Synthesis-of-organic-rectorite-with-novel-Gemini-surfactants-for-

Applied Surface Science 317(2014)35–42

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Applied Surface

Science

j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /a p s u s

Synthesis of organic rectorite with novel Gemini surfactants for copper removal

Guocheng Han a ,Yang Han a ,Xiaoying Wang a ,?,Shijie Liu a ,b ,?,Runcang Sun a ,c

State Key Laboratory of Pulp and Paper Engineering,South China University of Technology,Guangzhou 510640,China

Department of Paper and Bioprocess Engineering,State University of New York,College of Environmental Science and Forestry,1Forestry Drive,Syracuse,NY 13210,USA c

China Beijing Key Laboratory of Lignocellulosic Chemistry,Beijing Forestry University,Beijing 100083,China

a r t i c l e

i n f o

Article history:

Received 13May 2014

Received in revised form 31July 2014Accepted 14August 2014

Available online 23August 2014

Keywords:

Gemini surfactant Rectorite Copper Adsorption Regeneration

a b s t r a c t

Three novel Gemini surfactants were used to prepare organic rectorite (OREC)under microwave irradiation,in comparison with single-chain surfactant ester quaternary ammonium salt (EQAS)and cetyltrimethyl ammonium bromide (CTAB).The structure and morphology of OREC were characterized by XRD,BET,FT-IR,TEM and TGA.The removal of Cu 2+on OREC from aqueous solution was performed.The results reveal that Gemini surfactants modi?ed REC had larger interlayer distance and higher surface area than single-chain surfactants EQAS and CTAB,and the increasing amount or chain length of Gem-ini surfactants led to larger layer spacing and higher adsorption capacities.The adsorption behavior of Gemini surfactant modi?ed REC can be better described by Freundlich adsorption isotherm model,with a maximum adsorption capacity of 15.16mg g ?1.The desorption and regeneration experiments indicate good reuse property of Gemini modi?ed REC adsorbent.Therefore,this study may widen the utilization of Gemini surfactants modi?ed layered silicates.

1.Introduction

Heavy metals,natural components on the planet,are possibly the most widespread water contaminants causing environmental problems over decades.Copper is an ancient resource utilized by human for thousands of years [1–3],but copper cannot be degraded,resulting in harmful effects on living species via biological accumu-lation of food chain [4].Hence,minimization and removal of copper from aqueous environment is an important study.

Several techniques are available to remove copper from aque-ous solutions [5].Among them,adsorption is proposed to be the preferred treatment for its simplicity of operation,versatility,high ef?ciency and economy.Recently,high costs and environmental concerns associated with the use of conventional adsorbents have led to more and more investigation of low cost and eco-friendly adsorbents,such as the layered silicates.

Layered silicates are natural minerals with layered structure,high abundance,large surface area,high cation exchange capacity (CEC),chemical and mechanical stability [6].And rectorite (REC)

?Corresponding authors at:State Key Laboratory of Pulp and Paper Engineering,South China University of Technology,Guangzhou 510640,China.Tel.:+862087111861;fax:+862087111861.

E-mail addresses:xyw@https://www.360docs.net/doc/5d7712453.html, (X.Wang),sjliu@https://www.360docs.net/doc/5d7712453.html, (S.Liu).

is a typical kind of layered silicates consisting of alternate regu-lar (1:1)stacking of dioctahedral mica-like layer and dioctahedral smectite-like layer [7].The physical and chemical properties of lay-ered silicates determine the usefulness of layered silicates as ideal adsorbents for water treatment,soil decontamination,air puri?-cation and so on [6].However,narrow layer spacing,high surface energy and so on impair the adsorption capacity for contaminants and further application [8,9].To address this weakness,organic sur-factants have been employed to modify the layered silicates and convert the hydrophilic surface to a hydrophobic one [10].

