CHAPTER 2 CHLORINATED SOLVENT CHEMISTRY: STRUCTURES, NOMENCLATURE AND PROPERTIES

Transcription

1 APTER 2 LORINATED SOLVENT EMISTRY: STRUTURES, NOMENLATURE AND PROPERTIES David M. wiertny 1 and Michelle M. Scherer 2 1 University of alifornia at Riverside, Riverside, A 92521; 2 The University of Iowa, Iowa ity, IA INTRODUTION This chapter summarizes the principles of chlorinated solvent remediation, provides overviews of the biotic and abiotic reactions that can transform and detoxify these compounds, and discusses the remediation challenges posed by the properties and behavior of these compounds in the subsurface environment. 2.2 STRUTURE AND NOMENLATURE hlorinated solvents are organic compounds generally constructed of a simple hydrocarbon chain (typically one to three carbon atoms in length) to which at least one chlorine atom is covalently bonded. For the current discussion, chlorinated solvents will be further divided into three categories based upon common structural characteristics: chlorinated methanes, chlorinated ethanes and chlorinated ethenes. Examples from each solvent class are shown in Figure 2.1. Additional information pertaining to the nomenclature of these chemical species is provided in Table 2.1. hlorinated methanes represent the most structurally simple solvent class and consist of a single carbon center (known as a methyl carbon) to which as many as four chlorine atoms are covalently bonded. From the perspective of groundwater contamination, perhaps the most well known chlorinated methane is carbon tetrachloride (T). Also known by its International Union of Pure and Applied hemistry (IUPA) name of tetrachloromethane, T consists of a fully chlorinated methyl carbon. By IUPA conventions, the modifier of tetra serves as an indicator of the number of chlorine atoms bound to the carbon center. For chlorinated methanes other than T, hydrogen atoms usually make up the remainder of the substituents necessary to satisfy the methyl carbon s bonding requirements. Named in a similar fashion by IUPA, the chlorinated methanes with a lower degree of halogenation are trichloromethane (commonly referred to as chloroform [F]), dichloromethane (DM, more commonly called methylene chloride [M]) and chloromethane (M, also referred to as methyl chloride). hlorinated ethanes consist of two carbon centers joined by a single covalent bond. ommon groundwater pollutants from this class include 1,1,1-trichloroethane (1,1,1-TA) and 1,2-dichloroethane. In regards to the nomenclature associated with chlorinated ethanes, a similar convention to that used for chlorinated methanes is employed in which the prefix attached to chloroethane indicates the total number of chlorine atoms on the solvent molecule. ommon acronyms for this class follow the pattern in which the first letter (or series of letters) refers to the number of total halogen substituents (e.g., T for trichloro- or Te for.f. Stroo and.. Ward (eds.), In Situ Remediation of hlorinated Solvent Plumes, doi: / _2, # Springer Science+Business Media, LL

2 30 D.M. wiertny and M.M. Scherer dichloromethane (DM) carbon tetrachloride (T) 1,1,1 - trichloroethane (1,1,1-TA) 1,1,2,2 - tetrachloroethane (1,1,2,2-TeA) vinyl chloride (V) trichloroethene (TE) perchloroethene (PE) Figure 2.1. hemical structures of some common chlorinated solvents. tetrachloro-), the second letter refers to the halogen identity (e.g., for chlorine) and the last letter, in all cases A, refers to ethane. In addition, the numbers preceding the name or abbreviation indicate the location of the chlorine substituents on the two possible carbon centers. For example, 1,1,2,2-tetrachloroethane (1,1,2,2-TeA) possesses two chlorine atoms on each of its carbon centers, whereas the three chlorine atoms of 1,1,1-TA are all located on the same carbon. In certain instances, there can be more than one way in which the same number of chlorine atoms distribute themselves on the carbon centers, as is the case for 1,1,2-TA and 1,1,1-TA. These compounds, which share the same chemical formula ( ) yet differ in the sequence in which their atoms are connected, are referred to as structural isomers (Vollhardt and Schore, 1994). hlorinated ethenes (sometimes referred to as chlorinated ethylenes) also possess two carbon centers, but unlike chlorinated ethanes, these carbon atoms are joined by a carboncarbon double bond known as a p-bond (pi-bond) system. Another important difference between chlorinated ethanes and chlorinated ethenes is the maximum number of atoms bound to the carbon centers in each case. The double-bonded carbon centers in chlorinated ethenes can accommodate at most two halogen (or hydrogen) substituents, whereas the singlebonded ethanes can accommodate three halogen (or hydrogen) substituents. Examples of chlorinated ethenes that are important groundwater pollutants include tetrachloroethene, commonly referred to as perchloroethene (PE), and trichloroethene (TE). Another chlorinated ethene of note is the monochlorinated species that is most commonly referred to as vinyl chloride (V). The nomenclature associated with the chlorinated ethenes follows a similar convention to that used with the chlorinated methanes and ethanes

