Ecotoxicity study of titania (TiO2) NPs on two microalgae species: Scenedesmus sp. and Chlorella sp.
Introduction
Metal oxide NPs have been recently manufactured at the industrial level and have tremendous applications in water treatment, medicine, cosmetics, and engineering (et al., 2004, U.S. Environmental Protection Agency, 2005), for example aluminum NPs are already used in explosive combinations (Kaste and Rice, 2004). Titanium dioxide NPs are mostly used as photo-catalyst and UV absorbent in consumer products like sunscreen lotion and they also act as catalyst in sterilization (Fujishima et al., 2000, Meng et al., 2005, Pena et al., 2005). Since the applications of engineered metal oxide NPs increased in recent years, it is expected that metal oxide NPs such as titanium dioxide will find their way into the aquatic environment, where their fate and behavior are largely unknown (Maynard et al., 2006, Nowack and Bucheli, 2007). The photocatalytic activity of titanium dioxide NPs has been found to depend on the concentration, crystal structure and light intensity (Kim and Lee, 2005). The titania NPs were shown to cause toxicity to organisms by producing reactive oxygen species (ROS) upon interaction with UV light; leading to cell membrane damage (Yeung et al., 2009). Recently we have studied the effect of aluminum oxide NPs on Escherichia coli (Sadiq et al., 2009). Toxicity effect of alumina (Al2O3) nano-particle on bacteria was suggested to be due to the attachments of NPs to bacterial cells in 24–48 h growth period (Jiang et al., 2009).
Algal toxicity tests are extensively applied to assess the effects of hazardous substances in water since algae play an important role in the equilibrium of aquatic ecosystems, being the first level of the trophic chain to produce organics and oxygen. Ecotoxicity of titanium dioxide, zinc oxide and copper oxide NPs to algae such as Pseudokirchneriella subcapitata and Desmodesmus subspicatus was reported recently (Aruoja et al., 2009). A number of toxicity studies were reported with oxide NPs including titanium with pure cultures freshwater algae (Peller et al., 2007, Hartmann et al., 2010, Ji et al., 2010) and marine algae (Miller et al., 2010). As a part of our ongoing ecotoxicological assessment of manufactured NPs we have demonstrated the growth inhibitory effect of alumina NPs (Sadiq et al., 2011) for freshwater algae (72 h EC50 value, 45.4 mg/L for Chlorella sp.; 39.35 mg/L for Scenedesmus sp.).
With this background, the aim of the current investigation was to study the difference in (if any) toxic response of micron-sized and nano-sized titania particles towards microalgae isolated from aquatic environment.
Section snippets
Chemicals
Dry Titanium (IV) oxide-anatase nano-powder was procured from Sigma Aldrich, USA; (CAS number 637254). The supplier's data can be summarized as follows: 99.7% trace metal basis, particle size <25 nm, surface area: 200–220 m2/g. Micron-size titania powder was procured from Merck Ltd. (Mumbai, India) (CAS number 13463-67-7). The supplier's data can be summarized as follows: pigment white powder, Molar mass 79.87 g/mol, density 4.2 g/cm3 (20 °C).
Powdered X-ray diffraction analysis
To identify the crystal phases and to determine the
X-ray diffraction analysis
X-ray diffraction analysis was used to identify the crystal phases and to determine the crystallite size of each phase present in the spectra. Fig. 1 shows the X-ray diffractograms of nano-titanium dioxide. Seven diffraction peaks at 25.30°, 37.8°, 48.0°, 55.10°, 62.8°, 70.1° and 75.24° were observed, among the peaks 25.30° and 48.08° were the distinct peaks. The XRD pattern of nano-titania were matched with the database of Joint Committee on Powder Diffraction Standards (JCPDS) card file No.
Discussion
The physicochemical parameters including morphology, crystallinity, agglomeration state, surface active groups, play significant role in toxicity of the metal oxide NPs (Handy et al., 2008). The XRD peaks at 2θ=25.25° (1 0 1) and 48.0° confirm the characteristic facets for anatase form of TiO2, (Liu et al., 2005). In our results we also obtained similar peaks, which confirmed the presence of anatase phase of titania. In a related pervious study it has been observed that the crystal structure of
Conclusion
Titania particles exhibited toxicity, both in bulk and nano form, though it was remarkably more toxic in nano form. The variability in sensitivity between Scenedesmus sp. and Chlorella sp. towards titania NPs was observed. A concentration dependent growth inhibitory effect was evident through decrease in the chlorophyll content. The aggregation of the NPs in the test medium during test duration was confirmed through increase in hydrodynamic diameter. Algal species enhanced the aggregation
Acknowledgments
This study was financially supported by the Ministry of Environment and forests, Government of India and we also thank CAS (Center for Advanced Studies) in Botany, University of Madras, India for helping us in genus level identification of algal species. The authors also wish to express gratitude to Herbert Allen, Associate Editor, EES, and two anonymous reviewers for their constructive criticism and comments which helped to improve the technical quality of the manuscript to a great extent.
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