Ceria Hollow Spheres As an Adsorbent for Efficient Removal of Acid

Research Article pubs.acs.org/journal/ascecg

Ceria Hollow Spheres As an Adsorbent for Efficient Removal of Acid Dye Jing Hu,*,† Weijun Deng,†,‡ and Donghui Chen‡ †

School of Perfume and Aroma Technology, Shanghai Institute of Technology, 100 Haiquan Road, Fengxian District, Shanghai, 201418, People’s Republic of China ‡ College of Environmental Science and Engineering, Donghua University, 2999 North Renmin Road, Songjiang District, Shanghai, 201620, People’s Republic of China S Supporting Information *

ABSTRACT: Efficient removal of dye pollutants from water is of significant importance for environmental protection; thus, the development of advanced sorbent materials with high adsorption capacity is highly desirable. Herein, we report a large-scale synthesis of ceria hollow spheres (CHS) derived from the sol−gel reaction of cerium nitrate on polymeric templates. The obtained CHS have efficient adsorption performance to acid black 210 (AB 210), with a maximum adsorption capacity of 175.75 mg g−1, which is considerably bigger than those of the previously reported ceria-based adsorbents and more than 5 times those of powdered activated carbons (PAC) and ceria nanoparticles (CNP). KEYWORDS: Ceria hollow spheres, Adsorption, Acid dyes, Recyclability

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INTRODUCTION The global occurrence of dye pollutants in water resources has raised concerns about negative effects on aquatic ecosystems and human health. The wastewater discharged from textiles, leather, paper, printing, and plastic contains synthetic dyes in high concentration,1 which is over 7 × 105 tons of synthetic dye being produced annually, and more than 10−15% of them are released into the environment during their synthesis and application.2 Generally, dyes in effluent are both toxic and carcinogenic. For instance, direct discharge of leather dyeing effluent contains nonbonded acid dyestuffs3 and causes serious environmental problems due to high toxicity pollution.4 Particularly, dyes absorb, reflect sunlight entering the water stream, further lead to interference with the growth of bacteria, and finally impede on the ecological balance.5 AB 210 is an azo dye with sulphonyl and amino groups, which is used in dyeing leather, cotton, and woolen fabric with large quantities of consumption.6,7 It is toxic, recalcitrant, and has mutagenic and carcinogenic effects on aquatic biota and humans. Therefore, it is highly desirable to develop an economical solution to efficiently remove AB 210 from wastewater. To date, various approaches to remove dye pollution have been proposed, including flocculation,8 biodegradation, ozonation,9,10 electrochemical treatment,11 adsorption,12 and membranes.13 Specifically, to degrade AB 210, electrochemical oxidation,14 sonochemical decolourization,15 and biodegradation16,17 were usually used. Among these methods, adsorption techniques with inexpensive and efficient materials were © 2017 American Chemical Society

considered a simple and economical method for dye pollution removal from wastewater. Recently, leather tannery waste,18,19 metal-chelated membranes,20 and activated carbon21 were widely used as the absorbents to absorb AB 210. However, there are still some challenges that restrict the adsorption approach, including poor removal to many hydrophilic pollutants, a long time required for physical activation, or thorough washing for chemical activation for the current materials.22−24 Therefore, it is of great importance to explore new adsorbents with easy use. Inorganic hollow spheres have attracted tremendous interest in adsorption applications due to their well-defined shape, low density, large surface area, and functional characteristics.25−29 Lou et al.30 prepared urchin-like α-FeOOH hollow spheres in a glycerol−water system using FeSO4 as the precursor. This α-FeOOH hollow sphere exhibited excellent adsorption capacities for Congo red (275 mg g−1), As(V) (58 mg g−1), and Pb(II) (80 mg g−1). Yu et al.31 reported that Ni/Mg/Al layered double hydroxide hierarchical flower-like hollow microspheres have high adsorption capacities for Congo red and Cr(VI) (1250 and 103.4 mg g−1). In our previous work, we fabricated organic silica hollow spheres with excellent adsorption properties for heavy metal ions and methylene blue.32,33 Ceria, an inexpensive, sustainably produced rare earth, is likewise of Received: February 8, 2017 Revised: March 1, 2017 Published: March 13, 2017 3570

DOI: 10.1021/acssuschemeng.7b00396 ACS Sustainable Chem. Eng. 2017, 5, 3570−3582

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ACS Sustainable Chemistry & Engineering

