Engineered Nanomaterials as Emerging Contaminants in Water - Nanotechnologies for Water Environment Applications - page 578

from 40 kT to 18, 3, ~0, and -1 kT, respectively (Zhang, 2007). Among the metal oxide
NMs studied, only silica NMs retain a positive Φ Total, max value (i.e., repulsive interaction
likely to result in stable NMs). Thus, as ionic strength increases as rivers flow into
saline estuaries, NMs will likely begin to aggregate.

The rate of aggregation for NMs (assuming < 1 μm in diameter) can initially be
described best by perikinetic flocculation models:

dN 4 kT

− α N

2

(Eq. 16.8)

dt 3 μ

where N is the number concentration of NMs, k is Boltzmann's constant, T is
temperature, μ is viscosity and α is a “stickiness” factor. For larger sized colloids and
particles, orthokinetic flocculation and differential settling mechanisms lead to particle-
particle interactions and potential formation of aggregates (Crittenden et al., 2005).
However, for NMs the initial rate of particle aggregation should be controlled by
perikinetic flocculation and is related to the number of particles squared. The α value
ranges between 0 and 1, with 1 equating to every collision leading aggregation and 0
means no aggregation occurs even though a collision occurs. The magnitude of α
depends upon surface chemistry and correlates with the existence of a repulsive Total, max

value. So for higher repulsive Total, max values α lower values occur. Therefore, as NMs
in a water column move downstream and into an estuary the following will probably
occur:

1. NMs in rivers interact with NOM, becoming stable;

2. Salt gradients exist and increase the ionic strength in the water conveying the

NMs;

3. α values increase as ionic strength increases;

4. The rate of perikinetic flocculation increases as the NMs are destabilized; and

5. Larger NM aggregates form which have higher settling velocities (Equation

16.2) and begin to settle out of the water column as stream velocities decreases
within the estuary.

16.6.4 Removal of NMs during Water Treatment

Drinking water treatment studies have frequently used hematite, an iron
(hydr)oxide naturally occurring nearly spherical NM (d p ~ 100 nm), as a model or
representative aquatic colloid (e.g., (Buffle and van Leeuwen, 1992; Tiller and O'Melia,
1993b; Buffle and Leppard, 1995a, b; Pizarro et al., 1995; Findlay et al., 1996;
Wilkinson et al., 1997; Buffle et al., 1998)). Studies with variable ionic strength and
dissolved organic matter all conclude that hematite is stable (tends not aggregate) under