Characteristics, morphology, and stabilization mechanism of PAA250K-stabilized bimetal nanoparticles

Colloids and Surfaces A: Physicochem. Eng. Aspects 349 (2009) 137–144

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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

Characteristics, morphology, and stabilization mechanism of PAA250K-stabilized bimetal nanoparticles Yu-Hao Lin a , Hui-Hsin Tseng b , Ming-Yen Wey a , Min-Der Lin a,∗ a b

Department of Environmental Engineering, National Chung-Hsing University, Taichung 402, Taiwan Department of Occupational Safety and Health, Chung-Shan Medical University, Taichung 402, Taiwan

a r t i c l e

i n f o

Article history: Received 6 May 2009 Received in revised form 31 July 2009 Accepted 9 August 2009 Available online 14 August 2009 Keywords: Zero valent iron Bimetal PAA Aggregation Dispersion Morphology

a b s t r a c t Injection of bimetal (Pd/Fe) nanoparticles was evidenced as a promising treatment to remediate trichloroethene (TCE) polluted groundwater. However, the particles usually aggregate rapidly and result in a very limited migration distance. This study employed poly acrylic acid (PAA) for the synthesis of stable bimetal nanoparticles (SBN) by selecting appropriate PAA molecular weight and dosage to reduce particle aggregation. In addition, this study elucidated the SBN stabilization mechanism by analyzing morphology and characteristics. The results of TCE dechlorination and SBN stabilization experiments indicate that the bimetal nanoparticles modified with PAA250K exhibit better reactivity and stability characteristics than did other such nanoparticles. The SBN bind chemically onto PAA250K via the carboxylic group and bidentate pattern bridging, which could provide both electrostatic and steric repulsion to prevent particle aggregation. Therefore, the SBN exhibited a higher dechlorination effect than did bare bimetal. The morphology of SBN shown in the transmission electron microscopy (TEM) images exhibits capsule-like and wire-like structures instead of a single particle dispersed in the slurry. The results of this work can preliminarily elucidate the SBN’s morphology, dispersion, and size distribution. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Zero valent iron (ZVI) has been evidenced as a treatment material for chlorinated organic compounds and metal [1–3]. Nanoscale zero valent iron (NZVI) provides considerable reactive sites and great intrinsic energy, and thus exhibits high reactivity. And according to experimental results, the nano-sized bimetal (Pd/Fe), which few Pd deposits on NZVI surfaces as a catalyst, degrades trichloroethene (TCE) more rapidly, more completely, and with a higher yield of methane than NZVI [4–6]. However, the mobility of NZVI in aquifers is very poor owing to three factors: the density of iron, long-range magnetic attractive forces [7], and the high ionic strength in groundwater [8]. Therefore, the application of bimetal injection for aquifer remediation is still limited. Recently, research has taken various approaches to preventing the agglomeration of particles. Stable nanoparticles were synthesized by modifying the particle surface properties related to electrostatic and steric stabilization and by using surfactants and polymers such as starch, sodium dodecyl benzene sulfonate, and tween20 [7,9,10], or coated nanoparticles on delivery vehicles [11,12]. Generally, polymers such as carboxymethyl

∗ Corresponding author. Tel.: +886 4 222850509; fax: +886 4 22862587. E-mail address: [email protected] (M.-D. Lin). 0927-7757/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2009.08.007

cellulose, guar gum, and poly acrylic acid (PAA) provide steric stabilization that exhibit a larger magnitude of repulsion force than electrostatic repulsion [13], help to stabilize NZVI [14–17] and superparamagnetic ferrofluid [18] via carboxylate binding. PAA is a commercial, environmentally friendly material and has already been applied for stabilizing NZVI [11,15,19]. Nevertheless, there are few detailed investigations about the properties of PAA, such as (1) the stabilizing mechanisms of PAA modified bimetals, (2) the configuration between PAA and NZVI to prevent agglomeration, (3) PAA dose response on dechlorination reactivity, and (4) the characteristics of NZVI particle surfaces modified by PAA. Therefore, the objective of this study is to investigate the appropriate PAA molecular weight and dosage, on the basis of the performance of TCE dechlorination and stable bimetal nanoparticles (SBN) stability tests, for the synthesis of stable bimetal Pd/Fe nanoparticles. The current study investigated the effects of PAA on bimetal particle size and surface characteristics by using transmission electron microscopy (TEM), X-ray diffraction (XRD), and electron spectroscopy for chemical analysis (ESCA). Further, we elucidated the SBN stabilization mechanism and morphology by using TEM and Fourier transform infrared spectroscopy (FTIR) analysis. According to the results of the aforementioned characteristics-oriented analysis, the SBN’s morphology, dispersion, and size distribution can be preliminarily elucidated.

