Mechanism of bacterial

oxidation

Lecture # 4

Direct and indirect

mechanisms

• Direct bioleaching

mechanism is

the direct enzymatic oxidation of

sulfide minerals.

• The

indirect mechanism

is the non-

enzymatic metal sulfide oxidation by

iron(III) (Fe

3+

ions) combined with the

enzymatic oxidation of the resulting

iron(II) (Fe

2+

ions) ions.

Fe

2

+

Fe

3+

Microoragnis
ms

MeS

(a)Indirect bioleaching

„Non-contact”

(b) Direct bioleaching

„Contact”

Me S

„Contact” and „non-contact”

mechanisms”

Mechanisms of sulfides

dissolution

• Two different reaction mechanisms

control the dissolution of metal sulfides:

The

thiosulfate

pathway

The

polysulfide

pathway

Generally, leaching is achieved by
a combination of proton attack and
oxidation processes.

Thiosulfate pathway

Acid-nonsoluble metal

sulfides

• Metal sulfides such as pyrite (FeS

2

),

molibdenite (MoS

2

) and tungstenite

(WS

2

) are oxidized via electron

extraction by iron(III) ions.

First step

: Thiosulfate is liberated

Second step

: Free thiosulfate is

oxidized via tetrathionate

Third step

: Finally polythionates is

oxidized to sulfate

Acid-soluble metal sulfides

• Metal sulfides such as sphalerite (ZnS),

galena (PbS),arsenopyrite (FeAsS),
chalcopyrite (CuFeS

2

) and hauerite

(MnS) are dissolved by the combined
action of electron extraction and proton
attack.

• First step

: sulfide cation (H

2

S

+

)

formation.

• Second step

: dimeriza to disulfide (H

2

S

2

)

Jarosite precipitation

• 3Fe

3+

+ 2SO

42-

+6H

2

O+ Me=

MeFe

3

(SO

4

)

2

(OH)

6

+6H

+

Me= Na, K, NH

4+

Jarosite crystals

Sulfur precipitation

The significant
amounts of
elemental sulfur
(10-20%) may be
produced in the
absence of
sulfur-oxidizing
bacteria.

Mechanisms of

bioleaching

Sulfide

mineral

Direct
bioleaching

Indirect
bioleaching

Sulfur layer

Jarosite layer

Indirect
bioleaching

Bacteria

Indirect
bioleaching

Electrochemical mechanism of

bioleaching

S

0

Fe

3+

SO

4

2-

Fe

2+

e

-

FeS

2

CuFeS

2

H

2

O

O

2

, H

+

Cu

2+

H

2

O

A.ferrooxidans
A.ferrooxidans

A.thiooxidans

O

2

, H

+

e

-

Electrochemical mechanism

Dissolution occurs at local anodes.

The local anodes are the sites where
the cell is chmotactically attracted.

E

xtracellular

P

olymeric

S

ubstances

• Most leaching bacteria grow attached

to surfaces of sulfide minerals.

• More than 80% of microorganism cell

disappeared from solution within 24 h.

• The attachment process is

predominantly controlled by the
extracellular polymeric substances
(EPS), surrounding the cells.

EPS and biofilm

• The space between the cell wall and the

surface is filled with bacterial

extracellular polymeric substances

(EPS).

• Biofilm formation is important

mechanism of bioleaching minerals.

• The bacteria are able to adapt the

composition and amount of their EPS

according to leaching conditions.

Biofilm formation

Biofilm observation

The flow cell consisted of a rectangular chamber. The
dimensions of chamber were 40mm long,12mm wide and
0.5 mm deep. The coupon of pyrite was 20mm by 5 mm
wide. The flow rate was 30 ml/min.

Dissolution process

• Atomic force microscope (AFM) images

demonstrate that cells of

Acidithiobacillus ferrooxidans

preferentially (>80%) attach to the

sites with visible surface imperfections.

• The cells are attracted to dissolution

sites by their chemotactic sensory

system and determine the anodes and

cathodes on the metal sulfide surface.

Dissolution process

• The dissolution process occurs in the

EPS layer.

• The distance between the mineral

surface and microbial cell is 10 – 100
nm.

• The chemical reactions occur outside

the cells but still within the EPS layer
generated by microorganism.

Heap bioleaching

Bacteria oxidation

mechanism

(A)

(B)