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Kinetics pentachlorophenoeres

Kinetics and thermodynamics studies of pentachlorophenol adsorption on covalently functionalized Fe 3O 4@SiO 2–MWCNTs core–shell magnetic

microspheres

Lixin Zhou a ,Shengdong Pan b ,c ,Xiaohong Chen b ,c ,Yonggang Zhao b ,c ,Baobo Zou a ,Micong Jin b ,c ,?

Medical School,Ningbo University,Ningbo,Zhejiang 315211,China

Zhejiang Provincial Key Laboratory of Health Risk Appraisal for Trace Toxic Chemicals,Ningbo Municipal Center for Disease Control and Prevention,Ningbo,Zhejiang 315010,China c

Ningbo Key Laboratory of Poison Research and Control,Ningbo Municipal Center for Disease Control and Prevention,Ningbo 315010,China

h i g h l i g h t s

A novel Fe 3O 4@SiO 2–MWCNTs

adsorbent is synthesized and

characterized by SEM,XRD and VSM. The Fe 3O 4@SiO 2–MWCNTs adsorbent was used to absorb PCP in the aqueous solution.

Thermodynamic parameters showed that the adsorption of PCP was spontaneous,endothermic and random.

The Fe 3O 4@SiO 2–MWCNTs adsorbent has higher absorption capacities

compared with the other adsorbents.

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

Dispersion

a r t i c l e i n f o Article history:

Received 29April 2014

Received in revised form 10July 2014Accepted 12July 2014

Available online 21July 2014Keywords:

Fe 3O 4@SiO 2–MWCNTs PCP

Adsorption Kinetics

Thermodynamics

a b s t r a c t

A novel core–shell covalently functionalized Fe 3O 4coated SiO 2decorated multi-walled carbon nanotubes (Fe 3O 4@SiO 2–MWCNTs)adsorbent was synthesized and characterized by scanning electron microscopy,transmission electron microscopy,Fourier Transform Infrared Spectrum (FTIR),Thermogravimetry,X-ray diffraction and magnetic measurements.The pentachlorophenol (PCP)adsorption effect factors of initial concentration,contact time,solution pH and temperature were investigated in detail.It was found the absorption capacity was strongly depended on pH,and the maximum adsorption value for PCP (96.4mg/g)was obtained at pH 2.5.The equilibrium was attained within 30min,and the adsorption kinetics was better described by pseudo-second-order kinetic model.The data pointed out excellent ?t to Freundlich isotherm model.Thermodynamic parameters such as standard enthalpy (D H h ),standard entropy (D S h )and standard free energy (D G h )showed that the adsorption of PCP onto Fe 3O 4@SiO 2–MWCNTs was spontaneous,endothermic and random within the temperature range of 288–318K.The results indicate that the Fe 3O 4@SiO 2–MWCNTs adsorbent is a potential low-cost effective material for PCP removal from contaminated wastewater.

1.Introduction

Pentachlorophenol (PCP)has been widely used as a wood preservative,microbiocide,fungicide and herbicide for more than 100years.PCP has caused serious environment pollution especially in aqueous systems because of its bioaccumulation tendency and

https://www.wendangku.net/doc/0b11724146.html,/10.1016/j.cej.2014.07.060

?Corresponding author at:Zhejiang Provincial Key Laboratory of Health Risk

Appraisal for Trace Toxic Chemicals,Ningbo Municipal Center for Disease Control and Prevention,Ningbo,Zhejiang 315010,China.

E-mail address:jmcjc@https://www.wendangku.net/doc/0b11724146.html, (M.Jin).

toxicity including estrogenic and potentially carcinogenic effects. Many years ago,PCP had been listed in the priority pollutants list established by the US Environmental Protection Agency (EPA).Today,PCP can be detected in various degrees from all kinds of aqueous media,such as drinking water,river water, ground water and so on.It is necessary to develop an adsorbent with high adsorption capacity to remove pollutants from the water system.

