Environ. Sci. Technol. 2000, 34, 3462-3468
Cyclodextrins for Desorption and Solubilization of 2,4,6-Trinitrotoluene and Its Metabolites from Soil TAMARA W. SHEREMATA AND JALAL HAWARI* Biotechnology Research Institute, National Research Council of Canada, Montreal, Quebec, Canada H4P 2R2
Heptakis-2,6-di-O-methyl-β-cyclodextrin (DMβCD) and hydroxypropyl-β-cyclodextrin (HPβCD) (1% w/w solutions) were investigated for their ability to desorb 2,4,6trinitrotoluene (TNT), 4-amino-2,6-dinitrotoluene (4-ADNT), and 2,4-diamino-4-nitrotoluene (2,4-DANT) from two artificially contaminated soils (an organic rich topsoil and an illite shale). The DMβCD (which is highly surface active) was more effective than HPβCD (negligible surface activity) for desorption of the three nitroaromatic compounds (NACs) from the two soils. The efficiency of both CDs for NAC removal from topsoil decreased with increasing amino substitution (i.e. TNT > 4-ADNT > 2,4DANT), whereas for illite the efficiency generally decreased with increasing nitro substitution (i.e. TNT < 4-ADNT. 2,4-DANT). In general, the NAC removal efficiency increased with decreases in the sorption capacity constant (Kds). The two CDs were also assessed for their ability to remove TNT from a highly contaminated topsoil (5265 mg/kg) obtained from a former manufacturing facility that had been aged for 10-40 yr. The estimated association constants (Ks) were comparable to those obtained for solubilization of pure TNT into aqueous solution. This study demonstrates the effectiveness of CDs in decontaminating a variety of soils containing both high and low levels of TNT and some of its associated metabolites.
Introduction Soils contaminated with 2,4,6-trinitrotoluene (TNT) from activities in the munitions and defense industries is a worldwide environmental problem. In natural and engineered systems, it has been demonstrated that the nitro groups of TNT undergo reduction reactions to form amino derivatives that include 4-amino-2,6-dinitrotoluene (4ADNT), 2-amino-4,6-dinitrotoluene (2-ADNT), 2,4-diamino6-nitrotoluene (2,4-DANT), and 2,6-diamino-4-nitrotoluene (2,6-DANT) (1, 2). It has been demonstrated that TNT and its amino derivatives can be irreversibly sorbed by soil (1, 3-11). Irreversible sorption has been postulated as a possible explanation for poor mineralization of TNT in soil (1). Surfactants have been shown to be capable of desorbing TNT from soil (12) and to subsequently enhance the mineralization of TNT by Phanerochaete chrysporium (13). However, there are potential drawbacks to the use of surfactants for in and ex situ remediation. Potential operating problems include sorption of the surfactant by soil, precipitation of the surfactant, phase separation, and foaming * Corresponding author phone: (514)496-6267; fax: (514)496-6265; e-mail:
[email protected]. 3462 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 16, 2000
(14). In addition, if surfactants are to enhance the bioavailability of contaminants, then they must be compatible with the accompanying biological process. For example, nonionic surfactants have been shown to inhibit mineralization of phenanthrene in soil water systems (15). Cyclodextrins (CDs) are cyclic oligosaccharides formed from the enzymatic degradation of starch (16). CDs are considered nontoxic and used in pharmaceuticals and as food additives (17). One class of CDs is able to enhance the solubility of many organic compounds by forming inclusion complexes with “guest” molecules. It has been demonstrated in the laboratory and field that CDs are capable of enhancing the solubility of a number of common organic contaminants (18-20). Hydroxypropyl-β-cyclodextrin (HPβCD) was also shown to significantly enhance the transport of anthracene, pyrene, and trichlorobiphenyl through soil columns (21). In a further study, HPβCD increased the solubility of phenanthrene by 124 times and its removal rate by 6 times by a phenanthrene degrading isolate (22). In a pilot-scale field test, HPβCD increased the aqueous solubility of 12 target compounds by 100 to 20 000 times from a nonaqueous source in an aquifer (20). As discussed by McCray and Brusseau (20), CDs experience little or no sorption by soil and are not subject to precipitation, hence they are easily removed from the subsurface (which may be of regulatory and economic concern). CDs have also been shown to enhance the solubilization of the explosive hexahydro-1,3,5-trinitro-1,3,5triazine (RDX) from soil (23). Based on published data, it would appear that CDs are becoming comparable in cost with surfactants (24, 25). Particularly, it has been reported that the cost of β-CD is as low as $4/kg (25). In comparison, the cost of food grade surfactants, that were evaluated for subsurface remediation, was estimated to be $2.2/kg (assuming 1% inflation in converting to 1999 $) (24). A detailed comparison of the costs of surfactants and CDs for subsurface remediation was not found in the literature. However, since the cost of CDs has continuously decreased in recent years (24), investigations regarding their technical merit for subsurface remediation are justified. We recently demonstrated that significant sorptiondesorption hysteresis exists for TNT, 4-ADNT, and 2,4-DANT sorbed by natural and model soils (3). The purpose of the present study was to examine the ability of two CDs (heptakis2,6-di-O-methyl-β-cyclodextrin (DMβCD) and HPβCD) to desorb TNT, 4-ADNT, and 2,4-DANT from two artificially contaminated soils examined in our earlier study (an organic rich topsoil and an illite shale) (3). The DMβCD was selected because it is highly surface active, and the HPβCD was selected because it is commonly used and less expensive. We also studied the ability of the two CDs to solubilize TNT from a highly contaminated soil obtained from a former manufacturing facility, where contamination has existed for 10-40 yr. Desorption and solubilization data for soil were compared with the association constants that were estimated from CD-enhanced solubility of the three NACs.
