Effects of iron surface pretreatment on sorption and reduction kinetics of trichloroethylene in a closed batch system

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Water Research 39 (2005) 1037–1046

Effects of iron surface pretreatment on sorption and reduction

kinetics of trichloroethylene in a closed batch system

Chin Jung Lin

, Shang-Lien Lo

Research Center for Environmental Pollution Prevention and Control Technology, Graduate Institute of Environmental Engineering, Nation Taiwan University, 71 Chou-Shan Road, Taipei 106, Taiwan, ROC

Received 14 October 2003; received in revised form 24 May 2004; accepted 27 June 2004

Abstract

The decline of trichloroethylene (TCE) in a metallic iron–water system results from the combination of reduction reaction and sorption onto iron surfaces. Sorption, particularly by highly impure iron, accelerates the removal of TCE from the aqueous phase, but delays the prevalence of steady-state conditions. In this case, an overly high value of reaction rate constant in the design of a treatment system would be used. In this work, the effects of an iron surface with 8.0% C, 6.1% O and 0.8% Si separately following HCl-washing and H2-reducing pretreatment on sorption and reduction rates were examined. The amounts of both aqueous and sorbed TCE were measured using a modified solvent-extraction method. TCE sorption onto an iron surface, as quantified by the Langmuir sorption maximum, followed the trend H2-reduced Fe04HCl-washed Fe04untreated Fe0(0.887, 0.365 and 0.311 mg/g, respectively). Measurements of the concentration of sorbed TCE revealed that about 34–37% of the initial mass of TCE in the aqueous phase was removed by sorption by H2-reduced Fe0, 16–19% was removed by HCl-washed Fe0 and 13–16% was removed by untreated Fe0. A combination of new and previously reported data on cast iron’s capacity to sorb TCE revealed a linear relationship between this capacity and the C fraction in the surface of the iron, with the coefficient of determination (r2₎ exceeding 0.99. The first-order observed rate constants (kobs) of the reduction of TCE in contact with Fe0were obtained from the slope of a plot of total TCE loss rate (dCT=dt) versus the amount of TCE in the aqueous phase (Cw) using linear least-squares analysis. The kobs values were 0.080, 0.148 and 0.191 h1 for untreated, HCl-washed and H2 -reduced Fe0, respectively. Normalized to iron surface area concentration, the specific rate constants (kSA) were 2.37 103 , 2.31 103 and 5.62 103h1m2L, respectively. The results indicated that HCl-washing approximately doubled kobs, primarily because of the increase in the surface area of the iron, and it slightly decreased kSAdue to rapid corrosion during the rinsing process. Both the number of reactive sites and the sorption capacity per unit iron surface area through the H2-reducing pretreatment were increased due to the reduction of iron oxide layer and the carbonization of carbon-containing subjects on the iron’s surface. Hence, the H2reduction of cast iron promotes the removalof TCE from contaminated water by the concurrent sorption and reduction.

Keywords: Zero valent metal; Permeable reactive barriers; Iron surface pretreatment; Acid-washing process

1. Introduction

The use of zero-valent metal as an electron source for the reductive dechlorination of halogenated organic

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compounds in wastewater or groundwater has been shown over the past few years to be potentially very effective (Gillham and O’Hannesin, 1994;Matheson and

Tratnyek, 1994;Orth and Gillham, 1996;Agrawaland

Tratnyek, 1996). Elemental iron (Fe0_{) is the commonly}

chosen reactive metalbecause it is an inexpensive, nontoxic and natural material. Additionally, Fe0 effec-tively destroys more complex anthropogenic chemicals, such as pentachlorophenol (Kim and Carraway, 2000), pesticides (Sayles et al., 1997) and azo dyes (Nam and

Tratnyek, 2000). The typicalreaction between Fe0and

chlorinated hydrocarbons, denoted as RCl, under anaerobic conditions, in the absence of any other strong oxidants, such as carbonate, sulfate and nitrate, is as follows. (Matheson and Tratnyek, 1994; Agrawaland

Tratnyek, 1996):

Fe0þRCl þ Hþ_!_Fe2þ_þ_{RH þ Cl}

: (1)

The process is similar to that of the corrosion of iron by chlorinated hydrocarbons substituting for oxygen as the oxidant. In this process, chlorinated hydrocarbons are transformed mostly into benign compounds such as hydrocarbons and chlorides.

