Environ. Sci. Technol. 1997, 31, 472-478
Enhanced Solubilization and Destruction of Tetrachloroethylene by Hydroxypropyl-β-cyclodextrin and Iron G E O R G E O . B I Z Z I G O T T I , * ,† DAVID A. REYNOLDS,‡ AND BERNARD H. KUEPER‡ Mitretek Systems, Inc., 7525 Colshire Drive, McLean, Virginia 22102-7400, and Department of Civil Engineering, Queen’s University, Kingston, Ontario, Canada K7L 3N6
Hydroxypropyl-β-cyclodextrin (HP-β-CD) enhances the solubility of tetrachloroethylene (PCE) in water both in static and in flowing systems. HP-β-CD does not decrease the interfacial tension between PCE and water and, therefore, should not mobilize immiscible-phase PCE in the subsurface. Rates for the reaction of PCE with metallic iron were measured in HP-β-CD solutions under static conditions. In flowing systems, metallic iron removed PCE, and no other chlorinated ethylene species were observed in the column effluent. In several such systems, recycling of the HP-β-CD solution took place following the reaction with iron. No downward mobilization of the PCE pool in the generator column was observed. The solubility enhancement and reaction with metallic iron are consistent with reversible formation of a stoichiometric HP-β-CD/PCE complex. A theoretical treatment of the reaction rates of complexed PCE on an iron surface was developed. This treatment suggests that any material that enhances the solubility of low-solubility organic substances may slow down the rate of reaction in aqueous solution. In many cases, the rate retardation should equal the degree of solubility enhancement. The combination of HP-β-CD and iron metal appears to be a promising groundwater remediation technology.
Introduction Groundwater contamination by dense, non-aqueous-phase liquids (DNAPLs) such as chlorinated solvents, PCB oils, and certain pesticides is a common occurrence throughout industrialized areas of North America. DNAPL distributes itself in the subsurface in the form of disconnected blobs and ganglia referred to as residual as well as in larger accumulations referred to as pools (1). These liquids are more dense than water and, therefore, have the potential to migrate to great depths below the water table where they slowly dissolve into groundwater, giving rise to aqueous-phase plumes. Because groundwater velocities are relatively low at typical sites and because these liquids are relatively insoluble in water, the life span of DNAPL in the subsurface is expected to be on the order of 100 years or more (2). The toxic and persistent nature of DNAPL has prompted researchers to develop remediation technologies to remove residual and pooled DNAPL from the subsurface. One general * Corresponding author telephone: (703) 610-2115; fax: (703) 6101561; e-mail address:
[email protected]. † Mitretek Systems, Inc. ‡ Queen’s University.
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class of technologies, referred to as chemical flushing, includes the use of surfactants and alcohols to enhance the solubility of DNAPL and/or to physically displace DNAPL. While these technologies can offer the advantage of removing large amounts of DNAPL in a relatively short time frame, they generally bring about a lowering of DNAPL-water interfacial tension, which in turn may lead to vertical remobilization of pooled DNAPL. Such mobilization can lead to a worsening of the extent of contamination. One recent development in the remediation of chlorinated solvent DNAPLs involves enhanced solubilization using hydroxypropyl-β-cyclodextrin (HP-β-CD) (3). Cyclodextrins, first isolated in 1891 (4), are cyclic oligomers of glucose produced by the action of certain bacterial enzymes on starch (5). Cyclodextrins have a doughnut-shaped structure with a relatively hydrophilic exterior and a relatively hydrophobic interior “hole” (6). Low-polarity organic substances with a size and shape complementary to the hole form inclusion complexes with cyclodextrins (7). Cyclodextrins have found several other applications in the environmental area. β-Cyclodextrin was used to form inclusion complexes with pesticides and related compounds in order to suppress toxicity in activated sludge systems, leading to enhanced biological detoxification of industrial wastewater (8). Cyclopentanol enhances the