Engineering Superparamagnetic Iron Oxide Nanoparticles for Environmental Applications

Engineered, superparamagnetic iron oxide nanoparticles (IONPs) have drawn considerable research attention for a broad range of applications based on tunable size and shape, surface chemistries, and magnetic properties. For successful aqueous-based, environmental applications, it is necessary to overcome two, fundamental, yet interconnected challenges: 1) Design and synthesize material which provides information about or effectively induces a relevant reactive pathway at/to the targeted regimes/species, while 2.) Simultaneously, controlling particle stability in/at relevant environmental matrixes/interfaces. In this work, highly monodispersed, single domain, superparamagnetic IONPs, were developed via high temperature decomposition of iron carboxylate salts in an organic phase, with material size (8-40 nm) and morphology precisely controlled through the ratio of precursors, heating rates, reaction times, and addition of cosurfactants (additives). For aqueous evaluation and application, materials were rendered water stable through the development of 13 unique surface bilayer strategies, which were focused on a tunable series of ionic surfactants, varying in chain length, functional group, hydrophobicity, and surface charge. For each bilayer strategy, 8 nm, spherical IONP suspensions were fully characterized with regard to transfer efficiency (into the aqueous phase), aggregation kinetics (varying ionic types/strengths), and long-term aqueous stabilities. Further, for IONPs with oleic acid bilayer coatings, before and after surface (oxidative) aging, extensive evaluation of surface/collector deposition and release behaviors and processes over a range of water chemistries and surface types (hydrophilic vs. hydrophobic) are described using quartz crystal microbalance (QCM) based techniques, among others, highlighting critical aspects of interfacial dynamics for these systems. Finally, IONPs are demonstrated as a platform material for uranyl sorption and separation from water. Specifically, (bilayer) surface optimized, 8-30 nm monodisperse IONPs demonstrate ultra-high uranyl sorption capacities (\u3e50% by wt/wt in some cases). Synchrotron-based X-ray absorption spectroscopic (EXAFS and XANES) analyses indicate that particle size and stabilizing surface functional group(s) significantly affect binding mechanism(s) (e.g. redox based reactions)