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Review

Arsenic(III) and Arsenic(V) Removal from Water Sources by Molecularly Imprinted Polymers (MIPs): A Mini Review of Recent Developments

by
Athanasia K. Tolkou
1,2,
George Z. Kyzas
2 and
Ioannis A. Katsoyiannis
1,*
1
Laboratory of Chemical and Environmental Technology, Department of Chemistry, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece
2
Department of Chemistry, International Hellenic University, 65404 Kavala, Greece
*
Author to whom correspondence should be addressed.
Sustainability 2022, 14(9), 5222; https://doi.org/10.3390/su14095222
Submission received: 3 April 2022 / Revised: 23 April 2022 / Accepted: 25 April 2022 / Published: 26 April 2022
(This article belongs to the Section Environmental Sustainability and Applications)
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Abstract

:
The present review article summarizes the recent findings reported in the literature with regard to the use of molecularly imprinted polymers for the removal of arsenic from water and wastewater. MIPs are polymers in which a template is employed in order to enable the formation of recognition sites during the covalent assembly of the bulk phase, via a polymerization or polycondensation process. The efficiency of both arsenic species and the mechanism of removal are highlighted. The results have shown that under certain conditions, MIPs demonstrated arsenic sorption capacities of up to 130 mg/g for As(V) and 151 mg/g for As(III), while the regeneration ability was found to reach up to more than 20 cycles. The overall results showed that further development of MIPs could result in the formation of promising adsorbents for arsenic removal from waters. The use of MIPs for the removal not only of arsenic but also other inorganic contaminants is considered a very important topic, with great potential in terms of future applications in water treatment. The main advantage of these materials is that they are very selective toward the contaminant of interest. This enhanced selectivity is attributed to the incorporation of specific templates, which can then adsorb the contaminant of interest almost exclusively. Therefore, the main problem in adsorption processes is the competition for adsorption sites by other water components, for example, phosphates, nitrates, carbonates, and sulfates, which can be circumvented by the use of MI-type adsorbents.

1. Introduction

The occurrence of arsenic in water sources comprises a worldwide problem and is recognized as a human health threat [1]. In Southeast Asia (Bangladesh, Vietnam, Cambodia, and Laos) [2,3], the Indian Peninsula [4], and Northern Thailand [5], more than 100 million people are consuming drinking water with arsenic concentrations that are over the WHO recommended limit of 10 μg/L [6]. In South America (Chile and Argentina) [7] more than 2 million people are being exposed to elevated arsenic concentrations. Similarly, in Europe, many regions are affected by high arsenic concentrations (i.e., Hungary, Romania, Greece, and Spain) [8,9]. For example, in Northern Greece, more than 150,000 people are exposed to drinking water containing arsenic concentrations of between 20 and 250 µg/L [10]. Therefore, the removal of arsenic from groundwaters, even in cases where is used only for agriculture, is of vital importance. Several removal technologies [11,12] have been developed for water treatment in centralized plants, using iron coagulation—filtration and adsorption onto iron oxy-hydroxides being the most widely practiced technique [13,14,15,16,17].
Almost all the violations of the maximum allowable concentrations of arsenic have been observed in small communities with inhabitants of less than 10 × 103 people, particularly in less developed areas of the world. Methods developed for household use in Bangladesh and West Bengal comprise the 3-kolshi filters [18] and filter systems based on activated alumina [19]. Nevertheless, there is still a need to develop special treatment systems for arsenic removal at the household level that will not require frequent manual intervention or the use of chemicals for trivalent arsenic (As(III)) pre-oxidation; in the usual treatment methods, As(III) cannot be removed efficiently from water. This is due to the fact that As(III), when in an environment with pH values relevant to drinking water treatment, is present in its non-ionic form of arsenious acid, whereas pentavalent arsenic (As(V)) is present in its oxyanionic species, H2AsO4− or HAsO42−. Conventional treatments for arsenic removal include coagulation-flocculation using iron and aluminium-based salts [17], adsorption onto iron oxides [20] or activated alumina [21], ion exchange [22], membrane processes [23] (mainly nanofiltration) and reverse osmosis [24]. Almost all these processes are very efficient for removing the pentavalent arsenic form but, in the case of trivalent arsenic, there is a need for a peroxidation step to oxidize As(III) into As(V). A growing body of research [14,15,25] is, therefore, developing materials and technologies that can remove both As(V) and As(III) equally efficiently.
The oxidation of As(III) is primarily accomplished using chemical oxidants, such as KMnO4 [26], NaOCl, manganese oxides [26], or ozone [27,28]. These oxidants are very efficient for the oxidation of As(III) but might cause several secondary complications, which occur mostly from the presence of residuals or from the formation of side products, also inducing a substantial augmentation of the respective operational costs.
To overcome these problems and limitations, several methods have been developed that can remove arsenic without the use of oxidants for the transformation of As(III) [29,30], such as the use of biological oxidation or the use of specifically designed adsorbents that have a dual action, i.e., oxidizing As(III) and adsorbing the formed As(V), such as with the use of tetravalent manganese ferroxyte or the use of zero-valent iron [31] under oxidic conditions.
In this regard, several researchers have used specifically modified molecularly imprinted polymers for removing both As(III) or As(V) [32], depending on the type of imprinted template. In this way, they showed that this technique could efficiently remove either As(III) or As(V), or even mixtures of both species, as they normally exist in natural water sources. Furthermore, some researchers have used advanced modifications of MIPs by incorporating iron oxides into their structure [32] to improve the sorptive efficiencies of the new materials. The latter developments have rendered the use of MIPs a very promising technology for applications in water treatment and especially for arsenic removal from groundwaters or other water sources, such as rivers or lakes [33]. The importance of the use of MIPs for the removal of arsenic from groundwaters is that these materials are very selective toward the contaminant of interest. This superior selectivity is ascribed to the function of specific templates incorporated in the materials that can adsorb the contaminant of interest, i.e., arsenic, almost exclusively. Therefore, the main problem in these adsorption processes is the competition for adsorption sites by other water components, for example, phosphates, nitrates, carbonates, and sulfates, which is circumvented by the use of MIP-type adsorbents.
In the present review article, those studies that have used MIPs for arsenic removal are reviewed in detail, as well as mentioning several applied modifications, i.e., the type of monomer, cross-linker, and template. The mechanism and efficiency of removal for both arsenic species are highlighted. To the best of our knowledge, this is the first review appearing in the literature describing the use of MIPs for arsenic removal from water.

