Environ. Sci. Technol. 1991, 25,665-671 Society, 2nd ed.; Sallee, E. M., Ed.; American Oil Chemists' Society: Chicago, IL, 1973. McBain, J. W.; Richards, P. H. Ind. Eng. Chem. 1946, 38, 642. Saito, H.; Shinoda, K. J . Colloid Interface Sei. 1967, 24, 10. Tokiwa, F. J . Phys. Chem. 1968, 72, 1214. Moroi, Y.; Sato, K.; Noma, H.; Matuura, R. J . Phys. Chem. 1982,86, 2463. Carter, C. W.; Suffet, I. H. Enuiron. Sci. Technol. 1982,16, 735. Chiou, C. T.; Kile, D. E.; Brinton, T. I.; Malcolm, R. L.; Leenheer, J. A.; MacCarthy, P. Enuiron. Sci. Technol. 1987, 21, 1231.
(23) Ellis, W. D.; Payne, J. R.; McNabb, G. D. Treatment of Contaminated Soils with Aqueous Surfactants, EPA600/2-85-129; U.S. EPA: Cincinnati, OH, 1985. (24) Nash, J.; Traver, R. P. Field Evaluation of In-Situ Washing of Contaminated Soils with Water/Surfactants; EPA600/9-86/022; U.S. EPA: Cincinnati, OH, 1986. (25) Rickabaugh, J.; Clement, S.; Lewis, R. F. Surfactant Scrubbing of Hazardous Chemicals from Soil; Proceedings, 41st Purdue Industrial Waste Conference; Lewis Publishers: Ann Arbor, MI, 1986; pp 377-382.
Received f o r review July 5, 1990. Revised manuscript received November 2, 1990. Accepted November 7, 1990.
Laboratory Studies of Surfactant-Enhanced Washing of Polychlorinated Biphenyl from Sandy Material Abdul S. Abdul" and Thomas L. Gibson Environmental Science Department, General Motors Research Laboratories, Warren, Michigan 48090-9055
T o assess the suitability of an alcohol ethoxylate surfactant for washing contaminants from soils, the sorption of the surfactant by a sandy soil was studied and the extent of washing of polychlorinated biphenyls (PCBs) from the soil was evaluated. The surfactant adsorption is described by an S-shaped isotherm, consistent with a Langmuir-type monomolecular adsorption followed by adsorption of the surfactant micelles. After only 10 washings with water, the surfactant concentration in the effluent samples decreased from as high as 10000 mg/L to less than 60 mg/L. PCBs could be effectively washed from the sand by using surfactant solutions. After 20 washings about 66, 86, and 56% of the PCBs were washed from the columns by 5000, 10000, and 20 000 mg/L surfactant solutions, respectively. This is equivalent to a reduction in PCBs from 1728 mg/kg to about 614, 251, and 769 mg/kg, respectively. The mechanisms responsible for the PCB removal from the sand are presented and discussed. Introduction
Great effort and expense are currently required for disposal of wastes containing polychlorinated biphenyls (PCBs); a primary source is PCB-laden oil used in transformers and capacitors because of its excellent dielectric and fire-resistant properties. This oil has in some cases contaminated soil and groundwater with PCBs. PCBs are found as mixtures of chlorinated biphenyls having varying degrees of chlorination and are suspect carcinogens. Their use in most industrial applications has been banned since 1979 ( I ) . Because of the strongly hydrophobic nature of PCBs and their very low water solubility, PCBs migrate through soils very slowly; dissolved in water, their rate of migration could be less than a few centimeters per year (2, 3 ) . However, these same properties that limit the spread of PCBs also limit the extent of water washing of PCBs from contaminated soil. Technologies being used to clean up PCB-contaminated soils are costly and include excavation and disposal in a hazardous waste facility or excavation and incineration. More cost-effective methods are needed to clean up sites that are contaminated with PCBs. The feasibility of enhanced washing of PCBs from soils by an aqueous surfactant solution was assessed in this laboratory study. Previous laboratory studies have shown that aqueous solutions of commercially available surfactants washed 0013-936X/91/0925-0665$02.50/0
PCBs, petroleum products, and other organic compounds from sandy geologic material ( 4 , 5). However, several potential problems were identified concerning the use of aqueous surfactant solutions to clean contaminated soil in situ. The surfactant itself should be environmentally safe, in that it should not be toxic or hazardous, and it should be easily removed from the subsurface by anthropogenic or natural processes. Further, because of the surface-active properties of surfactants, they could disperse soil-clay particles; this could lead to clogging of the soil pore space and to the diversion of the surfactant solution from the contaminated zone. Therefore, it is expected that in situ surfactant washing could present difficulties in applying, containing, and recovering the surfactant(s). In a recent study, the effectiveness of each of 10 surfactants in washing an oil from a sandy aquifer material was evaluated (6). The surfactants included a t least one example from each of four main groups of commercial surfactants: (1)ethoxylated alcohols (nonionic), (2) ethoxylated nonylphenols (nonionic), (3) sulfates (anionic), and (4) sulfonates (anionic). From that study, a group of alcohol ethoxylate surfactants was judged to be promising for the in situ washing of petroleum products from hydrogeologic systems, because among the 10 