Supporting Information for. Mechanism of Arsenic Adsorption on Magnetite Nanoparticles from Water: Thermodynamic and Spectroscopic Studies

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1 Supporting Information for Mechanism of Arsenic Adsorption on Magnetite Nanoparticles from Water: Thermodynamic and Spectroscopic Studies Cheng-Hua Liu,, Ya-Hui Chuang, Tsan-Yao Chen, Yuan Tian, Hui Li, Ming-Kuang Wang, and Wei Zhang*,, Department of Plant, Soil and Microbial Sciences and Environmental Science and Policy Program, Michigan State University, East Lansing, Michigan 48824, United States Department of Engineering and System Science, National Tsing Hua University, Hsinchu 30013, Taiwan Department of Agricultural Chemistry, National Taiwan University, Taipei 10617, Taiwan *Corresponding author Bogue Street Room A516, East Lansing, MI 48824, United States. Tel.: Fax: Submitted to Environmental Science & Technology Content S1. Schematic of As Inner-sphere Complexes S2. Supplemental Materials and Methods S3. Supplemental Results and Discussions 19 pages, 7 figures, 4 tables, and 2 equations S1

2 S1. Schematic of As Inner-sphere Complexes Figure S1. Proposed inner-sphere complexes of AsO4 tetrahedron and AsO3 trigonal pyramid on FeO6 octahedral sites of iron oxides. 2 E = bidentate mononuclear edge-sharing complexes with As Fe distances (RAs-Fe) of ~2.8 Å, 2 C = bidentate binuclear corner-sharing complexes with RAs- Fe of ~3.3 Å, 1 V = monodentate mononuclear corner-sharing complexes with RAs-Fe of ~3.6 Å, and 3 C = tridentate hexanuclear corner-sharing complexes with RAs-Fe of ~3.5 Å S2

3 S2. Supplemental Materials and Methods Magnetite Nanoparticles Preparation. Magnetite nanoparticles (MNPs) were synthesized using a modified iron (II) and iron (III) coprecipitation method as described in the literature. 17 Deionized (DI) water was purged with N2 gas overnight before being used for solution preparations, and all chemicals used were of analytical grade. After placing 200 ml of mmol FeCl3 6H2O (Sigma-Aldrich) and 200 ml of mmol FeCl2 4H2O (Sigma-Aldrich) into a glass reaction vessel and mixing by mechanical stirring, the solution was heated to 80 C in a water bath. Then 50 ml of aqueous ammonia solution (28 30 wt%, Merck) was introduced into the mixed iron(ii)/iron(iii) solution with continuous stirring at 80 C for 30 min. In order to avoid Fe(II) oxidation, the synthesis process was carried out under N2 protection by purging N2 gas into both solution and headspace in the reaction vessel. After the reaction was completed, the black precipitates formed were collected by a magnet followed by washing with N2-purged DI water to remove residual salts. The collected solids were freeze-dried, transferred into a glass vial, and stored in a vacuum desiccator prior to use. During this process, transferring of solutions, suspensions and solids was done as quickly as possible to reduce air exposure time, followed by purging N2 gas for 10 seconds before the bottles were tightly capped or sealed to maintain anoxic conditions. Adsorbent Characterization. Structural and surface properties of synthesized MNPs were characterized as follows. X-ray diffraction (XRD) pattern of MNPs was obtained using a Rigaku Geigerflex diffractometer (Rigaku, Japan) from 20 to 70 (2θ) at a scanning rate of 2 min 1, and the XRD analysis was set at 45 kv and 15 ma with CuKα radiation (λ = Å). Specific surface area was determined from nitrogen adsorption isotherms using the Brunauer Emmett Teller (BET) method by a Micromeritics Tristar 3000 analyzer (Micromeritics, USA). S3

