Lecture Notes (PDF) - Aqueous and Environmental Geochemistry

Environmental GeochemistryDM Sherman, University of Bristol2007/2008Acid Mine DrainageEnvironmental GeochemistryDM Sherman, University of BristolWhat is Acid Mine Drainage?• Pollution associated withabandoned mines resultingfrom the oxidation of FeS 2(pyrite).• Very acid streamwaters withmobilized heavy metals.• Some AMD is natural (themining industry prefers that itbe called “acid-rockdrainage”)Page 1

Environmental GeochemistryDM Sherman, University of Bristol2007/2008The Rio Tinto (Spain)SW Spain, mining activity for > 3000 years.Pyrite (FeS 2 )Pyrite is the most commonsulfide mineral and occursassociated with hydrothermalore deposits and in coalderived from marine deposits.In pyrite theoxidation state ofsulfur is -1 (S 2-2)Page 2

Environmental GeochemistryDM Sherman, University of Bristol2007/2008Pyrite/FeOOH Stability(species in italics are aqueous complexes)Acid Generation by S 22-OxidationOxidation of S2-2 generates H +FeS 2 (s) + (7/2)O 2 (g) + H 2 O →Fe 2+ + 2SO2-4 (aq) + 2H + (aq)Oxidation of S 2- does not generate H +CuFeS 2 (s) + 4O 2 (g) → Cu +2 + Fe 2+ + 2SO 42-(aq)FeS(s) + 2O 2 (g) → Fe 2+ + SO 42-(aq)ZnS(s) + 2O 2 (g) → Zn 2+ + SO 42-(aq)Page 3

Environmental GeochemistryDM Sherman, University of Bristol2007/2008Kinetics of Pyrite Oxidation by OxygenFeS 2 (s) + (7/2)O 2 (g) + H 2 O →Fe 2+ + 2SO 42-(aq) + 2H + (aq)rate (moles/m 2 -s) = 10 !8.10 [O 2] 0.5[H ] 0.11At pH 2.0 in air-saturated conditions [O 2 ] = 10 -3.6 mol/kg, t 1/2 = 16 years for 0.05m 2 /g.Evaporite Deposits of FeSO 4 Under acidic conditions,abiotic oxidation of Fe 2+ isslow. In the Rio Tinto,FeSO 4. nH 2 O precipitatesupon evaporation.Page 4

Environmental GeochemistryDM Sherman, University of Bristol2007/2008Other minerals that form…Coquimbite: Fe 2 (SO 4 ) 3 .H 2 O pK = 3.58Jarosite: KFe 3 (SO 4 ) 2 (OH) 6 pK = 93.2Alunite: KAl 3 (SO 4 ) 2 (OH) 6 pK = 85.6Anglesite: PbSO 4 pK = 7.76Gypsum: FeSO 4 .2H 2 O pK = 4.59Szomolnokite: FeSO 4 .H 2 O pK 0.91Kieserite: MgSO 4 .H 2 O pK = 0.12A possible Mars Analogue?The chemical composition of the martiansoil suggests weathering of pyrite.Jarosite has been identified from theMars Rover Mossbauer spectrometer.Page 5

Environmental GeochemistryDM Sherman, University of Bristol2007/2008Fe 2+ Oxidation facilitated by BacteriaThiobacillus ferrooxidansis the most importantbacteria that facilitates theoxidation of pyrite bygenerating Fe 3+ 4Fe 2+ (aq) + O 2 + 4H + → 4Fe 3+ (aq) + 4H 2 OThiobacillus ferrooxidans is acidophillic, autotropic andfixes its carbon from atmospheric CO 2 .Kinetics of Pyrite Oxidation by Fe 3+ FeS 2 (aq) + 14Fe 3+ (aq) + 8H 2 O(l) →15Fe 2+ (aq) + 2SO 42-(aq) + 16H + (aq)rate (moles/m 2 -s) = 10 !8.58 [Fe 3+ ] 0.3[Fe 2+ ] 0.47 [H ] 0.32At pH 2.0 with [Fe] tot = 10 -2 and [Fe 2+ ]/[Fe 3+ ] = 0.01,t 1/2 = 150 days for 0.05m 2 /g.Page 6

