Pore-scale investigations of interactions between microorganisms and ionic strength: Implications for salt crystallization damage in porous media

Drying of building materials filled with salt-containing moisture is a common example of salt weathering [1]. Fluid flow, such as capillary uptake of water, and local climate changes stand out as key factors in salt weathering, substantially impacting the Earth's landscape and building infrastructure [2]. While microbial organisms are known to alter rock surfaces, some exhibit physiological capabilities that beneficially impact rock properties by producing biofilms, biocement and biogas [3]. Environmental factors such as temperature, relative humidity, and ionic strength of the medium influence microbial-induced products [4]. The impact of salt type, concentration, and ionic strength on microbially mediated reactions inside porous media is a largely unexplored phenomenon at the pore scale. Effective addressing of the respective challenges requires understanding the synergistic and counter effects of bacterial interactions and salt crystallization within the internal pore structure of rocks, influencing related pore-scale processes. In this study, we explored the response to the drying process in a range of porous materials, from PDMS transparent micromodels to sedimentary porous rocks containing brine solutions of various compositions in the presence and without bacterial solutions. We used Paracoccus denitrificans bacteria in our experiments. We specifically consider the case where air with different levels of humidity and at a constant temperature is exposed to one side of the porous media, forming a drying front—a defined interface separating liquid-saturated and partially gas-filled domains. High-resolution optical and confocal microscopy, Raman spectroscopy, and X-ray micro-computed tomography (µ-CT) were used to visualize and characterize bacteria-salt aggregates interactions in the porous media. Systematic investigations were carried out to understand how the interactions between salt crystallization and bacterial reactions depend on pore space morphology, type, and ionic strength of salt solutions. The findings highlight the potential of advanced 2D and 3D imaging techniques for enhanced understanding of the transport-crystallization coupling with bacterial activity through in-situ experiments and, hence, for constructing more accurate prediction models and conservation strategies.