Effective and Efficient Porous CeO2 Adsorbent for Acid Orange 7 Adsorption

A porous CeO2 was synthesized following the addition of guanidine carbonate to a Ce3+ aqueous solution, the subsequent addition of hydrogen peroxide and a final hydrothermal treatment. The optimal experimental parameters for the synthesis of porous CeO2, including the amounts of guanidine carbonate and hydrogen peroxide and the hydrothermal conditions, were determined by taking the adsorption efficiency of acid orange 7 (AO7) dye as the evaluation. A template−free hydrothermal strategy could avoid the use of soft or hard templates and the subsequent tedious procedures of eliminating templates, which aligned with the goals of energy conservation and emission reduction. Moreover, both the guanidine carbonate and hydrogen peroxide used in this work were accessible and eco−friendly raw materials. The porous CeO2 possessed rapid adsorption capacities for AO7 dye. When the initial concentration of AO7 was less than 130 mg/L, removal efficiencies greater than 90.0% were obtained, achieving a maximum value of 97.5% at [AO7] = 100 mg/L and [CeO2] = 2.0 g/L in the first 10 min of contact. Moreover, the adsorption–desorption equilibrium between the porous CeO2 adsorbent and the AO7 molecule was basically established within the first 30 min. The saturated adsorption amount of AO7 dye was 90.3 mg/g based on a Langmuir linear fitting of the experimental data. Moreover, the porous CeO2 could be recycled using a NaOH aqueous solution, and the adsorption efficiency of AO7 dye still remained above 92.5% after five cycles. This study provided an alternative porous adsorbent for the purification of dye wastewater, and a template−free hydrothermal strategy was developed to enable the design of CeO2−based catalysts or catalyst carriers.


Introduction
The rise of the synthetic dye industry led to a revolution in chemical technology in the mid to late 19th century. Synthetic dyes developed rapidly, production varieties increased, output soared and they basically replaced natural dyes in the 20th century. To date, synthetic dyes have been widely applied to the fields of textiles, papermaking, plastics, leather, rubber, paints, cosmetics, food, etc. [1,2]. The world is so beautiful and colourful with almost 700,000 tons of synthetic dyes; however, 10-15% of these are discharged into wastewater, resulting in water pollution [3][4][5]. In particular, many synthetic dyes, such as azo dye and benzidine dye, are not only toxic to aquatic organisms, but also carcinogenic and mutagenic to humans [6]. Therefore, many techniques have been applied to remove these dyes from aqueous solutions, such as adsorption [7,8], ultrafiltration [9], photocatalytic degradation [10], electrochemical degradation [11], advanced oxidation processes [12], biological processes [13], etc. Among these numerous physical, chemical and biological techniques, the adsorption method using porous materials is favoured in the (Ce 2 (CO 3 ) 3 ·8H 2 O) was generated immediately. Subsequently, the desired amount of H 2 O 2 (1~5 mL) was added to the above white suspension, and the white suspension promptly turned orange, then the suspension was stirred for 1 h and aged for 24 h.
The final CeO 2 product was obtained following a hydrothermal process. Typically, the above suspension was decanted into a 50 mL Teflon−lined stainless steel autoclave, which was heated and maintained for 24 h at a set temperature (120~200 • C). Note that distilled water was used to make a total volume of about 25 mL. Finally, the resulting pale yellow precipitate (CeO 2 ) was washed with distilled water and ethanol, then dried in air at 80 • C for 24 h.

Characterization
The phases of samples were examined using a DX−2700 X−ray diffraction (XRD, Dandong, China). The morphologies and microstructures of the CeO 2 samples were examined using a JSM−7500F scanning electron microscopy (SEM, JEOL, Tokyo, Japan) and a JEM−2100F transmission electron microscopy (TEM, JEOL, Tokyo, Japan). Nitrogen adsorption-desorption isotherms of the CeO 2 samples were measured on an ASAP2460 (Micromeritics, Norcross, GA, USA).

Adsorption of AO7 Dye
AO7, a typical azo dye, was selected as the model target to evaluate the adsorption capacity of the final porous CeO 2 product. First, AO7 aqueous solutions with different concentrations of 100~180 mg/L were configured as simulated wastewater, then 0.2 g as−obtained CeO 2 was dispersed into 100 mL AO7 solution with a desired concentration. The above mixture was stirred with a constant agitation speed of 200 rpm at room temperature, and the suspension was withdrawn at regular intervals. After the solid-liquid separation, the absorbance of the supernatant was measured at the absorption wavelength of 485 nm using an U−3900 ultraviolet−visible spectrophotometer (Uv−vis, Hitachi, Tokyo, Japan). The adsorption efficiency (η t ,%) and the adsorption amount (q t , mg/g) were calculated using Equations (1) and (2), respectively. The experimental data from the adsorption of AO7 dye onto porous CeO 2 were fitted according to the Langmuir (Equation (3)) [48] and Freundlich (Equation (4)) [49] isotherm models.
log q e = 1 n log C e + log K F (4) where C 0 (mg/L) is the initial concentration of AO7 aqueous solution, C t (mg/L) is the concentration of AO7 aqueous solution at a given time t, m (g) is the mass of porous CeO 2 absorbent (0.2 g), V (L) is the volume of AO7 aqueous solution (100 mL), K L and K F are the Langmuir and Freundlich adsorption constants, respectively. Moreover, the saturated adsorption amount (q m , mg/g) of AO7 could be obtained according to Langmuir linear fitting.
