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Review

A Review on Superadsorbents with Adsorption Capacity ≥1000 mg g−1 and Perspectives on Their Upscaling for Water/Wastewater Treatment

by
Kannan Karunakaran
1,
Muhammad Usman
2,* and
Mika Sillanpää
3,4,5,6
1
Department of Chemistry, Faculty of Arts and Science, Bharath Institute of Higher Education and Research, Chennai 600073, India
2
PEIE Research Chair for the Development of Industrial Estates and Free Zones, Center for Environmental Studies and Research, Sultan Qaboos University, Muscat 123, Oman
3
Department of Chemical Engineering, School of Mining, Metallurgy and Chemical Engineering, University of Johannesburg, P.O. Box 17011, Doornfontein 2028, South Africa
4
Department of Biological and Chemical Engineering, Aarhus University, Nørrebrogade 44, 8000 Aarhus C, Denmark
5
Zhejiang Rongsheng Environmental Protection Paper Co., Ltd., No.588 East Zhennan Road, Jiaxing 314213, China
6
Department of Civil Engineering, University Centre for Research & Development, Chandigarh University, Gharuan, Mohali 140413, India
*
Author to whom correspondence should be addressed.
Sustainability 2022, 14(24), 16927; https://doi.org/10.3390/su142416927
Submission received: 21 September 2022 / Revised: 4 December 2022 / Accepted: 14 December 2022 / Published: 16 December 2022
(This article belongs to the Special Issue Advances in Technologies for Wastewater Treatment and Reuse)
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Abstract

:
An adsorbent’s properties, its adsorption chemistry, and treatment efficiency are all interlinked for water/wastewater treatment. This critical review focuses on superadsorbents possessing ultrahigh adsorption capacities of ≥1000 mg g−1 for an efficient water/wastewater treatment. Using Google Scholar, we reviewed about 63 prominent studies (2017–2022) on superadsorbents to evaluate their preparation, characteristics, adsorption chemistries, and mechanistic interactions in the removal of aqueous inorganic and organic contaminants. The major contribution of this article is to present a series of perspectives on the potential upscaling of these adsorbents in real-scale water/wastewater treatment. The main findings are as follows: (1) the current literature analysis suggests that superadsorbents hold reasonable promise to become useful materials in water treatment, (2) there is still a need to perform extensive pilot-scale adsorption studies using superadsorbents under quasi-real systems representing complex real aqueous systems, and (3) the technoeconomic analysis of their upscaling in industrial-scale water/wastewater treatments still constitutes a major gap which calls for further studies. Moreover, the mass production and effective application of these superadsorbents are the major issues for real-scale water treatments.

1. Introduction

Today’s modern era of fast technological and industrial leaps continually yield a large panoply of services, production processes, and products. No doubt, these entities have enhanced the quality and standard of material life on the planet for a noteworthy portion of the global population. However, the accompanying dark side persists, and this facet constitutes the whole set of environmental contaminations. Among the most serious environmental concerns are land degradation, soil contamination [1], air pollution [2], water pollution [3], and their impacts on biodiversity [4]. Environmental health issues which follow downstream a pollution episode are equally serious concerns requiring deeper preparedness, more effective mitigation measures, and rigorous controls [5,6].
With specific regard to water pollution, there are hundreds of reports in the current literature which collectively compile a broad database of environmental micropollutants [7,8,9,10]. Their list is alarmingly long with respect to the numerous types of environmental pollutants detected in contaminated water and wastewater, including toxic metals [11], dyes [12], nutrients [3], and xenobiotics [13,14]. Xenobiotics can include an array of agrochemicals, pharmaceutical compounds, personal care products, and veterinary drugs [14]. Today’s society, more than ever, realizes the absolute need to control and eventually stop the release of these environmental pollutants into water [15,16]. Developing new and greener processes for integrating cleaner production schemes in existing pollution-causing processes and finding innovative water treatments are gaining attention.
Adsorption has been recognized as a prominent strategy to treat contaminated aqueous systems [17,18,19,20]. Adsorption removes the pollutants by binding them on the surface of suitable adsorbents, with the two phases collectively constituting either a liquid–solid or gas–solid interface [21]. The adsorption process has been regularly associated with the following advantages: easy operation, high efficiency, low energy requirements, adequate recovery and appreciable possibility for reuse, low handling costs, insensitivity to noxious environmental pollutants, and no dangerous by-product(s) formations [17,18,19,20,22]. The number of research articles and reviews in adsorption science, and with specific reference to adsorbents developed for scavenging aqueous pollutants, has grown in an almost inflationary manner in the past few decades. Over the years, different classes of adsorbents have been synthesized. A simple search in the literature using Google Scholar with the search words ‘adsorbent aqueous solution’ returned more than 800 publications dealing explicitly with an adsorbent for each of the custom time ranges of 2017–2017, 2018–2018, 2019–2019, 2020–2020, 2021–2021, and 2022–2022. At this point, discussing each class of adsorbents can become extremely demanding and redundant as well. There exist many reviews which have already compiled and discussed these different types of adsorbents [17,18,19,20,22,23,24,25].
Therefore, this article is intended to review a specific set of adsorbents examined for their respective performance in sequestering aqueous environmental pollutants. This set of adsorbents, which we designate as ‘superadsorbents’, exhibited ultrahigh superior adsorption capacities of 1000 mg g−1 and above for a range of organic and inorganic species in aqueous solutions. This is the first review that focuses exclusively on these adsorbents with strong adsorption efficiency irrespective of their nature. Superadsorbents have demonstrated significantly high adsorption capacities for different aqueous pollutants at the laboratory scale. Activated carbon remains apparently the most preferred adsorbent for industrial application in water purification and wastewater treatment [26,27].
Despite the existence of a large number of studies on the synthesis and applications of novel superadsorbents having the ultrahigh adsorption capacities of 1000 mg g−1 and above, there is no review exclusively discussing these superadsorbents. With the latter observation as an adequate motivation, this article first discusses the salient findings of recent research studies (2017–early 2022) on the preparation, physicochemical properties, adsorption chemistries, and mechanistic interactions of novel superadsorbents for various precursor pollutant species (Figure 1). A number of perspectives which can plausibly assist in selecting a few of these novel superadsorbents for potential real-scale water purification are then aired.
For the purpose of preparing this paper, articles published in different journals were carefully selected from Google Scholar. This search engine was preferred since it returned a large number of articles (over 1000) from many publishers in one search. The search words entered were ‘superadsorbents water treatment’ and ‘ultrahigh adsorption adsorbents water treatment’ from 2017 to early 2022. When selecting the articles, the term ‘superabsorbent’ also appeared recurrently. Articles which had the word ‘superabsorbent’ in their title and/or within the body of the text were also retained for discussion if the following two conditions were met.
First, the ‘superabsorbent’ material should have demonstrated adsorptive capacity in the uptake of a pollutant precursor or known environmental pollutant. Second, the corresponding adsorption capacity should have been at least 1000 mg g−1. For writing this article, 63 research articles [28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89] were perused. An overview of the superadsorbents, pollutants adsorbed, and Langmuir model maximum adsorption capacities reported in some of these articles is provided in Table 1. Table 1 highlights both the high degree of variability in the types of aqueous superadsorbent–pollutant systems which have been examined and the maximum adsorption capacity of the corresponding superadsorbent.

