Hydrogel Applicability for the Industrial Effluent Treatment: A Systematic Review and Bibliometric Analysis

In recent decades, hydrogels, as adsorption materials, have received important attention due to their characteristics and properties, such as mechanical strength, biocompatibility, biodegradability, swellability, and stimuli sensitivity. In the actual framework of sustainable development, it has been imperative to develop practical studies of hydrogels in the treatment of actual industrial effluents. Accordingly, the current work has, as its objective, to make evident hydrogels’ applicability in the treatment of actual industrial effluents. For this purpose, a bibliometric analysis and systematic review based on the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) method were conducted. The relevant articles were selected from the Scopus and Web of Science databases. Some important findings were that: (1) China is the leading country when it comes to hydrogel application in actual industrial effluents, (2) the motor studies are focalized on the treatment of wastewater by hydrogels, (3) the fixed-bed columns are suitable unit equipment for the treatment of industrial effluents of using hydrogels, and (4) the hydrogels show excellent adsorption capacities of ion and dye contaminants present in industrial effluents. In summary, since the implementation of sustainable development in 2015, the progress of practical hydrogel applications in the treatment of industrial effluent has been receiving more attention, and the selected studies demonstrate the implementation viability of these materials.


Introduction
The dumping of industrial wastewater, without previous treatment, into water bodies is an important source of environmental contamination. In particular, contamination through chemical products presents a public, environmental, and global preoccupation [1,2]. Discharging industrial effluents into rivers, lakes, rivulets, and wells could generate serious health problems for humans, animals, and the ecosystem [3].
Water consumption always means contamination of itself since when someone or some process (chemical, cosmetic, plastic, cement, agrochemical, textile, etc.) uses water, it will always be contaminated in the end; those industries generate big volumes of wastewater with different types of contaminants, which must be treated before dumping them in the environment [4][5][6][7]. Based on this premise, countries are aware of it and are taking political steps to protect hydric resources [8,9].
Over the years, countries have taken normative and legislative actions inside their territory, establishing and demanding the treatment of wastewater coming from any industrial sector before its final disposal in water bodies. For instance, Europe since the 1990s has developed committees for the generation of directives to implement zero wastewater discharge [4,10]. An important development on a global level happened on 25 September 2015 at a conference of the United Nations for sustainable development, where an agenda was established regarding production changes and waste management [11,12]. Among the Hydrogels are polymeric networks crosslinked and insoluble in water that are able to adsorb and keep a large amount of water in their structure. In addition, they are considered nontoxic materials, low cost, easy to prepare, eco-friendly and recyclable, and ideal for use as adsorbents of different elements or molecules. The tridimensional structure of hydrogels, whose polymeric chains and functional groups present a hydrophilic behavior able to adhere to some metallic ions or colorants for adsorption give them unique properties as adsorbent agents; hydrogels are principal candidates for contaminant removal from industrial effluents since their adsorption capacity could generate a large efficiency percentage; likewise, their control and reutilization give them advantages with respect to other adsorbents [25][26][27][28]. Most of the studies developed concerning hydrogels have focused on investigating the versatility of elements and compounds capable of being adsorbed by hydrogels, as well as improving their adsorption capacity under different environmental stimuli through the strengthening of their mechanical properties, incorporation of nanoparticles, and degree of biodegradability, among others. All of these studies have been conducted on a laboratory scale in a controlled environment with pre-established operating conditions. However, it is imperative to perform applicative studies to address the adsorption of the hydrogels in the treatment of current contaminants coming from industrial dumping since through these studies, the technical feasibility and environmental and economic impacts of hydrogels in the treatment of industrial pollutants can be evidenced. For these reasons, the present study carried out a systematic review to demonstrate the current state of the real applications of hydrogels in the industry and to show the audience the research and application scope that has been achieved in recent years.
Additionally, the present work has, as its target audience, professionals and researchers coming from the engineering area dedicated to the search for new technologies for the treatment of industrial effluents, which applies to the industrial level, to be viable in technical, economic, and environmental terms.
This article designed the research question based on the PICO strategy (P: problem; I: intervention; C: control; O: outcome) [29]. Consequently, the research question was: how effective are hydrogels as removal materials for the treatment of contaminants present in the actual industrial effluents? Furthermore, it established the goal: to evidence the applicability of hydrogels as adsorption materials to actual industrial effluents.

