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Magnetic Carbon Nanocages: An Advanced Architecture with Surface- and Morphology-Enhanced Removal Capacity for Arsenites Eleni Petala, Yiannis Georgiou, Vasilios Kostas, Konstantinos Dimos, Michael A Karakassides, Yiannis Deligiannakis, Claudia Aparicio, Ji#í Tu#ek, and Radek Zboril ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 16 May 2017 Downloaded from http://pubs.acs.org on May 17, 2017

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ACS Sustainable Chemistry & Engineering

Magnetic Carbon Nanocages: An Advanced Architecture with Surface- and MorphologyEnhanced Removal Capacity for Arsenites

Eleni Petala,a,b Yiannis Georgiou,c Vasilis Kostas,b Konstantinos Dimos,b Michael A. Karakassides,b* Yiannis Deligiannakis,c Claudia Aparicio,a Jiří Tuček,a and Radek Zbořila*

a

Regional Centre of Advanced Technologies and Materials, Departments of Physical Chemistry and Experimental Physics, Faculty of Science, Palacký University, 17. listopadu 1192/12, CZ77146 Olomouc, Czech Republic b

Department of Materials Science and Engineering, University of Ioannina, GR-45110 Ioannina, Greece c

Physics Department, University of Ioannina, GR-45110 Ioannina, Greece

* E-mail addresses: [email protected] (M. A. Karakassides); [email protected] (Radek Zboril)

1 ACS Paragon Plus Environment


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ABSTRACT Magnetic carbon nanocages (Mag@CNCs) have been synthesized via a green one-step process using pine resin and iron nitrate salt as a carbon and iron source, respectively. In order to produce Mag@CNCs, pristine materials have been carbonized at high temperature under inert atmosphere. The structural, textural, and surface properties of as-synthesized Mag@CNCs were studied employing microscopic, spectroscopic, and surface physicochemical methods. The obtained results showed that the new Mag@CNCs have significant surface area (177 m2 g–1) with both microporosity and mesoporosity. Moreover, the material exhibits a homogeneous distribution of core-shell-type magnetic nanoparticles within the carbon matrix, formed by iron carbide (Fe3C) and metallic iron (α-Fe), with sizes of 20-100 nm, surrounded by few graphitic layers-walls. Most importantly, Mag@CNCs were tested as absorbent for As(III) removal from aqueous solutions, showing a total As(III) uptake capacity of 263.9 mg of per gram of material at pH = 7, a record sorption capacity value among all previously tested iron-based materials and one of highest values among all reported sorbents so far. The adsorbed As(III) are anchored at the surface of Mag@CNCs, as demonstrated by high-resolution transmission electron microscopy and X-ray phtotoelectron spectroscopy measurements. The pH-edge As(III)adsorption experiments combined with theoretical surface complexation modeling allow a detailed understanding of the interfacial properties of Mag@CNCs, and hence the As(III) uptake mechanism. The analysis revealed that As(III) binds on two types of surface sites of Mag@CNCs, i.e., on carbon-surface species (≡CxOH2) and on Fe-oxide layer (≡FeΟH2) of nanoparticles. This exemplifies that the advanced morphology- and surface-driven synergistic properties of the Mag@CNCs material are crucial for its As(III)-uptake performance.


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KEYWORDS: Arsenite removal, porous magnetic carbon nanocages, iron carbide, zerovalent iron, graphitization of biomass.



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INTRODUCTION The evolution of human needs brought about a massive wave of over-consumption which has generated high environmental risks. Hence, water pollution and the effect of rising amount of waste are regarded as some of very important issues that have alerted the scientific community. Within the last decades, various technologies were investigated thoroughly and applied for water treatment. Carbon and iron-based nanomaterials were found to be very beneficial in many remediation aspects. Various types of carbon materials, with different physico-chemical features, have been investigated as promising adsorbents for removal of organic and inorganic contaminants from water such as heavy metals,1,2 dyes,3 and phenols and chlorophenols.4,5 Indeed, carbon materials can be easily modified and functionalized in order to increase their adsorption ability.6,7 Carbon nanocages (CNCs) are ranked among materials that have gained interest due to their attractive properties and ability to be involved in various applications as adsorbents, catalyst supports and electrode materials.8–10 CNCs have been described and presented as a new type of a material with a 3D spherical structure, consisting of a graphitic shell and a hollow interior with high surface area.11 The preparation of carbon nanocages has been achieved by different routes such as templating by using different metal particles,12–14 pyrolysis of iron and carbon source,10 and laser-induction complex heating evaporation synthesis.15 On the other hand, iron-based nanomaterials have high removal capacity towards many contaminants through adsorption and reduction phenomena. For instance, an excellent removal efficacy of iron-based nanomaterials has been reported for various heavy metals, e.g., Cr(VI),16,17 Pb(II),18 Co(II),19 As(V),20 and various organic compounds, e.g., azo dyes,21 pesticides,22 halogenated organic compounds,23 and pharmaceuticals.24 Moreover, they possess extraordinary magnetic features25, which is viewed as an appealing property for a cost-effective and easy