2. Preparation Methods

Polymerization [43,44,50,56], cross-linking and co-polymerization processes [45,53], grafting of metal oxide in polymers [59], pyrolysis [51], hydrothermal [49], solvothermal [74], and electrochemical processes [28] have been used to prepare superadsorbents. The ultrahigh adsorption capacity of the superadsorbent depends on the synthesis method, the attributes imparted during its preparation, and the specific reaction conditions. Thus, different materials yield different adsorption performances. Polymerization involves using precursors whereby the inclusion of an initiator, cross-linking species, and oxidant play a significant role in achieving a specific superadsorbent. Salama reported the synthesis of anionic hydrogel prepared by grafting of p-(3-sulfopropyl methacrylate) onto carboxymethyl cellulose [32]. Here, p-(3-sulfopropyl methacrylate) acted as the monomer, and ammonium persulfate and -N,N’-methylenebisacrylamide were the initiator and cross-linker, respectively [32]. The synthesized superadsorbent sorbed methylene blue from aqueous solutions at 1675 mg g−1 [32]. In another study, a bioinspired hydrogel-type superadsorbent, which possessed high porosity (specific surface area = 2.8031 m2 g−1 and 5914.66% swelling ratio with a microporous structure), had a high adsorption capacity of 2276 mg g−1 for methylene blue because of the presence of nano-layered polydopamine [46]. This ultrahigh adsorption capacity was attributed to the hydrophilicity of polydopamine and constituent functional groups on the superadsorbent [46]. Polymer-type sorbents have received attention because of their unique properties such as ease of synthesis, excellent redox behavior, high adsorptive tendency, and biocompatibility. Ballav et al. [36] reported that an L-cysteine-doped polypyrrole superadsorbent could efficiently remove Hg2+ ions from aqueous solutions. In that work, in situ oxidative polymerization of pyrrole and L-cysteine was undertaken in the presence of ammonium persulfate oxidant at 25 °C [36]. Elsewhere, a polyaniline/lignosulfonate superadsorbent, prepared by one-step polymerization at 0–4 °C, removed Acid Red dye 94 from aqueous solutions at a maximum sorption capacity of 10.56 g g−1 [50].
The electrochemical method is one of the most promising techniques because of its mild reaction conditions, high yield of products, low-cost equipment, and high efficiency in energy consumption. In this method, factors such as concentration of electrolytes, physiochemical nature of anodic and cathodic electrodes, and amount of applied current density or potential difference play important roles. Batch or small-scale electrochemical synthesis of adsorbent is being entertained more than pilot-scale production. This is due to issues related to electrode material stability, adsorption capacity, and regeneration of sorbent normally encountered at the pilot scale. Jung et al. [31] reported the electrochemical synthesis of a novel magnetic Fe3O4/γ-Al2O3 hybrid composite using Fe-Al electrodes. The applied current density was 24.87 mA cm−2. After electrolysis, the prepared samples were pyrolyzed at 873 K for 1 h [31]. The superadsorbent thus prepared had the highest adsorption capacity of 1501 mg g−1 (Sips isotherm model) at 25 °C for Acid Black 1 [31]. Similarly, a two-dimensional MoS2 superadsorbent was synthesized by ultrasound-assisted electrochemical exfoliation where the natural molybdenite was the cathode, Pt the anode, and aqueous Na2SO4 the electrolyte. This adsorbent could chemisorb aqueous Pb2+ ions at 1479 mg g−1 [28].

3. Physicochemical Properties

A thorough analysis of the literature shows that the physicochemical characteristics of the adsorbent(s) play an important part in determining their adsorption behavior(s) towards specific pollutants under a specific set of analysis conditions. Moreover, it is equally necessary to highlight that each adsorbent has its own specific set of physicochemical properties. These properties are developed during the synthesis of the adsorbents [24] and can be improved through certain chemical, physical, and/or biological functionalization/modifications [19,90]. Functionalization is performed to introduce the desired changes in the adsorbent material’s physico-chemical properties, including surface area, pore size, pore volume, surface charge distribution and behavior [91], surface functional groups [19], and degree of aromaticity [92]. Interestingly, the physicochemical characteristics of adsorbents are highly diverse and depend on the type of adsorbent and its synthesis conditions.
For example, carbonaceous adsorbents have extensively rich porous structures and large specific surface areas, and the latter constitute the essential morphological features of the adsorbent materials [93]. Adequately developed and adsorption-conducive pore size distributions give rise to favorable mass transfer paths, while large surface areas host more active adsorption sites [93] where pollutant molecules can compete and be adsorbed selectively. Another example is layered double hydroxides, which are usually ionic layered materials by reason of their structures consisting of positively charged brucite-like layers and abundant exchangeable anions within their galleries [94]. Yet another class of novel adsorbents are graphene-based materials. The incorporation of oxygen functionalities augments the hydrophilicity of graphene oxide, as a result of which it is rapidly dispersed to form stable colloidal suspensions in aqueous solutions [95]. The significantly large surface area (approximately 2630 m2 g−1), high aspect ratio for modification, and superior colloidal stability than other carbon-type materials make graphene oxide a suitable candidate for sustaining adsorption and surface reaction [95]. Likewise, the adsorption literature is populated with details of many other physicochemical properties of materials exhibiting adsorption potential for aqueous adsorbates. Examples of those properties include surface area [42,43,46], nature and distribution of functional groups [30,35,36,46,50], the type of crystal structure [96], average pore size [97], degree of exfoliation [98], degree of cross-linking and rigidity of solid network [99], response to a magnetic field and/or the presence of magnetic moieties [24], swelling properties [100], zeta potential [96], variations in the extent of aggregation [101], and ion exchange capacity [19,98].

