According to Lozinsky and Okay, Gels are systems of a polymer and an immobilized solvent where the macromolecular chains are linked together by chemical or physical connections to form a three-dimensional network (Lozinsky and Okay, 2014). When the used solvent is water, we're talking about hydrogels; when the solvent is an organic liquid, we're talking about organogels. Once the solvents are water–organic combinations, several intermediate versions are undoubtedly feasible. Gels are covalent gels, ionic gels, noncovalent or physical gels, or mixed variations based on the interchain connections between polymers forming the structure. The molecular structure of the individual polymers and the methodology of gel formation specify the overall morphology of the gel.

Although extensive research, the engineering and control of gel-based systems still pose some challenges owing to the inverse coupling of a set of network features. When the extent of crosslinking in gels is reduced to maximize the mesh size, the gels degrade more quickly. Furthermore, when processed in the swelled form, weakly crosslinked gels are delicate materials; they often break at very modest pressures due to the absence of the adequate energy dissipation pathway in the gel matrix. Furthermore, in many applications, the interaction rate of gels to different influences is not rapid enough. So, the kinetics of gel structural change requires the solvent's polymer system absorption or desorption, and it is a diffusive mechanism. This is a slow methodology that becomes slower as it approaches the critical point. As a result, in many current and proposed gel applications, the design of gels with strong mechanical properties and a quick response rate is critical. Interconnected pore structures are needed to convert the diffusive mechanism into convective to overcome this problem. Table 1 has been shown different techniques for macroporous gel formation.

According to basic technology, the monomer–crosslinker mixture is copolymerized by free-radical crosslinking in the presence of an inert material (named as porogen or diluent), which is soluble in the monomer mixture (Kanamori et al., 2007). A phase separation needs to occur through the network creation process to create macroporous structures, and the development of further crosslinks should fix the two-phase structure. The inert material is withdrawn from the network following polymerization, producing the pores inside the strongly crosslinked polymer network. This reaction-induced phase separation mechanism of porosity creation results in macroporous gels made up of agglomerates of polymer pieces of varied sizes. Although phase-separated microporous gels respond quickly to external stimuli, their aggregate-like shape eventually considerably loses mechanical characteristics (Sayil and Okay, 2001). Furthermore, the network chains do not behave like flexible polymer chains sensitive to external stimuli since a considerable amount of crosslinker is required to generate phase separation during gelation.

Cryogels are gel architectures that form in substantially frozen liquids or colloidal dispersions of components suitable for gelling, as shown in Figure 1. Solvent crystals are employed as porogen in the cryogelation process. Solvent crystals that have been defrosted succeed in large, interconnecting pores. Cryogelation is a cost and workforce economical way for producing macroporous gels compared to other methods (Plieva et al., 2011). Cryogels also have the advantage of being flexible in their production and functioning primarily with water as a solvent, enabling this type of material more cost-effective, and ecologically responsible (Rie et al., 2015). Moreover, as stated in a good study by Henderson et al., many parameters can be easily fine-tuned to create cryogels for their particular usage (Henderson et al., 2013). The type of cross-linker, polymer molecular weight, cryo-concentration, cryogelation temperature, and cooling rate are all variables that can be altered.

The main critical parameters impacting the distinctive features of the produced cryogels are the content and concentration of the starting solution, freezing temperature, cooling rate, type of crosslinker, the molecular weight of the polymer and incubation period. Adjusting any of the manufacturing variables throughout the cryogelation procedure will modify the morphology of the cryogel, such as polymer wall thickness, pore size, pore structure, and porosity (Bakhshpour et al., 2019; Akgönüllü et al., 2020b). The structural characteristics of the generated cryogels can be influenced by the number of monomers in the starting solution. The cryogel exhibits a decreased pore size as the starting concentration increases due to the creation of thicker sidewalls, resulting in a stiff and less spongy architecture. In general, the beginning monomer exists in a non-frozen phase, and its concentration influences the size of that phase, resulting in the creation of the cryogel's resultant polymeric wall. As a consequence, a bigger non-frozen component will develop, with a reduced porous structure and thicker pore sidewalls (Kirsebom et al., 2010).

