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Review

A Review of Symmetrical and Asymmetrical Research Outputs on Wastewater Treatment and Water Purification Through Sorption-Based Technologies

by
Abhijit Debnath
1,
Anurag Mishra
2,*,
Archana Pandey
3,
Prabhat Kumar Singh
1,
Yogesh Chandra Sharma
3 and
Rajnish Kaur Calay
2
1
Department of Civil Engineering, Indian Institute of Technology (BHU), Varanasi 221005, India
2
Department of Building, Energy and Material Technology, UiT, The Arctic University of Norway, 8515 Narvik, Norway
3
Department of Chemistry, Indian Institute of Technology (BHU), Varanasi 221005, India
*
Author to whom correspondence should be addressed.
Symmetry 2026, 18(5), 865; https://doi.org/10.3390/sym18050865
Submission received: 2 April 2026 / Revised: 16 May 2026 / Accepted: 18 May 2026 / Published: 20 May 2026
(This article belongs to the Special Issue Symmetry in Chemical and Systems Engineering: From Resource Recovery to Sustainable Design)
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Abstract

This review focuses on research outputs of water purification, wastewater treatment, metallic remediation, and sorption-based experimental studies. It aims to identify the leading nations contributing to these areas and identify the journals that have published the highest number of papers from 2010 to 2025, and centers on yearly publication trends. A thorough quantitative analysis was carried out to examine key characteristics of adsorbents derived from various materials, as well as symmetry and asymmetry of wastewater treatment for the removal of metallic pollutants. Key adsorption mechanisms—including ion exchange, surface complexation, electrostatic attraction, and pore filling—are discussed alongside the structural roles of symmetric (ordered) and asymmetric (heterogeneous) adsorbent architectures. Data was collected from the Scopus database, focusing on specific keywords like “metal,” “water,” “removal,” “adsorption,” “purification,” “drinking water,” “nano adsorbent,” etc. Among approximately 29,598 publications encompassing research papers, reviews, short communications, conference papers, and book chapters, China emerged as the leading publisher with 11,957 papers, trailed by India (4324 papers), the USA (1825 papers), Iran (1739 papers), Saudi Arabia (1484 papers), Egypt (1318 papers), and Republic of Korea (1194 papers). The bibliometric mapping of conventional adsorbents and nanomaterials used in sorption-based technologies was analyzed using VOSviewer, revealing major research clusters, research hotspots, networks, and evolutionary patterns in wastewater treatment and sorption-based water purification. This study indicates that several journals from Elsevier Ltd. and Springer Nature are leading the field with a large number of publications per year. The analysis reveals a consistent upward trend in the number of research publications in recent years. In sum, the bibliometric data provided highlights the growing relevance of these areas among academicians and acts as a catalyst for further research, motivating researchers to investigate new adsorbents or modifications that could improve adsorption performance while maintaining economic viability and efficiency.

1. Introduction

Wastewaters include a complex mixture of metals (with relatively high density, such as mercury, lead, cadmium, arsenic, chromium, nickel, etc.) and metal-based compounds (such as dyes, pharmaceutical products, pigments, herbicides, pesticides, etc.) that often originate from various industrial, agricultural, household, and municipal sources [1,2,3]. Persistent water contamination due to the presence of these toxic metals and associated species is a major global concern, as they have low degradation rates and tend to persist in the environment for extended periods [4,5,6]. One instance is the formation of inorganic mercury into toxic methyl mercury through the activity of bacteria present in water, sediment, and soil [7]. The ramifications of metal contamination (even at low concentrations) include the destruction of aquatic life, possible incorporation into food chains, and severe health effects on humans, such as organ dysfunction, neurological disorders, and carcinogenicity [7,8,9].
Over time, adsorption has emerged as one of the primary methods for eliminating hazardous metallic pollutants from wastewater due to its cost-effectiveness, ease of operation, reduced secondary pollution, and environmentally friendly nature [10,11,12,13,14]. The emergence of novel adsorbents for the symmetrical and asymmetrical treatment of wastewater containing metals is attracting considerable attention worldwide, as it is crucial to ensure clean, sustainable water. Adsorption processes are of great importance for various applications, especially in water and wastewater treatment. According to the Scopus database, searching from 2000, using the keyword “adsorbent” revealed around 153,857 articles, and using the keyword “adsorption” revealed 756,490 articles published till the year May 2026 across all subject areas of science, engineering, agriculture, and other professional studies, indicating significant development over the years. With the advancement of adsorption over time, novel adsorbents have been discovered and can be classified as hybrid, inorganic, or organic adsorbents. They may be found in several sizes, including nano and micro, and are available in membrane form. Adsorbents are derived from various natural and synthetic materials, which can be either pristine or physically or chemically modified, and even include composite materials. The commonly used adsorbents include activated carbon, zeolites, silicates, clay, hydrochar, biochar, metal–organic frameworks (MOFs), carbon nanomaterials, and polymers (including cellulose, chitosan, geopolymers, hydrogels, and aerogels), among others [15,16,17,18,19,20,21,22,23].
Several literature analyses explore the evolution of adsorbents for the removal of metals and their associated compounds, and the mechanisms of adsorption processes across diverse sources over time and space.
Bibliometrics is one such important tool that has been extensively used for assessing vast scientific research outputs both quantitatively and qualitatively [24]. Bibliometrics is concerned with quantifying external aspects of the literature and its references using statistical and mathematical methods. In contrast to qualitative evaluations, bibliometric analysis provides objective, reproducible, quantitative insights into emerging hotspots, publishing trends, cooperation networks, co-occurrence patterns, and structural imbalances. The articles published over the past decade demonstrate a notable prevalence of scholarly investigation and enthusiasm among researchers in the fields of water purification and its associated disciplines.
To the best of our knowledge, no thorough bibliometric analysis has yet mapped sorption-based water treatment from 2010 to 2015, identifying the nations, target metals, and those that have advanced the field, as well as areas where significant imbalances or knowledge gaps still exist. Several gaps have been identified from the literature and addressed in this study area: (i) no prior studies have systematically identified the transition of conventional adsorbents to advanced materials over 15 years or carried out a comparative assessment between conventional and nano-adsorbents; (ii) the geographic distribution of global research outputs in the area of water pollution has not been compared yet; (iii) previous reviews have not applied advanced mapping software such as VOSviewer for bibliometric mapping, including keywords and cluster analysis of adsorbent materials in this domain. The literature on water purification, wastewater treatment, metallic sorption, and nano-adsorbent-based studies from 2010 to 2025 was examined to provide a foundation for a deeper understanding of the global research landscape and the development of medium- to long-term strategies for this area.
This bibliometric review aims to quantify publication trends, leading nations, influential journals, and adsorbent types from 2010 to 2025 using descriptive statistics. The analytical software VOSviewer was employed for bibliometric mapping, including keywords, cluster analysis of adsorbent materials in this domain, target metallic contaminants, characterization techniques, and treatment technologies, to reveal the knowledge structure and research hotspots. This review paper also aims to highlight key findings on the efficiency and scientific advancements of several adsorbents derived from diverse sources for the adsorption of metals and related compounds. This review also discusses the types of adsorbents (pristine or physically/chemically modified, frameworks, composite materials, nano-adsorbents, biopolymers, etc.) in a strategic, tabular format for better understanding.
The factors studied included not only the numerical output of publications but also yearly outputs, mainstream journals, top contributing nations, research trends, and hotspots identified through word analytics of author keywords in this review. This paper also identifies symmetrical and asymmetrical patterns in the use of adsorbents and in research output across geographic, thematic, and methodological dimensions, thereby highlighting structural imbalances.
It is noteworthy that certain nations and their researchers have a dominant presence in publication counts; collaborative efforts among academics and institutions across different countries can provide more comprehensive approaches to addressing water purification difficulties.

