Environ. Sci. Technol. 2006, 40, 2778-2783
(3)
Classical Fenton oxidation of contaminants occurs via •OH, but in many soil systems Fenton reagents fail to degrade contaminants because of insufficient production of •OH, even with high doses. While the reasons for this are not well understood, it limits the choice of Fenton chemistry as a remediation alternative and makes it impossible to predict the effectiveness of Fenton reactions in a contaminated soil without first doing treatability studies. In some soils reductants are produced as intermediates instead of •OH (3). Also, H2O2 can degrade to release O2 (reaction 6), a scavenging reaction which does not produce •OH. Conventional Fenton’s chemistry maintains a pH near 3 to keep Fe3+ in solution, which is impractical in the presence of soils because of their buffering capacity. As a result, most ISCO applications use a modified Fenton (MF) treatment which produces •OH at a pH near neutral. Iron can be added as salts of Fe2+ or Fe3+ (4), or native iron-containing minerals can be used (5). Organic chelators are often added to increase the presence of Fe3+ in the aqueous phase (6). Common chelators include ethylenediaminetetraacetic acid (EDTA) and nitrilotriacetic acid (NTA). The results from recent studies with pure cultures of Xanthobacter flavus in aqueous systems support the often proposed hypothesis that •OH-mediated oxidation of contaminants coexists with aerobic biodegradation. Bu ¨yu ¨kso¨nmez et al. (7) showed that preacclimation of X. flavus FB71 to MF reactions sharply reduced the toxicity of •OH. Bu ¨ yu ¨ kso¨nmez et al. (8) showed that adding X. flavus to an MF system oxidizing tetrachloroethene (PCE) resulted in 10% greater PCE mineralization than stand-alone MF reactions. Properly demonstrating coexisting chemical and biological oxidation is difficult, since both are oxidative reactions. However, recently Howsawkeng et al. (9) tested PCE and oxalate (OA) as probe compounds to distinguish biological from chemical oxidation and then used them to demonstrate coexisting MF oxidation of PCE and biodegradation of oxalate (OA) by X. flavus in aqueous systems. The first objective of this study was to demonstrate coexisting •OH-mediated oxidation and aerobic metabolism in a natural soil. Simultaneous chemical and biological oxidation has not yet been documented for a native microbial consortium or for soil systems, which often behave differently than aqueous systems by not fostering the classical Fenton reactions. The same probe compounds used by Howsawkeng et al. (9) were tested to distinguish MF oxidation of added PCE from aerobic assimilation of OA in slurry reactors. The second objective of this work was to investigate the response of native soil microorganisms to doses of chemical oxidants in excess of that required to remove contaminants. Doses of Fenton reagents up to 20 times that required for 95% PCE oxidation were added to soil slurries, and the effects on PCE mineralization, aerobic OA assimilation, and heterotrophic plate counts were measured. The purpose of these experiments was to determine how sensitive soil microorganisms are to death and/or inactivation during overdosing of chemical oxidants, such as often occurs around injection wells during ISCO.
O2•- + Fe3+ f Fe2+ + O2
(4)
Experimental Section
•OH + H2O2 f HO2• + H2O
(5)
2H2O2 f H2O + O2
(6)
Materials. Spectrophotometric grade (99.9%) tetrachloroethene (PCE), 2-propanol (99.9%), and trichloromethane (TCM) (99.9%) were obtained from Acros Organics (Pittsburgh, PA). Oxalic acid (99.9%), nitrilotriacetic acid (NTA) (98.5%), Fe(III) sulfate, ammonium sulfate, potassium hydroxide (KOH), and monobasic and dibasic potassium phosphate were purchased from Aldrich Chemicals (Mil-
Effect of Fenton Reagent Dose on Coexisting Chemical and Microbial Oxidation in Soil ANNE-CLARISSE NDJOU’OU, JOSEPH BOU-NASR, AND DANIEL CASSIDY* De´partement de Ge´ologie et de Ge´nie Ge´ologique, l’Universite´ Laval, Sainte-Foy, QC, G1K 7P4, Canada
The ability of modified Fenton reactions to promote simultaneous chemical and biological oxidation in an artificially contaminated soil was studied in batch laboratory slurry reactors. Tetrachloroethene (PCE) and oxalate (OA) were used to distinguish chemical oxidation from aerobic heterotrophic metabolism. PCE was mineralized by Fenton reactions, but OA was not oxidized. Indigenous soil microorganisms did not degrade added PCE aerobically but readily assimilated OA. Fenton reactions were promoted at the natural soil pH (7.6) by adding H2O2 and Fe(III), with nitrilotriacetic acid (NTA) as a chelator, at a constant molar ratio of H2O2/Fe(III)/NTA of 50:1:1. The •OHmediated mineralization of PCE was demonstrated by adding 2-propanol (an •OH scavenger), which inhibited PCE oxidation. In subsequent dosing studies, PCE oxidation served as an indicator of Fenton reactions, while OA assimilation, dissolved oxygen (DO) concentration, and heterotrophic plate counts were indicators of aerobic microbial activity. Increasing Fenton doses to 20 times that required to achieve 95% PCE oxidation only delayed OA assimilation by 500 min and reduced plate counts by 1.5 log units g-1 soil. Results show that aerobic metabolism can coexist with Fenton oxidation in soils.
