Simultaneous Removal of Graphene Oxide and Chromium(VI) on the

Research Article pubs.acs.org/journal/ascecg

Simultaneous Removal of Graphene Oxide and Chromium(VI) on the Rare Earth Doped Titanium Dioxide Coated Carbon Sphere Composites Jian Wang,† Yu Liang,† Qingqing Jin,† Jing Hou,*,† Bei Liu,† Xin Li,† Wanying Chen,† Tasawar Hayat,‡ Ahmed Alsaedi,‡ and Xiangke Wang*,†,‡,§ †

College of Environmental Science and Engineering, North China Electric Power University, Beijing, 102206, People’s Republic of China ‡ NAAM Research Group, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia § Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou, 215123, People’s Republic of China S Supporting Information *

ABSTRACT: The potential coexistence of toxic metals and graphene oxide (GO) in the natural environment threatens human health. Herein, rare earth doped titanium dioxide coated carbon sphere composites (C@La-TiO2 and C@Ce-TiO2) were synthesized for the simultaneous removal of GO and Cr(VI) from wastewater. The results showed that a relatively high concentration of NaCl was beneficial to the binding of GO, whereas it was adverse to Cr(VI) removal. The removal capacity of C@La-TiO2 reached 383.3 mg/g for GO and 50.5 mg/g for Cr(VI) at pH 5.0. The adsorption process of GO and Cr(VI) on the composites was spontaneous and endothermic. Interestingly, the removal capacity of Cr(VI) on the composites increased significantly in the presence of GO, which was ascribed to the simultaneous adsorption of GO and Cr(VI) on composites and surface adsorbed GO for Cr(VI). However, in the presence of Cr(VI), the GO removal on the composites decreased prominently due to the competitive adsorption between GO and Cr(VI) on the composite surfaces. The interaction of GO was mainly dominated by electrostatic attraction and hydrogen bonding, whereas the removal of Cr(VI) was mainly attributed to outer-sphere surface complexation and electrostatic attraction. The findings can provide new insights into the simultaneous elimination of GO and heavy metal ions in natural aquatic environmental pollution cleanup. KEYWORDS: Graphene oxide, Cr(VI), C@La-TiO2 composite, C@Ce-TiO2 composite

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INTRODUCTION

However, many studies also found that GO was also one of the most toxic nanomaterials.11−13 Akhavan and Ghaderi12 demonstrated that the cytomembranes of Escherichia coli and Staphylococcus aureus could be destroyed by GO. Unfortunately, GO exhibited long-term retention in the liver, spleen, kidneys, and lungs, which might lead to obvious pathological changes such as inflammatory cell infiltration, granuloma formation, and pulmonary edema.13 Therefore, the efficient removal of GO from the natural environment is critical to decreasing its danger to human health. Ren et al.14 found that the removal of GO on Al2O3 was strongly affected by solution pH and the types of electrolytes. Also, Ca/Al-LDH and Mg/Al-LDH composites showed huge advantages in effective GO elimination by

Because of the potential risks to humans and animals, the problem of heavy metal pollution has caused wide public concern with rapid industry development.1−3 Among the most toxic heavy metals, chromium (Cr), especially Cr(VI), can cause many diseases such as arthritis, bronchitis, brain damage, dermatitis, and even cancer.4,5 Therefore, it is critical to eliminate Cr(VI) from wastewater to reduce its harm. As a significant man-made nanomaterial, graphene oxide (GO) has been studied widely for the uptake of Cr(VI) due to its unique structure and abundant oxygen-containing groups such as carboxyl groups and hydroxyl groups on its surfaces.6−8 For instance, Yan et al.9 found that the chitosan/GO composite was a versatile adsorbent for Cr(VI) removal from wastewater. The sorption capacity of Cr(VI) on the magnetic triethylene tetramine-GO ternary nanocomposite reached 180.1 mg/g.10 © 2017 American Chemical Society

