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 Cu ions before and after light irradiation was further investigated. Antagonistic effect was observed between the toxicity of pristine GO and co-existing Cu ions. After co-irradiation of GO and Cu 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 Cu ions. Two mechanisms were revealed for this finding: (1) Cu ions suppressed the photo-transformation of GO; (2) the toxicity of free Cu ions was decreased through the adsorption/retention of Cu ions and formation of Cu-based nanoparticles (e.g., CuO and CuS) on the photo-transformed GO. The provided data are helpful for better understanding the environmental process and risk of GO under natural conditions.