Effects of Aging and Oxidation of Palladized Iron Embedded in

Apr 20, 2009 - ... Environmental Protection Agency, 26 West Martin Luther King Drive, ..... below 0.2 mg/L), and aerobic (purged with air during the r...
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Environ. Sci. Technol. 2009, 43, 4137–4142

Effects of Aging and Oxidation of Palladized Iron Embedded in Activated Carbon on the Dechlorination of 2-Chlorobiphenyl H Y E O K C H O I , † S O U H A I L R . A L - A B E D , * ,† AND SHIRISH AGARWAL‡ National Risk Management Research Laboratory, U.S. Environmental Protection Agency, 26 West Martin Luther King Drive, Cincinnati, Ohio 45268, and Pegasus Technical Services, Inc., 46 East Hollister Street, Cincinnati, Ohio 45221

Received December 12, 2008. Revised manuscript received April 6, 2009. Accepted April 7, 2009.

Reactive activated carbon (RAC) impregnated with palladized iron has been developed to effectively treat polychlorinated biphenyls (PCBs) in the environment by coupling adsorption and dechlorination of PCBs. In this study, we addressed the dechlorination reactivity and capacity of RAC toward aqueous 2-chlorobiphenyl (2-ClBP), and its aging and longevity under various oxidizing environments. RAC containing 14.4% Fe and 0.68% Pd used in this study could adsorb 122.6 mg 2-ClBP/g RAC, and dechlorinate 56.5 mg 2-ClBP/g RAC which corresponds to 12% (yield) of its estimated dechlorination capacity. Due to Fe0 oxidation to form oxide passivating layers, Fe2O3/Fe3O4 (oxide-water interface) and FeOOH/FeO (oxide-metal interface), RAC reactivity decreased progressively over aging under N2 < H2O + N2 < H2O + O2 conditions. Considering nanoscale Fe/ Pd corrosion chemistry, the decline was quite slow at only 5.6%, 19.5%, and 32.5% over one year, respectively. Dissolved oxygen played a crucial role in enhancing 2-ClBP adsorption but inhibiting its dechlorination. The reactivity change could be explained with the properties of the aged RAC including surface area, Fe0 content, and Fe species. During the aging and oxidation, the RAC showed limited dissolution of Fe and Pd. Finally, implementation issues regarding application of RAC system to contaminated sites are discussed.

Introduction There have been enormous efforts to treat polychlorinated biphenyls (PCBs) in the environment (1). Many research groups have suggested a capping or barrier approach using adsorptive and/or reactive materials as a practical in situ remediation strategy for contaminated sites (1-5). Adsorptive materials such as activated carbon (AC) have been shown to effectively sequester PCBs desorbed from sediment and thus reduce their bioavailability in the aquatic environment (3-5). Meanwhile, reactive metal particles such as Fe, Fe/Pd, and Mg/Pd have been proven to electrochemically dechlorinate PCBs to lower congeners and eventually to bioavailable biphenyl (BP) (6-9). In spite of a concern regarding the cost of Pd as an efficient catalyst to speed up the reaction, * Corresponding author phone: (513) 569-7849; fax: (513) 5697879; e-mail: [email protected]. † U.S. Environmental Protection Agency. ‡ Pegasus Technical Services, Inc. 10.1021/es803535b CCC: $40.75