Conventional surfactants modi?ers,such as cetyltrimethyl ammonium bromide (CTAB),sodium dodecylbenzene sulfonate (SDBS)and polyvinyl alcohol (PVA),are widely used in modi?-cation of clays,most of which are single-chain surfactants [11].Gemini surfactants are considered as a new generation of surfac-tants with two hydrophilic and two hydrophobic groups in one molecule [12].Compared to other conventional surfactants,Gem-ini surfactants are superior ones exhibiting lower critical micelle concentration (CMC),higher ef?ciency in reducing the oil/water interfacial tension,better wetting,solubilizing and foaming [13].According to previous reports,Gemini surfactant modi?ed lay-ered silicates showed more spontaneous intercalation of surfactant,larger interlayer space,high surface area than conventional sur-factants,which resulted in more ef?cient adsorption capacity [14,15].

https://www.360docs.net/doc/5d7712453.html,/10.1016/j.apsusc.2014.08.087

36G.Han et al./Applied Surface Science317(2014)35–42

Table1

The detailed information of the obtained ORECs.

Samples Surfactants The amount of surfactant on the REC(×CEC)

With Br?/Cl?ions Without Br?/Cl?ions

(EQAS)-REC-0.2EQAS0.340.39

(EQAS)-REC-0.5EQAS0.520.58

(EQAS)-REC-3.0EQAS0.620.70

(CTAB)-REC-0.2CTAB0.550.70

(CTAB)-REC-0.5CTAB0.760.97

(CTAB)-REC-3.0CTAB0.78 1.00

(12-2-12)-REC-0.2Gemini12-2-120.290.40

(12-2-12)-REC-0.5Gemini12-2-120.340.45

(12-2-12)-REC-3.0Gemini12-2-120.440.59

(12-3-12)-REC-0.2Gemini12-3-120.330.38

(12-3-12)-REC-0.5Gemini12-3-120.410.47

(12-3-12)-REC-3.0Gemini12-3-120.540.62

(18-3-18)-REC-0.2Gemini18-3-180.280.31

(18-3-18)-REC-0.5Gemini18-3-180.380.43

(18-3-18)-REC-3.0Gemini18-3-180.430.48

However,less work has focused on the synthesis of Gemini surfactant modi?ed REC or the comparison between Gemini sur-factants and conventional single-chain surfactants.Therefore,the aim of present work was to modify REC with three Gemini surfac-tants and evaluate the adsorption behavior for copper ions on the OREC.Besides,the adsorption performance between Gemini sur-factant modi?ed REC and single-chain surfactant modi?ed REC and the relationship between chain length of Gemini surfactant and the adsorption capacities were also discussed.

2.Materials and methods

2.1.Materials and apparatus

Calcium Rectorite(REC)was supplied by Hubei Mingliu Inc., Co.(Hubei,China).The cation exchange capacity(CEC)value of the REC was determined to be70mmol100g?1by titra-tion and the surface area was7.8015m2g?1by BET method. Ester quaternary ammonium salt(EQAS,(ROOC)4N+X?)and cetyltrimethyl ammonium bromide(CTAB,C19H42NBr)were sup-plied by Shanghai Bio Science and Technology Co.,Ltd.(Shanghai, China).Gemini surfactants were purchased from Henan Titanin Chemical Technology Co.,Ltd.(Henan,China).They were1,3-bis(dodecyldimethylammonio)-ethane dibromide(C30H66N2Br2), 1,3-bis(dodecyldimethylammonio)-propane dichloride (C31H68N2Cl2),and1,3-bis(octadecyldimethylammonio)-propane dichloride(C43H92N2Cl2),designated as12-2-12,12-3-12and 18-3-18,respectively.REC and organic surfactants were used without any further puri?cation.The other reagents and solvents were of analytical grade.

An XH-100B microwave synthesis system was purchased from Beijing XiangHu Sci.-Tech.Dept.Co.,Ltd.(Beijing,China).The microwave power ranged from100W to850W,and the high-est reaction temperature was300?C.The reaction was performed inside the system under pulsed microwave irradiation at atmo-spheric pressure.

