Model of pyrite oxidation
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MODELS OF PYRITE OXIDATION- micro scale
liquid
Water film Product layer
a b c d e
Sulphide
gas waste rock
a b c d e
Influence of bacteria
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Influence of pH on Thiobacillus activity
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Oxygen dependence of bacteria
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Importance of bacterial catalysis on oxidation rate
++
-20
. Ca 2Cl 2(OH)2 H2O
-25 Lime -30
-35
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1
2
3
4
5
6
7
pH
Groundwater Quality
• Dependent on rock types and length of reaction times • Usually long enough to allow significant reaction • Higher dissolved substances than surface waters • Chemistry usually reflects composition of rocks in aquifer • Dolomites-Ca, Mg, HCO3 • Granites-Na, K, F, Sand Aquifer-low TDSrelatively inert rocks-rainfall determining factor ywu@
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Buffer reactions
CaCO3 + 2H+ Ca2+ + H2O + CO2(g) H2O + CO2(g) H2CO3 H2CO3 H+ + HCO3- H+ + CO32 FeS2 + 2CaCO3 + 3,75O2 + 1,5H2O Fe(OH) 3 + 2SO42+ 2Ca2+ + 2CO2 (open system) • 1 mole of FeS2 (64 g sulphur) is neutralised by 2 moles of CaCO3 (200 g) or 1 g sulphur: 3.125 g CaCO3 • FeS2 + 2CaCO3 + 3,75O2 + 3,5H2O Fe(OH) 3 + 2SO42+ 2Ca2+ + 2H2CO3 Closed system • 1 mole of FeS2 is neutralised by 4 moles of CaCO3, which results in a mass ratio of 1 g pyrite: 6.25 g calcite • • • •
0.1
0.1
0
0 0 1 2 3 4 5 6 7 pH 8 9 10 11 12 13 14
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Other reactions
• Dolomite CaMg(CO3) 2 + 4H+ = Ca2+ + Mg2+ + 2H2O + 2CO2 • Albite dissolution NaAlSi3O8(s) + H+ + 9/2H2O => Na+ + 2H4SiO4 + 1/2Al2Si2O5(OH)4(s) • Anorthite dissolution CaAl2Si2O8(s) + + H2O => Ca2+ + Al2Si2O5(OH)4(s) • K-feldspar dissolution KAlSi3O8(s) + H+ + 9/2H2O => K+ + 2H4SiO04 + 1/2Al2Si2O5 (OH)4(s) • Iron oxy-hydroxide dissolution Fe(OH)3(s) + 3H+ => Fe3++
碳酸平衡
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Distribution Diagram for the Carbonate System
1
1
0.9
0.9
0.8
0.8
0.7
0.7
Ionisation Fraction
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0.6
0.5 H2CO3 0.4 HCO3
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CO3
2-
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Pyrite (Fool’s Gold )
Detailed reactions
(1) FeS2 + 7/2 O2 + H2O => Fe 2+ + 2SO42- + 2H+ (2) Fe2+ + 1/4O2 + H+ => Fe3+ + 1/2 H2O (rate limiting step) (3) Fe3+ + 3H2O => Fe(OH)3 (yellow boy) + 3H+ (4) FeS2 +14Fe3+ + 8H2O => 15Fe2+ + 2SO42- + 16H+
Why should we study Hydrochemical Reactions?
0 Anhydrite Bassanite Gypsum Aragonite
(log Q/K)
-5
Antarcticite
Calcite Ca(OH) (c)
2
-10 Dolomite -15
Saturation, Min. w/ Ca
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Groundwater reacts with Soils and rocks
Water Reaction
SoilБайду номын сангаас and Rocks
Hydrogeochemistry
Water
Hydrochemistry Geochemistry ywu@
Groundwater Regime & Water Quality
unreacted pyrite ywu@ pyrite oxidation -oxygen consumed oxygen diffuses to oxidation layer liquid phase -oxygen concentration in equilibrium with the gas phase Gas phase between the waste rocks - oxygen supply by diffussion and advection
BOTTOM LINE = Sulphides +water +oxygen (+ bacteria) give acidity (4H+) + sulphates • If this is not neutralized it can result in heavy metal mobilisation which can have disastrous environmental consequences ywu@
Comparison of reaction rates
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Dissolve of Pyrite
3218BA1731:745m Malmesbury
• Oxidation of sulphide minerals, in the presence of oxygen, water and bacteria • Generally in South Africa the mineral pyrite (FeS2) is the primary cause. • Pyrite occurs as an associated or gangue mineral with gold, the base metals and also coal
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Flow & Reaction Time
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Overview of groundwater reactions
• Solid phase interactions Solution-precipitation and volatilization Adsorption including ion exchange • Enrichment • Neutralisation-acidbase reaction • Oxidation Reduction reactions (redox) • Adsorption including Ion exchange • Complexation •Hydrolysis reactions •Isotopic reactions • Decarbonation • Desulfation • Diogenic agency ywu@
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Buffer level of some common minerals
Mineral
Composition Buffer pH
Calcite CaCO3 5.5 - 6.9 Dolomite CaMg(CO3)2 5.3 - 6.8 Siderite FeCO3 5.1 - 6.0 Kaolinite Al2Si2O5(OH)4 3.7 - 4.3 Gibbsite Al(OH)3 3.7 - 4.3 Ferric hydroxide Fe(OH)3 3.3 - 3.7 Goethite FeO(OH) 2.1 - 2.2