Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi

Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb

K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga

Na Mg Al

Li Be

CLASSIFICATION OF METALS INTO FAMILIES WITHCOMMON CHEMICAL CHARACTERISTICS

I A II A

I B - VIII B (Transition Metals)

III A

IV A V A

BIOTRANSFORMATIONS OF METALS IN SOILSBIOTRANSFORMATIONS OF METALS IN SOILS

BIOMETHYLATION OF METALSBIOMETHYLATION OF METALS

Redox reactions of inorganic metal species

Conversion of inorganic to organic forms and vice versa

Redox reactions of inorganic metal species

Conversion of inorganic to organic forms and vice versa

1.

2.

3.

1.

2.

3.

1.

2.

3.

4.

5.

1.

2.

3.

4.

5.

Indirect effects of biological activityIndirect effects of biological activity

mineralization - immobilization

methylation - demethylation

mineralization - immobilization

methylation - demethylation

acidity - alkalinity

formation of microbial by-products which

undergo nonbiological redox reactions

acidity - alkalinity

formation of microbial by-products which

undergo nonbiological redox reactions

salt formation

complexing reactions

salt formation

complexing reactions

Hg is the most studied metal in regards to itsmethylation or alkylation

Hg is the most studied metal in regards to itsmethylation or alkylation

Monomethyl mercury ( volatile ) is the predominate

product at neutral pH

Monomethyl mercury ( volatile ) is the predominate

product at neutral pH

Higher under aerobic than anaerobic conditions

Inhibited by addition of sulfide

Higher under aerobic than anaerobic conditions

Inhibited by addition of sulfide

Higher microbial growth stimulates higher methylation ratesHigher microbial growth stimulates higher methylation rates

Temperature affects rates by affecting microbial growthTemperature affects rates by affecting microbial growth

BACTERIAL TRANSFORMATIONS OF METALS

Transformation Metal

Reduction

from Summers, A.O. and S. Silver. 1978. Microbial transformationsof metals. Ann. Rev. Microbiol. 32: 637-672.

As (V)Fe (III)Hg (I)Hg (II)

Mn (IV)Se (IV)Te (IV)As (III)

Fe (0)Fe (II)

Mn (II)Sb (III)

As (V)Cd (II)Hg (II)

Pb (II)Se (IV)Sn (II)Te (IV)RHg (II)

Oxidation

Methylation

Demethlyation

ESSENTIALITY OF HEAVY METALS

General Requirements of Heavy Metals by Microorganisms

Specific Requirements

Mettalloenzymes containing a fixed quantity of metal, bound as

an integral part of the enzyme

Metal-activated enzymes - metal is not an integral part of the enzyme

A) Active Site (Catalytic Role)

B) Structural Role

C) Both

1

2

2

1

Specificity of metal for activity to occur is very high for 1 and

much less for 2 .

Cu NO NH2

3

3- Nitrite Reductase

Alkaline PhosphataseZn R - O - P - OH ROH + P

=

OH

O

I

Protein Amino AcidsProteases

3 Co Vitamin B participates in synthesisof hemoglobin

12

42

- -Nitrate Reductase

Mo NO NO

2 4+Nitrogenase

N NH

5 Ni Urease

0 6 12 18 240 6 12 18 24 0 6 12 18 240 6 12 18 24

300

200

100

300

200

100

6000

4000

2000

6000

4000

2000

Fe2+

Fe2+Redox

Potential

RedoxPotential

Potential

Red

ox

Red

ox

Po

ten

tial(m

v)

Red

ox

Po

ten

tial

(mv)

500

400

300

200

100

500

400

300

200

100

So

lub

leF

e(

ug

/gso

il)

So

lub

leF

e(

ug

/gso

il)

2+

Without Rice StrawWithout Rice Straw With Rice StrawWith Rice Straw

Incubation Time ( days )Incubation Time ( days )

( Adapted from Pal, Sudhakar-Babik, and Sethunathan, 1979. Effects of benomylon iron and manganese reduction and

( Adapted from Pal, Sudhakar-Babik, and Sethunathan, 1979. Effects of benomylon iron and manganese reduction and redox potential in flooded soil. J. Soil Sci. 30:155-159 )redox potential in flooded soil. J. Soil Sci. 30:155-159 )

Reduction of iron in a waterlogged soil with andwithout the addition of rice straw ( 0.5%, w/w )Reduction of iron in a waterlogged soil with andwithout the addition of rice straw ( 0.5%, w/w )

Formation of water-soluble iron and aluminum

organic complexes

Formation of water-soluble iron and aluminum

organic complexes

FORMATION OF IRON PODZOLSFORMATION OF IRON PODZOLS

AlAl

Al

FeFe

Fe

FeFe

Fe

Al

Al

STAGE 1STAGE 1

STAGE 3STAGE 3

STAGE 2STAGE 2

Movement of the complexes into the B horizonMovement of the complexes into the B horizon

Precipitation ( mineralization ) of the complexes

in the B horizon

Precipitation ( mineralization ) of the complexes

in the B horizon

Al

Fe

Al

AlAl

Fe

Fe

( Reduction of the iron seems to increase the

movement of Fe into the B Horizon )

( Reduction of the iron seems to increase the

movement of Fe into the B Horizon )

The intensity of podzol formation depends on the relative

rates of movement into the B horizon vs. the rate of

The intensity of podzol formation depends on the relative

rates of movement into the B horizon vs. the rate of

mineralization of the Fe-Organic matter complex.mineralization of the Fe-Organic matter complex.

