Ind. Eng. Chem. Res. 1987,26, 261-268
data on alcohol-water-solvent mixtures are needed.
Acknowledgment We thank STVF (The Danish National Council for Scientific and Technical Research) for support of this acknowledges work. In addition E.A. Brignole Consejo Nacional de Investigaciones Cientificas y TBcnicas de la Reptiblica Argentina for the award of a fellowship. Registry No. H,CCH,OH, 64-17-5; (H,C),CHOH, 67-63-0; H,CCH&H,, 74-98-6; (H,C),CHCH,, 75-28-5.
Literature Cited Andersen, P. M. Ph.D. Thesis, Instituttet for Kemiteknik, Danmarks Tekniske Heriskole. Lvnebv. 1986. Brignole, E. A. Ph.D. Thesis, Instituttet for Kemiteknik, Danmarks Tekniske Herjskole, Lyngby, 1985. Brignole, E. A.; Skjold-Jerrgensen, S.; Fredenslund, A. Ber. 6 u n senges. Phys. Chem. 1984, 88, 801. Brignole, E. A., Skjold-Jmgensen, S.; Fredenslund, A. In Supercritical Fluid Technology;Penninger, J. M. L., Radosz, M., McHugh, M. A., Krukonis, V. J., Eds.; Elsevier: New York, 1985; pp 87-106. I
I
”
26 1
Busche, R. M. “Status and Future Opportunities: Separation Processes for Oxychemicals”, Presented at the NSF Workshop on Research Needs in Renewable Materials Engineering, Purdue University, Lafayette, IN, 1983. Gars, H. J. Ber. Bunsenges. Phys. Chem. 1984, 88, 894. Kobayashi, R.;Katz, D. L. Ind. Eng. Chem. 1953,45, 440. Kreim, K. Doctoral Thesis, Technical Universitat of HamburgHarburg, 1983. Kuk, M. s.;Montagna, J. c. In Chemical Engineering at Supercritical Fluid Conditions; Paulaitis, M. E., Penniger, J., Gray, R., Davidson, Ph., Eds.; Ann arbor Science: Ann Arbor, MI, 1983; p 101. Maiorella. B. L. Hvdrocarbon Process. 1982 (Aua), 95. Moses, J.‘M.; Goklen, K. E.; De Filippi, R. P. Presented at the AIChE Annual Meeting, Los Angeles, 1982, Paper 127c. Paulaitis, M. E.; Kander, R. G.; Diandreth, J. R. Ber. Bunsenges. Phvs. Chem. 1984,88. 869. Radosz, M. Ber. Bunsenges. Phys. Chem. 1984, 88, 859. Skjold-Jmgensen, S. Fluid Phase Equilib. 1984, 16, 317. Tsonopoulos, C.; Wilson, G. M. AIChE J . 1983, 29, 990.
Received for review September 20, 1985 Revised manuscript received May 23, 1986 Accepted August 11, 1986
Supercritical Extraction of Toxic Organics from Soils Basil 0. Brady, Chien-Ping C. Kao, Kerry M. Dooley, and F. Carl Knopf* Department of Chemical Engineering, Louisiana State University, Baton Rouge, Louisiana 70803
Robert P. Gambrel1 Department of Marine Sciences, Louisiana State University, Baton Rouge, Louisiana 70803
Supercritical fluid (SCF) extraction of organic hazardous waste from contaminated soils is a promising new technique for hazardous waste-site cleanup. T h e ability of SCFs to solubilize heavy molecular weight organics is well-documented. In this investigation, supercritical carbon dioxide (SC-COJ was used to extract polychlorinated biphenyls (PCBs), 2,2-bis(p-chlorophenyl)-l,l,l-trichloroethane (DDT), and toxaphene from contaminated topsoils and subsoils. An attractive feature of this process is that the C02, being virtually inert, leaves no solvent residue on the processed soil. Supercritical C 0 2a t 100 atm and 40 “C was continuously passed through a fixed bed of 10 g of soil. Approximately 70% of the D D T and 75% of the toxaphene can be leached from a topsoil contaminated with 1000 ppm D D T and 400 ppm toxaphene in under 10 min by using SC-C02 a t a rate of 0.7 g/s. T h e extraction of contaminated (with 1000 ppm PCBs) subsoil proved to be even more promising, with over 90% P C B extraction in under 1 min a t the same C 0 2 rate. Supercritical fluid (SCF) extraction of organic hazardous waste from contaminated soils is a promising new technique for hazardous waste-site cleanup. The ability of SCFs to solubilize heavy molecular weight organics is well-documented (Paulaitis et al., 1982; Groves et al., 1985). In this investigation, supercritical carbon dioxide (SC-C02) was used to extract PCBs, DDT, and toxaphene from contaminated topsoils and subsoils. An attractive feature of this process is that the COz, being virtually inert, leaves no solvent residue on the processed soil. Furthermore, the ease of separation of the extracted solute from SC-C02 results in the creation of a small waste volume of the now concentrated organic, improving the efficiency of subsequent treatment processes such as combustion. Typically in SCF extraction, a simple solvent gas, such as C02, is contacted with a solid or liquid phase at high pressure and moderate temperature. Slight changes in the temperature or pressure of the system can cause large changes in the density of the solvent and consequently in its ability to solubilize heavy nonvolatile waste compounds from the solid or liquid phase. For example, by manipulation of the system pressure, a nonvolatile can be extracted. Following a pressure letdown, generally to below 0888-5885/87/ 2626-0261$01.50/0
