MICROBIAL HYDROGENOTROPHIC DENITRIFICATION IN NUCLEAR WASTE REPOSITORY CONTEXT - EFFECT OF SOLID CEMENT PASTE PRESENCE

Author(s): Pierre Albina, Nadège Durban, Alexandra Bertron, Achim Albrecht, Jean- Charles Robinet, Benjamin Erable
Paper category: Proceedings
Book title: Proceedings of the Final Conference of RILEM 253-MCI Microorganisms-Cementitious Materials Interactions Volume II
Editor(s): Alexandra Bertron and Henk Jonkers
ISBN: 978-2-35158-207-7 (Set)
ISBN: 978-2-35158-210-7 (Volume 2)
e-ISBN: 978-2-35158-206-0
Publisher: RILEM Publications SARL
Publication year: 2018
Pages: 569-570
Total Pages: 308
Language : English

Abstract: France envisages to dispose high and medium level long-lived radioactive wastes into a repository settled 500 meters deep into the clay-rich Callovo-Oxfordian rock formation [1]. After repository closure, water will start percolating from the host rock, saturating the repository cells. Water will be charged with chemical compounds released from waste containers after they have lost their tightness. Alkaline pH (initially 13; dropping to 9 after several 10000’s of years) would be set by hydroxide ions leaching from the surrounding structural cement [2][3], and hydrogen will be generated by water radiolysis and steel corrosions [4]. Among the waste leachates, nitrate, released at varying concentrations from tens of mM to M, depending on waste type, possibly influences the mobility of some radionuclides [5]. Microbial life could reduce nitrate into gaseous nitrogen using electron sources such as organic matter or hydrogen [6]. The latter pathway, discussed here, is called hydrogenotrophic denitrification. The objective of the present work was to evaluate the fate of nitrate, in presence of hydrogen-oxidizing bacteria in the repository context (i.e. high nitrate concentration, alkaline pH, and presence of solid cement). The denitrification under hydrogenotrophic condition is not well studied in the repository context. To assess the effect of solid cement on microbial hydrogenotrophic denitrification, we used activated sludge as bacterial consortium to inoculate two bioreactors with and without the solid. No organic matter was added, nitrate was tested for concentrations ranging from 100 to 200 mM, pH oscillated between 9.5 and 10.2. The liquid medium was 1L of cement leachate (1g/L crushed cement mixed with water over 24h) supplemented with 50 mM carbonate as bacterial inorganic carbon source. Activated sludge collected from a wastewater treatment plant was cultivated in two separated batch reactors. Solid cement was added in one reactor as 9 triangular pastes (58g). The solid cement had been previously “aged”, by storing at ambient air over a year. Reactors were continuously flushed (1 atm, 5 mL/min) with hydrogen gas. The pH and nitrate concentrations were manually set in both reactors: from pH 9.5 and 100 mM (day 0) to pH 10.2 and 200 mM nitrate (day 20). Biomass level in the bioreactors was analysed by optical density, scanning electron microscope (SEM), and petri dishes enumeration. Nitrate and nitrite concentrations were analysed by high performance ionic chromatography. Results are presented on Figure 1, nitrate reduction kinetics are written in the graph over the black arrows.
Results showed that hydrogenotrophic denitrification was possible but slow in the studied pH and nitrate concentration ranges. From day 0 to day 20, nitrate reduction rate reached 17 and 56 mgN-NO3.L-1 .d-1 respectively with and without solid cement in the bioreactor. At day 21, with 200 mM nitrate and pH 10.2, nitrate reduction slowed down to 5 mgN-NO3.L-1 .d-1 for both bioreactors. The majority of the reduced nitrate remained as nitrite. Nitrate consumption was higher without cement at 100 mM nitrate and pH 9.5. The inhibition was probably caused by the slightly higher pH with solid cement. SEM analyses showed the perturbing effect of carbonation on biofilm formation. However, positives results in petri dishes inoculated from a solid cement paste sample indicated the presence of bacterial biofilm on solid cement. Despite the non-optimal conditions (absence of organic matter, high cement/liquid ratio) activated sludge was able to maintain nitrate reduction at nitrate concentrations and pH values expected in the later stages of the evolution of the radioactive waste repository. The presence of solid cement may slow down the bacterial activity for many thousands of years.

Online publication : 2018
Publication type : full_text
Public price (Euros) : 0.00