Joe S. Small

The kinetics and mechanisms of simulated British Magnox waste glass dissolution as a function of pH, silicic acid activity and time in low temperature aqueous systems

Abstract Dissolution of a simulated British Magnox waste glass is governed by two pH-dependent processes. At low pH, dissolution is governed by reactions occurring predominantly at non-Si sites and residual Si-rich gels develop at the glass surface as B, Al and modifier cations are selectively leached. Here, extensive proton promoted hydrolysis of BO and AlO bonds is coupled with hydration and ion exchange processes. Hydrolysis of siloxane bonds governs the rate of dissolution at high pH and the glass dissolves congruently as the silicate network breaks down extensively. Differences in the surface chemistries and morphologies of glass samples reacted in strongly acidic and highly alkaline media reflect the net effects of these processes. The rate of the congruent dissolution process is influenced by the activity of silicic acid. The results are compared with published data for other glass formulations and are discussed in the context of proposed kinetic dissolution models.

Alteration of sediments by hyperalkaline k-rich cement leachate: Implications for strontium adsorption and incorporation

Results are presented from 1 year batch experiments where K-rich hyperalkaline pH 13.5 young cement water (YCW) was reacted with sediments to investigate the effect of high pH, mineral alteration, and secondary mineral precipitation on (90)Sr sorption. After reaction with YCW, Sr sorption was found to be greater than 75% in all samples up to 365 days and 98% in a sample reacted for 365 days at 70 °C. Scanning electron microscopy analysis of sediment samples reacted at room temperature showed surface alteration and precipitation of a secondary phase, likely a K-rich aluminosilicate gel. The presence of Sr-Si(Al) bond distances in Sr K-edge extended X-ray absorption fine structure (EXAFS) analysis suggested that the Sr was present as an inner-sphere adsorption complex. However, sequential extractions found the majority of this Sr was still exchangeable with Mg(2+) at pH 7. For the sample reacted for 1 year at 70 °C, EXAFS analysis revealed clear evidence for ∼6 Sr-Si(Al) backscatters at 3.45 Å, consistent with Sr incorporation into the neoformed K-chabazite phase that was detected by X-ray diffraction and electron microscopy. Once incorporated into chabazite, (90)Sr was not exchangeable with Mg(2+), and chemical leaching with pH 1.5 HNO3 was required to remobilize 60% of the (90)Sr. These results indicate that, in high pH cementitious leachate, there is significantly enhanced Sr retention in sediments due to changes in the adsorption mechanism and incorporation into secondary silicate minerals. This suggests that Sr retention may be enhanced in this high pH zone and that the incorporation process may lead to irreversible exchange of the contaminant over extended time periods.

The Impact of γ Radiation on the Bioavailability of Fe(III) Minerals for Microbial Respiration

Conservation of energy by Fe(III)-reducing species such as Shewanella oneidensis could potentially control the redox potential of environments relevant to the geological disposal of radioactive waste and radionuclide contaminated land. Such environments will be exposed to ionizing radiation so characterization of radiation alteration to the mineralogy and the resultant impact upon microbial respiration of iron is essential. Radiation induced changes to the iron mineralogy may impact upon microbial respiration and, subsequently, influence the oxidation state of redox-sensitive radionuclides. In the present work, Mössbauer spectroscopy and electron microscopy indicate that irradiation (1 MGy gamma) of 2-line ferrihydrite can lead to conversion to a more crystalline phase, one similar to akaganeite. The room temperature Mössbauer spectrum of irradiated hematite shows the emergence of a paramagnetic Fe(III) phase. Spectrophotometric determination of Fe(II) reveals a radiation-induced increase in the rate and extent of ferrihydrite and hematite reduction by S. oneidensis in the presence of an electron shuttle (riboflavin). Characterization of bioreduced solids via XRD indicate that this additional Fe(II) is incorporated into siderite and ferrous hydroxy carbonate, along with magnetite, in ferrihydrite systems, and siderite in hematite systems. This study suggests that mineralogical changes to ferrihydrite and hematite induced by radiation may lead to an increase in bioavailability of Fe(III) for respiration by Fe(III)-reducing bacteria.

Fe(III) Reduction in the Subsurface at a Low-level Radioactive Waste Disposal Site

The low-level radioactive waste (LLW) repository located near to the village of Drigg, Cumbria is the principal site for disposal of LLW in the United Kingdom. To gain a better understanding of the potential microbial processes that could control radionuclide mobility in the LLW repository, we studied of a range of anaerobic processes in microcosms constructed from sediments at the site. Sediment samples taken from both former disposal trenches and the far-field region of the site were analyzed for the presence of Fe(II) and bioavailable Fe(III) using chemical extractions and assays. Near-field (trench) sediments were found to have undergone some microbial reduction of iron, with acid-extractable Fe(II) present and 16S rRNA genes affiliated to known Fe(III)-reducing microorganisms detected in clone libraries constructed by PCR using broad-specificity primers. In far-field material, no Fe(II) was detectable although poorly crystalline Fe(III) was present. No 16S rRNA genes affiliated with known Fe(III)-reducing microorganisms were observed in clone libraries prepared with broad-specificity primers, suggesting that they were not numerically dominant in the far-field sediments. Time-course microcosm experiments were set up by amending the near- and far-field material with 10 mM acetate. The utilization of nitrate, poorly crystalline Fe(III) and sulfate was monitored to identify the sequence of terminal electron accepting processes in the sediments. Analysis of the microbial community after Fe(III) reduction had terminated revealed a large increase in 16S rRNA gene sequences affiliated to known Fe(III)-reducing bacteria, with a close relative to Ferribacterium limneticum accounting for 55% of the community. 16S rRNA analysis of the sediments post-Fe(III) reduction revealed the presence of some Fe(III)-reducing microorganisms such as a close relative of Rhodoferax ferrireducens. Nitrate, iron and sulfate data were then modeled with a biogeochemical reactive transport computer model that includes representation of microbial growth kinetics, and which is used to simulate the evolution of the subsurface geochemistry at the repository site. The potential impact of these processes on radionuclide mobility at LLW sites is discussed.

