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ARTICLES Degradation of Trichloroethylene by Iron-Based Bimetallic Nanoparticles Yit-Hong Tee,† Leonidas Bachas,‡ and Dibakar Bhattacharyya*,† Department of Chemical and Materials Engineering and Department of Chemistry, UniVersity of Kentucky, Lexington, Kentucky 40506-0046 ReceiVed: October 14, 2008; ReVised Manuscript ReceiVed: March 16, 2009
Bimetallic nanoparticles of Ni/Fe and Pd/Fe were used to study the degradation of trichloroethylene (TCE) at room temperature. The activity for different iron-based nanoparticles with nickel as the catalytic dopant was analyzed using the iron mass-normalized hydrogen generation rate. Degradation kinetics in terms of surface area-normalized rate constant were observed to have a strong correlation with the hydrogen generated by iron oxidation. A sorption study was conducted, and a mathematical model was derived that incorporates the reaction and Langmuirian-type sorption terms to estimate the intrinsic rate constant and rate-limiting step in the degradation process, assuming negligible mass transfer resistance of TCE to the solid particles phase. A longevity study through repeated cycle experiments was conducted to analyze the effect of activity loss on the reaction mechanistic pathway, and the results showed that the attenuation in the nanoparticles activity did not adversely affect the reaction mechanisms in generating gaseous products, such as ethylene and ethane. 1. Introduction Reductive dechlorination of chlorinated organics using zerovalent metals such (Fe and Zn) has been well documented in the literature. The advances achieved in nanotechnology have stimulated further research in the synthesis of bimetallic nanoparticles having as a goal the enhancement of reaction kinetics and improvement in metal usage. Bimetallic systems consisting of a base metal, such as Fe0 as the reductant, and a second dopant metal, such as nickel or palladium as the catalytic agent have been synthesized. These bimetallic nanoparticles have reaction rates that are orders of magnitude higher than the corresponding monometallic nanoparticles. It is hypothesized that nanostructured particles with catalytic dopant are able to alter the dechlorination pathways as reported in the literature.1 Specifically, the dechlorination pathways of chloro-organics by zerovalent metals that include the reductive R/β elimination (dihalo-elimination), hydrogenolysis (hydrogen substitution of halogen), or hydrogenation (multiple bond cleavage) are theorized to be shifted to the catalytic reductive hydrodechlorination mechanism. Our previously reported results and degradation studies by others using bimetallic nanosystems (Ni/Fe, Pd/Fe, etc.) have demonstrated increased surface area-normalized rate constants with the formation of less-chlorinated side-products that accounted for less than 5 wt% of the total initial carbon balance.2-6 This indeed proved that the bimetallic nanoparticles have higher reactivity toward the destruction of chloro-organics than the bulk and monometallic systems. In many studies, the heterogeneous degradation of chlorinated organics has been modeled using a pseudo first-order reaction mechanism.1-3,7,8 The observed first-order mechanism was used * To whom correspondence should be addressed. Tel: 859-257-2794. Fax: 859-323-1929. E-mail:
[email protected]. † Department of Chemical and Materials Engineering, University of Kentucky. ‡ Department of Chemistry, University of Kentucky.
due to its simple and excellent agreement with the kinetic data obtained experimentally. The surface area-normalized reaction rate constant determined by the model assumed that the particle surface has similar sorption energy and reactivity in the degradation reaction. This assumption has neglected the importance of surface heterogeneity that consists of reactive and nonreactive sorption sites. Gotpagar et al.9 introduced a term called the fraction actiVe sites parameter and demonstrated the ability of their model to predict the intrinsic reaction rate by assuming the new parameter to be constant. Another study by Burris et al. has assumed that the sorption of TCE is to nonreactive sites, effectively separating the nonreactive sorption from the reactive sites responsible for the degradation process.10,11 The authors have also shown that the quasi-equilibrium sorption is nonlinear and can be fitted using a generalized Langmuir isotherm. The synergistic characteristics of bimetallic systems involved the oxidation of iron as the active reactant in generating the electrons and hydrogen necessary for the degradation process. The hydrogen generation reaction is represented by Fe + 2H+ f Fe2+ + H2. The electrons and hydrogen generated are then utilized by a second dopant metal to catalyze the reaction through the formation of active surface metal-hydride as a powerful reductant, represented as 2 M + H2 f 2M-H, where M can be nickel or palladium. The anaerobic oxidation of bulk iron in the presence of different co-solutes has been studied by Reardon, but the analysis lacks the kinetic data needed to explain the correlation between iron oxidation and the dechlorination process.12 The oxidative nature of Fe/B nanoparticles was established by Liu et al. in terms of total hydrogen generation.13 The authors demonstrated the catalytic effect of Fe/B nanoparticles in dechlorinating TCE by its ability to utilize the hydrogen generated in the oxidation process. The reported results showed the importance of active surface sites that