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2. Materials and methods 2.1. Material Reagent grade chemicals such as sodium borohydride, PdCl2 , and PAA with the various molecular weights (MW) of ca. 1800, 250,000, and 1,250,000 g/mol were purchased from Sigma–Aldrich Chemical Co. Ferric iron was obtained from Alfa Aesar. The original 35 wt% PAA solution concentration (MW of ca. 250K) was diluted to 3.5 wt% by means of adding deionized water. TCE (>99.5%) was obtained from Fluka. The PdCl2 solution was dissolved, and the water used in this study was tap water that underwent purification in a Millipore deionized (DI) water-purification system. 2.2. Synthesis of stable bimetal nanoparticles The SBN synthesis process employed in this work was modified from previous studies [20–23]. Before synthesis, DI water was purged with purified N2 for several hours to remove the dissolved oxygen. Equal volumes of 0.5 M NaBH4 and 0.09 M FeCl3 were mixed under an N2 (99.9%) atmosphere and stirred until hydrogen evolution ceased to form stabilized NZVI slurry. Then, the various stabilizer PAA was added to the FeCl3 precursor in order to yield a desired concentration of an Fe3+ –PAA complex. The NZVI slurry was loaded with 0.05% (w/w) catalyst Pd2+ , which was reduced onto the NZVI surface to form SBN slurry. The SBN slurry was then quantified to 2.0 Fe g/L and was applied directly to the TCE dechlorination experiment. For the analysis of SBN characteristics, the SBN slurry was separated magnetically and underwent impurity-removal washing, followed by freeze-drying overnight under vacuum. The dried SBN was deactivated to avoid spontaneous ignition and was then sealed under nitrogen gas prior to analysis. As to the PAA effect on iron surface characteristics, the fresh dried SBN powders were exposed to air for 5 days (these powders were then labeled “aged SBN”) and were analyzed by means of XRD and ESCA to interpret the conversion of surface iron oxide. 2.3. SBN characterization The SBN’s morphology and particle size were observed via TEM (JEOL JEM-1200CX II systems) which drops SBN slurry directly onto a Cu grid, and via an SEM test (FEI Quanta 200 FEG). We examined both the SBN’s hydraulic diameter and the SBN’s zeta potential by using a high-performance particle sizer (HPPS) and a potential analyzer (Zeecom ZC2000), respectively. The stability of SBN slurry was determined on the basis of three factors: the UV-sedimentation experiment, directly visible observation, and zeta potential measurement. The UV-sedimentation experiment, which was modified from literature [24], monitored the absorption ( = 508 nm) of 0.5 g/L SBN slurry for 3 h in a UV–vis spectrophotometer (Jasco V-530). The crystals and the elements of SBN were identified by means of XRD (Mac Science 18MPX), and SBN powder was compressed to form a film in ESCA (PHI 5000 Microprobe) and FTIR (Jasco 410) tests. 2.4. TCE dechlorination experiment The TCE dechlorination experiments were conducted in 250 mL serum bottles containing 100 mL bimetal slurry with 2 g/L concentration, with 150 mL of headspace, and were capped by means of Teflon MininertTM valves. A total of 200 ␮l of stock TCE solution (8657.9 mg/L in methanol) was spiked into the slurry so that there would be an initial TCE concentration of approximately 17 mg/L. Next, bottles were placed onto an orbital rotator and were mixed at 100 rpm at ambient temperature. At selected time intervals, the

Fig. 1. Different PAA MW modified bimetal in the same Fe/PAA molecular ratio for TCE dechlorination reactivity.

