THE GEOCHEMISTRY OF NATURAL WATERS

1. 1 THE GEOCHEMISTRY OF NATURAL WATERS STRUCTURES, PROPERTIES, AND OCCURRENCE OF ORGANIC COMPOUNDS IN NATURAL WATERS - I CHAPTER 6 - Kehew (2001) Properties of organic compounds

2. 2 LEARNING OBJECTIVES Understand the physical properties that affect how organic compounds partition among vapor, liquid, aqueous solution, and solid phases. Discover how these properties relate to possible remediation strategies. 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. 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.

3. 3 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 (i.e., 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. When an organic compound is present as a separate phase (i.e., 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.

4. 4 RELATIVE VOLATILITIES OF ORGANIC COMPOUNDS

5. 5 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 - non-polar compounds with low water solubility. Hydrophilic - polar compounds that readily dissolve in water.

6. 6 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. Non-polar covalent compounds will have very low solubilities in water, but will dissolve other non-polar covalent compounds. For example, toluene or benzene will dissolve in carbon tetrachloride, but they have low solubilities in water. 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. Non-polar covalent compounds will have very low solubilities in water, but will dissolve other non-polar covalent compounds. For example, toluene or benzene will dissolve in carbon tetrachloride, but they have low solubilities in water.

7. 7 SOLUBILITIES AS A FUNCTION OF MOLECULAR WEIGHT These data show that hydrocarbons with similar structures have solubilities that depend inversely on molecular weight. These data show that hydrocarbons with similar structures have solubilities that depend inversely on molecular weight.

8. 8 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 non-aqueous phase. Then, compounds with densities greater than that of water will sink (DNAPL) and those with densities less than that of water will float (LNAPL). 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 non-aqueous phase. Then, compounds with densities greater than that of water will sink (DNAPL) and those with densities less than that of water will float (LNAPL).

9. 9 HENRY’S CONSTANT - I 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 coefficient - 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, (CH3)2CO, as an example: (CH3)2CO(l) ? (CH3)2CO(g) (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 = pacetone, where pacetone is the vapor pressure. Solubility refers to the reaction: (CH3)2CO(l) ? (CH3)2CO(aq) (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: (CH3)2CO(aq) ? (CH3)2CO(g) (3) The equilibrium constant for this reaction is then given by K1/K2 = KH = pacetone/CW. 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. 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, (CH3)2CO, as an example: (CH3)2CO(l) ? (CH3)2CO(g) (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 = pacetone, where pacetone is the vapor pressure. Solubility refers to the reaction: (CH3)2CO(l) ? (CH3)2CO(aq) (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: (CH3)2CO(aq) ? (CH3)2CO(g) (3) The equilibrium constant for this reaction is then given by K1/K2 = KH = pacetone/CW. 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.

10. 10 HENRY’S CONSTANT - II The general definition of the Henry’s constant is An alternate way to define the Henry’s constant is 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, CA refers to the concentration of the organic compound in the air above the water. Note that, the definition of the Henry’s Law constant used by Kehew (2001) in Chapter 6 is just the inverse of that used in Chapter 3 to define the solubility of CO2 (i.e., KCO2 = aH2CO3/pCO2). 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 definition being used. In the second expression above, CA refers to the concentration of the organic compound in the air above the water. Note that, the definition of the Henry’s Law constant used by Kehew (2001) in Chapter 6 is just the inverse of that used in Chapter 3 to define the solubility of CO2 (i.e., KCO2 = aH2CO3/pCO2). 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 definition being used.

11. 11 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, A.W. (1995) Water Quality Data. Boca Raton, FL: Lewis Publishers, 397 p. 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, A.W. (1995) Water Quality Data. Boca Raton, FL: Lewis Publishers, 397 p.

12. 12 OCTANOL-WATER PARTITIONING COEFFICIENT The octanol-water partitioning coefficient, Kow, describes the partitioning of an organic compound between immiscible octanol and water. It is defined as Nonpolar organics prefer octanol (Kow high); polar organics prefer water (Kow low). The value of Kow is useful in the estimation of other parameters. For example, water solubility is related by Octanol is an alcohol with the formula CH3CH2CH2CH2CH2CH2CH2CH2OH which has a relatively low solubility in water (i.e., water and octanol are immiscible). The octanol-water 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. Non-polar 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. Octanol is an alcohol with the formula CH3CH2CH2CH2CH2CH2CH2CH2OH which has a relatively low solubility in water (i.e., water and octanol are immiscible). The octanol-water 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. Non-polar 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.

13. 13 ADSORPTION OF DISSOLVED ORGANIC COMPOUNDS - I The value of Kow 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 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 Koc, which is just the ratio of the mass of organic compound adsorbed to the mass of solute in 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 Koc, which is just the ratio of the mass of organic compound adsorbed to the mass of solute in solution.

14. 14 ADSORPTION OF DISSOLVED ORGANIC COMPOUNDS - II The value of Kow has been related to Koc by several relations of the type: (Karickhoff et al., 1979) (Schwarzenbach and Westall, 1981) Values of Kd can also be calculated from Koc if the weight fraction of organic carbon (ƒoc) is taken into account: A number of equations relating Kow and Koc have been developed. Moreover, values of Kd can also be estimated. The Kd is the partitioning coefficient between water and the total amount of soil. Normally, organic matter accounts for only a fraction of the total soil, so the conversion from Koc 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: S = Cs = KCn and Kd is the value of K when n = 1. A number of equations relating Kow and Koc have been developed. Moreover, values of Kd can also be estimated. The Kd is the partitioning coefficient between water and the total amount of soil. Normally, organic matter accounts for only a fraction of the total soil, so the conversion from Koc 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: S = Cs = KCn and Kd is the value of K when n = 1.