Oxidative Precipitation of Fe(II) in Porous Media: Laboratory Experiment and Numerical Simulation

The mixing of terrestrial groundwater and seawater forms a dynamic reaction zone at the land-ocean interface as they bear distinctive and active chemical species. The oxidative precipitation of the ferrous ion (Fe2+) was identified as one of the key processes in this reaction zone. Associated transport and transformation of Fe2+ contribute to the evolution of the coastal aquifer by continuously forming insoluble ferric oxyhydroxide and clogging the pore space of the permeable substrate. In this study, laboratory experiments and numerical simulations were conducted to investigate the combining effects of the flow regime and the hydrochemical compositions of the mixing fluids, primarily dissolved oxygen (DO), and Fe2+ on the precipitation distribution patterns and resulting flow reduction. Two distinctive precipitation distribution patterns were identified: a band-shaped precipitation plume under conditions with a low Peclet number (Pe) and/or a low DO/Fe2+ ratio and a wedge-shaped precipitation plume under conditions with a high Pe and/or a high DO/Fe2+ ratio. A permeability reduction resulted from the pore space clogging was observed in laboratory column experiments and verified by numerical simulations. The postreaction microscopy analysis of the sand specimens revealed the change in pore space morphology, providing the theoretical basis for the understanding of permeability decline during the groundwater-seawater mixing in coastal aquifers.