Chemical Engineering Journal 316 (2017) 893–902

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Chemical Engineering Journal

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Efficient removal of organic pollutants from aqueous media using newlysynthesized polypyrrole/CNTs-CoFe2O4 magnetic nanocomposites

http://dx.doi.org/10.1016/j.cej.2017.02.0371385-8947/� 2017 Elsevier B.V. All rights reserved.

⇑ Corresponding author.E-mail address: lixiaoli@lzu.edu.cn (X. Li).

Xiaoli Li a,⇑, Haijun Lu a, Yun Zhang b, Fu He b

aGansu Key Laboratory for Environmental Pollution Prediction and Control, College of Earth and Environmental Sciences, Lanzhou University, Lanzhou 730000, Chinab Institute of Biochemical Engineering & Environmental Technology, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China

h i g h l i g h t s

� Meso-CNTs-CoFe2O4@PPy magnetic composites were synthesized.� CNTs-CoFe2O4@PPy exhibited excellent adsorption capability and fast kinetics for anionic dyes.� CNTs-CoFe2O4@PPy is an efficient heterogeneous catalyst for activation of PMS.� CNTs-CoFe2O4@PPy/PMS could completely degrade cationic Methylene Blue within a very short time.� CNTs-CoFe2O4@PPy had high adsorption capacity, magnetic separation, catalytic ability and good reusability.

a r t i c l e i n f o

Article history:Received 25 November 2016Received in revised form 6 February 2017Accepted 6 February 2017Available online 8 February 2017

Keywords:CNTs-CoFe2O4@PPyOrganic dye removalAdsorptionCatalytic degradation

a b s t r a c t

In this study, the polypyrrole/CNTs-CoFe2O4 magnetic nanohybrid (CNTs-CoFe2O4@PPy) was preparedand then used as adsorbent and catalyst to remove anionic and cationic dyes. The hybrid material wascharacterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-raydiffraction (XRD), elemental analysis, BET surface area, Fourier transform infrared (FTIR) spectroscopyand vibrating sample magnetometer (VSM). Adsorption behaviors of the as-prepared composites foranionic dyes such as Methyl Blue (MB), Methyl Orange (MO) and Acid Fuchsin (AF) were conductedwhere the effects of solution pH, initial dye concentration, contact time and ionic strength were system-atically studied. The equilibrium adsorption data of MB, MO and AF were well fitted to the Langmuir iso-therm model, yielding the maximum monolayer adsorption capacity of 137.00, 116.06 and 132.15 mg/g,respectively. Adsorption kinetics was best described by the pseudo-second order model. The introductionof CoFe2O4 into CNTs-CoFe2O4@PPy hybrid could not only provide an effective magnetic separation per-formance, but also act as catalyst to activate peroxymonosulfate (PMS) for the degradation of cationicdye, Methylene Blue (MEB), from aqueous solution. The results showed that CNTs-CoFe2O4@PPy pre-sented higher catalytic activity, better stability and reusability for the decolorization of MEB, resultingin an almost complete removal of 50 mg/L MEB after 30 min with 1.0 g/L catalyst and 4 mmol/L PMS.The characteristics of high adsorption capacity, effective separation, excellent catalytic activity and goodreusability would make CNTs-CoFe2O4@PPy a promising candidate for wastewater treatment.

� 2017 Elsevier B.V. All rights reserved.

1. Introduction

Azo, anthraquinone and triphenylmethane dyes are the mostimportant classes of synthetic dyes due to their low cost, brightcolor and high stability, which are widely used in textile dyeing,paper, plastic, printing, publishing, clothing, leather, pharmaceuti-cal, food, cosmetic and other industries and are therefore commonindustrial pollutants. Owing to their stable and complex molecular

structure, most synthetic dyes are difficult to be biodegraded andhave posed a great challenge for decolorization and complete min-eralization. The discharge of these dyes into the receiving waterbody is extremely undesirable because they not only affect the aes-thetic merit and water transparency, but also reduce dissolvedoxygen concentration to cause anoxic conditions and subsequentlypose a threat to the aquatic life [1]. More importantly, these syn-thetic dyes exhibit high biotoxicity and may accumulate to toxiclevels under certain environmental conditions, causing carcino-genic and teratogenic effects on the public health [2,3].