Received: March 29, 2017 Revised: May 12, 2017 Published: May 15, 2017 5550

DOI: 10.1021/acssuschemeng.7b00957 ACS Sustainable Chem. Eng. 2017, 5, 5550−5561

Research Article

ACS Sustainable Chemistry & Engineering

Milli-Q water and ethanol three times and dried for 24 h at 60 °C to remove the redundant ethanol. The above product (0.4 g) was added to 60 mL of Milli-Q water, and ultrasonication was conducted for 30 min. Afterward, Ti(SO4)2 (0.8 g) and LaCl3·7H2O (0.2 g) were dissolved into solution and shaken for 30 min to get a uniform solution. Urea (5 g) was added to the suspension and shaken for 30 min. Then, the mixed solution was moved to the autoclave and kept for 12 h at 200 °C. The samples were collected by centrifugation, followed by washing with deionized water and ethanol and drying in a vacuum oven for 12 h at 60 °C. Finally, the products were calcined at 500 °C under a continuous N2 flow for 3 h. The obtained composites were cooled to room temperature and then used in the experiments. For comparison, CeCl3·7H2O was used to prepare the C@Ce-TiO2 composite by the same above dosage and steps. Characterization. The SEM images were achieved with a Hitachi S-4300 at a beam energy of 15.0 kV. The TEM images were acquired with JEM-1011. The XRD patterns were performed on a Scintag XDS-2000 diffractometer equipped with Cu Kα radiation (λ = 1.54 Å) at 2θ = 10−70°. A Nicolet Magana-IR 750 spectrometer was used to measure the FT-IR spectra in the range of 400−4000 cm−1. The N2−BET surface area was measured with a Quantachrome Instruments device (autosorb-iQ) at 77 K (BET method). The XPS spectra were acquired on a VG Scientific ESCALAB Mark II spectrometer. Batch Adsorption Experiments. The effects of ionic strength, pH, contact time, solid content, and temperature on GO and Cr(VI) adsorption to the composites were studied by batch technique. In a typical adsorption process, 1.0 mL of sample suspension (3.0 g/L), 1.0 mL of NaCl solution (0.06 M), 3.0 mL of Milli-Q water, and 1.0 mL of GO stock suspension were added to the polyethylene centrifuge tube. The pH was adjusted by the addition of a negligible amount of 0.01−1.0 M NaOH or HCl. After being oscillated at 150 rpm for 4 h to get the adsorption equilibrium, the solid was separated from the liquid phase by centrifugation. The residual GO concentration was measured by UV−vis spectrophotometer (UV-2600, Shimadzu) at a wavelength of 227 nm.14 The adsorption of Cr(VI) was carried out in a manner similar to the above steps, and the concentration of Cr(VI) was measured by inductively coupled plasma emission spectroscopy (ICPE-9000, Shimadzu). The removal percentage was calculated from the following formula:

electrostatic interaction and hydrogen bonding.15 In addition, quartz sand could remove GO from solution through a nonspontaneous endothermic process.16 Nevertheless, most of the existing materials are not suitable for GO elimination because of high cost or low efficiency, which may hinder practical application. Considering the extensive use of GO in the near future, it is inevitable that a certain amount of GO nanoparticles will be released into the natural environment during the processes of its production, transportation, and use, which may cause serious environmental pollution. In the past few years, titanium dioxide/carbon sphere composites have been investigated widely in different fields due to their excellent performance.17−21 Xiong et al.22 reported that a nanocrystalline titanium dioxide loaded carbon sphere could be used as a hydrogen storage material. A carbon-doped titania hollow sphere (THS), which had a unique hollow network three-dimensional structure, was prepared through the facile template method with high visible-light photocatalytic activity (i.e., higher than the commercial P25).17 Meanwhile, widespread attention was concentrated on the use of mixing rare earth metal composites in environmental pollution control, which can reduce the application of expensive rare earth metal ions and keep the high removal ability.23 For example, lanthanum-loaded granular ceramic was synthesized and applied for phosphorus elimination from wastewater.24 The adsorption capacity of fluoride on Fe−Al−Ce composites attained 178 mg/g, suggesting a promising application for fluoride removal from wastewater.25 However, to the best of our knowledge, the simultaneous removal of GO and Cr(VI) on the titanium dioxide/carbon sphere composite has not been reported yet, especially the research for using rare earth metal. Herein, we reported the synthesis of rare earth doped titanium dioxide coated carbon sphere composites (C@LaTiO2 and C@Ce-TiO2) by a simple hydrothermal method for simultaneous adsorption of Cr(VI) and GO from aqueous solutions. The physicochemical properties of the composites were carefully characterized by Fourier transformed infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The effect of different conditions (i.e., solution pH, contact time, ionic strength, solid content, and temperature) on GO and Cr(VI) removal were investigated, and the interaction mechanisms were discussed in detail from different characterizations. This research provided new insights into the simultaneous elimination of Cr(VI) and GO from wastewater, which broadened the practical applications of the composites in pollution management.

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removal percentage =

C0 − Ce × 100% C0

(1)

where Ce and C0 (mg/L) represent the equilibrium and initial concentrations of GO/Cr(VI). All experiments were carried out in duplicate, and the adsorption performances were assessed by the average data.

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RESULTS AND DISCUSSION Characterization. The surface morphology and shape were observed in SEM images. In Figure 1a, one can see that C@LaTiO2 showed a relatively uniform microsphere structure. Especially, the higher-magnification SEM image (Figure 1a, inset) clearly revealed the as-prepared C@La-TiO2 possessing a very smooth surface. Likewise, as shown in Figure 1b, it was found that the C@Ce-TiO2 also had a sphere-like morphology. Compared to C@La-TiO2, C@Ce-TiO2 also showed a relatively rough surface (inset in Figure 1b). Moreover, the structure of the composites was further observed by TEM images. Taking the C@La-TiO2 as an example, the TEM image (Figure 1c and the inset) indicated a smooth surface of a sphere, which was in good agreement with the SEM image. Crucially, the higher-magnification TEM image clearly demonstrated that carbon sphere was coated by TiO2, revealing a core−shell structure. To get further insight into the chemical composition of the composites, elemental mapping characterization was carried out. As presented in Figure 1d, it was apparent that the elements of C, O, Ti, and La were dispersed

