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Unit 2 Special English in Environmental ChemistryChapter 1 Introductions of Environmental Chemistry TheoriesPassage 1 Contaminant Transport and Transformation in the Environment (1)Because of the potential hazard that exposure to hazardous compounds poses to humans and the environment, the levels of toxic and carcinogenic substances in the environment have become important criteria for evaluating environmental quality. The amount of a material which enters the environment, though, is not always indicative of the amount that will be found there. The concentration of a contaminant at any point in the environment depends on the quantity added and the processes that influence its fate. This can include both transport and transformation mechanisms.Transport processes tend to move materials from one point to involve inter-media exchanges between atmospheric, aquatic, and soil environments, as well as movement within each of these media. Transport may be due to advective, dispersive, or diffusion processes.Associated with transport processes are partitioning processes, which dictate how much of the material will be in the gas, liquid, and solid phases. Partitioning is dependent on the compounds solubility, density, polarity, ionic state, and vapor pressure. Another form of partitioning is bioaccumulation, in which compounds are taken up by living organisms and concentrated in their tissues.Transformation processes within each media chemically alter the contaminants to new compounds that may have lower, equal, or greater toxicity. Transformations may be due to chemical, photochemical, or biological processes. The rates at which chemicals are transformed are critical to an understanding of the seriousness of a particular pollution incident. For example, a small spill of sodium acetate may have minimal environmental impact because it is rapidly biodegraded by bacteria in soil and water, whereas a small spill of creosote may have much greater implications because of its greater persistence in the environment.Many books have been dedicated to the topic of contaminant fate and transport in the environment (Clark, 1997; Hemond, 1994; Knox, 1993; LaGrega, Buckingham, and Evans, 1994; Schnoor, 1997; Thibodeaux, 1996). In the following sections, we will only superficially describe these processes so that the reader will better understand why avoiding introduction of these materials into the environment through pollution prevention techniques may be desirable. In addition, the reader will develop a better understanding of the process of establishing priorities for waste reduction.Before beginning this discussion, though, it is essential to describe how concentrations of materials in the environment are expressed.Contaminant ConcentrationsExcept when a major spill occurs, toxic materials in the environment are usually found in very low, although possibly still harmful, concentrations. Concentrations of materials can be expressed in several ways, depending on whether they are present in air, water, or soil. In most cases, chemical concentrations in aqueous solutions are expressed in terms of mass per unit volume, usually as mg/L (one-thousandth of a gram per liter). When concentrations are very dilute, as in ground-waters, they may be expressed as g/L (one-millionth of a gram per liter). In some cases, aqueous concentrations are expressed as parts per million (ppm) or parts per billion (ppb), a mass/mass designation, and considered equivalent to mg/L or g/L. Technically, this is incorrect because the unit basis is not equivalent. However, if the solvent is water, which has a density of 1.00 g/cm3 at 4 , a 1.0 mg/L solution does essentially equal 1.0 ppm.1.0 ppm=1 mg contaminant/106 mg media1.0 mg/L=1.0 mg contaminant/103 ml solventAssuming the solvent is water, with a density of 1.00 g/cm3 (or 1103 mg/mL),1.0 mg/L=1.0 mg contaminant/ (103 mL solvent) (103 mg/mL) = 1.0 mg contaminant/ (106 mL solvent) =1.0 ppmTherefore, for practical purposes,mg/L=mg/kg=ppmAt other temperatures, the density of water varies from 1.00 g/cm3 and this relationship does not hold. Therefore, it is usually safer to use mg/L units.Contaminant concentrations in soils or sludges are usually expressed on a mass/volume basis. The most common expression is mg contaminant/kg soil, which is equivalent to ppm.Air contaminant concentrations are expressed either on a mass/volume (g contaminant/m3 air) or a volume/volume (ppm) basis, whereppm=1 part contaminant by volume/106 part air by volumePartitioning Processes Compounds in the environment are rarely found in their pure form. Rather, they dissolve and diffuse through media, trying to achieve a minimum concentration difference with the surrounding material. Many factors govern the rate of dispersion of the compound, as described above. Other factors that govern dispersion are based on the tendency of a material to want to be associated with one phase as another. This division between two phases is termed partitioning. Partitioning is dependent on the properties of the compound and media it is contact with. There are many properties that can affect partitioning, but we limit this discussion to solubility, acid-base effects, adsorptive effects, and volatilization effects. 