Cleaning Up with Iron

Introduction

Since 1980, when Congress passed the first “Superfund” legislation to identify and clean up hazardous waste sites across the country, scientists and engineers have developed many innovative methods to remove contaminants from soil, surface water, and groundwater. Permeable reactive barriers (PRBs) are a good example of new technology that was created to solve environmental problems. A PRB is a wall built below ground to remove pollutants from contaminated groundwater. The walls are permeable, so water will flow through, but are made of reactive materials that will trap or detoxify pollutants. PRBs made of metallic iron are used to remove chlorinated organic solvents and heavy metals from groundwater. The chemical principle is simple—iron is a good reducing agent. It reduces toxic organic compounds and converts them to less harmful substances. The reaction of iron powder with organic redox indicators (dyes) demonstrates the “potential” of this method to reduce toxic organic compounds.

Concepts

• Groundwater remediation
• Permeable reactive barrier
• Oxidation–reduction
• Chlorinated organic solvents

Materials

Safety Precautions

Iron powder is a possible fire and explosion risk. Keep away from flames, sparks and other sources of ignition. Avoid breathing fine metal dust. Wear safety glasses or chemical splash goggles whenever working with chemicals, heat or glassware in the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Filter the heterogeneous reaction mixtures through a funnel to separate the iron powder. The iron powder may be disposed of in the solid trash according to Flinn Suggested Disposal Method #26a. The dye solutions may be disposed down the drain with plenty of excess water according to Flinn Suggested Disposal Method #26b.

Prelab Preparation

1. Prepare 20 ppm methylene blue solution: Add 1 mL of 1% methylene blue solution to 500 mL of distilled or deionized water in an Erlenmeyer flask or beaker. Stir the solution with a stirring rod to obtain a uniform concentration.
2. Prepare 20 ppm indigo carmine solution: In a 250-mL beakers, dissolve 0.25 g of indigo carmine in 100 mL of distilled or deionized water. Dilute 4 mL of this 0.25% solution to 500 mL with water in a 500-mL Erlenmeyer flask to obtain a 20 ppm solution. For best results, prepare this solution fresh the day of use. Note: Save the solution for use in future demonstrations.

Procedure

1. Weigh approximately 2 g of iron powder into a small weighing dish. Using a funnel or weighing paper, transfer the iron into a square-cut plastic bottle.
2. Pour the 20 ppm methylene blue solution into the bottle containing iron powder until the liquid is just about overflowing. (Remove as much air space or air bubbles in the liquid as possible.)
3. Cap the bottle and shake vigorously for 3–5 minutes. The bright blue dye solution will gradually fade and decolorize, and the resulting gray mixture will settle on standing to reveal a clear and colorless liquid and a bottom layer of iron.
4. The dye solution will remain colorless on standing for about 15 minutes before the blue color begins to slowly return due to air oxidation.
5. Using a new clear plastic bottle, repeat steps 1–4 to treat the 20 ppm indigo carmine solution with 5 g of iron powder.
6. The indigo carmine solution will gradually change from its initial blue color to green and then to yellow. The mixture will settle on standing to give a clear yellow solution and a bottom layer of iron.
7. The dye solution will remain yellow for about 15 minutes before gradually turning green and then blue again. The first traces of green (oxidized color) appear near the cap, where air can enter and then slowly diffuse throughout the liquid.

Student Worksheet PDF

12543_Student1.pdf

Teacher Tips

• This kit contains enough chemicals to perform the demonstration as written seven times. Six bottles are provided. To reuse the bottles for subsequent demonstrations, filter the mixtures as described in the Disposal section and rinse with distilled water.
• See a “Citizen’s Guide to Permeable Reactive Barriers” published by the U.S. Environmental Protection Agency for more information about this innovative technology. The publication may be downloaded from the EPA website at http://www.epa.gov/tio/pubitech.htm (accessed June 2006).
• Permeable reactive barriers are a passive technology, relying on the natural flow of water underground to clean groundwater. To illustrate the passive nature of PRBs, set up a series of three capped test tubes containing methylene blue solution. Add iron to the test tubes in sequence over a 3-day period.
• Methylene blue and indigo carmine are used in classic demonstrations, the blue-bottle reaction and the “stop-and-go” light, respectively, to illustrate reversible oxidation–reduction reactions. Other redox indicators that give interesting color changes include resazurin and dichloroindophenol.

