Colloids and Surfaces A: Physicochem. Eng. Aspects 481 (2015) 493–499

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Colloids and Surfaces A: Physicochemical andEngineering Aspects

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Preparation of amine-modified silica foams and their adsorptionbehaviors toward TNT red water

Shuchen Tua, Fengzhu Lva,∗, Pan Hua, Zilin Menga, Hongtao Ranb,∗∗, Yihe Zhanga,∗

a National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Beijing 100083, Chinab College of Science, Beijing Forestry University, Beijing 100083, China

h i g h l i g h t s

• Amine containing SiO2 foams are pre-pared for specific organic treatment.

• Secondary amine modified foamsshow excellent COD removal capac-ity.

• Adsorption behavior fit the pseudo-second-order dynamic and Langmuiradsorption models well.

• Adsorption bases on the attractionbetween amine groups on foam andthe negative organic pollutants.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:Received 11 March 2015Received in revised form 4 May 2015Accepted 10 May 2015Available online 10 June 2015

Keywords:Sio2 foamSilane coupling agentTNT red waterAdsorption model

a b s t r a c t

Secondary and primary amines containing silica foams (F–SiO2) were prepared by using silane couplingagents, (3-aminopropyl) triethoxysilane (KH-550) and bis(3-triethoxysilicyl propyl)-amine (KH-270), asmodifiers. Not only Brunauer–Emmett–Tell (BET) surface and pore size of F–SiO2 are greatly reduced aftermodification, but also increased chemical oxygen demand (COD) removal efficiency to trinitrotoluene(TNT) red water is obtained. Among the series of F–SiO2, the secondary amine modified silica foamspossess higher COD removal capacity. Controlled experiments indicate the adsorption mainly derivesfrom the attraction of amine groups toward the negative organic pollutants in TNT red water ratherthan the effect of the large BET surface and hydroxyl groups on the surface of F–SiO2. The adsorptionmodel studies show the pseudo-second-order dynamic adsorption model and Langmuir isotherm modelwell fit the experimental data. The prepared amine-modified F–SiO2 is an efficient adsorbents of organicpollutants in TNT red water.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

Mesoporous silicas are considered as promising materials forbimolecular loading [1,2], supports of catalyst [3], absorbents of

∗ Corresponding authors at: 29 Xueyuan Road, Haidian District, Beijing 100083,China. Fax: +86 10 82322345.∗∗ Corresponding author at: No.35 Qinghua East Road, Haidian District, Beijing100083, China.

E-mail addresses: zyh@cugb.edu.cn (F. Lv), xiaoran@bjfu.edu.cn (H. Ran),zyh@cugb.edu.cn (Y. Zhang).

wastewater [4] and greenhouse gases [5,6] due to their attractiveproperties, such as large surface area, tunable pore structure, andhigh thermal stability. In which mesocellular foams [7,8], meso-porous silica with textural (interparticular) mesoporosity [9], andhierarchical monoliths [10] show more efficient properties thanrelated bulk mesoporous silica materials.

In order to tailor the properties of mesoporous silica, meso-porous silicas have been functionalized with -NH2 [4,11], -COOH[12], -SO3H [13] and so on. Therefore, a series of solid-supportedsorbents, based on bulk mesoporous silicas including MCM-41,

http://dx.doi.org/10.1016/j.colsurfa.2015.05.0470927-7757/© 2015 Elsevier B.V. All rights reserved.

494 S. Tu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 481 (2015) 493–499

MCM-48, SBA-12, SBA-15, SBA-16, and KIT-6 have been preparedvia wet impregnation and systematically evaluated [11,14–17].