the system’s critical conditions, this same material can be completely precipitated from the solvent. Thus, the SCF phenomenon offers a unique opportunity for separation and recovery of “difficult to separate” materials in one processing stage. Supercritical fluid densities compare to liquid densities; however, their viscosities and diffusivities are intermediate to typical liquid and gas values of these properties. Thus, SCFs have the solvent power of liquids with better mass-transfer characteristics than typical liquids. Consequently, separation efficiencies for SCF extractions can be much higher than for liquid solvent extractions. In order to explain the application of SCF to the extraction of organics from soils, this paper is divided into four sections. The first section discusses the phase equilibria expected from both a C0,-PCB and a C0,-DDT mixture. The thermodynamic equations used to develop these semiquantitative phase diagrams are reviewed. The second section reviews the rate process of desorption from porous media, such as soils, into a supercritical phase. In the third section, the results of an experimental design employed to determine the effectiveness of SC-C02 extraction are presented. Finally, a lumped parameter model 0 1987 American Chemical Society
262 Ind. Eng. Chem. Res. Vol. 26, No. 2, 1987 '0°
r
7wr
P2IS,S2LIVI
1 .
2 - s,qv 3 - TP,
600
4
500
-
5 - S2LZV 6 - TPz 7 - O,(S2L,L2VI r
8-
600
-
L,L*V
500
1
2
3
S2L,V
400
\
Figure 1. P-T projection of the phase diagram for a PCB (C,,H,Cl,)-C02 mixture.
of the extraction process is employed to yield overall distribution coefficients characteristic of SCF extraction. For the extraction experiments, two soil types, a topsoil and a subsoil, contaminated by three organic waste mixtures, PCBs, DDT (and related compounds), and toxaphene, formed the basis of the experimental design. For the first tests, the soil used was a topsoil containing 12.6% organic matter and approximately 1000 ppm DDT and 400 ppm toxaphene; this test soil was obtained from an actual spill site where the DDT and toxaphene had penetrated the soil for a period of 10-12 years. For the next tests, uncontaminated topsoil (from an area near the contaminated soil) was spiked in the laboratory with 600 ppm DDT. In the final tests, a Cecil subsoil containing only 0.74% soil organic matter was spike with 1000 ppm PCBs (Aroclor 1254). The test soils were supplied by the LSU Wetland Soils and Sediments Laboratory. The unique properties of the individual soils and also certain environmental factors may affect extraction efficiencies; the effects of some environmental factors have been explored. For example, the effect of water in the soils was investigated by examining both dry and wet (20 wt % water) test soils. The effect of long-term exposure of the soil to the organic waste was examined through the use of both spill-site and lab-spiked topsoils. Finally, the effect of solute interactions was explored by testing spill-site soils contaminated with more than one organic waste.
Thermodynamic Considerations Phase diagrams for solutes (PCBs, p,p'-DDT) in equilibrium with supercritical solvents (CO,) provide insight into the solubility relations that may exist under various operating conditions. For simplicity, phase diagrams are discussed for binary systems only. The system consists of component 1,the supercritical solvent, and component 2, the solute to be dissolved. A complete phase diagram for a binary mixture is very complicated; however, for present considerations, a simplified P-T projection of the phase diagram, consisting of the vapor pressure curves for the pure components, the critical loci, and the S2-L-V equilibrium lines, is sufficient. Figure 1 shows such a P-T diagram for the C0,-PCB (as represented by the lumped isomers C12H5C15)system. This type of phase behavior is expected for a binary mixture in which there is immiscibility of the liquid phases
I
-
S,L,V TP,
\
Figure 2. P-T projection of the phase diagram for a p , p '-DDT-C02 mixture.
