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) reduct ion and immobilization using Fe(II) solutions within the sediments of the A tlantic 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 followin g exposure to Cr-free solutions indicated that simple pump-and-treat proced ures would require extended operation time to meet regulatory standards. In situ reduction was confirmed by the loss of Cr(VI) and Fe(II) from solutio n and a drop in pH compared to control systems. Batch and column-effluent C r(VI) decreased with increasing Fe(II), generally falling below detection l imits as the persistence of Fe(II) occurred. However, Cr-Dissolved, presuma bly 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, ma king 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 m ay be somewhat ineffective due to the enhanced migration of Cr, either Cr(I II) or Cr(VI), induced by the treatment solution.