solubilization of polynuclear aromatic hydrocarbons (PAHs) by cyclodextrins (9), and carboxymethyl-βcyclodextrin can be used to simultaneously remediate heavy metals and low-polarity organics (10). One potential drawback of the use of cyclodextrins for environmental remediation stems from the nature of inclusion complexes. Each molecule of organic compound removed via complex formation requires one cyclodextrin molecule. Thus, the cost of cyclodextrin required to remove all but small amounts of contamination is potentially prohibitive. An effective method of recycling would significantly enhance the attractiveness of cyclodextrins for environmental remediation. Another recent development in remediation of chlorinated solvents involves their reaction with metallic iron. Senzaki has published on the reductive degradation of TCE by iron powder (11). Preliminary lab tests showed that a mixture of industrial scrap iron filings and sand reduced the aqueous concentration of dilute chlorinated solvents through reductive degradation (12). Gillham and O’Hannesin studied iron filings and sand emplaced in the path of an aqueous phase plume of PCE and TCE and showed reduced aqueous solvent concentrations (13). Subsequent work characterized the rate and the products of the iron/TCE reaction (14). Tratnyek and Matheson studied the rates of the iron-chlorinated organic reaction, establishing that the dominant chemical reaction is Fe0 + RCl + H+ f Fe2+ + RH + Cl-, occurring at the iron surface (15). These studies have shown that metallic iron has excellent promise as a passive remediation technology; however, the time required to remediate groundwater with zero valent metals alone is limited by the low solubility of most chlorinated solvents. A method of enhancing solubility would make the iron technology attractive for contaminated sites where more rapid remediation is required (16). This paper will explore the combination of cyclodextrin solubility enhancement with metallic iron destruction of a chlorinated solvent. This combination appears to be a mutually beneficial one. Metallic iron holds promise as a means for cyclodextrin rejuvenation, and cyclodextrins have the potential to increase the aqueous solubility of chlorinated organics, thereby increasing the rate of mass delivery to the iron.
S0013-936X(96)00324-0 CCC: $14.00
1997 American Chemical Society
Materials and Methods General. All experiments reported in this study employed technical grade PCE obtained from Fisher Scientific Inc. Pharmaceutical grade HP-β-CD, R-cyclodextrin, β-cyclodextrin, and γ-cyclodextrin were provided by the American MaizeProducts Company, Hammond, IN. A natural silica sand (Ottawa sand obtained from K&E Sand and Gravel, Wyoming, Ontario) was employed in all column experiments. Two types of iron were used: a 100 mesh (0.15 mm) electrolytic iron powder, obtained from Fisher Scientific Inc., and a -8 to +50 mesh iron filing mixture, obtained from Peerless Metal Powders, Detroit, MI. All water used in the experiments was distilled and deaerated by applying a 28 mmHg vacuum to a 20-L flask. PCE-water interfacial tension was measured at 22 °C using a platinum ring tensiometer (ASTM D971). Analytical Procedures. Due to the presence of cyclodextrin in the aqueous matrix, direct injection methods for gas chromatography (GC) were not viable. Aqueous samples were analyzed with a combination of purge-and-trap GC and flame ionization detection. The analysis used a DB-wax column (J&W Scientific, Ltd.) in a Hewlett Packard 5890 GC in conjunction with an OI analytical purge-and-trap connected to a MPM-16 multiple purging module. Because of the non-standard nature of this analysis, method development was performed to ascertain the most stable configuration. The oven temperature was ramped from 45 °C for 3 min to 180 °C for 1 min at a rate of 8 °C min-1. The injector and detector ports were set at 180 °C. The parameters for the purge-and-trap system are a 12.0-min purge at 35 °C, a 1.0 min desorb at 180 °C, baking for 10.0 min at 180 °C, and transfer at 100 °C. The carrier gas used was ultra-high-purity helium (1 mL min-1), with nitrogen as the purge gas (19.32 mL min-1). Matrix spike (MS) recoveries between 90 and 110% of expected values and matrix spike duplicate (MSD) percent differences less than 10.3% were observed for all PCE analyses in the water/cyclodextrin/PCE system; MS/MSD matrices contained the same cyclodextrin concentrations as the analytical samples. A method detection limit of 6.9 ppb for PCE and a practical quantitation limit of 9.5 ppb for PCE in the water/cyclodextrin/PCE system were determined. Batch Tests. Batch tests were conducted in 40-mL EaglePicher pre-sterilized glass sample vials. The vials were fitted with Teflon-lined silicone septa to allow for repeated sampling. In the tests, 10 g of either the iron filings or iron powder was placed into the vial, which was then filled with a solution containing water, HP-β-CD, PCE, and 40 mg L-1 calcium carbonate (CaCO3). The calcium carbonate was added to the solutions to impart typical groundwater characteristics (13). Three sets of batch experiments were performed employing 0, 45, and 70 g L-1 HP-β-CD concentrations. Each set consisted of two or three replicates and a minimum of one control vial containing only aqueous solution. Aqueous solutions were prepared by adding the cyclodextrin and the CaCO3 to the water in a separatory funnel and adding a sufficient aliquot of PCE such that the solution could reach saturation. The funnels were placed in a vibratory shaker and allowed to equilibrate for 48 h. Samples were taken from the separatory funnels before the sample vials were filled to establish initial PCE concentrations. The vials were sampled immediately after filling to ensure no losses due to volatilization. The vials were then placed in a vibratory shaker for the duration of the tests. The vials were sampled a minimum of nine times during the course of each test by withdrawing a 5-µL sample with a gas-tight syringe. The withdrawn sample was immediately diluted in distilled water in a 5-mL glass vial, sealed with a Teflon-lined cap, and stored at 4 °C. All GC analyses were performed within 48 h of sample collection. Column Tests. The column test apparatus consisted of a 0.47 m long, 5 cm diameter glass generator column,
SCHEME 1. Reaction of Cyclodextrin-Complexed PCE with Solid Phase Iron
connected in series to a 1.0 m long, 20 cm diameter rigid PVC reaction column. The generator column was packed with two layers of silica sand. The lower layer consisted of 15 cm of silica sand passing a no. 70 mesh sieve and retained on a no. 100 mesh sieve (70/100 mesh sand). The sand was packed under a 30-cm water head in 2.5 cm lifts by pouring the sand into the top of the column and tamping it with a stainless steel rod. Above this layer, 10 cm of 30/40 mesh sand was packed using the same method. A total of 25 mL of PCE was then injected above the 70/100 mesh sand using a syringe to form a pool of DNAPL. The PCE pool did not mobilize into the 70/100 mesh sand because of the high capillary resistance provided by this material. The remainder of the column was then filled with 30/40 sand. The reaction column consisted of either a silica sand/iron powder mixture or a pure iron filings formulation. The column was packed under a water head in a manner similar to that of the generator column. The dimensions of the column and the employed flow rates produced an average retention time of 25.3 h. For the mixed component column, the silica sand and iron powder were mixed in small batches prior to emplacement and added quickly to prevent separation. Flow through the system was provided by two constant head tanks. The influent tank was fixed in place with the free surface approximately 30 cm above the top of the generator column. The effluent tank was constructed of a vented stainless steel box attached to a motorized pulley to allow for adjustment of the hydraulic gradient. All tubing used to connect columns and tanks was 1/4 in. PTFE. Sampling ports for determining aqueous concentrations were placed after the generator column and after the reaction column. The overflow from the effluent constant head tank was collected in a 20-L polypropylene container. In some experiments, the effluent from the reaction column was pumped back to the influent reservoir to once again cycle through the system. This was done to assess whether the presence of PCE complexed with HP-β-CD before dissolution would impair the efficiency of the system.
