Quantification and modelling of accumulation kinetics of nanomaterials in soil organisms under environmentally relevant conditions

Samenvatting

Chapter 1 introduced the objectives of the thesis and background information regarding the production of nanomaterials (NMs) and their release in the environment. The concepts of environmental and bio-mediated transformations of NMs were defined and their implication on the NM exposure assessment is explained. Silver nanoparticles (Ag-NPs) were selected as model NM because they are widely produced, undergo transformations that represent all the most relevant transformations of NMs in soil and specific analytical methods are available to quantify them at relatively low concentrations. A short review regarding previous toxicological studies of Ag-NPs in soil organisms and the importance of performing toxicokinetic studies was presented. Finally, selected model organisms (earthworms Eisenia fetida and Lumbricus rubellus) were briefly described. Chapter 2 reported a short-term (28 days) toxicokinetic study in E. fetida exposed to Ag-NPs, aged Ag-NPs (Ag2S-NPs), and AgNO3. A one-compartment model was applied to calculate separately the kinetic constants for uptake and elimination of particulate and ionic forms of Ag. The uptake and elimination rate constants for earthworms exposed to pristine Ag-NP or AgNO3 were not significantly different from each other. Uptake rate constants of (hardly dissolvable) Ag2S-NPs which resemble the environmental relevant form of Ag-NPs was significantly lower. spICP-MS analysis demonstrated that ~85% (average of both Ag-NP and AgNO3 treatments) of the Ag within the earthworms was present as ionic Ag, regardless of the actual form of Ag that the earthworms were exposed to. Indeed, the biogenic formation of particulate Ag (~10 % of total Ag accumulated overtime) in earthworms exposed to AgNO3 led to a kinetic pattern of particulate Ag body burden similar to pristine Ag-NPs. NP size analysis and imaging techniques showed evidences that the particles in the tissues were not the same as those to which worms were exposed, highlighting that biotransformation and/or biogenic formation took place also in the case of the Ag-NP exposure. Chapter 3 investigated the influence of dissolution on the uptake of metal NPs in earthworms by the use of bimetallic NPs. E. fetida specimens were exposed to Au core-Ag shell NPs (Au@Ag-NPs) and to a combination of Au-NPs, Ag-NPs, Ag and Au ions containing natural soil for 28 days. Our hypothesis was that Ag shell would dissolve partially or completely and that Au core would not interact with the exposure media and would therefore behave as a tracer of the particulate uptake. Analysis of earthworm tissues showed that concentrations of Ag in the earthworms were not statistically different in organisms exposed to the different forms of Ag. However, the concentration of Au in the earthworms exposed to HAuCl4 (ionic Au) exceeded around twenty times the Au concentrations in the exposures to particulate Au, which did not differ among each other. Mass measurements by spICP-TOFMS provided evidence that the uptake of the metals in their bimetallic particulate form represents approximately 5 % of the total metal amount. Size measurements by spICP-TOFMS showed that the Au core remained similar after the uptake, while the Ag shell increased in thickness suggesting that biotransformation processes took place at the surface of the NPs (e.g. aggregation, adsorption of Ag ions on the surface of existing particles). The study confirmed that dissolution is the main factor driving the uptake of (dissolving) metal NPs in earthworms. Additionally, different uptake patterns resulted from the co-exposure to Au and Ag-NP and Ag+, indicating that the Ag form can lead to different interactions with Au in the soil affecting the uptake in the earthworms. Chapter 4 presented a toxicokinetic study performed to assess the potential impacts of long-term exposure (nine months) on the uptake of pristine Ag-NP, aged Ag-NP (Ag2S-NP) and ionic Ag in earthworms E. fetida. The study was conducted with same species and conditions similar to the short-term experiment which was previously conducted for 4 weeks (chapter 2), in order to allow comparison between the two models. The accumulation of Ag in Ag-NP and AgNO3 exposed earthworms did not statistically differ after nine months exposure. In Ag2S-NPs exposed earthworms, the internalized concentrations were five times lower compared to the other treatments. The Ag concentrations in pore water did not reflect the uptake pattern and metallothionein concentrations were not different from the control group. The overall conclusion of this chapter was that even after a prolonged period of time the uptake kinetic rate constants of Ag-NP and AgNO3 were not statistically different, while the one of Ag2S-NP was statistically significant lower than the other treatments. Additionally, the short-term kinetic rate constants predicted the average bioaccumulation of pristine Ag-NP and AgNO3 in the earthworms exposed for nine months, while the bioaccumulation of Ag2S-NPs in earthworms was under-predicted somewhat. This was likely because the short-term did not take into account the late dissolution of Ag2-NPs. Ag bioaccumulation of Ag2S-NP could not be related to the concentrations of Ag measured at a specific time in pore water. Chapter 5 reports a study which demonstrated that earthworm bioturbation plays an important role in the vertical transport of Ag2S-NPs in soil. In the soil columns, daily rainfall from artificial rain water did not lead to displacement of Ag2S-NPs within 28 days indicating that in the case of hardly soluble metal NPs and unsaturated soil conditions, bio-mediated transport overcomes physical chemical transport. Bioturbation from L. rubellus was quantified by assessing the changes of the macro porosity in the soil columns. Results indicated that earthworms burrowing activity was not affected by the presence of Ag2S-NPs at the experimental concentrations. The study proposed a linear relationship between bioturbation rate and the abundance of earthworms that is applicable to future bioturbation studies. Chapter 6 presented an overall discussion of the results obtained in the thesis, and concluded with the implication of such findings in the risk assessment of NMs.