PHOTOCHEMICAL REDUCTION OF IRON OXIDE NANOPARTICLES AS A FUNCTION OF MINERAL SIZE

Reduction of ferric (hydr)oxides has been shown to be an important process in freshwater, oceanic, and atmospheric systems, all settings where nanocrystalline iron (hydr)oxides are abundant. Laboratory studies have attempted to relate photochemical reactivity to mineral structure and crystallinity, but the role of particle size has not yet been directly tested. Photochemical reduction of hematite has been found to be relatively inefficient; electron-hole pairs recombine after an average diffusive length of only a few nanometers. As the dimensions of the particles approach this length, trapping of electrons at the particle surface as Fe²⁺ is expected to compete significantly with recombination. In addition, changes in the geometric and electronic structure of nanoparticles should affect photochemical reactivity as a function of size. In this study, hematite nanoparticles have been synthesized with an average diameter of 9 nm. Ferrous iron production has been measured with and without irradiation with a UV lamp to determine photochemical reactivity using oxalate solutions (pH 4). Comparison with natural powdered hematite (150-200 nm size fraction) suggests the 9 nm particles are approximately 40 times more efficient at ferrous iron production under UV illumination than the larger hematite, and approximately twice as efficient when normalized to surface area. Further experiments, including those with 7 nm and 40 nm hematite particles, will investigate the release of metals and metalloids such as lead and arsenate as a result of irradiation, this related to transport and distribution of these metals in natural waters.