3.1 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 Figure 1(a) 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 (1997-2005), 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 (1997-2005). 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.
Characteristics such as high efficiency in the removal of a variety of pollutants, versatility in composition, low cost, low toxicity, regenerative capacity, among others presented by LDHs, added to the increasing disposal of large quantities and varieties of pollutants harmful to the environment, population growth and high demand for water resources, and the commitment to reduce environmental contamination, consequently generate an urgency in the development of new technologies in these areas and corroborate the growth trend observed.
3.2 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 1 (b) 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 2 (a) 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 Figure 2 (b). 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 Figure 2(b). 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 Figure 2(b), 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.
Table 1
The cooperation among countries/regions.
From
|
To
|
Frequency
|
China
|
USA
|
23
|
China
|
Saudi Arabia
|
17
|
China
|
Australia
|
14
|
China
|
Canada
|
10
|
China
|
Korea
|
10
|
China
|
United Kingdom
|
8
|
India
|
Korea
|
8
|
Saudi Arabia
|
Pakistan
|
7
|
China
|
India
|
6
|
Japan
|
Bangladesh
|
5
|
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 number of citations peaked in 2001 with average total citations of each article is 200 times. The average of total citations for each article reached 9.52 times per year. The article published by Liang (2017)(Liang et al., 2017) is the most cited.
3.3 Analysis of publications by institutions and authors
A total of 765 institutions contributed to the 663 retrieved publications on the topic. The most productive institution was the Chinese Academy of Sciences, which published 45 papers, followed by North China Electric Power University (22), Centre National de la Recherche Scienque CNRS (20), by China University of Geosciences (14) and Tsinghua University (14). The 20 institutions with the highest number of retrieved publications on the topic are summarized in Figure 4. Through the results it was found that 15 of the 20 most productive institutions are from China, the other 5 institutions are distributed among 4 countries: France, Saudi Arabia, India and Egypt.
A total of 2,910 researchers participated in the retrieved publications. The authors with the most publications on the topic are shown in Table 2. Researcher Wang XK appears with the highest number of publications (20, 3.01%), followed by Wang XX (13, 1.96%), Qian GR (11, 1.65%), Zhang XL (11, 1.65%), Yu SJ (10, 1.50%).
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.
3.4 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.
Table 2
Authors with the most publications on the theme.
Author
|
Documents
|
Citations
|
Quantity
|
Percent
|
Wang XK
|
20
|
3,01%
|
1246
|
Wang XX
|
13
|
1,96%
|
1099
|
Qian GR
|
11
|
1,65%
|
352
|
Zhang XL
|
11
|
1,65%
|
181
|
Yu SJ
|
10
|
1,50%
|
523
|
Kameda T
|
9
|
1,35%
|
73
|
Yan LG
|
9
|
1,35%
|
161
|
Yoshioka T
|
9
|
1,35%
|
73
|
Zhang J
|
9
|
1,35%
|
225
|
Chen H
|
8
|
1,20%
|
90
|
Hayat T
|
8
|
1,20%
|
593
|
Li J
|
8
|
1,20%
|
95
|
3.5 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.
Table 3
Journals that have published the most on the subject and their impact factors.
Journal
|
Documents
|
IF*
|
Quantity
|
Percent
|
Chemical Engineering Journal
|
44
|
6.637%
|
13.273
|
Applied Clay Science
|
38
|
5.732%
|
5.467
|
Journal of Hazardous Materials
|
35
|
5.279%
|
10.588
|
Chemosphere
|
27
|
4.072%
|
7.086
|
Journal of Cleaner Production
|
18
|
2.715%
|
9.297
|
Colloids and Surfacesa Physicochemical and Engineering Aspects
|
14
|
2.112%
|
4.539
|
Science of the Total Environment
|
14
|
2.112%
|
7.963
|
Environmental Science and Pollution Research
|
13
|
1.961%
|
4.223
|
Journal of Environmental Chemical Engineering
|
13
|
1.961%
|
5.876
|
Desalination and Water Treatment
|
12
|
1.810%
|
1.254
|
Journal of Materials Chemistry A
|
9
|
1.357%
|
7.393
|
*Impact Factor 2022.