Among the main applications of mesoporous silicas, removalof heavy metals for environmental cleanup is the essential one[18,19]. The earliest report on this topic is the removal of mer-cury in wastewater using propylthiol-functionalized mesoporoussilicas [18]. Since then, there have been a large number of worksdealing with metallic species, primarily cations, in wastewater[4,11]. Heavy metals are more efficiently adsorbed on mesostruc-tured silicas modified with amino groups [20–23]. For example,amine containing mesoporous silica, SBA-15, has been preparedto develop efficient adsorbents of Cu2+, Ni2+, Pb2+, Cd2+, and Zn2+

from wastewater [24]. In addition, SBA-15 functionalized with 3-aminopropyltrimethoxy-silane is studied as potential absorbent forCd2+, Co2+, Cu2+, Zn2+, Pb2+, Ni2+, Al3+ and Cr3+. The adsorptioncapacity and selectivity of the materials are investigated in mono-mental and multi-metal solutions. The amine-modified SBA-15adsorbent exhibits an excellent selectivity against sodium, potas-sium, and calcium [25]. But until now, reports on the organicpollutant treatment by mesoporous silica are few, only workson the organic pollutant removal by thiol and sulfonic acid-functionalized silica foams are reported [13]. Study on the organicpollutant removal by amine modified mesoporous silica based onthe composition of the wastewater has not been performed.

EP Geiannes group [26] has synthesized a more cost-effectivesilica foam with ultra large mesopores. The CO2 adsorption behav-ior of the silica foam after incorporation of polyetherimide hasbeen studied. In the present work the silica foam was modifiedwith amine-containing silanes and its specific removal behaviorsto trinitrotoluene (TNT) red water, which was produced during thepurification of crude TNT by sodium sulfate and contained largeamount of negative organic pollutants, was studied. Silica foammodified by secondary amine showed excellent chemical oxygendemand (COD) removal to the TNT red water. Under optimum con-ditions, its COD removal achieved 165.9 mg/g toward the TNT redwater with initial COD of 1800 mg/L due to the specific interactionof amine groups in the foam with the negative groups of the pollu-tants. The adsorption dynamic and isotherm models were used to fitthe data. The modified foams are excellent adsorbents of negativecharge containing organic pollutants.

2. Experimental

2.1. Materials

SiO2 foam was afforded by the group of EP Giannelis andits synthesis had been reported elsewhere [26]. Tetraethylorthosilicate (TEOS) was from Beijing Chemical Co. (BeijingChina). (3-Aminopropyl) triethoxysilane (99%, KH-550) and bis(3-triethoxysilicyl propyl)-amine (99%, KH-270) were purchased fromAladdin Industrial Corporation. TNT red water [27], which was red-dish brown and opaque, with a high concentration of dinitrotoluenesulfonates (2,4-dinitrotoluene-3-sulfonate and 2,4-dinitrotoluene-5-sulfonate) and high COD, was supplied by Dongfang ChemicalCorporation (Hubei Province, China). All the other reagents used inthis study were analytical grade, and distilled water was used toprepare the solutions.

2.2. Modification of SiO2 foam

The grafting of amine groups onto SiO2 foam was carried outas follows. 0.25 g of silica foam was introduced in a three neckflask with 30 ml of ethanol and 10 ml of water and stirred at roomtemperature for 1 h for complete dispersion, then oxalic acid wasdropped in to adjusted the pH of the dispersion to 4, followed by

dropwise addition of 0.5 ml of silane coupling agents (KH-550, KH-270) in half an hour. The dispersion was further stirred at roomtemperature and then 60 ◦C for 4 h, respectively. The solid was fil-tered and washed with distilled water and ethanol for two timesrespectively and dried in vacuum at 80 ◦C. In the target samplesthe weight ratio of initial silica foam and that from silane couplingagent component is approximate to 1.

The controlled sample, bulk SiO2 particles were prepared bysol–gel process. 15 ml of ethanol solution containing 7.5 ml of TEOSwas added dropwise to 100 ml of ethanol/water mixture (2/1) (con-taining 0.5 mol/l ammonium hydroxide) at a rate of 0.5 ml/min asthe solution was heated to 60 ◦C under stirring, where the reactionwas continued for 2 h. SiO2 particles were obtained after subse-quent steps of centrifugation, washing with water, ethanol, anddrying in vacuum at 70 ◦C

For clear understanding, bulk SiO2 particles, SiO2 foam, KH-550and KH-270 modified SiO2 foam were assigned as SiO2, F-SiO2, NH2-F-SiO2 and NH-F-SiO2, respectively.