EXPERIMENTAL SECTION

Chemicals. All chemicals (analytical grade) including LaCl3·7H2O and CeCl3·7H2O were purchased from the Sinopharm Chemical Regent Co., Ltd. (Beijing, China) and directly used without any purification. The GO was synthesized from graphite using a modified Hummer’s method.26 The GO was dispersed into Milli-Q water to prepare the stock GO suspension (300 mg/L), and K2Cr2O7 was used to prepare a Cr(VI) stock suspension (300 mg/L) at room temperature. Milli-Q water was used in all of the experiments. Preparation of Composites. The C@La-TiO2 composite was synthesized through a simple hydrothermal method. Briefly, 70 mL of Milli-Q water containing glucose (7 g) was shaken for 30 min to get a clear suspension and then was transferred into a stainless-steel Teflonlined autoclave (100 mL in capacity) and placed at 180 °C for 3 h. After centrifugation, the products were rinsed sequentially using 5551


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and 68.6° were well matched with the standard XRD pattern of TiO2 (JCPDS card no. 04-0477). The peaks at 29.0°, 38.0°, 44.6°, and 49.8° were in good agreement with La2O3 (JCPDS card no. 24-0554). The other diffraction peaks at 20.9°, 26.5°, 42.4°, 54.7°, and 59.9° were attributed to a carbon sphere (JCPDS card no. 23-0064). Similarly, Figure 2b indicated that the peaks at 25.4°, 37.8°, 62.7°, and 68.6° were in accordance with TiO2 (JCPDS card no. 04-0477). The peaks at 28.5°, 33.1°, 47.5°, and 56.3° were ascribed to CeO2 (JCPDS card no. 04-0593). Furthermore, the diffraction peaks at 20.9°, 26.5°, 54.7°, and 59.9° were attributed to a carbon sphere (JCPDS card no. 23-0064). More importantly, from the XRD analysis, it can be considered that there was almost no structure change of the composites before and after adsorption, revealing that the structures were not damaged during the adsorption process. The surface functional groups were further measured and characterized by FT-IR spectroscopy. As shown in Figure 2c, a strong peak of hydroxyl at 3404 cm−1 indicated the presence of O−H groups on the C@La-TiO2 surface.30 Notably, the small peak at 1635 cm−1 was attributed to O−H bending.31 The absorption peaks at 1473 and 1047 cm−1 corresponded to the asymmetric stretching vibrations of CO32−.32 In Figure 2d, the two peaks at 3387 and 1630 cm−1 were attributed to the O−H stretching and bending vibrations, respectively.33 The peak at 1042 cm−1 was assigned to the vibration of the CO32− bond.34,35 Absorption peaks observed in the low-frequency region were attributed to metal−oxygen−metal vibrations.36 Prominently, it was clear that the surface functional groups of the composites did not change obviously in the removal process. Adsorption Property. Influence of Ionic Strength and pH. The effect of pH on the adsorption behaviors of GO and

Figure 1. SEM images and high-magnification SEM images (inset) of C@La-TiO2 (a) and C@Ce-TiO2 (b). TEM image (inset) and highmagnification TEM image of C@La-TiO2 (c). Elemental mapping images of C@La-TiO2 (d).

uniformly on the C@La-TiO2 surface. Similarly, the elements of Ti, O, C, and Ce were also dispersed on the C@Ce-TiO2 surface (Figure S1). The XRD technique was utilized to determine the phase structures of as-prepared composites.27−29 In Figure 2a, one can see that the diffraction peaks at 25.4°, 36.9°, 48.1°, 62.7°,

Figure 2. XRD patterns of C@La-TiO2 (a) and C@Ce-TiO2 (b) before and after adsorption. The FT-IR spectra of C@La-TiO2 (c) and C@Ce-TiO2 (d) before and after adsorption. 5552


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Figure 3. Effect of pH and ionic strength on GO removal onto C@La-TiO2 (a) and C@Ce-TiO2 (b; GO initial concentration = 50 mg/L, m/V = 0.5 g/L, adsorption time = 24 h, T = 298 K). Effect of pH and ionic strength on Cr(VI) removal onto C@La-TiO2 (c) and C@Ce-TiO2 (d; Cr(VI) initial concentration = 30 mg/L, m/V = 0.5 g/L, adsorption time = 24 h, T = 298 K). GO removal on C@La-TiO2 and C@Ce-TiO2 as a function of solid content (e; GO initial concentration = 50 mg/L, adsorption time = 24 h, pH = 5.0 ± 0.1, I = 0.01 M NaCl, T = 298 K). Cr(VI) removal on C@La-TiO2 and C@Ce-TiO2 as a function of solid content (f; Cr(VI) initial concentration = 30 mg/L, adsorption time = 24 h, pH = 5.0 ± 0.1, I = 0.01 M NaCl, T = 298 K).

attraction played a significant role in the adsorption process.37,38 As shown in Figure 3b, a similar phenomenon can be clearly seen for GO removal on C@Ce-TiO2. The removal percentage of GO increased at pH 2.0−5.0 and decreased at pH 5.0−11.0, which was in good accordance with the GO removal on C@La-TiO2. This observation was also consistent with GO adsorption on Ca/Al-LDH and Mg/Al-LDH.15 The adsorption of Cr(VI) on C@La-TiO2 at different NaCl concentrations is shown in Figure 3c. It is interesting to note that the removal of Cr(VI) increased with pH increasing at pH 2.0−4.0 and decreased at pH > 4.0, indicating that the removal of Cr(VI) was strongly affected by pH. Likewise, the Cr(VI) removal on C@Ce-TiO2 showed the same trends (Figure 3d). The adsorption of Cr(VI) is interpreted from the relative distribution of Cr(VI) species in solutions and the C@Ce-TiO2 surface properties. The Cr(VI) ions are presented as negatively

