Abstract

The initial aggregation kinetic of oxidized SWCNTs (O-SWCNTs) under different solution chemistries was investigated by DLS, to quantify their stability. The O-SWCNTs showed good stability in aqueous solution with CFC of 0.16 M for NaCl. Results also indicated that the stability of O-SWCNTs were not sensitive to pH over therange from 3 to 8. The presence of HA could enhance the stability of O-SWCNTs dependent on the type of electrolyte present. For example, the CFC of NaCl increased from 0.16 in the absence of HA to 0.19, 0.23 and 0.25 M when the concentration of HA was 1, 5, and 20 ppm, respectively. However, the effect of HA on the aggregation of O-SWCNTs was negligible in the presence of CaCl₂ and AlCl₃. These findings indicated that the stability of engineered nanoparticles could be altered through the treatment process as well as after release into environment. The results also indicated that nanoparticles are likely to be effectively removed through the conventional coagulation and aggregation processes by the existing wastewater treatment facilities.

The effects of surface treatments on multi-walled carbon nanotubes (MWCNTs) adverse effects towards fresh water organism were assessed using C. dubia as in vivo organisms. Results from the present study indicated that the surface treatments methods had pronounced effects on the stability of carbon nanomaterials, and corresponding toxicity towards C. dubia. Ozone oxidation process significantly reduced toxic responses in all three toxic assays. This was attributed to the physical and chemical properties alternation through the oxidation process, such as the zeta potential, reduced the particle size and the aqueous stability. The changes of surface physical and chemical properties altered the corresponding negative effects on living organism. The O-MWCNTs exhibited lower toxic effects in 24 h acute test compared to ultrasound treated MWCNTs (US-MWCNTs) and Untreated-MWCNTs. The LC50 value of O-MWCNTs was one order of magnitude less than those of US-MWCNTs and Untreated-MWCNTs. Similar results were observed in the 3-brood reproduction test and sub-lethal growth test, i.e. to cause similar toxic response, the O-MWCNTs concentration was almost one order of magnitude less than that of US-MWCNTs. It was found that the US-MWCNTs had longer retention time inside the digestion tract compared to O-MWNCTs, indicating stronger interaction with the biological tissues. Results also suggested that the initial tube diameter had negligible influence on the toxicity.

The toxicity of two different nanomaterials were studied using C. dubia as in vivo organisms aimed at revealing the mechanisms causing adverse effects. Though Degussa TiO₂ P25 is highly photoactive whereas Al₂O₃ is considered as environmentally benign materials, C. dubia responsed to these two nanomaterials similarly in both reproduction and growth assay. Both materials inhibited the growth of C. dubia at 100 ppm and reduced the reproduction capability by 50% at concentration of 40 ppm. This implied that the generation of ROS might not be the key mechanism leading to the adverse effects observed for the invertebrate.

The exposure routes of nanomaterials were investigated using the resin embedding approach. It was found that C. dubia were exposed to nanomaterials by direct contact, inner body translocation and ingestion. Large amount of nanomaterials were adsorbed and ingested by C. dubia even within a 2 h exposure window. Other environmental stressors that could indirectly influence the fitness of C. dubia were also investigated. The presence of nanomaterials brought about particles aggregation and the depletion of algal cells, which were the major food source at the controlled testing environments. The algal cell density was reduced by nearly 3 orders of magnitude in the presence of 200 ppm of Al₂O₃ . Thus, the presence of nanomaterials could interrupt the C. dubia feeding process and increase the maintenance energy expenditure as well.

Result of dynamic energy budget (DEB) model analysis indicated that the energy assimilation and expenditure of C. dubia were dramatically disrupted by the presence of both nanomaterials. A negative correlation between the energy assimilated and the nanomaterials concentrations was observed. The energy assimilated by C. dubia was reduced from 11 to near 0 μg C animal^-1 day^-1 when exposed to 200 ppm of TiO₂ or Al₂O₃. The energy consumed for the maintenance increased with the increase of nanomaterials concentrations. The modeling results suggested that the energy allocation pattern was a function of nanomaterials concentrations. The energy allocated for growth and reproduction was significantly less in the presence of nanomaterials compared to the control condition. This energy disruption should be at least partially responsible for the adverse effects observed. The same mechanism was also observed in the CNTs toxicity studies. (Abstract shortened by UMI.)