Sorption of Fulvic Acids onto Titanium Dioxide Nanoparticles Extracted from Commercial Sunscreens: ToF-SIMS and High-Dimensional Data Analysis

Titanium dioxide nanoparticles (n-TiO2) are common ingredients of sunscreens and are often released into surface waters during usage. Once released, the surface chemistry of n-TiO2 changes by interacting with dissolved organic matter (DOM). In previous studies, these interactions were investigated using model n-TiO2 and; therefore, do not account for the complex composition of the coating of n-TiO2 aged in sunscreens. Taking advantage of a mild extraction method to provide more realistic nanoparticles, we investigated the potentials of time of flight-secondary ion mass spectrometry (ToF-SIMS) combined with high-dimensional data analysis to characterize the sorption of fulvic acids, as a model for DOM, on titanium dioxide nanoparticles extracted from ten different commercial sunscreens (n-TiO2 ⸦ sunscreen). Clustering analysis confirmed the ability of ToF-SIMS to detect the sorption of fulvic acids. Moreover, a unique sorption pattern was recognized for each n-TiO2 ⸦ sunscreen, which implied different fractionation of fulvic acids based on the initial specifications of nanoparticles, e.g., size, coating, etc. Furthermore, random forest was used to extract the most important fragments for predicting the presence of fulvic acids on the surface of n-TiO2 ⸦ sunscreen. Finally, we evaluate the potential of ToF-SIMS for characterizing the sorption layer

Investigation of interactions of TiO2 nanoparticles with dissolved components of surface waters under natural conditions

The booming global market of nanomaterials in the last few decades has led to the inevitable emission of these materials into aquatic environments; hence, understanding their physical, chemical, and biological transformations has become a big concern for environmental scientists. Despite a great deal of effort made to understand the mobility, fate, and risk assessment of e.g, TiO2 nanoparticles, it is still unclear if the obtained results, under lab-controlled conditions, can be generalized to realistic released nanoparticles in aquatic environments since the complex dynamics of environmental conditions are not completely reproducible under controlled conditions. In the present study, we proposed a new approach to expose TiO2 nanoparticles to environmental conditions of natural surface waters by making use of dialysis membranes as passive reactors. The function of these reactors is based on the permeability of the membrane to the dissolved matter of surface waters while TiO2 nanoparticles do not pass through the membrane. These systems benefit from the fact that although the complexity and temporal variability of most of the environmental parameters of surface waters are reproducible inside the reactors, colloidal and particulate interferences remain separated. Furthermore, no significant reduction in pore size i.e., membrane fouling is observed in dialysis bags after exposure to surface waters which validates the efficiency of the system. Taking advantage of these reactors to expose nanoparticles to surface waters, we investigated the influential physicochemical parameters of the surface waters on the formation of natural coating onto nanoparticles. Hence, dialysis bags were used to expose TiO2 nanoparticles, in situ, to ten different surface waters in the spring and summer of 2019. Due to the complexity of the natural dissolved matter of the surface waters as long as their low natural concentrations, we needed to use a combination of analytical techniques and multivariate data analysis to investigate the coatings. The initial findings were similar to the lab-controlled exposure studies in the literature showing pH, electrical conductivity, and Ca2+- Mg2+ concentration as the three most important parameters of surface waters controlling the formation of coatings. Nonetheless, we came across a phenomenon being overlooked under lab-controlled conditions; natural coatings are composed of not only organics (DOM: dissolved organic matter) but also inorganics (carbonate) which implies that their realistic coatings are more complex than what the previous studies described. The second part of this thesis focused on investigating the interactions of more realistic nanoparticles (extracted TiO2 nanoparticles from 11 sunscreens) with DOM. Using ToF-SIMS combined with high-dimensional data analysis, we tried to find a general DOM-sorption pattern among TiO2 nanoparticles since finding this pattern could have ultimately opened a way to assess the fate of (more) realistic nanoparticles in aquatic environments. Contrary to our expectations, the results showed a unique sorption pattern for each sunscreen controlled by the composition of the sunscreens implying that the sorption pattern of each sunscreen should be investigated individually. In the next step of this study, we used random forest to extract the most important fragments of DOM sorbed onto each sunscreen followed by an effort to assign these important masses to chemical fragments. Trying to provide a comprehensive understanding of interactions of the released n-TiO2 in aquatic environments, in future studies, we are going to expand our coating research to different types of TiO2 nanoparticles, such as extracted particles from paint, where the reaction media (surface waters) are covering a wide range of water parameters representative of various ecosystems. Making use of state-of-the-art techniques as long as multivariate data analysis, we will try to achieve a model describing the sorption mechanisms of dissolved matter of surface waters onto nanoparticles. Such studies can eventually lead us to a better understanding of the fate of the released nanoparticles under natural conditions