CLASSIFICATION OF METALS INTO FAMILIES WITH COMMON CHEMICAL CHARACTERISTICS

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1 CLASSIFICATI F METALS IT FAMILIES WITH CMM CHEMICAL CHARACTERISTICS I A Li II A Be III A a Mg I B - VIII B (Transition Metals) Al K Ca Sc Ti V Cr Mn Fe Co i Cu Zn Ga IV A VA Rb Sr Y Zr b Mo Tc Ru Rh Pd Ag Cd In Sn Sb Cs Ba La Hf Ta W Re s Ir Pt Au Hg Tl Pb Bi

2 BITRASFRMATIS F METALS I SILS Redox reactions of inorganic metal species Conversion of inorganic to organic forms and vice versa mineralization - immobilization methylation - demethylation Indirect effects of biological activity acidity - alkalinity formation of microbial by-products which undergo nonbiological redox reactions salt formation complexing reactions BIMETHYLATI F METALS Hg is the most studied metal in regards to its methylation or alkylation Monomethyl mercury ( volatile ) is the predominate product at neutral ph Higher under aerobic than anaerobic conditions Inhibited by addition of sulfide Higher microbial growth stimulates higher methylation rates Temperature affects rates by affecting microbial growth

3 BACTERIAL TRASFRMATIS F METALS Transformation Reduction xidation Metal As (V) Fe (III) Hg (I) Hg (II) Mn (IV) Se (IV) Te (IV) As (III) Fe (0) Fe (II) Mn (II) Sb (III) Methylation Demethlyation As (V) Cd (II) Hg (II) Pb (II) Se (IV) Sn (II) Te (IV) RHg (II) from Summers, A.. and S. Silver Microbial transformations of metals. Ann. Rev. Microbiol. 3:

4 = ESSETIALITY F HEAVY METALS General Requirements of Heavy Metals by Microorganisms 1 Mettalloenzymes containing a fixed quantity of metal, bound as an integral part of the enzyme A) Active Site (Catalytic Role) B) Structural Role C) Both Metal-activated enzymes - metal is not an integral part of the enzyme Specificity of metal for activity to occur is very high for 1 and much less for. Specific Requirements 1 Cu - itrite Reductase H Zn R--P-H Alkaline Phosphatase RH + PI H Proteases Protein Amino Acids 3 3 Co Vitamin B1 participates in synthesis of hemoglobin 4 - itrate Reductase - Mo 5 i Urease 3 itrogenase H4 +

5 Without Rice Straw With Rice Straw Soluble Fe ( ug/g soil ) Fe + Redox Potential Potential Redox Potential Fe Redox Potential (mv) Incubation Time ( days ) Reduction of iron in a waterlogged soil with and without the addition of rice straw ( 0.5%, w/w ) ( Adapted from Pal, Sudhakar-Babik, and Sethunathan, Effects of benomyl on iron and manganese reduction and redox potential in flooded soil. J. Soil Sci. 30: )

6 FRMATI F IR PDZLS STAGE 1 Fe Fe Al Al Fe Al Formation of water-soluble iron and aluminum organic complexes STAGE Fe Fe Al Fe Al Al Fe Fe Al Al Fe Al Movement of the complexes into the B horizon ( Reduction of the iron seems to increase the movement of Fe into the B Horizon ) STAGE 3 Precipitation ( mineralization ) of the complexes in the B horizon The intensity of podzol formation depends on the relative rates of movement into the B horizon vs. the rate of mineralization of the Fe-rganic matter complex.

7 A A 0 1 Weak illuviation A Intense illuviation Soil minerals Free M 3 B Soil minerals Free M 3 Intense mineralization of organo-mineral complexes Slow mineralization of organo-mineral complexes Iron podzol Humus illuvial podzol

8 CRRSI F METALS Corrosion is the destructive attack of a metal by a chemical or electrochemical reaction with its environment (rusting). Microorganisms contribute to corrosion processes in several ways: Through the formation of mineral acids, especially sulfuric acid Through the formation of organic acids By changing the electrode potential or E h of the environment By creating microgalvanic cells By depolarization surfaces through the oxidation of hydrogen By producing H S AERBIC CRRSI ( rusting in air ) Low levels under a microbial cell mass and higher levels adjacent to the cell mass causes a potential to form (a microgalvanic cell ) and electrons to flow from Fe ( metal ) to oxygen. VV V e e e Fe metal e e

9 Electrode Conventions Cathode Anode ions attracted half reaction direction of electron flow sign galvanic cations reduction into cell positive anions oxidation out of cell negative

10 Tubercule 4Fe(H) + H + 4e Fe+ 4H 4(H) 4e + H + (Cathode) (Cathode) 4Fe (Anode) Aerobic Corrosion Process of Iron. from Iverson, W.P Microbial corrosion of iron. pp , In J.B. ielands (ed) Microbial Iron Metabolism: A Comprehensive Treatus, Academic Press, ew York.

11 Anaerobic Corrosion of Metallic Iron 6H - S - 3Fe(H) FeS 3Fe + Fe + Desulfovibrio desulfuricans 8H 8e - Hydrogenase H S 4 + 8H S + 4H (sulfate reduction) 8H + + 8H- 8H cations anions 4Fe + aqueous medium cathode (metal surface) 8e - anode 4Fe from Zajic, J.E Microbial Biogeochemistry, p7, Academic Press, ew york.

