Effects of nano-zno/nano-tio 2 coexistence on nano-zno dissolution and toxicity

Transcription

1 Effects of nano-zno/nano-tio 2 coexistence on nano-zno dissolution and toxicity Kaiqi Fang (PI) and Tiezheng Tong Academic advisor: Prof. Jean-François Gaillard Department of Civil and Environmental Engineering, Northwestern University Introduction The early 21st century is witnessing the transition of nanotechnology from discovery to commercialization. Engineered nanomaterials (ENMs) are incorporated into numerous commercial products ranging from pharmaceuticals and cosmetics to alternative energy and electronic devices, leading to a global market projected to reach $3 trillion in As a result, diverse ENMs (such as nano-tio 2, nano-zno, nano-ag, carbon nanotubes and fullerenes) will be inevitably released into the environment 2-6, generating growing concerns about their potential toxicity to natural organisms. Considerable effort has been exerted to understand the environmental behavior and risks of ENMs. Information about the ecotoxicological effects of ENMs has been gathered for almost all the major ENMs and a broad range of taxa (e.g., bacteria, algae, plants, invertebrate, and vertebrate) However, these previous studies are mostly limited in scope to evaluate the fate or toxicity of only one nanomaterial at a time, and thus their results overlook the potential interactions between different ENMs. Consequently, little is known about the effects of co-contaminant interactions on ENM fate, transport, and the corresponding toxicity potentials. The primary study, sponsored by the National Science Foundation (NSF), addresses the fate and toxicity of nano-tio 2 and this complementary project supported by the Institute for Sustainability and Energy at Northwestern (ISEN), explores the chemical interactions between nano-tio 2 and nano-zno (Figure 1). In particular it investigates how nano-tio 2 affect nano- ZnO dissolution and toxicity. We chose these two semiconductor metal oxides because they are among the most extensively used ENMs in industry. According to studies by Muller et al. 12 and Gottschalk et al. 3, the annual production volume and predicted environmental concentrations of nano-tio 2 and nano-zno are much higher than those of other frequently studied ENMs (including nano-ag, carbon nanotubes and fullerenes). Therefore, coupled with their wide applications in personal-care products and industry, nano-tio 2 and nano-zno are the most likely to coexist and interact with each other in nature. The major findings obtained in this study are listed below. Figure 1. TEM micrographs of nano-tio 2 P25 (left) and nano-zno (right).

2 Major findings 1. Nano-TiO 2 surfaces control the concentration of dissolved Zn during nano-zno dissolution. We show that the adsorption of Zn 2+ onto nano-tio 2 (Evonik P25 was used as the representative of nano-tio 2 ) controls the concentrations of dissolved zinc ions in solution (e.g., when ZnCl 2 is dissolved in water) and also the concentrations of dissolved Zn released as a result of nano-zno dissolution (Figure 2A and 2B). However, in the case of the dissolution of nano- ZnO, [Zn] dis reaches time-dependent plateaux when the nano-zno is only partially dissolved, depending on the concentrations of nano-tio 2 present in suspension (Figure 2C). Figure 2. [Zn] dis from (A) 0.5 mg/l Zn in the form of ZnCl 2, (B) 0.5 mg/l nano-zno, and (C) 1 mg/l nano-zno in the presence of P25 with different concentrations. 0.5 mg/l and 1 mg/l of nano-zno represent fully- and partiallydissolved nano-zno in LMW after a 2d-incubation respectively, according to the stoichiometry. 2. The nano-zno/nano-tio 2 interaction is synergistically mediated by Zn (II) adsorption on nano-tio 2 and further dissolution of remaining nano-zno as a Zn 2+ reservoir. When nano-zno and nano-tio 2 coexist as particles in suspension, dissolved zinc is released from nano-zno and then partially adsorbed at the surface of nano-tio 2, as described by the following two reactions: The occurrence of Reaction 2 decreases the concentration of Zn 2+ and drives Reaction 1 to the right. Then the remaining nano-zno behaves as a reservoir of Zn 2+, which further dissolves and compensates the loss of [Zn] dis due to the adsorption by nano-tio 2. This explains the plateaux present in the [Zn] dis variations as a function of the concentration of nano-tio 2 P25 (Figure 2C). When the P25 concentration surpasses a certain threshold (e.g., 20mg/L in Figure 2C), however, the dissolution of remaining nano-zno is not sufficient to maintain the level of [Zn] dis and a decrease of [Zn] dis becomes visible. As shown in Figure 3, synchrotron-based X-ray adsorption spectroscopy confirms the coexistence of zinc as nano-zno and adsorbed on nano- TiO 2, and linear combination fittings of the XANES and EXAFS spectra consistently quantify their percentages. These results provide quantitative evidence to support our explanation from the molecular perspective.

