5水文地球化学过程Hydrochemicalprocess.pptx

Click to edit Master title style,Click to edit Master text styles,Second level,Third level,Fourth level,Fifth level,#,第,5,章 水文地球化学过程,环境与土木工程学院,地质灾害防治与地质环境保护,国家重点实验室,地下水科学研究与开发中心,成都理工大学,吴,勇,博士教授,2025/4/27 周日,Outline of topics,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,reduction/oxidation,solution/precipitation),Need to consider reactions to classify,predict migration,identify persistence and remediation,ywu,Water,Soils and Rocks,Hydrogeochemistry,Hydrochemistry,Geochemistry,Hydrogeological Process,Water,Groundwater reacts with Soils and rocks,Reaction,ywu,Groundwater Regime&Water Quality,ywu,Flow&Reaction Time,ywu,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,Why should we study,Hydrochemical Reactions?,ywu,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,HCO,3,Granites-Na,K,F,Sand Aquifer-low TDS-relatively inert rocks-rainfall determining factor,ywu,Comparison of reaction rates,ywu,Oxidation of sulphide minerals,in the presence of oxygen,water and bacteria,Generally in South Africa the mineral pyrite(,FeS,2,)is the primary cause.,Pyrite occurs as an associated or gangue mineral with gold,the base metals and also coal,Pyrite(Fools Gold),3218,BA1731:745m,Malmesbury,Dissolve of Pyrite,ywu,Detailed reactions,(1)FeS,2,+7/2 O,2,+H,2,O=Fe,2+,+2SO,4,2-,+2H,+,(2)Fe,2+,+1/4O,2,+H+=Fe,3+,+1/2 H,2,O (rate limiting step),(3)Fe,3+,+3H,2,O=Fe(OH),3,(yellow boy)+3H,+,(4)FeS,2,+14Fe,3+,+8H,2,O=15Fe,2+,+2SO,4,2-,+1,6H,+,BOTTOM LINE=Sulphides+water+oxygen(+bacteria)give acidity(4,H,+,),+sulphates,If this is not neutralized it can result in heavy metal mobilisation which can have disastrous environmental consequences,ywu,Model of pyrite oxidation,ywu,a unreacted pyrite,b pyrite oxidation-oxygen consumed,c oxygen diffuses to oxidation layer,d liquid phase-oxygen concentration in equilibrium with the gas phase,e Gas phase between the waste rocks-oxygen supply by diffussion and advection,Sulphide,Water film,Product,layer,MODELS OF PYRITE OXIDATION-micro scale,ywu,Influence of bacteria,ywu,Influence of pH on Thiobacillus activity,ywu,Oxygen dependence of bacteria,ywu,Importance of bacterial catalysis on oxidation rate,ywu,Buffer level of some common minerals,ywu,Buffer,reactions,CaCO,3,+2H,+,Ca,2+,+H,2,O+CO,2(g),H,2,O+CO,2(g),H,2,CO,3,H,2,CO,3,H,+,+HCO,3,-,H,+,+CO,3,2,FeS,2,+2CaCO,3,+3,75O,2,+1,5H,2,O,Fe(OH),3,+2SO,4,2-,+2Ca,2+,+2CO,2,(open system),1 mole of FeS,2,(64 g sulphur)is neutralised by 2 moles of CaCO,3,(200 g)or 1 g sulphur:3.125 g CaCO,3,FeS,2,+2CaCO,3,+3,75O,2,+3,5H,2,O,Fe(OH),3,+2SO,4,2-,+2Ca,2+,+2H,2,CO,3,Closed system,1 mole of FeS,2,is neutralised by 4 moles of CaCO,3,which results in a mass ratio of 1 g pyrite:6.25 g calcite,ywu,碳酸平衡,Distribution Diagram for the Carbonate System,ywu,Other reactions,Dolomite,CaMg(CO,3,),2,+4H,+,=Ca,2+,+Mg,2+,+2H,2,O+2CO,2,Albite dissolution,NaAlSi,3,O,8(s),+H,+,+9/2H,2,O=Na,+,+2H4SiO,4,+1/2Al,2,Si,2,O5(OH),4(s),Anorthite dissolution,CaAl,2,Si,2,O,8(s),+H,2,O=Ca,2+,+Al,2,Si,2,O,5,(OH),4(s),K-feldspar dissolution,KAlSi,3,O,8(s),+H,+,+9/2H,2,O=K,+,+2H,4,SiO04+1/2Al,2,Si,2,O,5,(OH),4(s),Iron oxy-hydroxide dissolution,Fe(OH),3(s),+3H,+,=Fe,3+,ywu,Importance of reaction kinetics,Although all these reactions can consume H,+,and thus buffer the system,the reaction rates of most 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),ywu,OPEN PIT,At field scale the neutralising minerals can buffer the system from acidity,PRECIPITATION,PRECIPITATION,PRECIPITATION,PRECIPITATION,WASTE,PILE,RIVER,VALLEY,UNDERGROUND MINE WORKINGS,TAILINGS BASIN,FILL,MATERIALS,WASTE,PILE,Lines of defence against acidity include,Natural waters alkalinity-pH as high as 8+,Carbonate minerals(Calcite/Dolomite)pH above 6,Silicates-react too slowly to act as buffer-pH falls to levels where heavy metals are mobilised etc,ywu,General results,Not 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,Staining,ywu,Outflow