Experimental Section Chemicals. TNT (>99% purity) was provided by Defense Research Establishment Valcartier (Valcartier, PQ), 4-ADNT (>99% purity) was purchased from Omega Inc. (Le´vis, PQ), and 2,4-DANT (>99% purity) was purchased from AccuStandard Inc. (New Haven, CT). The HPβCD and DMβCD (no reported purity) were purchased from Aldrich (Oakville, ON). The CD solutions were prepared daily. Soils. The agricultural topsoil and illite green shale used are described in detail elsewhere in terms of soil properties 10.1021/es9910659 CCC: $19.00
Published 2000 by the Am. Chem. Soc. Published on Web 07/08/2000
and mineralogy (3). However, the topsoil contained 8.4% organic matter, and the illite was a pure clay mineral. The topsoil was sterilized by gamma irradiation at the Canadian Irradiation Centre (Laval, Que.) with minimum and maximum doses of 35.4 to 40.4 kGy, respectively. Sterility was verified according to the methods of Trevors (26), and all manipulations with sterile topsoil were performed in a laminar flow biological hood to maintain aseptic conditions. A contaminated topsoil was obtained from a former explosives manufacturing site in south eastern Quebec in November 1992. The soil was contaminated between the early 1960s and late 1980s, hence it had been aged with TNT for quite some time. It contained 5265 mg/Kg (range of three replicates was 4952-5441 mg/Kg) of TNT, as determined by the EPA SW-846 Method 8330 described subsequently. The soil was analyzed for TNT at the same time that as the solubility enhancements using CDs (as described subsequently) were conducted. Sorption-Desorption. Batch sorption experiments were conducted in triplicate with topsoil and illite (3). The mass of each nitroaromatic compound (NAC) sorbed was calculated by difference. The supernatant was decanted, and the residual supernatant remaining with the soil pellet was measured gravimetrically. For topsoil, desorption was carried out by adding 15 mL of distilled water, 1% HPβCD (w/w in distilled water), or 1% DMβCD (w/w in distilled water) to the soil pellet (2 g topsoil). For illite, 15 mL of 0.1 N KCl or the 1% CD solutions (prepared in 0.1 N KCl) were added to 1 g of clay. The KCl was used to avoid deflocculation of the clay. The centrifuge tubes were wrapped in aluminum foil and agitated for 22 h to achieve equilibrium, as this time was previously determined to be more than sufficient to achieve equilibrium (3). Samples of the supernatant were retrieved for analysis, and the masses of NACs sorbed following desorption were calculated by difference. For each replicate, the % NAC removed from topsoil and illite by the two CDs and aqueous medium was calculated as the % difference between the mass of NAC sorbed following sorption and that sorbed following desorption (with correction for NAC in the residual supernatant). Control experiments that contained TNT, 4-ADNT, and 2,4-DANT in the absence of topsoil and illite were conducted for each NAC concentration. Results indicate that there were no losses from the system over the time periods studied. Sorption-desorption experiments for 4-ADNT and 2,4DANT (both initially at 2.74 mg/L) with sterile and nonsterile topsoil were also conducted. For desorption, 1% (w/w) HPβCD, 1% (w/w) DMβCD, and distilled water were employed. Following desorption, the NAC that remained with the soil pellet was extracted with acetonitrile, as described by EPA SW-846 Method 8330 (27). Briefly, the soil pellet was combined with 10.0 mL acetonitrile, vortexed for 1 min, and placed in a sonicator bath (20 kHz) that was cooled to 22 °C (Blackstone Ultrasonics, Jamestown, NY) for 18 h. After 30 min of settling, 5.0 mL of the supernatant was combined with 5.0 mL of 5 g/L CaCl2. The solutions were manually shaken and prepared for analysis after 15 min settling. Solubility Enhancements by Cyclodextrins. Solubility enhancements of TNT were used to estimate association constants of the inclusion complexes that form with the two CDs using the methods of Orienti et al. (28). Data analysis is described subsequently. An excess of TNT (1500 mg/L) was combined with increasing concentrations of CD (0, 0.1, 1.0, 2.0, and 4.0 w/w %). Solubility was measured over 12 d, and there were no increases after the first 2 d. Therefore, the solutions were equilibrated for 2 d at 23 °C, after which they were filtered through Millex HV 0.45 µm filter units (Millipore Corp., Bedford, MA). The filtrates were diluted 1:1 in acetonitrile for analysis. The same procedure was implemented for the field contaminated topsoil, where 1 g of soil was combined with 10 mL of the CD solutions.
