An Analytical Method to Determine Henry’s Law Constant For Selected Volatile Organic Compounds At Concentrations And Temperatures Corresponding To Tap Water Use

Henry’s law constants (H) are needed to model human exposure to Volatile Organic Compounds (VOCs) in Indoor air resulting from the use of tap water. This paper presents an experimental method to determine Hs for several common tap water pollutants at concentrations and temperatures used in household water. For 5 VOCs Henry’s law constants were obtained simultaneously over the 25° C to 45° C temperature range, providing data on H beyond the currently available data (up to 35° C). Henry’s law constants were obtained as the ratio of equilibrium concentrations of VOCs in air and water, using simultaneous sampling from sealed bottles kept at constant temperatures. Air and water samples were concentrated by a purge-and-trap method, thermally desorbed from a Tenax trap, and analyzed with a gas chromatograph with an electron capture detector (GC-ECD). Experimental results agreed well with available literature data.     

A review of Henry's law coefficients for chlorine-containing C1 and C2 hydrocarbons

Experimentally determined Henry's law coefficients of 18 chlorinated C(1) and C(2) hydrocarbons reported in the literature as a function of temperature and at the single temperatures 20 and 25 degrees C were compiled and converted to common units of concentration and pressure: K(H) (moldm(-3)atm(-1)). The individual values are plotted in the ln(K(H)) versus reciprocal absolute temperature coordinate frame, data not in harmony with others were deleted, and the resulting data sets treated by linear regression analysis to derive averaged parameters in the general equation ln(K(H))=A+B/T. The quality of the evaluation was further checked by comparison of values calculated from the resulting parameter values with averages obtained from the direct measurements at 20 degrees C. Good agreement was observed for 15 compounds, larger discrepancies arise only for chloroethane, 1,2-dichloroethane and hexachloroethane. In all three cases the data base is poor and needs to be improved. The results are used to derive heats of solution for the C(1) and C(2) chlorinated hydrocarbons in water, Gibbs energies of solution and standard Henry's law coefficients at 298.15K. Henry's law coefficients calculated from the ratio of solubility of the compound in water and the saturation vapor pressure of the pure compound reported by Sangster [Sangster, J.M., 2003. Henry's law constants for compounds stable in water. In: Fogg, P.G.T., Sangster, J.M. (Eds.), Chemicals in the Atmosphere - Solubility, Sources and Reactivity. Wiley, Chichester, West Sussex, England, pp. 255-397] provide good agreement with the experimental data in eight out of eleven cases treated.     

Aqueous solubility, Henry's law constants and air/water partition coefficients of n-octane and two halogenated octanes

New data on the aqueous solubility of n-octane, 1-chlorooctane and 1-bromooctane are reported between 1 degree C and 45 degrees C. Henry's law constants, K(H), and air/water partition coefficients, K(AW), were calculated by associating the measured solubility values to vapor pressures taken from literature. The mole fraction aqueous solubility varies between (1.13-1.60)x10(-7) for n-octane with a minimum at approximately 23 degrees C, (3.99-5.07)x10(-7) for 1-chlorooctane increasing monotonically with temperature and (1.60-3.44)x10(-7) for 1-bromooctane with a minimum near 18 degrees C. The calculated air-water partition coefficients increase with temperature and are two orders of magnitude lower for the halogenated derivatives compared to octane. The precision of the results, taken as the average absolute deviations of the aqueous solubility, the Henry's law constants, or the air/water partition coefficients, from appropriate smoothing equations as a function of temperature is of 3% for n-octane and of 2% and 4% for 1-chlorooctane and 1-bromooctane, respectively. A new apparatus based on the dynamic saturation column method was used for the solubility measurements. Test measurements with n-octane indicated the capability of measuring solubilities between 10(-6) and 10(-10) in mole fraction, with an estimated accuracy better than +/-10%. A thorough thermodynamic analysis of converting measured data to air/water partition coefficients is presented.     