3 Figure 3. The Zn K-edge XANES (A) and EXAFS (B) spectra of the nano-zno/p25 and ZnCl 2 /P25 mixtures as well as the powder standards for qualitative comparison; (C) EXAFS spectra (solid lines) and linear combination fits (open dots) of the nano-zno/p25 mixtures with different nano-zno concentrations. The residues of the fittings are shown below the spectra and the corresponding fittings. 3. A simple kinetic model is developed to predict the experimental data. In order to establish a predictive understanding of the nano-zno/nano-tio 2 interactions shown in this study, we developed a simple kinetic model that considers both the processes of nano-zno dissolution and adsorption of the released zinc on nano-tio 2. This model successfully predicts the trend and values of the [Zn] dis curves for both fully- and partially-dissolved nano- ZnO mixed with P25, in particular the [Zn] dis plateaux observed in Figure 2C (Figure 4). Figure 4. Comparison of experimental results and modeling prediction of [Zn] dis from nano-zno coexisting with P25. The purple solid line represents the best linear fit of the data points and the green solid lines represent the 95% upper and lower confidence limits. 4. Both nano-tio 2 and nano-zno are photoactive, but their phototoxicity is not additive. Nano-TiO 2 and nano-zno share similar band gaps and band edge energies 13. Both materials are used as photocatalysts 14 and their phototoxicity has been reported in the literature Therefore, we probed the co-toxicity of nano-tio 2 and nano-zno under simulated solar irradiation. As shown in Figure 5, both nano-tio 2 and nano-zno exhibited phototoxicity to bacteria E. coli, but their phototoxicity was not additive. For example, the presence of 1 mg/l nano-zno surprisingly eliminated the phototoxicity of 10 mg/l nano-tio 2 P25, and the presence of 25 mg/l nano-zno, which reduced bacterial viability by 40%, did not increase the phototoxicity of P25. We monitored the production of hydroxyl radical ( OH), which is believed

4 to cause nano-tio 2 phototoxicity 23, by using a selective probe HPF 21. Results show that the OH production did not correlate well to the phototoxicity of nano-tio 2 /nano-zno, indicating that other mechanisms may be responsible to explain the phenomena observed in Figure 5. Figure 5. Phototoxicity and OH productions of nano-tio 2 P25, nano-zno and nano-tio 2 /nano-zno mixtures. Environmental Implications According to a recent review article by Lowry et al. 7, nanomaterials are subjected to various physical, chemical, and biological transformations in natural environments, which determine their fate, transport and corresponding toxic potentials. As a variety of ENMs are commonly used in industry and consumer products and are then discharged into the environment, cocontamination of multiple ENMs is likely to occur, which creates complex interactions that can alter ENMs availability and exposure. In our study, the presence of nano-tio 2 is found to mediate both the dissolution of nano-zno and the concentration of zinc in solution. Considering the toxicity of nano-zno is largely attributed to the release of dissolved ionic zinc 24-27, our results indicate the potential importance of other ENMs such as nano-tio 2 or environmental surfaces in controlling the fate, mobility, and ultimately, the bioavailability of nano-zno. Accordingly, the incorporation of ENMs interactions into risk assessment models will lead to a more in-depth understanding of their environmental fate and ultimately, potential impacts on human and ecological health as a result of applications and discharges of diverse ENMs. Research outcomes and future work This study has led to one manuscript (which will be submitted to Environ. Sci. & Technol.) and one presentation at mainstream national conference (see appendix). Our ongoing and future work will be focused on: 1. The mechanisms of the co-phototoxicity of nano-tio 2 and nano-zno (shown in Figure 5). Various selective probes targeting different ROS species (e.g., OH, 1 O 2, and O 2 - ) will be used to systematically measure ROS produced by nano-tio 2 in the presence/absence of nano-zno. Also, the contact of bacterial cells with nano-zno/nano-tio 2 nanoparticles will be visually studied by transmission electron microscopy with a liquid holder. 2. The interactions between nano-tio 2 and nano-ag. It was actually proposed in this ISEN study. But due to the time- and labor-constraints, we invested most of our time and efforts on the interactions between nano-tio 2 and nano-zno, as shown above. Nano-Ag is widely applied as an antibacterial agent and Ag will undergo more complex physical and chemical transformations in the aquatic environments. Based on our experiences obtained from this ISEN grant, we will focus on the dissolution and photochemistry of nano-ag in the presence of nano-tio 2.