2.2.Preparation of OREC

The organically modi?ed REC samples were synthesized by using ion-exchange reaction between REC and organic surfactants (Gemini,CTAB or EQAS)under microwave irradiation as follows: REC powder(5g)was dispersed in a50%(v/v)isopropanol aque-ous solution.After stirring at600rpm under room temperature for about24h,the REC suspension was then put in microwave syn-thesis system(800W,80?C,60min).10mL50%(v/v)surfactant (Gemini,CTAB or EQAS)isopropanol aqueous solution was slowly dropped into the REC suspension during microwave heating,the amounts of surfactants were equivalent to0.2,0.5and3.0times than the CEC of REC.After vigorously stirring and heating for1h, the reaction products were collected by centrifugation and rinsed with isopropanol aqueous solution(50%,70?C)until no chloride or bromide anions were detected by AgNO3solution.Finally,the OREC products were obtained after freeze-drying at?40?C and screened with a100-mesh sieve.The detailed information of obtained ORECs is listed in Table1.

2.3.Characterization

XRD patterns of ORECs powder samples were measured by using a D8advance X-ray diffractometer(Bruker,Germany)and use a Cu target and K?radiation( =0.15418nm)at40kV and50mA at 20?C.The relative intensity was recorded in the scattering range (2?)of1–10?at a scanning rate of1?min?1.The d-spacing was calculated by using Bragg’s equation[Eq.(1)][16].

n =2d sin?(1) where n is an integer(n=1); is the wavelength of incident wave ( =0.15418);d is the spacing between the planes in the atomic lattice and?is the angle between the incident ray and the scattering planes.

The surface area and pore diameter of REC before and after mod-i?cation were characterized by BET method using a Micromeritics TriStar II3020instrument(Micromeritics,USA).

FT-IR spectra were conducted on a Nicolet FT-IR5700spec-trophotometer(Madison,USA)under dry air at room temperature by a KBr pellets method.The spectra were scanned from4000cm?1 to400cm?1at a resolution of4cm?1.

JEM-2010HR transmission electron microscopy(TEM)(JEOL, Japan)was used to observe the microstructure of REC and ORECs at an accelerating voltage of200kV.The TEM samples were prepared by dispersing the layered silicates in50%ethanol solution.A few drops of suspended layered silicates were placed on copper mesh grids,and then dried in an oven at50?C for10min.

2.4.Thermal behavior

Thermogravimetric analysis(TGA)was performed by a SDT-Q500thermobalance(TA,USA)to investigate the thermal stabilities of REC and ORECs.Approximately10mg ground sample was heated from room temperature to700?C at a heating rate of10?C min?1 in high-purity?owing nitrogen atmosphere(50mL min?1).

G.Han et al./Applied Surface Science317(2014)35–4237

2.5.Adsorption experiment

All experiments were conducted by mixing100mL of50mg L?1

CuSO4solution with0.2g of REC(or OREC).The pH values of ini-

tial solutions were about5.0.The experiments were performed in

temperature-controlled water bath at250rpm at room tempera-

ture for120min.At required time intervals,4mL of the reaction

solution was quickly sampled and?ltered through a0.45?m

hydrophilic polyethersulfone(PES)syringe?lter(Shanghai ANPEL,

China).The concentration of CuSO4solution was measured at the

analytical wavelength324.7nm for copper using atomic absorp-

tion spectrometer(AAS)(Jena,Germany).The amount of copper

adsorbed onto the adsorbent was determined by the difference

between the initial and remaining concentration of CuSO4solu-

tion.The adsorption capacity of adsorbed copper onto the layered

silicates can be evaluated by[Eq.(2)].

q e=(C0?C e)V

(2)

where q e is the amount of copper adsorbed on the layered silicates (mg g?1);C0and C e refer to the initial and equilibrium concentra-tions of CuSO4solution(mg L?1);while W and V are the mass of layered silicates used(g)and the volume of CuSO4solution(L), respectively.

Langmuir and Freundlich adsorption isotherm models were applied to analyze the adsorption behavior.The Langmuir adsorp-tion isotherm is based on the assumption of homogenous adsorption and can be expressed by[Eq.(3)].

q e=q m K L C e

1+K L C e

(3)

where q e is the amount of copper adsorbed on the layered silicates (mg g?1);q m is the maximum adsorption capacity of the layered silicates(mg g?1);C e refers to the initial and equilibrium concen-trations of CuSO4solution(mg L?1);K L is the Langmuir constant related to the adsorption energy(L mg?1).