Temperature and time of cryogel formation have a significant impact on the pore structure of the cryogel, including the shape, size, and density of the pores. Polymerization at lower freezing temperatures produces smaller and more frequent ice crystals, resulting in cryogels with smaller hole diameters. Furthermore, if the temperature is not low enough, the solution will be non-frozen, resulting in the creation of an undesired architecture. In the macrometer region, a temperature range of 10 to 21°C and a freezing time of 24 h is stated to give an ideal linked ice crystal structure (Bäcker et al., 2016).

The cooling rate is another parameter that affects the interconnected macroporous structure of the cryogel. Polymerization will proceed in the absence of porogens (solvent crystals) and a cryogel with smaller pores will be formed instead of a highly interconnected cryogel if cross-linking happens much faster than ice crystal formation. Lower cooling rates result in bigger ice crystal formation, resulting in a cryogel structure that is densely linked and macroporous (Memic et al., 2019).

The cross-linking process is carried out in one of two ways: chemical or physical. Covalent reactions and cross-linking of monomers or prepolymers are used to create chemical cross-linking. At the cryogelation temperature, chemically cross-linked cryogels are generated when reactive species (monomers, prepolymers) polymerize between ice crystals. Physical cross-linking, on the other hand, is caused by ionic or secondary interactions between polymer chains. Physical cross-linking can be used to create cryogel scaffolds, but the resulting macropores are typically tiny (<10 μm) with poor gel properties (like brittleness and stability) which can be limiting issues in some applications. In contrast, chemically cross-linked cryogels are generally more physically stable and have more defined and predictable features.

The viscosity and chain entanglement of a polymer can be affected by its concentration and molecular weight. Changes in any of these factors, therefore, can result in considerable variances in the final cryogel structure. Lower molecular weight polymers, in general, result in the creation of bigger holes within the final cryogel at a certain polymer concentration. The free water concentration in the polymer solution drops as the molecular weight of the polymer increases, resulting in smaller holes and thicker polymer walls inside the manufactured cryogels. Furthermore, increasing the polymer concentration at a constant molecular weight increases the availability of crosslinking groups while decreasing the availability of free water, resulting in smaller cryogel pore sizes (Plieva et al., 2011).

Cryogels formed from a singular precursor, such as a natural or synthetic polymer, are the basic (Lozinsky, 2002; Ulery et al., 2011). Polymers originating from plants or animals, such as polyesters, polysaccharides, and proteins, are examples of natural polymers (Volova et al., 2017). These polymers closely resemble the extracellular matrix's chemical and physical features. 3D architectural scaffolds made from naturally generated polymers have subsequently gotten a lot of interest for their ability to support mammalian cell development and proliferation in vitro and in vivo (Kumbar et al., 2014). Alginate (Textile, 2017), cellulose (Ciolacu et al., 2016), collagen (Brougham et al., 2017), chitosan (Ji et al., 2011; Biswas et al., 2016), gelatin (Barbetta et al., 2009; Demir and Bölgen, 2017), hyaluronic acid (Oelschlaeger et al., 2016), starch (Bagri et al., 2011) and silk (Ak et al., 2013; Yetiskin and Okay, 2017), as well as, rubber (Oztoprak and Hekimoglu, 2014), and their derivatives, have all been used successfully in cryogel manufacturing. Several synthetic polymers have variable physical and degrading features, making them suitable for a wide range of biotechnological applications (Kumbar et al., 2014). Because of their ability to a breakdown in biological conditions, poly(l-lactic) (PLLA) (Conoscenti et al., 2017), poly(ethylene glycol) PEG (Bäcker et al., 2016; Wang and Yang, 2018), poly(vinyl alcohol) (PVA) (Jeong et al., 2015; Ceylan et al., 2016; Göktürk et al., 2016), and poly(hydroxyethyl methacrylate) (PHEMA) (Bereli et al., 2012) are among the most commonly used synthetic polymers for the creation of cryogels. The main disadvantages of synthetic polymers in biotechnological applications are the lack of bioactivity and bioadhesive qualities. This drawback can be overcome by covalently connecting short synthetic peptides and/or numerous bioadhesive functionalities during cross-linking or combining them with natural polymers (Caló et al., 2016). Bäcker et al. (2016) describe the production and characterization of cryogel combinations containing polyhydroxyethyl methacrylate, alginate, gelatin, and poly(ethylene glycol) diacrylate (pHAG–PEGda) and their effects on cultivating the epithelial prostate cancer cell line LNCaP. Ingavle et al. (2015) investigated the binding efficiencies of protein A, monoclonal antibodies to the cryogel column by chemically coupling the affinity ligand (protein A) to reactive hydroxyl and epoxy-derivatized monolithic cryogels. Newland et al. (2015) used EDC/sulfo-NHS driven crosslinking of amino terminated starPEG and Alexa 647 tagged heparin to create cryogel microcarriers.

