Environmental Applications of Nanoscale and Microscale Reactive

Downloaded by CORNELL UNIV on June 11, 2017 | http://pubs.acs.org Publication Date (Web): December 20, 2009 | doi: 10.1021/bk-2009-1027.ch014

Chapter 14

Practical Applications of Bimetallic Nanoiron Particles for Reductive Dehalogenation of Haloorganics: Prospects and Challenges Teik-Thye Lim and Bao-Wei Zhu School of Civil and Environmental Engineering, Nanyang Technological University, Republic of Singapore

This chapter provides a brief review of the potential of using Pd/Fe nanoparticles to dehalogenate haloaliphatics and chloroaromatics, and discusses the recent findings on reactivities of Pd/Fe nanoparticles in various aqueous systems. The prevailing method of synthesis and characteristics of Pd/Fe are described. The role of Pd in the bimetallic particles in enhancing the reductive dehalogenation of chlorinated benzenes and halogenated methanes are discussed. The influences of various matrix species such as inorganic anions and amphiphilic molecules on the dechlorination process and kinetics are examined. Finally, challenges facing field application of the Pd/Fe nanoparticles are deliberated for the future research to address.

© 2009 American Chemical Society

245

Geiger and Carvalho-Knighton; Environmental Applications of Nanoscale and Microscale Reactive Metal Particles ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

246


Introduction Halogenated organic compounds (HOCs) including aliphatics and aromatics, are widely used as solvents in degreasing, cleaning and extracting, as synthetic intermediates for plastics, dyes and pesticides, and as fire retardant chemicals, moth repellents and deodorants. They are known toxic chemicals, and some are potential carcinogens. Due to their intensive usage in industries and households, and recalcitrance in natural environments, they are widespread contaminants found in soil, sediment, groundwater and surface water. Their continual releases into these environmental compartments have resulted in their increasing bioaccumulation in biota, threatening human and ecosystem health. To date, there are only a handful of effective technologies to degrade HOCs. One promising degradation pathway for HOCs is through abiotic reductive transformation that certainly reduces the degrees of halogenation and often enhances biodegradability of their intermediates and end products. In the last two decades, various types of zero-valent iron (ZVI) particles have been intensively investigated for their reactivities towards HOCs and other toxic oxidized organics and inorganics. Initially, the granular form of ZVI were introduced in permeable reactive barrier (PRB) systems to treat groundwater plume contaminated with chlorinated organics (1, 2). To improve over the PRB treatment system, nanoscale ZVI particles (nZVI) have been synthesized, and their field applications in treating contaminant source zone have been well documented (3). The nZVI particles offers several advantages compared to the granular ZVI for in-situ treatment of contaminated subsurface, such as higher reactivity due to greater density of reactive surface sites (and possibly with higher intrinsic reactivity too), low material cost, higher mobility, and flexible delivery into deep or stratified contaminated source zones in aquifer. In addition, the nZVI can be dispersed easily, and this allows its ex-situ applications in slurry or expanded bed reactors. To further enhance the reactivity of nZVI making the nanoparticles capable of hydrodechlorinating some persistent haloaromatics, the nanoscale ironbimetallic composite particle, has been synthesized in recent years. The bimetallic nanoiron particle (BNIP) has a noble metal (e.g., Pd, Pt, Ni, or Ag) deposited on the nZVI surface. These noble metals have lower hydrogen overpotentials compared to iron. In such BNIP composite system, the noble metal serves as catalyst to catalyze the reductive transformation of the oxidized organics, while the nZVI functions as electron donor or in some cases as catalyst too through its iron oxide shell (4). Among these catalytic metals, Pd and Ni are the most commonly used in BNIP in the past years. The BNIPs show enhanced reactivities towards various HOCs, including chloroaromatics (5-10). The various toxic organics that can be reductively transformed by the BNIP or bare (monometallic) nZVI are presented in Figure 1. In particular, the Pd/Fe particles with a Pd percentage as low as 0.01% (w/w) has been found still exhibiting catalytic hydrodechlorination of chloroaromatics that would otherwise inert towards the bare nZVI (9).



