水文地球化学过程Hydrochemicalprocess

1、1会计学水文地球化学过程水文地球化学过程Hydrochemicalprocess Water reacts with Soils and rocks Groundwater Regime & Water Quality Flow & Reaction Time Overview of groundwater reactions Most important indicator parameters in groundwater (pH,EC, alkalinity, Organic Indicators) Most important types of reaction (acid/base,

2、 reduction/oxidation, solution/precipitation) Need to consider reactions to classify, predict migration, identify persistence and remediationWaterSoils and RocksHydrogeochemistryHydrochemistryGeochemistryHydrogeological ProcessWater Solid phase interactionsredox)Adsorption Complexation Hydrolysis re

3、actionsIsotopic reactionsbrent Thu Mar 30 20001234567-35-30-25-20-15-10-50pHSaturation, Min. w/ Ca+ (log Q/K)AnhydriteAntarcticiteAragoniteBassaniteCa(OH)2(c)Ca2Cl2(OH)2.H2OCalciteDolomiteGypsumLimeGroundwater QualityOxidation of sulphide minerals, in the presence of oxygen, water and bacteriaGenera

4、lly 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 coalPyrite (Fools Gold )(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 =

5、Fe(OH)3 (yellow boy) + 3H+ (4) FeS2 +14Fe3+ + 8H2O = 15Fe2+ + 2SO42- + 16H+ 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 consequencesa unreacted pyriteb

6、pyrite oxidation -oxygen consumedc oxygen diffuses to oxidation layerd liquid phase -oxygen concentration in equilibrium with the gas phasee Gas phase between the waste rocks - oxygen supply by diffussion and advectiongasliquidwaste rocka b c d eSulphideWater filmProductlayerMineralCompositionBuffer

7、 pHCalciteCaCO35.5 - 6.9DolomiteCaMg(CO3)25.3 - 6.8SideriteFeCO35.1 - 6.0KaoliniteAl2Si2O5(OH)43.7 - 4.3GibbsiteAl(OH)33.7 - 4.3Ferric hydroxideFe(OH)33.3 - 3.7GoethiteFeO(OH)2.1 - 2.2 CaCO3 + 2H+ Ca2+ + H2O + CO2(g) H2O + CO2(g) H2CO3 H2CO3 H+ + HCO3- H+ + CO32 FeS2 + 2CaCO3 + 3,75O2 + 1,5H2O Fe(OH

8、) 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

9、 ratio of 1 g pyrite: 6.25 g calcite 碳酸平衡碳酸平衡0101234567891011121314pHIonisation Fraction01H2CO3HCO3-CO32-DolomiteCaMg(CO3) 2 + 4H+ = Ca2+ + Mg2+ + 2H2O + 2CO2Albite dissolutionNaAlSi3O8(s) + H+ + 9/2H2O = Na+ + 2H4SiO4 + 1/2Al2Si2O5(OH)4(s)Anorth

10、ite dissolutionCaAl2Si2O8(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 dissolutionFe(OH)3(s) + 3H+ = Fe3+ Although all these reactions can consume H+ and thus buffer the system, the reaction rates of mos

11、t of the minerals( apart from the carbonates) are slower than the sulphide oxidation rates Thus unless the acid production rate is very low ( low S material) only the carbonates prevent acidity ( generally)OPEN PITAt field scale the neutralising minerals can buffer the system from acidityPRECIPITATI

12、ONPRECIPITATIONPRECIPITATIONPRECIPITATIONWASTE PILERIVER VALLEYUNDERGROUND MINE WORKINGSTAILINGS BASINFILL MATERIALSWASTE PILELines of defence against acidity includeNatural waters alkalinity- pH as high as 8+Carbonate minerals (Calcite/Dolomite) pH above 6Silicates- react too slowly to act as buffe

13、r- pH falls to levels where heavy metals are mobilised etcGeneral resultsNot enough neutralising minerals Low pH water High sulphate Heavy metals Staining (yellow boy)Neutralized by co-existing minerals( common occurrence in SA) Neutral pH High sulphate High TDS StainingOutflow at a SA coal mineWhat

