Environ. Sci. Technol. 2008, 42, 4513–4519
Influences of Amphiphiles on Dechlorination of a Trichlorobenzene by Nanoscale Pd/Fe: Adsorption, Reaction Kinetics, and Interfacial Interactions BAO-WEI ZHU, TEIK-THYE LIM,* AND JING FENG School of Civil and Environmental Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Republic of Singapore
Received January 23, 2008. Revised manuscript received March 18, 2008. Accepted March 25, 2008.
The influences of amphiphiles on the catalytic dechlorination of 1,2,4-trichlorobenzene (124TCB) by the nanoscale Pd/Fe particles were comprehensively examined. The fresh and reacted Pd/ Fe particles were characterized with XRD, TEM, SEM, FTIR spectrometry, and goniometry. Adsorption of amphiphiles on the Pd/Fe particles, iron dissolution, and H2 evolution in the Pd/ Fe-water system were quantified to expound the influences of the various amphiphiles on the dechlorination process. The Langmuir-Hinshelwood model is used to elucidate the dechlorination kinetics, and it provides insight into the influence of amphiphiles on 124TCB partitioning to the interfacial layer andtheresultingdechlorinationrates.Therateconstantsincreased by a factor of 1.5-2.5 with the presence of cationic cetyltrimethylammonium bromide (CTAB). In the anionic sodium deodecyl sulfate (SDS) or nonionic nonylphenol ethoxylate (NPE) and octylphenolpoly (ethylene glycol ether)x (TX-100) surfactant solutions, the 124TCB dechlorination rates were slightly increased over those observed in ultrapure water. However, when concentrations of the surfactants were above their CMCs, the dechlorination rates decreased. The findings also showed that DPC (dodecylpyridinium chloride) and NOM (natural organic matter) might be the competitive H2 acceptors to 124TCB, and they significantly retarded its catalytic dechlorination by the Pd/Fe particles. CTAB at a concentration below the CMC appeared to be the most benign to the 124TCB dechlorination.
Introduction Zerovalent iron (ZVI) technology has been proven to be technically and economically viable to remove halogenated organic compounds (HOCs) from groundwater (1–4). It has also been proven that the reductive dehalogenation reaction involving HOCs and ZVI is a surface-mediated reaction, which is likely to be influenced by competitive adsorbates that are present in the system (5–8). Environmental amphiphiles, such as surfactants and natural organic matter (NOM), have affinity for both HOCs and ZVI surface sites and thus may affect their interactions either favorably or adversely. It is generally believed that amphiphiles may affect HOCs dechlorination in the iron-water system through various ways, including * Corresponding author tel: +65-6790-6933; fax: +65-6791-0676; e-mail:
[email protected]. 10.1021/es800227r CCC: $40.75
Published on Web 05/09/2008
2008 American Chemical Society
enhanced solubilization, enhanced sorption, competitive sorption, and electron transfer mediation (5). Surfactants are an important group of amphiphiles that have been extensively used in household, agriculture, and many industrial applications. In contaminated site remediation, surfactants are used to mobilize nonaqueous phase liquids. Although some efforts have been taken to investigate the influences of various surfactants on the HOC dechlorination reaction with ZVI, inconsistent findings were reported because of the complex surfactant types and different reaction conditions investigated. Sayles et al. (9) reported that nonionic Triton X-114 increased the reduction rate of DDT with ZVI 2-fold compared to that without the surfactant. The ZVI modified with a cationic surfactant hexadecyltrimethylammonium (HDTMA) has been shown to greatly increase perchloroethylene (PCE) reduction rates because of enhanced sorption of PCE on the ZVI surface (10–12). On the contrary, a study has reported that modifying the ZVI surface with an anionic surfactant SDS (sodium dodecyl sulfate) did not enhance the PCE reductive dechlorination (10). Many studies showed that the influences of surfactants on HOC dechlorination also depended on surfactant concentration. Loraine (13) found that TX-100 at concentrations below its critical micelle concentration (CMC) enhanced PCE dechlorination rate but at above CMC exhibited a reversed effect. He