Pop quiz: Name three sources of electrical power.

Transcription

1 Pop quiz: Nme three sources of electricl power. By Dvid C. Holzmn Before you red the title of this rticle, you my hve thought of lightning, especilly if you ve lredy red the other rticles in this issue. Perhps you know tht most of the electricity we use is produced in lrge generting plnts from turbines powered either by burning fossil fuel or using nucler rections. Or mybe you thought of common bttery tht utilizes chemicl rection to produce electricity. Perhps you even thought of solr pnels or windmills. You lmost certinly never thought of bcteri. Bcteri!!!??? It my seem hrd to believe tht bcteri could produce electricity, yet some bcteri hve mnged to do just tht. In Derek Lovley s lbortory t the University of Msschusetts Geobcter sulfurreducens displyed tlent tht enbled them to produce tiny electricl currents. Don t get too excited. We re not bout to use them to light up the skyline of mjor Americn city, but Lovley thinks it my be possible to hrness this power nd use it to help bring electricity to remote loctions. The feedstock for these bcteri is decying orgnic mtter the stuff of septic tnks. You probbly know tht n electric current simply consists of electrons tht re moving through or in conductor. When we use electricity to power motor or het the filment of light bulb, we don t ctully use up ny of these electrons. We just use some of their energy to do something we find useful. Bcteri produce electricity in much the sme wy tht bttery uses chemicl rections to produce electricity. Btteries hve two poles, or terminls. Your textbook or techer my hve referred to these s electrodes. One is positive, nd one is negtive. Electricity flows from one terminl to the other. For exmple, when you strt your cr, turning the key connects the two terminls of the bttery vi n electricl pthwy tht goes through the strter motor s coils. Current electrons immeditely flow from the negtive to the positive terminl. Here s how tht works. ChemMtters, APRIL

2 Like 60% of the btteries in the world, the cr bttery is wht is clled led cid bttery. At ech terminl, pltes re bthed in sulfuric cid. The pltes in the negtive pole, which supply electrons for the current, re mde of led. The pltes in the positive pole, which bsorb the electrons, re mde of led oxide. When the terminls re connected, t the negtive pole the led rects with the sulfuric cid to produce led sulfte nd electrons. Pb + SO 4 2 PbSO 4 + 2e Those extr electrons mke up the current tht flows to the positive pole, where the following rection tkes plce: PbO 2 + 4H + + SO e PbSO 4 + 2H 2O The terminls of Lovley s bcteril fuel cells re two smll slbs of grphite, form of crbon, connected by wterproof wire. We use slb of grphite typiclly one-fourth inch thick, sys Lovley. One terminl is plced in the mud t the bottom of body of wter, where the Geobcter re ubiquitous, Lovley sys. The bcteri grow nturlly on the surfce, forming highly stble colony clled biofilm, much like the colony tht grows on your teeth when you forget to brush them. The other terminl is plced in the cler wters bove. The bcteri on the node, the negtive terminl in the mud, feed off of decying plnt mteril. They oxidize cette from the orgnic mtter to crbon dioxide, lso producing hydrogen ions nd electrons. Normlly, iron minerls ct s electron cceptors for the bcteri, but in this cse, the grphite electrode plys tht role. The chemicl eqution for this is: CH 3COO + 2H 2O 2 CO 2 + 7H + + 8e The resulting current flows through the wire to the positive pole, the cthode, where the electrons combine with protons nd oxygen to produce wter: O 2 + 4H + + 4e 2H 2O In his experiments with the bcteril fuel cells, Lovley hs seen them produce electricity stedily for month (nd they could hve gone longer). But don t sell your shres in electric utilities. The current 65 millimps per meter squred of electrode surfce is not going to chrge Direct Electron Trnsfer to Electrode Anode Electron donor CO 2 CHEMISTRY IN THE COMMUNITY ACS H 2 O O 2 Cthode CESAR CAMINERO up the ntion. Croline Hrwood, microbiologist t the University of Iow, in Iow City, envisions using bcteril fuel cells to power sensing devices, tiny gdgets tht cn be used for things like recording tempertures in remote loctions for reserch on climte chnge. Lovley thinks it my hve the potentil to produce energy from wstes produced by frms nd homes. For exmple, some frms mke ethnol from corn for use s fuel, nd others convert orgnic wstes to methne. Geobcter could convert either mteril to electricity, n energy source with mny more pplictions, including cooking nd powering devices such s lights, rdios, nd computers. ALL PHOTOS COURTESY OF UNIVERSITY OF MASSACHUSETTS, AMHERST, ENVIRONMENT BIOTECHNOLOGY CENTER A scnning electron microscope imge of the bcteri (Rhodoferx ferrireducens) growing on grphite electrode. This working bcteri bttery from Lovley's lb cn power red light emitting diode (LED). To do tht, it would be necessry to increse the power output of the bcteril fuel cells. We ve lredy incresed the power output 5- to 10-fold, sys Lovley. We need to get mybe nother 10-fold increse. There re vriety of wys to pproch such improvements. One route hs to do with the pthwy of trnsfer of the electrons from Geobcter to the surfce of the electrode. There is biochemicl cscde s ech electron gets hnded off from one molecule to the next, until it reches the electrode. The question is, how cn tht cscde be mde to go fster? If reserchers knew wht enzyme ctlyzed the finl step of electron trnsfer, tht could help us in designing electrode mterils, sys Lovley. Efficiency gins could lso be mde by geneticlly engineering n orgnism tht produces more of the enzyme for whtever the limiting step is, sys Lovley. Towrd tht end, reserchers hve determined the complete genome of severl species of Geobcter. Microrrys, devices tht cn be used to determine wht genes re ctive in n orgnism t specific moment in time, llow scientists to uncover ech step of the cscde nd to determine which step is slowest, in other words, which step limits the speed of the entire rection. Bcteril fuel cells would be environmentlly benign. Although they would give off crbon dioxide s they generte power, unlike col- nd petroleum-powered mchines, the cr- 12 ChemMtters, APRIL

