Humic acid mitigated toxicity of graphene-family materials to algae through reducing oxidative stress and heteroaggregation
Graphene-family materials (GFMs) will be released into natural aquatic environments during their increasing applications, thus likely inducing adverse effects on aquatic organisms. This work systematically investigated the effect of natural organic matter on the toxicity of GFMs to algae (Chlorella pyrenoidosa). Toxicity antagonism was observed between humic acid (HA) and all the three types of GFMs, and the degree of antagonism in the presence of HA followed the order reduced graphene oxide (rGO) > graphene oxide (GO) > graphene (G). rGO showed the highest mitigation in membrane damage (29.3%) by HA in comparison with GO (22.9%) and G (28.4%), demonstrating that the reduction in membrane damage was a main mechanism for toxicity mitigation by HA. HA could alleviate GFM-induced membrane damage through decreasing oxidative stress, as confirmed by the lower intracellular reactive oxygen radical (ROS) and malondialdehyde content in the presence of HA. The decrease in direct contact between GFMs and algae was another reason for the membrane damage mitigation, and the direct contact was lowered by weakened GFM–algae heteroaggregation (for rGO and G) and enhanced steric hindrance (for GO, rGO and G). In addition, for GO, the nutrient depletion correction (e.g., Mg) was also responsible for the toxicity mitigation by HA, while HA did not correct nutrient depletion that was induced by rGO and G. These findings suggest that natural organic matter is of importance for better understanding the environmental risk of GFMs in aquatic environments.