1. Principles of Metal Sulfide Formation
Back to basics, always
Principles of Metal Sulfide Formation

2. Metal Sulfide in nature
interaction between an appropriate metal ion and biogenically
or abiogenically formed sulfide ion:
M2+ + S2- →MS
Biogenik
Abiogenik
bacterial sulfate reduction
from bacterial mineralization of organic sulfur-containing compounds

3. Solubility Products for Some Metal Sulfides
Because of their relative insolubility, the
metal sulfides form readily at ambient temperatures and pressures.

4. case of amorphous iron sulfide (FeS) formation1
The ionization constant for FeS
[Fe2+] = [H+]2/[H2S] x 10-19/10-21,96 = [H+]2/[H2S] x 1021,96 2
[S2-]= 10-21,96 [H2S]/[H+]2
The ionization constant for H2S 3
The constant for the dissociation of H2S into HS- and H+
[HS-][H+]/[H2S]= 10-6,96 4
The constant for the dissociation of HS- into S2- and H+
[S2-][H+]/[HS-]= 10-15
The following calculations will show that relatively low concentrations of metal ions, typical in some lakes, will form metal sulfi des by reacting with low concentrations of H2S. The ionic activities. in these calculations are taken as approximately equal to concentration because of the low concentrations involved. The following examines the case of amorphous iron sulfi de (FeS) formation.

5. LABORATORY EVIDENCE IN SUPPORT OF BIOGENESIS OF METAL SULFIDES

6. minimize metal toxicity for D. desulfuricans
Batch Cultures
cobalt sulfide on addition of 2CoCO3 · 3Co(OH)2,
nickel sulfide on addition of NiCO3 or Ni(OH)2
bismuth sulfide ,on addition
of (BiO2)2CO3 ·H2O,
reported that sulfides of Sb, Bi, Co, Cd, Fe, Pb, Ni, and Zn were
formed in a lactate-containing broth culture of Desulfovibrio desulfuricans to which insoluble salts of selected metals had been added.
Miller (1949,1950)
minimize metal toxicity for D. desulfuricans
Metal ion toxicity depends in part on the solubility of
the metal compound from which the ion derives
For instance, he found that bismuth sulfi de was formed on addition
of (BiO2)2CO3 ·H2O, cobalt sulfi de on addition of 2CoCO3 · 3Co(OH)2, lead sulfi de on addition
of 2PbCO3 · Pb(OH)2 or PbSO4, nickel sulfi de on addition of NiCO3 or Ni(OH)2, and zinc sulfi de
on addition of 2ZnCO3 · 3Zn(OH)2. The metal salt reactants were added as insoluble compounds to
minimize metal toxicity for D. desulfuricans. Metal ion toxicity depends in part on the solubility of
the metal compound from which the ion derives. Obviously, for a metal sulfi de to be formed from
another metal compound that is relatively insoluble, the metal sulfi de must be even more insoluble
than the source compound of the metal. Miller was not able to demonstrate copper sulfi de formation
from malachite [CuCO3 · Cu(OH)2], probably because malachite was too insoluble relative to
copper sulfi des in the medium. Miller (1949) also showed that with addition of Cd or Zn ions to the
culture medium, the yield of total sulfi de produced from sulfate by the bacteria in batch culture was
greater than in the absence of the added metal ions. This was because the uncombined sulfi de itself
becomes toxic to sulfate-reducers at high enough concentration.

