Modification of Portland cement matrix with diethyldithiocarbamate for technetium immobilization

Abstract

The paper considers the effect of sodium diethyldithiocarbamate (SDDC) addition on the immobilization of technetium in a Portland cement matrix. The leaching process was evaluated in a model solution that simulated the conditions of the future radioactive waste (RW) storage site at the Yeniseisky site. The results demonstrated that the addition of 1.0 wt % SDDC to the cement composite increased the incorporation of immobilized technetium into the cement matrix by 35% in comparison to the blank sample. Furthermore, the retention of technetium in the cement matrix was observed to be enhanced, with a retention of up to 90% observed on the 200th day of the experiment. Resulting materials fulfill the necessary technical characteristics for use as a matrix and engineered safety barrier in the concept of deep RW disposal. The addition of SDDC into cement results in microbial respiratory activity decreasing. In order to evaluate the mechanisms of technetium immobilization, the structure of the technetium compound with diethyldithiocarbamate (DDC) [Tc(C₅H₁₀NS₂)₄]TcO₄ is described for the first time. This compound contains a Tc(V) atom which coordinates four diethyldithiocarbamate C₅H₁₀NS₂ moieties through eight sulfur atoms to form a complex cation. The addition of SDDC in cement is presumed to result in a cascade of disproportionation reactions and the formation of stable Tc(IV) compounds, as it was evidenced by XANES spectroscopy.