until temperatures above the critical point of the more volatile component are reached (Rowlinson, 1969). Such phase behavior may be expected to occur for this system because the melting point of PCB (Aroclor 1254) is below the critical temperature of the solvent C02. The curves TP,-CP, and TP2-CP2 represent the vapor pressure curves for the pure liquids, beginning with their triple points (TP,) and ending with their critical points (CP,). The gas(or vapor)-liquid critical locus begins with CP, and ends at the critical end point K due to the formation of two immiscible liquid phases in equilibrium with the gas. The three-phase line (LlL2Vequilibrium line) ends at a quadruple point, Q1, where four phases (S2LlL2V) coexist. A t quadruple point Q1, three three-phase equilibrium lines appear-a SzL2Vequilibrium line which extends to the triple point of the PCB, a S2L,V equilibrium line which extends to the other quadruple point, Q2, where phases S1S2LlV coexist, and a S2LlL2equilibrium line which extends to elevated pressures. From the critical point of PCB (CP,), the vapor-liquid critical locus continues to elevated pressures where it may intersect the S2LlL2equilibrium line. For supercritical extraction, the region of interest is the area noted on Figure 1. At these conditions, the COz and Cl2H5Cl5exhibit a L-F equilibrium, here F implying a fluid phase which may be either a vapor or liquid phase. Figure 2 gives the pressure-temperature diagram for the mixture COP-p,p'-DDT. This type of phase behavior is expected for binary mixtures in which the melting point of the solute is above the critical point of the solvent, and the solubility of the solid solute in the liquid mixture is low (Rowlinson, 1969). Again the curves TP1-CP, and TP2-CP2 represent the vapor pressure curves for the pure liquids beginning with their triple points and ending with their respective critical points. The gas(or vapor)-liquid critical locus begins at CP, and ends at the lower critical end point (LCEP). There is a break in the critical locus between the lower critical end point and the upper critical end point (UCEP). At the LCEP, the vapor and liquid phases merge into a single fluid phase which is in equilibrium with solid p,p'-DDT. The region between the LCEP and the UCEP is a region in which pure solid p,p'-DDT is in equilibrium with a single homogeneous supercritical phase. The curve from the LCEP to low pressures is a three-phase equilibrium line, along which a gas, a liquid, and a solid p,p'-DDT phase is present. This three-phase line is interrupted by
Ind. Eng. Chem. Res. Vol. 26, No. 2, 1987 263 Table I. Physical Properties of p,p'-DDT and Aroclor 1254
Table 111. Vapor Pressures of Liquid Aroclor 1254
(C12H5C15)
(C12H5C15)
DDT
Aroclor 1254
354.49 938 21.5 675.5 0.469 382"
326.5 909 22.9 652 0.479 283b
222 2597.6'
192 1866d
p,p'-
T,"C ref Klincewicz and Reid, 1984 Klincewicz and Reid, 1984 Reid et al., 1977, pp 604-605 Reid et al., 1977, p 20 a: Weast and Astle, 1978 b: Hutzinger et al., 1974 Immirzi and Perini, 1977 c : Bidleman, 1984 d : Burkhard et al., 1985
25 56 75 112 144 200 270
P', calcd, mmHg 8.25 x 10-5 1.74 x 10-3 0.832 X lo-' 1.07 X lo-' 0.639 7.49 70.8
PI, exptl," mmHg 7.71 X 1.0 x 10-3 1.0 x 10-2 1.0 x 10-1 1 10 100
"Hutzinger et al., 1974.
Table 11. Vapor Pressure of Solid p,p'-DDT T , "C P s , calcd," mmHg Pa,exptl,b mmHg 2.3 X 10* 2.2 x 10-7 20 4.3 x 10-7 4.8 X lo* 25 9.3 x 10-7 9.6 X lo* 30 3.6 x 10-5 4.0 X 10+ 40 1.2 x 10-4 1.5 x 10-5 50 4.8 x 10-5 3.9 x 10-4 60 1.2 x 10-3 1.5 x 10-4 70 3.2 x 10-3 4.5 x 10-4 80 "In ( P s / P ' )= (AHf/R)[(l/Tm)- (1/2")],where P' is the vapor pressure of subcooled liquid, calculated from the Peng-Robinson equation of state. * Rothman, 1980.