also reported that below the CMC, SDS showed a negligible effect on the HOC reductive dechlorination, but the rate decreased at a high SDS concentration far above CMC possibly because of partitioning of the HOCs in mobile micelles. Tratnyek et al. (5) reported a continual decrease in the rate of nitrobenzene degradation by ZVI with cationic DPC (dodecylpyridinium chloride) as the DPC concentration increased (below CMC). NOM, or specifically, humic substances that comprise both humic and fulvic acids, are the most omnipresent natural amphiphiles in aquatic environments (14). Therefore, NOM constitutes another important aqueous matrix that can influence the dechlorination reaction by ZVI. It has been reported that NOM could decrease the HOC dechlorination rate because of competitive adsorption on reactive sites (15). However, NOM may involve in redox reaction in the iron-water system by acting as electron shuttles and thus accelerating the contaminant reduction rate (5, 16, 17). Besides, NOM can expose more available reactive surface sites by enhancing dissolution of the passive iron oxides film, thereby accelerating the contaminant reduction (17). Despites these previous studies, the current understanding on the influences of various amphiphiles on HOC dechlorination in the iron-water system is still vague and evolving due to the complex interactions among the ZVI surface and the cosolutes. Most of the previous studies are also based on microsized ZVI particles. Compared with a microscale ZVI, a nanoscale ZVI possesses a much higher specific surface area and denser reactive sites, and thus steric congestion in the vicinity of iron-water interface may be significant. In the case of catalytic dehalogenation of haloaromatics with Pd/Fe where Pd may be the only reactive site involved, the interaction between the amphiphiles and the Pd/Fe can be different from that reported for the ZVI systems. Thus far, the reports on the influence of amphiphiles on catalytic dechlorination of HOCs on metal surface are scarce (7). This study aimed at identifying the influences of the amphiphiles on the dechlorination of 1,2,4-trichlorobenzene (124TCB) by the nanoscale Pd/Fe particles. To gain insights into the mechanisms whereby the amphiphiles affect the dechlorination reaction, a kinetic model based on mechaVOL. 42, NO. 12, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Properties of the Surfactants
nistic considerations has been used to elucidate the reaction. A series of deliberate experiments were also carried out to provide further evidence on the mechanisms in play and their effects on surface-surfactant-contaminant interactions.
Experimental Section Synthesis and Characterizations. Nanoscale ZVI was synthesized by reducing Fe2+ with NaBH4, and the 0.1%(w/w) Pd/Fe was prepared by soaking the freshly prepared ZVI particles in an acetone solution of palladium acetate. The detailed procedure has been described in our previous report (18). Brunauer-Emmett-Teller (BET) surface area of the Pd/ Fe particles was determined with Quanta Chrome autosorb automated gas absorption system. XRD analyses of the fresh and reacted samples were performed using Bruker AXS D8 advanced X-ray diffractometer (λ ) 1.5418 Å). To identify the surface hydrophilicity of the fresh and reacted particles, contact angles between the Pd/Fe surface and the droplets of ultrapure water or surfactant solutions were measured by the sessile drop method using a goniometer (Contact Angle System OCA 20) with a microsyringe. Surface charges of the Pd/Fe particles were determined using the alkalimetric and acidimetric titration method (19). Transmission electron microscope (TEM) images were obtained with a JEM 2010 microscope. Scanning electron microscope (SEM) images were obtained through a Polaron LT7400 microscope. Fourier transform infrared spectroscopy (FTIR) spectra of the reacted samples (after rinse) were obtained with Spectrum GX FTIR system. The surface tension measurements were performed with a Kruss K10T maximum pull digital tensiometer using the du Nouy ring method with a Pt/Ir ring. Adsorption Isotherms. Adsorptions of NOM and surfactants (with their properties shown in Table 1) on the nanoscale Pd/Fe particles were quantified using the depletion method. To each 100 mL glass vial were added 0.1 g of Pd/Fe and 50 mL of amphiphile solutions with known initial concentrations (ranging from 0 to 10 CMC for surfactants and 0 to 1000 mg/L for NOM). The vials were shaken on an 4514
