## 4 Conclusions  
Crosslinked PEI and its composites have proven to be versatile materials used in multiple areas. The crosslinking reaction and other modifications of PEI are achieved via chemical and physical methods to make the material suitable for the specific applications. The crosslinking process enhances PEI's stability and adsorption capacity, which are crucial for environmental and industrial applications. PEI is effective at adsorbing heavy metals. Crosslinked PEI has an enhanced adsorption capabilities for metals like Cu2+, Pb2+, Au3+, and Pd2+. The adsorption capacities of the crosslinked PEI for metals can be tuned through the use of appropriate crosslinking reactions, as well as the manipulation of the reactions conditions. As described in the literature, the porosity of the resulting material will have a crucial role in improving the metal capturing tendency. In this regard, researchers have used templates for the synthesis of the materials whereby the templates are removed by storing the material in appropriate solvent (and or by treating with chemicals) and the template leaches out with the objective of improving porosity. The concentrations of the primary, secondary, and tertiary amines have also paramount importance in the behavior of the final product. Moreover, the basic amino groups in PEI enables it to interact with dyes such as methyl orange, Congo red, and various other organic molecules through electrostatic interactions, hydrogen bonding, and van der Waals forces. The strong interactions with these organic molecules make PEI a potential alternative for removing dyes from contaminated water. Crosslinked PEI is also used in the separation of value-added organic acids and aldehydes, such as furfural, 4-hydroxybenzoic acid, 4-hydroxybenzaldehyde, vanillin, and so on, from a lignocellulosic mixture of compounds. The fact that PEI is used to separate a particular organic compound that possess the necessary functional group from a mixture is interesting and can potentially have applications in a range of research areas, such as in organic synthesis and natural product chemistry, where the separation of compounds is part and parcel of the research. Reports have also revealed that crosslinked PEI has enormous potential in capturing CO2 from the environment. In addition to the adsorption of various pollutants, the adsorbates are also easily released using acid treatment, electrochemical processes, and other chemical treatments. The regeneration of the material without apparent degradation makes crosslinked PEI reusable and hence cost effective. Overall, due to the facile nature of the crosslinking reactions, modifications and its recyclability, PEI is a low-cost alternative for various adsorption applications. This review has attempted to bring together the major crosslinking reaction techniques reported in the literature. The mechanisms of the reactions and the characteristics of the final materials are discussed. For commercial applications, material recyclability is important for price affordability. The different ways reported to regenerate and reuse crosslinked PEI are discussed in detail. One technique discussed is by passing an electric current to alter the pHof the water and hence to facilitate the desorption of metal ions from a film of crosslinked adsorbent. The application of the materials in removing pollutants such as metal cations, CO\({}_{2}\), and dyes and the adsorption models are discussed.