the formation of a solid COz phase at the invariant quadruple point. The branch of the critical locus which begins a t the critical point of pure p,p'-DDT (CP,) is interrupted by a three-phase SzLzV equilibrium line at the UCEP. As noted by Rowlinson (1969), this type of freezing-point-depression curve arises from the low solubility of solid p,p'-DDT in the liquid mixture. For supercritical extraction, the region of interest occurs in the shaded area noted in Figure 2. At these conditions, COz and p,p'-DDT exhibit a Sz-F equilibrium, here F implying a fluid phase which may be either a vapor or a liquid phase. The semiquantitative representation of P-T projections, Figures 1 and 2, for the highly unsymmetric binary mixtures studied here can be obtained by using the PengRobinson equation of state (1976). The critical locus ( V = L ) for a binary mixture described by the Peng-Robinson equation of state can be calculated according to the algorithm proposed by Heideman and Khalil (1980). At the S2LV equilibrium, the following fugacity equalities must be satisfied: fl" f2"
(1)
= fll
= fzl =
fZOS
(2)
The solid phase is considered as pure solid 2 because the solubility of solvent 1 in solid solute 2 is negligible. The values off: and f / can be calculated directly from the Peng-Robinson equation of state. With the assumption that this equation is valid for calculating the fugacities of pure subcooled liquid (f201)and one additional equation,
the fugacity of pure solid 2 (fzo8)can be evaluated also. To construct the P-T projection, certain physical properties of the pertinent components were needed; these properties are given in Table I, together with the references to the original data or to the method used to estimate the data.
IO
Figure 3. Estimated PCB (ClZH5Cl5)solubility in CO,.
The comparisons between the experimental and calculated vapor pressures for p,p'-DDT and Aroclor 1254 given in Tables I1 and I11 demonstrate the limited reliability of the estimated physical properties and the limitations of using the Peng-Robinson equation of state to describe the phase behavior of heavy components of low volatility a t ambient temperatures. For solid p,p'-DDT, the calculated vapor pressures are higher than the experimental values by an order of magnitude. However, the calculated vapor pressures of C12H5C15are on the same order of magnitude as the vapor pressures of Aroclor 1254 as obtained by extrapolation of experimental values (Hutzinger et al., 1974). Normally the desorption rate of a heavy organic from a soil is proportional to its solubility in the vapor phase. The solubility of a compound of low volatility in a supercritical fluid near its critical point is orders of magnitude higher than the ideal solubility (the ratio of the vapor pressure of the organic to the total pressure). Figure 3 shows the estimated solubility of Aroclor 1254 (ClZH5Cl5) in C02 at 35,40, and 50 "C as calculated from eq 1-3. The enhancement factors (the ratios of real solubility to ideal solubility) a t 100 atm are estimated to be lo6, lo6, and lo3, respectively.
Desorption from Porous Media It is important to note that the semiquantitative P-T plots in Figures 1 and 2 represent the phase behavior of the heavy organics and COz in the absence of soil. In this section, the extraction of organics from porous media will be briefly reviewed in terms of selected factors influencing
264 Ind. Eng. Chem. Res. Vol. 26, No. 2, 1987
t
w 3 I 5 0 ,-
i "
l._0
-L,Et01
-
~
~
1
€102
I
Et03
i-
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PORE RADIUS l n m l
Figure 4. Cumulative pore-size distribution of the topsoil and the Cecil subsoil.