3 hlorinated Solvent hemistry: Structures, Nomenclature and Properties 31 (e.g., tetrachloroethene contains four chlorine substituents). The same is true for the acronyms commonly applied to this solvent class, only this time the last letter in all cases is E, which represents ethenes. The lone exception to this convention for acronyms is vinyl chloride, which is typically abbreviated as V. Table 2.1. Nomenclature for Selected hlorinated Solvents IUPA Name ommon Name Abbreviation/Acronym Molecular Formula hlorinated Methanes tetrachloromethane carbon tetrachloride T 4 trichloromethane chloroform F 3 dichloromethane methylene chloride DM 2 2 chloromethane methyl chloride M 3 hlorinated Ethanes hexachloroethane perchloroethane A 2 6 pentachloroethane ---- PA 2 5 1,1,1,2-tetrachloroethane ,1,1,2-TeA ,1,2,2-tetrachloroethane ,1,2,2-TeA ,1,2-trichloroethane ,1,2-TA ,1,1-trichloroethane methyl chloroform 1,1,1-TA ,2-dichloroethane ,2-DA ,1-dichloroethane ,1-DA chloroethane ---- A 2 5 hlorinated Ethenes tetrachloroethene perchloroethene PE 2 4 trichloroethene ---- TE 2 3 cis-1,2-dichloroethene cis-dichloroethene cis-de trans-1,2-dichloroethene trans-dichloroethene trans-de ,1-dichloroethene vinylidene chloride 1,1-DE chloroethene vinyl chloride V 2 3 Additional nomenclature is necessary in order to distinguish the possible isomers of dichloroethene. As with 1,1,1-TA and 1,1,2-TA, dichloroethene (DE) can exist as either of two structural isomers (1,1-DE and 1,2-DE). In addition, the p-bond system in chlorinated ethenes differs from the single carbon-carbon bond in chlorinated ethanes because it does not allow the halogen substituents to rotate freely in the plane perpendicular to the direction of the p-bond. onsequently, there are multiple spatial orientations for the two chloride substituents in 1,2-dichloroethene (Figure 2.2). One possibility is for the chlorine atoms to arrange themselves on the same side of the carbon-carbon double bond in a configuration known as cis. Alternatively, the chlorine atoms can be located on the opposite side of the p-bond system in a configuration known as trans. These two dichloroethenes, which are structurally identical but differ in the spatial arrangement of their chlorine substituents, are called conformational isomers (or simply conformers) (Vollhardt and Schore, 1994).

4 32 D.M. wiertny and M.M. Scherer cis-de trans-de Figure 2.2. onformational isomers of 1,2-dichloroethene. hlorinated methanes, ethanes and ethenes clearly do not encompass all types of chlorinated solvents that may be encountered at hazardous waste sites. For instance, chlorinated propanes, which possess three carbon atoms joined by single bonds, can represent important groundwater pollutants. Some examples of chlorinated propanes include 1,2-dichloropropane, which is regulated in drinking water by the U.S. Environmental Protection Agency (USEPA) (2003). Another example is 1,2,3-trichloropropane, which has been detected at more than 20 National Priorities List sites identified by the USEPA (ATSDR, 1992). Although such species are not the focus of subsequent portions of this chapter, the physical and chemical principles developed for chlorinated methanes, ethanes and ethenes can easily be extended to include these additional chlorinated solvents. Although this chapter is devoted to treatment strategies for chlorinated solvents, solvents with other halogen substituents (such as bromine or fluorine) are also frequently encountered in contaminated groundwater. A common example is 1,2-dibromoethane (also known as ethylene dibromide [EDB]), which was used as an additive in leaded gasoline (Baird and ann, 2005). Methanes, ethanes and ethenes with mixed halogen substituents can represent important environmental pollutants as well, as is the case for common disinfection byproducts bromodichloromethane (Br 2 ) and dibromochloromethane (Br 2 ). When necessary, key differences in the behavior and environmental fate of halogenated solvents with chlorine, bromine and fluorine substituents will be noted. 2.3 PROPERTIES The behavior of chlorinated solvents in the subsurface is controlled to a large extent by their physical and chemical properties. The properties considered most relevant to chlorinated solvent fate and transport in the subsurface are summarized in Table 2.2. In order to maintain some consistency among the values presented, the majority of the values were obtained from Mackay et al. (1993), one of the very few sources that contain data for all of the chlorinated methanes, ethanes and ethenes. In general, there is reasonable agreement between these values and several other summary tables available (e.g., Pankow and herry, 1996; Fetter, 1999; Schwarzenbach et al., 2003; hapter 1 of this volume). Table 2.2 is provided for purposes of discussion with regards to relevant trends in behavior and properties and is not intended as a set of values selected from a critical review of the literature. For a review of the primary literature, Pankow and herry (1996) is recommended because it provides a detailed review of the chlorinated solvent properties discussed herein as well as an excellent discussion of the history of production and industrial uses of chlorinated solvents. The following discussion of chemical and physical properties is organized around the major processes that impact the fate and transport of chlorinated solvents in the subsurface, starting with the process by which pure phase chlorinated solvents dissolve into groundwater, followed by their partitioning between the three phases present in the subsurface: aquifer solids, water and air. An overview linking these partitioning processes to the relevant chlorinated solvent properties is provided in Figure 2.3. The discussion concludes with an introduction to transformation reactions, which are discussed in greater detail in hapters 3 and 4.

