Effect of Heavy Metals on Dechlorination of Carbon Tetrachloride by Iron Nanoparticles

© Mary Ann Liebert, Inc.

Effect of Heavy Metals on Dechlorination of Carbon

Tetrachloride by Iron Nanoparticles

Hsing-Lung Lien,* _{Yu-Sheng Jhuo, and Li-Hua Chen}

Department of Civil and Environmental Engineering National University of Kaohsiung

Kaohsiung 811, Taiwan, Republic of China

ABSTRACT

Effects of heavy metals on the dechlorination of carbon tetrachloride by iron nanoparticles were investi-gated in terms of reaction kinetics and product distribution using batch systems. Removal of heavy met-als and the interaction between heavy metmet-als and iron nanoparticles at the iron surface were met-also exam-ined. It was found that Cu(II) enhanced the carbon tetrachloride dechlorination by iron nanoparticles and led to produce more benign products (i.e., CH4). Pb(II) may increase reduction rate slightly but also in-crease the production of more toxic intermediates such as dichloromethane. In comparison to the iron re-duction system without heavy metals, effects of As(V) were negligible while Cr(VI) decreased the dechlo-rination rate by a factor of 2. Removal of As(V) by iron nanoparticles behaved pseudo-first-order reaction kinetics but a fast initial removal followed by a slow subsequent process was found in the cases of Cu(II) and Pb(II). Scanning electron microscopy-energy dispersive X-ray (SEM-EDX) analysis indicated the de-position of heavy metals onto the iron surface. X-ray diffraction (XRD) analysis showed Cu(II) was re-duced to metallic copper and cuprite (Cu2O) at the iron surface while no reduced lead species was ob-served from the Pb(II)-treated iron nanoparticles. Limited data suggested an oxidized lead species formed. The enhanced dechlorination by Cu(II) can be attributed to the deposition of metallic copper and cuprite at the iron surface. This study suggests that implementation of iron nanoparticles rather than engineered bimetallic iron nanoparticles for remediation of mixed contamination with both chlorinated organics and heavy metals is sufficient.

Key words: arsenic; lead; copper; chlorinated methane; reduction; groundwater

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*Corresponding author: Department of Civil and Environmental Engineering, National University of Kaohsiung, No. 700, Kaohsiung University Rd., Kaohsiung 811, Taiwan, Republic of China. Phone: 886-7591-9221; Fax: 886-7591-9376; E-mail: [email protected].

INTRODUCTION

C

such as tetrachloroethylene and trichloroethylene have been in widespread use for several decades in industrial applications including the manufacture of herbicides, plastics, and solvents (Rammamoorthy and Ramma-moorthy, 1997). Many of them had been listed as

prior-ity pollutants by the U.S. Environmental Protection Agency (U.S. EPA). On the other hand, increasingly higher quantities of heavy metals are being released into the environment through anthropogenic activities, pri-marily associated with industrial processes, manufactur-ing, and disposal of industrial and domestic refuse and waste materials (U.S. EPA, 1997).

Remediation of contaminated groundwater using iron nanoparticles has been developed (Wang and Zhang, 1997). Iron nanoparticles with a diameter less than 100 nm can be delivered into hot spot through a direct injec-tion technology (Zhang, 2003; Schrick et al., 2004). Iron nanoparticles are capable of treating a wide variety of contaminants including chlorinated organic solvents (Lien and Zhang, 1999, 2001), heavy metals (Kanel et

al., 2005; Yuan and Lien, 2006), and other inorganic

con-taminants (e.g., perchlorate) (Cao et al., 2005).

Engineered bimetallic iron nanoparticles have widely been reported for improving the performance of monometallic iron nanoparticles (Schrick et al., 2002; Feng and Lim, 2005). Engineered bimetallic iron nanoparticles (e.g., Pd/Fe, Ni/Fe) can be synthesized by chemical reduction and precipitation.