Published on Web 04/20/2009

 2009 American Chemical Society

nanoscaling Pd and Fe particles (6, 7) and dispersing Pd particles uniformly to Fe surface without significant agglomeration (8, 10) can maximize the reactivity of Fe coupled with a small amount of Pd. Recently, we addressed the integration of the physical adsorption of PCBs with their chemical dechlorination on reactive activated carbon (RAC), where Fe/Pd bimetallic nanoparticles were impregnated into the mesopores of granular activated carbon (GAC) (8, 9). For the full scale environmental applications of RAC (11), our research activities have been geared toward (i) aging and oxidation of RAC, (ii) treatment of positional and highly chlorinated PCBs, and (iii) practical application aspects to contaminated sites. In this study, we particularly focused on the first issue to predict the dechlorination capacity and longevity of RAC. In the RAC system, PCBs adsorption capacity is closely related to the properties of AC such as surface area. The content of Fe0 (zerovalent iron, ZVI) as a primary electron donor determines dechlorination capacity and especially longevity (12). The adsorption ability, dechlorination capacity, iron oxidation, and aging of RAC are considered the most critical factors in determining the practical applications of the RAC system. The dechlorination chemistry of chlorinated compounds on the metallic systems has been extensively studied (6, 7, 12, 13). However, little attention has been given to their dechlorination capacity. Comparing empirical dechlorination results with an estimated dechlorination capacity based on the properties of the metal particles and nature of the reaction makes it possible to evaluate the effectiveness of various metallic systems with distinct properties. A previous study on the long-term performance of micron size iron fillings showed a slow decline in the dechlorination reactivity over several years and suggested an exponential decay of the reaction rate constant over 3 years (14). In cases of nanosize particles, more remarkable changes are expected in a short time due to their high surface area-to-volume ratio. Recently, Liu and Lowry found that the reactivity of nanosize ZVI particles is a function of exposure time to water and exhibits first order decay with respect to Fe0 content which decreases exponentially over 2 years with kobs ) 6.0 ( 2.1 × 10-3 d-1 and a half-life of 90-180 d (12). However, no attempts have been made so far to predict long-term performance of Fe/Pd bimetallic particles most probably because of much faster oxidation of Fe0 in the presence of Pd. The unique corrosion system of Fe/Pd promptly generates abundant electrons for direct dechlorination of PCBs and/or for H2 production followed by hydrodechlorination of PCBs (7), which is advantageous for treating high PCBs loading in a short period but might be disadvantageous for responding to long-term slow release of PCBs. The 9-12 nm Fe particles coupled with 2-3 nm Pd nanoislands on the RAC might oxidize more easily, causing the reactivity of RAC in terms of PCB dechlorination kinetics to decay faster. On the other hand, it was also reported that Fe oxide layers protect core Fe from the fast oxidation (15). As a result, the formation of oxide passivating layers instantly decreases PCB dechlorination kinetics, but eventually prolongs the longevity of RAC to dechlorinate PCBs. Nothing in the literature is reported on changes in the reactivity and properties of RAC over aging. The primary objective of this study was to test the effect of RAC aging on the dechlorination of aqueous phase 2-chlorobipehnyl (2-ClBP) selected as a representative lowermolecular weight PCB compound. Various oxidizing environments (N2, H2O + N2, H2O + O2), and the reactivity of the oxidized RAC were investigated to estimate its longevity and correlation with its physicochemical properties. We also VOL. 43, NO. 11, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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discuss the implementation of RAC system for the remediation of contaminated sites.

