Dissolution of Hematite Nanoparticle Aggregates: Influence of Primary Particle Size, Dissolution Mechanism, and Solution pH

The size dependent dissolution of nanoscale hematite,-(8 and 40 nm alpha-Fe2O3) was examined across a broad range of pH (pH 1-7) and mechanisms including proton- and ligand- (oxalate-) promoted dissolution and dark (ascorbic acid)- and photochemical (oxalate) reductive dissolution. Empirical relationships between dissolution rate and pH revealed that suspensions of 8 nm hematite exhibit between 3.3- and 10 fold greater reactivity per unit mass than suspensions of 40 rim particles across all dissolution modes and pH, including circumneutral. Complementary suspension characterization (i.e., sedimentation studies and dynamic light scattering) indicated extensive aggregation, with steady-state aggregate sizes increasing with pH but being roughly equivalent for both primary particles. Thus, while the reactivity difference between 8 and 40 nm suspensions is generally greater than expected from specific surface areas measured. via N-2-BET or estimated from primary particle geometry, loss of reactive surface area during aggregation limits the certainty of such comparisons. We propose that the relative reactivity of 8 and 40 nm hematite suspensions is best explained by differences in the fraction of aggregate surface area that is reactive. This scenario is consistent with TEM images revealing uniform dissolution of aggregated 8 nm particles, whereas 40 nm particles within aggregates undergo preferential etching at edges and structural defects. Ultimately, we show that comparably sized hematite aggregates can exhibit vastly different dissolution activity depending on the nature of the primary nanoparticles from which they are constructed, a result with wide-ranging implications for iron redox cycling.