Other than the reactivity of individualchlorinated hydrocarbons, Fe0 surface characteristics, such as specific surface area, impurity, crystallinity and mor-phology (Su and Puls, 1999) are the most significant factors that influence the rate of reduction reaction. A pseudo-first-order rate modelwith respect to the aqueous phase concentration (Eq. (2)) effectively de-scribes the disappearance of chlorinated solvents in contact with highly pure Fe0in a closed, well-mixed and anaerobic batch system.

dCw

dt ¼kobsCw; (2)

where Cw is the concentration of contaminants in the aqueous phase; t is reaction time (h), and kobs is the observed rate constant (h1_{). Then, k}

obs can be expanded to (Johnson et al., 1996)

kobs¼kSAasrm; (3)

where kSAis the speciﬁc rate constant (h1m2L); asis the speciﬁc surface area of Fe0(m2g1), and r_m is the mass concentration of Fe0_(gL1_{). Eq. (3) indicates that} the amount of available surface area is the most important factor that governs the reduction rate. Studies have demonstrated the linear relationship of kobsv.s rm for carbon tetrachloride (Matheson and Tratnyek, 1994), 1,2-dibromo-3-chloropropane (Siantar et al., 1996), nitrobenzene (Agrawaland Tratnyek, 1996) and trichloroethene (Su and Puls, 1999). Additionally, reducing the diameter of the iron particles to vary the asvalues also proportionally accelerates the apparent rate of dechlorination reaction (Siantar et al., 1996).

In fact, the disappearance of chlorinated hydrocar-bons from the aqueous phase in contact with Fe0 proceeds by concurrent sorption and reduction (Burris

et al., 1995; Kim and Carraway, 2000). Burris et al.

(1998) have demonstrated signiﬁcant sorption of TCE

and PCE by graphitic inclusions in a cast iron–water system. However, this effect has not usually been considered in laboratory experiments because of the use of highly pure Fe0. However, granular cast iron with a high content of impurities is practically used as a reductive medium in ﬁeld application when economical considerations are important. The presence of impurities leads to apparent reaction orders that differ from one another (Li et al., 1999). Burris et al. (1998)described the reaction rate modelwith accounting for sorption effect in a closed batch system:

dCT dt ¼ kobsC n w; (4) dCw dt ¼ kobsC n w dCs dt ; (5)

where CT, Cw, and Cs are the totalsystem, aqueous phase and sorbed concentrations, respectively, of con-taminants in milligram per vial and CT¼Cw+Cs; kobsis the reaction rate constant; n represents the reaction order. Eqs. (4) and (5) state that the decline in the concentration of target contaminants in the batch system was caused only by reduction reaction at the Fe0surface; however, the apparent concentration in the aqueous phase was reduced by the combination of sorption and reduction reaction. Regardless of the mechanism of sorption, Eq. (5) can be simpliﬁed as Eq. (2), based on the assumption that n equals unity. However, this assumption yields an overly high kobs value in the design of a treatment system. However, a materialthat can both sorb and reduce target con-taminants, acting as a reductive medium, could reduce the risk associated with the passing of contaminants through a system. Thus, an understanding of the removal, by cast iron, of contaminants in the aqueous phase by sorption and reduction, individually, is important for designing and evaluating the performance of the treatment system.

