Sequestration of Metal Cations with Zerovalent Iron ... - ACS Publications

Characterizations with high-resolution X-ray photoelectron spectroscopy. (HR-XPS) ... for concurrent sorption and reductive precipitation of metal ion...
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J. Phys. Chem. C 2007, 111, 6939-6946

6939

Sequestration of Metal Cations with Zerovalent Iron NanoparticlessA Study with High Resolution X-ray Photoelectron Spectroscopy (HR-XPS) Xiao-qin Li and Wei-xian Zhang* Center for AdVanced Materials and Nanotechnology, Department of CiVil and EnVironmental Engineering, Lehigh UniVersity, Bethlehem, PennsylVania 18015 ReceiVed: January 10, 2007; In Final Form: February 23, 2007

Applications of nanoscale zerovalent iron (nZVI) for removal of metal cations in water are investigated with the result that nZVI has much larger capacity than conventional materials for the sequestration of Zn(II), Cd(II), Pb(II), Ni(II), Cu(II), and Ag(I). Characterizations with high-resolution X-ray photoelectron spectroscopy (HR-XPS) confirm that the iron nanoparticles have a core-shell structure, which leads to exceptional properties for concurrent sorption and reductive precipitation of metal ions. For metal ions such as Zn(II) and Cd(II) with standard potential E0 very close to or more negative than that of iron (-0.41 V), the removal mechanism is sorption/ surface complex formation. For metals with E0 greatly more positive than iron, for instance Cu(II), Ag(I), and Hg(II), the removal mechanism is predominantly reduction. Meanwhile, metals with E0 slightly more positive than iron for example Ni(II) and Pb(II) can be immobilized at the nanoparticle surface by both sorption and reduction. The dual sorption and reduction mechanisms on top of the large surface of nanosized particles produce rapid reaction and high removal efficiency, and offer nZVI as an efficient material for treatment and immobilization of toxic heavy metals.

Introduction Nanoscale zerovalent iron (nZVI) is becoming an increasingly popular method for treatment of hazardous and toxic wastes and for remediation of contaminated soil and groundwater.1-13 In the U.S. alone, more than two dozens of pilot and full-scale projects have been completed since 2001 with more ongoing or being planned in North America, Europe and Asia.1 The nanoscale size allows the particles to overcome the limitations of gravitational force and advances the Brownian motion for particle movement and dispersion. The large specific surface area of the iron nanoparticles further foster enhanced reactivity for the transformation of recalcitrant environmental pollutants. Recent innovations in nanoparticle synthesis and production have resulted in substantial cost reductions and increased availability of the nZVI for large-scale field applications.1 Much published work has so far focused largely on applications of the electron donating or reductive properties of nZVI, especially for the degradation of halogenated organic contaminants. Iron with its relative low standard potential (E0, -0.41 V) is an effective and environmentally friendly electron donor. The reduction and dechlorination of organic solvents such as tri- and tetrachloroethenes serve as a remarkable example that persistent chlorinated hydrocarbons can be ably degraded by the ordinary iron metal:1,3,7,9

C2Cl4 + 6H+ + 5Fe f C2H6 + 5Fe2+ + 4Cl-

(1)

Transformation of a wide array of environmental contaminants including chlorinated methane, ethanes, ethenes and benzenes, organochlorine pesticides, chlorinated phenols, PCBs, organic dyes, and other compounds have been extensively documented.1-10 * To whom correspondence [email protected].

should

be

addressed.

E-mail:

In aqueous solution, iron reacts with water and oxygen and forms a layer of ferrous hydroxide and at the same time generates hydrogen gas:12

Fe(s) + 2H2O(aq) f FeOOH + 1.5 H2 (g)

(2)

