Environ. Sci. Technol. 1999, 33, 3432-3437
Role of Reductants in the Enhanced Desorption and Transformation of Chloroaliphatic Compounds by Modified Fenton’s Reactions R I C H A R D J . W A T T S , * ,† BRETT C. BOTTENBERG,† THOMAS F. HESS,‡ MARK D. JENSEN,† AND AMY L. TEEL† Department of Civil & Environmental Engineering, Washington State University, Pullman, Washington 99164-2910, and Department of Biological and Agricultural Engineering, University of Idaho, Moscow, Idaho 83843
The mechanism for enhanced desorption of chloroaliphatic compounds from a silty loam soil by modified Fenton’s reagent was investigated using a series of probe compounds of varying hydrophobicities. Hexachloroethane, which has negligible reactivity with hydroxyl radicals, was transformed more rapidly in modified Fenton’s reactions (g0.3 M hydrogen peroxide) than it was lost by gas-purge desorption, suggesting the existence of a non-hydroxyl radical mechanism. The addition of excess 2-propanol to scavenge hydroxyl radicals slowed, but did not stop, the desorption and degradation of hexachloroethane. In the presence of the reductant scavenger chloroform, hexachloroethane did not desorb and was not degraded, indicating that a reductive pathway in vigorous Fenton-like reactions is responsible for enhanced contaminant desorption. Fenton-like degradation of hexachloroethane yielded the reduced product pentachloroethane, confirming the presence of a reductive mechanism. In the presence of excess 2-propanol, toluene, which has negligible reactivity with reductants, was displaced from the soil but not degraded. The results are consistent with enhanced contaminant desorption by reductants, followed by oxidation and reduction in the aqueous phase. Vigorous Fentonlike reactions in which reductants and hydroxyl radicals are generated may provide a universal treatment matrix in which contaminants are desorbed and then oxidized and reduced in a single system.
Introduction The use of modified Fenton’s reactions has recently been investigated for the treatment of industrial wastewaters, groundwater, and surface soils. The fundamental Fenton’s reaction involves the addition of dilute hydrogen peroxide (H2O2) to a degassed solution of iron(II) (1):
H2O2 + Fe2+ f OH• + OH- + Fe3+
(1)
The reaction described by eq 1 provides near-stoichiometric generation of hydroxyl radicals (OH•), which react with many * Corresponding author phone: (509)335-3761; fax: (509)335-7632; e-mail:
[email protected]. † Washington State University. ‡ University of Idaho. 3432
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organic compounds, including highly halogenated alkenes and aromatics, at near diffusion-controlled rates of 109-1010 M-1 s-1 (2-4). Most waste treatment applications of Fenton’s reagent use a modification of eq 1 that is specific to a remediation or treatment objective. For example, chelators and pyrophosphate have been used to minimize iron precipitation and enhance catalysis at neutral pH (5-7). Photo-Fenton’s reactions have been used to enhance iron cycling and contaminant degradation (8, 9). Fenton-like catalysis by iron oxides is a modification employed to avoid the precipitation that occurs when soluble iron is used as a catalyst (10-14). Another modification of Fenton’s reagent, the addition of excess H2O2, has been shown to enhance the destruction of sorbed contaminants in the treatment of contaminated soils and groundwater (15). For example, Watts et al. (16) showed that, although hexachlorobenzene sorbed to silicon dioxide desorbed only 34% over 4 h, it was oxidized by a modified Fenton’s reaction to undetectable levels in 2 h. In a similar study, [14C]hexadecane, which did not desorb over 72 h from silica sand, was converted to [14C]CO2 in 82% yield within 24 h using high concentrations of H2O2 in a modified Fenton’s reaction (17). The ability of Fenton-like reactions to provide enhanced treatment of sorbed contaminants has been documented; however, the mechanism of enhanced desorption has not been established. Contaminants may be oxidized while in the sorbed phase, a phenomenon that has been documented in some biological transformations (18-20). Although Sedlak and Andren (21) showed that OH• is not capable of oxidizing sorbed organic compounds in a dilute Fenton’s reaction, a higher flux of OH• at the soil surface promoted by vigorous Fenton-like reactions may provide a mechanism for oxidizing sorbed contaminants. Hydroxyl radicals have also been shown to oxidize sorbed contaminants during TiO2-mediated photocatalysis (22). Alternatively, an OH• or non-OH• mechanism may exist in vigorous Fenton’s reactions that promotes the desorption of contaminants with subsequent transformation in the aqueous phase. The purpose of this research was to investigate the mechanism of enhanced contaminant treatment by Fenton-like reactions, with emphasis on distinguishing between the transformation of sorbed contaminants and the enhanced desorption of contaminants with subsequent transformation in the aqueous phase.