D/Max-γA rotating anode X-ray diffractometer at a scanning rate of 0.02° s−1 in a 2θ range from 10 to 80°. X-ray photoelectron spectroscopy (XPS, ESCALAB 250) was performed using monochromatized Al Ka X-rays (hv = 1486.6 eV) with a 500 mm spot size. The surface area was determined using the Brunauer−Emmett−Teller (BET) method. The pore size distribution was calculated from nitrogen desorption isotherm curves using the Brunauer−Joyner−Halenda (BJH) method. Adsorption Experiments. All adsorption experiments were carried out in the dark with glass vials covered by aluminum foil to prevent photocatalytic reactions. Typically, CHS (30 mg) were dispersed ultrasonically into 50 mL of AB 210 solution (100 mg L−1) for 10 min, and this vial was placed in a thermostatic shaker and agitated at 298 K at a shaking rate of 140 rpm. At given time intervals, the analytic sample was taken out and centrifuged at 12 000 rpm for 2 min to remove CHS. The concentration of AB 210 in the supernatant was determined at a wavelength of 460 nm by UV−vis spectroscopy (UV-1800, Shimadzu, Japan), from which the amount of AB 210 adsorbed was calculated. Adsorption equilibrium was established for 12 h with constant stirring. For the sake of comparison, the adsorption properties of PAC and CNP were monitored under equal conditions. The amount of AB 210 adsorbed at equilibrium Qe (mg g−1) was calculated from the following equation.

interest for removing dyes from water by means of adsorption.34 Although most ceria nanoparticles are relatively cheap and effective in removal of dyes such as Congo red35 and acid orange 7,36 they rarely show high adsorption capacities and show poor regeneration. We previously fabricated ceria hollow spheres (CHS) as a photocatalyst by adjusting the amount of precursor and ammonia, reaction temperature, and calcination temperature.37,38 Here, we developed a large scale fabrication of CHS with intact morphology. Because of high surface areas of mesoporous shells of hollow inorganic spheres,39 the obtained CHS can sequester AB 210 at a rate of more than 5 times that of PAC and ceria nanoparticles (CNP). Moreover, these CHS can be easily regenerated through a centrifugation−calcination procedure with a limited loss in adsorption performance. These results demonstrate that CHS as a novel absorbent has potential for effective water treatment.

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EXPERIMENTAL SECTION

Materials. Styrene (S, ≥ 99%), acrylic acid (AA, ≥ 98%), ammonium persulfate (APS, ≥ 98%), cerium nitrate hexahydrate (Ce(NO3)3·6H2O, ≥ 99%), absolute ethanol (EtOH, ≥ 99.7%), concentrated ammonia solution (25 wt % NH3 in water), sodium hydroxide (NaOH, 96%), hexamethylenetetramine (HMTA, 99.5%), and PAC (SBET = 936.237 m2 g−1) were purchased from Sinopharm Chemical Reagent Corp. CeO2 nanoparticles (CNP, 30 nm in mean diameter, SEM image and XRD pattern of CNP shown in Figures S1 and S2) were purchased from Jingrui Materials Company. Dye pollutant model compounds were obtained from commercial sources and used as received. All chemicals were used as received without further purification. Millipore water was used in all experiments. Synthesis of Ceria Hollow Spheres. CHS were fabricated using a polymeric template via a sol−gel strategy as follows: Colloidal poly(styrene-co-acrylic acid (PSAA) spheres were first synthesized using soap-free emulsion polymerization.40,41 Then, a PSAA colloidal suspension (2 g), H2O (25 mL), and 20 mL of cerium nitrate solution (0.434 g of Ce(NO3)3·6H2O in 20 mL of H2O) were added and stirred at 298 K for 2 h. Next, 20 mL of sodium hydroxide solution (0.02 g sodium hydroxide in 20 mL of water) was added automatically with a syringe over 1 h and kept at 298 K for 2 h. The sample was washed with absolute ethanol and water twice. Afterward, the suspension was centrifuged, dried, and finally calcinated at 773 K in the air for 3 h. Characterization Methods. The morphology and structure of CHS were observed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM images were obtained using a Philips XL 30 field emission microscope at an accelerating voltage of 10 kV. TEM images were taken on a Hitachi H-800 transmission electron microscope at 75 kV. Fourier transform infrared spectrometer (FTIR) spectra were obtained with a Nicolet Nexus 470 FTIR using powder-pressed KBr pellets. Nitrogen adsorption and desorption experiments were performed at 77 K on a NOVA 4000 gas adsorption analyzer (Quantachrome Corp.). The crystallinity of the sample after calcination was investigated by XRD on a Japan Rigaku

Qe =

(C0 − Ce)V W

(1) −1

where Qe is the adsorption capacity (mg g ) of the adsorbent at equilibrium, C0 and Ce (mg L−1) are the initial and equilibrium concentrations of solute, V is the volume of the aqueous solution (L), and W is the mass of the adsorbent used (g).42 The results shown here were the mean of three determinations. The recyclability was assessed in the following manner: CHS were recycled after adsorption equilibrium by centrifugation and further calcination at 773 K for 2 h. They were weighed for the next cycle of adsorption experiments. Six recycling cycles were conducted.