As a result, nZVI particles typically have a core-shell structure with the core being zerovalent iron (Fe0) and the shell as the iron oxides/hydroxides formed from the iron oxidation. This structure presents the nZVI with rich surface and redox chemistry and potentially unique properties for contaminant removal and transformation. Nonetheless, little has been published on the implications and applications of the core-shell nanostructure. The goal of this work is to explore the potential synergic effects of the core-shell iron nanoparticles for immobilization of metal ions. Recent research suggests that nZVI has metal removal capacity much higher than conventional sorbents such as activated carbon, zeolites and polymeric ion exchange resins.10,12-13 Mechanisms for metal removal by nZVI are seemingly complex and not well understood. Diverse and sometimes conflicting results have been reported in the literature.10,14-20 For example, Shokes et al.14 suggest that Cu(II) is reduced by ZVI since its standard potential is more positive than that of ZVI while the work by Wilkin et al.15 shows that adsorption of Cu(II) is the initial and the most rapid mechanism for copper uptake. For the removal of Pb(II) with nZVI, results by Ponder et al.10 suggest that Pb(II) is reduced to Pb(0). Other studies propose that Pb(II) is removed from aqueous phase by forming complex with inorganic ligands.14 It is also proposed that for metals with a standard potential more positive than that of iron, the main sequestration mechanism is electrochemical reduction.14 Experimental studies on the removal of Zn(II), Cd(II), Ni(II), Pd(II), Cu(II), and Ag(I) with nZVI are presented in this

10.1021/jp0702189 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/25/2007

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work. Systematic characterizations with high-resolution X-ray photoelectron spectroscopy (HR-XPS) on the nZVI structure and metal uptake are provided. Results from the HR-XPS work help answered three important questions: (1) which elements are present at the nanoparticle surface, (2) what chemical or valence states of these elements are present, and (3) how much of each chemical state of each element is present. Materials and Methods 1. Iron and Oxidized Iron Nanoparticles. Procedures used in the preparation of iron nanoparticles have been published previously.2 In brief, it entails slowly adding 1:1 volume ratio of 0.25 M sodium borohydride (0.25 M) into 0.045 M ferric chloride solution. The jet-black nanoparticle aggregates are collected by vacuum filtration and refrigerated in a sealed polyethylene container under 5% ethanol until use. The residual water content of the nanoparticles as used typically varies between 40 and 50%. Average size of the synthesized nZVI is 60 nm with 80% particles less than 100 nm.1 Oxidized iron nanoparticles are prepared by purging the above-described nZVI nanoparticles with pure oxygen. High purity oxygen at 175 mL/min is introduced into a 1000 mL nZVI solution with 5 g nZVI for 24 h at room temperature. A key advantage of this method is that it produces iron oxide particles with similar size as the nZVI. 2. Batch Experiments. Metal ion solutions are prepared by adding the desired amounts of metal salts to glass bottles (125 mL) and then filling in 100 mL DI water. The bottles are agitated thoroughly until the metal salts are completely dissolved. If needed, solution pH is adjusted by slow titration with 1M NaOH or HCl. Then iron nanoparticles (0-10 g/ L) are added. The batch bottles are sealed with screw caps and mixed on a shaker table (300 rpm) at ambient temperature. 3. Measurements of Metal Ion Concentrations. Concentrations of metal ions in the solution are determined with an atomic absorption (AA) spectrometer (Perkin-Elmer AA 100) with standard atomic absorption conditions. Samples are filtered with 0.45 µm syringe filters and diluted if needed. Calibration solutions with known metal ion concentrations (0.1-2500 mg/ L) are also prepared. 4. Images of the Nanoparticles. Images of the nanoparticles are recorded with a Philips EM 400T Transmission Electron Microscopy (Philips Electronics Co., Eindhoven, The Netherlands) operated at 100 kV. Samples are prepared by depositing a few droplets of dilute nZVI solution onto a carbon film (Ernest Fullam Inc., Latham, NY). The samples are then placed in a vacuumed hood till the ethanol is completely evaporated. 5. Characterization with X-Ray Photoelectron Spectrometer (HR-XPS). Surface analysis of nZVI is performed with a Scienta ESCA-300 HR-XPS. An Al rotating anode serves as the X-ray source generating a KRX-ray beam at 1486.6 eV. The X-ray beam is monochromatized using seven crystals mounted on three Rowland circles. The energy is analyzed using a high-resolution 300 mm mean radius hemispherical electrostatic analyzer and detected by a multichannel plate-CCD camera. Binding energies of the photoelectrons are correlated to the aliphatic adventitious hydrocarbon C(1s) peak at 284.6 eV. 6. Chemicals and Materials. Nickel chloride (NiCl2‚6H2O), cadmium acetate (Cd(CH3COO)2‚2H2O), zinc nitrate (Zn(NO3)2‚ 6H2O), silver nitrate (AgNO3), potassium dichromate (K2Cr2O7), lead acetate (Pb(CH3COO)2‚3H2O), cupric chloride (CuCl2‚ 2H2O), sodium hydroxide (NaOH), and hydrochloride acid (HCl) were obtained from Fisher Scientific. Ferric chloride