In order to investigate the thermal properties of the adsorption process, the Gibbs free energy change (∆G 0 , KJ/mol) and thermodynamic equilibrium constant (K 0 , L/g) were evaluated using Equation (5), while the entropy change (∆S 0 , J/mol·K) and enthalpy change (∆H 0 , KJ/mol) were obtained using the linear fitting of the Van't Hoff equation (Equation (6)) [50]. Meanwhile, to explore the kinetics characteristics of the adsorption process, the experimental data were evaluated using the pseudo−first−order (Equation (7)) and pseudo−second−order (Equation (8)) models, respectively [51]. The equilibrium adsorption amount (q e1,cal and q e2,cal , mg/g) and rate constant (k 1 , 1/h and k 2 , g/mg·h) could be evaluated using the plots of log(q e1,cal− q t ) vs. t and t/q t vs. t. Figure 1a shows the XRD pattern of the original white precipitate after adding guanidine carbonate to the Ce 3+ aqueous solution. As observed, the obvious diffraction peaks in Figure 1a were assigned to the standard orthorhombic Ce 2 (CO 3 ) 3 ·8H 2 O (JCPDS no. 38−0377), and this XRD pattern was similar to the commercial Ce 2 (CO 3 ) 3 ·xH 2 O powders [52] obtained in previous studies [53,54]. After following the addition of 5 mL 30% H 2 O 2 , the XRD pattern in Figure 1b displayed several well−resolved peaks that could be indexed to (111), (200), (220), (311), (400) and (331) planes of the standard CeO 2 with face−centred cubic structure (JCPDS no. 34−0394); however, its crystallinity was only 13.97% calculated using the X−ray diffraction method. Moreover, the diffraction peaks related to orthorhombic Ce 2 (CO 3 ) 3 ·8H 2 O were no longer present, which indicated the complete transformation of orthorhombic Ce 2 (CO 3  In order to investigate the thermal properties of the adsorption process, the Gibbs free energy change (ΔG 0 , KJ/mol) and thermodynamic equilibrium constant (K0, L/g) were evaluated using Equation (5), while the entropy change (ΔS 0 , J/mol·K) and enthalpy change (ΔH 0 , KJ/mol) were obtained using the linear fitting of the Van't Hoff equation (Equation (6)) [50]. Meanwhile, to explore the kinetics characteristics of the adsorption process, the experimental data were evaluated using the pseudo−first−order (Equation (7)) and pseudo−second−order (Equation (8)) models, respectively [51]. The equilibrium adsorption amount (qe1,cal and qe2,cal, mg/g) and rate constant (k1, 1/h and k2, g/mg•h) could be evaluated using the plots of log(qe1,cal−qt) vs. t and t/qt vs. t. Figure 1a shows the XRD pattern of the original white precipitate after adding guanidine carbonate to the Ce 3+ aqueous solution. As observed, the obvious diffraction peaks in Figure 1a were assigned to the standard orthorhombic Ce2(CO3)3•8H2O (JCPDS no. 38−0377), and this XRD pattern was similar to the commercial Ce2(CO3)3•xH2O powders [52] obtained in previous studies [53,54]. After following the addition of 5 mL 30% H2O2, the XRD pattern in Figure 1b displayed several well−resolved peaks that could be indexed to (111), (200), (220), (311), (400) and (331) planes of the standard CeO2 with face−centred cubic structure (JCPDS no. 34−0394); however, its crystallinity was only 13.97% calculated using the X−ray diffraction method. Moreover, the diffraction peaks related to orthorhombic Ce2(CO3)3•8H2O were no longer present, which indicated the complete transformation of orthorhombic Ce2(CO3)3•8H2O into cubic CeO2 under the oxidation of H2O2.    Figure 2a shows the XRD pattern of the samples obtained with different amounts of guanidine carbonate (4~16 mmol) and 5 mL 30% H 2 O 2 after hydrothermal treatment at 180 • C for 24 h. All patterns displayed several well−resolved peaks that could be indexed to (111), (200), (220), (311), (222), (400), (331) and (420) planes, which matched well with the standard CeO 2 (JCPDS No. 34−0394) pattern. Moreover, the diffraction peaks of the CeO 2 phase were complete and sharp, and no diffraction peaks of the impurity phase were observed, which suggested that pure CeO 2 with a face−centred cubic structure was successfully synthesized through the synthesis strategy used in this work. Moreover, the optimal amount of ammonium carbonate was determined using the adsorption efficiency of CeO 2 to AO7 dye in an aqueous solution under the same conditions. Figure 2b shows the corresponding adsorption histograms of AO7 dye onto CeO 2 synthesized hydrothermally at 180 • C for 24 h with different amounts of guanidine carbonate (4~16 mmol) and 5 mL 30% H 2 O 2 . When the initial concentration of the AO7 aqueous solution was 100 mg/L, the adsorption efficiency achieved a maximum value of 98.92% for the CeO 2 sample obtained with 4 mmol guanidine carbonate. With an increase in guanidine carbonate (6~12 mmol), the adsorption efficiency of AO7 by the as−obtained corresponding CeO 2 decreased gradually, but was still higher than 80%. When the addition amount of guanidine carbonate was higher than 12 mmol, the adsorption efficiency