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RESULTS AND DISCUSSION Synthesis, Morphology, and Structure of CHS. Scheme 1 briefly describes the synthetic procedure of CHS. PSAA colloids were dispersed evenly in water, and Ce3+ ions were adsorbed on the surface of the polymeric colloids with a great amount of −COO− by electrostatic interaction. After the addition of the aqueous solution of NaOH, Ce(OH)3 precipitated on the PSAA templates. At last, CHS were obtained by calcination at 773 K for 2 h. The initiator has the greatest effect on the final morphology of CHS. Generally, NaOH,43−45 HMTA,46,47 and urea48 can be used as the initiators for the sol−gel solution of ceria. During the fabrication process, HMTA was first used to prepare CHS. PSAA/Ce(OH)3 composite spheres with a diameter of 312 nm were obtained (Figure S3a). After the calcination, CHS are 281 nm in diameter with a smooth shell structure. CHS is hollow with dark spots in the SEM image (Figure S3b). Realizing the

Scheme 1. Schematic Process for Fabrication of CHS

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Figure 1. SEM (a), TEM (b), magnified TEM (c), and HRTEM (d) images of as-obtained CHS. Inset of a: magnified SEM image of the CHS surface.

The XRD pattern of CHS as shown in Figure 2a exhibits diffraction peaks at 2θ values of 28.6, 33.1, 47.5, 56.3, 59.1, 69.4, 76.7, and 79.1 ascribed to the (111), (200), (220), (311), (222), (400), (331), and (420) facets of cerianite CeO2 (JCPDS No. 34-0394). No impurities are detected, indicating the high purity of CHS,54 which is further confirmed by the FTIR spectra of PSAA spheres, composite spheres, and CHS in Figure 2b. The peaks at 3062 cm−1, 2961 cm−1, and 1602 cm−1 corresponding to the vibration absorptions of −CH, CH2, and CC groups, respectively, for PSAA spheres and composite spheres disappear in the FTIR curve of CHS. It indicates that all the organic segments have been completely removed by calcination. The O 1s XPS peaks mainly appear at 529.0 and 531.1 eV (Figure 2c), in which the former is CeO2 and the latter is CeO(OH)n.55 The Ce 3d XPS peaks appear at 882.1, 898.5, and 916.8 eV (Figure 2d). The peaks at 898.5 and 916.8 eV correspond to Ce 3d5/2 and Ce 3d3/2, indicating the presence of Ce4+.56 No peak at 885.2 eV, which is the typical peak of Ce3+, indicates an absence of Ce3+.57 Therefore, the main valence of surface Ce in CHS is +4, and the main form is CeO2. Figure 3a demonstrates the typical nitrogen adsorption/ desorption isotherms of CHS. The isotherms show typical typeIV curves according to IUPAC classification,58 revealing the characteristics of porous materials. CHS have a moderate surface area of 36.27 m2 g−1 and pore volume of 0.37 cm3 g−1. The pore size distributions from the desorption branch of the isotherms calculated by the BJH model as shown in Figure 3b displays the average pores size of 5.30 nm. The sharp peak at around 2 nm might correspond to the CHS voids, spaces among ceria nanoparticles of the shell, indicated by the white spots in Figure 1c. On the contrary, the BET surface area of

function of the initiator in the sol−gel system, we further investigated sodium hydroxide during fabrication. With manual addition in heating conditions (333 K), few intact CHS were obtained (Figure S3d). The composite spheres were intact in contrast (Figure S3c). This result was improved simply by automatically feeding sodium hydroxide diluted solution with a syringe under normal temperature conditions (273 K) as shown in Figure 1. The reason could be that the formation of Ce(OH)3 nanoparticles was slow enough to be captured by polymeric templates, and agglomeration was strictly controlled. Furthermore, dispersing agent PVP, surfactants help to stabilize ceria nanoparticles in solution.44,49,50 Citrate as a ligand assists with the formation of fine size ceria nanoparticles.46,51 With the improved method, a lower amount of sodium hydroxide was consumed, accounting for only 1/12 of that of HMTA. Additionally, energy was saved as the experiments were conducted under normal temperatures instead of 333 K in the other sol−gel systems. Therefore, our present work provides a costeffective approach for CHS scale-up production with readily available processing techniques. Figure 1a and b present typical SEM and TEM images of CHS derived from our large-scale approach. Obviously, CHS are homogeneous and have an average diameter of 290 nm, and rough shells are clearly observed in Figure 1a,b and especially in the inset of Figure 1a, which is necessary to form mesopore shells and characteristic for metal oxides.38,52,53 The white spots in Figure 1b distinguished from the surroundings verify that CHS have a hollow structure. The magnified TEM image in Figure 1c further confirms an intact hollow spheres structure with porous shells which are composed of nanoparticles about 6−7 nm in size. The shell thickness of CHS is around 26.5 nm, and the radius is 145 nm (Figure 1d). 3572


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Figure 2. (a) XRD pattern of CHS (the light green bars along 2θ axis are CeO2 standard card, JCPDS No. 34-0394). (b) FTIR curves of PSAA spheres, composite spheres, and CHS. XPS scans of O 1s (c) and Ce 3d (d) for CHS.

