February 2, 2007
Reactive Oxygen Species Responsible for the Enhanced Desorption of Dodecane in Modified Fenton’s Systems
By Corbin, Joseph F III; Teel, Amy L; Allen-King, Richelle M; Watts, Richard J
The enhanced treatment of sorbed contaminants has been documented in modified Fenton's reactions; contaminants are desorbed and degraded more rapidly than they desorb by fill-and-draw or gas- purge desorption. The reactive species responsible for this process was investigated using dodecane as a model sorbent. Hydroxyl radical, hydroperoxide anion, and superoxide radical anion were generated separately to evaluate their roles in enhanced dodecane desorption. Dodecane desorption from silica sand over 180 minutes was negligible in gas-purge systems and in the hydroxyl radical and hydroperoxide anion systems. In contrast, enhanced desorption of dodecane occurred in superoxide systems, with >80% desorption over 180 minutes. Scavenging of superoxide eliminated the enhanced desorption of dodecane in both superoxide and modified Fenton's systems, confirming that superoxide is the desorbing agent in modified Fenton's reactions. Conditions that promote superoxide generation in Fenton's reactions may enhance their effectiveness for in situ subsurface remediation of sorbed hydrophobic contaminants. Water Environ. Res., 79, 37 (2007).
KEYWORDS: desorption, Fenton's reagent, hydroperoxide anion, hydroxyl radical, superoxide.
In situ chemical oxidation (ISCO) is an increasingly popular technology for the remediation of organic contaminants in soils and groundwater. The most common ISCO processes-Fenton's reagent, permanganate, and ozone sparging-have the potential to rapidly oxidize biorefractory contaminants, such as perchloroethylene and trichloroethylene, at rates that are orders of magnitude faster than bioremediation or natural attenuation (Environmental security Technology Certification Program, 1999; Siegrist et al., 2001). Modified Fenton's reagent, the most widely studied of the ISCO processes, is based on the fundamental Fenton initiation reaction, in which the decomposition of hydrogen peroxide (H^sub 2^O^sub 2^) is mediated by iron(u) to generate hydroxyl radical (OH"), as follows (Haber and Weiss, 1934):
H^sub 2^O^sub 2^ + Fe^sup 2+^ [arrow right] OH^sup .^ + OH^sup - ^ + Fe^sup 3+^ (1)
In the standard Fenton's procedure, dilute hydrogen peroxide is slowly added to a degassed, well-mixed solution of excess iron(II) and organic substrate, providing near-stoichiometric generation of hydroxyl radicals (Walling, 1975). The traditional Fenton's reaction is typically modified for industrial waste treatment and ISCO applications. Modifications include the use of significantly higher concentrations of hydrogen peroxide (1 to 5 M) (Watts et al., 1990) and the use of alternative catalysts, such as iron oxides and oxyhydroxides (Miller and Valentine, 1995; Ravikumar and Gurol, 1994; Tyre et al., 1991; Watts et al., 1993, 1997), iron chelates (Pignatello and Baehr, 1994; Sun and Pignatello, 1992), and iron pyrophosphate complexes (Wang and Brusseau, 1998).
Many organic contaminants are hydrophobia and partition onto soils and aquifer solids. Because nearly all abiotic and biotic transformation processes occur in the aqueous phase, most soil and groundwater treatment processes are limited by the rate of contaminant desorption (Ogram et al., 1985; Watts, 1998). However, modified Fenton's reactions have been shown to treat sorbed contaminants more rapidly than their maximum fill-and-draw or gas- purge desorption rates (Gates and Siegrist, 1995; Watts et al., 1994). The potential to rapidly treat sorbed contaminants has made modified Fenton's reagent an important emerging remediation technology.
Although the enhanced treatment of sorbed contaminants has been documented in modified Fenton's reactions containing >0.3 M hydrogen peroxide, the species responsible for the enhanced treatment has not yet been identified (Watts and Stanton, 1999). While hydroxyl radical, generated in Fenton's reactions through eq 1, reacts rapidly with most soil and groundwater contaminants in the aqueous phase (Buxton et al, 1988; Dorfman and Adams, 1973; Haag and Yao, 1992), it is not reactive with sorbed contaminants (Sedlak and Andren, 1991). Watts et al. (1999) proposed that enhanced desorption in modified Fenton's reactions is mediated by a reductant or nucleophile, such as superoxide radical anion (O^sub 2^^sup .-^) or hydroperoxide anion (HO^sub 2^^sup -^). These species are generated in modified Fenton's systems through propagation reactions that are driven by elevated concentrations of hydrogen peroxide.
