Grantee Research Project Results
Final Report: Interactions of Natural Organic Matter with C60 Fullerene and their Impact on C60 Transport, Bioavailability and Toxicity
EPA Grant Number: R834093Title: Interactions of Natural Organic Matter with C60 Fullerene and their Impact on C60 Transport, Bioavailability and Toxicity
Investigators: Li, Qilin , Alvarez, Pedro J.
Institution: Rice University
EPA Project Officer: Hahn, Intaek
Project Period: January 1, 2009 through December 31, 2011
Project Amount: $399,995
RFA: Exploratory Research: Nanotechnology Research Grants Investigating Fate, Transport, Transformation, and Exposure of Engineered Nanomaterials: A Joint Research Solicitation - EPA, NSF, & DOE (2007) RFA Text | Recipients Lists
Research Category: Nanotechnology , Safer Chemicals
Objective:
The present research was originally proposed to investigate the impact of natural organic matter (NOM) on the transport, bioavailability and toxicity of C60 fullerene. During the investigation, solar irradiation, another important environmental factor, was found to greatly alter the physicochemical properties of fullerene nanoparticles (nC60), and strongly influence their stability and transport behaviors. Meanwhile, a number of studies on carbon nanomaterials show that much of the previously observed toxicity of nC60 is attributed to the organic solvents used in nC60 suspension preparation or their derivatives; CNTs were found to show stronger toxicity than nC60. Due to chemical similarity between C60 and CNTs, CNTs were hypothesized to undergo similar photochemical transformation. Therefore, we adjusted our focus to investigating the role of sunlight and NOM and expanded our scope of research to include CNTs. The new overarching objective of this investigation is to determine the role of sunlight and NOM in fate, transport and toxicity of carbon based nanomaterials including nC60 and carbon nanotubes in surface and ground waters.
Specifically, the project had three sub-objectives: 1) to investigate the photochemical transformation of nC60 and CNTs under sunlight and its impact on the physicochemical properties of nC60 and CNTs in realistic aqueous environments; 2) to determine the effect of photochemical transformation and NOM on the transport behaviors (i.e., aggregation and deposition) of nC60 and CNTs in surface water and ground water solution conditions; and 3) to determine the impact of photochemical transformation on the toxicity of nC60 and CNTs.
Summary/Accomplishments (Outputs/Outcomes):
Summary of Findings and Accomplishments
Over the past 4 years, we conducted extensive investigation following our proposed research approach. In this final report, we summarize the major findings of the research and present them according to the three sub-objectives proposed. Implications of this investigation will be discussed in a separate section.
Task 1: Photochemical transformation of fullerene nanoparticles and carbon nanotubes under environmentally relevant conditions
Our investigation reveals that photochemical transformation of aqueous fullerene nanoparticles (nC60) and carbon nanotubes occur at significant rates under UVA irradiation at intensity similar to that in sunlight. The transformation processes are mainly mediated by the self-generated reactive oxygen species (ROS), resulting in changes of surface chemistry of nC60 and CNTs depending on their initial surface oxidation state.
Photochemical transformation of aqueous nC60 under environmentally relevant conditions
UV lamps emitting light between 300 and 400 nm (peak at 350 nm) were used to simulate the UVA component in the solar spectrum. The irradiation intensity was set to be 2 mW/cm2, which was comparable to the UVA intensity at the ground level on a sunny day in Houston, TX.
(1) Photochemical transformation products and kinetics of nC60
The particle size (measured by dynamic light scattering [DLS]) and morphology (measured by transmission electron microscope [TEM]) remained unchanged over 21 days of UVA irradiation, suggesting negligible release of soluble phototransformation products from the nanoparticle surface. In addition, total organic carbon (TOC) concentration of the sample remained unchanged, suggesting that there was no mineralization of C60. However, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy results revealed oxygen-containing functional groups on the transformed nC60, and the photochemical reaction products had structures similar to that of fullerol (C60(OH)x) (Figures 1 and 2). XPS analysis with argon etching suggested that only the C60 molecules on nC60 particle surface were oxidized, while the core of nC60 remained intact (Figure 1).
The surface oxidation resulting from the photochemical reaction strongly affects C60 extraction efficiency by toluene. The pristine nC60 can be extracted with a recovery rate >98% using the Mg(ClO4)2-toluene extraction method and the concentration measured by UV absorbance at 335 nm agrees well with TOC measurement. After UVA irradiation, the Mg(ClO4)2-toluene extraction efficiency decreased dramatically (Figure 3). The kinetics of nC60 photochemical transformation were quantified by the loss of C60 concentration measured using the Mg(ClO4)2-toluene extraction method (Figure 3). In dark conditions, there was no reduction in extractable C60 concentration over 3 months. With UVA light, the extractable C60 concentration decreased with increasing irradiation time. Only 5% of the total C60 mass was extracted after 7 days. Based on the loss in the extractable C60 concentration, the apparent transformation kinetics can be described by a pseudo‐first order reaction with a reaction rate constant of 2.2 × 10‐2 h‐1, corresponding to a half life of 31 h. In the mean time, the TOC concentration and particle size distribution of the nC60 suspension remained constant, indicating no release of soluble compounds and no carbon mineralization over the same period of time. Small decrease (6.6%) in TOC was observed over prolonged irradiation time (21 days), suggesting possible mineralization in the long term.
(2) Role of O2 and NOM in the photochemical transformation
The surface oxidation of nC60 is attributed to photo activated reaction with dissolved oxygen in water. Under anoxic conditions, no notable changes in nC60 surface properties were observed. The photochemical oxidation reaction pathway was speculated to involve ROS, particularly 1O2 1.
NOM is known to both generate ROS upon photosensitization and reduce photodegradation through light screening and ROS scavenging. In this case, the presence of humic acid significantly inhibited the photochemical transformation of nC60 by light screening and scavenging of ROS. The inhibitive effect was more profound at higher humic acid concentration as shown in Figure 4.
(3) Long-term photochemical transformation
To further investigate the long-term photochemical fate of nC60, an irradiation experiment was carried out with fullerol as a surrogate for the intermediate photo-transformation products of nC60. Figure 5 presents the changes of UV-vis absorbance and TOC of fullerol upon UVA irradiation. Both absorbance and TOC decreased over time, suggesting mineralization of fullerol due to the photochemical oxidation. The pseudo-first order reaction rate constant is 1.8 × 10‐3 h‐1 for TOC reduction.