The solubilities of 4-ADNT and 2,4-DANT were determined in the absence and presence of DMβCD and HPβCD (4 w/w %) by combining an excess of the NAC (1100 mg/L). Due to the expense of these two NACs, a broader CD range was not studied. The solutions were equilibrated for 2 d, and samples were prepared for analysis as described above. Analytical Methods. The liquid NAC samples were analyzed using reversed-phase high-pressure liquid chromatography (HPLC) with an ultraviolet (UV) photodiode array (PDA) detector. An HPLC system (Waters Associates, Milford MA) consisting of a Model 600 pump, 717 Plus Autosampler, and a 996 PDA detector (λ ) 254 nm) was used. The system was outfitted with Millennium data acquisition software. Separations were performed on a Supelcosil LC-8 column (25.0 cm, 4.6 mm, 5 µm) (Supelco, Oakville, ON) at 35 °C with an isocratic mobile phase composed of 18% 2-propanol and 82% water at 1 mL/min. The two CDs had no effect on the UV spectra of the three NACs in samples diluted 1:1 in acetonitrile for the CD concentrations that were examined. Hence, all liquid NAC samples were diluted 1:1 with acetonitrile prior to analysis. The DMβCD and the HPβCD were also analyzed by the HPLC system described above with a refractive index detector (Mode 410). For the DMβCD, the Supelcosil LC-8 Supelco column described above was used. The mobile phase was 50% acetonitrile and 50% water, the column temperature was 30 °C, and the flow rate was 2 mL/min. For HPβCD analysis, a Supelcosil LC-NH2 column (25.0 cm, 4.6 mm, 5 µm) (Supelco, Oakville, ON) at 30 °C was used. The mobile phase consisted of 75% acetonitrile and 25% water and the flow rate was 2 mL/min. The injection volume was 50 µL for all cases.
Data Analysis Sorption-Desorption. The following Freundlich isotherm is adequate to describe equilibrium sorption and desorption data for TNT and its metabolites in soil (3, 4)
x ) K d s Cn m
(1)
where x/m is the mass of solute sorbed per unit mass of soil at equilibrium (µg/g), Kds is the sorption capacity constant (L/kg), C is the aqueous equilibrium phase solute concentration (mg/L), and n is a constant. The capacity constant for sorption is denoted as Kds, and for desorption it is denoted as Kdd. Typically, Kds < Kdd thereby indicating sorptiondesorption hysteresis. In addition to their ability to expedite the solubilization of contaminants, CDs have the potential to enhance the desorption of nitroaromatic compounds (NACs) from soil (23), thereby decreasing sorption-desorption hysteresis. Inclusion Complexes. Solubility diagrams are constructed by measuring the increase in solubility of a solute (S) with increases in concentration of an added ligand or complexing agent (L) (29). When the increase in S with L is linear, then the phase solubility diagram is classified as Type A. The stability of inclusion complexes can be obtained by such solubility diagrams (30). In particular, when the stoichiometry for inclusion complexes is 1:1, the following complexation reaction occurs in solution: K
CD + S 98 CD - S
(2)
The association constant (Ks) of such complexes is expressed as
Ks )
[CD - S] [S][CD]
VOL. 34, NO. 16, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
(3) 9
3463
FIGURE 1. Sorption-desorption isotherms for (a) TNT, (b) 4-ADNT, and (c) 2,4-DANT for nonsterile topsoil using 1% HPβCD, 1% DMβCD, and distilled water. where S is the concentration of free solute and is regarded as the aqueous solubility of the solute, So. Hence, the concentration of solute that complexes with CD is calculated as the difference between the total aqueous solute concentration and the free aqueous solute concentration (i.e. [CD - S] ) [St] - [So]). The concentration of free CD is equal to the concentration initially present minus that which is taken up as an inclusion complex with the solute (i.e. [CD] ) ([CDt] - ([St] - [So])). The stability constant can then be reduced to the following:
Ks )
(
)
St - So 1 So CDt - (St - So)
(4)
The Ks can be determined from the phase solubility diagram described above (28) using the following expression:
Ks )
slope So(1 - slope)
(5)
Results and Discussion Sorption-Desorption Isotherms for Topsoil. The sorptiondesorption isotherms for TNT, 4-ADNT, and 2,4-DANT for topsoil are shown in Figure 1. For each isotherm, the Freundlich constants (Kds, Kdd, and n) are summarized in Table 1. The linear constants (i.e. n ) 1) are reported when 3464
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 16, 2000