2. MIPs

2.1. Definition of MIPs

Molecular imprinting is a technique that can be defined as the production of ligand-selective recognition sites in polymers, into which a template is incorporated in order to enable the formation of recognition sites during the covalent assembly of the bulk phase using a polymerization or polycondensation process. Successive removal of the template is necessary for detection to occur in the spaces evacuated by the templating species. Correspondingly, molecularly imprinted polymer (MIP) has the ability to recognize and bind a specific molecule and is prepared by the molecular imprinting technique. These binding sites match the template in terms of size, shape, and functional group orientation and are able to rebind and extract it from complicated samples [32,34].
The development of reversible interactions with the template and polymerizable functionality may include one or more of the following interactions, as demonstrated in Figure 1 [35].
Successive polymerization in the presence of cross-linkers results in the formation of an insoluble matrix in which the template sites exist, which can promote recognition through steric, van der Waals, and even electrostatic interactions. The template is then removed from the polymer through disruption of the polymer–template interactions, along with extraction from the matrix [36]. The template, or analogs thereof, may then be selectively rebound by the polymer in the sites vacated by the template, the “imprints” [35,37].
For the synthesis of MIPs, five basic components are required: the substrate or template (imprinted molecule or molecule-guide), the functional monomer, the crosslinking monomer, the initiator, and the porogen (solvent). The template is known as the molecule (or compound), which is used to create the binding sites of the polymer, based on its shape and structure. The polymer will show increased selectivity for the molecules of this compound. The monomer should have a functional group (e.g.,-COOH,-NH2) so that it can interact with the molecules of the substrate to form either weak complexes or covalent bonds and to finally achieve molecular imprinting. The cross-linking monomer (“molecular glue”) is also a monomer but is multifunctional, since it contains more than one double bond, having the ability to polymerize in more than one direction. The cross-linking monomer is linked to the monomer by a covalent bond, usually on the side that does not react with the template [38]. Thus, the cross-linking monomer is a di- or multifunctional straight compound with varying chain lengths, which has the ability to form bridges between the macromolecular chains of the polymer, resulting in a hard and brittle final product. This also contributes to the formation of pores in the polymer mass, thereby facilitating the movement of template molecules and solvents to and from it. The initiator is a chemical compound that, upon thermal or photochemical degradation, generates free radicals that start the chain reactions of polymerization. Thus, this compound is capable of releasing free radicals either by heating or irradiating and thus contributes to initiating the reaction of polymer formation. Initiators are used in the compounds that mostly contain the group-N=N- in their molecule, the split of which gives radicals that cause the polymerization process to start up. Finally, the porogen is the solvent in which the polymerization takes place. It converts all the components of polymerization (template, functional monomer, cross-linking monomer, and initiator) in the same phase [38]. The porogen, therefore, characterizes the solvent in which the polymerization reaction will be conducted and it affects the ability of the solvent to create pores in the mass of the polymer (it creates passageways inside the monolith). These pores will form the passages of the template from and to the interior of the polymer particles so that the phenomenon of molecular recognition will take place.