TCE concentration was monitored by withdrawing 50 ␮l headspace sample and analyzed by GC/FID (China chromatography 9800) equipped with a DB-624 capillary column (JW, 30 m length by 0.32 mm internal diameter). The temperature of the injector port and the detector were set at 180 and 250 ◦ C, respectively, and the oven temperature was kept constant at 80 ◦ C. And the GC injection was carried out in the split mode. Calibration curves were prepared using a series of standard solutions of known TCE concentrations. Control tests were conducted and replicate reactors were analyzed for each type of bimetal slurry. 3. Results and discussion 3.1. TCE dechlorination and the stability of SBN slurry Three types of poly acrylic acid, namely PAA1.8K, PAA250K, and PAA1250K, whose molecular weights are 1800, 250,000, and 1,250,000 g/mol, respectively, were used in this work for the modification of bimetals. Fig. 1 illustrates the performances of TCE dechlorination tests for blank, bare bimetals (without PAA), and for modified bimetals. The figure reveals that the bimetals modified with PAA1.8K and PAA250K present higher dechlorination rates of TCE than bare bimetals. As to the performance of PAA1250K, the reactivity of bimetals was reduced owing to the bridging flocculation of PAA1250K long macromolecular chains [25]. The visible-observation results and the UV-sedimentation results for evaluating the stability of bimetal slurries are shown in Figs. 2 and 4, respectively. Owing to the strong van der Waals and magnetic attraction [24,26], the bare bimetals aggregated and deposited rapidly (within a few minutes), as indicated in Figs. 2 and 4(A). Such aggregate/deposit phenomena were significantly improved for bimetals modified with PAA1.8K and PAA250K, as shown in Fig. 4, indicating that the addition of PAA could help bimetals both from smaller particle sizes and prevent agglomeration. Since the bimetal modified with PAA250K in the same PAA/Fe molecular ratio exhibited better stability than PAA1.8K in the visible observation (Figs. 4(B) and (D)), PAA250K was employed in this study for improving the stability of bimetal slurry. Fig. 3 compares the performances of TCE dechlorination for bimetals modified with different dosages of PAA250K. As the PAA250K dosage increases from 2 to 20 vol.%, the TCE dechlorination decreases. Dosages more than 14 vol.% can be regarded as overdoses since their dechlorination effects are worst than those of bare bimetals. It may be due to that the excess PAA250K occupy more surface reactive sites of the bimetals and restrict the TCE

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Fig. 2. UV-sedimentation experiment of bimetal modified without and with various PAA250K doses.

diffuse onto the reactive sites, therefore result in decrease of TCE dechlorination. Overdoses may also cause bridge phenomena, which undermine slurry stability [25], resulting in particle aggregation, as shown in Fig. 2. The comparisons of the UV-sedimentation test (Fig. 2) and TCE dechlorination reactivity (Fig. 3) between 2 and 8 vol.% PAA250K dosages indicate that their differences are not significant. However, the visible observation illustrated in Fig. 4 shows that the bimetal slurry with 2 vol.% exhibited the sedimentation phenomenon on the top of the bottle on the third day (Fig. 4(C)). In contrast, 8 vol.% stabilized slurry remained suspended over an extended period (more than 5 days), as shown in Fig. 4(D). Therefore, regarding the stabilization of bimetal nanoparticles, 8 vol.% can be regarded as an adequate dosage for PAA250K.

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Fig. 3. Various PAA250K volume-doses for TCE dechlorination reactivity.

Another stability index of slurry is the particle surface zeta potentials. The SBN zeta potentials within different pH values shown in Fig. 5 illustrate that SBN synthesized at pH 8.7 has a −80 mV zeta potential, indicating the SBN surface was negatively charged due to the PAA ionization effect and provide electrostatic repulsion to prevent particle aggregation. Based on the aforementioned results, two factors may explain the phenomena that SBN exhibits higher dechlorination effects than bare nanoparticles. First of all, SBN contains more reactive surface area because, Fe(0) particles in SBN are smaller than those in bare nanoparticles, as shown in the SEM and TEM images in Figs. 6 and 7. Secondly, the UV-sedimentation experiment (Fig. 2) also indicates that the suspension of SBN is more stable than bare irons. Both of these two factors, namely Fe(0) size and suspension stability, will increase the probability of

Fig. 4. Visible observation of (A) bare bimetal slurry and modified with (B) PAA1.8K, (C) 2 vol.% PAA250K and (D) 8 vol.% PAA250K.

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Fig. 5. Zeta potential as a function of SBN solution pH.

