Highly Active and Stable Ni–Fe Bimetal Prepared by Ball Milling for

These ball-milled bimetals may also be used as the active materials in the permeable reactive barrier technologies for the treatment of COC-contaminat...
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Highly Active and Stable Ni−Fe Bimetal Prepared by Ball Milling for Catalytic Hydrodechlorination of 4-Chlorophenol Fuyuan Xu,† Shubo Deng,*,†,‡ Jie Xu,† Wang Zhang,† Min Wu,† Bin Wang,† Jun Huang,†,‡ and Gang Yu†,‡ †

POPs Research Center, School of Environment, Tsinghua University, Beijing, P. R. China, 100084 State Key Joint Laboratory of Environment Simulation and Pollution Control, Tsinghua University, Beijing, P. R. China, 100084

‡

S Supporting Information *

ABSTRACT: A novel Ni−Fe bimetal with high dechlorination activity for 4chlorophenol (4-CP) was prepared by ball milling (BM) in this study. Increasing Ni content and milling time greatly enhanced the dechlorination activity, which was mainly attributed to the homogeneous distribution of Ni nanoparticles (50− 100 nm) in bulk Fe visualized by scanning electron microscopy/energy dispersive X-ray spectrometry (SEM/EDS) with image mapping. In comparison with the Ni− Fe bimetal prepared by a chemical solution deposition (CSD) process, the ball milled Ni−Fe bimetal possessed high dechlorination activity and stability before being used up. Dechlorination kinetics indicated that the dechlorination rates of 4CP increased with increasing Ni−Fe dose but decreased with increasing solution pH. Solution pH had a significant effect on the dechlorination of 4-CP and the passivation of the Ni−Fe bimetal. The enhanced pH during the dechlorination process significantly accelerated the formation of passivating film on the bimetallic surface. The Ni−Fe bimetal at the dose of 60 g/L was reused 10 times without losing dechlorination activity for 4-CP at initial pH less than 6.0, but the gradual passivation was observed at initial pH above 7.0.

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INTRODUCTION Chlorinated organic compounds (COC) are known to be very toxic even at low concentrations.1 Reductive dechlorination over some bimetallic materials is an effective method to detoxify these contaminants and has been widely investigated by some researchers.2−12 In these bimetallic materials, the first metal with low standard redox potential such as Fe, Mg, Al, Zn, Sn, Si, etc. is an electron donor to reduce the COC,2−5 while the second metal (Cu, Ni, Ag, or Pd) with high standard redox potential promotes the reactivity via hydrogenation and accelerating corrosion,2,3,6,7 acting as a catalyst and accelerator. Among them, Fe−Pd is the most investigated bimetal due to the low cost of Fe and excellent hydrogenation activity of Pd,8,9 but the expensive Pd greatly restricts its wide application. The cheap Ni has been widely used as an alternative catalyst to dechlorinate COC recently.6,10,11 Normally, bimetals are prepared by chemical solution deposition (CSD), in which the second metal (catalyst) is deposited on the first metal by reduction reactions.2−4,6−15 Many bimetallic nanoparticles such as Pd−Fe, Ni−Fe, and Cu− Fe are synthesized by metal salt reduction with sodium borohydride, and the nanosized bimetals can significantly enhance the dechlorination.13 Some researchers report the synthesis, characterization, and application of Ni−Fe nanoparticles (supported on membranes, carbon nanotubes, etc.) for degradation of various chlorinated pollutants.14,15 However, excessive deposition of the second component can lead to the complete covering of the first metal, unfavorable for © 2012 American Chemical Society

