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Consequences of Biogeochemical Cycling……….. Dr. Diby Paul 1 2 Microbially influenced METAL corrosion 3 Microbially influenced METAL corrosion 4 Microbially influenced corrosion Damage occurs to metals structures submerged in water – hulls of ships, pipe lines that carry water/oil Sulfate reducing bacteria are involved SRB are active under strictly anaerobic conditions – utilize Sulfate as TEA When BIOFILMS form on metal surfaces, anaerobic microsites develop within biofilm SRB are common constituents of natural environments, it easily gets in to oil/water pipes 5 ROLE OF ‘SRB’ IN CORROSION PROCESS??? BIOFILM help to create an anaerobic environment SRB utilize H2 as an electron donor, thereby removing it from the environment and providing a driving force for the electrochemical reactions Step-1 Fe2+ + 2H2O Step-2 4H2 + SO42- Fe(OH)2 + H2 (spontaneous) H2S + 2OH- + 2H2O (SRB, eg. Desulfovibrio desulfuricans) Step-3 H2S + Fe2+ Overall: 2Fe2+ + SO42- FeS + H2 (spontaneous) + 2H2 FeS + Fe(OH)2 + 2OH6 How to control??? Coat the metal surfaces with bactericidal chemicals Mechanical and chemical disruption of biofilms 7 CONCRETE CORROSION 8 9 Microbes produce ACID METABOLITES The binding component of concrete are acid sensitive Fermentative bacteria produce ORGANIC ACIDS Chemoautotrophic SULFUR OXIDISING BACTERIA – sulfuric acid NITRIFYING BACTERIA – nitric acid Corrosion of concrete sanitary sewer pipes have been well documented 10 Concrete pipe Head space Anaerobic sulfide oxidation H2SO4 H2SO4 H2S Sewage pipe H2S Anaerobic sulfate reduction Liquid sewage Corrosion of concrete sanitary sewer pipe 11 In the liquid environment, anaerobic condition are created by microbial activity in the organic rich sewage Under anaerobic condition, SRB generate sulfide which is converted to the volatile H2S form and exchanged across liquid headspace interface In the aerobic and moist environment of the head space, sulfur oxidizing bacteria on the concrete wall, oxidize H2S to sulfuric acid Sulfuric acid react with calcium hydroxide binder in the concrete to form calcium sulfate with no binding capability H2SO4 + Ca(OH)2 CaSO4 + 2H2O 12 Over a 12 yr period of use, the thickness of pipe reduce from 88mm to 32mm CONTROL MEASURES??? Treat sewage liquid to prevent production of H2S – inhibit microbial growth Neutralize the surface of the concrete above the liquid level, by spraying with high pH solution. 13 ACID MINE DRAINAGE 14 15 PYRITE (FeS2) is a major source of sulfur in the lithosphere During mining of ore deposits, pyrite is exposed to oxygen and moisture and becomes the source of an acidic iron-rich leachate known as ACID MINE DRAINAGE The pH can be less than 2 & can seriously impair the quality of receiving waters 4FeS2 + 15O2 + 14H2O 4Fe(OH)3- + 8H2SO4 This process is initiated spontaneously, but as the pH drops, a sulfur and iron oxidizing bacterium, Thiobacillus ferroxidans begins to participate in the reaction 16 T. ferrooxidans attach directly to the pyrite crystal lattice and utilize FeS2 as an electron donor The autooxidation of Fe2+ is a very slow process The bacteria enhances the oxidation process aiding in an increase in the following reaction: 4FeS2 + 15O2 + 14H2O 4Fe(OH)3- + 8H2SO4 This leachate is highly acidic 17 Microbial desulfurization of coal Coal reserves can contain high levels of sulfur compounds (mainly as pyrite) Industrial burning of coal releases sulfur compounds as SO2 In the atm, SO2 combines with water to form H2SO4 – acid rain Most important S-compound in coal is FeS2 4FeS2 (pyrite) + 15O2 + 14H2O 4Fe(OH)3- + 8H2SO4 This reaction, aided by T. ferrooxidans removes S compounds from coal 18 METAL RECOVERY Microbially catalyzed oxidation of minerals is used in the commercial recovery of copper, uranium and gold from low grade ores As of 1989, more than 30% US copper and uranium production was microbially mediated Energy is provided to the microbe by the oxidation of Cu+ to Cu2+ 2Cu2S Chalcocite + O2 + 4H+ 2CuS + 2Cu2+ + 2H2O leached copper 19 Biomethylation of metals Some of the metals are environmental pollutants and can be toxic to higher animals Methylation occurs extensively in nature, Hg, being most extensively studied Microbes can also bind long carbon chains to metal – alkylation Methylation alters the toxicity of the element The most common form of mercury in the env is Hg2+ Microbes methyl;ate mercury in aerobic and anaerobic environments Sulfate reducing bacteria are major players in methylation The intra cellular agent of mercury ,Methylation is methylcobalamine (CH3CoB12) 20 CH3CoB12 + Hg2+ + H2O Methylcobalamine CH3Hg+ + H2OCoB12 methylmercury CH3CoB12 + CH3Hg + + H2O Methylcobalamine (CH3)2 Hg+ + H2OCoB12 dimethylmercury Mercury poisoning- minamata disease / cat’s dance disease 21 To atmosphere Hgo reduction Bioaccumulation in food chain methylation Hg2+ oxydation demethylation To atmosphere methylation CH3Hg (CH3)2Hg demethylation Spontaneous reaction HgS Microbially mediated reactions with Hg2+ in the env. 22 23