Highly-Efficient Arsenite [As(III)] Adsorption by a - ACS Publications

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Highly-Efficient Arsenite [As(III)] Adsorption by a [MIL-100(Fe)] Metal-Organic Framework: Structural and Mechanistic Insights Yiannis Georgiou, Jason A. Perman, Athanasios B. Bourlinos, and Yiannis Deligiannakis J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11247 • Publication Date (Web): 08 Feb 2018 Downloaded from http://pubs.acs.org on February 23, 2018

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The Journal of Physical Chemistry

Highly-Efficient Arsenite [As(III)] adsorption by a [MIL-100(Fe)] metal-organic framework: Structural and Mechanistic Insights Y. Georgioua, J.A. Permanb, A. B. Bourlinosa , Y. Deligiannakisa,* a

Physics Department, University of Ioannina, Ioannina 45110, Greece

b

Chemistry Department, University of South Florida, 4202 E. Fowler Avenue, Tampa,

Florida 33620, United States

* Corresponding author. E-mail address: [email protected] , tel. +3026 51008662

E-mail addresses: [email protected] (Y. Georgiou), [email protected] (J.A. Perman), [email protected] (A. B. Bourlinos).

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract MIL-100(Fe) metal-organic framework presents a high As(III) uptake capacity of 120 mg g-1. Mechanistic insight into the role of Fe-sites vs. carbon-sites on As(III) uptake is provided by a comparative study of a series of MIL-100(Fe) calcinated at 600, 800, 900 C. Using powder X-ray diffraction (PXRD), TEM, SEM and N2-porocimetry we have mapped the morphology evolution of the materials. FTIR, thermogravimetric analysis (TGA) combined with Electron Paramagnetic Resonance (EPR) show that non-calcined MIL-100(Fe) bears Fe3+ atoms, however after carbonization a porous carbon matrix is formed bearing Zero Valent Iron cores coated with a Fe-oxide layer, and iron carbide. The relative proportion of these phases depends on the calcination temperature i.e. 600, 800, and 900 C. A comprehensive Surface Complexation Model is presented, allowing a quantitative description of the As(III) adsorption on Fe-sites and carbon-sites. More specifically, As(III) uptake can be attributed to specific ≡FeOH sites, located inside the pores, and carbon ≡CxOH2 sites located on the surface. Confinement inside the pores is found to be responsible for the lateral interactions among the adsorbed [H3AsO3] species. The As(III) uptake of MIL-100(Fe) is 3 to 10-fold higher vs. pertinent adsorbent materials, such as graphite/graphite oxide, activated carbon, pyrolytic carbon, and comparable with MIL-101(Cr).

KEYWORDS: metal-organic framework, arsenite, adsorption, EPR, Surface Complexation Modeling


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1. Introduction Abatement of pollutants in drinking water, irrigation and industrial waters is a global concern. Arsenic is currently listed by the World Health Organization among the top ten major public health concerns in developing nations. The U.S. Environmental Protection Agency (USEPA)1, has set a limit of 10 g/L (10 ppb) or lower for arsenic contaminants in drinking water. Arsenic arises naturally from Earth’s dynamics i.e. during erosion and eruptions, but human sources such as industrial waste, ground water pumping and mining, additionally contribute to increasing arsenic levels. The global situation of arsenic contamination in water was assessed in 2008 by Amini et al.2 who showed the occurrence of non-negligible levels of arsenic in populated urban and rural areas3. Exposure to even small amounts of arsenic are associated with severe health conditions that may have adverse effects on sensitive peoplepregnant women and infants. Arsenic in aquatic environments may exist in its trivalent As(III) (arsenite) or pentavalent As(V) (arsenate) state depending on the aquatic environment’s pH. Both forms are toxic, yet As(III) is considered more toxic and more mobile than As(V)4. Various technologies and materials have been proposed for removal of arsenic, among them the most cost-effective technologies include surface binding to iron/iron oxide nanoparticles and/or amine-rich supports4–7. Immobilizing iron nanoparticles onto a bulk carrier can prevent their release into the environment while maintaining their reactivity8, whereby graphite/graphite oxide and activated carbon have supported iron-loadings for As(III) removal applications9–11. Previous work by our group and others have found that dispersion of Fe particles on various matrices improve As(III) uptake, the adsorption capacity being influenced by the characteristics of the Fe-particles, such as size, dispersion12, the type of solid-support as well as the electron-donation capacity of the matrix7,13. Metal-organic frameworks (MOFs)14–18, also 3 ACS Paragon Plus Environment


known as porous coordination polymers (PCPs)19–22, offeras adsorbentsnew opportunities in water purification technologies.