In summary, our results show the photochemical transformation of nC60 occurs at significant rates under UVA irradiation in the presence of dissolved oxygen. The nanoparticle surface undergoes oxygenation/hydroxylation, resulting in intermediate products similar to fullerol. Experiments with fullerol show potential mineralization of nC60 over long-term irradiation. This work was published in Environmental Science & Technology (Hwang YS, Li QL. Characterizing photochemical transformation of aqueous nC(60) under environmentally relevant conditions. Environ. Sci. Technol. 2010;44(8):3008-3013).
Photochemical transformation of carboxylated multi-walled carbon nanotubes (COOH-MWCNTs) under environmentally relevant conditions
Similar irradiation experiments were performed with COOH-MWCNTs. The UVA light intensity used was 2.4 mW/cm2. The mechanisms of the photochemical reaction were explored.
(1) Photochemical transformation of COOH-MWCNTs
Seven days of UVA irradiation of the COOH-MWCNT aqueous suspension significantly reduced the surface oxygen concentration of COOH-MWCNTs as shown in Figure 6. The changes in major oxygen-containing surface functional groups were studied by chemical derivatization in conjunction with XPS analysis. Most of the reduction in total surface oxygen was attributed to the removal of carboxyl groups. On the other hand, O[OH] and O[CO] remained fairly constant during the 7-day irradiation. These results suggest that significant changes in CNT surface chemistry are expected to occur in days under sunlight.
(2) Production of ROS by COOH-MWCNTs
COOH-MWCNTs were found to generate both 1O2 and ·OH in UVA light. The 1O2 generation was measured using furfuryl alcohol (FFA), and the pseudo-steady-state concentration of 1O2, [1O2]ss, was calculated to be 6.48×10-15 M, assuming FFA degradation follows pseudo-first-order reaction kinetics. This value is comparable to the [1O2]ss generated by carboxylated and polyethylene glycol functionalized SWCNTs (3.3×10-14 and 1.46×10-14 M, respectively) exposed to a UVA light source with higher intensity 2. Under the same irradiation conditions, terephthalic acid was employed as the ·OH probe. The reaction product, hydroxyterephthalic acid, was used to quantify the ·OH production kinetics. The pseudo-steady-state concentration of ·OH, [·OH]ss, was 2.37×10-19 M, much lower than previously reported [·OH]ss generated by carboxylated SWCNTs (1.58×10-15 M) with a stronger UVA source 2. A steep ROS concentration gradient was expected from the CNT surface to the bulk solution due to their short lifetime. Thus, the actual ROS production by COOH-MWCNTs is difficult to quantify as the majority of scavenger molecules are not located in the ROS diffusion zone at the vicinity of the nanotube surface 3, 4. The ROS could have much larger impact on the CNT surface chemistry than expected based on the measured concentrations in bulk solutions as they were generated right on the CNT surface.
(3) Effects of 1O2 and ·OH on COOH-MWCNT surface chemistry
To isolate effects of 1O2 and ·OH on COOH-MWCNT surface chemistry, COOH-MWCNT was reacted with externally generated 1O2 and ·OH separately. Rose Bengal was excited by visible light to produce 1O2, which subsequently reacted with the COOH-MWCNT. After 6 days of reaction, however, the surface oxygen of COOH-MWCNTs slightly increased from 10.5 ± 0.69% to 12.78 ± 1.38%; in contradiction to the observed decreased surface oxygen in the photochemical transformation experiments. Previous experimental and density functional theory calculation studies reported that 1O2 can interact with CNT sidewall, yielding surface oxides 5-7, which agree with our observation.
Hydroxyl free radicals were generated by direct photolysis of H2O2 with UVA irradiation. The total surface oxygen concentration along with the major oxygen-containing functional groups after reacting with ·OH were presented in Figure 7. The surface oxygen concentration of COOH-MWCNT gradually decreased as the reaction preceded and reached a plateau at around 5% O after 11 h of reaction. We speculate reactions between COOH-MWCNT and ·OH both generate and remove oxygen-containing functional groups, leaving holes on the sidewall. This dynamic process reached balance after 11 h at around 5% O. This hypothesis is supported by the Raman spectroscopy data (Figure 7c). Figure 7b clearly shows that COOH-MWCNTs continuously loose carboxyl groups upon reaction with ·OH, while concentrations of other oxygen-containing functional groups were not significantly changed. This observation agrees with the evolution of functional groups in the photochemical transformation of COOH-MWCNTs.
The change of surface structure also was studied by Raman spectroscopy. The intensity ratio of the D band and G band of CNT Raman spectra, ID/IG, is a sensitive indicator for the relative abundance of surface defects on CNTs. The ID/IG value sharply decreased from 0.915 to 0.779 in the first 2 h of reaction with ·OH and increased afterwards (Figure 7c). The non-monotonic change of ID/IG value also was observed in enzyme-catalyzed degradation of oxidized MWCNTs and was attributed to the exfoliation of highly oxidized graphitic lattice from the nanotube surface 8. The COOH-MWCNTs used in this study are modified from pristine MWCNTs by sulfuric/nitric acid treatment. It has been reported that a large portion of the carboxyl groups created by acid treatment is present on carboxylated carbonaceous fragments (CCF) on the nanotube surface 9. We speculate the initial decrease of ID/IG along with surface oxygen is caused by removal of carboxyl groups on CCF, leading to their destruction or exfoliation, which is known to significantly lower the D-band intensity as the inner graphitic sidewall is exposed 9. The following increase of ID/IG can be attributed to the formation of defects (e.g., functional groups and holes) on the graphitic sidewall. In the photochemical transformation experiment, the ID/IG of COOH-MWCNTs was reduced from 0.915 to 0.889 as surface oxygen concentration decreased from 10.50 ± 0.69 % to 8.58 ± 0.33 % after 7 days of UVA irradiation, consistent with the initial stage of reaction with ·OH.