Cyclodextrins: a new efficient absorbent to treat waste gas streams

Volatile Organic Compounds (VOCs) in the air provoke health and environmental concerns. This paper focuses on the absorption method to treat industrial polluted air loaded with VOCs. The key variable of this treatment being the choice of a suitable liquid absorbent, the aim of this research work is to investigate the effectiveness and the regeneration of a new potential family of absorbent: cyclodextrins (CDs). All CDs derivatives tested are able to decrease the Henry's law constant of toluene: a reduction of volatility up to 95% may be obtained, depending on CD nature and concentration. Moreover, absorption experiments show that beta-CD, which presents the highest absorption ability, is 250 time more efficient than water. The absorption efficiency is not totally correlated with static experiments, suggesting that, in addition to Henry's law constants and inclusion compounds stability, toluene diffusion into such solutions has to be taken into account. It is also to be noted that salt and pH variations seem to have little influence on the absorption capacity of CDs, which may be of great interest for industrial applications. Finally, since production of solid compounds was not observed during these experiments and since temperature decreases the capture ability in a drastic way, regeneration of the washing solution can be achieved by heating the solution in combination with air stripping.     

The transfer of trichloroethylene (TCE) from a shower to indoor air: experimental measurements and their implications

Experiments were performed to measure the transfer of trichloroethylene (TCE), a volatile organic compound (VOC), from tap water in showers to indoor air. In these experiments, the loss of TCE from tap water in the shower is based on the difference between influent and effluent concentrations. We have developed and previously published a three-compartment model, which we use to simulate the 24-h concentration history of VOCs in the shower, bathroom, and remaining household volumes resulting from the use of contaminated tap water. An important input to this model is the transfer efficiency of the VOC from water to air. The experiments reveal that the transfer efficiency of TCE from shower water to air has an arithmetic mean value of 61 percent and an arithmetic standard deviation of 9 percent. Analysis of the results shows that there is no statistically significant difference between the transfer efficiency measured with hot (37 degrees C) or cold (22 degrees C) shower water and that there is no statistically significant change in transfer efficiency with time during a 20-min shower. The implications for exposure assessment are considered.     

The transfer of trichloroethylene (TCE) from a shower to indoor air: experimental measurements and their implications

Experiments were performed to measure the transfer of trichloroethylene (TCE), a volatile organic compound (VOC), from tap water in showers to indoor air. In these experiments, the loss of TCE from tap water in the shower is based on the difference between influent and effluent concentrations. We have developed and previously published a three-compartment model, which we use to simulate the 24-h concentration history of VOCs in the shower, bathroom, and remaining household volumes resulting from the use of contaminated tap water. An important input to this model is the transfer efficiency of the VOC from water to air. The experiments reveal that the transfer efficiency of TCE from shower water to air has an arithmetic mean value of 61 percent and an arithmetic standard deviation of 9 percent. Analysis of the results shows that there is no statistically significant difference between the transfer efficiency measured with hot (37 degrees C) or cold (22 degrees C) shower water and that there is no statistically significant change in transfer efficiency with time during a 20-min shower. The implications for exposure assessment are considered.     

Determination of Henry's law constant for methyl tert-butyl ether (MTBE) at groundwater temperatures

The dimensionless Henry's law constant was determined for methyl tert-butyl ether (MTBE) at six temperatures (3, 5, 10, 15, 20 and 25 degrees C) by using a thermostatted flask (430 ml) containing an aqueous MTBE solution. The ratio between the gas phase and the water phase in the flask was approximately 1.7:1. The aim of this study was to acquire data needed to model the behaviour of MTBE at groundwater conditions. The dimensionless Henry's law constant at 10 degrees C is approximately 0.01 but is 0.03 at 25 degrees C. This is important for modelling MTBE because the variation of vaporization cannot be disregarded at groundwater temperatures. In a second experiment the water solubility of MTBE was determined to be 62.1 g/l at 5 degrees C and 35.5 g/l at 20 degrees C). The high solubility at low temperatures could cause MTBE plumes from spills (fuel accidents etc.) to spread rapidly.     

