IN-SITU REMEDIATION OF CR(VI)-CONTAMINATED GROUNDWATER USING PERMEABLE REACTIVE WALLS - LABORATORY STUDIES

Permeable-reactive redox walls, placed below the ground surface in the path of flowing groundwater, provide an alternative remediation appro ach for removing electroactive chemicals from contaminated groundwater . Four types of Fe-bearing solids, siderite [FeCO3], pyrite [FeS2], co arse-grained elemental iron [Fe-0], and fine-grained Fe-0, were assess ed for their ability to remove dissolved Cr(VI) from solution at flow rates typical of those encountered at sites of remediation. Batch stud ies show that the rate of Cr(VI) removal by fine-grained Fe-0 is great er than that for pyrite and coarse-grained Fe-0. Results from column s tudies suggest that partial removal of Cr(VI)) by pyrite and coarse-gr ained Fe-0 and quantitative removal of Cr(VI) by fine-grained Fe-0 occ ur at rapid groundwater flow velocities. The removal mechanism for Cr( VI) by fine-grained Fea and coarse-grained Fe-0 is through the reducti on of Cr(Vl) to Cr(III), coupled with the oxidation of Fe-0 to Fe(II) and Fe(III), and the subsequent precipitation of a sparingly soluble F e(III)-Cr(III) (oxy)hydroxide phase. Mineralogical analysis of the rea ctive material used in the batch tests indicates that Cr is associated with goethite(alpha-FeOOH). These results suggest th at Cr(lll) is re moved either th ro ugh the formation of a solid solution or by adsorpt ion of Cr(III) onto the goethite surface. The effective removal of Cr( VI) by Fe-0 under dynamic flow conditions suggests porous-reactive wal ls containing Fe-0 may be a viable alternative for treating groundwate r contaminated by Cr(VI).