The stability and defluoridation performance of MOFs in fluoride solutions

The stability of metal–organic frameworks (MOFs) in fluoride solutions was studied based on 11 water-stable MOFs (MIL-53(Fe, Cr, Al), MIL-68(Al), CAU-1, CAU-6, UiO-66(Zr, Hf) and ZIFs-7, -8, -9), and several factors were found to have large influence, such as the central metal activity, pore topology and coordination number. Furthermore, the defluoridation performance of UiO-66(Zr) was investigated comprehensively, which shows an adsorption capacity of 41.36   mg   g −1 that is higher than most of the reported adsorbents. The initial pH of fluoride solution has little effect in neutral range, while shows a negative effect when pH >9.0; the adsorption data of UiO-66(Zr) fit well with the Langmuir model, and pseudo-second-order kinetic model was found to be suitable for depicting the F − adsorption process, and finally, it was observed that the extent of the effect of co-existing ions is largely ion-dependent. On the basis of the systemic study, this work suggests that increasing the number of –OH groups is an efficient strategy to improve the defluoridation performance of MOFs, and MOFs are promising adsorbents for defluoridation in waste water.