Water solubility, an important parameter in determining the fate of a compound in the environment, is defined as the maximum amount of a chemical in solution and at equilibrium with excess chemical in pure water at specified ambient conditions (temperature, atmo- spheric pressure, and pH). Water solubility is usually expressed as weight/weight (ppm, ppb) or weight/volume (mg/L). Water solubility is generally not useful for gases, because their solubility in water is measured when the gas above the water is at a partial pressure of 1 atm.
Thus, the solubility of gases does not usually apply to environmental assessment, as their partial pressure of a gas in the environment is extremely low. Temperature and the solution pH may have a significant effect on water solubility, so these variables should be recorded with the water solubility value. If not specified, a pH of 7 and a temperature of 25C are assumed.
In general, highly soluble chemicals are more likely than poorly soluble chemicals to be distributed by the hydrological cycle; desorb from biomass, soils, and sediments; have relatively low bioconcentration potential; be more readily biodegradable by microorgan- isms; be less toxic to aquatic organisms; be less persistent; and be less likely to volatilize from water. Highly water-soluble chemicals are more likely to be transported and distributed by the hydrologic cycle than are relatively water-insoluble chemicals. Water solubility, along with vapor pressure, can be used to predict the extent to which a chemical will volatilize from water into air. Water solubility can also affect possible transformation by hydrolysis, photolysis, oxidation, reduction, and biodegradation in water. Finally, the design of most chemical tests and of many ecological and health tests requires precise knowledge of the water solubility of chemicals.
19.2.2 Dissociation Constant
The significance of the dissociation constant is the relationship between pKaand pH, and the resulting distribution of a substance in the environment. The degree of ionization of a substance at a particular pH will affect its availability to biological organisms; chemical and physical reactivity, and ultimate environmental fate. For example, an ionized molecule will generally have greater water solubility and will be less likely to partition to lipophilic substances than to its nonionized form. Ionic charge will also affect the potential of a molecule to participate in environmental ion-exchange processes such as those found in soil and sludge systems. Knowledge of the pKawill assist in designing appropriate adsorption and ecotoxicity studies, and in accurately interpreting the results from these studies.
TABLE 19.4 Acute Aquatic Toxicity Data and Their Interpretation
LC50 Inference
<1 mg/L (ppm) Very toxic
1–10 mg/L Toxic
10–100 mg/L Harmful
>100 mg/L Not toxic
Water solubility, along with vapor pressure, can be used to predict the extent to which a chemical will volatilize from water into air. Vapor pressure is the force per unit area exerted by a gas that is in equilibrium with its liquid or solid phase at a specific temperature.
Equilibrium vapor pressure can be thought of as the solubility of a chemical in air. The vapor pressure increases with an increase in temperature; thus, values are meaningful only if accompanied by the temperature at which they were measured.
Although the potential volatility of a chemical is related to its inherent vapor pressure, actual volatilization (or vaporization) rates will depend on environmental factors.Volatility is the evaporative loss of a substance to the air from the surface of a liquid or solid.
Volatilization is an important source of material for airborne transport and may lead to the distribution of a chemical over wide areas and into bodies of water (e.g., in rainfall) far from the site of release. Chemicals with relatively low vapor pressures, high adsorptivity onto solids, or high solubility in water are less likely to vaporize and become airborne than chemicals with high vapor pressures or less affinity for solution in water or adsorption to solids and sediments.
In addition, chemicals that are likely to be gases at ambient temperatures and that have low water solubility and low adsorptive tendencies are less likely to transport and persist in soils and water. Such chemicals are less likely to biodegrade or hydrolyze but are prime candidates for photolysis and for involvement in adverse atmospheric effects (such as smog formation and stratospheric alterations). On the other hand, nonvolatile chemicals are less frequently involved in significant atmospheric transport, so concerns regarding them should focus on soils and water.
For chemicals such as pharmaceuticals, there may be some concern about volatility from water. Although many pharmaceuticals are used in their nonvolatile salt form, in water this salt will dissociate and some of the nonionized species will be formed, depending on the compound’s pKavalue. While vapor pressure is a measure of the volatility of a chemical from itself, a more useful measure for environmental fate predictions is the Henry’s law constant.
Volatility is thus assessed through a determination of either Henry’s law constant for water (a volatility limit test) or vapor pressure (from solution or surface) for a volatile compound.
Some chemicals that have very low vapor pressure and low water solubility, such as DDT and polychlorinated biphenyls, volatilize to a significant extent. This phenomenon is considered responsible for the global spread of these materials. Vapor pressure and volatilization half-lives of selected chemicals are shown in Table 19.5.
