Environ. Sci. Technol. 1996, 30, 3199-3204
Moisture Effects on the Cleanup of PAH-Contaminated Soil with Dense Carbon Dioxide ARMIN SCHLEUSSINGER, BERND OHLMEIER, INGO REISS, AND SIEGFRIED SCHULZ* Institute of Thermodynamics, University of Dortmund, 44221 Dortmund, Germany
The cleanup of soils contaminated with organics by extraction with supercritical carbon dioxide can be influenced decisively by additional substances or entrainers. In most cases, the contaminated soil already contains water as a substance that can alter the extractibility of these contaminants. In particular, the effects of soil moisture, as a kind of discontinuous addition of water, on the extraction of polycyclic aromatic hydrocarbons (PAHs) from soil with supercritical carbon dioxide were examined. On the other hand, a continuous addition of water by humidifying the supercritical carbon dioxide was used. Both continuous and discontinuous addition of water elevates the extraction yield remarkably by altering the adsorption phenomena for spiked silt as well as for soil samples from a former gas plant site. Furthermore, two different purity grades of carbon dioxide were used to work out the influence of impurities in the technical carbon dioxide serving as entrainers. The improvement of the extraction yield by moisture indicates additionally that the extraction is limited by adsorption and not by diffusion effects. Otherwise, the water must decrease the extraction rate because of the formation of an additional phase. However, the contaminant is better accessible and transported faster out of the soil with water.
Introduction Supercritical carbon dioxide extraction is considered a promising technique for rapid and quantitative decontamination of polluted soil. A number of experimental and theoretical studies have recently been published showing the applicability of the SFE process for the cleanup of hydrocarbon contaminated soils (1-4). The solvent strength of a supercritical fluid is directly related to its density, allowing a simple adjustment of the solvating power of the supercritical fluid. PAHs were chosen as model substances because of their low vapor pressures, which renders their removal from soil. For a significant solubility in supercritical fluids of low volatile substances like PAHs, pressures up to 300 bar are required. Moreover, the temperature must be elevated up to 140 °C for a successful, quantitative de-
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1996 American Chemical Society
sorption from the soil (5). These conditions involve a high energy consumption for pressurizing and heating up the supercritical fluid. One way to lower the energy consumption, and with it the costs of such a supercritical extraction process, is the use of appropriate cosolvents. They maintain or improve, on one hand, the solubility for the solute in the supercritical fluid (6) and, on the other hand, the desorption behavior from the solid matrix (7). Nonpolar carbon dioxide is especially not the best solvent for polar or weakly polar substances, but polar cosolvents modify solubilities and desorptive forces effectively. The latter point is very important for achieving low residue concentrations in the treated soil. By extraction with pure supercritical carbon dioxide, a complete cleanup can only be accomplished for low molecular weight PAHs (8). It is known from analytical scale extraction of environmental samples that solvents such as methanol (7, 9-11) ethanol (9, 10), acetone (10, 11), or hydrocarbons (12) enhance the extraction efficiency. Disadvantages of these cosolvents for an industrial scale process are additional costs for the substances and the necessary plant hardware and possible residues of toxic cosolvents in the soil after depressurizing. However, water is found in nearly all soils due to the natural moisture and can increase the extraction yield without the mentioned disadvantages of other entrainers. The effects of water on the cleanup of organics were examined seldom and only insufficiently. Thus, one purpose of our research was to show the influence of water on the extraction of PAHcontaminated soil by dense carbon dioxide in general. This includes the different possibilities of entrainer feeding, discontinuously to the soil and continuously to the fluid. Spiked silt (average particle diameter 32 µm) from different sites was used to work out the influence of modifiers. The preparation and the origin of the samples led to slight differences in their extractibility, which has to be considered for the comparison of the results. Additionally, soil from a former gas plant site was extracted with different soil moistures.
Experimental Section The experiments were carried out in a lab-scale extraction facility as shown in Figure 1, which allows the treatment of 20 g of soil. The subcooled carbon dioxide is compressed by a diaphragm pump to the desired extraction pressure up to 340 bar. Afterwards, the solvent is led into a thermostatic controlled oven with the extraction vessel inside, where it is heated up to extraction temperature. After passing the autoclave filled with the soil, the PAH concentration in the solvent can be measured with a UVabsorption detector (Jasco UV 975) equipped with a highpressure cell. To prevent the entrainer from precipitation, the detector cell is heated up to 10 °C over the extraction temperature. To avoid damages of the cell, the extraction temperature is limited to 80 °C. The wavelength of 250 nm was chosen in accordance with analytical applications. After this, the solvent passes a filter, a pressure regulator, and a mass flow controller. After pressurization at the beginning of the experiment, the carbon dioxide can be lead through a bypass, allowing a calibration of the UV detector with the pure solvent. The extraction is started by closing the bypass and leading the solvent through the extraction vessel, where it becomes loaded with the contaminant. Entrainer can be
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FIGURE 1. Schematic of the apparatus for the supercritical fluid extraction of soil.