4. Adsorption Chemistries

The adsorption process is essentially a surface phenomenon whereby an adsorbate is bound to the surface of the adsorbent [102,103]. Adsorption mechanisms are mainly of the following three categories: chemisorption, which is brought about through the formation of chemical bonds [104]; physisorption, which can involve van der Waals forces [104]; and ion exchange [102]. The set of data obtained from adsorption mechanisms is of much significance when designing adsorption systems for handling real-scale process streams [102]. In an overwhelming majority, if not all, of adsorption-related research studies, reasonable attempts have been made to elucidate possible adsorption mechanisms using predefined mathematical models intended to portray the variations of equilibrium adsorption data.
These models can be linear and non-linear [102,105]. They can also be grouped as one-parameter isotherms (Henry’s isotherms), two-parameter isotherms (Hill–Deboer model, Fowler–Guggenheim model, Langmuir isotherm, Freundlich isotherm, Dubinin–Radushkevich isotherm, Temkin isotherm, Jovanovic isotherm, Elovich isotherm, Harkin–Jura isotherm, Flory–Huggins isotherm, Hill isotherm, Halsey isotherm, and Kiselev isotherm), three-parameter isotherms (Redlich–Peterson isotherm, Sips isotherm, Koble–Carrigan isotherm, Toth isotherm, Kahn isotherm, Radke–Prausniiz isotherm, Langmuir–Freundlich isotherm, and Jossens isotherm), four-parameter isotherms (Fritz–Schlunder isotherm, Baudu isotherm, Weber–Van Vliet isotherm, and Marczewski–Jaroniec isotherm), and five-parameter isotherms (Fritz and Schlunder derivation which approaches the Langmuir model) [105].
An adsorption isotherm describes the equilibrium relation between the concentration of adsorbate on the adsorbent and the quantity of adsorbate in solution at a constant temperature. An adsorption isotherm is constructed by studying the adsorption at different adsorbate concentrations followed by its mathematical modeling. An adsorption isotherm not only describes the extent of adsorption and the relationship between the concentration of adsorbate in solution and the adsorbent, but can also insignificantly contribute to reveal the underlying mechanisms and interactions between the adsorbate and adsorbent [106].
The pool of data from equilibrium adsorption isotherm models reveal the maximum adsorption capacity of an adsorbent for a specific adsorbate [102]. The maximum adsorption capacity is an essential parameter in the assessment and comparison of the adsorption performance of adsorbents within and across adsorption systems [102]. This is because the adsorption capacity gives an indication of the amount of adsorbate that can be scavenged from a specific volume of contaminated aqueous media. We say ‘within’ and ‘across’ adsorption systems from the following perspectives. ‘Within’ refers to a single adsorption system consisting of specific environmental conditions which can still be varied to explore the adsorption behaviors of the adsorbate(s)–adsorbent interactions. ‘Across’ refers to the comparative analysis of adsorption behaviors of two or more adsorption systems which may share a certain commonness in their specific environmental conditions, speciation of adsorbate(s), and properties of adsorbent.
A wide variety of adsorption isotherms have been applied for these superadsorbents (Table 1). Among them, Langmuir and Freundlich adsorption isotherms were widely used for the superadsorbents (Table 1). Though both of these models are two-parameter adsorption isotherms, they have different applications. The Langmuir isotherm model is assumed to be applicable for monolayer adsorption on homogenous sites. On the other hand, the Freundlich isotherm model is mainly applied to multilayer adsorption on heterogeneous sites. However, Langmuir adsorption isotherms were also found to fit better than Freundlich isotherms for the adsorption of cationic malachite green and anionic Congo red on calcium-rich biochar, having a heterogenous surface [34]. Therefore, it is necessary to select appropriate models considering the system’s complexity.
Besides adsorption isotherm modeling, adsorption kinetics analysis based on predefined mathematical equations has been frequently used to further understand the chemistry of an adsorbate–adsorbent interaction [107]. Adsorption kinetics data, particularly at initial time points, are highly important for accurate modeling and to derive logical conclusions [106]. Kinetic studies are intended to identify an appropriate contact time for an optimum equilibrium and adsorption process. The affinity between the adsorbate and adsorbent dictates the adsorption rate. For example, the adsorption of Pb2+ proceeded at a high rate, reaching equilibrium in 20 min on two-dimensional molybdenum disulfide [28], or may take 720 min when adsorbed onto esterified hydroxyapatite [48].
The kinetic models can be lumped as reaction-controlled kinetics or diffusion-controlled kinetics [107]. Typically, kinetic adsorption constant can be mathematically described by applying pseudo-first-order, pseudo-second-order, intraparticle diffusion, and Elovich models [106]. As reported in Table 1, the adsorption kinetics for a majority of the cited superadsorbents were better fitted by the pseudo-second-order model than the pseudo-first-order model. However, few studies reported that adsorption kinetics followed the intraparticle diffusion model [54,74]. Additionally, the significance of the intraparticle diffusion has been highlighted in few adsorption kinetics as the rate-limiting or rate-determining step [33,36,40,54]. For example, intraparticle diffusion has been noted as a rate-limiting process in the adsorption of amaranth dye onto Cu2O nano-composite, while the pseudo-second-order model exhibited the best fit for the adsorption kinetics [40].
Adsorption kinetic model elucidation for a specific adsorbate–adsorbent interaction has an important bearing in the design and scale-up of adsorption units [108,109]. For example, the effects of initial adsorbate concentration, adsorbent particle size, and stirring rate on the adsorption kinetics of methylene blue onto silica have been studied [108]. Among the many parameter-dependent kinetics findings, it was found that stirring rate and mass transfer coefficient shared a linear relationship [108]. Various kinetic models (such as the Yoon–Nelson model, Thomas model, and Bohart–Adams model) have been used in the analysis of laboratory-scale adsorption systems operated in the column configuration [109]. It should be noted that batch experiments allow optimizing the experimental conditions, while column studies, though complicated, are recognized as better representatives of field applications [110]. Based on the kinetic data compiled from such systems, the scale-up design of continuous adsorption columns can be worked out based on the breakthrough curve approach [109].
With these concepts in mind, the adsorption isotherm analysis, adsorption kinetics analysis, and adsorption selectivity analysis of the superadsorbents considered for this article are highly important components of adsorption studies. In fact, ‘selectivity’ is a relatively fresh aspect involved in the study of adsorption processes in aqueous media [111]. In a recent work, it has been indicated that the adsorption selectivity and capacity of electro-spun nanofibrous membranes, for example, for heavy metals depend significantly on the type and abundance of functional groups on the membranes’ surface [112]. In general, a high abundance of functional groups can be associated with higher adsorption capacity [112]. Yet, the extent to which the adsorption capacity of an adsorbent can shoot for a specific adsorbate will be determined by the specific interactions of the actual functional groups of the adsorbent with those of the adsorbate [113].