Experimental Section RAC Synthesis. The detailed description for the synthesis procedures and route of RAC containing 14.4% Fe and 0.68% Pd was reported previously (8, 9). The GAC (Hydrodarco 3000, Norit) was produced by steam activation of coal. Adsorption Isotherm. The experiment was based on a batch test, involving adsorption equilibrium of 2-ClBP between the aqueous and GAC solid phases. The detailed method was reported elsewhere (16), but slightly modified as noted in Figure S1 in the Supporting Information. Batch Reaction and Analytical Methods. 2-ClBP (Accustandard) was selected due to the simplicity of the system analysis and its highest solubility (5.9 mg/L) among PCBs. Unless otherwise specified, all experiments were based on sacrificial batch in anaerobic condition. One g of RAC (or GAC) was mixed with 10 mL of around 4.8 mg/L 2-ClBP dissolved in deionized water, and shaken in a gyroskaker at 60 rpm (standard condition). The detailed description for the batch experiment and analytical method was reported previously (8, 9). Only modifications made in each experiment are reported below. In general, initial solution pH at 6.0-6.5 increased immediately and stayed at 8.3-9.0 throughout the reaction. Due to the characteristic of sacrificial batch setup for solid phase PCBs extraction, triplicates were run only in limited cases, if statistically essential (note Supporting Information). Dechlorination Capacity. We investigated dechlorination capacity of RAC in the presence of an excessive amount of 2-ClBP to determine how much of the PCB can be dechlorinated per unit mass of RAC and how effective the Fe in RAC is to dechlorinate the PCB. In order to make PCB-rich condition, 0.1 g of RAC was added to 50 mL of 1057 mg/L 2-ClBP methanol/water (8:2) solution, resulting in RAC/2ClBP ratio at around 2. Supporting Information provides more detail on the use of cosolvent methanol and high 2-ClBP concentration. Short-Term Spiked 2-ClBP Treatment. To test short-term response of RAC to 2-ClBP spiking, reacted 2-ClBP solution under the standard condition for 3 d was replaced with a new 2-ClBP solution, without replacing AC materials. This was repeated 5 times (5 cycles). Long-Term Aging, Oxidation, and Reactivity. We investigated the longevity of RAC. RAC, once synthesized, was aged for one year under various environments: (i) stored in N2 condition, (ii) exposed to water under N2 condition (H2O + N2), and (iii) exposed to water open to air (H2O + O2). After aging, RAC was recovered and dried under anaerobic condition. The aged RAC was tested with 2-ClBP solution in the standard condition described above to elucidate changes in 2-ClBP adsorption and dechlorination kinetics for 24 h. To reduce complexities of data analysis, 4 h was selected as a reasonable time frame to compare adsorption and dechlorination efficiencies of aged RAC to those of fresh RAC (note Figure S2 in Supporting Information). Aerobic vs Anaerobic Conditions. To test the effect of dissolved oxygen in water on the reactivity of RAC, three different environments were set up in the standard condition but with 50 mL reaction volume under N2, anoxic open to air (initially N2 background and then open to air to maintain dissolved oxygen level below 0.1 mg/L), and aerobic purging with air (oxygen at around 2 mg/L). This experiment was triplicated. Analytical Method for PCBs and Fe/Pd. At each time interval, one reactor was sacrificed for the measurement of 2-ClBP and BP in the liquid and RAC solid phases. 2-ClBP and BP in the liquid phase were extracted with hexane (Fisher) while those in the RAC solid phase were extracted using 4138

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FIGURE 1. Adsorption and dechlorination of 2-ClBP on RAC for 30 d in the presence of an excessive amount of 2-ClBP for investigating 2-ClBP treatment capacity of RAC (insert), and fraction of 2-ClBP and BP partitioned to the liquid and solid phases after 30 d. automated Soxhlet (EPA Method 3541, note Supporting Information) (9). The sample was analyzed in a gas chromatograph (GC, HP 6890)/mass spectrometer (MS, HP 5973) (EPA Method 8270) (8). Liquid samples were taken, filtered with a 0.45 µm filter, and analyzed to measure the dissolution of metals, while RAC grains were analyzed to measure the mechanical loss of metals from RAC. Characterization of RAC. Fe and Pd were measured using inductively coupled plasma-atomic emission spectrometer (IRIS Intrepid, Thermo Electron Corporation). The Fe0 content was determined by X-ray photoelectron spectroscope (XPS, Perkin-Elmer model 5300). An X’Pert diffractometer (Philips) with Cu KR (λ ) 1.5406 Å) radiation was used for the X-ray diffraction (XRD) analysis to determine the crystal phase of Fe. A porosimetry analyzer (Tristar 3000; Micromeritics) was used to measure Brunauer, Emmett, and Teller (BET) surface area, pore size distribution, and porosity of RAC.

