In situ Cr(VI) reduction within coarse-textured, oxide-coated soil and aquifer systems using Fe(II) solutions

Batch and column techniques were used to evaluate the in situ Cr(VI) reduction and immobilization using Fe(II) solutions within the sediments of the Atlantic Coastal Plain. Remediation treatments included Cr-free groundwater and buffered and unbuffered Fe(II) solutions as either FeCl2 and FeSO4. The slow release of Cr(VI) from the Fe-oxide rich subsurface sediment following exposure to Cr-free solutions indicated that simple pump-and-treat procedures would require extended operation time to meet regulatory standards. In situ reduction was confirmed by the loss of Cr(VI) and Fe(II) from solution and a drop in pH compared to control systems. Batch and column-effluent Cr(VI) decreased with increasing Fe(II), generally falling below detection limits as the persistence of Fe(II) occurred. However, Cr-Dissolved, presumably Cr(III), exceeded regulatory limits due to the low pH (approximate to 3.0) induced by oxidation and hydrolysis of Fe(II). Acetate-buffered Fe(II) solutions maintained an elevated pH in the presence of Cr(VI) reduction, making the treatment effective at lowering Cr-Dissolved in batch evaluations. Acetate increased Cr(VI) mobility in columns ahead of the reactive Fe(II) front, suggesting that in situ reduction using soluble chemical additives may be somewhat ineffective due to the enhanced migration of Cr, either Cr(III) or Cr(VI), induced by the treatment solution.