In order to discern and quantify the effects of 1O2 and ·OH on COOH-MWCNT surface chemistry during UVA irradiation, two sets of inhibition experiments were conducted. The first set of inhibition experiments was carried out in anoxic conditions where the solution was purged with N2 for 1 h and sealed before UVA irradiation started. The production of 1O2 was largely suppressed in this condition. However, the surface oxygen concentrations of COOH-MWCNT after 14 days UVA irradiation in both aerobic and anoxic conditions were similar (8.37 ± 0.50% and 8.27 ± 0.15%, respectively). This finding indicates 1O2 does not play an important role in the photochemical transformation of COOH-MWCNTs, consistent with our earlier discussion. It also suggests that ·OH can be generated without the presence of O2, in contrast to a recently proposed pathway in which ·OH is generated through electron transfer from excited functionalized SWCNTs to O2 2. We speculate that ·OH can be formed through reaction between photo-generated holes and water molecules. To test our hypothesis, photochemical transformation experiments were carried out in the presence of a hole scavenger, Na2SO3 10. As shown in Figure 8, the phototransformation is inhibited in 10 mM Na2SO3 after both 3 and 7 days UVA irradiation, suggesting the inhibition of ·OH production. Na2SO3 scavenged photo-generated electron holes on COOH-MWCNT surface and consequently suppressed the ·OH production.
Based on these results, we propose that the photochemical transformation of COOH-MWCNTs in sunlight is mainly mediated by photo-activated self-generation of ·OH, reaction of which with COOH-MWCNT surface leads to reduction of surface oxygen concentration, mainly through removal of carboxyl groups. This part of the work has been summarized in a research article in preparation.
Task 2: Impact of sunlight and NOM on the transport behaviors of nC60 and CNTs in surface water and ground water systems.
Aggregation and deposition are two important mechanisms that remove nanoparticles from surface water and ground water. Therefore, we studied the impact of sunlight and NOM on the aggregation and deposition of nC60 and CNTs.
Impact of sunlight and NOM on the aggregation kinetics of nC60
Sunlight affects nC60 aggregation kinetics in two ways: (1) it directly affects nC60 aggregation behaviors by changing nC60 surface properties through photochemical oxidation reactions, and (2) it alters the interaction between nC60 and NOM, resulting in indirect impact on aggregation. The major findings are qualitatively summarized in Figure 9. The initial aggregation kinetics of nC60 suspensions were examined in various solution conditions using time resolved DLS. The aggregation rate was quantified using the rate of increase in the mean hydrodynamic diameter, and the aggregation kinetics under different solution conditions were compared using the attachment efficiency α, which was calculated using the aggregation rate in the solution of interest normalized by the rate in the diffusion-limited regime. Suwannee River humic acid (SRHA) was used as a model NOM in this study. This part of our work was published in Environmental Science & Technology (Qu XL, Hwang YS, Alvarez PJJ, Bouchard D, Li QL. UV irradiation and humic acid mediate aggregation of aqueous fullerene (nC(60)) nanoparticles. Environ. Sci. Technol. 2010;44(20):7821-7826).
(1) Direct impact of sunlight in the absence of NOM
From the colloidal stability curves, the critical coagulation concentration was determined and used to characterize the colloidal stability of nC60. Figure 10 summarized the critical coagulation concentration (CCC) of the pristine and UVA irradiated nC60 in the presence and absence of SRHA in NaCl and CaCl2. The aggregation behaviors of nC60 in NaCl can be qualitatively described by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, which considers the combined effect of electrostatic repulsion and van der Waals attraction. nC60 aggregation rate decreased with increasing UVA irradiation time during a 7-day period as shown in Figure 11. It is attributed to the UVA induced photochemical oxidation of nC60, which enhances its surface charge and hydrophilicity as discussed in the previous section. For both the pristine and UV irradiated samples, particle electrophoretic mobility became less negative with increasing NaCl concentration due to charge screening (Figure 12). The pristine nC60 and nC60 after 7 days irradiation (7DUV-nC60) were similarly charged at NaCl concentrations below 10 mM, while at higher NaCl concentrations up to 200 mM, the 7DUV-nC60 surface was notably more negatively charged, consistent with the higher colloidal stability. The difference in response to the charge screening effect of NaCl is attributed to the changes in nC60 surface chemistry. A close examination of the electrophoretic mobility data reveals that changes in particle electrophoretic mobility alone could not account for all the observed increase in particle stability after UV irradiation. Such inconsistency between the electrophoretic mobility and the stability of nanoparticles was attributed to the enhanced hydrophilicity of the irradiated nC60.
As described by the Schulze-Hardy Rule, cations of higher valence are more efficient in coagulation. Thus, the CCCs of CaCl2 for the pristine and 7DUV-nC60, 4.25 and 3.24 mM CaCl2, respectively, were markedly lower than those of NaCl (Figure 10). Contrary to the observations in NaCl solutions, the 7DUV-nC60 was less stable than the pristine nC60 in CaCl2 solutions. Comparison between the electrophoretic mobility of the pristine and the 7DUV-nC60 reveals that the 7DUV-nC60 was less negatively charged than the pristine nC60 over the entire tested range of CaCl2 concentrations (Figure 12), suggesting that Ca2+ was more effective in neutralizing negative charges on the 7DUV-nC60 surface than on the pristine nC60. The Schulze-Hardy Rule was not applicable to 7DUV-nC60 aggregation data, again suggesting Ca2+ specifically interacts with the 7DUV-nC60 surface, presumably with carboxyl groups formed from UVA irradiation.
(2) Indirect impact of sunlight in the presence of NOM
Humic acid can readily adsorb onto nC60 surface, alter their surface properties and mediate its aggregation. The adsorption of humic acid on nC60 strongly depends on the surface chemistry of nC60, which is a function of sunlight exposure time. SRHA at 1 mg/L greatly stabilized the pristine nC60 with the CCC increasing from 84 to 331 mM NaCl (Figure 10). The electrophoretic mobility of nC60 did not change notably after equilibrating with SRHA, suggesting that changes in electrostatic interactions were not responsible for the increase in stability. With 10 mg/L SRHA, no aggregation was observed at NaCl concentrations up to 1.5 M NaCl. This further demonstrates that steric hindrance rather than electrostatic repulsion is the main stabilizing mechanism in the presence of humic acid, because the surface charge should be effectively screened at such an elevated NaCl concentration. Humic acid has much less effect on the stability of 7DUV-nC60. The small influence of humic acid on the aggregation properties of 7DUV-nC60 is attributed to decreased adsorption of humic acid due to increased hydrophilicity of the nC60 surface after UV irradiation.
To better understand the influence of adsorbed humic acid on nC60 stability, adsorption of humic acid was examined. The low concentration of nC60 nanoparticles in the suspension makes the batch adsorption experiments difficult to carry out as the accuracy of the result largely depends on the difference between the initial and residual SRHA concentrations. To overcome this difficulty, centrifugal filters were used to concentrate both pristine and UV irradiated nC60 suspensions to 220 mg/L. Unlike pristine nC60, which showed significant adsorption of SRHA (q = 1209 mg/kg in 0.7 mg/L SRHA), UV-irradiated nC60 had no detectable adsorption of SRHA during 24 h of contact, consistent with the weak impact of humic acid on 7DUV-nC60 stability.