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orbital shaker for 1 h to achieve equilibrium (20). After centrifugation, the supernatant was analyzed for concentrations of the amphiphiles. Aqueous SDS, CTAB, and TX-100 concentrations were determined with a TOC analyzer (ASI-V Shimadzu) after dilution to TOC < 1000 mg/L with ultrapure water. Aqueous NPE and DPC concentrations were analyzed with a UV-vis spectrophotometer (UV-1700 PharmaSpec, Shimadzu) at wavelengths of 225 and 275 nm, respectively. All solution samples were filtered through a 0.2 µm filter before analysis and triplicate experiments were conducted to get the average values. Dechlorination Experiments. The batch experiment system was used to investigate the influence of the amphiphiles on the 124TCB dechlorination reaction by the Pd/ Fe. A 50 mg amount of the freshly synthesized Pd/Fe particles was added with 69.5 mL of surfactant or NOM solution to each 70-mL serum bottle. To account for adsorption loss and to ensure that the surfactant molecules remaining in the bulk solution could still form micelles, up to 10 CMC of the surfactant concentration was used. The reaction was commenced by spiking 0.5 mL of 124TCB methanol solution, resulting in an initial 124TCB concentration of 110 µM and a Pd/Fe loading of 0.71 g/L, and no headspace. These bottles were placed on the orbital shaker (250 rpm) at room temperature (23 ( 1 °C) in the dark. At each specific time interval, one of the bottles was sacrificed to withdraw 2 mL of aqueous sample by a gastight syringe with a 0.2 µm filter for analysis of the parent compound and its products. The amounts of 124TCB in the blank experiments remained relatively constant up to 30 h, which indicated insignificant leak or sorption to glass wall of the serum bottle, syringe, and filter. The initial and final pH values were also recorded. The whole series of experiments were repeated twice to confirm good reproducibility of the results. Concentrations of 124TCB and its daughter products were analyzed with a high performance liquid chromatograph (HPLC) using a Perkin-Elmer chromatograph equipped with a Model PE 785A UV/vis detector. The detection wavelength was set at 210 nm and the LC column was an Inertsil ODS-3
column, 25 × 4.6 mm with 4 µm diameter particle size silica. The mobile phase was acetonitrile/water (65:35), with a flow rate of 1.0 mL/min and an injection volume of 25 µL. Iron Release and H2 Evolution. To understand the surface-mediated reaction and the surface-amphiphilecontaminant interactions through a more rigorous approach, additional batch experiments were carried out to quantify iron release and H2 evolution. A 0.248 g amount of the freshly synthesized Pd/Fe and ultrapure water or surfactant solutions (at their respective CMCs) were poured into the 350-mL bottles leaving no headspace, resulting in an initial Pd/Fe loading of 0.71 g/L. At each specific time interval, 2 mL of sample was withdrawn through the syringe with a 0.2 µm filter paper for dissolved iron analysis. Iron concentration was measured by inductively coupled plasma-optical emission spectroscopy (ICP-OES, Optima 2000 DV). For the experiment with DPC solution, over a period of 170 h, milky oil-in-water emulsion was observed, indicating that DPC might have reacted with the Pd/Fe particles. The DPC solutions before and after reaction were extracted using n-hexane and compounds were identified by GC-MS (Agilent 6890 series GC, 5973 Network MS) equipped with an Agilent DB-5 column. H2 evolution was quantified in a batch experiment with 70-mL serum bottles. A 50 mg amount of the fresh Pd/Fe particles was added together with 60 mL of surfactant or NOM solution, leaving a 10 mL headspace initially filled with N2 gas. The bottles were sealed with Teflon-lined rubber septa and aluminum caps and shaken on the orbital shaker (250 rpm) in the dark. The gas evolved was gathered through silicone tubes to the inverted water-filled graded tubes, and the volumes of gas produced were recorded at appropriate time intervals over 48 h.
