A Short Review on Recent Advances of Hydrogel-Based Adsorbents for Heavy Metal Ions

: The growth of industry fulﬁlls our necessity and promotes economic development. How-ever, pollutants from such industries pollute water bodies which pose a high risk for living organisms. Thus, researchers have been urged to develop an efﬁcient method to remove toxic heavy metal ions from water bodies. The adsorption method shows promising results for the removal of heavy metal ions and is easy to operate on a large scale, thus can be applied to practical applications. Numerous adsorbents were developed and reported, among them hydrogels, which attract great attention because of the reusability, ease of preparation, and handling. Hydrogels are generally prepared by the cross-linking of polymers that result in a three-dimensional structure, showing high porosity and high functionality. They are hydrophilic in nature because of the functional groups, and are non-toxic. Thus, this review provides various methods of hydrogel adsorbents preparation and summarizes recent progress in the use of hydrogel adsorbents for the removal of heavy metal ions. Further, the mechanism involved in the removal of heavy metal ions is brieﬂy discussed. The most recent studies about the adsorption method for the treatment of heavy metal ions contaminated water are presented.


Introduction
Essential ions participate in a various biochemical process that helps to maintain biological activities in a living organism. Essential elements such as nitrogen, oxygen, hydrogen, sulfur, calcium, potassium, sodium, magnesium, phosphorus, chloride, and essential trace elements like fluorine, iodine, iron (Fe), lead (Pb), manganese (Mn), copper (Cu), chromium (Cr), molybdenum (Mo), selenium (Se), cobalt (Co), zinc (Zn), and vanadium (V) are necessary for living organisms to balance life functions [1][2][3]. Fluorine prevents dental cavities [4], calcium is beneficial for bone health [5], chromium helps potentiate the actions of insulin [6], iron is necessary for oxygen transport [7], copper and zinc play a vital role in metabolism [8], and iodine is advantageous in the regulation of thyroid hormones [2]. However, deficiency or low levels of these essential elements in living organisms leads to damage, ill effects, and alters functions of the body [9]. Furthermore, the presence of an excessive amount of essential metal ions, toxic metal ions, non-essential metal ions [10], or metalloids in unwanted places is considered a contaminant that affects humans and the environment. Contaminants or toxic metal ions include Zn, Mn, Cu, Pb, uranium (U), silver (Ag), strontium (Sr), tungsten (W), cesium (Cs), cadmium (Cd), nickel uranium (U), silver (Ag), strontium (Sr), tungsten (W), cesium (Cs), cadmium (Cd), nickel (Ni), Mo, arsenic (As), Cr, mercury (Hg), V, Se, aluminum (Al), and Co [1,11]. Lack of essential elements affects the uptake of non-essential elements, e.g., the presence of a high level of Zn will result in Cu deficiency [12]. A vitamin D deficiency will affect the deposition of calcium in bones. Fe deficiency will increase the uptake of Cd, which interferes with the metabolism of Zn essential metal ions [13]. Thus, a metal speciation analysis helps to identify the safe and dangerous limits of heavy metal ions in water systems. Generally, metal speciation is done using measurements such as atomic absorption spectrometry (AAS), gas chromatography coupled with high-performance liquid chromatography, electrochemical methods, and optical measurements [14][15][16][17]. AAS has been used for the speciation of As (III) and As(V) [18][19][20]. Other than these methods, recently, surface-enhanced Raman spectroscopy has been used to analyze the Cr(Vi) and Cr(III) ions in water [21].

Chemical Precipitation
Chemical precipitation is a method where heavy metal ions are separated from the contaminated water as sediments [29][30][31]. The precipitates are formed by the complexes of heavy metal ions with precipitation agents or coagulants which can be separated by filtration or centrifugation. The parameter which needs to be adjusted in the chemical precipitation method is pH. This method has advantages and disadvantages. The advantage of this method is that it is inexpensive. However, disposal of the large amount of sludge produced during purification requires additional costs.

Coagulation and Flocculation
The heavy metal ions are removed by adsorption, complexation, and co-precipitation from the water system [32][33][34]. It is a simple, integrated physicochemical process, and an inexpensive method. Complete removal of heavy metal ions is difficult, requires chemicals, and large amounts of sludge are produced during separation.