5. Mechanistic Interactions

It is important to adequately elucidate and understand the binding mechanism and nature of molecular interactions of a pollutant adsorbate onto a specific adsorbent. Being able to track the sequence of rate-limiting and non-rate-limiting steps at specific points within the adsorption process can plausibly allow the identification of areas of optimization of potential large-scale adsorption systems. As described elsewhere, the ‘mechanism of adsorption’ depicts the pathway by which the actual attachment of the pollutant species takes place onto the adsorption sites of the adsorbent, irrespective of molecule/ion diffusion [107]. ‘Attachment’ by way of an adsorptive route occurs spontaneously because the associated free energy is negative [107]. The force sustaining the attachment of the pollutant adsorbate is the total of many other forces which make up the total free energy of adsorption. Those driving forces can include intramolecular interactions and intermolecular interactions [107]. These interactions can also occur as a result of electrostatic attractions. These electrostatic attractions can be in the form of Coulombic attraction, covalent bonding, and dipole interactions. Surface complexation, oxidation reduction, isomorphic substitution, inner-sphere complexes, and ion exchange can also be other (competing) adsorbate uptake mechanisms (Figure 2) [54,107,114,115].
Mechanisms of adsorption are dependent on the nature of the pollutant and that of the adsorbent material. For example, adsorption can take less time to reach equilibrium onto non-porous adsorbents than porous materials owing to the different adsorption mechanisms [106]. Porous adsorbents rely mainly on pore filling along with other mechanisms including electrostatic attraction, surface precipitation, hydrogen bonding, cation exchange, π–π interactions, and n–π interactions. Likewise, Agathian et al. [42] highlighted the involvement of electrostatic interaction, hydrogen bonding, and complexation in the adsorption of Cr(VI) and Rhodamine 6G onto cotton fiber modified using poly-caprolactone/oxydianiline/V2O5 nano-particles. The super-loading of Pb2+ on 2D molybdenum disulfide was also attributed to the complexation of Pb2+ with intrinsic S or O atoms (as —S—Pb and —O—Pb complexation) together with electrostatic adsorption of positively charged Pb2+ onto negatively charged adsorbent [28]. Indeed, the nature of functional groups on adsorbent’s surface plays a vital role in determining the adsorption mechanism. For example, the higher adsorption of Hg2+ has been linked to the electrostatic interactions between positively charged Hg2+ and electron-rich functionality (—SH, —NH2 and —COOH) on the surface of L-cysteine-doped polypyrrole adsorbent [36]. These functional groups can strongly bind cationic pollutants. In another study, the FTIR characterization of GO intercalated layered double hydroxide after Pb2+ adsorption revealed an increase in the intensity of peaks [54]. This corresponded to 1120 cm−1 (C–O alkoxy group), 3460 cm−1 (O–H of COOH and/or H2O), and 1635 cm−1 (C=O and/or O–H bending), while the intensity of the peak corresponding to 791 cm−1 (M–O stretching vibration) decreased [54]. This has been attributed to the formation of —O—Pb-type complexation of Pb2+ with the carboxylic, epoxy, and hydroxyl groups in adsorbent structure. However, the involvement of different functional groups in adsorption in ion exchange was highlighted during the comparative adsorption of metal ions (Cd2+, Cu2+, Hg2+ and Pb2+) onto hyperbranched polyethylenimine functionalized carboxymethyl chitosan composite [47]. Moreover, FTIR spectral changes indicated the reaction of CN group with Hg2+ whereas the interaction of other three metal ions (Cd2+, Cu2+, and Pb2+) were prominent with —COO— group.
Thus, adsorption mechanisms can also vary according to the nature of the target pollutants. For example, Li et al. used maleylated-modified hydrochar to adsorb methylene blue and Cd2+ [49]. Their findings revealed that the adsorption of methylene blue occurred mainly through the π–π interaction, electrostatic interaction, and hydrogen bonding. However, predominant mechanisms for Cd2+ adsorption included surface complexation and ion exchange. Additionally, the co-existing ions and solution pH have substantial effects on adsorption extent and mechanisms, as reported by Qiu et al. [53] when two gels, namely sodium alginate/Ca/fiber hydrogel and cellulose nanofiber/chitosan aerogels, were used for the adsorption of methylene blue. Using a solution with pH lower than the point of zero charge led to the development of positive charges on the surface of the adsorbents repelling the pollutant. Here, the adsorption mechanism mainly involved the H-bond interactions between the nitrogen atoms in methylene blue and hydroxyl groups on the gels. However, when the solution pH was greater than the point of zero charge, gels’ surfaces were negatively charged, which led to electrostatic interactions as the adsorption mechanism. The solution pH can also affect the protonation/deportation of surface functional groups which could change the adsorption mechanisms [115]. However, recognizing the interactions at the adsorbent/adsorbate interface and the resultant adsorption mechanisms is a challenging task. Moreover, various mechanisms can be involved simultaneously and, therefore, identifying the extent of simultaneously involved interactions can also be a major challenge. To correctly depict the adsorption mechanisms, it would be highly rewarding to use sophisticated spectroscopic techniques along with an adequate interpretation of kinetic modeling.