Cr(VI) to the composites is illustrated in Figure 3. The removal percentage of GO on C@La-TiO2 increased gradually with the pH increasing from 2.0 to 5.0. The maximum adsorption was observed at pH = 5.0 (Figure 3a). Afterward, the GO removal declined slowly with the continued increase of pH. It is worth noting that the observed GO removal percentage maintained a high value (>90%) in a very wide pH range. The removal trends can be attributed to the surface electrical properties of C@La-TiO2. The surface charge of C@La-TiO2 was positive under acidic conditions due to the protonation reaction, which was conducive to binding the negatively charged GO through electrostatic attraction (Figure S2). While under neutral and alkaline conditions, the protonation decreased and the competition between GO and hydroxyl ions on the C@La-TiO2 surface led to the decline of GO removal. As previously reported, the influence of pH on GO removal further proved that electrostatic 5553


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ACS Sustainable Chemistry & Engineering charged species such as HCrO4− at pH < 4, CrO42− and HCrO4− at 4 < pH < 8, and CrO42− at pH > 8.39 Under acidic conditions, the electrostatic attraction between positively charged composites and HCrO4− ions dominates the adsorption process. With increasing pH, there are more hydroxyl ions in solution which can compete with CrO42−, resulting in a decrease of Cr(VI) removal, which was in good agreement with the adsorption of Cr(VI) ions on a polyaniline coated protonic titanate nanobelt composite.39 Moreover, the dissolution of metal ions (Ti, La, and Ce) was tested at different pH values. It should be pointed out that the leaching out of metal ions was not measured at 4.0 < pH < 11.0, and the partial dissolution of the composites was found at pH < 4.0. Considering the neutral wastewater under real conditions, it can be regarded that the composites have excellent stability and will not cause secondary pollution in practical applications. The influence of ionic strength on the removal of GO and Cr(VI) was also systematically studied. As illustrated in Figure 3, it was obvious that the removal of GO on C@La-TiO2 (Figure 3a) and C@Ce-TiO2 (Figure 3b) was affected by ionic strength under experimental conditions. More specifically, a relatively high concentration of NaCl was favorable for the binding of GO. Wu et al.40 found that a relatively high concentration of NaCl promoted the aggregation and deposition of GO by increasing the electrostatic interaction. Contrary to GO removal, a higher NaCl concentration was adverse to Cr(VI) adsorption (Figure 3c and d), revealing that the outer-sphere surface complexation rather than inner-sphere surface complexation dominated the interaction of Cr(VI) with the composites.41 This consequence was consistent with Cr(VI) adsorption on kaolinite.42 Influence of Solid Content. The dependence of GO and Cr(VI) removal on the composites at different solid contents is shown in Figure 3e and f. As shown in Figure 3e, the GO removal percentages on C@La-TiO 2 and C@Ce-TiO 2 increased with the increase of solid content. It is generally accepted that more surface functional groups are available, and then more sites are provided for the binding of GO with the increase of solid content. It is worth it to note that the removal percentages of GO on C@La-TiO2 and C@Ce-TiO2 exceeded 84% at a solid content of 0.1 g/L, indicating that the composites had high removal capacities for GO from aqueous solutions. Similar results were also found for Cr(VI) adsorption (Figure 3f). With the solid content increasing from 0.1 to 0.9 g/L, the removal of Cr(VI) increased from ∼9.9% to ∼41.8% on

C@La-TiO2 and from ∼6.5% to ∼36.5% on C@Ce-TiO2, suggesting that C@La-TiO2 had a better removal performance for Cr(VI) from wastewater as compared with C@Ce-TiO2. Adsorption Kinetics. The effect of contact time on GO and Cr(VI) adsorption is shown in Figure 4. From Figure 4a, it is clearly observed that the removal of GO on C@La-TiO2 and C@Ce-TiO2 increased obviously with an increase in time, and the adsorption reached equilibration in 60 min. In contrast to GO removal, the adsorption of Cr(VI) achieved adsorption equilibrium in about 6 h (Figure 4b). In order to give insight into the kinetic adsorption processes, the pseudo-first-order kinetic model and pseudo-second-order kinetic model were used to model the kinetic data. The pseudo-first-order kinetic model was expressed as43 ln(qe − qt ) = ln qe − k1t

(2)

