Section 3.2 Changing the Concentration of Harmful Chemicals in the Environment The best way to keep the environment safe is to prevent potentially harmful substances from entering it. However, this isn’t always practical because most human activities introduce potentially harmful chemicals into the environment.

Changing concentrations of Pollutants The concentration of pollutants in the environment can be changed using different techniques. The examples discussed here are: dispersion, dilution, biodegradation, phytoremediation, and photolysis

Dispersion The scattering of a substance away from its source. For example, suppose you are fertilizing your lawn and spill too much fertilizer in one spot. To prevent damage to the lawn in that spot, you could spread the fertilizer out over a larger area. In doing this, you would be dispersing the chemical.

Dilution Reduces the concentration of a pollutant by mixing the polluting substance with large quantities of air or water. For example, if you place one drop of bleach into a sink full of water, the molecules of the bleach will mix with the molecules of water, and the bleach will be diluted.

Dispersion and Dilution

Biodegradation Nature uses living things to clean the environment. Every autumn, leaves fall but they do not build up year after year because some of them decompose and become part of the soil. Organisms such as earthworms, bacteria, and fungi help the biodegradation of most organic substances, including many pollutants. “Bio” refers to living things and “degrade” means to break up.

                            Biodegradation Micro-organisms are especially important in the biodegradation of pollutants. Many different types of micro-organisms live in soil and water. Algae, bacteria, fungi, and protozoa are common

Bacteria Some bacteria grow and reproduce only when oxygen is present. They use oxygen for the process of aerobic biodegradation (“aero-” means “air”). Some bacteria need an anaerobic environment—one without oxygen (“an-” means “without”). They thrive where there is little or no oxygen; for example, deep within landfill sites.

Factors Affecting Biodegradation During the winter, biodegradation is slow, because temperature is one factor that affects the rate of biodegradation. Other factors include soil moisture, pH, oxygen supply and nutrient availability.

Phytoremediation Phytoremediation is a technique that can be used to reduce the concentration of harmful chemicals in the soil or groundwater. Plants have been used to clean up metals, hydrocarbons, solvents, pesticides, radioactive materials, explosives, and landfill leachates. The plants are able to absorb and accumulate large amounts of these chemicals and then when plants have matured, they are harvested, burned or composted.

Chernobyl Sunflowers                                                                                                                                                                 Figure 3.9 Sunflowers have been used at Chernobyl to remove radioactive substances from groundwater.

Photolysis Some substances degrade from exposure to light. Photolysis is the breakdown (lysis) of compounds by sunlight (photo).

Photolysis An example of photolysis is the formation of ozone. Nitrogen dioxide in the presence of light breaks down to form nitrogen monoxide and an oxygen atom. The oxygen atom then combines with an oxygen molecule to form ozone.

Photolysis Another example of photolysis is photodegradable plastic. Photodegradable plastic is made of chemicals that react when exposed to sunlight. In three months, the plastic becomes a fine powder that is easier to dispose of. (This type of plastic will only degrade if it is exposed to sunlight – if it is buried, it will last in its original shape for hundreds of years.)