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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 Cd
2+
, Pb
2+
, Cu
2+
, and Ni
2+
. In single metal systems, the formation of a BC-EPS corona shows little enhancement on Cd
2+
and Cu
2+
accumulation, but significantly elevated Ni
2+
and Pb
2+
adsorption: Ni
2+
sorption increased from 20 mg/g for pristine BCs to 30 mg/g, Pb
2+
sorption increased from below 25 mg/g to above 90 mg/g. In multiple metal systems, the binding distribution of Pb
2+
and Cu
2+
on BC-EPS corona appeared to be more extensive compared to that of Cd
2+
and Ni
2+
. 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 Pb
2+
and Ni
2+
, thus elevating the BC’s ecological risks owing extra amount of pollution loading.
Graphical Abstract