In CaCl2 solutions, however, the presence of 1 mg/L SRHA increased the stability of both pristine nC60 and 7DUV-nC60 as shown in Figure 10. The increased stability of 7DUV-nC60 indicates adsorption of humic acid on the 7DUV-nC60 surface, presumably facilitated by Ca2+. The enhanced humic acid sorption could be caused by two mechanisms:
(1) Ca2+ specifically interacts with the oxygen-containing function groups (e.g., carboxyl groups) on the 7DUV-nC60 surface, neutralizing surface negative charges. It also complexes with humic acid and reduces its molecular charge. Consequently, the electrostatic repulsion between the 7DUV-nC60 and humic acid is reduced, favoring humic acid adsorption.
(2) Ca2+ could form ionic bridges between humic acid and the oxygen-containing functional groups on the 7DUV-nC60.
Impact of sunlight and NOM on the deposition kinetics of nC60.
The impact of solar irradiation and humic acid, in either dissolved or surface-immobilized form, on the deposition kinetics of nC60 was investigated in NaCl solutions. The major findings are schematically summarized in Figure 13. Deposition kinetics of nC60 in various solution conditions was studied using both conventional packed-bed column experiments and a quartz crystal microbalance with dissipation monitoring (QCM-D). This part of the work was published in Environmental Science & Technology (Qu XL, Alvarez PJJ, Li QL. Impact of sunlight and humic acid on the deposition kinetics of aqueous fullerene nanoparticles (nC(60)). Environ. Sci. Technol. 2012;46(24):13455-13462).
(1) Solar irradiation hinders nC60 deposition on silica surface.
QCM-D experimental results suggest that deposition of pristine nC60 increases with increasing NaCl concentration in the reaction limited regime, with a critical deposition concentration (CDC) of 18 mM (Figure 14). UVA irradiation significantly enhances the stability of nC60 against deposition; consistent with our earlier finding that UVA irradiation enhanced the stability of nC60 against aggregation in NaCl solutions. The reduced deposition was attributed mainly to the surface oxidation of nC60 and the resulting increase in surface charge and hydrophilicity. nC60 underwent surface oxidation under UVA irradiation. The surface oxygen content of nC60 increased with increasing UVA irradiation time from 9.3 ± 0.3% at the pristine state to 11.3 ± 0.6% after 20 h and 14.2 ± 0.6% after 7 days of irradiation. Further deconvolution of the carbon XPS peaks revealed a significant increase of oxygenated carbon (C-O, C=O, or O-C-O) on UVA-irradiated nC60 surface. The addition of oxygen-containing functional groups increased nC60 surface charge as reflected by the increased electrophoretic mobility after irradiation, leading to higher electrostatic repulsion between nC60 nanoparticles and the silica surface. The attachment efficiency of the 7-day irradiated nC60 (7DUV nC60) was found to decrease at 300 mM NaCl and remain less than unity at higher ionic strength (Figure 14), deviating from the DLVO calculation, which suggests that the energy barrier disappears at 250 mM NaCl. The discrepancy indicates surface oxygen-containing functional groups can stabilize particles through non-DLVO forces. Lower than unity α values also were observed with UVA-irradiated nC60 in our previous study on nC60 aggregation. It can be attributed to structural forces, presumably hydration forces, which can stabilize particles at high ionic strength.
The attachment efficiency of pristine nC60 measured by packed-bed column experiments and QCM-D agree with each other very well (Figure 15), suggesting the QCM-D is a good alternative for the packed-bed column systems to study nC60 deposition behavior. QCM-D uses well defined collector surfaces, which avoids the heterogeneity of the porous media that often leads to discrepancies in deposition/retention data reported by different works. The measurement is much faster and more sensitive than column experiments and provides direct measurement of deposition kinetics.
(2) Humic acid adsorption hinders nC60 deposition.
As a major component of NOM, humic acid is known to adsorb on colloidal particles and stabilize them via electrosteric effects. In our study, dissolved humic acid was found to significantly reduce the deposition of nC60 mainly due to steric hindrance imposed by adsorbed humic acid molecules on nC60 surface; the reduction in deposition strongly depended on the type and adsorbed amount of humic acid (Figure 16). Adsorption experiments showed that Elliot humic acid (EHA) and Suwannee River humic acid (SRHA) had similar adsorption on pristine nC60 at 1 mg/L (Kd = 1648 and 1722 (mg/kg)/(mg/L), respectively). However, EHA, a soil humic acid, was much more efficient in stabilizing pristine nC60 than SRHA, an aquatic humic acid, likely due to its higher molecular weight, which results in stronger steric repulsion. In the presence of humic acid, nC60 attachment efficiency exhibited a complex, non-monotonous behavior as shown in Figure 16. It was attributed to the contradicting effects of ionic strength on humic acid adsorption by nC60 and nC60 surface potential. Increasing ionic strength screens the surface charge of nC60 and the silica surface, leading to lower electrostatic repulsion between them and hence increased deposition. Meanwhile, it also facilitates the adsorption of humic acid by nC60, which enhances the steric repulsion. The complex deposition behavior of nC60 in the presence of humic acid clearly demonstrates that quantification of NOM adsorption is crucial to accurate prediction of nC60 deposition. Both SRHA and EHA had negligible adsorption on the 7DUV nC60 surface at 1 mg/L due to increased nC60 surface hydrophilicity and negative charge after irradiation. Therefore, dissolved humic acid is expected to affect the deposition of UVA-irradiated nC60 to a much lesser extent.
(3) Surface-immobilized humic acid can enhance or hinder nC60 deposition depending on the complex interplay of attractive and repulsive mechanisms.
Suspended particles and sediments in surface water as well as the porous media in the vadose zone of the ground water aquifer could contain significant amount of NOM. This was simulated using humic acid immobilized on silica coated QCM crystal or sand collector surface. The deposition of pristine nC60 on the EHA-coated silica surface was lower than that on the bare silica surface, whereas the deposition of pristine nC60 on the SRHA-coated silica surface was similar to that on the bare silica surface (Figure 17a). DLVO calculation shows notably lower energy barrier on the EHA-coated silica surface, in contrast to the observed lower nC60 deposition rate. The discrepancy suggests that the reduced deposition is attributed to steric hindrance by the immobilized humic acid.
