Remediation of Perfluorooctane Sulfonic Acid (PFOS) using Supported Lipid Bilayers on Mesoporous-SiO2

Abstract

Removal of perfluoroalkyl substances (PFAS) such as perfluorooctane sulfonic acid (PFOS) from water relies on hydrophobic or polar interactions with the sorbants. Lipid bilayers contain polar headgroups such as zwitterionic phosphatidyl choline (PC) and a hydrophobic interior, which naturally incorporate the CF3-(CF2)m- hydrophobic/oleophilic tails of PFAS into the CH3-(CH2)n- lipid core, and the anionic headgroups (e.g. SO3- of PFOS or COO- for PFOA) into the PC headgroups. Liposomes consisting of 1,2-dipalmitoylphosphatidylcholine (DPPC) can incorporate PFOS at 1/1 DPPC/PFOS molar ratios and remain intact. However, liposomes cannot be separated from water. Therefore, supported lipid bilayers on micron-size mesoporous SiO2 with 20 nm pores were used to remove PFOS from the aqueous phase. Sorption capacity depends not only on pore volume and pore size of the nanoporous SiO2, but also on how much of the surface area of the pores can form the supported lipid bilayers (SLBs). For porous SiO2 with 20 nm diameter pores, specific surface areas of 320 m2/g, and a porosity of 78.8%, capacities between 363.5 and 474 mg PFOS per gram of (lipid + SiO2) = 0.63-0.95\ mmol\ PFOS\ per\ gram\ of\ (lipid\ +\ {\rm SiO}_2)\ were measured when ~ 35% of the surface was covered by SLBs. The sorption capacities were independent of pH between pH 3-pH 8 and ionic strengths between 0.1 mM and 10 mM NaCl. The sorption mechanism for the SLBs is rapid and the same as for that of PFOS insertion into lipids.