| Permeable reactive barriers (PRBs) are an in situ technology for remediation of contaminated groundwater. Most employ ZVI as the reactive medium, and although many achieve remediation goals, the performance of others is compromised by the precipitation of naturally-occurring solutes. Therefore, this research aimed to understand the precipitation of solids in these systems. Recent work has suggested the suitability of reduced iron sulfide (FeS) as a reactive material for PRB applications, so this research compared the solids production and hydraulic performance of pure ZVI and FeS-coated sands.;To better understand the factors associated with solids production and the potential for failure, a statistical analysis of data from field PRBs composed of ZVI was conducted. Based on this statistical analysis, a series of column experiments was conducted utilizing a simulated groundwater with high calcium (280 mg/L), total carbonate (420 mg/L), and chloride (405 mg/L) as a base solution, to which oxidants (0, 2, or 8 mg/L dissolved oxygen or 100 mg/L nitrate) were added.;Characterization of the aqueous phase in the ZVI column effluents indicated that both both calcium and carbonate were removed from solution. Production of gas bubbles was also observed. In the ZVI columns, increasing oxidant levels corresponded to higher hydraulic conductivity losses. Yet the spectroscopic analysis of solids produced and mass balances on the aqueous phase could not account for all of the hydraulic conductivity loss. Geochemical modeling of the systems was also used to estimate the potential for solids formation and gas production. Results of this modeling also suggested that hydraulic conductivity losses due to the gas phase may be a crucial component of permeability loss.;In the FeS columns, no calcium or carbonate was removed in the columns, no hydraulic conductivity loss was measured, and no solids were detected on the surface of the solids. Modeling of the FeS and ZVI systems on an equal mass basis indicated the potential for solids formation with FeS is much less than that with ZVI. Based on these hydraulic considerations, FeS may have significant advantages over ZVI for PRB applications. |
