Environmental Engineering Reference
In-Depth Information
developed a model and found that the levels of silver, TiO
2
ENPs, and carbon nanotubes
in freshwater were 0.3, 0.7, and 0.0005 μg/L, respectively. Kaegi et al. [23] also reported
the release of TiO
2
ENPs from facade paints by natural weathering. A concentration of
13.4 μg/L TiO
2
was detected in runoff waters, and a part of this comprised TiO
2
nanopar-
ticles. On the basis of the modeling study of ENP emissions in Switzerland, the predicted
environmental concentration of TiO
2
ENPs is 16 μg/L in surface water [22]. There are a
number of studies dealing with the aggregation and deposition of ENPs [24-26]. These
studies have been conducted nearly without exception under controlled test conditions by
adjusting the pH, ionic strength, and composition of monovalent and divalent ions, as well
as the concentration of dissolved organic matter.
Consequently, there is concern about the behavior and effects of the released ENPs in
the aquatic systems. This will largely depend on the hydrodynamic behavior, association
with larger sediment and natural colloidal particulates, binding with organic and metal
pollutants, exposure and uptake into biota, implications of ENP exposure for organ-
ism health, and ecosystem integrity [27]. The dispersion and bioavailability of the ENPs
could be affected by their interaction with aquatic colloids, such as natural organic matter
(NOM), humic substances, and salt ions. NOMs are generally adsorbed on the surface of
the ENPs through hydrogen bonding, electrostatic interactions, and hydrophobic interac-
tions [8]. NOMs are classiied into three major classes: (i) rigid biopolymers, such as poly-
saccharides and peptidoglycans from phytoplankton or bacteria; (ii) fulvic compounds,
mostly from terrestrial sources, originating from the decomposition products of plants;
and (iii) lexible biopolymers, composed of aquagenic refractory organic matter from a
recombination of microbial degradation products [28]. ENPs in aqueous ecosystems are
dispersed because of the electrostatic and steric repulsion of surface charge (positive/
negative) present on the particles. As the surface charges of the particle skew toward the
zero value, the repulsive forces between the particles are reduced and they ultimately
settle down. Because of agglomeration/aggregation, the physicochemical properties such
as surface charge, size, size distribution, surface-to-volume ratio, and surface reactivity
of ENPs become altered, thereby affecting their bioavailability and cellular responses.
Fulvic compounds and lexible biopolymers have a tendency to modify the ENPs' surface
charge, which leads to the aggregation and nonbioavailability of the particle. It has been
demonstrated that the humic acid coating of hematite reversed their charge from positive
to negative, leading to decreased attachment eficiencies from 1 to 0.01 mg/L to a sandy
soil [29]. This leads to the increased bioavailability and decreased agglomeration of the
hematite. However, the rigid biopolymers, such as polysaccharides and peptidoglycans
produced by phytoplankton and bacteria, coat the ENPs and increase their mobility and
bioavailability to the cell [30]. Apart from the NOMs, several other factors can also inlu-
ence the aggregation and bioavailability of the ENPs, e.g., salt ions, presence of hydropho-
bic surfactant or polar groups on the surface of ENPs, and others. Also, the biomolecules
such as proteins or polymers present in the ecosystem form a layer over the ENPs, named
as “corona,” which plays an important role in their biological fate. It is now well under-
stood that corona governs the properties of the “particle-plus-corona” compound in the
biological system [31,32].
Apparently, the formation of larger aggregates by high molecular weight NOM com-
pounds favors the removal of ENPs from the sediments and is likely to decrease their
bioavailability. However, solubilization by natural surfactants such as lower molecular
weight NOM compounds will tend to increase the mobility and bioavailability of ENPs.
Furthermore, it is now clear that ENPs can serve as transfer vectors for the pollutants
in the environment. The ENPs can control the bioavailability of compounds to the cells