Figure 1. XPS wide-scan survey of fresh nanoscale zerovalent iron (nZVI).

TABLE 1: Particle Surface Atomic Percentage Determined by XPS Fresh Fe 24 h (pH 5) 24 h (pH 7) 24 h (pH 8) 24 h (pH 11)

Fe

O

B

C

30.5 15.4 18.2 16.2 17.5

52.0 73.8 73.3 75.1 75.8

11.9 3.3 2.4 3.0 2.3

5.6 7.4 6.2 5.7 4.3

anhydrous (FeCl3) was purchased from Alfa Aesar. Sodium borohydride (NaBH4, 98%) was from Finnish Chemicals (Finland). All chemicals were used without further purification. Results and Discussion The Core-Shell Structure of nZVI. Figure 1 presents the XPS wide-scan of freshly prepared nZVI. The photoelectron peaks reveal that nanoparticle surface consists mainly of iron and oxygen, as well as small amounts of boron and carbon. Carbon on the nZVI surface is likely from the inadvertent exposure to carbon dioxide and potentially small hydrocarbon molecules in air, water, and on the glassware during the sample preparation and transfer. Boron at the surface is the oxidized boron (borate), is soluble and can be rinsed off with water.27 On the basis of the individual XPS survey of the four elements at the surface and their atomic sensitivity factors, surface compositions of nZVI under different pH conditions are summarized in Table 1. Oxygen and iron are clearly the two principal (>80%) elements on the surface. Detailed XPS survey for Fe 2p and O 1s regions are shown in Figures 2 and 3. For the Fe 2p region, photoelectron peaks at ∼711 and ∼725 eV correspond to the binding energies of 2p3/2 and 2p1/2 of oxidized iron [Fe(III)]. A smaller peak at 706.5 eV suggests the existence of zerovalent iron (Fe 2p3/2). The shoulder at 719 eV is likely the result of two overlapping components: the shakeup satellite 2p3/2 for oxidized iron and 2p1/2 for zerovalent iron.21-23 The presence of a relatively small amount of ZVI and large fraction of oxidized iron indicate extensive oxidation of iron at the surface. The photoelectron spectrum for the O 1s region (Figure 3) is decomposed into three peaks at 529.9, 531.2, and 532.5 eV, which represent the binding energies of oxygen in tO-, tOH, and chemically or physically adsorbed water (tOH2), respectively.21,24 These oxygen species are identical to the surface structures of iron oxides (e.g., goethite and hematite) in water. A literature survey suggests that the precise composition of the oxide shell depends on the fabrication processes and

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Figure 4. Photoelectron peak area ratios of total O vs total Fe. Figure 2. XPS survey of the Fe 2p region. The iron nanoparticles were recovered from aqueous solutions after 24 h.

TABLE 2: Removal of Metal Ions with Zerovalent Iron Nanoparticlesa metal ion

initial concentration (mg/L)

final concentration (mg/L)

removal efficiency

Cd(II) Ni(II) Zn(II) Cr(VI) Cu(II) Pb(II) Ag(I)

100 100 100 100 100 100 100

63.5 29.0 7.5 2.5 0.27 0.29 0.25

36.5% 71.0% 92.5% 97.5% 99.7% 99.7% 99.8%

a The initial concentration is 100 mg/L for each ion and nanoparticle loading is 5 g/L for all experiments. Reaction time is 3 h.

