一种高岭土基多孔硅材料的制备及其对Ca2+和Mg2+的吸附

李　健　李　岩　牟占军　张馨颖　郝贠洪　武朝军＊,

(1内蒙古工业大学化工学院，呼和浩特010051)

(2内蒙古工业大学土木学院，呼和浩特010051)

一种高岭土基多孔硅材料的制备及其对Ca2+和Mg2+的吸附

李健1李岩2牟占军1张馨颖1郝贠洪2武朝军＊,1

(1内蒙古工业大学化工学院，呼和浩特010051)

(2内蒙古工业大学土木学院，呼和浩特010051)

成功合成了一种高岭土基新型多孔硅材料(ASM)并以FTIR，XRD，FE-SEM和N2吸附-脱附进行了系统的表征。ASM的制备过程涉及两步：SiO32-提取和ASM的制备。SiO32-提取的最优条件为煅烧温度为960℃，NaOH浓度为20%，反应温度为90℃，反应时间为90 min，在此条件下SiO32-提取率为60.45%(w/w)。以此提取液为原材料，调整提取液中SiO32-的浓度为12 g·L-1，反应温度为90℃，反应时间为60 min，然后再搅拌2 h可制得ASM。以此ASM对Ca2+和Mg2+进行吸附研究，脱除率分别可达94.99%和62.32%。

多孔硅材料；制备；高岭土；吸附

0　Introduction

Silica material,a kind of white appearance, qualitative light,and fluffy amorphous porous material[1], can be expressed as SiO2·n H2O,wherein n H2O exists in the form of surface hydroxyl groups.The silica material was widely used in industry as reinforcing agent,white pigment and stabilizer[2].Currently,severalmethods were reported to prepare silica material, including chemical vapor deposition,precipitation process,sol-gel process,hydrothermal technique and dissociation of silicate minerals[3-4].Specially,Martinez et al.[5-6]had prepared amorphous SiO2by the sol-gel procedure,which presented high specific surface area, but the process parameters were difficultly controlled and only stayed at research stage.Therefore,silicate chemicals was paid more attention to reducing the cost of production by using inexpensive non-metallic minerals as silicon source.Recently,the silica material was produced from kaolin,chlorite,vermiculite,and chrysotile[7-9].Although the reserves of kaolin were largely and widely distributed,the further processing products from kaolin were less reported[10].As one of important fillers,kaolin was often used in many industries,including paper,rubber,cement,adhesives, ceramic industries,molecular sieves,and polyaluminum chloride production[11-12].Kaolin was rich in silica(Al2O3·2SiO2·2H2O,about 46.51%(w/w)in theory) and was an economically variable raw material for amorphous silica production[13].

In this work,kaolin was used as the raw material,which was calcined at high temperature and treated with sodium hydroxide solution.The optimum condition of the choosing alkaline fusion process was systematically investigated.Then the as-prepared SiO32-leachate was used as silica source,reacting with acid to produce the amorphous silica material (ASM).In the process,CO2gas was used as acid to synthesize high quality silica material.

The silica material with porosity and surface hydroxyl groups showed the possibility for adsorption of metal ions[14-17].Therefore,the performance of ASM for removal of Ca2+and Mg2+from aqueous solution were systemically studied.

1　Experimental

1.1Materials

Kaolin from Inner Mongolia autonomous region (China)was used as the initial raw material. Commercially produced CO2(purity＞99.9%),sodium hydroxide(purity＞97%),dibutyl phthalate(purity＞97.0%),calcium chloride anhydrous(purity＞96%), magnesium chloride(purity＞98%),and ethylenediaminetetra-acetic acid disodium salt dehydrate(purity＞99%)were used in this work.All the other reagents used were analytic grade.

1.2Extraction of SiO32-leachate from kaolin

Firstly,the raw kaolin was crushed,milled,and sieved,and then heated in a muffle furnace up to 400～1 000℃with a rate of 20℃·min-1for 2 h. Secondly,the calcined kaolin powder was mixed with NaOH solution(10%～25%)in a thermostat water bath at 90℃for 1～2 h.Thirdly,the suspension liquid was filtered and washed to obtain SiO32-leachate.To evaluate the SiO32-leaching rate,the concentration of SiO32-in the filtrate was determined with silico-fluoride natrium volumetric method.The principle was shown as follows:

The concentration of SiO2(g·mL-1)in the filtrate was calculated as the following formula:

c1:Concentration of HCl standard solution,mol·L-1;

c2:Concentration of NaOH standard solution,mol·L-1;

V1:Consumption of HCl standard solution,mL;

V2:Consumption of NaOH standard solution,mL;

V3:Consumption of HCl standard solution in blank test,mL;

V4:Consumption of NaOH standard solution in blank test,mL;

M:Molar mass of SiO2,g·mol-1.

