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 degrees C, 95 degrees C and 2 degrees C water to enhance the pore development and adsorption of Cr (VI) and Pb (II) from aqueous solutions. The activated carbons were characterized by Fouriertransform-infrared-spectroscopy, N-2-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 m(2)/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. (C) 2020 Elsevier B.V. All rights reserved.