SBN to contact with TCE and then enhance the TCE removal efficiency. 3.2. Characterization of SBN 3.2.1. Results of the TEM/SEM analysis The SEM image of the SBN is shown in Fig. 6. The single SBN appears spherical, sized about 40–60 nm, and clearly shows the growth of a fine chain-like film. The energy dispersive spectrometer (EDS) pattern indicates the presence of an approximate surface corrosion product of Fe and O. Nevertheless, the loading of Pd was too low to be discernible from the EDS spectrum. However, part of the surface appeared smooth, indicating that the iron was oxidized owing to either the deactivation process or the reaction with the dissolved oxide in the water. On the other hand, the TEM image of a single SBN shows that the size range of the spherical particles was from 5 to 50 nm and the iron particles were surrounded by shells between approximately 1 to 5 nm thick, as shown in Fig. 7(B). This finding is consistent with the literature proposing core–shell structure that would grow thicker with the iron oxidation process [27,28]. It is noteworthy that the image of Fig. 7(B) shows the emergence of several fairly smaller particles (<10 nm) agglomerated into the block beads and further into wire-like filaments along the PAA250K section. However, Fig. 7(A) presents a capsule-like object containing variously sized nanoirons via the PAA250K link, instead of single particle dispersing in slurry. It is reasonable to conclude that, regarding the SBN’s morphology,

Fig. 6. SEM and EDS image of SBN.

Fig. 7. TEM image of SBN in (A) capsule-like and (B) wire-like form.

the binding of bimetal particles to the PAA250K deliverer helped to reduce the SBN’s sizes and prevent the aggregation. 3.2.2. Results of the XRD/ESCA analysis The chemical composition and crystal structure of the SBN shown in Fig. 8 revealed that the major characteristic peaks at 2 = 44.8◦ , 65.32◦ , and 82.60◦ represent bcc Fe(0) (1 1 0), bcc Fe(0) (2 0 0), and bcc Fe(0) (2 1 1), respectively [12,27], and the loading of Pd was also too low to be discernible from the XRD test. Secondary peaks represent iron oxide [29], but the Fe3 O4 and ␥-Fe2 O3 peaks at 2 = 35.32◦ are not differentiable because their lattice parameters are very similar [30,31]. Therefore, we analyzed the conversion of iron oxide on the SBN surface by means of ESCA analysis (described later). The comparison of the aged SBN’s magnetite/maghemite peaks at 2 = 35.32◦ with bare bimetal indicates that no apparent variation of iron oxide was observed after exposure to air for 5 days. It can be concluded that the PAA250K can protect the surrounding Fe(0) from further corrosion and can prevent the bimetal from being oxidized in the air. The low-resolution scan of the ESCA survey, demonstrated in Fig. 9, identifies the predominant elements present on the nearsurface SBN region including oxygen, carbon, and iron, which are ascribed to the results of iron oxidation and PAA250K modification. The more intense Fe 2p3/2 region and O 1s of fresh and aged SBN shown in Fig. 10 was analyzed by curve fitting to elucidate the iron oxide transformation. Four feature peaks at 707.1, 710.6, 710.9, and 711.5 eV represent binding energy of different iron status on the fresh SBN surfaces, as shown in Fig. 10(A). Their relative area ratios are 3.5%, 51.3%, 0.7%, and 44.4%, which represent Fe(0), Fe3 O4 , FeOOH, and Fe2 O3 , respectively [32,33]. For binding energy of aged SBN, the corresponding relative area ratios are 1.8%, 76.1%, 0.0%, and

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Fig. 8. XRD pattern of (A) fresh and (B) aged SBN.

22.1%, as shown in Fig. 10(B). The active zero valent irons are still observed on the particle surface; this finding confirms that outer PAA250K could protect Fe(0) from oxidization, and the confirmation of this protection’s existence is consistent with the conclusion obtained from the XRD analysis results. A comparison of the area ratios of the above fresh and aged SBN suggest that Fe(0) and the transient state FeOOH would be oxidized to Fe3 O4 and would form a stable state. The O 1s analysis of the fresh and aged SBN shown in Fig. 11 could be decomposed into five curves with peaks at 530.0, 530.9, 531.2, 532.1, and 533.1 eV, which represent (1) the iron oxide [33], (2) the hydroxyl groups on the alkyl chain [34], (3) OH− [35,36], (4) the oxygen bonding to carbon [37], and (5) chemically or physically adsorbed water [2,38]. The energy peak at 530.0 eV increases with time due to iron oxidation, which we interpreted via our Fe 2p3/2 analysis (see Fig. 10). Generally, the PAA250K molecules possess multiple assignments including the hydroxyl (–OH), carbonyl (–CO–), and carboxylic acid (–COOH) groups, and previous research has suggested that the carboxylic acid groups are the most likely anchors coordinating organic molecules onto the metal surface

