Impacts
of Ionic Strength on Three-Dimensional Nanoparticle
Aggregate Structure and Consequences for Environmental Transport and
Deposition
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Abstract
The
transport of nanoparticles through aqueous systems is a complex
process with important environmental policy ramifications. Ferrihydrite
nanoparticles commonly form aggregates, with structures that depend
upon solution chemistry. The impact of aggregation state on transport
and deposition is not fully understood. In this study, small-angle
X-ray scattering (SAXS) and cryogenic transmission electron microscopy
(cryo-TEM) were used to directly observe the aggregate structure of
ferrihydrite nanoparticles and show how the aggregate structure responds
to changing ionic strength. These results were correlated with complementary
studies on ferrihydrite transport through saturated quartz sand columns.
Within deionized water, nanoparticles form stable suspensions of low-density
fractal aggregates that are resistant to collapse. The particles subsequently
show limited deposition on sand grain surfaces. Within sodium nitrate
solutions the aggregates collapse into denser clusters, and nanoparticle
deposition increases dramatically by forming thick, localized, and
mechanically unstable deposits. Such deposits limit nanoparticle transport
and make transport less predictable. The action of ionic strength
is distinct from simpler models of colloidal stability and transport,
in that salt not only drives aggregation or attachment but also alters
the behavior of preexisting aggregates by triggering their collapse