Results and Discussion Characterizations. The BET surface area of the Pd/Fe particles was around 27 m2 g-1. The XRD spectrum (Supporting Information, Figure S1) of the fresh Pd/Fe sample indicated cubic ZVI (R-Fe0). After reaction, peaks of iron oxides (magnetite or maghemite) emerged. The broad peaks reveal the existence of amorphous phases both in the fresh and reacted samples, and the phenomenon was the most obvious in the reacted sample after reaction in SDS solution. Spherical Pd/Fe particles aggregated into chains were observed through TEM (Figure S2a). The chain structure was also observed under SEM, and the morphology changed after reaction in various surfactant solutions (Figure S2). The contact angles between the ultrapure water drop and the tablets of compressed particles were 21° and 77°, for the fresh sample and reacted sample (sample after 7 days of reaction), respectively (Figure S3). This indicated that the fresh particles were hydrophilic, i.e., water wettable, but became increasingly hydrophobic when reacted with water. In the context of HOC degradation, the hydrophilic surface property is unfavorable because HOCs are usually hydrophobic compounds, but this can be improved by introducing amphiphiles. The surfactant molecules could lower the liquid-solid and liquid-air surface energies (as depicted in Figure S4), thus facilitating the access of the particle surface to substrate. Adsorption of the Amphiphiles onto the Nanoscale Pd/ Fe Particles. The adsorption isotherms for various amphiphiles, plotted on a log-log scale, are shown in Figure 1a. A typical four-region adsorption isotherm (Figure 1b) for a surfactant adsorption on a hydrophilic solid surface along with the postulated orientation and aggregation of the adsorbed surfactant molecules on the surface is also shown for comparison (20). In general, the surfactants were increasingly adsorbed by the Pd/Fe when their concentrations increased, driven by surfactant-solid electrostatic attraction
FIGURE 1. (a) Adsorption isotherms of SDS, CTAB, NPE, DPC, and NOM onto Pd/Fe sample (arrows indicate surfactant CMCs). (b) Schematic representation of a typical surfactant adsorption isotherm. and hydrophobic interactions between surfactant tails. Above CMCs, the amount of adsorbed surfactant approached a plateau and aqueous micelles formed. NOM adsorption probably occurred through ligand exchange mechanism of its charged moieties with the iron oxide film on Pd/Fe (17). The orientation of the adsorbed surfactant molecules and formations of admicelles (adsorbed amphiphile molecules) or hemimicelles on the surface can influence the amount and the ways a substrate (contaminant) is partitioned in the solid-water interfacial layer. This consequently determines whether the presence of the surfactant in the solid-water interface would adversely or beneficially affect the substrate interaction with the metal surface sites. For example, the adsorbed surfactant might inhibit the reductive dechlorination of a HOC by blocking the reactive sites, or promote the HOC dechlorination by enhancing the surface affinity for it. The orientation of the adsorbed surfactant molecules not only depends on the surfactant concentration but also the metal surface charge since the hydrophilic portion (head) of surfactants can be nonionic, cationic, or anionic. Figure S5 shows that the point of zero charge (PZC) of the nanoscale Pd/Fe was at pH 8.1, which is consistent with a previous report (21). It was observed that the PZC of the fresh particles shifted toward pH 9.0 with the presence of