3 At this Colordo pilot site, Geobcter food, cette, ws pumped into urnium(vi) contminted ground. In few months, the stimulted bcteri were ble to reduce 70% of the urnium into n insoluble form tht trps it in the soil nd keeps it wy from groundwter. bon dioxide they produce comes from plnt mteril, which would decy to crbon dioxide even if it were not used for electricity production. Thus, bcteril fuel cells would not dd crbon dioxide to the tmosphere, gs tht is ssocited with globl wrming. Lovley strted conducting experiments to test bcteril electricity production fter his progrm officer t the Office of Nvl Reserch sked him to look into it. The phenomenon of producing current from mrine sediments hd been known, but t tht time, no one knew how it worked or tht bcteri were involved. Besides producing electricity, the bcteri s tlent my prove useful for removing urnium from groundwter. Like the electrode in the Geobcter fuel cell circuits nd like-iron minerls, urnium cn serve s n electron cceptor. The electron reduces it from soluble form, urnium(vi), to n insoluble form, urnium(iv). Stimulting the growth of geobcter cn precipitte urnium out of contminted surfce wters before it gets into the groundwter, sys Lovley. We did field experiment in Colordo, where by dding cette to groundwter we could get Geobcter to grow. It worked relly well. This problem is huge, sys Lovley. The Colordo site hd been contminted by urnium mining. Other wters hve become contminted by urnium wstes tht hve been dumped into ponds during the mnufcture of wepons. But big problems my yield to tiny cretures tht hve proven to be verstile chemists. Yes, you cn. Success will require some extr reserch nd experimenttion. Here re some hints on wht you will need. Anerobic soil. Digging down in riverbnk, shllow pond, or other wetlnd to the blck stinking mud is the best option. Anything chrcterized s dirt' probbly doesn't hve enough orgnic mtter or nerobic bcteri to work well. A continer. Use mson jr or n qurium. Two wires, preferbly thick noncorrodible mrine grde. Two grphite cylinders for the electrodes. Pencil led is grphite, but it probbly doesn t hve enough surfce re. Try getting lrge pieces t n rt or specilty supply store. A voltge meter or smll clcultor to test your bttery (Ah! But think bout wht you might be ble to power if you were to hook series of these btteries together?) Some hints: The picture bove shows one possible bttery. It is mson jr hlffilled with mud nd then topped off with wter. One grphite electrode hngs in the wter connected to one of the wires. The second electrode is buried in the mud below nd connected to the second wire. A key point: You must crete wtertight connection between the grphite electrodes nd the wire. If you don t, you risk creting n unwnted electrochemicl rection involving the copper wire. Aqurium selnt might be helpful in your quest. Tht s it. The rest is up to you. If you hve success, tell us how you did it nd send pictures of your pprtus to [email protected]. We ll give the new ChemMtters 20-yer CD to the first five techers who send us pictures of working bttery. Dvid Holtzmn writes bout science nd technology in vriety of publictions nd lives in Lexington, MA. ChemMtters, APRIL

