Impact of Sunlight and Humic Acid on the Deposition Kinetics of Aqueous Fullerene Nanoparticles (nC₆₀)

Nanoparticle transport in natural settings is complex due to interactions with the surrounding environment. In this study, the impact of UVA irradiation and humic acid (HA) on deposition of aqueous fullerene nanoparticles (nC60) on a silica surface as a surrogate for natural sediments was studied using packed column experiments and quartz crystal microbalance with dissipation monitoring under various solution conditions. Surface oxidation of nC60 induced by UVA irradiation greatly retarded its deposition due to the increased negative surface charge and hydrophilicity. Dissolved HA, once adsorbed onto the nC60 surface, also hindered its deposition mainly through steric hindrance forces. The extent of this effect depended on the properties and the amount of HA adsorbed, which is a function of ionic strength and HA concentration. HA has limited adsorption on UVA-irradiated nC60 and is expected to play a less important role in its stability. HA immobilized onto the silica surface had a variable effect on nC60 deposition, depending on the complex interplay of Derjaguin−Landau−Verwey−Overbeek (DLVO) and non-DLVO interactions such as electrostatic interaction, steric hindrance, and hydrogen bonding as well as HA molecular conformation. These results highlight the importance of environment-induced changes in nC60 surface chemistry in its fate and transport in aquatic environments. ■ INTRODUCTION Buckminsterfullerene (C60) has drawn much attention due to its unique physicochemical properties such as antioxidative properties, photoactivity, hydrophobicity, the spherical cage structure, and high electron affinity, which could enable a wide range of potential applications including optics, cosmetics, and pharmaceuticals. The current and potential use in consumer products and large-scale production of C60 will inevitably lead to its release into the environment. Modeling studies predicted the concentrations of fullerenes at the order of magnitude of nanograms per liter to micrograms per liter in surface waters and micrograms per liter in wastewater treatment plant effluents. Because C60 as a molecule has extremely low solubility in water (1.3 × 10−8 ng/L ∼ 7.96 ng/L), the nanosized colloidal particle of C60 formed in the aqueous phase, usually referred to as nC60, is considered a more environmentally relevant form of fullerene. Many studies showed that nC60 can exert toxicity to various organisms such as bacteria, fish, and human cell lines, raising concerns regarding its potential impact to human and ecosystem health. Initial studies postulated that reactive oxygen species (ROS) generation is one of the major toxicity mechanisms involved. However, reports on the ROS generation of nC60 were contradictory. Several studies reported minimal to no ROS production by aqueous nC60 due to self-quenching. 10,11 Thus, it has been suggested that the toxicity of nC60 stems from ROSindependent oxidative stress. Unlike conventional contaminants such as dissolved organic compounds and heavy metals, the fate and transport of nanoparticles are largely controlled by aggregation and deposition processes. A number of studies have investigated the transport of nC60 in model porous media as well as natural soils using packed column experiments or quartz crystal microbalance with dissipation monitoring (QCM-D). These studies suggest that the properties of nC60 and the collectors, solution chemistry, and hydrodynamic conditions are the major factors governing the deposition process. The deposition of nC60 is qualitatively in good agreement with the classic DLVO theory and mostly happens in the primary minimum. However, the majority of past studies were carried out with pristine nC60 in simple electrolyte solutions. Once released into the environment, nC60 will interact with various environmental components, including sunlight and natural organic matter (NOM), which could alter its surface chemistry and consequently confound its environmental behavior. Ubiquitous in aquatic systems and soil, natural organic matter is a known key determinant of colloid transport. Dissolved humic acid has been shown to readily adsorb on nC60 and enhance nC60 colloidal stability in water. 25,26 On the other Received: July 18, 2012 Revised: November 5, 2012 Accepted: November 16, 2012 Article