## 3 Applications of Crosslinked PEI
### Adsorption of Heavy Metal Ions and Adsorption Models
Moreover, generally, the crosslinked materials at lower temperature gave a higher absorption capacity of the ions than the ones prepared at room temperature, which could probably be due to the difference in the available amine groups of the crosslinked materials. Generally, more than a 98% Hg\({}^{2+}\) removal efficiency was recorded from chloride solutions of Hg\({}^{2+}\)[17]. The mechanism of interaction of the crosslinked materials with the Hg\({}^{2+}\) proposed was an electrostatic interaction between an ammonium chloride salt found on the crosslinker with that of HgCl\({}_{4}\)\({}^{-}\), giving R\({}_{3}\)NH\({}^{+}\)HgCl\({}_{3}\)\({}^{-}\)[67]. Selenium is a micronutrient with a permissible level of 0.05 mg L\({}^{-1}\) in the environment. Higher concentrations of selenium in the environment, however, are toxic to aquatic life, which is a concern worldwide. There are several methods that have been studied to remove selenium from the environment, which include both physical and chemical methods. Sulfonated crosslinked PEI was used to remove particularly SeO\({}_{3}\)\({}^{2-}\) from mining wastewater [68]. The sulfonic acid group is attached on the PEI by heating a mixture of crosslinked PEI and 3-chloropropanesulfonyl chloride in tetrahydrofuran at 70 \({}^{\circ}\)C. It is reported that the sulfonated crosslinked PEI showed a removal efficiency of 80 and 81% at pH 3 and pH 8, respectively [68]. Importantly, the authors demonstrated that the material is reusable for a second batch experiment after desorbing the selenite using an acid solution. The regenerated PEI showed a 72% removal efficiency of SeO\({}_{3}\)\({}^{2-}\). PEI (MW = 10,000, 99%) was heated with 2,4,6-trichloro-1,3,5-triazine (TCT) at 70 \({}^{\circ}\)C with molar ratios from 10:1 to 2:1 in DMF to give a pale-yellow precipitate in a 35-85% of reaction yield (Scheme 9) [55]. The crosslinked polymers were characterized using spectroscopic techniques such as solid-state NMR and FTIR. The solid material was used to adsorb Au\({}^{3+}\) from an aqueous solution, which showed an adsorption efficiency that reached up to 95.6% within 10 s, and an adsorption capacity of 1073.0 mg g\({}^{-1}\) was recorded. This is a fast adsorption process. Moreover, the ion selectivity of the triazine-crosslinked polymer was also evaluated in a sample that consisted of Cu\({}^{2+}\), Cd\({}^{2+}\), Ni\({}^{2+}\), and Pb\({}^{2+}\) ions in addition to Au\({}^{3+}\). The study showed that 97.9% of the Au\({}^{3+}\) was selectively adsorbed in the presence of the competitive metal cations, showing the high affinity of the crosslinked material for gold. The adsorption efficiency of the crosslinked material for Cd\({}^{2+}\), Cu\({}^{2+}\), Ni\({}^{2+}\), and Pb\({}^{2+}\) was measured to be 1.79%, 3.06%, 1.52%, and 1.97%, respectively. Such excellent adsorption performance is mainly attributed to the complexation and mechanical stability of triazine-crosslinked PEI adsorbent. The regeneration of the adsorbent material was achieved using an HCl solution containing thiourea, which resulted in the release of 100% of adsorbed Au\({}^{3+}\) within a minute and up to 10 times, signifying that the crosslinked polymer is reusable. The regenerated polymer was again used in five consecutive adsorption experiments without showing performance degradation. To test whether the adsorption of gold was dominated by physical or chemical adsorption mechanisms, the adsorption experimental data were fitted in two classical kinetic models: pseudo-first-order (Equation (5)) (diffusion of the adsorbate) and pseudo-second-order (Equation (4)) (chemisorption). A larger correlation for the pseudo-second-order model indicates that the Au\({}^{3+}\) adsorption on the crosslinkedpolymer is more likely a chemisorption process, which might involve chelation and ion exchange [55]. \[\ln(q_{e}-q_{t})=\ln q_{e}-k_{1}t \tag{5}\]  
Au\({}^{3+}\) was also recovered from a water solution using EGDE-crosslinked PEI as an adsorbent (Scheme 4) in a reverse suspension reaction.