7. Desulfovibrio desulfuricans and Desulfotomaculum sp. (Clostridium
Desulfuricans). They grew them in lactate or acetate medium containing steel wool. The media were saline to simulate marine (near-shore and estuarine) conditions under which the
investigators thought the reactions are likely to occur in nature.
source of hydrogen for the bacterial reduction of sulfate
The
hydrogen resulted from corrosion of the steel wool by the spontaneous reaction,
Fe0 + 2H2O → H2 + Fe(OH)2
Baas Becking and Moore (1961)
used by the sulfate-reducers in the formation of hydrogen sulfide.
4H2 + SO H+  H2S + 4H2O
They succeeded in forming covellite from malachite where Miller (1950) failed, probably because they performed
their experiment in a saline medium (3% NaCl) in which Cl− could complex Cu2+, thereby
increasing the solubility of Cu2+.
The media were saline to simulate marine (near-shore and estuarine) conditions under which the
investigators thought the reactions are likely to occur in nature. They formed ferrous sulfi de from
strengite (FePO4) and from hematite (Fe2O3). They also formed covellite (CuS) from malachite
[CuCO3· Cu(OH)2]; argentite (Ag2S) from silver chloride (Ag2Cl2) and from silver carbonate (AgCO3);
galena (PbS) from lead carbonate (PbCO3) and from lead hydroxycarbonate [PbCO3·Pb(OH)2]; and
sphalerite (ZnS) from smithsonite (ZnCO3). All mineral products were identifi ed by x-ray powder
diffraction studies. Baas Becking and Moore (1961) were unable to form cinnabar (HgS) from mercuric
carbonate (HgCO3), probably owing to the toxicity of the Hg2+ ion. They were also unable to
form alabandite (MnS) from rhodochrosite (MnCO3), or bornite (Cu5FeS4) or chalcopyrite (CuFeS2)
from a mixture of cuprous oxide (Cu2O) or malachite and hematite and lepidochrosite. They succeeded
in forming covellite from malachite where Miller (1950) failed, probably because they performed
their experiment in a saline medium (3% NaCl) in which Cl− could complex Cu2+, thereby
increasing the solubility of Cu2+. The starting materials that were the source of metal were all relatively
insoluble, as in Miller’s experiments. Baas Becking and Moore found that in the formation of
covellite and argentite, native copper and silver were respective intermediates that disappeared with
continued bacterial H2S production.
Leleu et al. (1975) synthesized ZnS by passing H2S produced by unnamed strains of sulfatereducing
bacteria through a solution of ZnSO4. In one experiment, biogenic H2S formation and ZnS
precipitation by the biogenic H2S occurred in separate vessels. In a second experiment, biogenesis
of H2S and precipitation of ZnS occurred in the same vessel at an initial ZnSO4 concentration in
the culture medium of 10−2 M. The ZnS formed under either experimental condition was identifi ed
as a sphalerite–wurtzite mixture by powder x-ray diffraction. The presence of Zn directly in the
culture medium caused a lag in H2S production, which was not observed when H2S was generated
in a separate vessel.
ZnS from ZnCO3
Covellite (CuS) from Malachite [CuCO3.Cu(OH)2]
Galena (PbS) from PbCO3 and [PbCO3.Pb(OH)2]
Ferrous sulfide from FePO4 and Fe2O3
Argentite (Ag2S) from silver chloride (Ag2Cl2) and silver carbonate (AgCO3)
ZnS unable to form alabandite (MnS) from MnCO3 or Cu5FeS4 or CuFeS2 from a mixture of Cu2O or malachite and hematite and lepidochrosite.
unable to form cinnabar (HgS) from mercuric carbonate

8. COLUMN EXPERIMENT: MODEL FOR BIOGENESIS OF SEDIMENTARY METAL SULFIDES
The relatively high toxicity of many of the heavy metals for sulfate-reducing bacteria has been used
as an argument that these organisms could not have been responsible for metal sulfi de precipitation
in nature (Davidson, 1962a,b). However, in a sedimentary environment, metal ions will be mostly
adsorbed to sediment particles such as clays or complexed by organic matter (Hallberg, 1978), which
lessens their toxicity. Such adsorbed or complexed ions are still capable of reacting with sulfi de and
precipitating as metal sulfi des, as was shown experimentally by Temple and LeRoux (1964). They
constructed a column in which clay or ferric hydroxide slurry carrying adsorbed Cu2+, Pb2+, and
Zn2+ ions was separated by an agar plug from an underlying liquid culture of sulfate-reducers
actively generating hydrogen sulfi de in saline medium. They also tested clay that was carrying Fe3+
in this setup. They found that, in time, bands of precipitate formed in the agar plug separating the
slurry of metal-carrying adsorbent from the culture of sulfate-reducing bacteria (Figure 20.1). The
bands formed as upward-diffusing sulfi de ion species and downward diffusing, desorbed metal ion
species encountered each other in the agar. Differential desorption of metal ions from the adsorbent
and differential diffusion in the agar accounted for the discrete banding of the various sulfi des.
These results demonstrate that biogenesis of relatively large amounts of sulfi des in a sedimentary
environment is possible, even in the presence of relatively large amounts of metal ions. The main
requirement is that the metal ions are in a nontoxic form (e.g., adsorbed or complexed) or combined
in the form of insoluble mineral oxide, carbonate, or sulfate. As Temple (1964) pointed out,
syngenetic microbial production of metal sulfi de in nature is possible. Restrictions on the process,

9. Bioextraction of Metal Sulfide Ores by Complexation

10. acidophilic iron-oxidizing bacteria
oxidized by
Metal sulfide ores
acidophilic iron-oxidizing bacteria
an amount of acid-consuming constituents in the host rock
extracted by :
Penicillium sp.
mine-tailings pond of the White Pine Copper Co. in Michigan
Aspergillus sp.
complexing agents
unidentified metabolites
mobilization of copper in an oxidized mining residue by A. niger in a sucrose–mineral salts medium.
mobilize copper from sedimentary ores Czapek’s broth
contain : sucrose, NaNO3, cysteine, methionine, or glutamic acid
The chief mobilizing agents
act as acidulants as well as ligands of metal ions
gluconic and citric acids