5 Appendix: publications 1. Tong, T., Fang, K., Thomas, S. A., Binh, C. T. T., Kelly, J. J., Gaillard, J-F, and Gray, K. A. (2014) Effects of nano-tio 2/nano-ZnO interactions on nano-zno dissolution. Environmental Science & Technology, to be submitted. 2. Tong, T., Fang, K., Thomas, S. A., Binh, C. T. T., Kelly, J. J., Gaillard, J-F, and Gray, K. A. When nano meets nano : Effects of nano-tio 2/nano-ZnO interactions on nano-zno dissolution and photoactivity. The Second Sustainable Nanotechnology Organization Conference, November 3-5, 2013, Santa Barbara, CA.

6 References 1. Roco, M. C.; Mirkin, C. A.; Hersam, M. C., Nanotechnology research directions for societal needs in 2020: summary of international study. J Nanopart Res 2011, 13, (3), Benn, T. M.; Westerhoff, P., Nanoparticle silver released into water from commercially available sock fabrics (vol 42, pg 4133, 2008). Environ Sci Technol 2008, 42, (18), Gottschalk, F.; Sonderer, T.; Scholz, R. W.; Nowack, B., Modeled Environmental Concentrations of Engineered Nanomaterials (TiO 2, ZnO, Ag, CNT, Fullerenes) for Different Regions. Environ Sci Technol 2009, 43, (24), Kaegi, R.; Ulrich, A.; Sinnet, B.; Vonbank, R.; Wichser, A.; Zuleeg, S.; Simmler, H.; Brunner, S.; Vonmont, H.; Burkhardt, M.; Boller, M., Synthetic TiO(2) nanoparticle emission from exterior facades into the aquatic environment. Environ Pollut 2008, 156, (2), Kim, B.; Park, C. S.; Murayama, M.; Hochella, M. F., Discovery and Characterization of Silver Sulfide Nanoparticles in Final Sewage Sludge Products. Environ Sci Technol 2010, 44, (19), Kiser, M. A.; Westerhoff, P.; Benn, T.; Wang, Y.; Perez-Rivera, J.; Hristovski, K., Titanium Nanomaterial Removal and Release from Wastewater Treatment Plants. Environ Sci Technol 2009, 43, (17), Lowry, G. V.; Gregory, K. B.; Apte, S. C.; Lead, J. R., Transformations of Nanomaterials in the Environment. Environ Sci Technol 2012, 46, (13), Nel, A.; Xia, T.; Madler, L.; Li, N., Toxic potential of materials at the nanolevel. Science 2006, 311, (5761), Auffan, M.; Rose, J.; Bottero, J. Y.; Lowry, G. V.; Jolivet, J. P.; Wiesner, M. R., Towards a definition of inorganic nanoparticles from an environmental, health and safety perspective. Nat Nanotechnol 2009, 4, (10), Wiesner, M. R.; Lowry, G. V.; Alvarez, P.; Dionysiou, D.; Biswas, P., Assessing the risks of manufactured nanomaterials. Environ Sci Technol 2006, 40, (14), Navarro, E.; Baun, A.; Behra, R.; Hartmann, N. B.; Filser, J.; Miao, A. J.; Quigg, A.; Santschi, P. H.; Sigg, L., Environmental behavior and ecotoxicity of engineered nanoparticles to algae, plants, and fungi. Ecotoxicology 2008, 17, (5), Mueller, N. C.; Nowack, B., Exposure modeling of engineered nanoparticles in the environment. Environ Sci Technol 2008, 42, (12), Burello, E.; Worth, A. P., A theoretical framework for predicting the oxidative stress potential of oxide nanoparticles. Nanotoxicology 2011, 5, (2), Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W., Environmental Applications of Semiconductor Photocatalysis. Chem Rev 1995, 95, (1), Li, Y.; Zhang, W.; Niu, J. F.; Chen, Y. S., Mechanism of Photogenerated Reactive Oxygen Species and Correlation with the Antibacterial Properties of Engineered Metal-Oxide Nanoparticles. ACS Nano 2012, 6, (6), Ng, A. M. C.; Chan, C. M. N.; Guo, M. Y.; Leung, Y. H.; Djurisic, A. B.; Hu, X.; Chan, W. K.; Leung, F. C. C.; Tong, S. Y., Antibacterial and photocatalytic activity of TiO 2 and ZnO nanomaterials in phosphate buffer and saline solution. Appl Microbiol Biot 2013, 97, (12), Bar-Ilan, O.; Chuang, C. C.; Schwahn, D. J.; Yang, S.; Joshi, S.; Pedersen, J.; Hammers, R. J.; Peterson, R. E.; Heideman, W., TiO 2 Nanoparticle Exposure and Illumination during Zebrafish Development: Mortality at Parts per Billion Concentrations. Environ Sci Technol 2013, 47,