Environmental GeochemistryDM Sherman, University of Bristol2007/2008Pyrite Oxidation Summary(a) FeS 2 (s) + (7/2)O 2 (g) + H 2 O →Fe 2+ + 2SO 42-(aq) + 2H + (aq)Slow(b) 4Fe 2+ (aq) + O 2 + 4H + →4Fe 3+ (aq) + 4H 2 OVia Fe 2+ oxidizingchemolithoautotrophs(c) FeS 2 (aq) + 14Fe 3+ (aq) + 8H 2 O →15Fe 2+ (aq) + 2SO 42-(aq) + 16H + (aq)Fast, autocatalytic mechanismPrecipitation of FeOOHIron(III) (hydr)oxides precipitateout from solution when pH > 3.Goethite(α-FeOOH)Schwertmannite(Fe 8 O 8 (OH) 6 SO 4 )Page 7

Environmental GeochemistryDM Sherman, University of Bristol2007/2008Acid Mine Waste and Heavy MetalsAcid leaching and oxidation of galena (PbS), sphalerite(ZnS) and chalcopyrite (CuFeS 2 ) will release metalsinto solution.Secondary sulfates of Zn and Cu are quite soluble atlow pH. Secondary sulfates will limit the solubility ofPb:Anglesite: PbSO 4 (pK = 7.76)Beudantite: PbFe +3 (AsO 4 )(SO 4 )(OH) 6Beaverite: Pb(Fe,Cu) 3 (SO 4 ) 2 (OH) 6 Acid Mine Waste and Heavy MetalsAcid leaching and oxidation of galena (PbS), sphalerite(ZnS) and chalcopyrite (CuFeS 2 ) will release metalsinto solution.Metals concentrations (ppm) in Rio Tinto and Odiel RiverLocation pH Fe Cu Pb ZnBarrocal 2.5 5.8 415Cadea 2.58 5.6 64 0.5 1270Niebla 2.54 2.3 17 0.2 719The Rio Tinto/Huelva estuary provides up to 10% ofthe global riverine flux of Zn to the oceans!Page 8

Environmental GeochemistryDM Sherman, University of Bristol2007/2008Secondary Al phases from Acid WatersAcidic sulfate waters will react with primary aluminosilicateto give a complex series of Al hydroxy sulfates..3KAlSi 3 O 8 + 6H + + 2SO 4-2+ H 2 O→ 3Al(OH)(SO 4 ) + 6SiO 2 + 2K +Jurbanite3Al(OH)(SO 4 ) + K + + 3H 2 O= KAl 3 (OH) 6 (SO 4 ) 2 + SO 4-2+ 3H +AluniteKAl 3 (OH) 6 (SO 4 ) 2 + 3H 2 O = 3Al(OH) 3 + K + + 2SO 4-2+ 3H +GibbsiteSecondary Al phases from Acid WatersFlocculating Al(OH) 3Page 9

Environmental GeochemistryDM Sherman, University of Bristol2007/2008Remediation of Acid Mine Waste IA plausible method to deal with acid mine water would beto neutralize it using carbonate (Calcite, CaCO 3 ).2H + + CaCO 3 = Ca +2 + CO 2Trials of this method in CwmRheidol, Wales were notencouraging. A precipitate ofCaSO 4 formed on the surfaceof calcite and prevented theabove reaction.Sulphate-Reducing BacteriaThese bacteria live in soil andaquatic environments; they arestrict anaerobes and use(SO 4 ) 2- as an electron acceptor(“dissimilatory sulfatereduction”). Desulfovibrio and Desulfotomaculum are twoimportant genera.Page 10

Environmental GeochemistryDM Sherman, University of Bristol2007/2008Remediation of Acid Mine Waste IIArtificial wetlands createanaerobic conditions sothat Desulfovibrio andDesulfotomaculumbacteria can reduce(SO 4 ) 2- and generateHCO 3- :2CH 2 O + SO 42-= H 2 S + 2HCO 3-(sulfate reduction)H 2 S + Cd 2+ = CdS + 2H + (heavy metal precipitation)Summary• Acid Mine Drainage results from the oxidation ofpyrite.• Iron oxidizing, acidophyllic bacteria promoteoxidation of dissolved Fe 2+ to Fe 3+ .• Oxidation of pyrite by Fe 3+ is fast.• Remediation can be done with artificial wetlands topromote sulfate-reducing bacteria.Page 11