Cation Exchange of Natural Zeolites: Worldwide Research
Abstract
Research on natural zeolites (NZ) has increased over the years, showing potential in different areas, and many of them involve cation exchange (CE), considered one of the essential properties of NZ. This work aims to identify studies’ cognitive structure based on the cation exchange capacity (CEC) of NZ through bibliometric analysis to evaluate scientific production, growth trend, and visualization through bibliometric maps using the VOSviewer software. All types of documents and all languages indexed in Scopus from 1970 to 2020 were considered for the database, obtaining 703 documents. The results indicate an increasing trend in CE annual publications in NZ. This analysis shows the most influential authors such as Daković, Wang and Colella, while the countries that stand out are China, Turkey and the United States. Besides, the bibliometric maps made it possible to understand the intellectual structure of this academic discipline, identifying areas of current and potential interest in this field of studies such as its application in medicine, agriculture, catalysts, heavy metal removal, wastewater treatment (WWT), bioremediation and construction. Finally, these studies showed trends in science and technology studies favoring environmental remediation and human health.
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sustainability Article Cation Exchange of Natural Zeolites: Worldwide Research Fernando Morante-Carballo 1,2,3 , Néstor Montalván-Burbano 1,4 , Paúl Carrión-Mero 1,5,* and Nathaly Espinoza-Santos 1,5,* Citation: Morante-Carballo, F.; Montalván-Burbano, N.; Carrión-Mero, P.; Espinoza-Santos, N. Cation Exchange of Natural Zeolites: Worldwide Research. Sustainability 2021,13, 7751. https://doi.org/ 10.3390/su13147751 Academic Editors: Jorge Sanjurjo-Sánchez, Carlos Alves and Carlos Figueiredo Received: 19 April 2021 Accepted: 2 June 2021 Published: 12 July 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Centro de Investigación y Proyectos Aplicados a las Ciencias de la Tierra (CIPAT), Campus Gustavo Galindo, ESPOL Polytechnic University, Km 30.5 Vía Perimetral, Guayaquil P.O. Box 09-01-5863, Ecuador; [email protected] (F.M.-C.); [email protected] (N.M.-B.) 2Facultad de Ciencias Naturales y Matemáticas (FCNM), Campus Gustavo Galindo, ESPOL Polytechnic University, Km 30.5 Vía Perimetral, Guayaquil P.O. Box 09-01-5863, Ecuador 3Geo-Recursos y Aplicaciones (GIGA), Campus Gustavo Galindo, ESPOL Polytechnic University, Km 30.5 Vía Perimetral, Guayaquil P.O. Box 09-01-5863, Ecuador 4Department of Economy and Business, University of Almería, Ctra. Sacramento s/n, La Cañada de San Urbano, 04120 Almería, Spain 5 Facultad de Ingeniería Ciencias de la Tierra (FICT), Campus Gustavo Galindo, ESPOL Polytechnic University, Km 30.5 Vía Perimetral, Guayaquil P.O. Box 09-01-5863, Ecuador *Correspondence: [email protected] (P.C.-M.); [email protected] (N.E.-S.); Tel.: +593-99-826-5290 (P.C.-M.); +593-96-839-3048 (N.E.-S.) Abstract: Research on natural zeolites (NZ) has increased over the years, showing potential in different areas, and many of them involve cation exchange (CE), considered one of the essential properties of NZ. This work aims to identify studies’ cognitive structure based on the cation exchange capacity (CEC) of NZ through bibliometric analysis to evaluate scientific production, growth trend, and visualization through bibliometric maps using the VOSviewer software. All types of documents and all languages indexed in Scopus from 1970 to 2020 were considered for the database, obtaining 703 documents. The results indicate an increasing trend in CE annual publications in NZ. This analysis shows the most influential authors such as Dakovi´c, Wang and Colella, while the countries that stand out are China, Turkey and the United States. Besides, the bibliometric maps made it possible to understand the intellectual structure of this academic discipline, identifying areas of current and potential interest in this field of studies such as its application in medicine, agriculture, catalysts, heavy metal removal, wastewater treatment (WWT), bioremediation and construction. Finally, these studies showed trends in science and technology studies favoring environmental remediation and human health. Keywords: natural zeolites (NZ); ion exchange (IE); cation exchange capability (CE); bibliometrics 1. Introduction In 1756, the first natural zeolite was discovered. However, in the late 1950s, their commercial development began [ 1 – 3 ]. They were discovered in extensive and exploitable deposits, in tufaceous sedimentary rocks, formed due to the alteration of volcanic ash in marine and lake waters [ 3 ]. However, natural zeolites included in different geological environments and their classification varies according to the models and criteria of each author. Still, many agree on four: diagenetic, metamorphic, hydrothermal and magmatic [ 4 ]. NZ are hydrated aluminosilicate minerals [ 5 – 8 ] that have a porous structure with important physicochemical properties, such as CE, molecular sieving, catalysis [ 9 ] and high adsorption capacity (AD-C) [ 8 ]. They became beneficial industrial minerals with superficial and structural properties applied in industrial, agricultural [ 10 ], environmental [ 11 ] and biological technology [12]. NZ have microporous crystalline structure that allows the release and incorporation of water and cationic species because they consist of diameters adjusted through inlet ports Sustainability 2021,13, 7751. https://doi.org/10.3390/su13147751 https://www.mdpi.com/journal/sustainability
Sustainability 2021,13, 7751 2 of 26 of their internal structure, while the larger species are excluded, such as the ion sieving properties [ 13 ]. According to the studies in [ 8 , 14 ], zeolites conserve a structure in a threedimensional framework of tetrahedra of SiO 4 and AIO 4 , whose aluminum ion occupies the position in the center of the tetrahedron of four oxygen atoms, this substitution of AI 3+ by Si 4+ defines the negative charge in the network, which balanced with the exchangeable cation (Na, K and Ca, which are generally in a higher proportion than Mg, Ba, Sr, among others). These internal cations of zeolites can exchange with the cations in their environment and retain them in their internal network, causing the removal of cations such as copper, lead, cadmium, ammonium and certain radioactive cations. Different factors influence the IE behavior of NZ, such as the structure, the size and shape of the ions, the charge density of the anionic structure, the ionic charge and the concentration of the external electrolyte solution [ 15 ]. Due to the formation environment, zeolites present variability in their chemical composition and the CEC, between 0.6 and 2.3 meq/g [ 8 ]. The CEC and selectivity are specific according to the type of zeolite. A prior elemental analysis must perform to obtain the expected CEC to characterize synthetic zeolites [16]. There are around 70 types of NZ, and more than 260 synthetic zeolites