4 Transmission electron microscopy (TEM, JEOL JEM-2200FS, Japan) was performed for MNPs suspended in either methanol, 0.01 M NaNO3 solution, or solution containing 1 mm As(V) and 0.01 M NaNO3. Primary particle size distribution of MNPs was determined from 302 particles observed in 9 TEM micrographs of methanol-dispersed MNPs using ImageJ (version 1.46r) software (Wayne Rasband, National Institutes of Health, USA). As MNPs tend to form larger aggregates under experimental conditions, aggregate size distributions of 1g L 1 MNPs dispersed in 0.01M NaNO3 solution at 0 and 24 h post 30 min sonication were also measured by a Malvern Mastersizer 2000 (Malvern Instruments Inc., Westborough, MA, USA). Isoelectric points (IEPs) of MNPs with and without the adsorbed As (V) or As (III) were determined by measuring zeta potentials at a range of solution ph, using a Malvern Zetasizer Nano-ZS (Malvern, USA). The MNP samples for zeta potential measurements were prepared by mixing 50 mg MNPs with 50 ml of 0.01 M NaNO3 solution with or without 1 mm As(V) or As(III) at ph 5.0. After 30 min sonication, suspensions were shaken at 180 rpm for 24 h at room temperature (23 o C), and then MNPs were collected by a magnet followed by washing with DI water to remove residual As. After washing five times, the collected solids were re-suspended into 50 ml DI water to obtain 1.0 g L 1 As-free and As-adsorbed MNP suspensions. The suspensions were further diluted to a series of 10 ml 0.01 g/l MNPs suspensions at 0.01 M NaNO3 with ph adjusted between 2.0 to The suspensions were shaken on end-over-end shaker for 24 h. The final ph and zeta potential of suspensions were then measured, and the IEPs of As-free and As-adsorbed MNPs were determined. Adsorption Kinetics. For adsorption kinetic studies, 500 mg MNPs were weighed into the polypropylene bottles and then 250 ml of DI water was added. The suspensions were adjusted to ph 5.0 and placed into an ultrasonic bath to sonicate for 30 min. Subsequently, 250 ml of 1.5 mm S4

5 As(V) or As(III) solution in 0.02 M NaNO3 solution at ph 5.0 were added into the suspensions, achieving the initial As concentration of 0.75 mm, ionic strength of 0.01 M, and solid-liquid ratio is 1.0 g/l. Suspensions were horizontally shaken at 180 rpm in a shaking water bath at 25 C. At pre-determined time interval, 5-mL suspension was withdrawn and filtered through a 0.22 µm syringe filter with mixed cellulose esters membrane (Millipore, USA), followed by measurements of As concentrations in the filtrates by inductively coupled plasma optical emission spectrometry (ICP-OES, Perkin Elmer Optima 2000DV, Waltham, MA, USA). The pseudo-second-order kinetic model was used to fit the observed kinetic data. The linear form of pseudo-second-order kinetic model is given as: 18 t q t t 1 (S1) q k q e 2 2 e where k2 (g mmol 1 h 1 ) is the pseudo-second-order rate constant, qe (mmol g 1 ) is the amount of adsorbed As at equilibrium, and qt (mmol g 1 ) is the amount of adsorbed As at time t. The terms k2 and qe can be determined from the plot of t/qt versus t, and the initial adsorption rate h (mmol g 1 h 1 ) is defined as: 18 h k q 2 (S2) 2 e Sample preparation for X-Ray Absorption Spectroscopy (XAS) and X-Ray Photoelectron Spectroscopy (XPS) analyses. The wet-paste samples were prepared in a similar method as described for the adsorption experiments under anoxic conditions. Briefly, 250 mg MNPs samples were mixed with 125 ml DI water and the suspensions were sonicated for 30 min. After sonication, the suspensions were mixed with 125 ml of 2.0 mm As(V) or As(III) solution in 0.02 M NaNO3 at ph 5.0 to reach As concentration of 1.0 mm and ionic strength of 0.01 M NaNO3. The suspensions were shaken at 180 rpm for 24 h at 25 C, and then filtered through a 0.22 μm membrane filter using vacuum filtration. The collected solids were washed with 50 ml S5

