Ecotoxicological Assessment of Thermally- and Hydrogen-Reduced Graphene Oxide/TiO₂ Photocatalytic Nanocomposites Using the Zebrafish Embryo Model

Advanced oxidation processes (AOPs) have recently attracted great interest in water pollution management. Using the zebrafish embryo model, we investigated the environmental impacts of two thermally (RGOTi)- and hydrogen (H₂RGOTi)-reduced graphene oxide/TiO₂ semiconductor photocatalysts recently employed in AOPs. For this purpose, acutoxicity, cardiotoxicity, neurobehavioral toxicity, hematopoietic toxicity, and hatching rate were determinate. For the RGOTi, the no observed effect concentration (NOEC, mortality/teratogenicity score <20%) and the median lethal concentration (LC50) were <400 and 748.6 mg/L, respectively. H₂RGOTi showed a NOEC similar to RGOTi. However, no significant mortality was detected at all concentrations used in the acutoxicity assay (up to1000 mg/L), thus indicating a hypothetical LC50 higher than 1000 mg/L. According to the Fish and Wildlife Service Acute Toxicity Rating Scale, RGOTi can be classified as "practically not toxic" and H₂RGOTi as "relatively harmless". However, both nanocomposites should be used with caution at concentration higher than the NOEC (400 mg/L), in particular RGOTi, which significantly (i) caused pericardial and yolk sac edema; (ii) decreased the hatching rate, locomotion, and hematopoietic activities; and (iii) affected the heart rate. Indeed, the aforementioned teratogenic phenotypes were less devastating in H₂RGOTi-treated embryos, suggesting that the hydrogen-reduced graphene oxide/TiO₂ photocatalysts may be more ecofriendly than the thermally-reduced ones.