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Henry's Law Constants and Micellar Partitioning of Volatile Organic

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Henry's Law Constants and Micellar Partitioning of Volatile Organic 38 J. Chem. Eng. Data 2000, 45, 38-47 Henry’s Law Constants and Micellar Partitioning of Volatile Organic Compounds in Surfactant Solutions Leland M. Vane* and Eugene L. Giroux United States Environmental Protection Agency, National Risk Management Research Laboratory, 26 West Martin Luther King Drive, Cincinnati, Ohio 45268 Partitioning of volatile organic compounds (VOCs) into surfactant micelles affects the apparent vapor- liquid equilibrium of VOCs in surfactant solutions. This partitioning will complicate removal of VOCs from surfactant solutions by standard separation processes. Headspace experiments were performed to quantify the effect of four anionic surfactants and one nonionic surfactant on the Henry’s law constants of 1,1,1-trichloroethane, trichloroethylene, toluene, and tetrachloroethylene at temperatures ranging from 30 to 60 °C. Although the Henry’s law constant increased markedly with temperature for all solutions, the amount of VOC in micelles relative to that in the extramicellar region was comparatively insensitive to temperature. The effect of adding sodium chloride and isopropyl alcohol as cosolutes also was evaluated. Significant partitioning of VOCs into micelles was observed, with the micellar partitioning coefficient (tendency to partition from water into micelle) increasing according to the following series: trichloroethane < trichloroethylene < toluene < tetrachloroethylene. The addition of surfactant was capable of reversing the normal sequence observed in Henry’s law constants for these four VOCs. Introduction have long taken advantage of the high Hc values for these compounds. In the engineering field, the most common Vast quantities of organic solvents have been disposed method of dealing with chlorinated solvents dissolved in of in a manner which impacts human health and the groundwater is “pump & treat”swithdrawal of ground- environment. One particularly problematic class of solvents water from a contaminated aquifer usually followed by air consists of chlorinated organic compounds such as trichlo- stripping of the VOCs into the airshed or capture of the roethylene (TCE) and tetrachloroethylene (PCE). These VOCs on activated carbon. To design air-stripping proc- widely used compounds have low water solubilities, low esses, H for each compound must be known. Fortunately, biodegradation potentials, and densities greater than that c over the past decade, significant improvements in the of water (classified as DNAPLss dense nonaqueous phase accuracy and reproducibility of H measurements have liquids). As a result, spilled DNAPLs settle through the c been made, enabling improved design of air-stripping water table until they reach a confining layer where they systems (1-10). Unfortunately, while often referred to as serve as long term sources of contamination. Significant a remediation process, pump & treat installations are more chronic health effects have been associated with a number accurately viewed as plume containment processes, since of the chlorinated solvents resulting in regulated drinking many of these installations will require decades of opera- water concentrations which are orders of magnitude less tion and maintenance before the contaminant source is than the water solubilities of the compounds, generally in exhausted. the micrograms per liter (ppb) range. Consequently, chlo- rinated organic solvents receive a significant amount of In an effort to directly address the source of the con- attention in the environmental arena, from analytical tamination, researchers have been investigating the addi- chemistry through biochemistry to remediation engineer- tion of agents to enhance the solubility of contaminants - ing. (11 22). One such system which recently reached the field Many chlorinated solvents are classified as volatile demonstration stage is surfactant enhanced aquifer re- organic compounds (VOCs), exhibiting high vapor pres- mediation (SEAR). In SEAR processes, surfactants are sures as well as a tendency to participate in photochemical added at concentrations above that required to create reactions. The high vapor pressures, combined with low surfactant self-aggregates (micelles). That is, the concen- water solubilities, yield high vapor-liquid partition coef- trations are higher than the critical micelle concentration (cmc). These micelles act as high-capacity reservoirs where ficients as described by the Henry’s law constant (Hc) for aqueous samples: organic compounds accumulate, thereby creating a high apparent solubility of the organic compounds in the surf- C actant solution. Using hydraulic gradients, the aqueous ) g surfactant solution is made to flow through