2. Materials and Methods

We have conducted a comprehensive search for sorption-based studies using various keywords in the Scopus database spanning the years 2010 to 2025. We obtained a substantial number of results from 105 different journals indexed through Scopus, from several esteemed publications such as Elsevier (ScienceDirect), Springer Nature, Taylor and Francis, Royal Society of Chemistry (RSC), American Chemical Society (ACS), Wiley, MDPI, IWA Publishing, JSTOR, Hindawi, IOP Publishing, Nature, and more. We searched the Scopus database (Elsevier B.V., Amsterdam, The Netherlands), both in combination and separately, using various keyword sets to estimate the number of papers published in this specific area. Given below are the keywords employed in these searches, accompanied by the corresponding number of displayed results.
Scopus Database (search keywords; limit to 2010–2025)Results
metal AND water AND removal AND adsorption 29,598 [25]
water AND purification AND metal17,905 [26]
water AND purification AND treatment AND 4576 [27]
metal AND adsorption
metal AND removal AND drinking AND water 3280 [28]
drinking AND water AND metal AND adsorption2231 [29]
In addition, we searched for papers on nano-adsorbent-based remediation studies for removing metallic elements from water and identified 2066 research papers.
Scopus Database (search keywords; limit to 2010–2025)Results
nano adsorbent AND metal AND removal 2066 [30]
nano adsorbent AND water AND removal AND metal1484 [31]

