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Formation of natural coatings on nanoparticles in aquatic media is highly influenced by the dynamic composition of the medium. Hundreds of research articles were dedicated to this question and an in-depth understanding of the underlying mechanisms has been obtained for some materials. Nonetheless, it is still unknown to which extent these results obtained under controlled conditions in the laboratory can be extrapolated to environmental conditions. In this study, we used the recently introduced dialysis bag method to expose n-TiO2 in situ to 18 surface waters with contrasting physicochemical parameters by varying the sampling location and season and characterized the resulting natural coatings. ATR-FTIR and 2D-IR spectroscopy (thermal perturbation), thermogravimetry, and X-ray photoelectron spectroscopy analyses of the exposed nanoparticles revealed clear differences in the composition of the natural coatings in the presence of both organic (carboxylate and aromatic groups) and inorganic (carbonate) components. Subsequently, multivariate data analysis (correlation matrix, principal component analysis, and random forest) was used to explore the relationships between surface water parameters and the composition of the natural coating; the results enabled us to identify pH, electrical conductivity, and Ca2+–Mg2+ concentration as the three most important parameters for predicting the composition of the natural coating of TiO2 nanoparticles.

The fate of engineered nanoparticles in the aquatic-terrestrial transition zone is decisive for their effect in the environment. However, our knowledge on processes within this interface is rather low. Therefore, we used a floodplain stream mesocosm to enhance our understanding of the long-term distribution and biological effects of citrate-coated silver nanoparticles (Ag-NPs) in this ecosystem. Parallel to pulsed dosing of Ag-NPs, we observed fluctuating but successively increasing concentrations of aqueous Ag, 88–97% of which was categorized as particles. The remaining dissolved fraction was mainly complexed with natural organic matter (NOM). The major Ag fraction (50%) was associated with the uppermost sediment layer. The feeding activity of benthic amphipods was largely unaffected, which could be explained by the low Ag concentration and complexation of released Ag+ with NOM. According to our hypothesis, only a small nanoparticle fraction (6%) moved to the terrestrial area due to aquatic aging and enrichment of Ag-NPs in sediments and biota. Nanoparticle infiltration in deeper sediment and soil layers was also limited. We expect that a small fraction of nanoparticles remaining in the water for several weeks can be transported over large distances in rivers. The Ag-NPs accumulated in the top layer of sediment and soil may serve as a source of toxic Ag+ ions or may be remobilized due to changing physico-chemical conditions. Furthermore, the high enrichment of Ag-NPs on algae (up to 250 000-fold) and leaves (up to 11 000-fold) bears risk for organisms feeding on those resources and for the transfer of Ag within the food web.

Despite the numerous studies about the sorption of dissolved organic matter (DOM) onto nanoparticles, the extrapolation of laboratory results to environmental conditions is currently impossible. Indeed, the complex dynamics of DOM under variable environmental conditions are not completely reproducible under control conditions. In this study, we propose a different approach by exploring a method for exposing nanoparticles to realistic environmental conditions in natural river water by using dialysis membranes as passive reactors. Inside this reactor, the complexity and the temporal variability of a large number of environmental parameters (DOM structure and composition, temperature, inorganic ions, pH, etc.) are reproduced, while colloidal and particulate interferences remain separated. To verify this assumption, we determined the concentration of the water components and nanoparticles (n-TiO2, 20–50 nm) inside and outside the reactor before and after exposure to river water. In river water, more than 90% of the n-TiO2 nanoparticles remained inside the reactor while DOM retained its molecular composition/characteristics after passing through the membrane (DOC, fluorescence EEM, and FT-ICR MS). For most elements and anions, the concentrations inside and outside the reactor did not differ, indicating a good permeability for inorganic constituents (IC, ICP-OES); however, the concentrations of Al, Fe, Mn, and nitrate were lower. Membrane fouling, in terms of pore size distribution, was investigated using NMR relaxometry and AFM in fluid mode; no significant reduction in pore size was observed under the applied conditions during seven days of exposure. Finally, ATR-FTIR and CHNS analysis of n-TiO2 before and after exposure to the river water revealed that sorption of DOM occurred under field conditions. Therefore, we could demonstrate the validity and the potential of this method.

