The Geochemistry of Natural Waters
The Geochemistry of Natural Waters GEOL 464
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THE GEOCHEMISTRY OF NATURAL WATERS STRUCTURES PROPERTIES AND OCCURRENCE OF ORGANIC COMPOUNDS IN NATURAL WATERS I CHAPTER 6 Kehew 2001 Pages 166182 LEARNING OBJECTIVES I Understand the physical properties that affect how organic compounds partition among vapor liquid aqueous solution and solid phases I Discover how these properties relate to possible remediation strategies I Begin to learn how to classify and name organic compounds of relevance to aqueous geochemistry alkanes alkenes Some of the most dangerous contaminants in many natural waters are organic compounds such as chlorinated hydrocarbons polychlorinated biphenyls PCB S polycyclic aromatic hydrocarbons PAH s dioxins tert butyl ether the gasoline additive pesticides etc To understand how these chemicals behave in the environment and to develop effective remediation strategies we must understand the chemical and physical properties of organic compounds Also in Lecture 10 we saw that organic carbon is the most important electron donor in most natural waters Finally natural organic matter humic acids fulvic acids and others can play an important role in regulating metal mobility and the behavior of contaminant organic compounds Thus in this and subsequent lectures we will learn some aqueous organic chemistry VAPOR PRESSURE Vapor Pressure the pressure of a compound in a gas phase in equilibrium with the liquid form of the compound The vapor pressure depends on the temperature higher temperature greater vapor pressure Volatility the tendency of a compound to evaporate from a pure liquid of the compound Vapor pressure is a measure of volatility NAPLs NonAqueous Phase Liquids When an organic compound is present as a separate phase ie not dissolved in water or a NAPL its volatility is controlled by its vapor pressure Compounds with high vapor pressures may simply evaporate when they are spilled RELATIVE VOLATILITIES OF ORGANIC COMPOUNDS Volatility Vapor Pressure Compounds atm Volatile gt 10394 Light hydrocarbons halogenated hydrocarbons Semivolatile 104103911 Heavier hydrocarbons most compounds with O N S and P Nonvolatile lt 103911 Other organics WATER SOLUBILITY Solubility the mass or number of moles of an organic compound in a unit volume or mass of water Miscible completely soluble in water Immiscible relatively insoluble in water and separates to form a nearly pure organic phase when mixed with water Hydrophobic nonpolar compounds with low water solubility Hydrophilic polar compounds that readily dissolve in water FACTORS GOVERNING WATER SOLUBILITY 1 Polarity More polar organic compounds tend to have higher water solubilities Less polar organic compounds are less soluble In particular compounds containing oxygen and nitrogen tend to be more water soluble 2 Molecular weight Given similar structures compounds with higher molecular weights have lower aqueous solubilities They also tend to be less volatile Here we apply a principle that we learned in Lecture 1 Like substances dissolve like That is water is a polar covalent substance so it is most effective at dissolving ionically bonded e g NaCl CaSO4 or polar covalent e g acetic acid ethanol compounds Nonpolar covalent compounds will have very low solubilities in water but will dissolve other nonpolar covalent compounds For example toluene or benzene will dissolve in carbon tetrachloride but they have low solubilities in water SOLUBILITIES AS A FUNCTION OF MOLECULAR WEIGHT Compound Structure Molecular Solubility Weight 9 m3 Benzene 780 1780 Toluene mg 920 515 175 Oxylene 1060 These data show that hydrocarbons with similar structures have solubilities that depend inversely on molecular weight DENSITY The importance of density is in relation to the behavior of immiscible compounds in the subsurface Compounds with densities less than the aqueous phase will pool on the capillary fringe if enough of the compound is present LNAPL s or floaters Compounds with densities more than the aqueous phase will sink until an impermeable barrier is reached DNAPL s or sinkers The density of an organic compound relative to water determines whether it might form a DNAPL or an LNAPL First to form a separate NAPL phase at all the compound must be relatively insoluble in water and or be present in quantities sufficiently high to saturate the aqueous phase and form a second nonaqueous phase Then compounds with densities greater