The pseudo-second-order kinetic model was described as qt =

43

k 2qe2t 1 + k 2qet

(3)

where k1 is the pseudo-first-order rate constant, k2 is the pseudo-second-order rate constant, qt is the amount of GO or Cr(VI) adsorbed on solid phases at time t, and qe is the amount of GO or Cr(VI) adsorbed on solid phases after equilibrium. As illustrated in Figure 4, pseudo-first-order and pseudosecond-order kinetic models were applicable for simulation of the sorption data. For GO adsorption on C@La-TiO2 and C@CeTiO2, the experimental data were better simulated by the pseudosecond-order model than the pseudo-first-order model (Figure 4a). For the adsorption data of Cr(VI), the correlation coefficients of the pseudo-second-order model were higher than those of the pseudo-first-order model, indicating that the pseudo-second-order model simulated the kinetic adsorption of Cr(VI) well. Adsorption Isotherms and Thermodynamic Data. The adsorption isotherms of GO and Cr(VI) on the composites were carried out at three temperatures (i.e., 298, 313, and 328 K). Herein, the Freundlich model and Langmuir model were applied to simulate the adsorption isotherms to evaluate the interaction mechanism. The Langmuir model is applied to describe the adsorption at monolayer sites, which is described as23

qe =

Q 0Ceb 1 + Ceb

(4)

Figure 4. Pseudo-first-order (the dash lines) and pseudo-second-order (the solid lines) models for GO (a) and Cr(VI) (b) removal on the composites. GO initial concentration = 50 mg/L, Cr(VI) initial concentration = 30 mg/L, pH = 5.0 ± 0.1, I = 0.01 M NaCl, T = 298 K. 5554


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Figure 5. Adsorption isotherms in different systems: (a) GO removal on C@La-TiO2; (b) GO removal on C@La-TiO2 in the presence of Cr(VI) initial concentration = 10 mg/L; (c) GO removal on C@Ce-TiO2; (d) GO removal on C@Ce-TiO2 in the presence of Cr(VI) initial concentration = 10 mg/L; (e) Cr(VI) removal on C@La-TiO2; (f) Cr(VI) removal on C@La-TiO2 in the presence of GO initial concentration = 10 mg/L; (g) Cr(VI) removal on C@Ce-TiO2; (h) Cr(VI) removal on C@Ce-TiO2 in the presence of GO initial concentration = 10 mg/L (solid lines, Langmuir model; dashed lines, Freundlich model; pH = 5.0 ± 0.1; I = 0.01 M NaCl; m/V = 0.5 g/L; adsorption time = 24 h).

where qe is the adsorption capacity at saturation (mg/g), Ce denotes the final concentrations of GO or Cr(VI) in supernatant (mg/L), Q0 is the theoretical saturated sorption capacity (mg/g), b is the Langmuir sorption constant representing

The Freundlich model is suitable for multilayer adsorption, which is expressed as23 qe = K f Ce1/ n

(5) 5555


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ACS Sustainable Chemistry & Engineering Table 1. Langmuir and Freundlich Parameters for GO Adsorption Langmuir model different systems GO removal on C@La-TiO2

GO removal on C@La-TiO2 with C[Cr]initial = 10 mg/L

GO removal on C@Ce-TiO2

GO removal on C@Ce-TiO2 with C[Cr]initial = 10 mg/L

Freundlich model

T (K)

qmax (mg/g)

b (L/mg)

R2

Kf (mg1‑nLn/g)

n

R2

298 313 328 298 313 328 298 313 328 298 313 328

383.3 413.5 431.4 343.6 363.9 381.9 336.8 364.8 400.3 310.6 354.3 370.9

0.42 0.43 0.56 0.24 0.25 0.28 0.06 0.07 0.10 0.03 0.04 0.06

0.970 0.979 0.942 0.969 0.947 0.957 0.993 0.992 0.986 0.976 0.973 0.988

120.5 132.0 145.9 101.1 104.3 115.3 38.2 44.4 60.5 27.7 30.2 39.8

0.27 0.28 0.28 0.30 0.32 0.33 0.49 0.50 0.50 0.56 0.59 0.57

0.872 0.868 0.831 0.870 0.856 0.865 0.942 0.944 0.981 0.939 0.936 0.951

Table 2. Langmuir and Freundlich Parameters for Cr(VI) Adsorption Langmuir model different systems Cr(VI) removal on C@La-TiO2

Cr(VI) removal on C@La-TiO2 with C[Cr]initial = 10 mg/L

Cr(VI) removal on C@Ce-TiO2

Cr(VI) removal on C@Ce-TiO2 with C[Cr]initial = 10 mg/L

Freundlich model 2

T (K)

qmax (mg/g)

b (L/mg)

