## 3 Applications of Crosslinked PEI
### Crosslinked PEI for Dye Adsorption and the Separation of Organic Acids and Aldehydes
The crosslinked material was also used as a column packing material to separate methylene blue (MB) successfully from a mixture of MO and EY [49]. [PERSON] et al. prepared water-insoluble PEI/polyacrylamideonitrile nanofibrons films by crosslinking with ECH. The maximum adsorption of the material for MO reached 636.94 mg g\({}^{-1}\)[12]. The material has also demonstrated its reusability by maintaining a 75% MO adsorption capacity after four cycles of adsorption and desorption experiments. PEI has also been used as a crosslinker for a cellulose-based aerogel adsorbent consisting of cellulose acetoacetate and \(\beta\)-cyclodextrintrin for removing harmful dyes from water. The adsorption capacity of the material was tested toward MO and was found to reach up to 1013.11 mg g\({}^{-1}\) at 25 \({}^{\circ}\)C [95]. 3-Aminopropyltriethooxysilane (APTES) was grafted on multiwall carbon nanotubes (MWCNTs) by refluxing the two materials together and mixed with PEI and crosslinked with ECH to yield the crosslinked material (Scheme 13). The material produced was used to remove Reactive Yellow from water and found to have a 2.8 times higher adsorption capacity than that of MWCNT only, showing the superiority of the composite material for the adsorption of Reactive Yellow. The kinetic studies showed the removal of 99% of the dye molecules (50-100 mg L\({}^{-1}\)) in less than 15 min, demonstrating the effectiveness of the adsorbent to rapidly eliminate reactive dyes from aqueous solutions [96]. Lignocellulose biomass materials are important sources of value-added organic acids and aldehydes, such as furfural, 4-hydroxybenzoic acid, 4-hydroxybenzaldehyde, vanillin, and so on, by hydrolysis reactions [97]. Obtaining these compounds in pure form has been a long-standing challenge, and extraction, chromatography, and adsorption methods have been studied. [PERSON] et al. [98] studied glutaraldehyde-crosslinked PEI to selectively separate the lignocellulosic-derived organic acids and aldehydes without significantly adsorbing sugars. The study was performed on single, binary, and multiple solutes to understand the effects of solutes on the selectivity and adsorption capacity of the adsorbent. The adsorption efficacy of aldehydes over organic acids was achieved by adding inexpensive neutral salts as a composite material. An acid-base reaction is expected to take place between the amine functional group of PEI with an acidic functional group-possessing organic acid to form a salt in the interaction. The presence of other salts does not favor the adsorption of organic acids. An aldehyde functional organic compound, on the other hand, can be absorbed on PEI via a nucleophilic addition/elimination reaction between the aldehyde and the amine functional groups [98]. The recovery of the adsorbed molecules was achieved using a 1 mole/L HCl solution. Overall, a crosslinked-PEI-based adsorbent in the form of a powder, nanofibers, membrane or aerogel is well qualified to serve as a potential candidate for dye, organic acid and aldehyde removal from samples. Table 2 shows a summary of PEI-based crosslinked materials and their applications in dye adsorption from the environment. \begin{table}
\begin{tabular}{l l l l} \hline \hline
**PEI/Composite** & **Analytes** &
\begin{tabular}{l} **Removal Efficiency** \\ **(mg g\({}^{-1}\))** \\ \end{tabular} & **Ref.** \\ \hline PEI/Chitosan\(/\)Fe\({}_{3}\)O\({}_{4}\) & Congo red & 1876 & [93] \\ \hline PEI on a nylon microfiltration membrane & Sunset Yellow & 600 & [94] \\ \hline GDE-crosslinked PEI & Bovine serum albumin & 47.8 & [49] \\ \hline PEI/polyacrylonitrile nanofibrous films crosslinked & Methyl orange & 636.9 & [12] \\ with ECH & & & \\ \hline PEI crosslinked cellulose-based aerogel adsorbent & Methyl orange & 1013.1 & [95] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: PEI-based materials and their applications in removing dyes from samples.