Understanding mineral precipitation induced porosity clogging and being able to quantify its non‐linear feedback on transport properties is fundamental for predicting the long‐term evolution of energy‐related subsurface systems. Commonly applied porosity‐diffusivity relations used in numerical simulations on the continuum‐scale predict the case of clogging as a final state. However, recent experiments and pore‐scale modeling investigations suggest dissolution‐recrystallization processes causing a non‐negligible inherent diffusivity of newly formed precipitates. To verify these processes, we present a novel microfluidic reactor design that combines time‐lapse optical microscopy and confocal Raman spectroscopy, providing real‐time insights of mineral precipitation induced porosity clogging under purely diffusive transport conditions. Based on 2D optical images, the effective diffusivity was determined as a function of the evolving porous media, using pore‐scale modeling. At the clogged state, Raman isotopic tracer experiments were conducted to visualize the transport of deuterium through the evolving microporosity of the precipitates, demonstrating the non‐final state of clogging. The evolution of the porosity‐diffusivity relationship in response to precipitation reactions shows a behavior deviating from Archie's law. The application of an extended power law improved the description of the evolving porosity‐diffusivity, but still neglected post‐clogging features. Our innovative combination of microfluidic experiments and pore‐scale modeling opens new possibilities to validate and identify relevant pore‐scale processes, providing data for upscaling approaches to derive key relationships for continuum‐scale reactive transport simulations.

Borosilicate glasses are the favored material for immobilization of high-level nuclear waste (HLW) from the reprocessing of spent fuel used in nuclear power plants. To assess the long-term stability of nuclear waste glasses, it is crucial to understand how self-irradiation affects the structural state of the glass and influences its dissolution behavior. In this study, we focus on the effect of heavy ion irradiation on the forward dissolution rate of a non-radioactive ternary borosilicate glass. To create extended radiation defects, the glass was subjected to heavy ion irradiation using ¹⁹⁷Au ions that penetrated ~50 µm deep into the glass. The structural damage was characterized by Raman spectroscopy, revealing a significant depolymerization of the silicate and borate network in the irradiated glass and a reduction of the average boron coordination number. Real time, in situ fluid-cell Raman spectroscopic corrosion experiments were performed with the irradiated glass in a silica-undersaturated, 0.5 M NaHCO₃ solution at temperatures between 80 and 85 °C (initial pH = 7.1). The time- and space-resolved in situ Raman data revealed a 3.7 ± 0.5 times increased forward dissolution rate for the irradiated glass compared to the non-irradiated glass, demonstrating a significant impact of irradiation-induced structural damage on the dissolution kinetics.

Reactive transport modelling is a powerful tool to assess subsurface evolution in various energy-related applications. Upscaling, i.e., accounting for pore scale heterogeneities into larger scale analyses, remains one of the biggest challenges of reactive transport modelling. Pore scale simulations capturing the evolutions of the porous media over a wide range of Peclet and Damköhler number in combination with machine learning are foreseen as an efficient methodology for upscaling. However, the accuracy of these pore scale models needs to be tested against experiments. In this work, we developed a lab on a chip experiment with a novel micromodel design combined with operando confocal Raman spectroscopy, to monitor the evolution of porous media undergoing coupled mineral dissolution and precipitation processes due to diffusive reactive fluxes. The 3D-imaging of the porous media combined with pore scale modelling enabled the derivation of upscaled transport parameters. The chemical reaction tested involved the replacement of celestine by strontianite, whereby a net porosity increase is expected because of the smaller molar volume of strontianite. However, under our experimental conditions, the accessible porosity and consequently diffusivity decreased. We propose a transferability of the concepts behind the Verma and Pruess relationship to be applied to also describe changes of diffusivity for evolving porous media. Our results highlight the importance of calibrating pore scale models with quantitative experiments prior to simulations over a wide range of Peclet and Damköhler numbers of which results can be further used for the derivation of upscaled parameters.