Cryogels are very stable, capable of heavy metal removal, easy to prepare, economical to synthesize and can be manufactured in large quantities with good reproducibility. Separation and purification techniques based on these materials have received much attention recently. Owing to these advantages, they are widely used in many applications such as heavy metal removal, purification of biomolecules, drug delivery systems, antibacterial materials (Bakhshpour et al., 2018, 2020; Diken Gür et al., 2021).

Cryogels' characteristics, such as their elasticity, swelling, and interconnecting pore structures, can all play a part in determining their stability and, subsequently, their use. Cryogels typically offer improved mechanical characteristics, macroporosity, and swelling capability than other hydrogels, which has driven their use in different applications. Orakdogen and Boyaci presented about how to make pH-responsive nano-sized composite hydrogels and cryogels that have better mechanical properties (Orakdogen and Boyaci, 2016). Ari and Sahiner conducted the structural, thermal, and morphological characterization of inulin cryogels by following the previously published literature on hyaluronic acid (HA) cryogels (Ari and Sahiner, 2020). Cryogels may quickly return to their original shape and size following mechanical compressions, such as syringe injecting, because of these unique properties, making them a crucial consideration when designing materials for various specialized applications (Bencherif et al., 2012). A variety of approaches for determining these critical physical qualities and characterizations are covered below.

Cryogels can be used as a carrier for heterogeneous catalysts (Sahiner and Seven, 2014) or biocatalysts (Hedström et al., 2008), as a solid phase matrix in chromatography for the isolation or removal of (bio)macromolecules like enzymes (Efremenko et al., 2006), proteins (Bereli et al., 2011; Andaç et al., 2012), nucleic acid (Odabaşi et al., 2010), cells (Lozinsky, 2002) and contaminants (Plieva et al., 2003) as a drug carrier system (Kostova et al., 2011), or as a scaffold for tissue engineering (Plieva and Mattiasson, 2007; Savina et al., 2007; Dainiak et al., 2010) due to their unique structure. Cryogels offer effective delivery of nutrients and elimination of metabolomics waste in cell growth due to their tissue-like flexibility and good flow-through qualities.

In their review, Ertürk and Mattiasson (Ertürk and Mattiasson, 2014) provide an in-depth look at the various applications of cryogels in bioseparation. In summary, large pores, low-pressure drop, short diffusion route length, and brief residence periods of biomolecules in the column are all critical requirements for using cryogel matrices as stationary phases. Fluids are transported through macropores, while sorption and chemical reactions occur on the cryogel walls. Because of their wide pores, cryogels have a modest internal surface area. In comparison to packed-bed or monolithic columns, this reduces their binding capability. Several techniques have been devised, including graft polymerization on the interface of the gels (Yao et al., 2007; Bibi et al., 2013), sequential freeze-thawing procedure (Galaev et al., 2008), and the creation of composite cryogels using a particle embedding method to solve this problem (Erzengin et al., 2011; Koç et al., 2011).