surfactants studied the ethoxylate surfactants caused minimum dispersion of soil colloids, they showed high solubilization and dispersion of the low water solubility oil, they have low critical micelle concentrations, and they washed more than 80% of the oil from sandy material in batch washing studies. This study was carried out to evaluate the effectiveness of one of the previously selected alcohol ethoxylate surfactants to enhance the washing of Aroclor 1248 from a sandy soil. The specific objectives of this laboratory study were (1) to determine the adsorption of an alcohol ethoxylate surfactant on a sandy soil and the extent to which the adsorbed surfactant could be washed from the soil with water and (2) to evaluate the extent to which aqueous solutions of the surfactant could wash Aroclor 1248 (hereafter referred to as PCBs) from the soil. Materials and Methods
Materials. The sandy material used in this study was from the water table region (-6.5 ft deep) of a shallow
0 1991 American Chemical Society
Environ. Sci. Technol., Vol. 25, No. 4, 1991 665
Table I. Some Properties of the Sand Columns
SN70 Adsorption and Desorption Experiments column no.
aqueous SN70 conc, mg/L
mass of sand, g
column length, cm
bulk density (PJ,g/mL
pore vol, mL
1
1250 2500 5000 10000
348 272 348 272
46.0 30.2 46.0 30.2
1.49 1.40 1.49 1.40
93 78 93 78
2 3 4
Aroclor 1248 Decontamination Experiments
PCB conc, mg/L 1728 1728 1728 981 481
column no. 1
2 3 4 5
mass of sand, g
column length. cm
bulk density ( P b h g/mL
pore vol, mL
142 147
6.8 6.8 6.8 7.4 7.4
1.78 1.84 1.90
32 32 32 35 35
151 138
149
Outlet
4
Purge Line
n
1
Sample Line
Inlet
Figure 1. Schematic of column with sandy material.
groundwater system. The sand was air-dried and sieved, using a standard mechanical shaker, to collect the size fraction smaller than 500 pm, which represents the materials in sandy aquifers. This material was slowly packed into columns similar to that shown in Figure 1. To achieve a uniform porous medium, a continuous thin stream of the sandy material was poured into the columns with tapping. The columns were constructed from Pyrex glass tubes, which were fitted with porous end plates, screens, and ports. All connecting tubes, fittings, and stopcocks were made of Teflon. A variable-speed, cassette-type peristaltic pump (Manostat Co., Inc., New York) was used to pump aqueous surfactant solution or deionized water through the columns. Some properties of the packed sand beds in these columns are given in Table I. For the sorption experiments, the longer columns were 2.54-cm i.d. and the shorter columns were 2.86-cm i.d., while the columns used in the PCB washing experiments were 3.86-cm i.d. The average hydraulic conductivity of the sand in the columns was -5 X cm/s and the average organic carbon content of the sand was 0.5%. HPLC grade acetonitrile and water, methanol, and methylene chloride were purchased from EM Industries, Inc., Cherry Hill, NJ. Prepacked reverse-phase SPE (solid-phase extraction) columns used for concentrating 666
Environ. Sci. Technol., Vol. 25, No. 4, 1991
1.59
1.72
the surfactant in aqueous samples were purchased from J. T. Baker Chemical Co., Phillipsburg, NJ. The surfactant used was an alcohol ethoxylate [alkyl poly(oxyethy1ene glycol)], commercially known as Witconol SN70, from Witco Chemical Corp. (Houston, TX). The chemical composition of the surfactant is described by the formula RO(C2H4O).,Hwhere R = ClO-Cl2. Further, the surfactant has an average molecular weight of 392 and a specific gravity (25 "C) of 0.98; the pH for aqueous solutions varies from 6 to 8. The standard Aroclor 1248, manufactured by Monsanto Co., St. Louis, MO, was obtained from Accustandard, Inc., New Haven, CT. Methods. Quantification of Surfactant. A Varian 5020 HPLC equipped with a Model RI-3 refractive index detector and a DS-650 data system was used for measuring the aqueous concentrations of the surfactant. Chromatographic quantification of the surfactant was carried out by injecting aqueous samples (50-200 pL) on a Lichrosorb RP-18 reverse-phase column (EM Industries Inc.). The column was 125-mm length by 4-mm i.d. Elution was carried out by pumping acetonitrile and water (78:22 v/v) isocractically at a flow rate of 1.1mL/min. The peak areas of the chromatograms from the RI detector were integrated electronically and used to calculate the surfactant concentrations in the aqueous samples. For low aqueous concentrations of the surfactant, the samples were first concentrated on SPE octadecyl (C1J disposable columns and then injected into the HPLC. Quantification of PCBs. The concentrations of PCBs in aqueous samples were measured by using a Varian 5020 HPLC, a Zorbax ODS column (25-cm length by 4.6-mm i.d.; Du Pont, Wilmington, DE), and a Kratos Spectroflow 757 UV detector fitted with a 12-pL flow cell of 8-mm path length. Aqueous samples (25 pL) were directly injected on the column and eluted with acetonitrile and water (90:lO v/v). The absorbance of PCBs was measured a t a wavelength of 225 nm, and the concentration of PCBs in aqueous surfactant solutions was determined by comparison of peak areas with those from standard solutions (7).
