# Metachrylate-Based Polymeric Sorbents for Recovery of Metals from Aqueous Solutions
## 4 Polymeric Sorbents  
A great variety of homopolymers, copolymers, and polymer nanocomposites synthesized by free radical polymerization, radiation polymerization, graft polymerization, oxidation polymerization, dispersion/suspension, etc., have been used as metal ions sorbents. For example, membrane-supported crosslinked poly(acrylamide-2-methyl)propane sulfonic acid) hydrogel [17], amidoxine chelating polymer [18], conjugated polymers [19], aminated-glycidyl methacrylate polypropylene adsorbent [52], magnetic glycidyl methacrylate-based polymer grafted with diethylenetriamine [53] were used as heavy metal ions sorbents. Numerous research articles, reviews, and patents have been published regarding the synthesis and usage of polymeric sorbents for the sorption of precious metals [54; 55]. According to hard-soft acid-base (HSAB) theory, functional groups bearing S and N donor atoms can strongly interact with precious metals [56]. Therefore, polymeric sorbents selective for precious metals often possess groups such as thiourea [57; 58], thiazole [59], dithiocarbonate [60], amino [61], imino [62], etc. Numerous new functional polymeric sorbents selective for rare earth elements (REEs) have been designed in the last decade. For example, terpolymer of styrene-divinylbenzene and glycidyl methacrylate (GMA) with diglycolamic acid ligands was used for adsorption of neodymium (Nd(III)) and dysprosium (Dy(III)) ions [63]. [PERSON] et al. used methylene phosphonic groups grafted on poly(glycidyl methacrylate) with incorporated diethylenetriamine groups for adsorption of lanthanum (La(III) and yttrium (Y(III)) ions [64]. The properties of polymeric sorbents, such as functional groups and surface chemistry, particle size, porosity, hydrophobicity, polymer chain size, and molecular weight distribution, can be controlled by the synthesis and/or functionalization conditions and parameters [65]. In addition, polymeric sorbents can be regenerated and reused in a number of sorption/desorption cycles, which justifies the production costs. For example, hydrogels are very interesting polymeric materials, insoluble due to the presence of chemical or physical crosslinks but swellable. They could incorporate various functional groups and, thus, be used for heavy metal ions adsorption. [PERSON] et al. prepared polyethersulfone membranes with an integrated negatively-charged poly(acrylamide-2-methylpropane sulfonic acid) hydrogel [17]. An intramembrane diffusion model was used to describe Cu(II) and Cd(II) sorption kinetics. The calculated apparent diffusion coefficients were 6.26\(\cdot\)10\({}^{-10}\) m\({}^{2}\)/s for Cd(II) and 7.15\(\cdot\)10\({}^{-10}\) m\({}^{2}\)/s for Cu(II), i.e., 2-3 times larger than in commercial ion-exchange resins. Poly(2-hydroxyethyl acrylate-\(co\)-titaconic acid), P(HEA/IA), hydrogels synthesized using free radical crosslinking/copolymerization and used as Pb(II) sorbents from aqueous solutions [66]. It was observed that the parameters such as metal ions' initial concentration, pH, adsorbent dose, ionic strength, and temperature, strongly influenced the metal sorption. The best fit was obtained with the Redlich-Peterson isotherm and pseudo-second-order kinetic model. The maximum sorption capacities for Pb(II) ions were 392.2 and 409.8 mg/g for hydrogel samples with IA mole fractions of 2.0 and 10.0 P(HEA/2 IA) and P(HEA/10 IA), respectively. In multi-component system, selectivity decreased in the following order: Pb(II) > Cu(II) > Zn(II) > Cd(II) > Ni(II) > Co(II). Sorption/desorption experiments showed that the P(HEA/IA) hydrogels could be reused without significant loss after three adsorption-desorption cycles. Maximum desorption of 95.2 was observed for Pb(II) at 0.1 M HNO\({}_{3}\). Chelating polymer ligands (CPLs) recently became attractive as promising efficient sorbents for metal ions.