Preparation of CeO2@ SiO2 Microspheres By Non-Sintering Strategy

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Preparation of CeO2@SiO2 Microspheres By NonSintering Strategy For Highly Selective And Continuous Adsorption Of Fluoride Ions From Wastewater Fan Wang, Kaituo Wang, Yaseen Muhammad, Yuezhou Wei, Lin Shao, and Xinpeng Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b02643 • Publication Date (Web): 09 Aug 2019 Downloaded from pubs.acs.org on August 11, 2019

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ACS Sustainable Chemistry & Engineering

Preparation of CeO2@SiO2 Microspheres By Non-Sintering Strategy For Highly Selective And Continuous Adsorption Of Fluoride Ions From Wastewater Fan Wang†, Kaituo Wang*†, Yaseen Muhammad‡, Yuezhou Wei†, Lin Shao†, Xinpeng Wang†

† School of Resources, Environment and Materials, Guangxi Key Laboratory of Processing for Non-ferrous Metals and Featured Materials, Guangxi University, Daxue Road 100, Guangxi Nanning, 530004, P.R.China.

‡ Institute of Chemical Sciences, University of Peshawar, Grand Trunk Road, Peshawar, 25120, KP, Pakistan. *Corresponding author’s E-mail: [email protected]

Abstract High-performance composite materials with environment friendly and cost effective nature are attracting much attention in environmental pollution abatement. In this study, a novel CeO2@SiO2 adsorbent was prepared via a novel approach based on vacuum rotary evaporation and coprecipitation following a non-sintering strategy avoiding the application of high temperature calcination with much simplified mechanization. The fabricated CeO2@SiO2 was applied for the adsorption of F- ions from wastewater achieving much higher adsorption efficiency (257.7-363.9 mg/g (298-338 K) than many reported adsorbents. In addition to this, CeO2@SiO2 realized high selectivity for F- ions (residual concentration of less than 1.5 mg/L) under the presence of NO3-, SO42-, and Cl- as counter anions as simulated system. Concurrently, CeO2@SiO2 adsorbents also realized excellent adsorption performance in real seawater and tap water. The used adsorbent was easily regenerated via desorption in NaOH solutions and could be reused for four consecutive cycles with minimal loss in adsorption efficiency. This study provides an alternative, promising, cost effective, convenient and highly efficient CeO2@SiO2 microspheres adsorbent for the abatement of F- from wastewater, both in the presence as well as absence of counter anions. Keywords: CeO2@SiO2 adsorbent; fluoride ion; continuous adsorption; high selectivity; counter ion

Introduction Fluorine (F) is widely distributed in silicate minerals, from which a major portion is brought into the water during mineral processing, which is increasing with expanding related minerals industries. 1 ACS Paragon Plus Environment

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F in a concentration of 0.5-1.5 mg/L in drinking water is useful to human health[1,2] while beyond this, it is considered as pollutant[3]. With improving living standards, people have been paying more attention to the quality and safety of drinking water. In this regard more focus is dedicated to the depth of filtration of drinking water targeting fluoride ions (F-) via different approaches including ion-exchange[4,5], membrane filtration[6,7], adsorption[8] and chemical precipitation[9]. Among these, adsorption is ranked superior possessing merits of low-cost, ease of availability of adsorbent, simplified operation and high efficiency[10,11]. The main factors affecting the efficiency of an adsorbent and industrial applicability include large surface area, high selectivity, thermal properties, chemical and mechanical stability, and low production cost[12], and hence greater attention has been paid to designing adsorbent which can fulfill these demands. In this regard, graphene oxide[13] and carbon nanotubes[14] based composites have emerged as attractive carbon family attributed to surface chemical composition and high specific surface area (SSA). Metal oxide nanoparticles based adsorbents such as aluminium[15,16], iron[4,17], magnesium[18,19], zirconium[20], rare earth metal [21,22] and their composites are well known for F- removal from wastewater ascribed to their excellent adsorption properties compared to conventional adsorbents. In addition, non-metal oxide based adsorbents such as zeolite[23], biochar[24,25] and modified attapulgite[26] are equipped with intriguing properties like high porosity, large SSA and excellent ion exchange properties. However, these materials suffer from issues of low adsorption efficiency, post-adsorption separation problems, poor mechanical performance and crammed application conditions. Similarly, rare earth metal induced adsorbents though possess high efficiency in terms of F- removal[21,27], however their powdery or block bodied morphology and costly nature limit their practical applications. To handle these problems, the development of low-cost microspheres based adsorbent with easy recyclability is of crucial important for widened industrial applications. Among the rare earth elements, Cerium is the most abundant and cheapest one, while cerium oxide has been known for its outstanding adsorption[28,29], polishing[30] and catalytic[31] properties. Cerium oxide is almost insoluble in water and acids, which widens its utilization under a wide pH range and hence has been extensively applied for waste-water treatment. Kang et al. a examined the removal of F- over a series of cerium oxide with different morphologies[32]. Ankita et al. used cerium oxide and zinc oxide to prepare binary metal oxide which realized enhanced 2 ACS Paragon Plus Environment