These materials exhibit the optimum kinetic properties in fixed-bed applications due to their linked 3D porous structure with a wall thickness of only a few microns. Furthermore, cryogels' enormous pore sizes enable the creation of organic-inorganic cryogels for the selective removal of extremely hazardous metal and metalloid ions without obstructing solution flow in monoliths. The adsorptive performance of the cryogel adsorbents toward different heavy metals via batch and fixed-bed modes is discussed in the following sections.

Cadmium (Cd) a highly toxic heavy metal ion found naturally in the environment. Cd(II) ions removal is studied with a composite polymeric system. Cysteine based polymeric beads and cryogels were utilized for Cd(II) ions removal. Here, two different molar ratios of cysteine based functional monomer (1:1 and 2:1) were employed. SEM and FTIR were used for the structural characterization of the prepared cryogels. The adsorption studies of Cd(II) ions were obtained in different initial concentrations of Cd(II), pH, and temperature. According to these experiments, the authors reported 76.35 and 98.8 μmol/g maximum adsorption amount for 1:1 and 2:1 molar ratio ion-imprinted composite cryogels. In addition, the adsorption capacity of Cd(II) removal from synthetic wastewater was reported as 98.8% (Huseynli et al., 2020). Tabakli et al. (2015) developed particles embedding ion-imprinted cryogels for removal of Cd(II) ion. Ion-imprinted polymer-based cryogels are interesting adsorbents that have been synthesized for the specific removal of heavy metal ions from effluents. 2-hydroxyethyl methacrylate (HEMA) was used as a monomer, and N-methacryloyl-L-cysteine methyl ester (MAC) was used as a functional monomer in the particles system. Then, Poly(HEMA-MAC) particles were embedded into the cryogel network. They reported that these cryogels showed high removal capacity and affinity against Cd(II) ions. The experimental maximum removal capacity was calculated as 32.15 μg/g polymer. Finally, the selectivity of ion-imprinted particles embedded cryogels was examined using heavy metal ions such as Cd(II), Pb(II), and Zn(II). When the results were examined, it was reported that ion-imprinted-based cryogels recognized Cd(II) ions with higher selectivity. SEM image of Poly(HEMA-MAC) cryogels and selectivity result was shown in Figure 2.

In another study, Baimenov et al. (2020a) synthesized amphoteric cryogels by the use of free-radical co-polymerization of acrylate-based precursors (methacrylic acid and 2-acrylamido-2-methyl-1-propansulfonic acid) with allylamine at different ratios. They characterized the cryogels using FT-IR, SEM/EDX, XPS and zeta potential measurements. The prepared cryogels were used to remove Cd(II) ions from aqueous solutions at various pH and initial concentrations. Equilibrium studies revealed a maximum sorption capacity in the range of 132–249 mg/g. Leaching experiments indicated the stability of Cd(II) in the cryogel structure. They used these cryogels to compare commercially available adsorbents for the removal of Cd(II) from various water matrices such as tap water, river water and ultrapure water. As a result, under the experimental conditions used, the cryogels can be more effective adsorbents.

N-acetylcysteine attached poly(acrylamide-co-methyl methacrylate) [poly(AAm-MMA)] cryogels were synthesized by Evli et al., for sensitive removal of different heavy metal ions. For this purpose, they used the free radical cryopolymerization technique and then, the cryogels were modified using N-acetylcysteine. After preparing the cryogel, FTIR, SEM and EDX analyses were used for characterization studies. Removed amounts of heavy metal were demonstrated by stripping voltammetry and the heavy metal removal efficiency was found to be 90.74 for Cd(II). In addition, they showed that these cryogels were successfully used for the removal of heavy metals from environmental and biological media (Evli et al., 2020).

Nickel (Ni) is essential metal element and is defined as a component in a number of enzymes, participating in important metabolic reactions. However, Ni(II) ion intake over the permissible levels results in different types of disease such as pulmonary fibrosis, renal edema, skin dermatitis, and gastrointestinal distress (e.g., nausea, vomiting, diarrhea). It is therefore, essential to remove Ni(II) from wastewater before disposal (Marti, 2012).