11. Wenberg et al. (1971)
grew fungus in the presence of copper ore (sulfide or native copper minerals with basic gangue constituents)
addition of citrate
lowered the toxicity of the extracted copper
when the fungus was grown in the presence of the ore

12. obtained better results
grew the fungus in the absence of the ore
treated the ore with the spent medium from the fungus culture
by forming complexes
The organisms forms ligands
extracted the metals from the ores
more stable than the original insoluble form of the metals in the ores

13. MA : metal salt (mineral)
MA+ HCh → MCh + H+ + A−
MA : metal salt (mineral)
HCh : ligand (chelating agent)
MCh : the resultant metal chelate
A− : the counter ion of the original metal salt (S2−)
The S2− may undergo chemical or bacterial oxidation
(Chemical Processing, 1965)

14. Formation of Acid Coal Mine Drainage

15. Air, bacteria and moisture during mining
Acid Mine Drainage
Yellow boy in a stream receiving acid
drainage from surface coal mining.
An Enviromental problem in coal-Mining region
Degrades water quality > Mixing of acid mine water into natural in river
Polluted water for human consumption and industrial use
Air, bacteria and moisture during mining
Pyrite
Pyrite Oxidation
Propagation cycle
Initiator reaction
Formation of AMD

16. The breakdown of pyrite
Leads to the formation of sulfuric acid and ferrous iron
pH values ranging from 2 to 4.5
Sulfate ion concentrations ranging from 1,000 to 20,000 mg L−1 but a nondetectable ferrous iron concentration
The acid formed attack other minerals associated with the coal and pyrite, causing breakdown of rock fabric
Alumunium : Highly toxic

17. Pyrit Oxidation : Ferric ion oxidation
In AMD will be detectable some of acidophilic iron oxidizing thiobacilli. Acidithiobacillus ferrooxidans is involved, pyrite biooxidation proceeds
Pyrit Oxidation :
Ferric ion oxidation
Acidithiobacillus thiooxidans : Oxidized elemental sulfur (S0) and other partially reduced sulfur species : Intermediates in pyrite oxidation to sulfuric acid
Metallogenium-like organism that they isolated from AMD ( Walsh and Mitchell (1972) ) - pH drops below 3.5.

18. An early study by Harrison (1978)
Artificial coal spoil
Deposit into mound d= 50 cm l= 25cm on plastic tray
and migrated upward
Absorbed
Sampling
Inoculated : 20 L of an emulsion of acid soil, drainage water, and mud from a spoil from an old coal strip
mine
Microbial succession
in coal spoil
under laboratory conditions

19. Initial samples : The base of the mound
Heterotrophic bacteria.
2 weeks : The population density of ∼107 cells g−1
After 8 weeks : heterotrophs were still dominant
Between 12 and 20 weeks : The population decreased
Near the summit of the mound,
First 15 weeks : Heterotrophs predominated
Acidithiobacillus thiooxidans and Acidithiobacillus ferrooxidans
Higher pH values
Protozoans, algae, and arthropod
Metallogenium was not seen
pH had dropped from 7 to 5.
pH to just below 5 >> caused by a burst of growth by sulfur-oxidizing bacteria, >> then died off progressively.
The heterotrophic population increased again to just below 107 g−1.
The sulfur-oxidizing bacteria were assumed to be making use of elemental sulfur resulting from the oxidation of pyrite by ferric sulfate:
FeS2 + Fe2(SO4)3 → 3FeSO4 + 2S0
After 8 weeks : heterotrophs were still dominant
Between 12 and 20 weeks : The population decreased
Result...

20. NEW DISCOVERIES RELATING TO ACID MINE DRAINAGE
A fairly recent study of abandoned mines at Iron Mountain, California.
The ore body at Iron Mountain
various metal sulfides and was a source of Fe, Cu, Ag, and Au.
A signifi cant part of the iron was in the form of pyrite. The drainage currently coming
The distribution of Acidithiobacillus ferrooxidans and Leptospirillum ferrooxidans from a pyrite deposit
in the Richmond Mine, seepage from a tailings pile and AMD storage tanks outside this mine

21. Acidithiobacillus ferrooxidans
Occurred in slime-based communities at pH >1.3 at temperatures below 30°C
Affect precipitation of ferric iron but seemed to have
a minor role in acid generation
active role in generating ferric iron as an oxidizing agent
L. ferrooxidans
Abundant in subsurface slime-based communities.
Occurred in planktonic form at pH values in the range of 0.3–0.7 between 30 and 50°C

22. The Richmond Mine revealed the presence of Archaea
in summer and fall months: Archaea represented ∼50% of the total population
correlated these population fluctuations with rainfall and conductivity, (dissolved solids), pH, and temperature of the mine water
Ferroplasma acidarmanus, grew in slime streamers on the pyrite surfaces.
extremely acid-tolerant : pH optimum at 1.2
Its cells lack a wall
Archaean order Thermoplasmales