10.1021/jp809098z CCC: $40.75 2009 American Chemical Society Published on Web 05/06/2009
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are capable of utilizing hydrogen in the dechlorination reaction by comparing the Fe/B nanoparticles with the commercial nanoiron, where the later sample showed inactivity toward hydrogen utilization. Another study by Schrick et al. reported that the enhanced degradation kinetics of Ni/ Fe nanoparticles was accompanied by hydrogen generated by anaerobic iron oxidation of the bimetallic system.3 On the contrary, the nano and bulk iron, which had reaction rates that were 2 orders of magnitude lower than Ni/Fe, had significantly lower hydrogen generation. The objectives of this study are (1) to analyze the hydrogen generation rate by the anaerobic oxidation of nanoiron and different iron-based bimetallic nano systems (Ni/Fe) with and without degradation process, (2) to correlate the hydrogen generation rate with the surface area-normalized rate constant of TCE degradation by Ni/Fe nanoparticles, (3) to analyze the surface sorption effect on the degradation kinetics by an extraction study, (4) to derive a mathematical model for the determination of the intrinsic degradation rate, and (5) to determine the effect of deactivation on reaction kinetics and products formation through a cycle study. 2. Reaction Mechanisms and Kinetic Models 2.1. TCE Degradation Mechanism under Batch Study. Similar to the fundamental understanding of heterogeneous catalytic reactions, the batch degradation process of TCE includes (a) the transport of reactants from the bulk solution to the liquid-solid interface followed by adsorption to surface sites, (b) catalytic hydrodechlorination on the active surface sites, and (c) desorption of the products and transport back to the bulk solution. (a1): Adsorption of TCE from the solution phase to the particle surface forming a π-bonded adsorbate: kal
C2HCl3 + S h C2HCl3-S kal
(1)
(a2): Formation of a di-σ-bonded species from the π-bonded intermediate:
C2HCl3-S + S f C2HCl*3 -S2
(2)
(b): Reductive hydrodechlorination reactions on the surface by active nickel hydride (Ni-H) with the formation of ethane as the final product.
5 5 Fe + 5 H2O + 5 Ni f Fe2+ + 5 OH- + 5 Ni-H 2 2 (3) 3 3 Fe f Fe2+ + 3e2 2 -
the batch solution study is illustrated in Figure 1. According to the schematic, TCE is sorbed first (physical sorption and/or chemisorption) on the particle’s surface, followed by the degradation reaction. The main equation with reaction coupled with adsorption can be represented as:
mNi / Fe
-
(c): Desorption of product from the surface and transfer back to the bulk solution phase.
(6)
Combining the hypothesized surface-mediated catalytic mechanisms, the overall TCE degradation with iron oxidation is written as follows:
dqTCE dCaq ) -kintmNi / FeqTCE - Vaq dt dt
(8)
where Caq (µmol/L) and qTCE (µmol/g) is the TCE aqueous phase concentration and surface-sorbed TCE per mass of metal, respectively; kint (h-1) is the intrinsic reaction rate constant; Vaq (L) is the total reaction volume; and mNi/Fe (g) is the total metal mass used. In addition to the sorption on the metal surface according to (1), TCE can also form inactive-sorbed species on nanoparticles according to the following (Si denotes inactive vacant surface sites on the particles): ka2
C2HCl3 + Si y\z C2HCl3-Si
(9)
ka2
The TCE aqueous concentration can then be written as the following:
(4)
C2HCl*3 -S2+5 Ni-H + 3 e f C2H*6 -S2 + 5 Ni+3Cl (5)
C2H*6 -S2 f C2H6 + 2 S
Figure 1. Schematic for batch TCE sorption and degradation reaction by the Ni/Fe (Ni ) 20 wt%) nanoparticles.
Vaq
dCaq ) -ka1VaqmNi / FeCaqCV - ka2VaqmNi / FeCaqCV + dt k-a1mNi / FeqTCE + k-a2mNi / FeqSi (10)
The first two terms correspond to the sorption of TCE to the metal surface and, the remaining two terms represent desorption of surface-sorbed TCE back to the solution phase. The definition of the rate constants can be found in nomenclature. Transmigration of the inactive-sorbed TCE in (9) between adjacent reactive sites can occur on the metal surface, and such phenomena have been observed and reported in the literature:14
kint
C2HCl3 + 4 Fe + 5 H2O 98 C2H6 + 4 Fe2+ + 5 OH- + 3 Cl- (7) 2.2. Model Derivation. Derivation of the mathematical model for the heterogeneous degradation reaction of TCE in
kSi
C2HCl3-Si h C2HCl3-Si k-Si
(11)
If we assume that the transmigration process of (11) is rapid, we can write:
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mNi / Fe
Tee et al.
