Microbial degradation of bitumen

Summary A blown bitumen Mexphalte R 90/40 with a high content of saturated hydrocarbons was degraded by several microorganisms to the same extent. In batch cultures ofSaccharomycopsis lipolytica, maximal biodegradation was estimated to be about 9% w/w, 3.2·10^–3 g/cm² and 3.1·10^–3 cm of degraded bitumen. The Mexphalte R 90/40 degradation rate was closely coupled to biofilm formation. The microbial activity concerned predominantly the oxidation of saturated hydrocarbons. A direct distillation bitumen 80/100 with a low content of saturated hydrocarbons and a high content of aromatic hydrocarbons and resins was more resistant to biodegradation.     

Microbial legradation of bitumen

Summary Bitumen is commonly employed as a matrix for the long-term storage of low and intermediate level radioactive waste. As bitumen can be degraded by microbial activity, it is of great significance to determine the rates at which it may occur in nuclear waste repositories.Experiments have been carried out under optimal culture conditions using bitumen with a highly increased surface area. The potential of different microbial consortia to degrade bitumen has been examined. The investigations showed clearly that bitumen-degrading organisms are ubiquitous. In general the organisms formed biofilms on the accessible substrate surface area. Under oxic culture conditions a bitumen degradation rate of 20–50 g bitumen · m^–2· y^–1 leading to a CO₂ liberation of 15–40 l was observed. Anoxic conditions yielded a 100 times smaller degradation rate of 0.2–0.6 g bitumen · m^–2 · y^–1 and a CO₂ production of 0.15–0.45 l.Based on linear extrapolation the experimentally determined degradation rates would lead to a 25–70% deterioration of the bitumen matrix under oxic and 0.3–0.8% under anoxic conditions within 1000 years.     

Consequences of sulphate-reducing bacterial growth in a lab-simulated waste disposal regime

Summary Experiments are described which investigate corrosion of forged 0.2% carbon steel in the presence of sulphate reducing bacteria (SRB). Cultures of a thermophilic bacteriumDesulfotomaculum nigrificans were mixed with bentonite and synthetic groundwater to simulate a bacteria-contamined backfill, and placed in contact with carbon steel disc specimens in perspex cells at 50°C under anaerobic conditions. The rates of corrosion were monitored by electrochemical techniques, together with changes in near field redox potential. After 340 days the nature and extent of any corrosion was measured and the SRB content of the bentonite determined. Recovery of relatively large numbers of bacteria after about one year incubation in an alkaline (pH 9.5) medium confirmed the pH tolerance of the strain. Enhanced corrosion (three times the rate of the control) occurred in at least two of the five cells that contained SRB despite the nutritionally poor environment which existed in the bentonite gel.     

The Swedish final repository and the possible risk of interactions by microbial activities

Summary The final Swedish repository for low- and intermediate-level nuclear waste is described, and some of the possible problems caused by microbial activity during storage are discussed. Microbial degradation of bitumen constitutes one of the greatest risks in the silo part of a repository. The production of carbon dioxide due to both aerobic and anaerobic processes might lead to a decrease in the pH of the water, inducing corrosion of the metal construction and storage containers, with large amounts of hydrogen gas being produced. A risk assessment for the repository must thus take into account the various activities of microbes.     

Modelling of microbial degradation processes: The behaviour ofmicroorganisms in a waste repository

Summary In a repository for radioactive disposal the waste material is kept in place by several shells and boundaries to prevent a long term recycling of the material into the environment. Present investigations on various chemical and biological processes can be extrapolated into future centuries only with great uncertainty. Models may therefore be a good tool to forecast processes which may occur within the repository and to estimate whether the barriers present will prevent the leaching of waste material within a given time span. A mathematical model is described based on an experimental laboratory setup, a microcosm described by West et al.^19–22 simulating in a laboratory system repository conditions for a Swiss L/ILW repository. It includes microbial as well as physico-chemical processes. These simulations indicate that biological processes such as gas formation or proton release should also be included into the safety assessment of the repositories.     

Microbial influence on the sorption of137Cs onto materials relevant to the geological disposal of radioactive waste

Summary A major concern in the geological containment of radioactive wastes is the speed of movement of radionuclides from the repository, after their eventual leaching and release, into the geosphere and finally into the biosphere. Radionuclide sorption onto the host rock is an important retarding mechanism. Experimental evidence shows that the presence of microbes in this environment influences the sorption capabilities of the host rock. Their presence can decrease the amount of retardation of¹³⁷Cs, a common radionuclide in radioactive waste, by the solid phase. Sorption methods and data analysis procedures are presented and the implications for radioactive waste disposal assessments are discussed.