Microbial Degradation of Cellulosic Material and Gas Generation: Implications for the Management of Low- and Intermediate-Level Radioactive Waste

Abstract

Deep geologic repositories (DGR) in Canada are designed to contain and isolate low- and intermediate-level radioactive waste. Microbial degradation of the waste potentially produces methane, carbon dioxide and hydrogen gas. The generation of these gases increase rock cavity pressure and limit water ingress which delays the mobility of water soluble radionuclides. The objective of this study was to measure gas pressure and composition over 7 years in experiments containing cellulosic material with various starting conditions relevant to a DGR and to identify micro-organisms generating gas. For this purpose, we conducted experiments in glass bottles containing (1) wet cellulosic material, (2) wet cellulosic material with compost Maker, and (3) wet cellulosic material with compost Accelerator. Results demonstrated that compost accelerated the pressure build-up in the containers and that methane gas was produced in one experiment with compost and one experiment without compost because the pH remained neutral for the duration of the 464 days experiment. Methane was not formed in the other experiment because the pH became acidic. Once the pressure became similar in all containers after 464 days, we then monitored gas pressure and composition in glass bottle containing wet cellulosic material in (1) acidic conditions, (2) neutral conditions, and (3) with an enzyme that accelerated degradation of cellulose over 1965 days. In these experiments, acetogenic bacteria degraded cellulose and produced acetic acid, which acidity suppressed methane production. Microbial community analyses suggested a diverse community of archaea, bacteria and fungi actively degrading cellulose. DNA analyses also confirmed the presence of methanogens and acetogens in our experiments. This study suggests that methane gas will be generated in DGRs if pH remains neutral. However, our results showed that microbial degradation of cellulose not only generated gas, but also generated acidity. This finding is important as acids can limit bentonite swelling and potentially degrade cement and rock barriers, thus this requires consideration in the safety case as appropriate.

Enhanced microbial degradation of irradiated cellulose under hyperalkaline conditions

“… 2015 ) also suggest that the biodegradation of cellulosic materials and their alkaline hydrolysis products (for example, ISA) may nullify the complexation of radionuclides by organics, and therefore mitigate the enhanced solubility and mobility of the organic–radionuclide complex (Nuclear Decommissioning Authority 2010 ). Microbial fermentation of the organic material and the accumulation of acetate, hydrogen and carbon dioxide may lead to a drop in pH and to over-pressurisation of the GDF, which is in line with observations by Beaton, Pelletier and Goulet (2019 ) at lower pH conditions; however, these will be mitigated by the use of porous cementitious grout materials, which have been developed to absorb CO 2 gas and manage the generation of other gases (for example, H 2 ) generated by steel corrosion (Leupin et al . 2016 ; Radioactive Waste Management 2016 ).…”

Oxic and Anoxic Organic Polymer Degradation Potential of Endophytic Fungi From the Marine Macroalga, Ecklonia radiata

“…Bacteria account for 60% ( Sichert et al, 2020 ) to 90% ( Koop et al, 1982 ) of kelp detritus degradation. Kelp detritus degradation by fungal degradation has been largely disregarded ( Koop et al, 1982 ; Bengtsson et al, 2011 ; Van Erk et al, 2020 ), despite the dominant role of terrestrial fungi in refractory OM degradation ( Gooday, 1990a ; Tedersoo et al, 2014 ; Treseder and Lennon, 2015 ; Beaton et al, 2019 ). As has been reported previously, ( Ivarsson et al, 2016 ; Drake and Ivarsson, 2018 ; Barone et al, 2019 ) the current study demonstrates that, like bacteria, some fungi may be active under anoxic conditions and have the capacity to break down kelp cellulose in various habitats.…”

A Review on Bacterial Contribution to Lignocellulose Breakdown into Useful Bio-Products

“…Primarily, the bacteria hydrolyze the cellulose and convert it into cellobiose, then fermentation, which refers to the hydrolysis of the cellobiose, occurs and this produces carbon dioxide, hydrogen, and organic acids [35]. After this stage, the dominant bacteria utilize these secondary products to produce various useful products [36]. Anaerobic biodegradation is the breakdown of organic matter by microorganisms without the presence of oxygen via several metabolic connections [33].…”