Environ. Scl. Technol. 1984, 18, 890-893
Reduction of Low Molecular Weight Halocarbons in the Vapor Phase above Concentrated Humic Acid Solutions Jill Y. Callaway, Kasl V. Gabblta, and Vincent L. Vilker" Department of Chemical Engineering and National Center for Intermedia Transport Research, University of California at Los Angeles, Los Angeles, California 90024
rn The effect of humic acid on the air-solution distribution
coefficient of the low molecular weight halocarbons (LMHs) chloroform and trichloroethene was determined by using gas chromatographic measurements of vaporphase compositions in closed air-solution equilibrium flasks. Vapor-phase concentrations were measured as a function of time in the headspace above 10 w t % sodium humate solutions dosed with sufficient LMH to give about 0.2-1.0 ppm (volume) of LMH in the vapor. Vapor-liquid equilibrium was achieved in 1h or less in equilibrium flasks which were continuously agitated in a constant temperature bath at 22 OC. The ratio of the distribution coefficients ( g / K ) for the sodium humate solutions (K* = y * / x * ) relative to pure water ( K = y / x ) was found to be 0.39 and 0.79 for chloroform and 0.28 for trichloroethene. The two values for chloroform arise from differences that were observed between humic acids that were pretreated by different methods in order to eliminate volatile contaminants from the commercial "as-received" sodium humate solid particles. This enhanced solubility of LMH in aqueous humic acid solution may be similar to the effect observed by others for enhancement of low molecular weight hydrocarbon solubility in solutions of macroions like proteins and surfactants. There is increasing interest in predicting the fate of synthetic, volatile low molecular weight halocarbons (LMHs) from water bodies and soil-water systems. Both evaporative and degradative (biochemical, chemical, or photochemical) pathways are likely to be governed, in part, by the interactions of the LMHs with colloids made up of inorganic (e.g., finely dispersed clays) and soluble or insoluble organic (e.g., humic acids) fractions. significant reduction of volatilization rates of LMHs from aqueous solutions containing soil colloidal material (1,2)and from soil-water systems (3) has been previously demonstrated in laboratory experiments. Quantitative analysis of volatilization rates from dilute aqueous solutions of solutes like LMHs often proceeds from thermodynamic and mass transfer laws applicable to ideal solutions. The presence of colloids in the aqueous phase can influence the driving force for volatilization by changing the concentration of solute that is free to exchange with the vapor phase. Also, these colloids can affect volatilization rates by increasing the effective resistance to gas-liquid exchange through accumulation as a monolayer at the interface (surfactant effect) or inhibition of convection from bulk liquid to the interface (hydrodynamic effect). The magnitude of these possible effects in natural water bodies or soil-water systems has not been measured or estimated. For example, it is not known to what extent the air-water distribution coefficient for these solutes is changed when this two-phase exchange process occurs in a multiphase system like moist surface soil which is typically 25 vol % air, 25 vol % liquid, and 50 vol % solid. The interactions of clay and organic fractions of the soil matrix with many synthetic agricultural chemicals have been previously reviewed (4-6). Also, increasing soil organic content has been shown to be positively correlated 890
Envlron. Sci. Technol., Vol. 18, No. 11, 1984
with soil retentivity for several nonpolar and slightly polar compounds including chlorinated LMHs (7, 8). Humic acid is often a major constituent of the soil organic fraction and is known to exist in soluble and insoluble forms (9). A few reports have described interactions between soil organic colloids or humic acid and chlorinated hydrocarbons (10-14). These studies were concerned with the extent of uptake of the chlorinated hydrocarbon in aqueous solutions which were dilute with respect to both hydrocarbon (ppm levels) and humic acid (1000 ppm levels). Interactions in solutions that are dilute in LMH but high in humic acid concentration, and resulting effects on airsolution distribution coefficients, have not been reported. We are currently investigating the effect of high concentrations of interactive colloids typically found in soilwater systems on the air-water distribution coefficient for several volatile organic compounds. In this paper, we report results of the changes in vapor-phase concentrations of chloroform (CHCl,) and trichloroethene (TCE), at sub-ppm levels, in a multicomponent system of airwater-sodium humate (NaHu). The results have theoretical importance since they show that the distribution coefficient for air-aqueous sodium humate solutions is significantly changed from the air-pure water coefficient (Henry's constant). This is not surprising in a qualitative sense since (1)octanol-water distribution coefficients for these LMHs are relatively large, KoIw= 96 for CHC1, and 318 for TCE (15),and (2) colloidal electrolytes are known, in a quantitative way, to enhance the solubility of other hydrocarbon gases and vapors (16-18). The results have environmental importance since CHC1, and TCE are among the ubiquitous LMHs that persist in air, soil-water, and groundwater (19).