12 Anodic Reactions Cathodic Reactions Water AAERBIC CRRSI 4Fe + S H FeS + 3Fe(H) + H- + 4Fe 4Fe + 8e Fe + S FeS + - 3Fe + 6H 3Fe(H) + - 8H + 8e 8H (dehydrogenase) S + 8H S + 4H (sulfate reduction) 8H 8H - + 8H + ptimum Conditions for Anaerobic Corrosion 1 ph greater than 5.5 Eh less than 400 mv 3 Low concentration of free oxygen 4 High concentration of sulfate Treatments 1 Coating with an inert material Use of a bactericide 3 Cathodic protection

13 = = Active Methyl CH 3 H 3 C CH H CCH H C 3 H C 3 H CCH H = = C= C P H HH C H C= CH H H CH H CHCH = H CH 3 H C 3 Co IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII CH 3 CH 3 CH 3 CH 3 = CH CH CHCHCH H CH 3 CH 3 H CHCHCH = = HCCHCHCH H CH 3 S+ Active Methyl CH H H H H H S - ADESYL METHIIE H H METHYLCBALAMI Active Methyl H C = C = C C C CH 3 H CH CH H CH 5 C H CH CH CH CH CH - METHYLHYDRFLIC ACID Methyl Donors Involved in the Methylation Reactions of Metals

14 = = C H = = H H C= =C Caged Iron This sideophore, enterobactin, traps iron at the center of a six coordinate, octahedral complex. The complex has a high affinity for ferric iron, which can nevertheless be rapidly exchanged.

15 SIDERPHRES Low molecular weight, virtually Fe (III) specific ligands. Generally they are produced by aerobic or facultative aerobic bacteria and fungi. At ph 7.0 K for Fe (H) = 10 sp (Fe ) = 10 M Fe requirement for plants = 10 M -8 Types of Siderophores in Soils 1 Hydroxomates Citrate 3 Catechols 4 Amino acid composition

16 SIDERPHRES Assay Procedures 1 Chemical Procedures Biological Procedures Roles of Siderophores in Soils 1 Growth factor Iron chelation and transport 3 Antibiotic 4 itrification intermediate 5 Urease inhibitor

17 Heavy Metals TXICITY F HEAVY METALS 1 Toxicity brought about primarily by the metals affecting some enzymatic process A) Masking of catalytically active groups B) Protein denaturation C) Conformational changes D) Compete with activating metal ions for substrate and enzyme E) Reduce enzyme synthesis Microbial uptake A) Accumulation of metal ions in fungal spores or bacterial cell walls concentrating the metals in a specific food chain B) Uptake and excretion of organic acids, organic salts, or volatile organics more toxic than the original metal C) Reduction in diversity of microorganism strains D) Development of resistant strains

18 Heavy Metals TXICITY F HEAVY METALS 3 Soil factors affecting toxicity of heavy metals A) ph B) Base saturation (CEC) C) Amounts and properties of organic matter D) Interactions with other inorganic constituents E) ature of the assay substrate F) Redox

19 Average Percentage Inhibition of itrogen Mineralization -1 in Four Soils by Trace Elements (5 M g soil) Trace Element Element xidation State Percentage Inhibition Ag I 56.3 Hg Cu Cd Pb Mn Fe Zn Sn Co Cr Fe Al B As V Se As Mo W II III IV V V I Adopted from Liang and Tabatabai, Effects of trace elements on nitrogen mineralization in soils. Environ. Pollut. 1:

20 BILGICAL TRASFRMATI F METALS Adaptation and Tolerance Mechanisms by which Microorganisms Attain Tolerance Impermeability of the plasma membrane Concentration of metals in the cell walls Production of compounds which render the metal either less soluble or less available to the microorganism Detoxification through the formation of volatile metabolites ( Hg, Se, As )

21 REPLACEMET F THE TERM ' HEAVY METALS ' Instead divide the metals into Class A, B or borderline metals based on their relative ability to form various types of metal-ion / ligand complexes. CLASS A METALS F > Cl > Br > I ( Ligand preference order ) ~ > S = Se > As > > S ( Metal-binding donor atom sequence ) CLASS B METALS ( Large atoms ) I > Br > Cl > F ( Ligand preference order ) ~ Se = S > As > S > > Class A Metals - Alkali, alkaline earth, lanthanide, and actinide metals - Macronutients - Ionic character - Less toxic (primarily toxicity comes about by metal ion displacement ) Class B Metals - More traditional 'Heavy Metals' - Micronutrients - Covalent characteristics - Most toxic 1 3 Effective binding of SH and centers Displace borderline metals Can form stable organometallic complexes

22 H He Li Be B C F e a Mg Al Si P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co i Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr b Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba La Hf Ta W Re s Ir Pt Au Hg Ti Pb Bi Po At Rn Fr Ra Ac Class A Borderline Class B Lanthanides Ce Pr d Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Th Pa U p Pu Am Cm Bk Cf Es Fm Md o Lw Actinides A separation of metal and metalloid ions into three categories: Class A, Borderline, and Class B. Cu (I) and Pb (IV) are designated as belonging to the Class A and Cu (II) and Pb (II) as belonging to the Borderline category. from ieboer and Richardson, Environ Pollut. (Series B) 1: 3-6