the subsurface Hc (1) Cl region containing free phase contaminant to a series of withdrawal wells. The high solubilization capacity of the where Cg and Cl are the gas- and liquid-phase concentra- surfactant solution combined with targeting only the tions, respectively. Environmental scientists and engineers DNAPL source zone yields a dramatically smaller volume of stripping solution as compared to the volume of water * Corresponding author. E-mail: [email protected]. generally originating from a pump & treat process. Other 10.1021/je990195u CCC: $19.00 © 2000 American Chemical Society Published on Web 11/18/1999 Journal of Chemical and Engineering Data, Vol. 45, No. 1, 2000 39 Figure 1. Pictorial representation of micellar, extramicellar, and headspace partitioning of VOCs. compounds, such as salts or alcohols, may be added to paper, the equilibrium partitioning in closed systems manipulate the properties of the surfactant solution. (EPICS) headspace method has been used to determine In the aquifer and at extraction wells, the surfactant micellar partitioning of VOCs for several VOCs in the solution will, at times, contain contaminants at concentra- presence of surfactants and cosolutes over a range of tions far exceeding their aqueous solubilities. This ex- temperatures. tracted surfactant solution must subsequently be disposed of, treated for disposal, or treated for reuse. When no Experimental Methods surfactant is present, this system would be equivalent to Materials (the mention of trade names or commercial a standard pump & treat process on a small localized scale products does not constitute an endorsement or recom- and air stripping could be applied as before to remove the mendation for use). Solvents, VOCs, and other reagents VOCs. However, the addition of surfactants may create were purchased from Sigma/Aldrich. The VOCs included unmanageable foaming problems for direct vapor-liquid toluene (CAS# 108-88-3), TCE (CAS# 79-01-6), PCE (CAS# contact technologies such as air stripping, vacuum strip- 127-18-4), and 1,1,1-trichloroethane (TCA, CAS# 71-55-6). ping, or steam stripping. Other separation technologies, As listed in Table 1, five surfactants were studied in this such as pervaporation or liquid-liquid extraction, could be work. Aerosol MA 80-I surfactant (Cytec MA80), sodium applied to avoid foaming. dihexyl sulfosuccinate in a mixture of isopropyl alcohol To predict and assess the performance of processes (IPA) and water, was obtained from the manufacturer, removing VOCs from surfactant solutions, the partitioning Cytec Industries, who indicated that active surfactant behavior of VOCs between surfactant micelles, the extra- constituted 80% by mass of the delivered product. The IPA micellar aqueous phase, and the headspace must be evalu- content, determined by GC analysis, was 5% by mass. ated. In such systems, the “extramicellar” VOC may be only DOWFAX 8390 anionic surfactant was obtained from its a small fraction of the total VOC in the system, as shown manufacturer (Dow Chemical). The manufacturer indicated pictorially in Figure 1. It is this extramicellar VOC which that 35.6% by mass was active surfactant. Sodium dodecyl best defines process efficiency, redefining the vapor-liquid sulfate (SDS) was purchased from Sigma and was 99+% equilibrium relationship in air stripper designs. The total pure. Triton X-100 (ChemCentral), the only nonionic surf- amount of VOC accumulated in micelles will also play an actant included in this study, was also 99+% pure. A fourth important role in process design. While a significant anionic surfactant, Isalchem 145 4PO ether sulfate (IC145) number of literature references are available for ascertain- (also referred to as Alfoterra 145 by the manufacturer ing the Hc of VOCs in water, only limited information is CONDEA Vista) was 34.5 mass % active. Of these, two available for the partitioning of VOCs in surfactant solu- have been extensively studied in the surfactant literature tions (23-28). Methods for determining this partitioning (Triton X-100 and SDS) and three have been employed in include vapor pressure and headspace measurements, pilot or field demonstrations of SEAR processes (Cytec which are referenced to systems without surfactant. In this MA80, DOWFAX 8390, and IC145). 40 Journal of Chemical and Engineering Data, Vol. 45, No. 1, 2000 Table 1. Surfactant Properties ionic cmc MW surfactant trade name form chemical name (g/L) (g/mol) Aerosol MA 80-I anionic sodium dihexyl sulfosuccinate 7.1a 388 DOWFAX 8390 anionic disodium hexadecyldiphenyloxide disulfonate 0.426b 643 Triton X-100 nonionic t-octylphenoxypolyethoxyethanol 0.112c 628 SDS anionic sodium dodecyl sulfate 2.36d 288 e Isalchem 145 4PO ether sulfate anionic sodium (C14 or 15 branched alcohol) (propylene oxide)4 ether sulfate 0.0087 553 a Calculated from vendor product literature; cmc
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