Effects of physicochemical properties and structural heterogeneity on mineral precipitation and dissolution in saturated porous media

Abstract

Physicochemical properties, such as solute concentration, flow acidity and flow rate, regulate flow and reactive transport in porous media. Mineral precipitation – dissolution (PD) dynamically changes the pore structural heterogeneity and affects the physicochemical properties, yet the interplay between the PD process and structural heterogeneity remains unclear. This study took the precipitation of Ca2+ and CO32− (Ca2++CO32−⇌CaCO3) as an example to study the reactive transport behavior under different physicochemical conditions in saturated, heterogeneous-packed porous media. A series of column breakthrough experiments and numerical simulations of the coupled flow and PD processes were conducted. X-ray computed tomography (XCT) and pore morphology analysis were also applied to investigate the effects of calcite precipitation on pore structure. Results demonstrate that solute concentration, flow acidity, and flow rates in homogeneous columns significantly altered the time of the PD process to reach equilibrium, leading to immediate chemical environmental changes, and thus impact reactive transport behavior. Preferential flow paths in heterogeneous-packed columns could lead to early breakthroughs and thereby promoted the transport of calcium carbonate (CaCO3) in saturated porous media. XCT images revealed that surface roughness played an important role in Ca2+ precipitation, i.e., CaCO3 clustered more on concave surface than convex surface, reshaping a round coat outside grain particles. The CaCO3 precipitation could also narrow or even block the pore throats, and thus decrease the pore connectivity, which hinders the breakthrough behavior. Our study provided experimental evidence for a predictive understanding of Ca2+ reactive transport as well as new insights into the changes in soil structural heterogeneity and transport properties during the PD process.