Mechanism for the destruction of carbon tetrachloride and chloroform DNAPLs by modified Fenton's reagent

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Abstract

The destruction of a carbon tetrachloride DNAPL and a chloroform DNAPL was investigated in reactions containing 0.5 mL of DNAPL and a solution of modified Fenton's reagent (2 M H2O2 and 5 mM iron(III)-chelate). Carbon tetrachloride and chloroform masses were followed in the DNAPLs, the aqueous phases, and the off gasses. In addition, the rate of DNAPL destruction was compared to the rate of gas-purge dissolution. Carbon tetrachloride DNAPLs were rapidly destroyed by modified Fenton's reagent at 6.5 times the rate of gas purge dissolution, with 74% of the DNAPL destroyed within 24 h. Use of reactions in which a single reactive oxygen species (hydroxyl radical, hydroperoxide anion, or superoxide radical anion) was generated showed that superoxide is the reactive species in modified Fenton's reagent responsible for carbon tetrachloride DNAPL destruction. Chloroform DNAPLs were also destroyed by modified Fenton's reagent, but at a rate slower than the rate of gas purge dissolution. Reactions generating a single reactive oxygen species demonstrated that chloroform destruction was the result of both superoxide and hydroxyl radical activity. Such a mechanism of chloroform DNAPL destruction is in agreement with the slow but relatively equal reactivity of chloroform with both superoxide and hydroxyl radical. The results of this research demonstrate that modified Fenton's reagent can rapidly and effectively destroy DNAPLs of contaminants characterized by minimal reactivity with hydroxyl radical, and should receive more consideration as a DNAPL cleanup technology.

Introduction

The presence of dense non-aqueous phase liquids (DNAPLs) at contaminated sites represents significant environmental and public health problems. Approximately 60% of the sites on the National Priorities List (NPL) are contaminated with DNAPLs in the form of large pools, smear zones, or ganglia (U.S. EPA, 1993). Dissolution of DNAPLs into ground water is slow, providing a continuous source of contamination emanating from contaminated sites. Dissolution is usually the rate-limiting step in DNAPL remediation; therefore, DNAPLs may persist for decades, even when actively treated (Johnson and Pankow, 1992, Lemke et al., 2004).

Numerous approaches have been proposed for the in situ treatment of DNAPLs. Traditional technologies, such as bioremediation and pump and treat systems, are limited by dissolution and mass transfer rates and may require decades for complete DNAPL destruction (Stroo, 2003). Other technologies include thermal treatment (Hawthorne et al., 2000, Kuhlman, 2002, Kaslusky and Udell, 2002) and DNAPL mobilization methods, such as the addition of surfactants (Pennell et al., 1993), solvents (Kibbey et al., 2002), and dense brine water (Miller et al., 2000, Hill et al., 2001). Each of these processes has limitations: thermal treatment is expensive, and mobilization methods move the DNAPL but do not destroy it.

In situ chemical oxidation (ISCO) technologies have recently gained increased popularity for soil and ground water treatment, with 33% of recent cleanup efforts employing ISCO technologies (Hood, 2004). Trichloroethylene (TCE) and perchloroethylene (PCE) DNAPL destruction has been demonstrated using permanganate ISCO (Vella and Veronda, 1992, Schnarr et al., 1998, Nelson et al., 2001). Permanganate destruction of DNAPLs has been improved by the use of phase-transfer catalysts to enhance the penetration of permanganate into DNAPLs (Seol and Schwartz, 2000); however, this technology has also led to decreased subsurface permeability and clogging (Schroth et al., 2001).

Modified Fenton's reagent was one of the first ISCO processes studied (Watts et al., 1990), and it has recently been investigated for DNAPL destruction (Yeh et al., 2003, Watts et al., 2005). Modified Fenton's reagent is based on the standard Fenton reaction in which the decomposition of dilute hydrogen peroxide is initiated by iron (II), resulting in the generation of hydroxyl radical (OH•):Fe2+ + H2O2  Fe3+ + OH• + OH.