3. Results and Discussion

The total number of publications (TP) has steadily increased over the years, indicating growing interest and research activity in water purification and wastewater treatment. In 2010, over 600 articles were published, a number that then doubled in 2016 with over 1200 papers published. The cumulative number of publications has continued to rise over the years, reaching 3843 articles by 2025. It can be observed that a wide variety of adsorbents have been used in these studies over the years. Many common adsorbents, such as zeolites, bentonites, and clay-based materials, were reported only until 2014, suggesting their significance in adsorption studies. Figure 1 shows a comprehensive VOSviewer analysis (version 1.6.20) for advanced bibliometric mapping, including keywords, cluster analysis of adsorbent materials, target metallic contaminants, characterization techniques, and treatment technologies, all retrieved from Scopus. This reveals major research clusters, research hotspots, networks, and evolutionary patterns in wastewater treatment and sorption-based water purification.
Table 1 provides information about the adsorbents used in various studies, the total number of publications (TP), the country-wise distribution of publications, and the corresponding references for each year from 2010 to 2025. After removing the repetitive number of papers, approximately 29,598 papers were found to have been published on water purification, wastewater treatment, metal remediation, or sorption-based studies, as shown in the following Table 1.
Table 1 shows symmetrical research outputs on wastewater treatment and water purification through sorption-based technologies. A geographical symmetry in research contributions has been observed across countries such as Saudi Arabia, Iran, Egypt, the USA, and Republic of Korea, including the homogeneous development of adsorbents and the parallel contributions of conventional and nano-adsorbent technologies. Asymmetrical research outputs include the highest number of articles published by China (around > 10,000), followed by India. Also, the majority of studies have been focused on the removal of conventional heavy metals such as Cd, Pb, Ni, and Zn. The application of biochar and nanomaterials has been widely used since the post-2015 period, whereas natural absorbents have been studied less. The asymmetric scale also indicates that more than 95% of publications have been based on laboratory studies using synthetic solutions rather than on pilot-scale wastewater treatment studies. Also, asymmetric collaboration patterns were observed in intra-Asia and Asia–US/Europe, rather than between African unions or Middle Eastern countries.
Table 1 quantitatively summarizes adsorbent types and publication trends, providing a deeper understanding of wastewater treatment that requires examining the underlying adsorption mechanisms, how the different classes of adsorbents interact with metallic pollutants at the molecular level, and how symmetrical versus asymmetrical design features influence removal efficiency.
The efficacy of sorption-based technologies for metal removal is governed by several physicochemical mechanisms that often operate simultaneously. These include electrostatic attraction (between charged adsorbent surfaces and oppositely charged metal ions), ion exchange (replacement of exchangeable cations like Na+ or Ca2+ with toxic metal ions), complexation (coordination of metal ions with functional groups such as –OH, –COOH, or –NH2), surface precipitation (formation of metal hydroxides or carbonates at high local concentrations), and pore filling (physical entrapment within micropores or mesopores) [103,104,105,106,107]. Among these, complexation and ion exchange are typically dominant for chemically modified adsorbents, whereas pristine carbons and clays rely more on electrostatic interactions and physical adsorption.
The concept of symmetry and asymmetry in adsorbent design plays a crucial, under-discussed role in determining adsorption performance. Symmetrical adsorbents—such as uniformly structured zeolites, ordered mesoporous silicas (e.g., MCM-41, SBA-15), and metal–organic frameworks (MOFs) with regular pore networks—offer highly predictable diffusion pathways, reproducible binding sites, and facile modeling of adsorption isotherms. Their structural regularity facilitates uniform surface functionalization and enables consistent mass transfer behavior. For instance, the symmetrical cage-like pores in MOFs allow rapid and selective capture of Pb2+ and Cd2+ through size-exclusion effects combined with coordination chemistry [108].
Conversely, asymmetrical adsorbents—including many biochars, functionalized hydrogels, grafted polymers, and composite materials—exhibit heterogeneous surface chemistry, irregular pore-size distributions, and spatially variable functional-group densities. While this asymmetry complicates mechanistic modeling, it often yields superior practical performance due to multi-site synergistic effects. For example, magnetic nanocomposites with asymmetrically distributed thiol or carboxyl groups [109] can simultaneously capture multiple heavy metals via different binding mechanisms on the same particle. Similarly, asymmetrically modified cellulose or chitosan derivatives [110] exploit hydrophobic–hydrophilic gradients to enhance selectivity toward target contaminants. Thus, the choice between symmetric and asymmetric adsorbent architectures involves trade-offs between mechanistic clarity/regeneration consistency (symmetric) and high capacity and multipollutant removal (asymmetric).
Beyond mechanistic considerations, Table 1 also illustrates the potential applications of naturally occurring geological materials, including minerals, clays, geopolymers, and mining waste, for water treatment and decontamination. This diversity of adsorbents suggests ongoing efforts to develop sustainable, eco-friendly adsorption materials for purification. The table demonstrates a clear increase in adsorbent adsorption capacity through modifications/functionalization, highlighting the growing research efforts in water remediation via adsorption. As shown in the table, adsorbents, mainly pristine/native adsorbents, were used to remove pollutants from drinking water until 2015. Later, however, researchers shifted their attention to modified/functionalized adsorbents for improved efficiency. For instance, Li [73] prepared activated carbon from cassava sludge and found it as a highly potential adsorbent for the adsorption of chromium in potable water. Celik [74] designed metal-incorporated layered double hydroxides (LDH) and obtained splendid adsorption capacities of 378, 978, 332, 579, and 666 mg g−1 for Cu2+, Ag+, Cd2+, Pb2+, and Hg2+, respectively. In a study reported by Zuo [83], they fabricated activated carbon fibers abundant in S/N/O multiple active sites through hydrogen peroxide pretreatment and post-functionalization with thiadiazole, which helped them to remove lead with an excellent adsorption capacity of 29.05 mg g−1 [56]. Owing to their high surface area, flexible pore size, and pore surface, MOFs have also been widely used as adsorbents recently. Even today, biomass-based adsorbents are used to remove heavy metals from drinking water. For example, polycaprolactone-bound diatomite was used as an adsorbent for the removal of heavy metals [64]. Thus, there is a growing trend toward synthesizing new adsorbents, with a particular emphasis on treating drinking water.
The diverse approaches and materials employed to address the intricate challenges of water contamination affect the cost-effectiveness and commercial viability of the adsorbents used in water purification, wastewater treatment, and remediation studies from 2010 to 2025. Researchers have continuously worked to improve the sustainability and efficacy of water treatment processes, starting with the use of zeolite, kaolinite clay, biochar, peels, and AC during 2010–2017 and continuing with the more recent development of ligand-imprinted composite adsorbents, graphite pellets, sponges, xerogel, polymer-modified carbon, etc., until 2025.
Adsorbents with large surface areas and specific surface chemistries, including AC and modified biochars, have been developed to effectively adsorb a range of pollutants, including heavy metals, organic chemicals, and emerging contaminants such as pharmaceuticals. Large-scale applications might discover that traditional materials like zeolite and clay are attractive because of their availability and ease of accessibility, which may provide comparatively low-cost solutions. Modern adsorbents show better removal efficiencies (such as modified biochar, composites, geopolymers, carbon fiber, etc.), selectivity towards contaminants, and enhanced regeneration capabilities as a result of these efforts, which may offset higher initial costs that can be compensated by reducing adsorbent usage, process optimization, and disposal costs over the long run.