Materials and Methods
The present study was performed based on systematic review fundaments, and the relevant scientific evidence was evaluated and synthesized in accordance with PRISMA methodology (Preferred Reporting Items for Systematic Review and Meta-Analyses); likewise, the study realization lies in previous works with similar methodologies [30][31][32]. For the scientific article selections, principal databases about engineering were used: Scopus and Web of Science. In this way, the integrity and academic quality of the studies selected within a specific period of the years (2015-2022) were assured. The last consulted date was 26 October 2022. The literary research was conducted through a specific investigation using "titles"; likewise, the search codes were conformed to Boolean connectors for the prime source identifications. The keywords were compounded by the terms "Hydrogel" AND "Treatment OR Adsorption OR Water OR Tailing OR Effluent", and the exclusion of some terms was made "Drug OR Cancer OR Tumor OR Delivery" ( Table 2). The literary inquiry by "titles" allowed the recovery of the relevant articles, minimized sensibility loss, and gave thematic insight [33][34][35]. The retrieved articles from the literature search were analyzed using a bibliometric study (descriptive) to obtain metrics on publications, authors, countries, journals, and other parameters using the functions of the Bibliometrix R-package (http://www.bibliometrix. org (accessed on 13 November 2022) [36] of the free code software RStudio 4.2.1. [37] and through a systematic study based on the methodology PRISMA 2020 [38].
Subsequent to the bibliometric analysis, the articles' evaluations were made as a function of their pertinence to answer the investigative question. The Rayyan virtual platform was employed [39] as a first review of the articles regarding titles and abstracts, selecting articles whose topics were in concordance with the investigative question. Subsequently, an exhaustive review was made based on eligibility criteria, individual contribution studies, and aggregated results, which contribute new information about the main theme. All of this was done with the clear goal of minimizing bias.
The eligibility criteria (inclusion and exclusion) for the selection of relevant articles were made impartially and independently. The flow diagram for the included article selection in the systematic review based on the PRISMA methodology [40][41][42] is shown in Figure 1.

Results
The established study period (2015-2022) was chosen based on a significant global event, the United Nations Sustainable Development Summit, held in New York. This summit established guidelines for sustainable development regarding the three pillars, the

Results
The established study period (2015-2022) was chosen based on a significant global event, the United Nations Sustainable Development Summit, held in New York. This summit established guidelines for sustainable development regarding the three pillars, the environment, economy, and society.
These guidelines have environmental and social motivations because of global issues, such as global warming, famine, water scarcity, and discrimination. In this congress, 17 sustainable development goals were established for poverty eradication, hydric and biology sources protection, and human prosperity assurance without prejudice for the future generations' sources (Agenda 2030) [43].
Based on this, a bibliometric study was carried out to determine the direction of last year's studies and present the principal countries, authors, and journals regarding the development of hydrogels as adsorption materials. Likewise, through the bibliometric analysis, the impact of this milestone in the scientific community and how it has boosted the production of scientific articles were visualized; specifically for this study, articles related to the implementation of new technologies, such as hydrogels, for industrial wastewater treatment, were used.

Investigative Field Evolution Literature Development from 2000-2022
A relevant indicator of scientific advance regarding a specific theme is the number of published articles. Figure 2 shows the number of annual publications about the thematic "hydrogels applicative studies on the treatment of industrial wastewater" in the period from 2000 to 2022. In addition, this graphic arrangement allowed us to obtain quantitative information concerning scientific production and the annual increase rate.

Investigative Field Evolution
Literature Development from 2000-2022 A relevant indicator of scientific advance regarding a specific theme is the number of published articles. Figure 2 shows the number of annual publications about the thematic "hydrogels applicative studies on the treatment of industrial wastewater" in the period from 2000 to 2022. In addition, this graphic arrangement allowed us to obtain quantitative information concerning scientific production and the annual increase rate. The studies concerning hydrogels and their applications as adsorption materials in the treatment of industrial effluents had few relevant degrees and production for the first 10 years of the 21st century. However, at the beginning of 2012, the United Nations congregated a conference about sustainable development (Rio+20) [44] whose goal was concretizing a renewed political commitment for sustainable development, assessing the achieved progress and addressing new challenges, so that the 192 member countries signed a commitment to boost the established goals inside their territorial framework; as a consequence, the scientific production regard to hydrogels grew significantly, and this The studies concerning hydrogels and their applications as adsorption materials in the treatment of industrial effluents had few relevant degrees and production for the first 10 years of the 21st century. However, at the beginning of 2012, the United Nations congregated a conference about sustainable development (Rio+20) [44] whose goal was concretizing a renewed political commitment for sustainable development, assessing the achieved progress and addressing new challenges, so that the 192 member countries signed a commitment to boost the established goals inside their territorial framework; as a consequence, the scientific production regard to hydrogels grew significantly, and this fact was possible due to the characteristics of hydrogels that make them environmentally viable, economically attractive, and technologically efficient in technologies in concordance with the agreement established in Rio+20. The next years until 2015 maintained an important production of articles inside the scientific community for their properties and applications in different areas, such as cosmetics [45], personal care products [46], food [47,48], plastics [49,50], agriculture [51,52], sensors [53,54], etc. Afterwards, in 2016-2017 scientific article production fell slightly, and at the same time, the established agreements in the United Nations Sustainable Development Summit entered into force; however, this descent can be considered an impulse drive because in the subsequent years 2018-2022, studies on hydrogels as adsorption materials for the treatment of industrial effluents were in ascent, gaining a higher production than previous years.