894 X. Li et al. / Chemical Engineering Journal 316 (2017) 893–902

Therefore, efficient removal of synthetic dyes from aqueousenvironment has attracted considerable attention. Up to now, var-ious treatment techniques and processes such as coagulation/floc-culation, chemical oxidation, photocatalytic degradation,membrane separation, adsorption and biological treatment havebeen developed to remove both organic and inorganic compoundsfrom contaminated water. Among these methods, adsorption hasbeen proved to be one of the most attractive and widely usedmethods due to its special advantages including high efficiency,low cost, simple design, easy operation, reusing property andsludge-free process. A number of adsorptive materials includingactivated carbon, clay minerals, agricultural/industrial waste orbyproducts, natural/synthesized polymeric materials, etc. [4–9]have been reported for the removal of synthetic dyes from water.In view of pollutant removal process, the development of low-cost, effective and renewable adsorptive materials with largeadsorption capacity, high removal rate, rapid kinetics and goodselectivity is very necessary and important.

In recent years, carbon nanotubes (CNTs) have received greatattention due to their unique characters such as large specific sur-face area, hollow and layered nanosized structures, high mechani-cal strength and good chemical stability, which have been used as apromising adsorbent for the removal of organic and inorganic com-pounds [10–12]. In order to achieve highly effective compositeswith the enhanced adsorption capacity and selectivity, chemicalmodification of CNTs using organic polymers containing highlyreactive atoms can introduce different kinds of functional groupswhich can react with target pollutants through physical and chem-ical forces. Calcium alginate/carbon nanotubes, tannic acid/carbonnanotubes, chitosan/carbon nanotubes have been reported for theremoval of pollutants from water [13–15]. Polypyrrole (PPy) hasdrawn the attention of researchers for the potential applicationin supercapacitor [16], DNA extraction [17], microwave absorption[18] and pollutant removal [19] because of its high electrical con-ductivity, environmental stability and facile synthesis at low cost.Anions are required to maintain charge balance or electroneutral-ity of the polymer matrix during chemical polymerization of pyr-role, among which FeCl3 is commonly used as oxidant, Cl� ion isincorporated as the counter ion into the polymer matrix [20].Therefore, PPy exhibits excellent anion-exchange property andhas positively charged nitrogen atoms in the polymer chains[21], which have made it a promising material for the removal ofanionic pollutants from aqueous solution. Bhaumik et al. synthe-sized PPy/Fe3O4 magnetic nanocomposites via in situ polymeriza-tion of pyrrole monomer with a high performance for theremoval of Cr(VI) ions from aqueous solutions [22]. Xin et al. pre-pared the polypyrrole (PPy) nanofibers for the removal of a typicalanionic dye (methyl orange, MO) from aqueous solution andresults showed that the PPy nanofibers possessed excellent adsorp-tion capacity for MO [23].

However, there exist several defects in the use of nano-sizedCNTs, such as the serious aggregation, difficult separation, massloss during regeneration, which seriously restrict its practicalapplication in wastewater treatment field. In this regard, by com-bining CNTs and magnetic materials, novel types of magnetic com-posites that possess dual functionalities including the desirableadsorption performance and effective magnetic separation can beexpected. Cobalt ferrite (CoFe2O4) particles with unique propertieshave emerged as promising candidates for their potential applica-tion in a wide range of fields such as catalysis [24], supercapacitor[25], biomedicine [26,27], and environmental remediation [28,29].

In this study, CNTs-CoFe2O4@PPy nanocomposites were synthe-sized through a chemical oxidative polymerization of pyrrole in thepresence of CNTs-CoFe2O4. The resulting organic-inorganic mag-netic composites would be expected to display new physical andchemical properties and possess relatively high adsorption perfor-

mances, which make the composites exhibit synergic or comple-mentary behaviors. Another important reason for theintroduction of CoFe2O4 is that CoFe2O4 particles exhibit excellentcatalytic activity. Transition metal ions (Co2+, Fe2+, Mn2+, Ni2+, etc.)can activate peroxymonosulfate (PMS) to generate sulfate radicals(SO4

