Grantee Research Project Results

The project is progressing as scheduled. We have obtained expected results #1, 2, 4, and 5 and part of #3 and 6 described in the application. Objectives #1 and 2 have been achieved; tasks related to objective #3 are well underway.

 

In this report, we summarize the results on the photochemical transformation of nC₆₀ and carboxylated multi-walled carbon nanotube (MWCNT-COOH) under natural conditions and its impact on the mobility of these carbon-based nanomaterials. Data regarding the cytotoxicity of carbon nanotubes also are presented in this report.

 

 

The impact of solar irradiation and humic acid, in either dissolved or surface-immobilized form, on the deposition kinetics of nC₆₀ was investigated in this project year and the major findings are summarized in Figure 1. Deposition kinetic of nC₆₀ in various solution conditions was studied using both conventional packed-bed column experiments and a quartz crystal microbalance with dissipation (QCM-D). The affinity between nC₆₀ and silica surface was quantified by the attachment efficiency α, which was calculated using the deposition rate in the solution of interest normalized by the rate in the diffusion-limited regime.

 

Figure 1. Impact of solar irradiation and humic acid on the deposition of 
nC₆₀ nanoparticles.

 

Solar irradiation hindered nC₆₀ deposition on silica surface. Figure 2 shows that UVA irradiation (the major UV component in sunlight) can significantly enhance the stability of nC₆₀ against deposition; consistent with our earlier finding that solar irradiation enhanced the colloidal stability (i.e., stability against aggregation) of nC₆₀ 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 light. The surface oxygen content of nC₆₀ increased with increasing UVA irradiation time from 8.8 ± 1.5 % at the pristine state to 20.2 ± 3.7 % after 7 days of irradiation. XPS analysis also 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. In addition, 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, suggesting the surface oxygen functional groups can stabilize particles through non-DLVO forces. The hydrophilic surfaces of 7DUV nC₆₀ and silica can give rise to the repulsive hydration forces, which stabilizes 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, suggesting the QCM-D is a good alternative for the packed-bed column systems. QCM-D can avoid the heterogeneity of the porous media which often leads to discrepancies in deposition/retention data reported by different works. It also is less time and labor intensive.

 

Figure 2. Attachment efficiencies of pristine, 20-hour UVA-irradiated (20hUV) adn 7-day UVA-irradiated (7DUV) nC60 as a function of NaCl concentration.

Figure 3. Attachment efficiencies of pristine nC60 onto silica surfacea as a function of NaCl concentratin in the presence of dissolve Suwanee River humic acid or Elliott Soil humic acid.

 

 

Dissolved humic acid, once adsorbed on nC₆₀  surface, hindered its deposition. Natural organic matter (NOM) is ubiquitous in both aquatic and soil environments. As a major component of NOM, humic acid is known to readily adsorb onto various colloidal particles and stabilize these particles via electrosteric effects. In our case, dissolved humic acid significantly reduced the deposition of nC₆₀ mainly due to steric hindrance; and the reduction was closely related to the type and adsorption amount of the humic acid (Figure 3). Elloit humic acid (EHA), a soil humic acid, was much more efficient in stabilizing pristine nC₆₀ than Suwannee River humic acid (SRHA), an aquatic humic acid, likely due to its higher hydrophobicity and molecular weight. The more hydrophobic EHA tends to adsorb more on nC₆₀ surface and its bulky structure also aids in the steric repulsion. In the presence of humic acid, the nC₆₀ attachment efficiency exhibited a complex, non-monotonous behavior: the α vs. NaCl concentration curves determined in the presence of humic acid have an “N” shape. It was attributed to the interplay between the conflicting effects of ionic strength on humic acid adsorption by nC₆₀ and nC₆₀ surface potential. Increasing ionic strength will further screen the surface charge of nC₆₀ and the silica surface, leading to lower electrostatic repulsion between them. Meanwhile, it also facilitates the adsorption of humic acid by nC₆₀, which enhances the steric repulsion. Thus, in the presence of humic acid, the overall influence of ionic strength on nC₆₀ deposition will be determined by the net effect of these two mechanisms.

 

Surface-immobilized humic acid can enhance or hinder nC₆₀ deposition depending on the interplay of attractive and repulsive mechanisms. Surface-immobilized humic acid refers to the humic acid coated on the substrate surface. It was used to simulate nC₆₀ deposition on organic­ rich soils. The macromolecules of surface-immobilized humic acid can stretch into the solution, resulting in repulsive steric hindrance. Meanwhile, surface-immobilized humic acid was found to reduce surface potential of the silica crystal, facilitating deposition. The former mechanism dominatess at low ionic strength, thus the deposition of pristine nC₆₀ on ERA coated silica surface was lower than that on bare silica surface (Figure 4a). While at high ionic strength (> 60 mM), the repulsive steric hindrance diminishes as the conformation of humic acid becomes more compact. The reduced electrostatic repulsion led to higher deposition of 7DUV nC₆₀ on EHA coated silica surface (Figure 4b). However, 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.

