Fate and Transport of Nanomaterials in Aquatic Environments - Nanotechnologies for Water Environment Applications - page 520

photoreactions may alter the physical and chemical properties of NMs, and thus, alter
their behavior in aquatic environments. Photodegradation processes often involve direct
light absorption by the NMs followed by one of many decomposition mechanisms, a
process known as direct photolysis. For example, the photodegradation of CdSe NPs and
the associated release of free radicals in aqueous solution is well documented. For
example, CdSe QDs were found to be slowly photo-oxidized over 24 h to yield selenium
dioxide that slowly dissociated from the particle over a 96 h period (Bowen-Katari et al.,
1994). Surface photoreactions provide a pathway for NM transformation on soil surfaces
(USEPA, 2007). Another common mechanism, indirect photolysis, is also prevalent, in
which intermediate species, such as humic acids, some iron oxides, colored pigments, or
soil surfaces, absorb the light. The light energy is then transferred to oxygen or water,
resulting in the formation of oxidants such as hydroxyl radicals or singlet molecular
oxygen, species that react with organic contaminants (Zepp and Cline, 1977; Gohre and
Miller, 1986; Kieatiwong et al., 1990). For example, the ZnS cap of CdSe/ZnS water-
soluble semiconductor QDs can be oxidized slowly in the presence of air and water
under both light and dark conditions, giving SC>2 that desorbs into solution, producing
the free radical SC>2', a free radical that transforms to superoxide and then produces
hydroxyl free radicals that are known to nick plasmid DNA (Green and Howman, 2005).
As another example, photolysis plays a role in atmospheric aging of primary (e.g., oleic
acid, linoleic acid, cholesterol derivatives from cooking emissions) and secondary
organic aerosol particles (formed by condensation of low volatility products of
atmospheric oxidation of hydrocarbons) (Anthony et al., 2006). Humic substances are
known to photosensitize a variety of organic photoreactions on soil and other natural
surfaces that are exposed to sunlight.
Ion Exchange. Ion exchange is a specific category of adsorption. Ion exchange
occurs when the adsorbent charge deficiency can be neutralized more efficiently by ions
in solution than by those ions currently adsorbed. There are many factors affecting ion
exchange processes; only two of them will be discussed here. First, the surface charge
of NMs and subsurface media greatly affects the level of ion exchange processes. The
surface charge of clays and many other mineral surfaces is a function of pH. At an
intermediate pH, the surface exhibits a neutral charge; this pH is classified as the point
of zero charge (ZPC). The surface exhibits a net negative charge at a pH above the ZPC
of the mineral and a net positive charge at a pH below the ZPC. Thus, the surface charge
exhibited by a mineral or NM surface can be determined by evaluating the pH of the
solution relative to the ZPC of the mineral or the NM. Table 15.11 shows some ZPCs of
several common minerals and NMs. The ZPCs of NMs (or any minerals) can be shifted
to a lower or higher pH value if the formation of anionic negatively or catonic positively
charged surface complexes occurs. For example, both As (V) and As(III) can form
negatively charged inner-sphere complexes on TiC>2. With an increase in As
concentrations, the pHzpc of TiC>2 shifted from 5.8 (without As) to 5.2 (with 50 |ig As/L)
and 4.8 (with 100 |ig/L As) (see Chapter 5 for details).