Biochar has good adsorption ability to various contaminants. In this work, peanut shell, corncob, cotton stalks, and crayfish shell were pyrolyzed under three temperatures (300, 450, 600 °C) to obtain biochars for the removal of Ni2+. The biochars were further modified with 2 mol/L Na2S solution. Characterization results showed that the specific surface area and total pore volume of the modified biochars increased substantially. Among all the adsorbents, the modified corncob biochar (450 °C) showed the best Ni2+ adsorption. The adsorption kinetics followed the Elovich model with an equilibrium time of 24 h. The maximum capacity of the modified biochar reached 15.40 mg/g. The adsorption process was affected by pH, temperature, and coexisting ions. Increasing pH (under 7) provided more adsorption sites which enhanced adsorption capacity. Experimental results also indicated that the main adsorption mechanism of Ni2+ was ion exchange. Findings from this work suggest that modified biochar can be used as an effective adsorbent for the removal of Ni2+ from wastewater. Graphical abstract ᅟ.