desorption: the soil structure, the soil organic and water contents, and capillary condensation. As a first step toward understanding soil structures, the distributions of pore volumes within the two soil types were determined by mercury porosimetry. The pressure required to force the mercury into a differential pore volume is inversely proportional to the pore equivalent radius: -2y cos 4 r= (9.87) (4) P The results shown in Figure 4 indicate that the pore volume of the DDT-contaminated topsoil was dominated by voids (60-70% of the pore volume) of equivalent radius greater than 10 pm. The cumulative pore-size distribution gave a corrected voidage of 0.39 cm3 voidage/cm3 total. In comparison, the PCB-contaminated Cecil subsoil, Figure 4, showed a pore distribution dominated by pores of under 100-nm radius (50-60% of the pore volume). The cumulative pore-size distribution of the dried and ground soil material yielded a voidage of 0.47 cm3 voidage/cm3 total. The porosimetry results highlight the fact that the soils are quite porous, with similar void volumes. The two soil types differed significantly in their pore-size distributions, however. Further measurements revealed that spiked and nonspiked soils have equivalent distributions. Desorption of heavy organics from soils a t ambient conditions by volatilization or aqueous extraction has been discussed extensively (Deming, 1963; Kearney et al., 1964; Guenzi and Beard, 1970; Igue et al., 1972; Spencer and Cliath, 1970, 1972, 1973; Farmer et al., 1972). A heavy organic in soil may be partitioned in a number of ways: between the soil and water, soil and atmosphere, and even between a gas compartment within the soil and the soil itself. The rate of desorption of heavy organics to the atmosphere is controlled by many factors, such as a compound's vapor pressure, its solubility in the water present in most soils, and the chemical and physical properties of the soil such as its total organic matter content, porosity, pore-size distribution, clay content, water content, humidity, and temperature. The vapor pressure of a heavy organic in a soil may differ from the vapor pressure of the pure organic. This difference in vapor pressure can be attributed to adsorption of the compound on the soil, which reduces its chemical activity to below that of the pure compound. Thus, a larger heavy organic concentration in a soil is needed to give a vapor pressure equivalent to that of the pure compound. The water content of the soil greatly affects the observed vapor pressures of heavy organics. Water competes for adsorption sites on the soil. In the presence of nonpolar or weakly polar heavy organics, water is preferentially
adsorbed onto soil particles; hence, the presence of water weakens heavy organic soil bonding which in turn increases the organic's vapor pressure and enhances its rate of desorption. Adsorption of pesticides and other heavy organics from aqueous solution onto soils has been widely explored (Hamaker and Thompson, 1972; Green, 1974; Weed and Weber, 1974; Haque, 1975; Khan, 1978) but remains poorly understood. Only generalities are possible, but these generalities are pertinent because they may also apply to supercritical extraction. Soils which are low in organic matter normally contain sites that preferentially adsorb polar and ionic species. A soil that has a higher organic matter content exhibits a wide variety of sites that interact not only with polar and ionic compounds but with weakly polar and nonionic compounds as well. The dependence of chlorinated hydrocarbon adsorption on organic matter content is well-documented (Edwards et al., 1957; Kay and Elrick, 1967; Harris and Sans, 1967; Beal and Nash, 1969; Adams and Li, 1971). Of special interest in this work are several investigations into the adsorption of both DDT (Bailey and White, 1964; Yaron et al., 1967; Wershaw et al., 1969; Shin et al., 1970; Haque et al., 1974; Lichtenstein et al., 1977) and PCBs (Haque et al., 1974) from aqueous solution onto soils and soil fractions. The adsorption of organochlorine compounds was shown to increase considerably with increasing natural soil organic matter. Chlorinated hydrocarbons can be characterized as being hydrophobic compounds, and therefore the organic matter in the soil must have also been hydrophobic. Clay soils, such as a Cecil subsoil with a low organic matter content, typically have a very low adsorptive capacity for weakly polar hydrophobic contaminants such as DDT and PCBs. In contrast, there are few articles which treat the problems of either adsorption from or desorption into supercritical fluids. deFilippi et al. (1980) reported that supercritical C 0 2 can effectively extract a broad range of organic compounds from activated carbon. Picht et al. (1982) studied the desorption of three chemicals (acetic acid, phenol, and alachlor) from five adsorbents (two polymer resins, two carbonaceous resins, and activated carbon) into supercritical CO, at 55 "C and 150-210 atm. Adsorption at supercritical conditions has been treated more fundamentally in the work of Hayhurst et al. (1983) and Jones et al. (1959). Hayhurst et al. measured the adsorption isotherms of pure oxygen, nitrogen, and argon on three zeolites at temperatures from 308 to 338 K (far above the critical temperatures of the adsorbates) and pressures to 120 atm. The adsorbed gas molecules were considered to exist in the microporous volume of the zeolite in a state near to that of a liquid. The heats of adsorption were similar to extrapolated heats of condensation of an adsorbate molecule. Jones et al. (1959) measured the adsorption of C 0 2at 19,30, and 32 "C and at pressures from 1 to 80 atm within compressed porous plugs of lampblack (mean pore radius equivalent to 10 nm). The adsorption of C 0 2 at 19 and 30 " C was attributed to capillary condensation, and the shape of the COz adsorption isotherm at 32 "C, which is higher than the critical temperature of COz, was similar to that of the isotherms at 19 and 30 "C. At 32 "C the condensation was found to occur at 74 atm, which was approximately the "pseudo saturated vapor pressure" (75 atm), the pressure at which the density of COz increased rapidly. A t 40 "C this pseudo saturated vapor pressure occurs at a calculated value of 87 bar, implying that when C 0 2 at 40 "C and above 87 bar is in contact with a porous
Ind. Eng. Chem. Res. Vol. 26, No. 2, 1987 265
u
B - aiolout P - Pra%sure Gaups Pc-PIeSIuIe
P R - PII,*"I* R*~"IOtOI i~ ~ ~ ~ e e n m~p r oat u rt.