at a SA coal mine,What kind of hydrochemical process occurred in this picture?,ywu,.,The 10-90%concentration range of 23 acid drainages from coal mines throughout the United States taken from the,EPA effluent 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 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.For 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 US,ywu,Summary,AMD is caused by bacterially mediated oxidation of sulphide minerals,The“essential ingredients”are sulphides,oxygen,water and bacteria,Several mechanisms buffer the system,The carbonates are the most important of these due to reaction rate considerations.,ywu,Groundwater quality&Its influence factors,Vegetation also affects composition by selective uptake of ions.,Ions such as Cl,Na,Mg can accumulate in soils and be flushed into GW by recharge,In areas of higher evapotranspiration greater accumulation of salts,Higher recharge areas-more dilution of salts and closer correlation to rainfall,Other reactions:ion exchange,dissolution,formation water,contamination,Precipitation of over-saturated minerals seldom a significant factor-reduction in concentrations usually due to mixing/recharge etc,Rainwater chemistry important in GW studies and influence needs to be considered,ywu,ywu,Acids and Bases,Bronsted-Lowry theory,Acid is a proton donor.,Base is a proton acceptor,ywu,ywu,Strong Acids and Bases,Strong acids will completely deionize or dissociate 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 weak,ywu,Conjugate Pairs,ywu,pH Concepts,ywu,pH&pOH,ywu,Analysis Checks,Total dissolved solids-should be within 20%of summation of determined ions,Ion Balance(in milli equivalents),Should be within 5%(perhaps 10%),Does not hold for very high and very low pH-chelation,protonation,hydrolysis,At these values remember to bring H+or OH-into account,In very dilute waters rounding off can influence balance,ywu,Equilibrium Versus Kinetic Description of Reactions,In a closed system,the equilibrium point is a position of maximum thermodynamic stability,At equilibrium,there is no,chemical,energy to alter the relative distribution of mass between reactants and products,ywu,Theoretical approaches used to model chemical composition at equilibrium.,Give,no information,about how,long,it would take to reach equilibrium,nor,the reaction,pathways,that are involved.,Kinetic approach,is required to provide this information,ywu,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 flowing 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 using a kinetic approach,ywu,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,Redox very slow but catalysed by microorganism,Organic reactions are generally very slow.,Biodegradation of organic compounds is faster.,ywu,Relative times to reach equilibrium,ywu,Dissolution-Precipitation Reactions,Equilibrium Models of Reaction,Applicable to,Carbonates,Fe redox reactions,Ion exchange,Halites,Certain sulphates etc,ywu,General Formulation,Most are reversible reactions,Expressed using the general form,A+B,C+D,le Chateleirs principle applies,Any change will force the reaction in the direction that minimizes the change,ywu,Mass action laws,The law of mass action states that the rate of a chemical reaction is proportional to the,concentration,of the reacting substances.,IfA+2B,C,Rate,AB,2,orRate=kAB,2,For,reversible,reactions:,aA+bB,cC+dD,Forward rate=k,1,A,a,B,b,Backward rate=k,2,C,c,D,d,ywu,At,equilibrium,the,forward rate=backward rate,Thusk,1,A,a,B,b,=,k,2,C,c,D,d,and thus,ywu,The 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 k,2,=1.Thus for a solid that dissolves in water the equilibrium constant is equal to the concentration of the products:C,c,D,d,.,ywu,Solubility Product,partly soluble solids-assume equilibrium,Thus Ksp is defined(Solubility product),K,sp,is a constant at equilibrium,Values are given in tables in literature,ywu,The Solubility