Combined toxicity of biochar with nanoplastics or silver nanoparticles toward Chlorella vulgaris

Biochar is being increasingly used in the remediation of aquatic environments given its unique physicochemical properties. Biochar can coexist with various other nanoparticles in the environment and thus have a combined effect on susceptible organisms. In this study, we investigated the toxicity of corn stover biochar prepared at different temperatures to Chlorella vulgaris alone and in combination with polystyrene nanoplastics (PS NPs, PSCOOH NPs, and PS-NH2 NPs) or silver nanoparticles embedded in sodium citrate (CIT-Ag NPs). Inorganic phosphorus promoted algal growth, while small organic molecules were important for the biotoxicity of the biochar. Biochar affected algal cell growth through mechanisms such as disruption of cell membrane integrity, generating oxidative stress, and affecting chlorophyll a synthesis. Biochar mitigated the toxicity of nanoplastics/ silver nanoparticles (Ag NPs) to C. vulgaris by increasing the algal cell membrane integrity, reducing ROS production, and increasing the chlorophyll a content of algal cells. The conclusions from the study were i) the substantial binding of biochar and nanoplastics/Ag NPs is due to electrostatic interactions and mutual adsorption, thereby reducing the number of dispersed nanoparticles; ii) there is competitive adsorption of both biochar and nanoplastics/Ag NPs on C. vulgaris, with biochar being dominant. In summary, biochar possesses environmental biotoxicity, but it has a noticeable mitigating effect on other, more toxic nanomaterials and is therefore an important factor in the biosafety of biochar materials.