Magnetic Carbon Nanocages: An Advanced Architecture with Surface

Research Article pubs.acs.org/journal/ascecg

Magnetic Carbon Nanocages: An Advanced Architecture with Surface- and Morphology-Enhanced Removal Capacity for Arsenites Eleni Petala,†,‡ Yiannis Georgiou,§ Vasilis Kostas,‡ Konstantinos Dimos,‡ Michael A. Karakassides,*,‡ Yiannis Deligiannakis,§ Claudia Aparicio,† Jiří Tuček,† and Radek Zbořil*,† †

Regional Centre of Advanced Technologies and Materials, Departments of Physical Chemistry and Experimental Physics, Faculty of Science, Palacký University in Olomouc, 17. listopadu 1192/12, CZ-77146 Olomouc, Czech Republic ‡ Department of Materials Science and Engineering, University of Ioannina, GR-45110 Ioannina, Greece § Physics Department, University of Ioannina, GR-45110 Ioannina, Greece S Supporting Information *

ABSTRACT: Magnetic carbon nanocages (Mag@CNCs) were synthesized via a green one-step process using pine resin and iron nitrate salt as a carbon and iron source, respectively. To produce Mag@CNCs, pristine materials have been carbonized at high temperature under inert atmosphere. The structural, textural, and surface properties of assynthesized 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 a few graphitic layers-walls. Most importantly, Mag@CNCs were tested as absorbents for As(III) removal from aqueous solutions, showing a total of 263.9 mg As(III)-uptake capacity 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) species are anchored at the surface of Mag@CNCs, as demonstrated by high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy measurements. The pH-edge As(III)-adsorption experiments combined with theoretical surface complexation modeling allowed 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 (FeOH2) of nanoparticles. This exemplifies that the advanced morphology- and surfacedriven synergistic properties of the Mag@CNCs material are crucial for its As(III)-uptake performance. 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 overconsumption that 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 physicochemical 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 © 2017 American Chemical Society

be involved in various applications such 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 toward many contaminants through adsorption and reduction phenomena. For instance, an excellent removal efficacy of iron-based nanomaterials has been reported Received: February 8, 2017 Revised: May 15, 2017 Published: May 16, 2017 5782

DOI: 10.1021/acssuschemeng.7b00394 ACS Sustainable Chem. Eng. 2017, 5, 5782−5792

Research Article

ACS Sustainable Chemistry & Engineering 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 features,25 which is viewed as an appealing property for a costeffective and easy 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 the development of innovative systems.27 Magnetic composites have shown multifunctional properties and synergistic effects toward water treatment, combining different removal mechanisms, i.e., adsorption, 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 of 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 oxyhydroxideloaded cellulose beads, tested for arsenate and arsenite removal from water, were found to display a high efficiency at neutral pH (7.0).39 A ferrate(VI)-based environmentally friendly approach was also reported very effective for arsenic removal.40 Among these iron-based materials, zerovalent iron (ZVI) is widely viewed as an attractive remediation tool with a 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 synthesized reducing Fe(III) ions in situ onto a carbon matrix known as Starbon 300 with mesoporous features. As-developed materials were tested as adsorbents 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 hightemperature carbonization of pine resin and iron salt. Pine resin was 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 homogeneously dispersed magnetic nanoparticles surrounded by a few graphitic 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 environmentally friendly preparation protocol.

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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-(2-Hydroxyethyl)piperazine-1ethanesulfonic acid, N-(2-hydroxyethyl)piperazine-N-(2-ethanesulfonic acid) (HEPES), from Sigma-Aldrich, ethanol (EtOH, 99.5%) from Penta, and 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 was 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 labeled 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). 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 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 takeoff angle adjusted to 45° with respect to the surface 5783


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ACS Sustainable Chemistry & Engineering

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 of buffer solution in polypropylene tubes. pH-Edge. Generally, pH-dependent (pH-edge) experiments provide an in-depth analysis of the interfacial adsorption mechanisms, whereas 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 the equilibrium state was reached, 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 voltammetric 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.

normal. Final XPS spectra 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 curvefitting program (WinSpec) developed at the Laboratoire Interdisciplinaire de Spectroscopie Electronique, University of Namur, Belgium. 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 coppermesh TEM grid and air-dried at room temperature. Room-temperature 57Fe 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 50 mCi 57Co(Rh) γ-ray source. The recorded 57Fe Mössbauer spectrum was analyzed with the Lorentzian lines and the least-squares 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 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 was 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 time-evolution 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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RESULTS AND DISCUSSION Structural Characterization and Material Properties. The XRD pattern of Mag@CNCs sample is shown in Figure 1.