Ion-imprinted polymeric materials recently are used for prepared selective materials in the removal of heavy metals. Also, the different structure configuration of cryogels is presented as an effective alternative for this sensitive and selective removal due to its combination with ion-imprinted technology. In this aim, supermacroporous and high flexible cryogel columns were synthesized by Tamahkar et al., to remove Ni(II) ions. This study's remarkable combination of histidine based functional monomer and Ni(II) ions with the two different molar ratios optimized the adsorption capacity (Figure 3). The Ni(II)-MIP cryogels were prepared by cryo-polymerization of N-isopropyl acrylamide (NIPAm) and N-methacryloyl-l-histidine (MAH) was chosen as functional monomer. After synthesized the two types of ion-imprinted cryogel columns, the characterization studies were done with SEM, FT-IR and swelling tests. They reported 1.89 mg/g maximum adsorption capacity for type one cryogel column as named MIP1 and 5.54 mg/g for other (MIP2) cryogels column (Tamahkar et al., 2017).

Heavy metals ions are very harmful for environment if they are discharged into soils or natural water resources. It takes place in many applications in industry and therefore in our daily life. Papancea et al. prepared poly(vinyl alcohol) hydrogel membranes (PVA HG) for sorption and transport of metal ions (here Ni, Cu and Pb). For this purpose, physical crosslinking through the freezing/thawing method was used and was analyzed using aqueous nitrate solutions of copper, lead, and nickel, at wide concentrations ranging from 1 to 100 mM. The surface morphology and sorption effect of PVA HG were characterized by SEM, FTIR, swelling and loading degrees. They reported that analysis of adsoprtion and diffusion coefficients of 100 mM aqueous solutions of Ni²⁺, Cu²⁺, and Pb²⁺ was shown the diffusion process may be primarily defined by hydrodynamic models (Papancea et al., 2010). PVA membranes are synthesized by irradiation crosslinking or by PVA grafting with acrylic acid and styrene. PVA is a well-known biomaterial, and has also been used as a matrix for the removal of metal ions by biosorption or bioaccumulation.

The silver NPs (AgNPs) have unique antimicrobial features and they are one of the most broadly employed nanoparticles in a range of consumer goods including clothing, food packaging, medical tools, and cleaning agents. The common employ inevitably leads to AgNP release into the environment. AgNPs have potential adverse effects on human health and the environment (Sun et al., 2013). For this purpose, the Sarkaya et al., present a novel alternative method to prevent the problem of Silver releasing into the environment. Silver ions Ag(I) imprinted poly(hydroxyethyl methacrylate) (PHEMA)-based cryogels were prepared using N-methacryloyl-L-cysteine as functional monomer. They developed silver ions imprinted polymeric matrix and reported 49.27 mg/g maximum adsorption from an aqueous solution. They used photographic film to obtain the matrix's affinity as a natural Ag(I) ion source. Finally, the successful experiments were studied, and 72.8% recovery was demonstrated for Ag(I) imprinted matrix while this data was reported 0.62% for Ag(I) non-imprinted matrix. The reusability of the matrix was reported more than 10 times (Sarkaya et al., 2019). The mainly advantages of molecularly imprinted polymers (MIPs) are their high selectivity and affinity for the target molecule employed in the imprinting procedure (Akgönüllü et al., 2020a,c, 2021).

Arsenic (As) is a highly toxic metalloid and a natural environmental contaminant that humans are routinely exposed to in food, water, air, and soil (Hughes et al., 2011). Long-term arsenic-contaminated water exposure and ingestion can cause severe health effects, including neurological disorders, cancer, and skin lesions. The WHO designates a strict minimum limit of 0.01 mg/L in drinking water (Mandal and Suzuki, 2002; Mohan and Pittman, 2007). In aqueous systems, inorganic arsenic predominantly exists as oxyanions of arsenate (H₃AsO₄, +5 oxidation state) and arsenite (H₃AsO₃, +3 oxidation state). (Smedley and Kinniburgh, 2002; Türkmen et al., 2021). Arsenite (As(III)) ions are more mobile and toxic when compared to arsenate (As(V)) ions [119]. Several different methods for removing arsenic metalloids include ion exchange, adsorption, membrane filtration, and coagulation-flocculation. The adsorption method was used for removing arsenic due to the significant advantages such as high removal efficiency, simple process, and cost-effectiveness (Cheng et al., 2009).