Various treatment approaches for the removal of PCP have been developed,mainly including electrochemical oxidation[1], advanced oxidation[2],biological degradation[3,4],photocatalytic degradation[5],mechanochemical degradation[6]and absorption [7].Among these methods,the absorption approach is viewed as the most promising method since it can effectively remove pollutants from the contaminated system by the convenient design and operation.Many materials,such as allophanic soil[7], activated carbon[8],and carbon nanotubes(CNTs)[9]have been used to absorb PCP.Among these adsorbents,CNTs,a fascinating family of nanomaterials,have always attracted enormous attention in absorption pollutant aspects owing to its unique dimensional structure and high adsorption capacity.In recent years,many researches have been conducted on the adsorption different pollu-tants from various aqueous samples[10–17]and the results are more satisfying.Although the CNTs have the high adsorption capacity,tedious centrifugation separation process is dif?cult to overcome.Magnetic nanoparticles(MNPs),especially Fe3O4,has already attracted the eyeballs of many scientists in removal of pollutants from aqueous solution[18–20],because of the outstanding properties such as easy separation(due to their super-paramagnetic properties,magnetic particles can be easily retrieved from bulk solution by applying an external magnetic?eld without additional centrifugation or?ltration),ease of surface modi?cation (because of the abundant hydroxyls on the surface of iron oxide,it can be conveniently modi?ed with functional groups),good reco-verability(magnetic particles can usually be reused after appropri-ate rinse)[21].However,raw MNPs are susceptible to oxidation especially in acidic solutions[22]and are easy to aggregate in aqueous solution,which reduced the adsorption capacity[23].To compensate for these disadvantages,a core–shell structure is often introduced.For instance,the inner iron oxide core with outer shell of silica nanoparticles not only prevent oxidation and aggregation in solution but also improves their chemical stabilities[24],luckily, the magnetic silica particles can be easily modi?ed with various silane-coupling agents or compounds[21,25].Hence,in this work, a novel core–shell covalently functionalized magnetic Fe3O4coated SiO2decorated MWCNTs nanocomposite(Fe3O4@SiO2–MWCNTs) was synthesized and characterized.The prepared Fe3O4@SiO2–MWCNTs adsorbent was used to remove PCP from aqueous solution.The adsorption kinetics,effects of adsorption conditions, adsorption capability were investigated in detail.

2.Materials and methods

2.1.Materials

PCP was purchased from Aladdin company(Shanghai,China) with purity>99.8%.Iron chloride hexahydrate(purity>99%), tetraethoxysilane(TEOS),ammonium persulfate(APS).N-ethyl-N0-(3-(dimethylamino)propyl)carbodiimide(EDC,purity>98%) and N-hydroxysuccinimide(NHS,purity>98%)were obtained from TCI Tokyo Chemical Industry Co.,Ltd.(Tokyo,Japan).Metha-nol(LC/MS grade)and acetonitrile(LC/MS grade)were obtained from Thermo Fisher Scienti?c(USA).HPLC grade ammonium hydroxide was purchased from ROE scienti?c(USA).Multi-wall carbon nanotubes(>90%purity,outer diameter<8nm,length 30l m)were purchased from Nanjing XFNANO Materials Technol-ogy Corporation(Nanjing,China).Ultrapure water(18.2M X-cm) was obtained directly from a Milli-Q Plus water puri?cation system (Millipore Corporation,France).All other chemicals were analytical grade unless stated otherwise.

Stock solution of PCP(1000mg/L)was prepared by dissolving the powder in methanol.Different initial concentrations were prepared by diluting the stock solution in appropriate proportions by methanol and deionized water(1:1,v/v).

2.2.Synthesis of Fe3O4@SiO2–MWCNTs

In this study,the primary MWCNTs were pretreated as follows: 1.0g MWCNTs were immersed in a250mL three neck?ask with a mixture(1:1,v/v)of sulfuric acid(>98%)and nitric acid(70%).This mixture was sonicated at40°C for30min in an ultrasonic bath, then stirred and heated to re?ux at70°C for3h under dry nitrogen.Afterward,the reaction vessel was cooled down to room temperature,the solution was diluted with distilled water and rinsed for several times until the pH of the?ltered water was approximately6.0[26].The resulting MWCNTs were separated from the solution by?ltration and dried in vacuum for24h for 60°C.This treatment removed amorphous carbon and impurity. Surface carboxyl groups modi?ed MWCNTs(MWCNTs-COOH)with shorter length was obtained.

The synthesis of the novel Fe3O4was according to the literature [27]with some modi?cations.Firstly,an amount of 2.5g of FeCl3á6H2O was suspended in80mL of ethylene glycol in a 200mL beaker.Then,2.0g of polyethylene glycol were added into the beaker and dissolved with ultrasonication to prevent nanopar-ticles agglomeration[27].The mixture was vigorously stirring for 30min and then transferred into a Te?on-lined stainless-steel autoclave(100mL capacity).The autoclave was heated to and maintained at200°C for6h to improve the crystallinity of Fe3O4 and its magnetic property.After cooling to room temperature. The black magnetic Fe3O4particles were obtained and washed three times with methanol under ultrasonic conditions to remove the adsorbed solvent.

