Dual-Core Fe2O3@Carbon Structure Derived from Hydrothermal

Research Article pubs.acs.org/journal/ascecg

Dual-Core Fe2O3@Carbon Structure Derived from Hydrothermal Carbonization of Chitosan as a Highly Efficient Material for Selective Adsorption Xianbiao Wang,*,† Chuanliang Zhan,† Yan Ding,† Bing Ding,† Yongfei Xu,*,† Shengwen Liu,‡ and Huaze Dong§ †

School of Materials and Chemical Engineering, Anhui Jianzhu University, Hefei 230601, P. R. China Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, P. R. China § Department of Chemical and Chemical Engineering, Hefei Normal University, Hefei 230601, P. R. China ‡

S Supporting Information *

ABSTRACT: A dual-core Fe2O3@carbon structure was prepared by a hydrothermal treatment of iron salt and chitosan (CS) solution. The structure exhibits microsphere-like (∼850 nm in size) morphology with dual cores at two ends. The existence of CS plays an important role in the formation of the dual-core−shell structure. The unique structure was investigated to be a CS adsorption, and subsequent carbonization induced a selective etching process. Further, the special structure was proven to be a highly efficient material for selective adsorption. The dual-core structure facilitates the exposure of the Fe2O3 surface, and the carbonaceous shell gives plenty of functional groups for selective adsorption. More importantly, the selectivity was highly dependent on pH values. It was found that the adsorbent showed higher adsorption selectivity toward Cr(VI) at lower pH values, while the selectivity transferred toward Cu(II) at higher pH values by adsorption of Cu(II) and Cr(VI) in single-component solutions. In binary-component solution, the dual-core Fe2O3@C structure revealed adsorption selectivity for Cr(VI), with the highest separator factor [αCr(VI) Cu(II) ] = 1162, due to the strong electrostatic adsorption-coupled reduction interaction induced by the special structure of the adsorbent. This work not only gives deep insights into the understanding of the formation of the dual-core structure but also supplies a novel adsorbent with selective adsorption properties for water treatment. KEYWORDS: Dual-core, Fe2O3@carbon structure, Chitosan, Hydrothermal carbonization, Selective adsorption, Cr(VI) ions, Cu(II) ions

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INTRODUCTION

acidic solutions, resulting in instability as an adsorbent. Therefore, it is urgently needed to prepare a stable adsorbent derived from CS. Carbonaceous materials are good candidates for pH-stable adsorbents.9 In recent decades, producing functional carbonaceous materials from biomasses has attracted great interest, owning to vast and green resources. However, a conventional carbonization process usually involves harsh conditions, such as plasma processing,10 chemical vapor deposition,11 pyrolysis,12,13 and so on, leading to elimination of functional groups. Hydrothermal carbonization (HTC) is a newly developed mild approach, during which biomass is heated to around 180 °C in aqueous media. After the polymerization and carbon-

The U.S. Agency for Toxic Substances and Diseases Registry (ATSDR) classifies heavy-metal ions as the top 16th hazardous substance, as their high toxicity, carcinogenicity, and bioaccumulation can lead to various human disorders if humans exposed to heavy-metals-contaminated water.1,2 Therefore, numerous approaches have been adopted to remove them from aqueous systems, including chemical precipitation, reduction, ion exchange, membrane separation, and adsorption.3 One of the most effective methods is adsorption. Particularly, selective adsorption could remove the target molecules without the influence of other ingredients, exhibiting higher adsorption efficiency.4 Chitosan (CS) has been utilized as a selective adsorbent for removal of heavy metals due to its functional groups, such as amino groups and hydroxyl groups.5,6 For instance, CS was used for selective adsorption of Hg(II) and Cu(II).7,8 However, CS tends to dissolve in © 2016 American Chemical Society

Received: August 23, 2016 Revised: December 15, 2016 Published: December 29, 2016 1457

DOI: 10.1021/acssuschemeng.6b02034 ACS Sustainable Chem. Eng. 2017, 5, 1457−1467

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. (A) SEM image. (B) TEM image. (C−E) EDS mapping for (C) oxygen, (D) carbon, and (E) iron. (F) XRD pattern. (G) HRTEM image (inset: SAED pattern) of the product.

