Engineered Nanomaterials as Emerging Contaminants in Water - Nanotechnologies for Water Environment Applications - page 576

The concept of partitioning of pollutants to wastewater biosolids is a common
feature in USEPA models (e.g., EPIWIN). Typically the partition coefficients are
estimated from octanol-water partition coefficients (K OW ). K OW values generally relate
to the solubility of organic chemicals (Schwarzenbach et al., 1993). However, such
partition coefficients are not available for most NMs. Because NMs have surface charge
(Table 16.2) it is unlikely that they would prefer to partition into octanol, or as such have
very low K OW values. However, we have demonstrated that NMs (nC 60 , hematite,
quantum dots, nano-Ag) do partition into wastewater biosolids (Westerhoff et al., 2007).
This suggests some type of partition modeling may be a reasonable first approach for a
priori predicting the disposition of NMs aquatic systems. Perhaps K OW values should be
experimentally determined, or new partition models (e.g., lipids) considered.

16.6.2 Partitioning and Transport of NMs in Rivers

Partition coefficient models are widely employed to understand the fate of
environmental chemical pollutants. For example, octanol water partition coefficients are
used in several USEPA models to assess how organic chemicals bind with sediments,
accumulate in food chains, and to assess their relative toxicity (e.g., ECOSAR). If a
similar approach can be developed for NMs it may greatly accelerate our ability to
quantify potential exposure levels and ultimate disposition of NMS in the environment
without the use of more sophisticated mechanistic models (e.g., Equation 16.1). While
this may be a new field of study, this section considers how such models could be used
should they eventually be validated for NMs. One example is provided here for how
such models may be useful to assess interactions of NMs with suspended sediment.

A fate-and-transport model for NMs in a stream can readily be represented by the
following relationship, assuming that NM partitioning to suspended solids (SS), and
their associated settling out of the water column, is the removal mechanism for NMs:

∂ C ∂

( )

(Eq. 16.5)

V ⋅ = C ⋅ Q − V U ⋅ C + r ⋅ V

0 x SS

∂ t ∂ x

where C is the NM mass concentration (μg/L or ng/L); t is time (hr); Q is flow rate
(m 3 /hr); U x is advection velocity (m/hr); x is longitudinal distance (m); and V is volume
of a given river reach (m 3 ); r SS is reaction term (mg/L-hr) associated with partitioning of
NMs to suspended solids. The following expression incorporates the Stokes settling
velocity for suspended particles to estimate the rate of change in the solid-to-water phase
ratio ( r sw in kg/L) (Eq. 16.2) (Schwarzenbach et al., 1993):

[]

d r v

[] sw

sw S

= − × r (Eq. 16.6)

dt H