A lab-on-a-chip approach integrating in-situ characterization and reactive transport modelling diagnostics to unravel (Ba,Sr)SO4 oscillatory zoning

The co-precipitation of sulphate minerals such as celestine and barite is widely studied because their formation is ubiquitous in natural and anthropogenic systems. Co-precipitation in porous media results in crystallization of solid solutions yielding characteristics such as oscillatory zoning that are rarely observed in bulk solution or in batch experiments. In the past, the precipitation of compositionally-zoned (Ba,Sr)SO4 crystals was observed post-mortem in macroscopic silica gel counter-diffusion experiments. Their formation was originally explained by the difference in the solubility products of the end-members combined with diffusion-limited transport of solutes to the mineral-fluid interface, while a later study favored the idea of kinetically controlled reactions. With recent advances combining in-operando microfluidic experiments and reactive transport modelling, it is now possible to verify hypotheses on the driving forces of transport-coupled geochemical processes. We developed a “lab on a chip” experiment that enabled the systematic study of the nucleation and growth of oscillatory-zoned (Ba,Sr)SO4 crystals in a microfluidic reactor. The compositions of the solid solutions were determined by in-situ Raman spectroscopy. Our investigation shows (1) that the composition of the nucleating phases can be approximated using classical nucleation theory, (2) that the oscillatory zoning is not solely controlled by the limited diffusional transport of solutes, and (3) that nucleation kinetics plays a major role in the switch between different stoichiometric compositions. The zoning phenomena is governed by the complex interplay between the diffusion of reactants and the crystallization kinetics as well as other factors, e.g. surface tension and lattice mismatch.

Combining innovative experimental approaches and cross-scale reactive transport modelling for assessing coupled hydrogeochemical processes at interfaces in deep geological repositories for radioactive waste

Deep geological repositories with a multi-barrier concept are foreseen by various countries for the disposal of high-level radioactive waste. A reliable and consistent assessment of the safety of these repositories over time scales of some hundred thousand years requires an advancement of process understanding. Simulation tools need to be developed for a close-to-reality description of repository evolution scenarios. This is especially required to resolve the challenging task of comparing and assessing the safety of different repository concepts in different host rocks within the German site-selection process. The construction of underground galleries and geotechnical barriers in the host rock formation and the emplacement of nuclear waste packages will create perturbations induced by chemical, thermal and pressure gradients at the interfaces of the different barriers, leading to mineral dissolution and precipitation to achieve re-equilibration. Such coupled hydrogeochemical processes generate non-linear responses in transport and mechanical properties of barrier materials and host rocks, which have to be taken into account for a more rigorous assessment of repository system evolution. Reactive transport modeling (RTM) can be applied to investigate these perturbations and processes across temporal and spatial scales, from the micro-scale at interfaces via the repository near field to the entire repository system – information not accessible through experiments alone. Although RTM is capable of addressing highly complex hydrogeochemical phenomena, the application of RTM codes to real systems is impeded by the often simplified description of coupled processes. To enhance the predictive capabilities of reactive transport models and to gain fundamental insights into the coupling between solute and radionuclide transport properties (e.g., permeability and diffusivity) of porous media and dissolution/precipitation processes, we conducted experiments on “simplified” chemical systems combined with pore-scale and continuum-scale reactive transport modelling to study processes in isolation, with the final aim of improving conceptual approaches for process couplings implemented in reactive transport codes. In this context, we investigated the effects of coupled mineral dissolution and precipitation in porous media on changes in permeability using flow-through experiments conducted in a magnetic resonance imaging scanner, which enabled the in situ investigation of porosity evolution in combination with monitoring changes in permeability and mineralogy. Our observations showed that classical implementations in reactive transport codes such as the Kozeny–Carman equation (Carman, 1937) failed to reproduce the changes in permeability and that more sophisticated approaches are required (Poonoosamy et al., 2020a, b). Moreover, we developed a novel “lab-on-a-chip” setup, i.e., micronized counter diffusion reactors with in operando 3D Raman tomography (Poonoosamy et al., 2019, 2020c), which enables evaluation of the alteration in pore architecture and study of the effect of coupled mineral dissolution and precipitation on the diffusive transport of solutes and radionuclides in porous media. Our approach enables the development of process-based theoretical models which allow for improvements in RTM codes and for predicting the evolution of perturbed interfaces in waste repositories, thus building confidence in the predictive capabilities of reactive transport models and reducing uncertainties with respect to future repository evolution.

