Zeolitic imidazolate frameworks as capacitive deionization electrodes for water desalination and Cr(VI) adsorption: A molecular simulation study

In this study, molecular dynamics simulation is used to investigate the effect of metal atoms in Zeolitic imidazolate frameworks (ZIFs) on capacitive deionization (CDI) desalination using the same organic linker but with four different metal atoms as the surface of the electrodes in CDI desalination to simultaneously remove both salty and Cr(VI) ions in the starting period. The CDI performance is tested in terms of water flux and ion rejection. The effect of ZIF types on the surface interactions between the ZIFs and the salt solution is investigated and explained by radial distribution function (RDF) and ion distribution. The results show that ion adsorption in the ZIF electrode surface(s) occurs at its metal and nitrogen atoms, and ion rejection can be as high as 99.3% for salty ions in the starting period. However, the heavy metal ions Cr6+ and Cl- agglomerate constantly and the ion adsorption by the ZIFs deteriorates with time. It is also found that water flux is affected by the number of ions both at the entrance and in the middle of the nanochannel and ZIF hydrophilicity, which is influenced by the type of metal atoms used. Overall, this study uses molecular dynamics simulations to show that the metal atoms in ZIFs influence CDI desalination performance, which can help to screen ZIF materials for CDI applications. CdIF-1 performs best in the starting period for the four ZIFs tested.