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removal for F-[8]. These reports suggest that cerium oxide possesses superior efficiency than many reported adsorbents[33-38] for F- removal from wastewater, while further tuning of its morphological structure and properties can reveal great promise for industrial applications. Thus, herein we designed a new adsorbent by loading cerium oxide onto porous silica microspheres (75-150 μm) substrate via a novel vacuum rotary evaporation and co-precipitation strategy, and was subsequently tested in the adsorptive removal of F- from wastewater. The proposed strategy is novel and industrially more feasible attributed to non-sintering technique avoiding the application of high temperature calcination with much simplified mechanization. Attributed to good sphericity, uniform particle size distribution and mechanical, the prepared CeO2@SiO2 microsphere adsorbent exhibited large SSA and can be conveniently applied in packed beds to achieve unremitting F- removal from waste-water with easy and simplified recycling. Textural characterizations of the microspheres were achieved via advanced analytical techniques which helped in the elucidation of adsorption performance and experimental results of CeO2@SiO2 microspheres.

Experimental Materials Porous silica microspheres (75-150 μm) were self-made, while HCl and NaOH were obtained from Chengdu Kelong chemical reagent factory, China. NaF, CeCl3.7H2O, NH3.H2O and H2O2 were provided by Guangdong Guanghua Sci-Tech Co., Ltd. China. Tap water (Nanning, Guangxi) and real seawater (Beihai, Guangxi) were used as the real water sample for analyzing the adsorption efficiency of the prepared microsphere for F- removal. Distilled water (DW) was used throughout the experiments where required.

Synthesis of CeO2@SiO2 microspheres The synthesis of CeO2@SiO2 adsorbent by vacuum rotary evaporation and co-precipitation strategy is show in Fig. 1. 7.45 g CeCl3.7H2O was thawed in 111.75 ml DW, to which 2.27 g H2O2 was added so as to oxidize Ce3+ to Ce4+. Subsequently, 4 g of porous silica microspheres (D50=106.01 μm) were added to the Ce4+ solution and mixed at 25 oC. The mixture was placed in a rotary evaporator at 60 oC, which facilitated the impregnation and immobilization of Ce4+ solution into the pores of porous silica microspheres, which was followed by drying at reduced pressure. These porous silica microspheres containing Ce4+ was then added to a 100 ml solution of pH 9-10 3 ACS Paragon Plus Environment


(adjusted by NH3.H2O) and heated at 75 oC for 30 min followed by aging the mixture for 15 h at 25 oC.

The microspheres were washed with DW until no Cl- was detected in the washing fluid. Finally,

CeO2@SiO2 adsorbent (D50=116.63 μm) was collected by vacuum over drying the sample at 85 oC for 12 h.

Figure 1. Flow chart of CeO2@SiO2 microspheres adsorbent preparation.

Characterization of adsorbent The particle size distribution of SiO2 and CeO2@SiO2 microspheres was characterized via a laser diffraction particle size analyzer (HORIBA LA-960), while crystal structures were analyzed via Xray diffraction (XRD) (Rigaku SmartLab3kW diffractometer with Ni-filtered Cu (Kα) radiation) operated at 30 kV and 30 mA at a scanning rate 5°/min in a 2θ angular range of 0.5-80°. Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy were applied for surface morphological and chemical composition analysis of microspheres after gold coating the samples (Phenom ProX, Holland). Transmission electron microscopy (TEM) was performed on a Tian ETEM G2 80-300 (FEI Company). Surface area and pore size distributions of the microspheres were studied using Brunauer-Emmett-Teller (BET) approach (TriStar II, Micromertics Instrument Corp. USA). The zeta potential of CeO2@SiO2 microspheres under pH 2-8 was measured by a Zetasizer (NanoBrook Omni, brookhaven, USA). The solid content in the solution was 0.1 wt.%, while solution pH was maintained by NaOH or HCl (each 0.1 M). Functional group characterization were performed on fourier transform infrared (FT-IR) spectrometer (Shimadzu IRTracer-100) using KBr (1 %) disk method. The chemical composition and states of CeO2@SiO2 microspheres surface were studied by X-ray photoelectron spectroscopy (XPS) (ESCALAB 250XI, Thermo Fisher Scientific Inc., MA, USA). The concentrations of F- was tested by F- selective electrode (Shanghai

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San-Xin Instrumentation, China).

