The effects of humic acid on the toxicity of graphene oxide to Scenedesmus obliquus and Daphnia magna.

The wide production and application of graphene oxide (GO) has inevitably caused its release to the aquatic ecosystem. However, the influence of natural organic matter (NOM) on the toxicity of GO to aquatic organisms needs further investigation. In this study, we conducted several toxicity tests (i.e., acute toxicity and oxidative damage) with Scenedesmus obliquus (S. obliquus) and Daphnia magna (D. magna), as well as a chronic toxicity test with D. magna, to investigate the toxicity of GO with or without the presence of humic acid (HA). Our results showed that GO induced significant toxicity to S. obliquus and D. magna, and the median lethal concentrations (72 h-LC and 48 h-LC) for acute toxicity were 20.6 and 84.2 mg L, respectively, while the 21 d-LC for chronic toxicity to D. magna was 3.3 mg L. Additionally, HA mitigated the acute toxicity of GO to S. obliquus and D. magna by 28.6% and 32.3%, respectively, and mitigated the chronic toxicity of GO to D. magna. In the presence of HA, the decreased toxicity of GO was attributed to the alleviation of oxidative damage by HA to both S. obliquus and D. magna, the mitigation of surface envelopment to S. obliquus and the body accumulation in D. magna. Our study provides useful and basic biotoxicity data of GO with a consideration of its interaction with NOM which could aid in preventing an overestimation of the risks of GO to the natural aquatic environment.