Engineered Nanomaterials as Emerging Contaminants in Water - Nanotechnologies for Water Environment Applications - page 577

Suspended solids concentration (SS in kg/L) can vary between 1 and 1000 mg/L, but for
illustrative purposes a relatively high SS concentration of 50 mg/L will be selected.
Stream flow velocities are typically around 0.6 m/s. A representative settling velocity
( v S ) for SS with an average diameter of 25 μm and density of 1.25 g/cm 3 is 4.9 m/day.
Assuming a river depth (H) of 2 m and no resuspension then 50% of the SS would settle
out over a reach of approximately 15 km. The partition coefficient (Π) for neutral organic
pollutants is related to the fraction of organic carbon in the suspended sediment ( f OC ~
0.02) and K OW (Thomann and Mueller, 1987):

Π = 0 617 × f × K (Eq. 16.7)

OC OW

If this relationship is assumed suitable for NMs then < 5% of the NM would be
associated with the suspended sediment unless the NMs have a K OW above 1000 (log
K OW = 3). If NMs have a log K OW > 3 then partitioning to suspended sediment becomes
increasingly important for affecting NM removal from the water column. This approach
suggests information needs to be collected on partitioning between engineered NMs and
natural suspended sediment. Logistical hurdles need to be overcome to assess the
viability of conducting standard partitioning coefficient tests developed with organic
chemicals for determining K OW on NMs, because of issues like NM settling over time.
Perhaps new techniques to assess partition coefficients need to be developed particularly
for NMs. Whether or not this partitioning approach is more valid than discrete physico-
chemical models is debatable, but both approaches need to be investigated to develop
robust, yet simple to apply, fate and transport models for engineered NMs.

16.6.3 Nanomaterial Fate in Estuaries

As rivers flow into estuaries the salinity levels (i.e., conductance, total dissolved
solids) increase. Consequently, this leads to compression of electric double layers
around NMs and reduction in net energy of repulsion (i.e., Eq. 16.1). Compressed EDLs
destabilize NMs which leads to their aggregation into larger particles which can settle
out of the water column.

Several examples for how salts destabilize NMs are available in the literature
(Chen and Elimelech, 2007; Zhang et al., 2008-a, -b). However, these papers also
demonstrate extent to which salts destabilize NMs can be offset by the presence of
dissolved organic matter. For example, in the presence of 5 to 10 mg/L of nano-ZnO at
near neutral pH the net maximum energy barrier (Φ Total, max ) is approximately zero, which
indicates destabilized ZnO NMs will likely aggregate. Addition of 0.5, 1 and 5 mg/L
dissolved organic carbon (DOC) results in repulsive Total, max values 22, 40, and 53 kT,
respectively. So increasing DOC concentrations stabilizes the ZnO. In the presence of 1
mg/L DOC the addition of 0, 4, 8, 12, and 16 meq/L of Ca 2+ decreases Total, max values