Experimental investigation of the interplay between transverse mixing and pH reaction in porous media

pH-induced reactive transport in porous environments is a critical factor in Earth sciences, influencing a range of natural and anthropogenic processes, such as mineral dissolution and precipitation, adsorption and desorption, microbial reactions, and redox transformations. These processes, pivotal to carbon capture and storage (CCS) applications to groundwater remediation, are determined by pH transport. However, the uncertainty in these macroscopic processes' stems from pore-scale heterogeneities and the high diffusion value of the ions and protons forming the pH range. While practical for field-scale applications, traditional macroscopic models often fail to accurately predict experimental and field results in reactive systems due to their inability to capture the details of the pore-scale pH range. This study investigates the interplay between transverse mixing and pH-driven reactions in porous media. It focuses on how porous structure and flow rate affect mixing and chemical reaction dynamics. Utilizing confocal microscopy, the research visualizes fluorescently labeled fluids, revealing variations in mixing patterns from diffusive in homogeneous to shear-driven in heterogeneous media. However, pH-driven reactions show a different pattern, with a faster reaction rate, suggesting quicker pH equilibration between co-flowing fluids than predicted by transverse dispersion or diffusion. The study highlights the unique characteristics of pH change in water, which significantly influences reactive transport in porous media.