Answers to Questions

1. Describe the color and appearance of the methylene blue and indigo carmine solutions before and after mixing with iron powder. What are the colors of the oxidized form and the reduced form of each dye? Why does the original color of the dye solution gradually reappear upon standing?
   

   The methylene blue solution changed from blue (oxidized form) to colorless (reduced form) after mixing with the iron powder for about 5 minutes. Indigo carmine went through a series of color changes, starting out blue (oxidized form), then green, and finally yellow (reduced form). The original colors of the dye solutions gradually reappeared upon standing due to re-oxidation of the reduced forms by reaction with oxygen in air.

2. What characteristics of an organic redox indicator allow it to function as a model substrate for the reduction of pollutants using metallic iron?
   

   Redox indicators exist in at least two different oxidation states or forms that are different colors. The color of the indicator therefore provides a visible “clue” that the substrate has been reduced by reaction with metallic iron. This is a good model for the environmental fate of pollutants.

3. Predict how (a) the “mesh” or grain size of the iron particles and (b) the concentration of a pollutant in contaminated groundwater will affect the performance of a permeable reactive barrier.
   

   (a) Reducing the size of the iron particles in a PRB will increase the surface area of the reactive metal, which should increase the rate of the heterogeneous reaction with any possible contaminants in the groundwater. (b) As the concentration of a pollutant increases, the rate of the reaction with iron should also increase until all of the surface sites on the iron are occupied.

4. Chromate compounds are known human carcinogens and a serious environmental concern. Write a balanced chemical equation for the reduction of chromate ions (CrO₄^2–) to Cr³⁺ using iron metal in an acidic environment (in the presence of H⁺ ions and water).
   

   3Fe(s) + 2CrO₄^2– (aq) + 16H⁺(aq) → 3Fe²⁺(aq) + 2Cr³⁺(aq) + 8H₂O(l)

Discussion

The first full-scale permeable reactive barrier (PRB) was built in 1994. Since then, PRBs have been installed at more than 50 hazardous waste sites in the United States and Canada. PRBs are installed underground, beneath the water table, to clean up contaminated groundwater (see Figure 1). A barrier is built by digging a long, narrow trench and installing the reactive material in the natural flow path of the polluted groundwater. The advantages of PRBs for cleaning up groundwater are that they do not require pumps or expensive machinery, there are no energy costs to operate the barriers, and the process does not generate additional waste that would need to be disposed of in a landfill or by incineration.

There are three major classes of PRBs. Barriers are designed to (1) trap pollutant chemicals by adsorption, using charcoal; (2) precipitate dissolved pollutants or ions, using limestone; or (3) react with toxic chemicals and convert them into less harmful substances. Metallic (zerovalent) iron is the most important reactive chemical used in the third class of PRBs. Iron is inexpensive, readily available, and a good reducing agent, capable of reducing a wide range of organic and inorganic compounds in high oxidation states. So-called “iron walls” are commonly used to remediate groundwater contaminated with chlorinated organic solvents, such as trichloroethylene and perchloroethylene (dry-cleaning solvents), and are also effective for removing pesticides, nitrates, and chromates from water. The detoxification of trichloroethylene, a known carcinogen, occurs via a sequence of two-electron reductions and loss of chloride ions. The ultimate product is ethylene, which is easily biodegraded (Equation 1).

In this demonstration, redox dyes are used as model substrates to illustrate the ability of metallic iron to reduce organic compounds. The organic dyes are redox indicators that exist in two oxidation states, having different colors. The structures of the oxidized and reduced forms of methylene blue and indigo carmine, along with their colors, are shown in Figure 2.

References

This activity was adapted from Chemistry in the Environment, Flinn ChemTopic™ Labs, Volume 22; Cesa, I., Editor; Flinn Scientific Inc.: Batavia, IL (2006).