The deposition of 7DUV nC60 significantly increased on both EHA and SRHA-coated silica surface as shown in Figure 17b. We hypothesize that three mechanisms are responsible for the enhanced deposition: the diminished steric hindrance, the reduced collector surface potential, and the potential hydrogen bonding. The presence of surface-immobilized humic acid significantly reduced the surface potential of the silica surface, which partially offsets the effect of steric hindrance forces and gradually became the dominant effect with increasing ionic strength. The deposition of the 7DUV nC60 on humic acid coated silica surfaces is relatively insensitive to ionic strength, suggesting the reduced electrostatic repulsion is not the sole mechanism for the enhanced deposition. Another likely mechanism involved is hydrogen bonding between the oxygen-containing groups on the 7DUV nC60 surface and the oxygen and nitrogen-containing groups on surface-immobilized humic acid.
Impact of sunlight on the stability of carboxylated multi-walled carbon nanotubes
The objective of this part of study is to demonstrate the photochemical transformation of CNTs upon sunlight irradiation and determine its impact on physicochemical properties of CNTs as well as the subsequent influence on CNT aggregation and deposition. The photochemical transformation of carboxylated multiwall carbon nanotubes (COOH-MWCNT) in water was studied under UVA irradiation. The findings presented here were the initial steps toward understanding the phototransformation process of COOH-MWCNTs. A later study with more detailed information on mechanisms of the phototransformation process is presented in Task 1. Time-resolved DLS measurement and QCM-D were used to characterize the aggregation and deposition kinetics of COOH-MWCNTs before and after irradiation. Our results showed decreases in the surface oxygen content of the COOH-MWCNTs after UV irradiation. Furthermore, such photochemical transformation was found to have a significant effect on the aggregation and deposition behaviors of the COOH-MWCNTs. This work was published in Carbon (Hwang YS, Qu X, Li Q. The role of photochemical transformations in the aggregation and deposition of carboxylated multiwall carbon nanotubes suspended in water. Carbon 2013;55:81-89).
(1) COOH-MWCNTs generate 1O2 upon UVA irradiation
COOH-MWCNTs were found to generate 1O2 upon UVA irradiation, as reflected by the degradation of FFA, which followed pseudo-first-order reaction kinetics. Negligible FFA degradation was observed in the control experiments performed in the dark or in the absence of dissolved oxygen, suggesting the indispensable role of UVA light and dissolved oxygen in the generation of 1O2. The 1O2 production rate increased with increasing pH.
(2) Characterization of COOH-MWCNTs after UVA irradiation
The hydrodynamic diameter of the COOH-MWCNTs dispersed in deionized water (pH 6.0 ± 0.2) was 220 ± 10 nm, and remained unchanged during the 7 day UVA irradiation. TEM analyses also did not show noticeable changes in the physical properties (morphology and size) of the nanotubes after UVA irradiation, suggesting no physical damage of the nanotubes.
XPS analysis, however, revealed notable changes in surface chemistry of the COOH-MWCNT after UVA irradiation. The total atomic concentration of oxygen decreased after UVA irradiation, suggesting the loss of oxygen functional groups (e.g., -COOH and OH) from the surface of the nanotubes likely due to the attack by ROS generated under UVA irradiation. The percentage of di-oxygenated carbon (mostly C=O and O-C-O) decreased significantly after UVA irradiation, while that of underivatized carbon increased (Table 1). Because the initial COOH-MWCNTs are highly carboxylated, most of the lost di-oxygenated carbon is expected to be carboxyl functional groups.
(3) Impact of sunlight on the aggregation and deposition kinetics of COOH-MWCNTs
The aggregation behaviors of both the initial and irradiated COOH-MWCNTs can generally be described by the DLVO theory, in agreement with findings from previous studies. Electrostatic repulsion between COOH-MWCNT particles is the dominant factor controlling particle aggregation. The initial COOH-MWCNTs were stable at low NaCl concentrations, with a CCC of 226 mM (Figure 18a). After 7 days of UVA irradiation, however, the stability of the nanoparticles greatly decreased, with a CCC of 52 mM NaCl. The decreased stability is attributed to the loss of oxygen-containing functional groups on the surface of the irradiated COOH-MWCNTs as suggested by the XPS analysis, particularly carboxyl functional groups.
Over the entire range of NaCl, the irradiated COOH-MWCNTs were notably less negatively charged than the initial COOH-MWCNTs (Figure 18b), consistent with the decreased stability of COOH-MWCNTs after UVA irradiation. Considering that other oxygen-containing functional groups are unlikely to dissociate at pH 6, the dissociated carboxyl groups are expected to be the main contributor to the negative surface charge of both the initial and the UV irradiated MWCNTs.
Interestingly, in CaCl2 solutions, the CCC values of CaCl2 for the initial and irradiated COOH-MWCNTs were similar (Figure 19a). The EPM measurement (Figure 19b) also revealed that the initial and UV irradiated COOH-MWCNTs had similar negative surface charge over the entire range of CaCl2 concentrations tested (0.3 to 10 mM) despite the difference in surface oxygen content. These results suggest that Ca2+ was more effective in neutralizing negative charges on the initial COOH-MWCNT surface than on the 7-day irradiated COOH-MWCNT. The behavior of the initial COOH-MWCNTs deviated from the Schulze-Hardy Rule, with a CCCCaCl2/CCCNaCl ratio of 2-8.03. Because the Schulze-Hardy Rule is only valid for inert electrolytes, i.e., electrolytes that do not specifically interact with the particle surface, this difference indicates the loss of surface carboxyl groups during UV irradiation that can specifically complex with Ca2+.
Deposition rates of COOH-MWCNTs depend on both the mass transfer to the surface and the deposition attachment efficiency. The deposition rate of 7DUV COOH-MWCNTs was found to be three times higher than initial COOH-MWCNTs in 10 mM NaCl solution. Because the UVA irradiation did not change the initial particle size of COOH-MWCNTs, the mass transfer rates of initial and irradiated COOH-MWCNTs were considered to be the same at low ionic strength (< 20 mM) where aggregation did not occur. However, the surface potential of COOH-MWCNTs decreased after irradiation (Figure 18b), resulting in lower energy barrier. Thus, the higher deposition rate of irradiated COOH-MWCNTs at 10 mM NaCl can be attributed to their lower surface potential, which leads to higher attachment efficiency.