Furthermore, using natural resources such as fruit peels, agricultural waste, and biomass-derived biochar has not only provided affordable substitutes but also valorized waste and advanced circular-economy principles. In addition, synergistic effects are observed with composite adsorbents, where multiple substances are combined to take advantage of their combined properties, resulting in performance superior to that of any of their individual adsorbents. The entire cost structure is also affected by the scalability of manufacturing processes, with technological advancements that can be easily scaled up to industrial levels, often providing cost advantages. But despite these developments, problems with scalability, long-term stability, and compatibility with current treatment infrastructure persist, demanding further research and development.
Figure 2 represents year-wise numbers of papers published on the application of sorption-based technologies (conventional and nano-adsorbents) for water purification studies. China and India consistently dominate the number of publications, followed by the USA, Iran, Saudi Arabia, Egypt, etc., reflecting their large populations and significant water treatment challenges. In the case of China, it increases by a factor of two from 2013 to 2016 and then doubles again from 2018 to 2022. In the later years (2017–2025), Iran emerged as a significant contributor to the literature, especially in 2018, suggesting a growing research presence in the field during that period. It was observed that, from 2015–16 onward, the most publications used char/biochar/activated carbon as adsorbents for remediation studies. Their active research efforts suggest a commitment to addressing water quality and sustainability concerns.
Table 2 presents nanotechnology-based metal remediation studies using selected nano-adsorbents, the number of publications per country, and cited references. The number of publications on nanotechnology-based metal remediation studies exhibited a steady upward trend from 2010 to 2025, indicating escalating interest and scholarly investigation in this field. The number of papers published more than doubled over this timeframe. The total number of publications was much lower until the year 2013. However, the number of publications increased from 2014 onwards and doubled gradually from 2017 to 2023. China, in particular, has a significant presence in academic papers, while India, Iran, Egypt, and Saudi Arabia have also made substantial contributions in the field of nano-adsorbents. Over time, one can discern the production of many categories of nano-adsorbents, including graphene-based materials, magnetite nanorods, and functional mesoporous silica nanoparticles. The gradual transition from traditional materials to sophisticated nano-adsorbents throughout the years exemplifies the progression of metal remediation research, likely with improved efficiency and selectivity in capturing metal ions from various sources (Table 2).
The bibliometric mapping of nanomaterials used in sorption-based technologies was analyzed using the VOSviewer (Version 1.6.20) application, as shown in Figure 3. These additions move beyond statistical counting of studies to reveal major research clusters, research hotspots, adsorbents, networks, and evolutionary patterns in wastewater treatment and sorption-based water purification.
The effectiveness of nano-adsorbents has been demonstrated over the years for a range of contaminants, including heavy metals, organic pollutants, and dyes, often exceeding 85–90%. While the specific efficacies vary widely depending on the type of nano-adsorbent and the pollutants being addressed, in general, they exhibit considerable potential for environmental remediation. Adsorption effectiveness is determined by several parameters, including surface area, pore size distribution, surface chemistry, and adsorbent–contaminant affinity.
NPs with high surface areas and active sites for pollutant adsorption are metal oxide nanoparticles, which include iron oxide (Fe3O4) [170], titanium dioxide (TiO2), and zinc oxide (ZnO) with excellent sorption efficiency of up to 90%. Their high surface-to-volume ratio and paramagnetic qualities facilitate effective adsorption and separation.
Carbon-based materials, including graphene [161], carbon nanotubes (CNTs) [155], activated carbon (AC) NPs, and carbonaceous nanocomposites [156], constitute another family of nano-adsorbents with high sorption efficiency. Distinguished by their elevated surface area and microporous structure, these adsorbents exhibit remarkable adsorption capacity for a broad array of pollutants via hydrogen bonding, π-π interactions, and van der Waals forces, including heavy metals, organic pollutants, and novel contaminants such as pesticides.
Metal oxide and carbon-based nano-adsorbents, metal–organic frameworks (MOFs) [149,186], silica nanoparticles (NPs) [164], and magnetic nanoparticles (MNPs) [160,167,169] are all effective adsorbents for removing heavy metals, dyes, and organic pollutants from aqueous solutions and feature high porosity, tunable pore sizes, and diverse coordination networks, which enhance their effectiveness.
A critical factor in assessing the practical applicability of sorption-based technologies is the comparative cost of different adsorbents. Based on raw material sources, synthesis complexity, and processing requirements, adsorbents can be categorized into three cost tiers. Low-cost adsorbents (typically <100/kg) include agricultural wastes (e.g., rice husk, peanut shells, fruit peels), natural clays (kaolinite, bentonite), and unmodified biochars. These materials require minimal processing, are readily available, and are suitable for decentralized or resource-limited settings, although they often exhibit moderate adsorption capacities (20–100 mg/g). Medium-cost adsorbents (100–500/kg) include activated carbon, zeolites, chitosan derivatives, and chemically modified biochars. Their higher surface area and functional group density provide improved adsorption performance (100–300 mg/g) and moderate regenerability, making them viable for municipal and industrial applications. High-cost adsorbents (>500/kg) include metal–organic frameworks (MOFs), graphene-based nanomaterials, functionalized magnetic nanoparticles, and ordered mesoporous silicas. While these materials demonstrate exceptional selectivity and capacity (often >500 mg/g) with rapid kinetics, their complex synthesis routes, expensive precursors (e.g., organic ligands, noble metals), and limited scalability currently restrict their use to targeted remediation of high-toxicity contaminants or small-scale applications. However, high-cost adsorbents often offer superior reusability; for example, MOFs and magnetic nano-adsorbents can maintain 80–90% of their initial capacity over 5–10 regeneration cycles, thereby reducing long-term operational expenses. Thus, selecting an adsorbent for practical water treatment should consider not only upfront costs but also factors such as adsorption efficiency, regeneration potential, service lifetime, and the specific contaminant profile. A holistic cost–benefit analysis, rather than minimization of initial investment alone, is essential for successful real-world implementation of sorption-based technologies. The economic viability of nano-adsorbents for the treatment of pollutants involves several important aspects, including the cost and accessibility of raw materials, the overall synthesis cost, the potential for regeneration, and application compatibility, for which there have been several challenges regarding cost-effectiveness in the past. These factors have shaped the overall viability of nano-adsorbents over time. For example, although effective, amino-functionalized Fe3O4 @SiO2 and n-Alumina are relatively expensive due to the difficulty of their synthesis and the scarcity of their raw materials. As the sector developed, newer sorbents with higher efficiencies—though often at lower cost—emerged, such as modified biochar, magnetic NPs, and cellulose-based composites. As such, graphene-based MOFs demonstrated remarkable sorption efficacy; however, the high cost of graphene and the complex synthesis procedures hindered production.
Regeneration potential is one area where cost-cutting measures have been investigated. In the long term, sorbents with greater potential for reuse and regeneration—such as nZVI—provide greater economic benefits. Furthermore, the development of affordable nano-adsorbents has been enabled by readily available, inexpensive raw materials. Using natural materials such as cellulose and guanidine, sorbents such as cellulose–nZVI composites and mesoporous guanidine functionalized amorphous magnetic nano-adsorbents have shown remarkable efficiency at a reasonable cost. Furthermore, their applicability to specific applications might justify the higher prices of nano-adsorbents. In addition, after adsorption, the disposal or regeneration of spent adsorbents is a critical step. Used adsorbents can be regenerated using chemical eluents (acids, bases, or chelating agents) for 3 to 10 cycles, after which they require safe disposal. Regeneration cost typically ranges from 20 to 40 percent of the initial adsorbent cost. Spent adsorbents that cannot be regenerated should be stabilized by solidification or incineration, with recovered heavy metals recycled where feasible. Landfilling is the least expensive option but poses environmental risks if metals leach out.
