Methods for the investigation of the fate of engineered nanoparticles in the environment and complex matrices

In the past years, engineered nanomaterials (ENMs) have been increasingly used and incorporated in consumer products and it is likely that at different stages of their lifecycle they will enter the environment. There is a need for analytical techniques for the detection and characterization of ENMs in complex environmental systems, which can be applied both for monitoring purposes and alongside standard laboratory tests. In addition, similar to the standardized testing of the environmental fate of classic chemicals, new tests addressing endpoints relevant for nanomaterials have to be developed and implemented. Therefore the overall objectives of this thesis were to evaluate and investigate setups and methods for the assessment of the fate of nanomaterials in aquatic environmental systems. Specifically this comprised (i) the development, scientific justification and evaluation of a simplified standard test for the assessment of the dispersion stability of nanomaterials that would be applicable for most routine laboratories and (ii) the evaluation of analytical techniques on the applicability for detection and characterization of ENMs in a more complex system. The main findings can be summarized as follows:
Development of suitable test strategies for testing of dispersion stability
The presented test design showed that a compromise between scientific coherence on the one hand and practical requirements for the successful implementation in routine laboratories on the other hand could be achieved. Despite the necessary simplifications made to achieve these aims, a powerful tool, enabling a comparative assessment of ENM stability in different types of aquatic media reflecting the relevant composition of terrestrial surface waters was established. Furthermore, the principle of the test presented here, can be easily adapted to characterize ENM stability in different types of media, e.g. biological media for ecotoxicity testing. However, the determination of kinetic parameters is still limited within the presented test. In principle the presented test design should also be applicable to study hetero-agglomeration. In this case, the test media have to be modified / amended by the addition of particulate model phases representing typical natural nanoparticles present in environmental freshwater or other compartments, e.g. soil pore water.
Investigation and evaluation of more complex test designs and analytical methods
The applicability of Flow-Field-Flow Fractionation coupled UV/Vis, light scattering and inductively coupled plasma mass spectrometry (Flow-FFF-UV/Vis-MALLS-ICP-MS) was successfully demonstrated for the analysis of gold nanoparticles (AuNPs) in a complex environmental matrix containing natural nanoparticles (NNPs). However, the use of static light scattering to check separation performance was found to be of limited value for metallic NPs, as these are incompatible with currently available light scattering detectors. The combination of different detection systems allows in principle the investigation of the interactions between ENMs and NNPs. This has been demonstrated for the 30 nm AuNPs although this was not possible for the 100 nm AuNPs. The comparison of samples with different concentrations of natural organic matter (NOM) mitigated the common assumption that hetero-agglomeration would always dominate the fate of ENMs in environmental matrices.
The use of Flow-FFF for analysis of nanomaterial mixtures in complex matrices requires optimization of the parameters that influence the separation behavior. For this purpose the retention, recovery, and separation efficiency of two representative nanoparticles, were investigated against a parameter matrix of three different cross-flow densities, four representative carrier solutions, and two membrane materials. It was shown that the behavior of particles within Flow-FFF channels cannot be predicted or explained purely in terms of electrostatic interactions. Particles were irreversibly lost under conditions where the measured zeta potentials suggested that there should have been sufficient electrostatic repulsion to ensure stabilization of the particles in the Flow-FFF channel resulting in good recoveries. In general, careful adjustment of separation conditions can result in acceptable, but not ideal, separation for different stabilized materials. However, the optimized conditions will not be ideal for any particle and results have to be interpreted carefully e.g. with respect to size determination.