**4. Removal of heavy metal ions by SAHs**

The heavy metal ions are important and can be used up to certain permissible limit. But, when the concentration of these metal ions increases, then these become toxic and produce a huge number of disease in both terrestrial and aquatic animals. The accumulation of heavy metal ions in water bodies for a long time acts as a pollutant and undergoes in living bodies through different food chains. Therefore, the removal of these toxic heavy metal ions from wastewater is important and a hot area for researchers to work in. Various traditional adsorbents are used for the removal of these impurities from water. However, in the present the synthesis of SAHs has been increasing due to its easy synthesis route, better performance, selectivity, and recyclability with good efficiency toward absorption of heavy metal ions. A general sketch for entrapment of heavy metal ions by SAHs through swelling mechanism is presented in **Figure 4**.

SAH network consists of various functional groups (both positive and negative) as an active site for entrapping and removing of metal ions from the medium. Thus the researchers choose to test and work on the removal of these heavy metals from wastewater by incorporating opposite charge density on the network. The sorption process will be more if opposite charge exists on SAH network to that of heavy metal ions. As the charge nature on SAHs depends on the nature (cationic, anionic, and neutral) of monomers used for the synthesis process. So, for efficient removal of heavy metal ions, the hydrogels must have negative active sites for electrostatic interactions. In most of cases, the SAHs contain ▬OH, ▬COOH, and ▬SO3H functional groups as an active site for electrostatic interactions. Similarly, in some cases the sorption mechanism follows the ion exchange route for the removal of metal ions, and the salt group (▬COO<sup>−</sup> Na+ ) exchanges the heavy metal ions between the

**Figure 4.** *Swelling and sorption process of SAHs toward heavy metal ions [29].*

SAH network and that of wastewater. The heavy metal ion removal capacities of SAHs can be calculated by following Eq. (2):

$$\text{Removal capacity (mg/g)} = \frac{\text{(C}\_\text{o} - \text{C}\_\text{f}) \times V}{\text{m}\_\text{SM}} \tag{2}$$

where Co and Cf are the initial and final concentrations (mg/L) of heavy metal ions, V is the volume of the solution (L), and mSAH is the mass (g) of superabsorbent hydrogels used as an absorbent.

The effect of charge nature on hydrogels toward the removal of heavy metal ions was demonstrated previously by our group [12]. The synthesis of anionic [poly(methacrylic acid) (P(MAA))], neutral [poly(acrylamide) (P(AAm))], and cationic [poly(3-acrylamidopropyltrimethyl ammonium chloride) (P(APTMACl))] hydrogels was carried out to study the effect of charge nature on the removal of heavy metal ions. The prepared hydrogels have the potential to remove heavy metal ions selectively in the following orders such as P(MAA) > P(AAm) > P(APTMACl). Güçlü et al. [34] reported the synthesis of starch-graft-acrylic acid/montmorillonite (S-g-AA/MMT) superabsorbent composite hydrogels. These synthetic materials were used as a superabsorbent for the removal of Cu2+ and Pb2+. The removal affinity of Cu2+ was found to be greater than Pb2+. Similarly, Zhao and Mitomo [35] synthesized a physically cross-linked CMC-Chitosan hydrogels which were used for Cu2+ absorption from wastewater. The absorption of Cu2+ ions on hydrogels occurs due to electrostatic attraction between positive metal ions and that ▬COOH and ▬NH2 functional groups, which are the active sites of synthesized hydrogels. The maximum amount of Cu2+ ions removed by Chitosan-CMC was 169.5 mg/g. Thus the hydrogels can be used for the removal of Cu2+ ions. Yung et al. [36] synthesized SAH of sodium poly(acrylic acid) p(AANa) through single-step cost-effective techniques. This SAH was used to remove Cu2+ ions from aqueous solution. It was found that the maximum uptake capacity of Cu2+ ions was 243.91 mg/g, which they claim the highest uptake capacity mentioned in literature. Tang et al. [37] developed an interpenetrating network hydrogels for entrapping heavy metal ions. Especially the superabsorbents were reported by the author for efficient removal of Ni2+, Cr2+, and Cd2+ from aqueous solution. The study reveals that 102.34 mg/g of Ni2+ metal ions were removed, followed by chromium and cadmium divalent ions. The acrylic acid-based monomer SAH was synthesized by Bulut et al. [38] by dissolving AAc monomer in distilled water and then neutralized with NaOH solution, followed by dispersing the clay (bentonite) powder. The reaction was facilitated by adding MBA (as a cross-linker) and APS (as an initiator) into the reaction vessel and leads the reaction for 2 h at 70°C. The sample was reported by the author for the removal of heavy metal ions (Pb2+, Ni2+, Cd2+, and Cu2+) from aqueous medium. The maximum adsorption capacities of the sample as per reported were 1666.67, 270.27, 416.67, and 222.22 mg g<sup>−</sup><sup>1</sup> for Pb(II), Ni(II), Cd(II), and Cu(II) at 25°C, respectively. A novel poly(pyrrole) graft-based magnetic hydrogel nanocomposite (MHN) was reported by Hosseinzadeh and Tabatabai Asl [39]. It was mentioned that the sample is an effective adsorbent for the removal of lethal Cr(VI) ion from aqueous medium. The maximum absorption capacity of this sample was found to be 208 mg/g. Chauhan et al. [40] synthesized polycarboxylated starch-based hydrogels which were applied for the sorption of copper ions. This hydrogel is stimuli responsive in nature for the sorption of Cu2+ ions and strongly depends on the 3D network of hydrogels. The polycarboxylated hydrogel works as a chelating agent and causes chelation with Cu2+ ions. The 128 mg/g absorption capacity was reported by the author toward Cu2+ ions. Novel SAH nanocomposites were reported by Kaşgöz et al. [41] based on acrylamide (AAm)-2-acrylamido2-methylpropane sulfonic acid (AMPS) sodium