Figure 3. Nitrogen adsorption/desorption isotherms (a) and pore size distributions (b) of CHS and CNP.

CNP is as small as 12.95 m2 g−1 with an average pore size of 1.21 nm. This suggests that our CHS has promising potential for dye adsorption. Adsorption Activity. The adsorption capacity of CHS for acid dye removal from aqueous solution was investigated with AB 210, which is most frequently used in leather coloring and has the biggest consumption among leather dyestuffs. Figure 4a presents the AB 210 molecule structure. AB 210 is an amine dissulfonated triazo organic dye, CAS no. 99576-15-5. Dye pollution from the leather industry is normally the result of inefficient dyeing processes, which cause as much as 15% of the consumption and have concentrations up to 50 mg L−1 in

wastewater.59 As the final pH of leather dyeing is between 3.0 and 3.5, the initial pH for AB 210 adsorption is fixed as 3.5, close to the practical scenario. The fabricated CHS was aimed to be suitable to wastewater treatment in the leather industry without the need of preliminary pH adjustment. The initial concentration, volume, and CHS amount were set as 100 mg L−1, 50 mL, and 30 mg, respectively. PAC and CNP were used meanwhile for comparison. Figure 4b shows the adsorption activities of CHS, PAC, and CNP toward AB 210 as a function of time under continuous stirring at 298 K. After only 2 min of contact time, the concentration of AB 210 in aqueous phase with CHS declines to 28.35 mg L−1 (28.35%), and 3573


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Figure 4. (a) Chemical structures of AB 210. (b) Relationship between the removal efficiency and time for the adsorption of AB 210 by CHS, PAC, and CNP at an initial AB 210 concentration of 100 mg L−1. The inset is the digital image of the remaining AB 210 solution under different times. (c) UV−visible spectra of AB 210 solution absorbed by CHS. (d) Relationship between the removal quantity and time for the adsorption of AB 210 by CHS, PAC, and CNP. (e) Effect of time and initial concentration on the removal of AB 210 aqueous solution (30 mg CHS, pH = 3.5, T = 298 K).

further to 11.45 mg L−1 at a time of 60 min. After 240 min, the concentration of AB 210 is kept almost stable at 8.66 mg L−1, with a dye removal efficiency of 91.33%. In contrast, both PAC and CNP show quite a low ability of AB 210 removal with an efficiency of around 20%. The optical images in the inset of Figure 4b visually reveal that the color of AB 210 solution is changing remarkably from dark to light, further providing evidence for the effective adsorption by CHS. For the sake of a pure adsorption process, all experiments were conducted under dark conditions and all glass vials were covered with aluminum foil. Figure 4c exhibits adsorption spectra of AB 210 remaining in the vials during the CHS adsorption process. The curves have the same main absorption peak (λ = 460 nm), and no new

absorption band is found during the adsorption process, indicating that there are no new materials produced and only pure adsorption occurring. To better reveal the removal quantity, a calculation was conducted based on the data of Figure 4b. As shown in Figure 4d, the removal quantity of AB 210 by CHS at 240 min is as much as 152.54 mg g−1, which is almost 5 times those by PAC and CNP. To evaluate the adsorption of AB 210 in a wider range of concentrations, Figure 4e and Figure S4 investigate the adsorption of AB 210 concentration at 10, 50, 100, and 200 mg L−1. At lower concentrations, e.g., 10 and 50 mg L−1, all the removal efficiency reached more than 90% at a time of 60 min. While at concentration of 200 mg L−1, removal efficiency stayed lower at 50.90% at 120 min, and 3574


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Figure 5. (a) Adsorption isotherm of AB 210 on CHS. (b) Linear Langmuir isothermal model. (c) Freundlich isotherm model. (d) D-R isothermal model (30 mg CHS, T = 298 K).

removal quantity kept increasing until a final 173.60 mg g−1. Different amounts of CHS from 10 mg to 60 mg were separately added to 50 mL of 100 mL AB 210 solution at pH 3.5. The dye removal percentage increased with the increase of CHS amount (Figure S5). Even at 10 mg of CHS, the removal efficiency reached 71.98%. The obtained CHS has an efficient adsorption capacity of AB 210. Adsorption Isotherms. Adsorption isotherms were carried out to give general qualitative information on the adsorption capacity of adsorbents and distribution of adsorbates between the liquid phase and solid phase when the adsorption process reaches equilibrium.60 In Figure 5a, the adsorbed amount of AB 210 increased with an increase in the equilibrium concentration of AB 210. The maximum adsorption capacity in the uptake of AB 210 from aqueous solutions achieved 175.00 mg g−1. Langmuir, Freundlich, and Dubinin-Radushkevish (D-R) isotherm models are so far most frequently used for the interpretation of adsorption processes. The Langmuir isotherm model assumes that adsorption occurs at specific homogeneous sites within the adsorbent, which are identically forming a monolayer for adsorption. Once a site adsorbs a dye molecule, no further adsorption takes place at this site.61 The Freundlich adsorption isotherm model hypothesizes that the interaction between the dye molecules is not restricted to the monolayer. The isotherm explains that the sorption sites at heterogeneous