4 April 2004 Techer s Guide Bcteri Power

5 Student Questions Bcteri Power 1. Wht unusul bility do the bcteri Geobcter sulfurreducens disply? 2. Describe the bsic structure of bttery nd the generl method by which it produces electricity. 3. Describe how Derek Lovley sets up n pprtus to use nturlly occurring Geobcter sulfurreducens bcteri to produce electricity from body of wter. 4. Describe how the Geobcter sulfurreducens bttery produces electricity, including the chemicl rections tht occur. 5. Why re these kinds of electricity generting systems considered to be environmentlly benign? Copyright 2004, Americn Chemicl Society 2 ChemMtters, April 2004

6 Answers to Student Questions Bcteri Power 1. They re cpble of producing electricity. 2. Btteries use chemicl rections to produce electricity. They hve two poles lso clled terminls or electrodes. One is positive nd one is negtive. Chemicl rections occur t the two electrodes nd electrons flow from one terminl to the other. As the electrons flow between the two electrodes we cn use some of their energy to do something tht we find useful. 3. The terminls of the Geobcter sulfurreducens bttery re slbs of grnite bout one-fourth of n inch thick. One terminl is plced in the mud t the bottom of body of wter. Geobcter bcteri re nturlly present there. The other terminl is plced in the cler wter bove. The two terminls re connected by wterproof wire. 4. The bcteri on the terminl tht is in the mud feed off of decying niml mteril. They oxidize cette from the orgnic mtter to crbon dioxide, lso producing hydrogen ions nd electrons: CH 3 COO H 2 O 2 CO H e - The electrons relesed flow through the wire to the other terminl, where they combine with hydrogen ions nd oxygen to produce wter. O H e - 2 H 2 O 5. Although they produce crbon dioxide s they generte power, the crbon dioxide tht they produce comes from plnt mteril. This plnt mteril would decy to produce crbon dioxide nywy, so using the plnt mteril to produce electricity relly doesn t dd ny dditionl crbon dioxide to the environment. Crbon dioxide gs hs been ssocited with globl wrming. Copyright 2004, Americn Chemicl Society 3 ChemMtters, April 2004

7 Content Reding Mterils Ntionl Science Eduction Content Stndrd Addressed As result of ctivities in grdes 9-12, ll students should develop understnding. Science s Inquiry Stndrd A: nd bilities necessry to do scientific inquiry. Science s Inquiry Stndrd A: bout scientific inquiry. Physicl Science Stndrd B: of the structure of toms. Physicl Science Stndrd B: of the structure nd properties of mtter. Physicl Science Stndrd B: of chemicl rections. Physicl Science Stndrd B: of interctions of energy nd mtter. Life Science Stndrd C: of the cell. Life Science Stndrd C: of the interdependence of orgnisms. Life Science Stndrd C: of mtter, energy, nd orgniztion in living systems. Life Science Stndrd C: of the behvior of orgnisms. Erth nd Spce Stndrd D: of geochemicl cycles. Science nd Technology Stndrd E: nd bilities of technologicl design. Science nd Technology Stndrd E: bout science nd technology. Science in Personl nd Socil Perspectives Stndrd F: of personl nd community helth. Science in Personl nd Socil Perspectives Stndrd F: of nturl resources. Science in Personl nd Socil Perspectives Stndrd F: of environmentl qulity. Science in Personl nd Socil Perspectives Stndrd F: of nturl nd humn-induced hzrds. Science in Personl nd Socil Perspectives Stndrd F: of science nd technology in locl, ntionl, nd globl chllenges. History nd Nture of Science Stndrd G: of science s humn endevor. History nd Nture of Science Stndrd G: of the nture of scientific knowledge. History nd Nture of Science Stndrd G: of historicl perspectives. Bcteri Power Copyright 2004, Americn Chemicl Society 4 ChemMtters, April 2004

