Although they are poisonous to us and to most kinds of animals and plants, many of these toxic compounds serve as food to some types of microbes. This is very good news for us, since microbes can eliminate or neutralize many toxic compounds in the environment. Not all of the news is good - sometimes microbes make pollution problems worse, as is the case in acid mine drainage.
The Environmental Protection Agency (EPA) has put 1,290 of these waste sites on the National Priority List. Sometimes toxins stored at some of these sites leak out of their containers and make their way into ground water, soil, lakes and rivers. Accidents, such as train derailments or crashes involving tanker trucks transporting toxic wastes or oil spills from ships, can also release large quantities of these dangerous chemicals into the environment. Build-up of toxins can also result from release of small amounts over a large area and/or a long time. An example of this is the runoff of excessive fertilizers and pesticides applied to lawns and farm fields. Cleaning up these toxic wastes has cost $20 billion over the past 15 years in the United States. Cleanup of the rest of the toxins already in the environment (in the U.S.A.) is expected to cost between $100 and $500 billion (that is about $500 to $2500 per person in the United States.)
The amount of toxic wastes on the loose and causing trouble in the environment can be reduced in several ways including:
Although currently used the least, bioremediation may be the most economical and most efficient way to reduce toxic waste.
The most effective means of dealing with wastes is not to produce them at all. Some ways to do this are:
Use alternative process
For example, use ladybugs (instead of chemical pesticides) to kill aphids.
Stop use altogether
For example, use reusable cups instead of disposable Styrofoam cups
Reducing production of toxins is not always easy. Sometimes alternatives that are kinder to the environment are also more expensive to businesses. Sometimes the people who would have to pay more for cleaner alternatives are not the same people who live near the places that the toxins effect (many types of wastes and garbage are transported from cities and urban areas out to more rural areas in the country, or are moved from one state to another, or from wealthy countries with lots of industry to poorer countries). Sometimes it is hard to tell how bad the effects of toxins are, or how much less bad the effects of alternatives would be. Finally, sometimes people disagree about how much cleaner alternatives might be and how much more they might cost.
When reduction in production of toxic wastes is not possible, toxic compounds can be eliminated by several means:
Toxins can be buried (which often doesn't eliminate the problem, but instead hides or postpones the problem, leaving the problem for the children and grandchildren of the people who created it). Some toxins gradually degrade over long periods of time.
Wastes can be burned (which often releases toxic compounds into the atmosphere). Instead of always solving the problem, burning often transfers toxins from one area to another (for example, smoke from coal burned to provide power for industries in one area often drifts downwind to another state or country where it falls back to the ground as acid rain)
Toxins can be removed by microbial bioremediation. Bioremediation is the use of microbes to enhance the elimination of toxic compounds in the environment.
Bioremediation can take many forms. Some of the strategies for bioremediation include:
Improving microbes living conditions
Sometimes people can give natural bioremediation a helping hand. In some cases, bioremediation by naturally occurring microbes can be speeded up a great deal. This can be done by adding water to microbes that grow slowly in dry areas (deserts, for example), by adding oxygen to environments that have microbes that "eat" toxins very slowly if little air is present (such as in a swamp), or by adding chemicals (often fertilizers) to environments where there are only a few toxin-degrading microbes to begin with to help the microbes increase in numbers (such as on beaches where oil was spilled).
Adding microbes which have been selected
Some microbes are very good at degrading certain toxins. If microbes that degrade oil, for example, are not present in an environment where oil has been spilled, people can add oil-degrading microbes (grown in a lab) to that environment. Problems can arise, however, if the oil-degrading microbes aren't very well suited to life in their new home where the oil was spilled. For example, if the oil-degrading microbes were originally found in a freshwater pond, they might have trouble living on an ocean beach where oil was spilled because of the high salt concentration.
There are two broad categories of toxic compounds: organic and inorganic. The two types are different in terms of how microbes affect them and in terms of which kinds of microbes affect them. The ten most commonly found wastes are: lead, trichloroethylene, toluene, benzene, PCB's, chloroform, phenol, arsenic, cadmium and chromium.
Organic compounds are molecules containing carbon and hydrogen. These compounds are called "organic" because they are much more common in living creatures than in non-living substances. Many organic compounds can be degraded (or broken down) by living organisms. This process is called biodegradation.
Compounds that are biodegraded are not necessarily completely degraded as far as they can go (to water and carbon dioxide, for example). Some compounds are even more toxic after biodegradation! This is the case with the pesticide DDT, which after biodegradation is actually more toxic. Because of the confusion in terminology, the term "biomineralization" is used to refer to complete degradation of a compound to carbon dioxide and water and other inorganic compounds.
Often, when organic compounds are degraded, the carbon from the compound is used by the microbe for food.
Many compounds can be degraded by microbes, including the following:
polychlorinated biphenyls (insulator-coolants in electric power plant transformers)
1,1,1-trichloro-2,2-bis(p-chlorophenyl) ethane - an organic insect killer (pesticide)
Inorganic compounds contain no carbon. These include heavy metals such as selenium, arsenic, mercury and uranium; fertilizers, including nitrate and phosphate; and runoff from mining operations. Certain microbes can transform these compounds into other non-toxic or less toxic forms.
The following inorganic toxic compounds have microbes that can transform them in some way:
Nitrite - fertilizer runoff
Acid mine drainage
Humans have created many of these toxic chemicals. Some of these chemicals are found in small amounts in nature, but humans are responsible for accumulating many compounds not normally found in high concentrations in the environment, such as acid mine drainage, fertilizer, heavy metals, and oil.
A lot of pesticides ( over 1.1 billion pounds) costing a lot of money (over 25 billion dollars) are used every year in the United States, the world's leading consumer of pesticides. That's over four pounds of pesticide per person! Wow. One of these pesticides, 2,4-dichlorophenoxyacetic acid (2,4-D), is the most widely uses herbicide in the world. It is used on lawns, range land and in farming to kill unwanted broad-leaf plants.
In the United States, between 54 and 60 million pounds of 2.4-D are used annually making it the third most widely used herbicide after glyphosate (RoundUp) and atrazine.
One of the reasons 2,4-D is so widely used is that it does its job and then is easily degraded by microbes in the environment. In most agricultural soils, 2,4-D is degraded after 2 weeks. The reason it is so quickly degraded is because many different microbes can degrade 2,4-D. These are just some of the microbes that can degrade and mineralize 2,4-D:
This bacterium has a huge appetite for a broad range of chemicals, including 2,4-D.
This 2,4-D degrading bacterium can live only in high concentrations of salt.