Impact of Sunlight and Natural Organic Matter on the Fate, Transport, and Toxicity of Carbon Based Nanomaterials

The fast growing production of carbon based nanomaterials (CNMs) and their potential widespread use in consumer products raise concerns regarding their potential environmental risks. The present study investigated the role of photochemical transformation and natural organic matter (NOM) in the fate, transport, and toxicity of 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. UVA irradiation leads to oxygenation of nC60 surface and decarboxylation of carboxylated multi-walled carbon nanotubes (COOH-MWCNTs). The environmental transport of CNMs is significantly affected by their surface chemistry, concentration and species of electrolytes, and concentration and properties of co-existing NOM. In electrolyte solutions without NOM, the mobility of CNMs is largely decided by their surface chemistry, primarily the oxygen-containing functional groups. The mobility of nC60 greatly depends on its surface oxygen concentration, while the mobility of COOH-MWCNTs correlates well with the abundance of surface carboxyl groups. Humic acid, as a surrogate of NOM, can significantly enhance nC60 stability through steric hindrance. The extent of stabilization depends on the amount and properties of humic acid adsorbed. Humic acid has limited adsorption on UVA-irradiated nC60. Soil humic acid is more efficient in stabilizing nC60 than aquatic humic acid due to its higher molecular weight. Humic acid immobilized onto the silica surface can enhance or hinder nC60 deposition, depending on the complex interplay of attractive and repulsive forces. MWCNTs are more toxic to bacteria, Escherichia coli, than COOH-MWCNTs due to their higher bioavailability and oxidative capacity. Surface oxidation induced by ˙OH reduces the toxicity of MWCNTs. Oxidative stress is a major toxicity mechanism, which can be mitigated by adding antioxidants. Overall, this study highlights the critical role of environment-induced structural changes of CNMs in their potential environmental risks. It also provides structure-activity relationships for the assessment of CNM mobility and toxicity.