M2_{ Fe}0_{ M}0_{ Fe}2 _(M2

 e.g., Pd2_{, Ni}2_{) (1)} The deposition of a small amount of a second metal at the iron surface increased reaction rates significantly. For example, surface area-normalized reactivity of bimetal-lic Pd/Fe nanoparticles is one to two orders of magnitude higher than that of iron nanoparticles (Lien and Zhang, 2001). Iron serves as an effective electron donor while the enhancement of reduction rates has been attributed to (1) catalytic effects of second metals (e.g., Pd) through hydrogen reduction rather than direct electron transfer (Li and Farrell, 2000), (2) a galvanic corrosion leading to the increase of corrosion rates (Zhang et al., 1998), and (3) the prevention of an oxide formation at the iron surface (Schrick et al., 2002).

It is not an unusual case that the mixed contamination of heavy metals and chlorinated solvents occurs at the same site (Puls et al., 1999). The presence of heavy metals in contaminated sites could be capable of serving as the source of second metals resulting in the in situ formation of bimetallic iron nanoparticles when monometallic iron nanoparticles were implemented. Treatments of mixed con-taminants containing both chlorinated organic compounds and toxic metals using reactive iron have recently received attention (Jeong and Hayes, 2003; Dries et al., 2005). In this study, we focused on the effect of heavy metals on the influence of performance for iron nanoparticles in reacting with carbon tetrachloride. Heavy metals including As(V),

Cu(II), and Pb(II) were investigated systematically. Inter-face reaction between heavy metals and iron was examined by X-ray diffraction (XRD) and scanning electron mi-croscopy-energy dispersive X-ray (SEM-EDX) analysis. Hexavalent chromium, Cr(VI), was also selected to test its effect on the iron-mediated dechlorination.

MATERIALS AND METHODS Chemicals

All chemicals were reagent grade or above and used without further purification. Deionized water was used for preparation of all reagent solutions. Sodium arsenate 7-hydrate (Na2HAsO4  7H2O, 99%), copper (II) sul-phate (CuSO4, 99%) and lead (II) acetate trihydrate ((CH3COO)2Pb  3H2O, 99%) were purchased from J. T. Baker (Phillipsburg, NJ), Sigma (St. Louis, MO), and Aldrich (Milwaukee, WI), respectively. Sodium boro-hydride (NaBH4, 98%) and ferric chloride (FeCl3  6H2O, 98%) were obtained from Aldrich. Carbon tetra-chloride (99.5%) was purchased from SHOWA (Showa Chemical Co., Tokyo, Japan). Chloroform (99%) and dichloromethane (99.9%) were obtained from J.T. Baker. A standard gas mixture for GC analysis was ob-tained from Supelco (Bellefonte, PA), which conob-tained 1% each of ethane, ethylene, acetylene, and methane.

Synthesis of iron nanoparticles

Synthesis of iron nanoparticles was achieved by adding 1:1 volume ratio of NaBH4(0.25 M) into FeCl36H2O (0.045 M) solution. The borohydride to ferric iron ratio was 7.4 times higher than that of the stoichiometric re-quirement [Equation (2)]. Excessive borohydride was ap-plied to accelerate the synthesis reaction and ensure uni-form growth of iron crystals. Ferric iron was reduced by borohydride according to the following reaction: 4Fe3 3BH4 9H2O 4Fe0

 3H2BO3 12H 6H2 (2) The suspension was mixed vigorously under room temperature (22 1°C). The iron particles were then washed with large volume (1,000 mL/g iron) of Milli-Q water for at least three times and were used without further treatments unless indicated otherwise.