The Freundlich adsorption isotherm is based on the adsorption happened on heterogeneous surface and can be described by[Eq.

(4)].

ln q e=1

ln C e+ln K F(4)

where q e is the amount of copper adsorbed on the layered silicates (mg g?1);C e refers to the initial and equilibrium concentrations of CuSO4solution(mg L?1);K F and n are the Freundlich constants related to the adsorption capacity and intensity.

2.6.Desorption and regeneration experiment

For the desorption studies,the adsorbed copper ions were des-orbed in1mol ammonium acetate(NH4Ac)solution by shaking under room temperature for2h[17].After desorption,the solu-tions were?ltered and the trace metal contents were determined by AAS.The desorption rate can be calculated by[Eq.(5)]. Desorption rate

=the concentration of Cu 2+desorped by NH

Ac solution

initial concentration of Cu2+absorbed on adsorbent

×100%

(5)

For the regeneration studies,the copper-desorbed clay was washed with deionized water and dried in an oven at50?C. Thereafter,the clay was reused in next cycle of adsorption and desorption.The experiment was performed for three constructive cycles.

2θ (°)

Fig.1.XRD patterns of REC and OREC:(a)(EAQS)-REC;(b)(CTAB)-REC;(c)(12-2-12)-REC;(d)(12-3-12)-REC and(e)(18-3-18)-REC.

3.Results and discussion

3.1.Structural and morphology of the OREC

The X-ray diffraction(XRD)patterns of REC and ORECs modi?ed by surfactants are shown in Fig.1.An intense re?ection could be observed at2?=3.57?for unmodi?ed REC(corresponding inter-layer distance was2.40nm),which belonged to the d001plane of https://www.360docs.net/doc/5d7712453.html,pared to REC,the d001peak of ORECs shifted toward lower angle,indicating that the interlayer space expanded due to the intercalation of the surfactant(Gemini,CTAB or EQAS)under microwave irradiation.

As shown in Fig.1(c)–(e),the interlayer distances of Gemini modi?ed RECs increased with the increasing amount of surfactants from0.2CEC to3.0CEC.Besides,it can be found that the inter-layer distance of OREC was positively related to the chain length of Gemini surfactants,the interlayer distance of(18-3-18)-REC-3.0 reached3.26nm,while the interlayer distance of(12-2-12)-REC-3.0was only3.10nm.It can be seen in Fig.1(a)and(b)that,for

38G.Han et al./Applied Surface Science317(2014)35–42

Table2

The structural and surface parameters of REC and ORECs.

Samples d001(nm)S BET(m2g?1)BET pore size(nm)

REC 2.407.801525.73

(EQAS)-REC-3.0 2.798.015527.09

(CTAB)-REC-3.0 2.808.126831.10

(12-2-12)-REC-3.0 3.109.000731.44

(12-3-12)-REC-3.0 3.199.955832.12

(18-3-18)-REC-3.0 3.2610.490732.09

EQAS modi?ed RECs and CTAB modi?ed RECs,the interlayer dis-tances increased with the increasing amount of the surfactants. However,the interlayer distances of(EQAS)-REC-3.0(2.79nm)and (CTAB)-REC-3.0(2.80nm)were smaller than that of(12-2-12)-REC-3.0(3.10nm),(12-3-12)-REC-3.0(3.19nm)and(18-3-18)-REC-3.0 (3.26nm),suggesting that Gemini surfactants were more ef?cient than EQAS or CTAB in the modi?cation of REC,which is in agreement with previous study[15].

When the amount of surfactant was0.2CEC,the interlayer space of(12-2-12)-REC-0.2was2.56nm,suggesting a lateral-monolayer arrangement of surfactants in the interlayer,and similarly,the intercalation of Gemini surfactants in(12-2-12)-REC-0.5and(12-3-12)-REC-0.5may be arranged as a lateral-bilayer arrangement [8].For(18-3-18)-REC-0.2,there were two peaks in the XRD pat-tern,implying that the surfactants may be in a monolayer-bilayer arrangement since the intercalation reaction may be incomplete [15].And the interlayer space of(18-3-18)-REC-0.5and(18-3-18)-REC-3.0were3.15nm and3.26nm,respectively,which may reveal a pseudotrimolecular arrangement of surfactants,as shown in Scheme1.In summary,the interlayer space of OREC was related to the type and amount of surfactant,and its arrangement of in the interlayer.Speci?cally,the chain length of Gemini surfactant played an important role on the arrangement effect[18].

