Zero-valent iron emplacement in permeable porous media using polymer additions

At the Hanford Site in Washington, an extensive in situ redox manipulation (ISRM) permeable reactive barrier was installed to prevent chromate from reaching the Columbia River. However, chromium has been detected in several wells, indicating a premature loss of the reductive capacity in the aquifer. One possible cause for premature chromate breakthrough is associated with the presence of high-permeability zones in the aquifer. The potential emplacement of zero-valent iron (Fe-0) into high-permeability Hanford sediments to enhance the barrier's reductive capacity using shear-thinning fluids containing polymers was investigated in three-dimensional wedge-shaped aquifer models. Porous media were packed in the wedge-shaped flow cell to create either a heterogeneous layered system with a high-permeability zone between two low-permeability zones or a high-permeability channel surrounded by low-permeability materials. The injection flow rate, polymer type, polymer concentration, and injected pore volumes were determined based on preliminary short- and long-column experiments. The flow cell experiments indicated that iron concentration enhancements of at least 0.6% w/w could be obtained using moderate flow rates and injection of 30 pore volumes. The aqueous pressure increased by a maximum of 25 kPa during infiltration, but a decrease in permeability was not observed. Under optimal conditions, the 0.6% amended Fe-0 concentration would provide approximately 20 times the average reductive capacity that is provided by the dithionite-reduced Fe(II) in the ISRM barrier.