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separation from liquid media.26 Carbon and iron can be combined in composite materials, providing promising candidates for water treatment technologies. Combination of two components in a one single hybrid material, especially at the nanosized level, can result in a development of innovative systems.27 Magnetic composites have shown multifunctional properties and synergistic effects towards water treatment, combining different removal mechanisms, i.e., absorption, reduction and/or precipitation.28,29 A popular trend that attracted significant attention in the last years relies on exploitation of porous carbon materials made up from biomass precursors, through hydrothermal carbonization (high and low-temperature), template-directed synthesis (hard, soft, and dual template), and direct synthesis.30 Graphitization of biomass showed, in some cases, the potential of obtaining carbon nanocage structures.31,32 Merging carbon and iron in calcination processes proved the advanced properties of these systems in water treatment.33,34 Arsenic generally exists in natural waters in two forms, i.e., arsenite (As(III), AsO33−) and arsenate (As(V), AsO43−),35 and has been categorized as one of the most harmful elements for the aquatic and living systems even at low concentrations.36 Hence, the World Health Organization (WHO) recommends a maximum level of 10 ppb for arsenic in drinking water.37 Consequently, the need of efficient removal methods from any water source is essential and many materials have been tested including membranes, adsorbents, and nanoparticles.38 Along them, iron-based nanomaterials are regarded as prospective agents for arsenic removal. For instance, iron oxyhydroxide-loaded cellulose beads, tested for arsenate and arsenite removal from water, were found to display a high efficiency at neutral pH (7.0).39 Ferrate(VI)-based environmentally friendly approach was also reported very effective for arsenic removal.40 Among these ironbased materials, zero-valent iron (ZVI) is widely viewed as an attractive remediation tool with a



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high efficiency for As removal and low-cost/low-environmental hurdles related to iron handling.41–43 Thus, numerous attempts were designed and optimized to remove arsenic from waters by ZVI.41,44–46 For instance, adsorption of As(III) was reported to proceed in a rapid manner with ferric hydroxide formed via oxidation of Fe(0) surface promoted by dissolved oxygen. Sasaki et al.47 described removal of arsenite from water with ZVI displaying a maximum sorption capacity equal to 1.92 mg of As per g of ZVI (pH not stated). Furthermore, Kanel et al.41 determined a maximum As(III) adsorption capacity of 3.5 mg of As(III) per g of ZVI nanoparticles when treating As(III) species for 12 h at pH = 7. On the other hand, the deposition and stabilization of ZVI nanoparticles onto porous supporting materials were both identified as crucial prerequisites for effective treatment of arsenic-contaminated drinking water. A porous matrix can act as a size-controller to prevent aggregation, preserving thus the surface activity of ZVI nanoparticles. Recently, ZVI nanoparticles were synthetized reducing Fe(III) ions in situ onto a carbon matrix known as Starbon 300 with mesoporous features. As-developed materials were tested as absorbents for As(III) removal with a sorption capacity of 26.8 mg of As per g of the ZVI/Starbon 300 nanocomposite at pH = 7.27 Such type of nanocomposites offers an advantage of controllable loading of magnetic nanoparticles while retaining the highly porous nature of the carbon matrix for efficient adsorption kinetic mechanisms. Herein, we describe a simple, cost-effective, and environmentally friendly one-step synthesis of magnetic carbon nanocages (Mag@CNCs) with very high remediation ability. The synthesis of Mag@CNCs was achieved by a high-temperature carbonization of pine resin and iron salt. Pine resin has been chosen, among others, as an environmentally friendly reagent and as a natural carbon rich source based on monoterpenes. Mag@CNCs showed a significant surface area of 177 m2 g–1 and homogenously dispersed magnetic nanoparticles surrounded by few graphitic


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layers-walls with sizes of 20–100 nm. The as-synthesized Mag@CNCs were tested for removal of As(III) from aqueous solutions. Mag@CNCs material combines a superior As(III)-uptake capacity (263.9 mg g–1 at pH = 7) with an easy and environmental-friendly preparation protocol.

EXPERIMENTAL SECTION Materials and Chemicals. All the chemical reagents were used as purchased and were not subjected to any further purification procedures. Specifically, we used resin of pine tree, iron nitrate nonahydrate (Fe(NO3)3·9H2O), sodium meta-arsenite (NaAsO2), 2-(N-Morpholino) ethanesulfonic acid hydrate, 4-Morpholineethanesulfonic acid (MES hydrate) and 4-(2Hydroxyethyl)

piperazine-1-ethanesulfonic

acid,

N-(2-Hydroxyethyl)piperazine-N-(2-

ethanesulfonic acid) (HEPES), from Sigma-Aldrich, ethanol (EtOH, 99.5%) from Penta, hydrochloric acid (HCl), sodium hydroxide (NaOH), potassium nitrate (KNO3) and copper nitrate trihydrate (Cu(NO3)·3H2O) from Merck. A milli-Q Academic system from Millipore was employed for a production of ultrapure water. All solutions were prepared with chemicals of analytical grade and ultrapure milli-Q water with a conductivity of 18.2 µS cm–1. Preparation of Magnetic Carbon Nanocages (Mag@CNCs). In a typical procedure, 8 g of resin were dissolved in 15 ml of ethanol by heating at 60 °C and stirring for 30 min. Furthermore, 35 ml of 1.2 M iron nitrate solution in ethanol were added and stirred vigorously. The mixture was stirred at 60 °C up to the complete drying of the solvent. Next, the solid material was kept at 45 °C overnight. (sample labelled as RFe). Finally, the RFe sample was grinded and carbonized at 1000°C in a conventional muffle furnace under N2 flow (5 °C min−1, 15 min hold; sample labeled as Mag@CNCs).