The novel carbon cryogel/ceria composite for arsenic(III) adsorption process was developed by Arsic et al., They were performed adsorption kinetic, the effect of pH, and initial concentration of As(III) on removal. The rate was examined in a batch system. Carbon cryogel was synthesized by freeze-drying and carbonization in an inert atmosphere at 800°C. The characterization of carbon-cryogels by using a field emission (FE)-SEM showed that the homogeneous distribution of ceria on the surface of the carbon cryogel was achieved. The carbon cryogel/ceria and carbon cryogel specific surface area was calculated 614 m²/g and 620 m²/g, respectively. In this study, pH 5 was reported for the maximum adsorption capacity. Adsorption dose experiments on pure carbon-cryogel and carbon cryogel-ceria were carried out for the same amount of adsorbed arsenic(III) ions in the mass of 100 mg adsorbent. Compared with carbon-cryogel, the As(III) adsorption capacity of the carbon cryogel/ceria composite was increased by 20 times (Minovi et al., 2016). In another study, mesoporous carbon cryogel (CC) were prepared by Minovic. Resorcinol–formaldehyde (RF)-based cryogels were obtained using sol-gel polycondensation, and freeze-drying was used by t-butanol. The carbon cryogel adsorbent was used for the removal of As(III). They have investigated the adsorption of As(III) on carbon cryogel as a function of As(III) concentration and solution pH (pH range 2–11) (Gulicovski et al., 2015b).

Sahiner et al., developed poly[(3-Acrylamidopropyl)trimethylammonium chloride] [p(APTMACl)] cryogels for removing arsenate anions from an aqueous sample. It showed rapid swelling in water in about 7 secs. The maximum arsenate adsorption capacity was obtained about 120 mg/g for cryogels. According to the Langmuir and Freundlich adsorption isotherm models applied to adsorption arsenate anions by polymeric cryogels, the arsenate adsorption conformed to the Langmuir adsorption isotherm models (Sahiner et al., 2015).

A cryogels incorporated iron-aluminum double hydrous oxides (Fe-Al-Cryo) were synthesized by Kumar. These composite based cryogels were used for the adsorption of arsenic (III). Here, the Langmuir isotherm model was showed the best fit for As(III) adsorption. In addition, 24.6 mg/g maximum adsorption capacity was demonstrated. This study aimed to reduce the As(III) concentration in wastewater below 0.15 mg/L by synthesized cryogel without using any pretreatment step (Kumar et al., 2013).

Otero-gonzález et al. (2020) developed a novel nanostructured iron oxide macroporous cryogels for removing As(III). In the synthesis method, regardless of the nature of the copolymer forming the cryogel structure, iron oxide was allowed to disperse homogeneously in the cryogel reaction mixture. Here, monomer (AAm or HEMA) and crosslinker (MBAA or PEGDA) were used to prepare monolithic macroporous hybrid cryogels using co-polymerising under cryogenic conditions. Therefore, two different cryogels were synthesized by using AAm monomer and MBAA crosslinker and the other hand HEMA monomer using PEGDA crosslinker. An excellent mechanical property was reported for the acrylamide-based cryogel with 118 mg/g maximum adsorption capacity for As(III) and best fit with the Langmuir model.

Kurozumi et al. (2015) prepared polyacrylamide cryogels for removal of As(V). They synthesized the procedure by freezing monomer solution of acrylamide with cross-linker N, N-methylene bis(acrylamide) and polymerizing initiated by N,N,N′,N′-tetramethyl ethylenediamine, and ammonium peroxodisulfate at sub-zero temperatures. Then, Iron hydroxide oxide particles were embedded on the surface of the cryogel wall. The polyacrylamide cryogel adsorbent containing iron hydroxide oxide particles adsorbed many As(V) at low and high concentrations successfully. The synthesis procedure of polyacrylamide cryogels was demonstrated in Figure 4.

