Equilibrium Partitioning In Closed Systems Research Articles

Henry’s law constants of chlorinated solvents at elevated temperatures

Henry’s law constants for 12 chlorinated volatile organic compounds (CVOCs) were measured as a function of temperature ranging from 8 to 93 °C, using the modified equilibrium partitioning in closed system (EPICS) method. The chlorinated compounds include tetrachloroethylene, trichloroethylene, cis-1,2-dichloroethylene, vinyl chloride, 1,1,1-trichloroethane, 1,1-dichloroethane, 1,2-dichloroethane, chloroethane, carbon tetrachloride, chloroform, dichloromethane, and chloromethane. The variation in Henry’s constants for these compounds as a function of temperature ranged from around 3-fold (chloroethane) to 30-fold (1,2-dichloroethane). Aqueous solubilities of the pure compounds were measured over the temperature range of 8–75 °C. The temperature dependence of Henry’s constant was predicted using the ratio of pure vapor pressure to aqueous solubility, both of which are functions of temperature. The calculated Henry’s constants are in a reasonable agreement with the measured results. With the improved data on Henry’s law constants at high temperatures measured in this study, it will be possible to more accurately model subsurface remediation processes that operate near the boiling point of water.

Arsenic readily released to pore waters from buried mill tailings

At the McClean Lake Operation in the Athabasca Basin of northern Saskatchewan, the untreated acid raffinate solutions associated with U mill tailings contain up to 700 mg/L dissolved As. To reduce the concentration of As and other contaminants in acid tailing slurries at the JEB mill at McClean Lake, ferric sulfate may be added to the acid raffinates to assure that their molar Fe/As ratio equals or exceeds 3. Tailings slurries are then neutralized with lime to pH 4, and subsequently to pH 7–8. The neutralized tailings contain minerals from the original ore, which are chiefly quartz, illite, kaolinite and chlorite, and precipitated (secondary) minerals that include gypsum, scorodite, annabergite, hydrobasaluminite and ferrihydrite. Most of the As is associated with the secondary arsenate minerals, scorodite and annabergite. However, a few percent is adsorbed and/or co-precipitated, mainly by ferrihydrite. Of major concern to provincial and federal regulators is the risk that significant amounts of As might be released from the tailings to pore waters after their subaqueous disposal in the tailings management facility. A laboratory study was performed to address this issue, measuring readily desorbed As using a method known as equilibrium partitioning in closed systems (EPICS). The EPICS method was selected because it employs a leaching solution that, except for its As concentration, is identical in composition to the neutralized raffinate in contact with the tailings. Laboratory experiments and modeling results demonstrated that the As that could be readily released to pore waters is about 0.2% of the total As in the tailings. Long-term, such releases may contribute no more than a few mg/L of dissolved As to tailings pore waters.

SIFT-MS measurement of VOC distribution coefficients in human blood constituents and urine.

The new technique of selected ion flow tube-mass spectrometry (SIFT-MS) has been applied to the measurement of Henry's Law constants for the volatile organic chemicals o-xylene and trichloroethylene that both have low solubility in aqueous solvents. The method is validated by measurements in water at 298 K using the equilibrium partitioning in closed systems (EPICS) methodology in which the equilibrium headspace concentrations for the volatile organic compounds (VOCs) are measured in two sealed bottles containing different liquid volumes of very dilute solutions of the VOC. The range of solvents is then extended to human body fluids at 309 K including urine, saline, whole blood, red cells in saline, and plasma. The dimensionless distribution coefficients for these solvents vary markedly in the different fluids. For o-xylene they range from k(H) = 0.12-0.15 for water, saline, and urine; 0.53 for red cells in saline; 1.9 for whole blood; to 2.4 for plasma. For trichloroethylene the distribution coefficients range from k(H) = 0.070-0.091 for water, saline, and urine; 0.28 for red cells in saline; 0.35 for plasma; to 0.48 in whole blood. The very different solubilities of organic solvents in body fluids influence the uptake of solvents in workers exposed to VOCs. Some implications of these measurements are briefly discussed.

DETERMINATION OF THE HENRY'S CONSTANT OF VOLATILE AND SEMI-VOLATILE ORGANIC COMPONUDS OF ENVIRONMENTAL CONCERN BY THE BAS (BATCH AIR STRIPPING) TECHNIQUE: A NEW MATHEMATICAL APPROACH

Chemical transfer between environmental compartments plays a key role in an adequate command and control of pollutants. The gas-liquid partitioning equilibrium constant, better known as the Henry's constant (KH) represents a crucial parameter in order to determine the environmental fate of chemicals and therefore, an accurate determination at ambient conditions is extremely important to assess the mentioned process. Within the experimental dynamic methods, the batch air stripping technique (BAS) has as major drawback the equilibrium condition among phases, which is hardly achieved in open natural systems. The present work, based on previously published mathematical models (4, 22), centers in the development of a new mathematical approach to determine KH by means of the BAS in non-equilibrium conditions through experimental and theoretical determinations of volumetric mass transfer coefficients (KLa) for volatile (1,1-DCE, ethylbenzene, p-xylene and toluene) and semivolatile (1,1,2-TCE, 1,2-DCP, penhylmethylether (anisole) and naphthalene) organic compounds of environmental concern. In order to validate the approach, values obtained were compared to the KH determined through the static EPICS (Equilibrium Partitioning in Closed Systems) method, confirming the calculated KH and ratifying the new approach