Batch tests

For the study of heavy metal removal, experiments were carried out in a 250-mL high-density poly(ethyl-ene), (HDPE), vessel containing 2.5 g/L iron nanoparti-cles in 100 mL of metal ion solution at 22 1°C. Stock solutions of 1,000 mg/L metal ions were prepared from

reagent-grade chemicals in deionized water. Initial con-centrations of metal ions were 25 mg/L corresponding to 1 wt% of iron content in the batch tests. Reaction ves-sels were placed on a rotary shaker (50 rpm). The solu-tion pH was controlled only at the beginning of the re-action and adjusted to at pH 7.0 0.2. The experiments were conducted in duplicate to check the reproducibility of batch results. Acceptable variability for duplicate batch runs agrees within 10%.

For the study of carbon tetrachloride degradation, batch experiments were conducted in 150-mL serum bot-tles (Wheaton, Millville, NJ; actual volume was approx-imately 162 mL). For each batch bottle, 20-L methanol solution of carbon tetrachloride was spiked into a 100-mL aqueous solution. In a typical experiment, the initial concentration of carbon tetrachloride and individual metal ion was 30 and 25 mg/L, respectively. The metal loading of iron nanoparticles was 2.5 g/L. The serum bot-tles were then capped with a Teflon-faced septum and aluminum crimp cap and mixed on a rotary shaker (50 rpm) at room temperature (22 1°C). Control tests with-out iron nanoparticles were conducted with identical ex-perimental conditions and initial concentrations of reac-tants. Analyses of organic mass in the controls indicated that the mass varied by less than 5% over the course of a typical experiment. The experiments were conducted in triplicate. Acceptable variability for triplicate batch runs agrees within 15%.

Method of analyses

Analysis of heavy metals was carried out using an in-ductively coupled plasma-optical emission spectrometry (ICP-OES, Perkin-Elmer Optima 2000DV; Perkin-Elmer Inc., Norwalk, CT). The wavelength of As(V), Cu(II), and Pb(II) was set at 193.7, 324.8, and 220.4 nm, re-spectively. Prior to analysis, samples were filtered through 0.2-m filters and acidified with 3% HNO3. Analyses of duplicate samples indicated a relatively alytical error of less than 5% for metal concentration an-alyzed in the laboratory.

Concentrations of carbon tetrachloride and its inter-mediates were measured by a headspace-gas chromato-graph (GC) method. At selected time intervals, a 50-L headspace gas aliquot was withdrawn by a gastight sy-ringe for GC analysis. Headspace samples were analyzed by a HP4890 GC-FID equipped with a DB-624 capillary column (30 m 0.32 mm) (J&W Scientific, Santa Clara, CA). Temperature conditions were programmed as fol-lows: oven temperature at 45°C for 5 min, injection port temperature at 250°C, and detector temperature at 300°C. Concentrations of hydrocarbons were measured by a HP4890 GC-FID equipped with a GS-GASPRO capil-lary column (J&W, 30 m 0.32 mm). Temperature

con-ditions were programmed as follows: oven temperature at 35°C for 5 min, injection port temperature at 200°C; and detector temperature at 300°C. Concentrations of chlorinated methanes and hydrocarbons were determined by the external standard method using calibration curves. Calibration curves for each compound were made ini-tially and the variability was checked daily before anal-ysis ( 15%).

Solid-phase characterization

Characterization of iron nanoparticles was conducted using X-ray diffraction (XRD), scanning electron mi-croscopy (SEM), and a surface area analyzer. XRD mea-surements were performed using a X-ray diffractometer (Siemens D5000) with a copper target tube radiation (Cu K_) producing X-ray with a wavelength of 1.54056 Å. Samples were placed on a quartz plate and were scanned from 20° to 80° (2) at a rate of 2° 2/min. Morpholog-ical analysis of iron nanoparticles was performed by SEM using a Hitachi S-4300 microscope (Hitachi High-Tech Science Systems Corp., Ibaraki, Japan) with energy-dis-persive X-ray (EDX) analysis (at 10 kV). The specific surface area of iron nanoparticles was measured by Brunauer-Emmett-Teller (BET) N2 method using a COULTER SA 3100 surface are analyzer (Coulter Co., Miami, FL). Analysis of the specific surface area of iron nanoparticles was conducted in triplicate.