Theory of Reaction Rates of Complexed PCE on an Iron Surface The effect of cyclodextrins on the reaction rate of PCE and iron can be interpreted based on Scheme I. PCE is adsorbed to the iron surface, followed by the reaction of the adsorbed PCE to form products, initially TCE and chloride. In addition, aqueous PCE and cyclodextrin (CD) are in rapid equilibrium with a PCE-cyclodextrin complex (PCE-CD). A full treatment of the reaction rate of PCE and iron in the presence of CD is included in the Supporting Information for this paper; the derivation follows Spiro’s work on reaction rates for heterogeneous catalysis in solution (17). To summarize, we can write a combined rate equation that accounts for PCE in the bulk solution, the PCE-cyclodextrin complex, and reactant PCE on the surface, respectively:
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(
-
)
AmFe d d d [PCE]aq + [PCE-CD] + [PCE]ads ) dt dt V dt kAmFe [PCE]ads (1) V
where V is the volume of bulk solution, mFe is the mass of iron, A is the surface area per unit mass of the iron, and [PCE]aq and [PCE]ads denote aqueous and adsorbed PCE concentrations. We next account for the effect of cyclodextrin complexation. If aqueous PCE is in rapid equilibrium with the cyclodextrin-PCE complex, and assuming that [CD] is constant and much larger than [PCE], we can write the equilibrium expression for complexation where kassoc is the rate constant for the formation of the PCE-CD complex, kdiss is the rate constant for the dissociation of the PCE-CD complex, and Kd is the distribution coefficient: FIGURE 1. PCE-water interfacial tension.
kassoc [PCE-CD] ) [CD][PCE]aq ) Kd[CD][PCE]aq (2) kdiss If we assume equilibrium partitioning of PCE at the iron surface with Ks as the iron-solution partition equilibrium constant, we have the following relationship between [PCE]aq and [PCE]ads:
[PCE]ads ) Ks[PCE]aq
(3)
Substituting eqs 2 and 3 into eq 1 gives
-
(
)
AmFeKs d d d [PCE]aq + Kd[CD] [PCE]aq + [PCE]aq ) dt dt V dt kAmFeKs [PCE]aq (4) V FIGURE 2. Results for the reaction of PCE with -8 to +50 mesh iron filings at different hydroxypropyl-β-cyclodextrin concentrations.
Integrating eq 4 gives
(
ln
[PCE]aq
)
[PCE]aq,0
kAmFeKst )
(V(1 + Kd[CD]) + AmFeKs)
(5)
where [PCE]aq,0 is the initial concentration of aqueous PCE. Assuming the extent of adsorption of PCE is small, then V(1 + Kd[CD]) + AmFeKs ≈ V(1 + Kd[CD]). Incorporating the mass balance of PCE at initial conditions then gives
ln
(
[PCE]0
(1 + Kd[CD])[PCE]aq
)
kAmFeKst )
V(1 + Kd[Cd])
(6)
where [PCE]0 is the observed initial concentration of PCE. Finally, we combine eq 2 with eq 6 and then use the fact that when the aqueous phase is analyzed by purge-and-trap GC (as it was for this paper), the observed concentration of PCE, [PCE]obs ) [PCE]aq + [PCE-CD] to give the observed rate expression for the reaction of PCE on an iron surface in the presence of cyclodextrins
ln
(
[PCE]0
)
[PCE]obs
kAmFeKst )
V(1 + Kd[CD])
) kobst
(7)
Results Increasing concentrations of R-cyclodextrin, β-cyclodextrin, or γ-cyclodextrin causes a slight increase in the PCE-water interfacial tension, as shown in Figure 1. For studies of reactions of PCE and iron, HP-β-CD was used as the solubilityenhancing agent because the β-cyclodextrin/PCE inclusion complex precipitated from solution, whereas the HP-β-CD/ PCE inclusion complex remained in solution. HP-β-CD complexes are generally more water-soluble than inclusion complexes of underivatized β-cyclodextrin (18), and the
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surface activity of HP-β-CD differs only slightly from that of underivatized β-cyclodextrin (19). Reaction rates of PCE with metallic iron were measured by preparing solutions of HPβ-CD at 0, 45, and 70 g L-1 saturated with PCE. Saturation PCE concentrations ([PCE]0) gave a partition coefficient of 120 L mol-1, in reasonable agreement with the previously measured value of 90 ( 10 L mol-1 for PCE and HP-β-CD (20). The concentration of PCE in each solution was measured at various times ([PCE]obs) until the PCE concentration had decreased below 0.10 times the initial concentration, or until 24 days had elapsed. The results for the