H^sub 2^O^sub 2^ + OH^sup .^ [arrow right] HO^sub 2^^sup .^ + H2O (2)
HO2^sup .^ [Lef-right arrow] H+ + O^sub 2^^sup .-^ pK^sub a^ = 4.8 (3)
HO2^sup .^ + O^sub 2^^sup .-^ [arrow right] HO^sub 2^^sup -^ + O2 (4)
Identification of the desorbing species would provide a basis for elucidating the mechanism of enhanced desorption in Fenton's systems and aid in defining optimum process conditions for fullscale Fenton's ISCO. Therefore, the purpose of this research was to determine the reactive oxygen species responsible for the enhanced desorption of contaminants in modified Fenton's systems.
Materials and Methods
Materials. Purified silica sand, sodium hydroxide (NaOH) (>99.9%), potassium hydroxide (99.9%), and magnesium chloride (99.9%) were purchased from J. T. Baker (Phillipsburg, New Jersey). Sodium hydroxide was purified by adding 0.2 g magnesium chloride to 1 L of a 0.1 M sodium hydroxide solution, which was then stirred for 12 hours and passed through a 0.45-m membrane filter (Monig et al., 1983). Carbon adsorbent tubes (ORBO-32) were obtained from Supelco (St. Louis, Missouri), and n-dodecane (99%), dodecyl aldehyde (92%), potassium Superoxide, diethylenetriamine-pentaacetic acid (DTPA) (98%), sodium perborate, and sodium ascorbate were purchased from Sigma-Aldrich (Milwaukee, Wisconsin). Ethyl acetate (certified by the American Chemical Society, Washington, D.C.) was obtained from Fisher Scientific (Hampton, New Hampshire), and iron(ni) sulfate was purchased from EM Science (Gibbstown, New Jersey). Hydrogen peroxide was provided gratis by Solvay Interox (Deer Park, Texas). Doubledeionized water was purified to >18 Mohm-cm with a Barnstead Nanopure II ultrapure system (Dubuque, Iowa).
Probe Compound. Dodecane was used as a probe compound to evaluate enhanced desorption because of its high hydrophobicity and sorptivity (log K^sub OW^ = 6.44), high potential for gas-purge analysis (Henry's law constant = 24.2 atm-m^sup 3^/mole at 25C [Verschueren, 1983]), and low reactivity with Superoxide and hydroperoxide anion in aqueous solution (Bielski et al., 1985).
Adsorption of Dodecane. Silica sand was used because it is a simple and well-defined sorbent (Goss, 1992). Furthermore, use of an inorganic sorbent eliminates contaminant release resulting from soil organic matter destruction in modified Fenton's reactions, which can be a complicating factor in elucidating the reactive oxygen species. Vapor deposition was used to sorb dodecane to silica sand (Ong and Lion, 1991; Steinberg et al., 1999; Ungeret al., 1996). Dodecane was applied to 50 g of purified silica sand in 100-mL Pyrex media bottles to provide a concentration of 2.5 mmol/kg sand. The bottles were capped using aluminum foil as a liner and heated at 80C for 3 hours.
Gas-Purge Desorption. Gas-purge desorption was conducted in 100- mL media bottles fitted with one sparger for purging the reaction solution above the sand and a second sparger for purging the sand saturated with aqueous-phase reagents (Figure 1). The nitrogen flowrate to the aqueous-phase sparger was 500 mL/min. Gas production rates in the experimental systems (i.e., Fenton's and potassium Superoxide [KO^sub 2^]) were measured using a bubble meter attached to a port on the reaction bottle; the flowrate to the sand sparger was equal to the gas production rate in the parallel experimental reactions. An additional port on each bottle was used to hold an ORBO-32 gas adsorbent tube. The ORBO tubes were sampled and replaced at selected time points, then extracted with an ethyl acetate-water mixture (2:1 v/v), followed by analysis of the extract by gas chromatography.