2.2. Advantages and Applications

The produced polymers are feasible since they are comparatively cheap, straightforward to make, very robust, offer great selectivity with regard to the imprinted substance (target substance or template) and they can be reused without losing their effectiveness, exhibiting high resistance to extreme conditions (such as temperature, pH, solvents, pressure, etc.) [39,40]. Furthermore, owing to the substantial number of distinct monomers that are commercially accessible (more than 4000 polymerizable compounds), their properties can be set, rendering them a valuable class of affinity sorbents.
The technology of molecular imprinting is widespread and can be used in a variety of applications with growth potential [37,40], such as those shown in Figure 2.
  • Analytical techniques
    • Chromatography was the initial purpose for which imprinted polymers were employed. Separating the elements of complex mixtures is a vital step in the analysis and preparation of samples. Because they are fairly easy to make and have high selectivity for their template, MIPs are effective separation materials. MIPs have been used in many fields of chromatography, such as liquid chromatography (LC) and thin-layer chromatography (TLC) [41,42].
    • Another area of separation science in which MIPs are confirmed to be valuable is electrophoresis, where the analyte mixture is forced through a stationary phase by an electric field (instead of by gravity or pressure, as in chromatography).
    • Solid-phase extraction (SPE): molecular-imprint-based solid-phase extraction (MISPE) is presently the best-developed application in this scientific field with regard to the implementation of MIP-based technologies [43,44].
  • Membranes
Molecularly imprinted polymer membranes are intended to act as a barrier between two fluid phases. They might be proposed for the cleansing of the compound that passes exclusively from a feed solution to the permeate. Such membranes may be free-standing or supported on porous supports, and their porosity must be restricted so that transport through them is not purely governed by diffusion within the solution in the pores. Membranes with large (flow-through) pores can also be used as selective adsorbents for solid-phase extraction; thin films of imprinted material deposited on solid surfaces are also intended to act as sensing elements [45,46,47].
  • Catalysis
Catalysts are materials that can change the velocity of chemical reactions without being consumed. They typically accomplish that by decreasing the activation energy by developing a compound stabilizing an intermediate. To attain this with an MIP, a cavity has to be created where a crucial medium fits in and becomes stabilized. In addition, the cavities of an MIP can protect functional groups or orient two areas where a reaction should take place toward each other [48,49,50].
  • Drug delivery
Many drugs are successful against cancer but are not recommended for the treatment of human cancers because of dangerous side effects. Other medicines have little solubility in body liquids or are damaged by the immune system; therefore, they have no way to reach the area within the body where they are needed. Medicine delivery concerns materials that, upon incorporating the drug, may be delivered to the area in which they can offer the best treatment and are released in that specific place. MIPs (with additional requirements necessary for this objective) can reversibly incorporate a template molecule offering this capacity, which is highly desirable in drug delivery [19,51].
  • Drug discovery
The effectiveness of medicines is largely centered on their ability to attach to certain receptors and initiate or obstruct a reaction. Inhibition agents are important because they may be potential drugs or have other biological significance. It has been demonstrated that MIPs can be employed to find potential inhibitors for receptors. The use of MIPs as receptor mimics has allowed the screening of combinatorial libraries and the synthesis of drug candidates in the recognition sites of imprinted materials [52].
  • Biotechnology
The use of MIPs as an aid to biocatalytic processes has been suggested. These practices include shifting an unfavorable equilibrium in an enzyme-catalyzed process away from an unwanted side-product or toward enhancing the yield of the anticipated product, or the isolation of the anticipated product from a crude extraction or from a fermentation broth [53].
  • at
Surfaces with specified shapes and roughness were proved to enhance cell culturing. Because the size, density, and even surface chemistry of such patterns can be tuned in molecular imprinting, MIPs are attractive materials for this purpose [54].
  • Antibody mimics
The idea is to produce particles with a specific selectivity that could be given to patients as a substitute for natural antibodies produced in immune responses [55,56].
  • Crystallization
The structural determination of proteins is limited by their ability to form well-diffracting and large crystals. Because many proteins have abnormal shapes, they are difficult to crystallize. One very new application for MIPs is to initiate protein crystallization; it was shown that they can initiate protein crystal growth for various different proteins and they have also increased the speed of crystal formation significantly [57].
  • Other applications:
    • Food industry
      MIPs imprinted with methyl pyrazines have been proposed as useful vehicles for flavors and flavor profile analysis in the food industry [58,59].
    • Microfluidic devices
      The use of MIPs in microfluidics machines has also been suggested and a computational fluid dynamics model of the performance of such devices has been created. This procedure involves a broad range of flexibility by which to imprint different sets of biomolecules with a variety of structures, sizes, and physical and chemical characteristics [60].

2.3. Applications of MIPS in Water and Wastewater Treatment

Ensuring safe water to meet human demands is a major challenge of the 21st century. Universally, water provision struggles to meet the fast-growing demand, which is aggravated by increases in the human population, climate change, and worsening water quality [61,62]. Over the past few years, numerous surveys referred to the occurrence in water bodies of organic and inorganic micropollutants, such as endocrine-disrupting compounds, pharmaceuticals, and personal care products in surface waters, drinking water, and wastewater effluents. The occurrence of such compounds in the water caused a huge impact on the ecological balance and human health. Therefore, the treatment of water for the removal of such compounds has become a major issue. Conventional technologies have certain restrictions on their efficiency at removing micro-contaminants from waters, and there is a growing demand for innovative treatment processes and adsorbents that offer much greater efficiency [63]. Nanotechnology has a great capacity for improving the effectiveness of water and wastewater treatment [62].
Micropollutants are normally released into water bodies via effluent discharges from wastewater treatment plants and land applications, eventually contaminating both ground and surface waters [64]. The concentrations of these pollutants are normally at a low ppb level or even at ppt level. However, these compounds can accumulate in aquatic organisms and adversely affect their growth and reproduction. Several studies have recently compared the effectiveness of various treatments and removal processes for micropollutants [65]. Coagulation and flocculation are not very efficient at eliminating organic micropollutants. Activated carbon-based materials are efficient at removing hydrophobic organic contaminants [66]. However, activated carbon treatments are normally utilized for drinking water applications and not for wastewater treatment. Various parameters can affect removal efficiency, such as adsorption kinetics, adsorbate concentration, and water composition. Oxidation of micropollutants with strong chemical oxidants (e.g., ozone) and advanced oxidation processes (e.g., UV-H2O2) is also possible [67,68]. Nevertheless, the applications of such technologies may end up being the cause of byproducts that may be more toxic or estrogenic than the primary compounds (e.g., atrazine and deethylatrazine). In terms of membrane processes, high-pressure-driven processes such as reverse osmosis and nanofiltration offer exceptional advantages in organic pollutant elimination due to the high levels of removal that can be obtained, while avoiding the creation of side products. Their disadvantages include their high capital and operational costs.
Molecularly imprinted polymers (MIPs) comprise a novel material for water and wastewater treatment applications, offering high selectivity of compounds [64,69,70,71,72], providing great potential for the effective absorption of chemicals from water and air samples [73]. MIPs are suitable for the treatment of trace contaminants because they can be specifically designed to remove or detect one or a group of target compounds. This is an advantage over nonspecific technologies such as activated carbon, which may be consumed by the removal of large amounts of non-trace contaminants from the water [63]. Thus, MIPs have been used in water treatment to remove or detect substances harmful to human health, such as arsenate complexes, chromium, uranium, and pharmaceuticals [74], other metal ions [75,76], and toxic dyes [77].