remained basically unchanged. According to the above results, we concluded that the optimal addition amount of guanidine carbonate was 4 mmol for the synthesis of CeO 2 . Next, we investigated the influence of hydrothermal temperature on the phase composition of the samples and their adsorption efficiencies of AO7 dye. phase were complete and sharp, and no diffraction peaks of the impurity phase were observed, which suggested that pure CeO2 with a face−centred cubic structure was successfully synthesized through the synthesis strategy used in this work. Moreover, the optimal amount of ammonium carbonate was determined using the adsorption efficiency of CeO2 to AO7 dye in an aqueous solution under the same conditions. Figure 2b shows the corresponding adsorption histograms of AO7 dye onto CeO2 synthesized hydrothermally at 180 °C for 24 h with different amounts of guanidine carbonate (4~16 mmol) and 5 mL 30% H2O2. When the initial concentration of the AO7 aqueous solution was 100 mg/L, the adsorption efficiency achieved a maximum value of 98.92% for the CeO2 sample obtained with 4 mmol guanidine carbonate. With an increase in guanidine carbonate (6~12 mmol), the adsorption efficiency of AO7 by the as−obtained corresponding CeO2 decreased gradually, but was still higher than 80%. When the addition amount of guanidine carbonate was higher than 12 mmol, the adsorption efficiency remained basically unchanged. According to the above results, we concluded that the optimal addition amount of guanidine carbonate was 4 mmol for the synthesis of CeO2. Next, we investigated the influence of hydrothermal temperature on the phase composition of the samples and their adsorption efficiencies of AO7 dye.  Figure 3a shows the XRD patterns of the CeO2 samples synthesized at a set hydrothermal temperature of 120~200 °C for 24 h with 4 mmol guanidine carbonate and 5 mL 30% H2O2. As observed in Figure 3a, all XRD patterns displayed several well−resolved peaks that could be indexed to the standard face−centred cubic CeO2 (JCPDS No. 34−0394), and no impurity phases were detected. With an increase in hydrothermal temperature, the corresponding diffraction peaks of as−obtained CeO2 sharpened gradually and their intensities also increased, which indicated that hydrothermal temperature could improve the crystallization of CeO2. Figure 3b shows the corresponding adsorption histograms of AO7 dye onto CeO2 synthesized hydrothermally at a set hydrothermal temperature of 120~200 °C for 24 h with 4 mmol guanidine carbonate and 5 mL 30% H2O2. When the initial concentration of the AO7 aqueous solution was 100 mg/L, the adsorption efficiency of the CeO2 synthesized at 120 °C was only 72.18%. With an increase in hydrothermal temperature, the adsorption efficiency of AO7 by CeO2 increased significantly, and achieved a maximum value of 99.59% for the CeO2 synthesized hydrothermally at 200 °C. Interestingly, the adsorption efficiencies of the CeO2 samples synthesized hydrothermally at temperatures above 140 °C were higher than 96%. Based on the above analyses, we concluded that the optimal hydrothermal synthesis temperature for CeO2 was 200 °C. We would next determine the optimal addition amount of H2O2 for the synthesis of CeO2.  Figure 3a shows the XRD patterns of the CeO 2 samples synthesized at a set hydrothermal temperature of 120~200 • C for 24 h with 4 mmol guanidine carbonate and 5 mL 30% H 2 O 2 . As observed in Figure 3a, all XRD patterns displayed several well−resolved peaks that could be indexed to the standard face−centred cubic CeO 2 (JCPDS No. 34−0394), and no impurity phases were detected. With an increase in hydrothermal temperature, the corresponding diffraction peaks of as−obtained CeO 2 sharpened gradually and their intensities also increased, which indicated that hydrothermal temperature could improve the crystallization of CeO 2 . Figure 3b shows the corresponding adsorption histograms of AO7 dye onto CeO 2 synthesized hydrothermally at a set hydrothermal temperature of 120~200 • C for 24 h with 4 mmol guanidine carbonate and 5 mL 30% H 2 O 2 . When the initial concentration of the AO7 aqueous solution was 100 mg/L, the adsorption efficiency of the CeO 2 synthesized at 120 • C was only 72.18%. With an increase in hydrothermal temperature, the adsorption efficiency of AO7 by CeO 2 increased significantly, and achieved a maximum value of 99.59% for the CeO 2 synthesized hydrothermally at 200 • C. Interestingly, the adsorption efficiencies of the CeO 2 samples synthesized hydrothermally at temperatures above 140 • C were higher than 96%. Based on the above analyses, we concluded that the optimal hydrothermal synthesis temperature for CeO 2 was 200 • C. We would next determine the optimal addition amount of H 2 O 2 for the synthesis of CeO 2 . Figure 4a shows the XRD patterns of CeO 2 samples synthesized at 200 • C for 24 h with 4 mmol guanidine carbonate and different addition amounts of 30% H 2 O 2 (1~5 mL). All