**3. Properties of SAHs**

The SAHs show unique properties which are associated with the structural changes of their network. This makes the hydrogels versatile for various applications. Reversible swelling and deswelling transition is the most important property shown by SAHs. It occurs due to the hydrophilic nature of polymer chains, which is the presence of hydrophilic groups in SAHs and enables them to interact with water. So, the penetration or uptaking of water molecules inside the polymer network causes expanding or swelling of SAHs. The absorption property of SAHs toward water can be evaluated from the swelling ratio (Eq. (1)) and depends on the temperature of the medium. The SAHs composed of thermosensitive monomers like N-isopropylacrylamide (NIPAm), acrylamide (AAm), vinylcaprolactam, etc. show reversible swelling properties with temperature of the medium. At above and below the lower critical solution temperature (LCST), these SAHs behave differently. The temperature-sensitive hydrogel system was reported for the removal of metal-extractant complex at temperature below LCST, followed by desorption from p(NIPAM) network at temperature higher than LCST. The model system for this study was Cu2+ as a heavy metal ion, sodium n-dodecyl benzene sulfonate (SDBS), and p(NIPA) hydrogel, while a novel proposed temperature-swing solid-phase extraction (TS-SPE) technique has been introduced effectively [33]. The mechanism can be best seen in **Figure 3**.

Thakur et al. studied the absorbency and swelling of water by the dried hydrogel samples in aqueous solution at different pH values at room temperature. The % swelling was found to be 41,298% which is quite a greater value. The % swelling of the investigated samples was calculated by Eq. (1): % Swelling = \_

$$\text{9\% Swelling} = \frac{\text{W}\_{\text{s}} - \text{W}\_{\text{d}}}{\text{W}\_{\text{d}}} \times \mathbf{100} \tag{1}$$

**193**

**Figure 4.**

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

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

ions from wastewater.

presented in **Figure 4**.

ions, and the salt group (▬COO<sup>−</sup> Na+

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

swells the SAH network. Similarly, the process becomes reversed where the low pH of the medium causes the protonation of active sites, resulting in the disappearance of surface charge and the collapse of SAH network. The same results but with opposite trends can be observed for basic groups like ▬NH2 functional groups. The major application of pH-responsive hydrogels is used in drug delivery, removal of organic pollutants, dyes, and heavy metal ions from polluted and toxic water. In this chapter we are compiling the importance of SAHs in the removal of heavy metal

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

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

) exchanges the heavy metal ions between the

where Ws is swollen and Wd is dry weight of the SAH sample.

The SAHs consist of various different positive and negative functional groups as an active site also causes swelling and deswelling transitions by undergoing the ionization of active sites with respect to change in the pH of the medium. For example, ▬OH and ▬COOH groups in the network of the hydrogels undergo protonation and deprotonation at low and high pH of the medium, respectively. At high pH (pH > pKa of monomer used), the deprotonation causes the surface to become negatively charged, and an electrostatic repulsions occur, which in turn expands/

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

The SAHs show unique properties which are associated with the structural changes of their network. This makes the hydrogels versatile for various applications. Reversible swelling and deswelling transition is the most important property shown by SAHs. It occurs due to the hydrophilic nature of polymer chains, which is the presence of hydrophilic groups in SAHs and enables them to interact with water. So, the penetration or uptaking of water molecules inside the polymer network causes expanding or swelling of SAHs. The absorption property of SAHs toward water can be evaluated from the swelling ratio (Eq. (1)) and depends on the temperature of the medium. The SAHs composed of thermosensitive monomers like N-isopropylacrylamide (NIPAm), acrylamide (AAm), vinylcaprolactam, etc. show reversible swelling properties with temperature of the medium. At above and below the lower critical solution temperature (LCST), these SAHs behave differently. The temperature-sensitive hydrogel system was reported for the removal of metal-extractant complex at temperature below LCST, followed by desorption from p(NIPAM) network at temperature higher than LCST. The model system for this study was Cu2+ as a heavy metal ion, sodium n-dodecyl benzene sulfonate (SDBS), and p(NIPA) hydrogel, while a novel proposed temperature-swing solid-phase extraction (TS-SPE) technique has been introduced effectively [33]. The mecha-

Thakur et al. studied the absorbency and swelling of water by the dried hydrogel

Ws − Wd Wd

The SAHs consist of various different positive and negative functional groups as an active site also causes swelling and deswelling transitions by undergoing the ionization of active sites with respect to change in the pH of the medium. For example, ▬OH and ▬COOH groups in the network of the hydrogels undergo protonation and deprotonation at low and high pH of the medium, respectively. At high pH (pH > pKa of monomer used), the deprotonation causes the surface to become negatively charged, and an electrostatic repulsions occur, which in turn expands/

× 100 (1)

samples in aqueous solution at different pH values at room temperature. The % swelling was found to be 41,298% which is quite a greater value. The % swelling of

% Swelling = \_

where Ws is swollen and Wd is dry weight of the SAH sample.

*Extraction of metal-extractant complex onto p(NIPAM) hydrogel by TS-SPE technique [33].*

**3. Properties of SAHs**

nism can be best seen in **Figure 3**.

the investigated samples was calculated by Eq. (1):

**192**

**Figure 3.**

swells the SAH network. Similarly, the process becomes reversed where the low pH of the medium causes the protonation of active sites, resulting in the disappearance of surface charge and the collapse of SAH network. The same results but with opposite trends can be observed for basic groups like ▬NH2 functional groups. The major application of pH-responsive hydrogels is used in drug delivery, removal of organic pollutants, dyes, and heavy metal ions from polluted and toxic water. In this chapter we are compiling the importance of SAHs in the removal of heavy metal ions from wastewater.