The core–shell Fe3O4@SiO2microspheres were synthesized through a modi?ed stoker method.Brie?y,0.2g of the as-prepared Fe3O4microspheres were dispersed in a mixture of ethanol (100mL),water(30mL)and NH3áH2O(1.5mL)with the help of ultrasonication.Then,2mL of TEOS was added dropwise,the reac-tion was carried out under stirring for6h.The product was washed with distilled water and ethanol,and separated under a magnet for several times,and then dried under vacuum at60°C for6h prior to further use.

The Fe3O4@SiO2–NH2particles were prepared as follows:0.5g of Fe3O4@SiO2suspended in50mL of ethanol with0.1g of APS. Then the mixture was re?uxed at80°C for3h.The obtained Fe3O4@SiO2–NH2particles were then dispersed in100mL water, in which0.8g of MWCNTs-COOH particles was activated by 100mg of EDC and70mg of NHS.The mixture was subsequently stirred for1h to obtain core–shell structural Fe3O4@SiO2–MWCNTs. The preparation procedure was illustrated in Fig.1.

2.3.Characterization

The morphologies of the nanoparticles were observed by Scanning electron microscopy(SEM,type JEOL JSM-6330F)and Transmission electron microscopy(TEM,Hitachi,Japan)The wide-angle(10°–90°,40kV/30mA)powder X-ray diffraction (XRD)measurements were carried out by a Bruker Advance D8 X-ray diffractometer at room temperature.A commercial HH-15 model vibrating sample magnetometer(VSM,Lake Shore7410) was used at room temperature to characterize the magnetic

L.Zhou et al./Chemical Engineering Journal257(2014)10–1911

properties of Fe3O4@SiO2–MWCNTs.FTIR spectra were recorded on a Thermo Nicolet(NEXUS-470)FTIR spectrometer.Thermogravi-metry analysis(TGA)was conducted on a TG209F1instrument at a heating rate of20°C/min in a nitrogen atmosphere with a?ow rate of20mL/min.

2.4.Adsorption experiments

Adsorption experiments were carried out in100mL stoppered ?asks,each of which contained40mL of PCP.An amount of Fe3O4@SiO2–MWCNTs(0.02g)was added to each?ask and shaken at250rpm/min in a temperature-controlled shaker.The concen-tration of PCP in aqueous samples was measured by HPLC method as described in Section2.5.The amount of PCP adsorbed per unit mass of adsorbents was evaluated as Eq.(1):

q e ?

VeC0àC eT

e1T

where q e is the equilibrium adsorption capacity of Fe3O4@SiO2–MWCNTs(mg/g),C0is the initial concentration of PCP(mg/L),C e is the equilibrium concentration of the adsorbate(mg/L),V is the volume of the solution(L),and m is the weight of the adsorbent (g),the same hereinafter.

In pH studies,the initial pH of PCP solution with Fe3O4@SiO2–MWCNTs was adjusted to2.0–9.0by addition of0.1mol/L HCl or 0.1mol/L NH3áH2O.The initial concentration of PCP was 100.0mg/L.The suspension was separated by a magnet after agitated in a temperature-controlled shaker at25±1°C for 120min.The kinetic experiments were performed with contacting time ranging from1to180min at pH2.5for PCP.The PCP percent removal was calculated using Eq.(2):

removale%T?

C0àC e

?100e2T

Adsorption kinetics model including pseudo-?rst-order model and pseudo-second-order model,were applied to?t the experi-mental data.Two linear models are given as Eqs.(3)[28]and(4) [29],respectively.

logeq

àq tT?log q eà

k1t

2:303

e3T

k2q2

e4T

where k1(minà1)is the adsorption rate constant,k2(g/mg min)is the second-order rate constant.q e and q t(mg/g)are both the

12L.Zhou et al./Chemical Engineering Journal257(2014)10–19

amounts of analyte adsorbed on absorbent at equilibrium and at time t(min),respectively.

The adsorption isotherm studies were investigated with PCP initial concentrations ranging from1to100mg/L,under pH2.5 for PCP at25±1°C for120min.Two adsorption isotherms,Lang-muir model and Freundlich model were used to describe the equi-librium adsorption.The Langmuir’s isotherm model is represented by the following linear equation[30]:

C e q e ?