Hydroxyl radical is a powerful, relatively non-selective oxidant capable of degrading most contaminants of concern (Dorfman and Adams, 1973, Buxton et al., 1988, Haag and Yao, 1992). The reaction is usually modified for ISCO by the use of alternative initiators, such as iron (III) (Watts and Dilly, 1996), iron chelates (Sun and Pignatello, 1992) and iron minerals (Tyre et al., 1991, Watts et al., 1993, Ravikumar and Gurol, 1994, Miller and Valentine, 1995, Kwan and Voelker, 2003). Another modification is the addition of excess hydrogen peroxide, which promotes a series of propagation reactions (Walling, 1975, De Laat and Gallard, 1999) that result in the formation of the reactive species perhydroxyl radical (HO2•), superoxide radical anion (O2) and hydroperoxide anion (HO2):OH• + H2O2  HO2 + H2OFe3+ + H2O2  Fe2+ + HO2 + H+HO2  O2 + H+ (pKa = 4.8)HO2 + Fe2+  Fe3+ + HO2

Hydroperoxide, the conjugate base of hydrogen peroxide (pKa = 11.75), is a strong nucleophile (Edwards and Pearson, 1962, David and Seiber, 1999). Perhydroxyl radical is a weak oxidant that has minimal reactivity in aqueous systems (Afanas'ev, 1989). Superoxide is a nucleophile and a reductant and has been identified as the reactive species in modified Fenton's reagent responsible for the degradation of carbon tetrachloride dissolved in water (Smith et al., 2004).

Although hydroxyl radical-mediated oxidations have been the degradation mechanism most commonly associated with Fenton's reagent, hydroxyl radical reacts at diffusion-controlled rates and is not reactive with sorbed compounds (Sedlak and Andren, 1994, Watts et al., 1999), so it is not likely to react with non-aqueous phase contaminants (Sheldon and Kochi, 1981). However, rapid destruction of such DNAPLs has been demonstrated with modified Fenton's reagent in the laboratory for a TCE DNAPL (Yeh et al., 2003) and a carbon tetrachloride DNAPL (Watts et al., 2005). Such rapid destruction has also been observed in the field; for example, 94% of a 270-kg TCE DNAPL was destroyed at the Westinghouse Savannah River site using modified Fenton's reagent (U.S. EPA, 1998). Reactive oxygen species other than hydroxyl radical generated through propagation reactions are important in Fenton's ISCO (Watts et al., 1999, Watts and Teel, 2005) and may be responsible for the rapid treatment of DNAPLs. However, the destruction of DNAPLs by modified Fenton's reagent is complex and is affected not only by the rates of reaction of the contaminant with hydroxyl radical, superoxide, and other reactive oxygen species, but also by the rate of dissolution of the contaminant, the surface to volume ratio of the DNAPL, and many other possible factors (Stroo et al., 2003). The objective of this research was to determine the rates of DNAPL destruction in relation to the rates of DNAPL dissolution for chloroform and carbon tetrachloride, and to determine the reactive oxygen species responsible for the destruction of these DNAPLs.

Section snippets

Materials

Iron (III) sulfate [Fe2(SO4)3], magnesium hydroxide (> 99%), and sodium hydroxide (> 99%) were obtained from VWR (West Chester, PA). Carbon tetrachloride (99.9%), chloroform (trichloromethane) (99.9%), potassium superoxide (KO2), diethylenetriamine-pentaacetic acid (DTPA), decane (> 99%), and hexaketocyclohexane (HKCH) (97%) were purchased from Sigma-Aldrich (St. Louis, MO). Toluene (> 99%), ethanol (> 99%), and propanone (acetone) (99.6%) were purchased from Fisher Scientific (Hampton, NH). ORBO®

Carbon tetrachloride and chloroform DNAPL destruction by modified Fenton's reagent

The destruction of a carbon tetrachloride DNAPL and a chloroform DNAPL by modified Fenton's reagent is shown in Fig. 2A–B. The total mass lost from the carbon tetrachloride DNAPL was 84% and from the chloroform DNAPL was 62% over 24 h. After subtracting the masses of carbon tetrachloride and chloroform captured in the ORBO tubes and dissolved into the aqueous phase, the net destruction of the carbon tetrachloride DNAPL was 74% and the net destruction of the chloroform DNAPL was 44%. In parallel

Summary and conclusions

The destruction of carbon tetrachloride and chloroform DNAPLs by modified Fenton's reagent was investigated, and the reactive oxygen species responsible for their destruction was evaluated using systems that generate only (1) hydroxyl radical, (2) hydroperoxide, and (3) superoxide. A carbon tetrachloride DNAPL was effectively destroyed by modified Fenton's reagent at a rate 6.5 times that of gas purge dissolution. The carbon tetrachloride DNAPL was not destroyed by hydroxyl radical or

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