Membrane Filtration
Membrane filtration is a non-destructive separation method using a semipermeable barrier. Reverse osmosis is involved in this separation method [35][36][37]. This method requires very simple equipment, can be conducted in a small space, is effective at high concentrations, and no chemical is required. However, investment and maintenance are expensive, the design of the membrane filtration system can differ significantly, and selection of the membrane is required.

Chemical Precipitation
Chemical precipitation is a method where heavy metal ions are separated from the contaminated water as sediments [29][30][31]. The precipitates are formed by the complexes of heavy metal ions with precipitation agents or coagulants which can be separated by filtration or centrifugation. The parameter which needs to be adjusted in the chemical precipitation method is pH. This method has advantages and disadvantages. The advantage of this method is that it is inexpensive. However, disposal of the large amount of sludge produced during purification requires additional costs.

Coagulation and Flocculation
The heavy metal ions are removed by adsorption, complexation, and co-precipitation from the water system [32][33][34]. It is a simple, integrated physicochemical process, and an inexpensive method. Complete removal of heavy metal ions is difficult, requires chemicals, and large amounts of sludge are produced during separation.

Membrane Filtration
Membrane filtration is a non-destructive separation method using a semipermeable barrier. Reverse osmosis is involved in this separation method [35][36][37]. This method requires very simple equipment, can be conducted in a small space, is effective at high concentrations, and no chemical is required. However, investment and maintenance are expensive, the design of the membrane filtration system can differ significantly, and selection of the membrane is required.

Ion-Flotation
Ion flotation involves the removal of heavy metal ions by the complex formation between surfactant (with opposite charge) and target ions [38][39][40]. Bypassing gas bubbles in the wastewater, heavy metal ions complexes will float on the surface and thus contaminants are purified. The disadvantage of this method is that the initial investment, instrument maintenance, and operation cost are expensive.

Ion-Exchange
This is a physical separation process in which organic resins, inorganic three-dimensional matrix, and new-generation hybrid materials are used [41][42][43][44]. By stoichiometric chemical reaction, the metal ions in the solution will have ion exchange with resins and will change into a solid matrix with the same charge. The main advantage of this method is the regeneration of materials; however, only selective metal ions can be removed and it is an expensive method.

Electrochemical Treatment
In this method, heavy metal ions are removed using electrochemical treatment by three mechanisms: electrocoagulation, electroflotation, and electroxidation [45,46]. It is a low-cost method and extraction of metals can be achieved; however, the operational cost is high because of energy consumption.