the identified peaks in Figure 4a were assigned to the standard cubic CeO 2 (JCPDS No. 34−0394), no impurity phases were detected and the intensities of the diffraction peaks of all the CeO 2 samples were comparable. Figure 4b shows the corresponding adsorption histograms of AO7 dye onto CeO 2 synthesized hydrothermally at 200 • C for 24 h with 4 mmol guanidine carbonate and different addition amounts of 30% H 2 O 2 (1~5 mL). According to our previous adsorption experiment, the adsorption efficiencies of all the CeO 2 samples for AO7 dye were close to 100% when the initial concentration of the AO7 aqueous solution was 100 mg/L, so we increased the initial concentration of AO7 solution to 110 mg/L. As observed in Figure 4b, the adsorption efficiency of CeO 2 synthesized with 1 mL H 2 O 2 was 93.75%. The as−obtained corresponding CeO 2 synthesized with more H 2 O 2 exhibited a slightly better adsorption of AO7, reaching a maximum value of 96.43% for the CeO 2 synthesized with 4 mL H 2 O 2 . For the CeO 2 synthesized with 5 mL H 2 O 2 , its adsorption efficiency decreased, but remained higher than 90%. Combined with the analysis results of XRD and the adsorption experiment in Figures 2-4, the optimal experimental parameters for the synthesis of CeO 2 were determined by taking the adsorption efficiency of AO7 as the evaluation: 4 mmol of guanidine carbonate, 4 mL of 30% H 2 O 2 and a hydrothermal reaction at 200 • C for 24 h.   Figure 4b shows the corresponding adsorption histograms of AO7 dye onto CeO2 synthesized hydrothermally at 200 °C for 24 h with 4 mmol guanidine carbonate and different addition amounts of 30% H2O2 (1~5 mL). According to our previous adsorption experiment, the adsorption efficiencies of all the CeO2 samples for AO7 dye were close to 100% when the initial concentration of the AO7 aqueous solution was 100 mg/L, so we increased the initial concentration of AO7 solution to 110 mg/L. As observed in Figure 4b, the adsorption efficiency of CeO2 synthesized with 1 mL H2O2 was 93.75%. The as−obtained corresponding CeO2 synthesized with more H2O2 exhibited a slightly better adsorption of AO7, reaching a maximum value of 96.43% for the CeO2 synthesized with 4 mL H2O2. For the CeO2 synthesized with 5 mL H2O2, its adsorption efficiency decreased, but remained higher than 90%. Combined with the analysis results of XRD and the adsorption experiment in Figures 2-4, the optimal experimental parameters for the synthesis of CeO2 were determined by taking the adsorption efficiency of AO7 as the evaluation: 4 mmol of guanidine carbonate, 4 mL of 30% H2O2 and a hydrothermal reaction at 200 °C for 24 h.   All the identified peaks in Figure 4a were assigned to the standard cubic CeO2 (JCPDS No. 34−0394), no impurity phases were detected and the intensities of the diffraction peaks of all the CeO2 samples were comparable. Figure 4b shows the corresponding adsorption histograms of AO7 dye onto CeO2 synthesized hydrothermally at 200 °C for 24 h with 4 mmol guanidine carbonate and different addition amounts of 30% H2O2 (1~5 mL). According to our previous adsorption experiment, the adsorption efficiencies of all the CeO2 samples for AO7 dye were close to 100% when the initial concentration of the AO7 aqueous solution was 100 mg/L, so we increased the initial concentration of AO7 solution to 110 mg/L. As observed in Figure 4b, the adsorption efficiency of CeO2 synthesized with 1 mL H2O2 was 93.75%. The as−obtained corresponding CeO2 synthesized with more H2O2 exhibited a slightly better adsorption of AO7, reaching a maximum value of 96.43% for the CeO2 synthesized with 4 mL H2O2. For the CeO2 synthesized with 5 mL H2O2, its adsorption efficiency decreased, but remained higher than 90%. Combined with the analysis results of XRD and the adsorption experiment in Figures 2-4, the optimal experimental parameters for the synthesis of CeO2 were determined by taking the adsorption efficiency of AO7 as the evaluation: 4 mmol of guanidine carbonate, 4 mL of 30% H2O2 and a hydrothermal reaction at 200 °C for 24 h.  The morphology of the CeO 2 sample hydrothermally synthesized at 200 • C for 24 h with 4 mmol guanidine carbonate and 4 mL 30% H 2 O 2 is shown in Figure 5a. As observed, the CeO 2 featured equiaxed particles formed agglomerates. Moreover, the size value of the CeO 2 particles was demonstrated using a statistical analysis, and the size distribution histogram is shown in Figure 5b. As observed, it was clearly found that most of the CeO 2 particles were mainly concentrated at about 42.5 and 87.5 nm. Figure 5c shows the TEM image of a single CeO 2 particle, which revealed the porous structure and the many pores around the nanoparticles. Moreover, the high−resolution transmission electron microscope (HR−TEM) image in Figure 5d shows that these nanoparticles had lattice fringes with the same direction (see the yellow arrows in Figure 5d), indicating the single crystal structure of these nanoparticles.