Reaction kinetics

The reaction rate of both organic and inorganic cont-aminant removal was determined with pseudo first-order reaction kinetics:

 kaC (3)

where C is the concentration of contaminants in the aque-ous phase (mg/L); kais the observed first-order rate con-stant (h1); and t is time (h).

RESULTS AND DISCUSSION Characterization of iron nanoparticles

Figure 1 shows the SEM image of iron nanoparticles that are comprised of spherical particles assembled in chains. The average diameter of iron nanoparticles was in the range of 50–100 nm. The size of particles is con-sistent with previous studies and the observation of chain structure of iron nanoparticles is in agreement of the study conducted by Nurmi et al. (2005). A specific surface area of iron nanoparticles was in an average of 33.5 4.2 m2_{/g as measured by BET surface analyzer.}

dC

dt

Removal of heavy metals by iron nanoparticles

Removal of Cu (II), Pb(II), and As(V) by iron nanopar-ticles is shown in Fig. 2. Initial concentration of heavy metals was 25 mg/L, and the dose of iron was 2.5 g/L. The removal of As(V) can be fitted well by pseudo-first-order reaction kinetics [Equation (3)]. The observed rate

constant was 0.94 h1 (R2_{ 0.99) corresponding to a} surface-area normalized rate constant (kSA) of 11.2 mL/h/m2_{. This is a substantially high As(V) removal rate} for elevated level of arsenic compared to that obtained from most microsized iron particles at lower arsenic con-centrations (Su and Puls, 2001). However, a fast initial removal followed by a slow subsequent process was found in the case of both Cu(II) and Pb(II). Disappear-ance of more than 93% of total Cu(II) and Pb(II) occurred within 5 min. The first order reaction kinetics was sim-ply not fitted with the data. This result is consistent with studies conducted by Ponder et al. (2000). They sug-gested a physical mechanism was involved in this type of removal processes. Overall, the different removal be-havior among As(V), Cu(II), and Pb(II) suggests that the removal of these heavy metals involves different mech-anisms.

The disappearance of metal ions in the aqueous solu-tion was attributed to the deposisolu-tion at iron nanoparticle surface confirmed by SEM-EDX analysis. To ensure the deposition of heavy metals onto the iron surface, the con-centration of heavy metals conducted in surface analysis was 10 times higher than that used in batch kinetic ex-periments. Samples were taken when iron nanoparticles Figure 1. SEM image of the fresh iron nanoparticles.

Figure 2. Removal of heavy metals by iron nanoparticles. Initial concentration of metal ions was 25 mg/L and the dose of iron nanoparticles was 2.5 g/L. Insert shows the concentration change of Cu (II) and Pb(II) at a lower level.

reacted with 250 mg/L of metal ions for a day. As shown in Fig. 3, the EDX spectra indicated the existence of ar-senic, copper, and lead at the surface of reacted iron nanoparticles.

XRD analysis was further conducted to better under-stand the interaction between heavy metals and iron at the nanoparticle surface. The XRD analysis of fresh iron nanoparticles shows two major characteristic peaks at 44.8° and 65.2° degrees 2, indicating the presence of el-emental iron (Fig. 4A). A small amount of iron oxide (maghemite and/or magnetite) appeared in the fresh sam-ple. This could be attributed to a short exposure to air during the transport of fresh iron nanoparticles into a ves-sel before they were dried by nitrogen overnight.

