The natural clay mineral was activated using the acid treatment under ultrasonic conditions to improve its adsorption performance. The acid-activated clay (AAC) was investigated to remove Cr(VI) from wasted lubricant and U(VI) from aqueous solution by batch, spectroscopy, and modeling. Batch experiments showed that the isotherms and adsorption kinetics of Cr(VI)/U(VI) on AAC were satisfactorily followed by Frenudlich and the pseudo-second-order model, respectively. The decreased adsorption of U(VI) on AAC with the increase of ion strength indicated the outer surface complexation of U(VI) on AAC. The interaction mechanism of Cr(VI)/U(VI) on AAC was demonstrated to be surface complexation by oxygenated functional groups (e.g., hydroxyl) according to X-ray photoelectron spectroscopy (XPS) analysis. According to fitting of surface complexation models using three approaches, the adsorption of U(VI) on AAC at different pH can be better fitted by the double-layer model compared to the constant-capacitance model and triple-layer model. These findings are great important for the application of acid activated clay in the removal of heavy metals and radionuclides from the environment.