they provided a better fit of the data than the nonlinear case. Sorption-desorption hysteresis for TNT, 4-ADNT, and 2,4DANT is evident by the fact that the desorption isotherm lies above the sorption isotherm, and also since Kds < Kdd (95% confidence) for sorption-desorption of the three NACs with topsoil (Table 1). When 1% HPβCD was used to facilitate desorption, the resulting isotherms remain above the sorption isotherms for all three NACs. However, for TNT and 4-ADNT, the desorption isotherms lie below the sorption isotherm when 1% DMβCD was used for desorption. This is indicative of enhanced desorption and is reflected by the fact that Kdd < Kds when DMβCD was used to facilitate desorption from topsoil (Table 1). For 2,4-DANT desorption from topsoil using DMβCD, the desorption isotherm lies above the desorption isotherm where water alone was used (Figure 1c). Since NAC sorption was calculated by difference, it was thought that losses of 2,4-DANT by biotransformation processes were mistakenly attributed to sorption processes. This hypothesis is consistent with the fact that 2,4-DANT was biotransformed to 4-Nacetylamino-2-amino-6-nitrotoluene (4-N-AcANT) and 2-Nacetylamino-2-amino-6-nitrotoluene (2-N-AcANT) in nonsterilized topsoil (3). Hence, further experiments were conducted to determine the extent of 2,4-DANT desorption using 1% DMβCD, 1% HPβCD, and distilled water in sterile and nonsterile topsoil for an initial 2,4-DANT concentration of 2.74 mg/L. From these data, point estimates of Kdd were calculated and are reported in Table 2. For nonsterile topsoil, apparent Kdd’s were calculated using the values of n reported for nonsterile topsoil in Table 1. According to the values of Kdd for nonsterile topsoil (Table 2), it appears that 1% HPβCD had little effect on sorptiondesorption hysteresis and that 1% DMβCD increased the sorption-desorption hysteresis. For sterile topsoil, the Kdd’s were calculated using the values of n reported in Table 1 for nonsterile topsoil and also for the linear Freundlich constant (i.e. n ) 1). For sterile topsoil, there was reduced sorptiondesorption hysteresis, as indicated by the reduced Kdd (for both linear and nonlinear estimates) for the two CDs as compared to the values of Kdd for the case of distilled water. Therefore, the apparent increase in hysteresis illustrated in Figure 1c is an artifact caused by indigenous microbial activity in the soil. This was coupled with the appearance of 4-NAcANT in the aqueous phase and acetonitrile extract of the solid sorbed phases (Table 2). Following desorption of 2,4DANT from nonsterile topsoil, there was more 4-N-AcANT detected in the supernatant when CDs were used as compared to distilled water, there was also slightly more 4-N-AcANT extracted with acetonitrile from nonsterile topsoil following desorption with DMβCD compared to the case of distilled water. Furthermore, 4-N-AcANT was not detected in either the aqueous or in the solid phases in any of the replicates involving sterile topsoil. Therefore, it can be concluded that both CDs enhanced the desorption of 2,4DANT from topsoil (i.e. sterile conditions) but that CDs can also enhance its biotransformation to 4-N-AcANT (i.e. nonsterile conditions) and possibly other metabolites. Although there were no reports on the purity of the two CDs used in this study, according to Wang et al. (22), technical grade HPβCD can contain 3.5% propylene glycol that can act as a carbon source for a phenanthrene degrading isolate obtained from an estuary sediment. Results of the present study are consistent with the hypothesis that the CD (or an associated impurity) enhanced the activity of the indigenous microbial population and that this resulted in partial transformation of 2,4-DANT to 4-N-AcANT (and possibly other metabolites). Since the concentration of HPβCD did not change following 22 h contact with nonsterile topsoil (as determined for experiments for CD sorption by topsoil/illite
TABLE 1. Freundlich Sorption and Desorption Isotherm Parametersa topsoilb operationb sorption (Kds) desorption-aqueous medium desorption- 1% HPβCD (Kdd) desorption- 1% DMβCD (Kdd)
illitec
NAC
Kd, L/Kg
n
r2
TNT 4-ADNT 2,4-DANT TNT 4-ADNT 2,4-DANT TNT 4-ADNT 2,4-DANT TNT 4-ADNT 2,4-DANT
6.38 ( 1.15 7.91 ( 1.10 11.96 ( 1.16 12.01 ( 1.10 14.95 ( 2.06 36.58 ( 1.59 12.17 ( 1.07 10.04 ( 1.28 32.60 ( 1.43 2.08 ( 1.06 6.07 ( 1.07 50.25 ( 1.47
0.82 ( 0.06 0.70 ( 0.09 0.71 ( 0.13 0.82 ( 0.08 1.00d 0.64 ( 0.17 0.69 ( 0.08 1.04 ( 0.23 0.53 ( 0.13 0.84 ( 0.04 0.82 ( 0.06 0.61 ( 0.13
0.98 0.96 0.92 0.97 0.78 0.84 0.97 0.88 0.83 0.99 0.98 0.88
Kd, L/Kg
n
r2
223.63 ( 1.09 58.52 ( 1.30 19.73 ( 1.11 265.98 ( 1.05 87.56 ( 14.38 81.85 ( 7.59 224.93 ( 1.13 40.04 ( 5.85 18.56 ( 1.24 139.72 ( 1.06 26.84 ( 3.20 16.64 ( 1.29
0.47 ( 0.05 0.68 ( 0.18 0.81 ( 0.10 0.40 ( 0.02 1.00d 1.00d 0.43 ( 0.06 1.00d 0.33 ( 0.09 0.46 ( 0.04 1.00d 0.28 ( 0.11
0.97 0.84 0.96 0.99 0.72 0.93 0.95 0.84 0.82 0.98 0.87 0.70
a 95% confidence intervals are shown for values of K and n determined from 18 data points. b Aqueous medium was distilled water. c Aqueous d medium was 0.1 N KCl. d 95% confidence intervals are not shown for cases where the linear isotherm provided a better fit, since in these cases n was set to 1.00.