6. Milestones towards Real-Scale Water Purification

The present literature analysis of academic research has revealed many interesting aspects about the recent pollutant superadsorbents. First, it is of striking interest to observe the quality and extent of the integration of nanostructures in the design of these superadsorbents. It is an unequivocal inference that nanostructures already constitute many innovative developments in water science, water purification, and wastewater treatment technology. The advocacy for integrating nanomaterials into adsorption-oriented wastewater treatment is strong globally [116,117]. Second, the selectivity of these superadsorbents for one pollutant molecule provides an opportune stepping stone to focus on the formulation of pollutant-specific removal strategies. Yet, the formulation of such removal approaches will be a significant challenge because they will have to be designed for real contaminated systems.

6.1. Comprehensive Field-Scale Analysis

In such real aqueous systems, there is a high propensity for the selected superadsorbent to face multicomponent adsorbates. Thus, upscaling the application of one superadsorbent, which yielded adsorption capacities of over 1000 mg g−1 in controlled laboratory conditions, to large-scale water remediation systems will have to rope in more rigorous field-scale research and optimization. These field-scale analyses can potentially reveal the limitations of the superadsorbent when exposed to much more complex contaminated waters, which are subject to several externalities and uncertainties. Here, the term ‘externality’ refers to any of those processes and parametric variabilities that may not have been considered until then in the adsorption analysis for the sake of simplification. The term ‘uncertainty’ depicts an unforeseen occurrence which can disrupt a stable adsorption process and induce overshoots in parametric behavior.
One externality, for example, can be the influence of other competitive pollutants which might not have been tested with the selected superadsorbent earlier. Another set of externalities can be the presence, fluctuations, and subsequent influences of one stimulus, or more concomitantly acting stimuli (e.g., pH, temperature, alkalinity, light intensity, and microbial populations). One uncertainty in the system can emanate from shock loadings of the pollutant due to possible variations in flows. Hence, the shift from the laboratory to the actual water treatment plant, and the subsequent integration of the selected superadsorbent(s) within real-scale water treatment systems cannot actualize itself without reengineering and optimizing their pristine properties. Given there are many parameters involved and which interact to influence the adsorption efficacy, it can be highly well-timed to move ahead by applying a multi-objective optimization strategy. The transformative potential of appropriate artificial intelligence techniques can also be harnessed for the optimization process [118].

6.2. Fulfilling ‘Thrust’ Research Directions

It is indeed an impressive feature of an adsorbent to have adsorption capacities of 1000 mg g−1 and more. From a simplistic perspective, it implies that every unit of mass of the adsorbent can take up and hold within its structure an equal or significantly higher mass of a specific molecule. From a process operation perspective, such ultrahigh adsorption capacities can be expected to occasion more effective clean-up of contaminated waters given that a relatively low dosage of the superadsorbent can scavenge significant quantities of the target pollutant. There is quasi-unanimity within the research community working actively in the field of water-based adsorption on the fact that the different types of superadsorbents deploy extremely remarkable performances at the laboratory scale. Yet, there are certain highly critical considerations which, we believe, require a whole new thrust at the research and development level before definitively integrating a superadsorbent within a full-fledged adsorption process. They are:
(i)
Selection of ecofriendly and cost-effective raw materials for synthesizing the superadsorbent having commercial potential.
(ii)
(Optimization of the chemical stability, selectivity, mechanical strength, and resilience of the superabsorbent.
(iii)
Guarding the superadsorbent against microbial degradation.
(iv)
Better understanding the dominant adsorptive mechanistic interactions of superadsorbents.
(v)
Comprehensive life cycle-oriented toxicity analysis of the superadsorbents in the environment.
(vi)
Tuning the physicochemical features in superadsorbents that can diminish their overall toxicological footprint.
(vii)
Formulation and implementation of appropriate local, federal, regional, and/or international policies which regulate the ‘cradle-to-grave’ design, production, usage, and disposal of superadsorbents for large-scale water purification.
(viii)
Recovery and reuse of superadsorbents for enabling process-effective and cost-effective reuse cycles.
(ix)
Probing the scope for hybridizing superadsorbent-type water purification with other treatment technologies such as membrane filtration, photodegradation, bioremediation, and/or advanced oxidation processes.
(x)
Developing multi-functional adsorbents to apply hybrid treatment by relying on the same materials.