6 DI water five times on the filter to remove excess As. The top of filtration unit was sealed with Parafilm and purged with N2 during filtration process. Then, the filter uniformly covered with the collected solids was quickly transferred into a polyethylene (PE) bag. The PE bag was gently purged with N2 gas for a few seconds, and then the remaining gas was squeezed out before the bag was sealed. Finally, the PE bags containing the freshly prepared wet-paste samples were placed into a secondary bag with N2 gas protection, and then transported to the beamline BL17C1 for XAS measurement at National Synchrotron Radiation Research Center (NSRRC) within 24 h. The oven-dried samples were prepared in a similar adsorption manner as described for the wetpaste samples, but different post-adsorption treatment. After As adsorption under anoxic condition, the MNP solids were collected by a magnet followed by washing with DI water to remove residual arsenic. After washing five times, the collected solids were oven-dried at 40 o C overnight to remove residual water, and no N2 protection was taken during the drying process. Finally, these freshlyprepared dried samples were transported to both the beamline BL17C1 for XAS measurement and BL24A1 for XPS measurement at NSRRC within 24 h. XAS Data Collection. The As K-edge (11,867 ev) XAS spectra, including the X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), were collected at the wiggler beamline of BL17C1 in NSRRC. The electron storage ring was operated at 1.5 GeV with a stored current of 360 ma. The energy of the X-ray beam is selected by a fixed-exit double crystal monochromator with Si (111) crystals at BL17C1. The wet-paste samples within the sealed PE bag was mounted directly on an Al sample holder, and the ovendried powder sample mounted on an Al sample holder with Scotch tape. All XAS data were collected in the fluorescence mode at room temperature, using a Lytle detector equipped with Soller slits and Ge filter for screening scattering and fluorescence background. To obtain a full S6

7 XAS spectrum, the monochromator energy was scanned from to ev, using a step size of 0.6 ev in the near edge region. At least two XAS scans were collected for each sample to improve the signal-to-noise ratio. Energy calibrations were accomplished by recording a reference spectrum of sodium arsenate powder simultaneously with the samples. The maximum absorption edge of sodium arsenate reference was set to ev. No beam damage was observed during XAS data collection by comparing individual scans of each sample. XAS Data Analysis. The XAS data analysis was performed using the ATHENA and ARTEMIS interfaces to the IFEFFIT version program package, 19 following the procedures suggested in Kelly et al. 20 The raw XAS data was processed using ATHENA with data conversion, energy calibration, data alignment and merge, background subtraction, normalization, and Fourier transformation. E0 was set at the first inflection point in the spectra for all samples. The energycalibrated, aligned, and merged XAS spectra were normalized and background subtracted using a linear pre-edge function between -200 and -50 ev and a quadratic post-edge function between 150 and 800 ev, defined by the AUTOBKG algorithm. In order to analyze the As oxidation state of each sample, the normalized XANES spectra were fitted using linear combination fitting (LCF) analysis within a fitting range from 20 ev to 30 ev. The spectra of sodium arsenate and sodium arsenite aqueous solutions were served as standard spectra for LCF analysis. Precision of LCF 21, 22 analysis from XANES spectra was estimated to be 5%. The processed XAS spectra were further transformed from electron energy to photon-electron wave vector unit (k, Å 1 ) and weighted by k 3 to generate k 3 x(k) spectra, and then transferred to ARTEMIS for EXAFS data analysis. Fourier transformation was performed to produce radial distribution function (RDF) within selected k range, Å 1 for As(V)-adsorbed MNPs and Å 1 for As(III)-adsorbed MNPs, using Hanning function with a dk value of 1 and dr value S7

