Grantee Research Project Results

The project is progressing as scheduled. We have obtained expected results #1, 2 and 5 and most of #4 described in the proposal. Objective #1 has been achieved; tasks related to objective #2 are close to completion; tasks designed to achieve objective #3 have started.

 

In this report, we summarized the efforts and results to date about the impact of solar irradiation and humic acid on the aggregation and deposition behavior of nC₆₀ and carboxylated multi-walled carbon nanotubes.

 

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

 

Our study in the previous year revealed that UVA irradiation resulted in surface oxidation of nC₆₀, leading to changes in physicohemical properties of nC₆₀. In the current project year, we investigated the influence of such changes on the aggregation and deposition behavior of nC₆₀ in two ways: (1) direct impact on particle stability and (2) alter the interactions between nC₆₀ nanoparticles and other environment components like NOM, resulting in indirect impact on stability.

 

Aggregation of nC₆₀ suspensions in various solution conditions was characterized by monitoring the intensity weighted average particle size as a function chime via time-resolved dynamic light scattering (DLS) measurement. The initial aggregation rate was decided by linear regression of initial stage of the aggregation curve. The attachment efficiency α was calculated using the aggregation rate in the solution of interest normalized by the rate in the diffusion-limited regime. Electrophoretic mobility of nC₆₀ was measured by phase analysis light scattering.

 

 

 

(1) Aggregation Kinetics in NaCl solutions.

 

Figure 1 summarized the critical coagulation concentration (CCC) of nC₆₀ with and without 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 7-day period. It is attributed to the surface oxygen-containing functional groups introduced by UV irradiation. For both the pristine and UV irradiated samples, particle electrophoretic mobility became less negative with increasing NaCl concentration due to charge screening. The pristine nC₆₀ and nC₆₀ after 7 days irradiation (7DUV- nC₆₀) were similarly charged at NaCl concentrations below 10 mM, while at higher NaCl concentration 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 fact that electrophoretic mobility reflects the potential at the plane of shear, not the actual surface charge. Another potential explanation is that the surface oxygen functional groups increased nC₆₀ hydrophilicity and hindered hydrophobic interactions, as described by the extended DLVO theory. Humic acid readily adsorbs onto nC₆₀, altering its surface properties and potentially mediating colloidal aggregation. SRHA at 1 mg/L greatly stabilized the pristine nC₆₀, increasing the CCC from 84 to 331 mM NaCl. Negligible changes in electrophoretic mobility of nC₆₀ before and after contact with SRHA and nC₆₀ stability at NaCl concentrations up to 1.5 M NaCl in the presence of 10 mg/L SRHA suggest that steric hindrance rather than electrostatic repulsion is the main stabilizing mechanism. Humic acid has much less effect on the stability of 7DUV- nC₆₀, due to decreased adsorption of humic acid on the oxidized nC6o surface.

 

(2) Aggregation kinetics in CaCh solutions.

 

The CCCs of CaCh for the pristine and 7DUV- nC₆₀, 4.25 and 3.24 mM CaCh, respectively, were markedly lower than those of NaCl (Figure 1). Contrary to the observations in NaCl solutions, the 7DUV-nC₆₀ was less stable than the pristine nC6o in CaCh solutions. Comparison between the electrophoretic mobility of the pristine and the 7DUV-nC₆₀ reveals that the 7DUV­nC₆₀ was less negatively charged than the pristine nC₆₀ over the CaCh concentration range tested, suggesting that Ca²⁺ was more effective in neutralizing negative charges on the oxidized nC₆₀ surface than on the pristine nC₆₀. Schulze-Hardy Rule was not applicable to 7DUV-nC₆₀ aggregation data, suggesting Ca²⁺ specifically interacts with the 7DUV-nC₆₀ surface, presumably with the oxygen containing functional groups. QCM-D experiments showed that Ca²⁺ facilitated SRHA sorption onto 7DUV-nC₆₀ surface. As a result, the presence of 1 mg/L SRHA increased the stability of both pristine nC₆₀ and 7DUV-nC₆₀ in CaCl₂.