Purohit et al., developed a cost-effective and water-stable transition metal oxide doped cross-linked chitosan cryogel for highly sensitive removal of arsenic ions from potable water. The synthesized CuO–ZnO doped cross-linked chitosan cryogels were characterized by SEM, EDAX, XRD and FTIR. They reported that the adsorption kinetics fit the Freundlich isotherm model. The arsenic contamination was reduced to 76% after 2 h of contact with only 10 mg of adsorbent. They have cited it as a potential candidate for low-cost arsenic removal (Purohit et al., 2020).

The common goal of the studies given above is the social and economic importance of developing easy and cost-effective systems for the contamination of arsenic metalloids, which is a global concern.

Lead (Pb) has a low melting point, is soft and straightforward to operate, and is utilized in various applications, including pharmaceuticals, paint pigments, mascara, and batteries (Yari et al., 2015). Pb is also toxic in a trace amount and strongly affects ecosystems and the human body through various routes. Pb was taken up in the body through oral administration, inhalation, and other methods, is not easily excreted and accumulate in the body. This accumulated Pb can cause blood disorders, anorexia, gastrointestinal symptoms such as constipation, problems with the nervous system and kidney malfunction (Huang et al., 2011). Therefore, it is essential to remove Pb from water and wastewater.

Kim and Lee (2019b) developed a cryogel to absorb lead (Pb) ions from an aqueous solution. The poly(allylamine-co-methacrylamide-co-acrylic acid) [p(AA-co-MA-co-AAc)] cryogel and later functionalized with thiourea using methyl isothiocyanate. It contains amine groups and carboxyl groups. The functionalized p(AA-co-MA-co-AAc) cryogel exhibited the highest adsorption amount at pH 7 and 400 mg/L. Equilibrium isotherm data have been reported to fit the Langmuir isotherm better. The results show that the functionalized cryogel surfaces are uniform, and monolayer adsorption occurs. A new adsorbent for removing Pb ions from wastewater was developed by Park and Min (2012). They synthesized poly(vinyl alcohol)/hydroxyapatite (PVA/HAp) composite cryogel using water and freeze-thawing process. The maximum adsorption capacity of the HAp-based cryogels and PVP-based cryogel was 69 and 108 mg/g, respectively.

In another study, Kim et al., prepared a poly(allylamine-co-methacrylamide-co-acrylic acid) [p(AA-co-MA-co-AAc)] cryogel containing amine and carboxyl groups and then subsequently functionalized with thiourea using methyl isothiocyanate (Kim and Lee, 2019a). The functionalized p(AA-co-MA-co-AAc) cryogel was applied to the adsorption of Pb ions from an aqueous solution. They reported the amount of Pb adsorption. The functionalized p(AA-co-MA-co-AAc) cryogel was analyzed by FTIR and SEM. Also, they characterized pore shape, pore distribution, and specific surface area of the materials. They studied the effect of the solution pH and initial concentration on Pb adsorption. The functionalized p(AA-co-MA-co-AAc) cryogel exhibited the highest adsorption amount at pH 7 and 400 mg/L.

The great attention of 3D cryogels with interconnected macropore structures is increased owing to their immense application potential in wastewater cleanup. Zhong et al. (2021) fabricated a pure poly(vinyl imidazole) cryogel through an improved cryogelation method, adding the pre-freezing–thawing step, which reduced the temperature difference between the different positions of the precursor solution during the reaction. They reported that the prepared 3D cryogel featured a robust underwater elastic performance. The adsorption capacity of Ni(II), Cu(II), Zn(II), Pb(II), Co(II), and Cd(II) ions from aqueous solutions by cryogel was explored. They reported the specific adsorption of Cu(II), in which the adsorption capacity and removal efficiency reached 148 mg/g and 99.99%, respectively.