dechlorination. Moreover, the catalyst on the surface of first metal can easily flake off during the dissolution and erosion of the first metal, making the loss of dechlorination activity.16 Therefore, it is necessary to find new methods for the preparation of bimetals with high and stable dechlorination activity. In recent years, ball milling (BM) has been successfully used to prepare some metallic materials for hydrogen generation, hydrogen storage, and hydrogenation in different industries.17−19 Since hydrogenation reaction is also the mechanism for dechlorination of COC, BM should be effective in preparing the metallic materials with good performance for reductive hydrodechlorination. Moreover, the catalysts can be homogenously distributed in the bulk materials instead of the surface covering of materials prepared by CSD. To date, only Geiger's group has published several papers about the dechlorination of polychlorinated biphenyls in organic solvents over Mg and Mg−Pd bimetal prepared by BM.20,21 To our knowledge, no study has been reported to dechlorinate COC in aqueous solution on the cheap bimetals prepared by BM. The primary objective of this study is to prepare a costeffective Ni−Fe bimetal by BM for the dechlorination of COC in wastewater. 4-Chlorophenol (4-CP) was selected as a model Received: Revised: Accepted: Published: 4576

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different pHs (5.0, 6.0, 7.0 and 8.0), and the residual 4-CP concentrations after 60 min were measured in the successive ten cycles. All batch systems were carried out in conical flasks with ground-in glass stoppers, and the reaction solution was not protected or deoxygenated by inert gas. Each experiment was conducted in triplicate, and the standard deviations were calculated. 4-CP Analysis. After the dechlorination experiments, the solid was separated from the solution by a filter with a 0.45 μm nylon membrane, and the residual 4-CP concentration in solution was measured by HPLC. The analytical parameters, the equation for dechlorination percents, and pseudo-first-order kinetic model were described in Supporting Information.

pollutant to evaluate the dechlorination activity. The preparation of Ni−Fe bimetal was optimized, and the obtained bimetal was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and particle size analyzer. The dechlorination of 4-CP over the Ni−Fe bimetal prepared by BM and CSD was compared. The passivation of Fe-based materials significantly affects the dechlorination of COC, and the influences of solution composition and dissolved oxygen on the passivating film and dechlorination activity have been fully investigated.12,22 However, the effect of solution pH on the passivation and activity has not been clearly elucidated, and the underlying mechanism for the passivation should be further investigated. In our study, the effect of solution pH on the dechlorination and passivation of the Ni−Fe bimetal was studied in detail, and the possible reasons were revealed.

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RESULTS AND DISCUSSION Effects of BM Conditions on Dechlorination. During the BM process for the Ni−Fe bimetal preparation, the conditions including Ni content and milling time have important effects on the characteristics of the Ni−Fe bimetal and the dechlorination activity. Figure 1a shows the

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MATERIALS AND METHODS Materials. Iron (>99%, 325−400 mesh) and nickel (>99%, 325−400 mesh) powders were purchased from Beijing Xingrongyuan Technology Co., Ltd. 4-CP (>99%) and phenol (>99%) were obtained from Sigma-Aldrich Corporation. HPLC grade methanol was purchased from Fisher Chemical (USA). Other chemicals were reagent grade. Ni−Fe Bimetal Prepared by BM. BM was carried out in a planetary ball mill consisting of four grinding jars (100 mL each) at a rotation speed of 550 rpm without inert gas protection. The weight ratio of steel balls to metal powder was 37.2:1. The Ni and Fe mixture at different weight ratios was added into the jar and milled for different times. In order to remove the possible passivating film, 10 g of the bimetal prepared was first added in 100 mL of H2SO4 solution at pH 1.0 in a shaker at a speed of 220 rpm for 15 min and then washed 3 times by deionized water before use. The Ni−Fe bimetal prepared by CSD was described in the Supporting Information. Characterization. The scanning electron microscopy (LEO-1530, LEO, Germany) equipped with an energy dispersive X-ray spectroscopy (EDS) was used to characterize the morphology and elemental mapping of the Ni−Fe bimetal before and after BM as well as Fe and Ni contents in different positions. Ni contents in the bimetals prepared by CSD and BM were determined by inductively coupled plasma/atomic emission spectroscopy (ICP-AES, IRIS Intrepid, Thermo Electron Corporation, USA), and the bimetals were dissolved in aqua regia before determination. Brunauer−Emmett−Teller (BET) surface areas of the metallic samples were measured using N2 adsorption at 77 K with an automated gas sorption analyzer (Quantachrome, Corp., autosorb iQ, US). Dechlorination Experiments. All dechlorination experiments were carried out in 250 mL flasks containing 100 mL of 20 mg/L 4-CP solution and a certain amount of Ni−Fe bimetal, and the flasks were shaken at 220 rpm in a shaker at 25 °C. In the optimization of bimetal preparation, 10 g of Ni−Fe bimetal was added in 4-CP solution at initial pH 2.0, and the residual 4-CP concentration was measured at 90 min. In the dechlorination comparison of Ni−Fe bimetals prepared by BM and CSD, 8.5 g of Fe (different content of Ni) was placed in 4CP solution at initial pH 3.0, and the reaction time was 90 min. In the investigation of initial pH effect on the dechlorination, 6 g of bimetal was added in 4-CP solution at pH from 2.0 to 8.0, and 4-CP concentration, solution pH, and metal concentrations in solution were measured during 150 min. In the bimetal reuse experiments, 6 g of bimetal was added in 4-CP solution at

