**2. Synthesis of SAHs**

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

water and then maintain the water inside the gels for a long time [21].

of heavy metal ions from wastewater.

shown in **Figure 1**.

medium [28].

The swelling ability of these SAHs is due to the presence of physical and electrostatic interactions among polymer chains and water molecules. The presence of hydrophilic groups on polymer chains makes the network hydrophilic and sensitive toward environmental temperature [22]. The ionic groups like sulfonic (▬SO3H), amines (▬NH2), carboxyl (▬COOH), and hydroxyl (▬OH) [23] within the network are also responsible for the swelling of polymer networks by undergoing ionization. It generates the electrostatic repulsions and increases the osmotic pressure within the polymer network and increases the size of SAHs. Thus taking the advantage of this swelling ability, SAHs can be successfully applied for the removal

The two main processes involved in the removal of heavy metal ions from wastewater by SAHs are the diffusion and electrostatic attractions. The diffusion process causes the penetration of ions inside the SAH network through concentration gradient, while the ionizable groups set up an electrostatic attraction with heavy metal ions and remove them from the aqueous medium by attraction mechanism as

The hydrogels are responsive toward the external stimuli including pH [25], temperature [10, 11], intensity of light [26, 27], pressure fluctuation, etc. These stimuli in turn bring a volume phase transition with reversible property in aqueous

*Sorption mechanism, interaction, and regeneration of hydrogel network with heavy metal ions and removal* 

heavy metal ions from wastewater. Many traditional materials can be used for the removal of these pollutants from wastewater as an adsorbent like rice husk charcoal [17], saw dust [18], activated alumina, silica gels, activated carbon, and nut shells [19]. However, the limitation of traditional adsorbents demanded the introduction of novel materials with low cost, biocompatibility, biodegradability, easy synthesis, simple regeneration, and recycling with good efficiency. Thus, the introduction of SAHs makes possible the solution of aforementioned limitations. SAHs are the polymeric materials having 3D cross-linked polymer network (chemical or physical cross-linked) having the capability to absorb a large amount of water in its network, which is thousand times more than its dry state [20]. The dry state of SAHs is because of collapsing and dominancy of hydrophobic interactions of the polymer chains, which strongly depend on the nature and composition of the materials. But, when the dry SAH is in contact with water or any other solution (aqueous medium), it expands significantly to a considerable large size while retaining the water inside the 3D network. The better objectives to synthesize the SAH are to once absorb the

**190**

**Figure 1.**

*process [24].*

Different polymerization methods are used for the synthesis of SAHs, i.e., grafting, free radical, suspension, etc.; among them the free radical polymerization process is widely used. It is a single-step synthesis process which requires monomers (cationic, anionic, and neutral), cross-linkers, free radical initiators, and accelerators. Accelerators are required for the generation of free radicals from initiators. This is done by heat or using accelerating agents like N,N,N′,N′-tetramethylethylenediamine (TEMED), NaSO3, etc. Currently, our group synthesized SAH based on acrylic acid (AAc) and acrylamide (AAm) with different compositions having good thermal and mechanical properties by thermal free radical polymerization process [29]. Liu et al. [30] prepared a novel chitosan-g-poly(acrylic acid)/sodium humate SAH via grafting polymerization process using N,N′-methylenebisacrylamide (MBA) as a crosslinking agent and ammonium persulfate (APS) as an initiator. Similarly, Thakur and Arotiba [31] synthesized poly acrylic acid (p(AA)) grafted on sodium alginate (SA-cl-PAA) superabsorbent hydrogel. The grafted hydrogels were synthesized while grafting acrylic acid (AA) monomer on polymer sodium alginate in the presence of N,N′ methylenebisacrylamide (MBA) as cross-linking agent and potassium persulfate (KPS) as an initiator. The proposed mechanism of the reaction was reported by the author and shown in **Figure 2**. The selection of synthesis process is strongly related with the final application of materials.

A copolymer series of SAHs was synthesized and characterized by Mohana Raju and Padmanabha Raju [32] based on monomers acrylamide (AAm), sodium acrylate (SA), and calcium acrylate (CA) p(AAm-co-SA-co-CA) in the presence of MBA as a cross-linker and APS (initiator) and stirrer for 2 h at 80°C.

**Figure 2.** *Proposed mechanism for the formation of a SA-cl-PAA hydrogel network.*
