Heteroaggregation of CeO2 nanoparticles in presence of alginate and iron (III) oxide.

When manufactured nanoparticles are released to natural waters, heteroaggregation between nanoparticles and water compounds is expected to occur and play a key role in nanoparticle fate, transport and transformation. In this work, the heteroaggregation between CeO2 nanoparticles and Fe2O3 inorganic colloids, which represent the main inorganic fraction from Lake Geneva water, is studied. The heteroaggregation processes between CeO2, Fe2O3 and alginate in multiple water samples are investigated using zeta potential and z-average diameter measurements. The kinetics of heteroaggregation of individual components as well as mixtures of CeO2 nanoparticles and Fe2O3 colloids and alginate are studied using time resolved dynamic light scattering. The global attachment efficiency (αglobal) is calculated using data from kinetic experiments. αglobal for pristine CeO2 nanoparticles varied from 0.5 to 0.7 in lake and synthetic waters and is found around 1 for pristine Fe2O3 and mixture CeO2 and Fe2O3. Our findings demonstrate that heteroaggregation is highly dependent on environmental conditions and resulting electrostatic scenarios. No heteroaggregation at pH 8 between CeO2, Fe2O3 and alginate is observed in ultrapure water, because of electrostatic repulsions between negatively charged compounds. In synthetic and lake waters, the situation is opposite. Indeed, specific adsorption of divalent cations and presence of salt are found to promote heteroaggregation via cation bridging and screening effects. The kinetic experiments indicate that aggregation rate of pristine Fe2O3 is higher (89 nm/min in lake water) compared to pristine CeO2 nanoparticles (50 nm/min) and on the same level as mixture of CeO2 and Fe2O3 (96 nm/min). Low alginate concentration, 0.25 mg/L, has no effect on heteroaggregation in mixture of CeO2 and Fe2O3 in lake and synthetic waters. On the other hand, in natural water, the presence of higher alginate concentration, 2 mg/L, is found to reduce the heteroaggregation rate.

Exposure to sublethal concentrations of Co3O4 and Mn2O3 nanoparticles induced elevated metal body burden in Daphnia magna.

Despite the significant progress made in ecotoxicological research on nanoparticles (NPs), there is still very limited information available regarding the biological effects of certain types of NPs such as Co3O4 and Mn2O3. Only a couple of studies provide data on their impact on aquatic organisms whereas, alarmingly, these NPs have been proposed to have high toxicity potential. In addition, more data are needed to determine whether the adverse effects the metal NPs induce on aquatic organisms are rather due to their chemical or particulate nature. To address these open questions, the (sub)lethal effects of Co and Mn NPs in parallel with the respective soluble metal salts on Daphnia magna were studied. The aims of the current study were to i) assess the acute toxicity of Co3O4 and Mn2O3 NPs (primary size 10-30nm) to D. magna, ii) evaluate whether the acute NP exposure at sublethal concentrations influences D. magna post-exposure feeding behaviour and iii) quantify D. magna metal body burden after exposure and after the post-exposure feeding to estimate the potential of trophic transfer of metals. Flow cytometry and total reflection X-ray fluorescence spectroscopy were applied for feeding and metal body burden evaluations, respectively. CuO NPs (primary size 22-25nm) that are very toxic to D. magna were included in the study as a positive control. Since the release of metal ions is an important possibility for toxicity of metal NPs, soluble Co-, Mn- and Cu-salts were analysed in parallel. The solubilisation of Co3O4 NPs in the OECD202 assay conditions was 0.1% and Mn2O3 NPs 35%. Mn2O3 NPs also produced reactive oxygen species in abiotic conditions. However Co3O4 and Mn2O3 NPs were not acutely toxic to D. magna (48h EC50>100mg metal/L) at OECD202 assay conditions. The 48h EC50 values of soluble Co- and Mn-salts were 3.2mgCo/L and 41mgMn/L, respectively. Post-exposure feeding behaviour after 48h exposure to sublethal concentrations (≤10mg/L) of Co3O4 and Mn2O3 NPs differed from that of the unexposed (control) D. magna only at the highest exposure concentrations but was comparable to the feeding behaviour of the respective metal salt-exposed organisms. Upon 48h exposure, dose-dependent increase of D. magna total metal body burden in case of both the NPs and the soluble salts was observed. After 48h post-exposure feeding with algae C. reinhardtii (depuration): D. magna body burden remained elevated (up to 760-fold compared to the control organism) only in case of the NPs. This may indicate potential for trophic transfer of NPs/heavy metals and thus hazard for freshwater ecosystem.