5 hlorinated Solvent hemistry: Structures, Nomenclature and Properties 33 Table 2.2. Summary of Some Physical and hemical Properties of hlorinated Organic Solvents at 25 Degrees elsius ( ). Unless otherwise noted, all values have been taken from Mackay et al. (1993). Species Formula Weight (g/mol) arbon Oxidation State a Density (r) (g/ml) Solubility (S) (mg/l) Vapor Pressure (p o ) (torr) enry s Law onstant (K )( 10-3 atm m 3 /mol) Log (K ow ) Log ML c (K oc ) b (mg/l) hlorinated Methanes T IV F III , d DM II , M I ,235 4, NR e hlorinated Ethanes A III f NR PA II NR 1122-TeA I , NR 1112-TeA I , NR 111-TA , TA , DA I , DA I , NR A II , NR hlorinated Ethenes PE II TE I , cis-de , trans-de , DE , V I ,763 2, a Average value calculated using oxidation states for ¼þI and ¼ I. b When available, log(k oc ) values were obtained from Nguyen et al. (2005). c Source: USEPA (2003). d ML for total trihalomethanes, which is defined as the summed concentration of chloroform, bromoform (Br 3 ), bromodichloromethane (Br 2 ), and dibromochloromethane (Br 2 ). e NR ¼ Not regulated. f Reported vapor pressure for solid-phase hexachloroethane. Notes: atm -- atmosphere; g -- gram; K ow -- octanol/water partitioning coefficient; K oc -- soil organic carbon/water partitioning coefficient; L -- liter; ML -- maximum contaminant level; mg -- milligram; ml -- milliliter; mol -- mole. Water p S S, K OW, K O Dissolution Air p Figure 2.3. The three major phases present in the subsurface and the properties of chlorinated solvents that govern the partitioning between these phases. At room temperature (25 degrees elsius [ ]), most chlorinated solvents are colorless liquids with densities (r) greater than that of water (r solvent > 1 gram per liter [g/l]). Soil

6 34 D.M. wiertny and M.M. Scherer hlorinated solvents are typically discharged into the environment as pure organic liquids or as mixtures of several organic liquids. The process through which these organic phases are gradually released into groundwater is referred to as dissolution. For a chlorinated solvent, the extent of dissolution is controlled by the solvent s aqueous solubility (S), defined as the maximum amount of a chlorinated solvent that will partition into water at a given temperature (Lyman, 1982). Also referred to as saturation concentrations (Schwarzenbach et al., 2003), aqueous solubilities are typically reported with units of moles of chlorinated solvent per liter of water (molarity or M) or milligrams of chlorinated solvent per liter of water (mg/l, which is equivalent to parts per million [ppm]). Most chlorinated solvents can be classified as sparingly soluble in water, with aqueous solubilities generally on the order of several tens to hundreds of mg/l (Table 2.2). owever, their aqueous solubilities are high relative to their established USEPA MLs (Pankow and herry, 1996), which contributes to their prominence as groundwater pollutants. Another consequence of their limited solubility is their tendency to occur in the subsurface as a separate liquid phase at the base of an aquifer commonly referred to as dense nonaqueous phase liquid (DNAPL). Table 2.2 reveals the general solubility trend among chlorinated solvents- as the number of chlorine atoms on a compound increases, the aqueous solubility of that species decreases. This inverse relationship illustrates the influence that molecular size (specifically molar volume [orvath et al., 1999]) exerts on the miscibility of a chlorinated solvent in water. Environmental variables also can influence chlorinated solvent solubility. One such variable is temperature, although changes in the solubility of most chlorinated solvents are relatively minor over environmentally relevant temperature ranges (orvath, 1982). Another important variable is salinity; an increased concentration of dissolved salts results in a moderate decrease in chlorinated solvent solubility (Lyman, 1982). The presence of other organic chemicals (referred to as co-solutes) also can increase the saturation concentration of chlorinated solvents in water, behavior that is utilized for the treatment of chlorinated solvents during surfactant-enhanced aquifer remediation (SEAR) (e.g., Pennell et al., 1994; Fountain et al., 1996) Solid-Water Partitioning Partitioning of chlorinated solvents between aquifer solids and water plays an important role in contaminant fate and treatability because it affects the rate of transport in the subsurface. As a class, chlorinated solvents can be considered moderately hydrophobic; although they partition (or sorb) onto aquifer solids, their affinity for such processes is not as great as that for other organic pollutants such as polycyclic aromatic hydrocarbons (PAs) or polychlorinated biphenyls (PBs). A practical measure of a compound s hydrophobicity is the octanol-water partitioning coefficient (K ow ). For a two-phase system containing octanol and water, values of K ow are defined as the equilibrium concentration of the chlorinated solvent in octanol relative to its equilibrium concentration in water (Equation 2.1). K ow ¼ octanol (Eq. 2.1) water For laboratory investigations of hydrophobicity, octanol is chosen as a convenient reference solvent because it is immiscible with water. By definition, large values of K ow correspond to hydrophobic chemicals that are expected to sorb to soils and sediments more readily.