A A0 1

A2

B

Weak illuviationWeak illuviation Intense illuviationIntense illuviation

Soilminerals

Soilminerals

FreeM OFreeM O

2 3

Soilminerals

Soilminerals

FreeM OFreeM O

2 3

Intense mineralization oforgano-mineral complexesIntense mineralization oforgano-mineral complexes

Iron podzolIron podzol Humus illuvial podzolHumus illuvial podzol

Slow mineralization oforgano-mineral complexes

Slow mineralization oforgano-mineral complexes

Corrosion is the destructive attack of a metal by a chemical

or electrochemical reaction with its environment (rusting).

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:Microorganisms contribute to corrosion processes in several ways:

CORROSION OF METALSCORROSION OF METALS

Through the formation of mineral acids, especially sulfuric acid

Through the formation of organic acids

Through the formation of mineral acids, especially sulfuric acid

Through the formation of organic acids

By depolarization surfaces through the oxidation of hydrogen

By producing H S

By depolarization surfaces through the oxidation of hydrogen

By producing H S2

By changing the electrode potential or E of the environment

By creating microgalvanic cells

By changing the electrode potential or E of the environment

By creating microgalvanic cellsh

1.

2.

3.

4.

5.

6.

1.

2.

3.

4.

5.

6.

AEROBIC CORROSIONAEROBIC CORROSION( rusting in air )( rusting in air )

(a microgalvanic cell ) and electrons to flow from Fe ( metal )

to oxygen.

(a microgalvanic cell ) and electrons to flow from Fe ( metal )

to oxygen.

Low O levels under a microbial cell mass and higher O

levels adjacent to the cell mass causes a potential to form

Low O levels under a microbial cell mass and higher O

levels adjacent to the cell mass causes a potential to form2

V

VV

2O

ee

e

e

e

Fe metalFe metal

ElectrodeConventions Cathode Anode

ions attracted

half reaction

direction of electron flow

sign galvanic

cations

reduction

into cell

positive

anions

oxidation

out of cell

negative

4Fe

4Fe

Tubercule

(Cathode) (Cathode)

(Anode)

4Fe(OH)2

222+

4OHO + 2H O + 4e2 2- --- 4(OH) 4e + 2H O + O

2O 2O

Aerobic Corrosion Process of Iron.

from Iverson, W.P. 1974. Microbial corrosion of iron. pp 476-517, J.B. Nielands (ed)Microbial Iron Metabolism: A Comprehensive Treatus, Academic Press, New York.

In

Anaerobic Corrosion of Metallic Iron

cathode anode

Desulfovibrio

desulfuricans

Desulfovibrio

desulfuricansSO + 8H S + 4H O

2- 2-24 (sulfate reduction)

(metal surface)

aqueous medium

8e-

8e-

4Fe

4Fe

Fe

2+

2+

3Fe2+

23Fe(OH)

FeS2-S

6OH-

8H

28H + 8OH 8H O+ -

2OH-

Hyd

rog

en

ase

anions

cations

from Zajic, J.E. 1969. Microbial Biogeochemistry, p227, Academic Press, New york.

ANAEROBIC CORROSION

Anodic Reactions

Cathodic Reactions

Water

1 pH greater than 5.5

2 E less than 400 mV

3 Low concentration of free oxygen

4 High concentration of sulfate

1 Coating with an inert material

2 Use of a bactericide

3 Cathodic protection

4Fe + SO + 4H O FeS + 3Fe(OH) + 2OH4 2 22- -

2+4Fe 4Fe + 8e-

2-2+Fe + S FeS

22+ -3Fe + 6OH 3Fe(OH)

8H + 8e 8H (dehydrogenase)-

2- 2-2SO + 8H S + 4H O (sulfate reduction)4

28H O 8OH + 8H- +

+

Optimum Conditions for Anaerobic Corrosion

h

Treatments

METHYLCOBALAMIN

S - ADENOSYL METHIONINE

N - METHYLHYDROFOLIC ACID5

Methyl Donors Involved in the Methylation Reactions of Metals

CH CH NH2

CH3

CH

ActiveMethyl

NH2

O

2H C

C

C

C

CN

COOH

2 2N

N

C NH CH CH CH COOH

=

=

=

=

HCH3

CH3CH3

CH3

CH3

CH2

CH3

CH3

H C3

H C3

CH3

ActiveMethyl

NH

C

2

CH2

CH2

--

-

N

N N

N

= O

H NCCH22

Co

CH CNH22

=

O

H NCCH22

=

O

=

O

H

H

H

H

CH CH CNH2 2

=

O

22CH CH

NH

C=O

C O PH

3H C

O OHO

O

=O

HOH C2

H C3

N

N

CH CH CNH2 2

=

O

2

IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII

CH2

CH3

ActiveMethyl

NH

22

2

HOOCCHCH CH S

H

OHOH

O

H

N

N NH2

N

N

+

H H

NH

NH

HN

C = O

O

O

O

O

O

C

=

OC=

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.