At present, acid-washing pretreatment for virgin Fe0 is the available method for removing the passivating oxide layer and increasing the Fe0 surface area. The treatment of Fe0 with dilute HCl has shown a faster reduction rate compared to the untreated one (

Mathe-son and Tratnyek, 1994;Agrawaland Tratnyek, 1996;

Su and Puls, 1999). The explanations for the effects of

HCl-washing process on the apparent rate constants are

(Matheson and Tratnyek, 1994;Agrawaland Tratnyek,

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and leave clean reduced Fe0 free of nonreactive oxide or organic coatings; (2) to increase surface area due to corrosion etchings and pits; (3) to increase the density of highly reactive sites resulting from the locations of steps, edges, and kinks on the Fe0surface; (4) to accelerate the corrosion by sorbed H+_{and Cl}_. However, severaldrawbacks of this pretreatment method in practicalapplications have been shown and included: (1) the production of strongly acidic waste-water with a high concentration of iron ions; (2) the loss of about 15% mass of initialFe0 (Matheson and

Tratnyek, 1994); (3) the acceleration of corrosion by

sorbed H+ _{and Cl}_{. The above factors cause high} variability in kobs for different acid-washing processes and large restrictions on its application to in situ remediation.

In this study, the surface of Fe0was newly pretreated using reducing gas (20vol% H2/N2), before it was used to reductive by destroy the aqueous TCE, to reduce the oxide layer and to carbonize the carbon-containing subjects on the iron’s surface. This method offers many advantages over the acid-washing method for pretreating cast iron; for example, it produces no wastewater and simultaneously accelerates reduction and increases sorption capacity. The morphology and components of an exposed Fe0surface before and after the two pretreatments were identiﬁed using Scanning Electron Microscopy (SEM) and Energy Dispersion X-ray Spectroscopy (EDX), respec-tively. A simpliﬁed conceptual model consisting of the sorption/desorption and reduction of contami-nants by iron with graphite inclusions in a TCE– iron–water system, is proposed (Fig. 1). The amounts

of both aqueous and sorbed TCE mass were

measured as functions of time to evaluate the order of the reaction rate (n) and the constant (kobs) in Eq. (4). The purpose of this work was to elucidate the effects of the HCl-washing and the H2-reducing pretreatment of an iron surface with impurities on the reduction and sorption of TCE in a closed, anaerobic batch system.

2. Experimental Section 2.1. Chemicals

The chemicals used were trichloroethylene (99+%, Aldrich, Milwaukee, WI), n-hexane (HPLC grade, Fisher), N-[2-hydroxyethyl]piperazine-N0 -[2-ethanesul-fonic acid] acid (HEPES, Sigma) and iron powder (Wako Co., Japan). All aqueous solutions were made in water puriﬁed with a Milli-Q system (18.2 MO/cm). The desired concentrations of TCE in Ar-saturated water were prepared by dilution of a saturated TCE stock solution (1100 mg/L, 25 1C), which was made by stirring excess TCE with Ar-saturated water. Prior to use, the iron powders were hand-sieved to constrain particle size to a 325–400 mesh screen. The nominalpurity of the cast iron was 95%. However, the elemental analysis by EDX revealed 85.1% Fe, 8.0% C, 0.8% Si and 6.1% O by average weight on the iron’s surface.

2.2. Pretreatment methods for iron surface 2.2.1. Acid-washing process

The iron was pretreated by washing in Ar-sparged 10% HClwith periodic shaking for 30 min, then rinsed ﬁve times with vigorous shaking in Ar-sparged Milli-Q water to remove residualacidity and chloride, a method which is similar to that described previously (Matheson

and Tratnyek, 1994; Agrawaland Tratnyek, 1996;Su

and Puls, 1999). The HCl-washed iron was dried by

vacuum freeze-drying technique (200 103Torr and 56 1C for 24 h) before use.

2.2.2. H2-reducing process

The iron was heated in a ﬂow of H2/Ar (20 Vol%, 60 ml/min) from ambient to 400 1C, keeping at 400 1C for 4 h to completely reduce the iron oxide. After cooling down to room temperature, the ﬂow of H2/Ar was then replaced by He (60 ml/min) to purge the reduced sample Fe0 Products sorption desorption [TCE] aq [TCE] sorb * active site reduction C-containting subjects

Fig. 1. A simpliﬁed conceptual model consisting of sorption/desorption and reduction for TCE by iron with graphite inclusions in a closed batch system.