Figure 3. XPS survey of the O 1s region. The iron nanoparticles were recovered from aqueous solutions after 24 h.

also on environmental conditions.25-27 For example, the shell of R-Fe nanoparticles prepared by sputtering consists of either maghemeite (γ-Fe2O3) or partially oxidized magnetite (Fe3O4). Nanoparticles formed by the nucleation of metallic vapor also have γ-Fe2O3 or Fe3O4, but with relatively richer γ-Fe2O3 for smaller particles due to the relatively faster oxidation reactions. Meanwhile, nanoparticles produced by the reduction of goethite and hematite particles with hydrogen gas have a Fe3O4 shell. The presence of wustite (FeO) has also been suggested.26 As to the iron nanoparticles produced by the sodium borohydride reduction of iron salts, only limited information on the surface characterization has been published.27-29

Figure 5. XPS wide-scan survey of iron nanoparticles after 3 h exposure in a solution containing a mixture of NiCl2, Cd(CH3COO)2, Zn(NO3)2, AgNO3, K2Cr2O7, Pb(CH3COO)2, and CuCl2. The initial concentrations were 100 mg/L for each metal cation.

Based on our previous work,12 the oxide shell can be stoichiometrically expressed as FeOOH. In aqueous phase, Fe2+ is first formed at the surface and rapidly oxidized to Fe3+, which further reacts with OH- or H2O to form hydroxide or oxyhydroxide:

Fe3+ + 3OH- f Fe(OH)3

(3)

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Figure 6. High-resolution XPS survey of nZVI: (a) Zn 2p3/2, (b) Cd 3d5/2, (c) Cu 2p3/2, (d) Ag 3d5/2, (e) Hg 4f7/2, (f) Cr 2p3/2, (g) Ni 2p3/2, and (h) Pb 4f7/2. The initial concentrations were 1,000 mg/L for each metal cation. Reaction time was 3 h except for Pb and Hg, which had reaction times of 10 min and 1 h, respectively.

Subsequent dehydration generates FeOOH:

Fe(OH)3 f FeOOH + H2O

(4)

In spite of the extensive surface oxidation, a substantial portion (i.e., the core) of the iron nanoparticles can remain as metallic (zerovalent). The oxidation rate of iron decreases with the growth of the oxidized shell, which in turn “protects” or “conserves” the metallic core. Measurements with X-ray Absorption Near Edge Structure (XANES) suggests28 that even after 6 weeks in water, up to 42% of the total iron is still in the

state of zerovalent iron. For an iron nanoparticle with diameter at 50 nm, this corresponds to an oxidized iron shell of approximately 5 nm in thickness. Figures 2 and 3 also contain the XPS survey of iron nanoparticles after 24 h in solutions with pH from 5 to 11. Compared to fresh nZVI, much less Fe(0) is at the surface after 24 h (Figure 2), suggesting continued oxidation of iron. From the peak height and area comparisons, iron oxidation under neutral pH (pH 7 and 8) is less pronounced relative to the more acidic (ph 5) or alkaline (pH 11) conditions. At pH 11, the photoelectron peak of Fe(0) is barely visible.

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TABLE 3: Standard Electrode Potentials at 25 °C (Ref 35) E0 (V) barium (Ba) zinc (Zn) iron (Fe) cadmium (Cd) nickel (Ni) lead (Pb) copper (Cu) silver (Ag) mercury (Hg) chromium (Cr)

+ 2 e S Ba Zn2+ + 2 e- S Zn Fe2+ + 2 e- S Fe Cd2+ + 2 e- S Cd Ni2+ + 2 e- S Ni Pb2+ + 2 e- S Pb Cu2+ + 2 e- S Cu Ag+ + e- S Ag Hg2+ + 2e- S Hg Cr2O72- + 14H+ +6e S 2Cr3+ + 7H2O Ba2+