The BET surface area(S BET)and the average pore size of REC and ORECs are summarized in Table2.The S BET and pore size increased after intercalation of surfactants,and Gemini surfactant modi?ed RECs exhibited higher S BET and pore size than single-chain sur-factant modi?ed RECs,which is in agreement with the interlayer distance in XRD results.Hence,it shows that the chain length of surfactant in?uence the interlayer spacing and the surface area.

The FT-IR spectra of REC and ORECs are shown in Fig.2,describ-ing the major changes of REC before and after modi?cation by surfactants.After the intercalation of surfactants,the spectra of ORECs not only presented the characteristic bands of REC,but also some speci?c bands of the surfactants.All samples had a distinct band at3642cm?1,resulting from the hydroxyl stretching vibra-tion of the OH units within the REC.Meanwhile,a broad band of stretching vibration of the H O H occurred at3432cm?1,and the bands in the1638cm?1corresponded to the H O H bending vibra-tions of H2O.Low frequency bands in the spectra of REC and ORECs at1040cm?1,1036cm?1,524cm?1and470cm?1were attributed to different types of Si O stretching,Si O bending,Al O Si bend-ing and Si O Si bending vibration,respectively[19].Most band positions did not change after surfactants modi?cation,suggesting that the basic crystal structure of OREC remained the same as REC. Different from REC,the spectra of ORECs had two new bands at 2925cm?1and2853cm?1,corresponding to the CH2asymmetric stretching vibration as(CH2)and symmetric stretching vibration s(CH2)of surfactants,and the bands at1465cm?1and1385cm?1 belonged to the bending vibration of C H[8],the results fur-ther reveal that Gemini modi?ed RECs were successfully prepared. The spectra of Gemini12-2-12(Fig.2(d)),12-3-12(Fig.2(e))and 18-3-18(Fig.2(f))showed that the intensity of the characteristic bands enhanced as increasing the chain length.Additionally,the small band around3198cm?1existing in three OREC spectra repre-sented the stretching vibration of the N H units within the Gemini

Wavenumbers (cm-1)

Fig.2.FT-IR spectra of REC and OREC:(a)REC;(b)(EAQS)-REC-0.5;(c)(CTAB)-REC-0.5;(d)(12-2-12)-REC-0.5;(e)(12-3-12)-REC-0.5and(f)(18-3-18)-REC-0.5. surfactants,which were not possessed by EQAS or CTAB surfac-tants.Noteworthily,the bands at3432cm?1and1638cm?1in the spectra of OREC showed diminishing in intensity,which may be ascribed to the replacement of interlayer cations by the quaternary ammonium species[15].As a result,it reveals that the hydrophilic REC may transform to hydrophobic through modi?cation.

TEM images of REC and(18-3-18)-REC-0.5in two levels of mag-ni?cation are shown in Fig.3,evaluating the morphology of the clays before and after modi?cation.As can be seen from Fig.3,after intercalation of Gemini surfactants under microwave radiation, (18-3-18)-REC-0.5still exhibited well-ordered layered structure just like REC.It can be seen visually that the interlayer distance of (18-3-18)-REC-0.5(Fig.3(d))was enlarged obviously and the?eld of view was more clear than REC(Fig.3(b)).The layer spacing mea-sured by TEM analysis was about3.10nm,which is in agreement with XRD results and further con?rms the enlarged basal spacing of ORECs.

3.2.Thermogravimetric analysis

The thermal stability of REC and ORECs can be determined by the use of thermogravimetric techniques[20].Fig.4shows the thermo-gravimetric(TG)and derivative thermogravimetric(DTG)curves of REC,ORECs and the?ve selected surfactants.It is noted that ORECs modi?ed by EQAS,CTAB and Gemini surfactants had lower mass loss than REC below200?C,indicating less free water within OREC.