~
~
.
CO"f,Oll.,
Figure 5. Schematic of apparatus for supercritical soil extraction.
adsorbent, the CO, should partition between two phases, a free phase and a condensed phase. if we consider organics to be contained within the soil's micropore structure, the previous discussion indicates that a rigorous model of the SC desorption process must consider the sorbate to be partitioned between three phases: the free SCF phase, the condensed phase, and the adsorbed phase.
Experimental Methods Supercritical Soil Extractions. Two soils, a Cecil subsoil and a topsoil, were used. The soils were obtained in 1-2-kg batches, some initially free of DDT or PCB contaminants and some contaminated with DDT or PCBs. Some of the uncontaminated soil samples were then spiked with 1000 mg/kg of Aroclor 1254 (Chem Service, 99%) or 600 mg/kg of p,p'-DDT (Chem Service, 99%). They were air-dried and the initial PCB or DDT levels checked via EPA-approved test procedures (US EPA, 1980, 1982). The topsoil was obtained from a site at Lake Providence, LA. Its organic matter content was determined as 12.6 wt % by combustion train analysis. The Cecil subsoil was obtained from Simpson Experimental Station, Clemson University, and its organic matter content was determined as 0.74 wt % by wet chromic acid digestion. The subsoil consisted of approximately 35% sand, 35% silt, and 30% clay. Preparation of the lab-spiked soils was as follows. One liter of acetone and 1 L of water were added to a 4-L beaker with a magnetic stirrer used to maintain vigorous mixing. Then 500 g of finely ground soil was added to the liquid mixture to form a suspension. DDT or Aroclor 1254 in acetone was added from a buret to the acetone/ water/soil slurry to obtain the desired loadings. Stirring of the spiked suspension was continued for an additional 3 h, and then the slurry was placed in a forced draft oven operated at 35 "C to evaporate the water and acetone. Once dry, the lab-spiked soil was finely ground and mixed. A schematic diagram of the SC-C02extraction apparatus is given in Figure 5. Liquid COz (Liquid Carbonic, 99.9%) a t ambient temperature was fed to a diaphragm compressor (Superpressure Model 46-13421) and compressed to a pressure between 200 and 350 atm. The compressed CO, was stored in surge tanks to dampen any pressure fluctuations. From the surge tanks the CO, flowed at 0.7 g/s to a vertical tube where the pressure was controlled by a Tescom regulator (Model 44-1124) to f 5 psi. In the tube a fixed bed of the contaminated soil was contacted by the SC-CO,. The extraction pressure was monitored by a Heise digital pressure gauge (Model 710A). Both the SC-C02feed line and the tubular fixed bed were immersed in a constant-temperature bath maintained a t 40 f 1 "C. Downstream of the bed, two Autoclave micrometering valves were used to control the C 0 2 flow rate and reduce the pressure to atmospheric. The exhaust CO, passed through two filter traps to collect the precipitated contaminants and a dry test meter (Singer Model DTM200) to totalize the solute-free CO,. All wetted parts of
the apparatus were stainless steel, Teflon, or Viton. Sample Processing and Analysis. The residual concentration of PCBs or DDT in the soil was measured via EPA-approved test procedures (US EPA, 1980, 1982). Typically 5 g of soil was weighed to fO.O1 g into a cellulose extraction thimble, which was placed in a Soxhlet apparatus and extracted for 8 h with a 60/40 mixture of acetone/hexane (both solvents Baker Resi-Analyzed, >99.9%). The acetone/hexane extracts were then washed to remove acetone, and the aqueous phase was discarded. The hexane portion, containing the DDT or PCBs, was mixed with anhydrous sodium sulfate to remove moisture, evaporated to 5 cm3 by using a stream of dry nitrogen gas, and then passed through a Sep Pak Florisil cartridge followed by at least 10 cm3 of hexane and 5 cm3 of 6% diethyl ether-94% petroleum ether. This hexane-ether extract was evaporated until all of the ether was removed, and the samples were then diluted with hexane as necessary for analysis by gas chromatography. The gas chromatograph employed to analyze these samples was a Perkin-Elmer Model 3920B equipped with packed columns and two electron capture detectors. The carrier gas was 5% methane in argon at 70 cm3/min. The packed columns used were (1) a 6-ft-long, 1/4-in.