Formation of Nanoscale Biochar-EPS Corona: Adsorption and Toxicity

Biochar (BC) can be coated with microbial extracellular polymeric materials (EPS) to form BC-EPS corona in an aqueous environment, which may significantly alter its ecological toxicity caused by EPS-facilitated accumulation of environmental contaminants. This work examined the EPS corona formation on the surface of shrub branch BC and wheat straw BC, pyrolyzed at 450 °C and 650 °C, and evaluated its effect on the sorption of divalent metals Cd²⁺, Pb²⁺, Cu²⁺, and Ni²⁺. In single metal systems, the formation of a BC-EPS corona shows little enhancement on Cd²⁺ and Cu²⁺ accumulation, but significantly elevated Ni²⁺ and Pb²⁺ adsorption: Ni²⁺ sorption increased from 20 mg/g for pristine BCs to 30 mg/g, Pb²⁺ sorption increased from below 25 mg/g to above 90 mg/g. In multiple metal systems, the binding distribution of Pb²⁺ and Cu²⁺ on BC-EPS corona appeared to be more extensive compared to that of Cd²⁺ and Ni²⁺. The toxicity of BC-EPS increased by about 2 to 3 times in aquatic organisms compared to pristine BC, attributed to the higher accumulation of heavy metals in BC-EPS corona. In summary, BC-EPS corona enhanced BC’s sorption capacity for metals, especially for Pb²⁺ and Ni²⁺, thus elevating the BC’s ecological risks owing extra amount of pollution loading.