Experimental Procedures Adsorption Procedure. Surfactant. Batch and column adsorption experiments were conducted. In each batch test, 50 g of sand and 60 mL of aqueous solution having different initial concentrations of the surfactant were equilibrated by shaking for 24 h. Tests were done in duplicates, and at equilibrium, the aqueous concentration of the surfactant was determined. In the column tests, the surfactant dissolved in water was pumped a t -1.0
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Flgure 2. Adsorption of a nonionic alcohol ethoxylate surfactant on sandy material.
mL/min into the sand column. The effluent samples were collected in glass vials and were analyzed during the experiments to determine when adsorption equilibrium (equal influent and effluent concentrations) was reached. A t equilibrium, the pump was turned off momentarily (- 5-10 min) to prepare for the desorption phase of the experiments. Four separate experiments were carried out, one for each of the following aqueous surfactant concentrations: 10 000,5000,2500, and 1250 mg/L. These concentrations were selected to represent the expected range that would be used in the washing of contaminants from soil and aquifer systems. PCBs. Because of the very low water solubility of Aroclor 1248 (-50 ppb) the adsorption procedure used for the surfactant could not be used to contaminate the test soil with high levels of PCBs. Rather, the Aroclor 1248 was first dissolved in methylene chloride, which was added to the sand in a beaker to saturate the sand and pond on the surface. The sand was then stirred for -30 min, using a glass rod to uniformly distribute the PCB within the sand. The methylene chloride was then slowly evaporated in a vented flume cabinet by blowing a continuous stream of warm air across the surface of the sand. The air-dried contaminated sand was packed in vertical columns to simulate the downward flow through the unsaturated zone that would occur during the washing of contaminants from this zone. Five columns were prepared: three had 1728 mg/kg PCBs, and the other two 981 and 481 mg/kg PCBs (Table I). Washing Procedure. Surfactant. In preparation for a washing experiment, the influent solution was changed to deionized water and all tubing on the inlet side of the column was replaced. The washing solution (water) was then pumped at -1.0 mL/min through the column. Aqueous samples were collected in glass vials. Some of these samples were analyzed during the experiment to monitor the rate of washing of the surfactant from the sand. As changes in this rate became very small, the experiments were terminated. PCBs. For these experiments, the aqueous surfactant solutions were pumped at -1.0 mL/min into the top of the vertical columns; the solutions flowed through the contaminated sand and subsequently exited the column at its base. The rest of the procedure is similar to that described above for the surfactant. Three columns, each containing 1728 mg/kg PCB, were washed with 5000, 10 000, or 20 000 mg/L aqueous surfactant solution, while two columns containing 981 and 481 mg/kg PCB were washed with a 10000 mg/L aqueous surfactant solution.
