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The wide usage of ENMs in variety industrial applications and everyday products has led to the release of those materials to the environment, in particular to the aquatic systems.
The fate and behaviour of ENMs in the water body are defined by the intrinsic particle properties as well as by properties of surrounding media. Therefore, the goal of this thesis was to develop a mechanistic understanding of transformation processes of ENMs in various aquatic environments. In order to achieve this goal different types of materials and representative environmental media were used.
In Chapters III until VI, we concentrate our research on cerium dioxide (CeO2) NPs.
In Chapter III (Paper I) effect of different environmental factors such as water ionic composition, pH, presence of fulvic acid and dilution factors were investigated. CeO2 NPs released to natural water interact with NOM and form CeO2/NOM complexes. The outcome of the research showed that the way how those complexes are formed will influence the particle fate. Indeed, the gradual increase of FAs concentration led to the destabilisation of NPs and the formation of aggregates. However, the direct addition of FAs led to the formation of stable CeO2/FA complexes. It was also shown that formed complexes are stable with time and not affected by variation of pH, low ionic strength and dilution.
Once the CeO2/NOM complexes are formed it is of interest to investigate how they behave in natural water. Therefore, Chapter IV (Paper II) was dedicated to the investigation of the stability and the aggregation processes of uncoated and coated with FAs CeO2 NPs in natural environmental conditions. The results showed the environmentally relevant concentration of FA stabilises CeO2 NPs in ultrapure and in synthetic water. However, in natural Lake Geneva water, CeO2 NPs were found aggregated regardless the concentration of FAs. Therefore, the presence of natural water compounds play a key role in the stabilisation of NPs in natural water. The stability of CeO2 NPs coated with another type of NOM and stability of coating formed was investigated in Chapter V (Paper III). Alginate is a natural biopolymer which is widely used as a surrogate of natural polysaccharides. The stability of alginate coating around CeO2 NPs was tested in changing pH, ionic strength and in natural water. Our results demonstrated that when coating is formed it is persistent with time and in changing pH. The concentration of alginate plays crucial role in CeO2 stabilisation in ultrapure and in synthetic waters. In natural lake water heteroaggregation is observed, but is reduced by increasing the alginate concentration.
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One class of components in natural water which affects the stability of nanoparticles is inorganic colloids. Heteroaggregation between CeO2 NPs and Fe2O3 ICs in different environmental condition was thoroughly investigated in Chapter VI. We have found that no heteroaggregation is observed between NPs and ICs at low ionic strength due to the presence of electrostatic repulses. In synthetic and lake waters heteroaggregation is due to the modification of NP surface properties.
In Chapter VII (Paper IV and Paper V) the destabilisation of polystyrene micro- and nanoplastic particles in different conditions was investigated. The results indicate that particle surface properties, and more specifically the surface charge, define the affinity between plastics and water compounds. It was also shown that zeta potential measurements can be used to control the aggregation processes and to establish the optimal conditions to eliminate nano and micro particles from suspensions. The primary mechanism that is responsible for the particle destabilisation is charge neutralisation. It was also found that the aggregation rate of plastics in natural water is dependent on plastic concentration. Therefore, heteroaggregation will play an important role in the plastic environmental identity and removal from aquatic systems.
The research presented in this thesis showed that the stability of ENMs in environment is a complex problem and required interdisciplinary approach. In this thesis, we showed the effect of individual water compounds and their combinations on the stability of the selected ENMs and defined the mechanisms of ENM stabilisation and destabilisation. We also showed that behaviour of ENMs in synthetic water with representative composition of natural water is different from actual natural water.
Reflecting the complexity of natural aquatic systems compared to artificial environmental conditions. Future researches should pay more attention to environmentally relevant experimental conditions such as low concentration of ENMs on the level of ng/L and μg/L, long-term behaviour of ENPs in natural water, where sedimentation and possible resuspension of ENPs occurred and simulated as in real environmental system. More work should be also done in the direction of the development of universal ENM stability model which takes into account heteroaggregation attachment and collision efficiencies – parameters that could be obtained during explicit experiments between different water compounds and ENMs. Regarding micro- and nanoplastic particles there is still a lot of issues to explore since this domain of research just started to develop. Some of the knowledge gained in the field of ENM stability could be transferred to the field of
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nanoplastic stability in aquatic system, however, there are also differences regarding material properties, persistence in the environment and the effect on the leaving organisms. Until know most of the researches used model standard plastic particles which are easy to manipulate and understand regarding their transformation and interaction with other water components. In the future, plastic particles extracted from environment, particles which are used in products or aged particles should be used. The aging of plastic particles due to interactions with aquatic components or environmental factors and processes (UV, abrasion etc.) and the mechanisms of degradation of microplastic particles to nanoplastics are still unknown. One of the sources of micro- and nanoplastics to the environment is the municipal water treatment stations, therefore, the transformations of these particles in each step of the water treatment processes should be considered.
Another direction of research is the development of the detection and identifications methods for nanoplastics in the environment. For the moment, it is laborious and manual process which introduces many artefacts. In addition, due to the transformation and aging of environmental nanoplastics such as coating with NOM successful detection, for example using Raman spectroscopy or FTIR analysis, is limited.
Annexes
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Annex 1. Supporting information for the Chapter III
Annex 1.1. Effect of ionic strength
Influence of ionic strength on the CeO2 NP aggregation.
0.00 0.05 0.10 0.15 0.20 0.25 20
25 30 35 40 45 50 55 60 65