دانلود رایگان مقاله نرخ اکسیداسیون هیدروژن میکروبی و کاهش سولفات در سنگ Opalinus

abstract

Hydrogen gas (H2) may be produced by the anoxic corrosion of steel components in underground structures, such as geological repositories for radioactive waste. In such environments, hydrogen was shown to serve as an electron donor for autotrophic bacteria. High gas overpressures are to be avoided in radioactive waste repositories and, thus, microbial consumption of H2 is generally viewed as beneficial. However, to fully consider this biological process in models of repository evolution over time, it is crucial to determine the in situ rates of microbial hydrogen oxidation and sulfate reduction. These rates were estimated through two distinct in situ experiments, using several measurement and calculation methods. Volumetric consumption rates were calculated to be between 1.13 and 1.93 mmol cm
3 day
1 for H2, and 0.14 and 0.20 mmol cm
3 day
1 for sulfate. Based on the stoichiometry of the reaction, there is an excess of H2 consumed, suggesting that it serves as an electron donor to reduce electron acceptors other than sulfate, and/or that some H2 is lost via diffusion. These rate estimates are critical to evaluate whether biological H2 consumption can negate H2 production in repositories, and to determine whether sulfate reduction can consume sulfate faster than it is replenished by diffusion, which could lead to methanogenic conditions.

5. Conclusion

The present work demonstrates that microorganisms, especially sulfate-reducing microorganisms, are able to rapidly oxidize hydrogen in Opalinus Clay. This biological process can beneficial for the safety of geological disposal of nuclear waste. Indeed, hydrogen gas pressure build-up wrought by anoxic corrosion of steel might be reduced by the processes described in this study, resulting in a net improvement of the safety case. On the other hand, sulfatereducing bacteria are known for increasing steel corrosion through their activity, by producing sulfide (Muyzer and Stams, 2008). This implies that a careful repository design is needed to minimize this negative impact. For instance, microbial activity can be promoted in an iron-rich porous medium located somewhere between the canister and the host-rock. This would protect the latter from pressure build-up by consuming hydrogen, and would protect the steel canister by precipitating sulfide with Fe(II), precluding enhanced canister corrosion by sulfide. Nonetheless, further studies are needed to fully assess the impact of H2 consumption on repositories. The geometry of the repository and its resaturation history are also likely to be important parameters controlling the rate of hydrogen oxidation and the overall amount oxidized. Additionally, the availability of sulfate, the main electron acceptor for hydrogen oxidation, will significantly impact the consumption of H2. The concentration of sulfate in Opalinus Clay porewater ranges between 15 and 20 mM and could be locally depleted due to low hydraulic conductivity of the rock. When sulfate in the porewater becomes depleted and the conversion rate of hydrogen becomes limited by sulfate diffusion into the repository then methanogenesis could further reduce the hydrogen partial pressure in the repository backfill. This process has not been identified in situ and its occurrence remains an open question.