fed continuously to the solvent with a HPLC pump or directly with the soil. The latter was achieved by suspending the soil with a small amount of water and then drying it in an oven at 40 °C to the desired humidity. Because of the low solubility of water, its continuous feeding has to be done with a further autoclave filled with moistened sand for saturating the solvent before entering the extraction autoclave. The concentration q of the PAH in the soil was analyzed before and after the extraction. Therefore, a sample of the soil was extracted by ultrasonic with ethyl acetate and analyzed by gas chromatography. A comparison of this method with the traditional Soxhlet extraction revealed its good applicability and repeatability (13). The absorbance of either perylene or pyrene is proportional to the concentration in the carbon dioxide at low concentrations (law of Lambert-Beer). This allows the determination of the concentration c of the contaminant in the carbon dioxide from the UV signal Γ:
c(t) ) χΓ(t)
(1)
The required calibration factor can be calculated with the following equation derived from a mass balance for the extractor (14):
χ)
msoil(q0 - q(text))
∫
m ˘ CO2
text
0
(2)
Γ(t) dt
The numerator describes the mass of contaminant extracted, and the denominator describes the integration of the UV signal, which is illustrated by the area under the curve of the UV signal. The calibration factor was proved to be constant without change when extracting with water as an entrainer. For comparison, the extraction results are shown as extraction yield ξ, the ratio of extracted mass to the initial mass of the contaminant:
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ξ(t) )
∆mPAK(t) q0 - q(t) ) mPAK q0
(3)
which varies between 0 (no cleanup) and 1 (total cleanup). A measure for the extraction effort is the specific solvent mass Λ, the ratio of solvent mass to the mass of treated soil:
Λ)
mCO2
)
msoil
m ˘ CO2text msoil
(4)
which can also be interpreted as dimensionless extraction time. The course of concentration, derived from the timedependent absorption signal Γ, allows the calculation of the PAH concentration in the soil with the same mass balance. The combination of eq 2 and eq 3 leads to
∫ Γ(τ) dτ ) ∫ Γ(τ) dτ t
ξ(t) ) ξ(text
0 text
(5)
0
giving the extraction yield as function of time or specific solvent ratio respectively. ξ(text) is the measured extraction yield at the end of the extraction time text.
Results Silt was contaminated with perylene and extracted at a temperature of 70 °C and a pressure of 300 bar. These are mild conditions for the extraction in order to work out the effects of an entrainer. First pure carbon dioxide (99.995%) and technical carbon dioxide (99.5%) were compared to show the influence of the purity of the solvent. In a larger scale plant, only technical carbon dioxide should be used because of the high price of pure carbon dioxide. Figure 2 shows the concentration of perylene in the supercritical carbon dioxide over the specific solvent ratio Λ. The concentration was calculated using the signal of the UV detector and the calibration factor. When extracting with technical carbon dioxide, the concentration in the carbon
FIGURE 2. Influence of impurities on the extraction of perylene.
FIGURE 3. Extraction of perylene with pure and technical carbon dioxide.
dioxide rises to higher values, but after passing the maximum it falls quicker than for the extraction with pure carbon dioxide. The flatter curve for the extraction with pure carbon dioxide results in a lower extraction yield, since the area under the curve is proportional to the extracted amount of perylene. This is shown in Figure 3. The marked points are measured extraction yields after various extraction times. The lines were calculated with eq 5 using the measured signal from the UV detector. In accordance with Figure 2, more perylene is driven out in the first minutes of the extraction with technical carbon dioxide. So, further investigations will center on technical carbon dioxide because of its better results. These differences have also be taken into consideration for the theoretical investigation of the extraction process, i.e., solubility data in the literature are only available for pure carbon dioxide. These results lead to the conclusion that the impurities of the technical carbon dioxide have a decisive effect on the extraction. But in the technical carbon dioxide, various substances, mainly water, serve as entrainer. Because of this, we concentrated on the effect of water on the extraction of soil with technical carbon dioxide. First, the continuos addition of water by humidifying the supercritical carbon dioxide with the additional autoclave was examined; second, the moisture of the soil was adjusted. In both cases, the extraction yield increases as shown in Figure 4. The discontinuous addition of water by adjusting the moisture of the soil is more efficient for low specific solvent ratios as the extraction yield is higher
FIGURE 4. Effect of water on the extraction of perylene.