-o.d. column of 4% SE-30 and 6% OV-210 on Gas-Chrom Q, 80/100 mesh; and (2) a 6-ft-long, 1/4-in.-o.d.column of 3% OV-1 on Chromosorb W, 80/100 mesh. They were operated at 200 "C. DDT standards were prepared from standard-grade DDT obtained from the EPA Pesticides and Industrial Chemicals Repository. PCB standards were prepared by dilution of stock solutions of Aroclor 1254 in pesticidegrade n-hexane (Baker Resi-Analyzed, >99.9%). These laboratory standards were compared with standards from the EPA Repository for Toxic and Hazardous Materials (E105, PCB-Aroclor 1254, QAT). Sample blanks consisting of empty cellulose thimbles were analyzed by using the procedures described above to check for the possibility of reagent contamination. Quality control sediment samples containing various levels of Aroclor 1254 were available from the EPA Environmental Monitoring and Support Laboratory in Cincinnati, OH. For every large batch (20-30) of research samples extracted and analyzed, one sample of this standard reference material was extracted and analyzed by using the identical procedures used for the samples. Our criterion for acceptability was less than 15% variation of the extract concentration of this standard from the corresponding point of the calibration curve. Pore Size Distributions. These were measured by mercury penetration porosimetry using about 2-5 g of soil. The porosimeter was a Micromeritics Model 915-2 with a maximum pressure of 3400 atm. The mercury (Sargent-Welch, reagent grade) was cleaned prior to each experiment by washing with concentrated H N 0 3 followed by decantation.
Results The first test soil used to demonstrate the effectiveness of SC-CO, extraction (at 100 atm, 40 "C) was the DDTtoxaphene-contaminated topsoil (1000 mg/ kg of DDT). Extractions of this test soil showed that approximately 60-7070 of the DDT could be removed in approximately 10-20 min as shown in Figure 6. Longer time extractions did not show any improvement over this reduction. These data imply that a portion of the DDT is strongly bound to the soil and that SC-CO, at these conditions cannot extract this strongly bound DDT. In order to study the effects of long-term exposure of the soil to the hazardous organics, SC-CO, extraction data
266 Ind. Eng. Chem. Res. Vol. 26, No. 2 , 1987
09
;
w
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l
a
n
,
P L
04
h
-
L ---L____L-A
~
20
0
30 4c TIME I M IIYICII
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60
Figure 6. Temporal variation of relative soil concentration in the extraction of a DDT-contaminated, spill-site topsoil. The solid curve is the fit to the data by using the simple equilibrium model (eq 7).
0 1 -
.L I
L-_
10 "I
~
20
~. L. 40
30
-
-
50
L
-
-
60
.
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T I M E (Minutes)
Figure 9. Temporal variation of relative soil concentration in the extraction of wet (20 wt % water) PCB and DDT-contaminated, lab-spiked soils. The solid curve is the fit t a the data by using the simple equilibrium model (eq 7).
L
>
;2 7 Y
-
_. -L CO
30C
20C
COO
-1
_. 60:
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Figure 7. Temporal variation of relative soil concentration in the extraction of a DDT-contaminated, lab-spiked topsoil. The solid curve is the fit to the data by using the simple equilibrium model (eq 7).
both dry and wet test soils. In an attempt to match the natural condition of a moist soil, the DDT-toxaphenecontaminated topsoil and the PCB-contaminated subsoil were prepared with a 20 wt % moisture content. The wet soils were then extracted at the usual conditions. These results are given in Figure 9. When compared to the corresponding dry soil extractions (Figures 8 and 6), it is evident that with moist soils there was a slower rate of removal of the contaminants (especially in the case of the PCB-contaminated subsoil), but the final contaminant levels in the soils were comparable to those obtained from dry soil extractions.