Fig. 9. The ESCA survey of SBN particles.

to inhibit metal aggregation [21,37]. In terms of the O 1s-related results stemming from the SBN surface analysis, the dominant peaks at 530.9 and 532.1 eV imply that carbon was present on the SBN surface and confirm that SBN was adsorbed or bounded to PAA250K as well.

Fig. 10. High-resolution ESCA spectra for iron 2p3/2 of (A) fresh SBN and (B) aged SBN particles.

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Fig. 12. FTIR spectra of PAA, NZVI, bare bimetal and SBN.

Fig. 11. High-resolution ESCA spectra for SBN oxygen 1s of (A) fresh and (B) aged SBN particles.

3.3. The stabilization mechanism and the architecture of SBN The current study carried out FTIR tests on PAA250K, bare bimetal, and SBN to elucidate the SBN stabilization mechanisms. Fig. 12 clearly shows that there was no apparent assignment peak in the bare bimetal, and that the PAA250K presented a notable peak at 1720 cm−1 , referring to the frequency of free carboxylate (–COOH). (Other detailed assignment peaks are displayed in Table 1.) In contrast, SBN exhibited two new peaks at 1556 and 1396 cm−1 owing to the symmetric and asymmetric carboxylate vibration, and this finding indicates that the bimetal of SBN adsorbed onto PAA250K via chemisorption instead of via physisorption. These carboxylic stretching frequencies of PAA250K possess high affinity to mineral-

surface oxides [39], and were expected to shift significantly to symmetric and asymmetric carboxylate vibration wave lengths [21,40] depending on the pH value and the ionic strength of the solution [41]. The literature presents results similar to the current study’s results concerning PAA chemisorbed to the iron oxide [42]. However, according to the separation of frequency, , which is the difference between the symmetrical and asymmetrical carboxylate stretching frequencies, a set of patterns for identifying the bonding between PAA250K and particles can be proposed. These patterns are summarized as follows and the schematic views of bonding are illustrated in Fig. 13: (A) monodentate chelating, (B) bidentate chelating, and (C) bidentate bridging [43,44]. If  ranges from 200 to 320 cm−1 , the binding is governed by monodentate interaction; if  is less than 110 cm−1 , the binding is governed by bidentate chelating interaction; and if  ranges from 140 to 190 cm−1 , the binding is governed by bidentate bridging [41]. The SBN separation of frequency  equals to 160 cm−1 (1556–1396 cm−1 ) indicating that the bidentate bridging is the primary binding mechanism for connecting a PAA250K monomer to the particle surface, as shown in Fig. 14. Similar binding patterns were also observed in previous studies [40,42]. However, partially hydroxylated iron may contain negatively charged Fe–O− surface groups at the solution pH 8.7. As a result, hydrogen bonding could enhance strength of intermolecular between the iron particle and PAA250K, and thus hydrogen bonding could be regarded as secondary binding mechanisms. In the synthesis process of SBN, the PAA250K dissolved in a ferric precursor and presented an interlaced network. The ferric serves as a cross-linking agent and partially reacts to form the PAA250K-

Table 1 FTIR band assignments for PAA and SBN. Peak position (cm−1 )

Peak assignment

SBN (pH 8.7)

PAA alone (pH 2.0)

– 1556 1446 1396 1264

1720 – 1453 – –

–C O (free COOH) –COO– (asymmetric) –CH2 scissor –COO– (symmetric) –C–O

Fig. 13. Metal-carboxylate structure: (A) monodentate chelating, (B) bidentate chelating and (C) bidentate bridging.

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Fig. 14. The conceptualized profile of a possible binding mechanism between the particles of SBN and PAA250K.

