In this study, the sorption behavior of two important fission product radionuclides ((137)Cs and (140)Ba) onto sodium form of insolubilized humic acid (INaA) were investigated as a function of time, cation concentration and temperature, utilizing the radiotracer method. The sorption processes are well described by both Freundlich and Dubinin-Radushkevich type isotherms. Thermodynamic constants such as: free energy (Delta G(ads)), enthalpy (Delta H(ads)), entropy (Delta S(ads)) of adsorption were determined. It was found that Ba(2+) was adsorbed five times more than Cs(+) onto structurally modified humic acid and kinetic studies indicated that adsorption behavior of both ions obey the pseudo second order rate law. The effect of pH change on sorption was also examined. FTIR and solid-state carbon NMR ((13)CNMR) spectroscopic techniques were used to understand the structural changes during insolubilization process. Quantitative determination of adsorption sites was carried out using potantiometric titration. (c) 2009 Elsevier B.V. All rights reserved.