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Characterization Techniques. X-ray powder diffraction (XRD) patterns of all the samples studied were recorded on a PANalytical X’Pert PRO (The Netherlands) instrument in the Bragg– Brentano geometry with a Fe-filtered Co-Kα radiation (40 kV, 30 mA, λ = 0.1789 nm). Samples were placed on a zero-background Si slides and were then gently pressed to adjust the sample thickness to about 0.5 mm. XRD intensity was measured in the 2θ range from 10° to 105° in steps of 0.017°. The identification of crystalline phases and quantitative Rietveld analysis were performed using the High Score Plus software (PANalytical) that includes the PDF-4+ database. Raman spectra were collected employing a micro-Raman RM 1000 Renishaw spectrometer using a laser excitation line at 532 nm (Nd-YAG) in the range of 500–2000 cm–1. A laser power of ~10 mW was used on a 2 µm focus spot, avoiding thus a photodecomposition of the samples. X-ray photoelectron spectroscopy (XPS) measurements were performed under ultrahigh vacuum conditions with a base pressure of 5 × 10–10 mbar in a SPECS GmbH instrument equipped with a monochromatic Mg-Kα source (hν = 1253.6 eV) and a Phoibos-100 hemispherical analyzer. Samples were dispersed in H2O (1 wt.%); after short sonication and stirring, a minute quantity of suspensions was then drop cast on silicon wafers and left to dry in air before transferring to ultrahigh vacuum. The energy resolution was set to 0.3 eV with a photoelectron take-off angle adjusted to 45° with respect to the surface normal. Final XPS patterns were obtained as an average of 3 scans with an energy step of 0.05 eV and a dwell time of 1 s. All binding energies were referred to the C 1s core level at 284.6 eV. Spectral analysis involved a Shirley background subtraction and peak deconvolution with mixed Gaussian−Lorentzian functions in a least squares curve-fitting program (WinSpec) developed at the Laboratoire Interdisciplinaire de Spectroscopie Electronique, University of Namur, Belgium.


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The nitrogen adsorption–desorption isotherms were measured at 77.4 K on a Sorptomatic 1990, thermo Finnigan porosimeter. Prior to the measurement, the sample selected for the surface analysis was outgassed at 353 K for 18 h under a high vacuum (10–5 mbar). Specific surface area was determined by the Brunauer-Emmett-Teller (BET) method, securing that the pertinent consistency criteria were met, using adsorption data points in the relative pressure (p/p0) range of 0.01–0.35. The pore size distribution was calculated using the Barrett-Joyner-Halenda (BJH) model and adsorption branch. The total pore volume was estimated from the adsorbed amount at a relative pressure of 0.99; on the contrary, the total surface area and the micropore volume were determined using the V-t plot method considering the so-called carbon black equation proposed in the ASTM standard D-6556-01. Transmission electron microscopy (TEM) measurements were performed on a 80 kV Titan G2 60-300 transmission electron microscope equipped with an X-FEG electron gun, objective-lens image spherical aberration corrector, and chemiSTEM EDS detector. A drop of high-purity ethanol medium with ultrasonically dispersed particles was placed onto a Holey Carbon film supported by a copper-mesh TEM grid and air-dried at room temperature. Room-temperature

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Fe Mössbauer spectrum of the Mag@CNCs sample was measured in

transmission geometry employing a Mössbauer spectrometer working in constant acceleration mode and equipped with a 50mCi

57

Co(Rh) γ-rays source. The recorded

57

Fe Mössbauer

spectrum was analyzed with the Lorentzian lines and the least-square methods integrated in the MossWinn software program. The values of isomer shift were referred to α-Fe at room temperature. Batch Experiments for Arsenic Removal – Analytical Determination of As(III). The concentration of As(III) present in aqueous solution was assessed by a square-wave cathodic



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stripping voltammetry (SW-CSV) employing a Trace Master5-MD150 polarograph from Radiometer Analytica. SW-CSV is widely regarded as a technique well suited for analytical determination of As(III) at a low detection limit (0.5 µg L–1).48 Borosilicate glass cells purchased from Radiometer Analytica were used as measuring cells. A hanging mercury drop electrode (HMDE) acted as a working electrode with a drop diameter of 0.4 mm and produced by a 70 µm capillary. An Ag/AgCl electrode featuring a double liquid junction was employed as a reference electrode with a measuring electrode of Pt type. Samples were not purged with N2 gas, thus avoiding a loss of As(III).48 The solution was stirred at 525 rpm during the stripping step. For the measurements, aliquots of 8.3 mL were used, shifting at pH < 0.5 by 1.5 mL from 6.66 M HCl and final 2 M concentration in the electrochemical cell;49 then, 8 ppm of Cu(II) species were added. Next, As(III) concentration was assessed by SW-CSV with an accumulation potential and accumulation time of – 400 mV and 60 s, respectively. Specifically, As(III) was quantified from its signal appearing at – 670 mV.50 As(III) Adsorption Experiments. Kinetics. For the kinetic measurements, the As(III) uptake from aqueous solutions was investigated in batch experiments. To monitor the kinetics of As(III) adsorption using Mag@CNCs, 5 mg of Mag@CNCs was dispersed in 50 mL buffered aqueous solution in polypropylene tubes at pH = 7 and in the presence of 5 mg L–1 of As(III). The timeevolution of As(III) concentration was studied at various contact times from the interval from 0 to 1440 min. The solid was collected by centrifugation after reaching the end of each contact period and the solution was then analyzed to assess the As(III) concentration. The amount of arsenic, which was adsorbed at time t, q (mg As g–1), was calculated from the mass-balance between the initial As(III) concentration and As(III) concentration at time t; this provided to state the adsorption rates of As(III) onto the solid adsorbents.