Q0b

C e

e5T

where C e(mg/L)is equilibrium concentration of PCP in solution,q e (mg/L)is the amount of adsorbate adsorbed per unit mass of adsor-bent at equilibrium,Q0(mg/g)and b(L/mg)is the monolayer adsorption capacity and rate of adsorption,respectively.

The linearized form of the Freundlich adsorption isotherm is described as Eq.(6)[31]:

ln q

e ?ln K Ft

ln C ee6T

where K F(mg1à1/n L1/n/g)is a constant indicative of the relative adsorption capacity of the adsorbent and n is a constant indicative of the intensity of the adsorption and varies with surface heterogeneity.

To study thermodynamic experiments,?asks containing0.02g of Fe3O4@SiO2–MWCNTs and40mL of50mg/L PCP solution was agitated in an isothermal water bath shaker(Changzhou,China) with reciprocating motion at different temperatures15±1, 25±1,35±1,45±1°C.The absorption equilibrium was achieved after60min,shaking was stopped.The solution was treated with magnetic separation technology and the supernatant was?ltered through membrane?lter.The residual PCP concentration was detected by HPLC-UV.The absorbed capacity of the Fe3O4@SiO2–MWCNTs,q e(mg/g)at equilibrium,was obtained by the Eq.(1).

The thermodynamic parameters were determined by the fol-lowing equations[30]

ln K c?àD H h

D S h

e7T

D G h?àRT ln K ce8Twhere T is absolute temperature in Kelvin;R(8.314J/mol K)is the universal gas constant;K C is the ratio of adsorbate capacity on adsorbent at equilibrium(q e)to the remaining adsorbate concentra-tion in solution at equilibrium(C e);D H h and D S h are standard enthalpy and standard entropy;D G h is standard free energy.

2.5.Analytical method

High performance liquid chromatography(HPLC)equipped with2487dual k absorbance detector(Waters,USA)was used to analyze samples.The separation was performed on a Kromasil C18column(250mm?4.6mm,5l m,AkzoNobel Corporation, Nederland)with a mixture of acetonitrile and water(80:20,v/v) as the mobile phase at a?ow rate of1.0mL/min at25°C.Chroma-tographic data were acquired by ultraviolet(UV)detection at wavelength of254nm using the peak area by the external standard method for quanti?cation.Measurements were performed in three replicates and the averages of these replicates were reported.

2.6.Regeneration and reuse of Fe3O4@SiO2–MWCNTs

After adsorption experiments with50mg/L solution of PCP and 0.02g of Fe3O4@SiO2–MWCNTs,the PCP loaded Fe3O4@SiO2–MWCNTs was separated from the solutions by an attached magnet.The concentration of supernatant was measured using HPLC-UV, and the amount of PCP loading on the adsorbent(m ad)could be calculated as the equation m ad=(C0àC e)V ad.Desorption studies were carried out with5%ammonia/methanol,the suspensions were shaken for60min at25±1°C with a shaking speed of 250rpm/min.After desorption,the amount of PCP desorbed (m de)could be measured using HPLC-UV and calculated as the equation m de=C de*V de.The percent desorption was calculated using Eq.(9)and Fe3O4@SiO2–MWCNTs were washed with double-distilled water for reused for the next cycle.

desorptione%T?

m de

m ad

?100?

C de V de

eC0àC eTV ad

?100e9T3.Results and discussion

3.1.Characterization

In order to examine the microstructure of synthesized materi-als,the SEM and TEM images of Fe3O4@SiO2and Fe3O4@SiO2–MWCNTs were compared and illustrated in Fig.2.As shown in Fig.2a,the structure of Fe3O4@SiO2is spherical microparticles with an average diameter of about600nm,which showed a smooth and homogeneous surface morphology.However,the SEM image of Fe3O4@SiO2–MWCNTs was different from Fe3O4@SiO2.When the MWCNTs were coated on the surface of Fe3O4@SiO2,the surface morphology was greatly changed,which clearly con?rmed the presence of MWCNTs attached on the surface of the Fe3O4@SiO2 (Fig.2b)due to the uneven surface of Fe3O4@SiO2–MWCNTs.The same phenomenon could also be observed in TEM images in Fig.2c and d.

