Environmental Problems, Their Causes, and Sustainability 1

icals. Scientists are testing the use of cyclodextrin—atype of sugar made from corn starch—to remove toxicmaterials such as solvents and pesticides from contaminatedsoil and groundwater. To clean up a site, asolution of cyclodextrin is injected. After this molecular-spongematerial moves through the soil or groundwaterand attracts various toxic chemicals, it ispumped out of the ground, stripped of its contaminants,and reused.Some scientists and engineers consider biologicaltreatment as the wave of the future for cleaning upsome types of toxic and hazardous waste. Oneapproach is bioremediation, in which bacteria and enzymesare used to help destroy toxic or hazardous substancesor convert them to harmless compounds. Seethe Guest Essay by John Pichtel on this topic on thewebsite for this chapter.Another biological way to treat hazardous wastesis phytoremediation. It involves using natural orgenetically engineered plants to absorb, filter, and removecontaminants from polluted soil and water (Figure24-18). For example, researchers at the Universityof Georgia used genetic engineering to add extradetoxifying power to a cottonwood tree. They inserteda mercury-detoxifying gene of E. coli into the genomeof a common soil bacterium, which was incorporatedinto day-old cottonwood trees. After reaching maturitysuch trees can extract more mercury ions (Hg 2 )fromRadioactivecontaminantsOrganiccontaminantsInorganicmetal contaminantsPoplar treeBrake fernSunflowerWillow treeIndian mustardLandfillOilspillPollutedgroundwaterinDecontaminatedwater outPollutedleachateSoilSoilGroundwaterGroundwaterRhizofiltration Phytostabilization Phytodegradation PhytoextractionRoots of plants such assunflowers with dangling rootson ponds or in greenhousescan absorb pollutants such asradioactive strontium-90 andcesium-137 and variousorganic chemicals.Plants such as willowtrees and poplarscan absorbchemicals and keepthem from reachinggroundwater ornearby surface water.Plants such as poplarscan absorb toxic organicchemicals and breakthem down into lessharmful compoundswhich they store orrelease slowly into the air.Roots of plants such as Indianmustard and brake ferns canabsorb toxic metals such aslead, arsenic, and others andstore them in their leaves.Plants can then be recycled orharvested and incinerated.Figure 24-18 Phytoremediation. Ways that various types of plants can be used as pollution sponges to cleanup soil and water and radioactive substances (left), organic compounds (center), and toxic metals (right).(Data from American Society of Plant Physiologists, U.S. Environmental Protection Agency, and Edenspace)http://biology.brookscole.com/miller14551