A reliable explanation can be surface energy reduction of the clays after intercalation or adsorption by surfactants[8],being consistent with FT-IR results.

It can be seen from TG and DTG curves that T max(the temper-ature when the rate of weight loss reaches a maximum)of OREC was lower than that of REC,indicating the thermal stability of REC was also lower after modi?cation.Meanwhile,remain mass of REC at700?C was88.20%,while those were87.32%,86.65%and86.23% for(12-2-12)-REC-0.5,(12-3-12)-REC-0.5and(18-3-18)-REC-0.5,

G.Han et al./Applied Surface Science317(2014)35–42

Scheme1.The possible arrangement of Gemini surfactants in REC interlayer.

respectively.The result further proves that the thermal stability of Gemini surfactant modi?ed REC was lower than REC,and decreased with the increasing chain length of Gemini surfactants.

The DTG curve of REC(Fig.4(a))shows three mass loss steps[21], which were attributed to water desorption from REC,dehydration of water adsorbed by metal cations like Ca2+,and dehydroxyla-tion of the structural OH groups of REC,respectively.There was no signi?cant degradation process between200?C and500?C for REC,while the thermal degradation process of surfactants hap-pened between160?C and370?C(Fig.4(g)–(k)),so conclusion can be drawn that the degradation of OREC was mainly caused by the loss of the surfactants[22].Moreover,under the protection of silicate layer,the degradation of intercalated surfactants happened between300?C and500?C[8].In addition,the?rst two peaks in DTG curves of OREC(Fig.4(b)–(f))became broad,indicating less residual water and adsorption water within OREC.The above anal-ysis altogether con?rms that the surfactants were adsorbed or intercalated to the layered silicates.

The arrangement of surfactant in the interlayer of REC affected the degradation behavior of ORECs.It seems that lower degradation temperature was related to looser arrangement of the surfactant in the interlayer space[23].As depicted in DTG curves(Fig.4(d)–(f)), the onset degradation temperature of(18-3-18)-REC-0.5was lower than those of(12-2-12)-REC-0.5and(12-3-12)-REC-0.5,which

may Fig.3.TEM images of REC and OREC:(a,b)REC and(c,d)(18-3-18)-REC-0.5.

G.Han et al./Applied Surface Science 317(2014)35–42

M a s s (%)

D e r i v a t i v e (%/ °C )

Temperature (°C)

Fig.4.TGA curves (I)and DTG curves (II):(a)REC,(b)(EAQS)-REC-0.5;(c)(CTAB)-REC-0.5;(d)(12-2-12)-REC-0.5;(e)(12-3-12)-REC-0.5;(f)(18-3-18)-REC-0.5;(g)EQAS;(h)CTAB;(i)Gemini 12-2-12;(j)Gemini 12-3-12and (k)Gemini 18-3-18.

correspond to a looser arrangement (pseudotrimolecular arrange-ment)of Gemini surfactant molecules in the interlayer space of REC,while comparing with the bilayer arrangement of (12-2-12)-REC-0.5and (12-3-12)-REC-0.5.It may further prove the Gemini surfactants arrangement in REC,as shown in Scheme 1.

Furthermore,the surfactant amount on ORECs could be deter-mined by TG according to Eq.(6),and the results are listed in Table 1.

X =

m ×S ×10?2×100

(M ?y )×m ×(100?S )×10?2×70×10?3

S ×10

(M ?y )×(100?S )×70

(6)

where X is the surfactant amount on the OREC;m is the total weight of the OREC;S is the mass loss percentage of surfactant in the OREC and M is the molecular weight of the surfactant.y is 0(if all the Br ?or Cl ?remain)or 80(no Br ?,the molecular weight of Br is 80)or 160(no Br ?,the molecular weight of 2Br ?is 160),or y =71(no Cl ?,the molecular weight of 2Cl ?is 71).It is worth noting that y =0,80,160or 71is not possible to reach but it can help to work out the range of X by calculating theoretical maximum and minimum values.

