Modelling the release, dispersion and fate of nanoparticulate titanium dioxide from sunscreen products in recreational lakes

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Titanium dioxide nanoparticles (n-TiO2) represent an increasing threat for aquatic ecosystems due to their persistence and the direct release from sunscreens in recreational waters during the bathing season. However, their impact is still difficult to evaluate because, on one side, analytical methods for monitoring their concentration in environmental media have been developed only recently. On the other side, their fate in aquatic systems is still insufficiently understood. In particular, the distribution between the surface microlayer (SML), bulk water, sediments, plants, and plankton depends on various mechanisms that were investigated individually in laboratory experiments, but were rarely assessed under environmental conditions. Furthermore, the role of wind in the dispersion of nanoparticles present in the SML has not been investigated so far, although it is expected to contribute significantly to the spatial dispersion of hydrophobic n-TiO2. In this project, we propose to investigate the distribution of n-TiO2 in a typical recreational lake and, based on detailed empirical data, assess a reactive transport model describing their fate. We will quantify and characterize the input of sunscreens using surveys and samples among bathers and determine the wash-off rate of sunscreen from the skin under field conditions using dedicated experiments. The amount of n-TiO2 in the water phase, in the surficial water (hydrophobic films), and in aquatic organisms will be determined using a newly developed method relying on trace elements to correct for the natural TiO2-background. A sampling campaign at a high measurement frequency will be conducted to obtain empirical data on the dispersion rate due to convection and wind drift on the SML. The accumulation of anthropogenic n-TiO2 in the sediment will be determined by monitoring the concentration before and after the bathing season. The obtained data will be used to develop, test, and optimize fate distribution models taking into account spatial dispersion due to convection and wind together with the nanoparticles’ properties and the water chemistry. Functional assays will be carried out for determining the parameters required for the fate model. Attachment efficiencies will be determined using n-TiO2 extracted from sunscreen and coated naturally in lake water. Surrogate for natural colloids will be selected based on a detailed investigation of the colloids in the study lake and used as heteroaggregation partners in the functional assays. To parametrize the effect of wind on the SML, flume mesocosm experiments will be conducted to produce and test the stability of sunscreen SML under controlled aero- and hydrodynamic conditions. The results will enable us to determine the most relevant processes involved in the fate of n-TiO2 from sunscreens in recreational waters and, thereafter, open the door to more accurate ecological risk assessment of inorganic UV-filter in sunscreens.