Environmental Limits of Methanogenesis and Sulphate Reduction

The current proposed strategy for the disposal of intermediate-level radioactive waste (ILW) within the United Kingdom is through emplacement within a deep underground facility, termed a geological disposal facility (GDF). Anaerobic and highly alkaline (10.0<pH>13) conditions are expected to prevail within the near-field of a GDF, which will result in the chemical degradation of cellulose-bearing ILW. Isosaccharinic acids (ISA) and volatile fatty acids (VFA) are the major products of alkaline cellulose hydrolysis and their generation within an ILW-GDF will result in a range of organic carbon sources being present. The potential for these carbon sources to provide the substrates for methanogenesis and sulphate reduction under near-field conditions holds importance within a GDF. The generation of biogases such as 14CH4 from 14C-bearing waste could facilitate the transfer of radionuclides to the biosphere. The production of corrosive sulphide by colonising microorganisms could impact the integrity of engineered barriers used to prevent the transfer of radioelements to the biosphere.

The potential for methanogens and sulphate-reducing bacteria (SRB) to be active under ILW-GDF conditions is poorly understood. The work outlined in this thesis utilised anaerobic sediments from anthropogenic analogue sites to demonstrate the activity of methanogens and SRB under near-field conditions. The alkaline leachates generated in these sites result in high pore-water pH values equivalent with those expected to dominate an ILW-GDF (pH 11.0-13.0). In spite of these conditions, the incubation of cellulose in situ allowed a range of active microbial processes to be identified, including cellulose degradation by Fibrobacter species, sulphate-reduction by Desulfobacter and hydrogenotrophic methanogenesis by members of the order Methanomicrobiales. The formation of hydrophobic extracellular polymeric substances (EPS) and production of metabolic acids in situ facilitated microbial survival within these extreme environments.

Microcosms operating under methanogenic conditions at pH 10.0-11.0 developed from these alkaline sediments demonstrated high hydrogen consumption rates and were dominated by alkaliphilic Methanobacterium and Methanoculleus genera. Acetate was unable to be utilised as substrate by the associated methanogen communities under these conditions, however high acetate consumption rates were observed in pH 7.0-8.0 microcosms where the acetoclastic lineages Methanosarcina became more important. Sub-cultures of the alkaline methanogenic microcosms demonstrated the ability to utilise precipitated calcium carbonates as the sole carbon source for hydrogenotrophic metabolism at pH 10.0. Alkaliphilic Desulfonatronum biofilms grown on stainless steel surfaces developed from the alkaline sediment communities were capable of dissimilatory sulphate reduction at pH 11.0 using the products of alkaline cellulose degradation as the sole carbon and energy source. The sulphide produced by these biofilms induced the corrosion of stainless steel at pH 11.0 within 3 months.

The results outlined here suggest the colonisation of an ILW-GDF by methanogens will result in a population dependent on the hydrogenotrophic pathway, with acetate-derived methanogenesis being inhibited under these conditions. Furthermore, biofilms formed within the near-field facilitate the corrosion of steel materials by alkaliphilic SRB and enable microbial survival through the production of low pH niches. These findings can inform future safety assessments and gas generation modelling studies used to predict ILW-GDF performance.