14、 kind of hydrochemical process occurred in this picture ?Coal Mine Drainage Substance United States Illinois Kentucky Big Five Tunnel Drinking Water Standards Coal Al - 37 - 18 - 14000 Fe 0.6 220 57 50 500 50 0.3 16000 Mn 0.3 12 6.4 - 32 0.05 100 Cu 0.01 0.17 - - 1.6 1.0 19 Zn 0.03 2.2 - - 10 5.0 39

15、 Cd 0.01 0.10 - - 0.03 0.01 1.3 Pb 0.01 0.40 - - 0.01 0.05 16 As 0.002 0.20 - - 0.02 0.05 15 Ph 3.2 7.9 3.0 1.8 3.5 2.6 6.5 8.5 - So42- - 1300 500 12000 2100 250 - Ref. 1 2 3 4 5 6 .The 10-90% concentration range of 23 acid drainages from coal mines throughout the United States taken from the EPA ef

16、fluent limitations document (U.S. Environmental Protection Agency, 1982). .Median of 110 drainages from coal refuse disposal sites in Southern Illinois as compiled by Proudan et al. (1982). .Regional estimates from Caruccio (1979). .A typical metal mine drainage from the Front Range Mineral Belt of

17、Colorado collected by Wildeman and Laudon (1989). .Compiled from U.S. Government Printing Office (1988). For mine drainages, effluent limits in milligrams/Liter are: Fe, 7.0 daily maximum, and 3.5 monthly average; Mn, 4.0 daily maximum, and 2.0 monthly average; pH between 6.0 and 9.0 at all times. F

18、or the other substances in the table, there are no written restrictions (U.S. Environmental Protection Agency, 1982). .Average of U.S. coal compiled by Valkovic (1983).ACID MINE DRAINAGE in USSummary AMD is caused by bacterially mediated oxidation of sulphide minerals The “essential ingredients” are

19、 sulphides, oxygen, water and bacteria Several mechanisms buffer the system The carbonates are the most important of these due to reaction rate considerations. Groundwater quality & Its influence factorsTable 2.8 Chemical Composition of Groundwater (mg/l) Rhyolite Granite Gabbro Sandstone Shale Lime

20、stone Dolomite Schist SiO2 49 32 41 23 26 12.8 14.9 23.1 Al 0.62 0.18 0.2 0.1 3.6 0.09 0.13 0.1 Fe 0.32 0.29 0.62 0.74 1.7 0.4 1.1 0.5 Mn 0 0.02 0.06 0.06 3.1 0.06 0.07 0.08 Cu 0 0 0 0 0.04 0 0 0 Zn 0.07 0.06 0.03 0 0.09 0.01 0 0.03 Ca 8.4 38.1 25.7 53.2 114.4 71.3 62 40.4 Mg 2.2 8 26.3 20.8 53.7 19

21、.1 43.7 15.2 Na 20.7 51.2 14.3 51.1 194.3 12.9 27.4 22.4 K 2.3 3.7 9.1 4.3 5.3 2.2 1.8 3.1 HCO3 77 175 196 252 330 228 272 166 CO3 0 0 0 2.1 3 0 0.7 0 SO4 6.9 65.4 17.1 69 358.4 60.7 138.2 37.5 Cl 5.1 53.7 22.5 37.3 219 19.7 6.9 23.1 Fe 0.3 1.2 0.2 0.4 0.6 0.3 0.6 0.6 NO3 2.6 7.6 6.5 4.5 17.2 8.9 6.

22、3 4.4 PO4 0.1 0.07 0.03 0.02 0 0.09 0 0.01 TDS 175.61 436.52 359.64 518.62 1330.43 436.55 575.8 336.52 pH 7.2 7.1 7.5 7.5 7.2 7.5 7.7 7.1 Acid (proton donor)ProtonConjugate baseHClH+Cl-H2SO4H+HSO4-HSO4-H+SO42-HNO3H+NO3-H2OH+OH-NH4+H+NH3Acids and Bases Bronsted -Lowry theory Acid is a proton donor. B

23、ase is a proton acceptorBase (proton acceptor)ProtonConjugate acidOH-+H+H2OCN-+H+HCNCO32-+H+HCO3-HCO3-+H+H2CO3NH3+H+NH4+H2O+H+H3O+Acid 1+Base 2Base 1+Acid 2HClH2O=Cl-H3O+HNO3H2O=NO3-H3O+NH4+CO32-=NH3HCO3-H3O+OH-=H2OH2OH2ONH3=OH-NH4+Strong Acids and Bases Strong acids will completely deionize or diss

