Chromite Ore Processing Residue (COPR) is a waste product from historic chromium manufacturing. From the late 1800's to around 1970, hexavalent chromium (Cr(VI)) was produced from chromite ore by a high temperature, alkaline oxidation of the ore and subsequent extraction of sodium chromate with water. Lime (CaO) was used as the base, sodium carbonate was used as a source of both base and sodium ion, and, atmospheric oxygen was the oxidant. (Tinjum J, 2006, Mineralogical properties of chromium ore processing residue and chemical remediation strategies, Ph.D. Thesis (Civil Env. Eng) U. Wisc-Madison). The waste contained unreacted chromite ore, various alkaline calcium compounds, and other waste material. Some hexavalent chromium was still present, predominantly trapped in calcium compounds in the waste.
Millions of tons of the waste have been used as landfill material in many areas in the Eastern U.S. (predominantly in New Jersey and Maryland) as well as in Europe. Such waste is highly alkaline, and it contains hexavalent chromium as well as trivalent chromium. Hexavalent chromium leaches out of the waste causing environmental problems. Leaching hexavalent chromium may also render the waste “hazardous” under U.S. EPA regulations. In addition, the waste generates an alkaline leachate and can expand over time, causing heaving problems. (Moon DH et al., 2007, Long-term treatment issues with chromite ore processing residue (COPR): Cr6+ reduction and heave J Hazardous Mat 143:629-635). These environmental problems have driven the need to clean-up such landfill wastes.
Treatment of COPR has been problematic. Discussion of the problems associated with COPR disposal and treatment studies to remediate them have been conducted and reported for over a decade. (James BR, 1994, Hexavalent chromium solubility and reduction in alkaline soils enriched with chromite ore processing residue, J Environ Quality 23:227-233; James BR, 1996, The challenge of remediating chromium-contaminated soil, Environ Sci Tech 30:248A-251A; and, Tinjum, 2006). Treatment involves the reduction of hexavalent chromium to the more stable and less toxic trivalent form. While several common approaches exist to reducing hexavalent to trivalent chromium, none have been sufficiently successful with COPR. (Tinjum, 2006).
Treatment of materials contaminated with Cr(VI) involves reducing hexavalent chromium to the trivalent form (Cr(III)). Cr(III) is insoluble in neutral and moderately basic solutions due to the precipitation of Cr(OH)3 (or, if iron is present, as a mixed iron-trivalent Cr oxide). Several reducing agents are commonly used, including ferrous or elemental iron (Rai D et al., 1989, Environmental chemistry of chromium, Sci. Total Environ. 86:15-23; Palmer CD et al., 1991, Processes affecting the remediation of chromium-contaminated sites, Environ. Health Perspectives 92:25-40; Stanforth RR et al., 1993, In situ method for decreasing metal leaching from soil or waste, U.S. Pat. No. 5,202,033; James, 1994; 1996; Patterson RR et al., 1997, Reduction of hexavalent chromium by amorphous iron sulfide, Environ. Sci Tech 31:2039-2044; Fendorf S et al., 2000, Chromium transformations in natural environments: the role of biological and abiological processes in chromium (VI) reduction, International Geology Review 42:691-701; US EPA, In situ Treatment of Soil and Groundwater Contaminated with Chromium, EPA 625/R-00/004, Office of Research and Development, US EPA, Cincinnati Ohio. (2000)); and reduced sulfur species (Palmer and Wittbrodt, 1991; Patterson et al., 1997; Fendorf, et al. 2000; US EPA, 2000).
It is reported in Rai et al. (1989) (a review article on the environmental chemistry of chromium) that Cr(VI) can be reduced to Cr(III) by many reductants, including ferrous iron and sulfide. Palmer and Wittbrodt (1991) report that ferrous iron or sulfide can be used for reducing Cr(VI). Patterson et al (1997) reports the use of amorphous ferrous sulfide for reducing Cr(VI) in soils and water. The US EPA has stated that ferrous iron must be present for sulfide to reduce Cr(VI), and that iron sulfide needs to be present to reduce Cr(VI) in groundwater (US EPA 2000). Thus, treatment of Cr(VI)-contaminated material with ferrous iron, reduced sulfur species, or the combination of the two is a well-established concept.
Several reducing agents have been tried on COPR, such as ferrous iron (Geelhoed J S et al., Identification and geochemical modeling of processes controlling leaching of Cr(VI) and other major elements from chromite ore processing residue, Geochimica Cosmochimica Acta 66:3927-3942, (2002); Dermatas D M et al., 2006, Ettringite-induced heave in chromite ore processing residue (COPR) upon ferrous iron treatment, Environ Sci Tech 40:5786-5792; and, Moon 2007), reduced sulfur species (e.g. sulfide or polysulfide) (Wazne M et al., 2007, Assessment of calcium polysulfide for the remediation of hexavalent chromium in chromite ore processing residue (COPR), J Hazardous Mat 143:620-628; Tinjum, 2006; and Carlblom, L H et al., In-situ chemical reduction of hexavalent chrome at chromite ore processing residue sites, May 2008, Presented at Sixth International Battelle Conference on Remediation of Chlorinated and Recalcitrant Compounds, Monterey, Calif.), ferrous sulfate and sodium dithionate (Su C M et al., 2005, Treatment of hexavalent chromium in chromite ore processing solid waste using a mixed reductant solution of ferrous sulfate and sodium dithionate, Environ Sci Tech 39:6208-6216), manganese (II) (James 1994), metallic iron (Lai K C K et al., 2008, Removal of chromium (VI) by acid-washed, zero-valent iron under various groundwater geochemistry conditions, Environ Sci Tech 42:1238-1244), pyrite leachate (Chowdhury A, 2003, Method for stabilizing chromium-contaminated compounds, U.S. Pat. No. 6,607,474 B2 and Tinjum 2006), and organic reductants, such as acetic or ascorbic acid (James 1996).