Different adsorbents are suited for different application scenarios based on their properties, cost, and selectivity. For drinking water treatment, where safety standards are very strict, and contaminant levels are low, activated carbon, modified biochar, and chitosan-based adsorbents are preferred because they produce no toxic byproducts and can reduce metal concentrations to parts-per-billion levels. These materials are effective for removing lead, copper, and cadmium from drinking water sources. For industrial wastewater treatment, where metal concentrations are high and the matrix is complex, metal–organic frameworks, magnetic nanoparticles, and functionalized clays are more suitable. These adsorbents offer high capacity and rapid kinetics. For example, MOFs excel at removing mercury and lead from chemical industry effluents, while magnetic nanoparticles allow easy separation from high-volume wastewater streams. For mining effluents containing arsenic, chromium, and nickel, iron-based adsorbents such as nano zero-valent iron, iron oxide nanoparticles, and iron-modified biochar are most effective due to their strong affinity for oxyanions. These materials are also relatively inexpensive, making them suitable for large volumes of mining wastewater. For agricultural runoff and municipal wastewater containing mixed contaminants, low-cost adsorbents like unmodified biochar, rice husk ash, and natural clays are practical choices. While their adsorption capacity is moderate, their low cost and easy availability allow for disposal without regeneration, which is often more economical for dilute waste streams. Thus, the selection of an adsorbent must be guided by the specific application field, the target metal, the required effluent quality, and the operational scale.
In Table 2, it can be observed clearly that the most significant category among all is carbon-based nano-adsorbents, including carbon nano-tubes (CNTS), mesoporous activated carbon, biochar, graphene, and fullerenes. These carbon-based materials offer several benefits, including facile production processes, rapid advances in contaminant management, and the optimization and enhancement of preparation methods for pollutant removal. In addition, these materials exhibit notable characteristics, including high thermal stability and exceptional adsorption capabilities. Scientists shifted their focus to nano-adsorbents because of these distinctive advantages. Ahmad [139] synthesized a multifunctional hybrid nano-adsorbent for the removal of heavy metals. Nejad and Mohammadi [144] prepared functionalized magnetic nanoparticles and found them to be an efficient adsorbent for removing malachite green and Pb(II) from water. For effective separation of adsorbents from solutions, the adsorbent is affixed to the surface of a magnetic material, prompting researchers to synthesize magnetic nano-adsorbents [109]. Sun and colleagues conducted a study in which they synthesized a graphene oxide-based nano-adsorbent functionalized with N-containing compounds. The synthesized adsorbent was able to remove Cr(VI) with a maximum adsorption capacity of 564.7 mg/g [172]. Adsorbents derived from biobased nano-materials, such as chitin nanowhiskers and nanocellulose, have been employed for the elimination of heavy metals from potable water [110,187]. Hence, nanomaterials proved to be efficient adsorbents due to their extensive surface area, numerous sorption sites, capacity for low-temperature modification, minimal intraparticle diffusion distance, and adjustable pore size and surface chemistry [188].
The proliferation of scholarly publications in this domain highlights the substantial impact of nanotechnology in addressing environmental concerns and metal contamination. Moreover, compared with natural adsorbents, researchers mostly favored using natural adsorbents or low-cost materials due to their economic feasibility. This observation highlights the worldwide scope of scientific investigation and the dissemination of information across research communities, particularly in the domain of metal cleanup utilizing nano-technology.
This bibliometric review reveals symmetrical research outputs on wastewater treatment and water purification through sorption-based technologies. A geographical symmetry in research contributions has been observed across countries such as Saudi Arabia, Iran, Egypt, the USA, and Republic of Korea, including the homogeneous development of adsorbents and the parallel contributions of conventional and nano-adsorbent technologies. Asymmetrical research outputs include the highest number of articles published by China (around > 10,000), followed by India. Also, the majority of studies have focused on the removal of conventional heavy metals such as Cd, Pb, Ni, and Zn. The application of biochar and nanomaterials has been widely used since the post-2015 period, whereas natural absorbents have been studied less. The asymmetric scale also indicates that more than 95% of publications have been based on laboratory studies using synthetic solutions rather than on pilot-scale wastewater treatment studies. Also, asymmetric collaboration patterns were observed in intra-Asia and Asia–US/Europe, rather than between African unions or Middle Eastern countries. China emerged as the leading publisher with 11,957 papers, trailed by India (4324 papers), the USA (1825 papers), Iran (1739 papers), Saudi Arabia (1484 papers), Egypt (1318 papers), and Republic of Korea (1194 papers). The country-wise numbers of papers published using sorption-based technologies, up to 200, are shown in Figure 4.
Figure 5 presents a comprehensive compilation of academic journals, sourced from various publishers, that have published a substantial number of articles (over 90) during the period 2010 to 2025 in the field of water remediation and purification investigations. Top contributing journals with up to 100 publications in water purification, metal removal, and adsorption studies from 2010 to 2025 are: 1. Journal of Hazardous Materials (1335), 2. Chemosphere (1110), 3. Chemical Engineering Journal (899), 4. International Journal of Biological Macromolecules (723), 5. Journal of Environmental Chemical Engineering (719), 6. Environmental Science and Pollution Research (684), 7. Separation and Purification Technology (632), 8. Journal of Environmental Management (556), 9. Science of The Total Environment (519), 10. Desalination and Water Treatment (453), 11. Colloids and Surfaces A: Physicochemical and Engineering Aspects (437), 12. Environmental Research (428), 13. Water Science and Technology (417), 14. Journal of Colloid and Interface Science (406), 15. Water, Air, and Soil Pollution (402), 16. Rsc Advances (384), 17. Bioresource Technology (315), 18. Journal of Molecular Liquids (295), 19. Journal of Cleaner Production (282), 20. Water Research (275), 21. Journal of Water Process Engineering (267), 22. International Journal of Environmental Science and Technology (229), 23. Environmental Pollution (220), 24. Water Switzerland (217), 25. ACS Applied Materials and Interfaces (197), 26. Scientific Reports (192), 27. Environmental Technology United Kingdom (190), 28. Environmental Science and Technology (177), 29. New Journal of Chemistry (169), 30. Applied Surface Science (165), 31. Carbohydrate Polymers (160), 32. Molecules (155), 33. Polymers (144), 34. Materials (138), 35. Microporous and Mesoporous Materials (130), 36. Materials Today Proceedings (128), 37. Desalination (127), 38. Water Environment Research (122), 39. Ecotoxicology and Environmental Safety (117), 40. Environmental Monitoring and Assessment (115), 41. Environmental Technology and Innovation (114), 42. Journal of the Taiwan Institute of Chemical Engineers (111), 43. Journal of Chemical Technology and Biotechnology (109) 44. Journal of Materials Chemistry A (106), 45. Separation Science and Technology Philadelphia (105), 46. Biomass Conversion and Biorefinery (105), 47. IOP Conference Series Earth and Environmental Science (104), 48. ACS Sustainable Chemistry and Engineering (104), 49. Journal of Applied Polymer Science (102), 50. Langmuir (101), 51. Journal of Environmental Science and Health Part A: Toxic Hazardous Substances and Environmental Engineering (100), etc. Among the 51 peer-reviewed journals that were evaluated over 100 research publications, it was found that 25 of them were published by Elsevier, five were published by the American Chemical Society (ACS) and MDPI, four each were published by Springer and Taylor and Francis, three each were published by the Royal Society of Chemistry (RSC) and John Wiley & Sons, and one each was published by the International Water Association (IWA), Nature, IOP Science, and Desalination publishers.

