LESSON PLAN: Mercury in the Environment
By Jason McGraw, a high school science teacher

Subjects: Chemistry, geology, environmental science

Time: 1 to 2 class periods

Lesson Objectives:
To develop abilities necessary to do scientific inquiry.
To develop an understanding of chemical reactions, structures and properties of matter
To develop an understanding of geochemical cycles
To develop abilities in science and technology
To develop decision-making skills in natural and human-induced hazards

Competencies - Students will:

• Be informed of new EPA regulations on the emissions of mercury by coal-fire power plants.
• Identify a major human-related source of mercury in the environment.
• Identify regions in the United States that are affected most by mercury emissions.
• Summarize the dangers of mercury in humans.
• Calculate the percent composition of coal to determine the amount of dangerous contaminants present.
• View an analogous demonstration of how mercury is detected in a lab sample.

Overview:
In March, 2005, the Environmental Protection Agency announced new rules for mercury that power plants release into the air when they burn coal to make electricity. The plan, called the Clean Air Interstate Rule (CAIR), is designed to limit the tonnage of mercury that is released per year. The goal is to reduce mercury emissions from 48 tons per year to 38 tons per year in 2010 and 15 tons per year by 2018.

Coal-burning electric plants are the largest cause of avoidable mercury emissions. Other sources of mercury pollution in the environment include the disposal of batteries, thermometers, and gas meters. Not all mercury emissions are from industrial sources. Mercury is released into the air when volcanoes erupt and mercury-containing rocks contaminate ground water.

Mercury is dangerous to humans. High levels of mercury in a human can affect the nervous system and is especially harmful to unborn children in the womb. The most common way a person is exposed to mercury pollution is through eating fish. Lakes become contaminated when mercury compounds in the air mix with water vapor and fall as rain. The mercury is absorbed into the fatty tissues of fish, which pass that mercury on when the fish is eaten.

Coal is a solid hydrocarbon. It has 12 to 50 carbon atoms per molecule and over twice that number of hydrogen atoms. In addition, there are many impurities that are left over from the original organic material that made the coal. Oxygen, nitrogen (from plants), silicon (from shells), aluminum, iron, calcium (from bones), magnesium, sodium, potassium, and phosphorus are common in coal.

How did mercury get into the coal in the first place? Coal is a rock formed from plant material that has been compressed by overlaying rock for hundreds of millions of years. The three major contaminants in coal (mercury, arsenic, and sulfur) were not a part of the living organisms that made the coal. These elements seeped into the coal beds through ground water or during a time when the land was flooded with ocean water. Coal seams that are low in sulfur (termed "sweet" instead of "sour") and low in other contaminants had a layer of impermeable rock overlaying the coal bed.

Correlation to National Standards

Materials Needed (handouts provided in printer-friendly PDF format)

• NewsHour video segment: "EPA Limits Mercury Emissions," Mar. 15, 2005
• Article, "Mercury Contamination of Aquatic Ecosystems" (available at http://water.usgs.gov/wid/FS_216-95/FS_216-95.html)
• Handout 1, Hands-on Activity
• Handout 2, Percent Mercury in Coal
• A Ziploc bag of "coal atoms" for each lab station:
  - lots of small, plastic beads of two colors (one for H, one for C)
  - a small number of different colored plastic beads for other elements.
• A balance at each lab station with a range of 0.0 grams to 100.0 grams
• Several paper cups per lab station to hold beads
• Cold-vapor demo: Glass beaker, a strong light source such as a high-power flashlight or overhead, a small number of transparent beads and some opaque beads
• All students will need their calculator and a periodic table

To make these lesson plans better

Procedure
1. Have students watch "EPA Limits Mercury Emissions" for background on the new EPA rules. The students should think about these questions as they view the video:

• Why is mercury harmful to humans?
• How much of the mercury emitted is actually harmful?
• How will industry be able to reduce emissions by two-thirds?

After the video, ask the students for answers to these questions.

2. Then, ask students, "What is mercury oxide?" Only half of the mercury emissions are harmful to humans. The byproduct of burning coal, mercury oxide, causes nerve damage in humans. Try to write a reaction for elemental mercury becoming mercury oxide. (Hint: Hg loses 2 electrons to become Hg+2)
Answer: 2 Hg + O2 2HgO

3. Reading: How does mercury oxide go from the power plant smokestack to humans? Have students read the short article, "Mercury Contamination of Aquatic Ecosystems" by the USGS and answer the questions below.

• In the late 1980s, where was mercury found in the environment?
• If only 1 gram of mercury is deposited into a small lake each year, why does mercury appear in game fish? (hint: biomagnifications)
• Using figures 4 and 6, trace the path that mercury takes from the air to humans.
• Under what conditions will mercury spread faster through an ecosystem?
• How does inorganic mercury [mercuric oxide or HgO] turn into organic mercury [methyl mercury or CH3Hg+]? How is it transmitted from there?
• List three natural sources and three human-related sources for mercury to be in the environment.
• After the CAIR plan takes effect and coal-burning plants release less mercury, when will researchers see a significant difference in lakes?

4. Conduct the Hands-on Activity using the "Percent Mercury in Coal Handout" provided in the materials section of the lesson.

5. To end the lesson, conduct the following demonstration: Testing for mercury in coal and in drinking water.

Background:
The EPA approved method for detecting the amount of mercury in drinking water is the same method that coal mines use to rank their product. The general method is called "cold vapor atomic absorption spectrophotometry". This instrument takes advantage of mercury's tendency to absorb certain wavelengths of light. Here is an overview:

a. Mercury is separated from the sample of coal or drinking water.
b. Since mercury tends to bond with the element gold, a "gold trap" is often used to isolate the mercury vapors when the sample is heated. (Mercury is often used at gold mines in the same way to trap gold.) An alternate method is to heat the sample in a closed container with concentrated acid.
c. The mercury vapor is converted to HgO, a highly volatile compound.
d. Vapors are sent through the spectrophotometer, which shines a certain wavelength of light that will be absorbed by mercury. The more light that is absorbed, the more mercury in the sample.

Using this in class:
1. Prepare several different cups with mostly transparent beads and some opaque beads. The opaque beads represent mercury.
2. Shine a bright light underneath a glass beaker so that the light shines on the ceiling and it is easy to see. The larger the light and the beaker, the better the effect.
3. Slowly pour one cup of beads into the beaker until the bottom of the beaker is 1 bead deep. This represents the vapor coming from the test sample.
4. Repeat this demo using samples that increase in mercury contamination until the last sample blocks most of the light.

Correlation to National Science Education Standards:
http://www.nap.edu/readingroom/books/nses/html/

Science as Inquiry
Use technology and mathematics to improve investigations and communications
Physical Science
Chemical reactions
Structure and properties of matter
Life Science
Interdependence of organisms
Matter, energy, and organization in living systems
Earth Science
Geochemical cycles
Origin of the earth system
Science in Personal and Social Perspectives
Natural and human-induced hazards
Science and technology in national challenges

About the Author Jason McGraw graduated from Mississippi State University with an M.S. in geoscience. He teaches high school science in the Chicago area and has worked for several years as an industrial chemist. He frequently conducts workshops for teachers on science content and science literacy.

To find out more about opportunities to contribute to this site, contact Leah Clapman at extra@newshour.org.