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Isotherms. Adsorption isotherms for Mag@CNCs were recorded at pH = 4, 5.5, and 7 in the presence of 0 to 160 mg L–1 of NaAsO2, interacting with 0.1 g L–1 of Mag@CNCs suspended in 50 mL buffer solution in polypropylene tubes. pH-Edge. Generally, pH-dependent (pH-edge) experiments provide an in-depth analysis of the interfacial adsorption mechanisms, while the adsorption isotherms are used for determination of maximum uptake capacity. Here, pH-edge experiments were performed for an initial concentration of 5 mg L–1 (NaAsO2) and 0.1 g L–1 of Mag@CNCs suspended in a 50 mL buffer solution the pH of which was adjusted in the range from 4 to 8 in the polypropylene tubes. After metal addition, the suspension was left to equilibrate for 16 h for Mag@CNCs at room temperature while agitated by a magnetic stirrer. After reaching the equilibrium state, the suspension was then centrifuged at 3000 rpm for 10 min and the supernatant solution was analyzed to determine the As(III) concentration with the procedure as described above. Control experiments (without the presence of Mag@CNCs) confirmed no loss of initial As(III). The initial pH values of solutions were adjusted with tiny volumes of 1 M HCl and/or 1 M NaOH. Here, it should be emphasized that HCl does not react with As(III) in voltametric measurements. Thus, HCl is favored against the use of HNO3 which interferes with As(III).48,49 In adsorption isotherms, the pH drift of each suspension, derived from measuring the pH value at the beginning and at the end of incubation, was less than 0.2 pH units.

RESULTS AND DISCUSSION Structural Characterization and Material Properties. The XRD pattern of Mag@CNCs sample is shown in Figure 1. The material exhibits characteristic diffraction peaks that correspond to iron carbide (Fe3C, PDF 00-034-0001), metallic iron (α-Fe, PDF 01-087-0721 and



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γ-Fe, PDF 04-014-0264), and graphite (C, PDF 00-012-0212). In particular, the intense diffraction peak appearing at 52° and the two low-intensity diffraction maxima occurring at 77° and 100° can clearly assigned to presence of α-Fe crystalline phase, i.e., (110), (200) and (211) reflections, respectively, implying the formation of ZVI nanoparticles on the Mag@CNCs matrix during pyrolysis of RFe sample. In addition, the intense peak observed at ~30° can be ascribed to the (002) diffraction planes of graphite, while all other diffraction peaks observed in Figure 1 can be attributed to the reflections of Fe3C phase. The results of quantitative analysis, by Rietveld refinement method, of XRD pattern of Mag@CNCs are shown in Table S1 in Supporting Information. As clearly seen, iron carbide is the main iron phase of Mag@CNCs, i.e., ~28 wt.%, whereas α-Fe was calculated to be ~4.5 wt.% and γ-Fe only ~0.5%. These iron phases seem to form simultaneously with graphitic carbon during the stage of RFe pyrolysis, as evidenced from the XRD patterns of carbonized RFe sample at different temperatures (see proposed formation model in Figure 2a, XRD patterns in Figure S1 and derived phase composition in Table S2 in Supporting Information).

Figure 1. XRD pattern of the Mag@CNCs sample.


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High-resolution transmission electron microscopy (HRTEM) was employed to study the morphological characteristics of the mesostructured carbon as well as of the embedded magnetic iron nanoparticles. Figure 2b-d shows the obtained HRTEM images of the Mag@CNCs sample, where graphitic layers made of few graphitic sheets are visible, surrounding the iron-based particles (dark area). Mag@CNCs appear as iron/iron carbide particles surrounded by a few graphitic layers-walls with sizes of 20–100 nm which have caused the formation of many hollow graphitic particles (carbon nanocages (CNCs)). Iron particles with partial iron oxide layer were also observed. It is well known that the nucleation and growth of CNCs could be described similarly with the formation of carbon filaments, following the base-growth model.51,52 While the temperature increases, carbon is dissolved into iron, reaching the carbon solubility limit at a certain temperature. Iron carbide (Fe3C) is produced and acts as a catalyst for the formation of these carbon structures.31 Hence, part of carbon forms the Fe3C phase and the rest, partially, precipitates and crystallizes around these particles. As a result, graphitic sheets are formed as a cap and surround the iron carbide particles as depicted in Figure 2b-d. The whole procedure progressively continues while the surface of the metal particles is free for more hydrocarbon decomposition.53 These claims are supported by the study that was conducted for different heating temperatures (for details, see text in Supporting Information, Figures S1–S4, and Tables S2 and S3). Figure 2b,c depicts with colors the EDS elemental analysis including carbon, iron, and oxygen elements. It can be observed that iron particles, which are pushed off the carbon cage (see elemental analysis in Figure 2b), are covered by oxide-passivation layer, characteristic of zerovalent iron particles.54



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Figure 2. (a) Proposed model of formation of Mag@CNCs. (b-d) Representative HRTEM images of the Mag@CNCs sample with EDS elemental mapping (panel (b) and (c)) of selected areas showing Fe, O, and C.