ization process,14 carbonaceous materials are obtained. The HTC process exhibits remarkable advantages over the conventional ones, in that it not only retains the functional groups of biomass but also that it is economically and ecologically friendly.15 As the main chain of CS possesses amino and hydroxyl groups, CS and its derivatives show strong metal-chelating capability.16 So, it is anticipated that the carbon yielded from the HTC process would retain the same features when CS is chosen as precursor. Therefore, CS-derived carbonaceous materials are very suitable for direct adsorption of heavy metals from aqueous solution.16 For instance, CS-derived carbonaceous materials were applied for Cr(VI) removal.17 Given that the amino groups can be retained after HTC treatment, an amino-functionalized attapulgite clay nanoparticles adsorbent was fabricated for heavy-metal removal.18 However, the morphology of the adsorbents was not optimized. Morphology control is another important process for synthesis of adsorbent. Obviously, a larger specific surface area tends to have more adsorption sites. Carbonaceous materials with different morphologies can be prepared through the HTC process. Generally, the morphologies include regular spheres, which are synthesized without template.9,19 In addition, fiber structures or other shapes can also be prepared, depending on the kind of template used in the process.20−22 Iron oxide is an excellent adsorbent which has good performance for heavy-metals adsorption over a wide pH range.23 It can facilitate recovery of the adsorbent after the adsorption process owing to its higher density compared with

carbonaceous materials.24 However, the lack of surface functional groups has restricted iron oxide for selective adsorption. In this work, we present a novel dual-core Fe2O3@carbon structure formed through a HTC process, using CS as carbon precursor. The precursor solution was prepared by dissolving CS in FeCl3 aqueous solution. In the process, FeCl3 solution was not only applied to maintain acidic conditions for dissolution of CS but also supplied an iron source for the formation of iron oxide, which served as a hard template of the final structure. The final obtained Fe2O3@C structure exhibits a Brunauer−Emmett−Teller (BET) surface area of 59.6 m2/g and diameter of ∼850 nm. The special structure was formed through a CS carbonization-induced selective etching and carbon deposition process. Interestingly, the dual-core Fe2O3@ C structure has an advantage for selective adsorption, based on evaluation of the adsorption performance toward Cr(VI) and Cu(II) in single-component and binary-component solutions. The dual-core structure could enlarge the specific surface area of the adsorbent, in favor of adsorption. In addition, the dualcore iron oxide particles tend to expose their surface, which can serve as adsorption sites, and this plays an important role for selective adsorption. As a result, the adsorbent showed adsorption selectivity toward Cr(VI) or Cu(II) with varying pH values in single-component solutions. In binary-component solutions, the dual-core Fe2O3@C structure exhibited high adsorption selectivity for Cr(VI) at low pH value. 1458


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Figure 2. (A) Raman spectrum of the Fe2O3@C structure. (B) FTIR spectra of CS and the product after HTC treatment for 12 and 48 h (the Fe2O3@C structure). (C) Nitrogen adsorption−desorption isotherm. (D) TG and DTG curves of the Fe2O3@C structure.

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with an Al Kα X-ray source (hν = 1486.6 eV) operated with pass energy of 20 eV on a Thermo ESCALAB250 analyzer. Zeta potential was determined according to the principle of laser Doppler electrophoresis on a Malvern zetasizer nano instrument. Adsorption Experiments. All adsorption experiments were carried out at 25 °C. Cu(NO3)2·3H2O and K2Cr2O7 were used as sources of Cu(II) and Cr(VI), respectively. Typically, 10 mg of adsorbent was added into various concentration solutions (10 mL) under stirring. The pH value was adjusted by 0.1 M HNO3 or 0.1 M NaOH solutions. Adsorption was then carried out in a shaker bath for a certain time. After adsorption, the adsorbents were separated from the solution using a 0.45 μm syringe filter. The initial (C0, mmol·L−1) and the residual (Ct, mmol·L−1) heavy-metal ions’ concentrations were measured using inductively coupled plasma spectroscopy (ICP, IRIS Intrepid II ICP-OES). The adsorption amount (qt, mmol·g−1) after a certain adsorption time (t, h) was calculated according to the following equation:

EXPERIMENTAL SECTION

Preparation of Dual-Core Fe2O3@Carbon Structure. All reagents were of analytical quality and used without further purification. Typically, CS powder (1.00 g) was dissolved in 0.1 M FeCl3 aqueous solution (50 mL), and the mixture was stirred at room temperature for 1 h. The color of the solution turned to dark-red. Subsequently, the solution was transferred into a Teflon autoclave and sealed immediately. The autoclave was maintained at 180 °C for 48 h and subsequently cooled to room temperature. The obtained dark product was washed with water and ethanol several times and then airdried in an oven at 60 °C for 6 h. For comparison, products with different amounts of CS were also prepared. Characterization. The morphology of the microspheres was observed by using a scanning electron microscope (SEM, Sirion 200 FEG) and a high-resolution transmission electronic microscope (HRTEM, JEOL-2010), with energy-dispersive X-ray spectroscopy (EDS, Oxford, Link ISIS). The number-average diameter (Dn), weightaverage diameter (Dw), and particle distribution index (PDI) of the particles were calculated by the following equations with the diameters of at least 100 particles measured in the SEM images:

