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Microbial Solubilization and Immobilization of Toxic Metals for Contaminant Treatment

This study explores key biogeochemical processes involving microbial solubilization and immobilization of toxic metals, such as autotrophic and heterotrophic leaching, chelation by metabolites and siderospores, metal mobilization, sorption to cell components, transport and intracellular sequestration, precipitation, and leaching by fungi. Additionally, it investigates microbial transformations of metalloids, such as reduction to elemental forms, methylation, and metal precipitation by sulfate-reducing bacteria. The research aims to enhance understanding of these processes to improve the design and performance of bioremediation strategies.

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Microbial Solubilization and Immobilization of Toxic Metals for Contaminant Treatment

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  1. Microbial solubilization and immobilization of toxic metals: key biogeochemical processes for treatment of contamination C. White, J.A. Sayer, G. M. Gadd Department of Biological Sciences, University of Dundee, Dundee, UK

  2. Autotrophic and • heterotrophic leaching • Chelation by metabolites • and siderospores • Methylation Metal mobilization • Sorption to cell components • Transport and intracellular sequestration • Precipitation Metal immobilization

  3. Leaching by fungi Bioleaching : Dissolution of metals from their mineral source by organic acids produced by microorganisms Bacteria Fungi Carried out by acidophilic bacteria Do not tolerate high pH values Yarrowia lipolytica(citric) Mucor spp.(fumaric, gluconic) Rhizopus spp (lactic, fumaric, gluconic) Aspergillus spp (citric, oxalic, malic, tartaric,α-ketoglutaric, itaconic) Penicilliumspp(citric, tartaric, α-ketoglutaric, malic, gluconic)

  4. Mechanisms: • Metal cations may be directly displaced from the ore by hydrogen ions produced by the microorganisms resulting in a common acid leaching mechanism. • Compounds produced by the microorganism can bind metals into soluble complexes by chelation.

  5. Citrate  iron chelator, solubilizes Zn+2 from ZnO Oxalate  solubilizes phosphate, Al, Fe and Li, forms insoluble oxalates (mainly Ca oxalate)

  6. Some examples: P.simplicissimum leach Zn+2 in industrial filter dust by production of citric acid A.Niger  leach Cu from copper converter slag P.simplicissimum, A.Niger , P. notatum and Trichoderma viride  leach red mud

  7. Microbial Metalloid Transformations Metalloids: B, Si, Ge, As, Sb, Te and Po selenate selenite

  8. Metalloid transformations-Bioremediation • Reduction to elemental forms: removal of Se in sediments, soil and water • Methylation volatilization: in situ bioremediation of soil and water at Kesterton Res. • Simultaneous removal of NO3- and SeO4-2

  9. Metal Precipitation by Sulfate-reducing Bacteria(SRB) Low MW organic acids + SO4-2 ATP HCO3- + H2S Desulfovibrio desulfuricans www.sysbio.org/sysbio/ mechsensing/index.stm

  10. Immobilizing metals with SRB: Me+2 + S-2 MeS (s) Very stable under anaerobic conditions Optimum growth in pH 6-8 But sulfate reduction has been observed in pH of 3.25

  11. Using SRB for bioremediation: Acid Mine Drainage(AMD)

  12. What is AMD? 4Fe2+(aq) + 8SO42-(aq) + 8H+(aq) Whenwater discharges to surface: 4 Fe+2 + O2 + 10 H2O  4 Fe(OH)3(S) + 8 H+ Yellow Boy

  13. Remediation Options

  14. (Clarified liquor) (sludge)

  15. Sulfate-Reducing Permeable Reactive Zones (SR-PRZs) Mine Tailings Contaminated Water Remediated Water Aquifers Reactive Barrier 2 CH2O + SO4-2→ 2HCO3- + H2S Me+2 + S-2 → MeS

  16. Advantages Drawbacks • Low cost • In situ treatment • Low maintenance • Short lifetime • Poor performance • Long start-up time Understand role of microbial community Improve design of SR-PRZs SR-PRZs

  17. CH4 + CO2 Enzymatic Hydrolysis Polymers Methanogenesis CO2 Oligomers Alcohols Monomers H2 SO4 Sulfate Reduction Me2+ Acetate Organic Acids H2S CO2 MeS(s) CO2 Fermentation Hydrolytic processes Fermentative processes Sulfate-reduction processes Methanogenic processes Carbon Flow Dynamics

  18. Compare microbial inocula performance in sulfate reducing barriers Column Experiments • Identify and compare microbial communities PCR, Real Time PCR, DGGE, Cloning • Develop microbial inocula to optimize SR-PRZs performance Batch, column and field experiments Objectives

  19. Columns Set Up

  20. effluent DNA extraction DGGE 16S rDNA PCR Simulated mine drainage water Identification of microorganisms DNA sequencing Microbial Community Profiling

  21. Std. 0 1st wk 2 ndwk 3rdwk 4thwk Std. Band 5: Fermenter* Band 7: sulfate reducer* Band 9: Fermenter* Band 10: Fermenter* Band 17: fermenter or cellulolytic bacteria* Band 11: sulfate reducer* Band 21: sulfate reducer or fermenter* DGGE Gel Acclimated Inoculum Column * Putative based on DNA sequence similarity

  22. Simulated mine drainage water Organic matter Inoculum Batch Experiment • Inocula: • Dairy Manure • Material from 2 SR- bioreactors • Anaerobic Digester sludge • Twice acclimated inoculum

  23. Batch Experiment • Will help identify inocula with key capabilities • Inocula can be enriched or combined to enhance their performance • Improved inocula can be tested in column and field experiments that better represent the SR-PRZs conditions

  24. Conclusions • Inocula play important role in column/batch reactors performance • The wrong inoculum may not provide any advantage over no inoculum at all • A suite of molecular methods has been developed which will allow for evaluation of microbial communities in the field and also to guide in construction of ideal inocula

  25. What’s next? • Complete DNA analysis • Further test inocula in another batch experiment (this time inoculating with cells only to avoid matrix effects) • Combine inocula that showed good performance to create improved inocula • Test improved inocula in column experiments that better represent the conditions in the PRZs • Do field experiments. Evaluate effect of pH, temperature and stresses on microbial community

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