Results and Discussion
The morphology of the CeO2 sample hydrothermally synthesized at 200 °C for 24 h with 4 mmol guanidine carbonate and 4 mL 30% H2O2 is shown in Figure 5a. As observed, the CeO2 featured equiaxed particles formed agglomerates. Moreover, the size value of the CeO2 particles was demonstrated using a statistical analysis, and the size distribution histogram is shown in Figure 5b. As observed, it was clearly found that most of the CeO2 particles were mainly concentrated at about 42.5 and 87.5 nm. Figure 5c shows the TEM image of a single CeO2 particle, which revealed the porous structure and the many pores around the nanoparticles. Moreover, the high−resolution transmission electron microscope (HR−TEM) image in Figure 5d shows that these nanoparticles had lattice fringes with the same direction (see the yellow arrows in Figure 5d), indicating the single crystal structure of these nanoparticles. In order to further confirm the porous structure of CeO2, a N2 sorption experiment was performed, and the corresponding specific surface area, pore size and pore volume were determined. Figure 6a shows the N2 adsorption-desorption isotherm of the CeO2 hydrothermally synthesized at 200 °C for 24 h with 4 mmol guanidine carbonate and 4 mL 30% H2O2. Figure 6a shows that the N2 adsorption-desorption isotherm was similar to the Langmuir IV(a) type according to the IUPAC classification, and an obvious hysteresis loop was observed in the relative pressure (P/P0) range of 0.4~1.0, belonging to type H3 [55]. This isotherm was consistent with that of porous CeO2 in the reported literature [56][57][58], suggesting that the as−obtained CeO2 was a porous material with disordered mesoporous structures. The corresponding Barrett-Joyner-Halenda pore size distribution curve is shown in Figure 6b. The pore size presented a single distribution centred at about 2.5 nm, and the average pore size and pore volume were 6.2 nm and 0.129 cm 3 /g, respectively, using the Barrett-Joyner-Halenda analysis. Moreover, the specific surface area of mesoporous CeO2 was determined to be 86.8 m 2 /g using the Brunauer-Emmett-Teller method. In order to further confirm the porous structure of CeO 2 , a N 2 sorption experiment was performed, and the corresponding specific surface area, pore size and pore volume were determined. Figure 6a shows the N 2 adsorption-desorption isotherm of the CeO 2 hydrothermally synthesized at 200 • C for 24 h with 4 mmol guanidine carbonate and 4 mL 30% H 2 O 2 . Figure 6a shows that the N 2 adsorption-desorption isotherm was similar to the Langmuir IV(a) type according to the IUPAC classification, and an obvious hysteresis loop was observed in the relative pressure (P/P 0 ) range of 0.4~1.0, belonging to type H3 [55]. This isotherm was consistent with that of porous CeO 2 in the reported literature [56][57][58], suggesting that the as−obtained CeO 2 was a porous material with disordered mesoporous structures. The corresponding Barrett-Joyner-Halenda pore size distribution curve is shown in Figure 6b. The pore size presented a single distribution centred at about 2.5 nm, and the average pore size and pore volume were 6.2 nm and 0.129 cm 3 /g, respectively, using the Barrett-Joyner-Halenda analysis. Moreover, the specific surface area of mesoporous CeO 2 was determined to be 86.8 m 2 /g using the Brunauer-Emmett-Teller method. Figure 7 depicts the effects of the AO7 initial concentration (100~150 mg/L) on the adsorption efficiency of the porous CeO 2 hydrothermally synthesized at 200 • C for 24 h with 4 mmol guanidine carbonate and 4 mL 30% H 2 O 2 . Figure 7 shows that the adsorption of AO7 was rapid for all the initial concentrations of the AO7 aqueous solution at the early stages of adsorption reaction. The adsorption efficiencies within 10 min of contact achieved 97.5, 92.9, 91.2, 90.2, 89.4 and 86.2% at AO7 initial concentrations of 100, 110, 120, 130, 140 and 150 mg/L, respectively. As the adsorption reaction continued, the adsorption process was mostly complete within 30 min. In other words, the adsorption-desorption equilibrium between porous CeO 2 adsorbent and AO7 molecules was basically established within the first 30 min. The rapid and efficient adsorption of AO7 can be ascribed to the abundant porous structure of CeO 2 , which provides numerous adsorption sites for the AO7 molecule by increasing the effective contact area, and is helpful for transporting AO7 molecules to the adsorbent framework.   