The structure of Fe3O4@SiO2–MWCNTs could also be con?rmed by FTIR as shown in Fig.3.In the FTIR spectra of Fe3O4@SiO2–NH2 (Fig.3b),the characteristic absorptions of Si–O–Si groups at $1089cmà1,–NH2groups at$1597cmà1,and Fe3O4at $589cmà1.After the reaction with MWCNTs(Fig.3c),two new characteristic peaks at$1630and$1730cmà1arose compared to Fe3O4@SiO2–NH2,which could be assigned to–CONH–and –C@O,implying that CNTs were covalently linked to Fe3O4@SiO2–NH2,surface via amide reaction.The TG analyses(Fig.4)of Fe3O4,Fe3O4@SiO2–NH2and Fe3O4@SiO2–MWCNTs showed that the as-prepared materials were stable below120°C and the whole weight loss for them was increased from2%to31%after the temperature elevated to800°C,further con?rming the successful coating of CNTs on the surface of Fe3O4@SiO2–NH2.

Fig.5shows the XRD pattern of the Fe3O4@SiO2–MWCNTs, MWCNTs and Fe3O4.The strong diffraction peaks indexed to (220),(311),(400),(422),(511)and(440)showed the cubic structure characteristics of Fe3O4.The diffraction peaks for MWCNTs can be indexed by the characteristic(002)and(100) re?ections of graphite[32].In XRD pattern of Fe3O4@SiO2–MWCNTs,a new peak located at26.08°and43.08°could be owned to the CNTs,indicating that core–shell microspheres have been successfully synthesized without damaging the crystal structure of Fe3O4core during silica and MWCNTs coating.

The magnetic property of Fe3O4@SiO2and Fe3O4@SiO2–MWCNTs were studied by measuring magnetization at25°C.As the results shown in Fig.6,the corresponding magnetization of both Fe3O4@SiO2and Fe3O4@SiO2–MWCNTs increased with an increase in the magnetic?elds.The saturation magnetization (Ms)of Fe3O4@SiO2and Fe3O4@SiO2–MWCNTs were85.1emu/g (Ms1)and47.7emu/g(Ms2),respectively.Both of them showed negligible remanence and coercivity and typical superparamagnet-ism characteristics[17].The saturation magnetization decreased indicated Fe3O4@SiO2nanoparticles were coated by MWCNTs.

L.Zhou et al./Chemical Engineering Journal257(2014)10–1913

The magnetic separation for Fe 3O 4@SiO 2(Fig.7a and b)was faster

than that of Fe 3O 4@SiO 2–MWCNTs (Fig.7c and d),further implying the successful attachment of MWCNTs onto Fe 3O 4@SiO 2.3.2.Effect of pH on adsorption

The value of pH is a vital factor for the adsorption process,as it in?uences greatly the speciations of PCP and the surface charge of Fe 3O 4@SiO 2–MWCNTs in solution.The surface charge of Fe 3O 4@SiO 2–MWCNTs depends on the solution pH and its pH pzc (zero point charge),the surface is positively charged at pH pH pzc .The adsorbate is mainly in

protonated from at pH

p K a .Fig.8a showed the relationship between pH and equilibrium uptake capacities (q e )of PCP.

As can be seen from Fig.8,the highest PCP uptake was achieved at pH 2.5,the adsorption capacity of PCP was found to slightly decrease with pH increasing from 2to 2.5,while dramatically decrease from 96.4to 52.1mg/g with pH in ranging from 2.5to 9.The adsorption curve was demonstrated with a similar trend by Luo et al.[33].The reason can be explained as follows:PCP is a weak acid compound (p K a =4.75)and exists as either a neutral (protonated)or an ionized (charged)form [33,34],the speciation diagram for PCP can be prepared using the following reaction [35].

PCP $PCP àtH t

e10T

(a) (b)

SEM images of (a)Fe 3O 4@SiO 2,(b)Fe 3O 4@SiO 2–MWCNTs;TEM images of (c).Fe 3O 4@SiO 2,(d)Fe 3O 4@SiO 22500200015001000500

Wavenumber (cm -1)

, -CH 3)

v(C=O)

(N-H)

v(Si-O-Si)

(CO-NH)

v(Fe-O-Fe)

δδ(b)Fe 3O 4@SiO 2–NH 2,(c)Fe 3O 4@SiO 2–MWCNTs.