Results and Discussion Dechlorination Capacity. Based on current understanding, 1 mol of Fe0 is oxidized to release 2 moles of electrons by H+ in H2O via the most often assumed route to Fe2+ (eq 1). In the direct dechlorination (eq 2, pathway I), 2 moles of electrons dechlorinate 1 mol of mono PCBs (2-ClBP). However, this pathway is 2-3 fold slower than indirect Pdmediated hydrodechlorination (eqs 3 and 4, pathway II) (8). Fe0 f Fe2+(aq) + 2e+

-

(1) -

Ar2CL + H + 2e f Ar2H + Cl +

H2(g) T 2Had T 2H(aq) + 2e

-

PdH2 + Ar2Cl f Ar2H + PdClH

(2) (3) (4)

Two moles of electrons produce 1 mol of H2, which is able to dechlorinate 1 mol of 2-ClBP. In either pathway I or II, 1 mol of Fe0 can dechlorinate 1 mol of 2-ClBP. Assuming all Fe is in zerovalent state and completely oxidized and all electrons generated are used for the dechlorination reaction, 1 g of RAC with 14.4% Fe here can dechlorinate 488.1 mg of 2-ClBP (RAC:2-ClBP ) ∼2:0.98). Figure 1 shows the adsorption and dechlorination of 2-ClBP in the presence of an excessive amount of 2-ClBP (RAC:2-ClBP ratio of 2:1.06). Approximately 23.2% of the 2-ClBP provided disappeared from the liquid phase as a result of its adsorption at 122.6 mg of 2-ClBP/g RAC. Figure S1 in the SI shows 2-ClBP adsorption isotherm for GAC. The isotherm fitted to the Freundlich equation demonstrated high adsorption capacity of GAC at log KF (Freundlich sorption constant) of 5.274. The adsorption capacity of RAC was estimated to be slightly lower than that of GAC, based on

FIGURE 2. Short-term treatment of spiked 2-ClBP using GAC and RAC: (a) adsorption efficiency, based on 2-ClBP and BP concentration in the liquid phase, (b) apparent 2-ClBP dechlorination efficiency, based on BP accumulated on the RAC phase and actual 2-ClBP dechlorination efficiency, based on actual amount of BP formed in each cycle, and (c) Fe content remaining in RAC, showing detachment of Fe from RAC (Fe and Pd dissolution is shown in Figure S4 in SI). Reacted 2-ClBP solution for 3 d was replaced with a new 2-ClBP solution, without replacing AC materials. their surface area (8). Due to incomplete extraction of 2-ClBP and BP from the highly adsorptive RAC, the mass fraction of 2-ClBP and BP in RAC was estimated based on their relative fraction in extracted portion. The details have been described elsewhere (9). Their fractions are presented in Figure 1. The dechlorination was based on mass of 2-ClBP and BP observed in the overall system. After 8 d, 2-ClBP dechlorination reached a maximum point at 10.7%, corresponding to 56.5 mg 2-ClBP/g RAC. Dechlorination Yield and Implications. It is useful to introduce a term “dechlorination yield”, the ratio of the amount of dechlorinated PCBs to that of PCBs which can be theoretically dechlorinated by the system (eq 5). dechlorination yield ) observed capacity/estimated capacity (5) The yield here was calculated at 0.12. The low yield is possibly due to the overestimation of the capacity. Numerous undesired reactions occur, competing with the dechlorination reaction for electrons. In fact, Fe species are a mixture of Fe0 and Fe3+ since not all iron oxides are reduced to Fe0 during NaBH4 reduction as indicated previously by X-ray elemental mapping (8). Fe0 is also oxidized during the RAC synthesis via reductive deposition of Pd on Fe surface (8). Before Pd doping, Fe0 fraction among Fe species was around 62.1%. During 0.68% Pd deposition, the fraction of Fe0 should decrease theoretically to 57.4% based on eq 6, which is close to our observation of 57.2%. Fe0+Pd2+ f Fe2+ + Pd0V 0