4. Conclusions

This study provides a comprehensive review of symmetrical and asymmetrical outputs and prominent topics in water purification, wastewater treatment, metal remediation, and sorption research from 2010 to 2025. The present research examined several essential characteristics of retrieved documents using the Scopus package, including annual publications, publication sources, and highly cited publications. Researchers have observed a notable trend toward the use of various adsorbents in their investigations, suggesting an ongoing pursuit of novel and inventive materials in water treatment. Other emerging countries’ publications have also expanded rapidly, possibly because of their growing concerns about severe heavy metal contamination. China has dominated, with significant annual increases, followed by India, the United States, Iran, Saudi Arabia, Republic of Korea, and so on. Adsorbent efficiencies in water treatment and remediation studies highlight the dynamic and multifaceted nature of these processes, which are influenced by a range of intrinsic properties, environmental factors, and operational considerations aimed at preserving water resources and safeguarding public health and the environment. In the case of nanotechnology-based remediation, the most prolific and impactful countries were China, India, and Iran, according to the most papers published. The dominant metals that were removed by nano-adsorbents were Cd, Pb, Cu, Hg, and Ni. Mechanistic analysis reveals that symmetric adsorbents offer consistent diffusion and regeneration, whereas asymmetric designs provide synergistic multi-site binding and higher capacity for mixed contaminants. Proper management of spent adsorbents through regeneration or safe disposal adds 20 to 40 percent to the overall treatment cost, and this must be considered in lifecycle assessments. Considering cost, efficiency, and scalability, modified biochar emerges as the most recommended adsorbent for practical applications due to its low raw material cost, high adsorption capacity (often 200–500 mg/g for heavy metals), facile synthesis, and excellent reusability over 5–7 cycles, whereas MOFs and graphene-based materials, despite superior performance, remain limited by high production costs (>500/kg) and complex synthesis. Furthermore, the choice of adsorbent should be tailored to the application field: activated carbon and biochar for drinking water, MOFs and magnetic nanoparticles for industrial wastewater, iron-based adsorbents for mining effluents, and low-cost natural materials for agricultural runoff.
Thus, to develop environmentally friendly and economically feasible solutions for water treatment and remediation, an in-depth assessment of the economic considerations of adsorbents requires a holistic approach that accounts for factors such as raw material costs, production scalability, lifecycle costs, regulatory compliance, and technological innovation. Overall, the analysis highlights the dynamic nature of research in water purification, wastewater treatment, and remediation, with ongoing efforts to find innovative solutions and improve existing technologies to address water-related environmental issues. The diversification of adsorbents and the growing number of publications indicate that international collaboration among researchers, professionals, and legislators across diverse academic specializations is crucial for advancing this critical area.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/sym18050865/s1: The Supplementary File has been added to this manuscript, and the captions of the Supplementary Figures have been added below. Figure S1: Scopus database search page; Figure S2: Data Retrieve- Search Results (Scopus Database).

Author Contributions

Conceptualization, A.D. and Y.C.S.; methodology, A.D.; software, A.D.; formal analysis, A.M.; investigation, A.D. and A.M.; data curation, A.M. and A.P.; writing—original draft preparation, A.D.; writing—review and editing, A.P., P.K.S. and R.K.C.; visualization, P.K.S. and Y.C.S.; supervision, P.K.S. and Y.C.S. funding acquisition, R.K.C. All authors have read and agreed to the published version of the manuscript.

Funding

The research stay of Anurag Mishra at UiT Norway was funded by BRIDGE (Project No.: 322325), and the publication charges are paid by UiT, Norway.

Data Availability Statement

The authors declare that the data supporting the findings of this study are available within the paper and its Supplementary Files. The bibliographic data of the given numbers of articles published from the years 2010 to 2025 reported in this paper can be retrieved from the Scopus database with the following links: 1. 25,722 [25], 2. 17,905 [26], 3. 4576 [27], 4. 3280 [28], 5. 2231 [29], 6. 2066 [30], 7. 1484 [31]. It is also worth noting that these links may be updated when new articles are added to the Scopus database. The bibliographic data source is free and openly accessible after entering the search keywords listed in Section 2. A glimpse of the search results is shown in the Supplementary File (Figures S1 and S2).

Acknowledgments

The authors are sincerely thankful to the Department of Civil Engineering, IIT (BHU) Varanasi, and UiT Norway for providing guidance and support in carrying out this research.