surfactants. In the dechlorination experiments, consistent pH values of 8.0-9.0 were observed, indicating that the Pd/Fe particles probably possessed slightly more negative charges throughout the course of dechlorination reaction. VOL. 42, NO. 12, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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To elucidate the reaction kinetics on the amphiphilemodified Pd/Fe surface, Langmuir-Hinshelwood kinetic model is used for modeling the surface reaction (3, 13), in conjunction with the pseudo-first-order kinetic model. Neglecting mass transfer limitations in the batch experiment because of rigorous mixing, the following surface reaction processes are perceived. The dechlorination of the 124TCB substrate (P) must proceed via formation of a precursor complex (P-Pd/Fe) at the metal surface, followed by a ratelimiting reductive reaction on the surface to form products (eq 1). This can be inferred from our previous study (18) that found a large variation among the kobs values of 124TCB and the three dichlorobenzene isomers. The kinetics of such process can be described by eq 2, if the interspecies competitive effects are assumed insignificant (details of the model derivation are presented in Supporting Information). KA
FIGURE 2. (a) Dechlorination reaction time course for 124TCB with 0.1% Pd/Fe in NPE solution at CMC. Error bars represent the standard deviation for three independent tests. (b) Dechlorination of 124TCB with Pd/Fe in water with a surfactant at CMC, or 50 mg/ L NOM. Solid lines represent the simulated curves based on the Langmuir-Hinshelwood model. Dechlorination Reaction and Kinetics. Figure 2a shows the results of 124TCB dechlorination with the Pd/Fe in the NPE solution at CMC. The 124TCB concentration decreased exponentially with time, with concomitant appearances of intermediates (12DCB, 13DCB, 14DCB, and MCB) and end product (benzene). A similar pattern of dechlorination time courses was also observed in the solutions of other amphiphiles. For the chlorinated benzenes, our earlier study has found that their dechlorination reactions (in ultrapure water) cannot proceed without Pd (i.e., Pd is the only reactive site on Pd/Fe surface) (18). Thus, the dechlorination reaction is surface-mediated whereby the Pd-activated H* species attacks the π-bond of the benzene ring via electrophilic H addition. It is worth noting that the pseudo-first-order rate constant (kobs) widely used to express the rate of reaction kinetics in the ZVI literature is often determined based on the disappearance rate of the substrate in the bulk solution. However, for the substrate to be dechlorinated, the substrate must react with the surface reactive site. In the context of the present study where an amphiphilic cosolute was present, there were two possible mechanisms responsible for the substrate (124TCB) disappearance from the bulk solution (13): (i) dechlorination reaction on the surface reactive sites of Pd/Fe, and (ii) partitioning to the adsorbed amphiphiles without dechlorination by the Pd/Fe. Which one would be the predominant was dependent on the role of amphiphiles in the surface-liquid interfacial region. 4516
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kr
P + Pd ⁄ Fe 98 P - Pd ⁄ Fe 98 products
(1)
KAkrSt dC )C dt 1 + KAC
(2)