than that of water will sink DNAPL and those with densities less than that of water will oat LNAPL HENRY S CONSTANT The vapor pressure is only a measure of the volatility of a pure compound The volatility of an organic compound in an aqueous phase also depends on the solubility of that compound The important parameter describing this situation is the partitioning coefficient Partitioning coef cient the ratio of the abundances of a given compound in two phases in equilibrium Henry s Law constant the partitioning coefficient between a gas phase and liquid water We can only use the vapor pressure of an organic compound to assess its potential for evaporation if the compound forms its own pure phase If the compound is highly soluble in water then it might not readily evaporate even if its vapor pressure is quite high Vapor pressure refers to the following type of reaction using acetone CH32CO as an example CH32CO1 lt9 CH32COg 1 Thus the reaction refers to the change from liquid acetone to acetone vapor Because the liquid acetone is assumed to be pure its activity is unity so the equilibrium constant for reaction 1 is K1 puma where pacetone is the vapor pressure Solubility refers to the reaction CH32CO1 lt9 CH32COaq 2 The equilibrium constant for this reaction again assuming that liquid acetone has an activity of unity is K2 Cw where CW is the concentration of acetone in water If we subtract reaction 2 from reaction 1 we obtain CH32COaQ lt gt CH32COg 3 The equilibrium constant for this reaction is then given by KlK2 KH pumaCw Thus the Henry s Law constant combines information about vapor pressure and solubility and therefore is an appropiate measure of the volatility of an organic compound dissolved in the aqueous phase HENRY S CONSTANT The general definition of the Henry s constant is KH 2 i atm L mol39l CW An alternate way to define the Henry s constant is KH 2 gmol L rnol391 LW CW Henry s constant can be directly measured or it can be estimated from the vapor pressure and solubility of the compound of interest In the second expression above C A refers to the concentration of the organic compound in the air above the water Note that the de nition of the Henry s Law constant used by Kehew 2001 in Chapter 6 is just the inverse of that used in Chapter 3 to de ne the solubility of CO2 ie KC02 aHzc 03pCOz Depending on the source the Henry s Law constant can be written either way Thus it is a good idea always to be sure of the de nition being used Solubility mg L391 01 1 10 100 1000 10000 0001 a 001 E E 01 g KH 00001 I I 2 1 n 5 10 gt KH 001 100 KH 100 KH 1 Compounds most likely to partition into the gas phase highest KH are those with high vapor pressure and low solubility This diagram emphasizes the close relationship among solubility vapor pressure and the Henry s Law constant The units of KH employed in this diagram are not clear from Kehew 2001 You may determine the units by consulting the original source Hounslow AW 1995 Water Quality Data Boca Raton FL Lewis Publishers 397 p OCTANOLWATER PARTITIONING COEFFICIENT The octanolwater partitioning coefficient Kow describes the partitioning of an organic compound between immiscible octanol and water It is de ned as Km 2 g Smol L39s1 mol391 LW W Nonpolar organics prefer octanol K0W high polar organics prefer water K0W low The value of K0W is useful in the estimation of other parameters For example water solubility is related by logKow 730 0747logS Octanol is an alcohol with the formula CH3CH2CH2CH2CH2CH2CHZCHZOH which has a relatively low solubility in water ie water and octanol are immiscible The octanolwater partitioning coefficient KOW is therefore a measure of the preference of an organic compound for water vs a less polar organic solvent Polar organic compounds dissolve more readily in water than in octanol so their KOW values are low Nonpolar organic compounds dissolve more readily in octanol so their KOW values are high Octanol does not normally occur in significant concentrations in nature nor is it a common constituent of contaminated waters So why do we care about KOW We are not really interested in the value of KOW per se but it turns out that KOW is related to a number of other properties such as the solubility of the organic compound in water in which we do have an interest Values of KOW have been determined for a large number of organic compounds and we can use these values to predict the values of other parameters that might not be available Such applications of KOW will become apparent