TABLE 2. Partition Coefficients for 2,4-DANT Desorption from Nonsterile and Sterile Topsoila and 4-N-AcANT Measured in Supernatant and Soil Extractsb non-sterile topsoil
sterile topsoil
medium used for desorption
Kdd, L/kg nonlinearc
aqueous 4-N-AcANT, mg/L
extracted 4-N-AcANT, µg/g
Kdd, L/Kg nonlineard
Kdd, L/Kg linear (n ) 1)
distilled water
39.23
19.17
35.13
19.19
13.06
1% DMβCD
43.83
0.509 (0.127-1.155) 0.701 (0.184-1.509) 0.769 (0.033-1.571)
29.30
1% HPβCD
0.049 (0.005-0.101) 0.091 (0.020-0.196) 0.087 (0.007-0.180)
18.16
13.37
a Point estimates for C b Range of three replicates given in parentheses. c Values of n used to calculate K d for nonsterile topsoil initial ) 2.74 mg/L. d are reported in Table 1. d Values of n used to calculate Kdd for sterile topsoil are those obtained for nonsterile topsoil, Table 1.
that are described subsequently), it is unlikely that the amount of CD degraded was measurable. According to Szeijtli (17), a necessary prerequisite for the formation of inclusion complexes is that the molecular volume of the NACs (i.e. “guest” molecule) be less than that of the β-CD cavity (i.e. host cavity). However, slight solubility enhancements with HPβCD have been observed for the case of DDT, despite the fact that DDT has a larger molecular volume than the CD cavity (18). This behavior was attributed to partial entry of DDT within the CD cavity. The molecular volume of the three NACs studied and the 4-N-AcANT product were calculated according to Wang and Brusseau (18) and are summarized in Table 3. Since these values are all less than the interior volume of a β-CD cavity (also given in Table 3), the formation of inclusion complexes between the four NACs and the two β-CDs studied are theoretically possible. The molecular widths and lengths of the three NACs were calculated on the basis of bond lengths (Table 3). Although 4-N-AcANT is longer than the β-CD cavity, it may orient itself within the cavity since its width is less than that of the cavity. The effects of the two CDs on the desorption of 4-ADNT under sterile and nonsterile conditions were also studied since previous work indicates that this NAC can form microbially mediated acetylated products (13). However, for the time span examined, there were no differences between the sterile and nonsterile topsoil when distilled water, HPβCD, and DMβCD were used for desorption. Nonetheless, such biotransformations must be considered when evaluating the effectiveness of CDs for soil remediation. NAC Removal Efficiencies for Topsoil. The removal efficiency of the two CDs and water for removing each NAC from topsoil is depicted as a function of the initial mass of NAC sorbed in Figure 2. The % removal of 2,4-DANT is also
TABLE 3. Selected Properties of NACs and CDs Used in This Studya
compound
aqueous volume,b molecular width,d length,d solubility, 3 weight nm nm nm mg/L (at 25 °C)
TNT
0.258
227
0.71
0.69
4-ADNT
0.246
197
0.67
0.69
2,4-DANT
0.234
167
0.67
0.65
4-N-AcANT HPβCD DMβCD
0.305 0.346c 0.346
209 1331 1500
0.67 0.75 0.75
0.84 0.78 0.78
128 (124-130) 42 (38-46) 1597 (1584-1600) 570 000 (34)
a
Range of three replicates given in parentheses. b Calculated according to Wang and Brusseau (18). c For β-CD cavity (18). d Estimated on the basis of reported bond lengths (31). The width was calculated across the two ortho substituents on either side of the methyl group, and the length was calculated from the methyl group to the para substituent.