05 r
+
I
+ a
6 03"z
; Y
2
9
-
L
-
-
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200
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e 300
TIME lseconasl
Figure 8. Temporal variation of relative soil concentration in the extraction of a PCB-contaminated, lab-spiked Cecil subsoil. The solid curve is the fit to the data by using the simple equilibrium model (eq 7).
for a DDT lab-spiked (600 mg/kg) topsoil were obtained to compare to the previous data. These results are given in Figure 7. For the lab-spiked material, there was a more smooth decrease of DDT concentration vs. time, but again only -60-70% of the DDT was extracted. The extraction of the PCB-contaminated Cecil subsoil proved to be more effective. As is evident from the data in Figure 8,99% PCB removal was effected in under 1min of processing with SC-C02. Apparently there were no appreciable rate limitations in this extraction, and all the PCBs must have been weakly bound. As was explained in the section on desorption from porous media, soils with a low organic matter content (0.74 wt % for the Cecil subsoil) typically do not strongly adsorb nonpolar compounds such as PCBs. The effect of soil water content on the SC-C02extraction of the contaminated soils was investigated by extracting
Discussion of Results The rapid extraction of PCBs by SC-C02a t 40 "C and 100 atm contrasts sharply to the resistance of p,p'-DDT to extraction at these same conditions. Two possible explanations for this result are presented here. The first and most likely explanation focuses on the different physical and chemical characteristics of the soils. The PCB-contaminated Cecil subsoil was characterized by pores of under 100-nm equivalent radius (Figure 4) and contained minimal organic matter (0.74 wt %). Therefore, on the basis of the previous discussion of soil adsorption, this soil's adsorptive capacity for weakly polar PCBs should have been small. This was borne out by extraction data for a dry soil (Figure 8). However, the addition of water must have resulted in strong water adsorption on the soil's polar sites and capillary filling of the small pores. In the wet soil, PCBs would therefore have desorbed from a liquid-like capillary phase of somewhat higher affinity for PCBs, as evidenced by the extraction results of Figure 9. In contrast, the DDT-contaminated topsoil consisted of a significant amount of organic matter, 12.6 wt %, either contained within the soil matrix or present as colloidal material within the pores (note the relative absence of small pores in the topsoil distribution of Figure 4). As expected based on the previous discussion of soil adsorption, the p,p'-DDT was strongly adsorbed in the presence of hydrophobic organic matter; this conclusion is based on the slow removal rates of DDT from the topsoil (Figures 6 and 7) and also the presence of residual DDT which could not be removed. The addition of water had less effect on the DDT extraction (Figure 91, probably because most of the water could only penetrate the larger voids which did not contain DDT anyway. The DDT presum-
Ind. Eng. Chem. Res. Vol. 26,No. 2, 1987 267 ably was concentrated on the soil or in colloidal material within small pores. A second possible explanation for the p,p’-DDT and PCG extraction results may reside in the chemical structure of these compounds. For example, a typical pentachloro component of PCB is 2,2’,3,4,5’-pentachloro-l,1’biphenyl. The chlorines present are mildly electron withdrawing, but the compound as a whole is nonpolar and hydrophobic. CI
Table IV. Distribution Coefficients for SC-C02 Extraction of Contaminated Soils soil K , cm3/g of soil lab-spiked Cecil subsoil (PCBs) 9.6 wet lab-spiked Cecil subsoil (PCBs) 38 lab-spiked topsoil (DDT) 28 spill-site topsoil (DDT/toxaphene) 84 wet spill-site topsoil (DDT/toxaphene) 120
phases (where K is the distribution coefficient in cm3/g of soil), the solution of eq 5 is given by
A linear regression of the extraction data, In (Oleo) vs. t , allows the determination of the distribution coefficient ( K ) ,
CI
CI
1.1’- biphenol, 2.2’. 3.4.5’-
pentrchloro
-
The structure of p,p’-DDT shows that an acidic hydrogen is present on the carbon located between the rings. CI
cI
\I/
p,
PI-
CI
DDT
This mild acidity results from the two rings that are electron withdrawing and also the three electron-withdrawing chlorines present on the carbon p to the hydrogen. Because of this slightly acidic hydrogen, it is likely that p,p’-DDT can bond strongly with both hydrophobic and some of the hydrophilic groups present in and on the soil. The p,p’-DDT was adsorbed on a topsoil with a high organic content and therefore a high concentration of both types of groups. To test the possibility that the residual DDT is more strongly adsorbed than the DDT which was extracted, the extraction temperature of the DDT-contaminated topsoils could be elevated, for example, from 40 to 80 “C. Raising the temperature would have two effects: first, the equilibrium solubility of DDT in SC-C02 would be slightly reduced but, second, a t the higher temperature adsorbate-surface bonds are more easily broken and solubility in organic colloidal material is reduced. A simple preliminary approach to modeling the SC-C02 extraction data has been attempted. The contaminated soil bed is viewed as well-mixed with efficient contacting by the COz. Mass-transfer resistances in this case are considered to be negligible, with equilibrium being the only contributing factor to the extraction process. The finely ground soils used (