Measurement of Enhanced Desorption by Modified Fenton's Reagent and Specific Reactive Oxygen Species. After the gaspurge desorption of dodecane in deionized water was quantified, potential enhanced desorption by hydroxyl radical, Superoxide anion, and hydroperoxide was evaluated by generating each of these reactive oxygen species separately in dodecane-sand systems with simultaneous gas-purge desorption. All reactions were conducted in triplicate in 100-mL media bottles containing 50 g dodecane-spiked sand and 50 mL reaction solution with continuous nitrogen gas (N^sub 2^) purging in a constant temperature chamber at 20 2C, except superoxide reactions, which were conducted at 4 I0C-A set of triplicate reactors was prepared and sacrificed for each time point. Triplicate control reactions were conducted in parallel using deionized water in place of the reaction solution. For each set of reactors at each time point, the water was decanted, and the entire massof sand was extracted with ethyl acetate. In addition, the ORBO tubes were also extracted with ethyl acetate. Extracts were analyzed by gas chromatography to quantify residual dodecane concentrations.
Modified Fenton 's Reaction. Enhanced desorption in modified Fenton's systems was evaluated in separate reactors using initial concentrations of 0.5- or 3.0-M hydrogen peroxide at pH 3.0 catalyzed by 5-mM iron(III).
Enhanced Desorption by Hydroxyl Radicals. A standard Fenton's reaction was used for generating hydroxyl radicals as the sole reactant. The stoichiometrically efficient Fenton's system was based on the conditions provided by Babbs and Griffin (1989). To 50 mL of a 5-mM iron(II) solution at pH 3.0, 0.1 mL of 100-mM hydrogen peroxide was added every 10 minutes for 180 minutes.
Enhanced Desorption by Hydroperoxide. Hydroperoxide was generated by adding a 150-mM solution of sodium perborate at pH 10.0 to the reactors. Sodium perborate decomposes to hydroperoxide anion in water, as follows (David and Selber, 1999):
2H^sub 2^O + BO3^sup -^ [Lef-right arrow] HO2^sup -^ +B(OH)^sub 3^ (5)
Enhanced Desorption by Superoxide. Potassium superoxide in deionized water was used to generate superoxide radical anion as the sole reactive oxygen species based on the methodology of Marklund (1976). Reactions were conducted in triplicate and consisted of 1- or 3-M potassium superoxide, 0.1-M purified sodium hydroxide, and 1 mM DTPA (to bind and inactivate transition metals) at pH 14 to minimize the disproportionation of superoxide radical anion (Csanyi et al., 1983). The potassium superoxide was mixed into a 4C NaOH- DTPA solution; under these conditions, the half-life of superoxide radical anion was 110 minutes, which was quantified by measuring the rate of oxygen evolution using a manometer attached to a reaction vessel.
Analysis. Sand extracts and ORBO tube extracts were analyzed using a Hewlett-Packard 5890 gas chromatograph (Hewlett Packard, Palo Alto, California) fitted with a 0.53 mm (internal diameter) 15 m DB-1 capillary column and a flame ionization detector. The injector port and detector port temperatures were 200 and 300C, respectively. The oven temperature was 110C (isothermal). Solution pH was measured using a Fisher Accumet pH meter (Fair Lawn, New Jersey).
Results and Discussion
Desorption in a Gas-Purge System. Desorption of dodecane in the gas-purge system containing no reactants is shown in Figure 2. Over 95% of the dodecane remained sorbed to the silica sand over 180 minutes, which is expected because of the high hydrophobicity and sorptivity of dodecane (log K^sub ow^ = 6.44) (Pavlostathis and Jaglal, 1991). Watts et al. (2000) previously found minimal gaspurge desorption of dodecane from a surface soil over 50 hours.
Enhanced Desorption by Modified Fenton's Reagent. The enhanced desorption of dodecane by modified Fenton's systems containing 5-mM iron(III) and 0.5- or 3-M hydrogen peroxide is shown in Figures 3a and b. In the 0.5-M hydrogen peroxide Fenton's system, 45% of the dodecane was desorbed over 180 minutes (Figure 3a). A chromatographic peak with retention time equal to that of dodecyl aldehyde, the expected oxidation product, was not seen on chromatograms, although other unidentified peaks were found. In the 3-M hydrogen peroxide system, 61% of the dodecane was desorbed over 180 minutes (Figure 3b). These results show that desorption of dodecane is enhanced significantly in modified Fenton's reactions relative to its gas-purge desorption rate and that enhanced desorption and dodecane destruction increase with increasing hydrogen peroxide concentration. Such increasing destruction of an alkane is expected; Watts and Stanton (1999) found that increased mineralization of hexadecane occurred with increasing hydrogen peroxide concentration in a modified Fenton's system.