registered [ 17 ]. The most common forms are clinoptilolite, mordenite, phillipsite, chabazite, stilbite, analcime and laumontite, while offretite, paulingite, barrerite and mazzite are not often occurring [ 8 ]. Among them, clinoptilolite is the most abundant natural zeolite in nature and is widely used worldwide [8,18,19]. Some countries, e.g., Cuba, China, the United States, Russia, Japan, Italy, South Africa, Hungary and Bulgaria, have significant reserves with excellent production potential. However, the total amount of this mineral in the world is not exactly known [ 10 ]. In the Ecuadorian coastal region, zeolites were described for the first time in 1994 in the Cayo Formation by a group of Cuban researchers as being composed of marine volcanoclastic rocks [ 20 , 21 ], and their application has had a high impact on agriculture as fertilizer carriers [ 22 ]. In the outcrop area of the Cayo Formation, clinoptilolite, heulandite, mordenite, laumontite, analcime, stilbite, epistilbite, chabazite, thomsonite and erionite-type zeolites have been identified [21]. Due to the significant development of the CE and AD-C properties of NZ, its commercialization has shown significant progress, and it was considered a product that has great potential [ 23 ]. NZ most prominent commercial applications remove heavy metal ions from wastewater [ 24 – 26 ]. The selectivity of cations for CE varies according to the type of zeolite, since in the case of studies with clinoptilolite [ 27 – 29 ], it maintains a higher selectivity for Pb 2+ , while in other cases, clinoptilolite shows a higher selectivity for Cu 2+ [ 30 , 31 ] and Zn 2+ [ 32 ]. Likewise, studies have evaluated other types of zeolites, such as scolecite, with higher selectivity for Cu 2+ [ 33 ] and chabazite for Cu 2+ [ 34 ]. These variations in selectivity for one or another NZ cation are due to the Si/Al ratio of their structure, following the variability of the concentrations of their exchange cations. Likewise, NZ are used for the adsorption (AD) of ammonia in wastewater [ 35 , 36 ] and also of organic substances [ 37 , 38 ]. It is essential to highlight the reuse of NZ and its adsorbed components in agriculture as additional fertilizers to improve the quality and yield of crops [ 39 ]. Other possible uses of these minerals are in the construction industry [ 36 , 40 , 41 ], such as pozzolana cement [ 42 ], foamed geopolymers [ 43 ], oil spill cleaning [44], desiccants and gas-liquid separations [45], among others. Furthermore NZ can be reused, where contaminants are first removed and then recycled as a pozzolan addition for Portland Clinker [ 46 ]. In addition, studies have been performed for the regeneration of zeolites after being used for the removal of NH 4+ , using solutions of 1 N NaCl or KCl, replacing the exchange site with Na + or K + . This regeneration can increase its efficiency by raising the pH of the regeneration solution by adding lime [ 15 ]. Most zeolite applications have been implemented due to environmental concerns [ 47 ], mainly based on its CE properties, in areas such as nuclear wastewater [ 48 ], municipal and industrial wastewater [ 49 , 50 ], decontamination of mining effluents [ 51 ] and agricultural uses on contaminated soils [52].
Sustainability 2021,13, 7751 3 of 26 Therefore, the CE of NZ could be an economical, environmentally compatible and effective way for pollutant removal [ 53 ]. Besides, in recent decades, the application of NZ in medicine has begun to be inserted [ 54 ]. In 2004, the first study of the effects of clinoptilolite supplementation to treat immunodeficiency diseases was given [ 55 ]. Similarly, good results have been obtained from the application of NZ for the treatment of gastrointestinal and cancerous affections [56]. Likewise, studies have been carried out, such as the potential use of zeolites modified with Cu and Zn for the removal of ethylene and delayed ripening of the tomato fruit, improving its postharvest quality [ 57 ], as well as the possible use of chemically modified natural zeolites in the production of biodiesel [ 58 ], and the potential use in the pharmaceutical industry [ 59 ]. In a previous study [ 60 ] regarding zeolite research areas through a citation network analysis, IE scored very high. Therefore, it is essential to know how studies based on this field have developed over the years. However, there is no research from a bibliometric perspective on the CEC in NZ. Bibliometry gives the possibility of studying a specific research area to academics, using the analysis of citations, co-citations, geographical distribution and frequency of words to draw beneficial conclusions [ 61 ]. Bibliometric methods have been used to estimate scientific progress in various science and engineering disciplines as a standard research tool for systematic analysis [ 62 ]. Will it be possible to know, through the application of bibliometric analysis, the disciplines that had a more significant impact in this field of research? Who are the experts who stand out in this field of study? What has been the development of the intellectual structure of this field of research over time? This work aims to identify the different research lines that make up the cognitive structure of studies based on the CEC of NZ, performing a bibliometric analysis to evaluate scientific production, growth trend and visualization through bibliometric maps using the VOSviewer software contributing to the development of this field of scientific research. 2. Materials and Methods A literature review is very important for any research project [ 63 ]. The systematic review of the literature is a methodologically rigorous review of all the available research [ 64 ], making it possible to identify, evaluate and interpret the results with the most significant relevance of research for decision-making [ 65 , 66 ]. Bibliometric studies have a formal and rigorous process similar to the literature’s systematic review, which guarantees the quality of the information used [67]. Bibliometric analysis has become a necessary tool for measuring scientific progress in any study area [ 62 ]. Bibliometric studies evaluate scientific production’s quantity and quality [ 68 – 70 ]. Thus, sets of statistical and mathematical indices used are the so-called bibliometric indicators essential for the individual researcher and organizations [ 71 , 72 ]. These can be indicators of quantity, which evaluate productivity; quality indicators, which evaluate scientific production performance, and structural indicators, which evaluate the net between publications, authors and research areas [ 73 , 74 ]. Therefore, it is essential to establish a research methodology to carry out a bibliometric analysis in a research field, integrating knowledge and understanding its evolution and trends [75,76]. The methodology applied in this work developed in four phases: (1) Definition of search criteria; (2) Data collection; (3) Export and standardization of data; (4) Data analysis. Figure 1details the graphic scheme of the phases above, which were implemented in this work.