Conflicts of Interest

The authors have no relevant financial or non-financial interests to disclose. On behalf of all authors, the corresponding author states that there is no conflict of interest.

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Figure 1. Bibliometric mapping of conventional adsorbents used for wastewater treatment using sorption-based technologies using VOSviewer 1.6.20.
Figure 1. Bibliometric mapping of conventional adsorbents used for wastewater treatment using sorption-based technologies using VOSviewer 1.6.20.
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Figure 2. Year-wise numbers of papers published on the application of sorption-based technologies in the field of water purification or remediation studies.
Figure 2. Year-wise numbers of papers published on the application of sorption-based technologies in the field of water purification or remediation studies.
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Figure 3. Bibliometric mapping of nanomaterials used for wastewater treatment using sorption-based technologies using VOSviewer 1.6.20.
Figure 3. Bibliometric mapping of nanomaterials used for wastewater treatment using sorption-based technologies using VOSviewer 1.6.20.
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Figure 4. Country-wise global publication output using sorption-based technologies.
Figure 4. Country-wise global publication output using sorption-based technologies.
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Figure 5. Research journals published over 100 papers on the symmetry and asymmetry of wastewater treatment and water purification throughout the span of 2010 to 2025.
Figure 5. Research journals published over 100 papers on the symmetry and asymmetry of wastewater treatment and water purification throughout the span of 2010 to 2025.
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Table 1. Research output analysis on water purification, wastewater treatment, and remediation studies.
Table 1. Research output analysis on water purification, wastewater treatment, and remediation studies.
YearAdsorbents Used in Cited StudiesTP/YearCountry-Wise Highest Number of PublicationsReferences
2010Zeolite, kaolinite clay, chitosan-tripolyphosphate beads602China (154), India (94), USA (46)[32,33,34]
2011Wheat, Turkish illitic clay, FeO-coated sludge, peanut shell698China (199), India (111), USA (64)[35,36,37,38]
2012Dolomite adsorbent, switchgrass biochar, sludge biochar585China (171), India (94), USA (36)[39,40,41]
2013Zeolite, bentonite, palm shell AC, Mg-Al layered hydroxide680China (200), India (79), USA (58)[42,43,44]
2014Oak wood AC, zeolite, mango leaves817China (237), India (136), USA (66)[45,46,47]
2015Iron-coated zeolite, KMnO4-treated wood996China (323), India (138), USA (87)[48,49]
2016Fly ash, Ni-Fe layered double hydroxide, Mg-modified zeolite1266China (415), India (162), USA (96)[17,50,51]
2017Bentonite materials, ZnO-based montmorillonite, green polymer, biochar1419China (480), India (189), USA (116), Iran (99) [52,53]
2018Algal sorbent, apple peels, dendritic polymer, graphite oxide pellets, aluminum and zirconium-based xerogel, glutathione-functionalized melamine sponge, activated biochar1651China (665), India (171), Iran (121), USA (112)[54,55,56,57,58,59,60]
2019MgAl-layered hydroxide, iron–biochar composite, polypyrrole-based AC, Fe/Al/Ca-based adsorbent, microwave-functionalized rice husk and poly-aluminum chloride, iron-based sorption materials2085China (879), India (254), Iran (144), USA (132)[19,61,62,63,64]
2020Sodium alginate, Fe-Mn binary oxide biochar, functionalized straw absorbent, katoite (Ca3Al2(OH)12)2362China (1007), India (278), USA (167), Iran (135), Egypt (100), Republic of Korea (99)[65,66,67,68,69]
2021Modified biochars, polyethyleneimine modified straw hydrochar, OH-activated porous biochar, granular activated carbon, cerium–silver oxide composite incorporated with reduced graphene oxide, 2D hybrid LDH, activated carbon from cassava sludge2794China (1139), India (406), USA (165), Iran (158), Saudi Arabia (138), Republic of Korea (135), Egypt (124), Malaysia (104)[70,71,72,73,74,75,76]
2022Activated carbon fibers, magnetic metal–organic framework derivatives, metal composite oxide, ZnO immobilized on recycled polyethylene terephthalate3154China (1444), India (502), USA (189), Saudi Arabia (186), Iran (155), Egypt (146), Republic of Korea (145), Malaysia (126), Pakistan (104)[77,78,79,80,81,82,83]
2023Magnetic modified biochar, magnesium oxide-loaded mesoporous silica, bismuth and iron co-doped hydroxyapatite materials, polycaprolactone-bound diatomite filter, biomass-based removal, rice husk ash adsorbent, red mud-based geopolymer microspheres, functionalized cellulose, mesoporous activated carbon3165China (1413), India (522), Saudi Arabia (203), Egypt (170), Iran (167), USA (1603), Republic of Korea (131), Pakistan (117), Malaysia (116)[84,85,86,87,88,89,90,91,92]
2024Ligand-imprinted composite adsorbent, polyamide grafted eggshell, polymer-modified carbon, modified biochar, metal–organic framework, chitosan composite3481China (1549), India (550), Saudi Arabia (255), Iran (204), Egypt (200), USA (168), Republic of Korea (135), Pakistan (133) [93,94,95,96,97]
2025Chitosan, metal–organic frameworks, clay, biochar, zeolites, composites, alginate hydrogels, graphene-based aerogels 3843China (1673), India (616), Saudi Arabia (279), Egypt (214), Iran (209), USA (171), Pakistan (121), Republic of Korea (120)[98,99,100,101,102]