Specifically, in 2019, we did not appreciate a significant advance in hydrogels and their industrial applications; nevertheless, in the subsequent years (2020-2021), scientific production grew considerably, above all, in 2021. These affirmations are interesting due to the global situation, the COVID-19 Pandemic, in which schools, universities, development, and investigation centers were closed. At present (2022), 151 scientific articles have been registered, which is higher than previous years (2021: 132 articles, 2020: 105 articles, 2019: 86 articles).
On the other hand, the most relevant keywords for each study year were selected, through which a first image about the annual predominated theme in the period 2015-2022 was generated. Then, it was observed in the last seven years that the articles have two keywords that are the most recurrent: the first makes reference to the study object (hydrogels), and the second makes reference to the application of the study subject (adsorption). With these two terms, it was possible to make a first global concept about the thematic of hydrogels address in the selected articles.
In addition, particular keywords for each study year showed which specific study approach was developed in that year. Thus, in the 2015-2017 period, we had the following recurrent keywords: adsorption (41), hydrogel (41), kinetics (11), chitosan (7), heavy metal ions (6), swelling (6), adsorption kinetics (5), adsorption mechanism (5), and cellulose (5). The most relevant authors were as follows: Wang Y., Wang X., Zhang Y., Liu Y., Wang J, Li J., Wang H., Li Y., and Li X. The theme addressed by those articles in this period of years was focused on natural-based hydrogel synthesis and their ion heavy metal adsorption capacities. After this period of years, in 2018-2019, the published works were principally compounded by the following keywords: hydrogel (48), adsorption (43), chitosan (8), alginate (6), heavy metal ions (6), and methylene blue (5). The predominant authors in this lapse of time were Zhang Y., Cui W., Liu L., Liu Y., Zhu Y., Liang Y., Ma J., Wang X., An W., and Liu C. Based on the keywords, this period addressed themes similar to the period 2015-2017, except for the keyword methylene blue, which referred to the studies about industrial dyes. The remaining four years (2019-2022) of scientific production were represented by the following keywords: hydrogel (98), adsorption (85), chitosan (23), sodium alginate (17), graphene oxide (14), kinetics (9), desalination (7), polyacrylamide (7), water treatment (7), adsorption mechanism (6), composite hydrogel (6), Graphene (6), methylene blue (6), wastewater treatment (6), dye removal (5), and gelatin (5). The scientific development in this period of years was led by the following authors: Wang Y., Wang X., Zhang Y., Liu Y., Wang J., Li J., Wang H., Chen Y., Li Y., and Li X. This period of years represented the production of most scientific articles about the principal themes of this work; meanwhile, the most recurring keywords referred to natural and thermosensitive hydrogel synthesis reinforced with graphene oxides for treatment of wastewater contaminated with dyes, as well as water purification.

Global Evolution of the Most Relevant Countries, Sources, and Authors
In Figure 3, a three-field diagram (Sankey diagram) was presented to qualitatively observe the scientific publication density and the relationship between countries (left), journals (middle), and authors (right) in reference to the publications during the period 2015-2022. The network width about the publication density and the network route establishes the relationship between country, journal, and author. It was extracted from Figure 3 that the country that realized the most investigative production about hydrogel applications as adsorption materials of industrial contaminants was China, which produced almost 50% of published articles (570). Other relevant countries due to the productive mass of articles were India (61), Iran (49), USA (87), and Brazil (41). Likewise, the journals with the most density of published articles were the Chemical Engineering Journal (45), Journal of Environmental Chemical Engineering (30), and Desalination and Water Treatment (28 . On the other hand, in Figure 3, concerning the list of the most representative authors, Zhang Y. presented the highest number of produced articles (41), whose value of "articles fractionalized" was 7.2; additionally, the results regarding the authorship impact showed that Zhang Y. and Wang Y. had an ℎ equal to 17; however, Zhang Y. had a high score in to other impact indexes, such as the 41, the total number of citations = 1811, and publication number = 41. Figure 4 represents the complete spectrum of the author keywords as a function of term occurrences in the articles. The label size is proportional to the keyword density and frequency. Furthermore, two big conglomerates of keywords joined through networks and located on the top of the bibliometric map were observed, which present a high interrelationship among keywords that make up each conglomerate. Meanwhile, these two high sets were relatively founded near each other; on the other hand, three clusters were observed on the bottom of the map separate and away from the two principal sets of keywords. Additionally, these three clusters showed less density and frequency of keywords. It was extracted from Figure 3 that the country that realized the most investigative production about hydrogel applications as adsorption materials of industrial contaminants was China, which produced almost 50% of published articles (570). Other relevant countries due to the productive mass of articles were India (61), Iran (49), USA (87), and Brazil (41). Likewise, the journals with the most density of published articles were the Chemical Engineering Journal (45), Journal of Environmental Chemical Engineering (30), and Desalination and Water Treatment (28). Concerning the h-index, the Chemical Engineering Journal had the most value with an h index = 24, namely, at least 24 published articles in this journal were cited 24 times; other relevant journals with high impact respect to the h index were: Journal of Environmental Chemical Engineering (h index = 14) and Desalination and Water

