This study provides a one-step pyrolysis method for the preparation of nanosized zero-valent iron biochar. A spherical ZVI/Biochar composite (ZVIBC) was successfully prepared via carbonization of Fe3+-crosslinked sodium alginate. The effects of pH, Cd(II) initial concentration, adsorbent time, coexisting anions, and aging under air on the adsorption of Cd(II) by ZVIBC are studied. The results of physico-chemical characterization obtained for the ZVIBC loaded with Cd(II) (FTIR, XRD, XPS, and EDS) allowed to propose the mechanism of adsorption. The results showed that the pH was a strong impact on the adsorption of Cd(II) by ZVIBC，4 was selected as the optimum pH value and used throughout the following experiments. The Langmuir model better describe the adsorption of Cd(II) on ZVIBC with the maximum adsorption capacity of 240 mg/g. The main adsorption mechanisms were that the active functional groups (O—H、C—O、C=C、C=O、COO) formed complexes with Cd(II), and Cd(II) generated Cd(OH)2 precipitation with Fe2+ on ZVIBC surface.