Experimental Section Materials. Perchloroethylene (PCE; 99.9%) was purchased from Eastman Kodak Company. Hexachloroethane (HCA; 99%), hexachlorocyclopentadiene (HCCP; 99%), pentachloroethane (PCA; 95%), 1,1,1-trichloroethane (99%), and mixed isomers of 1,2-dichloroethylene (98%) were obtained from Aldrich. 1,1,1,2-Tetrachloroethane, 1,1,2,2-tetrachloroethane, 1,1,2-trichloroethane, and Fe2(SO4)3 were purchased from Sigma. Iron(III) was used as a catalyst to provide a superoxidedriven reaction (23). Chloroform, pentane, hexane, and carbon disulfide were purchased from Fisher Scientific, and toluene and 2-propanol were obtained from J. T. Baker. Hydrogen peroxide (50%, technical grade) was provided gratis by Solvay Interox. Double-deionized water (>18 MΩ‚cm) was purified with a Barnstead Nanopure II ultrapure system. The soil used was a Palouse loess silty loam sampled from a wheat field near Pullman, WA. Characteristics of the soil have been previously described (23). Probe Compounds and Scavengers. The probe compounds were selected based on their reactivities with oxidative or reductive species potentially present in Fenton10.1021/es990054c CCC: $18.00
1999 American Chemical Society Published on Web 08/20/1999
like reactions and their hydrophobicities (as measured by log Kow). PCE [log Kow ) 2.79 (24); kOH• ) 2.7 × 109 M-1 s-1 (2)] was used initially because of its high reactivity with OH• and its common occurrence as a groundwater pollutant. HCA [log Kow ) 4.28 (24)] was selected because it is structurally similar to PCE but is not reactive with OH•. To verify minimal reaction of HCA with OH•, a standard Fenton’s reaction was used (25). The results showed no loss of HCA relative to control reactions run in parallel. HCCP [log Kow ) 4.52 (24)] was chosen because it reacts rapidly with OH• [kOH• ) 3 × 109 M-1 s-1 (3)] and, based on preliminary data, showed negligible desorption from the Palouse loess soil. Toluene [log Kow ) 2.79 (24)] was used because it is highly reactive with OH• but not with reductants [kOH• ) 3 × 109 M-1 s-1; ke) 1.4 × 107 M-1 s-1 [2]). (The value of ke- is used here as a general predictor of reducibility.) 2-Propanol (kOH• ) 6 × 109 M-1 s-1[2]) was used as a OH• scavenger and chloroform [ke) 3 × 1010 M-1 s-1 (2)] was used as a reductant scavenger. The scavengers were added to the systems at 125 mM for scavenger:probe molar ratios of 5000:1. The concentrations of both scavengers were well below their respective water solubilities. Sample Preparation and General Procedures. Fentonlike reactions were conducted in sealed 40-mL batch borosilicate reactors containing 10 g of Palouse loess soil to which a probe compound was added in pentane. The pentane was allowed to evaporate, providing a probe concentration of 25 µmol kg-1, which is sufficiently low to assume that no nonaqueous phase liquids were present in the system. A set of triplicate reactors was prepared for each time point; at specific times, the reactions in one set were quenched, extracted, and analyzed for the probe compound. Modified Fenton-like reactions were initiated by adding H2O2 to the soil followed by iron(III), and the pH in each reactor was adjusted to pH 3 using 0.1 M H2SO4. The soil slurries contained sufficient buffering capacity so that the pH did not change over the course of the reactions. The H2O2 concentration most commonly used throughout the study was 0.6 M H2O2; however, it was varied from 0.06 to 2 M to evaluate the concentration effect on the transformation of the probe compound HCA. After the reactions were started, the reactors were capped with PTFE-lined septa; at varying time points, the reactions were quenched by the addition of 0.5 mL of concentrated H2SO4 (17), and each reaction was shake-extracted with hexane (decane for toluene) for 24 h. The extracts were then analyzed by gas chromatography. Triplicate control reactions were