Microbial degradation of cellulosic material under intermediate-level waste simulated conditions

Abstract

Under the alkaline conditions expected in an intermediate-level waste repository, cellulosic material will undergo chemical hydrolysis. This will produce hydrolysis products, some of which can form soluble complexes with some radionuclides. Analyses of samples containing autoclaved tissue and cotton wool incubated in a saturated solution of Ca(OH)2 ( pH > 12) confirmed previous reports that isosaccharinic acid (ISA) is produced from these cellulose polymers at high pH. However, when inoculated with a sediment sample from a hyperalkaline site contaminated with lime-kiln waste, microbial activity was implicated in the enzymatic hydrolysis of cellulose and the subsequent production of acetate. This in turn led to acidification of the microcosms and a marked decrease in ISA production from the abiotic alkali hydrolysis of cellulose. DNA analyses of microbial communities present in the microcosms further support the hypothesis that bacterial activities can have a controlling influence on the formation of organic acids, including ISA, via an interplay between direct and indirect mechanisms. These and previous results imply that microorganisms could have a role in attenuating the mobility of some radionuclides in and around a geological disposal facility, via either the direct biodegradation of ISA or by catalysing cellulose fermentation and therefore preventing the formation of ISA.

The Impact of Biofilms upon Surfaces Relevant to an Intermediate Level Radioactive Waste Geological Disposal Facility under Simulated Near-Field Conditions

“…With the exception of the pH 13 system, the removal of ISA from biotic systems was less than that observed in the abiotic systems indicating that microbiological activity was attenuating the ability of the NRVB to sorb ISA via a combination of biofilm formation and surface carbonation. The anaerobic degradation of ISA under alkaline conditions has previously been shown to be via a fermentation pathway to acetate, CO 2 , and hydrogen [14], resulting in acidification of the local environment [10]. The fermentation of ISA in the NRVB flow cells receiving pH 11 and 12 influents, resulted in a reduction in pH ( Figure S4) and formation of carbonates as indicated by an increase in inorganic carbon ( Figure S5).…”

“…This indicates that in order to propagate through cracks or fissures in the NRVB, microbes would require an initiation point of active microbial metabolism. Such an initiation point could for example be associated with an actively growing biofilm or a region of cellulosic waste that has acidified due to microbial activity [10]. In the presence of an active biofilm, the sorption of ISA to the NRVB was reduced by limiting the interaction of the NRVB with the solution.…”

“…Microbes which can survive under these harsh conditions and utilize ISA and other cellulose degradation products as a carbon source could affect the long term performance of such a facility. This could be through the metabolism of complexants [10,11], the modification of surface properties through the generation of biofilms [12], the catalysis of redox reactions and the carbonation of the cementitious materials [13]. Recent studies on industrial alkaline sites have shown that diverse microbial communities can exist at in situ pH values up to pH 12 [10,11].…”

“…This could be through the metabolism of complexants [10,11], the modification of surface properties through the generation of biofilms [12], the catalysis of redox reactions and the carbonation of the cementitious materials [13]. Recent studies on industrial alkaline sites have shown that diverse microbial communities can exist at in situ pH values up to pH 12 [10,11]. Microcosm experiments using sediments and biofilms generated in situ from this site have shown the ability of the local consortia to degrade both forms of ISA up to pH 11 [14,15].…”

In-Situ Biofilm Formation in Hyper Alkaline Environments

“…In addition recently isolated strictly anaerobic, ISA degrading bacteria also contain cellulosomes which would enable cellulose degradation . There was, however, no evidence of the acidification observed in some in-vitro studies (Bassil et al, 2015) indicating that the buffering capacity of all three sites was sufficient to counter act any acidification associated with fermentation processes.…”