in the next several slides ADSORPTION OF DISSOLVED ORGANIC COMPOUNDS The value of K0W can be used to estimate adsorption behavior Hydrophobic organic compounds do not interact electrically with surfaces of charged particles they are most strongly adsorbed to neutral solid organic matter The partitioning coefficient between solid organic matter and water is Cad g solute adsorbed g soilorganic carbon K 06 C W g solutem3 solution One of the properties that KOW can be used to predict is the adsorption of organic compounds onto naturally occurring solid organic matter Unlike aqueous ions hydrophobic organic compounds are not attracted to charged mineral surfaces Instead such compounds tend to be attracted to the neutral surfaces of solid organic matter The strength of this adsorption is measured by the partitioning coefficient Km which is just the ratio of the mass of organic compound adsorbed to the mass of solute in solution ADSORPTION OF DISSOLVED ORGANIC COMPOUNDS The value of K0W has been related to K0C by several relations of the type logKoc 021logK0w Karickhoff et al 1979 logKoc 049 07210gK0w Schwarzenbach and Westall 1981 Values of Kd can also be calculated from K0c if the weight fraction of organic carbon foe is taken into account Kd Koor06 A number of equations relating KOW and KOC have been developed Moreover values of Kd can also be estimated The Kd is the partitioning coef cient between water and the total amount of soil Normally organic matter accounts for only a fraction of the total soil so the conversion from K0C to Kd requires knowledge of the fraction of organic matter in the soil Here Kd is the same parameter introduced in Lecture 7 when we discussed Freundlich isotherms Recall that the Freundlich isotherm is given by the general equation SCSKC and Kd is the value of K when n l PARTITIONING OF ORGANIC COMPOUNDS How organic contaminants partition among the solid liquid and gas phases in natural waters is crucial to predicting how far they will migrate and in selecting remediation techniques To determine the partitioning one calculates the mass of the compound in one cm3 of soil containing both water and air in its pore spaces For the solids Ci 2 K de where CS is the concentration adsorbed to the soil Kd is the distribution coefficient cm3 g1 CW is the concentration in the aqueous phase 15 PARTITIONING OF ORGANIC COMPOUNDS II We can estimate Kd from the relation K d 06 focKow Once CS is determined for the compound the mass of the compound MS in a 1cm3 volume of soil is given by Ms Cspb where pb is the bqu soil density To calculate the mass of organic matter in water requires knowledge of the porosity and the volumetric water saturation PARTITIONING OF ORGANIC COMPOUNDS III The mass of water is given as MW 2 Cwnw where nW is the waterfilled porosity the product of the porosity and the volumetric water saturation The concentration of the organic compound in the gaseous phase is given by Ca 2 CWKH and the mass of the compound in air by Ma Cana where na is the airfilled porosity For an example application of these equations to determining the distribution of an organic compound between solid water and air in a soil see the answers to the problems in Kehew 2001 for this lecture PARTITIONING OF ORGANIC COMPOUNDS IV 2 ORGANIC MATTER Compound Air Water Solids 111Trichloroethane 5 5 90 KOW 300 111Trichloroethane 6 6 88 KOW 1479 Trichloroethene 16 46 938 20 C Trichloroethene 30 7 63 00 Acetone 007 85 15 18 This slide and the next reproduce the partitioning data given in Table 63 in Kehew 2001 I cannot reproduce all these numbers using the calculation method outlined above and the data given in the text Some of the numbers come out right and some do not Itherefore suspect that either 1 there are mistakes in Kehew 2001 andor 2 there are mistakes in the original reference In checking the original source I nd that Kehew 2001 has correctly quoted all the information so the mistake must be in Davis 1997 You should follow the method I outline in my solution to Kehew s 2001 Problem 1 from Chapter 6 PARTITIONING OF ORGANIC COMPOUNDSV 1 ORGANIC MATTER Compound Air Water Solids 111Trichloroethane 26 23 51 KOW 300 111Trichloroethane 35 31 34 KM 1479 Trichloroethene 14 35 52 20 C Trichloroethene 73 16 11 90 C Acetone 007 99 1 PARTITIONING OF ORGANIC COMPOUNDS VI In a soil with high organic matter content 111 TCA is partitioned almost entirely in the solid phases difficult to remove contaminant In a soil with low organic matter content 111 TCA is