Influence of light irradiance on the toxicity of graphitic carbon nitride to algae: Intensified nano-bio interactions driven by particle reassembly

Graphitic carbon nitride (g-C₃N₄) is increasingly applied in various fields involving water treatment, surely resulting in environmental release and ecological exposure. Nonetheless, the effect of light intensity on the toxicity of g-C₃N₄ and related mechanisms remain unexplored. Our study demonstrated that exposure to g-C₃N₄ sheets (1-3 µm, 0.5-10 µg/mL) at 10,500 lx induced significantly greater ultrastructure damage and growth inhibition (maximally 17.9 %) in freshwater algae (Chlorella vulgaris) in comparison to those at 1980 (6.45 %) and 5000 lx (11.9 %). Oxidative damage resulted from extracellular reactive oxygen species (ROS) was a predominant contributor to the toxicity. The increase of light irradiance down-regulated citrate cycle, amino acid, and fatty acid metabolisms, closely linked to the adverse effects triggered by g-C₃N₄. The distinct reassembly of g-C₃N₄ under varying light irradiances lead to differential nano-bio interactions with algae. At 10,500 lx, the introduction of negatively charged groups increased the hydrophilicity, strengthening the envelopment effect of g-C₃N₄ toward algae. The photoreactivity of g-C₃N₄ sheets was augmented due to bandgap reconfiguration, exacerbating the abiotic ROS generation. Our findings provide insights into the significant role and affecting mechanisms of light irradiance on the ecotoxicity of g-C₃N₄ sheets, which are critical for their ecological risk assessment in aquatic environments.