Batch experiments Adsorption performance of CeO2@SiO2 for F- was evaluated as removal efficiency (R), adsorption capacity (Qe) and desorption efficiency using Eq. (1-3) respectively.

R%  

Qe 

Ci  C e 100% Ci

V Ci  Ce  m

Desorption efficiency 

(1)

(2)

Ce  V 100% 1000  Qe  m

(3)

Where: Ci and Ce represent the initial concentration and equilibrium concentration of F- in solution (mg/L), respectively, V is volume of solution (L), M is the mass of adsorbent (g). Details about the adsorption experiments for F- removal by CeO2@SiO2 are shown in Table 1. Table 1 Various experimental conditions for the adsorption of F- over CeO2@SiO2.

Experimental conditions Adsorbent dosage pH Adsorption time

Initial concentration and temperature

Bed column experiment Competitive adsorption Desorption experiments

Operation Process 0.05-0.30 g CeO2@SiO2 adsorbent (at an increment of 0.05 g) were mixed with 50 mg/L (100 mL) NaF solutions (pH=3) at 1 h and 25ºC. pH of the solution was adjusted by 0.1 M HCl or NaOH in a range of 2-8, 0.15 g CeO2@SiO2 adsorbent were mixed with 100 mL of 50 mg/L NaF solution at 1 h and 25ºC. 0.15 g CeO2@SiO2 adsorbent were mixed to 100 mL of approximately 50 mg/L NaF solutions at 3 pH and 25ºC. At pH of 3, 0.15 g CeO2@SiO2 adsorbent were mixed with 100 mL of NaF solutions having concentration of 20 mg/L, 100 mg/L, 200 mg/L, 400 mg/L, 800 mg/L, 1000 mg/L, 2000 mg/L and 3000 mg/L and stirred for 1 h at 25 º C. For temperature optimization, other conditions were kept constant as mentioned above while temperature was varied from 45ºC to 65ºC. The glass column used possessed a diameter and length of 1 cm and 5 cm respectively, using 50 mg/L NaF solution, pH of 3 and 25ºC. The bed height was 1.0 cm in relation to the adsorbent mass (0.55 g) using a flow rate (0.1 mL/min) while the effluents were collected by a fractional collector. NaF solution of 100 mL (50 mg/L) and pH=3 was applied in combination with various concentration (100-500 mg/L) of Cl-, NO3-, SO42- and PO43- at an interval using an adsorbent dose of 100 mg/L. 0.15 g of used adsorbent were added to 100 mL of NaOH solutions (0, 0.01, 0.05, 0.1, 0.5 M) and was shaken at 160 shakes/min at 25ºC for 1 h and was analyzed for F- concentration. The well washed and dried desorbed adsorbents were mixed with F- solutions (50 mg/L) at pH=3, 160 shakes/min at 25ºC for 0.5 h, and was finally analyzed for F- concentration.

Results and discussions Characterizations of SiO2 and CeO2@SiO2 5 ACS Paragon Plus Environment


Figure 2. XRD (A), N2 adsorption-desorption isotherms (B), BJH pore size distribution (C) and SEM-EDS of: SiO2 (D) and CeO2@SiO2 microspheres (E).