6.3. Techno-Economic Maturity and Real-Scale Use ‘Worthiness’

The results reaped after addressing the above research and development challenges will enable the use of the selected superadsorbents in industrial-scale water treatment. For this, the market price of the superadsorbent will become an important design and operational parameter in industrial-scale water treatment. A first scenario, which may be less likely, is that the market price of such a superadsorbent is less than those of commercial activated carbons or activated carbon-based adsorbents. On the premise that the adsorption capacity of a ‘superadsorbent’ can be many times higher than those of certain pristine, functionalized, or commercial activated carbons for one target pollutant [70,87,119,120,121,122,123], the selection of the a ‘specialty’ superadsorbent can be a logical one in the first scenario. However, it is generally observed that activated carbon is the preferred adsorbent for industrial water treatment despite its high production cost [26,27].
In a second scenario, which is more plausible, the market price of a selected superadsorbent may be (significantly) more than those of commercial activated carbons or activated carbon-based adsorbents. The resulting, and quite challenging, research and development question is how to make this superadsorbent competitive. The need to make such a superadsorbent competitive and useful in real application anchors itself on the premise that some tangible and practical sense has to be absolutely made out of the immense pool of superadsorbents which have resulted from the extensive research efforts continually deployed in the related areas of chemical sciences and chemical engineering and innovation.
Technically, the superadsorbent can be further modified to enhance its overall adsorption performance and water remediation functionalities. Yet, then it may also mean that the cost of production can either decrease or increase. A decrease may become a possibility if there is a growing demand for such specialty chemicals and the materials used in their reengineering are benign yet markedly effective in enhancing their pristine (or latest) set of adsorption performances. An increase in the cost of production, and by ricochet on the market price of the superadsorbent, may occur if there is a low demand for the chemical and whose adsorption performance reengineering itself comes at higher costs. In a nutshell, there will practically always be an uncertainty in reaching a balanced trade-off among the techno-economic considerations and market forces intrinsically regulating the use, or not, of specialty superadsorbents in industrial-scale water treatment applications.
At present, the use of specialty superadsorbents is yet to gather its critical maturity and achieve complete worthiness for use in real-scale water treatment systems. Within the framework of the relevant policies (the existing ones or those to come in future), how fast and smoothly water treatment operators choose to shift to using and maintaining the use of the most suitable specialty superadsorbent(s) will inherently depend on many specific organizational priorities and competing techno-economic forces. The ensuing market-driven forces of supply and demand will then iteratively determine the micro- and macroeconomics of specialty superadsorbent production both at the research and development stages and at a probable ‘Specialty Superadsorbent Technology’ deployment level. How these technological and market-driven forces unleash will only be known when things start to move in the appropriate direction.

6.4. Priorities for Industrial Superadsorbent Production

From the above literature analysis, two observations can be made. First, the synthetic process for preparing the superadsorbent consists of more than one step and can involve different complex reactions [35,38,41,45,66,70,72,76,87]. For example, the reactions can involve crosslinking and gelation [87], co-precipitation, dispersion assisted by ultrasonication and radical polymerization [70], temperature-controlled polymerization in an inert atmosphere [72], self-stabilized precipitation copolymerization [61], and radiation grafting [41]. Second, the adsorption performance of the superadsorbent is examined at the laboratory scale in synthetic contaminated waters consisting of a few pollutants instead of real wastewater samples containing complex sets of pollutants.
Real contaminated waters contain many other pollutants which can actively compete for active adsorption sites. Therefore, if a specialty superadsorbent, which has been mass-produced, is to be deployed for industrial water purification applications, it will have to embody a few more properties besides having an ultrahigh adsorption capacity. These properties will have to include stability in adsorption performance amidst significant chemical complexity, mechanical robustness, and structural integrity under harsher conditions, significant resistance to potential in situ biodegradation, versatility in taking up a broader range of pollutants, and cost-effectiveness. Hence, besides the core economic considerations which will strongly influence the market behavior of specialty superadsorbents intended for use in industrial water treatment, their large-scale production systems will also necessitate intense research and engineering.
Once a versatile superadsorbent is selected for industrial water purification applications, having it retain an ultrahigh adsorption capacity and the desirable properties will plausibly need to become a priority for its mass production. When fulfilled adequately, this priority can contribute to the use of the specialty superadsorbent remaining technically effective and economically meaningful over a wide range of contaminated waters and adsorption-oriented water treatment systems. The specialty superadsorbent mass production process can be envisioned as being complex and state-of-the-art. It could be complicated due to the sequential operations and controlled parametric conditions required to construct the specialty superadsorbent and impart it with desirable properties. It should be state-of-the-art regarding in-line instrumentation, reactor designs, and the level of sophistication needed to house and control the complex reactions involved in the fabrication of the specialty superadsorbent, most expectedly nanostructured or nanocomposite in nature.
To the best of the authors’ knowledge, the count of industrial-scale mass production systems for producing specialty superadsorbent(s) for application in water purification appears close to nil. Indeed, there appears to be a marked paucity of data about the development and behavior of such superadsorbents’ mass production at the industrial scale. Ideally, one of the targets of this mass production will be to reach production to the tune of a few tens of kilograms of specialty superadsorbents per day, or a few tens of tonnes yearly, and hopefully more. Nevertheless, it is highly encouraging to note that there are a few organizations (for example, MetNano Limited, https://www.azonano.com/article.aspx?ArticleID=3938 (accessed on 21 September 2021), and the Centre for Nano Production and Micro Analysis at the Danish Technological Institute, https://www.dti.dk/specialists/nano-production-and-micro-analysis/36983 (accessed on 21 September 2021)) which are actively involved in the large-scale synthesis of nanoparticles for a wide range of other applications. In the December 2015 issue of Chemical Engineering (https://www.chemengonline.com/ and https://www.chemengonline.com/large-scale-production-carbon-nanotubes/, both websites were accessed on 23 September 2021), it was reported that in November 2015 Zeon Corp. (Tokyo, Japan) had started up the world’s first mass-production plant for producing high-grade (>99% purity) carbon nanotubes using the Super Growth method at its Tokuyama facility in Shunan City, Yamaguchi Prefecture, Japan (https://www.chemengonline.com/large-scale-production-carbon-nanotubes/) (accessed on 25 November 2022). Recently, Randon Companies, particularly Fras-le and the Randon Technology Center—CTR, discovered a novel and innovative method for producing niobium nanoparticles with enhanced properties on a large scale (https://www.fras-le.com/us-ca/noticias/randon-companies-announce-the-discovery-of-a-method-for-large-scale-production-of-niobium-nanoparticle/, accessed on 23 September 2021).
The EU-funded project BUONAPART-E stands as an initiative which has supported the advancement of the ‘upscaling and optimization of nanoparticle and nanostructure production’ using electrical discharges (https://cordis.europa.eu/article/id/190623-largescale-synthesis-of-nanoparticles (accessed on 21 September 2021)). The National Nanotechnology Initiative of the U.S. government offers a wide-ranging set of openings and resources which can be harnessed to promote technology transfer and commercialization in nanoscale research and development (https://www.nano.gov/, accessed on 31 March 2022). Through its nanotechnology centers, this initiative provides a broad platform for (i) fostering communication between academia and the business community on ongoing research activities, (ii) probing and developing opportunities for partnership and research collaborations, and (iii) making use of the available resources to advance nanotechnology research and development (https://www.nano.gov/, accessed on 31 March 2022) and nanomanufacturing (https://www.nano.gov/node/1570, accessed on 31 March 2022).
Interestingly, a deeper survey of the literature revealed two interesting studies which deal with the analysis of large-scale adsorbent production scenarios [124,125]. It has been found that poor energy efficiency of promising technologies can be a limitation for the large-scale production of bioadsorbents (viz., nanoparticles-modified chitosan microbeads) [124]. Specifically, the bioadsorbent production routes assessed had poor energy performance [124]. This performance was attributed to high water losses occurring in the bioadsorbent purification stages and to the separation units’ poor energetic efficiency [124]. The study eventually suggested deploying further research efforts in elucidating the environmental and economic impacts of the bioadsorbent production routes with a view to obtain additional sustainability-oriented indicators which could assist in selecting the most promising option for the specific system parameters assessed [124].
Recently, techno-economic and sensitivity analyses have been applied to examine production topologies of chitosan-based bioadsorbents (viz., chitosan microbeads and TiO2 nanoparticles-modified chitosan microbeads) [125]. The economic performance indicators garnered from the economic analysis demonstrated a significant potential to develop a chitosan market which can generate profit following the implementation of those production processes [125]. Interestingly, the sensitivity analysis revealed that the chitosan–TiO2 production process could absorb the higher variations in operating costs [125]. Such studies can surely serve to set the tone for deeper research and development analysis required for selecting and developing the blueprint for implementation the mass production of a specialty superadsorbent within the framework of a profitable business model.