��) and hydroxyl radicals (�OH) for efficient degradation oforganic pollutants [30,31]. Among which Co2+ is found to be themost effective catalyst of PMS activation for the production of sul-fate radicals [32]. Adsorption technique only can reduce the con-centration of target pollutant in liquid phase by transferring thetarget pollutant from aqueous solution to solid phase (onto thesurface of adsorbent). Actually there is no change in structureand chemical composition of target pollutant. In contrast,advanced oxidation processes (AOPs) based on the generation ofhighly reactive radicals such as SO4

��, �OH, O2�� and �OOH can achieve

high detoxification and TOC removal, which have been extensivelystudied for the removal of non-biodegradable organic pollutants. Inthis work, magnetic polymeric nanocomposites were prepared bycombining CNTs, PPy and CoFe2O4 through a two-step syntheticmethod. CoFe2O4 inside hybrid material could not only providean effective magnetic separation performance, but also act as cat-alyst to activate peroxymonosulfate (PMS) to oxidatively degradesynthetic dyes in water. Thus the resulting CNTs-CoFe2O4@PPywould possess adsorptive, magnetic and catalytic triple functional-ities, and make it an excellent candidate to remove organic pollu-tants. In addition, the stability of catalyst could be increased byimmobilizing CoFe2O4 particles within the matrix of PPy polymerand CNTs. Adsorption performances of as-prepared CNTs-CoFe2O4@PPy for anionic dyes, namely Methyl Blue (MB), MethylOrange (MO) and Acid Fuchsin (AF), from aqueous solution werecarried out as a function of solution pH, initial dye concentration,contact time and ionic strength. The experimental data were corre-lated by Langmuir and Freundlich isothermmodels. The adsorptionrates were determined and described by the pseudo-first order andpseudo-second order kinetic models. Further, the catalytic degra-dation of cationic dye, methylene blue (MEB), in water withCNTs-CoFe2O4@PPy as a heterogeneous catalyst of PMS was alsoinvestigated.

2. Materials and methods

2.1. Materials and chemicals

Multi-walled carbon nanotubes (CNTs) were purchased fromChengdu Organic Chemical Co. Ltd., Chinese Academy of Sciences(China). The detailed parameters supplied by the manufacturerare as follows: length: 10–30 lm; inner diameter: 2–5 nm; outerdiameter: <8 nm; purity: >95%; SSA: >350 m2/g. Pyrrole wasobtained from Shanghai Kefeng Industry&Commerce Co., Ltd(Shanghai, China). PMS available as a triple salt of the sulfate com-mercially known as oxone (KHSO5�0.5KHSO4�0.5K2SO4) wasobtained from Shanghai Aladdin Industrial Corporation (Shanghai,China). Anhydrous FeCl3, CoCl2�6H2O, sodium acetate (NaAc), ethy-lene glycol (EG) and other chemicals were of analytical grade andused as received without further purification. Stock solutions ofMB, MO, AF and MEB (1000 mg/L) were respectively prepared bydissolving MB, MO, AF and MEB in distilled water.

2.2. Preparation of CNTs-CoFe2O4@PPy magnetic composites

CNTs-CoFe2O4 nanomaterials were synthesized by a one-potsolvothermal method. In a typical experiment, 15 mmol of anhy-drous FeCl3, 7.5 mmol of CoCl2�6H2O and 1.33 g of CNTs wereadded into 70 mL of EG under ultrasonic dispersion, followed bythe addition of 7.2 g NaAc. The mixture was stirred vigorously for

X. Li et al. / Chemical Engineering Journal 316 (2017) 893–902 895

90 min under sonication and then transferred into a sealed Teflon-lined autoclave for thermal treatment at 200 �C for 8 h. After theautoclave was allowed to cool down to room temperature, the pre-cipitate was separated with a strong magnet, washed with distilledwater and absolute ethanol to remove residual reagents, andfinally dried at 70 �C for further use.

For the synthesis of CNTs-CoFe2O4@PPy nanocomposites, thetypical preparation process was described as follows. 2.5 g ofCNTs-CoFe2O4 were added into a three-neck flask with the capacityof 250 mL containing 100 mL of distilled water. The mixture wasstirred under ultrasonic treatment for 60 min, followed by additionof 8.0 mL of pyrrole. After stirring for 2 h, a known amount of FeCl3(2.0 g), which was previously dissolved in 50 mL of distilled water,was slowly added into the above mixture. The polymerizationreaction was maintained at room temperature for 24 h with con-stant stirring. The precipitate was magnetically collected, washedwith distilled water, and finally dried at 70 �C. The synthesis routeof CNTs-CoFe2O4@PPy was presented in Scheme 1.