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The structural properties of the developed material were also investigated employing Raman spectroscopy. Figure 3a shows the recorded Raman spectrum for the Mag@CNCs sample. Two fundamental vibrations are observed at 1345 cm−1 (FWHM ∼64 cm−1) and 1590 cm−1 (FWHM ∼63 cm−1), corresponding to the D and G bands, respectively.29 These bands are characteristic of all carbon materials showing a graphite structure with defects. The G band is related to the sp2hybridized carbons, derived from the E2g vibrational mode within aromatic rings.55 Moreover, the commonly mentioned defect band, the D band, originates from the sp3-hybridized carbons, particularly, from the breathing modes of six-atom rings; to activate them, a defect is required to emerge.56 Furthermore, a 2D band appears at 2697 cm−1 (FWHM ∼69 cm−1). The 2D band is derived from the interaction between the stacked graphene layers and its intensity gets proportionately higher as the number of the layers decreases. The value of the integrated D-to-G intensity ratio (ID/IG) was estimated to 0.89. This value is quite high in comparison with bulk graphite (i.e., ID/IG = 0.1−0.3 and FWHM ∼20 cm−1), indicating a rather low ordering and high defect quantity.



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Figure 3. (a) Raman spectrum of the Mag@CNCs sample. (b) C 1s core level XPS pattern of the Mag@CNCs sample. (c) Room-temperature

57

Fe Mössbauer spectrum of the Mag@CNCs

sample. (d) N2 adsorption-desorption isotherms of Mag@CNCs sample. The inset in panel (d) shows the pore size distribution derived from the BJH model.

Figure 3b shows the C 1s core level XPS pattern that was obtained and revealed a strong graphitic character of the material. The XPS pattern can be deconvoluted into six components using the mixed Gaussian−Lorentzian functions. The dominant peak appearing at a binding


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energy of 284.6 eV is ascribed to the C–C and C–H bonds. The second peak occurring at 285.7 eV corresponds to C–O and C–N bonds, while the third peak at 286.7 eV confirms the presence of the C–O–C bond from epoxide/ether groups. The peak at 287.8 eV is assigned to carbonyl functional groups (C=O), whereas the peak recorded at 288.8 eV is attributed to the presence of carboxyl groups (O–C=O).57,58 The peak at 283.5 eV is associated with the formed iron carbide (Fe3C) phase.59,60 Zero-field

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Fe Mössbauer spectroscopy was employed in order to probe the sample phase

composition identifying all iron-bearing phases. The

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Fe Mössbauer spectrum was recorded at

300 K and is depicted in Figure 3c while the respective values of the Mössbauer hyperfine parameters are listed in Table S4 in Supporting Information. The spectrum can be deconvoluted into three spectral components, i.e., two sextets and one doublet. The major sextet, with value of the hyperfine magnetic field equal to 20.6 T, can be well assigned to iron carbide (Fe3C) phase. As it is reported in the literature, Fe atoms in the Fe3C crystal lattice occupy two different sites. Namely,

57

Fe room-temperature Mössbauer spectrum of iron carbide should theoretically be

fitted with two sextets with slightly different values of the hyperfine magnetic field (20.5 T and 20.7 T) and identical isomer shift values (0.17 ± 0.02 mm/s).61,62 Considering that the Mössbauer hyperfine values of the two Fe3C sextet components are very close to each other, it was decided to use only one sextet for Fe3C. The minor sextet, with a higher value of the hyperfine magnetic field equal to 33.1 T and a zero isomer shift value, corresponds to α-Fe. Moreover, a Fe(III) doublet component can be identified in the spectrum with a relatively high quadrupole splitting value due to distorted octahedral polyhedra. This indicates that the nature of the iron(III) oxide phase in this sample is most probably amorphous. This amorphous iron oxide phase could be attributed to the shell that covers the surface of zerovalent iron nanoparticles. Calculating and



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considering the spectral areas of individual components, it is obvious that Fe3C is the predominant phase of iron with minor formation of iron/iron(III) oxide core/shell nanoparticles, in agreement with XPS data. The textural properties of Mag@CNCs were analyzed by the N2 adsorption–desorption isotherms (see Figure 3d). The material exhibits an adsorption isotherm similar of type IV, without welldefined steps, although the uptake of isotherm at p/p0 > 0.4 confirms the existence of mesopores. In contrast, the desorption branch of the hysteresis loop is significantly steeper compared to the slope of the adsorption branch, resulting in a triangular hysteresis loop of type H2 as described in the IUPAC classification. The surface specific area (SBET) of Mag@CNCs was calculated to be 177 m2 g–1 whereas the total pore volume (Vtot), determined using the Gurvich rule at p/p0 = 0.99, was found to be ~0.25 cm3 g–1. The observed uptake of the adsorption isotherm at low p/p0 values (~0.01) also suggests the existence of micropores. The adsorption data of the material was also analyzed employing the V-t plot method. From the slopes of the V-t plot, we calculated a total surface area and micropore volume to be equal to 147 m2 g–1 and 0.04 cm3 g–1, respectively. On the other hand, the triangular hysteresis loop of the desorption isotherm suggests a complex pore structure in which pore blocking/percolation phenomena are very important. According to the work by Thommes,63 conventional type H2 hysteresis occurs for a wide distribution of independent pores with the same or similar neck size, or in a network where the neck-size distribution is much more narrow compared to the size distribution of the main cavities. Besides, a step decrease in the desorption curve near to a 0.5 value of p/p0 indicates a mechanism of desorption from the pore body, involving mainly cavitation – a spontaneous nucleation and growth of nitrogen gas bubbles, suggesting that pore body empties while the pore neck remains filled. Besides, according to the BJH method and using the adsorption branch, the pore size