Gas–solid adsorption of selected volatile organic compounds on titanium dioxide Degussa P25

This paper focuses on the adsorption of gaseous trichloroethylene, toluene and chlorobenzene on the photocatalyst TiO 2 Degussa P25. An optimized EPICS (Equilibrium Partitioning In Closed Systems) methodology was used to study equilibrium partitioning. For the three compounds investigated, equilibrium adsorption was reached within 60 min of incubation. Adsorption isotherms, determined at a temperature ( T) of 298.2 K and relative humidities (RH) of 0.0% and 57.8% were found to be linear ( R 2>0.993, n=5), indicating that no monolayer surface coverage was reached in the concentration interval studied (0.02 mg l −1⩽C g⩽10.45 mg l −1 ). Within the linear part of the isotherm, the influence of both relative humidity and temperature was investigated in a systematic way and discussed from a thermodynamic point of view. Data analysis resulted in a double linear regression ln K=−( ΔU ads R −1) T −1+a RH+d(R 2>0.94,n=13) for 22% ⩽RH⩽90% and 283 K⩽T⩽313 K . The equilibrium adsorption coefficient K( l g −1) represents the equilibrium concentration ratio C solid ( mg g −1)/C g ( mg l −1) and Δ U ads is the internal energy of adsorption ( J mol −1) . At RH=0.0%, experimental K values were a factor 5–10 higher than those expected from the regression equation, indicating that another adsorption mechanism becomes important below monolayer surface coverage of TiO 2 by water vapour molecules. Since surface interactions are of primary importance in photocatalytic reactions, this paper contributes to a better understanding of the basic mechanisms of TiO 2 mediated heterogeneous photocatalysis and is an interesting tool for developing optimized mathematical models.

Comparative study of vapor-liquid phase equilibrium methods to measure partitioning coefficients of dissolved gases in hydrocarbon oils

This work presents a comparative study of three categories of vapor-liquid phase equilibrium methods (VLE methods) for determining the partitioning coefficients (K) of gases (H2, O2, N2, CH4, CO, CO2, C2H2, C2H4, C2H6, C3H6 and C3H8) in gas-oil systems: i) a direct vapor-phase calibration method (VPC), ii) three indirect methods such as equilibrium partitioning in closed system (EPICS), phase ratio variation (PRV) and phase ratio calibration (PRC) and iii) two alternatives of the multiple headspace extraction method (MHE). The methods were primarily tested for their capability of delivering in a single application all the coefficients needed for the external calibration of a static headspace-gas chromatographic procedure used to assess the species in oil samples. The results showed that the VPC approach is the most qualified method in terms of overall precision and accuracy for measuring the coefficients. Excellent results are also noted when the individual species are considered: an RSD better than 2% is measured for all species on three independent trials, except for H2, O2, N2 and CO where a slightly higher level of uncertainty is observed. The results showed also for the PRC method a quality of results very close to what is achieved with the direct method. It has to be noted that these findings do not exclude the possibility that the other VLE methods could deliver better results than seen in this comparison if applied under the optimal species conditions. However, it is almost impossible to meet these conditions in this range of K-values for the gas-oil systems and even if they could have been met, multiple method applications would have to be envisaged. Finally, the gas-oil partitioning coefficients obtained with the VPC method were further validated by demonstrating their dependence with the temperature.

Henry’s law constants derived from equilibrium static cell measurements for dilute organic–water mixtures

The relationship of pressure and composition in the Henry’s law regime has been experimentally measured in an equilibrium static cell for a set of binary organic–water mixtures. The solutes range from hydrophilic materials, such as alcohol to extremely hydrophobic components, such as toluene and 1,2-dichloroethane. The goal of this study is to determine the effective concentration range over which Henry’s law reasonably approximates the gas–liquid partitioning. With the goal of obtaining accurate values of Henry’s law constant, several methodologies are critically compared for the aqueous solutes examined experimentally. The apparatus employed can determine gas–liquid partitioning coefficients through a variety of methods including direct phase concentration ratios, equilibrium partitioning in closed systems (EPICS), and application of the coexistence equation for γ ∞. Results to date indicate a more complex d P/d x behavior in the dilute region than previously assumed; and Henry’s law constant may not strictly apply to hydrophobic materials until the solute concentration is so low that analytical detection is problematic.