XRD, BET and chemical composition analyses results of SiO2 and CeO2@SiO2 microspheres are provided in Fig. 2. The XRD patterns of SiO2 and CeO2@SiO2 microspheres shown in Fig. 2(A) suggest the former exhibited a broad mount near 22°, indicating the low crystallinity of SiO2 microspheres. After loading CeO2, the hump of SiO2 decreased and six diffraction peaks of CeO2 at planes (111), (200), (220), (311), (222) and (400) appeared which are ascribed to the face-centered cubic (fcc) crystal structure of ceria with the Fm-3m space group (JCPDS: No. 34-0394)[39,40]. Both SiO2 and CeO2@SiO2 have two obvious diffraction peaks at 0.5o and 0.7o, indicating that the two substances have ordered mesopore. Fig. 2(B) shows that the SSA of CeO2@SiO2 was much higher (86.13 m2/g) as compared to that of original SiO2 (68.34 m2/g), which could be attributed to the loading of CeO2 particles into the pores of SiO2 microspheres[41]. The micropore and mesopore volumes (N2-BJH) of SiO2 microspheres were 0.030 and 0.065 cm3/g while those of CeO2@SiO2 microspheres were 0.037 and 0.225 cm3/g respectively, which was coherent with the trend exhibited in Fig. 2(C), where the pore size of SiO2 was mainly concentrated at about 146 nm and 190 nm while that of CeO2@SiO2 microspheres converged at about 25 and 97 nm. SEM images in Fig. 2(D) revealed SiO2 and CeO2@SiO2 microspheres with proper and good sphericity, while EDS results concluded the presence of only oxygen and silicon in these samples. On the contrary, the EDS results 6 ACS Paragon Plus Environment

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of CeO2@SiO2 microspheres (Fig. 2(E)) showed high contents of cerium (54.09 wt.%) which confirmed the successful loading of CeO2 onto SiO2 microspheres. This result was consistent with the fact that particle size of SiO2 increased from D50=106.01 μm to D50=116.63 μm after loading CeO2 onto SiO2 microspheres (CeO2@SiO2 in Fig. 1). Elemental mapping of SiO2 and CeO2@SiO2, TEM, high resolution TEM (HRTEM) and EDS analysis results are shown in Fig. 3. Fig. 3(A) shows that SiO2 is amorphous and only contains silicon (63.00 wt.%) and oxygen (37.00 wt.%) elements, which is in good agreement with XRD results of SiO2 (Fig. 2A). Fig. 3(B) shows that inner surface of SiO2 microsphere contains cerium (57.11 wt.%) which further confirmed the successful loading of CeO2 onto SiO2 microspheres. From HRTEM image in Fig. 3B1, the 0.301 nm lattice spacings was indexed to peak (111) crystal facet of CeO2 (JCPDS: No. 34-0394)[39,40].

Figure 3. TEM pictures/EDS mappings of (A) SiO2 and (B) CeO2@SiO2, HRTEM image (B1) of CeO2@SiO2.

Adsorption experiments for F- removal Influence of adsorbent dose and pH The results for the influences of adsorbent dosage on the adsorption efficiency of CeO2@SiO2 microspheres for F- are presented in Fig. 4(A), which suggested increase in removal efficiency of Fwith increasing adsorbent dosage. At 0.15 g absorbent dosage, the removal of F- reached 98.9 %. However, this trend of adsorption amount was opposite to the removal efficiency, which gradually decreased with increasing adsorbent dosage. This is because at a constant F- concentration, active sites relative to F- concentration in the solution were excessive at higher adsorbent dosage, which led to decrease in the adsorption amount[42,43]. Therefore, 0.15 g/100 mL adsorbent dosage was 7 ACS Paragon Plus Environment


selected for onward experiments.

Figure 4. (A) Effect of CeO2@SiO2 microspheres dosage on F- removal; (B) Effect of pH on zeta potential of CeO2@SiO2 microspheres surface and F- removal efficiency

Fig. 4(B) shows the zeta potential and F- removal efficiency of CeO2@SiO2 microspheres in a pH range of 2-8. The surface of CeO2@SiO2 microspheres was positively and negatively charged under low pH and high pH respectively while pHPZC (chargeless) at pH=3.9[44]. Changes in surface charge of CeO2@SiO2 microspheres under different pH is shown via Eq. (4), where protonation and deprotonation on the surface of CeO2@SiO2 microspheres occur at low and high pH, respectively. The removal efficiency of CeO2@SiO2 microspheres for F- under pH 2-8 was consistent with their trend of zeta potential, which suggested that F- adsorption over CeO2@SiO2 microspheres occurred via electrostatic interaction[8]. The removal of F- by CeO2@SiO2 microspheres at pH=3 was the maximum, and hence was applied in onward experiments.