conducted in parallel with the addition of double-deionized water in place of H2O2. Additional control reactions were conducted with a Supelco ORBO-32 adsorbent tube inserted into the septa (26). The ORBO tubes were analyzed at the end of the 2-h reaction period, and no detectable off-gassing was found. Desorption Measurement. Gas purge (GP) desorption was measured in the same borosilicate glass reactors fitted with PTFE-lined stoppers through which the nitrogen purge gas was supplied into a fritted glass sparger that extended into the soil slurry (26-28). The stopper contained a port into which a Supelco ORBO-32 adsorbent tube was inserted. The reactors were prepared in the same manner as Fenton’s control reactions. GP measurements were started by initiating the nitrogen purge gas at a flow rate of 80 mL min-1. At sampling times, the adsorbent tube was taken from each reactor and immediately replaced with a fresh adsorbent tube. The tubes containing the captured probe were extracted for 30 min with carbon disulfide and analyzed by gas chromatography (28). Desorption measurements were conducted with and without scavengers, and no differences in desorption rates were found. GP desorption of PCE, HCA, and HCCP was confirmed by fill-and-draw methodology. Every 10 min over 2 h, 10 mL of water was drawn off and
replaced with fresh deionized water. Each aliquot of water drawn was extracted three times with hexane, and the extracts were analyzed for the chloroaliphatic compound by gas chromatography. Desorption of HCA and Toluene by Modified Fenton’s Reagent. Soil spiked with 25 µmol kg-1 HCA or toluene was treated with 0.6 M H2O2 and 5 mM iron(III), with a set of triplicate reactors prepared for each time point. At varying points, a set of the reactors was analyzed by filtering the aqueous phase through a 0.45-µm Millipore filter. The filtered aqueous phase was extracted three times, and the remaining soil was shake-extracted for 24 h. The extracts from each phase were analyzed by gas chromatography. Verification of Probe Degradation. Degradation of HCA and HCCP was confirmed by conducting parallel reactions and monitoring chloride release as the reactions proceeded. For each aliquot taken, the sample was diluted 2:1 and then analyzed using an ion-specific electrode. Parallel samples were analyzed without the addition of a probe compound to evaluate potential interferences from other species in the soil solution. No interferences were detected. Analysis of HCA Reduction Products. Fenton-like reactions were conducted with HCA as a probe compound without soil and with 2 M H2O2, 5 mM iron(III), and 250 mM 2-propanol. Aliquots were collected over 10 h, extracted with hexane, and analyzed by gas chromatography. Reduced products were evaluated by comparing retention times of degradation products to authentic standards of pentachloroethane, 1,1,1,2-tetrachloroethane, 1,1,2,2-tetrachloroethane, 1,1,2-trichloroethane, 1,1,1-trichloroethane, and mixed isomers of cis- and trans-1,2-dichloroethylene. Analysis. Extracts containing PCE, HCA, and HCCP were analyzed using a Hewlett-Packard 5890A gas chromatograph with a 0.53 mm (i.d.) × 15 m DB-5 capillary column and electron capture detection. The injector port and detector port temperatures were 180 and 350 °C, respectively. Initial temperatures were as follows: PCE, 40 °C; HCA, 80 °C; and HCCP, 140 °C. The program rate was 5 °C min-1. Residual toluene concentrations were analyzed on a HP 5890 gas chromatograph using a DB-1 capillary column with flame ionization detection (FID). An initial oven temperature of 80 °C with ramping to 180 °C at a rate of 10 °C min-1 was used. Hydrogen peroxide concentrations were followed by iodometric titration with 0.1 N sodium thiosulfate (29). Chloride ion analysis was performed using a Fisher Accumet chloride ion electrode paired with a double-junction reference electrode.
