Intra-NAPL diffusion and dissolution of a MGP NAPL exposed to persulfate in a flow-through system

•Persulfate/NAPL interactions restricted interfacial mass transfer.•Rate coefficients were ∼30% of those for an equivalent water-flushed system.•Intra-NAPL diffusion-based model developed for flow-through system.•NAPL composition, geometry, interphase mass transfer may limit intra-NAPL diffusion.•Treatment technologies involving aggressive flushing will decrease dissolution rate. Intra-NAPL diffusion is a critical process that can influence NAPL/water mass transfer. A series of physical model experiments was performed to investigate the role of intra-NAPL diffusion on the transient dissolution of a complex multicomponent NAPL subjected to persulfate treatment. To support these observations, a diffusion-based model was developed and calibrated using the experimental data. The experimental results indicated that while persulfate was able to completely degrade dissolved phase components, mass loss after ∼410 pore volumes of persulfate flushing was less than the no-treatment system. Intra-NAPL diffusion limitations were not observed in the physical model experiments. A comparison of experimental and simulated results indicated that processes related to persulfate/NAPL interactions restricted mass transfer, and yielded multicomponent mass transfer rate coefficients that were ∼30% of those estimated from an equivalent water-flushing experiment. Simulation results showed that a combination of NAPL composition and geometry, and interphase mass transfer rate can yield intra-NAPL diffusion limitations. Remedial technologies that rely on the aggressive flushing of reagents into NAPL zones may give rise to intra-NAPL diffusion limitations, which will directly affect treatment efficiency.