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distribution of the Mag@CNCs sample implies a presence of a variety of pore sizes between 2 and 12 nm (see inset in Figure 3d). Therefore, in agreement with TEM images, the adsorptiondesorption data reveal that the Mag@CNCs material can be described as a disordered and inhomogeneous pore system with pores having access to the external surface only through a narrower neck, as in the case of the ink-bottle pore.64–67 As(III) Remediation from Aqueous Solution. Adsorption kinetic data of As(III) onto Mag@CNCs in contact times ranging between 0 and 1440 min (24 h) are presented in Figure 4a. The plots represent the amount of As(III) adsorbed onto Mag@CNCs vs. time (min). Moreover, Figure 4b shows the As(III) uptake isotherms. The pH-edge experiments, Figure 4e, allow understanding of the interfacial mechanism of As(III)-uptake. The two-step kinetic, observed in Figure 4a, indicates a the existence of an initial fast As(III) phase followed by a lower phase. Taking into account our theoretical modeling in Figure 4e and Table 3, fast As(III) adsorption occurs at external carbon-sites (≡CxOH2 surface groups), while ≡CxOH2 and ≡FeΟH2 groups located at the internal voids of the Mag@CNCs matrix are responsible for the slow phase.68 After approximately 16 h of adsorption, the kinetics reaches equilibrium. At room temperature, the maximum As(III) uptake capacity by the Mag@CNCs was assessed by fitting the experimental data with the Langmuir adsorption isotherm (Eq. 1).

qe =

qm K adsCe 1 + KCe

(1)

where qm is the maximum As(III) adsorption (mg g–1), qe is the surface concentration or surface density in mg g–1, and Ce has units of mg L–1. Following the theoretical fit (Figure 4b, red line), 19 ACS Paragon Plus Environment


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maximum capacity was evaluated equal to qm = 263.9 mg g–1 at pH = 7 (for details, see Table 1). As(III) adsorption isotherms recorded at pH = 4 and pH = 5.5 (see Figure S5 and Table S5 in Supporting Information) show that the As(III) uptake varies with pH. At more acidic pH, As(III) is enhanced almost by 100% compared to pH = 7, i.e., qm = 356.6 mg g–1 at pH = 4 and qm = 321.9 mg g–1 at pH = 5.5, respectively (for details, see Table S5 in Supporting Information). This phenomenon is due to the higher concentration of ≡CxOH2 surface groups at acid pH, as fully analyzed in the “pH-Edge of As(III) Uptake and Theoretical Modeling of Adsorption” section hereafter and Figure 4e. Notice, however, that pH = 7 is pertinent for real water bodies; therefore, the As(III) capacity at pH = 7, i.e., 263.9 mg As(III) per gr material (see Table 1) is the value to be considered in most applications.

Table 1. Langmuir Isotherm Constants for As(III) Binding onto Mag@CNCs at pH = 7. 2

Sample

qm(mg g–1)

Kads

Adjusted R

Standard Error

Mag@CNCs

263.9

0.04

0.994

13.19


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Figure 4. (a) As(III) adsorption kinetics at pH = 7 for Mag@CNCs and (b) maximum As(III) adsorption capacity for Mag@CNCs at pH = 7. (c) Representative HRTEM image with EDS



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mapping showing the presence of arsenic after treatment of As(III)-contaminated aqueous solution. (d) As 3d XPS pattern of the Mag@CNCs sample after As(III) removal. (e) As(III) adsorption in the pH range from 4 to 8 for Mag@CNCs. The inset shows speciation of As(III) adsorption onto Mag@CNCs.

Comparison with Pertinent Literature Materials. Comparative maximum As(III)-removal capacities of by different materials, found in literature, are listed in Table 2. It is obvious that the developed Mag@CNCs exhibited significantly higher removal efficiency. After treatment, the presence of trapped arsenic was unambiguously confirmed by HRTEM with chemical mapping (see Figure 4c). Furthermore, XPS pattern of the sample after As(III) removal in the As 3d energy window was recorded and analyzed (see Figure 4d). Deconvolution with mixed Gaussian−Lorentzian functions revealed that As(III) species dominate the spectrum with an almost 90% contribution, while minor contributions from 42.3 to 43.8 eV are present and may rise from As(II) and As(I) species occurring as reduction products by ZVI nanoparticles.69 In other words, the As(III) removal by Mag@CNCs is achieved mainly by absorption process.

