Transformations of Nanomaterials in the Environment

Stacey M. Louie*,†; Rui Ma*,†; Gregory V. Lowry*,†    * Center for Environmental Implications of Nanotechnology, Carnegie Mellon University, Pittsburgh, Pennsylvania, USA
† Department of Civil and Environmental Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania, USA

1 Introduction

The Earth is awash with nanophase minerals produced in natural processes, for example, volcanic eruptions, sea spray aerosols, and continental mineral dust. Estimates of the annual production of nanophase minerals are as high as a million tons (Hochella et al., 2012). The increasing use of engineered nanomaterials (NMs) in commercial products translates into an increasing presence in the biosphere. Here, we define NMs as naturally occurring or engineered materials having at least one dimension in the nanoscale (ca. 1–100 nm). The extremely small sizes of these NMs result in a high percentage of its atoms at the surface, which can result in novel properties and reactivity compared to a larger size material with the same chemical composition (Auffan et al., 2009; Hochella et al., 2008). For example, gold nanoparticles (NPs) that are typically inert become catalytic as their size is decreased to a few nanometers (Haruta, 2004), and the surface area normalized rate of oxidation of Mn2 + by hematite NPs greatly increases when the particle size becomes less than about 10 nm (Madden and Hochella, 2005). The very high reactivity of NMs, either due to their large surface to volume ratio or unique reactivity with decreasing size, implies that these materials will readily transform in the environment. This indeed is the case for many NMs, and a poor understanding of these transformations complicates the forecasting of their fate, transport, reactivity, and toxicity in environmental systems.

Assessing the environmental and human health implications of engineered NMs requires an understanding of the potential exposure routes and toxicological effects from acute and chronic exposures. Research has been performed to determine the fate, transport, and toxic properties of engineered NMs, but much of this has focused on pristine or “as manufactured” NMs or where modifications of the materials are not assessed (Levard et al., 2013). Transformations of the NMs will affect their fate, transport, and toxic properties. For example, many metallic or metal oxide NPs (e.g., Ag, ZnO, CuO) may ultimately become sulfidized in the environment (Liu et al., 2011; Ma et al., 2013). Sulfidation of the metal or metal oxide NPs changes their aggregation state, surface chemistry and charge, as well as their ability to release potentially toxic ions such as Zn2 + or Ag+ (Levard et al., 2011a), and therefore their persistence and toxicity. Similarly, the interaction between gold NMs and humic substances (HS) including natural organic matter (NOM) results in a nanoscale coating of the NMs, resulting in greater colloidal stability at low pH (Diegoli et al., 2008). This coating process is analogous to the formation of protein coronas in mammalian systems (Cedervall et al., 2007), which dramatically changes aggregation, deposition, and toxic properties (Fabrega et al., 2009a; Li et al., 2008a). We currently lack sufficient knowledge of the types, rates, and extent of transformations expected for NMs in environmental and biological systems. By extension, we also fail to understand the impact of those transformations on the fate, transport, and toxicity of NMs, and potentially bias our conclusions about the toxicity or behavior of the materials by neglecting to adequately characterize NM transformations. To correctly forecast the environmental and human health risks associated with these materials, the environmental nanoscience research community must endeavor to broaden our knowledge of the transformations of NMs.

One goal of the current global nano environmental, health, and safety (EHS) research enterprise is to correlate the properties of engineered NMs with their risk, that is, potential for exposure and ability to produce a toxic effect in exposed organisms. For environmental exposures, for example, exposure of aquatic or sediment dwelling organisms, these risks cannot be determined without consideration of the complexity of natural systems (and therefore, transformations of the NPs, in particular). For example, some aquatic plants exposed to silver nanoparticles (Ag NPs) released exudates that interact with the Ag NPs and altered their aggregation state and ability to release Ag ion, Ag+ (Bone et al., 2012; Unrine et al., 2012). Since the toxicity of Ag NPs is correlated with the release of Ag+ and other dissolved Ag species (Levard et al., 2013; Xiu et al., 2012), the toxicity potential of Ag NPs will depend on the properties of the environment (e.g., type and number of plants) into which the Ag NPs are released. The research framework proposed by the U.S. National Research Council (NRC, 2012) recommends that nano EHS research focuses on identifying “critical elements of nanomaterial interactions” affecting exposure, hazards, and hence risks posed by engineered NMs. These critical elements include physical, chemical, and biological transformations that ultimately influence NM persistence, bioavailability or biouptake, reactivity, and toxicity. These are currently poorly understood and are not yet predictable from the NM and environment properties.

This chapter reviews the types of transformations that NMs undergo in biological and environmental media (with a focus on environmental media), and describes how these transformations affect the exposure potential and toxicity potential of engineered NMs, and the ability to measure them in biological and environmental media. It concludes with implications of these findings and potential future research to close existing knowledge gaps.

2 NM Transformations

Engineered NMs can be composed of a single material (e.g., Ag or Au), but often have a core–shell configuration (Figure 2.1). This core–shell configuration can be an engineered inorganic shell, for example, a quantum dot with a CdSe core and a ZnS shell. In nearly all cases, engineered NMs are designed with an organic coating (Table 2.1). This can be a surfactant or polymeric coating, which provides electrostatic or steric stabilization against aggregation (Phenrat et al., 2008) or can provide specific functionality or targeting properties (Farokhzad et al., 2004). Transformations of NMs in the environment affect the core, shell, or organic coating as described in this chapter.