-

-2.90 -0.76 -0.41 -0.40 -0.24 -0.13 0.34 0.80 0.86 1.36

The trends of elemental ratio as a function of solution pH are depicted in Figure 4. The O/Fe ratio after 24 h in water increases dramatically compared to that of the fresh nZVI, another indicator of iron oxidation. As expected, the XPS analysis also confirms that the relative abundance of tOH to tO- surges with the increasing pH (data not shown in the figure). Reactions of nZVI with Metal Cations. In one experiment, ZVI nanoparticles (5 g/L) are added to a solution containing a mixture of 7 metal compounds: NiCl2, Cd(CH3COO)2, Zn(NO3)2, AgNO3, K2Cr2O7, Pb(CH3COO)2, and CuCl2. The initial concentration is 100 mg/L for each metal ion. After 3 h, the residual metal ion concentrations in the solution are analyzed (Table 2). The removal efficiency of Cu, Ag, and Pb is very high (>99%). The efficiency for Cr and Zn removal is also impressive (92-97%). However, substantial amounts (39-64%) of Ni and Cd still remain in the solution. Separate experiments with single metal ions of Ni(II) or Cd(II) are also conducted in which the removal efficiency for both are much higher (>80%). It is possible that competition among the different metal cations may have resulted in the relatively low efficiency for Ni and Cd. The Ni removal is further addressed in this paper. Figure 5 presents the XPS survey of the ZVI nanoparticles after exposure to the metal ion mixtures for 3 h. In addition to Fe, O, and C, XPS peaks for five metals are evident: Zn (∼1020 eV), Cu (∼932 eV), Cr (576 eV), Cd (∼404 eV), Ag (∼368 eV), and Pb (∼138 eV).31-33 The peak for Ni is indiscernible (∼850 eV), likely due to the overlap with the background peaks and the weak signal-to-noise ratio. After 3 h of reactions, the total metal loading on the nZVI is approximately 119 mg/g nZVI. This result is consistent with our previous work on nickel removal indicating that nZVI has very high capacity for metal removal.12 The total capacity was experimentally determined to be 130 g Ni(II)/g Fe or 4.43 meq Ni(II)/g,12 which is many time higher than the best inorganic sorbents (e.g., zeolites) and well over 100% better than most polymeric cation exchange resins. The relatively high capacity can be partially attributed to the large surface area of the nanoparticles. In the following section, we demonstrate that the unique mechanisms for metal retention contribute to a large degree to the high efficiency. State of the Metals on the nZVI Surface. To achieve better XPS resolution and eliminate the interference among the metal ions, separate experiments with a single metal cation and higher initial metal concentration (1,000 mg/L) are also performed with the eight metals (Zn, Cd, Ni, Pb, Cu, Ag, Hg, and Cr). The ZVI nanoparticle concentration is 5 g/L for all experiments. After 24 h, the nanoparticles are collected for XPS characterization. For zinc, cadmium, and also copper, the binding energies of the metallic and ionic states are very close to each other. As a result, additional Auger lines are also taken with LMM for copper and zinc, and MNN for cadmium. The chemical states

Figure 7. A simplified band diagram of the iron/iron oxide/aqueous metal cations at pH 7. Values of the Fermi energies, EF (Fe) and EF (oxide), conduction, and valence band energies of the oxide layer, ECB and EVB, are from ref 37.

are then deducted from the two-dimensional plots of XPS binding energy versus Auger electron kinetic energy.31 In general, the states of the metals on the nZVI surface exhibit three distinctive responses: (1) The valence state of metals on the nZVI surface remains the same as the metal ions in solution. For example, the Zn 2p3/2 survey (Figure 6a) shows a peak at ca. 1021.6 eV. The Auger line confirms that this is Zn(II). Similarly, the Cd 3d5/2 survey (Figure 6b) presents a photoelectron peak centering at 404.7 eV. Additional Auger line survey proves that this comes from Cd(II). Both Zn(II) and Cd (II) are thus attracted to the iron surface by adsorption or surface complex formation, which may include electrostatic interactions and specific surface bonding. The bottom line is that there is no net electron transfer and hence no change on the metal valence state. (2) The valence state of the metals on the nZVI surface is in the reduced form. For example, Figure 6c is the XPS Cu 2p3/2 survey, presenting a photoelectron peak at 932.2 eV. Based on the additional Auger line survey, this peak is designated to the chemically reduced Cu(0). Likewise, the XPS survey of Ag 3d5/2, and Hg 4f7/2 has revealed only one valence state on the surfacesall represent the reduced metals, i.e., Ag(0), Hg(0) (Figure 6d,e). This group of metals is thus retained on the iron surface by chemical reduction and precipitation. Figure 6f also shows that Cr(VI) is completely reduced to Cr(III), confirming the observations of several previous studies.10,34 Since dichromate is an anion and has significantly different solution and surface chemistry, results from the Cr(VI) reduction will be addressed in a separate publication. (3) Both the reduced and ionic metals are on the nZVI surface. As shown in Figure 6 g, the Ni 2p3/2 survey offers two peaks at 852.1 and 855.5 eV. That is, both metallic Ni(0) (852.1 eV) and Ni(II) (855.5 eV) are present on the nZVI surface. Likewise

