Influence of Ligand Size and Chelation Strength on Zerovalent IronNanoparticle Adsorption and Oxidation Behavior in the Presence ofWater Vapor and Liquid Water
Aruni Gankanda, Nikki S. Rentz, Lauren F. Greenlee
The effectiveness of zerovalent iron (ZVI) nanoparticles in applicationsfrom water remediation to catalysis is intimately tied to adsorption and oxidationprocesses at the nanoparticle surface. Understanding water sorption and ZVI oxidation asa function of surface-sorbed organic ligand properties can provide new fundamentalinsights into tuning the reactivity of the nanoparticles. In this work, ZVI nanoparticleswere synthesized in the presence of four different organic ligand molecules: twocarboxymethyl cellulose polymers of different molecular weights and two phosphonatechelators with different known iron chelation strengths. The resulting ZVI nanoparticleswere similar in size (∼100 nm), and adsorption and oxidation behavior are compared onthe basis of the properties of the ligand sorbed to the surface of the ZVI nanoparticles.Adsorption and oxidation processes are studied via quartz crystal microbalance (QCM)measurements, where the change in nanoparticle mass is followed over time as thenanoparticles were exposed to varying levels of relative humidity in air or oxygenatedwater. A clear dependence was shown between measured change in mass and either chelation strength or polymer molecularweight. An increase in either the ligand size or the chelation strength reduced oxidation in oxygenated water. Ligand sizeresulted in an increase in water vapor adsorption. Reversible mass changes were observed for RH values≤50% and as a functionof ligand, suggesting water sorption, while irreversible mass changes were observed for RH values≥50% and suggest ZVIoxidation. QCM results were further corroborated with dynamic light scattering, zeta-potential measurements, and scanningelectron microscopy. Our results suggest that water adsorption on and oxidation of ZVI nanoparticles may be engineered to asuitable degree through a more thorough understanding of ligand−ZVI interactions.