This study aimed to investigate the effects of cypress cone biochar (RB) and its modifications (chitosan-coated biochar (CB), acid-modified biochar (AB), alkali-modified biochar (ALB), humic acid-coated biochar (HB), walnut shell residue extraction-coated biochar (WB), ball-milled biochar (BMB)) on the behavior of nickel (Ni) in Ni-spiked calcareous soil. The biochars were added separately to the soil at the rates of 1.5% and 3%, and the Ni behavior was investigated using methods of desorption kinetics and sequential extraction. The modification process significantly changed the specific area and functional groups of BMB, CB and HB compared to RB. Application of CB (26.3%), HB (28.7%), ALB (28.8%), BMB (30.7%), AB (31.9%) and RB (32.6%) showed the most effects on reduction of Ni mobility factor compared to control soil sample (38.8%). The results of Ni desorption showed high effectiveness of CB, BMB, ALB and HB, in reducing the Ni release compared to the control soil sample. Overall, our findings indicated that among the modified biochars, BMB, CB and HB showed a good performance in Ni stabilization, due to the simple and low-cost process of physical modification. BMB can be suggested as a suitable amendment for soil Ni immobilization.