Thematic Evolution in the Period 2015-2022
On the other hand, in Figure 3, concerning the list of the most representative authors, Zhang Y. presented the highest number of produced articles (41), whose value of "articles fractionalized" was 7.2; additionally, the results regarding the authorship impact showed that Zhang Y. and Wang Y. had an h index equal to 17; however, Zhang Y. had a high score in to other impact indexes, such as the g index = 41, the total number of citations = 1811, and publication number = 41. Figure 4 represents the complete spectrum of the author keywords as a function of term occurrences in the articles. The label size is proportional to the keyword density and frequency. Furthermore, two big conglomerates of keywords joined through networks and located on the top of the bibliometric map were observed, which present a high interrelationship among keywords that make up each conglomerate. Meanwhile, these two high sets were relatively founded near each other; on the other hand, three clusters were observed on the bottom of the map separate and away from the two principal sets of keywords. Additionally, these three clusters showed less density and frequency of keywords. Based on the sets obtained in the bibliometric map, five thematic groups were synthesized, which represent the principal thematic focus of the investigation:

Thematic Evolution in the Period 2015-2022
Cluster N°1: "Adsorption hydrogels". It includes the higher conglomerate of items whose keywords with higher density were hydrogels, adsorption, adsorption capacities, scanning electron microscopy, wastewater treatment, metal ions, and heavy metals.
Cluster N°2: "Hydrogels applied to the water treatment". It represents the second higher conglomerate whose principal keywords were hydrogel, water pollutant, pollutant removal, water pollutants, water purification, article, adsorption kinetics, and chemical.
Cluster N°3: "Biodegradable hydrogels". This group was made up of the following keywords: nanocomposites, cellulose, chitosan, and composite.
Cluster N°5: "Aqueous contaminants removal". It was formed by the removal of an aqueous solution. Based on the sets obtained in the bibliometric map, five thematic groups were synthesized, which represent the principal thematic focus of the investigation: Cluster N • 1: "Adsorption hydrogels". It includes the higher conglomerate of items whose keywords with higher density were hydrogels, adsorption, adsorption capacities, scanning electron microscopy, wastewater treatment, metal ions, and heavy metals.
Cluster N • 2: "Hydrogels applied to the water treatment". It represents the second higher conglomerate whose principal keywords were hydrogel, water pollutant, pollutant removal, water pollutants, water purification, article, adsorption kinetics, and chemical. Cluster N • 3: "Biodegradable hydrogels". This group was made up of the following keywords: nanocomposites, cellulose, chitosan, and composite.
Cluster N • 4: "adsorption nanoparticles". It was made up of 3 keywords: ions, water, and nanoparticles. Cluster N • 5: "Aqueous contaminants removal". It was formed by the removal of an aqueous solution.
Other visual tools regarding keywords and scientific study targeting generated the thematic map ( Figure 5) in which four differentiated categories were defined according to the locate quadrant.
Other visual tools regarding keywords and scientific study targeting generated the thematic map ( Figure 5) in which four differentiated categories were defined according to the locate quadrant. The three nodes located in the quadrant "Motor Themes" (high relevance degree and high development degree) were formed by "hydrogels and wastewater treatment", "adsorption and adsorption capacities", and "hydrogel and article". This quadrant contained principal studies whose relevancy for the investigative front was of a specialized type; those studies were well-developed and important for scientific progress.
The nodes located in the quadrant "peripheral themes" (high relevance degree and high development degree) were formed by "evaporation and purification". This quadrant contained peripheral studies whose investigation was according to the principal themes' guidelines; however, their scientific production was limited.
The localized nodes in the lower left quadrant "Emerging or declining themes" (low relevance degree and low development degree) were mainly structured by "release and behavior", "aqueous-solutions and methylene-blue", and "removal and aqueous-solution". The studies in this quadrant were considered emerging or declining, namely, they were in a consolidation phase; although much research has been done about those studies, a clear concept regarding those studies has not been made. Meanwhile, this quadrant was formed by studies whose theme was rejected because it was not important or relevant.
Finally, in the lower right quadrant, we found a node represented by the keywords "chitosan and water"; this quadrant was dominated by "basic and transversal themes" (high relevance degree and low development degree). The studies located in this section were those with high importance for a determined field of investigation, but they were not well developed by the scientific community.