R

298 313 328 298 313 328 298 313 328 298 313 328

50.5 56.1 59.3 67.8 76.4 85.8 43.6 49.0 53.8 49.4 62.9 75.3

0.04 0.03 0.02 0.04 0.03 0.01 0.04 0.03 0.03 0.02 0.02 0.03

0.951 0.962 0.982 0.961 0.979 0.997 0.975 0.971 0.973 0.989 0.982 0.975

enthalpy of adsorption and varies with temperature (L/mg), Kf (mg/g) and n are Freundlich adsorption coefficients, which are related to sorption capacity and the sorption intensity at a specific temperature, respectively. The model fitting of GO and Cr(VI) adsorption isotherms using Langmuir and Freundlich models is shown in Figure 5. The adsorption of GO on C@La-TiO2 increased significantly with the increase of temperature, revealing that higher temperature was beneficial for GO removal (Figure 5a). The parameters calculated from the Freundlich and Langmuir models are summarized in Table 1. It is clear that the Langmuir model simulated GO adsorption better than the Freundlich model, implying that the main mode was monolayer adsorption. According to the data calculated from the Langmuir model, the maximum adsorption capacity of GO on C@LaTiO2 reached 383.3 mg/g at 298 K. To investigate the simultaneous adsorption of Cr(VI) and GO, the GO removal on C@La-TiO2 was conducted at an initial Cr(VI) concentration of 10 mg/L (Figure 5b). The highest removal capacity of GO on C@La-TiO2 with C[Cr(VI)]initial = 10 mg/L was 343.6 mg/g. Obviously, the Cr(VI) ions can make the binding of GO compete with the oxygen-containing groups and active sites on the C@La-TiO2 surface, thereby resulting in a decrease of GO removal capacity. Similarly, the GO removals on C@Ce-TiO2 in the absence (Figure 5c) and presence (Figure 5d) of Cr(VI) were also investigated and compared. Compared with a GO removal capacity of 336.8 mg/g on C@Ce-TiO2 in the absence of Cr(VI), the sorption capacity of GO on C@Ce-TiO2 at C[Cr]initial = 10 mg/L decreased to 310.6 mg/g at 298 K.

Kf (mg

1−n

1.6 2.9 4.4 2.1 2.9 5.6 1.0 1.9 3.7 3.2 3.9 4.7

Ln/g)

n

R2

0.74 0.62 0.52 0.77 0.75 0.61 0.83 0.68 0.54 0.63 0.63 0.62

0.943 0.955 0.973 0.943 0.962 0.980 0.970 0.966 0.968 0.982 0.979 0.971

This phenomenon further demonstrated that Cr(VI) ions would compete with GO sorption to C@Ce-TiO2 from aqueous solutions. Correspondingly, the Cr(VI) adsorption on the composites in the absence/presence of GO was also studied. As shown in Figure 5e and f, the removal capacity of Cr(VI) on C@La-TiO2 increased with increasing temperature, indicating an endothermic process.44 As tabulated in Table 2, the Langmuir model described the adsorption of Cr(VI) on C@Ce-TiO2 and C@La-TiO2 as well. On the basis of Langmuir adsorption simulation, the maximum removal ability of Cr(VI) on C@La-TiO2 was 50.5 mg/g and increased obviously after the addition of GO (Figure 5f). Many reports had proved that GO was an efficient adsorbent for the elimination of Cr(VI) ions from solutions. The surface adsorbed GO on the composites could provide some functional groups and available sites to bind Cr(VI) ions, thus leading to the increase of Cr(VI) adsorption on the composites.45,46 The maximum removal capacity of Cr(VI) on C@La-TiO2 (50.5 mg/g) was much higher than that on C@Ce-TiO2 (43.6 mg/g). The ionic radius of La is larger than that of Ce, which is favorable for the binding of the negatively charged Cr(VI) ions via electrostatic attraction.47 Furthermore, the surface area of C@La-TiO2 (172.8 m2/g) is larger than that of C@Ce-TiO2 (159.5 m2/g), suggesting that C@La-TiO2 can provide more active sites for pollutant removal (Figure 6). As illustrated in Figure 5, the temperature played a decisive role in the adsorption process. Thermodynamic parameters can effectively reflect the relationship between the adsorption 5556


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Figure 6. N2 adsorption−desorption isotherms for C@La-TiO2 (a) and C@Ce-TiO2 (b).

Table 3. Thermodynamics Parameters for GO and Cr(VI) Removal ΔG (kJ/mol) adsorbate GO Cr(VI)

adsorbent

ΔH (kJ/mol)

ΔS (J/mol·K)

298 K

313 K

328 K

C@La-TiO2 C@Ce-TiO2 C@La-TiO2 C@Ce-TiO2

13.97 22.74 4.59 4.66

56.43 83.56 4.43 3.67

−6.57 −6.34 −8.23 −8.56

−6.89 −6.68 −8.65 −8.99

−7.23 −7.01 −9.06 −9.42

on the composites. From the TEM images, it is clearly observed that the adsorption of GO occurred on the composite surfaces (Figure S6c and d). Furthermore, the composites before and after adsorption were characterized by EDS analysis. As shown in Figure S7 and Table 4, compared to the bare C@La-TiO2,

process and temperature and provide some information about the ultimate uptake of GO and Cr(VI) onto the composites. To understand the thermodynamic feasibility and to evaluate the adsorption process properties, the corresponding thermodynamic data were calculated from the following equations:48 Kc =