U.S. Pat. No. 5,202,033 to Stanforth et al. report that hazardous wastes or soils containing Cr(VI) can be treated in-situ through the application of ferrous sulfate to reduce chrome. This method has been shown to be ineffective for COPR. (Geelhoed J S et al., 2003, Chromium reduction or release? Effect of Fe(II) sulfate addition on chromium (VI) leaching from columns of chromite ore processing residue, Environ Sci Tech 37:3206-3213).
Higgins T E (Process for the in-situ bioremediation of Cr(VI)-bearing solids) reports that in-situ bioreduction can be used for treating Cr(VI) containing solids, involving the steps of contacting the solids with bacteria, nutrients and water with the pH maintained between 6.5 and 9.5. (U.S. Pat. No. 5,562,588). However, this method would be inappropriate for COPR due to the highly alkaline nature of the COPR and the inherent toxicity of the metals in COPR towards bacteria.
U.S. Pat. No. 6,578,633 to Yen C Y entitled In-situ process for detoxifying hexavalent chromium in soil and groundwater and U.S. Pat. No. 6,955,501 to Yen C Y entitled In-situ process for detoxifying hexavalent chromium in soil and groundwater report a method for the in-situ treatment of Cr(VI) in soil and water by spreading a reducing agent on top of the contaminated area and adding water to infiltrate the reducing agent into the contaminated zone. Among the reducing agents mentioned are ferrous salts, sulfide salts, sodium thiosulfate and organic reducing agents. However, in-situ injection of ferrous sulfate has been reported to be ineffective for COPR due to the rebound effect. (Geelhoed et al, 2003). It is reasonable to conclude that other agents would also be ineffective for COPR.
US Publ. Application No. 2007/0088188 to Wazne et al. entitled Method of treatment, stabilization and heave control for chromite ore processing residues (COPR) and chromium contaminated soils reports adding acid to COPR to consume excess alkalinity so as to reduce the pH to below pH 10, and then adding a reducing agent to the COPR to reduce Cr(VI). While not being specific to these additives, Wazne et al. suggests using carbonated water as a source of acid, and ferrous iron, sulfide, or polysulfide as a reducing agent. The amount of alkalinity in some COPR would require large amounts of acid, such that the treated material would be turned into a slurry where a liquid acid is used. That also makes working with the material much more difficult since, under the US EPA regulations, landfilled solids must be free of liquids in order to pass the paint filter test.
US Publ. Application No. 2007/0098502 to Higgins T E et al., entitled In-situ treatment of in-ground contamination reports introducing ferrous iron and sulfide in a liquid state into the pores of COPR or a Cr(VI) contaminated aquifer. Insoluble ferrous sulfide that forms acts as an ongoing reducing agent for any Cr(VI) that may leach out of the COPR or pass through in the groundwater.
US Publ. Application No. 2007/0224097 to Chisick et al. entitled Methods of treatment of chromite ore processing residue report the use of sulfide ion and ferrous ion to reduce Cr(VI).
Current treatment processes fail to reduce sufficient hexavalent chromium in the waste to eliminate Cr(VI) so that it does not leach from COPR. Over time, concerning the treatment methods that have been tested in the field, chrome and alkalinity slowly leach out of the untreated areas resulting in increased pH and increased hexavalent chromium concentration, which is referred to as the “rebound effect.” It has been reported that ferrous iron is not a successful reductant for Cr(VI) in COPR because the high pH present in the COPR causes ferrous iron to precipitate as a hydroxide, which is unavailable for reducing Cr(VI). (Brown et al. 2008, and Geelhoed et al., 2003).
A further challenge associated with treating COPR is the lack of a reliable analytical tool to assess effective removal of hexavalent chromium from within the COPR matrix. For example, the alkaline digestion test (EPA SW846 method 3060A, a/k/a alkaline digestion test) now used by regulatory agencies does not accurately measure Cr(VI) remaining in the COPR since excess reductant present in a solid precipitate on the surface of COPR particles immediately reduces Cr(VI) released from the interior of the COPR particles during the alkaline digestion test. Because the excess reductant masks untreated Cr(VI) in the COPR particles, the alkaline digestion test overestimates the effectiveness of treating Cr(VI) in COPR and underestimates the compositional level of Cr(VI) remaining in the COPR particles. It therefore remains challenging to design effective treatment methods for treating Cr(VI) in COPR.