Environmental impact assessment of nanomaterials in the aquatic environment

Abstract

Nanotechnology is one of the fastest-growing technologies in the world. Silver nanomaterials (Ag NMs) and multi walled carbon nanotubes (MWCNTs) are the most commonly used NMs in consumer products for their antimicrobial properties and for their unique mechanical and electrical properties. Their use can lead to contamination of the aquatic environment. In this research study, the freshwater microalga Raphidocelis subcapitata was used to identify the impacts of NMs on the aquatic environment. The aim of this study was to carry out an evaluation of the toxicity of Ag NMs and MWCNTs via acute exposure to R. subcapitata. Moreover, two types of sonication – bath and probe – were used for comparison, to evaluate how suspension methods may affect NMs. DLS and TEM were used for NM characterisation. The probe-sonication dispersion method was a more effective process to prepare Ag NM suspensions since they were dispersed better, compared to bath sonication. However, the probe sonication was not an appropriate way for MWCNTs preparation. The OECD 201 test guideline was used to evaluate the toxicity of representative NM at 24, 48 and 72 h. Effects were assessed by using the following four different methods: chlorophyll extraction; optical density; protein content, and photosynthetic activity. The impact of the sonication method (bath or probe) used to prepare NM dispersions on the toxicity to algae was also evaluated. Overall outputs, based on chlorophyll extraction, demonstrated a high sensitivity of R. subcapitata exposed to Ag NMs and MWCNTs, which led to an increase in growth inhibition with increasing concentration of the tested materials. The chlorophyll extraction method was the best method for evaluating the algal biomass with a clear concentration response and no interference of the NMs with the method. Evidence for internalisation of Ag NMs into the R. subcapitata cells was shown for Ag NM dispersed using the probe-sonication methods, lower levels of Ag NM uptake were observed for bath sonicated Ag NMs. Finally, the role of oxidative stress in NM toxicity to algae was investigated. This highlighted an enhanced anti-oxidant defence system of R. subcapitata when subjected to NM suspensions prepared by probe sonication, as compared to bath sonication. In summary, this research has shown that Ag NMs are more toxic to R. subcapitata than MWCNTs and that the sonication method used to suspend the NMs affects toxicity, as Ag NMs show higher toxicity when dispersed by bath sonication, whereas MWCNTs are more toxic when dispersed via probe sonication. It is of the utmost importance to consider carefully NM dispersion approaches, particularly when devising standard methods. It is also very important to understand how dispersant NMs may be affected by different dispersion methods.