Surface reactivity, stability, and mobility of metal nanoparticles in aqueous solutions : Influence of natural organic matter and implications on particle dispersion preparation

Abstract: The growing development of nanotechnology has resulted in an increased use of nanoparticles (NPs) in various applications ranging from medicine, military, to daily consumer products. There is a concern that NPs can be dispersed into the environment in various ways, for example to air and water during manufacture, use, incineration or recycling of products and thus pose a risk to health and the environment. Risk assessments of NPs are hence necessary. One property of NPs, which may make them very useful and at the same time potentially harmful, is their small size (in nanometer range) and hence high surface area per NP mass.This study forms part of the National Mistra Environmental Nanosafety Research Program. The program provides an interdisciplinary platform for researchers from e.g. nanoscience, medicine, chemistry, material science, life cycle analysis, and social science. Specific aspects of this program involve characterization of NPs in different environmental settings, toxicity studies of aquatic organisms, integrated risk assessment of NPs, and societal dimensions of nanosafety. The contribution of this thesis within the program includes studies of stability and mobility of metal NPs and their extent of transformation/dissolution upon environmental interaction. Environmental risk assessments of NPs require a detailed understanding of how they change in terms of physical and chemical properties (charge, size, and surface oxide composition), important aspects for their stability, mobility, and reactivity in the environment. Generated data is highly relevant for the other activities of the Mistra Environmental Nanosafety program, e.g. to gain an improved understanding and design of particle dispersions and ecotoxicity studies, as any environmental interaction will result in the transformation/dissolution of the NPs and change the surface chemistry (e.g. adsorption of natural organic matter, changes in surface oxide properties), aspects that largely influence their speciation and potential toxicity.Common sonication protocols exist to prepare particle dispersions for different in vitro studies. The influence of key parameters stipulated by these protocols on the particle size, transformation/dissolution, and extent of sedimentation was investigated for bare metal NPs. Improved knowledge on these aspects is crucial for design and interpretation of results of NP-related investigations. Reactive metal NPs such as Cu and Mn NPs started to dissolve and release metals already during the probe sonication step of the stock solution, and that the presence of bovine serum albumin (often added as a stabilizing agent) enhanced this process. Even though prolonged sonication time i.e. increased delivered acoustic energy, reduced the size of formed agglomerates, sedimentation was still significant. As a consequence, administered doses from pipetted stock solutions were significantly lower (30-70%) than the nominal doses. The main reason behind the significant extent of agglomeration, with concomitant sedimentation, is related to the strong van der Waals forces prevailing between metal NPs. It is hence essential to determine the administrated dose of metallic NPs in e. g. nanotoxicological testing.Interactions between metallic NPs and natural organic matter (NOM) were studied in terms of stability, mobility and metal dissolution in order to mimic a potential exposure scenario. NOM was represented by humic acid (HA), a main component of organic matter in the environment, and by dihydroxybenzoic acid (DHBA), a small degradation product of NOM. Sedimentation of the Cu, and the Al NPs were slower in the presence of NOM in freshwater compared with freshwater only, whereas the effect of NOM was small for the Mn NPs. Stabilization was related to surface adsorption of NOM, which increased the steric repulsion between the particles, and in the case of HA also increased the magnitude of the zeta potential (resulting in increased electrostatic repulsion). Slight initial increase in particle stability wasobserved in freshwater containing DHBA, but after 24 h, sedimentation of the NPs was comparable to the conditions in freshwater only. The presence of HA (at a concentration of 20 mg/L) was found to stabilize the NPs in freshwater for more than 24 h. However, both the lower and higher HA concentration (2 and 40 mg/L) resulted in agglomeration of the Cu and Al NPs already within a few hours. Mn NPs were more stable in terms of sedimentation in freshwater at all three humic acid concentrations. This concludes that the concentration and type of NOM largely influence the stability of the studied metal NPs in solution. In contrast, SiO2 NPs were not influenced by the presence of NOM in terms of stability, most probably predominantly related to smaller attractive van der Waals forces and larger electrostatic repulsion (due to higher surface charge) compared with the metal NPs.Metal release from the Cu and Al NPs was enhanced in the presence of NOM, whereas no significant influence was observed for the Mn NPs. All metal NPs were dissolved relatively fast; 10% or more of the particle mass was dissolved within 24 h. Speciation predictions revealed rapid complexation between released Cu and Al in solution and NOM, reducing the bioavailability, whereas less complexation was evident for released Mn (as ions). In all, rapid agglomeration and sedimentation imply that any risks associated with the environmental dispersion of these metal NPs will be limited to the vicinity of their source. Mn NPs, having lower sedimentation rates than the Cu and Al NPs, and lack of solution complexation of released ions will likely have a relatively higher probability to be mobile and transported to other aquatic settings.