Figure 4B shows the XRD pattern for Cu(II)-treated iron nanoparticles taken after reacting with 250 mg/L Cu(II) for 12 h. Iron corrosion products (magnetite and/or maghemite) were found at the surface. Removal of Cu(II)

by iron nanoparticles involved the formation of two cop-per species, metallic copcop-per (Cu0_{) and cuprite (Cu}

2O), that were identified at the iron surface. Because the stan-dard reduction potential of Cu2/Cu0_{and Fe}2_/Fe0 cou-ples is 0.34 and 0.44 V, respectively (Table 1), metal-lic copper can be formed through the redox reaction:

Cu2 Fe0_{ Cu}0_{ Fe}2 ₍₄₎ The overall Eo _{for this reaction (E}o

rxn) is 0.78 V at 25°C, indicating a strongly favorable reaction from a thermodynamics perspective. As indicated in Fig. 2 that the removal of Cu(II) by iron nanoparticles is a very fast reaction, Equation (4) actually represents a synthetic step for making engineered bimetallic particles (e.g., Pd/Fe, Cu/Al, Ni/Fe) (Grittini et al., 1995; Fennelly and Roberts, 1998; Kim and Carraway, 2000; Lien and Zhang, 2002). The formation of Cu2O could be attributed to the reac-ENVIRON ENG SCI, VOL. 24, NO. 1, 2007 Figure 3. SEM-EDX spectra of heavy metal-treated iron

nanoparticles: (A) As(V), (B) Cu(II), and (C) Pb(II).

Figure 4. XRD patterns of (A) fresh, (B) Cu(II)-treated iron nanoparticles, and (C) Pb(II)-treated iron nanoparticles. Peaks are due to zero-valent iron (Fe), metallic copper (Cu), cuprite (Cu2O), magnetite/maghemite (Fe3O4/-Fe2O3) (), and

hematite (Fe2O3) (). Peaks assigned to -PbO2are indicated

by an open square symbol ().

A

B

C

A

B

C

tion between Fe(II) and Cu(II) in the aqueous solution (Maithreepala and Doong, 2004):

2Fe2 2Cu2 7H2O Cu2O

 2Fe(OH)3 8H (5) The XRD pattern for Pb(II)-treated iron nanoparticles taken after reacting with 250 mg/L Pb(II) for 24 h is shown in Fig. 4C. Iron corrosion products including hematite and magnetite and/or maghemite were detected. Theoretically, the reduction of Pb(II) to metallic lead should be thermodynamically favorable in the presence of zero-valent iron (Eo

rxn 0.32 V, Table 1). However, neither Pb0_{nor Pb(II) oxides (e.g., PbO, Pb(OH)}

2) were identified in the sample, which was inconsistent with the study conducted by Ponder et al. (2000). After carefully examining the XRD pattern, it was found that two minor peaks at 28.6° and 49.4° 2, which had a relative inten-sity of 11.4 and 6.4%, respectively, matched scrutinyite (-PbO2: 28.6°, 49.4°, and 32.7° 2). It is unusual to ob-serve no Pb0_{but Pb(IV) species in an iron reduction} sys-tem. No observation of Pb0_{might be due to low} deposi-tion amounts of Pb0_{that was unable to be detected by} XRD. Although the evidence to support the formation of Pb(IV) species is very limited in this study, the possibil-ity should not be ruled out. Recently, oxidation of As(III) to As(V) has widely been reported in the zero-valent iron system (Su and Puls, 2004; Kanel et al., 2005; Lien and Wilkin, 2005). It has been found that the zero-valent iron has the capacity for oxidation in the presence of oxygen, possibly through the formation of hydroxide radials (Joo

et al., 2005). In addition, the reductive activation of

dioxygen is another possible process for the oxidation of contaminants by zero-valent metals (Lien and Wilkin, 2002). In this study, batch reactors were sealed under am-bient conditions without purging with nitrogen. The oxy-gen in the reaction system may be a factor and further investigation is needed.