Figure 7 shows that the adsorption of AO7 was rapid for all the initial concentrations of the AO7 aqueous solution at the early stages of adsorption reaction. The adsorption efficiencies within 10 min of contact achieved 97.5, 92.9, 91.2, 90.2, 89.4 and 86.2% at AO7 initial concentrations of 100, 110, 120, 130, 140 and 150 mg/L, respectively. As the adsorption reaction continued, the adsorption process was mostly complete within 30 min. In other words, the adsorption-desorption equilibrium between porous CeO2 adsorbent and AO7 molecules was basically established within the first 30 min. The rapid and efficient adsorption of AO7 can be ascribed to the abundant porous structure of CeO2, which provides numerous adsorption sites for the AO7 molecule by increasing the effective contact area, and is helpful for transporting AO7 molecules to the adsorbent framework. The experimental data from the adsorption of AO7 dye onto porous CeO2 were fitted according to the Langmuir and Freundlich isotherm models, and the linear fittings results are shown in Figure 8a,b, respectively. The corresponding Langmuir (KL) and Freundlich (KF) parameters calculated are listed in the insets in Figure 8a,b. The Langmuir isotherm model showed higher associated correlation coefficients (R 2 = 0.9505) than that of the    Figure 7 shows that the adsorption of AO7 was rapid for all the initial concentrations of the AO7 aqueous solution at the early stages of adsorption reaction. The adsorption efficiencies within 10 min of contact achieved 97.5, 92.9, 91.2, 90.2, 89.4 and 86.2% at AO7 initial concentrations of 100, 110, 120, 130, 140 and 150 mg/L, respectively. As the adsorption reaction continued, the adsorption process was mostly complete within 30 min. In other words, the adsorption-desorption equilibrium between porous CeO2 adsorbent and AO7 molecules was basically established within the first 30 min. The rapid and efficient adsorption of AO7 can be ascribed to the abundant porous structure of CeO2, which provides numerous adsorption sites for the AO7 molecule by increasing the effective contact area, and is helpful for transporting AO7 molecules to the adsorbent framework. The experimental data from the adsorption of AO7 dye onto porous CeO2 were fitted according to the Langmuir and Freundlich isotherm models, and the linear fittings results are shown in Figure 8a,b, respectively. The corresponding Langmuir (KL) and Freundlich (KF) parameters calculated are listed in the insets in Figure 8a,b. The Langmuir isotherm model showed higher associated correlation coefficients (R 2 = 0.9505) than that of the The experimental data from the adsorption of AO7 dye onto porous CeO 2 were fitted according to the Langmuir and Freundlich isotherm models, and the linear fittings results are shown in Figure 8a,b, respectively. The corresponding Langmuir (K L ) and Freundlich (K F ) parameters calculated are listed in the insets in Figure 8a,b. The Langmuir isotherm model showed higher associated correlation coefficients (R 2 = 0.9505) than that of the Freundlich isotherm model (R 2 = 0.8615), which indicated that the Langmuir isotherm model was a better fit for modelling the AO7 adsorption onto porous CeO 2 . Moreover, the saturated adsorption amount (q m ) of AO7 was 90.3 mg/g according to the Langmuir linear fitting. Furthermore, Table 1 shows the relevant literature on the development of adsorbents for AO7 removal. Among the existing adsorbent materials, activated carbons are the most commonly used and effective adsorbents for the removal of pollutants because of their abundant channels and high specific surface areas [59][60][61]. However, the preparation process of activated carbons has several disadvantages, including high energy consumption, high costs and can easily pollute the environment. For these reasons, endeavours have been made to develop alternatives to activated carbons, such as low−cost fly ash [62][63][64] and agro−residue [65,66]; however, their adsorption capacities are limited except for the brown coal fly ashes [67]. Other materials reported in the literature [16,22,[68][69][70][71][72][73][74][75] exhibit satisfactory adsorption properties, especially 3D MgAl layered double hydroxide [75]. CeO 2 and its complexes were also among the sequences being investigated. Compared to the reported CeO 2 [42,76,77] and the porous CeO 2 in our previous studies [53,54,78], the porous CeO 2 in this work shows better adsorption capacity, but is lower than that of CeO 2 ·xH 2 O [40]. It is worth noting that CeO 2 with a porous structure not only has a potential application in the field of adsorption, but also in the fields of catalyst and catalysis carrier.