88
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Results and Discussion Adsorption and Desorption of the Surfactant. The results from the batch experiments are plotted in Figure 2 as the mass of surfactant adsorbed per mass of sand (Se, mg/ kg) versus the equilibrium aqueous concentration (Ce, mg/L) of the surfactant. The results are described by an S-shaped isotherm. The isotherm has a linear region for values of C, less than -500 mg/L, a region of decreasing slope of the isotherms for increases in C, up to -4100 mg/L, followed by a region of rapid increases in S, with further increases in C,. The first two regions of the Sshaped isotherm could be described by the Langmuir isotherm (8). Langmuir and S-shaped isotherms were previously observed for the adsorption of nonionic surfactants (9, 10). The Langmuir isotherm is a model for adsorption of a monomolecular layer of the surfactant, and the increase in adsorption beyond the monolayer is attributed to the adsorption of micelles ( I O ) . When the extent of adsorption of the surfactant is controlled by bonding of the hydrophobic group of the surfactant to hydrophobic surfaces, adsorption should increase as the organic carbon content of the soil increases ( 2 , I I ) and as the number of oxyethylene groups in the surfactant molecule decreases (10). Nonionic surfactants can also be adsorbed onto charged surfaces by hydrogen bonding between the oxyethylene groups and the surfaces. In this situation, the extent of adsorption will depend on the pH of the solution (9). Four sets of results from the column experiments are shown in Figure 3. Each set is for the adsorption of the surfactant from an aqueous influent concentration of either 1250, 2500, 5000, or 10000 mg/L (0.125, 0.25, 0.5, and 1.0 volume 70,respectively). The results are presented as the effluent concentration of the surfactant divided by its influent concentration versus the number of pore volumes of the solution flowing through the column. Each data point in Figure 3 represents the measured concentration of the surfactant in a sample volume of -0.25 of the column pore volume (see Table I). As expected, each set of results forms an S-shaped curve that is approximately symmetrical about a line parallel to they axis and drawn through the 0.5 relative concentration Environ. Sci. Technol., Vol. 25, No. 4, 1991
667
m
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Initial Concentration (vol %)
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0
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Figure 4. Percent mass of the surfactant retained by the sand during washing with water.
point on the breakthrough curve. The time before complete breakthrough (when the effluent and influent concentrations are equal and the system has reached equilibrium) and the resultant broadening of the curves were greater for lower influent concentrations of the aqueous surfactant solution. The number of pore volumes of solution needed to reach equilibrium increased from 1.5 to more than 5.0 as the influent surfactant concentration decreased from 1.0 to 0.125% (Figure 3). The mass of surfactant adsorbed on the sand a t equilibrium was determined from the area above the breakthrough curve and the equilibrium breakthrough concentration. The mass of surfactant adsorbed is 252,414, 519, and 997 mg for columns 1-4, respectively. The adsorption capacity S, (for a specific concentration) of the surfactant was calculated by using eq 1, where mi is the total mass
-
s, = [(mi- m,)/m,l/C,
(1)
of surfactant entering the column (g), m, is the total mass of surfactant leaving the column (g), m, is the mass of sand in the column (g), and C, is the equilibrium breakthrough concentration of the surfactant (g/mL). The values for S , are 0.74, 0.48, 0.38, and 0.28 for columns 1-4, respectively. The mass of surfactant lost from aqueous solution by adsorption on the sand was higher for higher influent concentrations of surfactant. In addition, the value of S , decreased from 0.74 to 0.28 as the aqueous concentration of the surfactant increased from 1250 to 10000 mg/L. When the values of S, are plotted on Figure 2 they also follow the trend of the S-shaped isotherm. The adsorbed surfactant was washed from the column sand by pumping deionized water through the column. The results from these desorption experiments are presented in Figure 4 as percent mass of surfactant retained by the sand versus the number of washings. Each data point in Figure 4 was calculated from the surfactant concentration in an aqueous sample. The sample volume was increased as the rate of surfactant removal approached steady state with increased washing. During the first two washings, when the adsorbed surfactant, was rapidly removed from the column, the sample volume required for analysis was as small as a tenth of a pore volume (see Table 668
Environ. Sci. Technol., Vol. 25, No. 4, 1991