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for 10 min. The H2-reduced Fe0 must be stored in a drying box.

2.3. Characterization of iron surface

Surface areas were determined by BET N2adsorption analysis on a Coulter SA3100 surface area analyzer (Coulter Co., Hialeach, FL). The morphology and size of the surface of the iron was viewed with SEM and localized elemental information from the chosen region with EDX in conjunction with SEM. Temperature programmed reduction (TPR) studies were performed to determine the required temperature for reducing iron oxide and the quantity of iron oxide with the apparatus similar to that described previously (Bond and Namijo, 1989). In that, a ﬂow of H2/Ar (20 vol%, 100 mL/min) was used as reducing gas. The oven temperature was programmed from ambient to 450 1C at 10 1C/min and keeping it at 450 1C for 1 h. The peak of H2consumption was assigned to Fe(III)-Fe0, represented as Eq. (6)

Fe2O3þ3H2!2Fe0þ3H2O: (6)

The quantity of H2 consumption was obtained by comparing the area of this peak to that of 1mL H2 (40.9 mmolat 1 atm, 298 K) passing through the reactor of TPR. Simultaneously, the total number of Fe2O3 atoms was calculated by multiplying by a factor (1/3), consistent with the stoichiometry of Eq. (6).

2.4. Batch experiments 2.4.1. Reactor system

All experiments as functions of time were performed with 15 mL serum bottles. In each bottle, 0.3 g iron and 14.9(70.1) mL Ar-purged buffered Milli-Q water (10 mM HEPES) were added with zero headspace. The use of HEPES as pH buffer avoids the effect of the variability in pH on the reaction rate over the duration of a typicalexperiment.Matheson and Tratnyek (1994) had demonstrated that HEPES interacted weakly with the iron and controlled the solution pH well. A 100 mL aliquot of TCE (1100 mg/L) was then added under the water level to cause the initial concentration of TCE at 7.3 mg/L. Immediately after TCE addition, the vials were capped with Teﬂon silicone septa and aluminum seals and then mixed on a rotary shaker (50 rmp) at room temperature (25 1C) in the dark.

2.4.2. Extraction

Aqueous phase and totalsystem concentration of TCE were determined by liquid–liquid and liquid–solid extraction using n-hexane as a solvent. Firstly, 5 mL aqueous solution was sampled from the reactor system by a gas-tight syringe through the septa, and simulta-neously another disposable needle of 13 mL He was

spiked through the septa to displace the liquid volume. Then, the 5 mL sample was extracted with 5 mL n-hexane by axial rotation on a roller drum at 20 rpm, and at room temperature in the dark for 15 min.Successive withdrawalof aqueous solution with the second gas-tight syringe was done for 9 mL aqueous solution, which was then thrown it away. Another syringe of 5 mL pentane was spiked through the septa to extract TCE of residualaqueous phase and sorbed phase in the vial. The extraction was achieved by axial rotation on a roller drum using the same extraction conditions.

2.4.3. Sample analysis

Two separate 0.5 mL samples of liquid–liquid and liquid–solid extraction were measured using a HP5890 GC equipped with a 30 0.53 mm (I.D) 3.0mm (thick-ness), DB-624 capillary column (J&W) and an electron capture detector operated in the splitless mode. Tem-perature conditions were programmed as follows: oven temperature at 50 1C; injection port temperature at 280 1C; detector temperature at 300 1C. Ultrapure nitrogen was the carrier gas for GC, and at a ﬂow rate of 4.16 mL/min. The method detection limit for TCE was 0.2 mg/L. The totalTCE mass (mg/vial) comes from the sum of both the measured masses in liquid-liquid and liquid-solid extraction. The sorbed TCE mass (CS, mg/vial) was determined by the difference of total and aqueous TCE mass as follows:

Cs¼CTCwVw (7)

where CT donates the totalmass; Cw is the aqueous phase concentration; VW is the volume of aqueous solution.