Materials and Methods Experimental and Analytical Apparatus. A 500-mL Erlenmeyer flask fitted with a 24/40 tapered ground glass stopcock (ungreased) provided a leak proof equilibration vessel from which vapor-phase sample could be withdrawn without disturbing the vapor-liquid distribution of the test LMHs (CHC1, or TCE). The end of the stopcock that opened into the room was fitted with a rubber septum. About 100 mL of the liquid (pure water or NaHu solution) was placed in the flask for each trial. Vapor samples for analysis were withdrawn by using 1-mL graduated disposable plastic syringes (Pharmaseal) fitted with Luerlock joints and 6-in. no. 22 needles. During sampling the stopcock was placed in the open position and the needle inserted through the septum. The stopcock was in the closed position except during sampling. The flasks were immersed in a continuously agitated waterbath at 22 "C nominal temperature. Variations in temperature (h0.5 OC) and pressure (h0.5 mmHg) were noted at each sampling interval during the experimental period. Preliminary experiments were performed to ensure that losses of LMH to glassware or sampling syringes did not affect measurement accuracy. Adsorption to glassware was shown not to be a source of error by checking that the air-solution distribution measurements did not change
0013-936X/84/0918-0890$01.50/0
0 1984 American Chemical Society
when the headspace volume/flask surface area ratio was varied. Adsorption to the syringes we used was shown not to be a source of error by refilling used syringes with room air, setting them aside for varying intervals up to 24 h, and injecting to the GC. No residual LMH in the syringes was ever found by this test. A Varian Model 3700 gas chromatograph (GC) with an automated sampling valve (0.25-mL capacity) and a s3Ni electron capture detector (ECD) were used to measure vapor sample CHC1, and TCE concentrations. The GC output was recorded by a Hewlett-Packard 3390A recorder/integrator. Carrier gas was ultrapure nitrogen (Mallinckrodt), and the flow rate of the carrier gas was 30 mL/min. The column was packed with 3% SP 1000 on chromosorb WHP 60/80 (first 5 cm) and 1% SP 1000 on Carbopack B 60/80 (remaining length) and had dimensions of 8 f t by l/* in. diameter. Oven and column temperatures were maintained at 180 OC; detector temperature was 240 "C, and injection port temperature was ambient. Linearity of the gas chromatograph response in the ranges of 0-0.9 ppm (volume) of CHC1, and 0-1.3 ppm (volume) of TCE was established using LMH-dosed water-air samples. Materials. Distilled-in-glass gas chromatography grade CHC13, TCE, acetone, methanol, methylene chloride, and pentane were used as purchased (Burdick &Jackson). The water was passed through a Barnstead purifying cartridge (no. D8921) to remove traces of chlorine-containing contaminants. Our experiments required accurate measurements of the total amount of LMH added to the closed flasks at the start of an equilibration run. Excessive volatilization losses from the flasks, which had to be open part of the time during which the pure LMH droplets were being added and slowly dissolving into the aqueous phase, did not permit us to achieve required accuracy at the 1ppm level. Therefore, methanol-stabilized dosing solutions were prepared by serial dilution to final concentrations of (2.234 i 0.004) X lo-' mol of CHC13/mL or (1.667 rt 0.007) X lo-' mol of TCE/mL. After addition of the LMH/methanol dosing solution to achieve target starting concentrations of about 1ppm of LMH in the flasks, the aqueous-phase methanol concentrations were only 0.05 vol % We have estimated that this small amount of methanol has an insignificant effect on LMH thermodynamic activity in the aqueous phase and therefore should not influence the air-solution equilibrium distribution determinations. Humic acid was purchased as the sodium salt from Aldrich Chemical Co., Milwaukee, WI. We measured the ash content to be 52% based on loss on ignition test (550 "C for 30 min). NaHu solutions (=lo wt %) were made by dissolving "as-received" or pretreated solid iri purified water. The sodium