Abstract
This study investigates the fate of nitrate and organic acids at the bitumenconcrete-steel interface within a repository storage cell for long-lived, intermediatelevel, radioactive wastes. The interface was simulated by a multiphase system in which cementitious matrices (CEM V paste specimens) were exposed to bitumen model leachates consisting of nitrates and acetic acid with and without oxalic acid, chemical compounds likely to be released by bitumen. Leaching experiments were conducted with daily renewal of the solutions in order to accelerate reactions. C-steel chips, simulating the presence of steel in the repository, were added in the systems for some experiments. The concentrations of anions (acetate, oxalate, nitrate, and nitrite) and cations (calcium, potassium, ammonium) and the pH were monitored over time. Mineralogical changes of the cementitious matrices were analysed by XRD. The results confirmed the stability of nitrates in the absence of steel, whereas, reduction of nitrates was observed in the presence of steel (production of NH4+). The action of acetic acid on the cementitious matrix was similar to that of ordinary leaching; no specific interaction was detected between acetate and cementitious cations. The reaction of oxalic acid with the cementitious phases led to the precipitation of calcium oxalate salts in the outer layer of the matrix. The concentration of oxalate was reduced by 65% inside the leaching medium.
Highlights
The interface was simulated by a multiphase system in which cementitious matrices (CEM V-paste specimens) were exposed to bitumen model leachates consisting of nitrates and acetic acid with and without oxalic acid, chemical compounds likely to be released by bitumen
Reduction of nitrate (NO3-) may occur (i) from abiotic processes, with iron from the steel and/or H2 acting as electron donors, and surface catalysis provided by the different types of steel and corrosion products present in the storage cell, and/or (ii) from biological catalysis through bacterial activity
This study aimed to explore the cementitious matrix-bitumen model leachates-steel interactions in the context of repository of long-lived, intermediate-level, radioactive wastes
Summary
The reactions may lead to the formation of nitrite (NO2-), gaseous nitrogen (N2) and/or ammonium (NH4+), depending on a variety of parameters not yet well understood, in concrete-dominated systems [6,7,8,9,10,11] Both types of nitrate reactions (abiotic and biotic) involve electron donors and numerous inorganic and organic candidates are available in the storage cell or in the host rock (organic acids, H2, zero-valent metals...) [5]. The overall project, of which this paper is a part, aims to investigate the reduction of nitrates within a system comparable to a “real” waste-cell where bacterial activity is likely to occur (denitrifying alkaliphilic bacteria) and notably to determine the phenomenology and kinetics of reactions and the role of the electron donor (i.e. organic acids released by bitumen, such as acetic or oxalic acids) in the reactions [9]. The investigation of bacterial denitrifying activity is the aim of the paper by Alquier et al [12], proposed in this conference
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