Bio-oxidation of airborne volatile organic compounds in an activated sludge aeration tank

An activated sludge aeration tank (40 x 40 x 300 cm, width x length x height) with a set of 2-mm orifice air spargers was used to treat gas-borne volatile organic compounds (VOCs; toluene, p-xylene, and dichloromethane) in air streams. The effects of liquid depth (Z), aeration intensity (G/A), the overall mass-transfer rate of oxygen in clean water (KLaO2), the Henry's law constant of the tested VOC (H), and the influent gaseous VOC concentration (C0) on the efficiency of removal of VOCs were examined and compared with a literature-cited model. Results show that the measured VOC removal efficiencies and those predicted by the model were comparable at a G/A of 3.75-11.25 m3/m2 hr and C0 of approximately 1000-6000 mg/m3. Experimental data also indicated that the designed gas treatment reactor with KLaO2 = 5-15 hr(-l) could achieve > 85% removal of VOCs with H = 0.24-0.25 at an aerated liquid depth of 1 m and > 95% removal of dichloromethane with H = 0.13 at a 1-m liquid depth.     

Salting-in and salting-out effects of ionic and neutral osmotica on limonene and linalool Henry's law constants and octanol/water partition coefficients

Foliar emission rates of plant-generated volatile monoterpenes depend on monoterpene partitioning between air, aqueous and lipid-phases in the leaves. While Henry's law constants (H pc, equilibrium gas/water partition coefficient) and octanol/water partition coefficients (K OW) for pure water have been previously used to simulate monoterpene emissions from the leaves, aqueous phase in plants is a complex solution of electrolytes and neutral osmotica. We studied the effects of dissociated compounds KCl and glycine and sugars glucose, sorbitol and sucrose with concentrations between 0 and 1M on H pc and K OW values for limonene and linalool. Linalool with ca. 1500-fold lower H(pc) (2.62 Pa m(3)mol(-1) for pure water at 30 degrees C) and ca. 30-fold lower K OW (955 mol mol(-1) for pure water at 25 degrees C) is the more hydrophilic compound of the two monoterpenes. H pc of both monoterpenes increased with increasing concentration of both ionic compounds and sorbitol, but decreased with increasing glucose and sucrose concentrations. The salting-out coefficients for H pc (kH) were ca. an order of magnitude larger for more hydrophilic compound linalool than for more hydrophobic limonene. For linalool, co-solutes modified H pc by 30-50% at the highest concentration (1M) tested. The effect of temperature on the salting-out coefficient of KCl was minor. As with H pc, K OW increased with increasing the concentration of KCl, glycine and sorbitol, and decreased with increasing glucose and sucrose concentrations. For limonene, co-solutes modified K OW by 20-50% at the highest concentration used. For linalool, the corresponding range was 10-35%. Salting-out coefficients for H pc and K OW were correlated, but the lipid-solubility was more strongly affected than aqueous solubility in the case of limonene. Overall, these data demonstrate physiologically important effects of co-solutes on H pc and K OW for hydrophilic monoterpenes and on K OW for hydrophobic monoterpenes that should be included in current emission models.     

Fate and transport of monoterpenes through soils. Part I. Prediction of temperature dependent soil fate model input-parameters

Monoterpenes are C10H(n)O(n') compounds of natural origin and are potentially environmentally safe substitutes for traditional pesticides. Still, an assessment of their environmental behaviour is required. As a first step in a theoretical study focussing on monoterpenes applied as pesticides to terrestrial environments, soil fate model input-parameters were determined for 20 monoterpenes with widely different structural characteristics. Input-parameters are the water solubility (S(W)), vapour pressure (P), n-octanol-water partition coefficient (K(OW)), atmospheric air and bulk water diffusion coefficients (D(A)air and D(W)water), first order biodegradation rate constants (k), and their temperature dependence. Values for these parameters were estimated or taken from previous experimental work. The quality of the estimations was discussed by focussing on their statistics and by comparison with available experimental data. From these properties, the air-water partition coefficient (K(AW), Henry's Law constant), the interface-water partition coefficient (K(IW)) and the organic matter-water partition coefficient (K(OM)) could be estimated with varying levels of accuracy. In general, little experimental data turned out to be available on biodegradation rate constants and on the temperature dependence of physico-chemical parameters.     

