Metal-containing nanoparticles derived from concealed metal deposits: An important source of toxic nanoparticles in aquatic environments

Highlights

• Many ore-related nanoparticles were found in the water samples.

• Concealed deposits were proposed to be important natural sources of nanoparticles.

• These ore-related nanoparticles happened to have potential ecological toxicity.

• Ore-related nanoparticles were firstly listed as a kind of toxic material.

Abstract

The potential environmental risks of engineered nanoparticles in aquatic environment have attracted considerable attention, but naturally produced nanoparticles have relatively been ignored, such as ore-related nanoparticles. To obtain more information about the natural ore-related nanoparticles, deep groundwater and well water samples were respectively collected in or around four major metal deposits in Inner Mongolia, China. These water samples were tested with high resolution transmission electron microscopy (TEM) and abundant metal-containing nanoparticles were found. Major ore-forming elements of corresponding metal deposits, such as Fe, Pb, Zn and Cu, and even associated elements, such as As, Sb, Sn and Cr, significantly contributed to the chemical compositions of these detected nanoparticles. Through comparison analyses, these metal-containing nanoparticles were shown to be originally from deep concealed metal deposits. They were the products of faulting and oxidation of ore minerals, and were transported long distances by water flow. Notably, these ore-related nanoparticles happened to have similar components with those nanoparticles of high environmental risks. Coupled with the analytical results of Atomic absorption spectroscopy (AAS) and inductively coupled plasma mass spectrometry (ICP-MS), it is recommended that the concentration limits of metal-containing nanoparticles should be considered in the safety assessment of drinking water. This is the first time, so far as we know, that naturally produced ore-related nanoparticles in the aquatic environment were listed as a kind of material with environmental risks. Considering the wide distribution of concealed metal deposits, more attention on related studies was urgently required for establishing specialized risk assessment and monitoring system.

Introduction

With the rapid development of nanoscience in the past decade, the applications of nano-materials in multiple industries have rapidly expanded (Banerjee et al., 2017; Cai et al., 2017; Dey et al., 2017; Gao et al., 2017; Parvanian et al., 2017; Priecel et al., 2016; Priyadarshini et al., 2016; Terzi et al., 2017; Wang et al., 2017). Subsequently, doubts concerning nanoparticle safety arose. Since 2003, top journals, such as Nature and Science, have published several articles indicating the potential negative effects of nanoparticles (Colvin, 2004; Nel et al., 2006; Schäfer, 2003; Service, 2008). After that, a number of studies demonstrated the adverse effects of nanoparticles to organisms, environment and ecosystems, especially concentrated on engineered nanoparticles, like SiO₂ nanoparticles (Gambardella et al., 2015), TiO₂ nanoparticles (Coccini et al., 2015) and iron oxide nanoparticles (Sadeghi et al., 2015). Besides, the toxicities of nanoparticles comprising heavy metals were especially prominent. For example, Bacchetta et al. (2017) showed that after 21-day exposures to ZnO nanoparticles and ZnSO₄ nanoparticles at low concentrations (0.3 mg/L), Daphnia magna showed mitochondrial swelling, increased autophagy vacuole numbers, and significantly inhibited reproduction. Zhao et al. (2017) found that CuO nanoparticles induced significant growth inhibition on Eichhornia crassipes, whose root caps and meristematic zone were severely damaged after exposure. Zamani et al. (2014) also observed the negative effect of PbS nanoparticles on aquatic algae. Furthermore, nanoparticles containing heavy metals may be more toxic than common engineered nanoparticles containing light metals. For examples, Zijno et al. (2015) found that ZnO nanoparticles can result in genotoxicity, inducing micronuclei and DNA damage in cells, whereas TiO₂ nanoparticles did not. Cells exposed to ZnO nanoparticles were not able to repair oxidative DNA damage but efficiently repaired after TiO₂ exposure. All of these studies effectively confirmed the risk of nanoparticles, especially these nanoparticles comprising heavy metals, like Pb, Zn and Cu.

Contrast to the extensive attention on the environmental risks of engineered nanoparticles, naturally produced nanoparticles attracted attention only recently. Though several studies addressed some different naturally-occurring nanoparticles (Sharma et al., 2015; Wimmer et al., 2018), the environment risks of metal-bearing nanoparticles related to metal deposits, however, have never been mentioned. In fact, previous researches about ore exploration have mentioned this kind of nanoparticles associated with natural metal deposits (Cao et al., 2015; Hu et al., 2015; Li et al., 2016; Wang et al., 2016), but never considered about their environmental influences. Given the original toxicity of their heavy metal composition, metal-containing nanoparticles associated with natural metal deposits, were regarded to have played important but underappreciated roles.

On the other hand, the ecological risks of excessive metal element concentration around metal deposits have always been the research hotspot. Numerous studies have detailedly investigated the concentration of ore-related elements in various media, such as surface water (Liu et al., 2010), surface sediments (Meng et al., 2015), atmosphere (Castillo et al., 2013), arable soils and associated food crops (Obiora et al., 2016), herbaceous species (Stefanowicz et al., 2016). Almost without exception, the pollution and risks of metal deposits were always evaluated through the excessive concentrations of metal elements but did not consider metal-containing nanoparticles. However, some previous studies have suggested that metal-containing nanoparticles may pose greater risks to organisms than metal elements at relatively lower concentrations, because the toxicity of metal-bearing nanoparticles not only originated from their chemical composition but also their nanoparticle form. For example, Yin et al. (2011) suggested that growth inhibition and cell damage can be directly attributed to the nanoparticles themselves. Wirth et al. (2012) indicated a nanoparticle-specific toxicity in biofilm viability loss. Rodhe et al. (2015) and Moschini et al. (2013) respectively showed the higher danger of Cu nanoparticles and CuO nanoparticles than that of dissolved coppers. Gil-Allué’s study (2015) on stream periphyton further demonstrated that a specific Ag nanoparticles inhibition effect on one of the extracellular enzyme (leucine aminopeptidase) was independent of dissolved Ag. These studies forced us to pay more attention on the contaminant of metal deposits that occurred in the form of nanoparticles.

Since aquatic environment has always been an important study subject in the environment-associated studies (Furtado et al., 2015; Wheatland et al., 2017; Ayotte et al., 2017), risk study of naturally produced ore-related nanoparticles contained in aquatic environment seems especially important. Thus, Inner Mongolia, a province of gathered polymetallic deposits in China, was chosen as the study area, and water (including deep groundwater and residential well water) was chosen as the study object. High resolution TEM was used to analyze the potential occurrence of nanoparticles in these water samples. This study aimed to elucidate the existence of naturally produced ore-related nanoparticles in the aquatic environment and their potential risks, to fill the void of environmental studies about naturally produced ore-related nanoparticles, further provided help for establish corresponding risk assessment and monitoring systems.