Analysis and characterization of
naturally occurring and engineered
nanomaterials in the environment are critical for understanding their
environmental behaviors and defining real exposure scenarios for environmental
risk assessment. However, this is challenging primarily due to the
low concentration, structural heterogeneity, and dynamic transformation
of nanomaterials in complex environmental matrices. In this critical
review, we first summarize sample pretreatment methods developed for
separation and preconcentration of nanomaterials from environmental
samples, including natural waters, wastewater, soils, sediments, and
biological media. Then, we review the state-of-the-art microscopic,
spectroscopic, mass spectrometric, electrochemical, and size-fractionation
methods for determination of mass and number abundance, as well as
the morphological, compositional, and structural properties of nanomaterials,
with discussion on their advantages and limitations. Despite recent
advances in detecting and characterizing nanomaterials in the environment,
challenges remain to improve the analytical sensitivity and resolution
and to expand the method applications. It is important to develop
methods for simultaneous determination of multifaceted nanomaterial
properties for in situ analysis and characterization
of nanomaterials under dynamic environmental conditions and for detection
of nanoscale contaminants of emerging concern (e.g., nanoplastics
and biological nanoparticles), which will greatly facilitate the standardization
of nanomaterial analysis and characterization methods for environmental
samples.
Iron sulfide nanoparticles (nano-FeS) have shown great potential for in situ remediation of Cr(VI) pollution by reducing Cr(VI) to the less soluble and toxic Cr(III). However, material oxidation that inevitably occurs during storage and application alters its reactivity. Herein, we show that partial oxidation of nanoparticulate mackinawite (FeS) significantly enhances its capability in sequestering Cr(VI). Oxidation of nano-FeS increases its binding affinity to Cr(VI), likely due to preferential inner-sphere complexation of Cr(VI) oxyanions to ferric over ferrous iron in mackinawite/lepidocrocite (FeS/γ-FeOOH) nanocomposites. A trade-off is that oxidation mitigates Cr(VI) reduction by lowering the electron-donating potential of the material and the electron transfer at a solution−material interface and consequently hinders the transformation of adsorbed Cr(VI) to Cr(III). Notably, the rate-limiting step of Cr(VI) sequestration transitions from adsorption to reduction during oxidation, as demonstrated with batch experiments coupled with kinetic modeling. Thus, an optimum oxidation degree exists, wherein the gain in the overall performance from enhanced adsorption overcompensates the loss from inhibited reduction, resulting in maximum sequestration of aqueous Cr(VI) as solid-phase Cr(III). Our findings inform better assessment and design of nanomaterials for Cr(VI) remediation and may be extended to interactions of other oxyanions with natural and engineered nanoparticles during oxidative aging.
Nanoplastics are an increasing environmental concern.
In aquatic
environments, nanoplastics will acquire an eco-corona by interacting
with macromolecules (e.g., humic substances and extracellular polymeric
substances (EPS)). Here, we show that the properties of the eco-corona
and, consequently, its ability to enhance the transport of nanoplastics
vary significantly with the surface functionality of nanoplastics
and sources of macromolecules. The eco-corona derived from the EPS
of Gram-negative Escherichia coli MG1655
enhances the transport of polystyrene (PS) nanospheres in saturated
porous media to a much greater extent than the eco-corona derived
from soil humic acid and fulvic acid. In comparison, the eco-corona
from all three sources significantly enhance the transport of carboxylated
PS (HOOC-PS). We show that the eco-corona inhibits the deposition
of the two types of nanoplastics to the porous media mainly via steric
repulsion. Accordingly, an eco-corona consisting of a higher mass
of larger-sized macromolecules is generally more effective in enhancing
transport. Notably, HOOC-PS tends to acquire macromolecules of lower
hydrophobicity than PS. The more disordered and flexible structures
of such macromolecules may result in greater elastic repulsion between
the nanoplastics and sand grains and, consequently, greater transport
enhancement. The findings of this study highlight the critical role
of eco-corona formation in regulating the mobility of nanoplastics,
as well as the complexity of this process.
Due to the extremely low solubility, mercury sulfide minerals, as the major environmental mercury sinks, are generally considered to be inert mercury species with minimal bioavailability. Here, we demonstrate that...
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