Zinc(II) ions under diffusive regime controls the stability of vaterite: Derivation of pore scale chemical dynamics near CaCO3 clogging interface via coupled micro-XRF/XRD/XANES and SAXS imaging.

Reactive transport of solutes in porous media involving dissolution/precipitation reactions may alter the physical properties of intact rocks, such as porosity and permeability. Such processes are difficult to predict and characterize due to heterogeneity at the pore scale, caused by the intimate coupling of chemical and transport processes. So far, experimental studies have mainly focused either on chemical aspects, using short time batch or microfluidic experiments, or on transport, using a porous medium in classical counter diffusion cells. The former approach offers a large flexibility in selecting and modifying parameters like pH, ionic strength, and element concentrations, and thereby allows the systematic derivation of chemical data such as mineral precipitation and dissolution at ex-situ conditions. The latter approach allows studying changes of (bulk) rock transport properties under realistic (in-situ) conditions over longer times, with limited knowledge of pore scale physical and chemical parameters, which may vary in space and time. Therefore, each of these methods limits a full characterization of the evolution of chemistry and transport properties in a realistic porous medium, particularly near the clogging zones that may form due to precipitation.

In this study, we present a methodology that allows the full chemical and physical characterization of reactive transport processes within a porous medium at the microscopic scale (1-10 um). We present experimental datasets that record, the chemical and physical evolution in 300 mm tapered glass capillaries filled with silica gel, in which precipitation of CaCO₃ polymorphs has been induced via counter diffusion of Na₂CO₃ and CaCl₂ reservoir solutions. Two counter diffusion systems, one containing 1 mM ZnCl₂ in the CaCl₂ titrant, the other without Zn, were prepared and evolved for three years. Optical microscopy and synchrotron-based techniques (micro XRF/XRT/XRD, micro XANES and SAXS) were used to characterize the system at selected times (1, 6, 12, 24, 36 months). In both experiments the precipitation of several calcite and/or vaterite crystal aggregates collectively reduced the gel porosity and formed clogging zones in the central part of the capillary. The data obtained for the Zn-free system show that vaterite crystals formed in the calcium rich part of the capillary (i.e., on the side of the CaCl₂ titrant) remain stable for the entire duration of the experiment without converting to the more stable calcite. In the Zn-doped system, the vaterite crystals remain stable for 1 year and start to dissolve afterwards. The carbonate ions released via vaterite dissolution did not lead to the formation of new CaCO₃ precipitates in the Ca-rich part of the capillary. Instead, a zinc hydroxy carbonate phase is formed.

Through this study, we show for the first time the evolution of a clogging zone in a porous medium at the microscopic scale over an extended period and under truly undisturbed in-situ conditions. Our results show that small concentrations of Zn²⁺ ions in the pore water have a strong effect on the precipitation/dissolution kinetics of CaCO₃ polymorphs, as well as on their crystal morphology.