Use of in-vitro experimental results to model in-situ experiments: bio-denitrification under geological disposal conditions

Abstract

Some of the low level radioactive wastes from reprocessing of spent nuclear fuels contain nitrates. Nitrates can be present in the form of soluble salts and can be reduced by various reactions. Among them, reduction by metal compounds and microorganisms seems to be important in the underground repository. Reduction by microorganism is more important in near field area than inside the repository because high pH and extremely high salt concentration would prevent microorganism activities. In the near field, pH is more moderate (pH is around 8) and salt concentration is lower. However, the electron donor may be limited there and it might be the control factor for microorganism's denitrification activities. In this study, in-vitro experiments of the nitrate reduction reaction were conducted using model organic materials purported to exist in underground conditions relevant to geological disposal. Two kinds of organic materials were selected. A super plasticizer was selected as being representative of the geological disposal system and humic acid was selected as being representative of pre-existing organic materials in the bedrock. Nitrates were reduced almost to N2 gas in the existence of super plasticizer. In the case of humic acids, although nitrates were reduced, the rate was much lower and, in this case, dead organism was used as an electron donor instead of humic acids. A reaction model was developed based on the in-vitro experiments and verified by running simulations against data obtained from in-situ experiments using actual groundwaters and microorganisms. The simulation showed a good correlation with the experimental data and contributes to the understanding of microbially mediated denitrification in geological disposal systems.

Surface and bacterial reduction of nitrate at alkaline pH: Conditions comparable to a nuclear waste repository

“…It should be noted that the low denitrification rates remain significant considering the several thousand years of life of a nuclear waste repository. Other studies carried out in a comparable context showed that microbial communities or pure strains such as Pseudomonas stutzeri could reduce millimolar nitrate concentrations with denitrification rates up to 0.8 mM h À1 (Spain and Krumholz, 2011;Masuda et al, 2013;Bertron et al, 2013a). These studies were carried out in a pH range of 6e8 and with a rich culture medium.…”