Biochar Nanoparticles Induced Distinct Biological Effects on Freshwater Algae via Oxidative Stress, Membrane Damage, and Nutrient Depletion

Long-term application of bulk biochar (BC) in soil or water environment could result in physical disintegration of BC into smaller particle sizes, including the formation of BC nanoparticles. The presence of nano-BC may pose a threat to aquatic organisms due to its nanoeffects and unique physicochemical properties. However, reliable assessments of the environmental risk presented by nano-BCs are limited, particularly the effects of nano-BC from various feedstocks on freshwater algae. In this study, three different types of nano-BCs were obtained from rice husk BC (RBC), wood sawdust BC (WBC), and corn straw BC (CBC) at a pyrolysis temperature of 350 °C (labeled as nano-RBC, nano-WBC, and nano-CBC, respectively). Analysis by transmission electron microscopy (TEM) and dynamic light scattering (DLS) showed that the particle sizes of the generated nano-BCs ranged from 177.5 ± 10.2 to 239.1 ± 34.4 nm. The ester ring (C–O–C) was present in only nano-RBC and nano-CBC, while phenolates were detected in only nano-WBC. Subsequently, the growth of algae Chlorella vulgaris was investigated along with its physiological indices after 72 h exposure to nano-BCs in an aquatic environment. Results showed that all nano-BCs inhibited algal growth in the order nano-WBC > nano-RBC > nano-CBC. Additionally, nano-BCs induced both oxidative stress and physical damage in algae, which was attributed to the overproduction of intercellular reactive oxygen species (ROS). The adsorption of nutrients on nano-BCs could be a key factor in their distinct biological effects on algae. These findings provide helpful reference information on the biological effects of nano-BCs on aquatic organisms, supporting the effective long-term management of BC treatment systems and the evaluation of appropriate BC feedstock for natural aquatic environment applications.