**195**

*Superabsorbent Hydrogels for Heavy Metal Removal DOI: http://dx.doi.org/10.5772/intechopen.89350*

different hydrogel samples are tabulated in **Table 1**.

Acrylamide (AAm)-2-acrylamido-2-methylpropane sulfonic

*Maximum absorption capacity of different hydrogel samples toward Cu2+ ions.*

acid (AMPS) sodium salt/clay

**Table 1.**

ions are discussed in this chapter.

the positive heavy metal ions and vice versa.

**5.1 Effect of pH**

salt along with clay and were synthesized through in situ copolymerization process. The synthesized materials were successfully used for removal of heavy metal ions including Cu(II), Cd(II), and Pb(II) from the aqueous medium. The absorption capacity of the samples was 1.07, 1.28, and 1.03 mmol/g for Cu(II), Cd(II), and Pb(II) ions, respectively. The comparative removal of copper divalent metal ions by

**Hydrogel Maximum adsorption** 

Starch-graft-acrylic acid/montmorillonite (S-g-AA/MMT) 130.1 [34] Chitosan-CMC 169.6 [35] Sodium poly (acrylic acid) p(AANa) 243.9 [36] Acrylic acid/clay 222.2 [38] Polycarboxylated starch p(CS) 128 [40]

**(mg/g) of copper ion**

81.66 [41]

**Reference**

The removal ability of SAHs toward heavy metal ions depends on the environmental conditions. The factors which directly affect the sorption of heavy metals

The modified polysaccharide superabsorbent (MPSA) hydrogel was synthesized

by Guilherme et al. [42] following free radical polymerization protocol by first treating KOH with AAc, followed by addition of known amount of AAm, modified gum arabic (MAG), and 84 mmol sodium persulfate (SPS) as an initiator. The synthesized SAHs were used as an ionic absorbent for the removal of two metal ions. It was found that the swelling of these SAHs is dependent on the pH of the medium. They studied the removal of Cu2+ and Pb2+ from aqueous medium, and it was found that the removal of heavy metals increases with the increase in pH value from 3 to 5. It is because of the deprotonation of carboxyl groups above the pKa value (4.0) present in hydrogel network. Thus the basic media generated anionic atmosphere which in turn established an ionic bond with positive metal ions in the medium and favor the process of removal. Another pH-sensitive hydrogels were reported by Peng et al., which were synthesized by grafting acrylic acid (AA) on hemicellulose having xylene. The samples were used for the removal of heavy metal ions like Pb2+, Cd2+, and Zn2+ from the medium. The maximum adsorption capacity of the sample for the heavy metal ions was reported by the authors as Pb2+ (859 mg/g), Cd2+ (495 mg/g), and Zn2+ (274 mg/g). It was also reported by the authors that the increase in the uptake capacity of the heavy metal ions from the solution by hydrogels was due to the increase in the pH value of the medium, which is due to the presence of ▬COOH group. The ▬COOH group deprotonated into ▬COO▬ at high pH value; thus more active sites are available on hydrogel network to remove

**5. Factor affecting the absorption of heavy metal ions by SAHs**


**Table 1.**

*Trace Metals in the Environment - New Approaches and Recent Advances*

SAHs can be calculated by following Eq. (2):

bent hydrogels used as an absorbent.

