In Situ Stabilization of Metal-Contaminated Groundwater by Hydrous Ferric Oxide:  An Experimental and Modeling Investigation

A potential method is investigated for remediation of metal-contaminated groundwater by in-situ emplacement of an adsorptive coating on the aquifer matrix. The coating is emplaced by sequentially injecting solutes that react as they mix in the aquifer to form a sparingly soluble solid with a high metal-adsorption capacity. Dissolved metals are removed passively as groundwater flows through the treated aquifer. The potential effectiveness of this method was demonstrated by sequentially injecting first ferrous sulfate and then oxygen solutions into a column of unconsolidated sand, producing a coating of hydrous ferric oxide (HFO) as the unretarded oxygen reacted with Fe(II) bound to the sand. The HFO delayed the breakthrough of Cr(VI) and As(V) by 8 and 30 pore volumes, respectively, relative to the unamended material. Attenuation of solutes by the unamended sand was reaction-rate limited, but coupled transport/equilibrium geochemical modeling matched well with the increased metal attenuation by the coating. Potential advantages of this method include the following: (1) coatings are emplaced preferentially in high-conductivity zones, reducing problems caused by aquifer heterogeneity; (2) surface disturbance is minimal; (3) regeneration of the coating is straightforward; (4) no hazardous material is generated; and (5) existing geochemical models can help extrapolate to larger scales.