1) Acid-base Ionization. Many acids and bases are used in industry, as are the salts of these acids and bases. These run the gamut from very strong acids and bases, such as sulfuric acid and sodium hydroxide, to very weak ones, such as carbonic acid and sodium sulfide. They may be in the form of gases, liquids, or solids. These materials may be very corrosive and may cause severe personal injuries and environmental damage. As will be seen, the degree of ionization of these materials may play a significant role in their transport and eventual fates. The classical definition of an acid is a compound that yields a hydrogen ion (H+) upon addition to water. A base yields a hydroxyl ion (OH-) upon addition to water. These definitions are fairly simplistic and not totally accurate, but they suffice for our purpose. In the case of a strong acid, the bond between the hydrogen atom and the anionic group is weak so that essentially all of the hydrogen atoms leave the molecules when the acid is placed in water. Ionization is essentially 100 percent complete. The same holds true for strong bases, where all of the base ionizes, liberating hydroxyl ions. Weak acids and bases hold on to their H+ and OH- groups better, and they only partially ionize. For a monoprotic acid (one that contains only one ionizable hydrogen), such as acetic acid, Arrheniuss theory of ionization states that the dissociation ratio can be described as HAc+Ac-H+Ac-/ HAc =Ka=1.7510-5 at 25 Where Ac- is used to denote the acetate ion (CH3COO-), H+ is the concentration of dissociated hydrogen ions in moles, Ac- is the moles of dissociated acetate ions, and HAc is the amount of undissociated acetic acid at equilibrium. The larger the value of, the dissociation constant, the greater is the ionization and stronger is the acid. Diprotic acids act in a similar fashion but have two dissociation constants, one for each ionizing hydrogen atom.Water acts as a weak monoprotic acid. Its dissociation can be depicted as H2OH+OH-H+OH-/ H2O =KaThe dissociation of water is very small, so the term H2O changes almost infinitesimally with repect to the ions. Consequently, it is usually considered a constant and the expression is rewritten as H+OH-=Ka H2O =KW=110-14 at 25Where Kw is the ionization constant of water. The hydrogen ion concentration present can be described in terms of its negative logarithm; this designation is termed the pH of the solution:pH=-log H+ pH=log (1/ H+)In the absence of any other materials besides H2O, and assuming that Kw is 110-14, H+=OH-=110-7. There fore, the pH of pure water is H+OH-=110-14 H+=110-7pH=-log 110-7 =7.0If the hydrogen ion concentration increases because of the addition of acidic materials, the pH will go down; if bases are added, the pH will go up. The pH of a solution, or its hydrogen ion concentration, has a direct bearing on the speciation of many contaminants added to water. For example, assume that a small quantity of pentachlorophenol (PCP) enters a body of water. PCP is an alcohol and as such has the ability to ionize by giving up the hydrogen from its alcohol group:The dissociation reaction can be written as C6Cl5OHC6Cl5O-+H+ pKa=4.74C6Cl5O-H+/ C6Cl5OH =10-4.74Where pKa is the negative logarithm of the ionization constant. Typical ionization constants for weak acids are listed in table 2-1. When the pH of the solution and the pKa are equal, 50 percent of the acid will have donated its ions to the solution and will exist as charged anionic species. Thus at pH 4.74 half of the PCP will be in the un-ionized form and half will be ionized. As the solution pH increases, the ratio will shift to a greater fraction being ionized. In general, a pH one unit above the pKa will result in 90 percent of the material being ionized, while a pH two units higher will cause 99 percent of the weak acid to be ionized. Thus for pentachlorophenol at a pH of 6.74 (slightly acidic), 99 percent of the PCP will be ionized. This can be seen clearly in Fig2-1, where the fraction of un-ionized PCP is plotted against pH. As will be seen later, this has a significant impact on its fate.Table 2-1 ionization constants for selected organic acids and basescompoundEquilibrium equationKapKaAcids Acetic Ammonium Carbonic Hydrocyanic Hydrogen sulfide Phenol2-4-DichlorophenolPentachlorophenol2-NitrophenolBases Ammonia Carbonate Calcium hydroxideCH3COOHH+CH3COO-NH4+H+NH3H2CO3H+HCO3-HCO3-H+CO32-HCNH+CN-H2SH+HS-HS-H+S2-C6H5OHH+C6H5O-C6H3ClOHH+C6H3ClO-C6Cl5OHH+C6Cl5O-C6H4(NO3)OHH+C6H4(NO3)O-NH3+H2ONH4+OH-CO32-+H2OHCO3-+OH-HCO3-+H2OH2CO3+OH-CaOH+Ca2+OH-1.810-55.5610-104.310-7(Ka1)4.710-11(Ka2)4.810-109.110-8(Ka1)1.310-13 (Ka1)1.210-101.410-81.810-56.210-81.810-52.1310-4(Kb1)2.3310-8(Kb2)3.510-2(Kb1)4.749.266.3710.339.327.0412.899.927.854.747.214.743.677.631.46Fig 2-1 typical distribution of pentachlorophenol 2) Solubility. The water solubility of a hazardous material often dictates its fate in the environment. Water solubility is defined as the maximum (or saturation) concentration of a substance that will dissolve in water at a given temperature. Solubility is very important because dissolved and undissolved fractions of a material act quite differently. For example, assume that naphthalene and anthracene, both polyaromatics, are added to water in an amount capable of producing 5 mg/L of each compound. Naphthalene has a water solubility of 32 mg/L, while anthracene has a solubility of only 0.031 mg/L. Essentially all of the naphthalene will dissolve into the water and its fate will largely be that of a dissolved compound, but only a very small fraction of the anthracene should dissolve in the water. The remainder will stay in the free form, sinking to the bottom of the container because of its greater density than water. Its fate will largely be determined by different factors than those for the naphthalene. The undissolved fraction of a material in water will float or sink, depending on its density. Undissolved fractions are called nonaqueous phase liquids (NAPLs). They will sink to the bottom of a container filled with water or to the bottom of a groundwater aquifer (Fig 2-2). Light NAPLs (LNAPLs) will float on the surface of water, whether in a container or in groundwater. In either case, both are very difficult to remove from the water. NAPLs may also volatilize or sorb onto