dqsi = 0 = -ksimNi / Feqsi + k-simNi / FeqTCE dt
(12) and we get:
(
qsi ) KsiqTCE, Ksi )
k-si ksi
)
(13)
Substituting eq 13 into eq 10 and upon further rearrangements, we obtain the following TCE aqueous concentration equation:
Vaq
dCaq ) -KAmNi / FeCaqCV + KBmNi / FeqTCE dt
(14)
with:
KA ) (ka1 + ka2)Vaq, KB ) (k-a1 + k-a2Ksi)
(15)
Combining eqs 8 and 14, we obtain the representative equation for degradation of surface-sorbed TCE:
mNi / Fe
dqTCE ) -(kint + KB)mNi / FeqTCE + KAmNi / FeCaqCV dt (16)
The surface-sorbed TCE can be characterized by a Langmuirian-type quasi-sorption isotherm written as the following (the sorption of aqueous TCE to the surface is termed quasi because of the simultaneous sorption and degradation reactions):
qTCE(t) )
KQCaq(t) 1 + KCaq(t)
(17)
where K (L/µmol) and Q (µmol/g) are the sorption parameter and maximum sorption concentration, respectively. Equation 16 is transformed into an aqueous phase TCE concentration by using eq 17. After some mathematical operations, the final equation becomes:
dCaq KA CV ) -(kint + KB)Caq(1 + KCaq) C (1 + dt K Q aq KCaq)2 (18) This is the general model derived for a batch degradation reaction with surface sorption and rapid transmigration of inactive-sorbed TCE to nearby active sites. We have assumed that the reaction is first-order with respect to the surface-sorbed TCE concentration. The pseudo first-order reaction model is widely used and reported in the literature for TCE studies.1-3,7,8 The assumptions used in deriving the model for the complex heterogeneous surface reactions are summarized as follows: (a) Mass transfer resistance between the aqueous and solid boundary layer is negligible. (b) Degradation reactions for TCE and any surface-sorbed intermediates are irreversible. (c) TCE dechlorination occurs on the surface of the particles before desorption of products to the bulk solution. (d) Reactions are at isothermal conditions. (e) There is an absence of inter- and intraspecies competitive effects. (f) Gaseous products such as ethylene and ethane are assumed to accumulate only in the headspace because of their low solubility at the aqueous phase. 3. Experimental Section 3.1. Materials. Granular sodium borohydride (NaBH4 ) 99.99%), nickel chloride (NiCl2.6H2O ) 99.99%), ultrapure
grade tris(hydroxymethyl)-aminomethane (purity ) 99.9+%), and denatured anhydrous reagent grade ethanol (water < 0.0003%) were purchased from Aldrich Chemical Co. Ferrous chloride (assay as FeCl2 · 4H2O ) 102.0), trichloroethylene (C2HCl3 ) 99.99%), sodium hydroxide solution (certified as 0.2490-0.2510 N), hydrochloric acid solution (certified as 0.2 N), nitric acid (trace metal grade), hexane (GC-MS grade), and deionized ultrafiltered water (DIUF) were from Fischer Scientific. All chemicals were used as purchased. Deoxygenated DIUF was prepared by heating at ∼60 °C and bubbling with N2 gas overnight. 3.2. Bimetallic Nanoparticle Synthesis. The synthesis procedure for the bimetallic Ni/Fe nanoparticles was similar to that reported in the literature.2 In brief, Ni/Fe nanoparticles with different nickel contents were synthesized using sodium borohydride as the reducing agent from an aqueous mixture of Fe2+ and Ni2+. The final dark colloidal particles were washed with ethanol and deoxygenated DIUF water followed by filtration. The nanoparticles were vacuum-dried at room temperature overnight and used immediately for the TCE study. BET surface area analysis for the prepared Ni/Fe nanoparticles was conducted using Micromeritics ASAP 2000 model. 3.3. Batch Degradation Study. A 120-mL EPA certified vial with a Mininert septum valve was used for the TCE and DCB batch degradation studies. Two initial aqueous TCE concentrations (10 and 500 mg/L) occupying a total volume of 40 mL with 80 mL headspace were used in the TCE study. In the study of the effect of pH on the hydrogen generation rate, the initial solution pH of 6.50 was adjusted to 5.0 and 8.0 using HCl or NaOH and tris(hydroxymethyl)-aminomethane (TRIS) as buffer. This organic buffer was chosen because of its reported weak interaction with ferrous ions in solution.1,13 Bimetallic Ni/Fe nanoparticles of 0.10 g were added to the TCE solution under nitrogen purging, and the container was immediately sealed with the Mininert valve. For the TCE cycle study, a new set of seven batches of TCE solutions were used for each of the cycle analysis. A volume of 4 mL of reacted sample from each of the TCE solutions at the end of the first cycle (120 min) was withdrawn, and another 4 mL of stock TCE solution (100 mg/ L) was added back into the vial for a fresh TCE initial concentration of 10 mg/L. A new septum vial was used at the beginning of each cycle study. The same procedure was used at the end of second cycle (140 min) for the third cycle analysis. 3.4. TCE Analysis. For the determination of the aqueous phase TCE concentration, a known amount of sample solution from the reaction vial was injected into another 42-mL total volume septum-sealed glass vial filled with 40 mL DIUF water at specific reaction time intervals. The instrumentation technique and equipment were similar to those reported in a previous TCE study. To determine the total TCE concentration (aqueous phase plus solid phase), a specific sample volume was withdrawn from the same reaction vial until 20 mL of solution was left in the container. A volume of 20 mL of hexane was added into the vial containing both the TCE solution and nanoparticles. The extraction process was allowed to run overnight. The total TCE concentration was determined using the hexane phase solution and analyzed by GC-ECD using Varian CP3800 equipped with an RTX-624 capillary column from Restek coupled with a CP8300 autosampler. A standard curve for chloride ranging from 1.00 to 50.00 mg/L was constructed from Ultra Scientific’s TCE standard (100.40 ( 0.50 mg/L in methanol). Periodic curvecheck was conducted at 15 and 25 mg/L Cl prepared from dilution of the same standard.