In summary, the figure of the thematic map gives a first impression of the direction of the selected studies during the period 2015-2022. The adsorption through hydrogels to the wastewater treatment and contaminated water body are those themes to boost and are located on the investigation front; meanwhile, the referent works regarding colorant removal (methylene blue), ionic compounds, and contaminates present in aqueous solutions were founded in the consolidation phase because they were emerging studies whose scientific production was beginning to take shape. In addition, the purification and The three nodes located in the quadrant "Motor Themes" (high relevance degree and high development degree) were formed by "hydrogels and wastewater treatment", "adsorption and adsorption capacities", and "hydrogel and article". This quadrant contained principal studies whose relevancy for the investigative front was of a specialized type; those studies were well-developed and important for scientific progress.
The nodes located in the quadrant "peripheral themes" (high relevance degree and high development degree) were formed by "evaporation and purification". This quadrant contained peripheral studies whose investigation was according to the principal themes' guidelines; however, their scientific production was limited.
The localized nodes in the lower left quadrant "Emerging or declining themes" (low relevance degree and low development degree) were mainly structured by "release and behavior", "aqueous-solutions and methylene-blue", and "removal and aqueous-solution". The studies in this quadrant were considered emerging or declining, namely, they were in a consolidation phase; although much research has been done about those studies, a clear concept regarding those studies has not been made. Meanwhile, this quadrant was formed by studies whose theme was rejected because it was not important or relevant.
Finally, in the lower right quadrant, we found a node represented by the keywords "chitosan and water"; this quadrant was dominated by "basic and transversal themes" (high relevance degree and low development degree). The studies located in this section were those with high importance for a determined field of investigation, but they were not well developed by the scientific community.
In summary, the figure of the thematic map gives a first impression of the direction of the selected studies during the period 2015-2022. The adsorption through hydrogels to the wastewater treatment and contaminated water body are those themes to boost and are located on the investigation front; meanwhile, the referent works regarding colorant removal (methylene blue), ionic compounds, and contaminates present in aqueous solutions were founded in the consolidation phase because they were emerging studies whose scientific production was beginning to take shape. In addition, the purification and evapo-ration studies through porous membranes showed important information for the principal thematic development, but they still did not have a highly relevant production. Figure 1 shows the decision flow for selecting the competent articles for the study. First, two specialized databases in the science and engineering areas were identified: Scopus and Web of Science; 496 pertinent articles were obtained from the former and 460 articles from the latter. Then, the 956 articles were gathered and saved in BIB and CSV formats for their verification, refinement, and selection. Afterward, the documents were uploaded to the free web tool Rayyan for their manual and automatic revision of duplicates, selection of pertinent articles based on titles, and abstract revision of each article (1st filter). The inclusion or exclusion of duplicated articles was made through the following criteria: direct elimination (>90%), titles and principal indices revision (70% < % < 90%) and exhaustive lecture of abstract (<70%). Afterward, the Excel 2016 tool was used as a second manual and automatic revision of duplicates; meanwhile, articles were selected based on title and abstract criteria (2nd filter). The results obtained were 125 articles in total for their later analysis based on eligibility criteria; meanwhile, 627 articles were excluded through titles and abstract revisions.

Systematic Analysis of the Selected Literature
After the title and abstract analysis and before the analysis by eligibility criteria, 22 articles were excluded because they could not be retrieved for their exhaustive evaluation. Then, an eligibility analysis was done on the 103 remaining articles, of which 32 articles were excluded based on language criteria, 12 by not being inside the range of the established years for the present study, and 36 were excluded based on article content criteria (study scope, investigative thematic, study strictness, investigation design); thereby, a group of 23 articles was consolidated as the final sample.
Below is presented a synthesis chart of the selected articles, with their objectives and contributions regarding hydrogel applications as a technology tool for the treatment of industrial effluents.

Discussion
Based on what was exposed in the synthesis chart (Table 3), below are some of the most relevant studies regarding the application in real cases. The design and synthesis of polyampholyte hydrogel strengthened with graphene oxide was done through polymerization with free radicals for the metallic ions' adsorption from practical wastewater. The mechanical characteristics of hydrogels give them properties such as easy regeneration and reutilization. Additionally, the results showed a great removal capacity of heavy metals from industrial effluents.  it also showed high yield and reutilization capacity through regeneration with methanol. Chitosan network (net-CS) hydrogels showed a high removal capacity of As and Fe from mining tailing, so that, this hydrogel is technically and economically preferable as removal materials by its low cost and easy operatively. Freundlich isotherm perfectly described the adsorption of chitosan network (net-CS) hydrogels; this model mentioned that the process showed heterogeneous adsorption; likewise, the pseudo-first order model suggested that the process was done by chemical adsorption. [59] T ( • C) = 25  The results showed the capacity of agarose hydrogels to eliminate surfactant wetting towards the distillation membranes during the treatment of dyes effluents. However, the raise in temperature detriments the efficacy of agarose hydrogels; it is recommended that the agarose hydrogel be changed to other ones, such as thermosensitive hydrogels.
