Photo-transformation of graphene oxide in the presence of co-existing metal ions regulated its toxicity to freshwater algae

Graphene oxide (GO) sheets are unstable in aqueous environments, and the effect of photo-transformation on GO toxicity to freshwater algae (Chlorella pyrenoidosa) was investigated. Our results demonstrated that GO underwent photo-reduction under 25-day sunlight irradiation, and the transformation was generally completed at Day 8. The toxicological investigation showed that 8-day sunlight irradiation significantly increased growth inhibition of GO (25 mg/L) to algal cells by 11.2%, due to enhanced oxidative stress and stronger membrane damage. Low molecular weight (LMW) species were produced during the 8-day GO transformation, and they were identified as two types of aromatic compounds, which played a crucial role in increasing toxicity. The combined toxicity of GO and Cu2+ ions before and after light irradiation was further investigated. Antagonistic effect was observed between the toxicity of pristine GO and co-existing Cu2+ ions. After co-irradiation of GO and Cu2+ ions for 8 days, their combined toxicity was unexpectedly lower or insignificant in comparison with the treatments of pristine GO, or pristine GO in the presence of Cu2+ ions. Two mechanisms were revealed for this finding: (1) Cu2+ ions suppressed the photo-transformation of GO; (2) the toxicity of free Cu2+ ions was decreased through the adsorption/retention of Cu2+ ions and formation of Cu-based nanoparticles (e.g., Cu2O and Cu2S) on the photo-transformed GO. The provided data are helpful for better understanding the environmental process and risk of GO under natural conditions. [Display omitted] •Photo-transformation significantly increased the GO toxicity to algal cells.•LMW species played a crucial role in increased toxicity.•After photo-transformation, the joint toxicity of GO and Cu2+ was decreased.•Cu2+ suppressed photo-transformation of GO.•Cu2+ was photo-transformed and formed to Cu2O and Cu2S nanoparticles on TGO surface.