+

3Fe + 4H2O f Fe3O4 + 8H + 8e

(6) -

(7)

Meanwhile, there is also a possibility of underestimating the capacity (the yield further decreases). In Fe0 oxidation, Fe0/Fe3O4 coupling might be more thermodynamically favorable than Fe0/Fe2+ coupling at pH above 6.0 (eq 7) (17). It was found that the oxide film formed at the Fe0/Fe oxide interface is magnetite (Fe3O4) while that at the Fe oxide/H2O interface is maghemite (Fe2O3) (18). Nurmi et al. (19) also observed only two oxides (Fe3O4 and Fe2O3) after exposure of ZVI to H2O or trichloroethene/H2O solution while a majority of the iron oxides formed during dechlorination of chlorobenzene on Fe/Pd is magnetite (other species might form in the transition) (20). Fe0/Fe3O4 coupling can generate 0.67 M more electrons per mole Fe0, compared to Fe0/Fe2+ (12). Many studies have shown the development of surface corrosion and authigenic precipitates (12, 18, 20, 21). Such precipitates mask the active redox sites for electron exchange. The corrosion products reduce the permeability of RAC by occupying available porous structure of RAC (discussed

below). Loss of some reactivity of RAC from passivation of the Pd surface might partly explain the low yield (22). In addition, immobilization of metal particles on a support material like GAC here is expected to lower the yield due to the mass transfer limitation of PCBs compared to unsupported dispersed type of Fe, while nanoscaling of the metallic particles increases the yield significantly. Consequently, the dechlorination yield suggested here might be an effective tool to compare the effectiveness of various metallic systems. Short-Term Treatability. Since an immediate decay in 2-ClBP adsorption and dechlorination kinetics was expected, we initially tested the response of GAC and RAC systems to short-term successive 2-ClBP spiking (Figure 2 and Figure S3 in SI). All 2-ClBP provided to GAC system in each cycle disappeared from the liquid phase within a day, resulting in its accumulation in the GAC phase. RAC exhibited similar 2-ClBP adsorption behavior. As expected, BP instead of 2-ClBP was accumulated in the RAC phase. Actual dechlorination in each cycle was stable at around 80%, while apparent dechlorination kept increasing since even nonreacted 2-ClBP absorbed to RAC in a certain cycle was further dechlorinated in the following cycle. During the 5 cycles under 60 rpm mixing condition, Fe content in RAC was very stable as shown in Figure 2c (dissolution will be discussed later). Although Pd was not reported due to its small initial content at 0.68%, Pd is expected to be as stable as Fe since Pd was reductively deposited to Fe surface (8). The mechanical stability of the metals was ascribed to their placement into GAC pores and Fe-C metal-support interaction described previously (8). Effect of Long-Term Aging and Oxidation. The reactivity of RAC aged under the conditions of N2, H2O + N2, and H2O + O2 (O2 level at 0.1-0.2 mg/L) for one year was compared with that of fresh RAC. As shown in Figure 3a, aged RAC under N2 showed no significant changes in its 2-ClBP adsorption ability. However, the presence of H2O and/or O2 during RAC aging affected the adsorption ability of ensuing RAC. Exposure to water for a short time temporarily decreased its adsorption ability, while exposure for a longer time eventually helped RAC to adsorb more 2-ClBP. As shown in Figure 3b, the reactivity of RAC decreased quickly in the case of RAC aged under more oxidizing environments: N2 < H2O + N2 < H2O + O2. The reactivity decreased by 5.6%, 19.5%, and 32.5% over one year aging, respectively. However, the decline was much slower than what we expected from nanoscale Fe/Pd corrosion chemistry. Considering RAC application to aquatic sediment in anaerobic or anoxic condition, we expect only 19.5-32.5% reactivity decrease over one year. Variation of Fe0 fraction among Fe species is shown in Figure 3c. Fe0 fraction decreased quickly in the presence of H2O and/or O2. The trend is consistent with the decrease in RAC reactivity shown in Figure 3b. These results VOL. 43, NO. 11, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Changes in (a) adsorption ability, (b) dechlorination reactivity, and (c) ZVI content due to the oxidation of Fe/Pd over one year aging under the conditions of anaerobic storing (N2), exposure to water in N2 environment (H2O + N2), and exposure to water open to air (H2O + O2). After RAC aging for the time period above, solid RAC was recovered, and then some of the aged RAC were analyzed for the measurement of Fe0 fraction, while some were sacrificially tested in 2-ClBP solution under anaerobic condition for 4, 8, and 24 h to study adsorption and dechlorination kinetics as shown in Figure S2 in SI. The results of aged RAC were normalized with those of fresh RAC.