Fe complex [18,40], in turn, improving both nucleation and particle growth. The interlaced PAA250K creates a diverse reaction pool that could inhibit iron particle growth and reduce the particle sizes. As the SBN formed, the particles are bound onto the PAA250K deliverer to form a capsule-like and wire-like morphology, which may depend on the pH and ionic strength of the solution. Therefore particles use the electrostatic and steric repulsion of PAA250K to steadily disperse in the slurry. The ferric cation plays a dual role in the SBN preparation process: the role of the ferric precursor for the formation of NZVI, and the role of the cross-linker. According to the results of the current study’s test regarding the capsule-like and wire-like structure sizes of SBN with approximately 150–600 nm in TEM images (Fig. 7), and the hydrodynamic diameters of 244 and 566 nm measured by HPPS, it can be concluded that the SBN particles are not singly dispersed in the slurry and that their sizes are distributed in a broad range. 4. Conclusion The current study used the polymer PAA250K to stabilize bimetal Pd/Fe nanoparticles (SBN). Both a test of the ESCA of O 1s and a test of the FTIR demonstrated that SBN bind chemically onto PAA250K via the carboxylic group and bidentate pattern bridging, which could provide both electrostatic and steric repulsion against particle aggregation for dispersal in slurry. The interlaced PAA250K comprised “nanoreactor” domains and a PAA250K–Fe complex to promote nucleation and inhibit the growth of particles, resulting in the production of dispersed and smaller iron sizes (<10 nm). Therefore, the SBN exhibited a higher dechlorination effect than bare bimetal. In addition, the XRD and ESCA results suggest that PAA could reduce the oxidization of Fe(0) in the air. On the SBN surface, the Fe3 O4 would be in a predominant state and the FeOOH would be in a transient state. For the SBN morphology, the SBN slurry presented capsule-like and wire-like structures in TEM images instead of single particles dispersed in slurry. References [1] W.A. Arnold, A.L. Roberts, Pathways and kinetics of chlorinated ethylene and chlorinated acetylene reaction with Fe(0) particles, Environ. Sci. Technol. 34 (2000) 1794–1805.