of calcite and dolomite are very similar in spite of their K,sp,values being very different 10,-8,calcite and 10,-17,dolomite,This is due to the way their mass action laws are expressed,K,sp,10,-4,compound considered,partially,soluble in normal water.,Solubility depends on,pH,temperature,ionic strength,common ion effect and complex formation,.,product of the,concentration,called ion product,Usually use IAP(activity rather than concentration),ywu,Solubility Index,If the ion product is=K,sp,the solution is saturated.,If the ion product is K,sp,precipitation occurs.,Solubilty Index,ywu,Solubility product Constants for Some Common Minerals,Halite10,1.54,Gypsum10,-4.58,Calcite10,-8.48,Dolomite10,-16.54,Fluorite10,-10.57,Quartz10,-3.98,Amorphous silica10,-2.71,ywu,FeS,2,+2CaCO,3,+3,75O,2,+1,5H,2,O,Fe(OH),3,+2SO,4,2-,+2Ca,2+,+2CO,2,ywu,Gypsum vs SO4 in coalmines,Over 8000 samples,Considered SI of waters,Plot SI of gypsum vs SO4,Gypsum precipitation forms an upper boundary for SO4,ywu,Important weak acid base reactions in natural water systems,ywu,Carbonate system,Carbonate minerals often present(calcite,dolomite),CO2 occurs from atmosphere-enriched in the soil,Dissolves to form carbonic acid,ywu,Carbonate system continued,Carbonic acid is a weak acid that dissociates to bicarbonate,H,2,CO,3,H,+,+HCO,3,-,Bicarbonate also dissociates to from carbonate in solution,HCO,3,-,H,+,+CO,3,2-,Neutralisation reaction can be written as follows,ywu,Carbonate system,3 equivalence points which can be determine by titration,(alkalinity titration),Three distinct species which are dominant at different pH values,ywu,pH dependence of species,In acid conditions pH 8 carbonate ions are dominant.,ywu,The reactions given in this table show that,CO,2,dissolved in water partitions,between H,2,CO,3,HCO,3,-,and CO,3,2,If the,pH,of the solution is,fixed,then the,mass law,equations allow us to calculate the,concentrations,of the individual,species,ywu,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,.,In,field,must,distinguish,between acid,neutral and alkaline waters,to,determine,what,elements,should be,analysed,for,ywu,Solubility of certain metals with pH,ywu,Examples,Most,water insoluble metal hydroxides,that are,basic,or,amphoteric,dissolve,in strong,acids,Fe(OH),3(s),+3H,3,O,+,Fe,3,+,+6H,2,O,(l),Al(OH),3(s),+3H,3,O,+,Al,3,+,+6H,2,O,(l),.,All,insoluble,Metal carbonates,dissolve in acid,solutions:,MCO,3,(s),+2H,3,O,+,M,2,+,+CO,2(g),+3H,2,O,(l),.,Metal sulphides,(e.g.CuFeS,2,or(Fe,Ni),9,S,8,)with,relatively large solubility products,are soluble in acid.,ywu,Fe,3,+,+3H,2,O,Fe(OH),3,+3H,+,Hydrolysis of metal ions to form hydroxides releases H,pH,CaCO,3,Ca,2,+,+CO,3,2,-,CO,3,2,-,+H,+,HCO,3,-,HCO,3,-,+H,+,H,2,CO,3,ywu,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 between 8 and 11,Metal carbonates,can be,precipitated,by adding,calcite,(expensive option)or,dolomite,ywu,Common metals vs pH,ywu,Amphoteric solubility,ywu,Oxidation-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,ywu,can be treated as electron transfer reactions,free or solvated electrons(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,ywu,Redox reactions are written as two half reactions as if the e,-,exist,Tables given in literature,For example:Fe,3,+,+e,-,Fe,2,+,andO,2,+4H,+,+4e,-,2H,2,O,ywu,Eh,is usually used in place of E,because it is measured with reference to the,h,ydrogen standard electrode,and is referred to as the,redox potential,it is especially useful in geochemistry.,u,electron activity,(pE or sometimes pe is used to express it as numbers,ywu,Variation of Oxidising Conditions With Depth Below Surface,ywu,Natural groundwater situation,Groundwater tends to Eh,0(reducing conditions).,Groundwater is isolated from the 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 O,2,is used and CO,2,builds up in the soil,ywu,Organic Oxidation in soil,A simple carbohydrate(CH,2,O)can be used to illustrate a