Sustainability 2021,13, 7751 4 of 26 Sustainability2021,13,xFORPEERREVIEW4of28 Figure1.Diagramofthemethodologyappliedinthisstudy. 2.1.DefinitionofSearchCriteria Thetermsionexchange,cationexchangeandnaturalzeolitewereconsideredcriteria forthesearch,whicharepartofthekeywords,andinturn,thewordsclayandsynthetic zeolitewereexcludedtoperformamorespecificsearch.Thesecriteriamadeitpossibleto compilethedatabasetoevaluatethisstudy,whichwaschosenbasedontheauthors’own researchexperienceandbibliographicreviewswhichallowedexcludingtermsfromsim‐ ilartopics(clay,syntheticzeolite)todefinethisfieldofstudy.Furthermore,througha previousstudy[60],cationicexchangeinzeoliteswasproposedasahighimpactresearch area. 2.2.DataCollection Studiesusingbibliometricmethodsrequiretheuseofaqualitydatabaseandcon‐ sistentinformation[77].Scopuswaselectedasamultidisciplinarydatabaseforthefol‐ lowingreasons:(i)avastcollectionofdocumentsinmostacademicdisciplines[78],in‐ cludingearthsciences[79];(ii)containsdocumentsthathaveundergonearigorouscon‐ tentandqualityselectionprocess;(iii)theuseofqualitystandardssuchasScimagoJour‐ nalRank(SJR)[80]and(iv)theabilitytoviewdata,performanalysesanddownloadin‐ formation[81,82]. TheanalyzeddatabaseconsistedofdocumentsindexedbyScopusfrom1970tothe present(22September2020).Forthisstudy,alltypesofdocumentsandlanguageswere included. Asearchstrategywasusedinwhichthedefinedcriteriawasconsideredinthetitles, abstract,andkeywords.Forthis,thefollowingsearchequationwasused:(TITLE‐ABS‐ KEY(“ionexchange”)ORTITLE‐ABS‐KEY(“cationexchange”)ANDTITLE‐ABS‐KEY (“naturalzeolit*”)ANDNOTTITLE‐ABS‐KEY(clay)ANDNOTTITLE‐ABS‐KEY(“syn‐ theticzeolit*”)).Theasteriskwasimplementedin“zeolit*”toallowallvariantsofthe searchterm[83],whichwouldincludea“zeolite*”and“zeolitic*”inthedatabasecol‐ lected,[60],resultingin710documentsfoundintotal. Figure 1. Diagram of the methodology applied in this study. 2.1. Definition of Search Criteria The terms ion exchange, cation exchange and natural zeolite were considered criteria for the search, which are part of the keywords, and in turn, the words clay and synthetic zeolite were excluded to perform a more specific search. These criteria made it possible to compile the database to evaluate this study, which was chosen based on the authors’ own research experience and bibliographic reviews which allowed excluding terms from similar topics (clay, synthetic zeolite) to define this field of study. Furthermore, through a previous study [60], cationic exchange in zeolites was proposed as a high impact research area. 2.2. Data Collection Studies using bibliometric methods require the use of a quality database and consistent information [ 77 ]. Scopus was elected as a multidisciplinary database for the following reasons: (i) a vast collection of documents in most academic disciplines [ 78 ], including earth sciences [ 79 ]; (ii) contains documents that have undergone a rigorous content and quality selection process; (iii) the use of quality standards such as Scimago Journal Rank (SJR) [ 80 ] and (iv) the ability to view data, perform analyses and download information [81,82]. The analyzed database consisted of documents indexed by Scopus from 1970 to the present (22 September 2020). For this study, all types of documents and languages were included. A search strategy was used in which the defined criteria was considered in the titles, abstract, and keywords. For this, the following search equation was used: (TITLE-ABS-KEY (“ion exchange”) OR TITLE-ABS-KEY (“cation exchange”) AND TITLE-ABS-KEY (“natural zeolit*”) AND NOT TITLE-ABS-KEY (clay) AND NOT TITLE-ABS-KEY (“synthetic zeolit*”)). The asterisk was implemented in “zeolit*” to allow all variants of the search term [ 83 ], which would include a “zeolite*” and “zeolitic*” in the database collected, [ 60 ], resulting in 710 documents found in total. 2.3. Data Export Subsequently, the database obtained from Scopus exported in CSV format (commaseparated values), which included all bibliographic information, abstracts, years, keywords and language used for the bibliometric analysis [61,84].