Hydrogels Adsorbents for Heavy Metal Ions
Hydrogels have a three-dimensional network structure that is capable of retaining water in their network structure. Polar or non-polar monomers are used for the preparation of hydrophilic or hydrophobic hydrogels. Hydrophobic hydrogels are prepared for specific applications. Generally, monomers, initiators, and cross-linkers are involved. Hydrophilic hydrogels are prepared using in situ polymerization of polar monomers. Polymerization techniques such as bulk polymerization, emulsion polymerization, and solution polymerization are used for hydrogel preparation.
Bulk polymerization is a simple technique in which only monomers and initiators are used. The reaction takes place at a high temperature and produces a highly viscous hydrogel because of a high degree of polymerization which causes a high concentration of monomers. This hydrogel will become soft in water [72]. An emulsion or inversesuspension polymerization is oil-in-water emulsion polymerization. Polymerization takes place in the colloidal particles that form in the earlier phase of the process. In this method, monomer, initiator, and cross-linker are used. Surfactants are used for the stabilization of particles which can be easily washed after the preparation of hydrogel particles [58,59]. Solution polymerization is where ionic or neutral monomers are cross-linked. The polymerization is generally initiated by UV-irradiation or by a redox initiator. Solvents are used and are removed after polymerization by immersion of the hydrogels in water, causing a heterogeneous phase separation to take place; thus, the hydrogels are purified [73]. The functional groups in the polymer network will involve ionic or coordination interactions.
These hydrogels are not dissolved in any solvents [74], are hydrophilic and have a unique three-dimensional structure [75], high swelling degree or ratio [58,59], one or more heteroatoms which are involved in the coordination with heavy metal ions [76,77], and can be reused [75,78,79].
The prepared hydrogels can be characterized using various techniques: crosslinking of polymer can be confirmed by Fourier transform infrared spectroscopy (FTIR) [75][76][77]; the chemical composition in hydrogels can be confirmed by X-ray photoelectron spectroscopy (XPS) [76]; optical microscope and scanning electron microscope (SEM) measurements will inform about morphology and porosity of hydrogel [76,77]; the strength of hydrogels can be studied using compress strength tests; differential scanning calorimetry (DSC) and thermogravimetry (TG) measurements will report the thermal behavior and stability of hydrogels [80]. The adsorption of heavy metal ions by hydrogels can be quantified using inductively coupled plasma optical emission spectroscopy (ICPOES) measurements [80].
The adsorption capacities or efficiency and percent of removal efficiency are calculated using the following equations: [58,59,77,81] Adsorption Efficiency or Capacity (Qe) A and B are the initial (before the adsorption experiment) and final (after the adsorption experiment) equilibrium concentrations of the heavy metal ions, respectively. V is the volume of the metal ion used for the adsorption experiment, and W is the weight of the dried hydrogel used. Using ICPOES, the concentrations (A and B) of heavy metal ions are measured.
The removal of heavy metal ions by hydrogels depends on many factors such as pH, metal ion concentration, the complexing ability of metal ions, ionic radius of metal ions, hydration energy of metal ions, hydrogel structure, swelling degree of hydrogel, uptake time, experimental conditions, and reactive sites available in hydrogels. These factors affect the interaction between hydrogels and heavy metal ions. The adsorption experiments are reported in single and multiple systems. The different hydrogel adsorbents used for the removal of heavy metal ions are tabulated in Tables 1 and 2. Some hydrogels which are used for the adsorption of heavy metal ions are regenerated and used many times. Figure 3 depicts the removal of heavy metal ions using hydrogel as adsorbents. The polymers will have functional groups such as carbonyl, hydroxyl, carboxylic acid, carboxylate group, and amine groups as shown in Figure 4. Ionic or electrostatic interactions will be involved between the functional groups in polymers and heavy metal ions to make complexation. Furthermore, the removal efficiency of metal ions depends on the affinity of functional groups in polymers towards metal ions. Typically, single-ion system hydrogels are added to a single heavy metal ion solution (Table 1) and multiple system hydrogels are added to a multiple/mixed metal ions solution ( Table 2). In both cases, hydrogels adsorbed the heavy metal ions. Thus, in this section, the affinity of polymers or effects of polymers towards the removal efficiency of specific or multiple heavy metal ions are discussed. Hydrogels were prepared by cross-linking 2-acrylamido-2-methylpropane sulfonic acid and 2-methacryloyloxy ethyl dimethyl-3-sulfopropyl ammonium hydroxide monomers using N, N-methylene bisacrylamide. The prepared hydrogels were used for the removal of heavy metal ions Fe(III), Hg(II), and Cr(III) [70].
