## 3 Applications of Crosslinked PEI
### Crosslinked PEI for Dye Adsorption and the Separation of Organic Acids and Aldehydes  
Polar dyes, such as reactive dyes, acid dyes, and direct dyes, are widely manufactured and used in various industries, including textile and paper [87]. Apart from the aesthetic issues that dyes can cause, the release of dyes into water bodies can have a significant  
Figure 12: Preparation of the porous PEI@CC electrode and working principle of the reversible copper uptake and electrochemical release, simplified on a segment of a carbon cloth (CC) fiber (figure adapted from ref. [32]). impact on aquatic life by hindering light penetration and hence disrupting the aquatic ecosystems [88]. This is a series concern worldwide that needs to be addressed. The presence of some dyes as low as <100 mg L\({}^{-1}\) in drinking water has adverse effects on human health, causing respiratory infections, hepatitis, vomiting, stomach aches, and even death [89; 90]. To address this problem, researchers have been exploring various methods for the removal of polar dyes from water using adsorbents such as chitosan and other biosorbents [91]. One promising approach involves the use of polymers as adsorbents. PEI is an effective material for dye adsorption due to its high density of amine groups, which can interact with various dyes through electrostatic interactions, hydrogen bonding, and van der Waals forces. For example, [PERSON] et al. crosslinked PEI with dichloroethane, glyoxal, or glutaraldehyde to yield a water-insoluble polymer and used it to adsorb methyl orange and its other homologues. The glutaraldehyde-crosslinked material showed a markedly increased binding affinity toward the dyes due to a higher degree of crosslinking compared to the other crosslinkers tested in the study [92]. [PERSON] et al. prepared an ECH-crosslinked composite of chitosan/PEI/Fe\({}_{3}\)O\({}_{4}\) using a facile one-pot synthesis approach where a mixture of FeCl\({}_{3}\)-6H\({}_{2}\)O, FeCl\({}_{2}\)-4H\({}_{2}\)O and chitosan was heated in a water-acetic acid mixed solvent at 90 \({}^{\circ}\)C under an inert gas atmosphere. The mixture was treated with aqueous ammonia, followed by the addition of PEI and the crosslinker. The final product was obtained after washing and freeze drying. The adsorption capacity of the prepared magnetic adsorbent toward Congo red (CoR) in aqueous solutions was investigated [93]. The chitosan/PEI/Fe\({}_{3}\)O\({}_{4}\) composite showed an enhanced removing capacity (1876 mg g\({}^{-1}\) at 40 \({}^{\circ}\)C) for CoR from aqueous solutions with an over 99.3% CoR removal efficiency. In a temperature-dependent adsorption experiment, the adsorbent performed at a relatively higher temperature (40 \({}^{\circ}\)C). The experimental adsorption kinetic data were better fitted by a pseudo-second-order model. [PERSON] et al. incorporated crosslinked PEI onto a nylon microfiltration membrane. The membrane soaked in PEI was treated with trimesoyl chloride (1,3,5-benzetricarbonyl trichloride) for an interfacial amide bond forming reaction (crosslinking reaction) to take place between the amine-rich PEI and the crosslinker. This composite membrane demonstrated strong dye adsorption capabilities. For example, the adsorption capacity for Sunset Yellow (SY) was measured to be 0.7 mg cm\({}^{-2}\). The nylon-crosslinked PEI composite showed an absorption capacity of 600 mg g\({}^{-1}\) for SY dye [94]. Glycerol diglycidyl ether (GDE) crosslinked PEI at \(-18\)\({}^{\circ}\)C in a cryopolymerization technique to yield a porous material (pore size > 100 \(\upmu\)m) to adsorb dyes having charged groups. The PEI cryogel modified with [PF\({}_{6}\)]\({}^{-}\) was used to adsorb bovine serum albumin (BSA), giving an adsorption capacity of \(47.8\pm 5.7\) mg g\({}^{-1}\). The material prepared was also successful at removing methyl orange (MO) and eosin Y (EY), with removal efficiencies reaching 98.5 and 98.6%, respectively, from an aqueous solution.