I n d . Eng. Chem. Res. 1987, 26, 2058-2062
2058
The Use of Entrainers in the Supercritical Extraction of Soils Contaminated with Hazardous Organics Kerry M. Dooley,t Chien-Ping Kao,t Robert P. Gambrel1,z and F. Carl Knopf*+ Department of Chemical Engineering and Department of Marine Sciences, Louisiana State University, Baton Rouge, Louisiana 70803
Supercritical fluid (SCF) extraction is a promising new technique for the cleanup of soils, sediments, and sludges that are contaminated with hazardous wastes. The ability of SCFs to solubilize heavy molecular weight organics is well-documented. In this investigation, supercritical carbon dioxide (SC-CO,) with a single entrainer, either methanol or toluene, is compared t o pure CO,; comparison is made on the basis of extraction rate and efficiency of removal of D D T from contaminated soils. T h e supercritical mixtures at 100 a t m and either 40 or 80 "C were continuously passed through a fixed bed of 10 g of soil. The most effective solvent system, SC-C02with 5 wt % (6.8 mol %) methanol at 40 "C and a flow rate of 0.7 g/s, was able t o leach approximately 95% of the D D T from the soil in under 5 min, as compared to either pure C 0 2 or C 0 2with 5 wt % (2.5 mol % ) toluene at the same conditions, which could only extract 70% in 10 min. Supercritical fluid (SCF) extraction has received much attention as a technique for separating relatively nonvolatile materials. Typically in SCF extraction, a solvent gas such as carbon dioxide, a t high pressure and moderate temperature, is contacted with a solid or liquid phase. Slight changes in the system temperature or pressure can cause large changes in the solvent density and consequently in its ability to solubilize relatively nonvolatile components. For example, at 200 atm and 35 "C, the density of C 0 2 approaches 0.8 g/cm3, and at these conditions a solute such as naphthalene would have a solubility some 10000 times that predicted if C02 behaved as an ideal gas. By taking advantage of these facts, a process can be envisioned whereby manipulation of the system pressure effects extraction of a nonvolatile material. A simple pressure letdown, to a pressure below the system critical conditions, can cause near complete precipitation of the relatively nonvolatile material from the solvent. In addition to the liquidlike densities of a typical SCF, viscosities and molecular diffusivities of SCFs are intermediate to typical liquid and gas values for these properties. For these reasons, the extraction efficiencies of SCFs are usually higher than those of liquids. The many advantageous properties of SCFs have opened up new technologies in environmental control. Successful efforts have included the use of SCFs in the regeneration of adsorbents contaminated with volatile organics using pure SC-C02 (Eppig et al., 1981), oxidation of organic contaminants in waste streams using SC-water (Modell, 1982);liquid-liquid extraction of waste streams again using pure COz as the extraction medium (Rignhand and Kopfler, 1983), the extraction/reaction comprising the generation of low sulfur chars from coals utilizing either SCtoluene or SC-alcohols (Vasilakos et al., 1985); and finally the regeneration with SC-COPof activated carbon used in the cleanup of liquid waste streams (deFilippi et al., 1980). This regeneration was not universally practical, due to the buildup of irreversibly bound organics. We are examining the capabilities of supercritical fluids to extract toxic chemicals such as DDT and PCB (Aroclor 1254) from soils, thereby providing a tool for cleaning up hazardous waste sites. Such removal offers the obvious advantage of the creation of a much smaller volume if further treatment such as combustion, biological degradation, or other disposal methods are desired. Department of Chemical Engineering. *Department of Marine Sciences.
0888-5885/87/2626-2058$01.50/0
This paper reports on the use of mixed solvents to remove DDT from contaminated topsoils. In particular, the use of SC-COPwith either methanol or toluene as an entrainer was explored. In order to explain the extraction process and the thermodynamics involved, this paper is divided into two sections. The first discusses background, including past results and the phase equilibria expected for ternary systems of C02-methanol-DDT and COPtoluene-DDT. The second section presents data showing the effectiveness of these systems for the extraction of DDT from a topsoil of high organic content. The most effective solvent, C02 with 5 w t % (6.8 mol %) methanol entrainer, is compared as an extraction solvent to pure SC-C02. There is a demonstrable enhancement in both % DDT extracted and extraction rate with a SCF plus entrainer as compared to a pure SCF.