The Transfer of Trichloroethyene (TCE) from a Shower to Indoor Air: Experimental Measurements and Their Implications

Experiments were performed to measure the transfer of trlchloroethylene (TCE), a volatile organic compound (VOC), from tap water In showers to Indoor air. In these experiments, the loss of TCE from tap water in the shower is based on the difference between Influent and effluent concentrations. We have developed and previously published a three-compartment model, which we use to simulate the 24-h concentration history of VOCs in the shower, bathroom, and remaining household volumes resulting from the use of contaminated tap water. An important input to this model is the transfer efficiency of the VOC from water to air. The experiments reveal that the transfer efficiency of TCE from shower water to air has an arithmetic mean value of 61 percent and an arithmetic standard deviation of 9 percent. Analysis of the results shows that there Is no statistically significant difference between the transfer efficiency measured with hot (37°C) or cold (22°C) shower water and that there Is no statistically significant change In transfer efficiency with time during a 20-mln shower. The implications for exposure assessment are considered.     

Effect of substrate Henry's constant on biofilter performance

Butanol, ether, toluene, and hexane, which have Henry's constants ranging from 0.0005 to 53, were used to investigate the effects of substrate solubility or availability on the removal of volatile organic compounds (VOCs) in trickle-bed biofilters. Results from this study suggest that, although removal of a VOC generally increases with a decrease in its Henry's constant, an optimal Henry's constant range for biofiltration may exist. For the treatment of VOCs with high Henry's constant values, such as hexane and toluene, the transfer of VOCs between the vapor and liquid phases or between the vapor phase and the biofilm is a rate-determining step. However, oxygen (O2) transfer may become a rate-limiting step in treating VOCs with low Henry's constants, such as butanol, especially at high organic loadings. The results demonstrated that in a gas-phase aerobic biofilter, nitrate can serve both as a growth-controlling nutrient and as an electron acceptor in a biofilm for the respiration of VOCs with low Henry's constants. Microbial communities within the biofilters were examined using denaturing gradient gel electrophoresis to provide a more complete picture of the effect of O2 limitation and denitrification on biofilter performance.     

Henry's law constants for hexachlorobenzene, p,p'-DDE and components of technical chlordane and estimates of gas exchange for Lake Ontario

The Henry's law constants (HLC) for trans- and cis-chlordane (TC, CC), trans-nonachlor (TN), hexachlorobenzene (HCB) and p,p'-DDE were determined by the gas-stripping method over a temperature range of 5-35 degrees C. The HLC variation versus temperature (K) was described by logH=m/T+b. Parameters of this equation were (with standard deviations) TC: m=-1524+/-158, b=6.58+/-0.54; CC: m=-1786+/-209, b=7.42+/-0.71; TN m=-2068+/-284, b=8.44+/-0.97; HCB: m=-3013+/-174, b=11.60+/-0.59 and p,p'-DDE: m=-2043+/-240, b=8.37+/-0.82. The HLCs (Pa m3 mol(-1)) at 25 degrees C (298.15 K) were: TC=29; CC=27; TN=32; p,p'-DDE=33 and HCB=35. These HLCs values were used to calculate fugacity ratios from paired air and water data from Lake Ontario, July 1998. The resulting fugacity ratios predict that volatilization was occurring for all compounds during that month.     

Henry's law constants and mass transfer coefficients for methyl bromide and 1,3-dichloropropene applied to Florida sandy field soil

Methyl bromide, a pre-emergent soil fumigant, is scheduled to be phased out in the US by 2005, with exceptions for critical use. Comparison of some of the physical constants related to distribution and retention for methyl bromide (MBr) to other fumigants yields a useful quantification of possible alternatives. In this study, the atmospheric and subsurface dissipation of methyl bromide as well as (Z)- and (E)-1,3-dichloropropene (1,3-D) isomers in Telone II were examined. The Henry's law constants of the three chemicals at soil temperature and their mass transfer coefficients for movement through an agricultural mulch of UV-resistant, high-density polyethylene (PE) were evaluated using field data. At the soil temperature of 16.4 degrees C, calculated Henry's law constant gave a fumigant ranking of MBr (0.21)>(Z)-1,3-D (0.041)>(E)-1,3-D (0.027). Since rapid subsurface distribution of a fumigant is highly dependent on the amount in the gas phase, the greater value for Henry's law constant implies faster distribution throughout the soil. After distribution through the soil, retention of the fumigant becomes imperative. Calculation of the fumigant's mass transfer coefficients through PE from field data gave a ranking of the three chemicals: MBr (1.08 cm/h)<(E)-1,3-D (3.25 cm/h)<(Z)-1,3-D (4.13 cm/h). With mass transfer coefficients of this magnitude, it was concluded that PE film was an inadequate barrier for retaining these fumigants in an agricultural setting.     