3. Arsenic Removal by MIPs

3.1. As(V) (Arsenate)

In the paper published by Önnby et al. [32], the elimination of As(V) via adsorptive retention from aqueous sources was investigated using 3 adsorbents. The adsorbents were: (a) aluminum nanoparticles (Alu-NPs, <50 nm), incorporated in amine-rich cryogels (Alu-cryo); (b) molecularly imprinted polymers (<38 μm) in polyacrylamide cryogels (MIP-cryo); and (c) thiol-functionalized cryogels (SH-cryo). These were assessed with regard to the material characteristics and arsenic removal in both batch test and continuous modes [32]. By designing a composite adsorbent combining cryogels with particles (at nano- and micro-scales), an increase in mechanical stability was observed. For the preparation of MIPs, As(V) was used as a template, with acrylamide (AAm) as a functional monomer, N,N′-methylene bis(a-crylamide) (MBAAm) as a cross-linking monomer, azobisisobutyronitrile (AIBN) as an initiator, methanol as a porogen, and bulk polymerization. The adsorptive removal of arsenic was tested in real wastewater; the adsorption amounts for the composites were 20.3 ± 0.8 mg/g for the adsorbent (Alu-cryo) and 7.9 ± 0.7 mg/g of adsorbent for (MIP-cryo), respectively. Composites worked well in the studied pH range of pH 2–8, with no significant changes to arsenic adsorption due to the pH change. For the materials used in this study, the Langmuir model fitted well with the obtained adsorption curves, with R2 values > 0.9. In experiments in real wastewaters that were spiked with arsenic(V), it was found that the co-ions (nitrate, sulfate and phosphate) affected arsenic removal less profoundly for MIP-cryogels than other cryogel materials [32], indicating the selectivity of the proposed material, which is its main advantage.
Another study synthesized a new arsenic-ion-imprinted polymer (As-IIP) for the removal of As(V) from water [78]. Na2HAsO4·7H2O was used as the template, 1-vinylimidazole (1-VIm), 4-vinylpyridine (4-Vp), and styrene were the functional monomers, ethylene glycol diacrylate (EDMA) was the cross-linking monomer, with azobisisobutyronitrile (AIBN) as an initiator, acetic acid-methanol as a porogen, and bulk polymerization at 60 °C overnight. The robust As-IIP sorbent demonstrated good reusability up to 20 cycles. The material was quite efficient over a working pH range of 5–7 [78]. Na2HAsO4·7H2O was also used as a template in another study [79]. Allylamine hydrochloride and 3-aminopropyl triethoxysilane (APTES) were employed as functional monomers, with N,N′-methylene bisacrylamide (MBA) as a cross-linking monomer, potassium persulfate (K2S2O8) as an initiator, and ethanol and aqueous solution as a porogen and for the surface imprinting technique were used in series. This innovative magnetic arsenate-imprinted polymer (mMIIP) was synthesized in order to investigate the selective adsorption performance of As(V). Compared with a non-imprinted magnetic polymer (NIP), it was found that this mMIIP showed high selectivity coefficients. The maximum adsorption capacity for As(V) was calculated as 7.13 mg/g [79].
Furthermore, a cysteine derivative-modified TiO2-doped ZnS nanoparticle was developed as a monomer (Cys@ZnS: TiO2 NPs) implemented for the synthesis of a novel, selective, and sensitive As(III) and (V)-imprinted membrane, i.e., a grafting-from-membrane (GFM) novel type [80]. The relative imprinted membrane was synthesized by mixing one of the monomers, acrylamide (AA), with membrane precursors, followed by the addition of a pre-polymer mixture, known as a membrane with a capping agent. The maximum adsorption capacity was found to be 151.0 and 130.0 mg/g for As(III) and (V), respectively, on the As(III)- and As(V)-imprinted GFMs. This comprises one of the case studies showing that the use of MIPs with the appropriate template can create very efficient adsorbents for As(V) and As(III) removal. Furthermore, after 25 regenerated cycles using 0.5 M HCl as the extraction solvent, the loss of initial adsorption capacity was only 3.1%. Additionally, the membrane also showed good antibacterial properties that can be used to prevent biofouling of the membranes [80].
Moreover, an imprinted polymer with nanopores (nanoMIP) has been used to remove arsenic from water [81]. Fluorescein was preferred as the chelating agent for As(V), combined with rhodamine B, dithizone, and EDTA. The formed fluorescein–As(V) complex reacted with the functional monomer, 4-vinyl pyridine (4-VP), followed by the addition of a cross-linking monomer, ethylene glycol di-methacrylate (EGDMA) in methanol. The polymer particles were fractionated using technical sieves. The adsorption capacity of nanoMIP for As(V) was 49.4 ± 1.9 mg/g at pH 7.0 ± 0.9, which is about eight times higher than with the commercially used methods [81]. The regeneration studies showed that the regenerated imprinted polymers were effective for 10 cycles by washing with 0.01 M HNO3.
Recently, IIP@SiO2@Fe3O4 was applied as a new nanoparticle for As(V) adsorption [82]. According to the results of this study, the optimum conditions were a pH of 5.3, a contact time of 102 min, and an adsorbent dose of 2.3 g/L. Under these conditions, an adsorption capacity of 104.7 mg/g was achieved through physical and spontaneous endothermic absorption, as shown in the respective thermodynamics study. IIP@SiO2@Fe3O4 was synthesized in order to obtain a larger surface area and more adsorption sites, due to SiO2 nanoparticles (NPs). Ethylene glycol dimethacrylate (EGDMA), 4-vinylpyridine (4-VP), azobisisobutyronitrile (AIBN), and As (NO3)5 were used as the ligand, cross-linking agent, functional monomer, initiator and template ion, respectively. The specific surface area of IIP@SiO2@Fe3O4 was 610.99 m2/g and the porosity was 0.1163 cm3/g. However, one drawback of this material was that regeneration studies using 0.5 M of EDTA solution showed that after 4 regeneration cycles, the removal efficiency dropped significantly and only accounted for about 59.7%.
Another attempt was based on the formation of a mesoporous As(V) ion-imprinted polymer (M-IIP), which was successfully prepared with “one-pot” co-polycondensation [83], by using 3-[2-(2-aminoethylamino) ethylamino] propyltrimethoxysilane (AAAPTS) as the functional monomer and diethylenetriamine (DETA) as the functionalized ligand. The synthesized M-IIP, proposed by Yin et al. in 2022 [83], provided an excellent adsorption capacity of 78.74 mg/g at a pH of 3 and showed no noticeable change with the increase in temperature. Moreover, after 6 cycles washed with dilute HCl, the adsorption capacity of M-IIP can be maintained for up to 93% of the first cycle. Furthermore, in a parallel study by Yin et al. in 2022 [84], an As(V) ion surface-imprinted polymer (S-IIP) was effectively formed on the surface of nano-SiO2, also using 3-[2-(2-aminoethylamino) ethylamino] propyltrimethoxysilane (AAAPTS) and applying it successfully in river water, tap water, and mineral water, with a recovery rate of between 98 and 100%. Below a pH of 3, the adsorption capacity could be up to 39.5 mg/g, which can be sustained at 75% after 4 adsorption–desorption cycles. Moreover, as the pH increased, the adsorption capacity decreased due to the various forms of arsenate in the different pH values and the variations of the surface charge of the adsorbent. Specifically, the adsorption capacity of 39.5 mg/g at pH 3 was about 12.1 mg/g at a pH of 7 [84].