The adsorption through hydrogels can simultaneously eliminate different heavy metals from industrial wastewater. Likewise, its efficacy was not prejudiced after 5 cycles of adsorption-regeneration. Thus, it was concluded GO/P (AANa-co-AM) hydrogels present a great operational yield in the treatment of industrial effluents. Modified Adams-Bohart model Fixed-bed columns based on chitosan hydrogel beads demonstrated high malachite green (MG) adsorption from industrial effluents; the operation time was around 3 h, which is a great candidate for its scaling to an industrial level. Furthermore, the studies done on fixed-bed columns regarding adsorption kinetics were explained by a modified Adams-Bohart model due to the complexity of mass transfer mechanisms. The results showed technical and economic viability regarding the utilization of CCx-CB hydrogels in fixed-bed columns for TC removal. Furthermore, economic results demonstrated a cost reduction when raw materials of industrial degree were utilized in the hydrogel preparations. The study results showed great removal capacity for MB adsorption from industrial effluents using ABH-1:3 hydrogels. Furthermore, the removal percentage remained up to 75%, even after 10 reutilization cycles. Likewise, ABH-1:3 hydrogels demonstrated efficiency and efficacy in textile industrial contaminant removal.  Fixed-bed column 99.86% iron H (cm) = 2 Nano-ZnO-loaded poly (acrylamide-co-itaconic acid) hydrogel (PAI) showed the elimination capacity of iron from Shankhini River to have minimum capital cost and great efficacy. Likewise, its column parameters make scaling easy to treat high volumes of industrial effluents. The studies demonstrated the removal capacity of methylene blue (MB) through GA hydrogels in the water purification process. Furthermore, GO addition to the adsorbent structure allowed hydrogel production to be large scale and low-cost, which gives them economic and technical viability for industrial scale applications. Based on the results, it was concluded that the complete elimination of water from biodiesel was achieved. Likewise, bed configuration presents great advantages due to the geometry of its plates, which could eliminate water content from different fuels without any worries about bed clogging.
[74] Adsorption efficacy of CH-LDH-ise hydrogels to Cr (VI) removal from water bodies and its reutilization capacity through fixed-bed column regeneration give it favorable characteristics of efficiency and fast ionic contaminants elimination from industrial effluents. In addition, an efficacy of nearly 100% Cr (VI) was observed through a treatment volume of 2250 bed volumes ** (BV). Thomas model The obtained results showed high removal efficacy of fluorides from water bodies using Y-GO-SA hydrogels in batch and fixed-bed column processes Likewise, batch studies demonstrated more efficacy than adsorption studies of fixed-bed columns. On the other hand, adsorption columns presented a reutilization capacity through regeneration cycles, which is essential in water potabilization. Zhou [56] presented in their investigation NH2-Starch/PAA hydrogel as adsorbents for heavy metal removal from industrial effluents of the Shuikoushan Smelting Plant, Hengyang, Hunan province, China. The adsorption process was done in a batch reactor to determine kinetic parameters and adsorption yield; likewise, two fixed-bed adsorption columns in series were done to evaluate practical hydrogels application; operation conditions are described in Table 3. Batch study results showed an adsorption capacity of 254 times dry hydrogel weight to the initial concentration of 180 mg/L Cd (II) and 1 g/L of adsorbent dose; on the other hand, the assays of actual effluents adsorption made in fixed-bed columns exhibited a relationship between adsorption percentage and adsorbent dose, so that to 4 g/L of adsorbent dose, the concentration of heavy metals reached lower values of 0.001 mg/L, except for Zn (21.95 mg/L) and TOC (14.35 mg/L).
Regarding bed efficacy, it was observed that the column can work continuously for a period of eight cycles reaching to volume treated of 2400 BV (27.1 L) and Cd (II) concentration was below 0.01 mg/L; each cycle finalization the second column took the first position, while the last put through a regeneration process. In this way, obtaining both columns in optimal conditions was achieved; in addition, a 1% advance in Cd (II) concentration in the breakthrough curve was observed after 228 BV. Column regeneration was made with a volume of 67.8 mL of HCl 0.1M solution, namely, when 2400 BV of wastewater was treated, 6 BV (67.8 mL) of eluent was necessarily used for column regeneration. Regarding the loss of hydrogel mass in the adsorption-regeneration process, the assays showed that loss achieved 1% of the total mass of the hydrogel, so that the sludge produced amount was insignificant.
