Houston, TX 77005
10:15 a.m. Wednesday, March 27, 2013
On Campus | Alumni
Carbon based nanomaterials (CNMs) possess remarkable mechanical, electrical, and optical properties, which enabled a wide range of promising applications. The fast growing production and potential widespread use in consumer products raise concerns regarding their potential risks to human health and ecosystems. The present study investigated the role of photochemical transformation and natural organic matter (NOM) in the fate, transport, and toxicity of CNMs including fullerenes and carbon nanotubes (CNTs) in natural aquatic systems, providing fundamental information for risk assessment and management. Photochemical transformation of aqueous fullerene nanoparticles (nC60) and CNTs occurs at significant rates under UVA irradiation at intensity similar to that in sunlight. The transformation processes are mediated by self-generated ROS, resulting in changes of surface structure depending on the initial surface oxidation state of CNMs. The nC60 surface will be oxygenated under UVA light. Contrarily, UVA irradiation reduces the surface oxygen concentration of carboxylated multi-walled carbon nanotubes (COOH-MWNTs) mainly through removing carboxyl groups. Self-generated •OH initially reacts with the carboxylated carbonaceous fragments on COOH-MWNT surface, resulting in their degradation or exfoliation. Further reaction between •OH and the graphitic sidewall leads to formation of defects including functional groups and holes. The environmental transport of CNMs is greatly affected by their surface chemistry, the concentration and properties of co-existing NOM, and the concentration and species of electrolytes. In electrolyte solutions without NOM, the mobility of CNMs is largely decided by their surface chemistry, primarily the oxygen-containing functional groups. Traditional DLVO theory serves as a good starting point for CNM mobility assessment. In NaCl solutions, surface oxygenation induced by UVA irradiation remarkably hindered nC60 aggregation and deposition on a silica surface due to the increased negative surface charge and hydrophilicity. In contrast, UVA irradiation enhanced nC60 aggregation in CaCl2 solutions due to specific interactions of Ca2+ with the negatively charged functional groups on irradiated nC60 surface. The mobility of COOH-MWNTs in NaCl solutions correlated well with the abundance of surface carboxyl groups. UVA irradiation reduces the surface carboxyl concentration of COOH-MWNTs, leading to increased aggregation and deposition. However, the surface potential and colloidal stability of COOH-MWNTs remain stable in CaCl2 solutions after UVA irradiation, suggesting that Ca2+ was more effective in neutralizing negative charges on the initial COOH-MWNT surface than on the irradiated samples. The humic acid, as a surrogate of NOM, has great impact on the transport of CNMs. Once adsorbed on the surface of nC60, humic acid can significantly enhance its stability through steric hindrance. However the stabilization effect depends on the amount and properties of humic acid adsorbed. Humic acid can readily adsorb on pristine nC60 through hydrophobic forces; while it has little sorption on UVA-irradiated nC60 as oxygen-containing functional groups are introduced onto nC60 surface during irradiation, enhancing its hydrophilicity. As a result, the presence of humic acid has much lower influence on the mobility of UVA-irradiated nC60 than the pristine ones. However, by neutralizing surface charge of both UVA-irradiated nC60 and humic acid as well as forming intermolecular bridges, Ca2+ can facilitate humic acid adsorption on UVA-irradiated nC60, resulting in enhanced colloidal stability in the presence of humic acid. Soil humic acid (e.g., Elliott Soil humic acid) is more efficient than aquatic humic acid (e.g., Suwannee River humic acid) in stabilizing nC60 due to its higher molecular weight, which results in stronger steric repulsion. Humic acid immobilized onto the silica surface can potential 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. MWNTs are more toxicity to bacteria, Escherichia coli, than COOH-MWNTs due to their higher bioavailability and oxidative capacity. Surface oxidation induced by •OH reduced the toxicity of MWNT, while reactions with •OH have little effect on the COOH-MWNT toxicity. Antioxidants such as glutathione can effectively inhibit the antibacterial activity of MWNTs. The present investigation reveals that fullerene nanoparticles and CNTs are dynamic in nature aquatic systems. Changes of their surface chemistry/structure due to interactions with sunlight and NOM significantly alter their fate, transport and toxicity in the environment. Thus a better quantitative understanding of environment-induced changes in CNM structure is crucial to assessing their potential environmental risks.