Surface Chemistry and Dissolution of α-FeOOH Nanorods and Microrods: Environmental Implications of Size-Dependent Interactions with Oxalate

Although recent evidence suggests that particle size plays an important role in the dissolution of iron from mineral dust aerosol, a fundamental understanding of how particle size influences the rate and extent of iron oxide dissolution processes remains unclear. In this study, surface spectroscopic methods are combined with solution phase measurements to explore ligand-promoted dissolution and photochemical reductive dissolution of goethite (alpha-FeOOH) of different particle sizes in the presence of oxalate at pH 3 and 298 K. Both X-ray photoelectron spectroscopy and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) revealed differences between alpha-FeOOH particles-in the nanometer-size range as compared to alpha-FeOOH particles in the micrometer-size range (nanorods and microrods, respectively). ATR-FTIR spectra showed a significant presence of surface hydroxyl groups as well as differences in surface complexes formed on nanorod surfaces. Furthermore, the saturation coverage of oxalate adsorbed on nanorods relative to microrods is similar to 30% less as determined from solution phase batch adsorption isotherms. Despite less oxalate uptake per unit surface area, the surface-area-normalized rate of oxalate-promoted dissolution was similar to 4 times greater in nanorod suspensions, suggesting this process is particle-size-dependent. Photochemical dissolution experiments revealed only a moderate increase in the rate of oxalate oxidation per gram of alpha-FeOOH with decreasing particle size. However, concentration profiles of photochernically generated Fe(II) and Fe(Ill) suggest differences in the dominant mechanisms controlling nanorod and microrod dissolution. Although loss of reactive surface area arising from oxalate-induced particle aggregation can contribute to size-dependent reactivity trends toward oxalate, our data, taken collectively, suggest unique surface chemistry of nanorods as compared to larger microrods. Results from ligand-promoted and photochemical dissolution experiments also highlight the important, and sometimes dominant, role that iron oxides on the nanoscale may play in iron mobilization relative to the larger oxide phases present in mineral dust aerosol.