reactions with -8 to +50 mesh iron filings are presented in Figure 2. For each experiment, regression lines for -ln([PCE]0/[PCE]obs) versus time were calculated, with the slope taken as the observed first-order rate constant, kobs. These rate constants are given in Table 1, along with the square of the Pearson product moment correlation coefficient and the number of observations for each regression. Average kobs values calculated for each HP-β-CD concentration are also given in Table 1. Calculated PCE concentration curves based on these values are also presented in Figure 2. In control experiments at each HP-β-CD concentration with no iron, [PCE] varied around [PCE]0 with a standard deviation of less than 2% and no significant trend. Column Test Results. The first column experiment employed a reaction column that consisted of 20% iron powder and 80% silica sand by weight and an eluant cyclodextrin concentration of 70 g L-1. The results of this test are presented in Figure 3. The post-treatment data are offset backwards in time to account for the delay caused by the reaction column retention time. The hydraulic gradient across the column was fixed at 0.037, resulting in an average reaction column retention time of 26.7 h. The 20/80 reaction
TABLE 1. Observed Rate Constants for Iron-PCE Reactiona [HP-β-CD]
kobs (h-1)
r2
n
0 0 45 45 70 70
0.011 0.013 0.00240 0.00246 0.000874 0.000865
0.935 0.930 0.942 0.971 0.777 0.819
8 7 9 9 11 11
av kobs (h-1) 0.012 0.00243 0.000870
a All rate constants were calculated from a least-squares regression of n measurements of [PCE]/[PCE]0 with time. Measurements were obtained over at least 24 days, or until [PCE]/[PCE]0 decreased below 0.10, whichever occurred first. All experiments employed 40 mL of PCE saturated HP-β-CD solution, 10 g of -8 to +50 mesh iron filings, and 40 mg L-1 CaCO3. All experiments were conducted at ambient temperature (23 °C) with vibratory shaking throughout.
FIGURE 4. Column test results for 45 g L-1 hydroxypropyl-βcyclodextrin eluant, treatment with 20% iron powder (100 mesh), with recycling initiated after 80 h.
FIGURE 3. Column test results for 70 g L-1 hydroxypropyl-βcyclodextrin eluant, treatment with 20% iron powder (100 mesh). column formulation resulted in a surface area to volume ratio for the iron of 0.195 m2 mL-1. The average peak concentration measured at the exit of the generator column during the first 45 h of the experiment was 1185 ppm, approximately 11% above the batch experiment findings of Dimitrievich (20) for a 70 g L-1 HP-β-CD solution. An approximate mass balance of the effluent using this value and a local equilibrium assumption dictates that it would require 34.7 L, or 58.4 h, to fully solubilize the 25-mL PCE pool. The increased time exhibited in the experiment was most likely a function of non-equilibrium mass transfer between immiscible-phase PCE and flowing aqueous solution due to a reduction in the specific area available for mass transfer (21). The average reduction in PCE concentration due to the reaction column was approximately 30%. This trend was reasonably constant throughout the duration of the test, indicating that the reduction of PCE concentration by the iron in these columns was not a function of the input PCE concentration. Finally, no downward mobilization of the PCE pool in the generator column was observed. The second test employed an eluant HP-β-CD concentration of 45 g L-1 and an average reaction column retention time of 25.5 h. The results of the experiments are presented in Figure 4. The average peak influent concentration during this test was 652 ppm, 26% lower than the batch test results of Dimitrievich (20). Recycling of the effluent of the reaction column into the influent constant head tank was initiated after approximately 82 h to assess the reversibility of the HP-β-CD/PCE complex. The results presented in Figure 4 indicate that PCE concentrations did not rise following the commencement of recycling and that the decrease in PCE concentration between prereaction and post-reaction samples was constant following recycling. It is therefore apparent that the recycling does not impair the PCE-iron degradation reaction.
