Behavior and effect of manufactured nanomaterials in the marine environment.

Estuary and coastal environments are a ‘‘sink’’ for contaminants, including manufactured nanomaterials (MNMs) (Klaine et al. 2008). MNMs may enter the marine environment via rainwater, wastewater, and river transportation. Harbor handling may also result in MNM leakage and deposition in harbor areas. Dumping wastes such as nanowaste, trash containing MNMs, and harbor-dredging, mudcontaining MNMs into the sea, atmospheric sedimentation, and others may all cause MNMs to enter the marine environment. Compared to those in fresh water, MNMs in marine environments may demonstrate different environmental behavior and effects. MNMs in a marine environment are less stable and more easily sink out of the water column than those in a freshwater environment. Multiwalled C nanotubes (MWCNTs) can disperse into an aqueous suspension in natural surface freshwater and remain stable for as long as 1 month after stirring (Hyung et al. 2007). However, we have found that MWCNTs sink to the bottom rapidly in seawater. Fullerene (C60) forms a faint yellow aqueous suspension in various experimental waters after stirring or with the assistance of a cosolvent that can remain stable for several weeks or months. This C60 aqueous suspension quickly sinks in seawater with 20% salinity; the sedimentation rate reaches 29% after mixing for 1 h (Blickley and McClennan-Green 2008). We similarly added C60 to neutral seawater, which was then stirred continuously. Although C60 formed a faint yellow suspension, it remained stable for only 48 h (Zhu and Cai 2011). Nanoscale titanium dioxide particles (nTiO2), zinc oxide particles (nZnO), and cerium oxide particles (nCeO2) were relatively stable in freshwater, and more than 90% of these nanomaterials remained in the water body even after standing for 6 h at 200mg/L concentrations (Keller et al. 2010). However, in seawater, less than 30% of them remained after 6 h. Consequently, the benthic ecosystem in those areas receiving elevated water column levels of MNMs may be affected. MNMs are more apt to agglomerate in larger average particle diameters in seawater than in freshwater. For example, nTiO2 rapidly agglomerates in seawater, and the average particle diameter of the agglomeration significantly increases with increased nTiO2 concentration and sedimentation time. At 10mg/L nTiO2, the agglomeration particle diameter rapidly increases from below 500nm to 1000 nm within 30min. At higher nTiO2 concentrations, the diameter rapidly increases from below 1000 nm to above 2000 nm within 30min (Keller et al. 2010). However, after entering freshwater, the average particle diameter of nTiO2 agglomeration is only slightly higher than 300 nm, which is far lower than the value in seawater. The diameter does not significantly change with increased nTiO2 concentration and sedimentation time within 6 h (Keller et al. 2010). The electric potential, electrophoretic mobility, and solubility of nanomaterials differ in freshwater and seawater. The electric potential of nTiO2 in natural seawater fluctuated between 0.0282 and 0.0148mV, whereas nTiO2 appeared relatively stable in natural freshwater. The electric potential of 15.88mV was obviously lower than that in seawater (Zhu and Cai 2011). The electrophoretic mobility of nTiO2 ( 2.40mm/s/V) in natural freshwater was also lower than that in seawater ( 0.04mm/s/V) (Keller et al. 2010). Moreover, approximately 3.5mg/L Zn2þ was released from 80mg/ L nZnO dissolved in seawater (Miller et al. 2010). In contrast, over 15mg/L Zn2þ was released from 100mg/L nZnO dissolved in freshwater (Franklin et al. 2007). The reason for this difference is unknown. Studies on the toxic effects of MNMs in marine species are limited. Toxicity of MNMs has everything to do with the category, concentration, particle diameter, suspension preparation method, and solubility in seawater of MNMs. Compared with those in freshwater, the different properties of MNMs in seawater may cause different toxic effects and mechanisms. For example, the mechanism of action of nZnO in freshwater green algae may be mainly by the Zn2þ it releases (Franklin et al. 2007). However, because the Zn2þ released by nZnO in seawater is less bioavailable than in freshwater, the toxic effects of nZnO in marine algae may be the compound toxic effects of nanoparticles and metal ions (Miller et al. 2010). nAg is widely used as an antibacterial agent, but it fails to have a significant influence on the diversity of natural bacterial colonies in river mouth sediments (Bradford et al. 2009). nTiO2 at concentrations up to 500mg/L was not acutely toxicity to zebrafish embryos in freshwater, but 10mg/L nTiO2 significantly reduced embryo hatching and increased the malformation rate of the abalone Haliotis diversicolor supertexta in seawater (Zhu et al. 2011). The possible ecological effects of MNMs in the marine environment should not be ignored. Acknowledgment—This study was supported by the National Natural Science Foundation of China (21107058), the Doctoral Education Funding of the Education Ministry of China (20100002120019), the Natural Science Foundation of Guangdong Province of China (S2011010005391), and the Basic Research Project of the SZSITIC Commission of Shenzhen, China (JC201005270289A).