 

Figure 4. Attachment efficiencies of (a) pristine, (b) 7-day UVA-irradiated nC₆₀ onto bare and humic
coated silica surface as a function of NaCl concentration.

 

 

 

UVA irradiation reduces surface oxygen concentration of MWCNT-COOH. The surface oxygen concentration of MWCNT-COOH decreased after UVA irradiation as suggested by XPS measurement, likely due to loss of carboxyl functional groups. MWCNT-COOH generates ¹O₂ and HO· upon UVA irradiation, attack of surface functional groups or the graphic carbon, which could be a potential mechanism of the transformation. TEM inspection and Raman spectra data showed no noticeable changes in the physical properties of MWCNT-COOH before and after irradiation, although XPS analyses revealed that di-oxygenated carbon (C=O and O-C-O) was significantly reduced by UVA irradiation, suggesting carboxyl functional groups were removed from the nanotube surface.

 

UVA irradiation reduces MWCNT-COOH stability. The aggregation and deposition kinetics of MWCNT-COOH were investigated using time-resolved dynamic light scattering (DLS) measurement and quartz crystal microbalance with dissipation monitoring (QCM-D), respectively. UVA irradiation greatly reduced the colloidal stability of MWCNT-COOH in NaCl solutions. The critical coagulation concentration (CCC) of MWCNT-COOH reduced from 175 mM to 52 mM NaCl after 7 days irradiation. It was attributed to the reduced surface potential of MWCNT-COOH caused by a UVA-induced deoxygenation process. The UVA irradiated samples were notably less negatively charged than the pristine MWCNT-COOH in NaCl solutions, consistent with the lower colloidal stability. The CCCs of the pristine and 7DUV MWCNT-COOH were 1.1 and 0.8 mM CaCh, respectively, which were markedly lower than those of NaCl. Both the electrophoretic mobility and the stability of MWCNT-COOH in CaCh solutions remain almost unchanged after UVA irradiation, contradictory to its behavior in NaCl solutions. It suggests that Ca²⁺ was more effective in neutralizing negative charges on the pristine MWCNT-COOH than on the 7DUV­ MWCNT-COOH. Consistent with the decreased colloidal stability, the irradiated MWCNT-COOH had notably higher deposition on silica surface than the pristine sample at low ionic strength (< 20 mM NaCl) mainly due to reduced surface potential as shown in Figure 5. However, as the electrolyte concentration was further increased, the deposition drastically decreased for both samples due to the concurrent aggregation of carbon nanotubes during the deposition, which greatly reduced their mass transport towards silica surfaces. The impact of concurrent aggregation on the deposition process was different for pristine and UVA-irradiated MWCNT-COOH due to their different colloidal stability. Aggregation had smaller impact on pristine MWCNT-COOH deposition because it is less prone to aggregation than UVA-irradiated samples. As a result, the deposition rate of pristine MWCNT-COOH gradually became higher than that of 7DUV MWCNT-COOH at ionic strength higher than 20 mMNaCl.

 

 

Figure 5. Deposition of the pristine MWNT-COOH and 
7-days UVA-irradiated MWNT- COOH in NaCl Solutions

 

 

 

Our preliminary experiments suggested the surface functionality of CNTs plays an important role in their toxicity. Pristine MWCNT was found to be much more toxic to bacteria, Escherichia coli, than MWCNT-COOH (Figure 6). Solar irradiation and humic acid were able to alter the surface chemistry of MWCNT-COOH. Thus, they are expected to change the toxicity of MWCNT-COOH as well. We currently are working on quantifying the impact of photochemical transformation and humic acid on the toxicity of MWCNT-COOH.

 

Figure 6. Loss of E. coli viability in the presence of 50 mg/L
carbon nanotubes after 3 h exposure

 

Our current results suggest a crucial role of the photochemical transformation processes induced by solar irradiation in the fate, transport and toxicity of carbon-based nanomaterials. nC₆₀ can gradually gain oxygen-containing functional groups when exposed to sunlight in surface water, which enhances its mobility in natural aquatic systems. While oxidized carbon nanotubes will gradually lose their functionality in sunlight, resulting in lower stability, they may settle out from the water column and potentially accumulate in sediments.

 

We will continue to investigate the impact of sunlight and NOM on the toxicity of carbon nanotubes. We hypothesize that the toxicity of carbon nanotubes stems from their ability to induce oxidative stress in cells. Carbon nanotubes with different irradiation time will be subject to the toxicity and oxidation stress tests, which include lipid peroxidation assay and glutathione oxidation assay. Also, the role of humic acid will be examined by adding predetermined amount of humic acid into the samples for toxicity and oxidation stress measurements.