Determination of Henry's Law Constants for Organosilicones in Acutal and Simulated Wastewater

The organosilicone compounds hexamethyldisiloxane (HMDS) and decamethylcyclopentasiloxane (D5) are used in consumer products that are disposed of down the drain and may be released to municipal wastewater treatment facilities. In such systems, their fate is determined largely by partitioning between air and water, which is affected by components of the aqueous phase. The Henry's law constant (H) is a commonly used expression for a compound's air−water partitioning behavior. The Equilibrium Partitioning in Closed Systems (EPICS) method of determination of H (dimensionless) was used to assess the fate of HMDS and D5 in wastewater systems. Three chlorinated hydrocarbons and toluene were used to validate the method. The pure water H is measured and reported for D5 and estimated for HMDS, which proved too volatile to be accurately determined using the EPICS technique. The apparent Henry's law constants (Ha) for all compounds were measured in raw wastewater and simulated wastewater solutions of KCl, humic acid, and resuspended secondary treatment sludge solids. A significant difference between pure water H and Ha in actual and simulated wastewater was observed for all compounds tested. In the simulated wastewater, relationships were observed for organosilicone air−water partitioning over ranges of suspended solids and humic acid and were best correlated to liquid-phase organic carbon content. An equilibrium partitioning model of the system is presented and used to estimate an organic carbon partitioning coefficient (Koc) and H for the organosilicones from the relationship between Ha and liquid-phase organic carbon content. Values of Koc were estimated at 24 000 for D5 and 29 000 for HMDS, with corresponding H values of 3.11 and 32.2 for D5 and HMDS, respectively. Observations from these experiments indicated that the components of natural wastewater can affect the volatility of the test chemicals by a significant amount.

A rapid determination method of the air/water partition coefficient and its application

The air/water partition coefficient ( K aw) and Henry's constant ( H) were measured by our own modified EPICS (Equilibrium Partitioning in Closed System) method, which uses two bubble columns. This method gives accurate K aw data much more rapidly than other methods. In order to establish this method, the influence of airflow rate and the volume ratios of water in two columns to partition coefficient were carefully examined. Then we have measured K aw for some n-alkanes, aromatic and chlorinated compounds. The experimental results of our modified EPICS method agreed very well with literature values. Besides the relationships between K aw and molar volume, vapor pressure and water solubility were also analyzed.

Sediment/water and octanol/water equilibrium partitioning of volatile organic compounds: temperature dependence in the 2–25°C range

The sediment/water ( K d) and octanol/water ( K ow) equilibrium partitioning coefficients have been investigated for volatile chlorinated and monocylic aromatic hydrocarbons in the 2 to 25°C temperature range. The equilibrium partitioning in closed systems (EPICS) method has been optimized to measure both equilibrium partitioning coefficients. Sediment/water equilibrium partitioning for a riverine sediment (organic carbon fraction 4.12%) proved to increase with increasing temperature, showing changes in enthalpy between 2.5 and 12.8 kJ·mol −1. A statistical approach showed that for 5 out of 12 compounds investigated this temperature dependence is significant at α=0.05. Next, the temperature dependence of the octanol/water equilibrium partitioning was investigated for 8 compounds. The octanol/water equilibrium partitioning coefficients increased with increasing temperature for all compounds, except for toluene. Changes in enthalpies of this process proved to be between −1.0 and 5.2 kJ·mol −1. However, the temperature dependence of this process was statistically not significant ( α=0.05). The log K d–log K ow relationship showed correlation coefficients between 0.89 and 0.90 at 2.3, 6.2, 18.6 and 25.0°C ( n=8). Slopes of the linear regression increased with increasing temperature. This was explained by the higher temperature dependence of sediment/water equilibrium partitioning process for compounds with higher K d coefficients.

Determination of Henry's law coefficients by combination of the equilibrium partitioning in closed systems and solid-phase microextraction techniques

This paper describes the determination of Henry's law coefficients by means of the EPICS (equilibrium partitioning in closed systems) technique in combination with SPME (solid-phase microextraction). The use of solid-phase microextraction-sampling allowed us to extend the possibilities of the equilibrium partitioning in closed systems technique with respect to the range of Henry's law coefficients which can be measured. Whereas the equilibrium partitioning in closed systems technique is limited to determine air–water equilibrium partitioning of volatile compounds with Henry's law coefficients of at least 0.06 (dimensionless), the current method allowed to measure coefficients between 0.0023 and 13.5. In this way Henry's law coefficients of 20 compounds, being in a range covering five orders of magnitude, were measured with relative standard deviations between 1.0 and 19.8% (mean standard deviation: 5.7%; median of standard deviations: 4.8%, n=99). Several types of compounds were examined i.e. aliphatic hydrocarbons, monocyclic and polycyclic aromatic hydrocarbons, chlorinated and fluorinated compounds, ethers and esters, biphenyl and N-containing compounds, including compounds for which availability of experimental Henry's law coefficients is limited. Measurement of the equilibrium partitioning in the 2 to 25°C range allowed to establish relations of Henry's law coefficient as a function of temperature.

Air-water partitioning coefficients of organics in dilute aqueous solutions

Henry's Law constants were measured for 45 chemicals spanning a wide range of chemical structures and volatilities. A static headspace method, Equilibrium Partitioning in Closed Systems (EPICS), was used to measure Henry's Law constant, and the batch air stripping method was used as a check. Measurements were conducted over a temperature range of 10–30°C, and the data were correlated with a temperature regression equation. An average precision of 10% was obtained for the EPICS runs, and the Henry's constants agreed well with the batch air stripping results and other reported values.