Figure 1. Effects of (a) Ni content (milling time = 1 h) and (b) milling time (Ni content = 15 wt %) on the dechlorination of 4-CP.

dechlorination of 4-CP on the Ni−Fe bimetals prepared with different Ni contents. It can be seen that 4-CP concentration decreased while phenol concentration increased with increasing Ni contents, and the total concentrations of 4-CP and phenol in solution were more than 96% of the initial 4-CP concentration, indicating that 4-CP was directly reduced to phenol in this process and their amounts adsorbed on the bimetal were negligible. The pseudo-first-order rate constants (kobs) increased with increasing Ni content (the plot for ln(C/ C0) versus t shown in Figure S1, Supporting Information), similar to the trend of dechlorination percents. The Ni−Fe bimetal with high Ni content had high dechlorination activity, while the pure iron possessed very low dechlorination percent (2.1%). The Ni−Fe bimetal can form some galvanic cells, and Fe passed electrons to the catalytic Ni, which played an important role in the increase of Fe corrosion and 4577

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Figure 2. SEM micrographs of the Ni−Fe bimetal (Ni content = 15 wt %) (a) before and (b) after 3 h milling, as well as the EDS (c) Fe mapping and (d) Ni mapping for the bimetal.

dechlorination activity.6 The low dechlorination efficiency over the pure Fe particles demonstrated that direct dechlorination of 4-CP was difficult to occur without the catalytic Ni. The addition of Ni significantly increased the dechlorination of 4CP due to the mechanism of catalytic hydrogenation. Some researchers have proposed that the adsorbed atomic hydrogen, rather than galvanic corrosion, is responsible for the enhanced reactivity of iron-based bimetallic reductants by CSD for 1,1,1trichloroethane.23,24 The Ni−Fe bimetal containing 15% Ni was adopted in the following experiments due to no significant increase of activity with increasing Ni content. The Ni−Fe bimetals were also prepared at different milling times, and the residual 4-CP concentrations decreased and kobs increased with increasing milling time (Figure 1b). The dechlorination percent was up to 99.3% when the milling time was increased to 3 h, much higher than 2.2% at 0 h. In consideration of activity and cost, the Ni−Fe bimetal with 15% Ni milled for 3 h was used in the following experiments. The above results indicated the importance of the BM process in the preparation of Ni−Fe bimetal for dechlorination of 4-CP. Ballmilling involves heavy deformation of the powders so that the material contains a high concentration of strain and defects.25 The presence of internal strain and dislocations in ball-milled Fe−Ti and Ni powders can substantially increase the adsorption of hydrogen in the materials.25,26 When only iron powder was milled for 3 h, the surface area increased from 0.17 to 0.50 m2/g, while the dechlorination percent increased from 0.89% to 14.3%, indicating that the enhanced dechlorination may be attributed to the strain and defects in ball-milled iron. Similarly, such effect may also occur on the Ni−Fe bimetal, and the enhanced sorption of hydrogen on the bimetal would improve the dechlorination efficiency with increasing ballmilling time. Ni−Fe Bimetal Characterization. The Ni−Fe bimetals before and after BM were observed by SEM. Before BM, Fe and Ni particles can be distinguished easily from the mixture according to their identification by the EDS (Table S1, Supporting Information) and morphologies (Figure 2a). The Fe particles exhibited the irregular shapes with smooth surface,