Introduction In situ chemical oxidation (ISCO) with Fenton reactions is a powerful tool for the destruction of many contaminants (1). Fenton chemistry involves the catalyzed decomposition of H2O2 by Fe2+ to form the hydroxyl radical (•OH) (reaction 1). If only Fe3+ is originally present, or the dose of Fe2+ is insufficient, Fe2+ required in reaction 1 can be generated via reactions 2-5 (2).
H2O2 + Fe2+ f •OH + OH- + Fe3+
(1)
Fe3+ + H2O2 f Fe2+ + HO2• + H+
(2)
HO2• T O2•- + H+
(pKa ) 4.8)
* Corresponding author phone: (418)656-7339; fax: (418)656-2131; e-mail:
[email protected]. 2778
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10.1021/es0525152 CCC: $33.50
2006 American Chemical Society Published on Web 03/16/2006
TABLE 1. Characteristics of the Soil after Being Passed through a 0.2 mm Sieve characteristic
valuea
pH moisture content (%) organic carbon content (%) sand (%) silt (%) clay (%) CEC (cmol 100 g-1) total carbonates (%) crystalline Fe (mg kg-1) amorphous Fe (mg kg-1) crystalline Mn (mg kg-1) amorphous Mn (mg kg-1) heterotrophic plate count (log CFU g-1)
7.6 12.5 0.68 37.6 41.3 21.1 14.2 3.6 2470 21 510 13 7.48 ( 0.21
a
Arithmetic mean ( standard deviation for four replicates.
waukee, WI). A stock H2O2 solution 50% was provided by Solvay (Houston, TX). Trypticase soy agar and Petri dishes were purchased from Fisher Scientific (Pittsburgh, PA). Soil. The silt loam used in these studies was obtained from the Kalamazoo Nature Center near Kalamazoo, MI and was selected because it had no history of contamination. It was dry sieved (0.2 mm) to remove all but the fine sand, silt, and clay fractions. This was done to disaggregate and homogenize the fines and to reduce sampling heterogeneities in the slurries caused by coarser particles. The characteristics of the sieved soil used in the studies are listed in Table 1. The sieved soil had a moisture content of 12.5%. The organic carbon content was quantified by trapping in KOH the CO2 evolved from combustion of the soil at 900 °C, followed by back-titrating unreacted KOH (10). Particle-size distribution was determined with the pipet method described by Gee and Bauder (11). Cation exchange capacity (CEC) was quantified according to the ammonium acetate method (12). The total carbonate content was determined with the simple titrimetric procedure (13). Crystalline and amorphous Fe and Mn oxides and oxyhydroxides were quantified with citratebicarbonate-dithionite extraction (14). Heterotrophic plate counts were quantified in quadruplicate with the spread plate method (9215C) (15) and reported as colony forming units (CFU) g-1 soil. Reactors. The slurry reactors consisted of 4 L roundbottom Pyrex glass vessels with fitted covers having five ports. The central port in the cover accommodated a propeller blade attached to a mechanical mixer rotating at 300 rpm. One port housed a YSI Instruments dissolved oxygen (DO) probe and another an Orion pH probe to allow continuous monitoring of DO and pH. A Supelco ORBO tube was fitted to the third port to allow PCE stripping to be quantified during the oxidation experiments. The fourth port was used to sample. In the dosing studies the sample port was left open after 100 min, when the PCE had already been oxidized, to enhance aeration of the slurry. Slurry. Each reactor received 0.9 kg of sieved wet soil, consisting of 0.8 kg dry soil plus 0.1 kg (0.1 L) of water from the soil moisture (Table 1). For killed controls, soil was autoclaved in the reactor vessel twice according to Wolf and Skipper (16) prior to adding autoclaved deionized water. The volume of water added to each reactor was 0.16 L, which along with 0.1 L of water from the soil moisture and 0.3 L of soil solids, resulted in 2 L of slurry having a solids concentration of 0.4 kg soil L-1. A water volume of 0.17 L was used to calculate the doses of all aqueous constituents. The appropriate volumes of solutions (all made in deionized water) were added, and the balance of the 0.16 L was made up with deionized water. Each reactor received 39 mL of a 1 M KH2PO4 solution and 261 mL of a 1 M K2HPO4 solution,
TABLE 2. List of the Doses of Fenton Reagents Tested in the Experiments with the Corresponding Name Used in the Text and the Appropriate Figure Fenton dosesa
Figure
Figure
none 10 mM H2O2 + 0.2 mM Fe(III)-NTA 20 mM H2O2 + 0.4 mM Fe(III)-NTA 50 mM H2O2 + 1.0 mM Fe(III)-NTA 100 mM H2O2 + 2.0 mM Fe(III)-NTA 200 mM H2O2 + 4.0 mM Fe(III)-NTA