Sustainability 2021,13, 7751 5 of 26 Once this information was obtained, a data cleaning process is required since they usually contain errors or incomplete data [ 85 , 86 ]. A manual review of authors’ data, journal titles or affiliations, types of documents, languages and year of publication was performed using Microsoft Excel software. In this data normalization, records without the author’s name available, language and type of document were found, which restored with their corresponding information, and those without data were eliminated. Finally, 703 documents were obtained and processed. 2.4. Data Analysis and Results In bibliometric studies, two types of analysis should be considered: the performance analysis of scientific production and the structure analysis [76,87]. The first allows evaluating the development of scientific production and its impact. The scientific production is examined based on the contribution of the most cited authors, countries, institutions, journals and cited documents [ 88 , 89 ]. Microsoft Excel was used for its versatility for the exploration and analysis of the information contained [90]. The second allows the analysis of the intellectual structure of the study field through recognized bibliometric networks, such as author occurrence maps, author citations and journals [ 91 , 92 ]. While the VOSviewer software was used to elaborate and visualize the bibliometric maps, it also allows the construction, exploration and graphic representation of two-dimensional maps of simple interpretation [ 93 , 94 ], a combination of three analyses used to understand the intellectual structure of this field of study. The analyses are: co-occurrence of author keywords, co-citation of cited authors and journals, which allow obtaining information at micro, meso and macro levels, respectively, of this structure [ 91 , 95 ]. These analyses require data pre-processing to eliminate errors and inconsistencies [ 85 , 86 ]. The VOSviewer has made a notable contribution to the development of bibliometric analyses in a wide variety of study areas: earth sciences [ 96 , 97 ], education [98,99], medicine [100,101] and food chemistry [102], among others. 3. Results 3.1. Performance Analysis 3.1.1. Scientific Production Analysis This analysis has been divided into four time periods: period I (1970–1990), period II (1991–2000), period III (2001–2010) and period IV (2011–2020) (Figure 2). It considered dividing the times into decades, as it granted the best representation to know the evolution of the field of study [ 103 ]. Likewise, two decades (1970–1990) have joined the beginning due to the small number of documents analyzed in these years. Additionally, Price’s Law, applied as an indicator in the analysis of this study’s productivity, reflects an elementary aspect of scientific production and exponential growth [ 104 , 105 ] (Table 1). A total of 703 documents were obtained, corresponding to 84.35% articles, 11.66% conference papers, 2.28% book chapters, 1.14% article reviews, 0.14% errata, 0.14% notes, 0.14% books and 0.14% editorials. Table 1. Growth trajectory of the scientific production of studies based on CE of NZ. Periods ND % CC % Price’s Law I 48 6.83% 751 3.61% y=1.11e0.0844x II 93 13.23% 4203 20.23% III 229 32.57% 11731 56.45% R2=0.8725 IV 333 47.37% 4095 19.71% Abbreviations: ND = Number of documents; CC = Citation count.
Sustainability 2021,13, 7751 6 of 26 Sustainability2021,13,xFORPEERREVIEW6of28 Figure2.Scientificproductivityfrom1970to2020oftheCEinNZ:Numberofdocumentsandcitations. Table1.GrowthtrajectoryofthescientificproductionofstudiesbasedonCEofNZ. PeriodsND%CC%Price’sLaw I486.83%7513.61%𝑦 1.11𝑒. II9313.23%420320.23% III22932.57%1173156.45%𝑅 0.8725 IV33347.37%409519.71% Abbreviations:ND=Numberofdocuments;CC=Citationcount. 1. PeriodI(1970–1990):OpeningoftheCEofNZ Thisperiodshowsthebeginningofthescientificproductionofthisfieldofstudy withgradualgrowth,inwhichthefirsttwodecadesgroupedsincebetweenthem,there wasnonotablegrowthinscientificproductionwithatotalof48documents,whichrep‐ resents6.83%ofthetotal,whichincluded40articles,7conferencepapersand1book, wherethehighestproductionrecordedwasbetween1984and1988.Inthisperiod,the citationsobtainedwere751(3.61%),witharticlesstandingout[106],inwhichtheydeter‐ minedtheimportanceoftheconditioningprocedureinzeolites,whichinfluencetheper‐ formanceforthecapacityandselectivityformetalions.Likewise,atthebeginningofthis researcharea,otherdocumentswererecordedthatcovertopicssuchastheeliminationof ammoniafromwastewaterusingclinoptilolitethroughitsselectiveIEprocess[107],and inthesameway,theeliminationofammoniumionsandphosphateandnutrientrecovery inwastewaterusingclinoptiloliteandKastelA510,ananionresinthathasADproperties [108],amongothers. 2. PeriodII(1991–2000):DevelopmentofCEofNZ Inthisperiod,anotableincreasewasobservedinasingledecadewith93documents (13.23%),amoresignificantnumberofarticle‐typepapers,including85articles,7confer‐ encepapersand1review.In1999wasthehighestproductionofthisperiodwith23doc‐ umentscontainingtopicssuchasevaluatingzeolites’potentialtoremoveheavymetals [109].Likewise,thenumberofcitationsincreasedconsiderablyto4203(20.23%),which indicatesthebeginningofinterestinthisfieldofstudy,highlightingtheresearchinwhich Figure 2. Scientific productivity from 1970 to 2020 of the CE in NZ: Number of documents and citations. 1. Period I (1970–1990): Opening of the CE of NZ This period shows the beginning of the scientific production of this field of study with gradual growth, in which the first two decades grouped since between them, there was no notable growth in scientific production with a total of 48 documents, which represents 6.83% of the total, which included 40 articles, 7 conference papers and 1 book, where the highest production recorded was between 1984 and 1988. In this period, the citations obtained were 751 (3.61%), with articles standing out [ 106 ], in which they determined the importance of the conditioning procedure in zeolites, which influence the performance for the capacity and selectivity for metal ions. Likewise, at the beginning of this research area, other documents were recorded that cover topics such as the elimination of ammonia from wastewater using clinoptilolite through its selective IE process [ 107 ], and in the same way, the elimination of ammonium ions and phosphate and nutrient recovery in wastewater using clinoptilolite and Kastel A510, an anion resin that has AD properties [ 108 ], among others. 2. Period II (1991–2000): Development of CE of NZ In this period, a notable increase was observed in a single decade with 93 documents (13.23%), a more significant number of article-type papers, including 85 articles, 7 conference papers and 1 review. In 1999 was the highest production of this period with 23 documents containing topics such as evaluating zeolites’ potential to remove heavy metals [ 109 ]. Likewise, the number of citations increased considerably to 4203 (20.23%), which indicates the beginning of interest in this field of study, highlighting the research in which AD experiments were carried out, where improved elimination of inorganic oxyanions from aqueous solution were obtained [6]. 3. Period III (2001–2010): Progress of CE in NZ In this following decade, the CE’s scientific production in NZ continues, with an exponential growth reaching 229 documents (32.57%), highlighting the number of articletype documents, including 196 articles, 25 conference papers, 4 book chapters, 3 reviews and 1 erratum. The most significant increase in citations was recorded in this period, obtaining 11,731 citations, representing 56.47% of the total, reaching the peak with 2457
