Influence of crystal topology and interior surface functionality of metal-organic frameworks on PFOA sorption performance

Perfluorooctanoic acid (PFOA) is an emerging persistent organic pollutant. This paper reports on the use of metal-organic frameworks (MOFs) as sorbents for the removal of PFOA from aqueous solutions. Specifically, we investigated the effects of topology and surface functionality on PFOA sorption and the uptake kinetics of MOF materials. Zeolitic imidazolate framework-7 (ZIF-7) and ZIF-8 share the same topology but differ in their organic ligands. ZIF-8 and ZIF-L comprise the same metal ion and organic ligand differ in their crystal structure. ZIF-8 and ZIF-L were used to evaluate the effects of MOF topology on their effectiveness as PFOA sorbents. The PFOA sorption performance of ZIF-7, ZIF-8, and ZIF-L was then compared with the performance of two commercialized sorbents, zeolite 13X and activated carbon. ZIF-8 and ZIF-L were shown to outperform the two commercial sorbents and the PFOA sorption capacity and kinetics of ZIF-L are comparable to the benchmark values achieved for PFOA. It appears that the interlayer spacing within ZIF-L plays a key role in sorption performance by reducing the structural restrictions found in most three-dimensional porous materials and thereby allowing for the faster diffusion of PFOA. The discoveries in this work could assist in the development of guidelines for the design of new high-performance sorbents for small-molecule pollutants in aqueous phase.