简介概要

Solvothermal synthesis of nano-CeO₂ aggregates and its application as a high-efficient arsenic adsorbent

来源期刊：Rare Metals2019年第1期

论文作者：Jing-Hua Pang Ying Liu Jun Li Xiao-Jiao Yang

文章页码：73 - 80

摘要：Aggregates of cerium dioxide nanoparticles（nano-CeO₂） were successfully prepared via a facile solvothermal process in this study. The crystallographic information and morphological information of nano-CeO₂ were systematically studied by X-ray diffraction（XRD）,transmission electron microscopy（TEM）, laser particle size analyzer（LA） and specific surface area and pore size analyzer during the solvothermal process. Among all the obtained samples, the 18-h solvothermal-prepared nano-CeO₂ aggregates show the best crystallinity and the largest specific surface area of 110.92 m²·g^-1. Owing to the high activity derived from the high specific surface area of the aggregates, the application as arsenic（As） adsorption was also studied. The adsorption efficiency of arsenic by nano-CeO₂ aggregates was established as the function of adsorbent dose, then pH value and at last adsorption time.The results indicate that the nano-CeO₂ aggregates show a high efficiency in removing arsenic from low As concentration solution, from which the nano-CeO₂ adsorbent could be easily separated. In addition, the adsorption kinetics is best fitted to pseudo-second-order model（R² = 0.99999）.

稀有金属(英文版) 2019,38(01),73-80

Solvothermal synthesis of nano-CeO₂ aggregates and its application as a high-efficient arsenic adsorbent

Jing-Hua Pang Ying Liu Jun Li Xiao-Jiao Yang

College of Materials Science and Engineering, Sichuan University

作者简介：*Ying Liu e-mail:liuying5536@scu.edu.cn;

收稿日期：11 January 2017

基金：financially supported by the Sichuan Province Science and Technology Support Program (No. 2014GZ0090);

Solvothermal synthesis of nano-CeO₂ aggregates and its application as a high-efficient arsenic adsorbent

Jing-Hua Pang Ying Liu Jun Li Xiao-Jiao Yang

College of Materials Science and Engineering, Sichuan University

Abstract：

Aggregates of cerium dioxide nanoparticles(nano-CeO₂) were successfully prepared via a facile solvothermal process in this study. The crystallographic information and morphological information of nano-CeO₂ were systematically studied by X-ray diffraction(XRD),transmission electron microscopy(TEM), laser particle size analyzer(LA) and specific surface area and pore size analyzer during the solvothermal process. Among all the obtained samples, the 18-h solvothermal-prepared nano-CeO₂ aggregates show the best crystallinity and the largest specific surface area of 110.92 m²·g^-1. Owing to the high activity derived from the high specific surface area of the aggregates, the application as arsenic(As) adsorption was also studied. The adsorption efficiency of arsenic by nano-CeO₂ aggregates was established as the function of adsorbent dose, then pH value and at last adsorption time.The results indicate that the nano-CeO₂ aggregates show a high efficiency in removing arsenic from low As concentration solution, from which the nano-CeO₂ adsorbent could be easily separated. In addition, the adsorption kinetics is best fitted to pseudo-second-order model(R² = 0.99999).

Keyword：

Nano-CeO₂; Large surface; Aggregates; Solvothermal; Removal of arsenic;

Received： 11 January 2017

1 Introduction

Nano cerium dioxide (nano-CeO₂) has been widely used in many fields such as capacitor [ 1, 2, 3] ,solid fuel cells [ 4, 5] ,ultraviolet shielding materials [ 6, 7] ,chemical-mechanical polishing (CMP) [ 8, 9] ,catalysts [ 10, 11, 12] and oxygen gas sensors [ 13, 14] .In recent years,the adsorption performance of nano-CeO₂ was studied by some research groups [ 15, 16] .The hexamethylenetetramine (HMT) as a precipitating agent was successfully established for the preparation of nanocrystalline CeO₂ particles in previous studies.It was found that CeO₂ nanoparticles presented a high level of lead removal ability and was also an excellent promising material to remove the trace chromium(Ⅵ) in purifying drinking water or reusing contaminated water [ 17] .Li et al. [ 16] synthesized hydrous ceria oxide (HCO)by a simple precipitation process,and the HCO demonstrated an exceptional arsenic removal performance on both trivalent arsenic[As(Ⅲ)]and pentavalent arsenic[As(Ⅴ)].Based on those studies,nano-CeO₂ powders have the great potential for water treatment in removing hazardous elements (e.g.,As and Cr) at present.Commonly,nanoparticles possessing high activity should be an efficient adsorbent.However,it is difficult to separate the nanoparticles from the treated solution due to the well suspension of those inpidual nanoparticles in the solution.Thus,this makes it dysfunctional to serve as a hazardous elements adsorbent.But a kind of suitable aggregate of nano-CeO₂ is an ideal adsorbent candidate not only due to their large specific surface area,but also due to the submicron size,which makes it feasible to be separated from the treated solution.