Temperature (?T h e r m o g r a v i t y (%)

1000

20406080

100

200300400500Fig.4.TG analyses of:(a)Fe 3O 4,(b)Fe 3O 4@SiO 14L.Zhou et al./Chemical Engineering Journal 257(2014)10–19

L.Zhou et al./Chemical Engineering Journal257(2014)10–1915 Photographs of(a)Fe3O4@SiO2–MWCNTs dispersed in aqueous solution and magnetic separation of Fe3O4@SiO2–MWCNTs(b)and Fe

Hence,the highest absorption capacity for PCP was at pH2.5.With increased in pH,the degree of deprotonation of PCP increases gradually and the PCPàaccount for more than95%of PCP when the pH is above6[35].The pH zpc of absorbent was found to be 4.1.At pH>pH zpc,the absorbent surface is negatively charged. Electrostatic repulsion force exists between the PCPàand the absorbent surface,which resulted in decreasing of absorption capacity.The PCPàwould weaken the formation of hydrogen bond between the hydroxyl of PCP and the carboxyl groups on the surface of Fe3O4@SiO2–MWCNTs.As pH increases,the main format of amine groups of Fe3O4@SiO2–MWCNTs surface might be–NH2, which was bene?cial to form hydrogen bond between absorbent and PCP rather than PCPà,which might effect on decreasing of absorption capacity.

3.3.Adsorption kinetics studies

Kinetic studies were carried out and the experimental results are presented in Fig.8b.The curve can be divided into three portions,which could be described by intraparticle diffusion model and indicated that the intraparticle process might be one of the rate-limiting steps for PCP removal by Fe3O4@SiO2–MWCNTs.At the initial stage,a large number of vacant surface sites were available for adsorption and a high af?nity of the interacting groups on the surface of the Fe3O4@SiO2–MWCNTs.Therefore PCP was adsorbed quickly,and after a lapse of time,the remaining vacant surface sites were dif?cult to be occupied due to repulsive forces between the solute molecules on the solid phase and in the bulk liquid phases[36].The exterior surface of the adsorbent was reached saturation,and the adsorption was reached equili-brium.Similar trend was observed in other literatures[37,38]. Thus,30min equilibration time was selected in subsequent batch experiments.

Utilization of appropriate kinetic models can offer useful infor-mation for understanding the underlying absorption mechanisms. Form this point of view,two models including pseudo-?rst-order model and pseudo-second-order model were applied to simulate the experiment kinetic data of PCP on Fe3O4@SiO2–MWCNTs adsorbent.The results are shown in Fig.9and Table1.The pseudo-?rst-order model gave poor?tting with low correlation coef?cient(R2=0.230)values and the calculated q e(cal)(4.94mg/ g)were signi?cantly lower than the experiment q e(exp)(35.58mg/ g).However,the pseudo-second-order model is more likely to pre-dict the behavior over the whole range of adsorption.The value of

correlation coef?cient(R2=0.998)were almost equal to unity.The experiment q e(exp)and calculated q e(cal)(35.29mg/g)were close together at50mg/L,which shows good agreement.Therefore,we can conclude that the pseudo-second-order model is more favorable the adsorption process of PCP on Fe3O4@SiO2–MWCNTs, Similar observations have been reported in the adsorption of PCP on Coconut shell[38].

3.4.Adsorption isotherms

In order to optimize the use of Fe3O4@SiO2–MWCNTs,it is important to establish the most appropriate adsorption isotherm. Thus the Langmuir and Freundlich isotherms were used to describe the equilibrium adsorption.

The Langmuir isotherm is often applicable to a homogeneous adsorption surface with all the adsorption sites having equal adsorbate af?nity,while Freundlich isotherm assumed that as the adsorbate concentration increases,the concentration of adsorbate on the adsorbent surface also increases[39].The values n>1 represent favorable adsorption conditions.In most cases,the exponent between1

The adsorption isotherms of PCP on Fe3O4@SiO2–MWCNTs are shown in Fig.10and all the correlation coef?cient(R2)and the constants obtained from the two isotherm models are summarized in Table2.The results showed that the Freundlich isotherm could effectively describe the adsorption data with R2=0.949,suggesting a better?t of the Freundlich isotherm rather than Langmuir isotherm(R2=0.855).The value of n(2.78)obtained from the

Table1

Pseudo-?rst-order model,pseudo-second-order model equation constants and correlation coef?cients for adsorption of PCP on Fe3O4@SiO2–MWCNTs at25°C.

