## 5 Future Directions  
The type of crosslinker and the reaction conditions affect the subsequent material properties, which in turn determine the adsorption capacity of the crosslinked PEI/composites for metal cations, dyes, and CO\({}_{2}\) and other pollutants. In this regard, several crosslinkers for PEI have been developed and used to crosslink PEI under different reaction conditions, resulting in different functional groups in the crosslinked material. However, there are not many comparative studies of performances of the materials prepared with different types of crosslinkers but under similar adsorption conditions. We believe that such comparative experimental studies will help us to better understand the adsorption mechanisms and the actions of different functional groups in the crosslinked material. Density functional theory (DFT) calculation can be used as a tool to predict materials' properties and the adsorption tendency of the materials toward different types of pollutants. A comprehensive study that includes both theoretical and experimental components will help us to understand materials' properties well. which will pave the way toward developing state-of-the-art materials for future commercialization. The stability and cost of crosslinked materials are other key aspects for commercial applications that need focus in the future. PEI comes from commercial sources with different architectures and molecular weights. In our review, we have identified that the literature is not clear on the effects of the architecture and molecular weight on the ensuing properties of the crosslinked materials, which need more work in the future.  
Moreover, our review article is based on materials prepared in small scale and tested at a laboratory level. Such studies may not necessarily reflect the performance of the materials when they are used at a larger scale. Such studies are lacking in the current literature, which we recommend researchers to work on in the future. Also, the deposition of PEI on a substrate and subsequent crosslinking have also been briefly reported in the literature. However, there are only few reports of PEI-based thin films and deposition techniques in the literature, which we believe future studies can focus on for future large-scale production. The use of such thin films in the adsorption of different types of metals should be studied. The impact of the presence of SO\({}_{2}\) on CO\({}_{2}\) adsorption efficiency is reported in the literature. The influence of the presence of other gases (including the presence of trapped water in the adsorbent) in CO\({}_{2}\) capture fefficiency has already been investigated. The study has shown that the presence of water together with a solid crosslinked PEI improves the performance of the material to capture CO\({}_{2}\). However, more comprehensive studies are needed in the future.  
Conceptualization, D.G. and M.R.A.; formal analysis, A.C., A.A.S., S.P., M.R.A. and D.G. investigation, A.C., A.A.S., S.P., M.R.A. and D.G.; data curation, A.C., A.A.S., S.P., M.R.A. and D.G.; writing--original draft preparation, A.C., A.A.S. and D.G.; writing--review and editing, A.C., A.A.S., S.P., M.R.A. and D.G. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
**Acknowledgments:** The authors would like to acknowledge the University of the South Pacific, Suva, Fiji. M. R. A. thanks the Australian Government through the Australian Research Council's Discovery Projects funding scheme (project DP210101243).  
**Conflicts of Interest:** The authors declare no conflicts of interest.