24、ociate while weak acids will only partly ionize or dissociate At equilibrium, the weaker acid and the weaker base predominate in concentration The conjugate base of a strong acid is weak The conjugate acid of a strong base is weakConjugate PairsAcidConjugate BaseStrong AcidsHClO4ClO4-Weak BasesHClCl

25、-H2SO4HSO4-HNO3NO3- H3O+H2OHSO4-SO42-Increasing AcidH3PO4H2PO4-Increasing BaseStrengthHC2H3O2C2H3O2-StrengthAl(H2O)63+Al(H2O)5(OH)2+H2SHS- NH4+NH3HCNCN-HCO3-CO32-HS-S2-H2OOH-Weak AcidsNH3NH2-Strong BasespH Conceptsp H = - lo g H+ = lo g 1 / H+ .H+pHOH-0.0110-220.0000000000010.00110-330.000000000010.

26、00000110-660.000000010.000000110-770.00000010.00000000110-990.000010.00000000000110-12120.01pH & pOHOHH10 Therefore 10K C25at tablesFrom OHHK Thus 1= is water ofactivity TheOHOHHKOHHOH 14-14wow227pH -logH=pH Since 10H And H10K Thus OHH mequilibriuAt +7214wI f Kw= H+ O H-th e n H+ = Kw/ O H-o r O H-

27、= Kw/ H+p H = - lo g H+ = lo g 1 / H+ .a n d p H + p O H = 1 4th u sp O H = 1 4 - p HAnalysis ChecksTotal dissolved solids- should be within 20% of summation of determined ionsIon Balance( in milli equivalents)Should be within 5% (perhaps 10%)Does not hold for very high and very low pH- chelation, p

28、rotonation, hydrolysisAt these values remember to bring H+ or OH- into accountIn very dilute waters rounding off can influence balanceIon Balance = cations - anions cations + anions x 100Equilibrium Versus Kinetic Description of Reactions In a closed system, the equilibrium point is a position of ma

29、ximum thermodynamic stability At equilibrium, there is no chemical energy to alter the relative distribution of mass between reactants and products Theoretical approaches used to model chemical composition at equilibrium. Give no information about how long it would take to reach equilibrium nor the

30、reaction pathways that are involved. Kinetic approach is required to provide this information Groundwater = partial equilibrium system. Equilibrium reaction is fast/ kinetic reaction is slow in relation to groundwater movement. These are relative concepts Equilibrium rarely reached in rapidly flowin

31、g and mixing surface waters. Equilibrium techniques only provide boundary conditions or best or worst case scenarios, showing the direction that changes will move toward. Irreversible reactions proceed in the forward direction until all the reactants are used up, such reactions are best described us

32、ing a kinetic approach Types of reactions and times Solute-solute and solute-water reactions are fast & homogeneous reactions. Acid base and complex reactions are homogeneous and therefore fast. Dissolution-precipitation:heterogeneous and widely ranging times. Surface reactions : relatively fast Red

33、ox very slow but catalysed by microorganism Organic reactions are generally very slow. Biodegradation of organic compounds is faster.Relative times to reach equilibriumDissolution - Precipitation Reactions Equilibrium Models of Reaction Applicable toCarbonatesFe redox reactionsIon exchangeHalitesCer

34、tain sulphates etcGeneral Formulation Most are reversible reactions Expressed using the general formA + B C + D le Chateleirs principle appliesAny change will force the reaction in the direction that minimizes the changeMass action laws The law of mass action states that the rate of a chemical react

35、ion is proportional to the concentration of the reacting substances. IfA + 2B CRate AB2 orRate = kAB2 For reversible reactions:aA + bB cC + dD Forward rate = k1 AaBb Backward rate = k2 CcDd At equilibrium the forward rate = backward rate Thusk1AaBb = k2CcDd and thusReactantsProductsBADCkk=Kbadc21The

36、 K value Concentrations of the reactants at equilibrium expressed in terms of the equilibrium constant K. The value of K depends on the units used (usually molarity) and on temperature. For most solid species k2 = 1. Thus for a solid that dissolves in water the equilibrium constant is equal to the c