2.3. Adsorption experiments

The adsorption experiments were carried out in 100 ml of con-ical flasks. Certain amounts of the adsorbents were separatelyintroduced into a series of conical flasks with 25 ml of TNT redwater. Then, the flasks were shaken in a SHA-BA water bath ata speed of 150 rpm for a certain time (5, 10, 20, 40, 60, 90, and120 min) at room temperature. Then, the samples were filtered anddried in vacuum at 70 ◦C for 8 h. For dynamic study, the amount ofNH-F–SiO2 used was 0.125 g toward 25 ml of TNT wastewater. Foradsorption isotherm study, 0.125 g of NH-F-SiO2 was added into25 ml of TNT wastewater with initial COD of 10,800, 5400, 2700,1800 and 1080 mg/L, respectively, for 1 h.

2.4. Characterization

The Fourier transform infrared (FT-IR) spectrum in the4000–400 cm−1 region was performed using a Perkin Elmer Spec-trum 100 FT-IR spectrometer. KBr was used as a backgroundmaterial and disks of sample/KBr mixtures were prepared to obtainthe FT-IR spectra. Transmission electron microscopy (TEM) imageswere obtained by a JEM-3010 of the Japanese electronics com-pany. Samples were prepared by dispersing the samples in ethanolunder sonication. A few drops of the dispersions were loaded ontoa carbon coated copper microgrid and dried in air. Field emissionscanning electron microscopy (FE-SEM) images were acquired froma LEO-1530 field emission scanning electron microscope operatedat 5 kV. N2 adsorption–desorption isotherms were obtained on amicromeritic instrument (Autosorb-IQ-2MP, USA) at liquid nitro-gen temperature. The silica foam and NH-F–SiO2 were degassedprior to analysis under high vacuum at 60 ◦C for at least 4 h.Their specific surface areas were determined using the multipointBrunauer–Emmett–Teller (BET) method. The pore size distribu-tions derived from the adsorption and desorption branches ofthe isotherms and calculated based on the Barrett-Joyner-Halenda(BJH) model.

The COD of the filtrates were analyzed by the COD rapid detec-tor (5B-6, Lian-Hua Tech. Co., China) with a precision of ±5% todetermine the adsorption efficiency of adsorbents. The adsorbedamount, qe (mg/g), by per unit mass of NH-F-SiO2 was calculatedbased on Eqs. (1):

qe = (CODO − CODe)V

W(1)

where CODo and CODe(mg/L) are the COD values of the initialTNT red water and treated red water after reaching equilibrium,respectively. V (L) is the volume of the TNT red water and W(g) is

S. Tu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 481 (2015) 493–499 495

Fig. 1. FT-IR spectra of KH-550 (a), KH-270 (b), F–SiO2 (c), NH2–F–SiO2 (d) andNH–F–SiO2 before (e) and after (f) adsorption of TNT.

the absorbent weight. At any time, the adsorbed qt (mg/g) by theNH–F–SiO2 can be calculated using a similar relationship based onEq. (1).

3. Results and discussion

3.1. Preparation of amine-modified SiO2 foams

For effective removal of the organics (2,4-dinitrotoluene-3-sulfonate and 2,4-dinitrotoluene-5-sulfonate) in TNT wastewater,secondary and primary amines containing silica foams were pre-pared using KH-550 and KH-270 as modifiers. Fig. 1 shows theinfrared spectra of KH-550, KH-270, F–SiO2 and their derivatives.The FT-IR bands in Fig. 1a and b appear at 2975 cm−1 and 2886 cm−1

are the asymmetric and symmetric stretches of C H in KH-550and KH-270, respectively. The bands at 1482 cm−1 and 1479 cm−1

