Humic acid-mediated reduction in toxicity of CoO NPs towards freshwater and marine microalgae in surfactant mixed medium.

The ever-increasing applications of CoO nanoparticles (NPs) have posed a serious concern about their discharge in the aquatic environment and ecotoxic implications. Being toxic towards aquatic species, the impact of other aquatic components such as dissolved organic matter (DOM), salinity, and surfactants are not studied sufficiently for their effect on the stability and ecotoxicity of CoO NPs. The present study aims at the influence of humic acid (HA) on the toxicity of CoO NPs in freshwater (C. minutissima) and marine (T. suecica) microalgae under surfactants mixed medium. The measure of % reduction in biomass and photosynthetic pigment were used as toxicity endpoints. Among various tested concentrations of HA, 25 mg/L HA was found suitable to minimize the NP's toxicity with or without the presence of surfactants. CoO NPs mediated reduction in biomass of C. minutissima was significantly minimized by the cumulative effect of HA with T80 (51.68 ± 4.55%) followed by CTAB (46.23 ± 5.62%) and SDS (42.60 ± 2.46%). Similarly, HA with T80 (26.93 ± 6.38%) followed by SDS (17.02 ± 6.64%) and CTAB (13.01 ± 3.81%) were found to minimize the growth inhibitory effect of CoO NPs in T. suecica. The estimation of chlorophyll - a content also indicated that microalgae treated with HA could maintain their photosynthetic ability more than control even in the co-presence of surfactants. Also, the reduced toxicity of CoO NPs were attributed to an increase in hydrodynamic sizes of HA-treated CoO NPs in both marine media (f/2) and freshwater media (BG11) due to increased aggregation and faster sedimentation of CoO NPs.