surfaces with greater affinity are occupied first. The DubininRadushkevish (D-R) isotherm model is usually applied to distinguish the physical and chemical adsorption by calculating its mean free energy of adsorption.62,63 Linear Langmuir, Freundlich, and D-R equations are described as eqs 1, 3 and 4, respectively. Ce C 1 = + e Qe Q mKL Qm

log Q e = log K f +

1 log Ce n

ln Q e = ln Q m − βε 2

(2)

(3) (4)

where Ce is the equilibrium concentration of AB 210 in the supernatant (mg L−1), Qe is the amount of AB 210 adsorbed per unit weight of CHS after equilibrium (mg g−1), Qm represents the maximum adsorption capacity of AB 210 per unit weight of CHS (mg g−1), KL is the Langmuir adsorption constant (L mg−1), Kf is a Freundlich constant ((mg g−1)(L mg−1)1/n) corresponding to adsorption capacity, 1/n is the heterogeneity factor, corresponding to a measure of adsorption intensity and surface heterogeneity,45,64 β is the D-R isothermal constant related to the free energy of adsorption (mol2 kJ−2), and ε is the Polanyi potential (ε = RT ln(1/Ce)). 3575


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ACS Sustainable Chemistry & Engineering The feasibility of the adsorption is expressed in terms of a dimensionless factor called separation factor (RL), which is defined by eq 5: 1 RL = 1 + KLC0 (5)

Table 2. Maximum Adsorption Capacities of Ceria Adsorbents with Different Morphologies to AB 210 and Other Acid Dyes

where C0 (mg L−1) is the initial AB 210 concentration. The calculated RL value indicates the shape of the isotherm to be either unfavorable (RL > 1), linear (RL = 1), or irreversible (RL = 0).65 The calculated RL values are between 0 and 1 at different initial concentrations of AB 210, indicating that the adsorption of AB 210 on CHS is favorable.64 In the Freundlich model, 1/n is 0.25, further confirming efficient adsorption of AB 210 on CHS. In the D-R model, β is 0.08825 mol2 kJ−2. Therefore, the mean free energy of adsorption (E = (2β)−0.5) is calculated as 2.38 kJ mol−1, indicating that the adsorption process of AB 210 on CHS was physical adsorption.63 As shown in Figure 5b,c,d, three isotherms fit adsorption isothermal data. Table 1 presents the parameters of each model.

parameter

Langmuir

Qm (mg g−1) KL (L mg−1) RL R2 log Kf (mg g−1) 1/n R2 Qm (mg g−1) β R2

Freundlich

D-R

value 175.75 0.6663 0.0029 0.9999 1.7174 0.2465 0.7697 144.43 0.0883 0.8911

± 0.31 < RL < 0.1305 ± 0.0764 ± 0.0468

± 0.0108

It is observed that the value of the correlation coefficient (R2) for the Langmuir model is highest in three models. The obtained data from experiments were well consistent with the equilibrium adsorption capacity calculated by the Langmuir model. This indicates that CHS have a uniform distribution of active sites for AB 210 molecule adsorption and have a monolayer adsorption behavior with high adsorption affinity and excellent saturation capacities. On the basis of the Langmuir equation, the value of Qm is calculated as 175.75 mg g−1 of AB 210, which is considerably higher than those of CNP (Figure S6, Table S1) and other ceria based adsorbents previously reported (Table 2). In particular, the Qm value of CHS is 5 times that of CNP. Adsorption Kinetics. To evaluate the adsorption rate and mechanism of the process, the kinetics of AB 210 adsorption on CHS were analyzed. Numerous kinetics models can be used to explain the behavior of adsorption. In this study, pseudofirst-order,68 pseudo-second-order,69 and intraparticle diffusion kinetics models70 were investigated. A linear form of three kinetics models was described eqs 6, 7, and 8, respectively. ⎛ k ⎞ log(Q e − Q t ) = log Q e − ⎜ 1 ⎟t ⎝ 2.303 ⎠

(6)

t 1 1 = + t 2 Qt Qe k 2Q e

(7)

Q t = k it

0.5

+C

dyes

Qm (mg g−1)

ref

ceria hollow spheres ceria nanoparticles powdered activated carbons ceria hollow sphere ceria multiple layers ceria ultrafine nanoparticles ceria ultrafine nanoparticles ceria ultrafine nanoparticles ceria nanoparticles ceria nanoparticles ceria nanotubes

acid black 210 acid black 210 acid black 210 acid orange 7 acid orange 7 methyl orange mordant blue 9 reactive orange16 acid orange 7 congo red congo red

