Molecular Simulation of Perfluorooctanoic acid (PFOA) Removal from Water
Per- and polyfluoroalkyl substances (PFAS) are an emerging class of persistent polluters that do not break down in nature due to strong carbon-fluorine covalent bonds that are present in their structures. Since they are soluble in water, PFAS compounds leads to spreading widely in surface and ground water sources, and persistent nature prevent their degradation. Hence, capturing PFAS from water with adsorbent materials is one of the promising options to clean water resources. In this thesis, among numerous types of adsorbent materials for the removal of PFAS contaminants, Metal Organic Frameworks (MOFs), calixarene-based porous polymers and all-silica zeolite Beta are investigated as a viable PFAS removal agent from water. By using molecular simulations, two calixarene-based porous polymers and their fluorinated versions, which are acquired by using fluorinated linkers instead, are investigated. Perfluorooctanoic acid (PFOA), which is one of the most widely encountered PFAS in water sources, was used as the probe molecule. The simulation results of calixarene-based porous polymers agreed with experimental results. Therefore, we investigated fluorinated MOFs that are synthesized by employing different methods. Our simulations show that fluorine functionalization by incorporating fluorinated anions as bridging ligands in MOFs creates specific sites that PFOA binds strongly; however, the same sites are also preferred adsorption sites for water molecules, which casts doubt on the potential of using this approach to develop efficient PFOA removing materials as they lack sufficient hydrophobicity. On the other hand, trifluoromethyl or fluorine substitution of the MOF ligands result in much higher hydrophobicity; however, pores fluorinated with this method should have the optimum size and shape in order to obtain high PFOA affinities. Likewise, post-
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synthetic fluorine functionalization of a MOF through grafting of perfluorinated alkanes can lead to a significant increase in PFOA affinity compared to the parent MOF.