FIGURE 5. Column test results for 70 g L-1 hydroxypropyl-βcyclodextrin eluant, treatment with 100% iron filings (-8 to +50 mesh), with recycling initiated after 135 h. It was of interest during the course of this experiment to assess whether or not the cyclodextrin would sorb to or degrade upon the iron present in the reaction column. Gravimetric analysis of the effluent was performed at two different intervals during the experiment with 70 g L-1 HPβ-CD; recoveries were 103.1 ( 0.9%, which would suggest that cyclodextrin was being conserved. The fact that recoveries were above 100% may suggest that some entraining of water was occurring within the cyclodextrin matrix. It was not thought, however, that this amount was significant given the relative precision of the procedure. The third column test consisted of a 70 g L-1 HP-β-CD eluant through a 100% iron filing reaction column with an average retention time of 23.8 h. The results of this experiment are presented in Figure 5. The initial peak concentration in this test was significantly higher (18%) than the previous 70 g L-1 eluant test. No explanation for this can be hypothesized at this point. The general form of the dissolution curve is also quite different than the previous test. Recycling in this test was begun after 135 h or after approximately 85% of the PCE had dissolved. This accounts for the increase in reaction column effluent concentrations seen after recycling begins because of the residence time in each column. The 100% iron column reduced the PCE concentrations by approximately 75%. This is thought to be a function of the larger iron surface area to volume ratio in the reaction column.
Discussion In batch reactions, if the cyclodextrin and iron are in excess of the PCE, the reaction is observed to be essentially firstorder in PCE. Equation 7 indicates that the observed rate
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TABLE 2. Observed Rate Constants for Iron-PCE Reaction, Normalized for Iron Surface Area ref
FIGURE 6. Influence of cyclodextrin concentration on observed firstorder rate constant in batch tests. constant is proportional to the partition equilibrium constant, Ks; the mass of iron per unit volume of bulk solution, mFe/V; and the surface area per unit mass of the iron, A; and is inversely proportional to the quantity 1 + Kd[CD], which reflects concentration of cyclodextrin present, and the cyclodextrin binding constant. In Figure 6, the observed firstorder rate constants, kobs, from Table 1 are plotted as a function of 1/(1 + 120 L mol-1 [CD]). The best-fit linear regression for these data has a slope (kAmFeKs/V) of 0.0134 h-1. Figure 6 shows that kobs decreases as [CD] increases; the theoretical derivation of eq 7 indicates that formation of the soluble PCE/ HP-β-CD complex accounts for this effect. Nonlinear sorption of PCE (22) to iron could explain some deviation from firstorder kinetics during the initial portion of the experiment. However, the relationship shown in Figure 6 tends to validate a linear sorption model that is used in the theoretical treatment derived above. Results obtained by other investigators are also consistent with eq 7 and thus with equilibrium partitioning of PCE to the iron surface. Johnson et al. (23) recently conducted an exhaustive examination of the literature on chlorinated compounds reduced by iron metal. On the basis of that examination, they determined an empirical rate equation equivalent to our eq 7. The literature, however, also contains kinetic and thermodynamic data on the behavior of PCE, TCE, and iron that lead to conflicting interpretations. Burris’ group reports that PCE and TCE adsorb to iron according to a generalized Langmuir isotherm and that the observed order of reaction with iron is 2.665 for PCE and 1.312 for TCE (22). We are presently unable to reconcile these results with our own results and the results of other groups that tend to support the equilibrium partitioning model for PCE on an iron surface. Table 2 presents first-order degradation rates for PCE normalized to the concentration of iron surface area from this work and several other investigations. The effect of cyclodextrin on the rate of reaction as described by eq 7 has some interesting parallels in the influence of surfactants on the rate of biodegradation (24). Volkering’s group developed a mathematical model assuming that surfactant micelles act as a separate pseudophase. Their experimental data matched the model prediction, indicating that the PAH substrate present in the micellar pseudophase was not readily available for degradation by microorganisms. Using their model, the effect of surfactant on the MichaelisMenten Km for oxygen uptake could be substantially accounted for by adjusting the total substrate concentration to the concentration of substrate in the aqueous pseudophase. This “correction” for “free” substrate concentration is analogous to the dependence of eq 7 on cyclodextrin concentration. Guha and Jaffe´ also formulated a mathematical model for the kinetics of phenanthrene biodegradation in the presence