175.75 35.29 12.95 22 25 113 101 97 63 18 122

this work this work this work 66 66 34 34 34 36 67 67

where Qt (mg g−1) and Qe (mg g−1) represent the amount of AB 210 adsorbed at time t and at equilibrium time, respectively; k1 (min−1), k2 (g mg−1 min−1), and ki (mg g−1 min−0.5) represent the rate constant of the pseudo-first-order model, the pseudosecond-order model, and the intraparticle diffusion model, respectively; and C is the intercept. A total of 10 mg and 30 mg of CHS with an AB 210 concentration of 100 mg L−1 were selected for studying adsorption kinetics with three modes. Figure 6a shows the removal efficiency at time t for different amounts of CHS. From Figure 6b,c,d and Table S2, it is found that the pseudosecond-order model best described the kinetic data for AB 210 adsorption on CHS because it possessed higher R2 values (>0.9997) than those of the other two models. These results indicated the rate in the experiments was chemical adsorption.71 Effect of pH. The pH of the aqueous solution is a key parameter in regulating the adsorption of charged moieties, by changing the surface charge of the adsorbent and ionization of the dyes. The effect of pH on the adsorption of AB 210 on CHS (100 mg L−1, 50 mL) was investigated in the pH range of 2−8. The pH of solutions was adjusted by using 0.1 M HCl and 0.1 NaOH. The removal efficiency of AB 210 increased with pH decrease, as in Figure 7a. Meanwhile, the zeta potential of CHS also had a similar trend (Figure 7b). When pH ≤ 7, CHS exhibited positive charge. This is helpful for the absorption of AB 210 with the negative charges (Qm = 175.75 mg g−1 at pH 3.5). As in the above discussion, the absorption ability of PAC for AB 210 was much lower than that of CHS. PAC is a well-known adsorbent for the dyes through van der Waals forces due to its large surface area ranging from 500 to 2000 m2 g−1.23,72 This proved that the electrostatic interaction plays an important role for the absorption of AB 210 by CHS. Effect of Temperature. Temperature has a significant influence on the adsorption process. Experiments were carried out at different temperatures of 298, 308, and 318 K (pH = 3.5, 50 mL, 100 mg L−1 of AB 210). An increase in temperature only slightly increased the adsorption (Figure S6). This behavior revealed an endothermic reaction. The higher temperature activates dye molecules, speeding up their dispersing rate. Recycling Ability. Different from nanoparticles, CHS after adsorption is ready to separate from solution by centrifugation and regeneration with calcination. Figure 7c further demonstrate CHS recycling ability via centrifugation−regeneration. After 6 rounds of recycling, the performance of CHS to AB 210

Table 1. Parameters of Three Isotherms Models isothermal model

adsorbents

(8) 3576


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Figure 6. (a) Relationship between the removal efficiency and time and plots of (b) pseudo-first-order kinetics, (c) pseudo-second-order kinetics, and (d) intraparticle diffusion models of AB 210 on CHS.

vibration of −NO2 group. For CHS, the peaks at 3400 and 1621 cm−1 are attributed to the absorbed H2O and hydroxyls in FTIR spectra. After the absorption of AB 210, the pronounced changes of the sulfonate vibration mode are observed. The symmetric and asymmetric peaks at 1136 and 1213 cm−1 are shifted to the higher wavenumber of 1143 and 1217 cm−1. The characteristic peak range of the SO3− group is broadening, which is attributed to the interaction between the sulfonate groups and CeO2. Furthermore, both the new absorption bands at 1173 and 1107 cm−1 of the −SO3H group appear, due to the hydration of AB 210 in aqueous solution. The shift of the association of the hydroxyl groups from 3415 to 3403 cm−1 and the disappearance of the −OH group at 1373 cm−1 show the formation of the hydrogen bond between AB 210 and CHS. The ratio of the intensities νas(SO3−)/νs(SO3−) = 0.63 for CHS-AB 210 is smaller than the one of AB 210 νas(SO3−)/ νs(SO3−) = 0.94. Deacon and Phillips proposed an empirical rule for the carboxylate compounds.73 The difference value between the asymmetric and symmetric vibrations (Δνas‑s) must be considered in helping to determine the type of bonding mechanism between this type of ligand and a central metal atom. This rule can be expressed as follows: Δνas‑s unidentate > Δνas‑s isolated > Δνas‑s bidentate.74,75 On the basis of this theory, Dohčević-Mitrović et al.34 confirmed that

