Oxidation behavior of zero-valent iron nanoparticles in mixed matrix water purification membranes

Abstract

Morphological changes resulting from the oxidation of zero valent iron (ZVI) nanoparticles were measured as an assessment of their mechanical robustness in mixed matrix membranes for water treatment applications. Upon oxidation from metallic iron to iron oxide hydroxide, FeO(OH), particles underwent a significant transformation in size and morphology from 100 nm diameter spherical particles to plate-like crystalline particles with a hydrodynamic diameter greater than 450 nm. Atomic force microscopy (AFM) was used to mechanically degrade the FeO(OH) crystallites during repeated imaging. To determine whether similar degradation would occur during water filtration in a mixed matrix membrane, force under standard membrane operating conditions was calculated. Such force calculations were used to compare the shear forces exerted during water flux in a mixed matrix membrane to the normal forces imparted by AFM. Analysis suggested that the oxidized ZVI nanoparticles will experience a 10⁻¹⁹ N maximum shear force in pore channels, much lower than the imaging forces in AFM, suggesting the mechanical stability of the particles during water remediation. Additional quartz crystal microbalance experiments were performed to confirm the mechanical stability of the oxidized iron nanoparticles in the flow environments of ultrafiltration. Taken together, the results of this study demonstrate that the mechanical properties of the nanoparticle composite membranes are such that minimal mechanical degradation of the nanoparticles will occur during water filtration.