given for the case of sterile topsoil for HPβCD and DMβCD (closed symbols in Figure 2c) for one initial sorbed concentration (approximately 11 µg/g). The Kds for the three NACs (Table 1) increases with amino substitution. In particular, Kds for 2,4-DANT is greater than that of 4-ADNT (95% confidence), and Kds for 4-ADNT is greater than that of TNT (80% confidence). Furthermore, the efficiency of NAC removal (Figure 2) decreases with amino substitution for DMβCD. As discussed previously, the only reactive constituent in the topsoil is its organic matter (8.4%), and the increase in Kds with amino group substitution may be due to irreversible interaction with quinoidal structures of soil organic matter (SOM) (3). The fact that as the amino group substitution VOL. 34, NO. 16, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3465
TABLE 4. Stability Constants of TNT, 4-ADNT, and 2,4-DANT for Complexation with HPβCD and DMβCD in Aqueous Systems and Slurries with Field Contaminated Soila compound
system
CD
Ks µM-1
TNT TNT 4-ADNT 4-ADNT 2,4-DANT 2,4-DANT TNT TNT
aqueous aqueous aqueous aqueous aqueous aqueous aqueous field contaminated soil aqueous field contaminated soil
4% HPβCD 4% DMβCD 4% HPβCD 4% DMβCD 4% HPβCD 4% DMβCD HPβCD HPβCD
14.5b 55.8b 254.1b 443.1b 123.3b 78.9b 18.1 ( 9.1c 15.2 ( 3.7c
TNT TNT
DMβCD DMβCD
66.4 ( 7.9c 62.9 ( 16.3c
a All determined at 23 °C. b Point estimates using eq 4 (n ) 3). Calculated using eq 5 and the slopes of the linear trend lines of St versus CDt (95% confidence intervals shown for 18 data points). c
FIGURE 2. Removal efficiencies of (a) TNT, (b) 4-ADNT, and (c) 2,4-DANT using 1% DMβCD, 1% HPβCD, and water for topsoil (note that for 2,4-DANT, data for sterile topsoil are also included as closed symbols). increased (i.e. 2,4-DANT > 4-ADNT > TNT), the efficiency of DMβCD for desorption decreased is consistent with irreversible interactions that may include quinoidal group interaction. The point estimates for the association constants (assuming 1:1 inclusion complex formation) for the two CDs and the three NACs in aqueous solutions (Table 4) indicate the following affinities for inclusion complex formation for both CDs: 4-ADNT > 2,4-DANT > TNT. This preference may be explained by the geometry of the NAC molecules in conjunction with a preference of amino groups to coordinate with ether groups within the CD cavity. For TNT, the width of the molecule (across the two ortho substituents) is similar to the width of the β-CD cavity (Table 3). Therefore, TNT may only be capable of entering the CD cavity on the side of its para nitro group. Furthermore, the nitro group may not coordinate as well with the ether groups of the interior of the CD cavity as compared to the amino groups in the para positions of 4-ADNT and 2,4-DANT. The difference in Ks of 4-ADNT and 2,4-DANT may be explained by a preference of the amino groups for coordination with the ether groups within the CD cavity in conjunction with molecular geometry. The 2,4-DANT may orient itself with the CD cavity on the side of its two amino groups and the 4-ADNT on the side of its one amino group. The 2,4-DANT would encounter steric hindrance in this orientation, as compared to 4-ADNT. 3466
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 16, 2000
Furthermore, based on the association constants (Ks) for TNT and 4-ADNT, complex formation with DMβCD is favorable to HPβCD; whereas, for 2,4-DANT, complex formation with HPβCD is favorable to DMβCD. This is in contrast to what was found for NAC desorption from topsoil by DMβCD where the removal efficiency was inversely proportional to sorption affinity (i.e. Kds). For HPβCD, the following trends for removal efficiencies were observed: 4-ADNT > 2,4-DANT-sterile ≈ TNT for similar levels of NACs initially sorbed. This trend is somewhat comparable to the association constants reported in Table 4 (i.e. 4-ADNT > 2,4-DANT > TNT). Therefore, for topsoil, removal efficiencies depended somewhat on association constants (Ks) for HPβCD (negligible surface activity) and on sorption affinity (Kds) for DMβCD (highly surface active). Sorption-Desorption Isotherms for Illite. The sorptiondesorption isotherms for the three NACs and illite are depicted in Figure 3, and the Freundlich constants are summarized in Table 1. There was sorption-desorption hysteresis for all NACs when 0.1 N KCl was used for desorption. For TNT, desorption hysteresis was slightly reduced by 1% HPβCD and was significantly reduced by 1% DMβCD. For the 4-ADNT, both CDs were highly effective in reducing desorption hysteresis, with DMβCD being slightly more effective. For 2,4-DANT desorption from illite, both CDs were nearly equal in reducing desorption hysteresis. NAC Removal Efficiencies for Illite. The CD enhanced removal efficiencies for NAC desorption from illite are depicted in Figure 4. For TNT, the sorption capacity constant (Kds of 223.63 L/Kg, Table 1) was high, and the % removal for both CDs was lowest for TNT (Figure 4). These results imply that sorption affinity is inversely proportional to removal efficiency for TNT and illite. The low removal efficiency of TNT from illite by both CDs may be explained by the formation of electron donor acceptor (EDA) complexes between the electron withdrawing nitro groups of TNT and the siloxane surface (9, 10). In addition, low affinity for coordination of the para nitro group with ether groups within the CD cavity may have contributed to the low removal efficiency. For 2,4-DANT, the removal efficiency increased with the initial sorbed concentration but ultimately reached the same levels as 4-ADNT that were relatively constant with the initial sorbed concentration. The fact that the 4-ADNT removal efficiencies were high and constant with surface loading may be related to the high values of the association constant (Ks) for 4-ADNT and the two CDs (Table 4). Solubility Enhancements for Field Contaminated Soil. The phase solubility diagrams for TNT in aqueous solution and for a highly contaminated soil obtained from the field
