Insight into ion exchange behavior of LDHs: Asynchronous chloride adsorption and intercalated ions release processes

Cl− adsorption and intercalated ions release processes during LDHs are highly intricate phenomena. In this study, ion exchange performance of LDHs with specific cation interlayers and intercalated anions (CaAl–NO3-LDHs, CaAl–NO2-LDHs, CaFeAl–NO3-LDHs and CaFeAl–NO2-LDHs) were extensively investigated. The results indicate that in the presence of low chloride concentration, the intercalated anions are prematurely released due to partial dissolution of LDHs in neutral deionized water and ion exchange with OH− in alkaline simulated concrete pore solution. Therefore, Cl− adsorption of LDHs and release process of intercalated anions in LDHs are not completely synchronized. Compared to partial LDHs dissolution, the accelerating effect of OH− exchange on intercalated anion release in simulated concrete pore solution is more pronounced. Further, higher stability of LDHs in testing solutions is mainly corresponding to higher binding energy between intercalation ions and cation interlayers, thus saturated Cl− adsorption content of NO3-LDHs with high binding energy is higher than NO2-LDHs with low binding energy, and saturated Cl− adsorption content of CaAl-LDHs with high binding energy is higher than CaFeAl-LDHs with low binding energy in testing solutions.