ReviewAdsorptive removal of different pollutants using metal-organic framework adsorbents
Graphical abstract
Introduction
The industrial revolution and fast industrialization developments, urbanization, and population expansions have contributed immensely to the detrimental effects of soil, water, and air pollutions [[1], [2], [3]]. To date, the origins of water, soil, and air pollutions are majorly from industrial wastes, mining, oil spill, nuclear waste leakage pesticides, etc. The huge amounts of contaminants discharged from households and industrial processes have instigated significant adverse effects on human life and eco-environmental societies [4]. Among such numerous pollutants are bio-toxins, heavy metals, COx, organics SOx, NOx, NH3, antibiotics, biological and chemical weapons, other industrial chemicals, food additives, veterinary, pharmaceutical, and personal care products [[5], [6], [7]]. Numerous physical, biological and chemical technologies were adopted in the past years for successfully controlling these pollutions [[8], [9], [10], [11], [12]]. Adsorption technology has been widely used among other technologies owing to its simplicity, ease of operation, and efficiency in removing pollutants of different types, and also it is not upshot in the secondary pollutants which produce harmful constituents [[13], [14], [15]].
Numerous toxic organic and inorganic chemicals have been discharged via the industrial wastes originating from painting, manufacturing, coating, mining, metallurgical, and nuclear industries, thus, causing the pollution of the air, water, and soil. The majority of the aforementioned pollutants and others including VOCs, CO2, NH3, H2S, Cd, Cr, Cu, Ni, Zn, etc., emanated from these industries have a notorious effect on the ecosystem [14]. Their presence in the ecosystems poses remarkable harm to the human and aquatic systems. Ineffective treatment (for instance wastewater) and management has caused widespread pollution of the water system, while global considerations largely highlighted water scarcity, water usage efficacy, and allocation matters [16]. Continuous water quality degradation worldwide has been observed to worsen global water scarcity [17]. The distributions of pollutants as controlled by the United States Environmental Protection Agency (USEPA) and also the imminent contaminations are presented in Fig. 1 [18].
Considering their effects on the ecosystem and the associated health menace, several strategies, and technologies have been employed including, filtration, chemical precipitation, coagulation/flocculation [[19], [20], [21]], etc. However, the techniques have associated shortcomings such as the necessity to use the complex instrument, space-consuming facility, huge maintenance costs, and so on [22]. Therefore, efficient and affordable methods of converting or separating these pollutants into substances that are non-toxic and environmentally friendly remains a big challenge. This makes the adsorption technology the only option capable of removing the water, soil, or air contaminants and the process is not energetic [3,23]. Combating the problems associated with the pollutant discharges requires the application of adsorption technology which uses the activated carbon (ACs) [[5], [6], [7]].
Adsorption method is a simple design, low/no cost, convenient operation, not sensitive to the toxicants, and reusability prospects. It makes use of wider ranges of adsorbents for efficient decontamination [[24], [25], [26]]. In view of its high sorption capacity, inexpensive ability, potential regeneration benefits, numbers of ACs [[27], [28], [29], [30], [31], [32], [33], [34], [35]] have been employed for the removal of different contaminations. However, the major prerequisite of this field is the selection of the novel adsorbents that can remove these pollutants without themselves becoming a serous pollutant than the one being removed [[36], [37], [38]]. For efficient adsorption processes, the adsorbents must have larger surface areas, pore volumes, and adequate functionalities. These factors remain determinants of successful adsorption processes [[5], [6], [7]]. Presently, carbonaceous/porous materials derived from activated carbons have been fabricated for effective adsorption processes, due to their various efficiency and distinctiveness for removing different contaminants from water, soil, and air [15,[39], [40], [41], [42]]. In fact, commercial activated carbon (CAC) has been shown to demonstrate high porosity, higher adsorption capacities, and thermal stabilities [[43], [44], [45], [46], [47], [48], [49], [50], [51], [52]]. It is efficient in cleaning up effluents and wastewater [1], where it is used to polish the influent before discharging into the receiving bodies [53]. Nevertheless, the adsorption of pollutants by CAC has some major restrictions; including the high cost of the AC, the need for regeneration after exhausting, and the loss of adsorption efficiency after regeneration [1,54]. These have significantly limited their further usages, thus paving way for other alternative adsorbents. In the search for such materials, some authors have utilized ACs derived from other low-cost adsorbents such as peat and agricultural materials for removing the heavy metal, dyes, etc., [[55], [56], [57], [58], [59], [60], [61], [62]]. However, the use of porous adsorbents (e.g aluminophosphates, ACs, zeolites, and clays, pillared clays, polymers, mesoporous oxides [[63], [64], [65]] for removing various contaminants from the ecosystem have been in high demands. These materials however, have issues of lower surface areas and tailorability problems which impede their satisfactory performances [23]. Similarly, the use of conventional photocatalysts, TiO2, and metal sulfides have problems of lower solar energy efficiency and poor photo-current quantum yield. This, therefore, imposes difficulties in post-separation and photocorrosion [[66], [67], [68]], thereby, leaving great demands for the development of the novel photocatalysts with the superior ability for decontamination purposes. The recent advancement in chemistry and related fields have resulted in the synthesis of other novel adsorbents like porous organic polymers (POPs), MOFs, covalent organic frameworks (COFs), and covalent triazine frameworks (CTFs) [[69], [70], [71], [72], [73]]. Adsorptive removal of different contaminants from the ecosystem using these adsorbents has been on the continuous rise while the adsorptive separation of these pollutants and the reversal of the same were also reported [74].