where KA (µM-1) is the Langmuir sorption coefficient of the substrate on reactive sites; kr (min-1) is the rate constant for the decay of the substrate at reactive sites; St (µM) is the abundance of reactive sites; and C (µM) is the substrate concentration in aqueous phase. This model assumes that (1) the substrate adsorbed onto a finite number of reactive sites that can be described by Langmuir isotherm, (2) the reductive dechlorination occurs at these sites following firstorder kinetics, and (3) the products are instantaneously released from these sites once formed. The solution to the ordinary differential equation (eq 2) is given in eq 3 where C0 (µM) is the initial substrate concentration. C)
Lambert W(KAC0e[-KA(krStt - C0)]) KA
(3)
The parameters kr and St cannot be determined independently and are evaluated together as krSt (µM · min-1). The parameters KA and krSt can be estimated by an optimization procedure using MATLAB (Supporting Information). Figure 2b compares the experimental results with the model-simulated results for the 124TCB dechlorination in the presence of various amphiphiles at concentrations as indicated. The model predictions are in reasonably good agreement with the experimental results. A summary of the simulated results for the dechlorination experiments conducted is listed in Table 2. For each experiment, the results were also analyzed with the pseudo-first-order kinetics and the initial rate constants (kobs) were determined with linear regression analysis, for comparison. In general, there is a reasonable consistency between the changes of kobs and krSt with changing amphiphile concentrations for all the experiments conducted, except for the ones with DPC (as discussed later). Indeed, it can be inferred that KAC , 1, and eq 2 can be approximated as a pseudo-first-order kinetic model. The KA values indicate the extent of 124TCB equilibrium adsorption on the Pd/Fe surface, which was subject to the influence of the surfactants or NOM present. In general, Table 2 shows that KA increases with concentrations of surfactants or NOM, except for SDS (at 1.25 CMC). A high KA value could be attributed to 124TCB partitioning to the adsorbed amphiphile molecules and admicelles, in addition to its direct adsorption onto the Pd/Fe surface site. Apparently the amphiphiles modified the hydrophilic Pd/Fe surface such that it became more accessible to the hydrophobic 124TCB. In the case of SDS, it could be the predominant partitioning of 124TCB in the SDS micelles in the bulk solution at 1.25
TABLE 2. Models-Derived Rate Constants for 124TCB Dechlorination with Pd/Fe in Various Amphiphile Solutions amphiphile
concentration
krSt (µM · min-1)a
KA (× 10-5 µM-1)b
kobs (min-1)c
nd
R2
control
0.1CMC 1CMC 1.25CMC
4571 ( 232 5314 ( 412 5882 ( 258 3346 ( 129
1.70 ((0.14) 2.20 ((0.16) 2.68 ((0.21) 1.30 ((0.15)
0.085 ( 0.013 0.123 ( 0.021 0.101 ( 0.019 0.045 ( 0.017
7 5 6 5
0.95 0.99 0.91 0.98
NPE
0.1CMC 1CMC 10CMC
6641 ( 514 6686 ( 462 1692 ( 62
1.64 ((0.12) 2.28 ((0.19) 3.12 ((0.23)
0.114 ( 0.022 0.153 ( 0.031 0.054 ( 0.012
6 6 6
0.98 0.98 0.96
TX-100
1CMC
4982 ( 235
1.87 ((0.15)
0.094 ( 0.021
7
0.98
CTAB
0.1CMC 1CMC 10CMC
8846 ( 391 11940 ( 742 1258 ( 116
1.86 ((0.21) 2.00 ((0.24) 2.75 ((0.26)
0.187 ( 0.025 0.233 ( 0.027 0.034 ( 0.011
7 7 7
0.95 0.97 0.96
DPC
0.1CMC 1CMC 5.7CMC
2205 ( 208 773 ( 129 735 ( 203
0.79 ((0.11) 1.91 ((0.17) 2.19 ((0.25)
0.017 ( 0.009 0.015 ( 0.008 0.016 ( 0.008
6 6 6
0.93 0.96 0.95
NOM
10 mg/L 50 mg/L 200 mg/L
3632 ( 425 377 ( 57 113 ( 21
1.62 ((0.13) 2.29 ((0.26) 2.81 ((0.17)
0.059 ( 0.013 0.009 ( 0.007 0.003 ( 0.001
7 7 7
0.96 0.98 0.86
SDS
a krSt values are for Pd/Fe dosage of 0.71 g/L. b KA is sorption coefficient for the substrate of 124TCB. pseudo-first-order initial rate constant for the 124TCB dechlorination reaction. d n is the number of data points.