M  OH 2  M  OH  M  O 

Low pH

(4)

High pH

Influence of absorption time Fig. 5 showing the influence of time on the adsorption capacity of CeO2@SiO2 microspheres for F- under the conditions mentioned in Table 1 suggests that highest adsorption capacity of CeO2@SiO2 microspheres for F- (32.24 mg/g) was saturated at about 45 min. As adsorption capacity revealed no marked variation after 1 h of adsorption, thus 1 h was opted in onward experiments. The removal phenomenon of F- by CeO2@SiO2 microspheres was further elaborated by pseudofirst (Eq. 5)[45], pseudo-second order (Eq. 6)[46] kinetic models and intra-particle diffusion model (Eq. 7)[47] to the experimental data.

Qt  Qe 1  e  k1t 

(5)


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t 1 t   2 Qt k 2Qe Qe 1

Qt  kid t 2  C

(6)

(7)

Where: Qt (mg/g) is the amount of F- adsorbed at time t. Qe (mg/g) is the amounts of F- absorbed at equilibrium time. k1 (min-1) and k2 (g.mg-1.min-1) show the pseudo-first and pseudo-second order rate constants respectively. kid (mg/(g.min1/2)) is the diffusion constant. Ci elaborates the thickness of liquid film. Comparison of the calculated results in Fig. 5 and fitting data in Table 2 suggests pseudo-secondorder kinetics for the adsorption of F- over CeO2@SiO2 microspheres. The calculated adsorption capacity by the pseudo-second-order kinetic model (32.68 mg/g) was in good agreement with the experimental one (Qe=32.24 mg/g). These results indicated chemical adsorption route for the adsorption of F- ion[42,43]. The intra-particle diffusion model fitting results in Fig. 5 for the adsorption of F- suggested a three staged adsorption process: (I) external adsorption, (II) intraparticle diffusion and (III) adsorption equilibrium. Furthermore, the fitting curves of the three stages were not passing through the origin, suggesting both, intra-particle diffusion and external adsorption were affecting the rate-controlling step of the adsorption process[48,49].

Figure 5. Effect of adsorption time on F- removal, fitting by various kinetic and diffusion models Table 2 Parameters of various models for the adsorption of F- on CeO2@SiO2 adsorbent.



Experimental Qe,exp

Pseudo-first order kinetic model

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Pseudo-second order kinetic model

Intra-particle diffusion model

mg/g

Qe.cal

k1(×10-4)

R2

Qe.cal

k2(×10-4)

R2

Kid

C

R2

32.24

12.18

538.0

0.989

32.68

162.6

0.999

1.99

18.53

0.987

Isotherms studies of F- adsorption The highest adsorption capacity of CeO2@SiO2 microspheres for F- in solution was calculated using temperature and initial concentration data mentioned in Table 1. The change in equilibrium adsorption amount with equilibrium concentration at variable temperatures is provided in Fig. 6, while the Langmuir (Eq. 8)[50], Freundlich (Eq. 9)[51] and Dubinin-Radushkevich (D-R) (Eq. 10)[52] isotherm models were applied to experimental results.

1 1 1   Qe Qmax k L Ce Qmax

(8)

1 log Qe  log k F  log Ce n lnQ e  ln Qmax   2

(9)

(10)

Where Qmax (mg/g) represents the highest adsorption capacity. kF ((mg/g)/(mg/L)1/n) represents adsorption equilibrium constants of Freundlich model. kL (L/mg) and n are constants of Langmuir model. β (mol2/J2) is linked to adsorption energy. ε (kJ/mol) represents potential energy of Polanyi and is determined by Eq. 11. R (J/(mol·K)) and T (K) represent molar gas constant and absolute temperature respectively. E (kJ/mol) is mean adsorption energy and is determined using Eq. 12.



  RT ln1  

E

1 2

1   Ce 

(11)

(12)


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Figure 6. Various isotherms models for the adsorption of F- on CeO2@SiO2 microspheres.

The solid dots in Fig. 6(A) indicate the experimental data while the fitting curves of Langmuir and Freundlich isotherm models are indicated by dashed lines. Fig. 6(A) suggests that with increasing equilibrium concentration correspondingly leads to increase in equilibrium adsorption amount at variable temperatures, and became linear once the adsorption sites reached saturation. The graph between lnQe vs. ε2 is linear in Fig. 6(B) with correlation coefficients of 0.911, 0.967 and 0.966 for 298 K, 318K and 338K, respectively, as shown in Table 3. This suggests that adsorption predominantly occurs via physiosorption when mean free energy is less than 8 kJ/mol, due to hydrogen bonding and electrostatic attraction[38]. This is in good agreement with data of zeta potential (Fig. 4(B)), which concludes that the mechanism for removal of F- is mostly governed by electrostatic attraction and chemical adsorption over CeO2@SiO2 microspheres. Table 3 Adsorption isotherm parameters of F- on CeO2@SiO2 microspheres. Temperature (K)