while Ni looked like the ears of rice with lots of small grains. The rice-ear-like particles disappeared, and the uniform particles were observed after milling for 3 h (Figure 2b). The Fe and Ni contents on the selected points of the bimetal were analyzed by EDS and shown in Table S1, Supporting Information. The Ni contents in different positions were very close to the theoretical value of 15%, indicating the very homogeneous composition in the milled bimetal. To further illustrate the distribution of Fe and Ni, the SEM-EDS mapping was employed to characterize the Ni−Fe bimetal. As shown in Figure 2c,d, Ni particles were dispersed homogeneously into Fe phase. The Ni particles were found to be in the size range of 50−100 nm in the enlarged micrograph in Figure 2d. Evidently, the homogeneous distribution of Ni in Fe phase was obtained after BM, leading to their close and effective contact. On the one hand, many bimetallic corrosion cells were easily formed by BM, speeding up the electron transfer of galvanic corrosion; on the other hand, the utilization efficiency of hydrogen was increased significantly due to the short route of migration on solid surfaces or diffusion in solution. Atomic hydrogen can readily transfer from where it is generated and adsorb to the Ni surface of the hydrogenation catalyst, preventing the formation and release of molecular hydrogen. This result revealed that the simple mixture of Fe and Ni had very low dechlorination activity since they did not contact closely, and the BM process can achieve the close and effective contact of Ni to Fe. Schrick et al.11 also mentioned the importance of close contact bimetals for dechlorination when the Ni−Fe nanoparticles prepared by the NaBH4 method were used to dechlorinate trichloroethylene. The particle sizes (D50, median diameter) of the Ni−Fe bimetal decreased from 42.1 to 16.6 μm after 3 h milling (Figure S2, Supporting Information), and the corresponding specific surface area increased from 0.17 to 0.49 m2/g. In Figure 1b, the dechlorination percent increased from 2.2% to 99.3% (45.1 times) when the milling time increased from 0 to 3 h. Although the specific surface area of the bimetal increased 1.9 times, the enhanced specific area was not the main reason for the significant increase of the dechlorination activity. Addition4578

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ally, the Ni−Fe bimetal was also analyzed by XRD (Figure S3, Supporting Information). After the milling, the Ni peaks (200) and (220) disappeared, and the Fe diffraction peaks were still present, but they were slightly shifted to lower angles and broadened significantly due to the refinement of grain size and the increase of internal strain,27,28 suggesting the formation of Ni−Fe solid solution. Other studies have confirmed the formation of solid solution of Fe−Ni when a ball mill was used to fabricate Fe and Ni alloys.29,30 For the ball milled Ni− Fe bimetal, the homogeneous distribution of nanosized Ni in the Ni−Fe solid solution was favorable for the formation of many galvanic cells, speeding up the electron transfer of galvanic corrosion. The close and effective contact of two metals in nanoscale enhanced the microcurrent between them because the contact resistance was low. Direct reduction of 4CP by galvanic corrosion on the Ni−Fe bimetal may occur, and this has been reported in the reductive dechlorination of COC on zerovalent iron and hybrid alloys.16,31 Activity and Stability Comparison of Bimetals Prepared by CSD and BM. The dechlorination activity of the Ni−Fe bimetals prepared by CSD and BM was compared in Figure 3. The dechlorination percent of 4-CP over the

Figure 4. Effect of acid washing on the dechlorination of 4-CP and Ni content on the Ni−Fe bimetals prepared by BM and CSD (dechlorination reaction: Ni−Fe dose = 60 g/L, 4-CP concentration = 20 mg/L, initial pH = 3.0, reaction time = 60 min).