37、oncentration of the products: CcDd.Solubility Product partly soluble solids-assume equilibrium Thus Ksp is defined (Solubility product) Ksp is a constant at equilibrium Values are given in tables in literature1ProductsReactantsProductsKsp The Solubility of calcite and dolomite are very similar in sp

38、ite of their Ksp values being very different 10-8 calcite and 10-17 dolomite This is due to the way their mass action laws are expressed Ksp 10-4, compound considered partially soluble in normal water. Solubility depends on pH, temperature, ionic strength, common ion effect and complex formation. pr

39、oduct of the concentration called ion product Usually use IAP ( activity rather than concentration)Solubility Index If the ion product is = Ksp, the solution is saturated. If the ion product is Ksp, precipitation occurs. Solubilty IndexKIAPlog=SIspSolubility product Constants for Some Common Mineral

40、s Halite101.54 Gypsum10-4.58 Calcite 10-8.48 Dolomite10-16.54 Fluorite 10-10.57 Quartz 10-3.98 Amorphous silica 10-2.71 FeS2 + 2CaCO3 + 3, 75O2 + 1, 5H2O Fe(OH) 3 + 2SO42- + 2Ca2+ + 2CO2Gypsum vs SO4 in coalmines Over 8000 samplesConsidered SI of watersPlot SI of gypsum vs SO4Gypsum precipitation fo

41、rms an upper boundary for SO4SI of Gypsum vs Sulphate Concentration-6-5-4-3-2-101010002000300040005000SO4 ( mg/l)SIImportant weak acid base reactions in natural water systemsReactionMass law equation-log K (25C)1. H2O = H+ + OH-K(H )(OH )w14.02. CO2+H2O = H2CO3K(H CO )(H O)CO23CO222P1.463. H2CO3= HC

42、O3-+H+K(HCO )(H )(H CO )13-+236.344. HCO3= CO32-+H+K(CO )(H )(HCO )23-+3-10.335. H2SiO3= HSiO3-+H+K(HSiO )(H )(H SiO )13-+239.866. HSiO3= SiO32-+H+K(SiO )(H )(HSiO )23-+3-13.1Carbonate system Carbonate minerals often present( calcite , dolomite) CO2 occurs from atmosphere- enriched in the soil Disso

43、lves to form carbonic acidH OCOH CO2223Carbonate system continued Carbonic acid is a weak acid that dissociates to bicarbonate H2CO3 H+ + HCO3- Bicarbonate also dissociates to from carbonate in solution HCO3- H+ + CO32- Neutralisation reaction can be written as followsCaCOH COCa2HCO32323Carbonate sy

44、stem 3 equivalence points which can be determine by titration (alkalinity titration) Three distinct species which are dominant at different pH valuespH dependence of species In acid conditions pH 8 carbonate ions are dominant. The reactions given in this table show that CO2 dissolved in water partit

45、ions between H2CO3, HCO3- and CO32 If the pH of the solution is fixed then the mass law equations allow us to calculate the concentrations of the individual species Natural waters have pH in the range 5 to 8. Mine waters have pH 3. Highly alkaline waters may be associated with waste disposal sites.

46、In field must distinguish between acid, neutral and alkaline waters, to determine what elements should be analysed forSolubility of certain metals with pHSolubilities of Metal Hydroxides as a Function of pHExamplesMost water insoluble metal hydroxides that are basic or amphoteric dissolve in strong

47、acids Fe(OH)3(s) + 3H3O+ Fe3+ + 6H2O (l) Al(OH)3(s) + 3H3O+ Al3+ + 6H2O (l) .All insoluble Metal carbonates dissolve in acid solutions: MCO3(s) + 2H3O+ M2+ + CO2(g) + 3H2O (l) .Metal sulphides (e.g. CuFeS2 or (Fe,Ni)9S8) with relatively large solubility products are soluble in acid. Fe3+ + 3H2O Fe(O

48、H)3 + 3H+ Hydrolysis of metal ions to form hydroxides releases H pH CaCO3 Ca2+ + CO32- CO32- + H+ HCO3- HCO3-+ H+ H2CO3 Many metal hydroxides are amphoteric optimum pH for their removal from solution can be found by experimentation Most metals can be precipitated as hydroxides by raising the pH to b