Isotherm model

Parameter

Langmuir

Qmax KL R2 n KF R2

298 285.7 0.0051 0.980 2.91 19.62 0.951

318 312.5 0.0119 0.995 2.81 22.52 0.945

338 370.4 0.0092 0.983 2.77 26.44 0.958

Qmax

197.8

245.2

288.2

β

2.2×10-7

1.8×10-7

1.2×10-7

E

1.50

1.65

2.08

R2

0.911

0.967

0.966

Freundlich

Dubinin-Radushkevich

Table 3 compiles the adsorption isotherm parameters which suggest that the Langmuir isotherm model better describes the adsorption of F- onto CeO2@SiO2 microspheres following a monolayer adsorption mechanism. In addition, the maximum experimental adsorption amounts (257.7 mg/g, 301.1 mg/g and 363.9 mg/g) are in good agreement with the theoretical values (285.7 mg/g, 312.5 11 ACS Paragon Plus Environment


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mg/g and 370.4 mg/g) at 298 K, 318 K and 338 K, respectively. The maximum adsorption amounts increased significantly with increasing temperature, which suggested endothermic nature of adsorption of F- onto CeO2@SiO2 microspheres[10]. The thermodynamic parameters for the adsorption of F- onto CeO2@SiO2 microspheres are compiled in Table 4. The Gibb’s free energy change (-20 kJ·mol-1＜∆G0＜0 kJ·mol-1) was negative which confirmed spontaneity and physical adsorption nature of the process. In addition, ∆G0 decreased with increase in temperature indicating positive effect of temperature on the adsorption process. The spontaneity of process was further cemented by the positive value of enthalpy change (∆H0=8.26 kJ·mol-1) before and after the adsorption of F- over CeO2@SiO2, which was consistent with the trend of effect of temperature with adsorption. Similarly, the negative value of entropy change in Table 4 suggested that CeO2@SiO2 microspheres changed structurally during the adsorption of F-, which increased the degree of disorderness at the solid-liquid interface[10]. From Table 5, the adsorption capacity of CeO2@SiO2 microspheres was about 5-8 times higher than similar CeO2/SiO2 adsorbent, and compared with the highly expensive dendrimer like chitosan beads and many other reported adsorbents shows advantageous adsorption capacity. This suggested the great promise for the industrial applications of the proposed CeO2@SiO2 microspheres for the treatment of F- from waste-water via direct packing in column beds.

Table 4 Thermodynamic parameters of F- adsorption on the CeO2@SiO2 microspheres. Temperature (K) 298

∆G0 (kJ·mol-1) -11.38

318 338

-12.57 -14.01

Thermodynamic parameters ∆H0 (kJ·mol-1)

∆S0 (J·mol-1·K-1)

8.26

65.80

a. ∆G0=-RTlnkd, kd=Qe/Ce b. ∆G0=∆H0-T∆S0[53] Table 5 Adsorption capacity of CeO2@SiO2 microspheres as compared with literature reports. Adsorbent type

Adsorption capacity(mg/g)

Reference

CeO2-ZrO2 nanocages

175

[2]

Ce-Zn adsorbent

194

[8]

CeO2/SiO2

46.36

[21]

Dendrimer like chitosan beads

17.47

[33]

DTAB/H2O2 solution-treated clay

53.66

[34]


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Nepheline prepared from kaolinite

125-183

[35]

La-modified pomelo peel biochar

19.86

[36]

153

[37]

Fe-La composite

27.42

[38]

Activated Cerium(IV) Oxide/SiMCM-41 Adsorbent

107.16

[54]

13.6

[55]