humate was completely solubilized at this concentration as judged by the absence of sediment or suspended material in the solutions. The solution pH was 9.65 after dissolution. Preliminary experiments involving a solution made from this as-received humate revealed that several volatile compounds were released into the vapor space above these solutions. Two of these compounds gave chromographic responses that corresponded with the retention times for CHC13and TCE. In order to confirm the presence of these contaminants in the NaHu, the LMHs were extracted from the as-received material with methylene chloride and the extract inspected by GC/MS. These analyses were performed with a Finnigan Model 4000 quadrupole mass spectrometer interfaced directly with a Finnigan Model 9610 GC. Similarities in the mass spectra intensities as functions of m / e for LMH-spiked solvent samples and the
.
Table I. Approximate Vapor-Phase CHCl, or TCE Concentrations above NaHu Solutions Before Addition of Dosing Solutions
NaHu preparation
pretreatment method
CHC13 or TCE vapor-phase concentration
NaHu-I
dry NaHu in vacuum 0.04 ppm (volume) of TCE desiccator for 1 h at 45 oc NaHu-I1 solution agitated in no CHC& or TCE detected open flask for 3 h at 35 "C NaHu-I11 pentane extracted 0.5 ppm (volume) of CHCl3, 0.9 ppm (volume) of TCE
NaHu extracts confirmed the presence of chloroform and TCE in the as-received commercial humic acid. We measured the approximate concentrations of these LMHs in the vapor from the bottle containing the as-received solids to be 30-50 pg of CHC13/g of air (7-12 ppm (volume)) and 2-3 pg of TCE/g of air (0.4-0.7 ppm (volume)). Attempts were made to eliminate the contaminants by either (1) evaporation of dry as-received solid NaHu in a vacuum desiccator for 1h at 45 "C, (2) volatilization from the NaHu solution during 3 h of agitation in an open flask at 35 "C, or (3) extraction of dry as-received solids with pentane. In the latter method, 20 g of the as-received material was mixed with 50 mL of pentane and refluxed in a 250-mL flask at 70-75 "C for 1 h. After the mixture was cooled to room temperature, the pentane was evaporated by gentle heating and extracted solid material used without further pretreatment. These three pretreated NaHu samples were used in determination of CHC1, or TCE partitioning by correcting the vapor-phase measurements for the amounts of these materials that were associated with the NaHu in blank (LMH undosed) solutions. Table I is a summary of the three forms of sodium humate which were used and shows approximate concentrations of CHC1, and TCE that were released to the vapor phase above the NaHu solutions before dosing with LMH. The LMH contaminants appear to have been eliminated by the solution agitation pretreatment. Surprisingly, the pentane-extracted material showed an increased release of both CHC1, and TCE. This is probably due to incomplete volatilization of these compounds during pentane evaporation since appreciable amounts of them were found in GC analysis of the postreflex pentane liquid. Experimental Procedures and Data Analysis. The distribution coefficients were determined by comparing the GC response (peak area) for a vapor-phase sample from an air-NaHu solution with the response for a vapor-phase sample for an air-purified water standard. The vaporphase measurements were made as a function of time for solutions that were continuously agitated at nominal temperature and pressure of 22 "C and 1 atm, respectively. Each trial consisted of duplicate LMH-dosed purified water-air standards, duplicate LMH-dosed NaHu solution-air samples, and a single undosed NaHu solution-air blank sample. Within a trial, water bath temperature was monitored to within f O . l "C and controlled to within f0.5 "C. Comparisons of vapor-phase concentrations were made for individual pairs of a NaHu solution sample and a standard for which the controllable parameters (ni,t,total moles of LMH in system; n,, total moles of vapor; nl, total moles of liquid, and time of sampling) were close to equal. The primary advantage gained by this pair analysis technique was minimization of errors in GC peak areas caused Environ. Sci. Technol., Vol. 18, No. 11, 1984