Background In previous work (Brady et al., 19871, we investigated the use of pure SC-COPto extract PCB's, DDT, and toxaphene from a contaminated topsoil (12.6% organic matter) and subsoil (0.74% organic matter). Experiments with SC-C02 continuously flowing through a fixed bed of 10 g of contaminated soils showed that over 90% of the PCBs (original contamination level 1000 ppm) could be extracted from the subsoil in under 1min using a C02flow rate of 0.7 g/s. However, only 70% of the DDT and 75% of the toxaphene could be leached from the topsoil contaminated with 1000 ppm DDT and 400 ppm toxaphene in 10 min a t the same COz rate. These results are not unexpected in that pure SC-C02,being a relatively nonpolar solvent, would show only van der Waals interactions with adsorbed compounds that may be strongly bound to polar sites on the topsoil. The presence of these sites is consistent with the high organic content of the topsoil. Mixed solvents, for example, CO, and a few weight percent of a polar entrainer, could result in a specific chemical attraction of adsorbate and extraction medium and thus enhance contaminant extraction from solids containing polar adsorption sites, such as activated carbon or topsoils of high organic content. Phase Diagrams The "entrainer" solvents used in this work are toluene and methanol. By use of the Peng-Robinson (1976) cubic equation of state, the triangular phase diagrams for the ternary systems C02-DDT-toluene and C02-DDTmethanol were constructed at 100 atm and two different 0 1987 American Chemical Society
Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987 2059
-
__ ___
40
c
SFE
--
VLE SLVE
---
80C SFE VLE SLVE
3 I C,H,l
I COLI
Figure 3. Predicted phase behavior (mole fractions) of the ternary system of C02-DDT-toluene a t 80 OC and 100 atm.
___ __
eo c SFE VLE SLVE
Figure 2. Predicted phase behavior (mole fractions) of the ternary system of C02-DDT-methanol at 40 "C and 100 atm.
conditions of temperature, namely, 40 and 80 "C, as shown in Figures 1-4. It is assumed that the solubilities of the solvents (CO, with either methanol or toluene) in solid DDT (component 2) are negligible. The solid phase is considered as pure DDT, and its fugacity, f,", in phase equilibria calculations was evaluated from
assuming the Peng-Robinson equation is valid for calculating the fugacity of the pure subcooled liquid (f201). The critical points are calculated according to the algorithm proposed by Heideman and Khalil (1980). At 40 "C and 100 atm, the addition of a small amount of toluene or methanol to CO, increases the DDT solubility. The solubility (mole fraction) of DDT in a mixed solvent of 95 wt % (97.5 mol %) CO, and 5 wt % (2.5 mol %) toluene is estimated from Figure 1as 2.5 X lo9 (-3.6 times the value in pure COP). If toluene is replaced by methanol of the same weight fraction, the estimated DDT solubility from Figure 2 is 0.01 (-14 times that in pure CO,). Therefore, a mixed solvent with 95 wt % (93.2 mol %) CO, and 5 wt % (6.8 mol %) methanol should be more effective in extracting DDT from soil than a mixed solvent with 95 wt % COz and 5 wt % toluene, although the latter
Figure 4. Predicted phase behavior (mole fractions) of the ternary system of C0,-DDT-methanol at 80 "C and 100 atm.