Reactive Transport Modelling of Chloride Ingress in Saturated Coral Aggregate Concrete

Utilizing coral aggregate concrete (CAC) for construction on remote islands can significantly reduce construction cost and period, CO2 emission, and consumption of non-renewable energy. The durability of reinforced CAC structures is critically influenced by their resistance to chloride attack. In this study, a reactive transport modelling was developed to investigate chloride ingress in CAC, in which a COMSOL-PHREEQC interface based on MATLAB language was established. The experiment from the literature was taken as a benchmark example. The results show that the developed numerical model can accurately predict chloride transport in CAC. Differing from ordinary aggregate concrete (OAC), Kuzel’s salt does not appear in cement hydrate compounds of CAC during chloride ingress. The numerical results indicate that the penetration depth of chloride in CAC gradually increases as the exposure time is prolonged. When CAC is exposed to an external chloride solution, the decrease in the pH of the pore solution affects the precipitation of Friedel’s salt, which is detrimental to the chemical binding of chloride.

Evolution of the Reaction and Alteration of Mudstone with Ordinary Portland Cement Leachates: Sequential Flow Experiments and Reactive-Transport Modelling

The construction of a repository for geological disposal of radioactive waste will include the use of cement-based materials. Following closure, groundwater will saturate the repository and the extensive use of cement will result in the development of a highly alkaline porewater, pH > 12.5; this fluid will migrate into and react with the host rock. The chemistry of the fluid will evolve over time, initially high [Na] and [K], evolving to a Ca-rich fluid, and finally returning to the groundwater composition. This evolving chemistry will affect the long-term performance of the repository, altering the physical and chemical properties, including radionuclide behaviour. Understanding these changes forms the basis for predicting the long-term evolution of the repository. This study focused on the determination of the nature and extent of the chemical reaction, as well as the formation and persistence of secondary mineral phases within a mudstone, comparing data from sequential flow experiments with the results of reactive transport modelling. The reaction of the mudstone with the cement leachates resulted in small changes in pH with the precipitation of calcium aluminium silicate hydrate (C-(A-)S-H) phases of varying compositions. As the system evolves, secondary C-(A-)S-H phases re-dissolve and are replaced by secondary carbonates. This general sequence was successfully simulated using reactive transport modelling.

Quantification of Carbon Cycling in a Large Aquifer Using Reactive Transport Modelling

Continental scale aquifers can store significant amounts of carbon as a result of immense water volumes, substantial concentrations of dissolved inorganic carbon (DIC) and its reactions with a matrix, thus contributing the global carbon storage and cycle. However, concentration of dissolved solutes may vary significantly over distances, which causes interpretative challenges and difficulties in process quantification. This occurs in the Guarani Aquifer System in South America, which is a subject of extensive research due to a significant strategic role in water supply. Dissolved CO2 is expected to dissociate and undergo reactions with aluminosilicate minerals, but it is unknown how much DIC may get immobilised in the aquifer. To quantify the processes, we performed reactive transport modelling which combines hydrological and geochemical information followed by global sensitivity analysis. We show that more than a half of the infiltrated CO2 may be consistently precipitated as CaCO3. The DIC concentrations across the aquifer depend primarily on the input carbon concentrations and the plagioclase hydrolysis rate, while other parameters including hydraulic conductivity, recharge rate and mineral stability are of the minor importance. We present how advanced modelling techniques may be used to interpret and quantify processes controlling water quality in continental scale groundwater systems.