Dn = Dw =

qt =

∑ niDi4

■

∑ niDi3 Dw Dn

(2)

where V (L) is the total volume of solution and W (g) is the weight of adsorbent. Competitive adsorption experiments were performed in binarycomponent solutions [Cu(II) and Cr(VI)] with the same initial concentrations.

∑ niDi ∑ ni

PDI =

(C0 − Ct )V W

RESULTS AND DISCUSSION Morphology and Structure. Figure 1A is the SEM image of the obtained product, which shows a uniform microsphere shape with a diameter of ∼850 nm. The conspicuous feature is the hollow in the middle of the microsphere. The TEM image (Figure 1B) further reveals that the microspheres have a dualcore−shell structure (see Figure 1B insets) and the cores are located at two ends of the microspheres with diameter of ∼300 nm. Further, the EDS mapping analysis implies that the structure is heterogeneous and contain C, O, and Fe element. The dual-core structure consists of O and Fe elements, as shown in Figure 1C,E, while the shell is composed of O and C elements, as observed in Figure 1C,D. Therefore, it can be deduced that the dual-core−shell structure is constructed with

(1)

X-ray diffraction (XRD) measurements were performed on an X’Pert Pro MPD diffractometer with Cu Kα radiation of 0.15419 nm. Raman spectra were recorded on a LabRam HR800 instrument with an excitation wavelength of 532 nm and a resolution of 0.6 cm−1. Fourier transform infrared spectroscopy (FTIR) measurements were conducted on a IR-750 spectrometer with an resolution of 4 cm−1, and KBr was used as background. Nitrogen adsorption isotherms were measured at 77 K on Micromeritics ASAP 2010 equipment. A simultaneous thermal analyzer (STA409PC) was used to determine the relative carbon content in the sample, which worked in temperature range of 30−900 °C with a heating rate of 10 °C/min under an air atmosphere. The surface elements of the sample were characterized by X-ray photoelectron spectroscopy (XPS), obtained 1459


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Figure 3. XPS spectra of Fe2O3@C structure: (A) C 1s, (B) O 1s, and (C) N 1s signals.

an iron oxide core and carbonaceous materials as the shell.25 The XRD pattern in Figure 1F verifies the iron oxide phase of the cores, which is well in accordance with the hexagonal hematite phase of α-Fe2O3 [JCPDS No. 33-0664]. Further, the selected area electron diffraction (SAED) pattern (Figure 1G, inset) implies the single-crystalline nature of the core. The HRTEM image of the core (Figure 1G) exhibits a plane spacing of 0.23 nm, which corresponds to the (006) plane of the hexagonal α-Fe2O3 phase. In addition, the Raman spectrum of the product also demonstrates the carbonaceous materials around the cores, as shown in Figure 2A. The two peaks at 1365 and 1585 cm−1 correspond to the characteristic D and G bands of carbon, which should be attributed to the amorphous carbon and graphitized carbon,26−28 respectively. A typical FTIR spectrum is shown in Figure 2B, which also exhibits the carbon-rich property of the Fe2O3@C structure. Similarly to the spectrum of CS in Figure 2B (black line), the spectrum of our obtained product (red line) displays a band of strong intensity between 3250 and 3550 cm−1 due to the overlapping signals of νOH and νNH.29 Besides, a clear signal at 1560−1640 cm−1 can be observed which is pertinent to primary amine.30 The wide peak in the range of 1100−1350 cm−1 is the overlapping signals of C−C and C−O groups.25 The absorbance at 1712 cm−1 corresponds to the stretching vibration of the −CO group. Therefore, the carbonaceous shell retains functional groups such as amino or hydroxyl groups after HTC treatment of CS. It is worth noting that the O and C elements are observed to be infiltrated over all the microspheres (Figure 1C,D) in the form of functional groups, indicating that the groups are packed on the product uniformly. On the basis of the functional groups, the Fe2O3@C structure could be a potential adsorbent for selective removal of heavy metals in water.22,27 Nitrogen adsorption−desorption measurements were carried out to determine the surface area of the obtained product, as shown in Figure 2C. The isotherm could be ascribed to type II,31 which indicates monolayer adsorption at low relative pressure and multilayer adsorption at high relative pressure. The BET surface area was calculated to be 59.6 m2/g.32 The relative carbon content in Fe2O3@C structure was estimated by thermogravimetric analysis (TGA) with air atmosphere (Figure 2D). A little mass was lost below 300 °C owing to the removal of surface hydroxyl groups and the adsorbed water. More than 80% of the total weight was lost at temperatures ranging from 300 to 600 °C, due to the consumption of carbonaceous components. Thus, the weight ratio of Fe2O3 cores was estimated to be 20%, while the carbonaceous shell was 80% of the weight. Furthermore, XPS analysis (Figure 3) was utilized to study the surface functional groups of the Fe2O3@C structure. As shown in Figure 3A, the C 1s spectrum consists of three peaks