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Figure 8. Comparison of nZVI (left column) and oxidized iron nanoparticles (right column): (a, b) TEM image, (c, d) XPS Fe 2p survey, (e, f) XPS Ni 2p3/2 survey, and (g, h) XPS Zn 2p3/2 survey.

the Pb 4f7/2 survey shows that Pb exists on the surface in two chemical states: 136.4 eV (Pb(0)) and 138.0 eV (Pb(II)). In other words, both sorption and reduction are in effect for the removal of Ni(II) and Pb(II). The above XPS results demonstrate that the ZVI nanoparticles can remove metal ions from the aqueous solution by multiple mechanisms, which, in our opinion are the direct results of the core-shell structure. The surface chemistry and adsorption

properties of ferrioxyhydroxide (FeOOH) in aquatic systems strongly depend on solution chemistry (e.g., pH) and have been extensively documented.35 In water, iron nanoparticles can have metal-like or ligand-like coordination properties depending on the solution chemistry. At low pH (pHzpc≈8) carries more negative surface charge and has higher affinity toward metal cations. The observed capacity for nickel removal by nZVI is well over 100% higher than that of the oxidized iron over the pH range tested. The core-shell structure and more specifically the reduction of nickel leads to the enhanced capacity for metal removal. Compared to conventional materials such as iron hydroxides and ion exchange resins for metal removal, the combined sorption and reduction mechanisms also result in faster reactions and less change of the removal efficiency to solution pH variation. The experimental results, especially the XPS data establish the manifold mechanisms for the metal uptake by nZVI. A conceptual model (Figure 10) is constructed to facilitate the understanding on experimental results. The availability of zerovalent iron in the core delivers the reducing power or electrons for the reduction and precipitation of some metal ions. The oxide shell offers the surface coordination and sorption properties of iron oxides and characteristic s-shape curve for cation removal as a function of pH. The combination of sorption and reduction, on top of the large available surface area and