C0(mg/L)q e(exp)(mg/g)Pseudo-?rst-order model Pseudo-second-order model

q e(cal)(mg/g)k1(minà1)R2q e(cal)(mg/g)k2(g/mg min)R2 5035.58 4.940.020.23035.290.020.998 16L.Zhou et al./Chemical Engineering Journal257(2014)10–19

Freundlich model was above one,representing that adsorption of PCP on Fe 3O 4@SiO 2–MWCNTs was favorable.Hence,the surface of Fe 3O 4@SiO 2–MWCNTs is a heterogeneity adsorption surface.With the PCP concentration increase,the absorption capacity of absorbent surface also increases.The results agreed with the works carried out by previous researchers which reported that the Freundlich model gave a better ?t than the Langmuir model on the adsorption of PCP using different adsorbents,such as allophanic soil [41],coir pith carbon [42],black carbon [33].3.5.Thermodynamic studies

To examine the effect of temperature,adsorption experiments were carried out for the initial concentration of 50mg/L at

different temperatures.The thermodynamic parameters that must be considered to determine the adsorption processes were changes in standard enthalpy (D H h ),standard entropy (D S h ),standard free energy (D G h )due to transfer of unit mole of solute from solution onto the solid–liquid interface.The D H h and D S h are calculated from the slope and intercept of the linear plot of ln K C versus 1/T (Fig.11),the results are listed in Table 3.

The calculated values of D G h ,D H h ,and D S h for adsorption of PCP on Fe 3O 4@SiO 2–MWCNTs are shown in Table 3.The negative value of D H °(à6.14kJ/mol)indicates the adsorption process of PCP is exothermic in nature,which is the same as the reported result for PCP on Carbon black by Dominguez-Vargas et al.[43].In general,the adsorption enthalpy for the physical adsorption is

Table 2

Langmuir isotherm,Freundlich isotherm equation constants and correlation

coef?cients for adsorption of PCP on Fe 3O 4@SiO 2–MWCNTs at 25°C.C 0(mg/L)

Langmuir isotherm Freundlich isotherm Q 0(mg/g)

b (L/mg)R 2K F (mg 1à1/n L 1/n /g)n R 250

122.10

0.20

0.855

30.98

2.78

0.949

Table 3

Thermodynamic parameters for adsorption of PCP on Fe 3O 4@SiO 2–MWCNTs.

T (K)D G h (kJ/mol)D H h (kJ/mol)D S h (J/mol k)288à0.73à6.14

à18.65

298à0.62308à0.44318

à0.16

L.Zhou et al./Chemical Engineering Journal 257(2014)10–1917

usually in the range of0toà42kJ/mol,while for the chemical adsorption is in the range ofà42toà125kJ/mol[44].Hence,the adsorption of PCP could also be considered as a physical adsorption in this study.The negative value of D S h(à18.65J/mol k)shows the af?nity of the Fe3O4@SiO2–MWCNTs for PCP in aqueous solution and may imply some structural changes in the adsorbate and adsorbent during the absorption process[36].The D G h of PCP is negative value,which indicates the process is spontaneous in nature and the spontaneity decreases as the temperature increased from15to45°C,which is in agreement with the experiment observation by Cea et al.[7].Likewise,the net value of D G h is between0and20kJ/mol.The results of the absorption of PCP at four experimental temperatures indicate that the adsorption process is spontaneous and feasible.

3.6.Adsorption mechanism

Three mechanisms had been proposed to interpret the adsorption behaviors of PCP on Fe3O4@SiO2–MWCNTs,based on the literatures,namely:p–p electron–donor–acceptor interaction, hydrophobic interaction,electrostatic interaction[45].

The p–p electron–donor–acceptor interaction was viewed as the primary mechanism for the enhanced adsorption of hydro-xyl-substituted aromatics(such as phenol)[46],which were supportive of their sorption af?nity[47].Lin et al.[47]result indicated that CNTs act as amphoteric adsorbents attracting both p-acceptors and p-donors to the surface.Hydroxyl is an electron-donating functional group[48]which can increase the p-donating strength of the host aromatic ring.Thus,carbon ring on the CNTs surface can form p–p bonds with hydroxyl-substituted aromatics [46],which increased the sorption af?nity of PCP on Fe3O4@SiO2–MWCNTs.In addition,the chlorine substituent plays an important role in absorption process.Chlorine is electron-withdrawing sub-stituent,which enhances the p–p interactions with the increase of the substituent number[8].PCP has?ve chlorine substituents in the molecule;therefore,the capacity of absorption of PCP is increased.