adsorption removal is only less than 5%. CHS is a promising absorbent for recyclability. In addition to AB 210, we also evaluated the ability of CHS to remove different dyes, including acid red 97, acid brown 97, active red X3B, and MO. The values of Qm are acceptable at 98.80 mg g−1, 135.59 mg g−1, 82.16 mg g−1, and 107.90 mg g−1, respectively, as indicated by Figure 7d. This demonstrates that CHS has effective performance as an adsorbent for dye removal. Adsorption Mechanism of AB 210 with CHS. The typical FTIR spectra of the as-obtained CHS, AB 210, and CHS-AB 210 for the sake of comparison are demonstrated in Figure 8a. For AB 210, the strong band at 3415 cm−1 is observed due to the association of the hydroxyl groups. The strong peaks at 1046 and 1136 cm−1 correspond to the symmetric stretching vibration of the SO3− group, while the weak absorption band at 1213 cm−1 is assigned to the asymmetric stretching vibration of the SO3− group. The bands at 1418 and 1373 cm−1 correspond to the stretching vibration of NN and the bending vibration of −OH. The obvious absorption band at 1615 and 1595 cm−1 belongs to the aromatic ring stretching vibration. The band at 1283 cm−1 is attributed to the stretching vibration of the C−N group. The peak at 1330 cm−1 is assigned to the symmetric stretching vibration of the −NO2 group, and that at 1492 cm−1 belongs to the asymmetric stretching 3577


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Figure 7. (a) Effect of different pH’s on equilibrium concentration of adsorption (AB 210 initial concentration = 100 mg L−1, amount of CHS = 30 mg, T = 298 K). (b) Zeta-potential of CHS as a function of pH. (c) Absorption effect on recycling times. (d) Qm of anionic dyes with CHS.

Figure 8. (a) FTIR spectra of AB 210, CHS, and CHS after adsorption of AB 210. (b) XPS scan of Ce 3d for CHS after adsorption and under control conditions. XPS scans of the O 1s (c) and N 1s peak (d) for CHS after adsorption.

than that of CHS-AB 210 (74 cm−1). The characteristic bidentate type bridge is obtained when two oxygen atoms of the SO3− group are linked with one or two Ce4+ cations.

Reactive Orange 16 (RO16), Methyl Orange (MO), and Mordant Blue 9 (MB9) form bidentate type bridges on the ceria surface. Here, the Δνas‑s of AB 210 (77 cm−1) is higher 3578


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Figure 9. (a) SEM image of CHS under control conditions. Inset is the inside view of CHS under control conditions. (b) SEM image of CHS after adsorption of AB 210. Inset is the inside view of CHS after adsorption of AB 210.

Figure 10. Schematic proposed view of AB 210 adsorption on CHS. (a) Before adsorption. (b) After adsorption. (c) Presentation of physical and chemical reactions of AB 210 adsorbed on the CHS surface. The inset of a is a digital picture before adsorption, and the inset of b is a digital picture after adsorption (CHS to AB 210, 50 mg L−1).

with the fitting peak at 399.3 eV of N−H···O groups from the N 1s peak (Figure 8d). The CHS adsorption performance and behavior also strictly correlate to the morphology and surface structure of shells. Large specific surface area contributes to adsorption capacity, which provides abundant functional groups and interacts with dye molecules. From the BET results, specific areas of CNPs and CHS were 12.95 m2 g−1 and 36.27 m2 g−1. Accordingly, the masses of AB 210 adsorbed per unit area by CNP and CHS at 12 h were calculated as 2.32 mg m−2 and 4.20 mg m−2. CHS have an 81.25% higher efficiency of AB 210 adsorption per unit area than CNP. This improvement of adsorption capacity could be contributed to the morphology of CHS due to the voids and pore sizes of CHS. As the XRD patterns of CHS and CNP overlap, they are assumed to possess the same instinct properties of ceria. The adsorption enhancement could be contributed to the morphology of hollow spheres and surface structure of shells.66 The molecule size of most dyes is less than 2 nm.76 The length of AB 210 is 3.29 nm, and the width is around 1.22 nm.18 The average pore size on the hollow sphere shell is 5.3 nm; thus the AB 210 molecules can freely enter inside CHS. Accordingly, each ceria nanoparticle in the shell can absorb AB 210 molecules. Numerous nanoparticles on the external and internal surfaces of CHS form monolayer adsorbing AB 210 molecules. In contrast, the average pore size of CNP with a 30 nm particle size is 1.21 nm. The paths of AB 210 entering inside the nanoparticles were hindered. As a result, the mass of

