Permeability of iron sulfide (FeS)-based materials for groundwater remediation

Iron sulfide (FeS) has been extensively assessed as a reactive medium to remove both metals and halogenated organics from groundwater. However, to address its suitability as a material for permeable reactive barriers (PRBs), its propensity for solids and gas production, which result in reduced permeability, must be evaluated. The reduction in permeability for sands coated with FeS (as mackinawite), under the anoxic conditions often encountered at contaminated groundwater sites, was examined through column experiments and geochemical modeling under conditions of high calcium and nitrate, which have been previously shown to cause significant permeability reduction in zero-valent iron (ZVI) systems. The column experiments showed negligible production of both solids and gases. The geochemical modeling predicted a maximum reduction in permeability of 1% due to solids and about 30% due to gas formation under conditions for which a complete loss of permeability was predicted for ZVI systems. This difference in permeability reduction is driven by the differences in thermodynamic stability of ZVI and FeS in aqueous solutions. The results suggest that geochemical conditions that result in high permeability losses for ZVI systems will likely not be problematic for FeS-based reactive materials.