is still a more effective solvent than pure C02. At 80 "C and 100 atm, the triangular phase diagram for CO,-DDT-toluene is shown in Figure 3. It is similar to the corresponding diagram a t 40 "C and 100 atm, except that the VLE region does not vanish a t a ternary critical point; rather it extends to the binary boundary of C02 and toluene and divides the one-phase region into two parts. One part, which is very rich in COz, is shown in the insert of Figure 3; from this insert it is seen that the solubility of DDT in pure CO, is 0.3 X (-4% of the solubility a t 40 "C and 100 atm) and the solubility of DDT in a mixed solvent of 95 wt % (97.5 mol %) CO, and 5 wt % (2.5 mol % ) toluene is well below 0.4 X low4. From a solubility standpoint, therefore, the lower temperature is better for the extraction of DDT from soil either with pure C02 or with a mixed solvent of 95 wt % COz and 5 wt % toluene. The same conclusion can be reached for the C02-DDT-methanol system, considering the phase diagram at 80 "C and 100 atm shown in Figure 4. Some mole fractions of the ternary system at the two operating conditions are summarized in Table I. The interaction parameter, lz,, was empirically determined by regressing VLE data for the binary systems of CO, plus toluene (Ng and Robinson, 1978) and CO, plus methanol (Semenova et al., 1979).
2060 Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987 Table I. Predicted Composition of SCF Phases for C02-Entrainer-DDT Systems mole fraction mole fraction (40 O C , 100 atm) (80 "C, 100 atm) system kl, ___ __0.07 J ' ~ = 0.7 x 10-3 COZ(1) + DDT(2) y z = 0.3 x 10-4 0 = 0.3 X DDT(2) + niethanol 1'2 = 0.391 DDl'(2i -t roluene 0 > ? = 0.155 = 0.429 _ _ _ _ _ _ l l l _
)*,
;m2
C 0 2 ( l )+ methanol(3) C 0 2 i i ) + toluene(3) CO, + IIDT + methanol
0.07 0.I
+ DDT + toluene
= 0.06
~3
= 0.027
~2
= 0.403
~g = 0.074 y3 = 0.009
XQ
x:!
= 0.055 = 0.014 = 0.032
12
= 0.1
~2 ~2
CO,
= 0.594 = 0.425
y3
xg
3'2 = 0.005 ~2
= 0.032
xg = 0.132 3'3 = 0.079 X Q = 0.115
y 2 = 0.4 X
= 0.14
X Z = 0.405 y 2 = 0.4 X
y 3 = 0.079 XQ
~g = 0.07 y 3 = 0.003
condition SFE SFE SFE VLE VLE SLVE
( L = V) critical pt SLVE ( L = V) critical pt
= 0.119
-
80: I O O O p g D D T l g
06
$011
0 -Pure s c - c o 2
Q" A
05
-
S C - C O p w i l h 5 w t % roluene S C - C O 2 w i t h 5 ~ 1 %methanol
2
0
c
2 04
b-
2 W V
0
03
V P
Prl,t"rl
GmOr
P:-
PIII,",,
Cart,*,,.,
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8
BIOlCUl
7
.
.99% extraction in less than 5 s. The actual extraction times shown in Figure 6 are 1-10 min; however and consequently, the actual distribution coefficient is larger. Some of the difference between the distribution coefficient for DDT in an actual soil and pure DDT may be due to the limitations of thermodynamic modeling by the Peng-Robinson equation; most of the difference, however, probably reflects the reduced activity of DDT adsorbed on the soil or associated with colloidal organic matter within the soil's pores. The presence of colloidal matter was indicated by the porosimetry data of Brady et al. (1987). Intraparticle diffusion also represents a feasible ratelimiting step. For this topsoil, a "diffusion time" [(I?/ 3 ) 2 / D ]of 1 min results in a computed diffusivity of cm2/s. This value is low for a solute in a liquid or supercritical fluid but not low for diffusion in a colloid. Therfore, the DDT-extraction process is limited by either or both of two factors: first, desorption from soil si';es or from colloidal organic material and, second, pore diffusion. An entrainer can lessen the first resistance by its effect on the desorption isotherm; entrainers could also have an effect on the diffusional resistance because they are capable of capillary condensation or absorption into a colloid, thereby altering the pore environment. Upcoming results of desorption equilibrium measurements in a static highpressure cell will be employed to more accurately model the SCF extraction process.