Table 2. As(III) Removal Capacity by Various Absorbents Reported in Literature. qm (mg g–1)

Material

Reference

γ-Fe2O3–TiO2 nanoparticles

33.0

70

magnetic carbon nanotubes

8.1

71

Nano zerovalent iron (NZVI)

7.5

44

NZVI/amine-rich graphitic carbon nitride (gC3N4)

76.5

72

NZVI/mesoporous carbon

26.8

27

83

73

Titanium oxide


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Mn3O4/CeO2 hybrid nanotubes

160

74

α-Fe2O3 nanoparticles

95.5

68

Fe3O4 nanoparticles

3.7

75

Iron oxide/graphene oxide

54.2

76

Iron hydroxide granulates

2.3

77

Iron-modified bamboo charcoal

7.3

78

Fe3O4/2D-carbon flakes

57.5

79

Magnetic carbon nanocages (Mag@CNCs)

263.9

This work

pH-Edge of As(III) Uptake and Theoretical Modeling of Adsorption. In order to understand the interfacial mechanism of As(III)-uptake onto Mag@CNCs, pH-edge experiments were performed. It is noted that the experiments in Figure 4e were conducted for a low initial As(III) concentration, not at the maximum of the adsorption isotherm. Generally, it is required to use low-sorbate concentrations for the pH-edge experiments as the goal is to map the molecular interfacial mechanism of sorbate binding, avoiding any potential interfering interactions to emerge like surface precipitation or lateral interaction occurring at high concentrations. As shown in Figure 4e, the As(III) adsorption is influenced by pH, reaching a maximum at acidic pH = 4, then declining at near neutral pH values and then an enhancement of As(III) adsorption is observed at more alkaline pH. These pH-edge data were modeled with the surface complexation modeling method27,50 which considers the interfacial and solution reactions, listed in details in Table 3. The symbol “≡” in Table 3 stands for surface species of Mag@CNCs. This model, as described in Table 3, assumes that (i) proton equilibria for As(III) and the surface of Mag@CNCs are described by reactions (1) to (7). Through these reactions, the solution pH determines the As(III) species as well as the surface species available at every pH value. (ii) 23 ACS Paragon Plus Environment


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Based on the present information from our XRD, TEM, XPS, and Mössbauer spectroscopy data, three types of Mag@CNCs80 surface sites are considered. These involve ≡CxOH2 sites from the carbon matrix,80 ≡FeΟH2 sites from the Fe-oxide layer,81,82 and ≡Fe3C sites. Among all the reactions, the reaction (8) is regarded as crucial for As(III) uptake, which entails that As(III) binds in its neutral form, i.e., H3AsO3, with the neutral ≡CxOH2 of Mag@CNCs80 and also with the Fe-oxide layer (≡FeΟH2), reaction (10), in good agreement with the work by Gupta et al.81 and the study by Su and Puls.82 Furthermore, the ≡Fe3C centers do not play very important role in the As-uptake by the Mag@CNCs material (see reaction (9)). All the stability constants derived upon fitting of the model to the data (see red symbols in Figure 4e) are listed in Table 3 (reaction 8-10). These reactions provide the minimum set of reasonable reactions that well describe the experimental pH-profile. The ensuing speciation (see inset in Figure 4e) shows that the strong affinity of the ≡CxOH2 sites controls the As(III) uptake capacity that occurs at lower pH values (pH = 4–6), while the ≡FeOH2 groups determine the As(III)adsorption capacity that occurs at higher pH values. Thus, it is the composite character of Mag@CNCs, i.e., carbon sites plus Fe-sites, which determine its As(III)-uptake efficiency at a wide pH-range. Yean et al.83 reported a magnetite material with a specific surface area of 90 m2 g–1 and adsorbing 9.3 As(III) molecules per nm2. Furthermore, by employing X-ray absorption near edge structure (XANES)-edge X-ray absorption fine structure (EXAFS) technique, Manning et al.84 studied in details the geometry of arsenic adsorption mechanism. They showed that H3AsO3 – that is the adsorbed molecule (not As(III) ion) – is anchored on the neutral surface sites of goethite in a perpendicular geometry, i.e., the triangular H3AsO3 molecule is “anchored” on the surface-FeOH(0) sites through one of the As–OH groups. This geometry allows packing of


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multiple H3AsO3 molecules per nm2. In our case, the density of H3AsO3 molecules (assuming a specific surface area of 177 m2 g–1 and an arsenic adsorption capacity of 264 mg g–1) amounts to 7–8 H3AsO3-molecules per nm2. This well agrees with a perpendicular anhoring of the H3AsO3 molecules on the Mag@CNCs surface.

