Heteroaggregation is a crucial step in the fate of natural and anthropogenic nanoparticles (NPs) in environmental waters: heteroaggregation between NPs and suspended natural organic or mineral matter can significantly influence the transport and bioavailability of NPs. In order to obtain a realistic estimate of the attachment coefficient on natural surfaces, empirical data is usually obtained through laboratory experiments under highly controlled conditions in which the complexity and dynamics of the environmental medium are lost. The concentrations used are generally several orders of magnitude higher than predicted for natural waters due to limitations in detection methods. This project aims to develop and apply a method for the simultaneous quantification and characterization of heteroaggregates under natural conditions and at realistic concentrations. Our concept is based on the dialysis bag method which allows to expose in situ an NP-partner aggregation system in natural waters. This method has been successfully used to characterize the surface coating formed on NPs under natural conditions. Its applicability to a system containing two types of particles will be tested in this project. The single counting technique by ICP-TOF-MS will be used in order to i) count particles at realistic concentrations (ng/L) and ii) determine the elemental composition of each NP. Thus, it will allow us to quantify the amount of heteroaggregates formed under natural conditions and to calculate an attachment coefficient specific to each target heteroaggregate (natural particles, FeOx, clays and NPTiO2). Thus, we will provide new information on the fate of nanoparticles in natural waters, which will help to understand geochemical cycles and the fate of emerging pollutants.