Dissolution of a Complex Borosilicate Glass at 60°C: The Influence of pH and Proton Adsorption on the Congruence of Short-Term Leaching

The short-term dissolution behaviour of a complex borosilicate glass has been investigated by controlled pH leaching, surface titration and leach rate temperature dependence experiments. The results indicate that the rates, congruence and mechanisms of dissolution vary significantly with pH. At low pH, dissolution occurs via a proton-promoted mechanism which results in enhanced release of B and many network modifying elements (relative to Si). At 60°C in high pH media, dissolution is essentially congruent. Here, dissolution is surface reaction controlled and occurs via a hydroxyl-promoted network dissolution process. Selective leaching is favoured at low and near-neutral pH. Congruent dissolution occurs in solutions of pH greater than that at the point of zero net proton charge.

Impact of the electron donor on in situ microbial nitrate reduction in Opalinus Clay: results from the Mont Terri rock laboratory (Switzerland)

At the Mont Terri rock laboratory (Switzerland), an in situ experiment is being carried out to examine the fate of nitrate leaching from nitrate-containing bituminized radioactive waste, in a clay host rock for geological disposal. Such a release of nitrate may cause a geochemical perturbation of the clay, possibly affecting some of the favorable characteristics of the host rock. In this in situ experiment, combined transport and reactivity of nitrate is studied inside anoxic and water-saturated chambers in a borehole in the Opalinus Clay. Continuous circulation of the solution from the borehole to the surface equipment allows a regular sampling and online monitoring of its chemical composition. In this paper, in situ microbial nitrate reduction in the Opalinus Clay is discussed, in the presence or absence of additional electron donors relevant for the disposal concept and likely to be released from nitrate-containing bituminized radioactive waste: acetate (simulating bitumen degradation products) and H2 (originating from radiolysis and corrosion in the repository). The results of these tests indicate that—in case microorganisms would be active in the repository or the surrounding clay—microbial nitrate reduction can occur using electron donors naturally present in the clay (e.g. pyrite, dissolved organic matter). Nevertheless, non-reactive transport of nitrate in the clay is expected to be the main process. In contrast, when easily oxidizable electron donors would be available (e.g. acetate and H2), the microbial activity will be strongly stimulated. Both in the presence of H2 and acetate, nitrite and nitrogenous gases are predominantly produced, although some ammonium can also be formed when H2 is present. The reduction of nitrate in the clay could have an impact on the redox conditions in the pore-water and might also lead to a gas-related perturbation of the host rock, depending on the electron donor used during denitrification.

Integrating Microbiology into the Drigg Post-Closure Radiological Safety Assessment

BNFL owns and operates the UK’s principal solid Low Level Radioactive Waste disposal site at Drigg in Cumbria, north west England. Drigg has been receiving waste since 1959 with approximately 900,000 m 3 of waste disposed of to date. Waste accepted for disposal at Drigg comes in a variety of forms including rubble, spoil, redundant equipment, scrap and process waste, and typically contains significant metallic and cellulosic components. The organic content of the waste means that microbial activity plays a significant role in the development of the repository environment. Consequently, microbial processes are integrated into many aspects of the Drigg Post-Closure Radiological Safety Assessment (PCRSA). This begins with the identification and screening of relevant features, events and processes, through supporting research, engineering designs and finally integration into radiological safety assessment modelling. This paper outlines how and where microbiology is integrated into the Drigg PCRSA and indicates areas of active research.

Testing of a near-field biogeochemical model against data from a large-scale gas generation experiment

A biogeochemical model that represents processes of metal corrosion, microbial degradation of cellulosic waste and mass transfer within a heterogeneous system has been used to represent processes of gas generation in a large-scale (20m 3 ) experiment that has studied degradation of typical nuclear reactor operating waste. The experiment has been in operation for a period of about eight years and has established a pattern of methanogenic gas generation. A “blind testing” approach has been used to develop the model of the experiment using independently derived kinetic data for corrosion and microbial processes. The model correctly represents the anaerobic conditions leading to methane generation during the course of the experiment. The overall rate of gas generation of the experiment is well represented, as is the composition of evolved gases and geochemistry of sampled liquids. The experiment and the model together build confidence in the ability to simulate processes of gas generation and variation in chemical conditions in heterogeneous repository environments.