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by electron capture detector drift over periods of several hours at these low LMH concentrations (-0.2 ppm (volume) in vapor samples). Also, this technique served to eliminate the effects of unknown determinant errors (which affect both NuHa solution sample and purified water standards) since the primary result of interest is the ratio of solution sample-to-purified water standard distribution coefficients. The air-NaHu solution distribution coefficient was calculated for each LMH, i = CHCIBor TCE, from the equation Ki* = Yi* -= xi*
ni,t*- yi*n,*
pt
Results and Discussion The ratios of our measured CHC1, or TCE distribution coefficient in NaHu solution systems (K*) to the distribution coefficient in air-purified water systems ( K ) are 892
Environ. Sci. Technol., Voi. 18, No. 11, 1984
h
0
0
" I
1
06
5 "
I1 A
0.2
i
A
i
A
A
1 (A1
0.0
1.0
where yi is the mole fraction of i in the air phase of the paired standard and n,, nl, and ni,tare total moles of vapor, liquid, and the LMH i in the standard. For experiments carried out with solutions made from NaHu samples NaHu-I and NaHu-111, Ai* was taken to be the GC response from the LMH-dosed solution minus the response from the undosed solution blank. The air-purified water distribution coefficient Ki was obtained from the measurements of Leighton and Calo (20); KCHCl = 193 and KTCE = 465 at 22 "C. Each equilibration flask contained typically 430 mL of vapor and 100 mL of liquid from which n,, n,* and nl, nl* were determined by using the ideal gas law and the molar density of water for these dilute systems. mol Typically 1 X lo-' mol of CHC13 (ncHCl,,J or 6 X of TCE (-cE,J were placed in the standard solution or the NaHu solution equilibratiopn flasks. The error for our Ki* determinations was calculated by using measured or estimated values of uncertainties in gas and liquid volume measurements, pressure, temperature, Ki, and GC measurements (Ai or Ai*). The latter includes effects of variability in flask vapor-phase sampling technique, sample GC-delivery technique, and GC-signal integration errors. Contributions to the total error in Ki* associated with uncertainties in volume measurement (=0.5%),temperature, or pressure variability were negligible. The uncertainty in Ki, reported to be &2.8% by Leighton and Calo (20), contributed less than 10% to the total error in Ki*. The uncertainty associated with a single GC measurement was only 1 3 % as determined by multiple measurements of standard samples but accounted for more than 90% of the total error due to comparing two (airwater and air-solution) or three (air-water, air-solution, and undosed air-solution) GC measurements for a single Ki* determination. For this reason, the largest errors are for determinations made with the pentane-extracted NaHu solution where three GC measurements were required and the blank correction (air-undosed solution) was largest.
0
0
6-04i 0
0.8
(1)
where xi* and yi* are mole fractions of i in the solution and air phase, respectively, and nl*, n,*, and ni,t*are the total moles of liquid, vapor, and LMH i in the closed equilibration flask. The air-phase mole fraction of i was determined by comparing the GC peak area (Ai*) with the peak area from the paired standard (Ai). Then
1
0
0.8
100
200
,
300 I
i1 t
1
0.6 Y
:* 0.4
o
0.0 0,2
100 TIME
,
MINUTES 200
300
Figure 1. Ratio of vapor-liquid partition coefficient for CHCl3 (A) or TCE (E) in air-10% NaHu solutions to partition coefficient in air-purified water (K" I K ) as a function of time in a closed well-stirred equilibration flask. Trace levels of volatile organics were removed by pretreating as-received NaHu by agitation of 10 wt % solutions in an open flask (NaHu-I1 preparation (0)in (A)), vacuum desiccation of dry NaHu in (E)), or pentane extraction (NaHu-111, (A) (NaHu-I preparation, (0) in (A) and (E)), Horizontal lines are simple arithmetic averages for each data set, and error bars show typical range of the error for each measurement within a data set.
