Effect of biogeochemical factors on the Np(V) mobility under the conditions of remote zone of deep liquid radioactive waste repositories

Under the conditions of remote zone of deep liquid radioactive waste repositories (Eh of pore waters from −60 to 186 mV), Np occurs in oxidation states (V) and (IV) on mineral particles incorporated in surrounding rocks. Np(V) can be additionally sorbed on newly formed Fe oxide-hydroxide phases and on Fe(II)-containing minerals owing to the sorption of the reduced forms. The fraction of Np forms tightly fixed on the rock increases with time. Shewanella putrefaciens microorganisms and sulfate-reducing bacteria isolated from deep liquid radioactive wastes repositories reduce Np(V) under anaerobic conditions. The addition to S. putrefaciens culture medium of the rock from the horizon in which the deep liquid radioactive waste repository is arranged leads to deposition of a significant fraction of Np(IV) and Np(V) on the microbial biomass and rock. In the presence of nitrates, the fraction of the reduced Np fixed on the rock considerably decreases. Sulfate-reducing bacteria reduce Np(V) with the released hydrogen sulfide and, probably, in a dissimilation process. The results obtained demonstrate the possibility of biogenic and abiogenic reduction of Np in deep liquid radioactive waste repositories, which will lead to a decrease in the Np(V) mobility.