Influence of size on reductive dissolution of six-line ferrihydrite

This work investigates size dependence of the kinetics of reductive dissolution of six-line ferrihydrite, ranging in average length from 3.4 to 5.9 nm. Empirical rate laws, activation energies, and pre-exponential factors were determined for freshly prepared aqueous suspensions and dried powders of each ferrihydrite sample. Mass-normalized initial rates of reductive dissolution are substantially faster for the freshly prepared suspensions than for reactions using the dried powders, which is consistent with a drop in reactive surface area upon drying. In addition, results demonstrate substantial differences between the empirical rate laws for the freshly prepared and the dried six-line ferrihydrite. Comparing surface-area-normalized rates of reductive dissolution reveals a small dependence on size for the freshly prepared ferrihydrite, no dependence on size for the dried ferrihydrite nanoparticles, and no statistically significant change in the activation energy for reaction in either case. In addition, X-ray diffraction and X-ray absorption studies revealed no size dependent changes in nanoparticle structure and electronic structure. However, the frequency of reagent - surface encounters, related to the pre-exponential factors increased significantly with decreasing particle size, consistent with the hypothesis that the ordering of water molecules at ferrihydrite nanoparticle surfaces mediates diffusion to the surface and is size dependent.