There are various ways to synthesize nano-CeO₂aggregates,including precipitation [ 18, 19] ,sol-gel method [ 20, 21] ,and solvothermal method [ 22, 23] .High purity and well crystallinity can hardly be achieved in the nano-CeO₂ prepared by precipitation method without calcination.And it is quite troublesome to separate the nano powders from the gels when prepared by sol-gel method.In order to get high-purity nano-CeO₂,calcination is indispensable in both the precipitation and sol-gel method.Compared with all these synthesis approaches,the solvothermal method has two significant advantages.One is that the phase,particle size and morphology of the CeO₂nanoparticles could be controlled during the solvothermal process,while the other one is that some inevitable severe agglomeration caused by high-temperature sintering could be avoided during the solvothermal process.Holding these two merits,the nano-CeO₂ prepared by solvothermal method could have a large surface and well crystallinity.

Large numbers of solvothermal experiments were carried out to synthesize nano-CeO₂ with different morphologies [ 24, 25, 26] in complex solution systems [ 27, 28, 29] .However,the nano-CeO₂ can hardly meet high purity if it was solely prepared by solvothermal method without calcination followed.In this study,the precursor was added into a simple solution of anhydrous alcohol without any other additives.The obtained nano-CeO₂ aggregates not only showed high crystallinity and purity together but also exhibited large specific surface area.And the prepared nano-CeO₂ was subjected to adsorption experiments using low concentration arsenic aqueous solution [ 8] .The results revealed that the as-prepared nano-CeO₂ could efficiently remove As even when As concentration is as low as0.01 mg·ml^-1.

2 Experimental

2.1 Synthesis and absorption experiments

In this study,all the chemical reagents were chemical pure and most purchased from Chengdu KeLong Chemical Co Ltd.Nano-CeO₂ was prepared by using cerium(Ⅲ) nitrate hexahydrate[Ce(NO₃)₃·6H₂O]as the source of cerium.At first,8 g Ce(NO₃)3·6H₂O was dissolved in the mixed solution of 200 ml deionized (DI) water and 100 ml anhydrous alcohol to form the cerium precursor solution.Then,2 ml hydrogen peroxide (H₂O₂,60%) was added into the cerium precursor solution.To improve the purity of the product powders,20 ml ammonia hydroxide (NH₃·H₂O)was added into the solution by a constant flow pump with the rate of 1 ml·min^-1.All the above reactions were performed in a 30℃water bath with a constant magnetic stirring rate of 400 r·min^-1.

After precipitation for 6 h with stirring,the precipitates were centrifuged and washed with DI water and anhydrous alcohol for several times.All the precipitates were weighed out and mixed with 37.5 ml anhydrous alcohol,which was transferred into a 50 ml Teflon-lined autoclave and heated at 180℃for a certain time.After the solvothermal process,nano-CeO₂ could be obtained by filtering and vacuum freeze drying at-55℃for 6 h.

The standard solution of As (concentration of1 mg·ml^-1,media 5%H₂SO₄,China Iron and Steel Research Institute Group) was prepared into 0.01 mg·ml^-1original solution.50 ml As original solution was used in each adsorption experiment,in which 50 mg as-obtained nano-CeO₂ was dissolved.The solution was magnetically stirred at a constant rate of 400 r·min^-1 for some certain period,and then the nano-CeO₂ powders and the As containing solution were separated by filter paper.The remaining As concentration was analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES).The adsorption capacity (q) of nano-CeO₂ for removing As was calculated by Eq.(1),while the removal rate of As(III)(R) by nano-CeO₂ was calculated by Eq.(2) [ 30] .

where C_i and C_f are the initial and final concentration of As(Ⅲ) in the solution (mg·L^-1),V is the volume (L) and m is the weight of the nano-CeO₂ particles (g).