Temperature dependence of the infinite dilution activity coefficient and Henry's law constant of polycyclic aromatic hydrocarbons in water

The water solubility of 9,10-dihydroanthracene was experimentally determined between 278.12 and 313.17 K. Determinations were carried out by an experimental procedure developed in our laboratory, which is a modification of the dynamic coupled column liquid chromatographic technique. The uncertainty of the experimental determinations ranged from +/- 0.50% to +/- 3.10%. These data, as well as the water solubility data of other five polycyclic aromatic hydrocarbons (PAHs) previously studied, were used to calculate the temperature dependence of the infinite dilution activity coefficient of 9,10-dihydroanthracene, anthracene, pyrene, 9,10-dihydrophenanthrene, m-terphenyl, and guaiazulene in water. Molar excess enthalpies and entropies at infinite dilution, at 298.15 K, were also derived. The temperature dependence of the infinite dilution activity coefficients was used, together with literature values of the vapor pressures of supercooled liquid PAHs (p(B)(sc)), to estimate their Henry's law constants (HLC). Only HLC for anthracene, pyrene, and 9,10-dihydrophenanthrene were calculated, since no p(B)(sc) data were available in the literature for 9,10-dihydroanthracene, m-terphenyl, and guaiazulene. From the observed temperature dependence of the Henry's law constants the enthalpy and entropy of the phase change from the dissolved phase to the gas phase were also derived for anthracene, pyrene, and 9,10-dihydrophenanthrene.     

Kinetics of catalytic oxidation of benzene, n-hexane, and emission gas from a refinery oil/water separator over a chromium oxide catalyst

With the advances made in the past decade, catalytic incineration of volatile organic compounds (VOCs) has become the technology of choice in a wide range of pollution abatement strategies. In this study, a test was undertaken for the catalytic incineration, over a chromium oxide (Cr2O3) catalyst, of n-hexane, benzene, and an emission air/vapor mixture collected from an oil/water separator of a refinery. Reactions were carried out by controlling the feed stream to constant VOC concentrations and temperatures, in the ranges of 1300-14,700 mg/m3 and 240-400 degrees C, respectively. The destruction efficiency for each of the three VOCs as a function of influent gas temperature and empty bed gas residence time was obtained. Results indicate that n-hexane and the oil vapor with a composition of straight- and branch-chain aliphatic hydrocarbons exhibited similar catalytic incineration effects, while benzene required a higher incineration temperature or longer gas retention time to achieve comparable results. In the range of the VOC concentrations studied, at a given gas residence time, increasing the operating temperature of the catalyst bed increased the destruction efficiency. However, the much higher temperatures required for a destruction efficiency of over 99% may be not cost-effective and are not suggested. A first-order kinetics with respect to VOC concentration and an Arrhenius temperature dependence of the kinetic constant appeared to be an adequate representation for the catalytic oxidation of these volatile organics. Activation energy and kinetic constants were estimated for each of the VOCs. Low-temperature destruction of the target volatile organics could be achieved by using the Cr2O3 catalyst.     