are the stretching vibration of C C and C N [28]. The characteris-tic peaks at 1443 cm−1 correspond to –NH- and -NH2 functionalgroups [29]. In the spectrum of F-Silica (Fig. 1c) the peaks at960 cm−1 and 800 cm−1 are ascribed to the asymmetric bendingand stretching vibration of Si OH, respectively [30]. The bandlocated at 1580 cm−1 is attributed to adsorbed water. The peakat 1094 cm−1 are assigned to the stretching vibration of Si O Siin the frame structure of SiO2 foam [28]. A strong broad peakat ∼3500 cm−1, which is attributed to the O H stretching vibra-tion of surface hydroxyl groups and water molecules adsorbed, isobserved for both F-Silica (Fig. 1c) and modified F-Silica (Fig. 1d,e)[8,28]. In the spectra of modified F-Silica, especially in that ofNH–F–SiO2 (Fig. 1e), bands associated to the asymmetric and sym-metric stretches of C H in silane coupling agents are observedand shift to lower wavenumber compared to F–SiO2, indicatingthe successful synthesis of NH2–F–SiO2 and NH–F–SiO2. Therefore,NH2–F–SiO2 and NH–F–SiO2 possess primary amine and secondaryamine respectively, in which NH–F–SiO2 is preferred with moreeffective COD removal efficiency toward TNT as long alkyl chainand more alkaline R2-NH groups exist in it.

3.2. Adsorption of organic pollutants in TNT red water bymodified F–SiO2

3.2.1. Adsorption of organic pollutants in TNT red water byNH2–F–SiO2 and NH–F–SiO2

Firstly, the optimum adsorption time and temperature forequilibrium are studied. Actually, temperature has little influ-

Fig. 2. COD removal of bulk SiO2, F–SiO2, NH2–F–SiO2 and NH–F–SiO2.

ence on adsorption. At room temperature, the adsorption time ofNH2–F–SiO2 and NH–F–SiO2 for equilibrium is all ∼20 min andtheir organic removal effeciency toward TNT red water are sum-marized in Fig. 2. The removal effeciency of NH–F–SiO2 to TNTred water with initial COD value of 1800 mg/L is 1008 mg/L andhigher than that of NH2–F–SiO2 (666 mg/L). So detailed character-ization, adsorption model and adsorption mechanism analysis ofNH–F–SiO2 on TNT red water are performed. Although the CODremoval efficiency of NH–F–SiO2 is high, but no obvious peaksrelated to organic pollutants are observed in the FT-IR spectrum ofNH–F–SiO2 after treatment of TNT red water (Fig. 1f) maybe due tothe masking effect of the strong free water vibration at ∼360 cm−1

and the Si O Si vibration at 1094 cm−1.

3.2.2. Adsorption models of NH–F–SiO2The adsorption dynamics describes the rate of adsorption, which

expresses the residence time at the solid-solution interface. Thepseudo-first-order and the pseudo-second-order models were usedto assess the adsorption of organic contaminants from TNT redwater onto adsorbents. Pseudo-first-order model presents as thefollowing:

dq

dt= k1(qe − q) (2)

where qe (mg/g) and q (mg/g) are the amount of organic contami-nants adsorbed at equilibrium and any other time. k1 (min−1) is theequilibrium rate constant and t (min) is the adsorption time. Whilethe pseudo-second-order model presents as the following form:

dq

dt= k2(qe − q)2 (3)

where k2 (g/mg min) is the equilibrium rate constant of pseudo-second-order sorption. qe (mg/g) and q (mg/g) are the amountadsorbed per unit mass of adsorbent at equilibrium and any othertime. The equation can be rearranged to give a linear form as fol-lowing:

t

qt= 1

(k2qe)2+ t

qe(4)

so that qe and k2 can be determined by a linear regression.Fig. 3 shows that F–SiO2 almost has no adsorption efficiency to

organics in TNT red water, while the adsorbing amount of organiccontaminants by NH–F–SiO2 increases rapidly in the first 20 min,and then increases steadily to reach equilibrium under the sameadsorption conditions. After equilibrium the adsorption amounts ofNH–F–SiO2 and F–SiO2 are 165.9 mg/g and 18.7 mg/g, respectively,

496 S. Tu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 481 (2015) 493–499

Fig. 3. Kinetics of contaminant adsorption on F–SiO2 (©) and NH–F–SiO2 (�). Thesolid line is pseudo-second-order fit to the observed data. Inserts are plots of t/qtagainst t for NH–F–SiO2.

as the initial COD of red water is 1800 mg/L. In comparison withF–SiO2, the adsorption capacity of NH–F–SiO2 is greatly enhanced.