Results and Discussion Enhanced Degradation of Sorbed PCE by a Vigorous Fenton’s Reaction. The loss of PCE during a Fenton-like reaction using 0.6 M H2O2 and 5 mM iron(III) is shown in relation to its GP desorption rate in Figure 1. The GP data were plotted as the residual on the soil in order to compare desorption rates to degradation rates. These results show that GP desorption resulted in 28% PCE loss over 2 h, while PCE was degraded to an undetectable concentration through the Fenton’s reaction over the same amount of time. These results are similar to Fenton-like desorption-degradation relationships previously reported in which the rate of Fentonlike degradations far exceeded the rate of GP desorption (16, 17). Hydrogen peroxide concentrations during the treatment of PCE are also shown in Figure 1. The H2O2 concentration dropped to 0.3 M within 10 min, after which the rate of PCE degradation decreased significantly. Similarly, Gates and Siegrist (30) noted that the in situ vadose zone remediation of trichloroethylene (TCE) was significantly more effective when H2O2 concentrations were in the range of 1-2% (0.30.6 M). VOL. 33, NO. 19, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Fenton-like degradation of PCE in relation to its gaspurge desorption [25 µmol of PCE (kg of soil)-1, 10 g of soil, and 10 mL of Fenton’s solution, T ) 20 ( 2 °C; Fenton-like reactors: 0.6 M H2O2 and 5 mM iron(III) at pH 3; control reactors: 5 mM iron(III) at pH 3; gas-purge reactors: nitrogen purge gas flow rate ) 80 mL min-1]. GP methodology represents the maximum natural desorption rate. In the GP procedure, PCE (H ) 0.0153 atm m3 mol-1) is removed rapidly from the solution through volatilization after it is desorbed, so the procedure provides a measure of the most rapid desorption possible without enhancing the mass transfer of the contaminant from the surface. The rapid rate of PCE degradation in vigorous Fenton-like reactions relative to its GP desorption rate may be due to (a) oxidation of the sorbed contaminant while on the surface of the soil or (b) enhanced desorption from the soil, followed by degradation in the aqueous phase. To distinguish between these two possible mechanisms, other probe compounds that have differing reactivities with OH• were used in similar Fenton-like reactions. Role of OH• in Enhanced Contaminant Desorption and Degradation under Vigorous Fenton-like Conditions. To investigate the potential mechanism of OH• directly or indirectly causing contaminant desorption, HCA was used as a probe because of its negligible reactivity with OH•. If OH• were responsible for the enhanced treatment of sorbed HCA, either directly or indirectly, the aqueous phase concentration of HCA would be expected to increase as the sorbed concentration decreased. The sorbed and filterable concentrations of HCA during the course of a vigorous Fenton-like reaction are shown in Figure 2. The sorbed HCA concentration decreased by 75% over the 2 h of the reaction. However, corresponding concentrations of HCA were not found in the aqueous phase, suggesting that the desorbed HCA had been degraded, a result that was unexpected based on the negligible reactivity of HCA with OH•. Chloride analysis showed that 0.59 µmol of Cl- was recovered over the reaction period of 2 h, while 0.15 µmol of HCA was degraded, corresponding to an average removal of 4 of the 6 chlorines per molecule of HCA and confirming degradation in vigorous Fenton-like reactions. To further evaluate the conditions promoting HCA degradation, Fenton’s reactions were conducted (combining sorbed and soluble measurements) with 5 mM iron(III) and H2O2 concentrations ranging from 0.06 to 2 M. Enhanced desorption and degradation of HCA was found at H2O2 concentrations above 0.3 M; the more vigorous the Fenton’s reaction, the greater the HCA degradation. To elucidate the role, if any, played by OH• in the degradation of HCA in vigorous Fenton-like reactions, the reaction with 0.6 M H2O2 and 5 mM iron(III) was repeated with the addition of excess 2-propanol as an OH• scavenger 3434
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FIGURE 2. Sorbed and filterable concentrations of HCA during modified Fenton’s reactions [25 µmol of HCA (kg of soil)-1, 10 g of soil, and 10 mL of Fenton’s solution, T ) 20 ( 2 °C; Fenton-like reactors: 0.6 M H2O2 and 5 mM iron(III) at pH 3; control reactors: 5 mM iron(III) at pH 3].