Another wastewater clean-up study was reported by Zhang et al. (2021) they developed a strategy to fabricate the poly(VIM) cryogel using micro fibrillated cellulose (MFC) as a reinforcing agent. Structural characterizations indicated that the poly(VIM)/MFC showed interconnected macropores of 12 ± 10 μm and high VIM contents. They reported batch adsorption with adsorption capacities of the poly(VIM)/MFC cryogel to Cu(II), Pb(II), Zn(II), Cd(II), Ni(II), and Co(II) were 87.4, 53.9, 48.6, 44.3, 23.8, and 20.1 mg/g, respectively. Dynamic adsorption displayed that the loaded Cu(II) ions could be rapidly adsorbed and hardly flow through the poly(VIM)/MFC cryogel, indicating the extremely fast adsorption at the flow mode.

Mercury (Hg) is one of the most toxic ion elements (Fu et al., 2019). Hg is highly poisonous even at trace levels for living organisms. It is frequently discharged into the aquatic environment through industrial wastewaters, including those produced by the fertilizer, mining, pharmaceutical industries, and electroplating and metal processing (Arshadi et al., 2018). Zhu et al. prepared SiO₂ cryogel through the sol-gel method and freeze-drying process. The SiO₂ cryogels were functionalized with sulfhydryl groups. The commercial water glass was chosen as the silica source to prepare silica cryogel, and 3-Mercaptopropyl-Trimethoxysilane (MPTMS) was used as the modifier. They reported that has been successfully synthesized high specific surface area silica cryogel. Four heavy metal ions including Cd(II), Zn(II), Cu(II), and Pb(II) were used to investigate the specific effect on the removal of Hg(II) by the SiO₂-SH cryogel. The removal rate of Hg always remained the highest compared to other ions. The theoretical maximum removal capacity from the Langmuir isotherm model was calculated to be 709.2 mg/g. They reported that the removal rate of Hg(II) was decreased from 99.96 to 81.95% after five adsorption/desorption cycles (Zhu et al., 2021).

Copper (Cu) as a heavy metal element is hazardous to human health and the environment. It is broadly employed in industry and is accumulating in their waste streams (Al-Harahsheh et al., 2015). Tang et al. (2020) developed cellulose nanofibril (CNF) cryogel beads for Cu(II) removal. The cellulose nanomaterial has been broadly employed in various fields such as wastewater treatment, energy storage, oil recovery, biomedical, agriculture to food additives. The highly carboxylated and chemically crosslinked CNF cryogel beads were synthesized by a one-step ester cross-linking protocol. The maleic anhydride (MA) was used as the crosslinker. The presence of carboxyl groups and ester linkages were characterized by FTIR and conductometric-potentiometric titration. The carboxylic was content of up to 2.78 mmol/g. They have also investigated the effect of CNF concentration on the morphology and wet mechanical strength of the crosslinked cryogel beads. The pH of 5.6 was selected as the optimum pH for conducting the adsorption studies. The maximum removal capacity of Cu (II) was found to be 82.17 mg/g. The results fitted the data better by the pseudo-second-order model. The maximum removal capacities of Cu(II) was determined from the Langmuir model were 60.92 mg/g and 84.12 mg/g for CNF-MA 1% and CNF-MA 2%, respectively. They reported that cryogel beads are robust and efficient to remove Cu(II) ions. In addition, the regeneration of beads was shown perfectly. As a result, the designed CNF-MA cryogel beads are promising adsorbents for heavy metal ions removal because of their quick removal rate, robust mechanical strength and, perfect reusability. Furthermore, catalyst and organic solvent were not needed.

Common cryogel and non- cryogel materials used in heavy metal removal are tabulated in Table 3.

The cryogels presented in this review displayed good performance with high removal capacity, quick absorption rates, and high selectivity, as well as pH insensitivity. Thus, they have considerable potential as an absorbent for the removal of heavy metal ions in wastewater containing multiple compounds.