7. Summary and Conclusions

This article presents a critical review on superadsorbents exhibiting ultrahigh adsorption capacities of 1000 mg g−1 and above for aqueous pollutants. The efficiency of these superadsorbents is strongly dictated by the synthesis method, characteristics, reaction conditions, and the pollutant type. Their range of adsorption chemistries and adsorption mechanisms vary significantly across the studied bench-scale aqueous systems and the tested materials/pollutants. Owing to their strong efficiency, finding suitable strategies to synthesize and/or improve the efficiency and stability of novel superadsorbents is a promising field. For this, it would be highly rewarding to look into cost-effective, efficient, and environmentally friendly raw materials that can be used to develop these superadsorbents. Solid wastes, being cost-effective and abundant, can be suitable candidates for the synthesis of superadsorbents. This would also facilitate the conversion of solid wastes into useful resources. Similarly, nanotechnology has found many applications in the development of superadsorbents, but a better understanding of the fate, ecotoxicity, and stability of nanomaterials is required. The development of multi-functional superadsorbents can also facilitate the integration of adsorption and other remediation strategies (such as advanced oxidation processes) for highly efficient wastewater decontamination. Moreover, a better understanding of the regeneration and reuse of spent adsorbents is also required.
The upscaling of the laboratory studies to field conditions remains a major challenge in the applications of superadsorbents in water/wastewater treatment. Indeed, a longstanding question resurges when the superadsorbents are viewed through the lens of practical application in real-world water purification systems. This question is simple: Which superadsorbent should be selected for use in a specific real-world steady-state water/wastewater treatment system? This question is highly legitimate because it raises a number of spin-off questions within the realm of realistic engineering applications in water purification. These spin-off questions include:
(i)
Why is there is a need for several synthetic methods for preparing superadsorbents?
(ii)
Is there any comprehensive method which is recommended for potential application in real-scale water treatment processes?
(iii)
Will the selected superadsorbent embody identical attributes at large-scale production?
(iv)
Will the selected superadsorbent deliver the expected adsorption behavior and performance in real contaminated aquatic systems which constitute hostile environments to its survival?
(v)
Which are the definitive techno-economic yardsticks to apply when actually selecting one specific superadsorbent?
These research and development questions constitute a clear call to the research, engineering, and industry communities involved in water purification. Ideally, the superadsorbent selected for scavenging pollutant molecules is envisioned as an ‘intelligent’ material which can self-regulate its properties and retain its ultrahigh adsorption behavior by ‘sensing’ variations in the key environmental conditions in its immediate micro-level and macro-level surroundings. These variations in the aqueous media originate from changes in pH, temperature, light intensity, magnetic field strength, and microbial stresses induced by a variety of microorganisms such as bacteria or viruses. One overarching response to these spin-off questions is that the selected superadsorbent needs to be versatile. Its versatility should include:
(i)
Its ability to withstand the harshness and complexity of different contaminated aqueous media in which it will be put to service;
(ii)
Its ability to self-clean and self-heal in the wake of the impacts those harsh environmental conditions (e.g., extreme fluctuations in pH, temperature, and hydrodynamic forces) can have on its structure;
(iii)
Its resilience to perform effectively and efficiently as an adsorbent until it reaches absolute exhaustion at the end of a predetermined number of regeneration cycles.
The future of ‘superadsorbents’ exhibiting adsorption capacities is promising, but more efforts in the science and engineering disciplines should be invested in developing viable adsorption-based water purification systems.

Author Contributions

Conceptualization, K.K., M.U., and M.S.; data curation, K.K.; writing—original draft preparation, K.K., M.U., and M.S.; writing—review and editing, K.K., M.U., and M.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

We are grateful to Ackmez Mudhoo (University of Mauritius) for his insightful contribution to this review.

Conflicts of Interest

The authors declare no conflict of interest.