8 Reding Strtegies These content frmes nd orgnizers re provided to help students locte nd nlyze informtion from the rticles. Student understnding will be enhnced when they explore nd evlute the informtion themselves, with input from the techer if students re struggling. If you use these reding strtegies to evlute student performnce, you my wnt to develop grding rubric such s the one below. Score Description Evidence 4 Excellent 3 Good 2 Fir 1 Poor 0 Not cceptble Complete; detils provided; demonstrtes deep understnding. Complete; few detils provided; demonstrtes some understnding. Incomplete; few detils provided; some misconceptions evident. Very incomplete; no detils provided; mny misconceptions evident. So incomplete tht no judgment cn be mde bout student understnding. Bcteri Power Wht re they? How do they work? Bcteril Fuel Cells Advntges Disdvntges nd Future Reserch needed Copyright 2004, Americn Chemicl Society 5 ChemMtters, April 2004

9 Anticiption Guides help engge students by ctivting prior knowledge nd stimulting student interest. If you hve time, discuss their responses to ech sttement before reding ech rticle. Students should red ech selection nd look for evidence supporting or refuting their responses. Evlute student lerning by reviewing the nticiption guides fter student reding. Directions for ll Anticiption Guides: In the first column, write A or D indicting your greement or disgreement with ech sttement. As you red, compre your opinions with informtion from the rticle. Cite informtion from the rticle tht supports or refutes your originl ides. Bcteri Power Me Text Sttement 1. Bcteri my be used to provide electricity for lrge cities. 2. Bcteril fuel cells work by producing sulfuric cid, the sme cid found in cr btteries. 3. Grphite electrodes re used in bcteril fuel cells. 4. Reserchers hve determined the genome of severl species of Geobcter so tht they cn develop more efficient bcteril fuel cell. 5. Bcteril fuel cells re hrmful to the environment. 6. Bcteri cn be used to clen up sites contminted by urnium mining. 7. The mterils needed for bcteril fuel cells re dngerous nd not suitble for student experiments. Copyright 2004, Americn Chemicl Society 6 ChemMtters, April 2004