To further demonstrate the adsorption mechanism of CHS for AB210, the surface elemental compositions of CHS and CHS-AB 210 were investigated using XPS. First, The Ce 3d XPS peaks (Figure 8b) before and after adsorption of AB 210 had no change and, with the absence of a peak at 885.2 eV, indicate that the main valence of surface Ce in CHS remained at +4 after adsorption and there was no catalysis reaction during adsorption. Second, after the adsorption of AB 210, as shown in Table S3, the total amount of O decreased from 61.24 to 35.96%, and the total amount of Ce decreased from 38.76 to 13.45%, while the C, N, and S atomic content dramatically increased, confirming the absorption of AB 210 on the surface of CHS. Furthermore, the decrease in the Ce/O atomic ratio was found to favor the bidentate linkage between the SO3− group and one or two Ce4+ cations. This was also proven by the high resolution of the O 1s peak (Figure 8c). Figure 8c shows that the one main peak with a small shoulder derived from the O 1s peak can be divided into two peaks by XPS differentiation imitation. The fitting peaks centered at a binding energy of 529 and 531.3 eV were attributed to CeO2 and CeO(OH)n, respectively.55 After the adsorption of AB 210, the O atomic content of the CeO2 group decreased from 77.56% to 60.95%, while that of CeO(OH)n increased from 22.44% to 34.05%. A new peak was centered at 532.2 eV, which was attributed to SO3−. This means there is coordination of some Ce4+ ions with the sulfites instead of the O2− ions and enhancement of the hydrogen bonds between CeO(OH)n and active groups of AB 210. Additionally, the hydrogen bonds can be demonstrated 3579


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ACS Sustainable Chemistry & Engineering AB 210 adsorbed per unit area by both types of CNPs is lower. The morphological features of CHS after adsorption were characterized by SEM images. Mechanical stirring may affect the morphological features and surface characteristics of CHS. The control experiment (in the absence of AB 210) was carried out under the same conditions. The surface of CHS in the control experiment was smooth (Figure 9a). After adsorption, it is clearly observed that the surface of the composite became rough with more white spots on the surface (Figure 9b). The inset of Figure 9a is the inside view of CHS in the control experiment, and the inset of Figure 9b is inside the view of CHS after adsorption. The inner surface of the CHS shell of inset of Figure 9a was smooth and that of the inset of Figure 9b rough. These demonstrated the abundant accumulation of AB 210 over the outer and inner surfaces of CHS after adsorption. Therefore, adsorption performance is greatly enhanced, and the behavior fits well to both the pseudo-second-order kinetic model and Langmuir model. An illustration of the adsorption mechanisms between CHS and AB 210 is shown in Figure 10. The large specific area, morphology, and surface structure of CHS shells enhanced the dye adsorption. Figure 10a presents the schematic view before adsorption of AB 210 solution. The blue spots in the beaker stand for AB 210 solution and the yellow hollow spheres for CHS. After adsorption (Figure 10b), AB 210 was adsorbed on the external and internal surfaces of CHS as AB 210 molecules can freely enter inside CHS through the pores and voids of shells. The physical and chemical reactions on the surface of CHS with AB 210 are summarized in Figure 10c. First, AB 210 can be adsorbed on CHS by electrostatic interaction. The interactions between sulfonate groups of AB 210 and CeO2 with bidentate type bridges also facilitate the adsorption. Furthermore, the hydrogen bond between AB 210 and CHS also is beneficial to the improvement of the adsorption. Finally, sulfite groups of AB 210 can directly coordinate some Ce4+ ions instead of the O2− ions.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. ORCID

Jing Hu: 0000-0001-9957-3906 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Key Research and Development Program Nanotechnology Specific Project (Grant No. 2016YFA0200300) and Natural Science Foundation of Shanghai (Grant No. 17ZR1429800) is appreciated.

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REFERENCES

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CONCLUSIONS In summary, we have developed a large scale fabrication of ceria hollow spheres (CHS) through a mild process. Because of the porous structure and the strong electrostatic interaction, these CHS exhibit efficient adsorption performance to AB 210 over the previously reported ceria-based adsorbents and powdered activated carbons and commercial ceria nanoparticles. Moreover, they still present good recyclability. The adsorption behavior matches very well to both pseudo-second-order kinetic and Langmuir models. The obtained CHS have promising potential for dye removal from wastewater.

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pH = 3.5, T = 298 K); effect of time and different amount of CHS on removal of AB 210 aqueous solution (50 mL, 100 mg L−1 AB 210 solution, pH = 3.5, T = 298 K); effect of temperature on AB 210 removal by CHS; parameters of three isotherm models for adsorption of AB 210 on CNP; parameters of three kinetic models for adsorption of AB 210 on CHS; the atom percentages of CHS and CHS-AB210 by XPS (PDF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00396. SEM images of ceria nanoparticles (30 nm in mean diameter); XRD pattern of ceria nanoparticles (30 nm in mean diameter); SEM images of composite and CHS from sol−gel system initiated by HMTA; SEM images of composite and CHS from sol−gel system with manually feeding sodium hydroxide; effect of time and initial concentration on removal of AB 210 aqueous solution (50 mL of AB 210 aqueous solution, 30 mg of CHS, 3580


Research Article

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Ceria Hollow Spheres As an Adsorbent for Efficient Removal of Acid

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