What effect does leachate recirculation at construction and demolition debris (C&DD) landfills have on the concentrations of chromium and nickel in leachate?  Thus far, this article series has explored the effects of leachate recirculation on iron, manganese, calcium, magnesium, arsenic, barium and strontium concentrations in C&D leachate and found either no significant trends or decreasing trends at each of the three study sites in Ohio. 

Chromium and nickel are transition metals that can be found associated with naturally occurring minerals or man-made materials.  Both of these metals are components in stainless steel, which is a common construction material that may be disposed of in a C&DD landfill.  For example,   type 304 stainless steel is 18% chromium and 8% nickel (Oakley, 1996).  Chromium is also used in electro-plating and in wood preservation (i.e. in chromated copper arsenate (CCA)-treated wood).  Nickel is a component of corrosion-resistant alloys, welding products, and nickel-cadmium batteries. 

Transition metals typically have more than one oxidation state.  Chromium has two common oxidation states at normal temperatures: chromium-3 (+3) and chromium-6 (+6).  Chromium-6, also known as hexavalent chromium, is a known carcinogen and can cause irritation and ulcers to the stomach, intestinal lining, or to the skin.  In contrast, chromium-3 is an essential part of the human diet and is commonly found is many foods such as fruits, vegetables, meat, grains, and yeast.  The Ohio Administrative Code (OAC) Rule 3745-400-21 requires analysis for total chromium, which includes both chromium-3 and chromium-6.  The United States Environmental Protection Agency (USEPA) established a Maximum Contaminant Level (MCL) of 0.1 mg/L for chromium in drinking water. 

The oxidation states possible for nickel range from -1 through +4, although +2 (nickel-2) is the most common in aqueous solutions.  Ingesting nickel in water has been shown to cause stomachaches in humans, as well as adverse effects to blood and kidneys.  There is no current USEPA Drinking Water Standard associated with nickel, though there is a Lifetime Health Advisory Level of 0.1 mg/L. 

In Ohio’s sand and gravel aquifer systems, chromium is reportedly non-detect in 99% of samples, with a maximum concentration reported of 0.050 mg/L (Ohio EPA Division of Drinking and Ground Waters, 2014).  Nickel concentrations in Ohio sand and gravel aquifer samples are as high as

0.269 mg/L, although 80% of samples are reported as non-detect (Ohio EPA Division of Drinking and Ground Waters, 2014).  The available C&D leachate data shows a maximum concentration of 0.2 mg/L for both chromium and nickel. 

Chromium and nickel are naturally occurring elements in rocks and minerals.  In Ohio soils, the average background concentration is 12.1 mg/kg for chromium and 14.4 mg/kg for nickel (Cox & Colvin, 1996). 

To determine if leachate recirculation causes increasing trends for chromium and nickel concentrations in leachate, I analyzed the leachate data using time-series graphs for visual trend analysis, and two different methods of trend testing using the statistical software Sanitas®.  The trend analyses presented below are based on 16 data points representing four years of sampling for Site A and Site C, and 29 data points for Site B representing 10 years of sampling results. 

The time series graphs (above) show the changes in the chromium and nickel concentrations at the three C&D landfill sites that recirculate leachate.  The concentrations are variable, but overall there are apparent decreasing trends for chromium and nickel at Sites A and B, but no apparent trends at Site C.   

The statistical trend analysis (left) for chromium and nickel in the recirculated leachate show statistically significant decreasing trends for chromium and nickel at Site B.  Site A and Site C do not show any statistically significant trends. 

Chromium and nickel appear to behave similarly, as shown by the overall similar shape of the time series plots.  Peaks in chromium concentrations appear correlated to peaks in nickel concentrations.  The overall concentrations vary between sites, which is most likely attributable to variation in the C&D waste stream.  A study of C&D leachate showed that compared with C&D leachate with no CCA-treated wood, leachate from a simulated landfill with 10% by weight CCA-treated wood showed chromium concentrations two orders of magnitude greater (Jambeck, Townsend, & Solo-Gabriele). 

As shown in the above trend analysis, leachate recirculation does not appear to be associated with increasing trends in chromium or nickel concentrations.  Instead, the metals concentrations actually decrease in one of the three study sites.  What chemical conditions can account for the observed trends?

As previously mentioned, chromium is present in two oxidations states: chromium-3 (Cr(III)) and chromium-6 (Cr(VI)).  There are a variety of stable ionic forms of chromium which can occur.  Reduced forms of Cr(III) include Cr³⁺, CrOH²⁺, Cr(OH)₂⁺, and Cr(OH)₄^- .  Oxidized forms of Cr(VI) include Cr₂O₇^2- (dichromate) and CrO₄^2- (chromate) (Hem, 1985).  In rocks and minerals, chromium is predominantly Cr(III).  Cr(III) is poorly soluble, and in reducing conditions will rapidly precipitate as chromium hydroxide (Cr(OH)₃).  The poor solubility of Cr(III) will maintain chromium concentrations in solution typically below 0.1 mg/L (Davis & Olsen, 1995).  In contrast, Cr(VI), which is the toxic form of chromium, is soluble.  In an oxidizing environment, Cr(III) will oxidize to Cr(VI) predominantly as chromate (CrO₄^2-).  Cr(VI) is also the form of chromium present in CCA-treated wood. 