10 Bcteri Power Bckground Informtion More bout Derek R. Lovley nd his work Although he is one of the most outstnding microbiologists in the country, Lovley never set out on tht creer pth. When he ws n undergrdute t the University of Connecticut in the 1970s, he ws not microbiology mjor. I ws minly interested just in environmentl science. I bsiclly wnted to get job working outdoors. Tht ws the level of my sophistiction t tht ge. He tended to equte microbes with disese nd microbiology with medicl reserch. Most people think of microorgnisms s pthogens, germs. Even lot of microbiologists. I remember being in grdute school; there were still some professors who seemed to be mzed tht microorgnisms were importnt in the environment. But tht s where whtever percent of bcteri live, in nturl environments. It s now being recognized tht they live not only on the surfce but lso very deep in the erth. Lovley points out tht the weight of the plnet s microorgnisms fr exceeds the totl weight of ll the plnt life on erth! Although the specific focus of the rticle centers round Lovley s work with Geobcter sulfurreducen bcteri nd their bility to generte electricity, Lovley s reserch extends to severl dditionl very significnt res. His generl focus is on the physiology nd ecology of novel nerobic microorgnisms. He pproches these t three levels--genetic, biochemicl, nd ecologicl. The rnge of his reserch projects is extremely impressive nd includes: microbil metbolism nd community structure in the deep subsurfce nd hydrotherml zones evolution of nerobic respirtion mechnisms of electron trnsport to Fe(III) nd humic cids nerobic bioremedition of petroleum-contminted subsurfce nd qutic hbitts bioremedition of metl contmintion The Dec issue of ChemMtters contined n rticle on hydrotherml vents nd gint tubeworms. Lovley nd co-resercher, Kzem Kshefi, discovered n extremophile (see ChemMtters, Dec. 1999) cptured from vent t the bottom of the Pcific Ocen tht cn survive temperture of C (266 o F) nd tht cn ctully grow nd reproduce t temperture of 121 o C (250 o F). This is well bove the norml boiling point of wter nd represents temperture t which it ws previously thought tht no orgnism could possibly survive, let lone thrive. But when this orgnism (s of yet unnmed but for obvious resons dubbed Strin 121 ) ws plced in n utoclve t 121 o C for 24 hours, it not only survived, but hd doubled in number. The previous extremophile record holder hd been Pyrolobus fumrii, which stops growing t 113 o C nd is killed in 121 o C utoclve fter bout n hour. The discovery of Strin 121 represents more thn just n interesting exmple of n unusul orgnism. The vent where Strin 121 ws discovered sits bout two miles under the ocen s surfce. Since no oxygen is vilble t tht depth, Strin 121 uses iron for its metbolism. The erly Erth ws iron-rich nd oxygen poor, nd it is speculted tht perhps erly forms of life might hve utilized metbolic mechnisms similr to tht used by Strin 121. The fct tht there is t lest one orgnism cpble of surviving such high tempertures in the bsence of oxygen lso increses the possibility tht life might exist on other plnets, either in our own solr system or elsewhere in the universe. As Lovley sys, No one hd ever seen bug like this before. Strin 121 ws obtined from blck smoker like the kind described in the ChemMtters rticle. John A. Bross of the University of Wshington in Settle nd his crew sent n unmnned Copyright 2004, Americn Chemicl Society 7 ChemMtters, April 2004

11 submrine to the ocen floor nd removed prt of the chimney with remotely controlled chinsw. It ws quickly trnsferred to n oxygen-free continer. Lovley ws ble to culture Strin 121 from prt of this chimney. Some generl informtion bout the metbolism of Geobcter sulfurreducen The geobcter sulfurredeucen discussed in the rticle represent only one of n entire fmily of geobctercee (Ltin for erth nd rod, which reltes to where they live nd their shpe) bcteri. At lest fifty species hve been isolted nd it is thought tht mny more my exist. The fmily ws discovered by Dr. Lovley. We re so ccustomed to biologicl systems tht utilize oxygen gs for their metbolic ctivities tht it my be somewht difficult to understnd biologicl systems tht use other reducible substnces. But the principle is reltively strightforwrd. There is nothing mgicl bout oxygen, but it is very suitble substnce upon which to bse metbolic ctivities. First, it is plentiful. Second, it is good oxidizing gent, mening tht it is redily reduced. Third, the products of reduction re typiclly crbon dioxide nd wter, esily relesed fter metbolism. Wht is importnt is tht the metbolic oxidtion process nd the ccompnying reduction relese energy the orgnism needs in order to survive. In principle, ny mteril tht is cpble of undergoing reduction might conceivbly be used. Geobcter sulfurreducen cn use iron(iii). Substnces tken from their surroundings, such s decying plnt mteril, re oxidized. The electrons removed, insted of being picked up by oxygen, re picked up by Fe(III) tht is contined in mterils like Fe(III) oxide. The process releses the energy which the orgnism needs to survive. Geobcter cn ctully sense the presence of iron in their environment nd will move towrds it. Genetic nd biochemicl studies hve shown tht Geobcter specificlly produce flgell nd/or pili (reltively short, hollow protein rods tht re importnt in binding the cell to solid surfces) in response to growth on insoluble electron cceptors such s Fe(III) nd Mn(III). Geobcter re nturlly present in soil nd mud, nd their growth cn be esily stimulted by simply dding n cette-contining mteril such s vinegr. In one experiment, Dr. Susn Childers set up series of microscope slides. In order to survive, the Geobcter would hve to move cross the slide to rech the metl required for their survivl, nd tht s exctly wht they did. They grew the required flgell nd swm towrds the metl source. Once they got there, they grew pili which llowed them to nchor themselves to the metl. To quote Dr. Childers, The Geobcter doesn t wste energy growing flgell or pili unless it genuinely needs them. But if it s not locted ner metl, it somehow senses tht it better get up nd strt moving, nd the gene tht governs the growth of flgell comes into ply. The process described in the rticle is slightly different. Insted of iron(iii) ccepting the electrons, the electrons re picked up by the grphite electrode nd then sent through wires to the wter s surfce, where they re picked up by oxygen in the presence of hydrogen ions. It turns out tht hyperthermophiles cn nerobiclly oxidize not only cette, but romtic compounds nd long-chin ftty cids s well. They cn lso reduce not only Fe(III) nd U(VI), s described in the rticle, but lso Mn(IV), Tc(VII), Au(III) nd some other metls. The potentil uses to which microorgnisms might be put is extremely brod. To help direct reserch efforts to res with higher probbility of success, computer models of genomes hve been creted in order to see how n orgnism might function in given environment. According to Lovley, this represents the strtegy of the future. We cn twek the genome by moving genes round. There is the potentil to design orgnisms to ccomplish certin tsk. Other studied Fe(III)-reducing microorgnisms exist, such s Shewnell nd Geothrix, but microorgnisms in the Geobctercee fmily represent the predominnt metl-reducing Copyright 2004, Americn Chemicl Society 8 ChemMtters, April 2004

