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A MIXED CHEMICAL REDUCTANT FOR TREATING HEXAVALENT CHROMIUM IN A CHROMITE ORE PROCESSING SOLID WASTE
Citation:
SU, C. AND R. D. LUDWIG. A MIXED CHEMICAL REDUCTANT FOR TREATING HEXAVALENT CHROMIUM IN A CHROMITE ORE PROCESSING SOLID WASTE. Presented at Annual Soil Science Society of America Meeting, Salt Lake City, UT, November 06 - 10, 2005.
Impact/Purpose:
To inform the public.
Description:
We evaluated a method for delivering ferrous iron into the subsurface to enhance chemical reduction of Cr(VI) in a chromite ore processing solid waste (COPSW). The COPSW is characterized by high pH (8.5 -11.5), high Cr(VI) concentrations in the solid phase (up to 550 mg kg-1) and dissolved phase (3-57 mg L-1). The dominant solid phases are forsterite (Mg2SiO4), brucite (Mg(OH)2), and hydrocalumite [Ca4(Al, Fe)2(OH)12X"6H2O), X = (OH)22-, SO42-, CrO42-]. The treatment method utilizes FeSO4 in combination with Na2S2O4 to inhibit oxidation and precipitation of the iron to avoid well and formation clogging during injection. Laboratory batch tests using a 0.05 M FeSO4 + 0.05 M Na2S2O4 solution indicated effective treatment of Cr(VI). Contrary to treatments with FeSO4 and FeCl2 alone, the combination of 0.05 M FeSO4 with 0.05 M Na2S2O4 resulted in both complete removal of Cr(VI) from solution and sustained Fe(II) concentrations in solution after a 24 h period. A field test involving injection of 5700 liters of a 0.07 M FeSO4 + 0.07 M Na2S2O4 solution into a COPSW saturated zone (pH 11.5) indicated no well and formation clogging during injection. A core collected 0.46 m from the injection well following injection indicated effective treatment of the solid phase Cr(VI) based on analysis of water, phosphate solution, and high temperature alkaline extracts. The FeSO4 + Na2S2O4 solution was also observed to impart a residual treatment capacity to the COPSW allowing for subsequent treatment of Cr(VI) liquid spikes; however, dissemination of the iron in the highly alkaline environment appeared to be impeded by the inability to sufficiently lower the pH with distance from the injection well to avoid precipitation of Fe(OH)2 and likely also FeCO3. Injection of a 0.2 M FeSO4 + 0.2 M Na2S2O4 solution into another COPSW saturated zone (pH 9) indicated much more effective dissemination of the injected iron.