Besides,hydrophobic interaction was an important factor contributed to absorption process.The adsorbate with higher hydrophobicity in aqueous solution has stronger tendency to be adsorbed and retain on the carbon surface[8].The solubility and log K o/w(K o/w:n-Octanol/water partition coef?cient)of PCP are 0.014g/L and 5.01–5.86,respectively.PCP is a hydrophobicity ionizable organic compound and is easy to congregate on surface of adsorbent due to relatively low solubility;hence,the capacity of absorption PCP was enhanced.

The electrostatic interaction was another reason that in?uenced adsorption,which mainly depend on the solution pH and pH zpc of absorbent,has been argued in the pH study.The absorption mechanism is complex,expect aforementioned factors,other factors might be also involved,such as hydrogen bonding[47], p–p dispersion forces[49].3.7.Regeneration and reuse of Fe3O4@SiO2–MWCNTs

The regeneration and reuse of adsorbents are likely to be a key factor in evaluating their potential applications.Desorption experi-ments were performed in5%ammonia/methanol solution with the purpose of reusing the adsorbents,the percentage of desorption was as high as91.9%,indicating that PCP has a good desorption ability from the Fe3O4@SiO2–MWCNTs.The results of four consecu-tive adsorption–desorption cycles were shown in Fig.12.It can be seen that the adsorption capacity of PCP on as-prepared adsorbent slightly decreased after every cycle of adsorption–desorption process.Therefore,Fe3O4@SiO2–MWCNTs was ef?cient and stable adsorbent for PCP removal.

https://www.wendangku.net/doc/0b11724146.html,parison with others adsorbents

A comparison of the maximum adsorption capacities of PCP on Fe3O4@SiO2–MWCNTs with other adsorbents in the literatures is shown in Table4.It can be seen that Fe3O4@SiO2–MWCNTs has a relatively high adsorption capacity than other adsorbents.It shows that the Fe3O4@SiO2–MWCNTs adsorbent is a suitable promising material for PCP removal from aqueous solutions.

4.Conclusions

A novel Fe3O4@SiO2–MWCNTs adsorbent has been prepared for the removal of PCP from aqueous solution.The factors of pH,con-tact time,initial concentration and temperature were studied.The adsorption kinetics was better described by pseudo-second-order kinetic model.The absorption capacity of the absorbent towards PCP was?tted well with the Freundlich model.Different thermo-dynamic parameters were obtained from the absorption process revealed that adsorption process was found to be exothermic. The negative values of D H h(à6.14kJ/mol),D S h(à18.65J/mol K) and D G h(à0.73toà0.16kJ/mol)show that the adsorption of PCP on Fe3O4@SiO2–MWCNTs is physical,random and spontaneous in nature within the experiment temperatures.

Acknowledgements

We would like to thank the National Natural Science Founda-tion of China(No.21377114),Ningbo Natural Science Foundation of China(Nos.2013A610242,2014A610283and2013A610243), Zhejiang Provincial Natural Science Foundation of China (LY12H26003,LY13B050003,LY14B070004),the Advanced Key Program of Agriculture and Social Development Funds of Ningbo, China(No.2011C11021),the Constructive Major Project for the Department of Health and Family Planning Commission–Zhejiang Province(2014PYA019)and Zhejiang Provincial Program for the Cultivation of High-level Innovative Health Talents for their ?nancial support of this research.

Table4

The maximum adsorption capacities(q max)of various adsorbents for PCP.

Adsorbent pH Isotherms q max(mg/g)K F(mg1à1/n L1/n/g)n Refs.

Fe3O4@SiO2–MWCNTs 2.5Freundlich127.4a30.98 2.78This study Flake-type chitosans 6.2Freundlich–0.40 1.57[50] Rhizopus oryzae ENHE4Freundlich–0.94 1.26[51] Coconut shell activated carbon–-Freundlich72.779.21b 3.12[38] Coir pith carbon–Langmuir 3.7––[42] Almond shell–Freundlich–0.080.53[52]

‘‘–’’:Not given.

a The adsorption capacity of Fe

O4@SiO2–MWCNTs with PCP initial concentrations100mg/L.

b(L/g)1/n.

18L.Zhou et al./Chemical Engineering Journal257(2014)10–19

Appendix A.Supplementary data

Supplementary data associated with this article can be found,in the online version,at https://www.wendangku.net/doc/0b11724146.html,/10.1016/j.cej.2014.07.060. References

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