Study on the characteristics of naturally formed TiO2 nanoparticles in various surficial media from China
Graphical abstract
Introduction
In recent decades, with the fast development of nanotechnology, the applications of nano-materials has expanded rapidly, such as in environment remediation (Cai et al., 2017), drug delivery (Dey et al., 2017), cancer diagnosis and treatment (Parvanian et al., 2017), catalytic applications (Priecel et al., 2016) and sensitive sensors (Terzi et al., 2017). Owing to their unique physical and chemical properties, nano-materials have enabled those related industries to achieve major breakthroughs. Among them, TiO2 nanoparticles, as one of the most common nanomaterials, has also found wide application in paints, ceramics, rubber, plastics, cosmetics and water purification (Warheit et al., 2007; Cui et al., 2014). As the use and discharge of engineered nanoparticles have increased, related studies concerning the safety and characteristics of engineered nanoparticle have subsequently emerged, but naturally formed nanoparticles are rarely reported (Hu and Cao, 2019a).
In fact, nature itself is a major producer of nanoparticles. Many natural activities can produce nanoparticles, such as mineralization (Palenik et al., 2004), oxidation and reduction (Hough et al., 2008; Griffin et al., 2018), fault activity (Wilson et al., 2005; Hu and Cao, 2019b), microbial actions (Lengke and Southam, 2006), volcanic eruptions (Lähde et al., 2013), etc. In other words, a fair number of nanoparticle reserves have naturally formed in the environment. Ti is one of the ten most abundant elements in the earth's crust; thus, it can be speculated that nanoparticles mainly comprising TiO2 should have a wide distribution in the natural environment via the transformation of natural activities. In fact, plenty of studies have calculated the environmental concentration of nanomaterials and suggested that TiO2 nanoparticles showed the highest concentrations for all regions among all of the nanomaterials (Mueller and Nowack, 2008; Gottschalk et al., 2009, Gottschalk et al., 2010, Gottschalk et al., 2013; Arvidsson et al., 2012). Gottschalk et al. (2009) pointed out that the concentrations of TiO2 nanoparticles could raise to 21 ng L−1 in surface water and 4 μg L−1 in effluents. However, naturally formed TiO2 nanoparticles have not gained the attention they deserve. To date, most researchers have conducted related research on engineered TiO2 nanoparticles (Buzea et al., 2007; Zhao et al., 2015), but there have been few reports on naturally formed TiO2 nanoparticles. Therefore, the characteristics of naturally formed TiO2 nanoparticles have not been revealed.
Thus, to obtain more information about naturally formed TiO2 nanoparticles, this study collected several different surficial media samples (including geogas, soil solid, underground water and organisms) from nine sampling sites distributed across five different provinces in China. A high-resolution TEM was used to observe the morphological, structural, and chemical composition characteristics of the TiO2 nanoparticles contained in these samples. Naturally formed TiO2 nanoparticles were firstly found to be widely distributed in various natural media. Based on this finding, further analyses on the occurrence state, growth process, geological significance and environmental influences of naturally formed TiO2 nanoparticles were conducted. This study can reveal the occurrence state and growth laws of TiO2 in the form of nanoparticles to some degree, and contribute to explaining the geochemical behavior of TiO2 nanoparticles in the natural environment.
Section snippets
Sampling sites
To collect TiO2 nanoparticles from different media in the natural environment as comprehensively as possible (such as from soil solids, underground water, geogas and organisms), many sampling sites were involved in this study. These sampling sites were distributed in five different provinces in China, mainly in deposit areas and fault zones, including the Bingba deposit in Guizhou Province, Fankou deposit and Shixing fault zone in Guangdong Province, Xiaozhai deposit in Yunnan Province,
Results
Plenty of TiO2 nanoparticles contained in different natural media were detected by the high-resolution TEM, and part of the obtained results is described in Table 1, Table 2, Table 3. The three data tables provide the EDS analytical results of representative TiO2 nanoparticles contained in the geogas, soil solid, underground water and organism samples. Overall, these TiO2 particles have various sizes and degrees of crystallinity, and they exist in the form of single particles or particle
Differences between the naturally formed and the engineered TiO2 nanoparticles
Zänker and Schierz proposed various technical methods for distinguishing engineered nanoparticles from natural nanoparticles (Zänker and Schierz, 2012). In this study, it can be observed that the characteristics of the naturally formed TiO2 nanoparticles obtained from the environment are indeed different from those of the engineered TiO2 nanoparticles synthesized via artificial pathways. Regarding the morphology, as the examples shown in Fig. 7, the TiO2 nanoparticles formed in the natural
Conclusion
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This study demonstrated for the first time that naturally formed TiO2 nanoparticles are widely distributed on the surface of the earth, including in soil solids, geogas, underground water and organisms. TiO2 nanoparticles in different natural media have similar characteristics; for example, single TiO2 nanoparticles usually appear as complete or incomplete polygons, whereas particle aggregations are mostly irregular in shape. Their size is generally at the nanometer or nearly nanometer scale.
Declaration of competing interest
None.
Acknowledgements
This research was supported by the National Natural Science Foundation of China (Grant nos. 41473040, 41873044 and 41030425). We thank the Instrument Analysis Centre of Yangzhou University and Suzhou University for helping with the microscopic analysis.
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