247

Figure 1. Reductively degradable organics with nZVI or BNIPs. Figure 2 shows a schematic illustrating the role of Pd in the Pd/Fe particle during reductive transformation of a chlorinated benzene. In the composite, Pd serves as hydrogen collector and subsequently catalyzes the hydrodechlorination reaction (11). It has also been suggested that a galvanic couple can form between the two metals in the bimetallic composite particles, and it is critical for the generation of the activated atomic hydrogen species (H*) (6, 7). Pd, the lower hydrogen overpotential metal in the iron/water system, could form a galvanic cell with iron. Thus, the corrosion rate of anodic Fe increased in the Pd/Fe system, resulting in higher hydrogen evolution and fresher iron surface condition (12). Zhang et al. (5) indicated that physically mixing of Pd and Fe particles produced no positive effects on trichloroethylene dechlorination. A close contact of the Fe with the catalyst metal in the bimetallic particle system is essential to obtain a positive effect, and through coating the catalyst on the iron surface or forming bimetal alloy can remarkly enhance reactivity of the system.

Synthesis of Pd/Fe Typically, BNIPs are synthesized by co-reduction of the ionic precursors of the two metals (6) or post-deposition of the second metal on the surface of the fresh nZVI (5, 13). The latter is commonly adopted to synthesize Pd/Fe in which Pd is only a minute amount in the bimetallic particle, and it will ensure Pd deposition on the nZVI surface, forming discrete islets. In this synthesis route, the nZVI is first synthesized by reduction with NaBH4, and the palladized nZVI is produced by soaking the freshly prepared nZVI particles in an acetone solution of palladium acetate. The synthesis processes are depicted in the following reactions: Fe(H 2 O) 4

2+

+ 2BH −4 + 2H 2 O → Fe 0 ↓ +2B(OH) 3 + 7H 2 ↑

Pd 2 + + Fe 0 → Pd 0 + Fe 2 +

(1)

(2)


248 Cl Cl

+ 3Cl-

Cl

H+

H* Pd e-

Fe2+

Fe0


H2

Fe2+

e-

H2O

Figure 2. Hypothesized dechlorination mechanism for a chlorinated benzenes with Pd/Fe particle. Samples with different Pd contents (1.0%, 0.5%, 0.2%, 0.1%, 0.05% and 0.01% w/w) have been synthesized by the authors (9, 14). The typical synthesis procedure is as follows. 20 mL of 12.41 g/L FeSO4·7H2O solution was added to a 70 mL bottle; 3.8 M NaOH solution was used to precipitate Fe(OH)2. The Fe(OH)2 formed was then reduced to ZVI when 20 mL 0.26 M NaBH4 was added dropwise. The ZVI particles formed were isolated by centrifugation. To remove the remaining NaBH4, the ZVI particles were rinsed with 50 mL water for each bottle. After discarding the rinsate, 10 mL acetone was added and mixed rigorously with the ZVI particles in each bottle, and then an appropriate amount of palladium (II) acetate was added. The Pd(II) would be reduced to Pdo as illustrated by eq. 2, and deposited on the ZVI.

Characteristics The actual Pd percents (w/w) in the synthesized Pd/Fe samples normally agreed with the stoichiometry illustrated in eqs. 1 and 2. The specific surface area of the various synthesized Pd/Fe particles with different Pd percentages was 27 ± 3 m2/g. The point of zero charge of the Pd/Fe was around pH 8.1. The Xray diffraction (XRD) patterns of the fresh sample corresponded to that of the body-centered cubic α-Fe0 while the XRD patterns of aged samples showed characteristic peaks associated with iron oxides (9). Through scanning electron microscope observation, the fresh Pd/Fe sample usually showed a homogenous chain-like texture, while the reacted sample exhibited platelet shaped crystals. Through transmission electron microscope (TEM), it could be confirmed that the fresh, spherical, Pd/Fe nanoparticles aggregated together forming chain-like structure (Figure 3a). The diameters of


249


individual particles were typically 5 to 80 nm. In the aged Pd/Fe sample, authigenic iron (hydr)oxides formed on the surface of the particles (Figure 3b).

(a)

(b)

Figure 3. TEM images showing (a) the fresh Pd/Fe sample and (b) the aged Pd/Fe sample. From X-ray photoelectron spectroscopy (XPS) analysis, the Fe 2p spectra of the fresh and reacted samples show a similar shape with binding energies of Fe 2p1/2 = 724.7 eV and Fe 2p3/2 = 710.9 eV (Figure 4a), corresponding to the oxidized iron. The two peaks at binding energies of 340.6 and 335.2 eV (Figure 4b) are associated with Pd0 deposited on ZVI (15). The Ar+ sputtering decreased the intensity of O and C peaks, while enhanced those for Fe and Pd. After Ar+ sputtering, the peak at Fe 2p3/2 = 706.9 eV and Fe 2p1/2 = 719.9 eV emerged for the fresh and reacted sample. This confirms the core/shell structure of the Pd/Fe particles, and is consistent with the results found on ZVI particles described by the previous researchers (16, 17). The XPS analysis indicates that thickness of the iron oxides shell increased after reaction while the ZVI core shrank concomitantly. The process of oxide film formation on the ZVI and its evolution has been discussed by Huang and Zhang (18) and Noubactep (19). They suggested that a stratified ZVI corrosion coating could be formed after reaction in water, for which the outer and middle layers usually comprised both FeOOH and Fe3O4, while the inner layer mainly consisted of Fe3O4. Thus, the film is multi-layered with density increasing from the outer surface towards the core as the oxyhydroxides become more aged and less porous (19).