49、etween 8 and 11 Metal carbonates can be precipitated by adding calcite (expensive option) or dolomite Common metals vs pHpH vs Concentration1.0E-091.0E-071.0E-051.0E-031.0E-011.0E+011.0E+0336912pHLog ConcentrationCuFeZnCdAmphoteric solubilityMetals Showing Amphoteric Solubility Characteristics Oxida

50、tion-Reduction Reactions redox reactions often mediated by organisms Without this catalysis these reactions would be very slow organisms use redox reactions as a source of energy, they are therefore usually autotrophic bacteria can be treated as electron transfer reactions free or solvated electrons

51、 (e-) do not exist in aqueous solutions. Oxidation is defined as an e- loss ( increase in oxidation state). Reduction is defined as an e- gain (decrease in oxidation state). (REG) These two always happen simultaneously because free electrons (e-) do not exist in aqueous solutions Redox reactions are

52、 written as two half reactions as if the e- exist Tables given in literature For example:Fe3+ + e- Fe2+ andO2 + 4H+ + 4e- 2H2O Eh is usually used in place of E, because it is measured with reference to the hydrogen standard electrode, and is referred to as the redox potential it is especially useful

53、 in geochemistry.uelectron activity (pE or sometimes pe is used to express it as numberspEF2. 3RTEhEh0.059 (at 25C and 1 atm osphere)Variation of Oxidising Conditions With Depth Below SurfaceNatural groundwater situationGroundwater tends to Eh 0 (reducing conditions). Groundwater is isolated from th

54、e atmosphere, any oxygen that is consumed by hydrochemical and biochemical reactions can not be replaced These oxygen consuming reactions usually take place in the soil where there is abundant organic matter from decaying plant debris so that O2 is used and CO2 builds up in the soilOrganic Oxidation

55、 in soil A simple carbohydrate (CH2O) can be used to illustrate a typical reaction: CH2O + H2O + O2 + 4H+ + 4e- = CO2(g) + 4H+ + 4e-+ 2H2OCH2O + O2 = CO2 + H2OCO2 + H2O = H2CO3 Must be combined with oxidation 1/2 reaction example combination with:O2(g) + 4H+ + 4e- = 2H2O yields: O2(g) + CH2O = CO2(g

56、) + H2O Common subsurface redox reactions ProcessEquationSulphide oxidation2O2 + HS-= SO42- + H+Iron oxidation O2 + 4Fe2+ + 4H+ = 4Fe3+ + 2H2ONitrification2O2 + NH4+ = NO3- + 2H+ + H2OManganese oxidationO2 + 2Mn2+ + 2H2O = 2MnO2(s) + 4H+Iron sulphide oxidation15/4O2 + FeS2(s) + 7/2H2O = Fe(OH)3(s) +

57、 2SO42-+ H+ These reactions consume dissolved oxygenNatural Redox ZoningProcessTypical reactionAerobic RespirationCH2O+O2(g)=CO2+H2ODenitrification CH2O+4/5NO3=2/5N2(g)+HCO3-+1/5H+2/5H2OManganese (IV) ReductionCH2O+2MnO2(s)+3H+=2Mn2+HCO3-+2H2OIron (III) ReductionCH2O+4Fe(OH)3(s)+7H+=4Fe2+HCO3-+10H2O

58、Sulphate ReductionCH2O+1/2SO42-=1/2HS-+HCO3-+1/2H+Methane Fermentation CH2O+1/2H2O=1/2CH4+1/2HCO3-+1/2H+Nitrogen FixationCH2O+H2O+2/3N2(g)+4/3H+=4/3NH4+CO2Redox zoningUsually in natural environments redox processes proceed from the highest energy yield downwards. Thus oxygen rich water that enters a

59、n aquifer rich in organic matter will first be freed from its DO, then from its nitrate followed by sulphate after which methane may be produced. Thus zones of increasing reduction are usually encountered as one moves downward through an aquiferRedox zoning The zones are often referred to as OXIC (at the water table) an the region below it containing DO in this region oxide minerals are stable. .ANOXIC the deep parts of the aquifer containing no DO, in this region sulphide minerals are stable. The anoxic zone is further divided into, post-oxic, sulphidic and methanic, dep