The data of Figures 3a and b demonstrate increased desorption as a function of hydrogen peroxide concentration, consistent with the results of Gates and Siegrist (1995), Watts et al. (1994), and Watts and Stanton (1999). The increased desorption at higher hydrogen peroxide concentrations is likely a result of the increased generation of one or more of the major reactive species in modified Fenton's reactions-hydroxyl radicals, superoxide, or hydroperoxide anion. To determine the reactant responsible for the enhanced desorption in modified Fenton's systems, a series of reactions was conducted, each generating only one reactive oxygen species.
Desorption by Hydroxyl Radicals. The standard Fenton's reaction was used to generate hydroxyl radicals as the sole reactive oxygen species in the dodecane-silica sand system. The reaction conditions were based on the stoichmetrically efficient conditions developed by Babbs and Griffin (1989). No significant desorption was observed in the Fenton's system over 180 minutes compared with gas-purge controls (Figure 4). The results of Figure 4 are consistent with the characteristics of the hydroxyl radical, which is short-lived and reacts at diffusion-controlled rates (Dorfman and Adams, 1973; Hagg and Yao, 1992); the findings are also consistent with those of Sedlak and Andren (1991), which indicate that this short-lived species is not reactive with sorbed organic contaminants.
Desorption by Hydroperoxide. In reactions using a 150-mM solution of sodium perborate to generate hydroperoxide in the dodecane- silica sand system, no significant desorption of dodecane occurred over 180 minutes compared with gas-purge controls (Figure 5). These data confirm that hydroperoxide is not the desorbing agent in modified Fenton's systems. The results are logical because, although hydroperoxide is a strong nucleophile, it recombines with protons at a diffusion-controlled rate, minimizing its reactivity in Fenton's systems.
Desorption by Superoxide. The addition of 1-M potassium Superoxide to the dodecane-spiked sand gas-purge system resulted in 22% dodecane desorption (Figure 6a), while the addition of 3-M potassium Superoxide provided 82% dodecane desorption (Figure 6b). These results demonstrate that Superoxide promotes enhanced dodecane desorption and suggest that Superoxide is the reactive species responsible for the enhanced desorption of other contaminants in modified Fenton's reactions.
To confirm the role of Superoxide in enhanced dodecane desorption, ascorbate was used as a Superoxide scavenger (k^sub O2^[black circle]- = 2.7 10^sup 5^ M^sup -1^ s^sup -1^) (Cabelli and Belski, 1983). The ability of ascorbate to scavenge Superoxide was confirmed by evaluating the transformation of 0.1-mM 1,4- benzoquinone(k^sub O2^[black circle]- = 9.0 10^sup 8^ M^sup -1^ s^sup -1^)(Bielski et al., 1985) in 3-M potassium Superoxide, with and without the addition of ascorbate. Without ascorbate, the benzonquinone was transformed to an undetectable concentration in 8 minutes; however, no measurable benzoquinone degradation occurred in the presence of 3-M ascorbate. Dodecane desorption in Superoxide systems (3-M potassium Superoxide) and in Fenton's reactions [1-M hydrogen peroxide and 5-mM iron(III)] was then evaluated with the addition of 3-M ascorbate. The concentrations of potassium superoxide and hydrogen peroxide used in the scavenging experiments were chosen to provide near-equal levels of dodecane desorption. In controls without scavenging, approximately 25% of the sorbed dodecane was trapped in ORBO tubes in both systems; however, no measurable dodecane desorption occurred in either system in the presence of excess ascorbate (Figure 7). The low desorption rate in the ascorbate-containing systems compared with the gas-purge controls in the other systems (Figures 2 to 6) may be the result of a lower Henry's law constant for dodecane in the less polar potassium superoxide-ascorbate system. The presence of ascorbate also lowered the efficiency of the hexane extractions, preventing quantification of the sorbed dodecane residuals.