CMC that limited the 124TCB surface-bound concentration. In addition, SDS was found to react with the Pd/Fe particles and resulted in formation of green-rust-like surface deposit, which might have adverse effects on HOC partitioning on surface. The extent of reductive reaction at the Pd/Fe reactive sites can be inferred from the krSt values. The krSt values in the cases of SDS, NPE, and CTAB at low concentrations (e1.0 CMC) are higher compared with the control. This was because the increased surface accumulation of 124TCB (facilitated by the surfactants) enhanced the interaction between 124TCB and the Pd/Fe. As the concentration of these surfactants increased above CMC, the reverse effect on the krSt and kobs values was observed. Other researchers (22) have attributed the decreased rates to the partitioning of the substrate into the hydrophobic interior of the surfactant micelles in bulk solution, resulting in the decreased adsorption of the substrate on the Pd/Fe surface. In this study, the KA values continue to increase even at above surfactant CMC (for the cases of NPE and CTAB) against the decreased krSt, as explained below. As the concentrations of surfactants increased above their CMC, more admicelles and hemimicelles were formed on the solid-water interface. This resulted in increasing 124TCB partitioning near the solid-water interfacial region and higher KA values. However, the increased admicelles and hemimicelles might have also blocked the Pd/Fe surface sites (e.g., Pd islets) and prevented their direct access by 124TCB, resulting in the decrease of krSt. This phenomenon was clearly evidenced in the NOM system, which showed up to 30 times reduction in the kobs values as compared with the control when the NOM concentrations increased to 200 mg/L. Apparently, the large NOM molecules, e.g., typically 1-1000 nm diameter for humic acids (23), adsorbed to the surface of the nanoscale Pd/Fe particles might have caused steric congestion and formed physical barrier to diffusion of the NOM-bound 124TCB to the metal surface. Similar phenomenon has been postulated in the study on TCE reduction with ZVI (8). Therefore, the coexistence of NOM, though could increase 124TCB partitioning in the solid-water interfacial region (as shown by increased KA), would inhibit the dechlorination reaction and result in significant decreases in krSt and kobs. However, the possibility that the reduced sulfur groups in the NOM might poison the Pd catalyst cannot be ruled out.
c
kobs is the
DPC at 0.1 CMC to 5.7 CMC adversely inhibited the 124TCB dechlorination with the Pd/Fe particles. This is in contrast to the increasing KA values. The formation of milky oil-in-water emulsion in the system suggested that DPC had reacted with the Pd/Fe, and this alternative redox reaction might be the main cause for inhibition of 124TCB dechlorination. Indeed, in the DPC system, the assumptions intrinsic to eq 2 are violated. Interaction of the Amphiphilic Molecules with Pd/Fe Surface. It appears that the accumulation of amphiphiles on the Pd/Fe surface might cause either synergistic or antagonistic effects on 124TCB dechlorination. To gain further insight into the interfacial interactions, the data of H2 evolution and iron release in the Pd/Fe-water systems were analyzed. When wetted, the Pd/Fe particles would react readily with water to produce H2 following eq 4 or 5 under anaerobic conditions (2). Fe0 + 2H2O f Fe2+ + 2OH- + H2 v
(4)
0
3Fe + 4H2O f Fe3O4 + 4H2 v
(5)
The iron corrosion resulted in formation of oxide film, release of Fe2+, and evolution of a stoichiometric amount of H2 gas (if H+ is the only electron acceptor present). Figure 3 and Figure 4 show the amounts of H2 evoluted and iron releases, respectively, in the solutions as indicated. More H2 gas was released from the systems of CTAB, NPE, SDS, and TX-100 solutions compared with the control and so were their 124TCB degradation rates (Table 2). In the cases of DPC and NOM solutions, negligible H2 was produced. Apparently, the amounts of iron releases did not correspond with the amount of H2 gas produced. In the DPC system, the excessive iron release (Figure 4) accompanied by formation of distinct iron oxides (Figure S1) might be caused by the presence of dissolved 622 mg/L Cl- dissociated from DPC molecules at 5.18 g/L. Chloride could promote dissolution of iron (hydr)oxides film by pitting corrosion (24). As mentioned earlier, it was found that DPC had reacted with the nanoscale Pd/Fe particles, resulting in formation of oil-in-water emulsion. GC-MS spectra of the n-hexane extraction of DPC solution before and after reaction (Figure S6) confirmed that DPC might have been transformed by the nanoscale Pd/Fe through catalytic hydrogenation, and the reaction is suggested as follows: VOL. 42, NO. 12, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Environmental Implications
FIGURE 3. H2 evolution from the Pd/Fe-water systems with different amphiphiles.
FIGURE 4. Amounts of dissolved iron released from the Pd/Fe particles in various amphiphile solutions.