The change in surface chemistry and consequently the colloidal stability of COOH-MWCNTs upon UVA irradiation indicates the importance of sunlight on the fate and transport of carbon nanotubes in natural aquatic systems. The photochemical transformation of the carbonaceous nanoparticle surface is not yet well understood. In our previous study, oxygen-containing functional groups were introduced onto underivatized fullerene nanoparticle surface after UVA irradiation, which contradicts the findings in this work. We speculate that the photochemical transformation depends on the original oxidation state of the carbon surface. Because the COOH-MWCNTs used in this study are highly oxidized, the ROS generated preferably attack the COOH groups, leading to oxidative decarboxylation.
Task 3: The impact of photochemical transformation on the toxicity of CNTs
The toxicity of CNTs is closely related to their physicochemical properties. As the surface chemistry of CNTs changes in the environment, their toxicity may change accordingly. However, little has been done to address this issue. In this study, the effects of sunlight on MWCNT ecotoxicity were investigated using Escherichia coli. Our earlier studies suggest the COOH-MWCNTs will gradually lose their carboxyl group under sunlight.
Recent studies suggested that CNTs are toxic to bacteria, with SWCNT exhibiting the stronger antimicrobial activity than MWCNT11. Previous investigations revealed that physical membrane perturbation is one of the major toxicity mechanisms12-14. As a result, direct contact between CNTs and bacteria is required for inactivation of the bacteria. A later study identified oxidative stress as another toxicity mechanism as suggested by the high level of oxidative stress-related gene expression13. However, another study concluded that oxidative stress does not play an important role in the antimicrobial activity of SWCNTs 14. Although the antimicrobial mechanism of CNTs is still not well understood, studies have shown that the physicochemical properties of CNTs have great influence on their cytotoxicity13-16. SWCNT exhibits much higher toxicity than MWCNT due to its smaller diameter13. For MWCNT, a correlation between bacterial cytotoxicity and physicochemical properties that enhance MWCNT-cell contact opportunities was found15. It also was reported that CNTs with higher toxicity are uncapped, debundled, short, and dispersed in solution. The effect of length on the CNT toxicity is still controversial. Two studies reported that short MWCNT and SWCNT possessed higher toxicity14, 15, while another study suggested that longer SWCNT was more toxic17. Metallic SWCNTs have higher toxicity than the semiconducting SWCNTs16.
Based on these studies, the overall toxicity of CNT is decided by a two-step process: (1) direct contact between CNT and bacteria (bioavailability) and (2) disruption of a specific microbial process/structure via disturbing/oxidizing a vital cellular structure/component (intrinsic toxicity).
(1) Pristine MWCNT has higher overall toxicity than COOH-MWCNT
Two sets of toxicity experiments, the solution assay and the filter assay, were used to examine the overall toxicity and intrinsic toxicity of MWCNTs respectively. The solution toxicity assay was carried out by mixing the E.coli and the CNT suspensions for 3 h and quantifying viable cells by plate-counting. The viability of the cells reflects the overall toxicity of the MWCNTs, which depends on both the bioavailability and the intrinsic toxicity. The filter toxicity assay was carried out by filtering E. coli on a MWCNT layer deposited on a membrane to ensure direct contact (bioavailability equates 100%). After filtration, the membrane coupon was immersed in saline solution (0.9% NaCl) and incubated at 37 degrees for 3 h. Then the viability of the bacteria was quantified by fluorescence staining.
The loss of cell viability in solution toxicity assay was shown in Figure 20; MWCNTs exhibited much stronger toxicity than COOH-MWCNT. MWCNTs are expected to possess stronger affinity to bacteria due to their more hydrophobic and less negatively charged surface as compared to COOH-MWCNT. MWCNTs also have higher oxidative capacity to disturb/oxidize certain cellular structure/components, which will be discussed in the following oxidative stress quantification tests, leading to higher intrinsic toxicity. As a result, the higher overall toxicity of MWCNT can be attributed to its higher bioavailability and intrinsic toxicity.
(2) Pristine MWCNT has higher intrinsic toxicity than COOH-MWCNT
The filter assay forces direct contact between bacteria and CNTs, thus the toxicity calculated from filter assay represents the intrinsic toxicity of the CNTs. Results of the live/dead assay showed that the loss in bacteria viability on the MWCNT surface was significantly higher than that on the COOH-MWCNT surface (Figure 21), indicating the MWCNT with little surface functional groups exhibits higher intrinsic toxicity to bacteria than the highly oxidized COOH-MWCNT. Quantitative inspection of the fluorescence images showed that 14.1 ± 7.1% were killed on the COOH-MWCNT coated filter, which is slightly higher than that of the control bacteria solution (without being filtered through the CNT layer), 8 ± 2.7%. The result demonstrates that the COOH-MWCNT has mild toxicity towards bacteria.
(3) UVA irradiation enhances the oxidative capacity of COOH-MWCNT
Oxidative stress is one of the proposed mechanisms for CNT toxicity. In this study, the oxidative stress of different CNT samples was quantified by their ability to oxidize glutathione, a common antioxidant in bacteria. 17.6 mM H2O2 was used as a positive control, which oxidized all the glutathione. MWCNTs showed strong ability to oxidize glutathione as shown in Figure 22. After 3 h exposure, all the glutathione was oxidized by the MWCNT, while only 16.8 ± 0.3% of the glutathione was oxidized by the COOH-MWCNT. The stronger oxidative capacity of MWCNTs correlates with its higher toxicity as measured by both solution and filter assays, indicating the toxicity of MWCNTs is related to their oxidizing capability. After 7 days of UVA irradiation, the oxidative capacity of COOH-MWCNTs increased.
Environmental Implications
This study contributes to the mechanistic understanding of the photochemical transformation of carbon nanomaterials (CNs) including fullerene nanoparticles and carbon nanotubes as well as the consequent impact on their fate, transport and toxicity. It also contributes to the knowledge on the role of NOM in the aggregation and deposition behaviors of fullerene nanoparticles. These findings provide fundamental understanding of the reactivity and mobility of CNs in natural aquatic systems, enabling better risk assessment and management. It addressed several key questions critical for predicting CN environmental risks as discussed below.
Are CNs reactive in natural aquatic systems? How and through which routes does sunlight exposure change CN structures?
