Solutions
1.2 Removal of Arsenic from Aqueous Solution
1.2.2 Sulfhydryl-Functionalized Magnetic Carbon Nanotube
1.2.2.2 Characterization of Adsorbents
Scheme 1.1 illustrates the synthesis process of GSH-functionalized MI- CNTs. Figure 1.1a displays the TEM images of APCNTs. It was found that entangled CNTs bundles were mixed with a high density of iron catalyst particles, as indicated by the black dots. High-resolution TEM (HRTEM) (the inset of Figure 1.1a) image reveals that the INPs are covered with carbon cages or shells with diameters ranging from 2 to 5 nm, and these cages consist of several graphitic layers. After the heat treatment, the entangled state of APCNTs was hardly changed. However, the carbon cages over the INPs have been completely destroyed, which has been previously reported [6, 10]. This is beneficial for the contact between As(III) and the INPs.
Furthermore, the size of the INPs obviously increased after heat treatment, which is due to the crystallization and growth of the nanoparticles at high temperature. After GSH functionalization, no obvious change was observed in the morphology of GMI-CNTs compared with MI-CNTs, as shown in Figures 1.1b and 1.S1. The lattice interplanar spacings of 0.286 and 0.242 nm correspond to the (220) and the (222) planes of Fe3O4, respectively, revealing the Fe3O4 NPs are polycrystalline. The interplanar spacings of 0.221 and 0.252 nm are characteristics of the (113) plane of α-Fe2O3 and the (311) plane of γ-Fe2O3, respectively.
Scheme 1.1 Illustration of Glutathione functionalized magnetic carbon nanotube (GMI-CNT) synthesis.
Figure 1.9d shows the XRD patterns of APCNTs, (magnetic carbon nanotubes)MI-CNTs, (oxidized magnetic carbon naotubes)OMI-CNTs, and GMI-CNTs. The peaks associated with the mixture of zero-valent Fe, γ- Fe2O3/Fe3O4 appeared after the heat treatment. Well-resolved diffraction peaks reveal the good crystallinity of Fe, γ-Fe2O3/Fe3O4. The peaks of C with a relatively high intensity and symmetry are clearly observed, which suggests that the graphite structure remained even after heat treatment.
Therefore, we can conclude that MI-CNT heterostructures were formed after the heat treatment. In our study, APCNTs were firstly treated by a two-step heat treatment. In the first step, the APCNTs were heated in the air at 400 °C for 60 min to destroy the carbon cages and oxidize INPs: Fe + O2→fFe2O3.
In the second step, the hybrids obtained from step one were heated at 850 °C for 60 min under Ar gas protection to remove the rest carbon by the redox reaction between C and Fe2 O3: C + Fe2O3 → Fe +COx (Scheme 1.2). After the oxidation step, the relative intensity of C peaks decreased, indicating large quantities of defects were generated during the oxidation process.
Moreover, the intensity of Fe peaks also obviously decreased because the zero-valent Fe was partially oxidized to α-Fe2O3, which was confirmed by the appearance of new peaks located at 33.152°, 49.496°, and 54.089° in the XRD pattern of OMI-CNTs. This result is consistent with the aforementioned HRTEM analysis, and the iron of GMI-CNTs can be identified as FeOx (x=0, 4/3, 3/2) (Figure 1.9b). The transformation of Fe states during the synthesis process is illustrated in Scheme 1.2. The XRD patterns of GMI-CNTs indicate that the functionalization process did not change the component of INPs on the surface of CNTs and would not inhibit the reaction between INPs and As(III).
Figure 1.9 TEM images of APCNTs and GMI-CNTs (a and b), HRTEM image of INPs on GMI-CNTs (c), and XRD patterns of prepared samples (d).
The inset of (a) shows INPs in APCNTs covered with carbon cages.
Scheme 1.2 The transformation of Fe species during the synthesis process.
XPS was employed to analyze the surface chemical composition, as shown in Figure 1.10. Typical survey scans of GMI-CNTs and OMI-CNTs are shown in Figure 1.10a. After functionalization, the new peaks of S and N appeared. For C 1s spectra in Figure 1.10b, the peak of typical graphitic carbon attributed to C 1s spectra was found at 284.6 eV. Other three peaks located at 285.4, 286.8, and 288.7 eV are assigned to C–N, C–O, and C=O, respectively. The S 2p peak was deconvoluted into two separate peaks at 163.8 and 165.1 eV, contributing to the –SH groups (Figure 1.10c) [38]. The
peak located at ~400.2 eV corresponds to the –NH– groups (Figure 1.2d) [39]. The O 1s spectra consist of three peaks at 530.1, 531.5, and 532.9 eV, which are assigned to Fe–O, C=O, and O–C=O, respectively [17]. A broad peak at 718.0 eV represents overlapping components for oxidized iron and zero-valent iron [40]. The Fe 2p spectra show two broad peaks of Fe 2p3/2 and Fe 2p1/2 with satellite peaks at ~711.0 and ~724.8 eV, respectively (Figure 1.11). The two peaks at 712.9 and 726.7 eV correspond to Fe2O3, while peaks at 710.8 and 724.3 eV represent Fe3O4. The surface analysis demonstrated the iron phase in GMI-CNTs was the mixture of zero-valent iron and iron oxides which is in accordance with the TEM and XRD analyses and confirmed that GSH molecules have been successfully grafted on the OMI-CNTs.