7 18. Bar-Ilan, O.; Louis, K. M.; Yang, S. P.; Pedersen, J. A.; Hamers, R. J.; Peterson, R. E.; Heideman, W., Titanium dioxide nanoparticles produce phototoxicity in the developing zebrafish. Nanotoxicology 2012, 6, (6), Miller, R. J.; Bennett, S.; Keller, A. A.; Pease, S.; Lenihan, H. S., TiO 2 nanoparticles are phototoxic to marine phytoplankton. Plos One 2012, 7, (1). 20. Tong, T.; Binh, C. T. T.; Kelly, J. J.; Gaillard, J.-F.; Gray, K. A., Cytotoxicity of commercial nano-tio 2 to Escherichia coli assessed by high-throughput screening: Effects of environmental factors. Water Res 2013, 47, Tong, T.; Shereef, A.; Wu, J.; Binh, C. T. T.; Kelly, J. J.; Gaillard, J.-F.; Gray, K. A., Effects of Material Morphology on the Phototoxicity of Nano-TiO 2 to Bacteria. Environ Sci Technol 2013, 47, Binh, C. T. T.; Tong, T.; Gaillard, J.-F.; Gray, A. K.; Kelly, J. J., Common freshwater bacteria vary in their responses to short-term exposure to nano-tio 2 in natural surface waters. Environ Toxicol Chem 2013, In press. 23. Cho, M.; Chung, H. M.; Choi, W. Y.; Yoon, J. Y., Different inactivation Behaviors of MS-2 phage and Escherichia coli in TiO 2 photocatalytic disinfection. Appl Environ Microb 2005, 71, (1), Franklin, N. M.; Rogers, N. J.; Apte, S. C.; Batley, G. E.; Gadd, G. E.; Casey, P. S., Comparative toxicity of nanoparticulate ZnO, bulk ZnO, and ZnCl 2 to a freshwater microalga (Pseudokirchneriella subcapitata): The importance of particle solubility. Environ Sci Technol 2007, 41, (24), Li, M.; Lin, D. H.; Zhu, L. Z., Effects of water chemistry on the dissolution of ZnO nanoparticles and their toxicity to Escherichia coli. Environ Pollut 2013, 173, Li, M.; Zhu, L. Z.; Lin, D. H., Toxicity of ZnO Nanoparticles to Escherichia coil: Mechanism and the Influence of Medium Components. Environ Sci Technol 2011, 45, (5), Rousk, J.; Ackermann, K.; Curling, S. F.; Jones, D. L., Comparative Toxicity of Nanoparticulate CuO and ZnO to Soil Bacterial Communities. PLOS One 2012, 7, (3).