Experimental investigations into the evolution of reactive fronts under radial injection conditions.

Reactive fronts form at interfaces where remediation agents encounter contaminant plumes in subsurface environments. The contaminant degradation is largely confined to this narrow mixing zone, but the transient reaction dynamics within it remain poorly understood. Existing models often assume uniform flow or constant dispersion, failing to capture how the spatial deceleration of radial velocities alters interfacial mixing. To address this, two-dimensional sandbox experiments were conducted by radially injecting potassium permanganate into a Naphthol Green B-dyed perchloroethylene (PCE) plume. Image analysis, advection-dispersion modeling, and breakthrough-curve measurements were combined to characterize reactive mixing zone evolution and product generation dynamics. The results indicate that (i) the 1/r decay of radial velocity drove a spatial transition from a proximal reaction-limited regime (Damköhler number, Da = 0.14-0.99) to a distal mixing-limited regime (Da = 3.98-27.38), producing a bimodal chloride breakthrough pattern; (ii) the sampling-based effective mixing-zone width exhibited a pronounced non-monotonic evolution, increasing rapidly during the early stage, reaching a maximum at approximately τ ≈ 0.10, and subsequently decreasing, with a more pronounced decline at higher injection rates; and (iii) the apparent reaction rate similarly increased during the early stage and then declined, while cumulative product mass continued to increase but its growth slowed markedly after τ ≈ 0.07. These results suggest that while higher injection rates enhance mechanical dispersion and initial migration, they can reduce local contact time, highlighting the need to balance physical mixing and reaction residence time in radial injection strategies.