FIGURE 4. XRD patterns of RAC after one year aging under (a) storing at N2 environment (N2), (b) exposure to water in N2 environment (H2O + N2), and (c) exposure to water open to air (H2O + O2). Only the predominant reference peaks for each oxide phase are presented. Other peaks correspond to XRD patterns of GAC background. suggest that Fe0 content determines the long-term performance of RAC. The oxidation of Fe is complex (12, 17-21). In anaerobic water, Fe corrosion follows eq 8. Reduction of water increases solution pH, leading to the precipitation of ferrous species such as ferrous hydroxide, which decomposes to FeO (eq 9). Then, ferrous oxide forms a layer on the Fe surface. However, the FeO is thermodynamically unstable and undergoes further oxidation (eq 10). Atomic hydrogen reacts to form molecular hydrogen and a layer of Fe2O3 builds up on the FeO layer. Another layer between the two layers is present, composed of Fe3O4 (eq 7). In addition, ferrous hydroxide may oxidize to goethite (FeOOH) (eq 11). Fe oxidizes faster in the presence of O2 (eq 12). Fe0 + 2H2O f Fe2+ + H2(g) + 2OHFe

2+

(8)

-

+ 2OH f Fe(OH)2 f FeO + H2O

2FeO + H2O f Fe2O3 + 2H

·

+

(9) -

(or 2H + 2e )

Fe2+ + 2H2O f FeOOH + 2H+ + 0.5H2(g) 0

2Fe + O2 + 2H2O f 2Fe

2+

+ 4OH

(10) (11) (12)

Changes in RAC Properties. To better understand the RAC oxidation above, changes in the properties of RAC were investigated after one year aging (Figure 4 and Table 1). Due to many diffraction peaks from the carbon background and various iron oxide phases, the peaks were too complicated 4140