MOFs are structurally designable porous materials self-

assembled from organic molecules coordinating with metal ions/clusters. Taken together the structural landscapes and its physicochemical properties are influenced by the judicious selection of reactants. So far, MOF-based applications are extensive, including chemical adsorption, storage, separations, catalysis, sensing and the remediation of contaminates in air and water23–26 .These applications arise, as MOFs can be adjusted to influence pore-size and -shape allowing

non-covalent interactions, such as hydrogen bonds, coordination with unsaturated metal centres, electrostatic, hydrophobic, -, and acid-base interactions for selective analyte recognition. For water treatment technologies, a growing number of water-stable MOFs under varying pH conditions27,28 have been elaborated29. Herein, the water stable MIL-100(Fe) (Materials Institute Lavoisier), and its carbonized products were investigated for its Fe-sites’ and C-sites’ role in remediating As(III) from an aqueous solution. MIL-100(Fe) is a robust MOF, notably resistant to high temperature, pH variation29 and shown to be biologically safe30. It’s currently used as drug delivery vessel, for artificial kidney function31–33, and shown useful at removing organic and inorganic pollutants from water (Table 1). Samakhvalov34 reviewed its adsorption-capacity for nitrogen or sulphur compounds that differentiate MOFs from classical adsorbents such as zeolites or activated carbons. MIL-100(Fe) can also be synthesized around magnetic nanoparticles i.e. to aid in its fast recovery from contaminated medium. Moreover, nanoparticles of Ag, Pd and Pt can be grown within MIL-100(Fe) aiding photochemical reduction of Cr(VI) to Cr(III) or degrade methyl orange in water using visible light35.

The high adsorption of the organoarsenic

compounds, p-arsanilic acid and roxarsone36, is encouraging for our selection to use MIL4 ACS Paragon Plus Environment

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100(Fe) in lieu of activated carbon for As(III) remediation(see Table 1).

ZIFs (zeolitic

imidazolate framework) MnO2@ZIF-8 and Fe3O4@MIL-101 have additional shown useful at remediating arsenic species from water37–39. It has been shown that carbonization of MOFs, specifically MIL-100(Fe), creates porous carbonaceous materials encapsulating metal nanoparticles such as -Fe or zero valent iron (ZVI). Supported ZVI12 has already shown to be effective in As(III) remediation reaching 26.8 mg g-1 when combined with activated carbon. In this context, herein the investigation on two classes of porous materials, namely, MIL-100(Fe) and Porous Carbon/ZVI derived from calcined MIL100(Fe), for their role in As(III) remediation are examined. The specific aims were to study the As(III) uptake capacity these different materials and provide a physicochemical parametrisation and theoretical model that consistently describes the role of each structural component on the As(III) uptake mechanism. For this purpose, powder X-ray diffraction (PXRD), transmission electron

microscopy

(TEM),

scanning

electron

microscopy

(SEM),

nitrogen

adsorption/desorption isotherms, Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA) have been used to characterize the material. Detailed study of the Fe-phases, using dual-mode electron paramagnetic resonance (EPR), has been carried out that allowed mapping of the Fe-phases formed during the carbonization of MIL-100(Fe). PLEASE INSERT TABLE 1 HERE