3. Results and discussion

Preliminary test was done using TPR to determine the required temperature for reducing iron oxides at the oven in flowing H2/Ar (20 vol%, 60 ml/min). The TPR profile of the iron oxides on the untreated iron surface was shown inFig. 2. A peak of hydrogen consumption appeared at 390–410 1C, which assigned to Fe(III)-Fe0_. Thus, the pretreatment for Wako iron in a flow of 20 vol% H2/Ar was set to be heated at 400 1C for 4 h

(Table 1).

3.1. Characteristics of iron surface

The characteristics of exposed iron surfaces, washed with dilute HCl, reduced by H2 and untreated, were compared. The speciﬁc surface areas obtained using N2 adsorption by BET analysis were 1.8, 3.4 and 4.9 m2/g

(Table 2) for untreated, HCl-washed and H2-reduced

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pre-treatment clearly increased the speciﬁc surface area. The morphology and contents of these three iron surface were analyzed using SEM and EDX. The H2-reduced

Fe0inFig. 3cyielded the same results as the untreated

Fe0 _{surface in} _{Fig. 3a}_{. However, the HCl-washed Fe}0 surface, depicted inFig. 3b, exhibited many folds. This change was due to pitting or etching corrosion by HCl

(Agrawaland Tratnyek, 1996), and increasing in the

surface area. Table 1 presents the relative mass percentage of elements on the iron surface, including Fe, C, Si and O. The mass percentage of C element on the H2-reduced Fe0surface was much higher than that on the surface of untreated Fe0. The source of the C mass on the H2-reduced Fe0 surface was the graphitic inclusions and the organic coating which were carbo-nized in ﬂowing 20 vol% H2/Ar. Accordingly, a greater C mass on the H2-reduced Fe0 surface than on the untreated Fe0surface corresponded to a higher surface

Fig. 3. The SEM and EDX images of iron surface at 5000 magniﬁcation. (a) untreated Fe0_{, (b) HCl-washed Fe}0_{, (c) H}

2-reduced Fe0.

0 100 200 300 400 500

Reducing Temperature, °C

H2 consumption, a.u

Isothermal

Fig. 2. The TPR proﬁle of untreated Fe0 _{in ﬂowing 20 vol%} H2/Ar, heating from room temperature to 450 1C at 10 1C/min and then keeping it for 1 h.

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area, even though no morphological change was observed. This fact was also evidenced by a dramatic difference between the TCE adsorption onto H2-reduced Fe0and that onto the untreated Fe0, considered in the next section. Additionally, the O fraction of HCl-washed iron surface components quickly rose to 2.7% (the rusted Fe fraction was 7.2%, see Table 2), due to corrosion during the rinsing process. In summary, the increase of the iron surface area (BET analysis) comes from the morphological change of HCl-washed Fe0 surface, but comes from the increase in the C fraction on H2-reduced Fe0surface.

The amount of H2consumed by the samples of iron (untreated, HCl-washed and H2-reduced Fe0) was obtained by TPR; the totalnumber of Fe2O3 atoms was calculated by multiplying by a factor consistent with the stoichiometry of Eq. (6). Then, the totalmass of Fe2O3, which is 30% O and 70% Fe, on the surface of the iron was normalized to the speciﬁc surface area in the units of milligrams per meter square. The total Fe mass on the iron surface was calculated by multiplying the totalO mass by the ratio of the mass percentages of Fe and O, given inTable 2. The fraction of the iron’s surface that had rusted is thus represented as the ratio of the Fe mass bonded to O atoms to the totalFe mass on the surface. Table 2 shows the relevant values. The fraction of untreated Fe0 that rusted reached 16.7%. This sample had been unsealed for 6 months and then stored in a drying box. After pretreatment by washing with dilute HCl, the iron quickly combined with the O atoms, by oxidation reaction, during the rinsing process. In contrast, few O atoms remained on the H2-reduced Fe0surface after reducing by H2.