shown in Figure 1 as a function of time elapsed after the liquid phase was dosed with the LMH. The relative constancy of this ratio with time for each data set suggests that equilibrium of the vapor-liquid partitioning was achieved in less than 1 h in these continuously agitated closed systems. The reduction of the ratio K*/K from unity reflects the extent of the decrease in the LMH vapor-phase concentration over a NaHu solution relative to the vapor-phase concentration over purified water. The data for CHCl, in Figure 1A indicate that the NaHu which was pretreated by pentane extraction reduced the distribution coefficient ratio (average K*/K = 0.39 for preparation NaHu-111) more than did the agitated-solution pretreated NaHu (average K*/K = 0.79 for preparation NaHu-11). The larger error shown for measurements made with the pentane-extracted material is due primarily to the correction for the CHC13response that was measured for the vapor phase above the undosed (blank) NaHu solutions. Figure 1B shows the reduction of the distribution coefficient ratio of TCE. These results do not show significant differences between the reduction in the distribution coefficient ratio measured for the NaHu solutions pretreated by vacuum desiccation (preparation NaHu-I) or by pentane extraction (preparation NaHu-111). The average value for each of these determinations was K*/K = 0.28. The larger error is again associated with measurements on the pentane-extracted material. These reductions in the equilibrium vapor-phase concentrations above NaHu solutions for CHC1, (20-6070
relative to pure water) and for TCE (70% redudion) imply that the driving force for volatilization from soil-water systems will be reduced relative to the volatilization driving force from water bodies. Further progress in developing predictive relationships for the equilibrium partitioning of the LMHs at other NaHu concentrations than those reported here are probably best approached by starting with the equality of the LMH i vapor-phase fugacity and its solution-phase fugacity resulting in the expression (21)
and the hydrophobic regions of the macroion. Such effects may also be relevant for our observed changes in the solubility of chlorinated LMHs in solutions of NaHu macroions. Acknowledgments We thank Ed Ruth of the UCLA Institute of Geophysics and Planetary Physics for assistance in the GC/MS analyses. We thank W. F. Spencer of the University of California, Riverside, for several helpful comments. Registry No. TCE, 79-01-6; CHC13, 67-66-3.
Literature Cited where Pi” is the vapor pressure of pure liquid i at system temperature, P is total pressure, and yi is the infinite dilution activity coefficient. Hi,sof the second equality is an effective Henry’s constant which reflects real solution behavior since Hi,s= Pi”Pyi. Either term in parentheses can be identified with the distribution coefficients Ki or Ki*. The decrease in Ki* relative to Ki which we have observed are due to solution-phase effects reflected through the infinite dilution activity coefficient yi. In general, this activity coefficient will be a function of SOlution composition, NaHu type and concentration, and type and concentration of LMH i solute. Solution composition in our experiments is altered (relative to pure water) principally by the addition of salts which are associated with the sodium humate. The solubility of nonpolar gases in electrolyte solutions is generally reduced relative to the solubility in pure solvent. If all of the 52 w t % ash associated with our NaHu samples was taken to be sodium chloride, the ionic strength of the 10 wt % NaHu solutions would be close to one. Then, the “salting-out” effect of the electrolyte alone (excluding effects of NaHu macroions) can be estimated to give K*/K = 1.3 (22). This is also consistent with a report of decreased chloroform solubility in solutions of copper salts (23). The solubility enhancement we report in