1.3Synthesis of the ASM

The ASM was prepared by precipitation method using the as-prepared SiO32-leachate reacting with CO2with flow rate of 100 mL·min-1at 90℃for 60 min under stirring at 200 r·min-1.Finally,the silica products formed after carbonation processes were thoroughlywashed with absolute alcohol and dried at 105℃for 10 h to attain the ultimate ASM.

1.4Characterization

Mineral composition of kaolin and calcined kaolin was analyzed using XRD analyses(Rigaku Ultima IV,Japan),which were collected on a powder X-ray diffractometer(Siemens D/max-RB)with Cu Kα (λ=0.154 06 nm)radiation and scanning rate of 0.05° ·s-1operated at working voltage of 40 kV and working current of 40 mA.Differential scanning calorimetry (DSC)measurements were conducted at a heating rate of 10℃·min-1from room temperature to 1200℃in N2atmosphere.The presence of kaolin and calcined kaolin special bands in the samples were confirmed through FTIR technology,using KBr as back ground (Nicolet,Nexus 670),in the region of 4 000～400 cm-1at room temperature.The morphology of ASM was analyzed by the scanning electron microscopy(SEM, Quanta FEG 650,China)with an accelerating voltage of 20 kV.The specific surface area of ASM was characterized by nitrogen gas adsorption-desorption at 77 K by using 3H-2000PS1/2 Specific surface and pore size analysis instrument.

1.5Adsorption of Ca2+and Mg2+

Different solutions of Ca2+and Mg2+were prepared by dissolving their chloride salts in deionized water. Mixing a certain amount of silica in 50 mL of prepared solution at 25℃and stirring at 250 r·min-1in shaking table[18].Then,the mixed solution was separated by filter.The Ca2+and Mg2+concentration in the filtrate were measured using EDTA titration method[19].The effect of response times,initial solution concentration, and adsorbent mass were systemically investigated.

2　Results and discussion

2.1Characterization of kaolin

The thermal analysis result of kaolin was shown in Fig.1.As illustrated in Fig.1,the sharp endothermic peak at 533℃represented the dehydroxylation process, in which the crystalline kaolin was turned into amorphous metakaolin[20].The sharp exothermic peak at 994℃indicated that the mullite(γ-Al2O3)was appeared in the calcination temperature range of 900 to 1 000℃.

Fig.1　DSC curve of kaolin

XRD patterns of kaolin and calcined kaolin were shown in Fig.2.From Fig.2,the crystal structure of kaolin was intact and orderly,narrow sharp,and good symmetry(PDF No.14-0164)[22].When calcination temperature was controlled at 400℃,the characteristic diffraction peaks of kaolin were still evident,although the strength of peak slightly decreased.However, when calcination temperature was controlled at 600～900℃,the kaolin diffraction peaks disappeared[23], indicating that the kaolin completely lost its original crystal structure and converted into amorphous metakaolin.At 1 000℃,the new peaks of mullite were obviously observed[24].

Fig.2　XRD patterns of raw kaolin and kaolin calcined at different temperatures

FTIR spectra of kaolin before and after calcination were presented in Fig.3and 4.As shown in Fig.3,the bands at 3 692 cm-1and 3 614 cm-1(Si-OH),3 431 cm-1and 1 629 cm-1(H-OH),1 030 cm-1, 1 100 cm-1,1 008 cm-1and 473 cm-1(Si-O),and 913 cm-1(Al-OH)and 540 cm-1(Si-O-AlⅥ)were typicalabsorption peaks of kaolin[25-26].From Fig.4,the peaks at 3 692 cm-1,3 614 cm-1,913 cm-1and 540 cm-1completely disappeared after calcined at 600℃, confirming that the dehydration reaction of kaolin was completed and the kaolin was converted into metakaolin[27].The bands at 1 030 cm-1,1 100 cm-1and 1 008 cm-1(Si-O)still existed after calcined at 400℃,but the bands changed into one broad peak at 1 068～1 196 cm-1when the calcination temperature changed from 500 to 900℃,which was due to the collapse of kaolin structure.As temperature increased to 930℃,the two new peaks at 565 cm-1(AlⅣ)and 739 cm-1(AlⅣ) appeared,indicating that the new γ-Al2O3phase was generated[26].

Fig.3　FTIR spectrum of kaolin

Fig.4　FTIR spectra of kaolin calcined at different temperatures

2.2Choice ofappropriate calcination temperature of kaolin

15 g kaolin at different calcination temperature reacted with 15%NaOH solution at 90℃for 90 min, then the influence of calcination temperature on leaching rate ofin kaolin was studied.leaching rate under different calcination temperature was shown in Fig.5.