The adsorption of organic contaminants from TNT red wateronto NH–F–SiO2 is fitted to adsorption dynamic models and thepseudo-second-order model fits the experimental data better (thesolid line in Fig. 3). The qe and k2 calculated based on the linearform of pseudo-second-order model (Eq. (4)) are 166.6 mg/g and0.1899 g/mg·min, respectively. The calculated qe (166.6 mg/g) wellfits to the experimental data (165.9 mg/g). The value of k2 showsan instantaneous removal of organic contaminants by NH–F–SiO2suggesting a great affinity of NH–F–SiO2 toward the organic con-taminants.

The plot of t/qt vs t could be used to assess the surface hetero-geneity of the adsorbents [31]. In this study, this plot of NH–F–SiO2(Fig. 3 insert) follows a perfect straight line (R2 = 1.00), suggestingthat the adsorption sites on NH–F–SiO2 towards organic contami-nants in TNT red water are homogeneous.

The adsorption isotherm can be used to express the functionalrelationship between the amount of contaminants adsorbed perunit weight of the adsorbent and the concentration of adsorbent ata fixed temperature under equilibrium conditions. The adsorptiondata of NH–F–SiO2 are fitted by Langmuir and Freundlich adsorp-tion models. The theoretical Langmuir isotherm is represented asfollows:

qe = QobCe

1 + bCe(5)

where qe is the amount adsorbed per unit mass of adsorbent atequilibrium (mg/g), and Ce is the equilibrium concentration (mg/L)of wastewater. Q0 is a Langmuir adsorption constant reflecting themaximum adsorption capacity (mg/g) of adsorbent and b is a con-stant representing energy of adsorption (L/mg).

The separation factor based on Langmuir adsorption isothermis a dimensionless constant called equilibrium parameter (RL), andit is defined to estimate the adsorption conditions. The equation isgiven by:

RL = 11 + bC0

(6)

The value of RL has three possibilities: 0 < RL < 1 indicates a favorableadsorption, RL > 1 shows an unfavorable adsorption, RL = 1 presentsa linear adsorption.

Another commonly used isotherm model is Freundlich isothermwhich assumes the surface heterogeneity and encompasses expo-

Fig. 4. Contaminant adsorption on NH–F–SiO2 (�). The dash line is Freundlichisotherm model and the solid line is Langmuir isotherm model.

nential distribution of the energies. The empirical Freundlichisotherm equation can be given by:

qe = KF C1/ne (7)

where KF ((mg/g) (L/mg) 1/n) and n (dimensionless) are the Fre-undlich constants, in which the constant n gives an idea forfavorability of the adsorption process [32].

Shown in Fig. 4, the Freundlich model fits (dash line) theresults in a big discrepancy (R2 < 0.989), while the Langmuir model(solid line) shows a better fit (R2 = 0.999) suggesting a surface-limited adsorption on NH–F–SiO2. The calculated Q0 based onLangmuir isotherm model is 1250 mg/g. The calculated values of RLis between 0.168 and 0.878, indicating that contaminant adsorptionon NH–F–SiO2 is favorable. The well fit to the Langmuir isothermillustrates that a monolayer adsorption takes place. Therefore, thewell-defined adsorption sites localize on the surface with the sameadsorption energy and no transmigration between the moleculesadsorbed and that in solution takes place.