Figure 1.10 XPS survey scans of the GMI-CNTs (a), the core peaks of C 1s (b), S 2p (c), and N1s (d) on the surface of GMI-CNTs.
Figure 1.11 XPS Fe2p spectra of GMI-CNTs.
The Fourier transform infrared (FT-IR) spectrum of OMI-CNTs and GMI- CNTs provides further evidence of the successful graft of GSH on OMI- CNTs (Figure 1.12a). The peak at ~3300 cm–1 corresponds to the O–H stretching vibration of adsorbed water or some other O–H containing groups, such as carboxyl [41]. The band at 1717 cm–1 attributes to the stretching vibrations of C=O of the carboxyl groups, which confirms the formation of carboxyl groups after the oxidation step [25, 42]. The peaks at 1654 and 1575 cm–1 indicate the formation of secondary amide on the OMI-CNTs resulting from the functionalization [43]. The strong peak at 568 cm–1 corresponds to the stretching vibration of large quantities of Fe–O. The FT-IR spectra also confirm that the GSH molecules are covalently bonded to OMI-CNTs.
Figure 1.12 FT-IR spectra (a) of OMI-CNTs and GMI-CNTs and TG-DSC (b) of GMI-CNTs.
The TGA indicated that the GMI-CNTs exhibit three main weight-loss peaks (Figure 1.12b). The total weight loss of GMI-CNTs is approximately 45% before 550 °C, indicating the loading ratio of INPs on GMI-CNTs reaches around 55%, which is much higher than many other iron oxide/CNTs based composites. The result is consistent with the strong Fe–O peak in the aforementioned FT-IR spectrum. A slight weight loss close to ~6% occurred below ~180 °C, which is due to the evaporation of adsorbed water and the elimination of carboxylic groups and hydroxyl groups on the GMI-CNTs [44, 45]. The second stage weight loss observed between ~180 and 400 °C is associated with the thermal decomposition of GSH on the OMI-CNTs [46, 47]. The rapid weight-loss region between ~400 and ~540 °C is attributed to the oxidation of CNTs. The TGA indicated that the stability of GMI-CNTs can meet the application needs of adsorbents in water treatment.
The SSA and pore parameters of GMI-CNTs were measured by nitrogen (77.4 K) adsorption/desorption experiments (Figure 1.13a). The SSA of MI- CNTs (299.4 m2·g–1) significantly increased by ~2.6 times, corresponding with the decrease in the average pore size from 11.03 to 5.01 nm (BJH). The micropore volumes of APCNTs calculated by the NLDFT kernel before and after heat treatment were both close to 0, indicating most surface area was attributed to mesopores. The meso-PV and micro-PV of MI-CNTs (0.778 cc·g–1) slightly changed compared with that of APCNTs (0.897 cc·g–1).
However, the pore size becomes smaller and more centralized, indicating the heat treatment can not only remove the carbon cages over the INPs but also produce much smaller and more uniform micropore/mesopore structures, which may finally result in the increase of SSA. After the functionalization,
the SSA (139.9 m2·g–1) and micro-PV/meso-PV (0.501 cc·g–1) of GMI- CNTs decreased compared with MI-CNTs, whereas the pore size was still uniformly distributed with an average pore size of 5.0 nm, which is beneficial for adsorption of pollutants. Although the SSA and meso-PV/micro-PV decreased after the functionalization, adsorption capacity of GMI-CNTs for As(III) is much higher than that of MI-CNTs, indicating the sulfhydryl groups of GSH play a very important role in the enhancement of adsorption capacity.
Figure 1.13 (a) N2 adsorption/desorption isotherms and pore size distribution (inset) of GMI-CNTs and (b) hysteresis loop of GMI-CNTs. The inset of (b) is the digital photograph of GMI-CNTs dispersed in water (1) and separated with magnetic separation (2).
The magnetization properties of GMI-CNTs were investigated at room temperature by measuring magnetization curves, as shown in Figure 1.13b.
The magnetization properties investigation showed the Ms of GMI-CNTs is 27.3 emu·g–1 with relatively low coercive force and remanence of 104.9 Oe and 2.4 emu·g–1, which can be beneficial for the reuse without reunite for magnetization. It can be found from the insets that the As(III)-loaded GMI- CNTs can be easily separated from water by using a magnet (the inset of Figure 1.13b). The concentration of residual CNTs in an aqueous solution was estimated nearly 0 g·L–1 by a UV–visible absorption-based approach [45].