Toxic effects and bioaccumulation of nano-, micron- and aqueous-Ag in the estuarine polychaete, Nereis (Hediste) diversicolor

Yi cong

Publikation: Bog/antologi/afhandling/rapportPh.d.-afhandling


Rapidly expanding growth in the field of nanotechnology has led to the development of numerous applications of nanomaterials in industrial and consumer products. Nanosilver is one of the most commonly used nanomaterials due to its effective antibacterial properties. However, there is increasing concern about the fate and potential risks of nanosilver for the aquatic environment after its eventual release via wastewater discharges.
In this thesis, dispersion and stability tests of commercially available nano (<100 nm)- and micron (2-3.5 µm and <45 µm)-Ag particles in two media (deionized water vs. filtered natural seawater) were firstly performed with the purpose to investigate the behavior of Ag particles in aqueous environments. A sediment exposure pathway was selected for the following toxicity experiments (I and II) as both Ag particles tended to precipitate in the water phase over time.
Due to the importance of fully characterizing nanoparticles for interpretation of toxicity results, the crystal structure, particle size and morphology of commercial nano (<100 nm, 20 and 80 nm)- and micron (2-3.5 µm)-Ag particles were determined in parallel to the toxicity studies. The characterization of hydrodynamic diameter and zeta potential was only carried out on nano-Ag in stock suspension (deionized water). However, we observed a clear difference of particle sizes between the manufacturer’s information and what we measured for both nano- and micron-Ag samples.
In toxicity experiment I, toxic effects of sediment-spiked nano-Ag100, micron2-3.5 or aqueous-Ag (AgNO3) on the sediment-dwelling polychaete, Nereis (Hediste) diversicolor were compared after 10 d of exposure, using mortality, growth, bioaccumulation and DNA damage (comet assay) as endpoints, with the purpose to decipher if effects were driven by the small size of particles, or other characteristics (i.e., ion release). The nominal concentrations used in all exposure scenarios were 0, 1, 5, 10, 25 and 50 µg Ag/g dry weight (dw) sediment.
In toxicity experiment II, toxic effects of sediment-spiked nano-Ag20, nano-Ag80 or aqueous-Ag (AgNO3) on N. diversicolor were investigated. Mortality, burrowing behavior, bioaccumulation, lysosomal membrane stability (neutral red assay) and DNA damage were used as endpoints as a result of 10 d of exposure, with the purpose to decipher if toxic effects were different between the two nano-Ag particle sizes. The nominal concentrations used in all exposure scenarios were 0, 5, 10, 25, 50 and 100 µg Ag/g dw sediment.
The overall results demonstrated that there was no significant growth in any of the Ag treatments or concentrations during 10 days. Burrowing behavior, which was assessed by burrowing time, was significantly affected in nano-Ag20 treatment compared to the aqueous-Ag, indicating an avoidance response of N. diversicolor after sediment Ag exposure. Ag was bioavailable and accumulated in N. diversicolor regardless of the form added to sediment in both toxicity experiments. Ag body burden increased in a concentration-dependant manner but there was no form-related difference in bioaccumulation of Ag in either experiment. There was a monotonic increase of Ag body burdens with increasing exposure concentrations in experiment I, which pattern however, was lack in experiment II. This was likely in part because of the wider size range and larger sizes of worms used in experiment II. Worm size (expressed as dry weight) significantly affected Ag body burden, such that smaller worms accumulated more Ag per body weight than larger worms. Lysosomal membrane stability of worm coelomocytes, which was measured by neutral red retention time (NRRT), decreased in a concentration-dependent manner in both nano- and aqueous-Ag treatments in toxicity experiment II, indicating increased permeability of lysosomal membranes. Worms exposed to sediment-spiked nano-Ag20 had significantly shorter NRRT than worms exposed to the aqueous-Ag. However, no significant difference was observed between nano-Ag20 and nano-Ag80 for either burrowing behavior or NRRT, which is likely attributed to their overlap in particle sizes. Tail moment and tail intensity (%), which are two commonly used indicators of DNA damage, increased significantly with increasing exposure concentrations after all three (nano-, micron-, and aqueous-) Ag treatments. Nano-Ag100 had significantly greater genotoxicity than micron-Ag2-3.5 and aqueous-Ag in experiment I and nano-Ag80 had significantly greater genotoxicity than aqueous-Ag in experiment II. Furthermore, the levels of DNA damage were comparable in nano-Ag100, nano-Ag20 and nano-Ag80 treated worms at the same exposure concentrations for both experiments, which were consistent with the evidence that they had a similar size distribution as indicated by TEM and DLS characterization. Overall, our study showed that nano-Ag treatments tended to be more toxic than the micron- and aqueous-Ag for tested endpoints except bioaccumulation. Such enhanced nano-size specific effects warrant further investigation and attention.
ForlagRoskilde Universitet
Antal sider166
ISBN (Trykt)978-87-7349-796-8
StatusUdgivet - 2011

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