3.3. Adsorption mechanism

3.3.1. BET variationThe surface area of the SiO2 foam and the modified silica

foams are analyzed by means of nitrogen physisorption. For theas-synthesized foam, the isotherm is of type IV and shows steephysteresis of type H1 at high relative pressures corresponding tothe mesostructure of the foams (Fig. 5a) [33]. The BET surface areaof F–SiO2 is 326 m2/g. The pore size distributes between 10 nm and30 nm with a medium diameter of 17 nm. The BET surface area ofNH–F–SiO2 is decreased to 109 m2/g (Fig. 5b). While the pores withdiameter of 17 nm are divided into two. One is with diameter of7.8 nm and the other is 3.1 nm. The large BET surface and pore sizedecrease of NH–F–SiO2 is due to the filling and cross-linking effectsof KH-270. After treatment of TNT red water, the BET surface ofNH–F–SiO2 is further decreased to 70 m2/g (Fig. 5c). The interest-ing thing is that the pores with diameter of 7.8 nm disappear, onlypores of 3.1 nm are left. These data indicate that the pores of 7.8 nmin NH–F–SiO2 are the adsorption place and matters in TNT red waterare absorbed in them. Although smaller pores contribute to largerBET surface, but smaller pores haven’t enough space for organicmolecules entering in. So porous materials with appropriate poresize is benefit of adsorption.

S. Tu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 481 (2015) 493–499 497

Fig. 5. Nitrogen adsorption and desorption isotherms (A) and BJH pore size distribution (B) of F–SiO2 (a) and NH-F-SiO2 before (b) and after (c) adsorption.

Fig. 6. SEM images of F–SiO2 (a) and NH–F–SiO2 (b).

Fig. 7. TEM images of F–SiO2 (a,b) and NH–F–SiO2 (c,d).

498 S. Tu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 481 (2015) 493–499

Scheme 1. Adsorption of NH–F–SiO2 on TNT red water.

3.3.2. Morphology of NH–F–SiO2The morphologies of F–SiO2 and NH–F–SiO2 are shown in

Figs. 6 and 7 . The SEM images (Fig. 6) show the surface of theformed NH–F–SiO2 becomes blur (Fig. 6b) compared to F–SiO2. Butthe outline of F–SiO2 is still clear which is more obvious in theTEM images. This phenomenon derives from the little diffractionand transmittance difference of organic chain (composition of themodification part) with the background.

3.3.3. Adsorption mechanismAs shown in Scheme 1, net structure with pore sizes of 7.8 and

3.1 nm form in NH–F–SiO2. So NH–F–SiO2 possesses large num-ber of hydroxyl and amine groups as well as large BET surface. Inorder to confirm the contribution of the above factors to adsorp-tion, the adsorption of bulk SiO2, which only has hydroxyl groups,and SiO2 foam with hydroxyl groups and large BET surface towardTNT red water are analyzed. The COD removal of bulk SiO2 andF–SiO2 are 180 mg/L and 279 mg/L respectively and all greatly lowerthan that of NH–F–SiO2 (1008 mg/L) which has not only hydroxylgroups and large BET surface, but also amine groups compared tobulk SiO2 and F–SiO2. So the contribution order of the above fac-tors to COD removal is amine group > hydroxyl group > BET surface.The large COD removal efficiency of NH–F–SiO2 derives from thespecific interaction between the positive -NH- charges with thenegative charges of dinitrotoluene sulfonates in TNT red water. Thisis also the reason why the adsorption isotherm fits the Langmuiradsorption model well.

4. Conclusions

Secondary and primary amines containing silica foams wereprepared using KH-550 and KH-270 as modifiers, which were con-firmed by FTIR analysis. N2 adsorption and desorption experimentsshow the BET surface and pore size of F–SiO2 are greatly reducedafter modification, but the as-synthesized NH–F–SiO2 with sec-ondary amine shows increased COD removal capacity to TNT redwater. The prepared amine-modified F–SiO2 is an efficient adsor-bent of organic pollutants in TNT red water. The adsorption showesa pseudo-second-order dynamic and a Langmuir adsorption behav-iors based on the dynamic and isotherm model studies. Controlledexperiments indicate the adsorption mainly derives from the elec-trostatic attraction between amine groups on silica foam towardthe negative organic pollutants rather than the effect of the largeBET surface and hydroxyl groups on NH-F-SiO2.

Acknowledgements

This study was financially supported by the excellent tutorsection of the Fundamental research Funds for the CentralUniversities (2-9-2013-49 and 53200859148), National HighTechnology Research and Development Program (863 Program2012AA06A109) of China, national key laboratory of minerals(09B003), China.

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