at 284.7, 286.0, and 288.5 eV, which are contributed by C−C, C−N/C−O, and CO, respectively.33 The O 1s signal (Figure 3B) at 533.3 eV is attributed to CO, C−OH, or C−O−C groups,34 while the peak at 531.6 eV is the signal of Fe−O,35 indicating that some of the Fe2O3 surfaces are exposed. The exposed Fe2O3 surface might be from the inner surface of the hollow structure. The N 1s signal in Figure 3C exhibits three peaks at 399.4, 400.5, and 401.6 eV, which belong to C−NH2/ C−N−C, N−C, and N−H groups, respectively.36,37 Measurement of the zeta potential of the dual-core Fe2O3@C was carried out at a series of pH values, as shown in Figure 4.

Figure 4. Zeta potential of the dual-core Fe2O3@C structure under different pH values.

The pH-zpc (zero point of charge) is ∼2.5, which means the product exhibits positive or negative charge when the pH value is lower or higher than 2.5, respectively. This could be attributed to the plentiful oxygen-containing groups and amino groups on the surface,9,22 which makes the dual-core Fe2O3@C structure suitable for selective adsorption.38 Influencing Factors. CS Amount. CS was investigated to be crucial to the formation of the dual-core Fe2O3@C structure, as exhibited in Figure 5. Without CS, an octahedron-like structure with an average diameter of 1650 nm was produced (Figure 5A). The corresponding XRD pattern in Figure S1 proved to be an α-Fe2O3 phase [JCPDS No. 33-0664]. With 0.1 g of CS introduced into the reaction, a clear etched tracing appeared around the middle of the products, as shown in Figure 5B. On further increasing CS to 0.5 g, middle-broken ellipsoid-shaped microspheres with a uniform size about 940 nm were produced, as shown in Figure 5C. Compared with the sample using 0.1 g of CS in the reaction, the microspheres were etched more severely. When the amount of CS further increased to 1.5 g, the hollow structure in the middle of the obtained product was similar to that of the sample with 1.0 g of CS (Figure 5D). Thus, an appropriate amount of CS is the key factor for developing this special structure. Reaction Time. In order to investigate the growth mechanism of the Fe2O3@C structure, detailed time-dependent experiments were conducted. Figure 6 exhibits morphologies 1460


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Figure 5. SEM images of the products prepared with (A) 0, (B) 0.1, (C) 0.5, and (D) 1.5 g of CS.

Figure 6. SEM and TEM images of products collected at (A) 2, (B) 5, (C) 12, and (D) 72 h.

Formation Mechanism. Accordingly, the proposed mechanism for formation of the dual-core Fe2O3@C structure could be illustrated as follows. At the beginning of HTC treatment, FeCl3 salt tended to undergo hydrolysis and produce the Fe2O3 phase.14,40,41 The reaction can be proposed as eq 3:

obtained at different reaction times. After HTC treatment for 2 h, the obtained shuttle-like products have uniform size (∼500 nm) and exhibit symmetrical sharp tips (Figure 6A). Further, the corresponding SAED pattern indicates their singlecrystalline structure. It is worth noting that a sharp tip should have a higher surface energy.39 As the reaction time was prolonged to 5 h, the sharp tips became blunt with increasing size (Figure 6B). When the reaction was further prolonged to 12 h, the SEM image (Figure 6C) shows that hollow structures were produced in the microspheres. The TEM image (Figure 6C, inset) exhibits a quasi dual-core−shell structure. Finally, the dual-core−shell structure was successfully obtained after 48 h HTC treatment (Figure 1B). If the HTC time was prolonged to 72 h, no obvious morphological change occurred (Figure 6D).