Considering, all these shortcomings and inherent problems; associated with the available adsorbents, MOFs stand unique based on their ease of tailorable pore sizes and structures, abundant active sites , and facile charge-separation under ambient light condition. These pave way for MOFs as good and alternative candidates. The interest in their suitability for adsorption technique in recent years is increasingly bridging research gaps in both environmental management systems, scientists, and engineers. MOFs, also known as porous coordination polymers, are assembled through the combination of clusters or metal ions with the organic ligands' coordination linkages. The utilization of MOFs has undergone a rapid development of being among the most active fields in coordination chemistry [75,76]. Over the years, MOFs have been known for several advantages; ranging from the higher surface area, diversified structures, tunable functional groups, to permanent porosity. These unique attributes made them suitable for a wider range of applications, such as separation [77,78], catalysis [79], storage [80,81], sensing [82,83], imaging [84], etc. Owing to the aforementioned prospects, many attentions from both material scientists, chemists, and/or environmentalists have shifted to the use of MOFs [[85], [86], [87]].
The crystallinity and porous nature of MOFs materials composing of organic linkers and metal ions or clusters connected by coordinative bonds, have resulted in their ultra-higher surface areas and porosities. These simple motifs for the combination of various organic linkers and metal nodes have led to the discovery and synthesis of about 70,000 MOFs [88,89] and more than 130,000 hypothetical MOFs [90] with unique topology and chemical composition [91]. Also, the higher chemical tunability over the metal cluster(s) and organic linker(s) makes the MOFs the superior candidates for wider ranges of applications in catalysis, storage, separation, and energy source for H2 and CH4 [[92], [93], [94], [95], [96], [97], [98]].
Though larger numbers of MOFs have been synthesized and experimented with to date, most of these synthesized MOFs are always neglected once another MOFs with superior performances are developed in the targeted applications. Generally, the authors have focused majorly on a few MOFs including HKUST-1, MIL-53, ZIF8, MOF-74, BUT-66, and UiO-66; owing to their exceptional qualities for many industrial applications. Therefore, recycling of the neglected MOFs is limited. Whereas, these MOFs can be recycled and modified for more noticeable enhancements in their properties which can perhaps render them to be better candidates for more applications than the original intention. Such interests should raise some questions of (but not limited to), why the pre-existing MOFs are not performing to their optimum capacities? What limited or impeded their possibilities from attaining the optimum potentials? What are the better experimental, optimization, and operating conditions?
