Natural organic matter (NOM) and
crystalline metal oxide nanoparticles
are both prevalent in natural aquatic environments, and their interactions
have important environmental and biogeochemical implications. Here,
we show that these interactions are significantly affected by an intrinsic
property of metal oxide nanocrystals, the exposed facets. Both anatase
(TiO2) and hematite (α-Fe2O3) nanocrystals, representing common engineered and naturally occurring
metal oxides, exhibited apparent facet-dependent adsorption of humic
acid and fulvic acid. This facet-dependent binding was primarily driven
by surface complexation between the NOM carboxyl groups and surficial
metal atoms. Thus, the adsorption affinity of different-faceted nanocrystals
was determined by the atomic arrangements of crystal facets that controlled
the activity of metal atoms and, consequently, the ligand exchange
and binding configuration of the carboxyl groups in the first hydration
shell of nanocrystals. Distinct facet-dependent fractionation patterns
were observed during adsorption of NOM components, particularly the
low-molecular-weight and photorefractory constituents. The molecular
fractionation of NOM between water and metal oxide nanoparticles was
dictated by the combined effects of facet-dependent metal complexation,
hydrophobic interaction, and steric hindrance and may significantly
influence the NOM-driven processes occurring both in aqueous phases
and at water–nanoparticle interfaces.
Growing evidence has suggested that
microbial biofilms are potential
environmental “hotspots” for the production and accumulation
of a bioaccumulative neurotoxin, methylmercury. Here, we demonstrate
that extracellular polymeric substances (EPS), the main components
of biofilm matrices, significantly interfere with mercury sulfide
precipitation and lead to the formation of nanoparticulate metacinnabar
available for microbial methylation, a natural process predominantly
responsible for the environmental occurrence of methylmercury. EPS
derived from mercury methylating bacteria, particularly Desulfovibrio
desulfuricans ND132, substantially increase the methylation
potential of nanoparticulate mercury. This is likely due to the abundant
aromatic biomolecules in EPS that strongly interact with mercury sulfide
via inner-sphere complexation and consequently enhance the short-range
structural disorder while mitigating the aggregation of nanoparticulate
mercury. The EPS-elevated bioavailability of nanoparticulate mercury
to D. desulfuricans ND132 is not induced by dissolution
of these nanoparticles in aqueous phase, and may be dictated by cell–nanoparticle
interfacial reactions. Our discovery is the first step of mechanistically
understanding methylmercury production in biofilms. These new mechanistic
insights will help incorporate microbial EPS and particulate-phase
mercury into mercury methylation models, and may facilitate the assessment
of biogeochemical cycling of other nutrient or toxic elements driven
by EPS-producing microorganisms that are prevalent in nature.
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...
Tiemannite nanoparticles available for microbial mercury methylation are formed during the co-precipitation of natural organic matter, divalent mercury and selenium.
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