250

(a)

Fe 2p1/2

Fe 2p3/2

Intensity (cps)

(1)

(2) (3)

(4) 735

730

725

720

715

710

705

700

(b) Pd 3d3/2

Intensity (cps)


Binding Energy (eV)

350

345

340

Pd 3d5/2

335

330

Binding Energy (eV)

Figure 4. (a) Fe 2p, and (b) Pd 3d X-ray photoelectron spectra of the fresh Pd/Fe sample surface: (1) Fresh sample, (2) Fresh sample after 5 min of Ar+ sputtering, (3) Aged sample, and (4) Aged sample after 5 min of Ar+ sputtering (Reproduced with permission from reference 9. Copyright 2007 ACS).

Reactivity of Pd/Fe Nanoparticle Reactivity Monochlorobenzene (MCB), dichlorobenzenes (DCBs), and 1,2,4trichlorobenzene (124TCB) dechlorination experiments in distilled water have been investigated with the freshly synthesized and aged Pd/Fe particles by the authors (9,20). The chlorinated benzenes could be completely reduced to benzene, following the pseudo-first-order kinetic model or LangmuirHinshelwood kinetic model. The reaction rates, when expressed with kSA (specific reduction rate constant, L·min-1·m-2), followed the order TCB < DCBs < MCB (9). With the bare nZVI, there was no dechlorination process observed, and the dechlorination rate increase almost linearly with Pd content up to 0.5%


251

100

Concentration Ratio (%)


Pd (9). This finding suggests that in dechlorination of the chlorinated benzenes, Pd is the only reactive site in the Pd/Fe particles. Figure 5 shows the result of 12DCB dechlorination reaction by 0.1% Pd/Fe, together with their simulated degradation curves using the pseudo first-order kinetic model. The DCB was dechlorinated to half of its initial concentration within 5 min. The DCB might be dechlorinated to benzene following a stepwise (DCB→MCB→B) or concerted (DCB→B) pathway. The dechlorination rates among the isomeric DCBs followed the order 14DCB > 13DCB ≥ 12DCB (9), which is consistent with the finding on gas phase catalytic hydrodechlorination of DCBs over Ni/SiO2 (21). The lower dechlorination rate constant of 12DCB compared with 14DCB was indicative of steric constraint effect.

80 60

Benzene MCB 12DCB Mass Balance

40 20 0

0

5

10

15

20

25

30

Time (min)

Figure 5. Dechlorination reaction time course for 12DCB with 0.1% Pd/Fe (Reproduced with permission from reference 9. Copyright 2007 ACS). For dehalogenation of chlorinated and brominated methanes, adding Pd over ZVI could significantly enhance the reductive dehalogenation rates (8,12). Table I shows a comparison of reductive dehalogenation rates of carbon tetrachloride (CTC), chloroform (CF) and carbon tetrabromide (CTB), with palladized and bare nZVI. The reduction rate is higher for CTC than CF. With 0.2%Pd/Fe and bare nZVI, the rates were faster for CTB than CTC. It is worth noting that in this study, although the increase in Pd content could shorten the half life of the reduction reaction for the halogenated methanes, the intermediate byproduct distribution was not significantly affected (12). On the other hand, excessive Pd might not be beneficial to the dehalogenation reaction.


252 Table I. Values of reduction rate constants of halogenated methanes with nanoscale Pd/Fe and Fe particles Particles

Rate Parameters kobs (h-1)

0.2% Pd/Fe


CTB

0.47

17.41

kSA (l h m )

1.61E-1

7.11E-3

2.63E-1

t1/2 (h)

0.07

1.47

0.04

56.04

0.89

>40

kSA (l h m )

8.47E-1

1.35E-2

>6.05E-1

t1/2 (h)

0.01

0.78

Environmental Applications of Nanoscale and Microscale Reactive

Recommend Documents