Influence of alumina coating on characteristics and effects of SiO2 nanoparticles in algal growth inhibition assays at various pH and organic matter contents

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 27.0 nm 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 (0–12 mg 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 ≥ 46 mg/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 220 mg/l. Addition of NOM decreased toxicity of both particles. For SiO2 NPs the 48 h 20% effect concentration of 21.8 mg/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 1000 mg/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.

Research Highlights
► An alumina coating on the surface of SiO2 NPs altered particle characteristics.
► Alumina coated SiO2 NPs aggregated and adsorbed PO43-, bare SiO2 NPs did not.
► Alumina coated SiO2 NPs showed lower toxicity than bare SiO2 NPs to green algae.
► Both particles showed a different pH dependent toxicity towards P. subcapitata.
► NOM decreased toxicity, with a linear relationship between NOM and ErCx.