Temperature dependencies of Henry's law constants and octanol/water partition coefficients for key plant volatile monoterpenoids

To model the emission dynamics and changes in fractional composition of monoterpenoids from plant leaves, temperature dependencies of equilibrium coefficients must be known. Henry's law constants (H(pc), Pa m3 mol(-1) and octanol/water partition coefficients (K(OW), mol mol(-1)) were determined for 10 important plant monoterpenes at physiological temperature ranges (25-50 degrees C for H(pc) and 20-50 degrees C for K(OW)). A standard EPICS procedure was established to determine H(pc) and a shake flask method was used for the measurements of K(OW). The enthalpy of volatilization (deltaH(vol)) varied from 18.0 to 44.3 kJ mol(-1) among the monoterpenes, corresponding to a range of temperature-dependent increase in H(pc) between 1.3- and 1.8-fold per 10 degrees C rise in temperature. The enthalpy of water-octanol phase change varied from -11.0 to -23.8 kJ mol(-1), corresponding to a decrease of K(OW) between 1.15- and 1.32-fold per 10 degrees C increase in temperature. Correlations among physico-chemical characteristics of a wide range of monoterpenes were analyzed to seek the ways of derivation of H(pc) and K(OW) values from other monoterpene physico-chemical characteristics. H(pc) was strongly correlated with monoterpene saturated vapor pressure (P(v)), and for lipophilic monoterpenes, deltaH(vol) scaled positively with the enthalpy of vaporization that characterizes the temperature dependence of P(v) Thus, P(v) versus temperature relations may be employed to derive the temperature relations of H(pc) for these monoterpenes. These data collectively indicate that monoterpene differences in H(pc) and K(OW) temperature relations can importantly modify monoterpene emissions from and deposition on plant leaves.     

Comparison of headspace and gas-stripping methods for determining the Henry''s law constant (H) for organic compounds of low to intermediate H

A computer-controlled headspace sampling and gas chromatographic system (HS-GC) was used to measure Henry's Law constant (H) for organic compounds. The HS-GC results, extrapolated to ambient temperature in Clausius-Clapeyron type equations, compared well with values obtained using a gas-stripping method at ambient temperature. The HS-GC method provided the temperature-dependence of H so that it can be calculated at any temperature, which is essential when comparing laboratory results with values of H derived from air/water distributions in the environment.     

Prediction of the temperature dependency of Henry's law constant using poly-parameter linear free energy relationships

The ability to predict the temperature dependence of air/water partitioning is important for the environmental fate modeling of organic pollutants. Here, literature data for the temperature dependence of air/water partitioning of some 200 compounds have been used to derive poly-parameter linear free energy relationships (pp-LFER) for predicting air/water partition coefficients at temperatures between 0 and 45 degrees C. The compounds used for calibrating the pp-LFERs span a range of almost 10 orders of magnitude in the partition constants and they cover a large variety of functional groups. Very good fits (r(2)>0.99) were obtained at all temperatures. Hence, these pp-LFERs should serve as a valuable tool for integrating the temperature dependence of air/water partitioning into fate modeling of organic compounds. In the environment one will quite often encounter a situation where water and air do not have the same temperatures. This situation is shortly discussed in the appendix.     

Henry's law constants of phenol and mononitrophenols in water and aqueous sulfuric acid

Phenols are widely present in the atmosphere and nitration probably in the aerosol phase leads to nitrophenols. Nitration by nitric acid in sulfuric acid can be rapid, but little is known of the process under atmospheric conditions. The Henry's law constants K(H)(dagger) of phenol and 2-, 3- and 4-nitrophenol were all measured by a bubble stripping method as: 2820mol kg(-1) atm(-1) (at 298K), 147mol kg(-1) atm(-1) (at 298K), 1.6x10(4)mol kg(-1)atm(-1) (at 308K) and 2.1x10(4)mol kg(-1) atm(-1) (at 308K), respectively. The Henry's law constant of phenol in sulfuric acid systems is lower by more than a factor of two at 1020mol kg(-1) atm(-1) (at 298K) in 40wt% sulfuric acid, which is in line with salting-out of oxygen-containing aromatic compounds in water-sulfuric acid systems. The Henry's law constants of 2- and 4-nitrophenol behave differently and are almost independent of sulfuric acid concentration. The variation of K(H)(dagger) with temperature (T) described in terms of -dln(K(H)(dagger))/d(1/T) does not to vary with sulfuric acid concentration, suggesting enthalpy of dissolution for phenol is independent of sulfuric acid. The series of Henry's law constants measured here can describe the equilibrium situation for phenols in careful determinations of phase partitioning in the atmosphere.