CAe Ce

ΔG° = −RT ln Kc

ΔS ° ΔH ° ln Kc = − R RT

Table 4. Element Changes of C@La-TiO2 and C@Ce-TiO2 before and after Adsorption

(6)

element

(7) different systems C@La-TiO2

(8) C@La-TiO2 + GO

where Kc is the thermodynamic equilibrium constant, CAe (mg/L) is the equilibrium concentration on the adsorbent, Ce (mg/L) is the equilibrium concentration in solution, T (K) is temperature in Kelvin, R is the ideal gas constant (8.314 J/(mol·K)), ΔG° (kJ/mol) is free energy change, ΔH° (kJ/mol) is the enthalpy change, and ΔS° (J/(mol·K)) is the entropy change. From Figure S5 and Table 3, the negative values of ΔG° suggested that GO and Cr(VI) adsorption on the composites was a feasible and spontaneous process. The ΔG° values became more negative at higher temperatures; for example, the ΔG° value of GO adsorption on C@La-TiO2 decreased from −6.57 kJ/mol (T = 298 K) to −7.23 kJ/mol (T = 328 K), revealing more efficient GO adsorption at higher temperatures.49 The positive values of ΔH° suggested the endothermic properties of the adsorption processes. In addition, the positive ΔS° values confirmed the increased randomness at the solid− water interfaces in the adsorption processes. Interaction Mechanism. In order to verify the adsorption mechanism, the GO and Cr(VI) removals on the composites were investigated by different techniques. As shown in Figure S6, the SEM images indicated that the morphology of the composites was influenced in the removal process. The C@La-TiO2 (Figure S6a) and C@Ce-TiO2 (Figure S6b) composites lost smooth surface and presented a rough surface after adsorption, which was attributed to GO and Cr(VI) adsorption

C@La-TiO2 + Cr(VI) C@La-TiO2 + GO + Cr(VI)

C@Ce-TiO2 C@Ce-TiO2 + GO C@Ce-TiO2 + Cr(VI) C@Ce-TiO2 + GO + Cr(VI)

Ti

La

C

O

Cr

atomic % weight % atomic % weight % atomic % weight % atomic % weight %

25.20 48.42 27.58 49.32 27.53 45.98 26.57 48.64

1.18 6.83 2.33 12.11 3.59 17.42 1.26 8.70

20.83 9.46 22.27 9.99 20.15 8.44 23.58 10.83 element

52.79 35.29 47.82 28.58 47.95 26.76 47.07 28.79

0.77 1.40 1.53 3.04

Ti

Ce

C

O

Cr(VI)

atomic % weight % atomic % weight % atomic % weight % atomic % weight %

24.79 47.52 27.74 53.88 30.48 55.63 27.91 53.63

1.54 8.65 1.05 5.50 0.98 5.26 0.63 3.53

20.99 10.09 21.52 10.88 20.71 9.94 21.36 10.31

52.68 33.74 49.69 29.75 47.39 28.30 49.44 31.48

0.44 0.87 0.66 1.05

the element content of C increased significantly after GO adsorption, indicating the successful loading of GO onto C@ La-TiO2. It is worth it to notice that the element contents of Cr(VI) and C increased more obviously after GO and Cr(VI) simultaneous removal, which were consistent with the batch experimental results. Similar results of GO and Cr(VI) adsorption on C@Ce-TiO2 were also observed. From Figure S8 and Table 4, it was found that the element content of Cr(VI) 5557


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Figure 7. XPS spectra: the wide scan of fresh and used C@La-TiO2 (a); the peak for Cr 2p (b); the high O 1s deconvolution of C@La-TiO2 (c), C@La-TiO2 + GO (d), C@La-TiO2 + Cr(VI) (e), C@La-TiO2 + GO + Cr(VI) (f); the wide scan of C@Ce-TiO2 (g); the peak for Cr 2p (h); the high O 1s deconvolution of C@Ce-TiO2 (i), C@Ce-TiO2 + GO (j), C@Ce-TiO2 + Cr (k), and C@Ce-TiO2 + GO + Cr (l).

groups participated in the binding of GO. A similar phenomenon was observed for the adsorption of GO on Ca/Al-LDH and Mg/Al-LDH.15 As illustrated in Figure 7e and Table 5, the peak position and the ratio of the peak area of O−H changed obviously after Cr(VI) adsorption. More importantly, the decrease of the peak area assigned to C−O indicated that the carbon sphere also played a significant role in the Cr(VI) adsorption process.52 It is also worthy to notice that the ratio of the peak area ascribed to C−O increased from 38.0% to 48.6% after GO adsorption (Figure 7d) and the C−O ratio increased from 38.0% to 40.3% in GO and Cr(VI) simultaneous adsorption (Figure 7f). The increase of the ratio of the peak area attributed to C−O decreased relatively in different systems, which was attributed to competitive interaction between GO and Cr(VI) with C@La-TiO2. In the same way, the peaks for O 1s, C 1s, Ti 2p, and Ce 3d can be seen for the C@Ce-TiO2 composites (Figure 7g). As shown in Figure 7h, one can find that a new peak at 578.3 eV appeared after adsorption, which was caused by Cr(VI). From Figure 7i, it was clear to see that the chemical binding energies positioned at 530.2, 531.0, and 532.5 eV were ascribed to M−O, O−H, and C−O, respectively.53 Notably, the ratio of the peak area of C−O increased from 18.6% to 26.0% and that of O−H decreased from 20.1% to 18.3%, which were ascribed to the GO adsorption on the surface of C@Ce-TiO2 (Figure 7j).