not as strongly partitioned into the solid phases and would be easier to remove Acetone is strongly partitioned into the aqueous phase I More mobile and may migrate far from source I Can be removed by pumpandtreat methods 20 PROPERTIES OF SOME COMMON GROUND WATER CONTAMINANTS Water Vapor solubility Pressure BP Density mg L39l mm Hg Compound C 25 C 252 10 C KH 25 C KOW 0105 Methylene 40 13182 20000 2609 1778 chloride Acetone 563 07899 00 1217 0000842 174 Carbon 768 15833 800 583 0807 6761 Benzene 801 088 1770 478 022 1349 Trichloro 873 14578 1100 376 0397 195 ethene Tetra 1213 1613 150 400 0928 400 chloroethene 2Butanone 796 07994 26800 3143 0001 182 This table is reproduced here for convenience It gives some properties of a few common ground water contaminants STRUCTURES OF COMMON COMPOUNDS AMMONIA Lewis Structure N I H Condensed Structural Formula NH3 Molecular Formula NH3 Formulas of organic compounds can be written in several different ways Here we use a familiar inorganic compound ammonia to illustrate The Lewis structural formula shows us all the details of the structural including the presence of a nonbonding pair of electrons on the nitrogen atom In the Lewis structure bonding pairs of electrons are denoted by a short straight line segment whereas nonbonding pairs are denoted by two dots In the case of ammonia the condensed structural formula and the molecular empirical formula are the same 22 STRUCTURES OF COMMON COMPOUNDS METHANE Lewis Structure H H C K H H Condensed Structural Formula Molecular Formula For methane the second two formulas are also the same 23 STRUCTURES OF COMMON COMPOUNDS ETHYLENE Lewis Structure H H CC H H Condensed Structural Formula Molecular Formula Ethylene illustrates the difference between the condensed structural formula and the molecular empirical formula The molecular formula simply tells us how many of each kind of atom is present but it tells us nothing about how these atoms are put together The condensed structural formula does not show the exact positions of the hydrogen atoms However it is understood that the Hatoms are each bonded to the carbon and not to each other for example 24 STRUCTURES OF COMMON COMPOUNDS METHANOL Lewis Structure H H C I H O39 39 H Condensed Structural Formula CH3OH Molecular Formula CH4O 25 STRUCTURES OF COMMON COMPOUNDS ACETIC ACID Lewis Structure If 3039 H C C jd H H Condensed Structural Formulas O H3C OH CH3COOH Molecular Formula 26 SOME DEFINITIONS Structural isomer two compounds having the same molecular empirical formulas but different structures Functional group a structural fragment of a molecule which identifies the compound to which it is attached as a member of a specific class of compounds For example alcohols are written as R OH where R represents a carbon atom or carbon chain Ethers are written as R OR and carboxylic acids are written as R COOH R represents a carbon atom or carbon chain that is different from R 27 ISOMERS EXAMPLE Hexane C6H14 H3C CH2 CH2 CH2 CH2 CH3 CH3 CH CH 2 methylpentane C6H14 H3C 1C6 CH3 H3C CH3 23dimethylbutane C6H14 CHCQ H3C CH3 CH3 lCH3 22dimethyl butane C6H14 H3CCHCCH 2 3 28 FUNCTIONAL GROUPS Alcohol or phenol R OH ethanol phenol H3C 0H CH2 OH Aldehyde ICH3 O CH 0 H H3C CH R C H 2methylpropanal 29 FUNCTIONAL GROUPS Primary amine R NHZ HzN CHZ n propylamlne CHZ39CH3 Secondary amine R NH R39 Hzc ethylmethylamine NH CH3 Tertiary amine H3O 39 CH3 trimethyla mine RNRu H3O 30 30 FUNCTIONAL GROUPS Carboxyllc aCId H3c CHz o CH2 C R o H OH O Butanoic acid Ester 0 CH2 H C C CH CH R O R 3 2 2 I O CHZ39CH3 O Amyl acetate Amyl acetate is the compound that is responsible for the taste of bananas 31 ALIPHATIC HYDROCARBONS Hydrocarbons compounds composed only of carbon and hydrogen Aliphatic compounds not containing a benzene ring consist of straight branched or cyclical chains of hydrocarbons Alkanes saturated hydrocarbons paraffins compounds containing only single covalent bonds between C atoms The general formula is CnHZn239 32 ALKAN E STRUCTURES Methane Ethane Propane H H If If t H H H H H Butane Pentane Hiii j HTTECH t t E I i i E It It 33 ALKYL SUBSTITUENT GROUPS CH3 CH239CH3 Propyl CH239CH239CH3 Isopropyl TH CH3 CH3 Butyl CH239CH239CH239CH3 H39CH CH Isobutyl CH2 CH CH3 secButyl T 2 3 CH3 CH3 CH3 tert Butyl H CH3 CH3 34 34 NAMING OF ALKANES 1 The general name for the saturated hydrocarbon is alkane 2 The longest chain of C atoms in a branched chain hydrocarbon is the parent chain and the root name of the compound is that of the parent chain 3 