The shell of Swietenia mahagoni was modified using sulfuric acid and ortho-phosphoric acid to improve the adsorption capacity for the removal of hexavalent chromium from aqueous solutions. The adsorbent characterization was done by using Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and thermogravimetric analysis (TGA). The adsorption of Cr(VI) using RSMS, SSMS and PSMS was investigated as a function of solution pH, adsorbent dosage, contact time, initial metal concentration and temperature in batch studies. The adsorption equilibrium data were well studied for adsorption isotherm models, kinetic models and thermodynamics. The adsorption data better followed the Langmuir model rather than the other models studied. An appreciable increase in the adsorption capacities of SSMS (47.61 mg/g) and PSMS (58.82 mg/g), was observed compared to RSMS (37.03 mg/g). The adsorption kinetic data were best described by a pseudo-second-order kinetic model. Thermodynamic parameters revealed the spontaneous, endothermic and increased randomness nature of the Cr(VI) adsorption process. Desorption analysis was performed with 0.1, 0.5 and 1 M concentrations of NaOH solutions. The results of the present study illustrated that RSMS, SSMS and PSMS can be used as effective adsorbents for the removal of hexavalent chromium from an aqueous system.