Stability and Interactions of Engineered Nanoparticles (ENP) in Aqueous Matrices (SIENA)
Final Report Abstract
Nanoscience and nanotechnology are part of modern life. Nanomaterials such as engineered nanoparticles (ENP) are fabricated with outstanding properties being substantially different from those of the same materials on the micrometer length scale. Thus, ENP find broad use in technical, industrial, medical, food and life science applications. In all probability, ENP will be unintentionally released into the environment. Especially for aquatic systems, little is known on their occurrence, fate and impact. ENP emitted into the environment are unlikely to be persistent, but to undergo alterations and ageing, leading to camouflaged species or eve new products. The main focus of the project was on the molecular interactions of inorganic ENP with refractory organic matter (ROM) and inorganic water constituents, which are ubiquitous in environmental and technical aquatic systems. As ENP, various silver ENP and superparamagnetic iron oxide nanoparticles (SPION) were used. In particular, questions regarding the stability (dissolution and agglomeration) and mobility of ENP were addressed, considering various environmental matrices. For this purpose, analytical methods for the characterisation and detection of inorganic ENP had to be identified, implemented and optimised. The results obtained should contribute to better understand the fate and the environmental behaviour of ENP, to derive further research needs and concepts regarding the ecological safe production and application of ENP and to optimise techniques for their separation from aqueous systems. The results obtained clearly indicate that ROM plays a key role regarding the stability, mobility and toxicity of ENP in aqueous systems. The application of different analytical methods and instruments gave clear evidence that the presence of ROM leads to a coating of ENP and thus, to changes of their initial properties. In particular, the impact of ROM quality (humic acid, HA; fulvic acid, FA) and ion type (sodium/potassium; magnesium/calcium) on the stability of silver ENP could be differentiated. In general, divalent cations had a greater destabilising effect on ENP than monovalent cations. It could be observed that at low ionic strength (< 50 mmol/L), the presence of ROM increased the stability of silver ENP. Here, HA was more stabilising than FA most likely due to a thicker adsorbed layer of HA onto AgNP (“coating”) leading to a greater steric stabilisation. However, at high ionic strength (> 50 mmol/L) and in the presence of HA, divalent cations (Ca2+, Mg2+) acted as bridging cations leading to the agglomeration of HA-coated AgNP (including complex formation between Ca2+/Mg2+ and HA). Regarding the different behaviour of Ca2+ and Mg2+ in the presence of NOM, Mg2+ had a stronger destabilising effect than Ca2+. These results diverge from experiments involving silver ENP without ROM, which showed that Ca2+ was more destabilising than Mg2+. The difference can be attributed to ROM-metal complexation: The equilibration formation constants for Ca-ROM complexes are higher than for Mg-ROM complexes. As a result, in comparison to Mg2+, a larger proportion of Ca2+ is complexed with (free) ROM and a smaller proportion is engaged in neutralising the surface charge, leading to a higher stability of the silver ENP. As far as the toxicity of silver ENP is concerned, the presence of ROM led to a clear detoxification, most likely due to a coating of the particles with ROM and a resulting delayed dissolution (release of silver ions) and due to molecular interactions of silver ions with ROM (e.g. reaction with thiol groups). ROM also enhanced the mobility of silver ENP and SPION in porous media. This could be derived from a set of column experiments. Apart from classical breakthrough tests, the application of magnetic resonance tomography (MRT) allowed the visualisation of the transport behaviour and the distribution pattern of various SPION directly in the water saturated porous media (spatial and temporal information). The results revealed that besides ROM, particle surface properties (functionalisation, zeta potential) are key parameters affecting the mobility of the particles. This approach can be transferred to other porous systems and contributes to a better understanding of particle transport in environmental and technical porous media. Regarding the development and application of analytical methods to characterise and detect ENP in environmental aquatic systems, it became evident that an application of different, independent methods is highly recommended to meet the limitations of the individual analytical methods and instruments. Furthermore, to meet the low concentrations of ENP in the environment, highly sensitive instruments are of paramount importance. The results of this this project contributed to a better understanding of the molecular interactions of ENP in aqueous systems and their environmental fate, and thus to assess their environmental impact. In particular, the environmental behaviour of ENP will be highly dependent on the water quality. ROM might play a future role in green production of ENP (e.g. as stabiliser). Urgent need is seen in the further standardisation of analytical methods and in the definition of standard reference materials for assessing ENP in technical and environmental systems. Delay, M., Abbt-Braun, G. and Frimmel, F. H.: Nanos meet humics – von synthetischen Nanopartikeln und natürlicher organischer Materie. 2014, Aktuelle Wochenschau der Gesellschaft Deutscher Chemiker (GDCh), http://www.aktuelle-wochenschau.de/main-navi/aktuelle-wochenschau-2014/kw26-nanos-meet-humics-von-synthetischen-nanopartikeln-und-natuerlicher-organischer-materie.html
Publications
- Nanoparticles in aquatic systems. Analytical and Bioanalytical Chemistry. 2012, 402 (2):583-592
Delay M., Frimmel F. H.
(See online at https://doi.org/10.1007/s00216-011-5443-z) - Einfluss von organischer Substanz und Ionenstärke auf das Verhalten synthetischer Nanopartikel in wässrigen Systemen. Jahrestagung 2013 der Wasserchemischen Gesellschaft – Fachgruppe in der Gesellschaft Deutscher Chemiker, Goslar. 2013. Tagungsband, 131-136, ISBN 978-3-936028-76-8
Delay, M., Schwegmann, H., Abbt-Braun, G., Frimmel, F., Horn, H.
