Engineered Nanomaterials as Emerging Contaminants in Water - Nanotechnologies for Water Environment Applications - page 572

with heterogeneous mixtures of naturally occurring colloids and particles (i.e.,
suspended solids) or how unique engineered properties (e.g., magnetic NMs) affect their
behavior in the environment.
16.5.2 Effect of Salts and Dissolved Organic Matter (NOM) on NM
Stability
Various constituents in water affect the stability of NMs in water (i.e., tendency
for NMs to aggregate together). Two of the more important constituents are salts
(common anions/cations such as sodium, potassium, calcium, chloride, sulfate) and
dissolved organic matter (e.g., humic substances). Stable NMs in water tend not to
aggregate, or do so very slowly. NMs with zeta potentials of > -10 mV tend to be stable
in water because a net repulsive energy barrier occurs between the NMs as they
approach each other. NMs with zeta potentials between +5 mV to -10 mV are less stable
in water, and will tend to aggregate over time, depending upon their size and
concentration. Thus, zeta potential is a good indicator of NM stability in water, and
their tendency to aggregate together.
Representative data illustrating the effect of NOM and salts on zeta potentials for
three NMs are illustrated in Figure 16.7. The isoelectric points (pH IEP ) of hematite, TiO 2
and SiO 2 are 8.4, 5.2 and 1.8, respectively (Zhang et al., 2008-a). Consequentially at pH
7.8 hematite has a positive zeta potential while TiO 2 and SiO 2 have negative surface
charges in the presence of 10 mM KCl (Figure 16.7). The addition of NOM (same pH
and ionic composition) reduces the zeta potential of hematite and TiO 2 (Figure 16.6) and
most other metal oxide NMs. However NOM does not bind strongly to SiO 2 and
consequentially does not alter the zeta potential of SiO 2 . The presence of a divalent
cation (calcium) instead of a monovalent cation (sodium) begins to offset the reductions
in zeta potential induced by NOM sorption to the NM surface. This occurs because
calcium complexes NOM, allowing bridging and charge neutralization of the NMs.
Similar results have been observed for fullerenes in the presence of NaCl, CaCl 2 or
MgCl 2 with TOC from the Suwannee River NOM (Chen and Elimelech, 2007). Thus
the ability of NMs to sorb NOM is important and affects their stability in water.
16.6 Examples of NM Fate in Engineered and Natural Systems
This section considers three scenarios, related to Figure 16.5, to demonstrate how
knowledge of the detection, surface chemistry, biogeochemical processes and transport
processes can be applied to estimate potential NM exposures to aquatic ecosystems and
humans.