2. Materials and Methods 2.1. Synthesis and reagents All materials were used as received from the providers, iron powder, trimesic acid, 40% hydrofluoric acid, sodium meta-arsenite (NaAsO2), 2-(N-Morpholino)ethanesulfonic acid hydrate, 4-Morpholineethanesulfonic acid MES hydrate & 4-(2-Hydroxyethyl) piperazine-1ethanesulfonic acid, N-(2-Hydroxyethyl)piperazine-N-(2-ethanesulfonic acid) HEPES, used for pH buffering, were obtained from Sigma-Aldrich, while HCl, NaOH, KNO3 and Cu(NO3)∙3H2O were obtained from Merck. Ultrapure water was produced by a milli-Q Academic system, Millipore. All solutions were prepared with analytical grade chemicals and ultrapure milli-Q water with a conductivity of 18.2 μS cm-1. A Carbolite Type 301 tube furnace was used for the carbonization of MIL-100(Fe). Synthesis of MIL-100(Fe) followed the procedure outlined by Horcajada et al.40 where 826.5 mg of iron powder and remaining reagents to scale are loaded into an acid digestion vessel, heated at 10 C/min and held for 24 hours at 150 C. The vessel was allowed to cool to room temperature prior to opening and the contents were then carefully decanted using a magnet to avoid unreacted metallic iron entering the washing phase. The orange crystalline powder was then thoroughly washed using a filtration set-up with copious amounts of warm water (>100 mL, 50 C), warm EtOH/H2O (1/1 v/v) and warm ethanol (100 mL) to remove unreacted ligand prior to drying the sample in a convection oven at 60 C for 12 hours. Carbonization of MIL-100(Fe) at 600, 800, and 900 C was performed under an inert Ar atmosphere with all heating rates set to 5 Cmin-1 and sample exposure for 3 hours after


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achieving the set point before returning to room temperature. The flow of argon was set to 90 cm3min-1 during heating and cooling. 2.2.Characterization Powder X-ray diffraction (PXRD) data was collected at room temperature using a Bruker D8 Advance theta-2theta diffractometer with copper radiation (Cu K,  = 1.5406 Å) and a secondary monochromator operating at 40 kV and 40 mA; whereby samples were measured between 3 and 40 for MIL-100(Fe), and between 20 and 90 2 for the carbonized samples. Thermal gravimetric analysis (TGA) was performed a TA Instruments TGA Q50 from room temperature to 1000 C at a 5 C/min rate using the default stepwise isothermal mode. Nitrogen adsorption/desorption isotherms were performed on a Micromeritics ASAP 2020 surface area analyzer at 77 K. Preparations of MIL-100(Fe) and carbonized MIL-100(Fe) samples for surface area analysis were carried out by evacuating the sample at 150 C for 3 hours under vacuum of less than 10 mHg. FTIR spectra were collected using a Perkin Elmer FT-IR Spectrometer Spectrum Two (UATR Two). Dual-Mode Electron Paramagnetic Resonance (EPR) spectra were recorded with a Bruker ER200D spectrometer at room temperature (RT) and liquid nitrogen (77K), equipped with an Agilent 5310A frequency counter. A Dual-Mode cavity was employed (Brucker ER 200D-SRC) that allows measurement of EPR signal with the microwave field (B1) polarized either perpendicular vs. external magnetic field (B) i.e. symbolically noticed “B1┴B”, or B1 parallel to B that so-called parallel-mode “B1//B”. The perpendicular-mode is the usual mode of operation of the EPR spectrometers that detects only half-integer spins41,42 e.g. such as Fe3+(S=5/2 or 1/2). On the other hand, the parallel-mode is well-suited for detection of integer 7 ACS Paragon Plus Environment


spins e.g. such as S=2 states of Fe2+ 43. Thus, in the present work, the Fe-states were studied by parallel and perpendicular EPR mode for a detailed understanding of the redox-state of the Fe in the studied materials. Scanning electron microscopy (SEM) images were captured on a Hitachi SU6600 microscope operating in the secondary electron mode and using an accelerating voltage of 5 kV. Tunneling electron microscopy (TEM) images were captured on a JEOL 2010F microscope operating at 160 kV with a point-to-point resolution of 0.19 nm. 2.3. Analytical determination of As(III) The concentration of As(III) in aqueous solution was determined by square wave Cathodic Stripping Voltammetry (SW-CSV) using a Trace Master5-MD150 polarograph by Radiometer Analytica. SW-CSV is well suited for analytical determination of As(III)7,12,44,45 with a low detection limit (0.5μg L-1). The measuring cells were borosilicate glass cells obtained from Radiometer. The working electrode was a hanging mercury drop electrode (HMDE) with a drop diameter of 0.4 mm generated by a 70 μm capillary. An Ag/AgCl electrode with a double liquid junction was used as the reference electrode with a Pt measuring electrode. Importantly, samples were not purged with N2 gas in order to avoid loss of As(III)12. During the stripping step the solution was stirred at 525 rpm. For the measurements we used aliquots of 8.3 mL shifting at pH 1700 m2g-1 with a pore volume of 0.823 cm3g1

at 0.95 P/Po

40,46

that is similar to previously reported results (see Figure S3).

