The optimization of nickel ion (Ni2+) removal in aqueous solutions with various factors (initial Ni concentration, solution pH, adsorbent dosage, contact time), as affected by raw cow bone (RBO) and its biochar (bone char: BC; produced by pyrolysis processes at 500 °C and a residence time of 4 hours) as adsorbents was investigated by a three-level Box–Behnken model (BBM) under response surface methodology (RSM). A total of 29 experimental runs were set for each adsorbent, and the experimental data were fitted to the empirical model. To understand the Ni2+ adsorption processes better, the properties of RBO and BC were characterized using Fe-SEM, FT-IR, BET, XRD, and CHNS elemental analysis techniques. The BC characteristics showed that pyrolysis increased the specific surface area (by 100 times) and phosphate functional groups, but decreased the carbonate functional groups, and yielded a more irregular and rougher morphological surface compared to RBO. Based on BC's superior ion exchange mechanisms and physical electrostatic adsorption compared to RBO, the removal efficiency of Ni2+ by BC was higher in aqueous solutions. The numerical optimization of BBM revealed that the optimum removal by BC (82.56%) was obtained at an initial Ni2+ concentration of 30.79 mg L−1, pH of 6.99, adsorbent dose of 4.87 g L−1, and contact time of 57.82 min, with the desirability of "1". BC can be effectively used for Ni removal from Ni-contaminated aqueous solutions; still, the application of modification methods (e.g., physical and chemical activation) may be necessary to help remove more Ni2+ by BC.