The microbial impact on U, Pu, Np, and Am immobilization on aquifer sandy rocks, collected at the deep LRW injection site

In this work the analysis of biogeochemical factors influencing on immobilization of Np, U, Pu and Am on rocks taken from a depth of 450 m in a deep repository of liquid radioactive waste of the Siberian Chemical Combine is carried out. It was found that the development of microbial processes on aquiferous sandy rocks leads to a significant increase in the sorption efficiency of Np and U due to the formation of microbial biofilms and new biogenic mineral phases (Fe sulfides, Fe carbonates, amorphous iron oxides). The high values of Pu immobilization did not deteriorate and the effect on quantitative Am immobilization was negligible, but it led to an increase in the low-desorbed fraction. It is important to note the role of amorphous SiO 2 , formed after the biogenic dissolution of the clay fraction, in the distribution of U, Np and Am bond forms on rock samples. Development of microbial processes can lead to a redox potential decreasing, which leads to the formation of biogeochemical barrier zones for actinides in reduced oxyhydroxide forms. Thus, complex multifactorial models of radionuclide migration should be based not only on the Kd model and various hydrodynamic parameters of facilities, but also on data obtained from detailed laboratory investigation of the influence of microbial processes on Kd taking into account geochemical conditions determining the intensity of biological processes in the water-rock system. • Here we describe the biogeochemical role in actinides immobilization in deep aquifers. • U and Np immobilization largely depends on biogenic processes than Pu and Am. • The microbial transformation resulted in increasing all Ac fractions in strongly fixed forms. • Biogenic FeS and biofilms are the most important factors of Ac immobilization. • The microbial NO 3 reduction is the key factor for biogeochemical redox barrier set-up.