FIGURE 3. Fenton-like degradation of HCA with and without scavenging [25 µmol of HCA (kg of soil)-1, 10 g of soil, and 10 mL of Fenton’s solution, T ) 20 ( 2 °C; Fenton-like reactors: 125 mM 2-propanol or chloroform in scavenger experiments, 0.6 M H2O2, and 5 mM iron(III) at pH 3; control reactors: 5 mM iron(III) at pH 3]. (Figure 3). While the presence of 2-propanol slowed HCA degradation, the degradation rate was still greater than the GP desorption rate. These results suggest that the presence of OH• partially promotes HCA degradation (perhaps indirectly) but is not entirely responsible for HCA degradation and desorption. Because HCA is characterized by perhalogenation, it has a greater thermodynamic potential to transform via reductive processes than oxidative processes (31). Recent results have documented non-OH• reductive mechanisms in a number of advanced oxidation processes (AOPs) (32, 33). Similar reductive pathways may also exist in modified Fenton’s systems, explaining the degradation of HCA. Evaluation of a Reductive Pathway in Fenton-like Reactions. The Fenton-like treatment of HCA using 0.6 M H2O2 and 5 mM iron(III) with and without the addition of chloroform as a reductant scavenger is also shown in Figure 3. These results show undetectable HCA degradation in the presence of excess chloroform, indicating that the HCAtransforming species was scavenged by chloroform and, therefore, was likely a reductant. The transformation of HCA and the corresponding formation of the reduction product pentachloroethane (PCA), using 2 M H2O2 and 5 mM iron(III) with the addition of 250 mM 2-propanol to scavenge OH•, is shown in Figure 4.
FIGURE 4. Fenton-like degradation of HCA with and corresponding production of PCA (25 µM HCA, 10 mL of Fenton’s solution, T ) 20 ( 2 °C, 2 M H2O2, 5 mM iron(III), and 250 mM 2-propanol at pH 3). Recovery of PCA was not quantitative, which may have been the result of the rapid conversion of PCA to other products (34). Other products detected in low (0.1-2 µM) concentrations included PCE, 1,1,2-trichloroethane, and cis- and trans1,2-dichloroethylene, indicating that the non-OH• transformations proceeded through both elimination and dehydrohalogenation reactions. The formation of these reduced products confirms the presence of a reductive pathway in vigorous Fenton-like reactions. More detailed product studies are proceeding. Coexisting oxidative-reductive reactions have been documented in a number of AOP systems. Glaze et al. (33) showed that PCE was degraded by both oxidative and reductive pathways in TiO2-mediated photocatalytic reactions, and Peyton et al. (32) found that 2,4-dinitrotoluene was reduced in UV/ozone reactors, a system previously thought to be only oxidative. The results shown in Figures 2-5 indicate that similar coexisting oxidation-reduction reactions occur during vigorous Fenton-like reactions. When high concentrations of H2O2 are used in modified Fenton’s systems, several reactions proceed beyond the initiation reaction listed in eq 1:
H2O2 + OH• f HO2• + H2O HO2• T H+ + O2•-
pKa ) 4.8
HO2• + O2•- f + HO2- + O2
(2) (3) (4)
where HO2• is the perhydroxyl radical, O2•- is the superoxide anion, and HO2- is the hydroperoxide anion. Superoxide and hydroperoxide anions are potential reductants in modified Fenton’s systems. The perhydroxyl radical is not a significant reductant; however, the pKa for its distribution with superoxide is 4.8, so some superoxide would be present at pH 3 (2). The superoxide anion has been shown to reduce quinones (35), nitrobenzenes (36), and nitrogen heterocycles (37), carbon tetrachloride, and chloroform (38). The hydroperoxide anion is a reductant and a nucleophile that has been documented to reduce organic compounds and metal complexes via one-electron transfers (39). Hydroperoxide has been shown to reduce benzoquinones (39) and 5-aminophthalazine-1,4-dione (40). Other reductants have been implicated in AOP systems; quinones have been shown to be important electron-transfer agents in Fenton and photo-Fenton systems (41), and ethoxy and alkoxy radicals have been shown to be important in H2O2-ozone and titanium dioxide-mediated photocatalysis systems (32, 41).