FIGURE 5. Influence of moisture on the extraction of perylene from spiked silt.
than those for the extraction with humidified carbon dioxide. For higher specific solvent ratios, this effect decreases and the curves cross each other, since the water is driven out. This effect is more significant for methanol as entrainer (14). Because of the higher solubility of methanol in supercritical carbon dioxide, it is extracted faster than water, and thus the extraction is not as efficient. Similar results were measured for pyrene-spiked soil, but not as clear as described before because of the higher volatility of pyrene. In additional studies, the effect of varying the soil moisture was examined. Figure 5 shows the extraction yield for several water contents of the soil. The extraction yield is more than doubled when extracting humidified silt with a moisture between 0.03 and 0.10 g of water/g of soil. In this region, the optimum moisture for the extraction is found. If the water content of the soil is higher than 0.10 g/g, the extraction yield decreases. Similar results could be found for the cleanup of soil from a former gas plant site with an averaged particle diameter of only 15 µm. Table 1 gives the initial concentration of the analyzed PAH. By increasing the water content of this soil, the contamination can be reduced remarkably as can be seen in Figure 6. As a parallel to Figure 5, an optimum moisture could be found at 0.12 g/g for this soil. However, the optimum moisture differs, since it depends on the type of soil and its particle size. Table 1 summarizes the relative enhancement of the extraction
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TABLE 1
Initial Concentration of Soil from Former Gas Plant Site and Enhancement of Extraction Yield with 0.12 g/g Water (text ) 130 °C; pext ) 330 bar; Λ ) 30 g/g) PAH
q0 (ppm)
enhancement ∆ξ (%)
pyrene chrysene benzo[b+k]fluoranthene benzo[a]pyrene sum of analyzed PAH
21.6 2.8 8.4 1.6 34.4
110 150 161 428 176
FIGURE 7. Comparison of the extraction of dry and moistened gas plant soil.
FIGURE 6. Influence of moisture on the final soil concentration of several PAH from a gas plant soil; text ) 130 °C; pext ) 330 bar; Λ ) 18 g/g.
yield (100% corresponds to the extraction yield of dry soil). Benzo[b]fluoranthene and benzo[k]fluoranthene are determined together as benzo[b+k]fluoranthene because their peaks in the GC chromatogram could not be separated from each other. In Figure 7, the course of extraction for the dry soil and this soil with the optimum moisture are compared. Especially for benzo[a]pyrene, as one of the worst contaminants, the moisture enables a virtually complete cleanup, even at this low extraction temperature.
Discussion While extracting PAH with supercritical carbon dioxide, the extraction curve measured with the UV detector reveals almost the same shape, even when adding entrainer. This curve can be divided into two stages at the maximum of the signal. The first stage reveals a virtually linear gradient, because the carbon dioxide that is pressurized in the extraction autoclave is loaded with the contaminant while flowing slowly through the extractor. This part of the extraction is considered as the solubility and mass transfer controlled period. However, even in the maximum of the extraction curve, the solubility of the PAH in the supercritical carbon dioxide is not reached. After passing the maximum, the transport of the PAH in the supercritical carbon dioxide diminishes since now the transport becomes more and more limited by the desorption of the remaining stronger adsorbed PAH from the soil. The necessary extraction time for the decontamination up to permissible levels of PAH contamination increases. The increase of the extraction yield when using technical carbon dioxide for the cleanup of contaminated silt reveals
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that the impurities improve the cleanup as the maximum of the extraction curve is higher than that for the extraction with pure carbon dioxide. As several substances contribute to these effects, it cannot be elaborated if they result from an increase of density or an improvement of the desorption of the contaminants. Thus, water can serve as a polar entrainer, which is very poor miscible in the supercritical carbon dioxide. At these extraction conditions, the equilibrium concentration of water in carbon dioxide is 1.1 mol % according to the data of Wiebe (15). Therefore, the solubility is only slightly enhanced due to the higher density of the mixture. The major effect of water as an entrainer is the influence on the adsorption. Adsorption isotherms cannot be measured because no equilibrium between solid and supercritical phase could be reached due to the conditions in the equipment. Because of this, the relation between fluid concentration and soil concentration are shown as operating lines for higher extraction times. The operating lines were obtained by fitting the parameters of the Freundlich isotherm to experimental data of the experiments. The isotherm suggested by Freundlich (7) has the following form:
q ) KFc1/n
(6)
This isotherm was chosen because results of other investigators and the modeling of the process suggest this nonlinear shape. The Freundlich isotherm does not describe the solubility limit as, for example, the BET isotherm. The lack of this feature can be neglected here because the solubility limit is not reached, even at the end of the extraction. In Figure 8, several operating lines are describing the adsorption conditions for the extraction of dry silt and humidified carbon dioxide. Table 2 gives the resulting parameters. For comparison, the operating line for the extraction of contaminated silt by carbon dioxide modified with methanol was added. However, the operating lines indicate that after the addition of water for the same concentration in the soil, the PAH concentration in the supercritical phase is higher. This leads to the conclusion that the contaminant is more accessible in moist silt than in dry silt and is driven out quicker. Furthermore, water and methanol, which were valued as effective modifiers for the supercritical fluid extraction in earlier studies (7), have comparable effects on the extraction.
