Adsorption of heavy metals on modified orange biomass has been studied. This biomass was treated with NaOH and CaCl₂ to improve its adsorption properties. Kinetic and thermodynamic studies of the adsorption of Cd2+, Ni2+, Cu2+, and Zn2+ were performed at different operating conditions, including competitive adsorption studies with binary metallic mixtures. Results show that this adsorption process was endothermic where an ion exchange mechanism played a relevant role. Adsorbent effectiveness decreased in binary solutions, indicating a strong antagonistic adsorption behavior caused by counter-ions. This antagonistic adsorption was highly dependent on the counter-ion type and its concentration. Multicomponent adsorption of Cu2+ ions was not significantly affected by the presence of other metallic counter-ions, whereas the adsorption of Ni2+ could be totally suppressed by the other ions. The modeling of binary adsorption isotherms was successful using the modified Langmuir equation, which outperformed the Ideal Absorbed Solution Theory-Sips and modified Redlich-Peterson models.