FIGURE 3. Sorption-desorption isotherms for (a) TNT, (b) 4-ADNT, and (c) 2,4-DANT from nonsterile illite using 1% DMβCD, 1% HPβCD, and 0.1 N KCl. with increasing CD concentrations are depicted in Figure 5. For pure TNT in aqueous solution, it is evident that the relative solubility (actual solubility divided by initial solubility or St / So from eq 4) increased substantially with increasing quantities of DMβCD (Figure 5b), but the increase in relative solubility was less pronounced for HPβCD (Figure 5a). Since the solubility increased linearly with increasing CD concentration (i.e. CDt versus St, from eq 4) and the slope of the two curves is less than unity, the 1:1 inclusion complex formation is a reasonable assumption (29). The association constants (Ks), calculated from eq 5 and the slopes of the solubility diagrams (St versus CDt), are summarized at the bottom of Table 4. From this, it is evident that TNT complex formation with DMβCD was more favorable than HPβCD. Furthermore, the apparent stability constants for the field contaminated soil are comparable to those for aqueous systems for both CDs despite the fact that the field contaminated soil had been aged for 10-40 yr. This implies that for soils containing high levels of TNT, extraction will be limited by TNT solubility and that this may be enhanced by the use of CDs. In particular, Hundal et al. (6) found that when solid phase TNT was present in artificially contaminated soil, the concentration present in various extracting solvents (i.e. 3 mM CaCl2 and CH3CN) was limited by the solubility of TNT in the respective solvents. For the present system, the total concentration of TNT in the soil slurries of the
FIGURE 4. Removal efficiencies for (a) TNT, (b) 4-ADNT, and (c) 2,4-DANT using 1% DMβCD, 1% HPβCD, and 0.1 N KCl. experiments involving the field contaminated soil was calculated as 526 mg/L, which far exceeds the aqueous solubility of TNT (128 mg/L, Table 3). Hence, phase solubility diagrams may be used to predict the efficiency of cyclodextrins for such highly contaminated soils in the field. Sorption of Cyclodextrins by Soil. The DMβCD performed consistently better than the HPβCD for desorption of all NACs from illite and topsoil and for solubilization of TNT from the field contaminated topsoil. This may stem from the high surface activity of DMβCD as compared to the case of HPβCD that has negligible surface activity (16). This is also consistent with the fact that HPβCD was not sorbed by either illite or topsoil, whereas DMβCD was sorbed by topsoil (2.2 w/w % of total CD mass) and illite (9.9 w/w % of total CD mass). These results are consistent with those of Ko et al. (32) where it was reported that HPβCD was not significantly sorbed by kaolinite. Enhanced Desorption of NACs by Cyclodextrins. Although it has been demonstrated that the presence of HPβCD can retard sorption of contaminants (i.e. anthracene, pyrene, and trichlorobiphenyl) by soil (21), the present study demonstrates that both HPβCD and DMβCD were able to facilitate desorption. Ko et al. (32) suggested that HPβCD was able to desorb phenanthrene from a model sandy aquifer by flushing with HPβCD solution. This was based on a onedimensional numerical model assuming a Kds of 0.411 L/Kg for the distribution of phenanthrene between aquifer solids and the aqueous phase. However, it is unlikely that there VOL. 34, NO. 16, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3467
Literature Cited
FIGURE 5. Phase-diagrams for solubilization of pure TNT in solution and for a highly contaminated soil from the field (i.e. 5265 mg/kg) using (a) HPβCD and (b) DMβCD (vertical bars denote range of three replicates). was extensive sorption of phenanthrene by the aquifer solids considering that such a small value of Kds was assumed. In particular, Huang et al. (33) reported Kds values that were 3-4 orders of magnitude larger than the value assumed by Ko et al. (32) for five EPA reference soils. Cyclodextrins for Remediation of Soil Contaminated by NACs. This study demonstrates that CDs can be used to enhance the solubilization of NACs from highly contaminated soil and that their efficiencies can be accurately predicted from solubility phase diagrams. An interesting property of methylated CDs (i.e. DMβCD) is that increased methylation increases CD solubility in cold water (16). No explanation for the unusual inverse relationship between solubility and temperature was given (16). Although this property may be undesirable in pharmaceutical applications since precipitation will occur at higher temperatures, it is an added benefit for in situ remediation since subsurface temperatures are approximately 10 °C. Furthermore, the fact that CDs were able to desorb TNT from soil is indicative of their potential application for soil remediation at lower levels of contamination. The DMβCD was generally more effective than HPβCD. Both CDs enhanced the biotransformation of 2,4DANT to 4-N-AcANT in nonsterile topsoil. This last point must be considered when evaluating the effectiveness of CDs for soil remediation. Further research will focus on using these cyclodextrins to enhance the solubility of TNT and its metabolites in soil, that may in turn enhance their availability for biodegradation.