7 hlorinated Solvent hemistry: Structures, Nomenclature and Properties 35 More pertinent for describing processes in the subsurface are values of K oc, which represent a measure of a chemical s equilibrium partitioning between water and the organic carbon fraction of aquifer solids (Equation 2.2). K oc ¼ organic carbon (Eq. 2.2) water Accordingly, a key factor controlling the extent of chlorinated solvent sorption is the organic carbon content of the subsurface material and the dissolved organic matter. Often times, values of K oc can be estimated using linear correlations developed between log(k ow ) and log(k oc ) for a given pollutant class. In Table 2.2, values of both K ow and K oc generally increase as the number of chlorine substituents on a compound increases. These larger values of solid-water partitioning coefficients will result in slower rates of subsurface transport. An inverse relationship between aqueous solubility and K ow (or K oc ) values is also observed in Table 2.2; chemicals with limited aqueous solubilities generally prefer to partition into a phase such as octanol or soil organic matter rather than associate with water Air-Water Partitioning hlorinated solvents are relatively volatile compounds. Accordingly, air-water partitioning is expected to take place when contaminated groundwater comes into contact with air, as is the case in unsaturated subsurface zones (e.g., the vadose zone). In such instances, the equilibrium partitioning between air and water is typically described by enry s Law, which is applicable to dilute solutions of a chlorinated solvent in water. The enry s Law constant, K, relates the equilibrium concentration of the chlorinated solvent in air to its equilibrium concentration in water (Equation 2.3). K ¼ air (Eq. 2.3) water By definition, large K values indicate a chemical s preference to partition from water into air, although additional chemical properties and several environmental factors will also influence the volatility of a species (Thomas, 1982a). In Table 2.2, K values are reported with units of atm m 3 /mol, but K values also are commonly reported with alternative units that depend upon the conventions used to report the chlorinated solvent s concentrations in air and water. Unlike reported values of S, K ow and K oc, the K values presented in Table 2.2 do not reveal any significant trends within or across the different classes of chlorinated solvents Solid-Air Partitioning The last chlorinated solvent partitioning process to consider is that between aquifer solids and air, a topic covered in detail by Thomas (1982b). As with volatilization between air and water, several chemical and environmental factors are at play in solid-air partitioning processes (Thomas, 1982b), but our mechanistic understanding of this process is rather limited. One noteworthy variable is the vapor pressure (p ) of a chlorinated solvent, which represents the maximum attainable concentration of a chlorinated solvent in air (Schwarzenbach et al., 2003). ompounds with high values of p (which has units of torr or atm) tend to partition more