The

thermogravimetric data revealed high stability of MIL-100(Fe) up to 300 C under air or nitrogen atmosphere (Figure S4). The initial mass-loss up to 300 C (~ 29%) is attributed to the loss of physically

adsorbed

water.

This

loss

of

water

gives

the

molecular

formula

Fe3O(H2O)2F[C6H3(CO2)3]2nH2O for MIL-100(Fe) where n is approximately equal to 14.5 molecules of adsorbed water molecules per metal cluster in its hydrated state. PLEASE INSERT FIGURE 3 HERE Carbonized MIL-100(Fe) samples: The thermal calcination of MIL-100(Fe) in argon produces the carbonized materials that significantly differ in physical properties i.e. as recognized by microscopy and spectroscopy measurements.

The crystalline appearance as seen from the

scanning electron micrographs, see Figure S4, is maintained for the sample heated to 600 C but is transformed to a spherical morphology with higher agglomeration for samples heated to 800 and 900 C. Transmission emission micrographs of carbonized MIL-100(Fe) which, shown in Figure 3, exhibit a carbonaceous shell coating iron nanoparticles. Noticeable, the TEM images show a distribution of nanoparticle diameters from ~50 to 200 nm. Nitrogen isotherms for the carbonized MIL-100(Fe) show a dramatic loss in porosity displaying a type IV isotherm with surface areas equal to or less than 211 m2g-1 for the three samples (Figure S5). This is expected i.e. as the organic ligands begin to graphitize/carbonize with the formation of iron nanoparticles during heating under inert atmosphere.

This

graphitization of the ligand is also evidenced in the Raman spectra (Figure 4). The characteristic peaks describing the ratios between the sp3 (ID) and sp2 (IG) carbon atoms are observed, while the 11 ACS Paragon Plus Environment


2D peak at ~2680 cm-1 was absent from the spectrum of MIL-100(Fe) carbonized to 600 C implying the presence of an amorphous carbon phase. The 800 and 900 C MIL-100(Fe) samples have higher amounts of graphitic carbon phase. PLEASE INSERT FIGURE 4 HERE The propensity of remaining organic functional groups i.e. such as carbonyl moieties, are removed with increasing calcination temperature as shown the infrared spectra (Figure S6). Note the loss of peak around 1633cm-1. PLEASE INSERT FIGURE 5 HERE The iron species formed in the carbonized MIL-100(Fe) were identified using XRD and dual mode EPR. XRD analysis identifies -Fe (Zero Valent Iron, ZVI), iron carbide (Fe3C), magnetite, wuestite and graphitic carbon in the respective materials (See Figure 5). Furthermore XRD shows that calcination induces formation of zero-valent iron particles. PLEASE INSERT FIGURE 6 HERE EPR spectra were recorded in the perpendicular (, Figure 6A) and parallel mode (//, Figure 6A insert figure). Perpendicular-mode EPR43,47 is the usual mode-of-operation in EPR spectrometers that detects half-integer spins i.e. in our case S=5/2 from Fe(III) states, or S=1/2 and from radicals. When integer-spin systems are of interest i.e. Fe(II)(S=2) in the present case, one has to use the parallel-mode, where the microwave field is polarised parallel to the externally applied magnetics field7,43. EPR spectroscopy, see Figure 6, reveals that exposure of the calcined materials at ambient atmosphere leads to formation of an oxidised Fe-layer. Formation of a thin Fe(III) oxide layer on Fe(0) is a common phenomenon, confirmer also by our group7,12,42. As explained


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in previews work7,12,42 the asymmetric line shape of the EPR signal observed on M900 is indicative of a thin Fe-oxide layer

Highly-Efficient Arsenite [As(III)] Adsorption by a - ACS Publications

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