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Chapter 21 - Microorganisms and Metals

Chapter 21 - Microorganisms and Metals. Objectives Know the common toxic metals and the main sources that they come from Know factors affecting metal bioavailability in the environment Know why are metals are toxic to microorganism and their mechanisms for resistance

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Chapter 21 - Microorganisms and Metals

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  1. Chapter 21 - Microorganisms and Metals • Objectives • Know the common toxic metals and the main sources that they come from • Know factors affecting metal bioavailability in the environment • Know why are metals are toxic to microorganism and their mechanisms for resistance • Be able to discuss some general approaches for metal remediation

  2. Top 20 Hazardous Substances List ATSDR/EPA 2003 • Arsenic • Lead • Mercury • Vinyl Chloride • Polychlorinated Biphenyls (PCBs) • Benzene • Cadmium • Polycyclic Aromatic Hydrocarbons • Benzo(a)pyrene • Benzo(b)fluoranthene 11. Chloroform 12. DDT 13. Arochlor 1254 14. Arochlor 1260 15. Dibenz[a,h]anthracene 16. Trichloroethylene 17. Chromium (+6) 18. Dieldrin 19. Phosphorus, white 20. Chlordane Five of the top 20 EPA hazardous substances are metals.

  3. Metals in the Environment * Required metals

  4. Common metal contaminants found in Superfund sites Metal Occurrence (%) Lead (Pb) 71 Arsenic (As) 60 Zinc (Zn) 57 Nickel (Ni) 50 Mercury (Hg) 47 Barium (Ba) 46 Cadmium (Cd) 30

  5. Metals and Metalloids of Concern Quantities Produced and Uses • Arsenic- As 43,000 tons/yr (1995) • used in: insecticides, herbicides, seed additives, wood • preservatives, desiccants, ceramics, glass (0.2-1%) additives • Cadmium- Cd 14,500 tons/yr (1995) • used in: battery-powered cellular telephones, camcorders, • personal computers, pigments, stabilizers, coatings and alloys • Cobalt- Co 18,500 tons/yr (1994) • used in: alloys, nuclear industry, pigment in glazes, UV • protectant in eye protective equipment, paint additive, catalyst • in the petroleum industry. • Lead- Pb 1,510,000 metric tons/yr in the US (2002) • (a large portion is recycled) over half of lead is used by the • auto industry in batteries. Other uses include manufacture of • cable sheathings, sheet, pipe foil and tubes, solders,alloys, • ammunition, and paints.

  6. Mercury- Hg 10,000 tons/yr (1980) • major uses include electrical apparatus, the electrolytic • preparation of chlorine and caustic soda, the manufacture of • mildew-proof paint and in industrial and control instruments. • Nickel- Ni 875,00 tons/yr (1995) • used in alloys, plating, batteries, magnets, electrical • contacts,electrodes, spark plugs, machinery parts, and as a\ • catalyst.

  7. 2. pH high pH bioavailability metal phosphates/carbonates low pH bioavailability free ionic species 3. redox potential high Eh bioavailability free ionic species low Eh bioavailability metal phosphates/carbonates/sulfides Metals in the Environment • Total metal vs. bioavailable metal • Factors that affect metal bioavailability 1. metal sorption by soil (organic matter, clay minerals, metal oxides)

  8. Bioavailable vs. total cadmium in soil – example Note that a very small fraction of the total metal is actually bioavailable (defined here as soluble in water). The majority of the metal is sorbed or precipitated.

  9. Despite the fact that low amounts of metals in the environment are bioavailable, as the graphs below demonstrate, it does not take much metal to induce toxicity.

  10. Effect of increasing Pb on toxicity

  11. Metal Toxicity

  12. Metal Resistance Mechanisms

  13. General remediation approaches for metals These are based on mechanisms of metal resistance and include: • Removal of metals from wastestreams using biomass as a • sorbent • In situ precipitation of metals by creating anaerobic conditions • Removal of metals from soil using metal-complexing agents, • e.g. biosurfactants • Volatilization of metals, e.g. Selenium • Phytostabilization of metals, e.g. mine tailings

  14. Example of a successful bioremediation • Zinc smelter (100 yrs old) with 135 mg/L zinc and 1300 mg/L sulfate in groundwater. • (Budelco in the Netherlands) • Cleanup was mandated, choices included: • Ion exchange – good Zn removal, no sulfate removal, costly • Liquid membrane extraction – good Zn removal, no sulfate removal, costly • Bioremediation using SRBs The Solution After pilot-scale testing, a commercial plant with an 1800 m3 bioreactor was constructed to treat 6000 m3 of groundwater per day ( 55-gallon drum every 3 seconds).

  15. Three effluents are generated: • Solid sludges that are returned to the smelter to recover the precipitated Zn. • Liquid containing 80% sulfur mostly as H2S or S0. This is passed into an aerobic fixed film bioreactor. Here sulfate oxidizers convert H2S to S0. • Gas that contains 40% H2S, 60% CH4, and a small amount of CO2. The H2S is removed by passing through a zinc sulfate solution, and the CH4 is burned. • Aqueous effluent design criteria • Zinc < 0.3 mg/L Original 135 mg/L • Cadmium < 0.01 mg/L • Sulfate < 200 mg/L Original 1300 mg/L

  16. ZnSO4 solution discharge CH4 flare Biofilter scrubber H2S + CH4 vapor vapor Tilted plate settler H2S liquid SFF Groundwater SO42-, Zn2+ UASB S0 liquid O2 discharge sandfilter ZnS solids S0 + biomass ZnS + biomass sludge Solids to zinc smelter What would you add as an electron donor in the UASB??

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