[2] X.-Q. Li, J. Cao, W.-X. Zhang, Stoichiometry of Cr(VI) immobilization using nanoscale zero valent iron (nZVI): a study with high-resolution X-ray photoelectron spectroscopy (HR-XPS), Ind. Eng. Chem. Res. 47 (2008) 2131–2139. [3] Y. Liu, H. Choi, D. Dionysiou, G.V. Lowry, Trichloroethene hydrodechlorination in water by highly disordered monometallic nanoiron, Chem. Mater. 17 (2005) 5315–5322. [4] W.-X. Zhang, C.-B. Wang, H.-L. Lien, Treatment of chlorinated organic contaminants with nanoscale bimetallic particles, Catal. Today 40 (1998) 387–395. [5] C.-B. Wang, W.-X. Zhang, Synthesizing nanoscale iron particles for rapid and complete dechlorination of TCE and PCBs, Environ. Sci. Technol. 31 (1997) 2154–2156. [6] H.-L. Lien, W.-X. Zhang, Nanoscale Pd/Fe bimetallic particles: catalytic effects of palladium on hydrodechlorination, Appl. Catal. B: Environ. 77 (2007) 110–116. [7] T. Phenrat, N. Saleh, K. Sirk, H.-J. Kim, R. Tilton, G. Lowry, Stabilization of aqueous nanoscale zerovalent iron dispersions by anionic polyelectrolytes: adsorbed anionic polyelectrolyte layer properties and their effect on aggregation and sedimentation, J. Nanopart. Res. (2007). [8] D.W. Elliott, W.X. Zhang, Field assessment of nanoscale bimetallic particles for groundwater treatment, Environ. Sci. Technol. 35 (2001) 4922–4926. [9] F. He, D. Zhao, Preparation and characterization of a new class of starchstabilized bimetallic nanoparticles for degradation of chlorinated hydrocarbons in water, Environ. Sci. Technol. 39 (2005) 3314–3320. [10] S.R. Kanel, D. Nepal, B. Manning, H. Choi, Transport of surface-modified iron nanoparticle in porous media and application to arsenic(III) remediation, J. Nanopart. Res. 9 (2007) 725–735. [11] B. Schrick, B.W. Hydutsky, J.L. Blough, T.E. Mallouk, Delivery vehicles for zerovalent metal nanoparticles in soil and groundwater, Chem. Mater. 16 (2004) 2187–2193. [12] L.B. Hoch, E.J. Mack, B.W. Hydutsky, J.M. Hershman, J.M. Skluzacek, T.E. Mallouk, Carbothermal synthesis of carbon-supported nanoscale zero-valent iron particles for the remediation of hexavalent chromium, Environ. Sci. Technol. 42 (2008) 2600–2605. [13] N. Saleh, H.-J. Kim, T. Phenrat, K. Matyjaszewski, R.D. Tilton, G.V. Lowry, Ionic strength and composition affect the mobility of surface-modified Fe0 nanoparticles in water-saturated sand columns, Environ. Sci. Technol. (2008). [14] N.-D. Ahfir, A. Benamar, A. Alem, H. Wang, Influence of internal structure and medium length on transport and deposition of suspended particles: a laboratory study, Transport Porous Media (2008). [15] S.R. Kanel, R.R. Goswami, T.P. Clement, M.O. Barnett, D. Zhao, Two dimensional transport characteristics of surface stabilized zero-valent iron nanoparticles in porous media, Environ. Sci. Technol. 42 (2008) 896–900. [16] A. Tiraferri, K.L. Chen, R. Sethi, M. Elimelech, Reduced aggregation and sedimentation of zero-valent iron nanoparticles in the presence of guar gum, J. Colloid Interface Sci. 324 (2008) 71–79. [17] A. Tiraferri, R. Sethi, Enhanced transport of zerovalent iron nanoparticles in saturated porous media by guar gum, J. Nanopart. Res. (2008). [18] C.-L. Lin, C.-F. Lee, W.-Y. Chiu, Preparation and properties of poly(acrylic acid) oligomer stabilized superparamagnetic ferrofluid, J. Colloid Interface Sci. 291 (2005) 411–420. [19] S.R. Kanel, H. Choi, Transport characteristics of surface-modified nanoscale zero-valent iron in porous media, Water Sci. Technol. 55 (2007) 157–162. [20] H.L. Lien, W.X. Zhang, Nanoscale iron particles for complete reduction of chlorinated ethenes, Colloid Surf. A: Physicochem. Eng. Aspects 191 (2001) 97–105.

144

Y.-H. Lin et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 349 (2009) 137–144

[21] F. He, D. Zhao, J. Liu, C.B. Roberts, Stabilization of Fe–Pd nanoparticles with sodium carboxymethyl cellulose for enhanced transport and dechlorination of trichloroethylene in soil and groundwater, Ind. Eng. Chem. Res. 46 (2007) 29–34. [22] G.N. Glavee, K.J. Klabunde, C.M. Sorensen, G.C. Hadjipanayis, Chemistry of borohydride reduction of iron(II) and iron(III) ions in aqueous and nonaqueous media. Formation of nanoscale Fe, FeB, and Fe2B powders, Inorgan. Chem. 34 (1995) 28–35. [23] G.C.C. Yang, H.-C. Tu, C.-H. Hung, Stability of nanoiron slurries and their transport in the subsurface environment, Sep. Purif. Technol. 58 (2007) 166–172. [24] N. Saleh, K. Sirk, Y. Liu, T. Phenrat, B. Dufour, K. Matyjaszewski, R.D. Tilton, G.V. Lowry, Surface modifications enhance nanoiron transport and NAPL targeting in saturated porous media, Environ. Eng. Sci. 24 (2007) 45–57. [25] S. Liufu, H. Xiao, Y. Li, Adsorption of poly(acrylic acid) onto the surface of titanium dioxide and the colloidal stability of aqueous suspension, J. Colloid Interface Sci. 281 (2005) 155–163. [26] T. Phenrat, N. Saleh, K. Sirk, R.D. Tilton, G.V. Lowry, Aggregation and sedimentation of aqueous nanoscale zerovalent iron dispersions, Environ. Sci. Technol. 41 (2007) 284–290. [27] J.T. Nurmi, P.G. Tratnyek, V. Sarathy, D.R. Baer, J.E. Amonette, K. Pecher, C. Wang, J.C. Linehan, D.W. Matson, R.L. Penn, M.D. Driessen, Characterization and properties of metallic iron nanoparticles: spectroscopy, electrochemistry, and kinetics, Environ. Sci. Technol. 39 (2005) 1221–1230. [28] J.E. Martin, A.A. Herzing, W. Yan, X.-Q. Li, B.E. Koel, C.J. Kiely, W.-X. Zhang, Determination of the oxide layer thickness in core-shell zerovalent iron nanoparticles, Langmuir 24 (2008) 4329–4334. [29] S.R. Kanel, B. Manning, L. Charlet, H. Choi, Removal of arsenic(III) from groundwater by nanoscale zero-valent iron, Environ. Sci. Technol. 39 (2005) 1291–1298. [30] L. Del Bianco, A. Hernando, M. Multigner, C. Prados, J.C. Sanchez-Lopez, A. Fernandez, C.F. Conde, A. Conde, Evidence of spin disorder at the surface–core interface of oxygen passivated Fe nanoparticles, J. Appl. Phys. 84 (1998) 2189–2192. [31] T.C. Rojas, J.C. Sánchez-López, J.M. Greneche, A. Conde, A. Fernández, Characterization of oxygen passivated iron nanoparticles and thermal evolution to ␥-Fe2 O3 , J. Mater. Sci. 39 (2004) 4877–4885.