Cerium impregnated chitosan composite

Cerium-doped bone char This work

257.7-363.9

Adsorption mechanism By comparing Fig. 2(E) with Fig. 7(A and B), it could be found that F- with 6.13 wt.% was evenly distributed on the surface of CeO2@SiO2 microspheres. The XPS spectra of CeO2@SiO2 microspheres (pre and post adsorption of F-) in Fig. 7(C) suggested a new peak at 684.8 eV (F1s)[29] for post F- adsorption CeO2@SiO2 microspheres, confirming the adsorption of F-. The FTIR patterns in Fig. 7(D) suggested bands at 464, 800 and 1095 cm-1 ascribed to Si-O bending vibration of SiO2[56,57], while that at 1629, 1638 and 3448 cm-1 corresponded to O-H bending vibrations of surface-adsorbed water[57,58]. The change in the position of O-H bending peak before and after adsorption was attribute to the substitution of OH- by F-. This was consistent with the highresolution XPS spectra of O1s in Fig. 7(E and F) where the O1s signal before and after adsorption can be deconvoluted into two components. The proportion of M-OH (532.6 eV) was 88.18 % and 86.21 % before and after adsorption of F-, respectively. Table 6 suggest an increase in the pH of the solution from 3.02 to 3.19 after adsorption of F-, which was due to the of substitution OH- group by F- over CeO2@SiO2 microspheres, and hence further strengthened the proposed mechanism based on chemical adsorption and ion exchange for F- adsorption over CeO2@SiO2 microspheres[59].



Figure 7. SEM scans/EDS maps of CeO2@SiO2 microspheres (A and B) after F- adsorption (C) XPS and (D) FTIR spectra of CeO2@SiO2 microspheres before and after adsorption of F-; (E), (F) High-resolution O1s XPS spectra of pre and post F- adsorption CeO2@SiO2 microspheres. Table 6 Variation in solution pH in pre and post F- adsorption. Solution pH

Before adsorption After adsorption 3.02

3.19

Dynamic adsorption and desorption experiment The bed column test was performed to evaluate the dynamic adsorption influence of F- on CeO2@SiO2 microspheres under the experimental conditions mentioned in in Table 1 (Bed column experiment). The breakthrough curve and dynamic desorption curve are plotted in Figure 8(A) and 14 ACS Paragon Plus Environment

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(B) respectively, while the F- breakthrough curve was fitted using Thomas model (Eq. 13)[60].

Ct  C0

1 1 e

 q m   KT  0 C0t    Q   

(13)

Where Q (mL/min) shows the flow rate of F- solution. q0 (mg/g) indicates F- amount absorbed at equilibrium time t (min). m (g) is the mass of the adsorbent. KT (mL.mg-1.min-1) represents the adsorption rate constant. Fig. 8(A) suggests a classical S shaped curve. The concentration of F- in the effluent began to exceed 1.5 mg/L at 3800 min while the breakthrough time was 8000 min. The maximum dynamic adsorption capacity determine by Thomas model fitting was 49.7 mg/g (R2=0.967) (0.1 mL/min flow rate). In addition, according to the mass balance, the actual dynamic adsorption amount at a flow rate of 0.1 mL/min was 45.3 mg/g. Then, 0.5 M NaOH was used to explore the dynamic desorption experiment under constant set of other conditions constant. The results from this experiment shown in Fig. 8(B) suggested that the concentration of F- in the effluent was 49.98 mg/L in the first 400 min which decreased gradually with the passage of time and reached to less than 1 mg/L after 2000 min.

Figure 8. (A) Breakthrough curve fitted by Thomas model; (B) Dynamic desorption curve of CeO2@SiO2 adsorbent.

Competitive adsorption Waste-water contains a variety of anions alongside F- including PO43-, SO42-, NO3- and Cl-[61], which could affect F- adsorption. Thus, the influence of coexisting anions on CeO2@SiO2 microspheres adsorption efficiency for F- was tested under the conditions mentioned in Table 1 (Competitive adsorption). The experimental results in Fig. 9(A) reveal that F- removal over 15 ACS Paragon Plus Environment


CeO2@SiO2 microspheres first decreased and then stabilized with increasing the concentration of various anions. At 200 mg/L concentration of PO43-, SO42-, Cl- and NO3-, F- removal decreased by 42.5 %, 17.0 %, 9.1 % and 13.3 % respectively. This could be due to the competition of counter anions for active sites on CeO2@SiO2 microspheres with increasing their concentrations, thus hindering the effective communication between F- and adsorbent, thus decreasing adsorption efficiency. From Fig. 9(A), the competitive order of the counter anions was: NO3- ＜Cl- ＜SO42- ＜ PO43-. In addition, the adsorption of F- in real seawater and tap water was investigated and the results provided in Fig. 9(B) suggested the removal percentages of F- as 98.6 %, 84.5 % and 84.2 % in pure water, tap water and real seawater, respectively. These results conclusively suggested that the newly designed CeO2@SiO2 microspheres realized outstanding adsorption efficiency for the removal of F- from both, tap as well real sea water, in the presence as well as absence of counter anions, which poses great promise for its industrial application in waste-water treatment plants.