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

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 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 be 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 the 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 5784


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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 energydispersive X-ray spectroscopy (EDS) elemental mapping (panels b and c) of selected areas showing Fe, O, and C.

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

Figure S1 and derived phase composition in Table S2 in the Supporting Information). 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 a 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 that 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 5785


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

whereas 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 57Fe Mössbauer spectroscopy was employed in order to probe the sample phase composition identifying all iron-bearing phases. The 57Fe Mö ssbauer spectrum was recorded at 300 K and is depicted in Figure 3c, whereas 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, room-temperature 57Fe Mössbauer spectrum of iron carbide should theoretically be fitted with two sextets with slightly different values of the hyperfine magnetic field (20.5 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

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 oxidepassivation layer, characteristic of zerovalent iron particles.54 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 proportionally 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. Figure 3b shows the C 1s core level XPS spectrum that was obtained and revealed a strong graphitic character of the material. The XPS spectrum can be deconvoluted into six components using the mixed Gaussian−Lorentzian functions. The dominant peak appearing at a binding 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, 5786


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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 mapping showing the presence of arsenic after treatment of As(III)-contaminated aqueous solution. (d) As 3d XPS spectrum 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.

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 the pore body empties while the pore neck remains filled. Besides, according to the BJH method and using the adsorption branch, the pore size 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 adsorption−desorption 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

iron nanoparticles. Calculating and 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 well-defined 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 specific surface 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 5787


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ACS Sustainable Chemistry & Engineering 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 an understanding of the interfacial mechanism of As(III)uptake. The two-step kinetics, observed in Figure 4a, indicates the existence of an initial fast As(III) removal phase followed by a slower 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), whereas CxOH2 and FeOH2 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 =

Table 2. As(III) Removal Capacity by Various Adsorbents Reported in Literature

γ-Fe2O3−TiO2 nanoparticles magnetic carbon nanotubes nano zerovalent iron (NZVI) NZVI/amine-rich graphitic carbon nitride (gC3N4) NZVI/mesoporous carbon titanium oxide Mn3O4/CeO2 hybrid nanotubes α-Fe2O3 nanoparticles Fe3O4 nanoparticles iron oxide/graphene oxide iron hydroxide granulates iron-modified bamboo charcoal Fe3O4/2D-carbon flakes magnetic carbon nanocages (Mag@CNCs)

(1) −1

where qm is the maximum As(III) adsorption (mg g ), 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), maximum capacity was evaluated equal to qm = 263.9 mg g−1 at pH = 7 (for details, see Table 1). As(III) adsorption Table 1. Langmuir Isotherm Constants for As(III) Binding onto Mag@CNCs at pH = 7 sample

qm (mg g−1)

Kads

adjusted R2

standard error

Mag@CNCs

263.9

0.04

0.994

13.19

33.0 8.1 7.5 76.5 26.8 83 160 95.5 3.7 54.2 2.3 7.3 57.5 263.9

reference 70 71 44 72 27 73 74 68 75 76 77 78 79 this work

nanoparticles.69 In other words, the As(III) removal by Mag@CNCs is achieved mainly by adsorption process. pH-Edge of As(III) Uptake and Theoretical Modeling of Adsorption. 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 that 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 r1 to r7. Through these reactions, the solution pH determines the As(III) species as well as the surface species available at every pH value. (ii) 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 FeOH2 sites from the Fe-oxide layer,81,82 and  Fe3C sites. Among all the reactions, the reaction r8 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 (FeOH2), reaction r10, 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(III)-uptake by the Mag@CNCs material (see reaction r9). All the stability constants derived upon fitting of the model to the data (see red symbols in Figure 4e) are listed in Table 3 (reactions r8−r10). These reactions provide the minimum set of reasonable reactions that well describe the experimental pHprofile. The ensuing speciation (see inset in Figure 4e) shows

qmK adsCe 1 + KCe

qm (mg g−1)

material

isotherms recorded at pH = 4 and pH = 5.5 (see Figure S5 and Table S5 in the 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 the 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 in 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 g material (see Table 1) is the value to be considered in most applications. Comparison with Pertinent Literature Materials. Comparative maximum As(III) removal capacities of various materials, found in the literature, are listed in Table 2.27,44,68,70−79 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 spectrum 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 almost 90% contribution, whereas 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 5788


