ObjectiveIn this lesson we will answer the following question:
- What are the characteristics of sulfur?
- How does sulfur contribute to acid rain and acid mine runoff?
Reading AssignmentRead Chapter 26 in Simplified Procedures for Water Examination.
Sulfur is an element which is found in nature as a pale yellow solid. The element is abbreviated as "S" and has an atomic number of 16.
You are probably familiar with sulfur's "rotten egg odor." This smell is typically present when sulfur is bonded to hydrogen, as in the gas hydrogen sulfide (H2S).
The most important elements in biology are carbon, hydrogen, and oxygen since these elements make up such a large percentage of both plant and animal bodies. However, our bodies require many trace elements - including sulfur - without which we could not survive.
Sulfur makes up about 0.25% of the human body. In our bodies, sulfur is primarily used to make proteins, but it also contributes to elasticity in cells and minimizes arthritis.
Plants also require sulfur, using it as a nutrient in many of the ways we do. Farmers use sulfur for pH adjustment and as a fungicide, as well as in fertilizers.
The Sulfur Cycle
The majority of sulfur on earth is found in rocks where it may be present in its elemental form or as salts. However, the important sulfur from a biological perspective consists of several other forms of sulfur which can be found in the air, soil, and water. The table below lists the most important sulfur compounds in nature.
H2S Hydrogen Sulfide
These compounds change form and move from place to place as part of the sulfur cycle. The picture below shows a simplified version of the movement of sulfur between the earth and air.
Sulfate is the integral sulfur compound from a plant's point of view since this is the only sulfur-containing compound which plants can take in. Some of the sulfate in the soil comes from the weathering of minerals, but the majority enters the soil when sulfur dioxide in the air turns into sulfuric acid and falls as rain.
Sulfur dioxide can enter the air naturally when hydrogen sulfide is released from the ground and water. However, the majority of sulfur dioxide in our air results from coal-burning power plants, which we will discuss in the next section.
Despite sulfur's importance to plants, sulfur deficiencies are not a common problem for today's farmers. Since the beginning of the industrial revolution, the burning of coal and petroleum has introduced vast quantities of sulfur dioxide into the air. The sulfur dioxide reacts with water and oxygen to form sulfuric acid which falls to earth as acid rain.
Acid rain is any form of precipitation which contains sulfuric and/or nitric acid and has a pH below 5.6. Acid rain changes the pH of streams, harming aquatic life. It also damages trees and may be instrumental in the current worldwide decline of amphibians.
Scientists first began to recognize the effects of acid rain the middle of the nineteenth century. The United States began to pass legislation regarding air pollutants in 1955. As the laws became more strict and the levels of pollutants decreased, the acid rain problem in the United States also lowered.
Currently, acid rain is still a problem in some portions of the country. On the other hand, sulfur dioxide emissions have become low enough in other areas so that farmers are beginning to see sulfur shortages which harm their crops. The problem seems to be that sulfur concentrations are greatest in cities rather than in farmland where plants grow.
Scientists have come up with a few possible solutions to the sulfur problem. Some scientists suggest creating more wind in cities to move the sulfur out to nearby farmland. A more efficient suggestion is to use hydrazine fuel cells. These fuel cells can be used in electric cars which do not emit much pollution. The hydrazine is generated remotely using coal, with the sulfur byproduct being captured and sold to farmers who can use it on their crops.
Acid Mine Drainage
Iron pyrite, or fool's gold, is a compound containing both iron and sulfur. This mineral is often found around coal seams and can be exposed during coal mining operations. Tailings, the rocks which are discarded during mining, often contain a large amount of iron pyrite.
Acid mine drainage
When iron pyrite in the tailings reacts with oxygen and water, sulfuric acid is formed. Surface and ground water running through the tailings pick up this sulfuric acid, along with heavy metals such as iron, copper, lead, and mercury. This combination of water, sulfuric acid, and metals is known as acid mine drainage.
Current laws require mining companies to prevent acid mine drainage from entering nearby streams. However, abandoned mines continue to leach acid mine drainage into the environment, causing a variety of environmental problems. The sulfuric acid in the runoff lowers the pH of the streams, causing the same problems that are caused by acid rain. The heavy metals in the water can also reach levels in which they are toxic to aquatic life.
Many methods have been used to prevent or correct acid mine drainage, including active treatment, limestone channels, and wetlands. We will consider each of these methods below.
Active treatment takes the form of adding lime or some other alkaline chemical to the water. Although active treatment is very effective in treating acid mine drainage, it creates a great deal of sludge and uses large amounts of chemicals and power. Although a few mines currently treat the water leaving their land in this manner, active treatment is usually prohibitively expensive.
Limestone channels take advantage of limestone's natural buffering capacity. As acid mine drainage is allowed to flow through a channel lined with limestone, the acidic water dissolves calcium carbonate from the limestone and the calcium carbonate raises the pH of the water. A variation on this technique involves diverting water into a well which contains crushed limestone.
Wetlands can also be designed to treat acid mine drainage. These wetlands are typically planted with species such as Sphagnum moss or cattails which will take up sulfur out of the water. Since the water passes through the wetlands very slowly, heavy metals tend to precipitate out in the wetlands. In some wetlands, limestone is added to raise the pH of the water.
Sulfur is an element which is naturally found in a variety of compounds. Sulfur is important to both plants and animals since it is a component of proteins and can change the pH of rain and surface water. The sulfur cycle shows how sulfur moves between the atmosphere and the earth.
Extra sulfur can be released into the environment from burning coal or from mining, resulting in acid rain and acid mine drainage. Techniques exist to alleviate the environmental problems resulting from this extra sulfur.
"Acid Mine Drainage." March 15, 2004. Environmental Literacy Council.
"Acid Mine Drainage: Chemistry." February 5, 2002. Exploring the Environment: Water Quality. Wheeling Jesuit University/NASA Classroom of the Future.
"Acid Rain." January 3, 2005. U.S. Environmental Protection Agency.
Corbin, Katie. "Biogeochemical Cycles." Virginia Tech, Blacksburg.
Fleming, J.R., and B.R. Knorr. 1999. "History of the Clean Air Act." American Meteorological Society.
"The Science of Acid Mine Drainage and Passive Treatment." Pennsylvania Department of Environmental Protection.
"Sulfur Cycle." 1998-2005. Lenntech Water Treatment and Air Purification. Rotterdamseweg, Netherlands.
Complete the interactive exercises in Assignment 17.
Print the assignment out first and become familiar with the exercise before completing it online. You can do the exercise online and get credit for it, or print it out and send to the instructor. It will require the Flash flayer to view, which should already be installed on your machine.
Read Chapter 26 in Simplified Procedures for Water Examination.
Answer the questions in the Quiz 17. When you have gotten all the answers correct, print the page and either mail or fax it to the instructor.