solids at a greater rate than the dissolved fraction. In some cases, the undissolved material may form an emulsion in the water, particularly if any turbulence is imposed in the water. The water solubility of a compound is controlled by a number of factors, especially its size and structure. Table 2-2 summarizes some generalized relationships between a molecules properties and its water solubility. These are only generalizations, though, and there are many exceptions to these rules. Solubilities of selected compounds cited in the text are listed in Appendix B.Fig 2-2 Migration and fate of NAPLs and DNAPLs in the subsurface. (Adapted from La Grega, Buckingham, and Evans, 1994)Table 2-2 factors affecting solubility of organic compoundsAs molecular size increases, solubility decreasesAs molecule polarity increases, solubility increasesAs the number of double or triple bonds increases, solubility decreasesThe order of decreasing solubility is AliphaticAromaticCycloaliphaticAs branching increases, solubility decreasesAs halogenation increases solubility decreasesAs temperature increases, solubility increasesPolar substituents such as carboxyl, amine, alcohol, and nitrite increase the solubility of the base compoundAcid/base salts are more soluble than the undissociated species Materials that are highly polar are more soluble in water. Polar compounds, in general, are compounds that can have a charge upon dissociation. A polar material such as phenol has a water solubility of 82000 mg/L at 20, whereas benzene, which is structurally similar to phenol but without the polar hydroxyl group, has a solubility of 1780 mg/L. The solubilities of materials of similar structure but with varying sizes tend to decrease as the molecular size increases, as shown in Table 2-3.Table 2-3 water solubility of selected normal aliphatics of the same classcompoundMolecular weightWater solubility, mg/LButanePentaneHexaneHeptaneOctaneNonaneDecanedodecane58.1272.1586.18100.20114.23128.26142.29170.3461.039.610.92.01.460.1220.0210.005 Adsorption involves the movement of one material into another, for example the dissolution of oxygen into water. Adsorption involves the condensation and attachment of one material onto the surface of another material, for example, the accumulation of a toxicant on the surface of an activated carbon particle. Here we concentrate on adsorptive properties. Sorption is an equilibrium process in which compounds partition between the liquid (or in some cases gaseous) phase and a solid surface, based on their affinity for the two phases. It is an equilibrium process, so some of the material will be found in each phase. The stronger the affinity for the solid, the less will remain in solution. Sorption occurs when the net sorbent-solute attraction overcomes the solute-solvent attraction. (Note that the solute is the contaminant being sorbed, the sorbent is the solid surface doing the sorbing, and the solvent is the water.) There are three general types of adsorption: physical, chemical, and electrostatic. Physical adsorption is due to the weak forces attraction between molecules, or van der Waalsforces. This type of adsorption is fairly weak and is easily reversible if the contaminant concentration in the liquid phase decreases. It is usually the dominant mechanism for sorption of organics to solid surfaces. Chemical adsorption involves much stronger forces, resulting from bonding interactions between the organic compound and constituents on the solid surface. These bonds are relatively irreversible. Electrostatic adsorption is caused by electrical attraction between the adsorbate and the surface. It is of particular importance for ionic materials such as metals and their salts. Materials of opposite charge are attracted to one another. The greater the charge on the ion, the greater attraction. For example, Al3+ will be attracted to a negatively charged surface much more strongly than will be Na+. For ions of equal charge, smaller ions will be bound more tightly than larger ones because of their higher charge-to-mass ratio. Partitioning of a material between two phases contributes, to a large degree, to the ultimate fate of the material. A contaminant in aqueous solution that is discharged into a river will be partitioned between the water phase and any solid material present (suspended solids, bottom sediments, fish, etc.). And its future movement will be dictated by movement of the solid rather than that of the contaminant will partition between the groundwater and the soil. The movement of a material that has a strong tendency to sorb to solids may be significantly impeded by its rate of volatilization or transformation in the environment. Generally, sorbed materials are less available for volatilization, biodegradation, or photochemical attack than are dissolved species. Sorption effects can also be used to remove a toxic material from water. Many organics have a stronger affinity for the surfaces of activated carbon than they do for the water phase that they are in. by bringing the water into contact with activated carbon, the toxic organic will transfer from the water phase onto the surface of the activated carbon, leaving a relatively contaminant-free water.The tendency for a material to sorb to a surface is dependent on the characteristics of the solid surface, the hydrophobicity of the solid and the sorbent, and the charge on the solid and the sorbent. Since adsorption is a surface phenomenon, the rate and extent of adsorption are related to the surface area of the solid involved. Smaller particles, such as clays, will have a greater tendency to sorb organics than sand particles because of clays greater specific surface area (particle surface area/particle volume). Activated carbon is particularly suited for adsorption