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13 22 32 this work 12 12 13 13 this work this work this work 12 33 33 34
AmFe/V kKs kobs, (h-1) (m2 mL-1) (mL m-2 h-1) type of experiment 0.19a 0.0557b 0.038 0.0134 0.030 0.019 1.65e 0.356f 0.070f 0.068f 0.060 0.00542 0.0379 0.00075
0.078 0.33 0.410c 0.275 0.016c 0.016c 0.14 8.8 3.22 0.195 0.195
2.5 0.17 0.093 0.049 1.9 1.2 5.8d 0.19 0.11 0.36 0.35
batch batch batch batch column column column column column (100-70) column (20-70) column (20-45) batch batch batch batch
a Calculated as 0.693 divided by t from Table 1 of ref 13. b λ from 50 a Figure 9 in ref 21. c Value obtained from ref 23. d Calculated as 0.693 divided by the normalized t50 from Table 3 of ref 13. e Calculated as 0.693 divided by t1/2 from 47 cm/day experiment in Figure 6a of ref 13. f Calculated from ln([PCE]/[PCE] ) divided by average retention time 0 and multiplied by (1 + Kd[CD]) to correct for the influence of HP-β-CD on the reaction rate.
of surfactant and observed that the fraction of micellar-phase phenanthrene that is bioavailable falls as surfactant concentration increases (25). Other groups have made similar qualitative observations. Six surfactants inhibited biodegradation of 14C-labeled phenanthrene as measured by output of 14CO2 (26), although the authors attributed the effect to disruption of membranemediated cellular events or destruction of membrane integrity. Aronstein et al. also observed reduced levels of biodegradation of PAHs at higher surfactant concentrations (27). They attributed this to surfactant toxicity, although at surfactant concentrations close to or below the critical micelle concentration, biodegradation was actually enhanced. Laha and Luthy also observed inhibition of PAH biodegradation by nonionic surfactants (28). We now have three rigorously derived treatments which give similar results, eq 7 in this paper for cyclodextrins that function via formation of stoichiometric inclusion complexes and the treatments of the data in refs 24 and 25 for surfactants that function via non-stoichiometric mass transfer to a micellar pseudophase. We also note the widespread qualitative observation of such effects in surfactant-based systems. In such cases where solubility is enhanced by the presence of an additional pseudophase, i.e., the cyclodextrin complex or the micellar interior, the increase in solubility is balanced by a corresponding decrease in the rate of a degradation reaction in aqueous solution. Important exceptions to this phenomenon are found in systems where the degradation rate is limited by mass transfer (29) and in certain biological systems where solubilized substrate can interact directly with cells (30), including surfactants naturally produced by microorganisms specifically to metabolize nonpolar substances (31). Other exceptions could occur in systems in which the solubility-enhancing agent is functionalized in such a manner as to also increase the rate of reaction, although we could find no examples of such systems in the literature. The column test results presented in Figure 3 demonstrate that peak PCE concentrations of approximately 1185 ppm were produced by the generator column during the first 25 h of operation. A gradual decrease in concentrations is produced after this initial period in response to a decrease in PCE-water interfacial area available for mass transfer. The post-treatment curve demonstrates that an approximately