The landfill environment is typically reducing due to anoxic conditions and prevalence of biological and chemical decay reactions.  Recirculation of leachate, depending on the method of recirculation, can expose the leachate to oxidizing conditions due to aeration at the surface.  The change to oxidizing conditions will likely lead to increased dissolution of chromium as Cr(VI) near the surface.  However, as the recirculated leachate infiltrates back into the landfill, the oxygen will be consumed, the environment will change back to predominantly reducing conditions, and the soluble and mobile Cr(VI) will be reduced to Cr(III) and rapidly precipitated from solution.  Therefore, the act of recirculation may actually decrease the concentration of chromium in the leachate, as is reflected in the data.

In fact, recirculation of the leachate can act to enhance decay of the C&D waste, at least as it pertains to chromium sources.  The aforementioned reactions will act to mobilize chromium as chromate near to the top of the landfill and move it down into the lower portions of the landfill, where reduction of the Cr(VI) to Cr(III) and resultant precipitation of chromium hydroxide sequesters the chromium.  While the reduction of Cr(VI) to Cr(III) is fast, the oxidation of Cr(III) is slow, reducing the likelihood of remobilizing chromium once precipitated, even with continued recirculation (Suthersan, Horst, & Ams, 2009). 

Additionally reduced iron (Fe(II)) in the leachate can react with Cr(VI), reducing it to Cr(III), leading to mixed Fe(III) and Cr(III) hydroxides (Suthersan, Horst, & Ams, 2009).  Iron and chromium hydroxides have an even lower solubility than chromium hydroxide, which can support even lower concentrations of chromium in the leachate. 

While nickel, like chromium, also has multiple oxidation states, in solution nickel is predominantly present as Ni²⁺.  The concentration of nickel is controlled by the solubility of the predominant nickel minerals and the pH.  Once in solution, nickel tends to co-precipitate with iron oxides and manganese oxides (Hem, 1985).    As discussed in Article #2 of this series, “Changes in Iron and Manganese Concentrations in C&DD Leachate with Recirculation”, both iron and manganese exhibit decreasing concentrations in the recirculated leachate, and the precipitation of oxidized minerals at the surface was listed as a potential cause of the observed decrease.   Precipitation of iron and manganese oxides with co-precipitated nickel can remove the nickel from leachate temporarily.  However, because nickel concentrations are governed by mineral solubilities, unless the source becomes depleted, nickel concentrations are unlikely to decrease with recirculation. 

In the case of chromium and nickel, leachate recirculation does not support increasing trends.  In fact, the chemical behaviors of both of these constituents in the landfill environment, as discussed in this article, appear to inhibit increasing concentrations associated with leachate recirculation.

References

Agency for Toxic Substance and Disease Registry. (2005, August). Nickel, CAS # 7440-02-0. Division of Toxicology ToxFAQs. U.S. Department of Health and Human Services.

Agency of Toxic Substances and Disease Registry. (2012, October). Chromium - ToxFAQs : CAS # 7440-47-3. U.S. Department of Health and Human Services.

Cox, C. A., & Colvin, G. H. (1996). Evaluation of Background Metal Concentrations in Ohio Soils. Columbus: Cox-Colvin & Associaties, Inc. Environmental Services.

Davis, A., & Olsen, R. L. (1995). The Geochemistry of Chromium Migration and Remediation in the Subsurface. Ground Water, 759-768.

Hem, J. D. (1985). Study and Interpretation of the Chemical Characteristics of Natural Water. U.S. Geological Survey.

Jambeck, J. R., Townsend, T., & Solo-Gabriele, H. (n.d.). Leachate Quality from Simulated Landfills Containing CCA-Treated Wood.

Oakley, D. a. (1996). Nickel and Chromium in Ground Water Samples as Influenced by Well Construction and Sampling Methods. Ground Water Monitoring and Remediation, 93-99.

Ohio EPA Division of Drinking and Ground Waters. (2014). Major Aquifers in Ohio and Associated Water Quality.

Smith, D. B., Cannon, W. F., Woodruff, L. G., Solano, F., & Ellefsen, K. J. (2014). Geochemical and Mineralogical Maps for Soils of the Conterminous United States: U.S. Geological Survey Open-File Report 2014-1082. Reston: U.S. Geological Survey. Retrieved from http://dx.doi.org/10.3133/ofr20141082

Suthersan, S., Horst, J., & Ams, D. (2009). In Situ Metals Precipitation: Meeting the Standards. Ground Water Monitoring and Remediation, 29(3), 44-50.

Previous Articles in Series:

Introduction to the Effects to Leachate Recirculation Series

Changes in Iron and Manganese Concentrations in C&D Leachate with Recirculation

Changes in Calcium and Magnesium Concentrations in C&D Leachate with Recirculation

The Effects of Leachate Recirculation on Arsenic Concentrations in C&D Leachate

The Effects of Leachate Recirculation on Barium and Strontium Concentrations in C&D Leachate

Up next:

Changes in Selenium and Vanadium Concentrations with C&D Leachate Recirculation

Date Published: January 16, 2019