Figure 1 is with respect to pure water and therefore suggests that the LMH-NaHu macroion interaction more than compensates for the salting-out effect. The effects of LMH interactions with NaHu macroions in vapor-solution partitioning are probably similar to the effects of other macroions on volatile hydrocarbons such as methane, ethane, propane, and n-butane that have been previously reported (16-18). The solubility of these gases was found to be either enhanced (salting-in effect) or diminished (salting-out effect) in aqueous solutions that were up to 1m in concentration of tetralakylammonium ions, R4N+ (where R = CH3, CzH5,n-C3H7,and n-C4H,) (17). Solubility depended on the particular volatile species, identity of R, R4N+ concentration, and temperature. Enhancement of the solubility of ethane in solutions of sodium alkyl sulfates was found to occur only when the concentration of these surfactants were higher than their critical micelle concentration, and the degree of enhancement depended on surfactant molecule chain length (18). The solubility of these gases has also been found to increase (relative to pure water) in 5 wt % solutions of the proteins bovine serum albumin and hemoglobin, and in 1.8 wt % solutions of sodium lauryl sulfate (16). In these studies, gas solubility was throught to be affected through either changes in the structure of the medium brought about by the presence of the macroion or direct hydrophobic interactions between the nonpolar gas molecules
(1) Mackay, D. In “Aquatic Pollution: Transformation and Biological Effects”; Hutzinger, 0.; Van Lebjveld, 1, H.; Zoeteman, B. C. J., Eds.; Pergamon Press: Oxford, 1978; pp 175-178. (2) Mackay, D.; Shiu, W. Y.; Sutherland, R. P. Environ. Sei. Technol. 1979,13, 333-337. (3) Dilling, W. L.; Tefertiller, N. B.; Kallos, G. J. Environ. Sci. Technol. 1975. 9. 833-838. (4) Bailey, G. W.; White, J. L. J . Agric. Food Chem. 1964,12, 324-332. ( 5 ) Hamaker, J. W.; Thompson, J. M. In “Organic Chemicals in the Soil Environment”; Goring, C. A. L;Hamaker, J. W., Eds.; Marcel Dekker: New York, 1972; Vol. I, Chapter 2. Spencer, W. F.; Farmer, W. J.; Jury, W. A. Environ. Toxicol. Chem. 1982, 1, 17-26. Chiou, C. T.; Peters, L. J.; Freed, V. H. Science (Washington, D.C.) 1979,206, 831-832. Karickhoff, S. W.; Brown, D. S.; Scott, T. A. Water Res. 1979, 13, 241-248. Wershaw, R. L.; Burcar, P. J.; Goldberg, M. C. Environ. Sci. Technol. 1969, 3, 271-273. Porter, L. K.; Beard, W. E. J . Agric. Food Chem. 1968,16, 344-347. Pierce, R. H., Jr.; Olney, C. E.; Felbeck, G. T., Jr. Geochim. Cosmochim. Acta 1974, 38, 1061-1073. Choi, W.-W.; Chen, K. Y. Environ. Sei. Techol. 1976, 10, 782-786. Carter,C. W.; Suffet, I. H. Environ. Sci. Technol. 1982,16, 735-740. Perdue, E. M.; Wolfe, N. L. Environ. Sei. Technol. 1982, 16, 847-852. Babari, T. A.; King, C. J. Environ. Sci. Technol. 1982,16, 624-627. Wishnia, A. Proc. Natl. Acad. Sei. U.S.A. 1962, 48, 2200-2204. Weng-Yang, W.; Hung, J. H. J . Phys. Chem. 1970, 74, 170-180. Bolden, P. L.; Hoskins, J. C.; King, A. D. J . Colloid Interface Sci. 1983, 91, 454-463. National Academy of Sciences “Chloroform, Carbon Tetrachloride, and Other Halomethanes: An Environmental Assessment”; Washington, DC, 1978. Leighton, D. T.; Calo, J. M. J . Chem. Eng. Data 1981,26, 382-385. Prausnitz, J. M. “Molecular Thermodynamics of FluidPhase Equilibria”; Prentice-Ha. New York, 1969; Chapter 6. Charpentier, J.-C. Adv. Chem. Eng. 1981, 1 1, 1-133. Kennish, J.; Roe,D. K. J . Phys. Chem. 1983,87,5158-5166.
Received for review March 12,1984. Accepted June 11,1984. The information in this document has been funded wholly or in part by the U.S. Environmental Protection Agency under Assistance Agreement CR-807864-02 to the National Center for Intermedia Transport Research. The document does not necessarily reflect the views of the Agency, and no official endorsement should be inferred.
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