The volatility of chemicals in aqueous or soil systems is also influenced by the chemical’s rate of movement through water to the water–air interface, the chemical’s water solubility, TABLE 19.5 Vapor Pressure and Volatilization Half-Lives of Selected Chemicals
Compound
Vapor Pressure at 20C (torr)
Volatilization Half-Life (min)
Chloromethane 3700 27
Dichloromethane 362 21
Carbon tetrachloride 90 29
1,1,2-Trichloroethane 19 21
1,1,2,2-Tetrachloroethane 5 56
Hexachloroethane 0.4 45
the chemical’s tendency to sorb to soil and sediments, the amount of soil water; the evaporation rate of soil water; the depth to which chemical is incorporated into soil; and the wick effect that brings water and dissolved chemicals to soil surfaces.
19.2.4 UV/Vis Spectrum
The ultraviolet–visible (UV/Vis) absorption spectrum is a quantitative measure of the ability of a substance to absorb radiation in the electromagnetic spectral region between 290 and 800 nm. It is generally measured with a spectrophotometer and presented as a function of wavelength or wavenumber.
The UV/Vis spectrum of a compound may be used to evaluate a compound’s suscepti- bility to being degraded by light. In general, a compound possessing absorbance maxima above 300 nm may be susceptible to direct photodegradation. The UV/Vis spectrum for a particular compound will not definitively determine the photodegradability potential, and additional photolysis testing would be required to determine the extent and the rate at which a compound will be degraded. Because UV/Vis absorbance maxima may shift with changes in pH for ionizable compounds, the UV/Vis spectra for these materials are determined at different pH levels. A typical UV/Vis spectrum is shown in Figure 19.2.
19.2.5 Partitioning
Knowledge of the distribution or partitioning behavior of a chemical between phases in the environment is essential to evaluating its potential environmental fate. Partition coefficients of various sorts are determined and used to estimate how a chemical may partition into lipids, fats, or organisms; sorb to particulates such as soils, sediments, biomass, and sludges;
partition into air; or distribute among the various environmental compartments.
In addition, they can be used to predict the bioconcentration potential in aquatic and terrestrial organisms and to estimate the amount of sorption to soils, sediments, biomass, and sludges. These processes are major factors in determining the movement of chemicals in the biosphere. The principal distribution coefficients are:
1. The octanol/water distribution (or partition) coefficient—KowandDow 2. The organism/water distribution coefficient—the bioconcentration factor 3. The soil (sediment)/water distribution coefficient—Koc
10000 20000 30000 40000 50000 60000 70000 80000 90000 100000
500 400
300 200
100 0
Wavelength (mmicrons)
Absorbance
0
FIGURE 19.2 Sample UV/visible spectrum.
5. The air/water distribution coefficient—Henry’s law constant(H).
Octanol/Water Distribution Coefficient, Kow Although other systems have been used to measure distribution (or partition) coefficients between a chemical and an organic solvent, such as hexane/water or benzene/water, it has become customary in environmental fate assessment to use then-octanol/water system.n-Octanol is considered to be a good medium for simulating natural fatty substances. Then-octanol/water system is widely used as a reference system, and many data using this system have been reported. The coefficient is designated asP,Pow,K, orKow. The logarithmic values of the coefficient generally range from about 1 to 6 and are designated as logP, logPow, logK, or logKow. LogKowvalues>3 may indicate the propensity of the chemical to absorb or adsorb to sediments and soil and to bioaccumulate in fatty tissue.
The octanol/water distribution coefficient,Dow, is defined as the ratio of the concentration of a chemical in two phases,n-octanol and water, when the phases are in equilibrium with one another and the test chemical is in dilute solution in both phases. The n-octanol/water distribution coefficient is given by dividing the concentration of the compound inn-octanol by the concentration in water.
For ionizable compounds,Dow is usually determined at pH values of 5, 7, and 9. For environmental risk assessments, the value at pH 7 is generally used. LogDowvalues less than 1 indicate that the chemical is unlikely to bioconcentrate or adsorb significantly onto organic particles. Log Dow values equal to or greater than 4 indicate that the chemical may bioconcentrate or adsorb significantly. The logDowvalues for selected chemicals and the way these values are interpreted is shown in Table 19.6.