The XRD analysis for As(V)-treated iron nanoparti-cles exhibited only iron oxide species such as maghemite and/or magnetite (data not shown) and no crystal phases

of arsenic species deposited at the iron surface was found. Reduction of As(V) to metallic arsenic (As0_{) is unlikely} (Melitas et al., 2002) because such reaction is thermo-dynamically unfavorable under ambient conditions (Eo

rxn 0.94 V, Table 1). However, as shown in Fig. 3, EDX analysis indicated the deposition of arsenic at the iron surface suggesting the removal of As(V) with iron nanoparticles was a complicated process that could in-volve surface adsorption, precipitation, and coprecipita-tion (Nikolaidis et al., 2003).

Dechlorination of carbon tetrachloride by iron nanoparticles in the presence of heavy metals

Figure 5A shows the carbon tetrachloride degradation with iron nanoparticles in the absence of heavy metals. Ap-proximately 96% of carbon tetrachloride was dechlorinated and produced several intermediates. Chloroform appeared as the major byproduct accounting for 40% of the carbon tetrachloride lost. The yield of dichloro-methane and methane was about 19 and 14%, respectively. Trace amounts ( 2% in total) of hydrocarbon such as ethane, eth-ylene, and acetylene were detected. Observed rate constant was determined to be about 0.13 h1(R2_{ 0.98).}

In the presence of Cu(II), an increase of carbon tetra-chloride degradation rate was found (Fig. 5B). Approxi-mately 96% of carbon tetrachloride was degraded within 12 h. Chloroform (41%) peaked at 2 h and gradually decreased. Methane appeared immediately after carbon tetrachloride was added to the aqueous solution and continued to accu-mulate after the disappearance of carbon tetrachloride.

Figure 5C shows the dechlorination of carbon tetra-chloride by iron nanoparticles in the aqueous solution containing with 25 mg/L of Pb(II). Dechlorination rate was slightly increased by the existence of Pb(II) and, again, chloroform, dichloromethane, and methane were observed. However, unlike the results shown in Fig. 5A and B, dichloromethane (47%) became the major prod-uct in this test. Both chloroform (23%) and methane (8%) accumulated relatively steady during the course of the re-action.

Table 1. The standard reduction potential of varius heavy metals (Lide, 1993).

Species Reaction Standard reduction potential (V)

Fe2 Fe2 2e→ Fe0 _0.447 Cu2 Cu2 2e→ Cu0 _0.3419 Pb2 Pb2 2e→ Pb0 _0.1262 As5 AsO43 4H2O  5e→ As0 8OH 1.39 Cr6 Cr2O72 14H 6e→ 2Cr3 7H2O 1.232 Cr3 CrO2  2H2O  3e→ Cr0 4OH 1.2

Effects of heavy metals on carbon tetrachloride dechlorination

Effects of heavy metals on the degradation of carbon tetrachloride in terms of reaction rates and product

dis-tribution are shown in Figs. 6 and 7, respectively. Dechlo-rination of carbon tetrachloride followed pseudo first-or-der reaction kinetics, which had R2_{values between 0.91} and 0.98. It was found that both Cu(II), and Pb(II) en-hanced the dechlorination rate of carbon tetrachloride by iron nanoparticles while the effect of As(V) on the car-bon tetrachloride dechlorination was negligible (Fig. 6). Among them, Cu(II) exhibited the best promoting effect, which increased the dechlorination rate by a factor of 2 compared to the reaction system with iron nanoparticles alone. It is worthy of note that Cr(VI) decreased the degradation rate of carbon tetrachloride by iron nanopar-ticles. In comparison to the iron reduction system in the absence of heavy metals, dechlorination of carbon tetra-chloride was two times slower in the presence of 25 mg/L of Cr(VI). This is consistent with previous studies (Schlicker et al., 2000; Dries J. et al., 2005) suggesting a competitive effect between the strong oxidant Cr(VI) and carbon tetrachloride in reaction with zero-valent iron. Although Cr(VI) is readily reduced to Cr(III) (Eo

rxn 1.68 V, Table 1), Cr(III) cannot be reduced to Cr0 (Eo

rxn 0.75 V, Table 1). Therefore, the formation of bimetallic Cr/Fe structure onto the iron surface is un-likely.

