5 research outputs found
Simulating Multiwalled Carbon Nanotube Transport in Surface Water Systems Using the Water Quality Analysis Simulation Program (WASP)
Under the Toxic Substances Control
Act (TSCA), the Environmental Protection Agency (EPA) is required
to perform new chemical reviews of nanomaterials identified in premanufacture
notices. However, environmental fate models developed for traditional
contaminants are limited in their ability to simulate nanomaterials’
environmental behavior by incomplete understanding and representation
of the processes governing nanomaterial distribution in the environment
and by scarce empirical data quantifying the interaction of nanomaterials
with environmental surfaces. In this study, the well-known Water Quality
Analysis Simulation Program (WASP) was updated to incorporate particle
collision rate and particle attachment efficiency to simulate multiwalled
carbon nanotube (MWCNT) fate and transport in surface waters. Heteroaggregation
attachment efficiencies (α<sub>het</sub>) values derived from
sediment attachment studies are used to parametrize WASP for simulation
of MWCNTs transport in Brier Creek, a coastal plain river located
in central eastern Georgia, and a tributary to the Savannah River.
Simulations using a constant MWCNT load of 0.1 kg d<sup>–1</sup> in the uppermost Brier Creek water segment showed that MWCNTs were
present predominantly in the Brier Creek water column, while downstream
MWCNT surface and deep sediment concentrations exhibited a general
increase with time and distance from the source, suggesting that MWCNT
releases could have increasing ecological impacts in the benthic region
over long time frames
Aggregation Kinetics and Transport of Single-Walled Carbon Nanotubes at Low Surfactant Concentrations
Little is known about how low levels of surfactants can
affect
the colloidal stability of single-walled carbon nanotubes (SWNTs)
and how surfactant-wrapping of SWNTs can impact ecological exposures
in aqueous systems. In this study, SWNTs were suspended in water with
sodium dodecylsulfate (SDS) as a surface-active dispersing agent.
The effect of SDS concentration on SWNT suspension stability was investigated
with time-resolved dynamic light scattering (TRDLS) initial aggregation
studies utilizing both monovalent (Na<sup>+</sup>) and divalent (Ca<sup>2+</sup>) cations. The critical coagulation concentration (CCC) values
increased with SDS concentration for the Na<sup>+</sup> treatments,
but the Ca<sup>2+</sup> treatments were less sensitive to SDS concentration
changes. Longer term stability studies with SDS concentrations orders
of magnitude below the SDS critical micelle concentration demonstrated
that SWNTs remained suspended for over six weeks in a surface water.
Transport studies in a freshwater sediment similarly showed a SDS
concentration-dependent mobility of SDS–wrapped SWNTs in that
SWNTs showed a relatively greater retention at lower SDS concentrations
(0.001%–0.05% w/v) than at a higher SDS concentration (0.1%).
It is hypothesized that the stability and mobility of SWNT suspensions
is directly related to the surface coverage of SDS on the SWNT surface
that simultaneously increases electrosteric repulsion and decreases
surface chemical heterogeneity. Overall, these studies demonstrate
that low levels of surfactant are effective in stabilizing and mobilizing
SWNTs in environmental media
Colloidal Properties and Stability of Graphene Oxide Nanomaterials in the Aquatic Environment
While
graphene oxide (GO) has been found to be the most toxic graphene-based
nanomaterial, its environmental fate is still unexplored. In this
study, the aggregation kinetics and stability of GO were investigated
using time-resolved dynamic light scattering over a wide range of
aquatic chemistries (pH, salt types (NaCl, MgCl<sub>2</sub>, CaCl<sub>2</sub>), ionic strength) relevant to natural and engineered systems.
Although pH did not have a notable influence on GO stability from
pH 4 to 10, salt type and ionic strength had significant effects on
GO stability due to electrical double layer compression, similar to
other colloidal particles. The critical coagulation concentration
(CCC) values of GO were determined to be 44 mM NaCl, 0.9 mM CaCl<sub>2</sub>, and 1.3 mM MgCl<sub>2</sub>. Aggregation and stability of
GO in the aquatic environment followed colloidal theory (DLVO and
Schulze-Hardy rule), even though GO’s shape is not spherical.