Ni content was up to 80% when 94% of bimetal was washed off. Since Fe was more reactive in acid solution than Ni, more Ni was left in the bimetal. The nanosized Ni grains uniformly distributed in Fe phase would flake off and aggregate when the surrounding Fe was dissolved in solution. In contrast, the dechlorination percents on the bimetal prepared by CSD decreased significantly with increasing weight loss. When the bimetal loss exceeded 45.3%, the dechlorination percent was below 6.5%. The deposited Ni on Fe surface by CSD was easy to flake off, making the material ineffective for dechlorination. It is also noticed that the Ni contents in the residual bimetal increased with increasing weight loss, but the enhanced Ni content was not favorable for the dechlorination of 4-CP on the bimetal prepared by CSD as the flaked Ni did not closely contact Fe. Evidently, the homogeneous Ni−Fe bimetal prepared by BM possessed high and stable dechlorination activity for 4-CP before being used up. Effect of pH on Dechlorination and Passivation. Solution pH not only affects the dechlorination of 4-CP and the dissolution of Fe and Ni but also influences the formation of iron hydroxide precipitate and the passivation. The effects of Ni−Fe dose and initial 4-CP concentration on the dechlorination kinetics were investigated, and it can be found that the high dechlorination percents and surface-area-normalized rate constants (kSA) were achieved at the high Ni−Fe dose and low initial 4-CP concentration (Figures S4 and S5, Supporting Information). Actually, the Ni−Fe dose and initial 4-CP concentration directly influenced the variable pH during the dechlorination process (Figure S4 and S5, Supporting Information), resulting in the different dechlorination efficiency. To further investigate the pH effect on the dechlorination and passivation of the bimetal, the dechlorination kinetics of 4-CP and the pH change profile at different initial pH were studied. As shown in Figure S6 (Supporting Information), the dechlorination percents of 4-CP decreased at the same reaction time when the initial pH increased from 2.0 to 8.0. The calculated kSA indicated that low pH resulted in fast dechlorination of 4-CP (Figure 5a). At low pH, more Fe was dissolved in solution (eq 1), and more H2 and H+ were adsorbed and then transformed to highly reductive atomic hydrogen on the catalyst (Ni) surface (eqs 3, 4), where the chlorine of 4-CP was replaced by atomic hydrogen (eq 5). Of course, the disadvantage is that more Fe was dissolved in acidic solution. Some researchers also reported that the dechlorination velocity of CCl4, trichloroethene, and 1,1,1-trichloroethane decreased with increasing pH.32−35 The obtained kSA values for

Figure 3. Dechlorination of 4-CP over the Ni−Fe bimetals prepared by CSD and BM at the reaction time of 90 min.

bimetal prepared by CSD first increased and then decreased with increasing Ni content, and the bimetal with 3.87% Ni had the highest dechlorination percent (90.5%) for 4-CP. Excessive deposition of Ni would cause the complete covering of Fe particles, unfavorable for the dechlorination. In contrast, the dechlorination percent of 4-CP on the ball milled Ni−Fe bimetal increased with increasing Ni content in the range of 0− 15%, and the dechlorination percent was over 96% when the bimetal contained more than 10% Ni. Actually, the surface coverage is more important for dechlorination activity of bimetals.24 The Ni surface coverage on the Ni−Fe samples by BM should be close to the weigh percent, much lower than that prepared by CSD at the same Ni weight percent. Therefore, the Ni−Fe bimetal by BM should have higher dechlorination percent for 4-CP than that by CSD if they have the same Ni surface coverage. To evaluate the stability and longevity of the Ni−Fe bimetals prepared by BM and CSD, 1 mol/L H2SO4 solution was used to flush the bimetals, and the residual powder was washed by deionized water and used to dechlorinate 4-CP. As shown in Figure 4, the bimetal prepared by BM kept the high dechlorination percents from 96% to 100% when it lost weight from 0% to 94.1%. It is notable that the Ni contents in the residual bimetal increased with increasing weight loss, and the 4579

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oxygen (eqs 7, 8) since the solubility product of Fe(OH)3 is much lower than that of Fe(OH)2.39 Fe2 + + 2OH− → Fe(OH)2 ↓

