New insights into the arrangement pattern of layered double hydroxide nanosheets and their ion-exchange behavior with phosphate

As an ion-exchange type material, layered double hydroxides (LDH) can effectively remove phosphate from wastewater to prevent eutrophication. The exchange reaction occurs only between LDH lamellae, which is directly related to the arrangement of LDH nanosheets. Consequently, understanding the effect of LDH's arrangement structure on ion-exchange properties is essential for its application in phosphate sequestration, which, however, is still missing. Here, we used oven-drying and freeze-drying methods to prepare two types of LDH with different arrangement patterns (O-LDH and F-LDH). A new method was established to quantitatively evaluate the ion-exchange process. Results show that the chemical functional groups of O-LDH and F-LDH were similar, but O-LDH nanosheets exhibited a bent and disordered stacking morphology, while F-LDH was arranged in parallel. The disordered structure enabled O-LDH to form pore sizes and volumes up to 6.86 and 2.96 times that of F-LDH, respectively. Besides, the phosphate capture capacity of O-LDH was 5.01 mg/g larger than that of F-LDH, and the kinetic rate was 24.0%-26.7% faster than F-LDH. The disordered stacking of O-LDH enabled it to exchange ions in all spatial direction, and its larger pore space facilitated this process. Moreover, the bending cross section of O-LDH provided additional ion-exchange channels, which reduced the ion transport distance and increased the exchange rate. This work reveals the relationship between the stacking pattern of LDH nanosheets and their ion-exchange behavior, which provides new insights and methods for solving similar ion-exchange problems.