3.2. As(III) (Arsenite)

Regarding the development of MIPs intended specifically for the removal of As(III), several studies can already be found in the literature.
In the study by Alizadeh et al. [85], a brand-new approach was recommended for the formulation of As(III)-imprinted polymer by utilizing As(C4H5O2)3 as a template [85]. Precipitative polymerization was applied to synthesize a nano-sized As(III)-imprinted polymer. Methacrylic acid (MAA), hydroquinone and ethylene glycol dimethacrylate (EDMA) were used as a functional monomer and cross-linking agent, respectively, with azobisisobutyronitrile (AIBN) as an initiator and acetonitrile as a porogen. The polymerization reaction took place at 60 °C and lasted for 24 h. It was validated that the arsenic was identified as As(III) by the selective cavities of the synthesized IIP. The FT-IR figure revealed that the arsenic was extracted by the IIP sites in the form of As(III), where it was stabilized via coordination bonding with the carboxylic acid functional groups of the sites [85].
In another study, ion-imprinted polymer (As(III)-IIP) was manufactured using allylthiourea as a functional monomer and As(III) as a template for removing As from water [86]. The synthesis of As(III)-IIP as a sorbent was prepared via bulk polymerization. Ethylene glycol dimethacrylate (EGDMA) and 2, 2′-azobisisobutyronitrile (AIBN) were used as the cross-linking monomer and the initiator, respectively. The total capacity was 0.0679 mmol/g for 25 mg/L of initial arsenite. At pH 7, a high percentage of removal (90%) was achieved. The novelty of As(III)-IIP is that it is able to recover the arsenite from aqueous media [86].
Furthermore, an As(III)-imprinted polymer was effectively produced by using 1-vinyl imidazole as a ligand for the removal of arsenic from the natural environment [87]. Methacrylic acid (MAA) was used as the functional monomer, with ethylene glycol dimethacrylate (EGDMA) as the cross-linking monomer, and 2, 2′-azobisisobutyronitrile (AIBN) as the initiator. Maximum adsorption was achieved at pH 7 and the reusability evaluation showed that As(II)-IIP can be used up to 6 times, with only slight adsorption decay. Similarly, a novel MGO-As(III)-IIP [88] was prepared by using As(III) as a template, with Fe3O4/GO as a support, methacrylic acid (MAA) as a functional monomer, ethylene glycol dimethylacrylate (EGDMA) as a cross-linker, and azobisisobutyronitrile (AIBN) as an initiator. MGO-As(III)-IIP exhibited an adsorption capacity of 49.42 mg/g at a pH of 6 and great reusability after five cycles of regeneration and reuse when using 0.5 M NaOH solution as the desorption agent.
Last, but not least, the development of ion-imprinted magnetic nanoparticles for As(III) and As(V) removal from wastewater was recently studied [89], using N-methacryloyl-l-cysteine (MAC) as a monomer with As(III) and As(V) as the template molecules, ethylene glycol dimethylacrylate (EGDMA) as the cross-linker, and 2-hydroxyethyl methacrylate (HEMA) as the initiator. The resulting MAC-As(III)/As(V) complex displayed an efficient adsorption removal capacity of 76.83 mg/g for As(III) and 85.57 mg/g for As(V). Moreover, the IIP nanoparticles showed great removal ability in a wide pH range of 4–8. Ten adsorption-desorption cycles were repeated ten times using 0.01 M HNO3 for washing, in order to show the reusability and stability of the IIP-As(III) and IIP-As(V). At the end of ten cycles, no significant reduction in removal capacity was observed [89].