2.2 Characterization

The crystallographic information of the prepared nanoCeO₂ was collected by an X-ray diffractometer (XRD,DX-2700,China) over the 2θrange of 20°-80°,operating at40 kV and 30 mA.And the distribution of particle size and the average particle size (d₅₀) of all the samples were studied by a laser particle size analyzer (HELOS-RODOS/M,German).The specific surface area and pore structure were characterized by a specific surface area and pore size analyzer (Kubo-X1000,China).The microstructure of the nano-CeO₂ was examined by a transmission electron microscope (TEM,Tecnai G2 F20 S-TWIN,America).

In absorption experiments,surface morphology information and chemistry information were investigated by scanning electron microscope (SEM,JSM-7500F,Japan)and energy-dispersive spectroscopy (EDS),respectively.The functional groups adsorbed on the nano-CeO₂ were studied by Fourier transform infrared spectroscopy (FTIR,Nicolet 6700,America).The concentration of remaining As in the solution was determined by an inductively coupled plasma atomic emission spectrometer (ICP-AES,VG PQExCeII,America).

3 Results and discussion

3.1 Morphology and structure of nano-CeO2aggregates

XRD patterns of the obtained nano-CeO₂ with different solvothermal processing time are shown in Fig.1.It could be observed that the characteristic peaks of the cubic fluorite CeO₂ (JCPDS No.34-0394) become shaper and more intensive as the reacting time increases.The diffraction peaks of the precursor without any reacting time exhibit broad bump like curve only except for three parts of mild slope amplitude denoting the (111),(220) and (311) crystal planes.After 2-h solvothermal reaction,the intensity of(111),(220) and (311) crystal planes noticeably becomes stronger with the appearance of (200) crystal plane on XRD pattern.Prolonging the reaction time to 6 h,these diffraction peaks show no further obvious change.While when the reaction time increases to 10 h,all these peaks exhibit higher intensity with the occurrence of the (222)crystal plane peak.Furthermore,the (400) and (331) peaks also emerge on the XRD pattern.With the 18-h solvothermal reaction time,all the diffraction peaks become shaper and higher,especially the intensity of the diffraction peaks of (400) and (331) crystal plane at high 2θangle of 69.40°and 76.68°increases evidently,indicating the improvement in crystallinity of the nano-CeO₂.Moreover,no additional phase is detected in any obtained sample,indicating that high purity nano-CeO₂ could be prepared via this process [ 31] .After 20-h solvothermal reaction,these diffraction peaks show no further obvious change.Consequently,it may be concluded that a good crystallinity phase of CeO₂ could be obtained during 18 h.Besides,Fig.1 also shows XRD pattern of the precursor calcinated in air for 2 h.Obviously,the crystallinity of solvothermal sample is better than that of the calcinated one.In addition,the specific surface area of calcinated one is only 60.03 m²·g^-1,which means that the active nanointerface of solvothermal sample is better than that of the calcinated one.

Fig.1 XRD patterns of CeO₂ prepared with different solvothermal time and calcinated sample

Further investigation was conducted to understand the effects of reaction conditions on the crystallite sizes.The results evaluated depending on Debye-Scherrer equation are shown in Fig.2 [ 32] .Obviously,the crystallite size of solvothermal sample is increasing with the increase in reaction time.The specific surface areas and porous micro structure of the CeO₂ nanostructure were analyzed by N₂ adsorption-desorption measurements at 77 K.All the samples exhibit the identical hysteresis loop;therefore,only the result of 18-h-treated sample is shown here.It can be seen from Fig.3 the significant hysteresis loop which indicates the existence of micropores in the particles [ 33] .

Figure 4 shows SEM image of the obtained CeO₂sample with 18-h solvothermal process.The nanoparticles show granular facet,relatively regular and small particles clinging together to form a weakly agglomerated spheroidal structure.The aggregates formed by these small particles present microporous structure corresponding to the one in Fig.3.