Figure 7 shows the product distribution from the car-bon tetrachloride degradation by iron nanoparticles with various heavy metals. The concentration of carbon tetra-chloride and its products were measured when reactions completed during a period of 24 h. It was found that in-dividual heavy metals had distinctive effects on the prod-uct distribution of carbon tetrachloride dechlorination. In the presence of Cu(II), significant amounts of methane (34%) and less amounts of chloroform (8%) were pro-duced, although incomplete recovery of carbon still

ac-ENVIRON ENG SCI, VOL. 24, NO. 1, 2007 Figure 5. Dechlorination of carbon tetrachloride by iron

nanoparticles. (A) in the absence of heavy metals, (B) in the presence of Cu(II), and (C) in the presence of Pb(II).

Figure 6. Effects of heavy metals on rates of the carbon tetraachloride dechlorination by iron nanoparticles.

A

B

counted for about 40%. However, the presence of Pb(II) led to the accumulation of dichloromethane (48%). There was no significant difference between the reaction sys-tem with iron nanoparticle alone and that with both iron nanoparticles and As(V) for the product distribution. Un-like the above-mentioned metals, Cr(VI) decelerated degradation rates and resulted in a relatively large amount of carbon tetrachloride remaining unreacted (14%). It also caused the accumulation of chloroform (44%) in the reaction system.

Because methane is relatively benign, whereas di-chloromethane is more toxic than carbon tetrachloride, the presence of Cu(II) in the iron reduction system ex-hibited a positive effect on the degradation of carbon tetrachloride. Although Pb(II) may enhance the dechlo-rination rate, the accumulation of significant amounts of dichloromethane limited the benefit of Pb(II) in the iron reduction system.

The enhanced dechlorination of carbon tetrachloride by iron nanoparticles with Cu(II) can be attributed to the for-mation of bimetallic structure. The XRD analysis indicated metallic copper was deposited at the iron surface (Fig. 4b). The bimetallic structure leads a galvanic corrosion taking place readily at the surface to facilitate the electron trans-fer for the surface-mediated dechlorination of carbon tetra-chloride. Moreover, metallic copper is known as a mild hy-drogenation catalyst (Satterfield, 1991). It is effective for most of the elementary reactions that are required in cat-alytic dehalogenation (Yang et al., 1997). Studies have demonstrated bimetallic Cu/Fe accelerate reduction rates (Fennelly and Roberts, 1998; Kim and Carraway, 2000). In

addition, the formation of Cu2O at the iron surface may serve as a reductant beneficial to the carbon tetrachloride degradation (Maithreepala and Doong, 2004).

Implication to in situ remediation

Considering the frequent occurrence of both chlori-nated organic solvents and heavy metals, the develop-ment of technologies for treatdevelop-ment of mixed contami-nants at the same site is necessary. The iron nanoparticle has shown its potential for treating both types of conta-minants. In this study, we demonstrated that iron nanoparticles can form bimetallic iron nanoparticles in the presence of certain heavy metals such as Cu(II) that resulted in increasing dechlorination rates and producing more benign products. Because Cu(II) is an inorganic contaminant commonly found at a contaminated site, the remediation of chlorinated organic solvents using iron nanoparticles rather than engineered bimetallic iron nanoparticles may be sufficient. However, heavy metals such as Cr(VI) and Pb(II) may cause negative effects that decreased reaction rates and/or produced more toxic in-termediates. Careful examination of the effects of heavy metals is necessary before implementation of iron nanoparticles for remediation of mixed contamination.

ACKNOWLEDGMENTS

The authors would like to thank National Science Council (NSC), Taiwan ROC, for supporting this work through the NSC project grand NSC 93-2211-E-390-006. Figure 7. Effects of heavy metals on the product distribution for the carbon tetrachloride dechlorination by iron nanoparticles.

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