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Sustainability 14 16927 g001 550
Figure 1. Major focuses of the present review on superadsorbents.
Figure 1. Major focuses of the present review on superadsorbents.
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Figure 2. A schematic illustration of the predominately reported interaction mechanisms involved in the removal of various pollutants by adsorption (adapted with permission from Ref. [115]).
Figure 2. A schematic illustration of the predominately reported interaction mechanisms involved in the removal of various pollutants by adsorption (adapted with permission from Ref. [115]).
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Table
Table 1. Maximum adsorption capacities of selected examples of pollutant superadsorbents.
Table 1. Maximum adsorption capacities of selected examples of pollutant superadsorbents.
SuperadsorbentTarget PollutantsMaximum Adsorption
Capacity (mg g−1)		Ref.
Two-dimensional molybdenum disulfidePb2+1479[28]
SBA-15/polyamidoamine dendrimer hybridAcid Blue 621428[29]
Polyamidoamine dendrimer-modified carbon nanotubesPb2+, Cu2+4870 (Pb2+), 3333 (Cu2+)[30]
Fe3O4/γ-Al2O3 hybrid compositeAcid Black 11959[31]
Carboxymethyl cellulose-g-poly(3-sulfopropyl methacrylate) hydrogelMethylene blue1675[32]
Poly(acrylic acid)-based nanocomposite hydrogelMethylene blue2100[33]
Calcium-rich biocharMalachite green, Congo red12,502 (Malachite green),
20,317 (Congo red)		[34]
Non-covalent functionalized graphene oxide with an organic gelatorMalachite green, Eriochrome blue black R2687 (Malachite green),
1189 (Eriochrome blue black R)		[35]
L-cysteine-doped polypyrroleHg2+2042[36]
Magnetite single-walled carbon nanotubes-CoSHg2+1666[37]
Magnetic MnFe2O4 and CuFe2O4-2-aminobenzoic acid–phenylenediamine nanocompositeMethylene blue7089[38]
MIL-Ti metal–organic frameworksBasic Red 46, Basic Blue 411296 (Basic Red 46), 1257 (Basic Blue 41)[39]
Nanoparticles-composed Cu2O microspheresAmaranth7627[40]
Poly-acrylic acid-g-Corn huskMethylene blue1682[41]
Cot-g-PCL-Schiff baseRhodamine 6G2121[42]
Reduced graphene oxide–CoFe2O4 ferrite–polyaniline nanocompositeU6+2430[43]
Poly(2-acrylamido-2-methyl-propanesulfonic
acid-co-acrylic acid) hydrogel spheres		Methylene blue and Pb2+4625 (methylene blue)
and 4312 (Pb2+)		[44]
pH-responsive resin containing glycine
and maleic acid		Methylene blue2101[45]
Bioinspired catecholamine/starch compositeMethylene blue2276[46]
Hyperbranched chitosan compositeHg2+1722[47]
Esterified nanohydroxyapatite nanocrystalsPb2+2398[48]
Maleylated modified hydrocharMethylene blue1155[49]
Polyaniline/lignosulfonateAcid red 9410,560[50]
Saccharina japonica macroalgae-derived biocharCrystal violet1719[51]
PAF-1@cellulose nanofibrilBisphenol A1000[52]
Sodium alginate/Ca/fiber hydrogelsMethylene blue1335[53]
Graphene oxide intercalated layered double hydroxidePb2+1062[54]
p(AETAC-co-NVP) hydrogelsMethyl orange1992[55]
Partially hydrolyzed polyacrylamide-grafted Arabic gumMethylene blue2300[56]
Polyacrylamide/chitosan/Fe3O4 composite hydrogelsMethylene blue1603[57]
Dual-functionalized microporous organic
network		Methylene blue, malachite green and crystal violet2564 (methylene blue), 3126 (malachite green), 1114 (crystal violet)[58]
TiO2 nanoparticles dispersed in chitosan-grafted polyacrylamide matrixSirius yellow K-CF1000[59]
Microtube and microsphere porous carbon synthesized from poly(ε-caprolactone-b-4-vinyl benzyl chloride) triarm block copolymer with ZnCl2Malachite green1684[63]
Poly((methacryloylamino)propyl trimethylammonium chloride-co-vinylimidazole) quaternized hydrogelsEriochrome black T, methyl orange1818 (Eriochrome black T), 1449 (methyl orange) [64]
Poly(m-phenylenediamine) microspheresHg2+1499[65]
Poly(vinyl alcohol)/potassium humate/guar
gum-based interpenetrating network hydrogel		Methylene blue1166[66]
Manganese Prussian blue analogue/graphene oxide compositeCiprofloxacin1826[68]
Maleic acid and glycine-based pH-responsive
resin		Pararosaniline hydrochloride1534[69]
Villi-like poly(acrylic acid)-based hydrogelMethylene blue2286[72]
Amine functional cyclotriphosphazene submicrospheresMethyl orange1244[73]
KIUB-MOF-1 (Co-based metal organic framework)Methyl orange, methylene blue and malachite green15,610 (methyl orange), 14,721 (methylene blue), 5083 (malachite green)[76]
β-cyclodextrin modified hydrogelMethylene blue2638[77]
Magnetic sulfur-doped graphene-like carbon-supported layered double oxideMethyl orange1456[79]
Pod-inspired MXene/porous carbon microspheresCrystal violet2744[80]
β-cyclodextrin and magnetic graphene oxide modified porous composite hydrogelMethylene blue, Safranine T2802 (methylene blue),
1470 (Safranine T)		[70]
CuS/cellulose compositesHg2+1040[81]
Functional chitosan-based hydrogel nanocomposites (2 wt % of montmorillonite clay loading)Basic Red 461813[84]
Sodium alginate grafted poly (N-vinyl formamide-co-acrylic acid)-bentonite clay hydrogelMethylene green2108[85]
Carrageenan and itaconic acid-based hydrogelMethylene blue, crystal violet2439 (Methylene blue),
1111 (Crystal violet)		[87]
MgAl layered double oxideCd2+, Cu2+ and Pb2+1422 (Cd2+), 1135 (Cu2+),
1336 (Pb2+)		[89]
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Karunakaran, K.; Usman, M.; Sillanpää, M. A Review on Superadsorbents with Adsorption Capacity ≥1000 mg g−1 and Perspectives on Their Upscaling for Water/Wastewater Treatment. Sustainability 2022, 14, 16927. https://doi.org/10.3390/su142416927

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Karunakaran K, Usman M, Sillanpää M. A Review on Superadsorbents with Adsorption Capacity ≥1000 mg g−1 and Perspectives on Their Upscaling for Water/Wastewater Treatment. Sustainability. 2022; 14(24):16927. https://doi.org/10.3390/su142416927

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Karunakaran, Kannan, Muhammad Usman, and Mika Sillanpää. 2022. "A Review on Superadsorbents with Adsorption Capacity ≥1000 mg g−1 and Perspectives on Their Upscaling for Water/Wastewater Treatment" Sustainability 14, no. 24: 16927. https://doi.org/10.3390/su142416927

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