Understanding of the interplay of generally known colloidal transformations under conditions of test media (TM) used during cultivation of organisms and biological effect (=ecotoxicological) studies is still limited, although this knowledge is required for an adequate interpretation of test outcomes and for a comparison among different studies. In this context, we investigated the aggregation and dissolution dynamics of citrate-stabilized silver nanoparticles (Ag NPs) by varying the composition of three TM (ASTM, SAM-5S, and R2A, used during bioassays with Daphnia magna, Gammarus fossarum, and bacterial biofilms, respectively) in the presence and absence of two types of natural organic matter (NOM), namely, Suwanee River humic acid (SRHA) and seaweed extract (SW). Each original test medium induced reaction-limited aggregation of Ag NPs, and aggregation increased from R2A to SAM-5S and ASTM. In addition to the differences in aggregation dynamics, the concentration and speciation of Ag(I) differed between the three TM, whereby SAM-5S and ASTM are comparable with respect to the nature of the aggregation process but clearly differ from the R2A medium. Furthermore, Cl−, mainly present in SAM-5S, induced NP stabilization. The release of silver ions from Ag NPs was controlled by the presence of NOM and organic constituents of TM and by interactions with Cl− and Br−. The degree of aggregation, formation of interparticle cation–NOM bridges or stabilization was larger for Ca2+ than for Mg2+ due to the stronger ability of Ca2+ to interact with citrate or NOM compared to Mg2+. These observations and the dependence of aggregation rates on the particle concentration renders the interpretation of dose–response relationships challenging, but they may open perspectives for targeted ecotoxicological testing by modifications of TM composition.

The demand to quantify the elemental composition of very small sample amounts and/or of samples which form artefacts during conventional sample preparations is increasing. Example applications are the quantification of engineered metal(loid) based nanomaterials in environmental samples, e.g. (i) the direct analyses of engineered nanoparticle (ENP) suspensions showing broad particle size distributions which are not suitable to be applied via the spray chamber in ICP-MS analyses, (ii) measurements of single invertebrates and tissue of selected organs which were exposed to ENPs, and (iii) whole plants or plant parts e.g. from Lemna sp. The use of imaging based high resolution methods like atomic force microscopy or environmental scanning electron microscopy creates the need to quantify the elemental composition of the visualised objects as directly and exactly as possible, at very low limits of detection. With this study the authors present a method/concept for the multi-element quantification of analytes from ENPs in complex matrices with different degrees of complexity by graphite furnace electrothermal vaporisation coupled to inductively coupled plasma quadrupole mass spectrometry equipped with collision/reaction cell (GF-ETV-ICP-QMS).

Freezing and melting of H(2)O and D(2)O in the cylindrical pores of well-characterized MCM-41 silica materials (pore diameters from 2.5 to 4.4 nm) was studied by differential scanning calorimetry (DSC) and (1)H NMR cryoporometry. Well-resolved DSC melting and freezing peaks were obtained for pore diameters down to 3.0 nm, but not in 2.5 nm pores. The pore size dependence of the melting point depression DeltaT(m) can be represented by the Gibbs-Thomson equation when the existence of a layer of nonfreezing water at the pore walls is taken into account. The DSC measurements also show that the hysteresis connected with the phase transition, and the melting enthalpy of water in the pores, both vanish near a pore diameter D* approximately equal to 2.8 nm. It is concluded that D* represents a lower limit for first-order melting/freezing in the pores. The NMR spin echo measurements show that a transition from low to high mobility of water molecules takes place in all MCM-41 materials, including the one with 2.5 nm pores, but the transition revealed by NMR occurs at a higher temperature than indicated by the DSC melting peaks. The disagreement between the NMR and DSC transition temperatures becomes more pronounced as the pore size decreases. This is attributed to the fact that with decreasing pore size an increasing fraction of the water molecules is situated in the first and second molecular layers next to the pore wall, and these molecules have slower dynamics than the molecules in the core of the pore.