This investigation reveals both fullerenes and CNTs undergo photochemical transformation at significant rates under UVA irradiation at intensity similar to that in sunlight. Thus, they will be reactive in natural aquatic systems and their physicochemical properties can be dynamic. This finding has significant implications for risk assessment because the environmental behaviors of CNs are sensitive to their physicochemical properties. The phototransformation processes of nC60 and COOH-MWCNTs are mainly mediated by photo-activated self-generated ROS, leading to oxygenation or deoxygenation depending on their initial surface oxidation state. From the application perspective, the dynamic nature of CNs when exposed to UVA light and ROS will limit their potential applications in photocatalytic processes such as advanced oxidation for water and wastewater treatment.
How are the mobility and toxicity of CNs related to their properties (i.e., structure-activity relationships)?
Establishing structure-activity relationship is a major step towards modeling nanoparticle fate and transport in the environment. In this work, the impact of several physicochemical properties of CNs on their environmental behaviors was investigated. The stability of CNs against aggregation and deposition is largely determined by their surface oxygen-containing functional groups in simple electrolyte solutions without NOM. The mobility of nC60 is a function of the surface oxygen concentration. On the other hand, the mobility of COOH-MWCNTs correlates well with the abundance of surface carboxyl groups. Phototransformation influences the transport of CNs through either introducing or removing surface oxygen-containing functional groups. MWCNT is more toxic to bacteria, Escherichia coli, than the highly oxidized COOH-MWCNT due to its higher oxidative capacity and bioavailability. UVA irradiation enhanced the oxidative capacity of COOH-MWCNT.
Are traditional theories of colloid science applicable to CN nanoparticle transport studies?
The size of CN nanoparticles studied falls in the size range of colloids (1 to 1000 nm), the interactions of which are usually described by the DLVO theory. In this study, the aggregation and deposition kinetics of nC60 and CNTs with different surface properties were quantified using DLS, QCM-D and packed-columns in solutions varying in ionic strength, ionic composition and humic acid concentration. In most conditions without NOM, the results qualitatively agree with the prediction of the DLVO theory. Non-DLVO forces such as steric hindrance, hydrophobic effects, ionic bridging and hydration forces also were found to contribute to the aggregation and deposition behaviors in certain conditions.
The role of NOM in transport of CNs in the environment.
NOM is one of the most important controlling factors in the environmental fate and transport of CNs. Once adsorbed on the surface of nC60, humic acid molecules can greatly enhance CN stability through steric hindrance. The stabilization effect depends on the amount and properties of humic acid adsorbed. Sunlight-induced surface oxidation of nC60 reduces humic acid sorption and hence its role in nC60 stability and transport. Soil humic acid is found to be more efficient than aquatic humic acid in stabilizing nC60 due to its higher molecular weight. Humic acid in the immobilized phase can either enhance or hinder nC60 deposition, depending on the interplay of attractive and repulsive forces.
Conclusions:
Our investigation reveals that the physicochemical properties of aqueous fullerene nanoparticles and carbon nanotubes can be dynamic in natural aquatic systems due to their interactions with sunlight and NOM; such interactions significantly alter their environmental fate, transport and toxicity. Thus, a better quantitative understanding of environment-induced changes in CN structure is crucial to assessing their potential environmental risks.
Physicochemical properties of nC60 and CNTs can be dynamic in natural aquatic systems.
- Photochemical transformation of nC60 and carboxylated multiwalled CNTs (COOH-MWCNTs) occurs at significant rates under UVA irradiation at intensity similar to that in sunlight.
- The transformation is caused by photogenerated reactive oxygen species (ROS) and leads to oxygenation or decarboxylation of the nanocarbon surface depending on its initial surface oxidation state.
- Surface oxidation of nC60 occurs under UVA irradiation in the presence of oxygen, resulting in intermediate products similar to fullerol. The photochemical oxidation reaction initially is limited only to the surface of the nC60 nanoparticles; while long-term exposure can potentially lead to its degradation and mineralization.
- UVA irradiation reduces the surface oxygen concentration of COOH-MWCNTs mainly through ·OH-mediated reactions. ·OH initially reacts with the carboxylated carbonaceous fragments on CNT surface, resulting in their destruction or exfoliation. Further reaction between ·OH and the graphitic sidewall leads to formation of defects including functional groups and holes.
- The presence of NOM hinders the phototransformation of nC60 due to light screening and ROS scavenging.
Changes in surface chemistry due to interactions with sunlight and NOM significantly alter the environmental fate and transport of CNs.
- The environmental transport of CNs is strongly affected by their surface chemistry, the concentration and properties of NOM, and the ionic concentration and composition of water.
- In electrolyte solutions without the presence of NOM, mobility of CNs is mainly decided by their surface chemistry, especially the oxygen-containing functional groups.
- Surface oxidation induced by UVA irradiation hinders nC60 aggregation and deposition on silica surface in NaCl solutions because of the increased negative surface charge and hydrophilicity; it enhances nC60 aggregation in CaCl2 solutions because of specific interactions of Ca2+ with the negatively charged oxygen-containing functional groups on UV-irradiated nC60 surface.
- The colloidal stability of COOH-MWCNTs correlated well with the abundance of surface carboxyl groups. COOH-MWCNTs with more surface carboxyl groups have higher surface charge and consequently higher stability.
- UVA irradiation reduces the surface carboxyl concentration of COOH-MWCNTs, leading to increased aggregation and deposition on a silica surface in NaCl solutions. However, the surface potential and colloidal stability of COOH-MWCNTs remain unchanged in CaCl2 solutions after UVA irradiation.
- Dissolved humic acid, once adsorbed onto nC60 surface, hinders its deposition mainly through steric hindrance forces. The extent of this effect depends on the properties and amount of humic acid adsorbed, which is a function of ionic strength and humic acid concentration. Humic acid has limited adsorption on UVA-irradiated nC60 and is expected to play a less important role in its stability.
- Humic acid immobilized onto the silica surface can potentially enhance or hinder nC60 deposition, depending on the complex interplay of DLVO and non-DLVO interactions such as electrostatic interaction, steric hindrance, and hydrogen bonding as well as humic acid molecular conformation.
Environment-induced transformation changes the toxicity of CNTs.
- MWCNTs are more toxic to the bacterium Escherichia coli than COOH-MWCNTs because of their higher bioavailability and oxidative capacity.
- UVA irradiation enhances the oxidative capacity of COOH-MWCNTs.
