Engineered Nanomaterials as Emerging Contaminants in Water - Nanotechnologies for Water Environment Applications - page 574

First, dissolution of nano-Ag depends upon the chemical form of the nano-Ag
and water conditions (pH, ionic composition). In our laboratory, addition of commercial
nano-Ag (Aldrich Inc.) to distilled water results in gradual accumulation of ionic silver,
as measured by an ion selective electrode, over a period of hours to weeks. For other
types of NMs, their dissolution potential will depend upon their crystal structure and
associated conditional stability constants (K sp ). For example, titanium dissolution from
titanium dioxide is probably less likely to occur than silver from nano-Ag because of the
low K sp for TiO 2 in water under typical conditions found in rivers and lakes (Hem,
1992).

Second, discrete settling velocity of nano-Ag can be calculated from the Stokes
settling velocity ( v S ):

( ρ − ρ ) d

2 g

p L p

(Eq. 16.2)

v =

S

18 μ

where ρ is the density of nano-Ag (ρ p assumed 2 g/cm 3 ) and water (ρ L is 1 g/cm 3 ), d P the
diameter of nano-Ag (assume 500 x 10 -7 cm), g is gravitational acceleration (981 cm/s 2 )
and μ is dynamic viscosity (0.014 g/cm-s). This results in a v S rate of 9.7 x 10 -6 cm/s ( v S
= 3.4 x 10 -9 cm/s if nano-Ag were present as distinct primary NMs ( d P ~ 10 nm) instead
of aggregates ( d P ~ 500 nm)). Calculated v S values are considerably lower than the
design value of 0.01 m/s for many wastewater sedimentation systems. Consequently,
discrete settling of nano-Ag is unlikely to be a significant removal mechanism. Of
course flocculant settling conditions may increase the overall removal of nano-Ag in
primary clarifiers. However, in the absence of experimental data this is difficult to
assess, but removal of nano-Ag during flocculant settling depends upon the tendency for
nano-Ag to favorably interact electrochemically with settling biological flocs (i.e., low
repulsive energy barrier).

Third, partitioning of nano-Ag onto wastewater biomass (floc) would result in
the removal of nano-Ag as the biosolids settle out in sedimentation clarifies, for which
WWTPs are explicitly designed. Figure 16.8 illustrates a typical activated sludge and
clarification system. A fraction of the settled biosolids is recycled to maintain a high
cellular biomass concentration in the activated sludge reactor to facilitate decomposition
of organics and nutrient removal. Non-specific partitioning of nano-Ag (or dissolved
ionic silver) onto activated sludge biomass concluded the data could be fit with a
Freundlich isotherm:

/ 1 n

q = kC (Eq. 16.3)