 

 

3. Influence of humic acids on nC₆₀ deposition kinetics

 

 

The impact of dissolved and surface immobilized humic acids on the deposition of nC₆₀ was investigated using a combination of packed-bed column experiments and the quartz crystal microbalance with dissipation monitoring (QCM-D) technique under similar hydrodynamic conditions. The deposition attachment efficiencies of nC₆₀ on bare silica surface measured by QCM-D were consistent with that from column experiments. Dissolved SRHA was found to have two important effects on nC₆₀ deposition. First, it stabilized nC₆₀, resulting enhanced mass transfer towards the silica surface at high ionic strength. Second, it reduced the deposition attachment efficiency mainly through steric hindrance effects as shown in Figure 2. The interactions between nC₆₀ and surface immobilized humic acids were controlled by electrostatic and steric hindrance forces. Deposition of nC₆₀ was facilitated at low ionic strength, which was consistent with the reduced surface charge of silica surface after coating. While in 10 mM NaCl, surface immobilized humic acids hindered deposition kinetics through steric hindrance effects. At high ionic strength, the steric hindrance effects diminished as the humic acid structure became more compact. Dissolved humic acid was found to play a more important role in nC₆₀ deposition comparing to surface immobilized humic acids.

 

These findings suggest solar irradiation and NOM could play a critical role in the aggregation and deposition behavior of nC6o and consequently its environmental fate and transport.

 

2. Photochemical transformation of multiwalled carbon nanotubes (MWCNT) and its impact on MWCNT stability.

 

 

We also examined the photochemical transformation of oxidized multi-walled carbon nanotubes (OMWCNT) in water and the subsequent impact on their aggregation and deposition behaviors. The photochemical transformation of OMWCNT nanoparticles was investigated under UVA (300 - 400 nm) irradiation, the main component of UV light in solar irradiation. Time-resolved dynamic light scattering (DLS) measurement and quartz crystal microbalance with dissipation monitoring (QCM-D) were used to study the .initial aggregation and deposition kinetics. Characterization of the physicochemical properties of the OMWCNT before and after irradiation revealed notable decrease in surface oxygen and negative surface charge after UV irradiation, likely through decarboxylation of the nanotube surface. ¹O₂ was generated during irradiation and the production increased with increasing pH. The pristine OMWCNT has a CCC of 175 mM NaCl. After 7 days of UVA irradiation, however, the stability of the nanoparticles in NaCl decreased drastically, with a CCC of 52 mM NaCl. The aggregation rate increased with increasing UV irradiation time during the 7-day period. Such decrease in stability can be attributed to the decrease of oxygen-containing functional groups on the surface of the irradiated OMWCNTs as proved by XPS analysis. For both the pristine and UV irradiated samples, particle electrophoretic mobility became less negative with increasing NaCl concentration due to charge screening. However, the pristine OMWCNT was notably more negatively charged than the 7day irradiated OMWCNT, consistent with the higher colloidal stability. Cations of higher valence are more efficient in coagulation. As a result, in CaCl₂ solutions, the CCCs of the pristine and 7DUV-OMWCNT were l .l and 0.8 mM CaCl₂, respectively, which were markedly lower than those of NaCl. Contrary to the observations in NaCl solutions, the stability of OMWCNT before and after UV irradiation was similar. Comparison between the electrophoretic mobility of the pristine and the 7DUV-OMWCNT reveals that both were similarly negatively charged over the entire tested range of CaCh concentrations, suggesting that Ca²⁺ was more effective neutralizing negative charges on the pristine OMWCNT surface than on the 7DUV-OMWCNT.

 

Consistent with the decreased colloidal stability, the irradiated OMWCNTs had notably higher deposition rate than the pristine OMWCNTs in NaCl solutions. The surface potential of UV irradiated OMWCNT was lower than that of pristine OMWCNT in NaCl due to the loss of carboxyl functional groups during the irradiation. Thus UV irradiated OMWCNT experienced less electrostatic repulsion than pristine OMWCNT, which led to higher deposition rate. At higher ionic strength, significant aggregation of OMWCNT took place during the deposition process, which greatly hindered the convective-diffusive transport of OMWCNT towards silica crystal surface. Since the aggregation rates of pristine and UV irradiated OMWCNT were quite different at ionic strength less than their CCCs, the impact of aggregation on their deposition rates was also different. Pristine OMWCNT was much more stable than the UV irradiated sample. Aggregation had smaller impact on the deposition of pristine OMWCNT than the UV irradiated OMWCNT due to its slower aggregation rate and resulting higher diffusion coefficient. As a result, the deposition rate of pristine OMWCNT gradually reversed to higher than UV irradiated sample as ionic strength increased.