Sustainability 2021,13, 7751 7 of 26 citations in 2004. In 2010, the review document was published [ 8 ], which obtained the most significant impact in this decade with 1147 citations; in this document, the development of NZ as adsorbents used in water and WWT were reviewed. Other documents registered in this period were the article that deals with the use of agricultural and agrochemicals of clinoptilolite [ 52 ], and the importance of NZ applications for environmental use, which shows the potential of the zeolite as adsorbent material [110]. 4. Period IV (2011–2020): The advance of CE in NZ Finally, there is the highest scientific production in Period IV with 333 documents, which is equivalent to 47.37% of the total, with 272 articles, 12 book chapter, 43 conference papers, 4 reviews, 1 note and 1 editorial. While in the types of languages, the English language had the most significant influence with 313 documents. However, documents were also recorded in Chinese, Portuguese, Persian, Polish, Portuguese, Russian, Spanish and Turkish. Additionally, in the last decade, 4095 citations (19.71%) were registered, highlighting the article [ 11 ], which shows the importance of the different applications of NZ based on their CE properties, making a brief review of the literature on the application of NZ in environmental remediation. 3.1.2. Country Contribution The activity, productivity and impact of scientific research can be promoted through research contribution. Therefore, it is necessary to have regular quantitative monitoring of supplies and results through bibliometric studies [ 111 ]. Seventy-one countries have developed a contribution to this field of study. Table 2shows the scientific production of the 15 leading countries in this study area, where China leads the table with 70 documents, followed by Turkey and then the United States. In addition, these countries are among the largest producers of zeolites, with China standing out with 1,700,000 tons, the United States with 380,000–430,000 tons and Turkey with 150,000 tons [ 4 ]. However, according to the number of documents/number of citations [ 75 ], Australia, with 29 documents is the most cited country with 80.5 citations/documents, while the Russian Federation, with 24 documents is the least cited country with just 3 citations and 8 documents. Table 2. Top 15 countries with the highest scientific citations. Rank Country ND CC CA 1 China 70 2695 38.5 2 Turkey 58 2752 47.4 3 United States 54 2359 43.7 4 Italy 48 1262 26.3 5 Japan 38 830 21.8 6 Mexico 37 932 25.2 7 Iran 34 970 28.5 8 Serbia 31 657 21.2 9 Australia 29 2334 80.5 10 South Korea 27 785 29.1 11 Romania 24 193 8.0 12 Russian Federation 24 90 3.8 13 Greece 22 1123 51.0 14 Croatia 21 1213 57.8 15 Slovakia 18 241 13.4 Abbreviations: ND = Number of documents; CC = Citation count; CA = Citation average. Moreover, this contribution of countries was visualized through a co-authorship network map (Figure 3) using VOSviewer software, where the nodes represent the countries that develop this field of study, and their size, according to the number of documents they need and the thickness of the lines they interconnect, represents the collaboration’s strength [ 92 ]. For elaborating the map, the countries with at least 5 contributions were
Sustainability 2021,13, 7751 8 of 26 used for better visualization, giving 38 countries. The nodes with the largest size, such as China, Turkey, United States and Italy, have a more significant contribution of documents: 70, 58, 54, 48, respectively. With the highest contribution, China is in cluster 2 (green color) and has a close relationship with South Korea, India and Jordan. Continuing with Turkey, which is in second place in scientific production, it is closer to Poland, Brazil and Slovakia; that is, it has a good relationship. While the United States, which is in cluster 1 (red), has a close relationship with Mexico, the United Kingdom and Slovakia. At the same time, Hungary has less research collaboration with other countries. Sustainability2021,13,xFORPEERREVIEW8of28 Table2.Top15countrieswiththehighestscientificcitations. RankCountryNDCCCA 1China70269538.5 2Turkey58275247.4 3UnitedStates54235943.7 4Italy48126226.3 5Japan3883021.8 6Mexico3793225.2 7Iran3497028.5 8Serbia3165721.2 9Australia29233480.5 10SouthKorea2778529.1 11Romania241938.0 12RussianFederation24903.8 13Greece22112351.0 14Croatia21121357.8 15Slovakia1824113.4 Abbreviations:ND=Numberofdocuments;CC=Citationcount;CA=Citationaverage. Figure3.ThenetworkofcollaborationbetweencountriesinthefieldofstudyofCEofNZ. Figure 3. The network of collaboration between countries in the field of study of CE of NZ. 3.1.3. Performance of Sources For this analysis, the performance of sources of all types of documents were considered in this study. Table 3shows the 15 most prominent sources based on the number of published documents and their percentage, with 184 documents representing 26.29% of the total. The number of citations and performance indicators were shown for each source, such as H-index, SJR 2019. Leading the Top 15 was the Journal of Hazardous Materials with 30 documents representing 4.29%. It has an H 260 index, SJR 2.010, and the document that stands out the most in this journal corresponds to [ 112 ] with 287 citations related to the use of the NZ for the removal of ammonia from an aqueous solution. Continuing was the journal Microporous and Mesoporous Materials with 24 documents (3.43%); 966 citations present an H-index of 151 and SJR 0.999. Then, the sources that take possession are Studies in Surface Science and Catalysis,Science and Technology of Separation and Desalination and Water Treatment.