FIGURE 5. Fenton-like degradation of PCE with and without scavenging by chloroform [25 µmol of PCE (kg of soil)-1, 10 g of soil, and 10 mL of Fenton’s solution, T ) 20 ( 2 °C; Fenton-like reactors: 125 mM chloroform in scavenger experiments, 0.6 M H2O2, and 5 mM iron(III) at pH 3; control reactors: 5 mM iron(III) at pH 3]. The potential for generating reductants in Fenton-like reactions has a number of implications for applied research published over the past 10 years. Most degradations in modified Fenton’s systems have been interpreted as oxidation by OH•; however, a combined oxidation-reduction mechanism may account for some of the transformations observed at higher H2O2 concentrations. For example, the degradation of organochlorine insecticides (10), pentachlorophenol (15), trichloroethylene (30), nitrophenols and nitrobenzene (42), and 2,4,6-trinitrotoluene (43, 44) probably involved Fentonlike reductions. Many halogenated and nitro-substituted contaminants, such as PCE and nitrobenzene [kOH• ) 3.9 × 109 M-1 s-1; ke- ) 3.7 × 1010 M-1 s-1 (2)], react with both OH• and reductants at near-diffusion-controlled rates; therefore, their degradation in vigorous Fenton-like reactions may proceed through parallel oxidations and reductions. The results of Figures 2-5 show that a reductant, in addition to OH•, is an important reactant in the vigorous conditions commonly used in the Fenton-like treatment of contaminated soils. Because PCE potentially reacts by both oxidative and reductive pathways, the reactions shown in Figure 1 were repeated, but with the addition of excess chloroform as a reductant scavenger. The results (Figure 5) show that chloroform quenched approximately 80% of the PCE transformation. Furthermore, the PCE transformation rate in the presence of excess chloroform closely followed the GP desorption rate. These results suggest that, when reductants are scavenged from the vigorous Fenton’s system, PCE is transformed by OH• oxidation alone following its naturally occurring rate of desorption into the aqueous phase:
PCEsorbed f PCEaq
(5)
PCEaq + OH• f products
(6)
Because no detectable enhanced desorption occurred when reductants were scavenged from the Fenton’s system, the data of Figure 5 provide further evidence that reductants are the species promoting the enhanced desorption of PCE. Based on the results of the scavenger-probe relationships presented in Figures 3 and 5, the following proposed mechanism is consistent with PCE desorption, oxidation, and reduction in a vigorous Fenton’s system: VOL. 33, NO. 19, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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PCEsorbed + OH• f no reaction
(7)
PCEsorbed + Xreduced f PCEaq + X
(8)
PCEaq + OH• f products
(9)
PCEaq + Xreduced f products + X
(10)
where Xreduced ) O2•-, HO2-, e-aq, quinones, alkoxy radicals, or other reductant. Enhanced Desorption of Toluene. To confirm the role of a reducing species and to determine whether desorption precedes degradation or occurs simultaneously with it, toluene was used as a probe compound in a vigorous Fenton’s reaction. Toluene is characterized by minimal reactivity with reductants [ke- ) 1.4 × 107 M-1 s-1 (2)], and a 5000:1 molar excess of 2-propanol was added to the reaction to scavenge OH•, minimizing the oxidation of toluene [kOH• ) 3 × 109 M-1 s-1 (2)]. Sorbed and soluble concentrations of toluene, measured separately at time points over the course of 2 h, are shown in Figure 6. The concentration of sorbed toluene decreased by approximately 60% as compared to deionized water controls, while the concentration of filterable toluene increased approximately 60% over deionized water filterable controls. These data confirm that a non-OH• species is responsible for the enhanced desorption of hydrophobic compounds in vigorous Fenton-like reactions. In addition, the results indicate that compound degradation is not required for desorption, suggesting that desorption occurs first, followed by degradation in the aqueous phase. Treatment of Sorbed HCCP. To confirm the proposed mechanism outlined in eqs 7-10, scavenging experiments