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ZnO nano fate


The 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 natural systems.


In water, asignificant 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).