Sustainability 2021,13, 7751 9 of 26 Table 3. Top 15 of the most prominent sources by the number of documents in this field of study. Sources ND % CC CS SJR HI Journal of Hazardous Materials 30 4.29% 2220 13.1 2.010 260 Microporous and Mesoporous Materials 24 3.43% 966 7.7 0.999 151 Studies in Surface Science and Catalysis 13 1.86% 54 0.5 0.124 60 Separation Science and Technology 12 1.71% 473 2.6 0.374 73 Desalination and Water Treatment 12 1.71% 67 2.7 0.327 51 Water Science and Technology 12 1.71% 479 2.9 0.471 131 Water Research 11 1.57% 1671 14.5 2.932 285 Journal of Environmental Science and Health—Part A Toxic/Hazardous Substances and Environmental Engineering 11 1.57% 149 2.8 0.478 67 Clay Minerals 10 1.43% 49 3.1 0.428 65 Handbook of Natural Zeolites 10 1.43% 20 - - - Journal of Colloid and Interface Science 9 1.29% 1571 11 1.450 225 Environmental Engineering and Management Journal 8 1.14% 105 1.8 0.322 33 Journal of Radioanalytical and Nuclear Chemistry 8 1.14% 130 2.3 0.360 65 Separation and Purification Technology 7 1.00% 651 8.3 1.209 155 Applied Clay Science 7 1.00% 384 7.6 1.069 119 Sum of Top 15 184 26.29% 8989 Total documents of the study field 703 100% 20,780 Abbreviations: ND = Number of documents; % = Contribution percentage; CC = Citation count; CS = CiteScore; SJR = SCImago Journal Rank; HI = H-index. 3.1.4. Author Contribution In general, in 2054 documents, authors dedicated to studies related to the NZ CE were obtained. Table 3shows the first 15 authors with the highest contribution of documents in the study area. Leading the investigation was Dakovi´c with 12 documents, followed by Colella and Gennaro with 10 and 9 documents, respectively. However, Stevens showed a higher number of citations than 262 with 8 documents in this Top 15 concerning the number of documents. Figure 4shows an author contribution map using the bibliographic coupling analysis. There is a bibliographic coupling between two publications when a third publication is cited by both publications [ 113 ]. This bibliographic coupling relationship between the two publications will be more significant than the number of references they have in common [ 93 ]. For this visualization, a minimum number of contributed documents were established, 5 for each author, resulting in 42 authors grouped in 6 clusters. The nodes represent the authors who contributed to this field of study; the size depends on the number of documents; the thickness of the line with which they interconnect represents the strength of collaboration. Dakovi´c, Colella, Gennaro, Raji´c, Dong and Inglezakis, among others, are found in the most significant nodes, which coincide with the Top 15 of the authors in Table 4. Table S1 shows the Top 15 of the authors’ contributions from a different perspective, such as the number of citations, where new authors appear with fewer documents referring to the field of study but have a more significant influence on citations. Leading the investigation was Wang, with only 2 documents but 1431 citations. Then there is Peng and Donat, with 1147 and 1065 citations, respectively.
Sustainability 2021,13, 7751 16 of 26 must be considered for the publication of articles [ 173 , 174 ]. In the journal co-citation network analysis, if two journals are co-cited, at least one article from each journal must be present in the references of a citing article [175]. Table 8shows the 15 journals that lead to the highest number of co-citations. For elaborating the bibliometric map, a minimum number of 20 citations per journal was considered, obtaining a total of 67, which grouped into 5 clusters. Figure 7shows the 68 nodes representing the journal names, grouped by color for each cluster, connected by co-citation links. Table 8. The Top 15 journals with the highest number of co-citations. Journal Co-Citation Journal of Hazardous Materials 812 Water Research 737 Microporous and Mesoporous Materials 534 Chemical Engineering Journal 312 Journal of Colloid and Interface Science 292 Environmental Science & Technology. 243 Applied Clay Science 225 Desalination 218 Water Science Technology 206 Sep. Purif. Technol. 204 Zeolites 141 Chemosphere 139 Bioresour. Technol. 136 Separation Science and Technology 127 American Mineralogist 105 Sustainability2021,13,xFORPEERREVIEW18of28 Figure7.Journals’co‐citationnetworkmap. Cluster1(redcolor)“SynthesisandcharacterizationofNZ”composedof25nodes, highlightingthejournalMicroporousandMesoporousMaterials(534co‐citations;H‐index 151),mainlycoveringthetopicsofnovelanddistinctiveaspectsofporoussolids,suchas synthesisandphysical‐chemicalcharacterization,amongothers.Theyarecontinuingwith otherrepresentativejournalssuchasAppliedClayScience(225co‐citations;H‐index119) andZeolites(141co‐citations;H‐index43). Cluster2(greencolor)“Applicationsinwaterquality”,groups17nodes;amongthe newspapersthatstandoutthemostinthisgroupisWaterResearch(737co‐citations;H‐ index285),whichincludestopicsofscienceandtechnologyaspectsaboutwaterquality andmanagement.ItwasfollowedbyEnvironmentalScience&Technology(243co‐citations; H‐index373)andWaterScienceandTechnology(206co‐citations;H‐index131). Cluster3(bluecolor)“Applicationsinhealthandenvironment”with12nodes,con‐ tainedthejournalwiththehighestnumberofco‐citations,JournalofHazardousMaterials (812co‐citations;H‐index260),whichpublishesissuesrelatedtotheunderstanding,im‐ pactassessment,andmitigationofthedangersandrisksthatcertainmaterialscangener‐ ateforhealthandtheenvironment.Likewise,otherjournalswereChemicalEngineering Journal(312co‐citations;H‐index198)andDesalination(218co‐citations;H‐index169) standout. Then,cluster4(yellowcolor)“Technologicalapplications”,wasoneoftheminor groupsmadeupofsevennodes,wherethemostprominentjournalsareJournalofColloid andInterfaceScience(218co‐citations;H‐index169),TotalEnvironmentalScience(72co‐cita‐ tions;H‐index224)andAppliedSurfaceScience(56co‐citations;H174index). Finally,cluster5(purplecolor)“NZcatalysis”,withonlysixnodes,isledbyJournal ofCatalysis(85co‐citations;H‐index231),CatalysisToday(58co‐citations;H‐index201)and AppliedCatalysisB:Environmental(31co‐citations;H‐index229). Figure 7. Journals’ co-citation network map. Cluster 1 (red color) “Synthesis and characterization of NZ” composed of 25 nodes, highlighting the journal Microporous and Mesoporous Materials (534 co-citations; H-index 151), mainly covering the topics of novel and distinctive aspects of porous solids, such as
Sustainability 2021,13, 7751 17 of 26 synthesis and physical-chemical characterization, among others. They are continuing with other representative journals such as Applied Clay Science (225 co-citations; H-index 119) and Zeolites (141 co-citations; H-index 43). Cluster 2 (green color) “Applications in water quality”, groups 17 nodes; among the newspapers that stand out the most in this group is Water Research (737 co-citations; H-index 285), which includes topics of science and technology aspects about water quality and management. It was followed by Environmental Science & Technology (243 co-citations; H-index 373) and Water Science and Technology (206 co-citations; H-index 131). Cluster 3 (blue color) “Applications in health and environment” with 12 nodes, contained the journal with the highest number of co-citations, Journal of Hazardous Materials (812 co-citations; H-index 260), which publishes issues related to the understanding, impact assessment, and mitigation of the dangers and risks that certain materials can generate for health and the environment. Likewise, other journals were Chemical Engineering Journal (312 co-citations; H-index 198) and Desalination (218 co-citations; H-index 169) stand out. Then, cluster 4 (yellow color) “Technological applications”, was one of the minor groups made up of seven nodes, where the most prominent journals are Journal of Colloid and Interface Science (218 co-citations; H-index 169), Total Environmental Science (72 cocitations; H-index 224) and Applied Surface Science (56 co-citations; H 174 index). Finally, cluster 5 (purple color) “NZ catalysis”, with only six nodes, is led by Journal of Catalysis (85 co-citations; H-index 231), Catalysis Today (58 co-citations; H-index 201) and Applied Catalysis B: Environmental (31 co-citations; H-index 229). 4. Discussion This work shows an increase in scientific research on CE in NZ by the increased demand for adsorbent materials and low-cost IE, used for energy development, pollution control and metal removal, among other applications [27]. In the analysis of scientific production, a coefficient of de R2= 0.8725 was obtained according to Price’s Law, and the field of study of the CE in NZ is exponential (Figure 2). In period I of the opening of the CE in NZ, two decades were analyzed (1970–1990), since there is a lower amount of production (6.83%), observing the beginning of investigations about the properties of IE in NZ and its different applications, as well as WWT, agricultural use, paper product and cement, among others [ 176 ]. However, the NZ did not achieve this success despite the different applications’ proposals because their development was affected by commercial efforts that tried to sell NZ without sufficient studies for their intended use. Due to this, Mumpton, in 1988, proposed the determination of properties of IE, AD, hydration, catalysis and reaction mechanisms for the implementation of marketing strategies for the company [177]. The increase in scientific research is reflected over the years, obtaining in period II (1991–2000) 13.23% in a single decade. The beginning of interest in this field of study shows the significant increase in the number of citations (20.23%), highlighting the importance of the ability to control the properties of NZ at the molecular level through the discovery of new materials and advances in technology that improve these processes [ 178 ]. Likewise, in period III (2001–2010), 32.57% of the production was recorded, and the highest number of citations was obtained (54.47%). Period IV (2011–2020), corresponding to the most recent time period, the highest amount of scientific production was obtained with 333 documents (47.37%), highlighting the application of NZ in environmental remediation [ 11 ]. Its application in medicine is beginning to have a greater interest [ 179 ], such as its benefit in nutrition due to improved supply of minerals [56]. In these 50 years of research in CE in NZ, it was observed that the most significant contribution to scientific production corresponds to articles (84.45%), and in terms of language, English (92.46%) dominates this study field. Furthermore, the potential of the different types of zeolites and their application, as well as clinoptilolite [ 180 ], mordenite, chabazite [ 181 ] and phillipsite [ 182 ] have been evaluated. Clinoptilolite stands out for its significant presence and ideal structure for the AD and IE processes [52].
