This study reports the combination of new statistical physics models and density functional theory (DFT) for the analysis and understanding of the adsorption of heavy metals on a flamboyant biomass-based adsorbent. Single and ternary isotherms of the adsorption of Pb2+, Hg2+ and Zn2+ ions on an adsorbent obtained from the pyrolysis of flamboyant biomass were determined experimentally at pH 5 and 298-313 K. Experimental studies showed that the adsorption of Hg2+ on flamboyant char was higher than those of Zn2+ and Pb2+ in both single and ternary solutions. A strong antagonistic effect for the multicomponent adsorption of Zn2+ was identified due to the presence of other competitive metal ions in the ternary solution. The endothermic adsorption mechanism involved in both mono-and multi-metallic solutions was explained via the integration of the results from the adsorbent physicochemical characterization and calculations using statistical physics models and density functional theory. Theoretical studies showed that the single-compound adsorption of heavy metals correlated with the calculated DFT binding energies of oxygen-containing functionalities of flamboyant surface. On the other hand, the electronegativity and the thermodynamic stability of the metal - active site complexes determined the adsorbent selectivity and adsorption capacities in the simultaneous removal of these heavy metals. In particular, the carboxylic functional groups of flamboyant char were the main active sites involved in the adsorption of these heavy metals in both mono- and multi-component aqueous solutions. These new findings contribute to a deeper understanding of the mechanisms involved in the multicomponent adsorption of relevant water pollutants such as heavy metals using carbon-based adsorbents.