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Abstract: Silica nanoparticles (NPs) belong to the industrially most important NP types. In a previous study it was shown that amorphous SiO2 NPs of 12.5 and 27.0nm are stable in algal growth inhibition assays and that their ecotoxic effects are related to NP surface area. Here, it was hypothesized and demonstrated that an alumina coating completely alters the particle–particle, particle–test medium and particle–algae interactions of SiO2 NPs. Therefore, stability and surface characteristics, dissolution, nutrient adsorption and effects on algal growth rate of both alumina coated SiO2 NPs and bare SiO2 NPs in OECD algal test medium as a function of pH (6.0–8.6) and natural organic matter (NOM) contents (0–12mg C/l) were investigated. Alumina coated SiO2 NPs aggregated in all media and adsorbed phosphate depending on pH and NOM concentration. On the other hand, no aggregation or nutrient adsorption was observed for the bare SiO2 NPs. Due to their positive surface charge, alumina coated SiO2 NPs agglomerated with Pseudokirchneriella subcapitata. Consequently, algal cell density measurements based on cell counts were unreliable and hence fluorescent detection of extracted chlorophyll was the preferred method. Alumina coated SiO2 NPs showed lower toxicity than bare SiO2 NPs at concentrations ≥46mg/l, except at pH 6.0. At low concentrations, no clear pH effect was observed for alumina coated SiO2 NPs, while at higher concentrations phosphate deficiency could have contributed to the higher toxicity of those particles at pH 6.0–6.8 compared to higher pH values. Bare SiO2 NPs were not toxic at pH 6.0 up to 220mg/l. Addition of NOM decreased toxicity of both particles. For SiO2 NPs the 48h 20% effect concentration of 21.8mg/l increased 2.6–21 fold and a linear relationship was observed between NOM concentration and effective concentrations. No effect was observed for alumina coated SiO2 NPs in presence of NOM up to 1000mg/l. All experiments point out that the alumina coating completely altered NP interactions. Due to the difference in surface composition the SiO2 NPs, which had the smallest surface area, were more toxic to the alga than the alumina coated SiO2 NPs. Hence, surface modification can dominate the effect of surface area on toxicity. [Copyright &y& Elsevier]

The influence of pH (6.0–9.0), natural organic matter (NOM) (0–10 mg C/L) and ionic strength (IS) (1.7–40 mM) on 14 nm CeO2 NP aggregation and ecotoxicity towards the alga Pseudokirchneriella subcapitata was assessed following a central composite design. Mean NP aggregate sizes ranged between 200 and 10000 nm. Increasing pH and IS enhanced aggregation, while increasing NOM decreased mean aggregate sizes. The 48 h-ErC20s ranged between 4.7 and 395.8 mg CeO2/L. An equation for predicting the 48 h-ErC20 (48 h-ErC20 = −1626.4 × (pH) + 109.45 × (pH)2 + 116.49 × ([NOM]) − 14.317 × (pH) × ([NOM]) + 6007.2) was developed. In a validation study with natural waters the predicted 48 h-ErC20 was a factor 1.08–2.57 lower compared to the experimental values. [Copyright &y& Elsevier]

Cerium dioxide nanoparticles (CeO2 NPs) are increasingly being used as a catalyst in the automotive industry. Consequently, increasing amounts of CeO2 NPs are expected to enter the environment where their fate in and potential impacts are unknown. In this paper we describe the fate and effects of CeO2 NPs of three different sizes (14,20, and 29 nm) in aquatic toxicity tests. In each standard test medium (pH 7.4) the CeO2 nanoparticles aggregated (mean aggregate size approximately 400 nm). Four test organisms covering three different trophic levels were investigated, i.e., the unicellular green alga Pseudokirchneriella subcapitata, two crustaceans: Daphnia magna and Thamnocephalus platyurus, and embryos of Danio rerio. No acute toxicity was observed for the two crustaceans and B. rerio embryos, up to test concentrations of 1000, 5000, and 200 mg/L, respectively. In contrast, significant chronic toxicity to P. subcapitata with 10% effect concentrations (EC10s) between 2.6 and 5.4 mg/L was observed. Food shortage resulted in. chronic toxicity to B. magna, for wich EC10s of ≥8.8 and ≤20.0 mg/L were established. Chronic toxicity was found to increase with decreasing nominal particle diameter and the difference in toxicity could be explained by the difference in surface area. Using the data set, PNECaquaticS ≥ 0.052 and ≤ 0.108 mg/L were derived. Further experiments were performed to explain the observed toxicity to the most sensitive organism, i.e., P. subcapitata. Toxicity could not be related to a direct effect of dissolved Ce or CeO2 NP uptake or adsorption, nor to an indirect effect of nutrient depletion (by sorption to NPs) or physical light restriction (through shading by the NPs). However, observed clustering of NPs around algal cells may locally cause a direct or indirect effect. [ABSTRACT FROM AUTHOR]

The ecological risk assessment of chemicals including nanoparticles is based on the determination of adverse effects on organisms and on the environmental concentrations to which biota are exposed. The aim of this work was to better understand the behavior of nanoparticles in the environment, with the ultimate goal of predicting future exposure concentrations in water. We measured the concentrations and particle size distributions of CeO(2) nanoparticles in algae growth medium and deionized water in the presence of various concentrations and two types of natural organic matter (NOM). The presence of natural organic matter stabilizes the CeO(2) nanoparticles in suspension. In presence of NOM, up to 88% of the initially added CeO(2) nanoparticles remained suspended in deionized water and 41% in algae growth medium after 12d of settling. The adsorbed organic matter decreases the zeta potential from about -15 mV to -55 mV. This reduces aggregation by increased electrostatic repulsion. The particle diameter, pH, electric conductivity and NOM content shows significant correlation with the fraction of CeO(2) nanoparticles remaining in suspension., (Copyright © 2010 Elsevier Ltd. All rights reserved.)

Cerium dioxide nanoparticles (CeO2 NPs) are increasingly being used as a catalyst in the automotive industry. Consequently, increasing amounts of CeO2 NPs are expected to enter the environment where their fate in and potential impacts are unknown. In this paper we describe the fate and effects of CeO2 NPs of three different sizes (14, 20, and 29 nm) in aquatic toxicity tests. In each standard test medium (pH 7.4) the CeO2 nanoparticles aggregated (mean aggregate size approximately 400 nm). Four test organisms covering three different trophic levels were investigated, i.e., the unicellular green alga Pseudokirchneriella subcapitata, two crustaceans: Daphnia magna and Thamnocephalus platyurus, and embryos of Danio rerio. No acute toxicity was observed for the two crustaceans and D. rerio embryos, up to test concentrations of 1000, 5000, and 200 mg/L, respectively. In contrast, significant chronic toxicity to P. subcapitata with 10% effect concentrations (EC10s) between 2.6 and 5.4 mg/L was observed. Food shortage resulted in chronic toxicity to D. magna, for wich EC10s of > or = 8.8 and < or = 20.0 mg/L were established. Chronic toxicity was found to increase with decreasing nominal particle diameter and the difference in toxicity could be explained by the difference in surface area. Using the data set, PNEC(aquatic)S > or = 0.052 and < or = 0.108 mg/L were derived. Further experiments were performed to explain the observed toxicity to the most sensitive organism, i.e., P. subcapitata. Toxicity could not be related to a direct effect of dissolved Ce or CeO2 NP uptake or adsorption, nor to an indirect effect of nutrient depletion (by sorption to NPs) or physical light restriction (through shading by the NPs). However, observed clustering of NPs around algal cells may locally cause a direct or indirect effect.