The evolutionary history of latex, a widespread chemical defense against insect herbivores, is not fully understood, yet a more detailed understanding of the fossil record of latex could help answer important evolutionary questions. This is, however, hampered by the difficulty of recognizing fossil latex and our still incomplete comprehension of the processes preserving latex. The best-studied fossil latex comes from the middle Eocene Geiseltal lignites in Germany, where fibrous laticifer mats, called “monkeyhair,” are preserved in the severely degraded remains of some ancient trees in the brown coals. Laticifers are specialized elongate cells that carry latex throughout the plant. In previous studies, researchers have hypothesized that these fossil laticifers are preserved through natural low temperature vulcanization of rubber within the latex. Here, we report the results of Raman spectroscopic study on Geiseltal laticifers to identify the vulcanization of natural rubber and on spatially associated carbonaceous material to test various Raman carbon geothermometers for their accuracy for low-thermal-maturity samples. Raman spectra of the fossil laticifers are virtually identical to that of rubber (cis-1,4-polyisoprene) with additional bands demonstrating sulfur vulcanization. Raman spectra from the surrounding lignite and existing Raman-based carbon thermometers, currently calibrated down to about 100 °C, clearly indicate that these samples were never exposed to temperatures higher than the surrounding lignite. These results directly validate the previous hypothesis of fossilization through natural vulcanization. Moreover, this work demonstrates that Raman spectroscopy is a rapid, non-destructive method for reliably identifying and characterizing fossil latex and that further development and calibration of the carbon thermometer may allow quantitative temperature measurements for low-thermal-maturity carbonaceous material.

The disposal of spent nuclear fuels and high-level radioactive wastes in deep geological repositories represents one of the greatest scientific-technical and societal challenges of our times. Most disposal concepts rely on a multibarrier system, consisting of a combination of engineered materials, geotechnical and geological barriers to provide a safe containment of the radioactive waste to protect humans and the environment against dangers arising from ionizing radiation. A reliable safety assessment of a deep geological repository over assessment time scales of several 100 000 years requires a profound and comprehensive understanding of the complex coupled physical (thermal, hydraulic, mechanical), chemical and biogeochemical (THM/CB) processes that govern the long-term evolution of the repository system. As a result of thermal and chemical gradients at the interfaces of different components and materials of the multi-barrier system (e.g. interfaces between metallic waste containers and bentonite backfill or between structural concrete and clay host rock), mineral dissolution and precipitation reactions are promoted; thus the (local) porosity, the volume filled with gas and/or water, can increase or decrease leading to changes in the macroscopic transport properties of the respective media. Although a reduction of the porosity (porosity clogging) appears to be desirable to inhibit radionuclide migration, it can also be detrimental, particularly in the case of gas pressure build-up due to canister corrosion or bacterial activity. So far, porosity clogging at barrier interfaces and associated consequences on solute or gas transport remain poorly understood; currently used mathematical descriptions of porosity clogging in reactive transport codes usually fail to capture respective experimental observations (Chagneau et al., 2015; Deng et al., 2021). In this context, we are developing a “lab-on-a-chip” set-up, which combines time lapse optical microscopy imaging and in operando Raman spectroscopy (Poonoosamy et al., 2019, 2020) to determine (i) whether complete clogging is possible and permanent, (ii) which parameters control the porosity clogging and (iii) which changes in transport properties of porous media are induced due to porosity clogging. Our approach comprises micronized counterdiffusion experiments with in situ visualization and monitoring of the evolution of mineralogy and microstructure/pore architecture with time. Complementary pore scale modelling will be used to derive key relationships that describe changes in transport properties due to mineral precipitation-induced porosity clogging. This approach will help to improve reactive transport codes and their predictive capabilities thus enhancing confidence and reduce uncertainties in long-term predictions, leading to more realistic descriptions of the evolution of complex repository systems.