8 36 D.M. wiertny and M.M. Scherer readily between air and sediments (and similarly, between air and water), and empirical relationships have been developed to estimate the rates at which such partitioning processes occur (Thomas, 1982b). Values of p tend to decrease with increasing chlorination, although exceptions to this generalization are frequently observed (e.g., compare the p values for chloroethane and 1,1,2-trichloroethane in Table 2.2) Transformation Reactions Not included in Figure 2.3 is an additional critical pathway that impacts chlorinated solvent fate in groundwater, that of transformation reactions. Rates and products of transformation reactions will depend upon many of the chemical and physical properties discussed above, as well as the average oxidation state of carbon in the chlorinated solvent (Table 2.2). The carbon oxidation state is a measure of the number of electrons associated with the carbon atoms in a chlorinated solvent; this value ranges from I to +IV for the chlorinated solvents listed in Table 2.2. The more negative the oxidation state, the more electrons associated with the carbon atom. A positive oxidation state (e.g., carbon tetrachloride with a +IV) corresponds to a species in a highly oxidized form that is prone to reduction (gaining electrons). On the other hand, chlorinated solvents with more reduced carbon centers, such as vinyl chloride ( oxidation state of I), are more susceptible to being oxidized (losing electrons). From a practical sense, transformation reactions are often classified as either biotic or abiotic. Biotic reactions are typically those that involve microbial processes associated with bacterial metabolism, whereas abiotic reactions are defined as those processes that involve another chemical species. The distinction, however, can become blurred when discussing chemicals such as biological exudates or minerals formed as a direct result of microbial activity or as an indirect result of biological modification of a chemical environment. The classification does, however, provide a convenient organizational structure for discussing the principles of chlorinated solvent remediation, and it has been adopted for use by the authors in hapter 4. hapter 3 discusses microbially driven processes, including cometabolic reductive reactions, oxidative metabolism, and dehalorespiration. hapter 4 describes the important abiotic processes for chlorinated solvents, including sorption, volatilization and transformation reactions such as substitution, elimination, oxidation and reduction. hapter 5 examines the practical challenges for site remediation that result from the properties and behavior of chlorinated solvents. REFERENES ATSDR (Agency for Toxic Substances and Disease Registry) Toxicological profile for 1,2,3-trichloropropane. U.S. Department of ealth and uman Services ATSDR Public ealth Service, Atlanta, GA, USA. Accessed January 11, Baird, ann M Environmental hemistry, 3rd ed. W.. Freeman and ompany, New York, NY, USA. 652 p. Fetter W ontaminant ydrogeology, 2nd ed. Prentice-all, Inc., Upper Saddle River, NJ, USA. 500 p. Fountain J, Starr R, Middleton T, Beikirch M, Taylor, odge D A controlled field test of surfactant-enhanced aquifer remediation. Ground Water 34: orvath AL alogenated ydrocarbons: Solubility-Miscibility with Water. Marcel Dekker, New York, NY, USA. 889 p.

9 hlorinated Solvent hemistry: Structures, Nomenclature and Properties 37 orvath AL, Getzen FW, Maczynska Z IUPA-NIST solubility data series 67. alogenated ethanes and ethenes with water. J Phys hem Ref Data 28: Lyman WJ Solubility in Water. In Lyman WJ, Reehl WF, Rosenblatt D, eds, andbook of hemical Property Estimation Methods: Environmental Behavior of Organic ompounds. McGraw-ill, New York, NY, USA, pp Mackay D, Shiu WY, Ma K Illustrated andbook of Physical-hemical Properties and Environmental Fate for Organic hemicals. Lewis Publishers, helsea, MI, USA. Nguyen T, Goss K, Ball WP Polyparameter linear free energy relationships for estimating the equilibrium partition of organic compounds between water and the natural organic matter in soils and sediments. Environ Sci Technol 39: Pankow JF, herry JA Dense hlorinated Solvents and Other DNAPLs in Groundwater: istory, Behavior, and Remediation. Waterloo Press, Portland, OR, USA. 525 p. Pennell KD, Jin M, Abriola LM, Pope GA Surfactant enhanced remediation of soil columns contaminated by residual tetrachloroethylene. J ontam ydrol 16: Schwarzenbach RP, Gschwend PM, Imboden DM Environmental Organic hemistry. John Wiley & Sons, Inc., oboken, NJ, USA p. Thomas RG. 1982a. Volatilization from Water. In Lyman WJ, Reehl WF, Rosenblatt D, eds, andbook of hemical Property Estimation Methods: Environmental Behavior of Organic ompounds. McGraw-ill, New York, NY, USA, pp Thomas RG. 1982b. Volatilization from Soil. In Lyman WJ, Reehl WF, Rosenblatt D, eds, andbook of hemical Property Estimation Methods: Environmental Behavior of Organic ompounds, McGraw-ill, New York, NY, USA, pp USEPA (U.S. Environmental Protection Agency) Ground Water and Drinking Water. National Primary Drinking Water Standards. EPA-816-F USEPA, Office of Water, Washington, D, USA. Vollhardt KP, Schore NE Organic hemistry. W.. Freeman and ompany, New York, NY, USA p.

10