dose 0 dose 1 dose 2 dose 5 dose 10 dose 20
2 4a
a
4b 4c
A constant molar ratio of H2O2/Fe(III)-NTA of 50:1 was used.
which buffers at a pH of 7.6, the natural pH of the soil. Each reactor received a dose of 1 mM (NH4)2SO4, which along with the phosphate provided macronutrients for OA assimilation. The internal temperature was not monitored, but the laboratory temperature varied between 21 and 24 °C. Probe Compounds (PCE and OA). PCE was chosen as a probe compound for chemical oxidation since it is known to be very reactive with the •OH (17, 18). The slurry received 160 mg (100 µL) of PCE. The added PCE was allowed to mix for 72 h before beginning the oxidation experiments, with the sampling port open to ensure aerobic conditions. This mixing period allowed time for the PCE to dissolve in the aqueous phase and adsorb to the soil, though some PCE was lost via volatilization during this time. The calculated PCE concentration in the slurry with no losses would be 200 mg kg-1, whereas initial PCE concentrations for the oxidation experiments ranged from 126 to 154 mg kg-1. OA was used as a probe compound for aerobic biological assimilation because it has a negligible reactivity with •OH (17), and in fact it often accumulates as a partial oxidation product of PCE oxidation by •OH in abiotic systems (18, 19). Howsawkeng et al. (9) observed negligible oxidation of OA in MF experiments in aqueous systems. OA was added to the slurry to provide a calculated initial concentration of 200 mg L-1. Measured initial concentrations of OA varied from 162 mg L-1 to 187 mg, probably due to biodegradation and/or adsorption. Modified Fenton Reactions. Fe(III) chelated with NTA in a molar ratio of 1:1 was used as a catalyst, which is effective for modified Fenton reactions (9). A 1 M stock solution of Fe(III)-NTA was prepared by first dissolving NTA in deionized water and then adding an equimolar amount of ferric sulfate and adjusting the solution to a pH of 7.6 with KOH. The appropriate volume was then added to the slurry to achieve the desired dose. The Fenton oxidation experiments were initiated by adding the desired dose of H2O2 from the 50% stock solution directly to the slurry. A constant molar ratio of H2O2/Fe(III)-NTA of 50:1 was used for all the different Fenton doses tested. Table 2 lists all the doses of H2O2 and Fe(III)-NTA tested, along with the corresponding name used in the text and the figure illustrating the data for that test. The Fenton doses were chosen based on results from background tests indicating that dose 1 provided greater than 95% PCE oxidation. For control experiments an equivalent volume of deionized water was added instead of the H2O2. Reductant and •OH Scavenging Studies. Separate studies were conducted to determine whether the chemical oxidation of PCE observed was mediated by •OH or by reductants. Trichloromethane (TCM) is an effective scavenger of reductants (20), and 2-propanol scavenges •OH (17). TCM and 2-propanol were therefore added to separate reactors with Fenton reagents to determine the effects of the two on MF oxidation of PCE. The doses of Fenton reagents used were 20 mM H2O2 and 0.4 mM Fe(III)-NTA (dose 2 in Table 2), and the molar ratio of scavenger to PCE was 50:1. VOL. 40, NO. 8, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Concentrations of PCE and Cl- with modified Fenton oxidation (50 mM H2O2 + 1.0 mM Fe(III)-NTA, or dose 5) and in a control with no H2O2 added. Sample Handling and Preparation. PCE was extracted from duplicate 30 mL slurry samples with 3 mL of hexane by mixing in 50 mL screw-cap test tubes on a wrist action shaker for 6 h. After centrifuging, the hexane was extracted with a syringe and placed in 2 µL vials for PCE analyses. PCE in the contents of ORBO tubes was extracted in the same fashion using 2 mL of hexane and 15 mL of added deionized water. OA and Cl- were analyzed in filtrate from slurry samples passed through 0.20 µm Whatman filter. Analyses. OA was quantified spectrophotometrically in duplicate in filtrate samples according to the method described by Zerwekh et al. (21). A mixture of oxalate oxidase, 3-methyl-2-benzothiazoline hydrazone, 3-(dimethylamino) benzoate, and peroxidase yields a purple indamine dye with an absorbance at 590 nm. Chloride ion (Cl-) concentrations were measured in duplicate in filtrate samples with a Thermo Orion chloride ion specific electrode and a 720APlus meter. Values of pH in the slurry were measured with the same meter and a pH probe. PCE was analyzed in duplicate in hexane extracts using a Hewlett-Packard 6890 gas chromatograph (GC) with electron capture detection and a DB-1 fused-silica capillary column (15 m × 0.317 mm i.d., 0.25 µm film thickness). The oven temperature started at 40 °C for 5 min and was increased at 5 °C min-1 to a final temperature of 130 °C. The injector and detector temperatures were 240 °C and 350 °C, respectively. PCE concentrations are reported as mg kg-1 soil.