12 microorgnisms in wide diversity of subsurfce sediments. The mechnism by which they ccess Fe(III) oxides nd trnsfer electrons onto Fe(III) surfces differs significntly from tht of these other microorgnisms. Furthermore, Geobctercee contins high percentge of genes tht re devoted to sensing its environment nd then llowing it to regulte its metbolism in response to the environmentl conditions. Connections to Chemistry Concepts No mtter how technologiclly or chemiclly complex it might be, the bsic concept behind the opertion of ny electrochemicl cell, or bttery, is quite simple. Oxidtion-reduction rections involve the trnsfer of electrons from one substnce to nother. When redox rection occurs in beker, for exmple, the rectnts mke contct with ech other, nd the electrons re trnsferred directly. Slightly oversimplified, the reson the trnsfer occurs is bsiclly becuse the electrons re in lower energy stte when they re on one substnce vs. being on the other substnce. The substnce tht loses electrons is sid to be oxidized while the substnce gining the electrons is sid to be reduced. Oxidtion nd reduction lwys occur together. An electrochemicl cell is simply technologicl trick tht forces the electrons to trvel through wire (or other conductor) in order to get from the substnce oxidized to the substnce tht is going to be reduced. As the electrons trvel through the wire we cn use some of their energy to light light bulb, power lptop computer, strt cr, or whtever. For exmple, let s consider very simple electrochemicl cell, one tht utilizes the rection between copper metl nd silver ions to produce copper ions nd metllic silver: Cu(s) + 2 Ag + (q) Cu 2+ (q) + 2 Ag(s) The copper metl is being oxidized nd the silver ions re being reduced. If we simply dip piece of copper metl into solution contining silver ion, the electron trnsfer will occur directly nd we won t be ble to mke much use of the energy of the electrons tht re being trnsferred. But suppose we plce copper electrode into solution tht contins some Cu(NO 3 ) 2, which in solution will exist s Cu 2+ nd NO 3 -. We ctully don t need the Cu 2+, since it isn t one of the rectnts, but we do need some ions tht won t rect directly with the Cu electrode, so Cu(NO 3 ) 2 will do fine (we could hve used NNO 3 or ny other slt tht wouldn t rect with the Cu electrode). In nother beker we plce silver electrode in solution of AgNO 3, which will contin Ag + nd NO 3 - ions. Here we ctully don t need n electrode mde of Ag, since Ag isn t rectnt. We could hve used ny nonrective electrode, like grphite or pltinum. Now let s plce the two bekers next to ech other nd connect the two electrodes with conducting wire: Copyright 2004, Americn Chemicl Society 9 ChemMtters, April 2004