2Fe3 + + 3H 2O → Fe2O3 ↓ + 6H+

(3)

During this stage, the abundant amino groups of CS were prone to construct a stable chelation effect with Fe3+ ions,29,42 leading to adsorption of CS chains onto a certain crystal plane of Fe2O3 and anisotropic crystal growth of Fe2O3. Then, a shuttle-like morphology was obtained (Figure 6A). Without CS, only octahedron-like particles were obtained according to the crystal growth habit of Fe2O3.14 Therefore, CS plays an important role in the formation of the final dual-core Fe2O3@C structure. 1461


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ACS Sustainable Chemistry & Engineering Next, polymerization and carbonization of the CS chain take place to produce carbonaceous materials, which is similar to the HTC process of carbohydrates.43,44 The carbonization of CS was verified by FTIR analysis, as shown in Figure 2B. After hydrothermal treatment, the νCH signal of CS at 2870 cm−1 disappears, demonstrating the polymerization and carbonization of CS.25 In the early stage, the carbonaceous materials prefer to cover the tips of the shuttle-like Fe2O3 to minimize the surface energy,39,45 leading to formation of carbon-covered Fe2O3. Meanwhile, as a feature of biomass carbonization, H+ ions were released along with the process of carbonization,43 resulting in an Fe2O3 etching effect. Interestingly, the acid etching process prefers to dissolve Fe2O3 from the middle of the shuttle due to the tip covering effect. The carbon deposited on the tips protects them from etching. Meanwhile, the etching process could be demonstrated by the etched tracing and the hollow structure in our obtained product with different CS dosages, as illustrated in Figure 5B−D and Figure 1A. Subsequently, the carbonaceous materials were kept covering the structure to replace the dissolved Fe2O3.26 After HTC treatment for 12 h, the middle of the Fe2O3 was almost completely dissolved, leading to the formation of a quasi-dualcore Fe2O3@C structure (Figure 6C). Comparing the FTIR spectra (Figure 2B) of the product after HTC treatment for 12 and 48 h, the decrease of Fe−O vibration signals (475 and 553 cm−1) and increase of organic peaks (such as primary amine, 1560−1640 cm−1) verified the replacement of etched Fe2O3 with carbonaceous materials.14 Finally, the dual-core Fe2O3@C structure was successfully prepared after reaction for 48 h (Figure 1B). The process of etching would cease as the reaction reached equilibrium, due to the decreasing of CS concentration during the HTC process. When the reaction time was further prolonged to 72 h, a similar morphology was observed (Figure 6D), indicating that the reaction equilibrium was reached within 48 h. As a result, the formation mechanism could be described as a CS adsorption and carbonization-induced selective etching process, as illustrated in Scheme 1. Selective Adsorption Performance. As discussed in the above sections, the dual-core Fe2O3@C structure could be a potential adsorbent owing to its having plenty of functional groups on the surface.9,22,27 Herein, we evaluate the selective adsorption performance of the sample by selective adsorption of Cr(VI) and Cu(II) in single-component and binarycomponent solutions. In Single-Component Solutions. The selective adsorption performance of the dual-core Fe2O3@C adsorbent in singlecomponent solutions was studied in Cr(VI) and Cu(II) solutions with the same initial concentrations (1.0 mmol/L) under different pH values. In consideration of the precipitation of Cu(II) in basic conditions,4 we carried out the experiments in acidic solutions with the range of pH values 2−5. It is worth noting that the adsorbent exhibited good stability under acidic conditions. Figure 7 exhibits the equilibrium adsorption capacities of Cr(VI) and Cu(II) at different pH values. As the pH value increases from 2 to 5, the equilibrium adsorption capacity for Cr(VI) decreases, while that for Cu(II) increases sharply. The maximum adsorption capacity of the dual-core Fe2O3@C adsorbent for Cr(VI) and Cu(II) occurs at pH values of 2 and 5, respectively. The adsorbent shows high adsorption selectivity toward Cr(VI) at pH values of 2 and 3, but toward Cu(II) when the pH is increased to 4 and 5. Therefore, the prepared

Scheme 1. Schematic Illustrations for the Formation of the Dual-Core Fe2O3@Carbon Structure

Figure 7. Effect of pH on the adsorption of Cr(VI) and Cu(II) in single-component solutions (C0 = 1.0 mmol/L).