8 of 0.5. The EXAFS analysis was then performed by fitting the experimental spectra to the theoretical calculations. All spectra were fitted for interatomic distance shift (ΔR), degeneracy of a path (N), Debye Waller factor (σ 2 ), and energy shift (ΔE) across the R range from 1.0 to 4.0 Å. One fixed amplitude reduction factor (S0 2 ) of 0.95 and one fitted ΔE were applied to all scattering paths. Theoretical As O and As Fe scattering paths were calculated with FEFF8 23 extracting from the structural model of scorodite and tooeleite. The goodness of fitting was evaluated by R-factor, for which less than 0.05 is considered to be a reasonable fit. 20 XPS Data Collection. The synchrotron-based XPS analyses were performed using the widerange spherical grating monochromator beamline and a SPEC Phoibos150 energy analyzer (energy resolution < 0.3 ev) at the beamline BL24A1 in NSRRC. The energy analyzer was operated with a pass energy of 20 ev and a dwell time of 0.1 s. The MNP samples were pressed onto an indium foil (Sigma-Aldrich) and mounted on a stainless steel sample holder by double-sided carbon tape. The sample holder was then transferred into an ultra-high vacuum chamber and MNP samples were scanned under a pressure of to Torr. The instrument work function was referenced to the Au4f7/2 peak for metallic gold at a binding energy (BE) of 84.0 ev. The photon energy was set to 700 ev for survey scans and 275 ev for high-resolution scans. Survey scan analyses were performed with a step size of 0.1 ev in an energy range of ev. High-resolution scan was carried out with a step size of 0.05 ev in an energy range of ev for Fe3p and As3d XPS spectra. Each high-resolution scan was performed twice. The potential effects of beam damage during XPS data collection was excluded by testing with longer irradiation time (around 40 min) and different photon energy ( ev) at the same spot position. XPS Data Analysis. Indium was serviced as internal standard and the XPS spectra were charge-calibrated to the In 3d5/2 peak at BE of ev. 24 The Fe 3p and As 3d region of XPS S8

9 spectra were further analyzed using XPSpeak 4.1 software. For peak curve fitting, Shirley function was applied to subtract background from 61 ev to 51 ev for Fe3p peak and from 50 ev to 40 ev for As 3d peak. The Fe 3p peak was deconvoluted into Fe(III) and Fe(II) components via the curve fitting. The Fe 3p peak was fitted by fixing BE of 55.7 ev for Fe(III), 25 BE of 54.9 ev for Fe(II), 25 Gaussian/Lorentzian ratio of 40/60, and asymmetry factor of 0.4 for both peaks. 26 The full width at half maximum (FWHM) of Fe(III) and Fe(II) peaks was fitted to be 2.60 for Fe(III) and 1.60 for Fe(II) from As-free MNPs sample, and the determined FWHM values were fixed for the curve fitting of As-adsorbed MNPs samples to investigate the change of Fe oxidation states after arsenic adsorption and air exposure. The As 3d peak was deconvoluted into As(V) and As(III) components. The As 3d peak was curve-fitted with the BE of 44.8 ev for As(V), BE of 43.5 ev for As(III), Gaussian/Lorentzian ratio of 30/70, asymmetry factor of 0, and the FWHM of 1.15 ev. 27 The As 3d5/2 and As 3d3/2 components of the As 3d doublet peaks were constrained by the intensity ratio of 3:2 and spin-orbit splitting of 0.7 ev. 27 Finally, the precision of XPS analysis was estimated to be 5%. 28 S9

10 S3. Supplemental Results and Discussion Figure S2. XRD pattern of (a) powder synthesized MNPs and (b) standard magnetite (PDF ). S10

11 Figure S3. TEM image of MNPs: (a) suspended in methanol, (b) suspended in 0.01M NaNO3, and (c) suspended in 1 mm arsenate and 0.01M NaNO3 after 24-h adsorption experiment; (d) Particle size distribution of MNPs in methanol from TEM image analysis. S11

12 Figure S4. Particle size distribution of 1 g L 1 MNPs suspension in 0.01M NaNO3 after 30 min sonication (0 h) and after 24 h shaking. Figure S5. Zeta potential of As-free and As-adsorbed MNPs dispersed in 0.01 mm NaNO3 solution. S12