4. Critical Comparison

In an attempt to remove As(V) from potable water, mine effluent, industrial effluent, or other fluids and, generally, in water treatment (in both industrial and residential wastewater treatment) [74], several studies have been conducted, taking advantage of the properties of MIPs and preparing different materials, such as polymer beads [90], resins [63], molecularly imprinted polymer cryogels (MIP-cryo) [32], monoliths [32,78], magnetic arsenate-imprinted polymer (mMIIP) [79], and membranes. Similar to the removal of As(V), there have been fewer studies so far on the removal of As(III), in which the researchers have prepared molecularly imprinted polymer beads and membranes [77].
Table 1 summarizes the main characteristics of molecularly imprinted polymers (MIPs) in their applications regarding the removal of As(V) and As(III) from water. As it turns out, it is only in recent years that their application has been developed in terms of the removal of As(III), showing the emerging nature of these materials for use as an arsenic removal technology.
From the above studies, it seems that the technology of MIPs has been used to remove arsenic from environmental water samples and water solutions in the form of both As(V) and As(III), by the preparation of several materials. These materials have different advantages and disadvantages, and their use depends on their application. The properties of these materials appear to be influenced by various factors, such as the template, the functional and cross-linking monomers, the initiator, the porogen, their relative proportions, the polymerization technique, and the polymerization conditions. The main features used for the creation of MIPs, as described in this review, are classified in Figure 3. As depicted, there are several functional monomers used, but the most commonly used cross-linking monomers are ethylene glycol di-methacrylate (EGDMA) and N,N′-methylene bisacrylamide (MBA), while for the initiator, 2,2′-azobisisobutyronitrile (AIBN) and ammonium persulfate (APS) are most frequently utilized [91].
From the studies that have been carried out so far, it seems that the greater adsorption capacity (130 and 151 mg As/g for As(V) and As(III), respectively) was exhibited by the As(III)- and (V)-imprinted membranes, designed by “grafting from” and imprinting technology [80]. However, optimizations are required in ready-prepared materials for enhancing the removal of As(III), since MIPs seem to be very promising materials with a wide range of applications.
In particular, molecularly imprinted polymer beads for As(V) removal from potable water, mine effluent, industrial effluent, or other fluids, have been reported in previous research [90] with quite promising results. Regarding this study [90], W/O/W polymerization (400 r/min, 323–343 K for 3 h) has been applied, using 1,12-dodecanediol-O,O′-diphenyl-phosphonic acid (DDDPA) and 4-vinylpyridine (4-VP) as functional monomers, TRIM as a cross-linking monomer, AIBN as an initiator, toluene as a porogen, and Span 80 as an emulsifier, to form a novel mixed As(V)-Cr(III)-ion co-imprinted polymer (MICIP). The obtained results seem to be quite promising, with 200 nm clusters of near nano-size-imprinted polymer beads, quite a large surface area (309.2 m2/g), and satisfactory pore and micro-pore volumes (0.687 cm3/g and 0.03 cm3/g, respectively). The pore width was 1.75 nm and the adsorption capacity was 12 mg/g, with a pH = 5, demonstrating the satisfactory application of MIPs. Cr(III) ions contributed, as synergetic ions, to increasing the imprinting of arsenate on the ion-imprinted polymer.
Molecularly imprinted polymer resins have been reported for the removal of As(V) from a water medium, e.g., drinking water, lakes, streams, irrigation runoff, industrial effluent, mine waste, etc. and in water treatment (both industrial and residential wastewater treatment). Particularly in the work of the authors of [63], suspension polymerization (stirring at 320 rpm, 80 °C for 5 h) has previously been applied, using styrene as a functional monomer, DVB as a cross-linking monomer, AIBN as an initiator, and aqueous phase as a porogen. The prepared material seems to be in the form of resin beads with a permanent porous structure, even in their dry state. The particle size of the resin beads was between 250 and 841 nm for 97% of them, a bit larger than in previous research. Therefore, it seems that UV polymerization with the specific conditions and proportions of these specific compounds leads to more efficient molecularly imprinted resins.
Furthermore, MIP-cryogels have been reported regarding the removal of As(V) from water solutions [32], in which AAm and MBAAm were used as a monomer solution. By constructing a composite adsorbent combining cryogels with particles (at both nano- and micro-scales), an increase in mechanical stability was observed. The MIP-cryogels have a macroporous structure (20–100 μm), and the adsorption capacity was found to be 7.91 ± 0.7 mg/g. Furthermore, it was found that co-ions (nitrate, sulfate, and phosphate) affected arsenic removal less for MIP-cryogels than other cryogel materials. However, the development of MIP-cryogels is needed, considering their low adsorption capacity.