dual-core Fe2O3@C adsorbent exhibited good adsorption selectivity, which can be tuned by varying the pH value of the solution. Further, the adsorption capacities of Cr(VI) and Cu(II) onto the dual-core Fe2O3@C structure were studied at their optimized pH values, as shown in Figure 8. Kinetic curves in Figure 8A show that both heavy-metal ions reach adsorption equilibrium after 12 h. Equilibrium adsorption isotherms (Figure 8B) indicate that Cr(VI) species have higher adsorption capacity (∼2.0 mmol/g, pH 2) than that of Cu(II) (∼1.25 mmol/g, pH 5). To investigate the adsorption isotherms and adsorption behavior deeply, Freundlich46 and Langmuir47 adsorption models were utilized to analyze the equilibrium adsorption isotherms of Cr(VI) and Cu(II) onto the dual-core Fe2O3@C adsorbent, as shown in Figure 8C,D and Figure S2A,B. Obviously, the adsorption isotherm of Cr(VI) is subject to the Freundlich model,46 which describes adsorption on a heterogeneous surface, or qe = KFCe1/ n

(4) −1

−1

where qe (mmol·g ) and Ce (mmol·L ) are the adsorption capacity at equilibrium and equilibrium concentration, respectively. KF (mmol1−1/n L1/n g−1) and n are the parameters related to adsorption capacity and adsorption intensity, 1462


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Figure 8. (A) Kinetic adsorption curves (C0 = 0.6 mmol/L) and (B) equilibrium adsorption isotherms for Cr(VI) (pH = 2) and Cu(II) (pH = 5) onto the dual-core Fe2O3@C adsorbent in single-component solutions. (C) Corresponding plots of lg qe ∼ lg Ce[Cr(VI)] and (D) Ce/qe ∼ Ce[Cu(II)].

Figure 9. (A) Kinetic adsorption curves of Cr(VI) and Cu(II) on the dual-core Fe2O3@C adsorbent in binary-component solution with an initial concentration of 0.53 mmol/L (pH = 4). (B) Corresponding plots of pseudo-second-order adsorption model fit. (C) Adsorption isotherms of Cr(VI) and Cu(II) at different pH values [yellow spheres denote Cr(VI); blue cubes denote Cu(II)]. (D) Corresponding separation factors of Cr(VI) to Cu(II) (αCr(VI) Cu(II) ) at different pH values and initial concentrations.

respectively. The corresponding plots of lg qe ∼ lg Ce present a good linear relationship, as show in Figure 8C. In contrast, the adsorption isotherm of Cu(II) can be well described by the Langmuir model,47 which usually illustrates chemical adsorption on uniform surfaces, or qe =

q0KLCe 1 + KLCe

are illustrated in Figure 8D, implying a good linear relationship. The fitted parameters are listed in Table S1. The maximum adsorption amount of Cu(II) (1.54 mmol· g−1) is higher than in some reported literatures, such as Fe3O4 nanoparticles with gum arabic,48 grafted silica,49 activated slag,50 and so on. As for the adsorption capacity toward Cr(VI), which is about 104 mg/g (∼2 mmol/g), the dual-core composite particles rank in the front groups according to a review paper.3 In Binary-Component Solutions. The adsorption selectivity was also evaluated through competitive adsorption in binary-

(5)

where q0 denotes the maximum adsorption amount (mmol·g−1) and KL is a constant (L·mmol−1) referring to the affinity in the process of adsorption. The corresponding plots of Ce/qe ∼ Ce 1463


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Figure 10. XPS spectra of the dual-core Fe2O3@C structure after adsorption of Cu(II) and Cr(VI): (A) Cu 2p, (B) N 1s, (C) Cr 2p, and (D) O 1s signals.

in Figure 9C. Similar to the adsorption in single-component solution, no significant adsorption of Cu(II) occurred on the adsorbent at a pH value of 2. Meanwhile, the adsorption amount of Cr(VI) increased obviously as the initial concentrations were increased. When the pH values of the binary-component solutions increased to 3, 4, and 5, the adsorption of Cu(II) increased sharply, and the adsorption of Cr(VI) slightly decreased. Thus, it can be concluded that lower pH values benefit adsorption of Cr(VI) and higher pH values benefit adsorption of Cu(II), which agrees well with the phenomenon in singlecomponent solutions. However, differently, the adsorption amounts of Cr(VI) are higher than those of Cu(II) within pH 2−5 in our experiments, implying that the presence of Cr(VI) showed an inhibitory effect on the adsorption of Cu(II) in the binary system. A separation factor56 was utilized to investigate the adsorption selectivity in binary-component solution, defined as

component solutions containing Cr(VI) and Cu(II) with the same initial concentrations. The kinetic competitive adsorption experiment was first carried out, as shown in Figure 9A. For both Cr(VI) and Cu(II), the adsorption amounts increase along with adsorption time and then reach adsorption equilibrium after 12 h. Different from adsorption in single-component solution, the adsorption amount of Cr(VI) is higher than that of Cu(II), indicating that Cr(VI) species are more competitive than Cu(II) in binarycomponent solutions. Furthermore, both of these kinetic adsorption behaviors could be well described by the pseudosecond-order model (Figure 9B), implying chemical-bondingrelated interactions between the adsorbent and heavy-metal ions:51−53 1 1 t = + t 2 qt qe kqe

