In the present study, cypress cone raw biochar was modified using walnut peel extract under the chemical modification process and its performance optimization on nickel (Ni) removal in aqueous solutions was affected by various factors was evaluated using the RSM. Contrary to expectation, chemical modification caused a decrease in the specific surface area and a decrease in effective functional groups in Ni removal (functional groups containing oxygen and C-C) compared to the non-modified biochar sample. The investigations of Ni removal by modified biochar also showed a significant reduction of Ni removal by this adsorbent in the same conditions compared to the non-amended sample, so that the average amount of Ni removal in the same conditions in the non-modified adsorbent was 1.8 times more than modified biochar. The investigation of the factors affecting the removal of Ni in two modified and non-modified adsorbents showed that with the increase of the initial concentration of Ni, the removal of Ni had a decreasing trend; while with the increase of other factors, Ni removal showed a relatively increasing trend. The results of the present study show that the modification and engineering of biochars by the mentioned chemical method, with the effect of reducing the physical absorption (reduction the specific surface area) and chemical adsorption (reduction of the intensity of the active groups affecting the adsorption), caused the reduction of Ni removal efficiency compared to the raw sample, which challenges the hypothesis of constant increase of heavy metal removal efficiency by chemical modification of biochar.