Microbially mediated reduction of Se(VI) to a Se(0)/Se(-II) solid phase in high pH sediments; relevance to radioactive waste disposal.

Selenium (79Se) is a fission product, found in intermediate level waste (ILW), due for disposal in a deep geological disposal facility (GDF). Globally, cement is a candidate material for waste encapsulation and backfill in many GDF designs for ILW as the high pH conditions imposed by this material favour the immobilisation of most priority radionuclides. However, radioactive Se oxyanions show appreciable mobility in Ca-rich alkaline systems, which is further enhanced in the presence of organic waste components that are often disposed with ILW. However, organic waste materials, such as gluconate, also have the potential to stimulate indigenous microorganisms, which in turn may impact the fate of Se oxyanions (and other radionuclides), for example via enzymatic reduction. Here, we used a multidisciplinary approach, including x-ray absorption spectroscopy (XAS) and 16S rRNA gene sequencing, to study the impact of an alkaline microbial community, stimulated by gluconate on the fate of Se(VI) in sediments analogous to cementitious ILW.