3.2. TCE sorption onto cast iron

Sorption of TCE from solution onto the iron surface was observed by simultaneously measuring the amount

of both sorbed and aqueous TCE mass, described by the Langmuir isotherm. The form of the Langmuir isotherm is represented by

S ¼ KLSmC ð1 þ KLCÞ

; (8)

where S is the amount sorbed on the iron surface at equilibrium (mg/g); C is the equilibrium aqueous concentration (mg/L); Sm is the sorption capacity (mg/ g); KL is the sorption intensity (L/mg). The sorption isotherms for TCE onto various iron surfaces (un-treated, acid-washed and H2-reduced Fe0) are presented

in Fig. 4 and well described by the linear form of

Langmuir isotherm (Eq. (9)) with coefﬁcients of determination (r2_{) exceeding 0.97} C S ¼ 1 KLSm þ C Sm : (9)

Table 3 indicates that the observed maxima were

0.311, 0.365 and 0.889 mg/g for TCE mass onto untreated, acid-washed and H2-reduced Fe0, respec-tively. All these observed values are larger than the value of 0.116 mg/g of Fisher cast iron (3.1% C) reported by

Burris et al. (1995). TCE sorption onto iron was

increased proportionalto the carbon fraction of iron on the surface (Fig. 5), but not related to the fraction of the iron’s surface that had rusted (Table 1). In consequence, TCE predominantly was sorbed onto the carbon-containing subjects rather than iron oxide on the iron’s surface, consistent with the conclusion proposed

by Burris et al. (1998). Obviously, the H2-reducing

pretreatment for iron dramatically increased the TCE sorption, attributed to the increase in the carbon content on the iron’s surface. The highly TCE sorption also gave evidence that the sharp increase in the speciﬁc surface area of H2-reduced Fe0, 4.9 m2/g, compared with the untreated one, 1.8 m2/g, mainly attributed to the N2 adsorption by the carbon content on the iron’s surface. Thus, the reasonable speciﬁc surface area of exposed H2

-Table 1

element analysis on the iron surface by EDX

Elemental Name Untreated Wako Fe0(n ¼ 5)a HCl-washed Wako Fe0(n ¼ 4) H2-reduced Wako Fe 0

(n ¼ 4)

Elemental wt% Elemental wt% Elemental wt%

Fe 85.172.4b _86.7_73.2 _82.0_73.3 C 8.070.8 8.371.1 15.172.6 Si 0.870.1 2.270.4 2.070.4 O 6.170.7 2.870.2 0.970.1 Total100 100 100 a

The value of n refers to the number of random analyses for the iron surface at 5000 magniﬁcation. b_Average

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reduced Fe0_{was 1.8 m}2_{/g the same as that of untreated} Fe0.

3.3. Kinetics of reduction of TCE by Cast iron

As presented inFig. 6, both totaland aqueous TCE mass declined substantially. The rate of loss of total TCE mass obtained using acid-washed Fe0 _resembled that of H2-reduced Fe0, and both losses were signifi-cantly faster than that obtained using untreated Fe0_. This finding implies acid-washing or H2-reducing pre-treatment enhances the destruction of TCE in a metallic iron–water system. The steep slope of the curve for the aqueous TCE loss due to H2-reduced Fe0persisted for a few hours, and then declined over the long-term as the reaction continued because a very large amount of TCE mass was initially eliminated by concurrent reduction and sorption, finally, the sorbed TCE mass was released