(Ksp = 4.87 × 10−17) (6)

4Fe2 + + 2H2O + O2 → 4Fe3 + + 4OH−

Fe3 + + 3OH− → Fe(OH)3 ↓

(7)

(Ksp = 2.79 × 10−39) (8)

At high solution pH, the passivating film is formed easily, inhibiting the contact of target compounds with the reactive sites and blocking the electron transfer.32 At initial pH above 6.0, the highest pH values exceeded 9.0 in the reaction process, accelerating the formation of passive film by iron hydroxide. Some formed Fe (II) and Fe (III) precipitates covered on the particle surface and retarded Fe dissolution, resulting in the slow dechlorination and even the complete stop of reaction. The dissolved Fe and Ni concentrations in solution at different initial pHs were measured. The concentrations of Fe (from 26.58 to 1.64 mg/L) and Ni ions (from 1.19 to 0.48 mg/ L) decreased with the increase of initial pH from 2.0 to 8.0 (Figure 6). On the basis of the reaction mechanisms (eqs 1−5), Figure 5. Effect of initial pH on (a) the kSA in the dechlorination kinetics of 4-CP and (b) the pH change profile in the reaction process.

4-CP were compared with other reported bimetals (Table S2, Supporting Information). The ball milled Ni−Fe bimetal had much higher kSA or kobs values for 4-CP than that by CSD, even the Pd−Fe nanoparticles. The pH values during the dechlorination reaction increased fast within the initial 10 min and then gradually reached the highest pH (Figure 5b); pH decreased with further increasing time and finally kept relatively stable values after 90 min. The lower the initial solution pH, the lower was the final solution pH. In the initial stage, Fe was dissolved in solution and 4-CP was dechlorinated due to the following reactions.6,36 Fe0 + 2H+ → Fe2 + + H2

(in acidic solution)

Figure 6. Effect of initial pH on the concentrations of dissolved metal ions in solution after 60 min reaction.

(1)

the molar amount of effective Fe for the dechlorination of 4-CP should be equal to that of the reduced 4-CP, and the concentrations of effective Fe in solution dissolved for dechlorination were also shown in Figure 6. Apart from the initial pH 2.0, the concentrations of Fe ions in solution were lower than the effective Fe ion concentrations for dechlorination, implying that some dissolved Fe ions were precipitated in the reaction process. More Fe ions were precipitated at higher solution pH, and about 78.6% of Fe ions were precipitated at initial pH 8.0. Considering the ineffective dissolution of Fe (the produced H2 was released in eq 1 and not involved in the dechlorination), the precipitation proportion was even higher. The more iron hydroxide that was produced, the easier the passivating films formed on the bimetal surface. The ion products of Ni2+ and OH− at initial pH below 8.0 ranging from 9.3 × 10−20 to 3.1 × 10−16 was less than the solubility product of Ni(OH)2 (Ksp = 5.48 × 10−16),39 indicating that no Ni(OH)2 occurred. It is well-known that iron hydroxide may deposit on material surface and form passivating films, which could hinder the transport of the chlorinated molecules and decrease the reaction rate.40 To evaluate the passivation and stability of the bimetal in the dechlorination of 4-CP, this bimetal was used repeatedly for 10 times at different initial pHs. As shown in

Fe0 + 2H2O → Fe2 + + H2 + 2OH− (in neutral or alkaline solution)

(2)

Ni

H2 ⎯→ ⎯ 2H* Ni

(3)

2H+ + 2e ⎯→ ⎯ 2H*

(4)

RCl + 2H* → RH + Cl− + H+

(5)

These reactions caused the increase of pH due to the consumption of H+ or generation of OH−, making the solution pH increase. With the increase of Fe2+ and OH− concentrations, the precipitation Fe(OH)2 occurred (eq 6) when the ion product of Fe2+ and OH− was higher than the solubility product. However, the precipitation rate was lower than the generation rate of Fe2+, and solution pH still increased until the generation rate of Fe2+ decreased to its precipitation rate. The solution pH decreased due to the fast consumption of OH− via the formation of iron hydroxide precipitate (eq 6).37 Finally, the overall reaction equilibrium of dissolution and precipitation of Fe(OH)2 was achieved at relatively stable pH.38 In addition, Fe(OH)3 can easily form, especially in the presence of dissolved 4580

