Fate and Transport of Nanomaterials in Aquatic Environments - Nanotechnologies for Water Environment Applications - page 488

-Rec-Br --•-Cur-Br
•Rec-sh —«^Cur-sh
-Rec-ds ----Cur-ds
Particle diameter
Figure 15.3 Collision function calculated from rectilinear (Rec) and curvilinear (Cur)
collision models. Br, sh and ds refer to Brownian motion, fluid shear and differential
sedimentation, respectively (adapted from Logan, 1999).
15.2.2.2 Transport Processes in Sediments
Transport processes acting on NMs and associated sediment particles include
sedimentation, deposition, burial, resuspension, bioturbation, and downstream transport.
The processes of deposition and burial create a long-term environmental sink for NMs
that are hydrophobic and recalcitrant (resistant to degradation). From a global
perspective, the ultimate sink is the bottom sediment of the oceans. Once the NMs
become buried in this sediment, they will probably remain resident there until it is
transformed, regardless of the time span.
NMs without aggregation, and those adsorbed by small particles (e.g., silts,
clays, and colloids) are more likely to remain suspended in the water column than larger
sediment particles, especially in rivers where advection (transport with the current) and
turbulence (rapid mixing) can affect particle sedimentation. Aggregated NMs may settle
down in the same way as larger sediment particles. In rivers, these processes result in a
sorting of bed sediment particles by size as the water moves downstream. In addition,
there are spatial variations in bed sediment particle size even within a given reach of the
stream. For example, NMs tend to accumulate and remain in depositional areas within
the stream. Such depositional areas tend to occur in the low-energy zones of surface