Alex Yiannakakais, Guy Chammas, and Gary Walter

When a company is faced with a chromium contamination problem, whether due to a sudden accidental release, waste disposal at an on-site landfill, or just to previous waste management practices, their options may appear at first glance to be limited - and rather depressing. A recent US Environmental Protection Agency (EPA) fact sheet¹ suggests four options for treating chromium in soils (see box). The first three - isolation or removal technologies-are usually expensive and may not be practical or reduce overall liability. The fourth option - creating an insoluble form of the metal in-place, thus reducing or eliminating it’s potential for migrating into groundwater - may be both the lowest-cost and lowest-liability option.

Chromium Chemistry and Toxicity

Chromium mobility is dependent on it’s solubility in water, soil moisture, or other carrying fluid. The pH and oxidation-reduction (redox) potential are the two measurable parameters that allow us to determine the form (speciation) chromium is likely to exist in, and hence it’s mobility. Chromium commonly speciates into a reduced trivalent form Cr⁺³ and an oxidized hexavelent form, Cr⁺⁶. Cr⁺⁶ usually occurs as the chromate or dichromate anion, which is soluble and mobile under aerobic (oxidizing) and neutral to alkaline pH conditions. Cr⁺³ on the other hand, occurs primarily as a nearly-insoluble hydroxide and is stable over a fairly wide redox and pH range (Figure 1). As a solid Cr⁺³ can persist in neutral and basic environments, and generally will not re-oxidize even in the presence of free oxygen.

The EPA lists Cr⁺⁶ as a carcinogen and as toxic through inhalation and ingestion exposure pathways. Cr⁺³ is not classified as a carcinogen and it’s toxicity is significantly less than Cr⁺⁶. Because, however, EPA does not differentiate between Cr⁺³ and Cr⁺⁶ in setting cleanup standards, additional effort usually must be made to verify not only chromium speciation but also it’s stability.

In Situ Chromium Remediation Strategies

Historically, remediation of chromium contamination has relied on excavation/encapsulation for soils or pump and treat for groundwater. Recently, natural geochemical and microbiological processes that reduce Cr⁺⁶ to Cr⁺³ are becoming the method of choice for the reasons discussed above. In-situ strategies which have been proposed or tested include:

• elemental or ferrous iron³
• sulfides, sulfates, or organic matter⁴
• sulfur dioxide (discussed below)

The EPA4 has helped establish criteria to identify natural processes that may attenuate Cr⁺⁶ in the subsurface, and verify that they are effective (see side bar). The essential criterion is demonstrating that in-situ conditions favor Cr⁺³ over Cr⁺⁶ and that, once reduced, chromium will remain immobile. A strong oxidant, such as manganese dioxide is able to reoxidize Cr⁺³ to Cr⁺⁶, so it is important to demonstrate that the conditions in the release area do not support oxidation, and that future site activities will not create conditions for remobilization. The following case histories illustrate two successful strategies for leaving chromium in place.

Demonstrating Natural Attenuation, Metal Etching Plant

Approximately 3,000 pounds (lbs) of acidic Cr+6 solution at a metal etching plant in the southern U.S. had seeped into soil adjacent to the plant. Following soil and groundwater sampling to establish the extent of chromium migration and to conclude that groundwater hadn’t been impacted, we applied EPA-proposed methods⁵ to evaluate the potential for natural attenuation of Cr⁺⁶. We demonstrated that the soils were effective at reducing Cr⁺⁶ to Cr⁺³ and maintaining the stability of the Cr⁺³ by measuring:

• the potential for native soil to reduce Cr⁺⁶ to Cr⁺³ and to oxidize Cr⁺³ to Cr⁺⁶
• the redox potential of the groundwater, and
• ferrous and ferric iron speciation in groundwater.

This information was used to establish residual levels of chromium that could be left in place without adversely impacting groundwater. We obtained an agreement of no further action for this site.

In-Situ Remediation, Chromium Disposal Pit

Several years ago we conducted what may have been the first full-scale in-situ Cr+6 reduction in soil, using sulfur dioxide injected into soils within and beneath a pit used for disposal of spent acidic Cr⁺⁶ solutions. Approximately 10,000 lbs of Cr⁺⁶ were disposed of in the pit. Excavation and off-site disposal cost were estimated at $2 million. Costs of capping and in-situ reduction to immobile Cr⁺³ were estimated at less than 10% of excavation. Although the high alkalinity of the natural soils neutralized the acidity in the pit, natural chromium reduction was not likely because of the soils’ high pH and low organic content. Sulfur dioxide gas injection was bench tested and shown effective at chromate reduction. The reaction stoichiometry suggested a 1:1 or 3:2 ratio of sulfur dioxide injected to chromate reduced. Injection took place over a 3-week period (Figure 2). Progression of the sulfur dioxide front was monitored using temperature probes and gas detection meters. Reduction was complete (100%) in the treated area. As a final protective measure, we engineered and constructed a capillary barrier soil cover to minimize water infiltration into the treated zone. The state approved the remedial results and issued a no further action determination, pending groundwater monitoring and maintenance of the soil cap.

Applicability

The stabilization through reduction, may apply to several other metallic and semi-metallic elements.¹ Arsenic, cadmium, lead, and possibly selenium are potential candidates, however, their long-term stability in reduced form in the natural environment is uncertain and would require site-specific studies.

References

¹EPA. 1997. Engineering Bulletin, Technology alternatives for the remediation of soils contaminated with As, Cd, Cr, Hg, and Pb. EPA/540/S-97/500.

²Bodek, I, WJ Lyman, WF Reehl, and DH Rosenblatt. 1998. Environmental inorganic chemistry. Pergamon Press, NY.

³James, BR. 1996, The challenge of remediating chromium-contaminated soil. ES&T 30(6):248A-251A.

⁴Palmer, CD and PR Wittbrodt. 1991. Processes affecting the remediation of chromium-contaminated sites. Environmental Health Perspectives 92:25-40.

⁵Palmer, CD and RW Puls. 1994. Natural attenuation of hexavelent chromium in ground water and soils. EPA/540/S-94/505.