Sustainability 2021,13, 7751 18 of 26 A total of 71 countries obtained have contributed to this field of study (Figure 2), highlighting China with 70 documents and 1695 citations, where the use of zeolitic tuffs as cement additives is popular [ 183 , 184 ], with a production of 1,700,000 tons [ 4 ]. Followed by Turkey, whose presence is widespread, with an estimated 50 billion tons of NZ reserve, clinoptilolite being an essential mineral in this country [ 185 ], with a production of 150,000 tons [ 4 ]. China has had a greater collaboration with South Korea, Japan, Jordan and Turkey, corresponding to group 2, which are important producing countries of NZ [ 17 ]. Likewise, the commercial use of NZ has been developing in the United States, Italy, Mexico, Bulgaria and Germany [ 27 ]. The United States demonstrated a close relationship with Mexico and the United Kingdom (cluster 1), while Italy is a little further away in cluster 5. There is also Hungary, which is the country with a minor collaboration in research with others. The analysis of the authors will verify the contribution of 2054 researchers, highlighting Dakovi´c with 12 documents, 374 citations and H-index 23; among their works, the study of the MZ predominates, evaluating its potentiality and application [ 186 – 189 ], demonstrating NZ’s higher efficacy in AD. However, it is curious to observe that among the authors that lead the Top 15 according to the number of citations, that they have a minimum value of documents, such as Wang, S., with 2 documents, 1431 citations and Hindex 113, and Peng with 1 document, 1147 citations and H-index 18, standing out for his work related to the use of NZ as AD for the treatment of drinking water and wastewater [ 100 ]. The number of citations depends on several factors, such as the article’s quality, the impact factor of the journal, the author’s reputation, and the broad scope of the field of study [ 190 ]. Therefore, it can verify that the work of Wang, S., stands out for its relevant reputation in citations, despite having a minimal number of documents than Dakovi´c, who has had a more significant contribution to scientific research in this field of study. Concerning the analysis of this field of study’s intellectual structure, we have found some relevant data exposed below. The co-citation network analysis of cited authors (Figure 6) shows that these researchers grouped in clusters are related [ 152 ]. According to the number of co-citations, the most significant node represents Colella (284). Moreover, it has a more significant relationship with the other authors, not only with the members of cluster 5 (purple) “Properties and reactions of the CE in NZ”, but also with authors from other clusters such as Pansini who leads cluster 2 (green color) “Applications of CE and characterization of NZ”, contributing with an investigation of the use of chabazite for the elimination of lead from water [ 191 ]. That is, the close relationship not only implies belonging to the same cluster but also the size and proximity to which they are. Cluster 1 (red color) “Environmental applications” is grouped by Wang, S., Wang, Y., Ho and Turan, among others; their research shows a relationship for the use of NZ for removal of contaminants [ 139 – 141 ]. While cluster 2 (green color) presents a close relationship with cluster 3 (blue color), “Applications of clinoptilolite in CE”, in which topics related to the characterization and applications of NZ in CE stand out [ 50 ]. Cluster 7, “MZ process” (orange color), only shows a relationship with cluster 5 (purple color) “AD of metals”; these groups are the ones with a lower occurrence, showing studies related to the AD of zinc (Zn ), copper (Cu) and lead (Pb) by IE, giving a higher removal efficiency for Pb and Cu ions than for Zn ions [117]. In the analysis of the network of journal co-citations, the journals that have had the most significant influence in this field of study are evidenced (Table 8), highlighting the Journal of Hazardous Materials (812 co-citations; H-index 260), Water Research (737 cocitations; H-index 285) and Microporous and Mesoporous Materials (534 co-citations; H-index 151). Anthropogenic activities generate many pollutants to the environment related to environmental impact. NZ are presented as an alternative for the decontamination of the environment, using their properties, such as their high CEC, related to risk mitigation of hazardous materials. Additionally, it can show that the clusters are wholly differentiated because the analysis considers the number of co-citations obtained by each journal on the topic of CE in NZ (Figure 7). Moreover, in the analysis of sources’ performance,
Sustainability 2021,13, 7751 19 of 26 these journals’ influence in this scientific field can be corroborated due to their academic contribution, standing out in the Top 15 (Table 3). One of the limitations of this study was using a single database (Scopus) since this could omit specific significant contributions in this field of study published in other databases. Furthermore, only NZ was used, excluding the various studies on CE in synthetic zeolites. 5. Conclusions NZ have a wide field of applications due to their exceptional properties, mainly their high CEC. Through the analysis of the intellectual structure of this scientific field, it was possible to distinguish different lines of research related to CE in NZ, such as heavy metals removal [ 192 , 193 ], nutrient recovery [ 124 , 194 ], WWT [ 195 , 196 ], soil treatment [ 52 , 197 ], construction materials [ 43 , 198 ], nutrition and health [ 56 ] and feed additives [ 52 ], among others. Furthermore, NZ can be modified [ 14 , 134 ] and regenerated [ 199 , 200 ], increasing their efficiency in their CEC. About the analysis of scientific production, in period II (1991–2000), a notable rebound in this scientific field’s development begins (Figure 3); this is mainly due to the continuous discovery of new materials that have allowed improvements in processes and the development of new technologies. This field of study shows a growing trend in the scientific production of the CE in NZ, obtaining a total of 703 documents, which correspond primarily to articles (84.35%), conferences (11.66%), and to a lesser extent, other types of documents (3.98%). Most of these documents have been published in English. A total of 71 countries registered have contributed to this field of study, highlighting China with 70 documents, Turkey with 58 and the United States with 54, which are also part of the countries with the highest NZ production. Regarding the contribution of authors, a total of 2054 documents were obtained. According to the number of documents, Dakovi´c leads with 12 documents and 374 citations, while Wang, S., with 2 documents and 1431 citations stands out for the number of citations. In the analysis of the intellectual structure, the researcher, Colella, obtained the highest number of co-citations (284) and has a more significant relationship with the other authors, contributing with research on the evaluation of the properties, reactions and equilibrium of the CE in NZ, and its various applications. Among the sources that had a more significant influence on the field of study of the CE in NZ are Journal of Hazardous Materials (812 cocitations; 30 documents), Water Research (737 co-citations; 11 documents) and Microporous and Mesoporous Materials (534 co-citations; 24 documents). Regarding the analysis of future trends, the studies carried out by the CE on NZ have shown a trend in science and technology studies to benefit environmental sustainability and human health, considering NZ as economical, ecological, reusable and recyclable material. Such are the challenges of zeolites as catalysts, filter materials, medicines, pollutant removal, energy production, construction industry, agricultural and livestock uses. Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/su13147751/s1, Table S1: Top 15 of the most cited documents. Author Contributions: Conceptualization: F.M.-C., N.M.-B., P.C.-M., and N.E.-S.; methodology: N.M.-B., F.M.-C., and N.E.-S.; investigation: N.M.-B., N.E.-S., and F.M.-C.; writing—original draft preparation: N.M.-B. and N.E.-S.; writing—reviewing and editing: N.M.-B., F.M.-C., P.C.-M., and N.E.-S.; supervision: F.M.-C. and P.C.-M. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable.
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