=O

O

=

=

SIDEROPHORES

Low molecular weight, virtually Fe (III) specific ligands. Generally they

are produced by aerobic or facultative aerobic bacteria and fungi.

At pH 7.0

Types of Siderophores in Soils

K for Fe (OH) = 10sp-38

3

-173+(Fe ) = 10 M

-83+Fe requirement for plants = 10 M

1 Hydroxomates

2 Citrate

3 Catechols

4 Amino acid composition

SIDEROPHORES

Assay Procedures

Roles of Siderophores in Soils

1 Growth factor

2 Iron chelation and transport

3 Antibiotic

4 Nitrification intermediate

5 Urease inhibitor

1 Chemical Procedures

2 Biological Procedures

TOXICITY OF HEAVY METALS

1 Toxicity brought about primarily by the metals affectingsome 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

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

2 Microbial uptake

Hea

vy

Met

als

TOXICITY OF 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) Nature of the assay substrate

F) Redox

Hea

vy

Met

als

Average Percentage Inhibition of Nitrogen Mineralization

in Four Soils by Trace Elements (5 M g soil)-1�

Trace Element

Element Oxidation StatePercentageInhibition

Hg IICuCdPbMnFeZnSnCo

44.827.325.318.517.815.013.812.36.8

Cr IIIFeAlBAs

V IVSe

Ag I 56.3

As V

Mo V IW

Adopted from Liang and Tabatabai, 1977. Effects of trace elementson nitrogen mineralization in soils. Environ. Pollut. 12:141-147.

18.017.315.510.53.0

11.85.5

27.07.0

5.3

Adaptation and ToleranceAdaptation and Tolerance

Mechanisms by which Microorganisms Attain ToleranceMechanisms by which Microorganisms Attain Tolerance

Impermeability of the plasma membrane

Concentration of metals in the cell walls

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

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 )

Detoxification through the formation of

volatile metabolites ( Hg, Se, As )

1.

2.

3.

4.

1.

2.

3.

4.

BIOLOGICAL TRANSFORMATION OF METALSBIOLOGICAL TRANSFORMATION OF METALS

REPLACEMENT OF THE TERM ' HEAVY METALS 'REPLACEMENT OF THE TERM ' HEAVY METALS '

CLASS B METALSCLASS B 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

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 )F > Cl > Br > I ( Ligand preference order )

O > S = Se N > As O > N > SO > S = Se N > As O > N > S~

( Metal-binding donor atom sequence )( Metal-binding donor atom sequence )

( Large atoms )( Large atoms )

I > Br > Cl > F ( Ligand preference order )I > Br > Cl > F ( Ligand preference order )

Se = S > O As > N S > N > OSe = S > O As > N S > N > O~

Class A Metals Class B MetalsClass A Metals Class B Metals

- Alkali, alkaline earth,

lanthanide, and actinide metals

- Macronutients

- Alkali, alkaline earth,

lanthanide, and actinide metals

- Macronutients

- Ionic character

- Less toxic (primarily toxicity

comes about by metal ion

displacement )

- Ionic character

- Less toxic (primarily toxicity

comes about by metal ion

displacement )

- More traditional 'Heavy Metals'

- Micronutrients

- Covalent characteristics

- Most toxic

- More traditional 'Heavy Metals'

- Micronutrients

- Covalent characteristics

- Most toxic

1

23

1

23

Effective binding of SHand N centersDisplace borderline metalsCan form stable organometalliccomplexes

Effective binding of SHand N centersDisplace borderline metalsCan form stable organometalliccomplexes

Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lw

Lanthanides

Actinides

H

Li

Na

K

Rb

Cs

Fr

Be

Mg

Ca

Sr

Ba

Ra

Sc

Y

La

Ac

Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr

Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe

Hf Ta W Re Os Ir Pt Au Hg Ti Pb Bi Po At Rn

B C N O F Ne

Al Si P S Cl Ar

He

Class A

Borderline

Class B

A separation of metal and metalloid ions into three categories:Class A, Borderline, and Class B. Cu (I) and Pb (IV) are designatedas belonging to the Class A and Cu (II) and Pb (II) as belonging tothe Borderline category.

from Nieboer and Richardson, 1980. Environ Pollut. (Series B) 1: 3-26