Environmental Problems, Their Causes, and Sustainability 1

AIRWINDSPRECIPITATIONWINDSPRECIPITATIONHg and SO 2Hg 2+ and acidsHg 2+ and acidsIncineratorHuman sourcesCoalburningplantElementalmercuryvapor(Hg)PhotochemicaloxidationInorganicmercuryand acids(Hg 2+ )Inorganic mercuryand acids(Hg 2+ )DepositionRunoff of Hg 2+ andacidsDepositionVaporizationWATERDepositionSmall fishLarge fishBIOMAGNIFICATIONIN FOOD CHAINPhytoplanktonZooplanktonElementalmercury liquid(Hg)SettlesoutOxidationInorganicmercury(Hg 2+ )SettlesoutBacteriaand acidsBacteriaOrganicmercury(CH 3 Hg + )SettlesoutSEDIMENTFigure 24-26 Cycling of mercury in aquatic environments, in which mercury is converted from one form to another.The most toxic form to humans is methylmercury (CH 3 Hg ), which can be biologically magnified inaquatic food chains. Some mercury is also released back into the atmosphere as mercury vapor.Humans are exposed to mercury in two ways.One is by inhaling vaporized elemental mercury (Hg)or particulates of inorganic mercury (Hg 2 ) salts (suchas HgS and HgCl 2 ). The other is eating fish contaminatedwith methylmercury (CH 3 Hg ), which is verytoxic and can be biologically magnified in food chainsand webs. The greatest risk is brain damage from exposureto low levels of methylmercury (CH 3 Hg ) infetuses and young children whose nervous systemsare still developing.Mercury is released naturally from rocks, soil, andvolcanoes and by vaporization from the ocean. However,50–75% of mercury emissions are believed tocome from human activities, mostly coal burning andto a lesser degree waste incineration. Mercury is alsoreleased into the air when it is used to help extractgold and silver from ores, in some chlorine manufacturingprocesses, and when a fluorescent light bulbbreaks.These human-related sources emit elemental mercury(Hg) vapor and particles of inorganic mercury(Hg 2 ) salts into the atmosphere. Within hours to days,these forms of mercury fall back to the earth’s land andaquatic systems in rain, snow, or as dry particles. Somefalls out locally and some in downwind areas.Once moderately harmful inorganic mercury ions(Hg 2 ) enter an aquatic system, bacteria often convertit to highly toxic methylmercury that can be biologicallymagnified in food chains and webs (Figure 24-26).This explains why high levels of methylmercury are oftenfound in the tissues of sharks, swordfish, kingmackerel, tilefish, and albacore (white) tuna feeding athigh trophic levels in food chains and webs. In 2004,the Food and Drug Administration (FDA) and the U.S.556 CHAPTER 24 Solid and Hazardous Waste