Ta ble 2 Sp eciﬁc surfa ce area and the fr action of rusted Fe 0 on the surfa ce with diff erent pretrea tme nt Surfac e area (m 2/g) (1) H2 consum ptio n of 0.45 g vario us iron sam ple (m mol) (2 ) H2 consum ptio n per unit expo sed iron surfa ce area (mole/m 2) ð3 Þ¼ ð2 Þ ð1 Þ 0 :45 Fe 2 O3 mas s per un it expo sed iron surfa ce area (mg/m 2) ð4 Þ¼ ð3 Þ 10 6 55 :8 10 3 3 Th e mas s o f Fe-fraction of Fe 2 O3 pe r unit exposed iron surfa ce area (W rust, F e ) (mg/ m 2) ð5 Þ¼ð 4 Þ 70% Tot alFe mas s per unit expo sed iron surfa ce area (W surf,Fe ) (mg/m 2) (6) W rust ;Fe W surf ;Fe 100% ð7 Þ¼ ð5 Þ ð6 Þ Un treated Fe 0 1.8 112.5 139.0 7.39 5.17 31.02 16.7 HCl-washed Fe 0 3.4 96.5 56.4 3.35 2.35 32.39 7.2 H2 -r educed Fe 0 1.8(4.9 ) a 18.6 22.9 1.22 0.85 33.73 2.5 aThe valu e in the bracket is the surfa ce area of H2 -r educed Fe 0 measu red by BET ana lysis, but that mai nly comes from carbon-c ontain ing subjec ts rathe r than the exposed ele mental Fe 0. Hydrogen-reduced Fe Acid-washed Fe Untreated Fe

Equilibrium AqueousTCE mass, mg/L

Sorbed TCE mass, mg/g

0.8 0.6 0.4 0.2 0 0 10 20 30 40

Fig. 4. Sorption isotherms for TCE on the iron with different pretreatments. Fitted lines are based on the Langmuir equation using the parameters inTable 3.

Table 3

Fitted langmuir parameters for sorption of TCE by iron with different pretreatment

Untreated Fe0 HCl-washed Fe0 H2-reduced Fe 0

Sm(mg/g) 0.311 0.365 0.887 KL(L/mg) 0.035 0.046 0.054

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into the aqueous phase in a rate-limiting process. Measurements of the concentration of sorbed TCE over 2 h indicated that approximately 34–37% of the initial mass of TCE was removed from the aqueous phase by

sorption by H2-reduced Fe 0

, 16–19% was removed by acid-washed Fe0and 13–16% was removed by untreated Fe0.

The loss of total TCE mass was ﬁtted with an nth-order rate equation (Eq. (1)), and then converted to Eq. (10) with the naturallogarithm

ln dCT dt

¼n ln Cwþln kobs; (10)

where CT and Cw donate the totaland aqueous TCE mass in milligrams per vial, respectively. n was obtained by plotting lnðdCT=dtÞv.s ln Cwin a linear regression.

Fig. 7 shows the resulting values of n; kobs and the

coefficients of determination (r2_{). The orders of} degra-dation reaction (n) were 1.7, 1.5 and 1.4 for untreated, acid-washed, and H2-reduced Fe0, respectively. The orders of degradation reaction are not relatively close to unity, reflecting the complexity of the reaction mechanisms. The order of degradation reaction was assumed to unity to facilitate comparison among reaction rate constants for the various irons (untreated, acid-washed and H2-reduced Fe0). Hence, Eq. (4) was simplified to Eq. (11) which differs from the first-order mode (Eq. (2)) that has been commonly used in other studies

dCT

dt ¼kobsCw: (11)

The ﬁrst-order observed rate constants (kobs) of the dechlorination of TCE in contact with Fe0 _were obtained from the slope of a plot of total TCE loss rate

0 4 8 12 16 20 24 C content,% 0 0.2 0.4 0.6 0.8 1

Sorbed TCE mass, mg/g

Y = 0.046X-0.027, r 2 = 0.996

Fig. 5. Relationship between sorbed TCE mass and the C content on the surface of the iron. The data of (3.1, 0.116) is quoted fromBurris et al. (1995).

Residual

aqueous

or total TCE mass (mg/vial)

total TCE mass, Hydrogen-reduced Fe total TCE mass, Acid-washed Fe total TCE mass, Untreated Fe

aqueous TCE mass, Hydrogen-reduced Fe aqueous TCE mass, Acid-washed Fe aqueous TCE mass, Untreated Fe 0.1 0.08 0.06 0.04 0.02 0 0 10 20 30 40 50 60 Time, hr

Fig. 6. The loss of total and aqueous TCE as function of time in contact with various iron surfaces with different pretreat-ment. The initialmass of TCE: 0.11 mg/vial. The initial concentration of TCE was 7.3 mg/L.