fitting. Furthermore, Table 1 shows the relevant literature on the development of adsor-bents for AO7 removal. Among the existing adsorbent materials, activated carbons are the most commonly used and effective adsorbents for the removal of pollutants because of their abundant channels and high specific surface areas [59][60][61]. However, the preparation process of activated carbons has several disadvantages, including high energy consumption, high costs and can easily pollute the environment. For these reasons, endeavours have been made to develop alternatives to activated carbons, such as low−cost fly ash [62][63][64] and agro−residue [65,66]; however, their adsorption capacities are limited except for the brown coal fly ashes [67]. Other materials reported in the literature [16,22,[68][69][70][71][72][73][74][75] exhibit satisfactory adsorption properties, especially 3D MgAl layered double hydroxide [75]. CeO2 and its complexes were also among the sequences being investigated. Compared to the reported CeO2 [42,76,77] and the porous CeO2 in our previous studies [53,54,78], the porous CeO2 in this work shows better adsorption capacity, but is lower than that of CeO2•xH2O [40]. It is worth noting that CeO2 with a porous structure not only has a potential application in the field of adsorption, but also in the fields of catalyst and catalysis carrier.    In order to determine the effect of the solution pH on the removal of AO7 dye onto porous CeO 2 adsorbent, adsorption experiments, with varying pH levels of the AO7 aqueous solution in the range 1~7, were performed. As shown in Figure 9, with an increase in pH, the adsorption efficiency increased and reached its maximum when the pH value was about 3; the adsorption efficiency decreased with a continued increase in pH gradually. Moreover, a lower pH was conducive to the adsorption reaction. A possible reason for this could be that there were more available protons on the CeO 2 surface at a lower pH, thereby increasing the electrostatic attraction between the negatively charged AO7 dye anions and positively charged CeO 2 , and causing an increase in adsorption. In contrast, the number of OH − ions increased at higher pH values, which resulted in ionic repulsion between the negatively charged CeO 2 surface and the anionic AO7 dye molecules. Considering the complexity associated with adjusting the pH of solution, as well as the possible environmental pollution risks, the subsequent adsorption experiments were carried out without pH preadjustment.
The experimental data from adsorption at different temperatures were fitted using the Van't Hoff equation, and the fitted linear curve is shown in Figure 10, while the thermodynamic parameters including K 0 , ∆G 0 , ∆H 0 and ∆S 0 are calculated and summarized in Table 2. Table 2 shows that K 0 values decreased with an increase in temperature, which implies that the adsorption of AO7 molecules on the porous CeO 2 surface was dominated by physical adsorption. The negative ∆G 0 values at specified temperatures indicated that the adsorption reaction was spontaneous and favourable, while the negative ∆H 0 value indicated that the adsorption reaction was exothermic. Furthermore, the negative ∆S 0 value indicated that the three−dimensional motion of the AO7 molecules in solution transformed into two−dimensional motion on the CeO 2 surface. Moreover, a high associated correlation coefficient (R 2 = 0.9973) was obtained, confirming the reliability of the thermodynamic fitting result. The experimental data from adsorption at different temperatures were fitted using the Van't Hoff equation, and the fitted linear curve is shown in Figure 10, while the thermodynamic parameters including K0, ∆G 0 , ∆H 0 and ∆S 0 are calculated and summarized in Table 2. Table 2 shows that K0 values decreased with an increase in temperature, which implies that the adsorption of AO7 molecules on the porous CeO2 surface was dominated by physical adsorption. The negative ∆G 0 values at specified temperatures indicated that the adsorption reaction was spontaneous and favourable, while the negative ∆H 0 value indicated that the adsorption reaction was exothermic. Furthermore, the negative ∆S 0 value indicated that the three−dimensional motion of the AO7 molecules in solution transformed into two−dimensional motion on the CeO2 surface. Moreover, a high associated correlation coefficient (R 2 = 0.9973) was obtained, confirming the reliability of the thermodynamic fitting result.   The experimental data from adsorption at different temperatures were fitted using the Van't Hoff equation, and the fitted linear curve is shown in Figure 10, while the thermodynamic parameters including K0, ∆G 0 , ∆H 0 and ∆S 0 are calculated and summarized in Table 2. Table 2 shows that K0 values decreased with an increase in temperature, which implies that the adsorption of AO7 molecules on the porous CeO2 surface was dominated by physical adsorption. The negative ∆G 0 values at specified temperatures indicated that the adsorption reaction was spontaneous and favourable, while the negative ∆H 0 value indicated that the adsorption reaction was exothermic. Furthermore, the negative ∆S 0 value indicated that the three−dimensional motion of the AO7 molecules in solution transformed into two−dimensional motion on the CeO2 surface. Moreover, a high associated correlation coefficient (R 2 = 0.9973) was obtained, confirming the reliability of the thermodynamic fitting result.   The adsorption kinetics of AO7 molecules onto the porous CeO 2 surface was tested using the pseudo−first−order and pseudo−second−order kinetic models; the linear fitting curves are shown in Figure 11. The kinetic parameters were calculated by plotting log(q e − q t ) vs. t (Figure 11a) and plotting t/q t vs. t (Figure 11b), which are listed in Figure 11a,b as the insets. As observed in Figure 11, the pseudo−second−order model exhibited a better linear relationship than that of the pseudo−first−order, which was also supported by the higher correlation coefficients (R 2 = 0.99997) of the pseudo−second−order model than that of the pseudo−first−order model (R 2 = 0.87878). Combined with thermodynamic analysis, it can be concluded that the AO7 adsorption process involved not only physical adsorption, but also chemical adsorption.