13 Since the two electrodes re now connected, one might ssume tht the electrons could trvel from the Ag electrode to the Cu electrode. If this were to occur, then oxidtion would be occurring t the Ag electrode, nd reduction would be occurring t the Cu electrode. We hve bsiclly broken the entire rection into wht re clled two hlf-rections. If these two hlf-rections were to occur we would hve: At the Ag electrode: Ag(s) Ag + (q) + e - At the Cu electrode: Cu 2+ (q) + 2 e - Cu(s) But there is one mjor problem. If these two hlf-rections were to occur, the two solutions would no longer be electriclly neutrl. Positive ions (Ag+) would be produced in the AgNO 3 solution while positive ions (Cu 2+ ) would be removed from the Cu(NO 3 ) 2 solution. This would leve the AgNO 3 solution with net positive chrge nd the Cu(NO 3 ) 2 solution with net negtive chrge. This is not llowed. Solutions must be electriclly neutrl t ll times. To solve this problem we cn use something clled slt bridge. There re other wys to solve this problem s well, but this is one good wy. A slt bridge is nothing more thn U-tube filled with solution tht contins unrective ions. A solution of something like NNO 3 will work well. The ends of the U-tube re plugged with cotton. This keeps the solution from flling out or mixing with the solutions in the bekers, but still llows ions to move in nd out of the U-tube. Once the slt bridge is in plce the cell looks like this: Copyright 2004, Americn Chemicl Society 10 ChemMtters, April 2004

14 Now the cell will work. As Ag + ions re produced in the left-hnd beker they cn migrte into the slt bridge, keeping the AgNO 3 solution electriclly neutrl. As Cu 2+ ions re removed from the right-hnd beker, N + ions cn migrte into the solution to replce them nd keep tht solution electriclly neutrl lso. And most importntly, s the electrons flow from the Ag electrode to the Cu electrode, we cn use some of their energy. Tht ws the whole ide behind building this electrochemicl cell in the first plce. And tht s wht the Geobcter re bsiclly doing. They re oxidizing cette from orgnic mtter in the mud. The electrons relesed in the oxidtion process trvel from one grphite electrode t the bottom of the pond to nother electrode plced in the cler wter bove, where they re picked up by oxygen molecules nd hydrogen ions. As the electrons trvel, we cn use some of their energy. Possible Student Misconceptions The rticle briefly mentions tht Geobcter cn reduce U(VI), soluble form to U(IV), n insoluble form. It is conceivble tht some students my drw couple of invlid conclusions from these sttements. First, it isn t urnium tht is soluble or insoluble. It is compounds of urnium. Wht the soluble form nd insoluble form refer to in this instnce is tht the compounds typiclly formed by U(VI) in the environment re slts of the UO 2 2+ ion, which re generlly soluble, while U(IV) forms insoluble UO 2. Some students my even incorrectly think tht in some wy this is eliminting or reducing the rdioctivity. This, of course, is not the cse t ll. Rdioctivity is nucler phenomenon, nd no chemicl process cn remove or reduce it. But by chnging the oxidtion stte of urnium from VI to IV, it trnsforms it into insoluble compounds tht precipitte out rther thn remin in solution. These compounds cn then be collected nd tken to proper storge site insted of remining in solution where they hve the potentil of migrting to other res. Becuse the rticle discusses the use of Geobcter sulfurrenducens to produce electricity, students my ssume tht this is the only bcterium tht is cpble of engging in this kind of ctivity. This is hrdly the cse. See Bckground Informtion. Demonstrtions nd Lessons Ntionl Public Rdio conducted n interesting interview with Dr. Lovley nd Brbr Methe, coresercher. It runs to just over 30 minutes nd provides nice science for the generl public discussion of Dr. Lovley s work nd its potentil. Audience members cll in with questions, which rnge from firly knowledgeble nd thoughtful to some tht my be bit off-bse. It might be interesting to hve students listen to this brodcst nd then rect to both its content nd the Copyright 2004, Americn Chemicl Society 11 ChemMtters, April 2004