The results shown in Figure 7 confirm that superoxide generated in a potassium superoxide system desorbs dodecane and that superoxide is the desorbing species in modified Fenton's systems. Therefore, the enhanced treatment of sorbed contaminants previously reported (Gates and Siegrist, 1995; Watts et al., 1994,1999) was likely a result of the interaction of superoxide with sorbed species. Sorption of apolar compounds, such as dodecane, to mineral surfaces is driven primarily by van der Waals forces (Schwartzenbach et al., 2003). The vapor deposition method used for preparing the sorbed dodecane was conducted under dry, heated conditions, which ensures that the dodecane was adsorbed to the mineral surface with little hydration before the initiation of the desorption experiments. The adsorbed dodecane concentration constituted approximately two molecular layers of surface coverage, based on the assumption that dodecane and the mineral surface were not hydrated, an average sand diameter of 0.2S mm, and a dodecane total surface area determined from its liquid density (0.75 g/cm^sup 3^) and molecular weight (170.3 g/mol) with a spherical shape. As a result, the experiments began with sand grains that were oil-wet, a condition which inhibits water from adsorbing to the surface when the treated sand is transferred into the water-filled reactors. Superoxide has been shown to be solvated by apolar solvents (Afanas'ev, 1989; Bielski et al., 1985) and therefore likely partitions into the dodecane layers at the mineral surface. Because superoxide can participate in hydrogen bonding on the mineral surface, it may displace dodecane from the surface adsorption sites, thus facilitating dodecane desorption into the aqueous phase. Superoxide-mediated desorption from titanium dioxide surfaces to aqueous solution has been documented by Konaka et al. (1999), and such desorption has been implied as an important step in photocatalytic transformation of organic compounds i\n solution (Feitz and Waite, 2003). Once the dodecane is desorbed, it would proceed through oxidation reactions with hydroxyl radical in the aqueous phase.
The results of this research provide evidence that modified Fenton's reagent transforms contaminants through multiple pathways for the remediation of soils and groundwater. In addition to its ability to oxidize contaminants reactive with hydroxyl radical, modified Fenton's treatment can transform contaminants not reactive with hydroxyl radical (Smith et al., 2004; Teel and Watts, 2002), rapidly destroy dense non-aqueous-phase liquids (Watts et al., 2005; Yen et al, 2003), and desorb hydrophobic contaminants. The use of surfactants has been proposed to enhance the Fenton's treatment of hydrophobic contaminants in soils and groundwater. Although some success has been seen with the use of surfactants in the treatment of sorbed contaminants (Li et al., 1997), their addition, which results in a demand on hydrogen peroxide, is not necessary, because superoxide effectively desorbs contaminants.
The generation of superoxide at effective concentrations will improve Fenton's remediation systems for the treatment of sorbed contaminants found in soil and groundwater systems. Based on the results showing that superoxide is the desorbing species in modified Fenton's reactions, the process chemistry can be modified to generate more or less superoxide, depending on contaminant sorptivity, resulting in more efficient use of hydrogen peroxide and minimizing treatment costs. By optimizing modified Fenton's reactions to promote enhanced desorption, the contaminant rebound that is common in the remediation of contaminated sites may be minimized. The use of such a rational basis for system design will improve the stoichiometry of Fenton's ISCO processes, which are reagentintensive relative to bioremediation and monitored natural attenuation, and will ultimately lead to more effective full-scale treatment.
Funding for this research was provided by the Strategic Environmental Research and Development Program (Arlington, Virginia), through grant no. CU-1288, and the National Science Foundation (Arlington, Virginia), through grant no. BES-001314.
Submitted for publication March 30, 2006; revised manuscript submitted June 1,2006; accepted for publication June 30,2006.
The deadline to submit Discussions of this paper is April 15, 2007.
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Joseph F. Corbin III1, Amy L. Teel2, Richelle M. Allen-King3, Richard J. Watts4*
1 Graduate Student, Department of Civil and Environmental Engineering, Washington State University, Pullman, Washington.
2 Research Scientist, Department of Civil and Environmental Engineering, Washington State University, Pullman, Washington.
3 Associate Professor, Department of Geology, University at Buffalo, State University of New York, Buffalo, New York.
4* Professor, Department of Civil and Environmental Engineering, Washington State University, Pullman, Washington 99164-2910; e- mail: [email protected]
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