The influence of surface coating on PFAS filter performance

Abstract

Novel molecules containing a silane anchor, a hydrocarbon segment, and a polyethylene glycol (PEG) head have been developed to functionalize membranes for capture of per and poly-fluoroalkyl species (PFAS). The membranes have been experimentally tested against PFAS-contaminated water. Based on previous experimental testing and molecular modeling, the length of the hydrophobic PEG segment was identified as a potential feature to improve filter performance, particularly against low molecular-weight, short-chain PFAS. In our previous work, silane molecules featured an eight-carbon hydrocarbon segment and a segment with four consecutive PEG monomers. In this work, versions of the silane molecules with both shorter and longer PEG segments were synthesized and tested. We report results from tests measuring the dynamic filtration ability of aluminum-oxide hydroxide membranes functionalized with amphiphilic molecules against PFAS at part-per trillion and part-per billion concentration levels. Filters utilizing silanes with shorter PEG segments outperformed those with longer PEG segments, particularly against short-chain PFAS. Molecular dynamics simulations using metadynamics sampling methods were employed to construct free-energy interaction profiles, and interaction energies between several different contaminant species and a silane brush layer. Decomposition of the interaction free energy into the molecular interaction contributions to the potential energy (enthalpy) highlights the influence of PFAS length, PEG length, and brush geometry on the overall PFAS-brush interaction. Experimental measurements indicate that membrane performance depends on the length of the PEG segment in the silane brush. Combined experimental and simulation results indicate that both the intermolecular interaction strength and the density of the surface coating depend on the length of the PEG segment. The experiment and simulation results also show that both molecular architecture and membrane processing impact the overall filter performance.