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J. Phys. Chem. C, Vol. 113, No. 22, 2009 9457 headspace at different time intervals (5 and 10 h), and its concentration was analyzed using the H2 standard curve. Neglecting the solubility of hydrogen in water, percent errors between the initial and time-lapsed value were determined to be less than 5%. For the TCE cycle study, gaseous degradation products, such as ethylene and ethane, were analyzed using another calibration curve constructed using standard gas (1% of acetylene, carbon monoxide, carbon dioxide, ethane, ethylene, methane, and the balance nitrogen) from Scott Specialty Gases. 4. Results and Discussion 4.1. Hydrogen Generation by Ni/Fe Nanoparticles. Figure 2 shows the total amount of hydrogen generated by Ni/Fe nanoparticles with different nickel content normalized with iron mass under anaerobic condition. The results represent the sum of the gas phase and aqueous phase hydrogen gas produced:
Figure 2. Total hydrogen generation normalized with iron for different bimetallic Ni/Fe nanoparticles versus time under anaerobic aqueous solution. Ni/Fe ) 0.1 g, volume ) 40 mL, headspace ) 80 mL, initial pH ) 6.5.
H2 ) H2(g) + H2(aq)
where the gas phase hydrogen, H2 (g), is determined directly from the headspace GC-TCD analysis, and the aqueous phase hydrogen, H2 (aq), is obtained as follows:
H2(aq) ) (7.515 × 10-6)H2(g)RTMH2O / VHS
Figure 3. Iron-normalized hydrogen generation of iron nanoparticles with (open diamond symbols) and without acid treatment (solid diamond symbols) under anaerobic condition. Nanoiron ) 0.1 g, volume ) 40 mL, headspace ) 80 mL, initial pH ) 6.5.
3.5. Hydrogen Gas Analysis. Quantification of hydrogen evolved due to the oxidation of nanoiron and bimetallic nanoparticles (Ni/Fe) was conducted by GC-TCD analysis using an Agilent 6890N Network GC System interfaced with ChemStations software and equipped with a Carboxen 1004 micropacked column from Supelco. A Hamilton airtight lock syringe was used to withdraw 1 mL of headspace volume from the vials and injected directly to the manual injection port. The total moles of H2 generated was correlated with a five-point calibration curve constructed using standard gas of 1% H2 in nitrogen (analytical accuracy ) ( 0.02%) from Scott Specialty Gasses. To check the accuracy of the calibration curve, a known mass of Fisher electrolytic iron was digested in a 120 mL-vial with 40 mL of 1.0 M nitric acid. A volume of 1 mL of the headspace sample was analyzed, and the total H2 generated was determined using the standard curve. The theoretical value and calculated results using the standard curve were within 96.50 ( 0.50% accuracy. Imbalance may be due to the formation of surface impurities, such as oxides upon exposure of Fisher Fe in the air. Leakage through the Mininert septum vial was checked by injecting a known volume of H2 standard gas into the vial filled with 40 mL of deoxygenated water and 80 mL headspace. The headspace H2 gas was redrawn from the
(19)
(20)
where 7.515 × 10-6 mol kg-1 kPa-1 is the solubility of hydrogen in water at 100 kPa at 25 °C; R is the gas constant; T is the room temperature; and MH2O and VHS are the mass of the total solution and headspace volume, respectively. Solid phase hydrogen due to the diffusion and entrapment of H2 by the nanoparticles is assumed to be negligible in short-term analysis (2-h duration). This assumption is justifiable since the solubility of H2 in metal particles is relatively low (
