Environmental Engineering Reference
In-Depth Information
32.2.1.1 Dissolution
A large number of NMs may be composed of or can contain constituents that can undergo chemical
phenomena such as reduction and/or oxidation in aquatic environments resulting in their dissolution. The dissolution process
of an NM is of great importance since it strongly affects the stability of the NM and, therefore, directly influences its long-term
use in all solution-based applications. An understanding of the NM's dissolution mechanism will lead to a much more rational
safer-to-design efficient synthesis route for high-quality and safer NMs. Despite the importance of a NM's dissolution, very
little is known about its underlying mechanism and reaction pathways under environmentally relevant conditions.
In Ag NMs, the oxidation of Ag
0
to Ag
I
results in the NMs' dissolution and subsequent release of bactericidal Ag
+
[4]. Sulfur
and selenium, major components of quantum dots (QD), can also undergo oxidation resulting in the release of soluble toxic
metal ions such as Cd or Pb [5]. Moreover, the use of stabilizers that have a binding capacity through metal ions can also pro-
mote the dissolution of these photoluminescent particles [6]. Photoreactions can be very important transformation processes
because they can affect the NM's coatings, oxidation state, generation of reactive oxygen species (ROS), and persistence. NMs
such as TiO
2
and ZnO are innately photoactive, potentially producing ROS when exposed to sunlight [7], and promoting disso-
lution mainly in the case where NMs contain ZnO particles. NMs that constitute Ag, Zn, and Cu are mainly affected by disso-
lution and sulfidation because they form partially soluble metal oxides and they have a strong affinity for inorganic and organic
sulfide ligands. The dissolution of these NMs results in the release of toxic cations [8] such that their persistence is reduced but
their toxicity is increased. Evidently, complete dissolution of metal-containing NMs allows the prediction of their impact using
existing models for metal ion speciation and their effects.
The quantification of the dissolved ions from a NM dispersed in an environmental compartment is a complicated issue
because the dissolution process is most often not in equilibrium. The real-time kinetic measurement of the dissolution process
limits the storage of whole unfractionated samples for subsequent ion analysis using appropriate analytical techniques, because
the dissolution rate may be fast or not attain the equilibrium during the experimental time.
32.2.1.2 Aggregation
Once in the environment, NMs will interact with naturally occurring bio- or geomacromolecules,
including proteins, polysaccharides, and natural organic matter (NOM), which will significantly affect their surface chemistry.
Homo- and heteroaggregation are dynamic processes that can lead to a decrease in the available surface area of the materials,
thereby decreasing their reactivity. However, this decrease is dependent on the particle number, size distribution, and the fractal
dimensions of the aggregate [9].
The diffusion of the particles in dispersion is driven by the Brownian motion, where the temperature and particle number
concentration determine the particle-particle collision frequency. The particles affect each other by attractive and repulsive
forces such as Borne repulsion, diffuse double-layer potential, and van der Waals attraction that act on different length scales.
The extended Derjaguin-landau-Verwey-Overbeek (DlVO) theory [10] weights the attractive and repulsive forces acting on
two closely adjacent particles. However, this theory is only applicable if there is no interference with these diffusive or attractive
forces, including the effects of particle shape, charge heterogeneity, and surface roughness that may influence the collision
efficiency [11]. The addition of salt ions to the medium results in an important effect on the homoaggregation of particles: some
of the salt ions will accumulate in the electrostatic diffuse double layer (which consists of a layer of charge at the surface of a
particle and the electric field generated by the charged surface), screening some of the surface charge of the particles. The result
is a decrease in the electrostatic diffuse double layer and, consequently, a decrease in the repulsion forces and an increase in the
aggregation between the particles. Multiple charged cations and anions such as Ca
2+
and PO
4
3−
may also promote the aggregation
of NMs [12]. The presence of highly negatively charged natural colloids in natural environmental systems is also known to have
a key effect on the aggregation of positively charged NMs. The effect exerted depends on the concentration of the NOM: a low
concentration of NOM may lead to a reduction in the surface charge of the particles, an intermediate NOM concentration to a
net neutral surface charge, while elevated concentrations can provide the particles with a net negative surface charge by charge
reversal, stabilizing the colloidal dispersion. However, the outcome of such colloidal dispersions composed by NOM and NMs
is not easily predicted, and needs to be analyzed experimentally by performing acid-base titrations and electrophoretic mobility
measurements. In fact, it is known that NOM may provide both charge and steric stabilization [13] of the NMs, although they
may also result in bridging flocculation [12]. Steric stabilization can occur even at higher levels of ionic strength [13a], where
the DlVO theory would predict strong aggregation. For NMs stabilized with higher-molecular-weight (MW) polymeric coat-
ings, mixed polymer-NOM layers may form on NMs, but depending on the nature of the coating polymer, interactions with
NOM may be minimal [14]. NOM effects are complex and difficult to predict; however, it is extremely important to explore
these interactions since NOM concentrations are typically orders of magnitude higher in concentration than NMs, and so are
likely to substantially modify their properties and behaviors.
As for the quantification of the dissolution process of a NM, the determination of the aggregate sizes in an environmental
compartment may also require real-time kinetic measurements, because the aggregation rate can be fast or the aggregate's size
distribution may not reach equilibrium within the experimental time window. The size of a NM will affect its bioavailability
