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

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

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

case of amorphous iron sulfide (FeS) formation 1 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.

LABORATORY EVIDENCE IN SUPPORT OF BIOGENESIS OF METAL SULFIDES

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.

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 + SO42- + 2H+  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

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,

Bioextraction of Metal Sulfide Ores by Complexation

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

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 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

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)

Formation of Acid Coal Mine Drainage

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

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

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.

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

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...

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

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

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