(6)

where k (g·mmol−1·h−1) is the adsorption rate constant. The fitted parameters are listed in Table S2. Further, in order to investigate the selective adsorption process of Cr(VI) and Cu(II) in a binary system, Weber’s intraparticle diffusion model (eq 7)54 was applied to illustrate the steps involved in the adsorption process: qt = k ipt 1/2 + h

α21 =

qe1Ce2 Ce1qe2

(8)

where qe1 (mmol/g) and Ce1 (mmol/L) are the equilibrium adsorption capacity and equilibrium concentration toward constituent 1, and qe2 (mmol/g) and Ce2 (mmol/L) are the equilibrium adsorption capacity and equilibrium concentration toward constituent 2, respectively. The separation factors for Cr(VI) to Cu(II) [αCr(VI) Cu(II) ] in binary-component solutions with different pH values and initial concentrations are shown in Figure 9D. At a pH value of 2, the separation factors increased with increasing initial concentrations from 0.09 to 1.9 mmol/L. The highest separation factor (1162) was achieved at an initial concentration of 1.9 mmol/L and then decreased to 475 at an initial concentration of 2.7 mmol/L due to the saturated equilibrium adsorption at that concentration (2.7 mmol/L). It was found that the separation factors rapidly decreased from pH 2 to 3, 4, and 5. As a result, it could be concluded that the dual-core Fe2O3@C adsorbent exhibits high adsorption selectivity toward Cr(VI) species in binary-component

(7) 1/2

where kip (mmol/(g·hr )) and h are constants related to the intraparticle diffusion rate and the thickness of the boundary layer at each adsorption stage, respectively. The qt ∼ t1/2 plots of the kinetic adsorption curves of Cr(VI) and Cu(II) in binarycomponent solution are shown in Figure S3. Both of the plots exhibit multilinearity, indicating that three adsorption steps occur.55 The first line is associated with external surface adsorption or instantaneous adsorption, the second step is an intraparticle diffusion process, and the third step is the final equilibrium stage.55 The fitted parameters are listed in Table S3. To be more specific, competitive equilibrium adsorption experiments were performed with different pH values, as shown 1464


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competitive adsorption capacity than Cu(II) in binarycomponent solutionsw55 with the presence of the dual-core Fe2O3@C structure. More importantly, redox reaction in the adsorption of Cr(VI) causes stronger affinity between the adsorbent and Cr(VI) species,57 leading to higher adsorption capacity (Figure 9C) and inhibitory effect on the adsorption of Cu(II) in the binary system.20 As a result, the dual-core Fe2O3@C adsorbent exhibited selectivity toward Cr(VI) species in a binarycomponent solution.

