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EC number: 215-222-5
CAS number: 1314-13-2
processes (e.g., speciation,dissolution,
absorption, precipitation, complexation, etc.)that
control the fate, environmental distribution, and bioavailability of
zinc and bulk ZnO, as the free metal ion,are
the same for nano-ZnO. In addition to these processes, the formation of
larger particles throughaggregation/agglomeration
also influences the fate of nano-ZnO. As such, the
potential for environmental distribution is ultimately dependent on the
solubility/dissolution and agglomeration of nano-ZnO particles in
increase in solubility with decreasing particle size of ZnO has been
reported, suggesting that increased surface area is directly associated
with kinetic rates of dissolution. Size-dependent dissolution of
nano-ZnO particles of varying size demonstrates that smaller particles
have enhanced dissolution compared to larger ones, even though these
particles form similar-sized aggregates in test solution (Ma et al.,
2013; Pipan-Tkalec et al., 2010; Wang et al., 2009). In addition, the
solubility of nano-ZnO particles is also influenced by water quality
characteristics such as pH, temperature, and the presence of organic
matter. Miao et al. (2010) demonstrated that free zinc ion (Zn2+)
concentrations increased by nearly 100-fold as pH decreased from 9 to 7.
Similarly, the pH influence together with ionic strength also explained
accelerated dissolution in seawater due to the presence of additional
anions (chloride and sulfate) that serve as ligand binding for Zn2+.
Furthermore, nano-ZnO dissolution was decreased at higher temperatures
(37 degrees C compared to 20 degrees C; Reed et al., 2012; Majedi et
al., 2014), which is supported by the negative enthalpy of ZnO
dissolution. Finally, the influence of natural organic matter on ZnO
dissolution may promote ZnO dissolution by providing additional
chelating agents in solution (Ma et al., 2013).
Although the fate of nano-ZnO particles
in benthic systems has not be investigated, behavior in soils has
received considerable attention. Because nano-ZnO is present in
different phases within the soil matrix (soil, soil water) much of the
existing work on the behavior of aqueous media will also apply relative
to transport and bioavailability (Tourinho et al., 2012). In soil, the
solubility of ZnO NPs has been shown to be very similar in comparison
with bulk ZnO (Milani et al., 2010; Kool et al., 2011). Gimbert and
co-workers (2007) studied the particle size distribution of nano-ZnO
suspensions extracted from a high pH soil and reported that ZnO
particles were found to quickly equilibrate between the aqueous and
solid phases, and concentrations were stable during the experimental
period. However, similar to water, the formation of agglomerations of
nano-ZnO particles, and their interactions with soil particles and soil
pore water, occur over time (Lin and Xing, 2008; Zhu et al., 2009;
Frankin et al., 2007; Hooper et al., 2011).
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