This study aims to more accurately quantify the contribution of each adsorption mechanism, in order to eliminate the influence of pickling on the heavy metal sorption capacity of biochar. Taking the cotton stalk as the raw materials, the specific procedure was as follows. Firstly, slow pyrolysis was selected to produce the biochar at the temperatures of 350 and 550 ℃. The pickling (HCl+HF) was then used to remove the influence of salt and silicon oxide on the sorption capacity. After that, the demineralized biochar was prepared from the cotton stalk. Taking the Pb2+ in the aqueous solution as the research object, a sorption experiment was carried out using the biochar and demineralized biochar to quantify the contribution of each sorption mechanism. Scanning electron microscope (SEM), energy dispersive spectrometer (EDS), X-ray photoelectron spectrometer (XPS), X-ray diffractometer (XRD), and Fourier transform infrared spectrometer (FTIR) were applied to characterize the microscopic morphology and physicochemical properties of all biochar samples before and after Pb2+ sorption. The FTIR peaks of all biochar samples before and after Pb2+ sorption indicated that the carboxyl and phenolic hydroxyl had participated in the sorption process through complexation. Boehm titration was used to detect the content of oxygen-containing functional groups (carboxyl, phenolic hydroxyl, and lactone) in all biochar samples before and after Pb2+ sorption. Furthermore, the Pb2+ sorption capacity of complexation was evaluated using the pH difference of equilibrium solution. The reason was that there was a significant decrease in the pH value of equilibrium solution during the complexation of oxygen-containing functional groups with Pb2+. The contribution rates of pickling and complexation between cotton stalk biochar and Pb2+ were determined for the actual Pb2+ sorption capacity, combing with the oxygen-containing functional groups. An Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) was used to detect the concentration of ions in the solution. The sorption capacity caused by ion exchange was then calculated by the net release of K+, Na+, Ca2+, and Mg2+ in the solution before and after Pb2+ sorption. As such, a quantitative dataset was achieved for the sorption capacity and contribution of each sorption mechanism. The results show that five mechanisms were involved in the sorption process, including the precipitation, ion exchange, π-electron interaction, complexation, and physical sorption. The effect of physical sorption was very weak to be ignored. The contribution of precipitation and π-electron interaction increased, whereas, the contribution of ion exchange and complexation decreased significantly, with the increase of pyrolysis temperature. Ca2+ and Mg2+ were dominated in the ion exchange adsorption, accounting for more than 95%. Consequently, the inorganic components were greatly contributed to the sorption of Pb2+ by cotton stalk biochar, where the contribution rates of precipitation and ion exchange were not less than 70.6% in the diverse adsorption mechanisms. This finding can provide a theoretical basis for the quantitative analysis of the heavy metal sorption mechanism of biochar/modified biochar. A feasible technical approach can be served as the resource utilization of waste cotton stalk, as well as the prevention and control of heavy metal pollution in water and soil.