The negligible release of H2 gas (Figure 3) provides further evidence to support this conclusion. Similarly, in the NOM solution, the low H2 evolution could be due to NOM serving as H2 acceptor, for which the H2 produced from ZVI corrosion is transferred to NOM with the presence of Pd. Alternatively, with the presence of a Pd site, the NOM could serve as competitive electron acceptor to H+, inhibiting H2 formation. In the experiments, the brownish color of the NOM solution gradually turned colorless with elapsed time, indicating transformation of NOM in the Pd/ Fe-water system (Figure S7 and discussions in Supporting Information). It has also been reported that NOM can accept electrons and H2 transferred by Fe(III)-reducing microorganisms (16). Besides being a redox active species, NOM is able to form inner-sphere surface complex with nanoscale ZVI, and this induces competitive sorption with the substrate (25). The large NOM molecule might also block the limited reactive sites (Pd). Therefore, the 124TCB dechlorination was significantly inhibited in the NOM solutions (Table 2). In the SDS system, the reacted Pd/Fe particles appeared green with fluffy morphology after drying but turned yellow after exposure to air. It was postulated that a material similar to green rust (FeII4FeIII2(OH)12SO4 · yH2O) was formed, with the OSO3- portion of SDS molecules oriented to the Pd/Fe particle surface. The FTIR spectra (Figure S8) of the reacted particles (after rinse) showed the presence of SDS on the reacted Pd/Fe. Examinations of the particles under SEM and EDX also showed the surface coverage by SDS molecules (Figure S2). 4518
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Because of the complex interaction between the amphiphiles and substrate, and their competitive or synergistic interactions with metal surfaces, understanding which mechanism that predominates in the system is challenging. The use of an appropriate kinetic model, such as the LangmuirHinshelwood kinetic model, and with several testing protocols could help gaining insight into the roles of amphiphiles in the catalytic reduction of the substrate with Pd/Fe. In the present study, several possible adverse effects of surfactants were identified. Certain surfactant moieties (e.g., sulfate group) could react with the Pd/Fe sites, either passivating the Fe surface by forming secondary minerals or accelerating its nonspecific corrosion, and slowing the dechlorination of the target compound. The type of complementary anions (e.g., halides) of the surfactant molecules may be critical to the longevity of the ZVI, and this effect has to be discriminated from that caused by the surfactant molecule per se in the future study. Typically, at a high surfactant concentration such as above the CMC, the surfactant tends to inhibit access to the surface sites by the hydrophobic substrate by partitioning the substrate into its hydrophobic interiors. At a concentration below CMC, the redox-inert surfactants show beneficial effects on the HOC dechlorination rate, attributable to the surfactant-enhanced partitioning of the hydrophobic substrate on the interfacial film. Thus, the use of surfactants to disperse the nano-ZVI particles for injection into treatment zone or as corrosion inhibitors may not always lead to compromising the ZVI reactivity. Among the various surfactants investigated in the present study, cationic surfactant CTAB at a concentration below CMC appeared to be the most benign to the catalytic dechlorination of 124TCB by the Pd/Fe. NOM is ubiquitous in natural water systems. In the Pd/Fe system, NOM appears to be an inhibitor to dehalogenation of chlorobenzenes because of the several aforementioned phenomena. In this context, its role as electron shuttles that promotes reduction of aliphatic HOCs in the ZVI/water systems as reported by previous researchers (7, 8) is not pertinent in the present study. Because the reaction between the chlorobenzenes and Pd/Fe occurred on Pd, steric congestion around the nanosized particle could be another reason for the dechlorination inhibition occurred on the nanosized particles at high NOM concentrations.
Acknowledgments The support from the Nanyang Technological University is acknowledged. The support came in the forms of project grants MS05-11 and MS06-11, and Ministry of Education’s scholarship.
Supporting Information Available Chemicals, derivation of the kinetic model, XRD patterns, TEM, SEM-EDX images, and FTIR spectra of fresh and reacted samples in various surfactant solutions, water-air surface tension, contact angles, influences of surfactants on surface charge as a function of solution pH, UV-vis spectra of NOM solution before and after reaction with Pd/Fe particles, GCMS results of DPC solution before and after reaction with the Pd/Fe. This information is available free of charge via the Internet at http://pubs.acs.org.
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