Stability of uncoated and fulvic acids coated manufactured CeO2 nanoparticles in various conditions: From ultrapure to natural Lake Geneva waters.

Understanding the behavior of engineered nanoparticles in natural water and impact of water composition in changing conditions is of high importance to predict their fate once released into the environment. In this study we investigated the stability of uncoated and Suwannee River fulvic acids coated CeO2 manufactured nanoparticles in various environmental conditions. The effect of pH changes on the nanoparticle and coating stability was first studied in ultrapure water as well as the variation of zeta potentials and sizes with time in presence of fulvic acids at environmental pH. Then the stability of CeO2 in synthetic and natural Lake Geneva waters was investigated as a function of fulvic acids concentration. Our results indicate that the adsorption of environmentally relevant concentrations of Suwannee River fulvic acids promotes CeO2 stabilization in ultrapure water as well as synthetic water and that the coating stability is high upon pH variations. On the other hand in natural Lake Geneva water CeO2 NPs are found in all cases aggregated due to the effect of heterogeneous organic and inorganic compounds.

Effects of pH and fulvic acids concentration on the stability of fulvic acids--cerium (IV) oxide nanoparticle complexes.

The behavior of cerium (IV) oxide nanoparticles has been first investigated at different pH conditions. The point of zero charge was determined as well as the stability domains using dynamic light scattering, nanoparticle tracking analysis and scanning electron microscopy. A baseline hydrodynamic diameter of 180 nm was obtained indicating that individual CeO2 nanoparticles are forming small aggregates. Then we analyzed the particle behavior at variable concentrations of fulvic acids for three different pH-electrostatic scenarios corresponding to positive, neutral and negative CeO2 surface charges. The presence of fulvic acids was found to play a key role on the CeO2 stability via the formation of electrostatic complexes. It was shown that a small amount of fulvic acids (2 mg L(-1)), representative of environmental fresh water concentrations, is sufficient to stabilize CeO2 nanoparticles (50 mg L(-1)). When electrostatic complexes are formed between negatively charged FAs and positively charged CeO2 NPs the stability of such complexes is obtained with time (up to 7 weeks) as well as in pH changing conditions. Based on zeta potential variations we also found that the fulvic acids are changing the CeO2 acid-base surface properties. Obtained results presented here constitute an important outcome in the domain of risk assessment, transformation and removal of engineered nanomaterials released into the environment.

Manufactured Nanoparticle Behavior and Transformations in Aquatic Systems. Importance of Natural Organic Matter.

Major concerns to elucidate the fate of nanomaterials and manufactured nanoparticles in aquatic systems are related to the lack of data on nanoparticle transformations under relevant environmental conditions. The present article discusses some of the important physicochemical processes controlling the behavior of manufactured nanoparticles in aqueous systems by focusing on their interaction with natural organic matter, which is expected to play a crucial role when adsorbing at the nanoparticle surface. The precise knowledge and consequences of such adsorption processes are important not only to predict the nanoparticle stability and dispersion state but also to evaluate their chemical reactivity and ecotoxicology. Most importantly, findings indicate that the presence of natural organic matter, at typical environmental concentrations, can induce significant disagglomeration of large nanoparticle agglomerates into small fragments. Such a result constitutes an important outcome with regard to the risk associated with the possible transformation and redispersion of large assemblies containing manufactured nanoparticles.