15 posed questions. Dr. Lovley nd Dr. Methe hndle ll the questions nicely nd never insult or tlk down to the udience. The progrm cn be ccessed t: Connections to the Chemistry Curriculum This rticle connects prticulrly strongly with the topic of oxidtion-reduction nd electrochemicl cells (see Connections to Chemistry Concepts). There re dditionl connections to the concept of oxidtion sttes nd how they ffect the solubility of metllic compounds. The biologicl connections re obvious. Suggestions for Student Projects 1. Although most orgnisms utilize oxygen for their metbolic processes, in principle ny substnce tht redily undergoes reduction could conceivbly be the bsis for n orgnism s metbolic requirements. It might be both chllenging nd fun to hve knowledgeble nd cretive students consider different possible reduction rections (from tble of reduction potentils, for exmple) nd then try nd crete theoreticlly vible metbolic system tht might be used by n orgnism. Some considertions would include the vilbility of the requisite mterils, the energy relese tht might be nticipted, nd the properties of the metbolic products. 2. Dr. Lovley s reserch includes studies of wht re clled extremophiles (see Bckground Informtion). ChemMtters published two rticles tht connect to Dr. Lovley s work. The Dec issue contined n rticle on hydrotherml vents, nd the Dec issue contined n rticle on extremophiles. Students could lern more bout Dr. Lovley s reserch in these res, red both of the ChemMtters rticles nd the ccompnying Techer s Guide mterils nd then prepre report tht connects ll of the informtion. 3. The use of microbes for bioremedition extends to mny more res thn re mentioned in the rticle. For exmple, microbes cn be used to clen up gsoline spills. Every dy enormous quntities of gsoline re pumped into Americn motor vehicles. Inevitbly some of this ends up in the environment. It hs been demonstrted tht microbes cn be used to clen up much of this mess, nd reporting on how this is done nd the progress which hs been mde to dte might provide good topic for student report. Anticipting Student Questions 1. Since Geobcter rect with iron nd urnium in the soil could they lso rect with n undesirble metl like mercury, Hg, to remove it from the soil? It hsn t been done yet, but it might be possible. Geobcter contin genes similr to other orgnisms tht cn remove mercury. 2. Could the kind of process described in the rticle be ptented? It lredy hs been ptented. 3. Is there dnger tht by feeding these bcteri so they become prevlent enough to ccomplish some useful ctivity we run the dnger tht they my grow out-of-control? Not relly. When you remove the cette upon which they feed, their growth drops off. 4. Since these bcteri cn feed on iron, is there dnger tht they might feed off the iron in the hemoglobin in our blood nd kill us? No. Geobcter hve no genes for pthogenicity. They cnnot ttck the blood. Copyright 2004, Americn Chemicl Society 12 ChemMtters, April 2004

16 5. Could Geobcter be used to clen toxic substnces from humns? No. They cnnot grow in the humn body. If we wnted to try nd get them to ccomplish something like this we d hve to lter their genome. 6. Is there microorgnism for everything? No, but the potentil tht exists is wesome. For exmple, we hve dded chemicls to the environment, such s PCBs, tht do not nturlly exist. Nevertheless, orgnisms hve emerged tht re cpble of biodegrding these kinds of mterils. Websites for Additionl Informtion nd Ides Ntionl Public Rdio contins n interview with Derek Lovley nd Brbr Methe from the Institute for Genomic Reserch. The interview nd the ccompnying cll-in questions from listeners runs bit over 30 minutes. It cn be ccessed t: A much more thorough scientific pper published by Dr. Lovley nd Dniel R. Bond on electricity production by Geobcter sulfurreducens cn be found t: Check out n rticle on Plugging into the power of sewge from The New Scientist mgzine. Copyright 2004, Americn Chemicl Society 13 ChemMtters, April 2004