PRECIPITATION OF HYDROUS FERRIC OXIDE NANOPARTICLES AND THEIR MORPHOLOGICAL AND STRUCTURAL TRANSFORMATION

Precipitation of iron oxide nanoparticles and films on mineral surfaces can significantly influence the fate and transport of toxic metal contaminants and potentially alter the porosity and permeability of geo-media. Therefore, more accurate quantitative and qualitative information about the mechanisms and kinetics of nanoparticle development at surfaces is required. However, direct in situ observations of nanoparticle development have been challenging because of lack of proper tools.

In this work, we have used a time-resolved simultaneous small angle x-ray scattering (SAXS)/grazing incidence (GISAXS) setup for real-time monitoring of water-mineral interfacial reactions. The size, shape, and distribution of hydrous ferric oxide nanoparticles on quartz surfaces as well as in solutions and their growth modes were monitored as a function of exposure time, ionic strength, and the presence of arsenate. Nanoparticles formed preferrentially along steps rather than terraces due to highly reactive dangling bonds at steps. Under aqueous conditions, newly formed nanoparticles did not exhibit any facets. However, clear facet formation of ferric oxide nanoparticles at surfaces were observed when they were dried overnight. This implies the importance of in situ observations of hydrous oxide nanoparticles. In the absence of any other metal ions, the earliest nuclei sizes of iron oxides are 1.7 ± 0.5 nm. On the other hand, the presence of arsenate ions significantly influenced nanoparticle sizes and crystallinities of hydrous nanoparticles and altered the kinetics of the ferric oxide nucleation and growth. For comparison, we conducted complementary measurements of their morphology with atomic force microscopy for interfacial observations, as well as dynamic light scattering and high resolution transmission electron microscopy for particle observations in solutions.

Using an environmental in situ time-resolved SAXS/GISAXS setup, this study provides more accurate depiction of evolving nanoparticle distributions and topology at an active interface without dehydration. Our findings have implications not only to hydrous ferric oxide containing biogeochemical systems such as acid mine drainage (AMD) but also to any type of precipitation processes at solid-water interfaces in environmental research.