It can be found from Table 1that the amount of the intercalated surfactant on the ORECs gradually increased with the increasing dosage of surfactant,being consistent with XRD results.It shows that the amount of single-chain surfactants on the REC was larger than that of Gemini surfactants.However,as every molecular of Gemini surfactant has two chains,so Gemini surfactant modi?ed REC still have more surfactant chains than single-chain surfactant modi?ed REC.

3.3.Adsorption of Cu 2+on OREC

The adsorption behavior of Cu 2+on REC and ORECs at different time was studied under room temperature.The results are shown in Fig.5.For ORECs,a two-stage kinetic behavior can be observed:rapid initial adsorption in a contact time of about 5min,followed by a second stage with much lower adsorption rate.The adsorption approached equilibrium at about 60min.These results followed the general process of metal adsorption on minerals,during which adsorption onto reactive sites takes place in the initial rapid stage and the followed slower stage may result from adsorption onto less reactive sites [24].Meanwhile,REC presented a low adsorption rate from beginning to end.Obviously,all the ORECs exhibited better adsorption capacities than REC.Gemini surfactant modi?ed RECs exhibited better adsorption capacities than that of single-chain sur-factants,and as the amount or chain length of Gemini surfactants increased,the adsorption capacities of ORECs improved.(18-3-18)-REC-3.0with the largest interlayer distance had the best adsorption performance.

Fig.6shows the adsorption isotherms of Cu 2+on REC and ORECs.It can be seen that the increasing initial Cu 2+concentration led to improved adsorption capacity.Generally,as the chain length of Gemini surfactant increased,the adsorption capacity of Cu 2+increased.Moreover,the Gemini surfactant modi?ed RECs exhib-ited better than single-chain surfactant modi?ed RECs on the Cu 2+removal under different concentration.Table 3shows the ?tting parameters of Cu 2+adsorption isotherms on REC and ORECs.Both

q e (m g ·g -1)

Time (min)

Time (min)Ti

me (min)Ti

me (min)

Fig.5.Cu 2+adsorption on REC and ORECs.

G.Han et al./Applied Surface Science 317(2014)35–42

Table 3

Langmuir and Freundlich isotherm constants of REC and ORECs for Cu 2+.

Adsorption model

Parameters

REC

(EQAS)-REC-3.0

(CTAB)-REC-3.0

(12-2-12)-REC-3.0

(12-3-12)-REC-3.0

(18-3-18)-REC-3.0Langmuir model q m (mg g ?1)7.229.9510.7713.1613.8715.16K L (L mg ?1)0.0020.040.040.070.060.06R 2

0.9919

0.8649

0.8628

0.8279

0.8467

0.8516Freundlich model

1/n

0.690.190.190.150.160.16K F (L mg ?1)0.05 3.20 3.41 5.38 5.21 5.83R 2

0.9743

0.9896

0.9859

0.9830

0.9865

0.9873

Table 4

The adsorption capacity for Cu 2+on various adsorbents.

Adsorbents

q m (mg g ?1)

T (K)

References GMZ bentonite 6.23 6.5293[25]

Ca-rectorite

7.22 5.0298This study Multi-walled carbon nanotubes/fulvic acid 8.25

6.0

293

[26]

EQAS modi?ed rectorite 9.95

5.0

298

This study

Acid-activated kaolinite 10.10

2.0

303

[27]CTAB modi?ed rectorite

10.77 5.0

298

This study

Ca-montmorillonite 12.63 5.0298[28]

Gemini modi?ed rectorite 15.16

5.0

298

This study

Palygorskite

17.40 5.0295[29]Graphene oxide/Fe 3O 418.26 5.3293[30]Bentonite-polyacrylamide composites

19.89

5.0

293

[31]

Modi?ed multi-walled carbon nanotubes

38.60

6.0

300

[32]

Langmuir model and Freundlich model can well describe Cu 2+adsorption process on ORECs.From the analysis of coef?cient R 2of isotherm correlation,the adsorption behavior of REC can be better described by Langmuir adsorption isotherm model,indicat-ing a homogenous surface.And Freundlich adsorption isotherm model can better describe the isothermal adsorption process on ORECs.Therefore,these results imply that the surface of Gem-ini surfactant-modi?ed REC was heterogeneous,which may result from the adsorbed surfactants on the surface and inside the layers after modi?cation.The theoretical maximum adsorption capacity of Gemini modi?ed REC was 15.16mg g ?1.