Acknowledgments We thank the Natural Sciences and Engineering Research Council and the National Research Council (NRC) of Canada for a Fellowship to T.W.S. and the Department of National Defence for their continued interest in this work. The authors would also like to thank Louise Paquet and Annamaria Halasz for helpful discussions and technical support. This is NRC publication No. 43312. 3468
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 16, 2000
(1) Daun, G.; Lenke, H.; Reuss, M.; Knackmuss, H.-J. Environ. Sci. Technol. 1998, 32, 1956-1963. (2) Gorontzy, T.; Drzyzga, O.; Kahl, M. W.; Bruns-Nagel, D.; Breitung, J.; von Loew, E.; Blotevogel, K,-H. Crit. Rev. Microbiol. 1994, 20(4), 265-284. (3) Sheremata, T. W.; Thiboutot, S.; Ampleman, G.; Paquet, L.; Halasz, A.; Hawari, J. Environ. Sci. Technol. 1999, 33, 40024008. (4) Selim, H. M.; Iskandar, I. K. Sorption-Desorption and Transport of TNT and RDX in Soils; CRREL Report 94-7; U.S. Army Corps of Engineers, Office of the Chief of Engineers: 1994. (5) Li, A. Z.; Marx, K. A.; Walker, J.; Kaplan, D. L. Environ. Sci. Technol. 1997, 31, 584-589. (6) Hundal, L. S.; Shea, P. J.; Comfort, S. D.; Powers, W. L.; Singh, J. J. Environ. Qual. 1997, 26, 896-904. (7) Pennington, J. C.; Patrick, W. H., Jr. J. Environ. Qual. 1990, 19, 559-567. (8) Comfort, S. D.; Shea, P. J.; Hundal, L. S.; Li, Z.; Woodbury, B. L.; Martin, J. L.; Powers, W. L. J. Environ. Qual., 1995, 24, 11741182. (9) Haderlein, S. B.; Weissmahr, K. W.; Schwarzenbach, R. P. Environ. Sci. Technol. 1996, 30, 612-622. (10) Haderlein, S. B.; Schwarzenbach, R. P. Environ. Sci. Technol. 1993, 27, 316-326. (11) Xue, S. K.; Iskandar, I. K.; Selim, H. M. Soil Sci. 1995, 160(5), 317-327. (12) Taha, M. R.; Soewarto, H.; Acar, Y. B.; Gale, R. J.; Zappi, M. E. Water, Air, Soil Pollut. 1997, 100, 33-48. (13) Hawari, J.; Halasz, A.; Beaudet, S.; Paquet, L.; Ampleman, G.; Thiboutot, S. Appl. Environ. Microbiol. 1999, 65(7), 2977-2986. (14) Deshpande, S.; Shiau, B. J.; Wade, D.; Sabatini, D. A.; Harwell, J. H. Water Res. 1999, 33, 351-360. (15) Laha, S.; Luthy, R. G. Environ. Sci. Technol. 1991, 25, 19201930. (16) Fro¨mming, K.-H.; Szejtli, J. Cyclodextrins in Pharmacy; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1994. (17) Szejtli, J. In Cyclodextrins and their Industrial Uses; Ducheˆne, D., Ed.; Editions de Sante´: Paris, France, 1987; Chapter 5. (18) Wang, X.; Brusseau, M. L. Environ. Sci. Technol. 1993, 27, 28212825. (19) Wang, X.; Brusseau, M. L. Environ. Sci. Technol. 1995, 29, 23462351. (20) McCray, J. E.; Brusseau, M. L. Environ. Sci. Technol. 1998, 32, 1285-1293. (21) Brusseau, M. L.; Wang, X.; Hu, Q. Environ. Sci. Technol. 1994, 28, 952-956. (22) Wang, J.-M, Marlowe, E. M.; Miller-Maier, R. M.; Brusseau, M. L. Environ. Sci. Technol. 1998, 32, 1907-1912. (23) Hawari, J.; Paquet, L.; Zhou, E.; Halasz, A.; Zilber, B. Chemosphere 1996, 32, 1929-1936. (24) Sabatini, D. A.; Knox, R. C.; Harwell, J. H. Surfactant-Enhanced DNAPL Remediation: Surfactant Selection, Hydraulic Efficiency, and Economic Factors; Environmental Research Brief; United States Environmental Protection Agency, EPA/600/5-961002; National Risk Management Research Laboratory: Ada, OK, 1996. (25) McCoy, M. Chem. Eng. News 1999, March 1, 25-28. (26) Trevors, J. T. J. Microbiol. Methods 1996, 26, 53-59. (27) US EPA. Method 8330: Nitroaromatics and Nitramines by High Performance Liquid Chromatography (HPLC), Test Methods for Evaluating Solid Waste, SW-846 update III, Part 4: 1(B); Office of Solid Waste: Washington, DC, 1997. (28) Orienti, I.; Fini, A.; Bertis, V.; Zecchi; V. Eur. J. Pharm. Biopharm. 1991, 37, 110-112. (29) Higuchi, T.; Connors, K. Adv. Anal. Chem. Instrum. 1965, 4, 117-212. (30) Szejtli, J. Cyclodextrin Technology; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1988. (31) Lide, D. R. CRC Handbook of Chemistry and Physics, 72nd ed.; CRC Press: Boca Raton, FL, 1991; Section 9. (32) Ko, S.; Schlautman, M. A.; Carraway, E. R. Environ. Sci. Technol. 1999, 33, 2765-2770. (33) Huang, W.; Yu, H.; Weber, W. J., Jr. J. Cont. Hydrol. 1998, 31, 129-148. (34) Uekema, K.; Irie, T. In Cyclodextrins and their Industrial Uses; Ducheˆne, D., Ed.; Editions de Sante´: Paris, France, 1987; Chapter 10.
Received for review September 15, 1999. Revised manuscript received April 10, 2000. Accepted May 10, 2000. ES9910659