References:
- Hou, W. C.; Jafvert, C. T., Photochemistry of Aqueous C(60) Clusters: Evidence of (1)O(2) Formation and its Role in Mediating C(60) Phototransformation. Environ. Sci. Technol. 2009, 43, (14), 5257-5262.
- Chen, C. Y.; Jafvert, C. T., The role of surface functionalization in the solar light-induced production of reactive oxygen species by single-walled carbon nanotubes in water. Carbon 2011, 49, (15), 5099-5106.
- Latch, D. E.; McNeill, K., Microheterogeneity of singlet oxygen distributions in irradiated humic acid solutions. Science 2006, 311, (5768), 1743-1747.
- Hassett, J. P., Chemistry - Dissolved natural organic matter as a microreactor. Science 2006, 311, (5768), 1723-1724.
- Dukovic, G.; White, B. E.; Zhou, Z. Y.; Wang, F.; Jockusch, S.; Steigerwald, M. L.; Heinz, T. F.; Friesner, R. A.; Turro, N. J.; Brus, L. E., Reversible surface oxidation and efficient luminescence quenching in semiconductor single-wall carbon nanotubes. J. Am. Chem. Soc. 2004, 126, (46), 15269-15276.
- Chan, S. P.; Chen, G.; Gong, X. G.; Liu, Z. F., Oxidation of carbon nanotubes by singlet O-2. Phys. Rev. Lett. 2003, 90, (8).
- Alvarez, N. T.; Kittrell, C.; Schmidt, H. K.; Hauge, R. H.; Engel, P. S.; Tour, J. M., Selective Photochemical Functionalization of Surfactant-Dispersed Single Wall Carbon Nanotubes in Water. J. Am. Chem. Soc. 2008, 130, (43), 14227-14233.
- Zhao, Y.; Allen, B. L.; Star, A., Enzymatic Degradation of Multiwalled Carbon Nanotubes. J. Phys. Chem. A 2011, 115, (34), 9536-9544.
- Salzmann, C. G.; Llewellyn, S. A.; Tobias, G.; Ward, M. A. H.; Huh, Y.; Green, M. L. H., The role of carboxylated carbonaceous fragments in the functionalization and spectroscopy of a single-walled carbon-nanotube material. Adv. Mater. 2007, 19, (6), 883-+.
- Reber, J. F.; Meier, K., PHOTOCHEMICAL PRODUCTION OF HYDROGEN WITH ZINC-SULFIDE SUSPENSIONS. J. Phys. Chem. 1984, 88, (24), 5903-5913.
- Kang, S.; Mauter, M. S.; Elimelech, M., Microbial Cytotoxicity of Carbon-Based Nanomaterials: Implications for River Water and Wastewater Effluent. Environ. Sci. Technol. 2009, 43, (7), 2648-2653.
- Kang, S.; Pinault, M.; Pfefferle, L. D.; Elimelech, M., Single-walled carbon nanotubes exhibit strong antimicrobial activity. Langmuir 2007, 23, (17), 8670-8673.
- Kang, S.; Herzberg, M.; Rodrigues, D. F.; Elimelech, M., Antibacterial effects of carbon nanotubes: Size does matter. Langmuir 2008, 24, (13), 6409-6413.
- Liu, S. B.; Wei, L.; Hao, L.; Fang, N.; Chang, M. W.; Xu, R.; Yang, Y. H.; Chen, Y., Sharper and Faster "Nano Darts" Kill More Bacteria: A Study of Antibacterial Activity of Individually Dispersed Pristine Single-Walled Carbon Nanotube. ACS Nano 2009, 3, (12), 3891-3902.
- Kang, S.; Mauter, M. S.; Elimelech, M., Physicochemical determinants of multiwalled carbon nanotube bacterial cytotoxicity. Environ. Sci. Technol. 2008, 42, (19), 7528-7534.
- Vecitis, C. D.; Zodrow, K. R.; Kang, S.; Elimelech, M., Electronic-Structure-Dependent Bacterial Cytotoxicity of Single-Walled Carbon Nanotubes. ACS Nano 2010, 4, (9), 5471-5479.
- Yang, C. N.; Mamouni, J.; Tang, Y. A.; Yang, L. J., Antimicrobial Activity of Single-Walled Carbon Nanotubes: Length Effect. Langmuir 2010, 26, (20), 16013-16019.
Journal Articles on this Report : 5 Displayed | Download in RIS Format
Other project views: | All 13 publications | 5 publications in selected types | All 5 journal articles |
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Hwang YS, Qu X, Li Q. The role of photochemical transformations in the aggregation and deposition of carboxylated multiwall carbon nanotubes suspended in water. Carbon 2013;55:81-89. |
R834093 (2009) R834093 (2010) R834093 (Final) |
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Qu X, Hwang YS, Alvarez PJJ, Bouchard D, Li Q. UV irradiation and humic acid mediate aggregation of aqueous fullerene (nC60) nanoparticles. Environmental Science & Technology 2010;44(20):7821-7826. |
R834093 (2009) R834093 (Final) |
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Qu X, Alvarez PJJ, Li Q. Impact of sunlight and humic acid on the deposition kinetics of aqueous fullerene nanoparticles (nC60). Environmental Science & Technology 2012;46(24):13455-13462. |
R834093 (2009) R834093 (2010) R834093 (Final) |
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Qu X, Alvarez PJJ, Li Q. Applications of nanotechnology in water and wastewater treatment. Water Research 2013;47(12):3931-3946. |
R834093 (Final) |
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Hwang YS, Li Q. Characterizing photochemical transformation of aqueous nC60 under environmentally relevant conditions. Environmental Science & Technology 2010;44(8):3008-3013. |
R834093 (Final) |
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Supplemental Keywords:
fullerene nanoparticles, carbon nanotubes, nanomaterials, natural organic matter, UVA, sunlight, irradiation, transport, fate, bioavailability, toxicity, photochemical transformation, aggregation, deposition, stability, ROS, reactive oxygen species, MWCNTs, SWCNTs, Health, Scientific Discipline, Water, Environmental Chemistry, Health Risk Assessment, Risk Assessments, Biochemistry, Drinking Water, Engineering, Chemistry, & Physics, fate and transport, human health effects, carbon fullerene, epidemelogy, nanotechnology, other - risk assessment, particle exposure, community water system, ambient particle health effects, human exposure, engineered nanomaterials, toxicity, nanomaterials, water quality, cellular responses, drinking water contaminants, biochemical research, human health risk, drinking water systemRelevant Websites:
Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.