If the molecular structure of a chemical is known, it is often possible to estimate this parameter. This is because the partition phenomenon exhibits a reasonable additive- constitutive property. That is, the partition coefficient can be considered as an additive function of the partition coefficients of component parts of the molecule, particularly if the components are nonpolar. This has led to the development of models that can be used to estimate the logKow, which in turn can be used in environmental fate models for predicting other physical properties or distribution coefficients. The logKowcan also be estimated from the water solubility of a chemical. For example, one such relationship cited in the U.S. Food and Drug Administration’sEnvironmental Assessment Technical Assistance Handbook9is given by
logKowẳ5:000:67 logS
TABLE 19.6 LogDowof Selected Chemicals and Fate Inferences
Compound LogDow Inference
Amoxycillin 1.56 Will not sorb significantly
Granisetron 0.15 (pH 7) Will not sorb significantly Cimetidine 0.198 (pH 7) Will not sorb significantly
Paroxetine 1.32 (pH 7) Will not sorb significantly
Pranlukast 2.69 (pH 7) Will sorb moderately
Nabumetone 3.13 Will sorb strongly
whereSis the aqueous solubility inmmol/L. Then-octanol/water distribution coefficient is corrected for the ionization of the compound so that only the concentration of the nonionized species is considered. This corrected coefficient is designated asKow, and is given by
KowẳDowð1ỵ10abs pH-pKaị
Log Kow is often represented as log P. For nonionizable chemicals, logPẳlog Dow. For large, ionizable chemicals such as pharmaceuticals, logPdiscounts the potentially significant solubility of the ionized species in the octanol layer. For these molecules, use of logDowis preferred:
. Dow: partitioning from water into octanol, corrected for degree of ionization
. Kow: partitioning from water into octanol, uncorrected for degree of ionization Bioconcentration Factor The tendency of some chemicals to move through the food chain, resulting in higher and higher chemical concentrations in the organisms at each level of the food chain has been termed biomagnification or bioconcentration. From an environmental point of view, this phenomenon becomes important when the acute toxicity of the chemical is low and the physiological effects go unnoticed until the chronic effects become evident. For this reason, prior knowledge of the bioconcentration potential of new or existing chemicals is desirable. The bioconcentration factor (BCF) is given by
BCFẳconcentration in organism concentration in water log BCFẳlog10BCF
BCFs can be estimated from the logKowusing a number of regression equations. One such relationship cited in theEnvironmental Assessment Technical Assistance Handbook9is given by
log BCF0:79 logKow0:40
where BCF is the bioconcentration factor andKowis the octanol/water partition coefficient.
This relationship is most appropriate for relatively inert compounds that do not undergo rapid biotransformation in the body. Degradable chemicals usually have a lower BCF value because elimination is enhanced.
Biomass/Water Partition Coefficient, Kd Since many organic chemicals are treated in wastewater treatment plants, the tendency of the chemical to sorb to the biosolids in such plants is an important factor that needs to be evaluated. The biosolids/water distribution coefficient,Kd, is the ratio of the concentration of a chemical in two phases, biosolids and water, when the phases are in equilibrium with one another and the test chemical is in dilute solution in both phases. Biomass or sludge adsorption studies are generally run at a biomass concentration of 2500 mg/L. This approximates the biomass concentrations found in typical
the compound during treatment:
Fraction adsorbedẳ KdS
ẵ1ỵKdS
whereSis the solids/water ratio (g/mL). In some instances, a value forKdmay be estimated from a sorption/desorption isotherm study using activated sludge (sorption of a compound onto the biomass in sludge). A controlled biomass adsorption study may be conducted, monitoring depletion of parent compound as a function of initial biomass concentration [as measured by total suspended solids (TSS)] and of time. The data are then fit to a Freundlich equation:
logðx=mị ẳlogKỵ ð1=nịlogCe
where
logðx=mị ẳlogarithm of the amount of chemical sorbed per amount of adsorbent at equilibrium logCeẳlogarithm of the amount of chemical in solution at equilibrium
KẳFreundlich adsorption coefficient
nẳa constant describing the degree of nonlinearity of the isothermðwhennẳe1;the F reundlich constantKfcan be used as an adsorption distribution coefficient;Kdị If a plot of log (x/m) vs. logCegives a straight line, the slope of the line is the 1/nlinearity term and the intercept is logKd.
Soil/Water Partition Coefficient, Koc Methodology similar to that described above may be used to determine the soil/water partition coefficient. AKdis determined from isotherms generated using soils or sediments. TheKocmay then be calculated from
Kocẳ ðKd=%organic carbonị 100 or
Kocẳmgchemical dissolved at equilibrium=g solution
In general, the rate of movement of organic chemicals through soil is inversely correlated with sorption. Compounds having aKocvalue of around 1000 (logKocẳ3) are quite tightly bound to organic matter in soil and are considered immobile, whereas those with aKocvalue below 100 (logKocẳ2) are moderately to highly mobile.