I) and increased to more than a pore volume after about five washings. The surfactant was rapidly removed from the columns during the first two washings, after which the percent mass retained curves approached the x axis asymptotically. Further, the rate of removal depended on the initial mass of surfactant adsorbed on the sand. The columns with lower initial concentrations of surfactant showed higher percent mass retained on the sand at each stage of washing. The results indicated that as the equilibrium aqueous surfactant concentration increased by a factor of 8 from columns 1-4 the amount of adsorbed surfactant increased by a factor of -4 (from 251.9 to 997.0 mg), and the percent mass of surfactant retained on the sand after washing decreased by a factor of -2. After 10 washings, only 10.2 (25.2 mg) and 5.270 (51.8 mg) of the initial mass of surfactant remained in the columns, which had breakthrough concentrations of 1250 (0.125%) and 10000 (1.0%) mg/I,, respectively. Thus, the percent mass retained was lower for higher breakthrough concentrations, although the actual mass of surfactant remaining in the column after the same number of washings was higher. For each experiment, the effluent concentration decreased steadily as washing progressed. For 10 washings, the concentration of surfactant in effluent from the column that had the lowest breakthrough concentration decreased from 1250 to 49.2 mg/L, while for the column having the highest breakthrough concentration the decrease was from 10000 to 57.2 mg/L. Although the surfactant is water soluble and most of it could be readily washed from the sand, its complete removal from the soil systems by washing with water may not be practical or cost effective. However, several previous studies have shown that ethoxylate surfactants can be biodegraded. Complete degradation under anaerobic conditions was observed for concentrations up to 1000 mg/L (121, and more than 90% biodegradation was observed under aerobic conditions (13,14). While degradation rates are slower a t lower temperatures, the rates change only slightly among alcohol ethoxylates having less than 55% branching and less than 100 oxyethylene units/mol (13). Toxicity effects of alcohol ethoxylate surfactants on plants and aquatic life have been previously studied (12-14). While there is some evidence of toxic effects from low levels of some alcohol ethoxylate surfactants on aquatic life, the consensus is that the products of degradation are not toxic, at least at low concentrations (14). Nonetheless, because physical, chemical, and biological conditions differ significantly among sites, each site could pose unique problems and limitations; therefore, tests should be carried out with the site material to understand, among other factors, the adsorption, desorption, and biodegradation of the specific surfactant(s) to be used. In a field setting it will be necessary to control the surfactant application to avoid its spread away from the zone of interest and to capture and recover all the leachate and surfactant from the subsurface. With regard to field applications, the higher the concentration of the surfactant in the washing solution, the greater will be its loss to the subsurface material. Also, a greater effort could be needed to remove all the adsorbed surfactant from the system after the site is remediated. Both the surfactant loss and its subsequent recovery could increase cost. On the other hand, the lower the surfactant concentration the larger the number of washings that would be needed to satisfy the adsorption capacity of the soil and to reach equilibrium conditions a t which the surfactant would be most effective. This increase in the
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Figure 5. Washing of Aroclor 1248 from sand columns with 0.5, 1.0, and 2.0 YO (v/v) aqueous surfactant solutions.
Figure 6. Washing of sand columns having different initial concentrations of Arocior 1248 with a 1.0% (v/v) aqueous surfactant solution.
number of washings will also increase cost. Selection of the aqueous concentration of the surfactant in the washing solution will likely be based mainly on the concentration needed to maximize the removal of the contaminant(s) of interest. Decontamination of Aroclor 1248. A main objective of this study was to evaluate the extent to which aqueous solutions of an alcohol ethoxylate surfactant can wash Aroclor 1248 (PCBs) from sandy material. The results presented in Figure 5 are from three column experiments, in which the initially clean sand was contaminated with 1728 mg of Aroclor 1248/kg of sand. Each of the columns was washed with either 2.0, 1.0, or 0.5% (20000, 10 000, or 5000 mg/L, respectively) aqueous surfactant solution to evaluate the extent of PCB washing from the sand. The results presented in Figure 5 are for percent mass of Aroclor 1248 remaining in the sand columns versus the number of washings. Each data point was calculated from the concentration of Aroclor 1248 in an effluent sample of aqueous surfactant solution. The concentration of Aroclor 1248 in the aqueous samples was determined by comparing the total peak area under the chromatogram for the aqueous sample with that for a standard solution. No significant differences between the chromatograms (relative peak heights) were observed when standard Aroclor 1248 and the numerous aqueous samples analyzed for this study were compared. For the same sample the total peak areas were repeatable within 2-3%. The percent mass of Aroclor 1248 remaining in the sand column was rapidly reduced during the first 7-10 washings (one washing equals one pore volume of the sand column), after which the removal rate decreased significantly. For example, washing with the 1.0% aqueous solution removed 76% of the PCBs during the first 10 washings, while only another 13.5% was removed for an additional 18 washings. Of the three surfactant concentrations studied, the 1.0% solution was the most effective in removing the PCBs from the sand. After 20 washings, about 66,86, and 56% of the Aroclor 1248 were removed by the 0.5, 1.0, and 2.0% solutions, respectively. Further, after the same number of