-6 -5 -4 -3 -2 ln(Cw) -8 -7 -6 -5 -4 -3 ln(-dC T /dt) Untreated Fe HCl-washed Fe Hydrogen-reduced Fe n = 1.41 r 2_{= 0.91} n = 1.48 r 2_{= 0.96} n = 1.69 r 2_{= 0.90}

Fig. 7. Reaction order plots for TCE in contact with various iron surfaces including untreated Fe0, HCl-washed Fe0and H2 -reduced Fe0.

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(dCT=dt) vs TCE concentration in the aqueous phase, Cw in mg-TCE/vial, using linear least-squares analysis.

Fig. 8 shows the resulting values of kobs and the

coefﬁcients of determination (r2_{). The k}

obsvalues were 0.080, 0.148 and 0.191 h1 _{for untreated, acid-washed} and H2-reduced Fe0, respectively. Washing with dilute HCl approximately doubled the rate constant of the reduction of TCE by Fe0. Moreover, the increase due to the H2-reducing pretreatment exceeds that due to HCl-washing pretreatment. Both the pretreatments of the iron surface clearly increased kobs. However, the rate of loss of total TCE mass (dCT=dt), depended not only on kobs but also Cw, and trended toward slack in H2 -reduced Fe0_{-water system because the sorbed TCE was} slowly released in the final reaction period. Normalized to iron surface area, the specific rate constants (kSA) were 2.37 103, 2.31 103 and 5.62 103h1 m2L, respectively. Notably, a specific surface area of 1.8 m2/g, the same as that of the untreated Fe0, was used to calculate kSA for H2-reduced Fe0, because the morphology of its surface was similar to that of untreated Fe0, using SEM analysis. These findings indicated that acid-washing approximately doubled kobs mainly because of an increase in the Fe0surface area, but it slightly decreased kSA because of the rapid corrosion during the rinsing process. After H2-reducing pretreatment, both kobsand kSA were increased due to the increase in the number of reactive sites per unit surface area. Meanwhile, the carbonization of carbon-containing subjects increased the sorption capacity for TCE.

4. Conclusion

Signiﬁcant sorption of TCE by highly impure iron was observed. Sorption capacity was increased by HCl-washing and H2-reducing pretreatment of the iron, resulting from the increase in the C fraction in the iron’s surface. The results revealed that approximately 34–37% of the initialmass of TCE was removed from the aqueous phase by sorption by H2-reduced Fe0, 16–19% was removed by HCl-washed Fe0and 13–16% was removed by untreated Fe0_{. HCl-washing} approxi-mately doubled kobs, mainly because of the increase in the surface area of iron, but it slightly decreased kSA because of rapid corrosion during the rinsing process. H2 reduction reduces the fraction of rusted iron, increases the number of the reactive sites and increases the sorption capacity in relation to those values obtained when iron is, as is common, pretreated by acid washing. Additionally, no wastewater or sludge was generated. Therefore, pretreatment by H2reduction is a promising alternative means of promoting the removal of TCE from contaminated water.

Acknowledgement

The authors would like to thank the National Science Council of the Republic of China for ﬁnancially supporting this research under Contract No. NSC 92-2211-E-002-063.

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0 0.04 0.08 Cw, mg/vial 0 0.004 0.008 0.012 0.016 0.02 -dC T /dt, mg/vial/h Untreated Fe HCl-washed Fe Hydrogen-reduced Fe Kobs =0.08 r 2 =0.86 K_obs=0.148 r 2 ₌_0.97 K_obs=0.191 r 2 ₌_0.93

Fig. 8. Observed reaction rate constant plots for TCE in contact with various iron surfaces including untreated Fe0_, HCl-washed Fe and H2-reduced Fe0.

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