Removal of Tc-99(VII) by organo-modified bentonite

Abstract

(99)Technetium is one of the largest components of nuclear waste material. In aqueous solutions Tc-99 is present as the pertechnetate oxoanion, (TcO4-)-Tc-99, which is highly soluble and mobile in groundwater under oxidizing conditions, thus posing a major environmental concern. In this study, a series of organo-modified bentonite clay samples was tested as adsorbents for the removal of radioactive (TcO4-)-Tc-99 from aqueous solution at room temperature. The influence of the hexadecyl trimethylammonium (HDTMA)/bentonite ratio on the adsorption ability of the organobentonites was investigated. It was found that the adsorbent in which the exchangeable cations were only partially substituted with HDTMA showed very low affinity toward pertechnetate. On the other hand, the adsorbents in which the HDTMA loading exceeded the cation exchange capacity (CEC) value had high efficiency. The behavior of such adsorbents was best described by a Freundlich isotherm, while for the sample in which the HDTMA loading was equal to the CEC, the linear adsorption isotherm was the most appropriate. The difference in adsorption performance of several adsorbents was discussed from the point of view of the interlayer arrangement of the HDTMA. The kinetics of pertechnetate adsorption on HDTMA-bentonites was tested and analyzed using different surface reaction- and diffusion-based kinetic models. For all the investigated adsorbents, the most appropriate kinetic model was the pseudo-second-order kinetics model. The obtained adsorption capacities were and HDTMA-bentonites with HDTMA loadings exceeding CEC value should be regarded as very promising adsorbents for the remediation of Tc-99 polluted waters.