cobo et al. 2012;.
The detection of the most commonly used keywords in the retrieved documents, limited to the minimum number of 5 occurrences, are presented in Fig. 7. Each point represents a node in the network, with the nodes being equivalent to the keywords. The larger the node, the greater the number of links made by these terms. The lines between the nodes indicate the co-occurrence between the keywords, and the thickness of the line is proportional to the quantity with which this co-occurrence is perceived.
Visibly, the terms "adsorption", "lamellar double hydroxides", and "water treatment" stand out among the others as the terms that have the largest numbers of links to other keywords. The term "adsorption" is the most   highlighted by the retrieved documents with a total of 199 occurrences, which reflects its central position in this highfrequency search field. The top 20 retrieved keywords and their respective occurrence numbers are summarized in Table 4. The co-occurrences between the keywords did not highlight a specific trend in the retrieved papers on the topic, as no significantly thick lines were observed between the terms. These results may indicate that research trends in this area are growing more diverse. The propensity for research on the topic can be identified by analyzing the different groups into which the keywords fall. In all, the terms are comprised in 7 groups identified by distinct colors. Of these, four main groups were observed, chosen by the largest number of keywords included.
The first group, colored yellow, comprises a total of 12 keywords and focuses on the determination of the parameters related to the adsorption process due to its conjunction with terms such as "kinetics", "thermodynamics", "isotherm" and "equilibrium". The second group, blue in color, has 17 keywords, and the presence of the terms "composite", "biochar", "memory effect", "magnetic separation of metal ions" and "regeneration" indicate that    this group focuses mainly on the production of composite materials and on the properties of LDHs. The third group, colored green, with 13 keywords, focuses on the methods of synthesis and composition of materials, with the presence of the terms "precipitation", "selective adsorption", "calcination", and "engineering". The fourth group, with red coloring, has 14 keywords, and deals with the efficiency of LDHs in the removal of different classes of pollutants due to the adjection of the terms "organic pollutants", "dyes", heavy metals, among others. Some terms such as "phosphate", "phenol", "metal ions", "dyes", "pesticides", among others, used to refer to the wide range of pollutants and/or contaminants present in water, and the terms "biochar", "photocatalysis", "nanomaterials" among others, which refer to the structure and composition of LDHs are found in more than one group in Fig. 7. The analysis took into consideration only the distinct terms present in the groups in order to understand the different guidelines of papers retrieved on the topic. Some papers dealing with the different research guidelines related to the topic are summarized in Table 5. Table 5 concisely shows the diversity in the composition of LDHs, as well as their versatility in removing a variety of contaminants in aqueous solution. Furthermore, it can also be seen that the coprecipitation method is the most commonly used for syntheses, a fact elucidated by being a simple method and easy to manipulate in the laboratory (Chang et al. 2005). In this method, the formation of the lamellar structure of the hydroxides occurs in a simple step of precipitation of the bi-and trivalent cations with the interlamellar anion, using the addition of alkaline aqueous solution, in batch ). The number of cycles, a very important data for knowledge of the regeneration capacity and, which can be used for industrial application, is little reported in the researched works. Li et al. (2020)   Just as the adsorption capabilities in pure LDHs are widely investigated, new work involving the production of LDH composites is under increasing development, demonstrating excellent performances in enhancing contaminant removal. In (Țolea et al. 2021), LDH -Mg 3 Al was used to support Methyl Trialkyl Ammonium Chloride and increase the adsorption capacity of the new adsorbent material, furthermore, in the same work, two synthetic routes were also investigated and compared for their adsorption capacity as shown in Table 5. Thus, the study identified that functionalization of Mg 3 Al with Methyl Trialkyl Ammonium Chloride (Mg 3 Al-IL-US) increased the adsorption capacity from 143 mg/g to 217 mg/g and presented maximum adsorption capacity of 648 mg/g for sample (Mg 3 Al-IL-COS) obtained by cosynthesis. In the Cl-LDH chloride intercalated LDH-MgAl a slight improvement in phosphate removal was presented, with the value of 63.2 mg/g, when compared to the materials with glycerol (Gly-Cl-LDH) and alanine intercalated (Ala-Cl-LDH), with removal capacity of 55.8 and 58.2 mg/g, respectively (Zhang et al. 2022). However, in the same work, characterization results showed that compared to MgAl-CL, Gly-Cl-LDH and Ala-Cl-LDH have higher porosity and higher specific surface area, in addition to the larger interlayer space for samples with intercalated glycerol and alanine.

Insights and Prospects
The adsorption process is considered the best technique over other contaminated water treatment strategies for its simplicity of operation and universality for common organic and inorganic contaminants (Crini 2005;Jiang et al. 2018;D. H. S. Santos et al. 2020). However, the production of adsorbent materials with high dispersion of active sites that allow the maximum utilization of their potential, tunability of the composition and electronic state of the adsorption sites on an atomic scale for greater selectivity for a specific pollutant and Stable active sites that guarantee reuse and performance for long-term use has been the major bottleneck in the application and development of this technology (Chen et al. 2022;Keyikoglu et al. 2022).
Recent progress in nanoscience and nanotechnology has allowed researchers to design nanosized materials with the desired structure and functionality, and in this sense a growing interest in the use of HDLs for water remediation as an adsorbent has been observed (Jiang et al. 2018). These materials present their unique characteristic structures, such as (i) tunability of the elemental composition of the host layer where the metallic cations reside, (ii) high ion exchange capacity due to the interchangeability of interlayer anions and (iii) controllability of the interlayer distance and dimensions of the material by incorporation of suitable anions (Crini 2005;Jiang et al. 2018;Keyikoglu et al. 2022). In addition to the type of metal cation, the ratio of divalent and trivalent (M 2+/ M 3+ ) can be regulated.
The variety of different types of HDLs with a wide range of elemental compositions that can be produced and its association with other materials has increased interest in research using these materials. It is necessary to understand more objectively what all the aspects involved cause in the material obtained and how this product can be used in its most satisfactory way.

Conclusion
The survey obtained from the searches in the scientific bases, culminated in the observation of the rising interest of the development of research and technologies that use layered double hydroxides as adsorbent material in the treatment of contaminated water. From 1997 to 2021, a total of 663 papers related to the topic were published in scientific journals indexed to the Web of Science database. A significant increase in the production on the topic was observed in the last 5 years. China was the country that presented the largest number of retrieved publications on the subject, and also the country that has cooperated the most with other nations in the development of new documents, this is important information, since the issue of water pollution affects the whole world. The bibliometric analysis also indicated that the most productive authors were Wang XK, Wang XX, and Qian GR. The journals that published the most on the topic were Chemical Engineering Journal, Applied Clay Science, Journal of Hazardous Materials, and Chemosphere. In addition to the main focus and research trends in the area by using the keywords most frequently found in the retrieved documents were "adsorption". "lamellar double hydroxides" and "water treatment". Through the bibliometric analysis it was possible to verify that LDHs are promising adsorptive agents for the removal of a wide diversity of pollutants present in contaminated water. However, some researches point to the development of composite materials and/or materials with adjusted composition in order to improve even more the efficiency of these materials. Concerning the study regarding data searches on the use of LDHs functionalized to other materials, few works were reported, with examples of the use of biochar, quantum dots of carbon and polymers as support for LDHs for the treatment of aquatic effluents. Thus, considering the response that data visualization tools can offer the field of science, the authors recommend further research into studies of LDH composites for the removal of pollutants in aquatic matrices, with emphasis on their adsorption capacity.

Data Availability
Data available on request from the authors.