significantly increased from the absence of GO to the presence of GO. This observation is consistent with the results in other references.50,51 To gain the further insight into the possible interaction mechanism, the XPS spectra of the as-prepared composites before and after GO and Cr(VI) adsorption were measured and analyzed (Figure 7), and the relevant binding energies of various elements in the composites are tabulated in Table 5. According to the XPS analysis, one can see that the multifarious peaks such as O 1s, C 1s, Ti 2p, and La 3d were observed in C@La-TiO2 before and after adsorption (Figure 7a). After Cr(VI) adsorption, a distinct peak at 578.3 eV was found in C@La-TiO2 as compared with the pristine sample, demonstrating the successful capture of Cr(VI) (Figure 7b). As illustrated in Figure 7c, the strong high resolution of the O 1s spectrum of C@La-TiO2 was decomposed into three characteristic components located at 530.0 (M−O, M represents Ti, La, or Ce), 530.6 (O−H), and 532.3 eV (C−O).36 However, the position and intensity of the three peaks changed remarkably after GO adsorption (Figure 7d). For instance, the ratio of the peak area attributed to C−O increased from 38.0% to 48.6%, which was ascribed to the GO adsorption on the surface of C@La-TiO2. Meanwhile, the peak position of O−H changed to 531.1 eV, and the ratio of the peak area decreased from 18.9% to 17.5%, revealing that hydroxyl 5558


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and binding sites to form surface complexes with Cr(VI) ions. However, in the presence of Cr(VI), the GO removal capacity decreased due to the competitive adsorption between GO and Cr(VI). The removal of GO was mainly attributed to electrostatic attraction and hydrogen bonding. The adsorption of Cr(VI) was mainly attributed to electrostatic attraction and outer-sphere surface complexation. This work provided new insights into the co-removal of GO and potential toxic metal ions in natural aquatic environmental pollution cleanup.

Table 5. High Deconvolution of O 1s Spectra of the Composites before and after Adsorption different systems C@La-TiO2

C@La-TiO2 + GO

C@La-TiO2 + Cr(VI)

C@La-TiO2 + GO + Cr(VI)

C@Ce-TiO2

C@Ce-TiO2 + GO

C@Ce-TiO2 + Cr(VI)

C@Ce-TiO2 + GO + Cr(VI)

component

peak position (eV)

area %

M−O O−H C−O M−O O−H C−O M−O O−H C−O M−O O−H C−O M−O O−H C−O M−O O−H C−O M−O O−H C−O M−O O−H C−O

530.0 530.6 532.3 530.1 531.1 532.3 529.9 531.4 532.4 530.0 531.6 532.5 530.2 531.0 532.5 530.1 531.0 532.5 530.1 530.8 532.4 530.1 531.0 532.5

43.1 18.9 38.0 33.9 17.5 48.6 50.7 15.9 33.4 43.8 15.9 40.3 61.3 20.1 18.6 55.7 18.3 26.0 59.4 12.6 28.0 49.2 17.6 33.2

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00957. Figures S1−S8 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Phone: 86-10-61772890. Fax: 86-10-61772890. E-mail: [email protected]. *Phone: 86-10-61772890. Fax: 86-10-61772890. E-mail: [email protected] or [email protected]. ORCID

Xiangke Wang: 0000-0002-3352-1617 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (91326202, 21577032), the Fundamental Research Funds for the Central Universities (JB2015001, JB2016166), the National Special Water Programs (2015ZX07203-011, 2015ZX07204-007), the Jiangsu Provincial Key Laboratory of Radiation Medicine and Protection, and Priority Academic Program Development of Jiangsu Higher Education Institutions are acknowledged.

Figure 7k displayed that the ratio of the peak area attributed to O−H decreased from 20.1% to 12.6%, suggesting that hydroxyl groups played key roles in Cr(VI) binding on the composites. As previously reported, the trapping Cr(VI) was mainly dominated by surface complexation through the coordination of Cr(VI) with oxygen-containing groups, which was in line with the results of the influence of ionic strength.54 In addition, the high resolution of O 1s for GO and Cr(VI) simultaneous adsorption on C@Ce-TiO2 is illustrated in Figure 7l. From Table 5, it can be seen that the ratio of the peak area attributed to C−O increased and the ratio of the peak area assigned to O−H decreased after the adsorption, which was similar to the GO and Cr(VI) co-removal on C@La-TiO2. According to the above-mentioned analysis, one can conclude that the potential mechanism of GO interaction with the composites was mainly driven by electrostatic attraction and hydrogen bonding. Correspondingly, the electrostatic attraction and outer-sphere surface complexation governed Cr(VI) adsorption onto the composites.

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REFERENCES

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CONCLUSION In this study, rare earth doped titanium dioxide coated carbon sphere composites were synthesized via a simple hydrothermal method for GO and Cr(VI) simultaneous removal. The results indicated that the GO and Cr(VI) removal on the composites was dependent on pH. Interestingly, a relatively high concentration of NaCl was beneficial to the binding of GO, and adverse to Cr(VI) removal. The adsorption processes of GO and Cr(VI) to the composites were spontaneous and endothermic. The removal capacities of C@La-TiO2 reached 383.3 mg/g for GO and 50.5 mg/g for Cr(VI) at pH 5.0. The sorption capacity of Cr(VI) on the composites increased significantly in the presence of GO, which was ascribed to the surface adsorbed GO to provide more surface functional groups 5559


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Simultaneous Removal of Graphene Oxide and Chromium(VI) on the

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