Each substituent group attached to the parent chain is given a name and a number The number indicates the carbon atom of the parent chain to which the substituent is attached 35 NAMING OF ALKANES 4 If the same substituent occurs more than once the number of each carbon of the parent chain on which that substituent occurs is given In addition the number of times the substituent group occurs is indicated by a prefix di tri penta or hexa 5 If there is one substituent the parent chain is numbered from the end that gives the substituent the lowest number If there are two or more substituents the parent is numbered from the end that gives the lowest number to the 1st substituent encountered 6 If there are two or more different alkyl substituents they are listed in alphabetical order 36 CYCLOALKANES Cycloalkanes alkanes joined together in rings of various sizes The root name is the name of the corresponding straightchain alkane with the prefix cyclo If there is one substituent it is unnumbered but if there is more than one they all must be numbered Polygons with the same number of sides as carbon atoms can be used as a shorthand THs 13dimethylcyclopentane HzcCKCHZ CH239 CH3 37 37 ALKENES AND ALKYNES Two of three types of unsaturated hydrocarbons Alkenes hydrocarbon chains containing one or more double bonds General formula CnH2n Alkynes hydrocarbon chains containing one or more triple bonds General formula CnH2n2 To name alkenes determine the longest carbon chain containing the double bond and change the suffix from ane to ene The position of the double bond is specified by numbering the chain from the direction that gives the lowest number The number refers to the carbon that the double bond follows Similar rules for allqnes 38 38 EXAMPLES H3C CH2 CH H3C CH c CH2 CH2 CH2 CH2 zCH H cm 1hexene 2hexene H3C CH CH CHZ CH zCH3 3hexene CH3 C H2C Q C CH CH3 1butyne 2butyne 39 THE GEOCHEMISTRY OF NATURAL WATERS MINERAL WEATHERING AND MINERAL SURFACE PROCESSES III SORPTION AND ION EXCHANGE CHAPTER 4 Kehew 2001 Pages 107128 LEARNING OBJECTIVES Learn about sorption distinguish among adsorption absorption and ion exchange Understand why minerals acquire surface charge and what the implications are Learn about sorption isotherms Learn to deal quantitatively with ion exchange Investigate the role of ion exchange in natural and contaminated waters IS SOLUBILITY THE ONLY CONTROL ON SOLUTE CONCENTRATIONS I The answer is no Solubility often controls the concentrations of major solutes such as Si Ca and Mg and some minor or trace solutes such as Al and Fe However for many trace elements sorption processes maintain concentrations below saturation with respect to minerals In other words sorption is a means to remove solutes even when the solution is undersaturated with any relevant solids In the preceding two lectures we have learned about solubility controls on natural water compositions Both congruent and incongruent dissolution can exert control on the concentrations of major solutes and some trace solutes as well Nevertheless the concentrations of many trace elements are controlled by a collection of processes collectively called sorption Sorption processes involve the removal of solutes from solution into or onto a solid These processes may occur even though the solution is not saturated with any mineral containing the solute of interest For example the concentration of a trace element such as Cd may be limited by sorption onto the surface of a clay or iron oxyhydroxide mineral even though the solution is undersaturated with respect to all minerals of which Cd is an essential constituent Sorption processes are important because they retard the movement of contaminants through aquifers Sorption processes are expected to play a dominant role in retaining radionuclides near nuclear waste repositories should the primary waste form be breached and come in contact with ground water Most repository designs provide for back lling of metal canisters containing nuclear wastebearing borosilicate glass with clays Sorption onto clay and other mineral surfaces should help retard the migration of radionuclides into the biosphere Finally sorption processes also are important in uncontaminated natural waters recall the Madison Aquifer example in Lecture 4 where ion exchange occurred along ow path 2 Thus an understanding of sorption processes is of paramount importance to aqueous geochemistry DEFINITIONS I Sorption removal of undersaturated solutes from solution onto minerals I