3.5 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 Figure 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; 'transversal and general themes') (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, 2011).
The detection of the most commonly used keywords in the retrieved documents, limited to the minimum number of 5 occurrences, are presented in Figure 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 high-frequency 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.
Table 4
Main keywords and their number of occurrences.
Keywords
|
Occurrences
|
Adsorption
|
199
|
Layered Double Hydroxides
|
129
|
Layered Double Hydroxide
|
67
|
Water Treatment
|
56
|
Wasterwater Treatment
|
54
|
Kinetics
|
18
|
Hydrotalcite
|
30
|
Sorption
|
25
|
Fluorede
|
23
|
Wasterwater
|
17
|
Phosphate
|
19
|
LDH
|
18
|
Biochar
|
15
|
Photocatalysis
|
17
|
Methyl Orange
|
16
|
Cr(VI)
|
13
|
Removal
|
13
|
Dyes
|
8
|
Equilibrium
|
6
|
Arsenic
|
15
|
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 Figure 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 (Silva et al., 2021). 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) (A. Li et al., 2020), evaluated the recyclability of the ZnAl adsorbent using sodium hydroxide for the adsorption-desorption experiments and, identified that the removal rate of the Congo red dye decreases from 98.01% (1st cycle) to 67.11% after four times of recycling.
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 Tolea et al. (2021) (Țolea et al., 2021), LDH -Mg3Al 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 Mg3Al with Methyl Trialkyl Ammonium Chloride (Mg3Al-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 (Mg3Al-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.
Table 5
Efficiency of LDHs of varied composition obtained by different synthesis methods, used as adsorbents in the removal of different pollutants.
Composition
|
Pollutant
|
Efficiency/adsorption capacity
|
Synthesis method
|
Regeneration cycles
|
References
|
MgFe-Cl
|
Chromates Sulfates
|
100% 93%
|
coprecipitation
|
-
|
(Matusik and Rybka, 2019)
|
MgFe- CO3-
|
Molybdenum
|
39.9 mg/g
|
coprecipitation
|
-
|
(Golban et al., 2019)
|
MgFe-Cl
|
Nitrate
|
18.17 mg/g
|
coprecipitation
|
-
|
(L. C. Santos et al., 2020)
|
MgAl-Cl
|
Bright Yellow
|
115.00 mg/g
|
hydrothermal
|
-
|
(Pourfaraj et al., 2017)
|
CaAl-NO3
|
Diy Sunset Yellow
|
398.41 mg/g
|
coprecipitation
|
-
|
(De Sá et al., 2013)
|
MgAl-CO3-
|
Methyl orange
|
197.62 mg/g
|
urea hydrolysis
|
-
|
(Zaghloul et al., 2020)
|
ZnAl- NO3
|
Congo Red diy
|
625.00 mg/g
|
coprecipitation
|
4
|
(Li et al., 2020)
|
ZnFe-NO3
|
Methyl orange Methyl blueMalachite green
|
230.68 mg/g 133.29 mg/g 57.34 mg/g
|
coprecipitation
|
-
|
(Mahmoud et al., 2021)
|
Mg3Al-NO3 Mg3Al-IL-US Mg3Al-IL-COS
|
Diclofenac
|
143 mg/g 217 mg/g 648 mg/g
|
coprecipitation ultrasson cossynthesis
|
-
|
(Țolea et al., 2021)
|
MgAl-Cl Gly-Cl-LDH Ala-Cl-LDH
|
Fosfate
|
63.2 mg/g 55.8 mg/g 58.2 mg/g
|
coprecipitation
|
-
|
(Zhang et al., 2022)
|
3.6 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 (M2+/M3+) 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.