In addition, MIP-monoliths have been reported [32,78] for the separation, recovery, and removal of As(V) from environmental water samples and water solutions, by two studies. Bulk polymerization (60 °C for 48 h) has been applied, using 4-OHBPH and 2-Vp as the functional monomers, EDMA as the cross-linking monomer, AIBN as the initiator, and methanol as the porogen. The monolith was heterogeneously distributed, and its particle size was <38 μm. It produced a satisfactory surface area equal to 80.23 m2/g and the adsorption capacity was found to be 6.94 ± 0.4 mg/g. Moreover, bulk polymerization (60 °C overnight) has been applied using 1-VIm, 4-Vp, and styrene as the functional monomers, EDMA as the cross-linking monomer, AIBN as the initiator, and acetic acid-methanol as the porogen [78]. The monolith was a white rigid solid, and the particles had a non-spherical appearance. The formation of As-imprinted cavities at MIPs and the rough surface of NIPs was observed, with a lack of comparable porous structure. In terms of selectivity, as well as imprinting effects, compared with the non-imprinted polymer (NIP), MIPs with 1-VIm exhibited superior analyte recognition for the As ion among 23 competing elements, and it appeared to offer a 25-fold enhancement in adsorption capacity, equal to 0.00362 mg/g. Therefore, it seems that bulk polymerization with the specific conditions and proportions of these specific compounds leads to more efficient molecularly imprinted monoliths.
Previous research has reported that magnetic arsenate molecularly imprinted polymer (mMIIP) can be used for the selective adsorption of As(V) [79]. For this reason, the surface imprinting technique was applied (stirred and refluxed for 16 h at 70 °C), using two different amine-containing functional monomers (allylamine hydrochloride and APTES), with MBA as the cross-linking monomer, K2S2O8 as the initiator, ethanol-aqueous solution as the porogen, and tetraethyl orthosilicate-modified magnetic Fe3O4 nanoparticles as a support. It was found that coupling allylamine hydrochloride as a co-functional monomer and MBA as a cross-linker can increase the surface roughness of the material. The BET surface area of mMIP was 12.766 m2/g. The pore volume of the material was found to be 0.058 cm3/g, and the pore width was found to be quite large, equal to 1.752 nm. It was found that the synthesized mMIIP could selectively and efficiently adsorb As(V) species in the presence of competing ions, including Ca2+, Mg2+, Cd2+, Zn2+, Cu2+, Mn2+, Ni2+ and As(III), and had a high selective As(V) adsorption efficiency; the maximum adsorption capacity for As(V) was quite satisfactory, this being calculated as 7.53 mg/g within the pH 4–6 range.
Moreover, molecularly imprinted membranes have been reported for As(V) in water purification and for removing As(III) from realistic samples (hot spring water, urine, and arsenic-based pharmaceutical samples) [85]. In particular, the first prepared molecularly imprinted nanoparticles with precipitation polymerization (60 °C for 24 h), using MAA and hydroquinone as the functional monomers, EDMA as the cross-linking agent, AIBN as the initiator, and acetonitrile as the porogen; then, a membrane electrode was constructed by the dispersion of As(III)-imprinted or non-imprinted polymer nanoparticles of THF, adding DNP or DBP as a plasticizer and PVC powder to the mixture. The sensor was used for arsenic determination in some real-life samples. High-level selectivity and a low detection limit were shown for the developed As(III) sensor. The proposed sensor was found to work well under laboratory conditions and showed satisfactory results when used as an approach for the determination of arsenic amounts in a real sample. The selectivity of the developed sensor to As(III) was shown to be satisfactory; however, the lifetime of the sensor needed to be improved.
Recently, nanoMIPs, prepared by the templating of fluorescein and As(V) complex in 4-VP-co-EGDMA polymer via molecular imprinting, were reported for As(V) in aqueous solutions. The adsorption capacity of nanoMIPs for As(V) was 49.4 mg/g at pH 7.0 [81]. The nanoMIPs were also tested with real water samples, i.e., groundwater, surface water, and estuary samples, and the adsorption capacity was 40, 36, and 29 mg/L, respectively. The BET surface area of the nanoMIPs was 390 m2/g, which is 40% higher compared to the non-imprinted polymers (NIP), and the pore volume was found to be 0.141 cm3/g. In addition, the highest surface area was provided by some very recently presented MIPs, i.e., a mesoporous As(V) ion-imprinted polymer (M-IIP) [83], which is 523.9 m2/g, for an IIP@SiO2@Fe3O4, it is 610.99 m2/g [82], and for a MAC-As(III)/As(V) complex, 980 m2/g.
On the other hand, regarding As(III) removal, a pre-polymerization complex of As(III), with 1-vinylimidazole as a ligand [87], offers the most effective, accurate, and specific binding sites for the As(III) ion. In addition, a synthesized ion-imprinted polymer (As(III)-IIP), with As(III) as a template and allylthiourea as a functional monomer, exhibited a high percentage removal of 90% [86].
Finally, according to our adsorption–desorption experiments, in order to evaluate the regeneration efficiency of MIPs, we concluded that after 4–10 cycles of regeneration and reuse, as mainly reported elsewhere [81,82,83,84,87,88,89], the loss of initial adsorption capacity was slight. There are two studies showing that the regenerated imprinted polymers were effective for 20–25 cycles [32,80]. In most cases, an acidic solution was used as the desorption agent, i.e., HCl [80,83,84,87] and HNO3 [81,89], and only in a few cases was NaOH [88] or EDTA [82] used.