were repeated using HCCP as the probe compound, which is reactive with both OH• and reductants. However, HCCP is more hydrophobic than PCE [log Kow ) 4.52 (24)] and showed negligible desorption over 2 h based on GP desorption and fill-and-draw methodologies (Figure 7). In contrast, HCCP degraded approximately 80% over 2 h in vigorous Fenton’s reactions. HCCP degradation was confirmed by Clanalysis; the results were analogous to Cl- release by HCA with approximately 80% Cl- released from HCCP. In the presence of excess chloroform to scavenge reductants, HCCP degradation was indistinguishable from the deionized water controls, indicating that a reductive species was required for HCCP desorption and degradation. With the addition of excess 2-propanol to scavenge OH•, some degradation occurred, resulting in approximately 40% loss over 2 h. These data are in agreement with the HCA-2-propanol data shown in Figure 3 and indicate that a reductive species is required for the enhanced desorption and degradation seen with these compounds in vigorous Fenton-like reactions. Some studies have also shown the enhanced treatment of organic compounds sorbed on inorganic sorbents, which indicates that destruction of soil organic matter is not the mechanism of contaminant release (16, 17). Contaminant desorption is often the dynamic that limits the treatment of soils and groundwater. For example, hundreds of pore volumes of clean water over decades may be required to achieve low contaminant levels in pumpand-treat groundwater remediation (45). The use of vigorous Fenton-like reactions to treat sorbed contaminants in soils and groundwater has a number of advantages over other soil and groundwater treatment processes, including (a) a combined enhanced desorption-degradation process that can be conducted in situ or ex situ and (b) coexisting oxidative and reductive mechanisms that may provide the potential to treat a wider range of contaminants than can be treated by OH• mechanisms alone. The potential for generating both oxidants and reductants in modified Fenton’s reactions has important ramifications 3436
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FIGURE 6. Sorbed and filterable concentrations of toluene during modified Fenton’s reactions in the presence of excess 2-propanol [25 µmol of HCA (kg of soil)-1, 10 g of soil, and 10 mL of Fenton’s solution, T ) 20 ( 2 °C; Fenton-like reactors: 125 mM 2-propanol, 0.6 M H2O2, and 5 mM iron(III) at pH 3; control reactors: 125 mM 2-propanol and 5 mM iron(III) at pH 3].
FIGURE 7. Fenton-like degradation of HCCP with and without scavenging [25 µmol of HCCP (kg of soil)-1, 10 g of soil, and 10 mL of Fenton’s solution, T ) 20 ( 2 °C; Fenton-like reactors: 125 mM 2-propanol or 125 mM chloroform in scavenger experiments, 0.6 M H2O2, and 5 mM iron(III) at pH 3; control reactors: 5 mM iron(III) at pH 3]. in waste treatment. A Fenton’s system that generates both species has the potential to desorb contaminants, oxidize reduced contaminants (e.g., monocyclic aromatic hydrocarbons, alkenes, PAHs), and reduce oxidized contaminants (e.g., carbon tetrachloride, 1,3,5-trinitrobenzene). Furthermore, many degradation products that are relatively unreactive with OH• may be transformed by reductants in the system, which may enhance the potential for contaminant mineralization by Fenton-like reactions. Therefore, vigorous Fenton-like reactions in which reductants are generated may provide a universal treatment matrix in which contaminants are rapidly desorbed from solids and sludges, followed by transformation through both oxidative and reductive mechanisms.
Acknowledgments Funding for this research was provided by the U.S. Environmental Protection Agency through Assistance Agreement R826163-01 and by the National Science Foundation through Grant BES-9613258. The authors thank Dr. Glenn C. Miller for critical review of the manuscript.
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Received for review January 19, 1999. Revised manuscript received June 14, 1999. Accepted July 12, 1999. ES990054C
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