When compared with other kinds of absorbents (Table 4),the adsorption capacity of Gemini modi?ed REC was lower than

q e (m g ·g -1)

C e (mg ·L -1

)

https://www.360docs.net/doc/5d7712453.html,ngmuir and Freundlich adsorption isotherm for Cu

on REC and ORECs.

Table 5

The desorption rate and regeneration performance of REC and ORECs.

Samples

Desorption rate (%)

Regeneration

performance (mg g ?1)

Cycle 1

Cycle 2

Cycle 3

REC

82.370.400.380.33(EQAS)-REC-3.081.86 4.66 3.92 3.61(CTAB)-REC-3.080.83 5.18 4.70 4.28(12-2-12)-REC-3.082.737.807.567.24(12-3-12)-REC-3.083.289.008.187.85(18-3-18)-REC-3.0

82.90

10.10

9.20

8.51

graphene oxide/Fe 3O 4,bentonite-polyacrylamide composites and modi?ed multi-walled carbon nanotubes.However,as a low-cost clay adsorbent,Gemini modi?ed REC had higher copper removal ability than GMZ bentonite,acid-activated kaolinite,Ca-montmorillonite,multi-walled carbon nanotubes/fulvic acid,re?ecting its potential application in adsorption of heavy metal pollutants.

The adsorption behavior may be explained as follows.On one hand,with the increase of the amount or chain length of Gem-ini surfactants,the interlayer spacing (from XRD results)of OREC was enlarged,and higher surface area (from BET results)can be obtained,leading to a better adsorption capacity than REC [15].On the other hand,it may because the surface energy (from FT-IR and TGA results)of OREC was reduced after modi?cation,and the lay-ered silicates with surfactants on the surface and inside the layers (from FT-IR and BET results)showed more adsorption sites,making it easier to adsorb heavy metal ions.

3.4.Desorption and regeneration

The desorption of Cu 2+from REC and ORECs with NH 4Ac were carried out and the results are shown in Table 5.It can be seen that NH 4Ac had satisfying desorption capacity that more than 80%of Cu 2+could be desorbed from REC and ORECs.

The regeneration performance of REC and ORECs after each cycle of adsorption and desorption is listed in Table 5.The regeneration performance of three Gemini modi?ed RECs for copper removal decreased but still have 8.51mg g ?1,7.85mg g ?1,7.24mg g ?1after three cycles of adsorption and desorption,respectively,about 75%,79%and 77%of the ?esh adsorbent,showing their favorable reusability.It can be found that Gemini modi?ed RECs exhibited better regeneration performance than single-chain modi?ed RECs and REC (each of them was about 60%),which may be related to their larger interlayer distance and higher surface area.Thus,the Gemini modi?ed REC adsorbents were reusable and feasible for copper removal.

4.Conclusions

In this study,rectorite (REC)has been modi?ed by three Gem-ini surfactants under microwave irradiation.The interlayer space

of REC expanded due to the intercalation of Gemini surfactants and increased with the increase amount and chain length of

42G.Han et al./Applied Surface Science317(2014)35–42

Gemini surfactants.Correspondingly,the chain length and amount of Gemini surfactants also affected the arrangement of the inter-calary surfactant in the interlayers of REC.Due to the larger interlayer distance and higher surface area,the Gemini surfactant modi?ed RECs exhibited better adsorption capacity for copper ions than pure REC,single-chain surfactant modi?ed RECs and some other clay adsorbents.The adsorption behavior of Gemini modi-?ed RECs can be better described by Freundlich isotherm model and the adsorbent showed good reuse property.These results con-?rm that Gemini surfactant modi?ed REC is a promising and more competent adsorbent in heavy metal wastewater disposal.

Acknowledgements

This work was?nancially supported by Program for New Cen-tury Excellent Talents in University(NCET-13-0216),Science& Technology Project of Guangzhou City in China(No.2012J2200018) and the Fundamental Research Funds for the Central Universities, SCUT(No.2014ZG0011).

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