5. Conclusions

The development of molecularly imprinted polymers for arsenic removal from groundwater has been reviewed in the present article. The results of the reviewed papers have shown that appropriately synthesized MIPs can be an adsorbent that under certain circumstances can remove both As(III) and As(V) species from natural waste or wastewater. This depends on the monomer, the template, and the cross-linker that will be used. The best results reported in the literature have indicated that with Cys@ZnS:TiO2 NPs and acrylamide as the monomer and N,N′-methylene bisacrylamide (MBA) as the crosslinker, the maximum sorption capacity for As(III) and As(V) reached 151 and 130 mg/g, respectively, and the material could be reused for up to 25 cycles with only a slight loss of sorption capacity. These results are very promising and further research in this field could lead to the development of extremely efficient and selective adsorbents for the water treatment industry.

Author Contributions

Conceptualization, A.K.T., I.A.K. and G.Z.K.; methodology, A.K.T., I.A.K. and G.Z.K.; validation, I.A.K., G.Z.K.; formal analysis, A.K.T.; I.A.K. and G.Z.K.; investigation, A.K.T., I.A.K. and G.Z.K.; resources, A.K.T., I.A.K. and G.Z.K.; data curation, A.K.T.; writing—original draft preparation, A.K.T., I.A.K. and G.Z.K..; writing—review and editing, A.K.T., I.A.K. and G.Z.K.; visualization, A.K.T., I.A.K. and G.Z.K.; supervision, A.K.T., I.A.K. and G.Z.K. All authors have read and agreed to the published version of the manuscript.

Funding

The financial support received for this study from the Greek Ministry of Development and Investments (General Secretariat for Research and Technology) through the research project “Intergovernmental International Scientific and Technological Innovation-Cooperation. Joint declaration of Science and Technology Cooperation between China and Greece” with the topic “Development of monitoring and removal strategies of emerging micro-pollutants in wastewaters” (grant No. T7DKI-00220) and the support is gratefully acknowledged.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All data analyzed during this study are included in this published article.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Preparation of MIPs using the molecular imprinting technique.
Figure 1. Preparation of MIPs using the molecular imprinting technique.
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Figure 2. Applications of MIPs in several scientific sectors.
Figure 2. Applications of MIPs in several scientific sectors.
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Figure 3. Classification of the main features used for the creation of MIPs, as described in this review.
Figure 3. Classification of the main features used for the creation of MIPs, as described in this review.
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Table
Table 1. Summary of the main characteristics of molecularly imprinted polymers (MIPs) in their application to the removal of As(V) and As(III) from water.
Table 1. Summary of the main characteristics of molecularly imprinted polymers (MIPs) in their application to the removal of As(V) and As(III) from water.
TemplateMonomersInitiatorPolymerization TechniqueResultsRef.
FunctionalCross-LinkingAdsorption
Capacity
(mg As/g) *		Surface Area (m2/g)pHRegeneration
(Cycles/Washing Solution)
As(V)AAm 1MBAAm 9AIBN 14-7.980.22–8-[32]
As(V)1-Vim 2, 4-VP 3, styreneEGDMA 10AIBN 14Bulk0.00362-5–720c[78]
As(V)-Cr(III)4-VP 3DDDPA 11AIBN 14W/O/W emulsion12.03095-[90]
As(V)PAA HCl 4, APTES 5MBA 12K2S2O8Surface imprinting technique7.1312.84–6-[79]
As(V)4-VP 3EGDMA 10AIBN 14-49.4-710c/0.01 M HNO3[81]
As(V)4-VP 3EGDMA 10AIBN 14-104.761154c/0.5 M EDTA[82]
As(V)AAAPTS 6DETA 13CTAB 15“One-pot”
co-poly-condensation		78.7452436c/dilute HCl[83]
As(III)MAA 7EGDMA 10AIBN 14Precipitation polymerization--6.5-[85]
As(III)allylthioureaEGDMA 10AIBN 14bulk5.1-7-[86]
As(III)MAA 7, 1-Vim 2EGDMA 10AIBN 14Microwave-assisted3.5-76c/1 M HCl[87]
As(III)MAA 7EGDMA 10AIBN 14-49.42-65c/0.5 M NaOH[88]
As(III)FT 18EGDMA 10Toluene---7.5-[92]
As(III) and As(V)Cys@ZnS:TiO2 NPs, acrylamideMBA 12APS 16-151 (As(III))
130 (As(V))		--25c/0.5 M HCl[80]
As(III) and As(V)MAC 8EGDMA 10HEMA 17-76.8 (As(III))
87.6 (As(V))		980510c/0.01 M HNO3[89]
1 AAm: acrylamide; 2 1-Vim: 1-vinylimidazole; 3 4-VP: 4-vinilpyridine; 4 PAA HCl: polyallylamine hydrochloride; 5 APTES: 3-aminopropyl triethoxysilane; 6 AAAPTS: 3-[2-(2-aminoethylamino) ethylamino] propyl-trimethoxysilane; 7 MAA: methacrylic acid; 8 MAC: N-methacryloyl-l-cysteine; 9 MBAAm: N,N′-methylene bis(acrylamide); 10 EGDMA: ethylene glycol di-methacrylate; 11 DDDPA: 1,12-dodecanediol-O,O′-diphenyl-phosphonic Acid; 12 MBA: N,N′-methylene bisacrylamide; 13 DETA: diethylenetriamine; 14 AIBN: 2, 2-azobisisobutyronitrile; 15 CTAB: cetyltrimethylammonium bromide; 16 APS: ammonium persulfate; 17 HEMA: 2-hydroxyethyl methacrylate; 18 FT: (2, 6-difluorophenyl) thiourea. * represents the quantity of adsorbate (As concentration (mg/L)) per mass of the adsorbent (g) used.
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Tolkou, A.K.; Kyzas, G.Z.; Katsoyiannis, I.A. Arsenic(III) and Arsenic(V) Removal from Water Sources by Molecularly Imprinted Polymers (MIPs): A Mini Review of Recent Developments. Sustainability 2022, 14, 5222. https://doi.org/10.3390/su14095222

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Tolkou AK, Kyzas GZ, Katsoyiannis IA. Arsenic(III) and Arsenic(V) Removal from Water Sources by Molecularly Imprinted Polymers (MIPs): A Mini Review of Recent Developments. Sustainability. 2022; 14(9):5222. https://doi.org/10.3390/su14095222

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Tolkou, Athanasia K., George Z. Kyzas, and Ioannis A. Katsoyiannis. 2022. "Arsenic(III) and Arsenic(V) Removal from Water Sources by Molecularly Imprinted Polymers (MIPs): A Mini Review of Recent Developments" Sustainability 14, no. 9: 5222. https://doi.org/10.3390/su14095222

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