PRINCIPLES OF METAL SULFIDE FORMATION

Back to basics, always

• Metal Sulfide in nature

M2+ + S2- →MS

interaction between an appropriate metal ion and biogenicallyor abiogenically formed sulfide ion:

Biogenik Abiogenik

bacterial sulfate reduction

from bacterial mineralization of organic

sulfur-containing compounds

• Solubility Products for Some Metal Sulfides

Because of their relative insolubility, themetal sulfides form readily at ambient temperatures and pressures.

• case of amorphous iron sulfide (FeS) formationThe ionization constant

for FeS [Fe2+][S2-]= 10-191

The ionization constant for H2S

[S2-]= 10-21,96 [H2S]/[H+]2

2

The constant for the dissociation of H2S into

HS- and H+

[HS-][H+]/[H2S]= 10-6,96

3

The constant for the dissociation of HS- into

S2- and H+ [S2-][H+]/[HS-]= 10-15

4

[Fe2+] = [H+]2/[H2S] x 10-19/10-21,96 = [H+]2/[H2S] x 1021,96

LABORATORY EVIDENCE IN SUPPORT OF BIOGENESISOF METAL SULFIDES

Batch Cultures

Miller (1949,1950)

reported that sulfides of Sb, Bi, Co, Cd, Fe, Pb, Ni, and Zn wereformed in a lactate-containing broth culture of Desulfovibrio desulfuricans to which insoluble salts of selected metals had been added.

bismuth sulfide ,on additionof (BiO2)2CO3 ·H2O,

cobalt sulfide on addition of 2CoCO3 · 3Co(OH)2,

nickel sulfide on addition of NiCO3

or Ni(OH)2

minimize metal toxicity for D. desulfuricans

Metal ion toxicity depends in part on the solubility ofthe metal compound from which the ion derives

Baas Becking and Moore

(1961)

Desulfovibrio desulfuricans and Desulfotomaculum sp. (ClostridiumDesulfuricans). 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 theinvestigators thought the reactions are likely to occur in nature.source of

hydrogen for the bacterial

reduction of sulfate

Thehydrogen resulted from corrosion of the steel wool by the spontaneous reaction,Fe0 + 2H2O → H2 + Fe(OH)2

used by the sulfate-reducers in the formation of hydrogen sulfide.

4H2 + SO42- + 2H+ H2S + 4H2O

Ferrous sulfide from FePO4 and Fe2O3

Covellite (CuS) from Malachite [CuCO3.Cu(OH)2]

Argentite (Ag2S) from silver chloride (Ag2Cl2) and silver carbonate (AgCO3)

Galena (PbS) from PbCO3 and [PbCO3.Pb(OH)2]ZnS from ZnCO3

unable to form cinnabar (HgS) from mercuric carbonate

ZnS unable to form alabandite (MnS) from MnCO3 or Cu5FeS4 or CuFeS2 from a mixture of Cu2O or malachite and hematite and lepidochrosite.

They succeeded in forming covellite from malachite where Miller (1950) failed, probably because they performedtheir experiment in a saline medium (3% NaCl) in which Cl− could complex Cu2+, therebyincreasing the solubility of Cu2+.

COLUMN EXPERIMENT: MODEL FOR BIOGENESIS OF SEDIMENTARY METAL SULFIDES

BIOEXTRACTION OF METAL SULFIDE ORES BY COMPLEXATION

acidophilic iron-oxidizing bacteriaMetal sulfide oresoxidized by

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.unidentified metabolites

mobilize copper from sedimentary ores Czapek’s broth contain : sucrose, NaNO3, cysteine, methionine, or glutamic acid

complexing agents

mobilization of copper in an oxidized mining residue by A. niger in a sucrose–mineral salts medium.

The chief mobilizing agents

gluconic and citric acids

act as acidulants as well as ligands of metal ions

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

obtained better results

grew the fungus in the absence of the ore

treated the ore with the spent medium from the fungus culture

The organisms forms ligands

extracted the metals from the ores

by forming complexes

more stable than the original insoluble form of the metals in the ores

MA : metal salt (mineral)HCh : ligand (chelating agent)MCh : the resultant metal chelateA− : the counter ion of the original metal salt (S2−)The S2− may undergo chemical or bacterial oxidation

(Chemical Processing, 1965)

MA+ HCh → MCh + H+ + A−

FORMATION OF ACID COAL MINE DRAINAGE

Acid Mine DrainagePyrite Air, bacteria and

moisture during mining

Pyrite Oxidation

Formation of AMD

Initiator reaction

Propagation cycle

• 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

• 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.

An early study by Harrison (1978)

Inoculated : 20 L of an emulsion of acid soil, drainage water, and mud from a spoil from an old coal stripmine

Microbial succession in coal spoil under laboratory conditions

Result...

Between 12 and 20 weeks : The population decreased

After 8 weeks : heterotrophs were still dominant

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