Chronic toxicity of core-shell SiC/TiO2 (nano)-particles to Daphnia magna under environmentally relevant food rations in the presence of humic acid.

To date, research on the toxicity and potential environmental impacts of nanomaterials has predominantly focused on relatively simple and single-component materials, whilst more complex nanomaterials are currently entering commercial stages. The current study aimed to assess the long-term and size-dependent (60 and 500 nm) toxicity of a novel core-shell nanostructure consisting of a SiC core and TiO₂ shell (SiC/TiO₂, 5, 25, and 50 mg L^-1) to the common model organism Daphnia magna. These novel core-shell nanostructures can be categorized as advanced materials. Experiments were conducted under environmentally realistic feeding rations and in the presence of a range of concentrations of humic acid (0.5, 2, 5, and 10 mg L^-1 TOC). The findings show that although effect concentrations of SiC/TiO₂ were several orders of magnitude lower than the current reported environmental concentrations of more abundantly used nanomaterials, humic acid can exacerbate the toxicity of SiC/TiO₂ by reducing aggregation and sedimentation rates. The EC₅₀ values (mean ± standard error) based on nominal SiC/TiO₂ concentrations for the 60 nm particles were 28.0 ± 11.5 mg L^-1 (TOC 0.5 mg L^-1), 21.1 ± 3.7 mg L^-1 (TOC 2 mg L^-1), 18.3 ± 5.4 mg L^-1 (TOC 5 mg L^-1), and 17.8 ± 2.4 mg L^-1 (TOC 10 mg L^-1). For the 500 nm particles, the EC50 values were 34.9 ± 16.5 mg L^-1 (TOC 0.5 mg L^-1), 24.8 ± 5.6 mg L^-1 (TOC 2 mg L^-1), 28.0 ± 10.0 mg L^-1 (TOC 5 mg L^-1), and 23.2 ± 4.1 mg L^-1 (TOC 10 mg L^-1). We argue that fate-driven phenomena are often neglected in effect assessments, whilst environmental factors such as the presence of humic acid may significantly influence the toxicity of nanomaterials.