Biochar derived from agricultural byproducts is increasingly being recognized as a promising adsorbent material for wastewater remediation. This research is evaluating the effect of chemical treatment solutions on activating the sorption capacity of biochar derived from cotton stalks. The surface characteristics of chemically modified biochar (CMB) were investigated with scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (SEM-EDS) and Fourier Transform Infrared (FTIR) spectroscopy. Using batch adsorption experiments, biochar pretreated with sulfuric acid (SAB), oxalic acid (OAB), sodium hydroxide (SHB) and the untreated biochar (UTB) were subjected to assess their removal efficiency of heavy metal ions (Pb2+, Cd2+, Ni2+ and Co2+) as compared with active coal (AC). Biochar showed a higher sorption capacity of heavy metal ions as compared with AC. Biochar pretreated with acid solutions (SAB and OAB) exhibited a low removal efficiency of heavy metal ions as compared with other pretreatment solutions. SHB was the most effective product for adsorption of Pb2+ (44.64 mg g-1), Cd2+ (0.648 mg g-1) and Ni2+ (6.20 mg g-1); however, UTP showed the highest adsorption capacity of Co2+ (0.522 mg g-1). These results are confirmed with FTIR and SEM-EDS analyses. Sorption equilibrium isotherms could be described by the Langmuir model with a correlation coefficient higher than 90% in most cases.