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Table 3. Equilibrium Equations and Optimized Constants of Reactions for As(III) Binding onto [email protected] log K

reaction

reference

solution reactions

H 2O ↔ H+ + OH−

(r1)

−14.0 ± 0.2

85

−9.2 ± 0.2

35, 86

protonation of As(III)

H3AsO3 ↔ H+ + H 2AsO3−

(r2)

H 2AsO3 ↔ H+ + HAsO32 −

(r3)

−21.2 ± 0.2

Mag@CNCs

≡Cx OH + H ↔ ≡ Cx OH 2

(r4)

≡Fe3C + H+ ↔ ≡ Fe3CH

(r5)

≡FeOH + H+ ↔ ≡ FeOH 2

(r6)

≡FeOH ≡ FeO− + H+ (r7) As(III) adsorption reactions sorption of As(III) onto Mag@CNCs ≡Cx OH 2 + H3AsO3 ↔ ≡ Cx OH 2 − [H3AsO3 ]

≡Fe3C + H3AsO3 ↔ ≡ Fe3C − [H3AsO3 ]

(r8)

(r9)

≡FeOH 2 + H3AsO3 ↔ ≡ FeOH 2 − [H3AsO3 ]

(r10)

6.0 ± 0.2

87−89

7.1 ± 0.2

90

5.7 ± 0.2

this work

−7.7 ± 0.2

this work

8.6 ± 0.2

this work

0.3 ± 0.2

this work

3.0 ± 0.2

this work

Diffuse layer model (25 °C): specific surface area, 177 m2 g−1 (Mag@CNCs); concentration of suspended solid, 0.1 g L−1 (Mag@CNCs); concentration of electrolyte, 0 mol L−1 (Mag@CNCs); constant capacitance, 22 μF cm−1 (Mag@CNCs). a

that the strong affinity of the CxOH2 sites controls the As(III)-uptake capacity that occurs at lower pH values (pH = 4−6), whereas 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 determines 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 detail the geometry of arsenic adsorption mechanism. They showed that H3AsO3, 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 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 anchoring of the H3AsO3 molecules on the Mag@CNCs surface.

step 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 carbon-rich source, giving the advantage of flexibility of the presented route. Such a preparation method results in Mag@CNCs with a significant surface area of 177 m2 g−1 and homogeneously dispersed magnetic nanoparticles surrounded by a few graphitic layers-walls with sizes of 20− 100 nm, which 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 of 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 (FeOH2) 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 the Supporting Information), similarly as in the case of waterdispersible superparamagnetic magnetite−graphene hybrid materials designed for As(III) and As(V) removal.91 Hence, the present study stimulates the potential of converting biomass

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CONCLUSIONS In the present work, porous carbon nanocages (Mag@CNCs) containing magnetic iron species, such as zerovalent iron and iron carbide nanoparticles, were synthesized using a green one5789



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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 The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00394. 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 spectra 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 roomtemperature 57Fe 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 (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M. A. Karakassides). *E-mail: [email protected] (Radek Zbořil). ORCID

Michael A. Karakassides: 0000-0003-4344-0375 Jiří Tuček: 0000-0003-2037-4950 Radek Zbořil: 0000-0002-3147-2196 Author Contributions

All authors contributed to writing the paper and have approved the final version. Notes

The authors declare no competing financial interest.

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REFERENCES

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ACKNOWLEDGMENTS

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 Palacký University in Olomouc, Czech Republic (Project No. IGA_PrF_2016_010). The authors thank Jana Stráská, Dr. Ondřej Tomanec, and Martin Petr (all from the Regional Centre of Advanced Technologies and Materials, Faculty of Science, Palacký University in Olomouc, Czech Republic) for TEM, HRTEM, and XPS measurements, respectively. The authors also thank Ivo Medřı ́k (Regional Centre of Advanced Technologies and Materials, Faculty of Science, Palacký University in Olomouc, Czech Republic) for the assistance with the conduction of the experiments. 5790


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