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to explain in detail. In general, iron oxide phases including Fe2O3, Fe3O4, and FeO were found in aged RAC under H2O and/or O2 environment. Obvious peaks for goethite were seen in the case of H2O + O2 exposure. Even in visual inspection of aged RAC, red brown rust corresponding to Fe2O3 · H2O (or Fe(OH)3) was formed under the most oxidizing condition (H2O + O2) while RAC aged under H2O + N2 had greenish black color for Fe3O4 and Fe(OH)2 (note Table 1). Based on the results and eqs 7-12, Fe particles composition was speculated to be various oxide layers, Fe2O3/Fe3O4 (oxide-water interface), and FeOOH/FeO (metal-oxide interface) on ZVI (19-21). The formation of iron oxyhydroxides and other iron oxides may be beneficial to the immobilization of certain contaminants through sorption and coprecipitation as observed in Figure 3a (23). The formation of oxide passivating films could explain the slow decreases in the reactivity and Fe0 fraction of RAC (Figure 3b and c) over aging, which is much slower than that of even nanosize Fe reported by Liu and Lowry (12). First, microscopically focusing on individual 9-12 nm Fe particle, the Fe oxide layer shields the core ZVI or less oxidized Fe to prevent fast and direct oxidation of the core Fe part (called shielding effect). Second, macroscopically focusing on each 2-3 mm RAC grain, Fe particles closely located in the grain boundary oxidize more preferentially than those in the grain core (i.e., inner pores of RAC). The oxides formed in the grain boundary grow (will be discussed later) and protect the grain core, preventing fast oxidation of Fe particles in the core. This protecting effect is a characteristic of this supported type Fe/Pd, which is distinct from highly dispersed neat metal particles with high susceptibility to oxidation. The formation of Fe oxide layer apparently decreases 2-ClBP dechlorination kinetics on RAC. However, it eventually prolongs the longevity of RAC to dechlorinate 2-ClBP. This is an effective feature of RAC response to low level of PCBs slowly released from contaminated sites (e.g., aquatic sediment) over time. Based on the reduction in surface area and pore volume of RAC over aging (Table 1), the iron oxide particles grew slightly, implying that the Fe0 core shrank while the Fe oxides shell became thicker. Coincidently with the result in Figure 3, the structural changes were more significant in cases of more oxidizing condition. Corrosion of Fe under H2O + O2 resulted in 20.1% reduction in surface area and 14.5% decrease in pore volume. In addition to loss of its reactivity, a problem of Fe aging is that the oxide films do not firmly adhere to the metal surface. However, in this supported Fe/ Pd system, minimal amounts of Fe and Pd were detached as summarized in Table 1. After one year aging even under H2O

TABLE 1. Changes in Structural Properties, Fe/Pd Content, and Fe species of RAC due to its Oxidation over One Year Aging under the Conditions of Anaerobic Storage (N2), Exposure to Water in N2 Environment (H2O + N2), and Exposure to Water Open to Air (H2O + O2) exposure condition

surface area (m2/g)

pore volume (cm3/g)

pore size (nm)

Pd (%)

Fe (%)

Fe0 % in Fe

Fe oxidesa

fresh N2 H2O+N2 H2O+O2

358 348 301 286

0.352 0.322 0.319 0.301

4.66 5.55 6.10 6.23

0.68 0.64 0.63 0.61

14.4 14.3 13.9 13.4

57.2 46.2 33.4 26.5

Fe2O3 Fe2O3 Fe2O3, Fe3O4 Fe2O3, Fe3O4, FeOOH

a Due to heterogeneous nature of RAC, the result might be slightly different from overall formulation of Fe species in RAC. Iron oxides in water are hydrated and thus Fe2O3 is Fe2O3 · H2O (red brown hydrous ferrous oxide, Fe(OH)3) and Fe3O4 is Fe3O4 · H2O (black hydrated magnetite or ferrous ferrite, Fe2O3 · FeO).

FIGURE 5. Fe and Pd leaching due to the oxidation of RAC over one year aging under H2O + N2 and H2O + O2 shown in Figure 3. + O2, 14.4% Fe and 0.68% Pd contents in RAC were reduced only to 13.4% and 0.61%, respectively. Fe and Pd Leaching. Figure 5 shows Fe and Pd leaching properties of RAC during one year oxidation (note Figure S4 in SI). The presence of O2 slightly facilitated Fe and Pd dissolution due to the fast Fe oxidation (eq 12). Fe dissolution was negligible at below 0.2 mg/L. Concentration of Pd was slightly high but stable at 0.8 mg/L (the dissolved Pd was confirmed to come from Pd2+ unwashed from GAC during its synthesis rather than metallic Pd deposited on Fe). The amounts correspond to only 0.3-0.7 × 10-5 of Fe and 1.0-1.1 × 10-3 of Pd in fresh RAC, respectively. The pH of reaction solution at 6.5 immediately increased to pH 8.9 (eqs 1 and 2) at which Fe species exists as iron oxides (eqs 9-11). Moreover, in this supported Fe system, iron oxides, if existing, most probably stay on RAC rather than being present in solution as detached form (note Figure 2c). Oxygen Effect on RAC Performance. As shown in Figure 6, aerobic condition was beneficial to 2-ClBP adsorption.