The changes of the average particle size and the surface area of aggregates both as a function of time are shown in Fig.5.It can be seen from Fig.5 that the average particle size of the aggregates dramatically decreases within 2 h,and then stays around 0.664μm after 2 h.And the specific surface area of the aggregates firstly increases remarkably in 2 h,and then decreases to 163.65 m²·g^-1 when prolonging the solvothermal process time to 6 h.And then it undergoes a sharp drop with the reacting time raising to10 h and keeps almost unchanged thereafter.These results indicate that the solvothermal process is advantageous for the dispersion of the precursor powders at the early stage.

TEM images of the nanoparticles are shown in Fig.6.It could be seen that the particle size together with themorphology of the nanoparticles changes with reacting time.Before the solvothermal process,the nanoparticles are hexagonal in shape,and the particle size is measured as～4.8 nm.After 10 h,the shape of some nanoparticles transforms to spheroidal shape to reduce the surface energy,while the rest keep hexagonal morphology.And the particle size increases to～7.1 nm,indicating that the fusion and preferred orientation [ 32] phenomenon happens in this 10-h period.When the solvothermal process is prolonged to 18 h,nearly all the nanoparticles exhibit spherical shape with bigger particle size,which can be observed in Fig.6c.Even,the size of some particle is～10 nm.This implies that the evident fusion and preferred orientation phenomenon take place in this stage.This result agrees with the changing of particles size calculated from XRD results.

Fig.2 Crystallite size of CeO₂ prepared from different solvothermal processes

Fig.3 Nitrogen adsorption-desorption isotherms

Fig.4 SEM image of 18-h solvothermal-treated sample

Fig.5 Average particle size and surface of CeO₂ prepared from different solvothermal processes

Figure 7 is the schematic diagram showing the evolution from HCO to nano-CeO₂ during solvothermal process.The process could be pided into three stages.At first,before the high-pressure period,the particle size of the HCO in this stage is～4.8 nm,and they present hexagon in shape.The HCO nanoparticles have high surface energy in this stage,which causes serious agglomeration.In the second stage,when the nanoparticles are under high pressure within 2 h,they tend to be more dispersive and smaller compared with the precursors.And the specific surface area of the nanoparticles increases in this stage.The evolution occurred in this stage could be attributed to the intensive Brown movement of the nanoparticles in high-energy solvothermal environment.In the third stage,when the solvothermal reacting time exceeds 2 h,the nanoparticles keep growing and become more dispersive according to the results in Fig.5.However,the size of the nano-CeO₂aggregates decreases together with their specific surface area being bigger in this stage.With the reacting time prolonging to 10 h,the morphology of the nanoparticles transforms to spheroidal shape.The crystallinity of the nano-CeO₂ would continue to get improved with the reacting time further increasing to 18 h.Based on the experimental results with reaction time,it could be concluded that 18 h is the most favorable reacting time for preparation of nano-CeO₂ aggregates.

3.2 Effects of CeO2 dose,pH and adsorption mechanism of As

To determine a suitable CeO₂ dose for adsorption of As,the effect of CeO₂ dose on As adsorption was studied with the identical adsorption time of 1 h.It can be clearly seen in Fig.8a that the adsorption capacity increases steeply with the increase in CeO₂ dose from 0.1 to 0.5 g·L^-1.And the equilibrium is reached with the CeO₂ dose of 1 g·L^-1,indicating that approximately 1 g·L^-1 CeO₂ is fitted for the removal of 50 ml As original solution (0.01 mg·ml^-1).

In order to investigate the effect of pH,experiments were conducted at room temperature of 25℃.About50 mg CeO₂ was added to 50 ml solution with an As initial concentration of 0.01 mg·ml^-1.From Fig.8b,it can be observed that the adsorption capacity of CeO₂ has certain stability in the pH range of 3-11.Even at pH 11,the removal rate of As remains a relatively high value of92.5%.At the same time,it could be seen that the adsorption capacity is more stable in acidic conditions than in alkaline.Besides,previous literature had reported absorption behavior of As under neutral conditions [ 16] .For these reasons,the further experiments were conducted in solution at pH 3.

