Abstract
Intermediate level radioactive waste (ILW) generally contains a heterogeneous range of organic and inorganic materials, of which some are encapsulated in cement. Of particular concern are cellulosic waste items, which will chemically degrade under the conditions predicted during waste disposal, forming significant quantities of isosaccharinic acid (ISA), a strongly chelating ligand. ISA therefore has the potential to increase the mobility of a wide range of radionuclides via complex formation, including Ni-63 and Ni-59. Although ISA is known to be metabolized by anaerobic microorganisms, the biodegradation of metal-ISA complexes remains unexplored. This study investigates the fate of a Ni-ISA complex in Fe(III)-reducing enrichment cultures at neutral pH, representative of a microbial community in the subsurface. After initial sorption of Ni onto Fe(III)oxyhydroxides, microbial ISA biodegradation resulted in >90% removal of the remaining Ni from solution when present at 0.1 mM, whereas higher concentrations of Ni proved toxic. The microbial consortium associated with ISA degradation was dominated by close relatives to Clostridia and Geobacter species. Nickel was preferentially immobilized with trace amounts of biogenic amorphous iron sulfides. This study highlights the potential for microbial activity to help remove chelating agents and radionuclides from the groundwater in the subsurface geosphere surrounding a geodisposal facility.
Highlights
The policy of the UK Government is to dispose of long-lived intermediate level waste (ILW) via engineered deep underground geological disposal facilities (GDFs)[1]
No studies to date have addressed the fate of radionuclide-isosaccharinic acid (ISA) complexes, such as of Ni-ISA, which could potentially form under repository conditions, and have the potential to influence the properties of components in a GDF16,36
In the presence of the medium only, approximately 90–91% Ni remained in solution, whilst in the presence of ISA the solubility of Ni was increased to approximately 100%, regardless of the initial Ni concentration
Summary
The policy of the UK Government (and those of other nuclear nations) is to dispose of long-lived intermediate level waste (ILW) via engineered deep underground geological disposal facilities (GDFs)[1]. Other scavengers for metals and radionuclides, could be Fe(III) oxyhydroxides present in a GDF from the corrosion of steel under aerobic conditions in the “open phase of a GDF”, or naturally occurring Fe(III)-bearing minerals in the wider circumneutral pH geosphere (or “far field”) These Fe(III)-bearing minerals may further play a role as alternative electron acceptors, if bioavailable, sustaining ISA metabolizing anaerobes in and around a GDF. We have used a microbial inoculum obtained from an alkaline, Ca2+-rich lime kiln site, which is an analogue for a cementitious GDF and known to contain ISA-degrading microorganisms[30,34] This inoculum was used to investigate the fate of Ni during ISA biodegradation under Fe(III)-reducing conditions possibly relevant to the wider geosphere of a GDF, including identifying the role of key Fe(II)-bearing biominerals in Ni immobilization
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