Kaolin was converted into metakaolin dehydrated after 600℃.The transformation process was shown as follows: When the calcination temperature was higher than 930℃,the amorphous SiO2and inert γ-Al2O3increased significantly resulting in the evident increasing ofleaching rate(Fig.5):

Fig.5　Silica leaching quantity under different activation temperature of kaolin

At 1 100℃,a small amount of mullite was generated making theleaching rate decrease gradually:

Comprehensive the above,the suitable calcination temperature was confirmed at 930～960℃.

2.3Effect of reaction conditions on leaching rate

The effect of NaOH concentration onleaching rate was shown in Fig.6.From Fig.6,the SiO32-leaching rate increased with the increasing NaOH concentration from 10%to 25%.When the NaOH concentration was less than 20%,chemical balance was in favor of generating the sodium metasilicate with the increasing chemical reaction rate and the diffusion velocity increasing[28-29].When the NaOH concentration continued to increase by 25%,the SiO32-leaching rate sharply decreased due to a side reaction intensifies.So the optimum NaOH concentration of 20%was chosen.

Fig.6　Effect of alkalinity on leaching rate

Temperature was an important factor,which might affect the chemical reaction process[30].When alkali concentration was 20%,the effect of reaction temperature and time onleaching rate was shown in Fig.7and 8.As shown in Fig.7,theleaching rate was gradually increased when temperature increased from 80 to 90℃and reached the maximum of 60.45%at 90℃.Theleaching rate would not significantly increase even if the reaction temperature was extended,so the optimum reaction temperature was set at 90℃.As shown in Fig.8,the optimum reaction time was 90 min.

Fig.7　Effect of reaction temperature on leaching rate

Fig.8　Effect of reaction time on leaching rate

2.4Characterization of the ASM

FTIR spectra of the ASM were shown in Fig.9. The bands at 3 427 and 1 637 cm-1were separately assigned to the stretching vibration and bending vibration of adsorbed water[31].The peaks at 1 092 and 800 cm-1corresponded to asymmetric and symmetric Si-O stretching vibration,respectively.The sharp absorption peak at 470 cm-1was assigned to the bending vibration of Si-O-Si.

Fig.9　FTIR spectrum of the ASM

In the XRD pattern of the ASM(Fig.10),a wide shape and low intensity peak at 2θ=20°～25°[32-33]indicated that the ASM was amorphous.

FE-SEM was utilized to study the morphology and size distribution of the amorphous silica.Aggregations consisting of small particles were shown in Fig. 11.From Fig.11(a),ASM were mostly in the form of aggregates.After amplification for further observation (Fig.11(b)),it could be found that the aggregates were form by small spherical particles.And the average size of these spherical particles was 51 nm.

Fig.10　XRD pattern of the ASM

Fig.11　FE-SEM images of the ASM

As shown in Fig.12,the adsorption-desorption curve showed that ASM was a typical mesoporous materials.When the relatively pressure was low,the single molecular layer adsorption occurred.And capillary condensation had occurred when the pressure was high,this led to a jump in the adsorption isotherms.Additionally,the BET surface area of ASM was 127 m2·g-1and the average pore diameter of ASM was 3.968 nm.

Fig.12　Nitrogen adsorption-desorption isotherm of the ASM

2.5Removal of Ca2+and Mg2+

The effect of adsorbent dose(2,4 and 6 g·L-1) on the removal of Ca2+(200 mg·L-1)was presented in Fig.13(a).From Fig.13(a),a rapid decline of Ca2+concentration was observed from 0 to 10 min and the adsorption equilibrium was obtained from 10 to 60 min.With the increasing adsorbent dosage,Ca2+concentration in the solution decreased gradually.When the adsorbent dose was 4 g·L-1,Ca2+concentration dropped to 10.02 mg·L-1,adsorption rate reached 95%and adsorption quantity reached 47.50 mg·g-1.

The effect of adsorbent dose(4 and 8 g·L-1)on the removal of Mg2+(100 mg·L-1)was presented in Fig.13(b).From Fig.13(b),the Mg2+concentration rapidly declined from 0 to 10 min and the adsorption equilibrium reached after 6 h,demonstrating that the capability of the ASM on adsorbing Mg2+was weakerthan Ca2+.When the adsorbent dose was 8 g·L-1,Mg2+concentration dropped to 37.68 mg·L-1,adsorption rate reached 62.32%and adsorption quantity reached 7.80 mg·g-1.

