Developed Multi-Functional Hydrogel for Metal Ion Removal

`High rates of metal and chemical consumption in industrial processes have resulted in a significant amount of waste, containing high levels of hazardous heavy metals in water resources. Because of their bio-accumulative, and toxicity, heavy metals cause massive environmental pollution. Heavy metals are toxic to almost all living species and are able to accumulate in the food chain, whereas excessive concentrations of other metals, such as magnesium and calcium, also have an influence on toxicity in humans and other animals. Therefore, such toxic metals must be removed by water purification. On that note, adsorption is an excellent approach to get rid of metals and metal ions. Hydrogel is an attractive adsorbent that can be used to eliminate contaminants from water bodies.<br/>Water purification multi-functional hydrogel was synthesized from poly(acrylic acid) polyacrylamide copolymer and modified with urea to improve the adsorption efficiency of the hydrogel by increasing the active metal ion binding sites. Synthesized hydrogel (PAA-co-PAAm-modified with Urea) was characterized using FTIR, XRD, and TGA.<br/>FTIR study confirmed polymerization, and urea modification in the hydrogel. The peak acquired due to the stretching vibration of the N—C—N bond was verified by the modification of the hydrogel with urea. The hydrogel's amorphous nature was confirmed by XRD analysis, which was revealed by broad peaks. On the TGA thermogram, there were five distinct stages of thermal decompositions related to weight losses of physically bound water, chemically bonded water, or lattice water, loss of –NH₂ as ammonia, release of H₂O, CO₂, and acrylic acid due to the breakdown of the C-C backbone and subsequent degradation of organic groups,<br/>The developed hydrogel had the absorption capacities for the metal ions Cd(II), Co(II), Cu(II), Fe(III), Mn(II), Zn(II), and Mg(II) ions of 67.54 mg/g, 69.81 mg/g, 50.98 mg/g, 97.28 mg/g, 45.37 mg/g, 60.74 mg/g, 25.41 mg/g, and 67.09 mg/g respectively. Adsorption isotherm models and kinetic models of adsorption were also explored in the synthesized hydrogel. In kinetic determination, pseudo-first order, pseudo-second order, and particle diffusion kinetics models were investigated. In the pseudo-second-order kinetic model, adsorption of Cd(II), Co(II), Cu(II), Fe(III), Mn(II), and Mg(II) ions were well behaved. This suggests the adsorption was chemisorption, but the adsorption of Zn(II) ion was fitted to pseudo-first-order kinetics, indicating physisorption.<br/>Freundlich and Langmuir isotherm models were investigated in the isotherm inquiry. The Freundlich isotherm model was used to fit the adsorption of Cd(II), Co(II), Cu(II), Fe(III), Mn(II), and Mg(II) ions, confirming that multilayer adsorption occurred. Nevertheless, the adsorption of Zn(II) ion followed monolayer adsorption and was fitted to the Langmuir isotherm model.<br/>High amount of N functional groups of the hydrogel has increased the metal ion binding capacity and thus has yielded an excellent hydrogel for metal ion removal endeavors.