Table 3. Equilibrium Equations and Optimized Constants of Reactions for As(III) Binding onto Mag@CNCs.$ Reaction

Log K

Reference

Solution reactions -(1)

+

H2O ↔ H + OH-

Protonation of As(III) (2) H3AsO3 ↔ H++ Η2AsO3(3) H2AsO 3 ↔ H++ ΗAsO32-

85

– 9.2 ± 0.2

35,86

– 21.2 ± 0.2

Mag@CNCs (4) ≡CxOH+ H ↔ ≡CxOH2 (5) ≡Fe3C + H+ ↔≡Fe3C +

(6) ≡FeOH + H ↔≡FeΟH2 -

– 14.0 ± 0.2

87–89

6.0 ± 0.2 7.1 ± 0.2 5.7 ± 0.2

This work

– 7.7 ± 0.2

This work

8.6 ± 0.2 0.3 ± 0.2

This work This work

3.0 ± 0.2

This work

90

+

(7) ≡FeOH ↔ ≡FeO + H

As(III) adsorption reactions Sorption of As(III) onto Mag@CNCs (8) ≡CxOH2+ H3AsO3 ↔ ≡CxOH2-[H3AsO3] (9) ≡Fe3C+ H3AsO3 ↔ ≡Fe3C-[H3AsO3] (10) ≡FeOH2 + H3AsO3 ↔ ≡FeΟH2-[H3AsO3] $

2

–1

Diffuse layer model (25 °C): Specific surface area: 177 m g (Mag@CNCs ); concentration of –1 –1 suspended solid: 0.1 g L (Mag@CNCs ); concentration of electrolyte: 0 mol L (Mag@CNCs ); –1 constant capacitance: 22 µF cm (Mag@CNCs).

CONCLUSIONS In the present work, porous carbon nanocages (Mag@CNCs) containing magnetic iron species, such as zero-valent iron and iron carbide nanoparticles, were synthesized using a green one-step 25 ACS Paragon Plus Environment


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synthetic method. The material was synthesized by carbonization (1000 °C, inert atmosphere) of pine resin, an environmentally friendly reagent containing iron nitrate both as precursor for magnetic species and as a catalyst for the graphitization process. It is worth mentioning that the overall process could be extended, with simple modifications, to any kind of biomass or carbonrich source, giving the advantage of flexibility of the presented route. Such preparation method results in Mag@CNCs with a significant surface area of 177 m2 g–1 and homogenously dispersed magnetic nanoparticles surrounded by few graphitic layers-walls with sizes of 20–100 nm, which have caused the formation of many hollow graphitic particles or CNCs. The material showed extremely high efficiency for As(III) removal with a maximum adsorption capacity equal to 263.9 mg g–1 at pH = 7, a value considerably higher compared to those or previously tested sorbents reported in literature. The interfacial mechanism of As(III)-uptake onto Mag@CNCs studied by pH-edge experiments revealed that As(III) binds in its neutral form, i.e., H3AsO3, with the neutral surface species, i.e., ≡CxOH2, of Mag@CNCs and Fe-oxide layer (≡FeΟH2) too, suggesting the synergistic and advanced properties of the magnetic composite. In addition, the developed Mag@CNCs hybrid is highly magnetic and can be thus readily separated from the aqueous solution by a relatively weak external magnetic field (see Figure S6 in Supporting Information), similarly as in the case of water-dispersible superparamagnetic magnetite-graphene hybrid materials designed for As(III) and As(V) removal.91 Hence, the present study stimulates the potential of converting biomass into advanced materials with extraordinary beneficial uses in water treatment technologies in an easy and cost-effective way.


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ASSOCIATED CONTENT Supporting Information. XRD patterns of the RFe samples carbonized at different temperatures, phase composition of the RFe samples heated up to different temperatures, representative TEM images of the RFe samples carbonized at different temperatures, Raman spectra of the synthesized materials, C 1s core level XPS patterns of the RFe400 and RFe800 sample, maximum As(III) adsorption capacity for Mag@CNCs at pH = 4 and 5.5, photo demonstrating the ability of magnetic separation for the Mag@CNCs sample, phase composition of the Mag@CNCs sample with the values of the lattice parameters of individual phases identified during XRD analysis, values of the Mössbauer hyperfine parameters derived from the room-temperature

57

Fe Mössbauer spectrum of the Mag@CNCs sample, XPS analysis of C

atoms components, and Langmuir isotherms constants for As(III) binding onto MCNCs at pH = 4.0 and 5.5. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors * E-mail addresses: [email protected] (M. A. Karakassides); [email protected] (Radek Zboril) Author Contributions All authors contributed to writing the manuscript and have approved the final version. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS



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The authors acknowledge the support from the Ministry of Education, Youth and Sports of the Czech Republic (LO1305) and the assistance provided by the Research Infrastructure NanoEnviCz, supported by the Ministry of Education, Youth and Sports of the Czech Republic under Project No. LM2015073. This work was further supported by the Internal Grant of Palacky University in Olomouc, Czech Republic (Project No. IGA_PrF_2016_010). The authors would like to thank Jana Stráská, Dr. Ondřej Tomanec, and Martin Petr (all from the Regional Centre of Advanced Technologies and Materials, Faculty of Science, Palacky University in Olomouc, Czech Republic) for TEM, HRTEM, and XPS measurements, respectively.


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Table of Content Graphics

Magnetic Carbon Nanocages: An Advanced Architecture with Surface- and MorphologyEnhanced Removal Capacity for Arsenites

Eleni Petala, Yiannis Georgiou, Vasilis Kostas, Konstantinos Dimos, Michael A. Karakassides, Yiannis Deligiannakis, Claudia Aparicio, Jiří Tuček, and Radek Zbořil

Synopsis: Magnetic carbon nanocages, synthesized via sustainable green protocol, shows enhanced arsenite adsorption capacity due simultaneous adsorption on carbon-surface species and Fe-oxide layer of nanoparticles.


Magnetic Carbon Nanocages: An Advanced ... - ACS Publications

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