FIGURE 8. Influence of entrainer on the operating lines for the extraction of perylene from silt with technical carbon dioxide (fitted to Freundlich isotherm). TABLE 2
Parameters for Operating Lines in Figure 8 entrainer
KF (ppm(1-1/n))
n (-)
methanol (7.2 mol %) water (1.1 mol %)
47.82 7.58 11.53
3.08 1.07 1.41
Similar results are reported by Thibaud et al. (16) and Chiou et al. (17), who measured the adsorption isotherms of volatile organics on soil at different moistures. They found that, with increasing humidity of the soil, the sorption of organic vapors on the soil decreases. These results can be explained by a theory suggested by Valsaraj et al. (18). The authors proposed that the polar water molecules occupy the polar adsorption sites of the soil and displace the less polar organics. In contrast to the model proposed in refs 16 or 19 for volatile organics, PAHs are virtually insoluble in water. Thus, the amount of PAH in the water phase can be neglected. According to Valsaraj et al. (18), the soil moisture can be divided into three regions of dry, damp, and wet soil. The dry region refers to low moistures, where the small amount of water has no effect on the adsorption of the organic compound. The damp region is characterized by a higher water content that does not suffice to form a monolayer coverage of water molecules on the soil particles. Nevertheless, the influence on the adsorption of the organic compounds is remarkable. With higher water content, the region of wet soil, nearly all adsorption sites are occupied by water. The effects are more measurable the lower the volatility of the organic compound is. However, accurate boundaries of water content for these regions cannot be specified as they depend on the different soils and the amount of organic matter in it. While extracting silt with a water content higher than 0.1 g of water/g of soil, the extraction yield decreases again. One possible explanation for this can be the agglomeration of the soil particles at a certain moisture. The contaminants are less accessible in these larger objects, and the contaminants have to be transported by diffusion over longer distances. The cleanup of soil by supercritical carbon dioxide extraction can be influenced decisively by the use of water as an additional substance. Shorter extraction times can
be achieved by adding water continuously or discontinuously. As the extraction time effects directly on the remediation costs, the extraction time has to be optimized. Additionally to an optimization of pressure and temperature, the feeding of entrainers can be used to increase the cleanup. In case of the discontinuous addition of water to the soil, the water is extracted with the contaminants. Thus, the effect of water diminishes slowly due to the low solubility of water in the supercritical fluid. Since this occurs only for high extraction times, in most cases a discontinuous addition of water by adjusting the soil moisture is sufficient for influencing effectively the extraction. Even in a technical-scale plant, water accumulates in the solvent cycle leading to a moistened solvent. When using water as an entrainer, in most cases the discontinuous addition by adjusting the soil moisture is sufficient. Because of the very low miscibility of water in supercritical carbon dioxide, the water content of the soil remains in the region of effective desorption. Further equipment for continuously humidifying the supercritical carbon dioxide or entrainer feeding can be avoided. The optimum water content of the soil depends on the nature of the soil. For different types of soil, we found the optimum in the range between 8 and 14%. This fact has to be considered for the realization of the soil pretreatment for an extraction plant in industrial scale.
Nomenclature Symbols c PAH concentration in the supercritical phase q PAH concentration on the soil parameter of the Freundlich isotherm KF m mass m ˘ mass flow n parameter of the Freundlich isotherm t time Greek Letters ξ yield of extraction Λ specific solvent ratio Γ absorption χ calibration factor Subscripts ext extraction 0 initial state
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(12) Dobbs, J. M.; Wong, J. M.; Johnston, K. P. J. Chem. Eng. Data 1986, 31, 303. (13) Michel, St. Ph.D. Thesis, University of Dortmund, 1991. (14) Lu ¨ tge, C.; Schleussinger, A.; Schulz, S. Proc. 3rd Symp. I.S.A.S.F. 1994, 3, 429. (15) Wiebe, R. Chem. Rev. 1941, 29, 475. (16) Thibaud, C.; Erkey, C.; Akgerman, A. Environ. Sci. Technol. 1993, 27, 2373. (17) Chiou, C. T.; Shoup, T. D. Environ. Sci. Technol. 1985, 19, 1196. (18) Valsaraj, K. T.; Thibodeaux, L. J. J. Hazard. Mater. 1988, 19, 79.
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(19) Pennell, K. D.; Rhue, R. D.; Rao, P. S. C.; Johnston, C. T. Environ. Sci. Technol. 1992, 26, 756.
Received for review December 5, 1995. Revised manuscript received July 3, 1996. Accepted July 3, 1996.X ES950914Z X
Abstract published in Advance ACS Abstracts, October 1, 1996.