Sorbate the species removed from solution I Sorbent the solid onto which solution species are sorbed I Three types of sorption I Adsorption solutes held at the mineral surface as a hydrated species I Absorption solute incorporated into the mineral structure at the surface I Ion exchange when an ion becomes sorbed to a surface by changing places with a similarly charged ion previously residing on the sorbent 4 The three different types of sorption processes de ned above cannot always be distinguished clearly in practice However it is useful to make these distinctions in theory When it is not clear exactly which of these processes is occurring the general term sorption should be used It should also be kept in mind that not all authors de ne these processes in exactly the same way as Kehew 2001 Consult Figure 427 in Kehew 2001 to make the distinction between adsorption and absorption clearer ACQUISITION OF SURFACE CHARGE I I In general solutes interact with mineral surfaces because the latter have acquired electrical charge I Two ways to acquire charge I Substitution for a cation in a mineral by one of lesser positive charge This type of charge is considered to be xed I Reactions involving functional groups on the mineral surface and ions in solution surface complexation This type of charge is variable and dependent on solution pH The main reason that ions are attracted to mineral surfaces is because these surfaces generally have an excess charge that is acquired in one of two ways In the first way of acquiring charge a less highly charged ion e g Al substitutes in the crystal lattice of a mineral for a more highly charged ion e g Si Such substitution leads to an excess negative charge on the mineral surface Because the ionic substitution causing the charge imbalance takes place within the mineral structure the charge imbalance is permanent or fixed The second way of acquiring charge is via the formation of surface complexes ie the formation of a bond between reactive atoms on the mineral surface and ions in solution In ensuing slides we will investigate these mechanisms further ACQUISITION OF SURFACE CHARGE II I Only 21 clay minerals eg smectites vermiculite can acquire significant fixed charge through ionic substitutions Substitution of divalent cations for trivalent cations in octahedral sites and of trivalent cations for tetravalent cations in tetrahedral sites results in a deficiency of positive charge or a net negative fixed charge on the surface This negative charge can be balanced by the sorption of cations from solution We saw in Lecture 4 that ll clay minerals such as kaolinite do not generally exhibit much ionic substitution Thus ll clay minerals do not possess fixed surface charge but as we will see they may acquire variable surface charge through surface complexation reactions On the other hand many 21 clay minerals such as smectite and vermiculite do exhibit extensive ionic substitutions and these can lead to charge imbalance For example if divalent ions Mg2 Mn2 Fe2 substitute for trivalent ions Fe3 Al in the octahedral layers or if trivalent ions A1 substitute for Si4 in the tetrahedral layers a fixed excess negative charge will exist on the surfaces of these clays The negative charge is balanced by sorption of cations ACQUISITION OF SURFACE CHARGE III I Silica tetrahedra near the outer surface of a 21 clay mineral are arranged in such a way to present a plane of oxygen atoms siloxane surface I Siloxane cavities occur at regular intervals on the surface and serve as reactive sites for the formation of surface complexes with cations I Complexes can be formed with either hydrated or dehydrated cations An alternative way to acquire surface charge is through surface complexation reactions Surface complexes are analogous to aqueous complexes The reactive sites for complex formation on silicate surfaces are cavities formed by oxygen atoms and hydroxyl groups Siloxane cavity formed by surface oxygens and hydroxyls in silicate minemls From Sposito 1989 The Chemistry of Soils Oxford University Press ACQUISITION OF SURFACE CHARGE IV Innersphere surface complex a surface complex formed directly between a dehydrated cation no surrounding water molecules and the siloxane cavity The bond formed is very strong Outersphere surface complex a surface complex formed between a hydrated cation and the siloxane cavity The bond formed here is much weaker than in the outersphere case Outer sphere complex ions exchange more readily with ions in solution