[32] Y.-P. Sun, X.-q. Li, J. Cao, W.-x. Zhang, H.P. Wang, Characterization of zero-valent iron nanoparticles, Adv. Colloid Interface Sci. 120 (2006) 47–56. [33] J.F. Moulder, W.F. Stickle, P.E.B.K.D. Sobol, Handbook of X-Ray Photoelectron Spectroscopy, Phys. Electron. (1995). [34] K. Asami, K. Hashimoto, The X-ray photo-electron spectra of several oxides of iron and chromium, Corrosion Sci. 17 (1977) 559–570. [35] J. Cao, X. Li, J. Tavakoli, W.-X. Zhang, Temperature programmed reduction for measurement of oxygen content in nanoscale zero-valent iron, Environ. Sci. Technol. 42 (2008) 3780–3785. [36] X.-Q. Li, W.-X. Zhang, Sequestration of metal cations with zerovalent iron nanoparticles—a study with high resolution x-ray photoelectron spectroscopy (HR-XPS), J. Phys. Chem. C 111 (2007) 6939–6946. [37] Y.-P. Sun, X.-Q. Li, W.-X. Zhang, H.P. Wang, A method for the preparation of stable dispersion of zero-valent iron nanoparticles, Colloid Surf. A: Physicochem. Eng. Aspect 308 (2007) 60–66. [38] X.-Q. Li, D.W. Elliott, W.-X. Zhang, Zero-valent iron nanoparticles for abatement of environmental pollutants: materials and engineering aspects, Crit. Rev. Solid State Mater. Sci. 31 (2006) 111–122. [39] S. Chibowski, M. Wisniewska, Study of electrokinetic properties and structure of adsorbed layers of polyacrylic acid and polyacrylamide at Fe2 O3 –polymer solution interface, Colloid Surf. A: Physicochem. Eng. Aspect 208 (2002) 131–145. [40] R. Baigorri, G.-M.J. Maria, G.-G. Gustavo, Supramolecular association induced by Fe(III) in low molecular weight sodium polyacrylate, Colloid Surf. A: Physicochem. Eng. Aspect 292 (2007) 212–216. [41] N. Wu, L. Fu, M. Su, M. Aslam, K.C. Wong, V.P. Dravid, Interaction of fatty acid monolayers with cobalt nanoparticles, Nano Lett. 4 (2004) 383–386. [42] L.J. Kirwan, P.D. Fawell, W. vanBronswijk, An in situ FTIR-ATR study of polyacrylate adsorbed onto hematite at high pH and high ionic strength, Langmuir 20 (2004) 4093–4100. [43] L.J. Kirwan, P.D. Fawell, W. vanBronswijk, In situ FTIR-ATR examination of poly(acrylic acid) adsorbed onto hematite at low pH, Langmuir 19 (2003) 5802–5807. [44] F. Jones, J.B. Farrow, W. van Bronswijk, An infrared study of a polyacrylate flocculant adsorbed on hematite, Langmuir 14 (1998) 6512–6517.