However, despite the fascinating properties of MOFs for a wider range of applications, their stabilities have been of great concern. Many MOFs are unstable under humid air conditions [74,82,99,100], while many of them with the hydrophilic groups or atoms on their pore surface preferentially adsorb water molecules over aromatic hydrocarbons [99]. Therefore, the selection of adsorbent for any adsorption system requires serious consideration of the omnipresent water vapor presence in any industrial stream. Because, many unfavorable conditions such as the organic ligand, metal ion, metal-ligand coordination geometry, and pore surface hydrophobicity are major limitations and threats to the stability of MOFs. Therefore, the instability aspects of some MOFs under these conditions limit their fascinating properties for a wider range of applications. The chemical stabilities of the MOFs have to do with their abilities to retain their structures under conditions that are unfriendly [74]. The chemical stabilities of MOFs are majorly controlled by two main factors: the external factor (i.e. the operating environment(s)) and internal factors (i.e. structures of the MOFs). Some MOFs have been reported to be unstable as their frameworks become compromised on exposure to moisture in air or water which are ever available components in most industrial processes. Such structural deformation has limited the desirability of most MOFs for a wider scope of applications. Therefore, the instability of these MOFs can be attributed to limiting factors such as the organic linkers, hydrophobicity, metal ion clusters, geometry, surface area, and other operating conditions [[100], [101], [102], [103], [104]]. The susceptibility of the bonds coordinating the frameworks has been also reported to contribute to their stability [105]. Besides this, competitive binding remains another major factor controlling the efficacy of MOFs for the sequestration of different pollutants, particularly under toxic industrial chemical conditions. Therefore, this limitation requires MOFs with the higher selectivity for binding the target molecules perhaps to achieve reasonable efficiency. Also, the introduction of hydrophobic groups via the post-synthesis functionalization can be considered to enhance the selectivity for such pollutant over water.
This review, therefore, aimed at presenting some advancement in the use of MOFs for effective adsorptive removal of different contaminants from the ecosystems. Beginning from the introduction, we gave some useful insights and hints into what is to be done to most of the past synthesized MOFs that have been neglected once more MOFs with superior performances were discovered. In addition, this study also gave some vital insights into the stability of MOFs for the removal of different contaminants from the ecosystems with the corresponding adsorption mechanisms, while the milestones of synthesis and application of MOFs in water pollutant remediations are not left out. To furtherance the effective and practical applicability of MOFs, major frequently unnoticed factors were proposed as the challenges and prospects to foster better improvements in the removal of different pollutants.
Section snippets
Metal-organic frameworks
Metal-organic frameworks are novel adsorbents that have gained the attention of researchers globally owing to their unique structural diversity from the coordination bonds; existing between the inorganic metal atoms (acting as nodes) and organic ligands (acting as linkers). The porous hybrid solids could possess inorganic parts with large dimensionalities that could give rise to chains (1D), layers (2D), and even frameworks (3D) which are called MOFs [[106], [107], [108]]. The inorganic parts
Insight into the stability of MOFs
Recently, the research on the MOFs adsorbents with crystalline, highly ordered, and porous materials for remediating various pollutants has been reported as one with extraordinary performances [76,178]. In par with nanomaterials, MOFs are suitable for diverse applications. Nevertheless, the poor stability of the MOFs in water deters their usages in the adsorptive removal of pollutants in water. Various milestones of synthesis and application of MOFs in remediating water pollution are presented
Adsorption of heavy metal onto MOFs
The occurrence of heavy metals associated contaminants in the environments has raised a lot of global concern owing to their possible related acute and chronic poisons in both humans and aquatic life [101,125,178]. These poisons are the results of heavy metal (including Ag, As, Cd, Cr, Cu, Fe, Hg, Pb, and Zn) accumulation in foodstuffs and water [101,125,202] Their presence in water pose detrimental effects on the growth and development of living organisms, thereby, causing the biochemical
Challenges and future prospects
For any adsorbent to be applicable for higher adsorption capability, the material is expected to possess a reasonable higher porosity and surface area. Conversely, some as-synthesized MOFs suffer from low surface area, thus sometimes undermining their utilization. To overcome such limitations, adsorbents modifications via physical and chemical activation is required - this is a popular method for the development of vastly porous carbons from various precursors. Hitherto, numerous methods were
Conclusion
Up till now, enormous interests focusing on the rapid expansion in the construction of novel MOF adsorbents are increasing owing to the size, composition, shape control abilities, and structural diversities of MOFs and also their auspicious adsorption properties. These adsorbents possess the larger specific surface areas and the exceptional surface functional groups required for effective adsorptive removal of different pollutants from the soil, air, and water. MOF adsorbents have wider ranges
Declaration of Competing Interest
The authors declare no conflict of interest.
Acknowledgments
The authors acknowledge the platform to carry out this work given by the Department of Pure and Applied Chemistry, Faculty of Pure and Applied Sciences, Ladoke Akintola University of Technology Ogbomoso, Oyo State, Nigeria, and the Department of Chemical Sciences, Lead City University, Ibadan, Nigeria. The corresponding author acknowledges the award received from The World Academy of Sciences (TWAS) in partnership with South Africa's National Research Foundation (NRF) and Department of Science
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