Table 2. Research output on nanotechnology-based water purification, wastewater treatment, and remediation studies (2010–2025).
Table 2. Research output on nanotechnology-based water purification, wastewater treatment, and remediation studies (2010–2025).
YearSelected Nano-Adsorbents UsedTP/YearCountry-Wise Number of PublicationsReferences
2010Amino-functionalized Fe3O4 @SiO2, n-alumina, n-TiO2, iron-doped AC, n-HMO, n-HFO18China (8), Iran (4)[103,104,105]
2011nZVI, polyrhodanine-encapsulated magnetic NPs, thiol-functionalized silica nano hollow sphere29China (10), Iran (7), India (2)[106,107,111]
2012Fe2O3 NPs, nanocomposite-based XG-g-PAM/SiO2, maghemite NTs, chitosan-grafted polyacrylamide51China (15), India (15), Iran (6)[112,113,114,115]
2013N-doped porous carbon, magnetite nanorods38Iran (9), China (8), India (8) [116,117]
2014Mercury nano-trap, hydrated zirconium oxide (HZO)/polystyrene hybrid adsorbent68India (16), China (15), Iran (9)[118,119,120]
2015Silica NTs, SiO2 magnetic NPs, nickel oxide nano catalyst75Iran (21), China (17), India (14) [121,122,123]
2016FeO NPs, nZVI, Fe2O3 NPs, nZV-Zeolite and Montmorillonite100China (31), Iran (23), India (19)[124,125,126,127]
2017Sulfonated magnetic NPs, mercaptoamine-functionalized silica-coated magnetic NAs, magnetic FeO NP–multiwalled carbon NT composites, guar gum–nano-ZnO122China (24), Iran (22), India (21) [128,129,130,131]
2018Graphene oxide NCs, biochar-immobilized nZVI, sulfonated graphene oxide NA, magnetic Zr-MOF NCs115China (41), Iran (24), India (16)[132,133,134,135]
2019nZVI composite, biochar-modified Mg/Al layered double hydroxide, graphene-based NAs, bi-functional nano-adsorbent, β-cyclodextrin and polyethyleneimine bi-functionalized magnetic NA, rod-shaped Ca–Zn@Chitin composite, metal–organic frameworks, molecularly imprinted magnetite nanomaterials151China (50), India (30), Iran (30), Egypt (12)[136,137,138,139,140,141,142,143]
2020Fe/Mn oxide micro NPs, metal oxide chitosan magnetic nanocomposites, bimetallic NCs, epoxy-triazinetrione-functionalized magnetic NPs, 2D water-stable zinc-benzimidazole framework nanosheets, poly (methacrylate citric acid) NA, ionic liquid-assisted mesoporous silica-graphene oxide NC, molybdenum-disulfide-functionalized MXene NC, nano iron oxide185China (47), India (46), Iran (26), Egypt (11), USA (11)[144,145,146,147,148,149,150,151]
2021Functional mesoporous silica NPs, 1D and 2D NCs, magnetic ion-imprinted chitosan NPs, magnetite-based polymeric NC, PEG/ Fe3O4/GO-NH2 NAs, magnetic chromium ferrite NC, zero-valent iron-decorated graphene oxide/activated carbon NC, multi-walled carbon NTs, hydrophobic fluorous metal−organic framework NA188China (60), India (38), Iran (31), Egypt (20)[108,152,153,154,155,156,157,158,159]
2022Magnetic iron oxide NA, mercapto-grafted magnetic graphene oxide, rutile phased titania NPs–acid-modified kaolinite clay, mesoporous guanidine functionalized Santa Barbara amorphous magnetic NA, superparamagnetic magnetite NA, mesoporous organic silica NA, La-anchored Zr-based metal–organic frameworks223China (76), India (55), Iran (26), Egypt (19), Saudi Arabia (18)[109,160,161,162,163,164,165,166]
2023Dolomite-quartz@Fe3O4 NC, carboxyl and thiol-functionalized magnetic NAs, graphene nanoflakes, iron oxide NPs, thiol-rich hydrogel adsorbent; PHPAm/Fe O @SiO-SH polymer NC, nanocellulose-based NA, nano-ceria biochar NC216China (64), India (50), Iran (27), Saudi Arabia (17), Egypt (17) [110,167,168,169,170,171,172]
2024AC–nanocrystal composites, MOF–alginate particles, chitosan-nano CuO composite, zeolitic imidazolate framework nanohybrid, nanosilica–chitosan porous microbeads, cellulose–nZVI composite252China (72), India (57), Saudi Arabia (30), Egypt (28), Iran (26)[173,174,175,176,177,178]
2025Carbon nanomaterials,
nanocellulose-based aerogels, ZnO-MXene nanocomposites, nanosilica microspheres, cellulose nanofibers, magnetic silica nanoparticles, glycosylated activated clay carbon spheres, Mixed Metal Nano-Oxides (MMNOs)		235China (69), India (61), Iran (30), Egypt (25), Saudi Arabia (25)[179,180,181,182,183,184,185]
n: nano; NPs: nanoparticles; TP: total number of publications/year; NAs: nano-adsorbent; NTs: nanotubes; NCs: nanocomposites.
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Debnath, A.; Mishra, A.; Pandey, A.; Singh, P.K.; Sharma, Y.C.; Calay, R.K. A Review of Symmetrical and Asymmetrical Research Outputs on Wastewater Treatment and Water Purification Through Sorption-Based Technologies. Symmetry 2026, 18, 865. https://doi.org/10.3390/sym18050865

AMA Style

Debnath A, Mishra A, Pandey A, Singh PK, Sharma YC, Calay RK. A Review of Symmetrical and Asymmetrical Research Outputs on Wastewater Treatment and Water Purification Through Sorption-Based Technologies. Symmetry. 2026; 18(5):865. https://doi.org/10.3390/sym18050865

Chicago/Turabian Style

Debnath, Abhijit, Anurag Mishra, Archana Pandey, Prabhat Kumar Singh, Yogesh Chandra Sharma, and Rajnish Kaur Calay. 2026. "A Review of Symmetrical and Asymmetrical Research Outputs on Wastewater Treatment and Water Purification Through Sorption-Based Technologies" Symmetry 18, no. 5: 865. https://doi.org/10.3390/sym18050865

APA Style

Debnath, A., Mishra, A., Pandey, A., Singh, P. K., Sharma, Y. C., & Calay, R. K. (2026). A Review of Symmetrical and Asymmetrical Research Outputs on Wastewater Treatment and Water Purification Through Sorption-Based Technologies. Symmetry, 18(5), 865. https://doi.org/10.3390/sym18050865

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