From biochar to functions: Lignin induced formation of Fe3C in carbon/Fe composites for efficient adsorption of tetracycline from wastewater

•Carbon/Fe adsorbents were successfully prepared by carbonizing lignin-doped MOFs.•Carbon-Fe3C/lignin exhibited excellent adsorption capacity on tetracycline and Cr(VI).•Chemical adsorption and physical adsorption were mainly adsorption mechanisms.•Carbon-Fe3C/lignin can be easily recovered through magnetic separation.