On the way to accomplishing SDG and according to the agreement of sustainable development proposed in 2015 and started in 2016, biodegradable hydrogels accomplish an important role in eliminating environmental contaminants; therefore, in recent years, the utilization of monomers or polymers based on natural sources, such as cellulose, chitosan [77], guar gum, starch, and organic wastes, have been studied for their synthesis in hydrogels and subsequently, application in wastewater treatment [78][79][80].
According to what was exposed, Ma et al. [61] made synthesis studies of hydrogels based on waste cotton fabric (WCFs) dissolved into NaOH/urea to former a cellulose viscous paste with activated groups (Cellulose/PAM DNHs), likewise, this study made applicative essays to heavy metals adsorption (Cd (II), Cu (II), Pb (II), Zn (II) y Fe (II) from Minmetals Copper Industry located in Hengyang, Hunan province, China). Those studies were made in batch reactors and fixed-bed columns, and the last showed great advantages compared to other adsorption equipment, such as a low-cost index and relatively easy scaling from design and operation parameters. The studies showed high heavy metals adsorption efficacy of employing continuous adsorption columns for many operation cycles (Table 3); however, when outlet effluent concentrations in columns exceeded the permissible limits (0.1 mg/L Cd (II), 1.0 mg/L Cu (II), 1.0 mg/L Pb (II), 4.0 mg/L Zn (II), and 10 mg/L Fe (II)), column regeneration was carried out for its reutilization in the next cycle. To maintain the columns' activity for a prolonged time, the study determined that treatment volume must be delimited to work below 42 BV (1932 mL); likewise, this work parameter allowed the elimination of the ions present in the solution in an efficacious way; the operation time achieved pH = 5.0 and T = 25 • C in 420 min. The elution of saturated columns was carried out by washing with 0.1 M HCL to a volume of 2 BV (92 mL).
Among the industries that emit the most pollutants through effluents and emissions, the mining-metallurgical industry is considered one of the main tailings and mineral sludge generators. Burillo et al. [59] conducted characterization studies of tailing and mining studies located in an acid mine drainage site near Arroyo San Pedro (San Luis Potosi, Mexico); likewise, a chitosan network (net-CS) was synthesized for Fe (III) and As (V) removal. The process was performed in batch reactors with an operation time of 50 h. The results showed that the present tailings were adsorbed satisfactorily, with a basic pH and environment temperature, by chitosan hydrogels (0.786 mg/g As (V) y 76.85 mg/g Fe (III)); in addition, the equilibrium studies (Freundlich isotherm) and kinetics studies (Pseudo-first order) manifested information regarding the adsorption nature, highlighting that the adsorption was a heterogeneous process governed by chemical adsorption.
According to the mentioned studies, Zhou et al. [60] worked on the adsorption of actual effluents from the Smelting Plant located in Hunan, China. The heavy metal removal from industrial effluents was carried out through fixed-bed columns packed with Jute/PAA hydrogels; likewise, equilibrium, kinetics, and the influence of principal variables, such as pH, initial concentration, initial dose, and temperature, were made. The obtained results showed that adsorption equilibrium was achieved in just 10 min using 1 g/L of adsorbent when contaminants were present at initial concentrations of 40 mg/L Cd 2+ and 40 mg/L Pb 2+ . The studies of fixed-bed columns revealed that effluents, after the adsorption process, contained a metallic ion concentration below 0.001 mg/L in an operation time of 2 h using 4 g/L of adsorbent. The regeneration of adsorption columns was carried out by 50 BV (565 mL); in other words, the adsorption columns were treated with 2900 BV (32.8 L) of industrial effluents, and 50 BV (565 mL) of eluent was needed to regenerate them.
Lin et al. [64] studied distillation membrane wetting for the treatment of wastewater of superficial low tension coming from the dyeing industry. This wetting was principally occasioned by surfactants (sodium dodecyl sulfonate (SDS), Tween20, and Tween85), which reduced the purification efficiency of the membranes. For this reason, a film of agarose hydrogel adhered to the Teflon membrane. The results showed that the distillation membrane did not present wetting during the first 24 h when the initial concentration of surfactant was 10 mg/L. Nevertheless, when the surfactant achieved a concentration above the critical micelle concentration (CMC), it was observed that surfactant penetration toward the membrane, at a slow pace and low way. The disadvantages observed in the anti-wetting distillation membrane system (Teflon membrane + agarose hydrogel film) were the operation flow, which was reduced by 70% when the hydrogel film was used; the temperature of operation, which showed a reduction of repulsion efficiency when it worked to high temperatures, this happened because the thermal stability of agarose hydrogels decreased when the operation temperature increased; furthermore, specifically in this study case, the membrane had a temperature gradient, which was the motor force of the operation (T cold = 21 • C, T hot = 60 • C). Thus, the application of this hydrogel is not recommended when it is worked to very high temperatures. On the other hand, micelle presence allowed the surfactant to penetrate through the hydrogel film and caused membrane wetting. A viable alternative regarding hydrogel selection could be the implementation of thermosensitive hydrogels, such as N-isopropylacrylamide (NIPAAm) and N-acryloylpyrrolidine [81].