Conclusion This paper reported the use of SC-C02 with either toluene or methanol as an entrainer, for the extraction of DDT from contaminated soils. The extraction efficiency was shown to be a strong function of both the entrainer selected and the extraction temperature. A supercritical
2062 Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987
mixture of COPplus 5 wt % toluene showed no improvement over COz alone, with only -75% removal of the DDT possible; the residual DDT was strongly adsorbed on the soil. However, near complete removal of DDT was possible using SC-COz with 5 wt % methanol a t 40 O C . Raising the temperature from 40 to 80 "C slowed the extraction rate for each mixed solvent system, indicating that the increased energy available for bond breaking is not sufficient to offset the more unfavorable desorption isotherm. The efficiency of the C02-methanol system remained unchanged with a large variation in flow rate, from 1 to 0.1 g/s, showing that external mass-transfer resistances are insignificant a t these conditions. As an entrainer can affect both the desorption isotherm and pore diffusional resistances, desorption equilibrium measurements are necessary to accurately model the SCF extraction process. These results will be reported in a subsequent paper.
V = vapor phase or molar volume, cm3/mol VLE = vapor-liquid equilibrium W = soil weight, g x = mole fraction in the liquid phase y = mole fraction in the vapor phase Greek Symbol 0 = soil-phase solute concentration, g/g of soil
Acknowledgment
CeHSCH3, 108-88-3.
This study was supported in part by Grant CR-809714 from the U.S. Environmental Protection Agency. This support does not signify that the contents necessarily reflect the views and policy of the Agency; mention of trade names or commercial products does not constitute endorsement or recommendation for use. We acknowledge the experimental assistance of Dianne Leach.
Nomenclature C = fluid-phase solute concentration, g/cm3 D = diffusivity, cm2/s F = fluid phase f = fugacity, atm A@ = heat of fusion, cal/mol K = distribution coefficient, cm3/g of soil k = interaction parameter L = liquid phase P = pressure, atm Q = SCF volumetric flow rate, cm3/s R = gas constant, 1.987 cal/(mol.K) or 82.06 (atm.cm3)/ (mo1.K) S = solid phase SFE = solid-fluid equilibrium, the fluid phase is either liquid or vapor SLVE = solid-liquid-vapor equilibrium T = temperature, K t = time, s
Subscripts i = species
m = melting point 0 = initial value 1 = supercritical solvent 2 = solute 3 = entrainer Superscripts
01 = pure subcooled liquid os = pure solid Registry No. DDT, 50-29-3; COz, 124-38-9; CH,OH, 67-56-1;
Literature Cited Brady, B. 0.;Kao, C.-P.; Gambrell, R. P.; Dooley, K. M.; Knopf, F. C. Ind. Eng. Chem. Res. 1987,26, 261. deFilippi, R. P.; Krukonis, V. J.; Robey, R. J.; Modell, M. US.EPA Report 600/2-80-054, 1980; EPA Office of Research and Development; Research Traingle Park, NC. Eppig, C. P.; deFilippi, R. P.; Murphy, R. A. U S . EPA Report 600/2-82-067, 1981; EPA Office of Research and Development, Research Triangle Park, NC. Heideman, R. A.; Khalil, A. M. AZChE J. 1980, 26, 769. Modell, M. US.Patent 4 338 199, 1982. Ng, H.-J.; Robinson, D. B. J. Chem. Eng. Data 1978, 23, 325. Peng, D. Y.; Robinson, D. B. Ind. Eng. Chem. Fundam. 1976,15,59. Ringhand, P. H.; Kopfler, F. C. 186th National Meeting of the American Chemical Society, Washington, D.C., Aug 28-Sept 3,1983. Semenova, A. I.; Emel'yanova, E. A.; Tsimmerman, S. S.; Tsiklis, D. S. Russ. J . Phys. Chem. 1979, 53, 1428. US.EPA Interim Methods for the Sampling and Analysis of Priority Pollutants in Sediments and Fish Tissue; U.S. EPA, Environmental Monitoring and Support Laboratory: Cincinnati, OH, 1980. US.EPA Organochlorine Presticides and PCB's-Method 608; US. EPA Environmental Monitoring and Support Laboratory: Cincinnati, OH, 1982; 600/4-82-057. Vasilakos, N. P.; Dobbs, J. M.; Parisi, A. S. Ind. Eng. Chem. Process Des. Deu. 1985, 24, 121.
Received f o r review August 18, 1986 Accepted July 7, 1987