washings the concentration of Aroclor 1248 in the sand decreased from 1728 mg/kg to about 614, 251, and 769 mg/kg by washing with the 0.5, 1.0, and 2.0% solutions, respectively. The 1.0% aqueous solution was just as effective in washing the PCBs from the sand for lower initial concentrations of PCBs. Figure 6 shows results for washing when the initial concentrations of Aroclor 1248 were 481, 981, and 1728 mg/kg. The three sets of results in Figure 6 are similar in that there was initial rapid removal of the PCBs followed by a slower removal rate after about the first eight washings. After 15 washings the values for percent mass PCBs removed were 79, 88, and 82 for the experiments with initial PCB concentrations of 481, 981, and 1728 mg/kg, respectively. The differences in these results may reflect variations in the initial distribution of PCBs in the columns and the effect of the small differences in the bulk densities (Table I) of the sand on flow through the columns. A higher bulk density means closer packing of the sand and more dead-end or isolated pores. These dead-end pores could resist permeation and washing by the surfactant solution. As the curves in Figure 6 are extrapolated to beyond 25 washings the differences in the three sets of results become less significant. The 1.0% aqueous surfactant solution was found to effectively wash Aroclor 1248 from the sandy geologic material. This effective washing of the PCBs from the sand could be explained by the increased solubilization of the PCBs in the surfactant micelles (6,9,15). As the aqueous surfactant concentration is increased from zero, a critical concentration (CMC) is reached a t which the surfactant molecules become arranged into structural units (micelles) having hydrophobic interiors and hydrophilic exteriors. The CMC for the surfactant used in this study is -0.1% (6);therefore, the surfactant concentrations in the washing solutions were greater than the CMC. Adsorption of the hydrophobic PCBs into the hydrophobic interior of the micelles increased the apparent solubility of the PCBs and enhanced their transport through the sand. Another mechanism is the surfactant-enhanced dispersion and suspension of the PCBs from the sand and their transport
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Environ. Sci. Technol., Voi. 25, No. 4, 1991
669
by the bulk solution. This dispersion and suspension is caused by the detergency of the surfactant and its associated low interfacial tension (-30 dyn/cm for the surfactant; -75 dyn/cm for water). Previous laboratory studies have shown significant increases in the solubility of organic compounds in aqueous surfactant solutions, including polyoxyethylene alkylphenyl ethers (16) and petroleum sulfonate surfactants (17). Those studies indicate that the magnitude of solubilization is dependent on the cloud point, temperature, and oxyethylene chain length of the surfactant solution (16). Kile and Chiou (17) concluded that the increase in solubility is similar to that caused by natural humic substances (18, 19) and can be described by the following equation: S,* = S,(l XK,,) (2)
+
where S,* is the apparent solubility in water, S, is the intrinsic water solubility of the solute, X is the concentration of the surfactant, and K,, is the partition coefficient of the solute between the surfactant and water. In a system containing soil, water, oils, and contaminants, mathematical description of the enhancement of the contaminant’s solubility in water becomes very difficult and predicting the extent of washing of the contaminant from the soil system may be highly inaccurate. Edwards and Luthy (20) provides some discussion of these interactions. The improved washing by the 1.0% solution relative to that by the 0.5% solution could be due to the greater detergency of the more concentrated surfactant solution and the expected higher solubilization of the PCBs (9). However, this concept cannot explain the poor washing of the PCBs by the 2.0% solution. The results in Figure 2 show that the adsorption of the surfactant on the sandy material increases rapidly at concentrations above 4000 mg/L. This increased adsorption and the expected increase in the number of micelles a t higher aqueous concentrations may have led to pore clogging, so that smaller pores did not contribute to flow and the PCBs in these pores could not be washed out. The increased adsorption of the surfactant would also change the chemistry of the soil surface and could increase the adsorption of PCBs and reduce their mobility and extent of washing. Note that the 2.0% solution was as effective as the 1.0% solution during the first washing, when its concentration would have been less than 2.0% due to its adsorption on the sand. Also, the pores would become clogged only after about one pore volume of solution passed through. In addition, the higher adsorption of the surfactant on the sand from the 1.0% solution would cause greater narrowing of the pores, which could enhance clogging. Note that only after the fifth washing did the 0.5% solution become more effective than the 2.0% solution in washing the PCBs from the sand. Intermittent washing seems to enhance the removal of the PCBs from the sand. The experiments were not designed to study any effects of intermittent washing; however, the experiments were carried out on two consecutive days, with about five to seven washings on the first day. The results in Figures 5 and 6 show an increase in the slope of the percent mass retained curves within the range of five to seven washings, indicating that the concentration of PCBs in the first washing on the second day was higher than that in the last washing of the first day. This improvement in washing may have been caused by the opening of some previously closed pores between the sand particles as flow was stopped and restarted. Also, PCBs that diffused overnight from closed and unwashed pores into the opened pores may have contributed to the observed increase in PCB concentration in the first washing of the second day. Intermittent washing was previously 670
Environ. Sci. Technol., Vol. 25, No. 4, 1991
proposed to enhance the removal of contaminants from aquifer systems (21).
