Environmental Problems, Their Causes, and Sustainability 1

contaminated soil than conventional cottonwoods.Various plants have been identified as potential “pollutionsponges” to help clean up soil and water contaminatedwith chemicals such as pesticides, organicsolvents, radioactive metals, and toxic metals such aslead, mercury, and arsenic. Figure 24-19 lists advantagesand disadvantages of phytoremediation. Studythis figure carefully.Another way to detoxify hazardous waste is witha plasma torch. Plasma—a fourth state of matter—isan ionized gas made up of electrically conductive ionsand electrons. Passing electrical current through a gasto generate an electric arc and very high temperaturescan create a plasma. This process can be carried outcontinuously in a plasma torch somewhat similar to awelding torch.High temperatures from the torch can decomposeliquid or solid hazardous organic material into ionsand atoms that can be converted into simple molecules,cleaned up, and released as a gas. They can alsoconvert hazardous inorganic matter into a moltenglassy material that encapsulates toxic metals andkeeps them from leaching into groundwater.AdvantagesSmallMobile. Easy tomove to differentsitesProduces no toxicashT rade-OffsPlasma ArcDisadvantagesHigh costProduces CO 2 andCOCan releaseparticulates andchlorine gasCan vaporize andrelease toxic metalsand radioactiveelementsFigure 24-20 Trade-offs: advantages and disadvantages ofusing a plasma arc torch to detoxify hazardous wastes. Pick thesingle advantage and disadvantage that you think are the mostimportant.T rade-OffsAdvantagesEasy to establishInexpensiveCan reducematerial dumpedinto landfillsProduces little airpollution comparedto incinerationLow energy usePhytoremediationDisadvantagesSlow (can takeseveral growingseasons)Effective only atdepth plant rootscan reachSome toxic organicchemicals mayevaporate from plantleavesSome plants canbecome toxic toanimalsFigure 24-19 Trade-offs: advantages and disadvantages ofusing phytoremediation to remove or detoxify hazardous waste.Pick the single advantage and disadvantage that you think arethe most important.One approach is to feed solid or liquid waste materialinto a reaction chamber where the heat of theplasma breaks down organic molecules such as oil,solvents, and paint into their basic atoms, which thenrecombine into gases.Another approach is to drill a small hole into acontaminated site such as an old landfill, insert thetorch, and turn it on. The process is repeated over agrid pattern to decontaminate an entire area. Unlikeincineration, the plasma process produces no toxic ashthat must be disposed of safely.Figure 24-20 lists the advantages and disadvantagesof using a plasma arc torch to detoxify hazardouswaste.What Are the Advantages and Disadvantagesof Burning and Burying Hazardous Waste?Last ResortsHazardous waste can be incinerated or disposedof on or underneath the earth’s surface, but this canpollute the air and water.Hazardous waste can be incinerated. This has thesame mixture of advantages and disadvantages asburning solid wastes (Figure 24-13). Two major disadvantagesof incinerating hazardous waste is that itreleases air pollutants such as toxic dioxins and producesa highly toxic ash that must be safely and permanentlystored.552 CHAPTER 24 Solid and Hazardous Waste