Modification of activated carbon is considered as an effective process to enhance its adsorptive characteristics. The KOH activated carbons prepared from elm tree waste were treated by a novel and green method using thermal tension by successive washing with 80 °C, 95 °C and 2 °C water to enhance the pore development and adsorption of Cr (VI) and Pb (II) from aqueous solutions. The activated carbons were characterized by Fourier-transform-infrared-spectroscopy, N₂-adsorption/desorption and scanning-electron-microscopy. The simulation of thermal stress using a finite-element-analysis implied that the quick temperature changes might cause the formation of cracks leading to enhancement of porosity. The BET surface area was increased from 465 to 1085 m²/g after post-modification of the activated carbon. The maximum adsorption capacity was achieved 232.56 and 113.63 mg/g for Pb (II) and Cr (VI), respectively. The thermal tension could be used as an efficient process to improve the adsorptive properties of the activated carbons.