Results and Discussion Testing PCE and OA as Probe Compounds. To verify that PCE and OA were reliable probe compounds to distinguish chemical from biological oxidation in the slurries, it was first necessary to demonstrate that (1) PCE was chemically oxidized by the MF reagents added to the slurry but that OA was not, and (2) OA was aerobically consumed by soil microorganisms but not PCE. Figure 1 shows concentrations
of PCE and Cl- during MF oxidation with dose 5 (see Table 2) compared with a control, in which H2O2 was replaced with deionized water. The results show that the added PCE was degraded by the MF reactions. PCE values decreased by over 98% within 60 min, and the accompanying release of Cl- indicates that the PCE was degraded. The molar ratio of Cl-/PCE oxidized was 3.74, which is consistent with Clrecoveries from other authors reporting PCE mineralization (18, 22). Volatile PCE losses were 2.2%. These results show that PCE was a reliable probe for MF oxidation. The ability of the native soil microorganisms to assimilate OA under aerobic conditions is shown in Figure 2. In a killed control, OA concentrations did not measurably decrease in the slurry, whereas in a biologically active system OA was completely taken up within 60 min. During the time of OA removal in the active system, dissolved oxygen (DO) concentrations decreased from approximately 7 mg L-1, the DO saturation concentration in the slurries, to nearly 2 mg L-1. After the OA was completely removed, DO levels increased to the saturation concentration. Consumption of O2 accompanied by OA removal shows that OA was aerobically assimilated in the active system. In contrast, the killed reactor showed no OA uptake and the DO (not shown) remained at the saturation level. Although not verified by microbiological analyses, it can be reasonably assumed that the native soil microorganisms were responsible for the observed activity because plate count data for the soil indicate that they were present in large numbers (Table 1). PCE was not removed significantly in either reactor. The findings in Figure 2 are not surprising, as PCE is typically biodegraded under anaerobic conditions (23), while OA is a constituent of the citric acid cycle and is therefore assimilated aerobically by many microorganisms. Verification of •OH as Responsible for PCE Oxidation. Scavenging studies were conducted to determine whether the chemical oxidation of PCE observed could be attributed to •OH, according to reaction 1. PCE can be degraded by oxidants and reductants (18) and recent studies by Watts et al. (3) show that reductants such as the superoxide anion (O2•-) and the hydroperoxide anion (HO2-) are produced in some MF-soil systems. Figure 3 illustrates PCE oxidation in the slurry with and without the scavengers. Dose 2 (20 H2O2 mM + 0.4 mM Fe(III)-NTA) was used in these experiments. TCM had no effect on PCE oxidation, whereas adding 2-propanol resulted in nearly complete inhibition of PCE oxidation. These results demonstrate that the oxidation of PCE was caused by •OH, as predicated by classical Fenton chemistry. Simultaneous Biological and Chemical Oxidation with Different Fenton Doses. The probe compounds were then used to distinguish chemical oxidation from biodegradation in the slurries. Increasing doses of H2O2 and Fe(III)-NTA (in a constant molar ratio of 50:1, Table 2) were added to different slurry batches in order to investigate the response of soil
FIGURE 2. PCE, oxalate (OA), and dissolved oxygen (DO) concentrations in a biologically active reactor and a killed (autoclaved) control. The DO for the killed control is not shown but remained at approximately 7 mg L-1 throughout. 2780