Metals 2021, 11, x FOR PEER REVIEW 6 of 13 monomers using N, N-methylene bisacrylamide. The prepared hydrogels were used for the removal of heavy metal ions Fe(III), Hg(II), and Cr(III) [70].  The polyacrylic acid hydrogel was used to remove Cu(II) and Ni(II) ions from water [77]. 2-acrylamido-2-methyl-1-propane sulfonic acid-based magnetic responsive hydrogels were used to remove heavy metal Cd(II), Co(II), Fe(III), Pb(II), Cr(III), Ni(II), and Cu(II) ions. [82] Hydrogels from Brazilian vegetal species were reported as adsorbents for Mg(II), Cr(III), Fe(III), and Zn(II) ions [83]. El-Hag Ali. A. et al. [84] reported the chelating poly(vinyl pyrrolidone/acrylic acid) hydrogels for the removal of Fe(III), Cu(II), and Mn(II) ions. A magnetic-vinyl pyridine-based hydrogel removed U and thorium metal ion from aqueous environments [85]. Controlled hydrolysis of polyacrylamide resulted in polyacrylamide-polyacrylic acid hydrogel used for the recovery and separation of Cu(II)   The polyacrylic acid hydrogel was used to remove Cu(II) and Ni(II) ions from water [77]. 2-acrylamido-2-methyl-1-propane sulfonic acid-based magnetic responsive hydrogels were used to remove heavy metal Cd(II), Co(II), Fe(III), Pb(II), Cr(III), Ni(II), and Cu(II) ions. [82] Hydrogels from Brazilian vegetal species were reported as adsorbents for Mg(II), Cr(III), Fe(III), and Zn(II) ions [83]. El-Hag Ali. A. et al. [84] reported the chelating poly(vinyl pyrrolidone/acrylic acid) hydrogels for the removal of Fe(III), Cu(II), and Mn(II) ions. A magnetic-vinyl pyridine-based hydrogel removed U and thorium metal ion from aqueous environments [85]. Controlled hydrolysis of polyacrylamide resulted in polyacrylamide-polyacrylic acid hydrogel used for the recovery and separation of Cu(II) Further, ionic monomers are used to prepare hydrogels and the ionic unit promotes the interaction of polymers with heavy metal ions. As(V) was removed efficiently using cationic hydrogel (poly(3-acrylamidopropyl)trimethyl ammonium chloride) with 99.7% removal efficiency. Sulfonic acid-based hydrogels are used to remove Cd(II), Co(II), Cu(II), and Fe(III) ions [88]. Hydroxyethyl methacrylate-based hydrogels are used for the selective removal of Fe(II), Cu(II), and Cr(VI) ions in decreasing order [89]. Magnetic propane sulfonic acid-based hydrogels showed selective removal capability of toxic metal ions, Cu(II), Cd(II), Fe(II), and Pb(II) ions [90]. After the removal of heavy metal ions, the hydrogels are recovered and reused for many cycles. In the case of polyvinyl alcohol reinforced with multiwalled carbon nanotubes hydrogel used to remove Pb(II) ion, the prepared hydrogel was reused for four cycles with an adsorption efficiency of more than 80% after four cycles [91]. Poly(acrylic acid-co-hydroxyethyl methacrylate)-based hydrogels were reported as selective adsorbents for Pb(II) ions among Pb(II), Cu(II), and Zn(II) ions [76]. Chitosan-based hydrogels, which are prepared by crosslinking of chitosan, polyacrylic acid, and N, N'-methylene bisacrylamide, removed Cr(VI) ions effectively with 94.72% [92]. Waste biomass hydrogel of soybean-poly acrylic acid has been used to remove Cr(III) ion [93]. One review reports the problem of arsenic and the materials used for the removal of arsenic [94]. Molybdate impregnated chitosan beads remove As(V) effectively [95]. Table 1. Adsorption capacity and removal efficiency of heavy metal ions using different adsorbents in single metal ion systems.

Conclusions
Contamination of water bodies with heavy metal ions needs urgent action. Among existing methods such as chemical precipitation, ion exchange, electrochemical treatment, membrane filtration, coagulation, ion flotation, and adsorption; adsorption is a simple, effective, and cost-effective method. The methods used for the preparation of hydrogel adsorbents were presented. The selected research focused on the improvement of adsorbents for the efficient removal of heavy metal ions and the plausible interactions between adsorbents and heavy metal ions is also briefly discussed. This review discussed hydrogels as an effective adsorbent for heavy metal ions. However, a few important aspects should be focused on in the development of hydrogel adsorbents for heavy metal ions. The hydrogels are recycled and reused for many cycles; however, the reported results show a decrease in removal efficiency with cycles. Thus, research should focus on the development of hydrogel adsorbents that maintain removal efficiency. Furthermore, hydrogels mostly remove a specific or few metal ions and complete removal of metal ions is difficult. Hence, additional research should focus on the development of effective adsorbents for the simultaneous and complete removal of multiple heavy metal ions so that adsorbents can be used for the practical application towards the removal of multiple heavy metal ions.

Conflicts of Interest:
The authors declare no competing financial interest.