Figure 9. (A) Effect of nature and concentration of counter anions on the removal of F- by CeO2@SiO2; (B) removal efficiency of CeO2@SiO2 microspheres for F- from different real wastewater.

In order to further expand the range of CeO2@SiO2 microspheres in practical applications, the adsorption of F- under different simulated working conditions was studied as summarized in Fig. 10 using F-, PO43-, SO42-, NO3- and Cl- solutions of 20, 180, 700, 100 and 20000 mg/L concentration, respectively. Fig. 10 suggests that the concentration of F- decreased gradually with the passage of time. At 180 mg/L PO43- concentration, F- concentration was balanced after 45 min with an equilibrium concentration of about 5 mg/L. The concentration of F- in NO3-, SO42-, and Cl- solution was < 1.5 mg/L after 15, 30 and 45 min, which are much lower than the standards set by environmental protection agency and world health organization for drinking water[3]. This further implied that the newly designed CeO2@SiO2 microspheres can be used as an efficient alternative for treating wastewater for F- ions at a much lower cost, easy operational procedure, and meeting 16 ACS Paragon Plus Environment

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up well with the health standards on industrial level.

Figure 10. Change in F- concentration with time under different simulated working conditions.

Desorption and reusability performance

Figure 11. (A) Desorption performance of F- at different NaOH concentrations; (B) Influence of multiple cycle on the adsorption amount of CeO2@SiO2 adsorbent.

Desorption and recyclability properties are important indexes for evaluating the industrial applicability of an adsorbent, which affect both, process efficiency as well cost. The desorption process of F- on CeO@SiO2 microspheres using water and NaOH was evaluated. The data in Fig. 11(A) suggested that desorption efficiency in water (4.05 %) gradually increased and reached maximum (97.7 %) at NaOH concentration of 0.5 mol/L. Furthermore, Fig. 11(B) indicated that CeO2@SiO2 microspheres exhibited good regeneration performance, and the adsorption capacity still remained fairly high (19 mg/g) after the fourth consecutive reuse, which can further extend its feasibility for industrial treatment of F- containing wastewater.

Conclusions In this work, a new type of CeO2@SiO2 microsphere adsorbent was prepared using porous SiO2 17 ACS Paragon Plus Environment


microspheres as support via a simplified, cost effective and environment friendly novel nonsintering method. The newly designed CeO2@SiO2 adsorbent was tested in the adsorptive removal of F- from simulated as well real waste-water via both, static and dynamic experiments. Adsorption kinetics well fitted with pseudo-second-order kinetic model indicating the chemical nature of the process, involving the replacement of OH- groups by F- as confirmed by XPS analysis. The single layer adsorption was confirmed by the best fitting of data with Langmuir isotherm model. Thermodynamic parameters confirmed that the adsorption of F- over CeO2@SiO2 microspheres was endothermic and spontaneous. Competitive adsorption tests revealed that the competitive order of the counter anions was: NO3- ＜Cl- ＜SO42- ＜PO43-. CeO2@SiO2 microspheres can be directly loaded with excellent adsorption and desorption effects in column experiments. The newly designed CeO2@SiO2 microspheres can be envisaged of potential applications in the industrial treatment of F- containing wastewater.

Author Contributions The experiments were designed by F. W. and K.T. W.; Most of the experiments were performed by F. W. and only a small part of the experimental test were done by K.T. W.; F. W., K.T. W., Y.Z. W., L. S. and X.P. W explored and analyzed the experimental data; K.T. W. wrote the paper; Y. M. helped revise the paper. Corresponding Author *E-mail: [email protected]. Conflict of Interest There are no financial interests to declare for this study.

Acknowledgments F. W. and K.T. W. contributed equally to this work. The authors are thankful for the financial support from the Guangxi Natural Science Fund (Grant: 2018GXNSFBA281064) and the Chinese Natural Science Fund (Grants: 51772055, 21566006 and 11675102).

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deposited strong alkaline anion exchange fiber. J. Appl. Polym. Sci. 2017, 45855, DOI 10.1002/app.45855.

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Brief synopsis: The CeO2@SiO2 microspheres was prepared via vacuum rotary evaporation and co-precipitation and non-sintering strategy with high selectivity and capacity for F- ions removal.


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Preparation of CeO2@ SiO2 Microspheres By Non-Sintering Strategy

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