SAH network and that of wastewater. The heavy metal ion removal capacities of

Removal capacity (mg/g) = \_\_\_\_\_\_\_\_\_\_\_

where Co and Cf are the initial and final concentrations (mg/L) of heavy metal ions, V is the volume of the solution (L), and mSAH is the mass (g) of superabsor-

The effect of charge nature on hydrogels toward the removal of heavy metal ions was demonstrated previously by our group [12]. The synthesis of anionic [poly(methacrylic acid) (P(MAA))], neutral [poly(acrylamide) (P(AAm))], and cationic [poly(3-acrylamidopropyltrimethyl ammonium chloride) (P(APTMACl))] hydrogels was carried out to study the effect of charge nature on the removal of heavy metal ions. The prepared hydrogels have the potential to remove heavy metal ions selectively in the following orders such as P(MAA) > P(AAm) > P(APTMACl). Güçlü et al. [34] reported the synthesis of starch-graft-acrylic acid/montmorillonite (S-g-AA/MMT) superabsorbent composite hydrogels. These synthetic materials were used as a superabsorbent for the removal of Cu2+ and Pb2+. The removal affinity of Cu2+ was found to be greater than Pb2+. Similarly, Zhao and Mitomo [35] synthesized a physically cross-linked CMC-Chitosan hydrogels which were used for Cu2+ absorption from wastewater. The absorption of Cu2+ ions on hydrogels occurs due to electrostatic attraction between positive metal ions and that ▬COOH and ▬NH2 functional groups, which are the active sites of synthesized hydrogels. The maximum amount of Cu2+ ions removed by Chitosan-CMC was 169.5 mg/g. Thus the hydrogels can be used for the removal of Cu2+ ions. Yung et al. [36] synthesized SAH of sodium poly(acrylic acid) p(AANa) through single-step cost-effective techniques. This SAH was used to remove Cu2+ ions from aqueous solution. It was found that the maximum uptake capacity of Cu2+ ions was 243.91 mg/g, which they claim the highest uptake capacity mentioned in literature. Tang et al. [37] developed an interpenetrating network hydrogels for entrapping heavy metal ions. Especially the superabsorbents were reported by the author for efficient removal of Ni2+, Cr2+, and Cd2+ from aqueous solution. The study reveals that 102.34 mg/g of Ni2+ metal ions were removed, followed by chromium and cadmium divalent ions. The acrylic acid-based monomer SAH was synthesized by Bulut et al. [38] by dissolving AAc monomer in distilled water and then neutralized with NaOH solution, followed by dispersing the clay (bentonite) powder. The reaction was facilitated by adding MBA (as a cross-linker) and APS (as an initiator) into the reaction vessel and leads the reaction for 2 h at 70°C. The sample was reported by the author for the removal of heavy metal ions (Pb2+, Ni2+, Cd2+, and Cu2+) from aqueous medium. The maximum adsorption capacities of the sample as per reported were 1666.67, 270.27, 416.67, and

for Pb(II), Ni(II), Cd(II), and Cu(II) at 25°C, respectively. A novel

poly(pyrrole) graft-based magnetic hydrogel nanocomposite (MHN) was reported by Hosseinzadeh and Tabatabai Asl [39]. It was mentioned that the sample is an effective adsorbent for the removal of lethal Cr(VI) ion from aqueous medium. The maximum absorption capacity of this sample was found to be 208 mg/g. Chauhan et al. [40] synthesized polycarboxylated starch-based hydrogels which were applied for the sorption of copper ions. This hydrogel is stimuli responsive in nature for the sorption of Cu2+ ions and strongly depends on the 3D network of hydrogels. The polycarboxylated hydrogel works as a chelating agent and causes chelation with Cu2+ ions. The 128 mg/g absorption capacity was reported by the author toward Cu2+ ions. Novel SAH nanocomposites were reported by Kaşgöz et al. [41] based on acrylamide (AAm)-2-acrylamido2-methylpropane sulfonic acid (AMPS) sodium

(Co − Cf) × V

mSAH (2)

**194**

222.22 mg g<sup>−</sup><sup>1</sup>

*Maximum absorption capacity of different hydrogel samples toward Cu2+ ions.*

salt along with clay and were synthesized through in situ copolymerization process. The synthesized materials were successfully used for removal of heavy metal ions including Cu(II), Cd(II), and Pb(II) from the aqueous medium. The absorption capacity of the samples was 1.07, 1.28, and 1.03 mmol/g for Cu(II), Cd(II), and Pb(II) ions, respectively. The comparative removal of copper divalent metal ions by different hydrogel samples are tabulated in **Table 1**.