Li and Zhang reaction sites, explains the observed high capacity and efficiency of metal removal by the nZVI. Acknowledgment. Research described in this work has been supported by the Pennsylvania Infrastructure Technology Alliance (PITA) and by the US Environmental Protection Agency (EPA STAR Grants R829625 and GR832225). References and Notes (1) Li, X. Q.; Elliott, D. W.; Zhang, W. Crit. ReV. Solid State 2006, 31, 111. (2) Wang, C.; Zhang, W. EnViron. Sci. Technol. 1997, 31, 2154. (3) Zhang, W. J. Nanopart. Res. 2003, 5, 323. (4) Nutt, M.; Hughes, J.; Wong, M. EnViron. Sci. Technol. 2005, 39, 1346. (5) Doyle, J.; Miles, T.; Parker, E.; Cheng, I. Mcirochem. J. 1998, 60, 290. (6) Lien, H.; Zhang, W. J. EnViron. Eng. 1999, 125 (11), 1042. (7) Lowry, G. V.; Johnson, K. M. EnViron. Sci. Technol. 2004, 38, 5208. (8) Uegami Kawano, J.; Okita, T.; Fujii, Y.; Okinaka, K.; Kakuya, K.; Yatagai, S. (Toda Kogyo Corp.). U.S. Patent 20030217974A1, 2003. (9) Zhang, W.; Wang, C.; Lien, H. Catal. Today, 1998, 40 (4), 387. (10) Ponder, S. M.; Darab, J. G.; Mallouk, T. E. EnViron. Sci. Technol. 2000, 34, 2564. (11) Elliott, D.; Zhang, W. EnViron. Sci. Technol. 2001, 35 (24), 4922. (12) Li, X. Q.; Zhang, W. Langmuir 2006, 22, 4638. (13) Cao, J.; Zhang, W. J. Hazardous Materials 2006, 132, 213. (14) Shokes, T. E.; Moller, G. EnViron. Sci. Technol. 1999, 33, 282. (15) Wilkin, R. T.; McNeil, M. S. Chemosphere 2003, 53, 715. (16) Herbert, R. B., Jr. Mineral. Mag. 2003, 67, 1285. (17) Dries, J.; Bastiaens, L.; Springael, D.; Agathos, S. N.; Diels, L. EnViron. Sci. Technol. 2005, 39, 8460. (18) Khudenko, B. M.; Gould, J. P. Water Sci. Technol. 1991, 24, 235. (19) Powell, R. M.; Pils, R. W.; Hightower, S. K.; Sabatini, D. A. EnViron. Sci. Technol. 1995, 29, 1913. (20) Cantrell, K. J.; Kaplan, D. I.; Wietsma, T. W. J. Hazard. Mater. 1995, 42, 201. (21) Allen, G. C.; Curtis, M. T.; Hooper, A. J.; Tucker, P. M. J. Chem. Soc., Dalton Trans. 1974, 14, 1525. (22) Joyner, D. J.; Johnson, O.; Hercules, D. M. J. Am. Chem. Soc. 1980, 102, 1910. (23) Grosvenor, A. P.; Kobe, B. A.; Biesinger, M. C.; McIntyre, N. S. Surf. Interface Anal. 2004, 36, 1564. (24) Abdel-Samad, H.; Watson, P. R. Appl. Surf. Sci. 1997, 108, 371. (25) Kuhn, L. T.; Bojesen, A.; Timmermann, L.; Nielsen, M. M.; Mφrup, S. J. Phys: Condens. Matter 2002, 14, 13551. (26) Signorini, L.; Pasquini, L.; Savini, L. et al, Phys. ReV. B 2003, 68, 195423. (27) Nurmi, J. T.; Tratnyek, P. G.; Sarathy, V.; Bear, D. R.; Amonette, J. E.; Peacher, K.; Wang, C.; Linehan, J. C.; matson, D. W.; Penn, R. L.; Driessen, M. D. EnViron. Sci. Technol. 2005, 39, 1221. (28) Sun, Y. P.; Li, X. Q.; Cao, J.; Zhang, W.; Wang, H. P. AdV. Colloid Interface Sci. 2006, 120, 47. (29) Liu, Y.; Choi, H.; Dionysiou, D.; Lowry, G. V. Chem. Mater. 2005, 17, 5315. (30) Ponder, S. M.; Darab, J. G.; Bucher, J.; Caulder, D.; Craig, I.; Davis, L.; Edelstein, N.; Lukens, W.; Nitsche, H.; Rao, L.; Shuh, D. K.; Mallouk, T. E. Chem. Mater. 2001, 13, 479. (31) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. In Handbook of X-ray Photoelectron Spectroscopy, 1st ed.; Muilenberg, G. E., Ed.; Perkin-Elmer Corporation (Physical Electronics Division): 1979. (32) Yuan, G.; Seyama, H.; Soma, M.; Theng, B. K. G.; Tanaka, A. J. EnViron. Sci. Health 1999, A34, 625. (33) Chada, V. G. R.; Hausner, D. B.; Strongin, D. R., et al. J. Colloid Interface Sci. 2005, 288, 350. (34) Pratt, A. R.; Blowes, D. W.; Ptacek, C. J. EnViron. Sci. Technol. 1997, 31, 3348. (35) Stumm, W.; Morgan, J. J. Aquatic Chemistry, 3rd ed.; John Wiley & Sons, Inc.: New York, 1996. (36) Park, K. T.; Klier, K.; Wang, C. B.; Zhang, W. X. J. Phys. Chem. B 1997, 101, 5420. (37) Balko, B.; Tratnyek, P. G. J. Phys. Chem. B 1998, 102, 1459.