Discernible Orientation for Tortuosity During Oxidative Precipitation of Fe(II) in Porous Media: Laboratory Experiment and Micro-CT Imaging

In the mixing zone, where submarine groundwater carrying ferrous iron [Fe(II)] meets seawater with dissolved oxygen (DO), the oxidative precipitation of Fe(II) occurs at the pore scale (nm~μm), and the resulting Fe precipitation significantly influences the seepage properties at the Darcy scale (cm~m). Previous studies have presented a challenge in upscaling fluid dynamics from a small scale to a large scale, thereby constraining our understanding of the spatiotemporal variations in flow paths as porous media evolve. To address this limitation, this study simulated subsurface mixing by injecting Fe(II)-rich freshwater into a DO-rich saltwater flow within a custom-designed syringe packed with glass beads. Micro-computed tomography imaging at the representative elementary volume scale was utilized to track the development of Fe precipitates over time and space. Experimental observations revealed three distinct stages of Fe hydroxides and their effects on the flow dynamics. Initially, hydrous Fe precipitates were characterized by a low density and exhibited mobility, allowing temporarily clogged pathways to intermittently reopen. As precipitation progressed, the Fe precipitates accumulated, forming interparticle bonding structures that redirected the flow to bypass clogged pores and facilitated precipitate flushing near the syringe wall. In the final stage, a notable reduction in the macroscopic capillary number from 3.0 to 0.05 indicated a transition from a viscous- to capillary-dominated flow, which led to the construction of ramified, tortuous flow channels. This study highlights the critical role of high-resolution imaging techniques in bridging the gap between pore-scale and continuum-scale analyses of multiphase flows in hydrogeochemical processes, offering valuable insights into the complex groundwater–seawater mixing.