Many studies on the adsorptive properties of AC materials for phenolic compounds revealed that O2 promotes their adsorptive capacity and this phenomenon is attributed to the oligomerization of phenolic compounds on the surface of AC materials (24). On the other hand, the aerobic condition was detrimental to 2-ClBP dechlorination by rapid formation of oxide passivating layer. Most importantly, iron corrosion in water is initiated by eq 12 and/or eq 8, depending on the presence of O2. Oxygen, as an electron acceptor, inhibits direct dechlorination of 2-ClBP, while more H2 for the fast hydrodechlorination of 2-ClBP is produced in anaerobic condition (eq 4). Implementation Issues. The results on aging and oxidation, reactivity change, and metal leaching of RAC are promising. For its sound-good full-scale application to contaminated sites, however, some technical challenges and concerns should be addressed. First, treatment of highly chlorinated and positional PCBs (beyond 2-ClBP) is of interest. The tri-, tetra, penta-, and hexa-chlorinated biphenyls are the most problematic in real world treatment. Despite the relative resistance of substituted chlorines in PCBs, such a bimetallic system was reported to be highly effective to treat higher PCB congeners (25). Second, Fe oxidizes more slowly when coupled with a more active metal (Zn, Al, Mg) while it oxidizes faster when coupled with a less active metal (Cu, Pd, Co, Ni). The amount, type, and subsequently the cost of the second metal may vary in response to the characteristics of the contaminated site and purpose of treatment. For example in RAC, Pd as a catalyst governs mainly the dechlorination kinetics while Fe as an electron donor determines the dechlorination capacity and longevity. It is well-known that a very small amount of Pd, even much less than 0.1%, works effectively (6, 7). Pd dissolution is limited due to the spontaneous redeposition of Pd (if any in form of Pd2+ in solution) to Fe surface (6, 8). In addition to nanoscaling of Fe and Pd particles as discussed in the

FIGURE 6. (a) Adsorption of aqueous 2-ClBP and (b) solid phase conversion of 2-ClBP to BP under anaerobic (N2 background), anoxic (initially N2 background and then open to air to maintain oxygen level below 0.2 mg/L), and aerobic (purged with air during the reaction to keep oxygen level at around 2 mg/L) conditions. The error bars are the standard deviation of triplicated results. VOL. 43, NO. 11, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Introduction, these properties of Pd improve the feasibility of the use of RAC in full-scale environmental applications. Finally, future research should investigate negative or positive effects of coexisting species such as natural organic matter (NOM) and dissolved ions on the effectiveness of RAC. For example, NOM protects hydrophobic PCBs and shields reaction sites in RAC while its functional groups also act as soluble electron carriers to transfer electrons from Fe/Pd to PCBs.

Acknowledgments This research was funded and conducted by the National Risk Management Research Laboratory of U.S. Environmental Protection Agency (EPA), Cincinnati, OH. This paper has not been subjected to internal policy review of the U.S. EPA. Therefore, the research results do not necessarily reflect the views of the agency or its policy. Mention of trade names and commercial products does not constitute endorsement or recommendation for use. We recognize the support of Dr. Gautham Jegadeesan and Mr. Eric Graybill of Pegasus Technical Support Inc. for ICP-AES analysis and GC/MS analysis, respectively. Donation of the activated carbon from Norit Americas Inc. is appreciated.

Supporting Information Available Supplementary descriptions for some experimental sections, 2-ClBP adsorption isotherm (Figure S1), fresh RAC reactivity (Figure S2), spiked 2-ClBP treatment (Figure S3), and Fe/Pd leaching (Figure S4). This material is available free of charge via the Internet at http://pubs.acs.org.

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