Dissolution of Hematite
Nanoparticle
Aggregates: Influence of Primary Particle Size, Dissolution Mechanism,
and Solution pH
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Abstract
The size-dependent dissolution of nanoscale hematite
(8 and 40
nm α-Fe<sub>2</sub>O<sub>3</sub>) 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 nm 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<sub>2</sub>–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