Another important subject in this context is the global agreement about the management of the environment, specifically for this study, water management (SDG N • 6), which mentions that it is necessary to guarantee water availability for all people, necessitating its sanitation and purification. According to that, He et al. [76] carried out studies about treatments for water bodies contaminated with fluorides F − ; a series of new hydrogels of Y-GO-SA were synthesized as adsorbents, and the studies were carried out in batch reactors, and fixed-bed columns. The results showed a high adsorption capacity (288.96 mg/g F − ) in 24 h of operation time; however, the adsorption presented efficiency problems of removal due to the presence of phosphate ions; this could be explained by the ionic charge of PO 3− 4 and F − , which competed for the activated sites of the hydrogel internal structure; this selectivity depends on ionic molecule size (the smaller the ion size, the higher the ion adsorption will be by hydrogels).
On the other hand, the studies of adsorption columns provided high adsorption results (152.3 mg/g F − ), although they were lower than the studies made in batch reactors; furthermore, modeling studies were done to understand the adsorption dynamic, as a function of outlet solution from the fixed-bed column to determine the breakpoint, the breakthrough curve, and the exhaustion point; with that information, the adsorption efficacy as a function of time can be studied. Likewise, the breakthrough curve can determine the efficacy decrease of the column, namely, this curve is elicited by its regeneration time through washing with HCl. Concerning the fluoride adsorption, Thomas's model fitted so well to experimental data that it had a continuous flow. Other applicative works have presented studies about adsorption process modeling in fixed-bed columns based on the Thomas model [55,65,69,70]; nevertheless, not all adsorption processes can be described by this model; for instance, the adsorption studies of 137 Cs y 134 Cs using PVA-alginate encapsulating Prussian blue-graphene oxide (PB-GO) hydrogel beads [62] showed that adsorption dynamics were fitted best by The Yoon-Nelson Mathematical Model and Thomas model. However, other important indicators such as Chi-square values (χ 2 ) and average percentage errors (APE), which evaluate the fitness of the breakthrough model, showed that the Yoon-Nelson Mathematical Model (0.08-0.17 and 12.64-42.69) presents good agreement with the experimental data compared to the Thomas model (0.08-0. 22 and 14.36-45.50).
Other works, such as Song et al. [68], studied the adsorption of Malachite green through fixed-bed columns, and their results demonstrated that the kinetics process was fitted by the Adams-Bohart model [68].
The hydrogels as purification materials of water bodies have achieved important relevance due to their adsorption properties to eliminate contaminants, easy operation control, fast regeneration, reutilization, and low operative cost. These statements were supported by the study of Sharma and Tiwari [72], who presented the treatment of contaminated water bodies (Shankhini River, Dantewada, Chhattisgarh, India) through adsorption of Fe 2+ y Fe 3+ with nano-ZnO-loaded poly (acrylamide-co-itaconic acid) hydrogel (PAI). The study was done using fixed-bed adsorption columns and batch reactors. The removal efficacy in both systems achieved values upper to 99.0% of iron adsorption; likewise, the study evidenced that the columns could work in many cycles of adsorption-regeneration without manifesting a detectable loss of hydrogel efficacy. The design and operation parameters could be easily scaled to the removal of contaminants present in high wastewater volumes. On the other hand, the operation and capital costs were lower than the implementation of other removal technologies of contaminants, such as (1) membranes, (2) ozonation, and (3) electrodialysis; thus, practical and economic viability was evidenced for iron removal from rivers, underground water, or industrial effluents by PAI hydrogels.

Conclusions
The implementation of global goals to use, manage, and sanitize hydric resources has promoted the development of scientific studies about hydrogel utilization as adsorption materials for the treatment of industrial effluents; as a consequence, the production of scientific articles regarding the removal of industrial contaminants through batch reactors and fixed-bed adsorption columns has increased in recent years. Likewise, the easy operational control of equipment, low operational cost, low maintained cost, and straightforward scaling for the treatment of high volumes of wastewater have been fundamental to determine the viability of this new treatment methodology of industrial effluents.
The present study allows visualization of the current state of hydrogel implementation in the industry, and its advantages and disadvantages against diverse operation conditions. Likewise, the synthesis chart allows us to obtain design and operation data about unit equipment used in the adsorption of industrial contaminants.
On the other hand, it was mentioned that the study scope limited the information searching to other languages, such as Chinese, since the bibliometric and systematic studies showed that China is the country where the most scientific advances have been developed regarding hydrogel application in the treatment of actual industrial effluents.