Conclusion The selection of the alcohol ethoxylate surfactant used in this study was based on its ability to solubilize and disperse oil, its beneficial inability to disperse soil-clay particles, and its being biodegradable. The fact that this surfactant can also effectively wash PCBs from sandy material in laboratory columns makes it a good candidate for in situ soil washing studies. However, actual success in the field will depend on site-specific conditions such as the spatial variation of hydrologic and biologic properties and of material composition. Higher organic carbon content of the soil will cause greater adsorption and associated loss of the surfactant to the soil and more efforts to wash the surfactant out after site remediation. Also, when possible, the optimum surfactant concentration should be determined in laboratory tests using representative soil samples from the site, since the extent of soil dispersion by the surfactant and the resulting clogging of the soil pores will be site specific. Getting the washing solution to permeate the contaminated zone and recovering the leachate, particularly in the unsaturated soil zone, are expected to be very challenging. Meeting such challenges is a prerequisite for successful site remediation by in situ surfactant washing. Acknowledgments We appreciate the support provided by Devi N. Rai and Carolina C. Ang in conducting the adsorption and desorption experiments and for analyzing the samples from those experiments. Registry No. Witconol SN70, 39387-65-0; Aroclor 1248, 12672-29-6.
Literature Cited
U.S. Environmental Protection Agency. Polychlorinated Biphenyls Manufacturing Processing Distribution in Commerce and Use Prohibitions. Fed. Regist. 1979, 44, 31504. Abdul, A. S.; Gibson, T. L.; Rai, D. N. Statistical Correlations for Predicting the Partition Coefficient for Nonpolar Organic Contaminants Between Aquifer Organic Carbon and Water. Hazard. Waste Hazard. Mater. 1987,4, 211. EPRI. Physiochemical Measurements of Soil a t SolidWaste Disposal Sites, EPRI EA-4417, Project 2485-3, Final Report, 1986. U.S. Environmental Protection Agency. Treatment of Contaminated Soils with Aqueous Surfactants. Report EPA/600/2-85/129, 1985. American Petroleum Institute. Test Results of Surfactant Enhanced Gasoline Recovery in a Large-Scale Model Aquifer. API Publication No. 4390, 1985. Abdul, A. S.; Gibson, T. L.; Rai, D. N. Selection of Surfactants for the Removal of Automatic Transmission Fluid From Sandy Aquifer Material. Ground Water 1990, 28, 920. Gibson, T. L.; Abdul, A. S.; Rai, D. N. Measurement of Toxic Organic Contaminants in Ground Water Studies by High-Performance Liquid Chromatography. Hydrol. Sei. Technol. 1986, 2, 61. Weber, W. J., Jr. Physicochemical Processes for Water Quality Control; Wiley-Interscience: New York, 1972. Rosen, M. J. Surfactants and Interfacial Phenomena; Wiley-Interscience: New York, 1978. Narkis, N.; Ben-David, B. Adsorption of Non-ionic Surfactants on Activated Carbon and Mineral Clay. Water Res. 1985, 19, 815. Chiou, C. T.; Porter, P. E.; Schmeddling, D. W. Partition Equilibria of Nonionic Organic Matter and Water. Enuiron. Sei. Technol. 1983, 17, 227.
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Wagener, S.; Schink, B. Anaerobic Degradation of Nonionic and Anionic Surfactants in Enrichment Cultures and Fixed-Bed Reactors. Water Res. 1987, 21, 615. Kravetz, L. Biodegration of nonionic ethoxylates. JAOCS 1981,58A. Turner, A. H.; Abram, F. S.; Brown, V. M.; Painter, H. A. The Biodegradability of Two Primary Alcohol Ethoxylate Nonionic Surfactants Under Practical Conditions, and the Toxicity of the Biodegradation Products to Rainbow Trout. Water Res. 1985, 19, 45. Wershaw, R. L. A New Model for Humic Materials and Their Interactions with Hydrophobic Organic Chemicals in Soil-U'ater Sediments-Water Systems. J . Contam. Hydrol. 1986, 1 , 29. Saito, H.; Shinoda, K. The Solubilization of Hydrocarbons in Aqueous Solutions of Nonionic Surfactants. J . Colloid Interface Sci. 1967, 24, 10. Kile, D. E.; Chiou, C. T. Effects of Some Petroleum Surfactants on the Apparent Water Solubility of Organic Compounds. Enuiron. Sci. Technol. 1990, 24, 205. Abdul, A. S.; Gibson, T. L.; Rai, D. N. Use of Humic Acid
Solution To Remove Contaminants From Hydrogeologic Systems. Enuiron. Sci. Technol. 1990, 24, 328. (19) Chiou, C. T.; Malcolm, R. L.; Brinton, T. I.; Kile, D. E. Water Solubility Enhancement of Some Organic Pollutants and Pesticides by Dissolved Humic and Fulvic Acids. Enuiron. Sei. Technol. 1986, 20, 502. (20) Edwards, D. A,; Luthy, M. Nonionic Surfactant Solubilization of Polycyclic Aromatic Hydrocarbons in Aqueous and Soil/Water Systems. In Proceedings of the 1990 Environmental Engineering Conference;ASCE: Washington, DC, 1990; pp 286-289. (21) Keely, J. F.; Palmer, C. D.; Johnson, R. L.; Piwoni, M. D.; Enfield, C. D. Optimizing Recovery of Contaminant Residuals by Pulsed Operation of Hydraulically Dependent Remediations. Proceedings of the Petroleum Hydrogens and Organic Chemicals in Ground Water Conference, NWWA/API, 1987; p 91.