CCC values of GO were lower than reported fullerene CCC values and
higher than reported carbon nanotube CCC values. CaCl<sub>2</sub> destabilized
GO more aggressively than MgCl<sub>2</sub> and NaCl due to the binding
capacity of Ca<sup>2+</sup> ions with hydroxyl and carbonyl functional
groups of GO. Natural organic matter significantly improved the stability
of GO in water primarily due to steric repulsion. Long-term stability
studies demonstrated that GO was highly stable in both natural and
synthetic surface waters, although it settled quickly in synthetic
groundwater. While GO remained stable in synthetic influent wastewater,
effluent wastewater collected from a treatment plant rapidly destabilized
GO, indicating GO will settle out during the wastewater treatment
process and likely accumulate in biosolids and sludge. Overall, our
findings indicate that GO nanomaterials will be stable in the natural
aquatic environment and that significant aqueous transport of GO is
possible
Deposition and Release of Graphene Oxide Nanomaterials Using a Quartz Crystal Microbalance
Interactions of graphene oxide (GO)
with silica surfaces were investigated
using a quartz crystal microbalance with dissipation monitoring (QCM-D).
Both GO deposition and release were monitored on silica- and poly-l-lysine (PLL) coated surfaces as a function of GO concentration
and in NaCl, CaCl<sub>2</sub>, and MgCl<sub>2</sub> as a function
of ionic strength (IS). Under favorable conditions (PLL-coated positive
surface), GO deposition rates increased with GO concentration, as
expected from colloidal theory. Increased NaCl concentration resulted
in a greater deposition attachment efficiency of GO on the silica
surface, indicating that deposition of GO follows Derjaguin–Landau–Verwey–Overbeek
(DLVO) theory; GO deposition rates decreased at high IS, however,
due to large aggregate formation. GO critical deposition concentration
(CDC) on the silica surface is determined to be 40 mM NaCl which is
higher than the reported CDC values of fullerenes and lower than carbon
nanotubes. A similar trend is observed for MgCl<sub>2</sub> which
has a CDC value of 1.2 mM MgCl<sub>2</sub>. Only a minimal amount
of GO (frequency shift <2 Hz) was deposited on the silica surface
in CaCl<sub>2</sub> due to the bridging ability of Ca<sup>2+</sup> ions with GO functional groups. Significant GO release from silica
surface was observed after adding deionized water, indicating that
GO deposition is reversible. The release rates of GO were at least
10-fold higher than the deposition rates under similar conditions
indicating potential high release and mobility of GO in the environment.
Under favorable conditions, a significant amount of GO was released
which indicates potential multilayer GO deposition. However, a negligible
amount of deposited GO was released in CaCl<sub>2</sub> under favorable
conditions due to the binding of GO layers with Ca<sup>2+</sup> ions.
Release of GO was significantly dependent on salt type with an overall
trend of NaCl > MgCl<sub>2</sub> > CaCl<sub>2</sub>
Photochemical Transformation of Graphene Oxide in Sunlight
Graphene
oxide (GO) is promising in scalable production and has
useful properties that include semiconducting behavior, catalytic
reactivity, and aqueous dispersibility. In this study, we investigated
the photochemical fate of GO under environmentally relevant sunlight
conditions. The results indicate that GO readily photoreacts under
simulated sunlight with the potential involvement of electron–hole
pair creation. GO was shown to photodisproportionate to CO<sub>2</sub>, reduced materials similar to reduced GO (rGO) that are fragmented
compared to the starting material, and low molecular-weight (LMW)
species. Kinetic studies show that the rate of the initially rapid
photoreaction of GO is insensitive to the dissolved oxygen content.
In contrast, at longer time points (>10 h), the presence of dissolved
oxygen led to a greater production of CO<sub>2</sub> than the same
GO material under N<sub>2</sub>-saturated conditions. Regardless,
the rGO species themselves persist after extended irradiation equivalent
to 2 months in natural sunlight, even in the presence of dissolved
oxygen. Overall, our findings indicate that GO phototransforms rapidly
under sunlight exposure, resulting in chemically reduced and persistent
photoproducts that are likely to exhibit transport and toxic properties
unique from parent GO