Air/Water Distribution Coefficient, H The Henry’s law constantHrepresents the ratio of the equilibrium concentration of a chemical in air to its concentration in water.Hcan be measured directly or estimated from the equilibrium vapor pressure and the water solubility:
Hẳ16:04PM TS
wherePis the equilibrium vapor pressure of pure chemical in mmHg,Mthe gram molecular weight of the chemical,T the temperature in K, andS the solubility in water in mg/L.
Chemicals with values ofHless than107are less volatile than water and would be considered nonvolatile in the environment. Rates of evaporation of organic chemicals from water can be estimated from
K1ẳ 221:1
ð1:042=Hỵ100:0ịM1=2
whereHis the Henry’s Law constant; andMis the molecular weight of the test chemical.K1 can then be used to determine the evaporation half-life (t1/2) in minutes:
t1=2ẳ0:6931ðd=K1ị wheredis the solution depth.
Thus, a chemical such asp,p-DDT, which has a very low vapor pressure (7.16107 torr), also has very low solubility in water (0.0017 mg/L). The combination of these properties leads to a not-insignificant Henry’s law constant and indicates that the chemical is likely to have measurable volatilization from water in the environment. The global dissemination ofp,p-DDT is attributed to this mechanism. Under environmental conditions, the actual volatility rates will be influenced by a number of factors, including the rate of dispersion of the chemical away from the evaporative site by wind; the degree of persistence of the chemical in the environment, that is, the extent to which the chemical will undergo photodegradation; oxidation; hydrolysis; or biodegradation before volatilization.
Example 19.2 A new compound in development has the attributes listed in Table 19.7.
What can you say about the fate of this compound?
Solution From Table 19.1 we can see that the compound has comparatively low water solubility, and together with the hydrolysis data, we might conclude that it would be stable for reasonably long periods of time in water. Together with aDowof 2.9, we would also conclude that the compound is likely to sorb to biomass in a wastewater treatment plant, or to partition to fatty substances in environmental organisms, including humans. However, based on its UV/Vis absorbance, we might want to see if photolysis may play a role as a depletion mechanism.
Additional Points to Ponder If you were a synthetic organic chemist, what might you do to this compound to make it less stable in the environment? What are the drawbacks in doing this?
TABLE 19.7 Compound Attributes
Property Result
Water solubility 0.5 mg/L
UV/Vis absorbance 350 nm and 480 nm
Hydrolysis Stable for 3 months
LogDow 2.9
AND DEPLETION MECHANISMS 19.3.1 Hydrolysis
Hydrolysis is a common reaction occurring in the environment and therefore represents a potentially important degradation and depletion pathway for many classes of compounds.
Hydrolysisrefers to the reaction of an organic chemical (RX) with water:
RXþHOHÐROHþHX
In aqueous systems, rates of hydrolysis usually depend only on the concentration of the organic chemical because water is present in such excess that its concentration does not change during the reaction and thus does not affect the reaction rate. The half-life (t1/2) is defined as
t1=2ẳ0:6931 k
wherekis the rate constant observed. Rates of hydrolysis depend on a number of factors that change seasonally and slowly in the aquatic environment, such as the pH, temperature, and concentration of the chemical. However, hydrolysis rates are independent of many rapidly changing factors that normally affect other degradative processes, such as the amount of sunlight, the presence or absence of microbial populations, and the oxygen supply.
Hydrolysis data are important in the design and interpretation of other environmental fate and effects tests. If a substance is extremely susceptible to hydrolysis, loss of the compound must be taken into account in tests such as aquatic toxicity and photodegradation. While many hydrolysis products are more polar than the parent compound, this is not always the case. Toxicity tests may need to be carried out on hydrolysis products.
19.3.2 Photolysis
Photolysis represents a second potentially important depletion mechanism, but in water it is generally a less common reaction than hydrolysis. Photolysis is a process whereby chemicals are altered directly as a result of irradiation, or indirectly through interaction with products of direct irradiation. Photolysis experiments may be carried out using a single- or multiple- wavelength light source (e.g., a mercury vapor light) or in direct sunlight between April and October. Tests carried out in direct sunlight are preferred over single-wavelength light studies.
Direct photolysis involves the absorption of light in the UV/Vis region by a molecule with a resultant increase in the molecular energy level. The increased energy then trans- forms the molecule chemically into one or more products. Molecules whose UV/Vis spectrum does not show absorption in the UV-Vis region (290 to 800 nm) will not undergo direct photolysis.
Indirect photolysis results from a chemical that may either receive energy for degradation from another chemical that has absorbed sunlight (sensitizer) and functions as a catalyst; or may react with products formed through direct photolysis (reactive species). Indirect photolysis in the aquatic compartment is not well documented. In the atmospheric compartment, the hydroxy radical has been implicated as the most important reactive