− qt) vs. t (Figure 11a) and plotting t/qt vs. t (Figure 11b), which are listed in Figure 11a,b as the insets. As observed in Figure 11, the pseudo−second−order model exhibited a better linear relationship than that of the pseudo−first−order, which was also supported by the higher correlation coefficients (R 2 = 0.99997) of the pseudo−second−order model than that of the pseudo−first−order model (R 2 = 0.87878). Combined with thermodynamic analysis, it can be concluded that the AO7 adsorption process involved not only physical adsorption, but also chemical adsorption. To examine the reproducibility of the porous CeO2 absorbent in this work, five adsorption-desorption cycles were performed, in which a NaOH aqueous solution (0.6 mol/L, 20 mL) was employed as an eluant to desorpt AO7 molecules from the CeO2 surface. Figure 12 showed the adsorption histogram of five successive adsorption-desorption cycles. It was observed that the adsorption efficiency in the first adsorption-desorption cycle could reach 99.8%. The regenerated porous CeO2 adsorbent still exhibited a satisfactory uptake capacity, and the adsorption efficiency for AO7 remained at more than 92.5% after five cycles. The excellent adsorption properties and reproducibility of the porous CeO2 in this work suggested that they were suitable as a promising absorbent for dye removal in water. To examine the reproducibility of the porous CeO 2 absorbent in this work, five adsorption-desorption cycles were performed, in which a NaOH aqueous solution (0.6 mol/L, 20 mL) was employed as an eluant to desorpt AO7 molecules from the CeO 2 surface. Figure 12 showed the adsorption histogram of five successive adsorption-desorption cycles. It was observed that the adsorption efficiency in the first adsorption-desorption cycle could reach 99.8%. The regenerated porous CeO 2 adsorbent still exhibited a satisfactory uptake capacity, and the adsorption efficiency for AO7 remained at more than 92.5% after five cycles. The excellent adsorption properties and reproducibility of the porous CeO 2 in this work suggested that they were suitable as a promising absorbent for dye removal in water.

Conclusions
A porous CeO2 adsorbent was successfully synthesized through a wet chemical process at room temperature, combined with a hydrothermal process in which Ce(NO3)3•6H2O (cerium source), guanidine carbonate (precipitating agent), H2O2 (oxidizing agent) and H2O (inorganic solvent) were used only as starting reagents without an additional template. The optimal experimental parameters were determined by taking the adsorption efficiency of AO7 dye as the evaluation: 4 mmol of guanidine carbonate, 4 mL of 30% H2O2 and a hydrothermal process at 200 °C for 24 h. The porous CeO2 hydrothermally synthesized at 200 °C for 24 h, with 4 mmol guanidine carbonate and 4 mL 30% H2O2, possessed an excellent adsorption capacity for AO7 dye. The adsorption-desorption equilibrium between CeO2 and AO7 molecules could basically be established within the first 30 min; in particular, the adsorption efficiencies within 10 min of contact could achieve 97.5% at an AO7 initial concentration of 100 mg/L. The saturated adsorption amount of AO7 dye was 90.3 mg/g according to fitting the experimental data with the Langmuir model. Moreover, while the CeO2 adsorbent could be recycled by using a NaOH aqueous solution, the removal percentage still reached 99.8% after the first cycle and remained above 92.5% after five consecutive adsorption-desorption cycles.

Conclusions
A porous CeO 2 adsorbent was successfully synthesized through a wet chemical process at room temperature, combined with a hydrothermal process in which Ce(NO 3 ) 3 ·6H 2 O (cerium source), guanidine carbonate (precipitating agent), H 2 O 2 (oxidizing agent) and H 2 O (inorganic solvent) were used only as starting reagents without an additional template. The optimal experimental parameters were determined by taking the adsorption efficiency of AO7 dye as the evaluation: 4 mmol of guanidine carbonate, 4 mL of 30% H 2 O 2 and a hydrothermal process at 200 • C for 24 h. The porous CeO 2 hydrothermally synthesized at 200 • C for 24 h, with 4 mmol guanidine carbonate and 4 mL 30% H 2 O 2, possessed an excellent adsorption capacity for AO7 dye. The adsorption-desorption equilibrium between CeO 2 and AO7 molecules could basically be established within the first 30 min; in particular, the adsorption efficiencies within 10 min of contact could achieve 97.5%