30% reduction in PCE concentration occurs in the reaction column, independent of influent concentrations. The PCE half-lives reported by Gillham and O’Hannesin (13) suggest that near 100% PCE degradation should have occurred for the specified reaction column residence time. Our results indicate that the presence of the cyclodextrin slows the rate of PCE degradation with iron. However, even extrapolation of the 20% iron column experiments back to zero cyclodextrin concentration gives a percentage degradation of 80-90%. It should be noted, however, that the half-lives in ref 13 are based on lower initial PCE concentrations, slightly lower groundwater velocities, and slightly higher iron surface areas than ours. Whether or not these differences are sufficient to account for the difference in extent of PCE degradation remains to be determined. Figure 4 presents the pre- and post-treatment data for a 45 g L-1 HP-β-CD solution in the same reaction column as that employed in Figure 3. The reduced input HP-β-CD concentration results in lower peak PCE concentrations at early time but a greater percentage of reduction in PCE concentrations following treatment. Recycling of the posttreatment solution does not impair the degree of PCE degradation. Figure 5 presents the results of the 70 g L-1 HP-β-CD solution using 100% iron in the reaction column. The higher iron surface area results in a greater percentage of PCE degradation as compared to the previous two column tests, both before and after recycling of the post-treatment solution. The increase in PCE concentration seen following the start of recycling is due to the fact that the reaction column effluent at the start of recycling contains generator column solution from the early stages of the dissolution process when PCE-water interfacial area was high. The half-lives reported in Gillham and O’Hannesin (13) again suggest that complete degradation of the PCE should have taken place. In conclusion, the batch test results are in excellent agreement with eq 7, derived from first kinetic principles. The batch test results and the theoretical kinetic treatment also agree qualitatively with the column test results, namely, a smaller percentage degradation of PCE occurs in the column at the higher HP-β-CD concentration, all other conditions being the same (see Figures 3 and 4). Similarly, a larger percentage degradation occurs at higher iron surface area per unit volume (see Figures 3 and 5). The column tests performed here demonstrate that HP-β-CD is capable of delivering higher amounts of PCE mass to iron as compared to dissolution of PCE into water alone. The tests also demonstrate that while complete degradation of PCE does not occur in the presence of HP-β-CD for the iron surface areas and residence times employed, a significant decrease in PCE concentration does occur for both non-recycling and recycling of the post-treatment column effluent. Future work will be directed toward studying the relationship between kobs and the surface area (A) and iron mass (mFe) terms and selecting iron surface areas, cyclodextrin concentrations, and residence times to allow a greater degree of PCE degradation in the presence of iron.
Acknowledgments The authors wish to thank the American Maize-Products Company for providing hydroxypropyl-β-cyclodextrin, Ms. Bettina Longino of Queen’s University for performing interfacial tension measurements, and Prof. Philippe Lamarche of Royal Military College, Kingston, Ontario, for use of a gas chromatograph instrument. Financial assistance was provided, in part, by a grant from the University Consortium Solvents-In-Groundwater research program through financial contributions from Boeing, Ciba Geigy, Eastman Kodak, General Electric, MITRE, Motorola, PPG, and the United Technologies Corporation.
Supporting Information Available Additional text (including equations and references) and one table on the theory of reaction rates of complexed PCE on an iron surface (5 pp) will appear following these pages in the microfilm edition of this volume of the journal. Photocopies of the Supporting Information from this paper or microfiche (105 × 148 mm, 24× reduction, negatives) may be obtained from Microforms Office, American Chemical Society, 1155 16th St. NW, Washington, DC 20036. Full bibliographic citation (journal, title of article, names of authors, inclusive pagination, volume number, and issue number) and prepayment, check or money order for $15.00 for photocopy ($17.00 foreign) or $12.00 for microfiche ($13.00 foreign), are required. Canadian residents should add 7% GST. Supporting Information is also available to subscribers electronically via the Internet at http://pubs.acs.org (WWW) and pubs.acs.org (Gopher).
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Received for review April 9, 1996. Revised manuscript received October 2, 1996. Accepted October 4, 1996.X ES960324G X
Abstract published in Advance ACS Abstracts, December 15, 1996.