13 Adsorption Kinetics. The kinetics of As(V) and As(III) adsorption on MNPs was investigated to determine the As adsorption rate and equilibrium time. The As adsorption kinetics at ph 5.0 are shown in Figure S6, indicating that As(V) and As(III) have a similar adsorption pattern. As(V) and As(III) adsorptions on MNPs occurred rapidly during the initial 5 min, followed by a slower adsorption that gradually reached an adsorption plateau at 120 min. The majority of adsorption (more than 80%) occurred within the first 5 min. The rapid adsorption process may have been enhanced by the large surface area of MNPs, which provided more accessible adsorption sites for both As species. The slower adsorption may have occurred because most available adsorption sites were occupied, which decreased collision efficiency of As to the MNP surface due to repulsion between free As and adsorbed As. The fitted parameters of the pseudo-second-order model and coefficient of determination (R 2 ) are summarized in Table S1. While the initial adsorption rate (k2) of As(V) was faster than that of As(III), the adsorption magnitude was similar. The high R 2 values (R 2 > 0.999) indicate that the pseudo-second-order model was suitable to describe the As(V) and As(III) adsorption kinetics on MNPs, and the adsorption processes were controlled by chemisorption. 29 Table S1. Fitted Parameters of Pseudo-second-order Kinetic Model to the Adsorption Kinetics of As(V) and As(III) on MNPs Arsenic species qe,exp (mmol g 1 ) qe,cal (mmol g 1 ) k2 (g mmol 1 h 1 ) h (mmol g 1 h 1 ) As(V) R 2 As(III) S13

14 Figure S6. Adsorption of As on MNPs as a function of time (initial [As] = 0.75 mm, ph = 5.0, temperature = 25 C, ionic strength = 0.01 M NaNO3, solid-liquid ratio = 1 g L 1 ). Solid line represents the fitting result of pseudo-second-order kinetic model. S14

15 Figure S7. Adsorption of As on MNPs at different ph (a) and ionic strength (b) (initial [As] = 1.0 mm, temperature = 25 C, solid-liquid ratio = 1.0 g L 1 ). S15

16 XAS and XPS Analysis Results. Table S2. Linear Combination Fitting Analysis Results of XANES Data for As-adsorbed Samples. Sample As(III)(%) As(V)(%) As(V)-MWP 2 98 As(V)-MOD As(III)-MWP 96 4 As(III)-MOD Table S3. Results of Shell-by-shell Fitting of EXAFS Data for Arsenic-adsorbed MNPs Samples a Sample Atomic path CN R (Å) σ 2 (Å) ΔE (ev) R-factor As(V)-MWP- As-O 3.9 (0.4) 1.69 (0.00) (0.002) 14.5 (1.8) μmol m -2 MSAs-O-O As-Fe 1.5 (0.4) 3.35 (0.02) (0.002) As(V)-MOD- As-O 3.5 (0.7) 1.69 (0.01) (0.001) 14.5 (1.1) μmol m -2 MSAs-O-O As-Fe 1.4 (0.9) 3.39 (0.06) (0.002) As(III)-MWP- As-O 2.9 (0.4) 1.79 (0.01) (0.001) 13.0 (1.7) μmol m -2 MSAs-O-O As-Fe 3.8 (0.5) 3.49 (0.03) (0.008) As-Fe 1.0 (0.7) 3.76 (0.05) (0.004) As(III)-MOD- As-O 3.2 (0.6) 1.74 (0.01) (0.001) 9.3 (1.7) μmol m -2 MSAs-O-O As-Fe 3.8 (1.0) 3.39 (0.09) (0.009) As-Fe 1.3 (0.8) 3.67 (0.11) (0.005) a CN = coordination number; R = interatomic distance; σ 2 = Debye Waller factor (disorder parameter); ΔE = difference in threshold energy; R-factor = goodness of fitting; MSAs-O-O = multiple scattering. The amplitude reduction factor (S0 2 ) was fixed of Fitted parameters are shown with standard error in parentheses. Constrained parameters are shown without a parenthesis. The value after labels is the surface coverage. S16

17 Table S4. Fitting Parameters and Relative Peak Areas of Peak Components for Fe 3p and As 3d XPS Spectra Sample Electron shell Component BE (ev) a FWHM (ev) b Peak area (%) As-free MNPs Fe 3p Fe(III) Fe(II) As 3d As(V) 44.8 c 1.15 As(III) 43.5 c 1.15 As(V)-MOD Fe 3p Fe(III) Fe(II) As 3d As(V) 44.8 c As(III) 43.5 c As(III)-MOD Fe 3p Fe(III) Fe(II) As 3d As(V) 44.8 c As(III) 43.5 c a BE = binding energy; b FWHM = full width half maximum; c BE of As 3d5/2 peak. The As 3d spin orbit-split doublet peaks were fixed a ratio of 3:2 at 0.70 ev. S17

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