Perfluorooctane sulfonate (PFOS) removal with Pd0/nFe0 nanoparticles: Adsorption or aqueous Fe-complexation, not transformation?

PFOS removal at 6 and 21days (d) when reacted with palladium coated zero valent iron nanoparticles (Pd0/nFe0 NPs) was quantified as a function of pH (3.6, 5.5, and 8.7) and temperature (22, 45 and 70°C). PFOS concentrations were measured in aqueous phases and NP extracts. The greatest PFOS removal occurred at 6 d with the lowest pH and highest temperature; however, recovered PFOS increased at 21 d. Furthermore, neither F- nor SO42- (from SO32- cleavage) generation was observed indicating the absence of PFOS transformation. X-ray diffraction analysis of PFOS-reacted NPs at 45°C revealed generation of FeO(OH) on the NPs at 21 d, which was concomitant with subsequent increases in PFOS recovery. Under anaerobic conditions, strongly adsorbing Fe(OH)3 is formed which can then transform to less sorptive FeO(OH) via Fe2+ catalyzed transformation. In the process of exploring causes for apparent PFOS removal with Pd0/nFe0 NPs, PFOS as well as other perfluoroalkyl acids (PFAAs) were found to form aqueous-phase complexes with Fe(II/III) which can reduce their quantifiable levels. PFOS-Fe complexation was greater with Fe(III) and increased with Fe concentration and decreasing pH. Complexation was also greater for longer chain perfluoroalkyl acids and complexation of PFNA was greater than for PFOS.