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FIGURE 3. PCE oxidation with and without scavengers. Trichloromethane (TCM) was used to scavenge reductants and 2-propanol to scavenge •OH (Fenton dose 2 ) 20 H2O2 mM + 0.4 mM Fe(III)-NTA). microorganisms to high doses of Fenton reagents. The purpose of the dosing studies was to determine how sensitive soil microorganisms are to high oxidant doses, which are encountered near injection wells in ISCO systems. High doses are used to deliver oxidants to the subsurface because the cost of chemicals is usually much less than that of adding more injection wells (24). The lowest dose (dose 1) was chosen because previous work (data not presented) showed it capable of removing 95% of the added PCE. Doses ranging up to 20 times greater than dose 1 were used, and the effects on PCE oxidation, PCE volatilization, OA assimilation, heterotrophic plate counts, and pH were studied. Figure 4 shows the time profiles for concentrations of PCE, OA, and DO, and for pH values over a 600 min period for doses 1, 5, and 20. Table 3 lists the results for PCE removal, Cl- yield per mol of PCE oxidized, and heterotrophic plate counts after 600 min in the slurry reactors for the same experiments. It is clear from Table 3 that all doses resulted in greater than 95% PCE removal. Despite increased intensity of bubbling observed after adding increasing doses, volatile losses of PCE did not increase. This can be explained by higher concentrations of oxidants with increasing dose, resulting in more rapid PCE destruction. The yield of Cl- per mol of PCE oxidized only varied between 3.7 and 3.8, indicating that PCE was mineralized and that stripping was not a significant mechanism of PCE removal in the reactors. Figure 4 shows that pH values remained stable at approximately 7.6, the natural pH of the soil, for even the highest dose. The soil was naturally phosphate buffered due to its high carbonate content (Table 1), but a supplemental phosphate buffer at a pH of 7.6 was also added. Figure 4 shows that chemical oxidation of PCE and aerobic uptake of OA coexisted in the slurries, though increasing doses progressively delayed the onset of OA assimilation. The DO concentrations also show that O2 was released from the Fenton reagents at early time (reaction 6) and can be used to monitor the duration of the chemical oxidation reactions in the slurry. With dose 1 (Figure 4a) OA assimilation and PCE oxidation were truly simultaneous. Removal of both compounds was coincidental and was nearly complete within 100 min. It is interesting to note that OA assimilation with dose 1 occurred without the decrease in DO observed in Figure 2 and Figure 4, parts b and c. The reason for this is that O2 released from the MF reactions provided sufficient DO to support simultaneous OA assimilation without the DO dropping below saturation levels (7 mg L-1). However, doses 5 and 20 (Figure 4, parts b and c, respectively) showed progressive delays between the end of PCE oxidation and the onset of OA degradation, which resulted in OA assimilation being accompanied by temporary decreases in DO concentrations below 7 mg L-1. The potential for O2 release to be used in aerobic biodegradation represents a significant advantage which a simultaneous treatment system has over
FIGURE 4. Concentrations of PCE, oxalate (OA), and dissolved oxygen (DO), and pH values in modified Fenton systems with different doses: (a) dose 1 ) 10 mM H2O2 + 0.2 mM Fe(III)-NTA; (b) dose 5 ) 50 mM H2O2 + 1.0 mM Fe(III)-NTA; (c) dose 20 ) 200 mM H2O2 + 4.0 mM Fe(III)-NTA. one in which biological and chemical oxidation are separate in time. Table 3 shows that increasing the dose of H2O2 and Fe(III)-NTA 20 times (from dose 1 to dose 20) reduced VOL. 40, NO. 8, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 3. Results for PCE Removal, Cl- Yield per Mole of PCE Oxidized, and Heterotrophic Plate Counts after 600 min in the Slurry Reactors for the Different Doses of Fenton Reagents Tested Fenton dose dose 0 dose 1 dose 2 dose 5 dose 10 dose 20
PCE PCE mol PCE removed stripped oxidized Cl-/mol PCE plate counts (%) (%)b (%)a oxidized (log CFU/g)c 2.1 97.1 99.2 99.8 99.9 99.9
2.5 1.2 2.4 1.4 2.1 2.0
0 95.9 96.8 98.4 97.7 97.9
n/a 3.7 3.7 3.8 3.7 3.7
7.51 ( 0.18 7.42 ( 0.20 7.49 ( 0.15 7.5 ( 0.16 6.82 ( 0.19 6.04 ( 0.16
a Based on initial measured PCE concentrations for each reactor. PCE oxidized ) total PCE removed - PCE stripped. c Arithmetic mean ( standard deviation for four replicates. b