Effect of contact time and initial concentrations of metal ions on the adsorption capability of the ASM are shown in Fig.14.As could be seen in Fig.14,the concentration of metal ions declined with the prolonging contact time.The adsorbent dose on adsorbed Ca2+and Mg2+were 4 and 8 g·L-1,respectively.The experimental results in Fig.14(a)showed that the adsorption mainly took place within 10 min.Fig.14(b)showed that just half of Mg2+were adsorbed within 10 min, and then the concentration of Mg2+gradually declined from 10 to 360 min until the adsorption reached equilibrium after 6 h.The adsorption was due to the porous structure and surface hydroxyl of the ASM.An increase of the initial Ca2+and Mg2+concentrations lead to an increase in the adsorption capacity.When the initial Ca2+concentration increased from 50 to 300 mg·L-1in Fig.14(a),the adsorption capacity of Ca2+on adsorbent changed from 11.2 to 64.0 mg·g-1.When the initial Mg2+concentration increased from 50 to 200 mg·L-1in Fig.14(b),the adsorption capacity of Mg2+on adsorbent changed from 4.75 to 9.50 mg·g-1. Maximum sorption capacities for Ca2+and Mg2+adsorption using various natural adsorbents were listed in Table1to compare with the results from the present work.Maximum sorption capacities by ASM was 64.0 mg·g-1for Ca2+and 9.50 mg·g-1for Mg2+respectively, namely higher than those reported in Table 1.The results also showed that the ASM owned a higher removal capacity for Ca2+than Mg2+.

Fig.13　Effect of contact time and adsorbent dose on the adsorption capability

Fig.14　Effect of contact time and initial concentration of metal ion on the adsorption capability

The increasing in initial solution concentration results in an increase in removal capacity,which could be attributed to the increase of driving forcethat caused by the increase of concentration gradient. Surface adsorption and chemical deposition could be considered as two important driving forces for the removal of Ca2+and Mg2+[44-45].On the one hand,ASM contained a large number of surface silicon hydroxyl silanol,and hydrogen of surface silicon hydroxyl could be free since ionization[46].This made the surface of ASM present negative charge in aqueous solution,and further promoted the ASM adsorbing Ca2+and Mg2+ions by electrostatic interaction[44].On the other hand, a large number of hydroxyl groups at the ASM surface could complex with Ca2+and Mg2+ions on the ASM surface[45].It was this way that the ASM have a higher adsorption capacity to Ca2+and Mg2+ions.

Table1　Maximum sorption capacity of some adsorbents for Ca2+and Mg2+

3　Conclusions

In summary,the optimum synthetic condition of as-prepared SiO32-leachate was confirmed as follows: calcination temperature of 960℃,alkali concentration of 20%,reaction temperature of 90℃and reaction time of 90 min.Under the optimum condition,the SiO32-leaching rate could arrive at 60.50%.The main factors influencing the SiO32-leaching rate is the calcination temperature.The ASM was prepared from the SiO32-leachate with 100 mL·min-1CO2,and 12 g·L-1silica at 90℃for 60 min,and aged for 2 h.The ASM showed strong adsorption capability to Ca2+and Mg2+,and the removal rate of Ca2+(200 mg·L-1)and Mg2+(100 mg· L-1)on the ASM(4 g·L-1and 8 g·L-1,respectively) arrived at 94.99%and 62.32%,respectively.

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Amorphous Silica Material Prepared from Kaolin and Its Adsorption Properties to Ca2+and Mg2+

LI Jian1LI Yan2MOU Zhan-Jun1ZHANG Xin-Ying1HAO Yun-Hong2WU Zhao-Jun＊,1
(1College of Chemical Engineering,Inner Mongolia University of Technology,Huhhot 010051,China)
(2College of Civil Engineering,Inner Mongolia University of Technology,Huhhot 010051,China)

A novel amorphous silica material(ASM)was successfully prepared from kaolin and characterized by several techniques,including Fourier transform infrared spectroscopy(FTIR),X-ray diffraction(XRD),field emission scanning electron microscopy(FE-SEM),and N2adsorption-desorption.The preparation procedure of ASM involved two aspects:SiO32-leaching from kaolin and preparation of ASM.The optimum SiO32-leaching condition was confirmed at calcination temperature of 960℃,NaOH concentration of 20%,reaction temperature of 90℃and reaction time of 90 min.Under this condition,SiO32-leaching yield arrived at 60.45%(w/w).The ASM was then easily prepared from SiO32-leaching at the silica concentration of 12 g·L-1,reaction temperature of 90℃,reaction time of 60 min and aging time of 2 h.The removal behavior of Ca2+and Mg2+revealed that the removal rate of Ca2+and Mg2+on the ASM could arrived at 94.99%and 62.32%,respectively.

amorphous silica material;preparation;kaolin;adsorption

O611.4

A

1001-4861(2016)11-2049-09

10.11862/CJIC.2016.254

2016-06-05。收修改稿日期：2016-09-20。＊通信联系人。E-mail：nmzhjwu@163.com