Fig.6 TEM images of cerium dioxide with different solvothermal time

Fig.7 Nanoparticles dispersion mechanism in anhydrous alcohol

Fig.8 Effect of a CeO₂ dose and b pH on As adsorption

Before and after adsorbing process,the nano-CeO₂ was characterized by SEM and EDS,as shown in Fig.9.The morphology of nano-CeO₂ changes little by comparing the insets SEM images,showing high stability of the nanoCeO₂ due to the high crystallinity.Furthermore,As and S were detected in EDS,as seen from Fig.9b,giving more solid proof to the adsorption of As and S on the nano-CeO₂surface.

Figure 10 shows FTIR spectra of the nano-CeO₂ before and after adsorbing process.Before adsorption,FTIR spectrum of the nanoparticles present (1) H-O-H stretching (3430.1 cm^-1) and bending (1633.4 cm^-1) vibrations of H₂O,(2) bending vibrations of hydroxyl groups on metal oxides (Ce-OH) at 1047.1 cm^-1.After that,there is a new characteristic peak of at about 1182.5 cm^-1.Thus,combined with the results of Fig.9,it could be easily concluded that SO₄^2-is absorbed on nano-CeO₂.However,As-O group is not found in FTIR spectrum,while As is actually detected together with S in EDS from Fig.9.Therefore,it could be deduced that As is adsorbed through SO₄^2-on the nano-CeO₂.

Fig.9 SEM images (inset) and EDS spectra of nano-CeO₂ a before and b after adsorption at 25℃for 1 h with CeO₂ dose of 1 g·L^-1

Fig.10 FTIR spectra of CeO₂ (1) and adsorbed CeO₂ nanoparticles(2)

3.3 Kinetic study

Studying the kinetic model would be helpful on understanding the mechanism of arsenic adsorption.The kinetics of As ion adsorption by nanoparticles is presented in Fig.11a.The adsorption amount of As increases sharply before the first 20 min and then levels off after 60 min.The equilibrium is reached in approximately 60 min and lasts till 300 min.The pseudo-second-order model was shown as follows [ 34] :

Fig.11 a Effect of time on adsorption capacity of As and b pseudo-second-order kinetics

When t→0,the initial sorption rate (h) could be expressed as follows:

where q_e (mg·g^-1) and q_t (mg·g^-1) denoted the amount of As adsorbed at equilibrium and adsorbed at time t,respectively.h (mg·min·g^-1) was the initial sorption rate,and k (g min mg^-1) was the rate constant of the adsorption process.The application of the pseudo-second order by plotting t/q_t versus t is shown in Fig.11b.And the value of coefficient of determination (R²) for the pseudo-secondorder kinetic model is 0.99999.Therefore,the system under study could be appropriately described by the pseudo-second-order model.These results suggest that the adsorption kinetics is controlled by chemisorption [ 34] .Moreover,it only needs 230 s to remove 80%As (removal rate=80%) according to the fitted equation.

where Y and X represented t/q_t and t.

Based on the analyses of IR,EDS and kinetic study,it is concluded that the adsorption of As on nano-CeO₂ aggregates is completed through SO₄^2-.The combination of nano-CeO₂ and As is a chemical process implemented by SO₄^2-.Thus,the adsorption process of As on nano-CeO₂aggregates is a chemisorption.For their high surface area,the prepared aggregate is an excellent absorbent.

4 Conclusion

In summary,the surface and size of nano-CeO₂ aggregates depend on the movement and morphology evolution of nanoparticles with time.Based on the experimental results with reaction time,it is concluded that the most favorable condition of preparing nano-CeO₂ is 180℃solvothermal treatment for 18 h.After 18 h,10-nm spherical aggregating particles (d₅₀=0.644μm) with large surface area(110.92 m²·g^-1) were successfully prepared.In addition,in the solvothermal process,the calcination process could be avoided and higher quality nano-CeO₂ could be obtained.

Moreover,in the present investigation,the nano-CeO2aggregate is an efficient adsorbent which could be applied in wide pH range (pH 3-11),and it could be easily separated from the treated solution.When it was used in pH 3acid solution,the removal rate of arsenic could reach 80%in 230 s.And the kinetic study reveals that the adsorption process of As on nano-CeO₂ aggregates is a chemisorption indirectly implemented by SO₄^2-.