Received f o r review July 23, 1990. Revised manuscript received September 20, 1990. Accepted Nouember 5, 1990.
Intercomparison of a Range of Primary Gas Standards of Carbon Monoxide in Nitrogen and Carbon Dioxide in Nitrogen Prepared by the National Institute of Standards and Technologyt and the National Physical Laboratory Ernest E. Hughes,$,§Arthur J. Davenport,lI Peter T. Woods,Il and Walter L. Zielinski, Jr.*S$
Gas and Particulate Science Division, Center for Analytical Chemistry, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, and Division of Quantum Metrology, National Physical Laboratory, Teddington, TW11 OLW, UK Measurements were carried out by the National Physical Laboratory (NPL) in the United Kingdom and the National Institute of Standards and Technology (NIST) in the United States to intercompare the primary gravimetric gas standards developed by these two laboratories. These intercomparisons involved analyses of a set of CO and C 0 2 NIST Standard Reference Materials by the NPL, and of a similar set of NPL primary standards by the NIST. In each case, the exact concentrations of the exchanged sets were unknown to the analyzing laboratory. The CO and COz standards ranged in nominal concentration from 8% to 10 ppm and from 8% to 0.5%, respectively. The analyses were carried out using primary gravimetric standards, which were independently produced by the two laboratories. The mean difference between the values assigned by the supplying laboratory and the values determined by the analyzing laboratory was less than 0.2 70 relative for both sets of CO and C 0 2standards. The results confirmed that standards produced by NPL and NIST have a high degree of consistency, and that measurements obtained using these standards for measurements of levels of atmospheric CO and C 0 2 may be directly intercompared.
Introduction An increasing number of national laboratories around the world are preparing primary gas standards in order to provide a basis for ensuring the quality and intercomparability of measurements of pollutant gases in ambient
* Present address: Food and Drug Administration, Division of Drug Analysis, 1114 Market St., Room 1002. St. Louis. MO 63101. 'Formerly, National Bureau of Standards. National Institute of Standards and Technology. Deceased. National Physical Laboratory.
* J
0013-936X/91/0925-0671$02.50/0
air. These primary standards, prepared by absolute gravimetric methods, are used to accurately determine and certify the concentrations of certified gas mixtures. These are then used as reference standards for the calibration of instruments employed for monitoring atmospheric pollution and air quality, and for the development and evaluation of improved monitoring methods. Applications involving certified gas standards include measurements of pollutant gas emissions from stationary and mobile sources, assessments of air quality in urban and workplace environments, and quantitative evaluations of changes in the trace gas composition of the atmosphere. To ensure that such measurements are comparable throughout the world, it is necessary that gas standards prepared by different national laboratories agree with one another. One example where international uniformity is required is for measurements of gaseous pollutants produced by vehicles and aircraft, since these generally are manufactured for the international market. The predominant gaseous pollutants emitted from these sources include carbon monoxide, carbon dioxide, oxides of nitrogen and sulfur, and hydrocarbons. Reference standards for each of these gases are prepared a t concentrations ranging from 10% to several parts per million (ppm) in a balance gas of nitrogen or air. The National Institute of Standards and Technology (NIST), formerly the National Bureau of Standards, in the United States has been preparing a wide range of gas standards since the late 1960s. The National Physical Laboratory (NPL) in the United Kingdom has been engaged in a similar program since 1976. Both laboratories supply certified gas reference standards in order to provide the basis for measurement quality assurance and to demonstrate intercomparability of measurements made by industry and government organizations. These standards are certified by direct comparison to the primary gravi-
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Published 1991 by the American Chemical Society
Environ. Sci. Technol., Vol. 25, No. 4, 1991 671