solutions owing to their more competitive adsorption capacity. Importantly, the selectivity can be controlled by tuning the pH values. Structure-Induced Selective Adsorption Property. The selective adsorption performance of the dual-core Fe2O3@C structure is related to their special structure, that is, the functional carbon-containing groups (−CO, −C−O, −C−C, etc.) and amino groups from the carbonaceous shell and the dual-core and hollow structure which facilitate exposure of the Fe2O3 surface. As we know, Cu(II) exists as cations in aqueous solution, while Cr(VI) species exist in the form of HCrO4− and Cr2O72− anions in acidic conditions.2−9 From the mentioned zeta potential of the adsorbent (Figure 4), the surface charge is positive at a pH value of 2 and becomes negative in at pH values higher than 3. Thus, no obvious adsorption of Cu(II) occurred at pH 2, and then it increased along with increasing pH values (Figures 7 and 9C), owing to the electrostatic interaction effect.8,57 As a result, the adsorption of Cr(VI) decreased (Figures 7 and 9C). The electrostatic and chelation adsorption of Cu(II) can be demonstrated by the XPS spectrum of the dual-core Fe2O3@C structure after adsorption of Cu(II) (Figure 10A). The peaks at 934.3 and 954.3 eV are attributed to the Cu 2p3/2 and Cu 2p1/2 signals, respectively.58,59 The N 1s signal at 400.2 eV (Figure 10B) is close to the Cu−N bonding,60 indicating that the Cu(II) ions are chelated with amino groups.8 In terms of lower pH value (pH = 2), the amino groups were protonated to −NH3+ groups,61 resulting in no obvious adsorption of Cu(II). Further, the Langmuir adsorption isotherm (Figure 8D) and pseudo-second-order kinetic adsorption (Figure 9B) of Cu(II) also proved the chemical bonding related chelation interaction. For comparison, the adsorption of Cr(VI) is an electrostaticadsorption-coupled reduction mechanism.42 The exposed Fe2O3 surface62 and protonated amino groups9 on the surface of adsorbent at low pH values can serve as adsorption sites for Cr(VI) species. Meanwhile, the adsorbed Cr(VI) species are partially reduced to less toxic Cr(III) in an acidic environment.57 The Cr 2p signal in Figure 10C verifies the existence of Cr(III). The peaks of Cr 2p3/2 (577.20 eV) and Cr 2p1/2 (587.11 eV) correspond to Cr(III) and Cr(VI) species on the adsorbent, respectively, which gives strong evidence of the redox reaction of Cr(VI) through the adsorption procedure.9,22 Thus, it is understandable that the adsorption of Cr(VI) still occurs at pH values between 3 and 5. Further, the O 1s spectrum of the adsorbent (Figure 10D) after adsorption consists of three peaks at 531.2, 532.1, and 533.5 eV, which correspond to Fe−O, HCrO4−/Cr2O72−, and −CO/C−OH/ C−O−C groups, respectively.9,34,35 In addition, the O 1s signal shifts to lower binding energy after adsorption (from 532.9 to 531.5 eV), indicating the coordination of Cr(VI) with oxygencontaining groups of the dual-core Fe2O3@C structure.63 Therefore, it is reasonable to assume that the interaction of Cr(VI) with the surface of the adsorbent is heterogeneous, leading to a Freundlich adsorption isotherm (Figure 8 B,C). In terms of kinetic adsorption of Cr(VI) and Cu(II) (Figure S3), the first step is mainly caused by electrostatic interaction. The adsorption procedures are then dominated by the reduction mechanism and chelation adsorption for Cr(VI) and Cu(II), respectively. At last, the steps reach adsorption equilibrium in the third stage. The constants kip in the three steps for adsorption of Cr(VI) are higher than those of Cu(II) (Table S3), demonstrating that Cr(VI) species have more

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CONCLUSION A dual-core Fe2O3@carbon structure was prepared by hydrothermal carbonization treatment of aqueous precursor solution containing chitosan and FeCl3. The iron salt (FeCl3) not only enhanced dissolution of chitosan (CS) but also supplied iron source for the cores. CS plays an important role in the formation of the dual-core−shell structure. The adsorption, carbonization of CS, and subsequent selective etching process was proposed for the formation of such special structure. The shell is composed of carbonaceous materials with plenty of functional groups, such as −CO, −C−O and the retained amino groups from CS. The obtained dual-core Fe2O3@C structure exhibited selective adsorption performance by evaluation of its capacity to remove Cu(II) and Cr(VI) in single-component and binary-component solutions. The adsorption selectivity could be controlled by tuning the pH values of the solutions. The highest separation factor (αCr(VI) Cu(II) ] = 1162) was achieved at pH 2, depending on the electrostatic interaction coupled chelation of Cu(II) and reduction of Cr(VI) process, induced by the special structure of the adsorbent.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02034. Figures S1−S3 and Tables S1−S3, showing XRD of the product prepared without chitosan; plots of Ce/qe ∼ Ce[Cr(VI)] and lg qe ∼ lg Ce[Cu(II)] for equilibrium adsorption isotherms of Cr(VI) and Cu(II) in singlecomponent solutions; Weber’s plots of the kinetic adsorption curves of Cr(VI) and Cu(II) in binarycomponent solution (C0 = 0.53 mmol·L−1, pH = 4); fitted parameters of equilibrium adsorption isotherms of Cr(VI) and Cu(II) in single-component solutions; parameters for the adsorption of Cu(II) and Cr(VI) in binary-component solutions by the pseudo-second-order model fitting; and fitting results of Weber’s plot for Cr(VI) and Cu(II) adsorption in binary-component solution (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Xianbiao Wang: 0000-0003-3746-9163 Notes

The authors declare no competing financial interest. 1465


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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 21371005, 21001002) and the Excellent Talent Foundation for Young Scholars in Universities of Anhui Province (Grant No. gxyqZD2016152).

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Dual-Core Fe2O3@Carbon Structure Derived from Hydrothermal

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