# Metachrylate-Based Polymeric Sorbents for Recovery of Metals from Aqueous Solutions
## 3 Natural Polymers, Inorganic Materials, and Polymer/Inorganic Composite as Metal Ions Sorbents  
Natural materials, such as aluminosilicates and clays [38], have been used as metal ions adsorbents due to their favorable characteristics, such as high ion exchange capacity, chemical inertness, and low toxicity) [39]. [PERSON] et al. tested Cu\({}^{2+}\) and Zn\({}^{2+}\) sorption on different natural materials (clays and clay minerals), natural-modified (activated carbon and alumina), and synthetic (zeolite, titanium dioxide (TiO\({}_{2}\)), and ion exchange resin) [38]. [PERSON] et al. investigated sorption performances of zeolite and hydroxyapatite (HAP) towards Cu(II) ions [40]. The effect of zeolite particle size and sorption parameters such as initial meal concentration, agitation speed, and adsorbent mass on Cu(II) sorption kinetics was examined. The volumetric mass transfer coefficient (\(k_{\theta}\)) and effective diffusion coefficient (\(D_{\mathit{eff}}\)) using single resistance mass transfer models were calculated. Their results showed that the controlling step in the case of sorption with HAP was only pore diffusion, while sorption with zeolite sorption was governed by film diffusion within the first sorption stage (up to 10 min), followed by diffusion inside the pores. As a continuation of their research, [PERSON] et al. examined the possibility of enhancing of sorption performances of biogenic HAP (BHAP) for lead, copper, nickel, cadmium, and zinc (Pb, Cu, Ni, Cd, and Zn) by the corresponding functionalization, i.e., condensation reaction of surface hydroxyl groups in BHAP and hydroxyl groups from caffeic acid (CA) and 3,4-dihydroxybenzoic acid (3,4-DHBA) [41]. The sorption results showed selectivity towards Pb ions from mixed equimolar solutions of investigated metal ions (Pb, Cu, Ni, Cd, and Zn ions) by all investigated sorbents. It was observed that the sorption capacities of functionalized BHAP were higher in comparison with unmodified ones. [PERSON] et al. used agroindustrial waste, i.e., unmodified lemon peel, to test Cu(II) removal efficiency from aqueous solutions and wastewater [13]. The effects of sorption parameters (pH, adsorption time, initial ion concentration, and adsorbent dose) on sorption efficiency were studied in batch experiments. The maximum Langmuir adsorption capacity was 13.2 mg/g at an optimum contact time of 15 min. A high value for Cu(II) ions removal efficiency (89%) from mining wastewater at natural pH (pH 3.0) was observed. One of the most commonly known natural polymeric sorbents is chitosan, which originates from crustacean shells (crabs and prawns) [42]. The presence of -NH\({}_{2}\) and -OH groups on the polymeric chains provides chelating and reaction sites [43]. The main advantages of chitosan are the high density of functional groups, easy functionalization, non-toxicity, biocompatibility, biodegradability, etc. Chitosan can be used as raw material,as well as after physical (preparation in the shape of membranes, fibers, and spherical beads of different sizes and porosities) or chemical modifications (impregnation, crosslinking, graft polymerization, and composite preparation) [43; 44]. Since chitosan is soluble in most diluted minerals and organic acids, it has to be chemically stabilized by crosslinking. [PERSON] et al. modified chitosan by incorporating epichlorohydrin (via a covalent crosslinking reaction) and triphosphate (via an ionic crosslinking reaction), and is used for Cu(II), Cd(II), and Pb(II) ions adsorption and desorption [45]. The optimum adsorption pH values were 6.0 for Cu(II), 7.0 for Cd(II), and 5.0 for Pb(II). The adsorption process was best fitted with the pseudo-second-order and Langmuir isotherm models. Maximum Cu(II), Cd(II), and Pb(II) ions adsorption capacities were 130.72, 83.75, and 166.94 mg/g, respectively. The best desorption was observed with nitric and hydrochloric acid.