The Biodegradation of Isosaccharine Acids under Hyperalkaline Conditions

One of the current proposed strategies for the disposal of intermediate level radioactive waste (ILW) is that of an underground facility termed a geological disposal facility (GDF). Under the anoxic alkaline conditions (10.5<pH>13) expected to develop within an ILW-GDF cellulose will undergo chemical degradation to a range of cellulose degradation products (CDP). The major components of which will be the isosaccharinic acids (ISA). ISA is of particular interest with regards to the long term performance of a GDF due to its ability to complex radionuclides altering their solubility and thus the long term retention of such radionuclides. The removal of these complexants by microbial action could therefore have an impact upon the long term performance of an ILW-GDF.

The biofilm mode of existence is able to increase the resistance of microbes to environmental stresses and could be a mechanism for microbial survival under ILW-GDF conditions. Recent research has shown the ability of microbes from an ILW-GDF anthropogenic analogue site to degrade ISA under hyperalkaline conditions, however, no studies have yet looked at the ability of alkaliphilic biofilms to survive under these conditions whilst using ISA as a carbon source. The aim of this work was to ascertain the ability of alkaliphilic biofilm consortia to survive and degrade ISA under near field conditions and to further investigate the mechanisms of this survival.

Using cotton bait, biofilm was able to be grown in-situ at an analogue site located at Harpur Hill, Buxton, UK. Enrichment of this biofilm using CDP driven microcosms under both sulphate reducing and methanogenic conditions at pH 11 showed the degradation of the α-ISA, β-ISA and xylo ISA types through a fermentation pathway to acetate and hydrogen. Investigation into the morphology of these microcosms revealed the microbes to existing within flocs or aggregates of EPS which had a complex EPS composition. Microelectrode profiling of these flocs revealed areas of lower pH than the external pH to be present within the interior of these flocs, a finding which was attributed to microbial survival under the alkaline conditions.

Floc cultures were able to form thick dense biofilm within sand columns which enhanced the rate of ISA degradation and facilitated the production of methane and sulphide under relevant conditions. Micro-electrode pH profiling again demonstrated low pH areas within the biofilm which contributed towards biofilm survival and ISA degradation up to pH 13. Biofilms were able to impact ILW-GDF relevant surfaces with evidence of carbonation and EPS substances found upon NRVB, steel and graphite surfaces.

The ability of biofilms to form under ILW-GDF conditions could facilitate microbial survival under ILW-GDF conditions and have an impact upon the long term performance of an ILW-GDF. This could be through the carbonation of NRVB surfaces, the blocking of pore throats, gas production, microbial induced corrosion related to sulphide production and through the removal of radionuclide complexants.