Planar optical chemical sensing and imaging of heterogeneous pH and oxygen fronts during reactive transport in porous media: Applications to acid mine drainage neutralization and zero-valent iron permeable reactive barrier treatment

Reactive transport governs the spatial organization of geochemical conditions in porous media and thereby controls contaminant mobility, reaction pathways, and remediation efficiency, yet remains difficult to resolve experimentally under dynamic flow conditions using conventional point-based measurements. Here, fluorescence-based planar optical sensor foils were developed and applied to obtain non-invasive, high-resolution two-dimensional (2D) measurements of pH and dissolved oxygen (DO) gradients during reactive transport in porous media. Planar pH and oxygen optodes were calibrated over environmentally relevant ranges and deployed in a 2D flow-through tank under controlled hydraulic conditions. Application to acid mine drainage (AMD) neutralization and zero-valent iron (ZVI) permeable reactive barrier analogues revealed internally structured geochemical gradients that were not captured by bulk influent and effluent measurements alone. In AMD systems, limestone-amended media generated evolving pH fronts associated with carbonate dissolution and alkalinity consumption. In ZVI systems, persistent oxygen depletion zones reflected sustained redox reactions linked to iron corrosion. The imaging approach directly resolved spatial heterogeneity in treatment-zone behavior, reaction-front development, and coupled flow-reaction dynamics. These results demonstrate that planar optical chemical sensing provides a powerful experimental framework for resolving spatial heterogeneity and reaction-front development during reactive transport in porous media. • Planar optodes image pH and oxygen during reactive transport in porous media. • Reactive fronts organize strong pH and redox heterogeneity under flow. • AMD neutralization shows migrating, vertically structured pH fronts. • ZVI barriers form persistent oxygen-depleted redox zones under flow. • Imaging reveals treatment-zone structure missed by effluent measurements.