heterotrophic plate counts by 1.5 log units g-1 soil (from 7.5 to 6). Figure 4 suggests that in addition to a reduction in counts of culturable individuals, increasing doses also caused temporary inactivation of the remaining microorganisms (500 min for the highest dose, Figure 4c). It is expected that the MF reactions in the slurries would result in some degree of microbial death and/or inactivation, because •OH reacts with biological molecules at diffusion-limited rates (17). Although it was not monitored in these studies, high temperatures may also have contributed to microbial death and inactivation at high doses. Temperature should increase with increasing dose because Fenton reactions are exothermic. This study is the first to document coexisting biological and chemical oxidation in soil with the native microorganisms, though it has been reported in aqueous systems with a pure culture (7-9). Other research has reported modified Fenton oxidation of contaminants in slurries followed by addition of waste-activated biomass to achieve biological oxidation (25), which is impractical for in situ applications. The results from this study are also encouraging in that they demonstrate how robust soil microorganisms can be when exposed to high doses of •OH-producing chemicals. Even with an oxidant dose 20 times greater than that required to oxidize 95% of the PCE, plate counts remained above 106 CFU g-1 soil (Table 3), and the remaining microorganisms regained their aerobic metabolic activity within a matter of hours (Figure 4). These findings suggest that aerobic biodegradation of contaminants and/or chemical oxidation products takes place in ISCO systems, even adjacent to injection wells where concentrations are the highest. This has implications for long-term health of the host soil and for the fate of readily biodegradable intermediates in pore waters formed from •OH-mediated oxidation of organic contaminants, such as oxalate, formate, and dichloroethane (18, 19, 26, 27). Because of the long use of •OH for water disinfection, biological processes have traditionally been considered incompatible with Fenton reactions. Early efforts to inject H2O2 into aquifers to stimulate in situ aerobic bioremediation were later recognized as having promoted modified Fenton reactions (28, 29). Bu ¨ yu ¨ kso¨nmez et al. (7) reported that microorganisms in aqueous systems could survive low levels of •OH and that preacclimation to •OH reduced its toxicity dramatically. Arienzo (30) found that phospholipid levels (a surrogate measure of viable biomass) were either unaffected or increased after MF treatment of a trinitrotoluenecontaminated soil. Ndjou’ou and Cassidy (31) showed that MF oxidation of hydrocarbons in soil did not significantly reduce plate counts of native hydrocarbon-degraders. Even conventional Fenton treatment (pH ) 2) of a soil in a slurry reactor only reduced bacterial counts from 108 to 104 mL-1 after 1 day (32). In contrast, Schrader and Hess (25) reported 2782
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a reduction in counts of soil bacteria of nearly 106 g-1 soil after Fenton oxidation in soil slurries. The findings from this and other recent work suggest that advanced oxidation systems in the future should be designed to incorporate coexisting biodegradation reactions. For example, dose 1 of Fenton reagents in this study provided simultaneous biodegradation and chemical oxidation while using O2 released from the Fenton reactions for aerobic activity (Figure 4a). Although the slurries used in these experiments allowed ideal control over reaction conditions, the same processes are likely at work during in situ treatment. It seems increasingly obvious that ISCO systems owe a significant component of their contaminant removal to biodegradation. These processes need to be better understood and quantified so they can be incorporated into designs. This work can also be used to develop hybrid ex situ treatment reactors for soils, sludges, and industrial wastewaters, which incorporate abiotic and biological contaminant destruction.
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Received for review December 15, 2005. Revised manuscript received February 7, 2006. Accepted February 8, 2006. ES0525152
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