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promising method to prepare some effective composite metals for the catalytic dechlorination of some COC. Many bimetallic materials prepared by CSD may also be prepared by BM, and they have the significant implications for wastewater treatment to dechlorinate COC. These ball-milled bimetals may also be used as the active materials in the permeable reactive barrier technologies for the treatment of COC-contaminated groundwater. To avoid the passivation of the Ni−Fe bimetal, solution pH should be controlled under acidic conditions.

Figure 7, the bimetal kept relatively stable dechlorination percents when it was reused for 10 times at initial pH 5.0 and

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ASSOCIATED CONTENT

S Supporting Information *

Fe and Ni contents in the bimetal, particle size distribution, XRD spectra, effects of pH, bimetal dose, and initial 4-CP concentration on the dechlorination, kSA comparison for different bimetals. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 7. The dechlorination activity of the Ni−Fe bimetal at different initial pH in the successive use for 10 times (each reaction time = 60 min).

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pH 6.0, but the dechlorination percent decreased in the seventh experiment at initial pH 7.0, and the dechlorination percent was only 51.4% in the tenth cycle. When the initial pH increased to 8.0, the bimetal lost the dechlorination activity fast in the third and fourth cycle. Evidently, the Ni−Fe bimetal can keep high dechlorination activity at initial pH below 6.0, while the formed passivating film on the bimetal surface at initial pH above 7.0 significantly decreased the dechlorination efficiency and even completely stopped the dechlorination reaction. Actually, both the precipitation and dissolution processes were involved in the dechlorination of 4-CP on the bimetal. The iron precipitates formed at initial pH above 3.0 since the pH increased to above 8.0 (Figure 5b). Since the dissolution velocity of Fe was faster than that of the adsorbed iron precipitants at low pH, the firmly passivating film was not formed, resulting in the stable dechlorination activity in the successive use. With increasing pH, the dissolution of Fe became slow; especially, more iron precipitates would form and deposit on the bimetal surface, making less active surfaces available for dechlorination. When the stable passivating film was formed on the bimetal surface, the complete deactivation occurred and the bimetal lost the dechlorination acitivity. The compositions of the passive films formed by iron precipitates are very complex, and they include Fe(OH)2, Fe(OH)3, Fe3O4, and Fe2O3.37,41,42 When H2SO4 solution at pH 1.0 was used to regenerate the passivated Ni−Fe bimetal in this study, the dechlorination activity was completely recovered, indicating that the passivating film was easily removed by acid solution and acid washing is an effective method to solve the passivation problem. In general, solution pH had significant effects on the dechlorination of 4-CP and passivation of Ni−Fe bimetal. Low pH can cause the fast dechlorination but result in more ineffective dissolution of bimetal. The increasing pH accelerated the formation of iron precipitates, and passivating film was formed when solution pH exceeded above 9.0 in the reaction process. Therefore, it is crucial to control solution pH in a reasonable range to balance the dechlorination, passivation, and lifetime of bimetal. Environmental Implication. The Ni−Fe bimetal prepared by BM had high catalytic dechlorination activity for 4-CP and can be used repeatedly at pH below 6.0 while no passivation occurred. The high and stable dechlorination of 4-CP was attributed to the homogeneous distribution of nanosized Ni catalyst into Fe phase. This study demonstrated that BM was a

AUTHOR INFORMATION

Corresponding Author

*Phone: +86-10-62792165; fax: +86-10-62794006; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National High-Tech Research and Development Program of China (project no. 2009AA063902), National Nature Science Foundation of China (project no. 50838002), and China Postdoctoral Science Foundation (grant no. 20100480286) for the financial support. Additionally, the analytical work was supported by the Laboratory Fund of Tsinghua University.

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dx.doi.org/10.1021/es203876e | Environ. Sci. Technol. 2012, 46, 4576−4582