2.3. Characterization

The morphology and surface characteristics of CoFe2O4, CNTs,CNTs-CoFe2O4 and CNTs-CoFe2O4@PPy were observed with a JEOLJSM-6701F scanning electron microscopy (SEM). Transmissionelectron microscopy (TEM) images were performed with a TECNAIG2 TF 20 transmission electron microscope. Elemental analysis wasperformed with a PerkinElmer 2400 CHN analyzer. The Brunauer–Emmett–Teller (BET) specific surface area was determined at 76 Kby the N2 adsorption–desorption technique using a MicromeriticsChemisorb 2750 surface area analyzer. Pore volume and pore sizewere determined by the Barrett–Joyner–Halenda (BJH) method.The crystal structure of CNTs, CoFe2O4, CNTs-CoFe2O4 and CNTs-CoFe2O4@PPy were examined by the X-ray diffraction (XRD, RigakuD/MAX-2400 X-ray diffractometer with Ni-filtered Cu Ka radia-tion). Fourier transform infrared spectrum (FTIR) of CNTs-CoFe2O4@PPy was collected in KBr pressed pellets on a NicoletNEXUS 670 infrared spectrometer. The FTIR spectrum wasrecorded over the range of 400–4000 cm�1. Magnetic properties

CNTs

Co2+

Co2+

Co2+

Fe3+Fe3+

Fe3+

Fe3+ Fe3+

Fe3+

200oC 8hEG, NaAC

NH

CNTs-CoFe2O4

SO4-

CO2+H2O OH

Dye

Adsorption

Catalytic degradation

Intermediates

Scheme 1. Synthesis route of CNTs-Co

of CoFe2O4, CNTs-CoFe2O4 and CNTs-CoFe2O4@PPy nanocompos-ites were analyzed using a vibrating sample magnetometer (VSM,LAKESHORE-7304, USA) at room temperature.

2.4. Batch adsorption experiments

Batch adsorption experiments were carried out in Erlenmeyerflasks on a temperature controlled shaker with a shaking speedof 200 rpm at 20 �C. 0.025 g of CNTs-CoFe2O4@PPy were added into25 mL of dye solutions with the initial dye concentrations rangingfrom 50 to 500 mg/L at natural pH. The flasks were shaken for 12 h.

The effect of initial pH was carried out by dispersing 0.025 g ofCNTs-CoFe2O4@PPy into 25 mL of 50 mg/L dye solutions with dif-ferent pH values and stirred for 12 h. The initial pH values of dyesolutions were adjusted to the required values in the range of2.0–9.0 with 1.0 mol/L HCl or 1.0 mol/L NaOH solutions.

To investigate the adsorption rate of three dyes, 0.1 g of CNTs-CoFe2O4@PPy was added into 100 mL of dye solutions with initialconcentration of 100 mg/L at natural pH. The obtained mixturewas shaken at room temperature. The suspension samples weretaken out at specific time intervals and filtered for the determina-tion of dye concentration.

Effect of ionic strength on adsorption capacity of CNTs-CoFe2O4@PPy was investigated by changing the initial concentra-tion of NaCl from 0.2 to 2.0 mol/L in dye solutions; the initial dyeconcentration was fixed at 100 mg/L and the adsorbent dosagewas fixed at 1.0 g/L. The mixture was shaken in a temperature-controlled shaker at 20 �C for 12 h. The residual concentration ofdye was determined by using a UNICAM UV300 spectrophotome-ter (Thermo Spectronic, USA).

The amount of dye adsorbed per unit mass of the adsorbent, Qe

(mg/g), was calculated by the following equation:

Qe ¼ðC0 � CeÞV

mð1Þ

where C0 (mg/L) is the initial dye concentration in the solution; Ce(mg/L) is the final or equilibrium concentration of dye in the solu-

FeCl3

Dye

NN NH H

HN

n

H

CNTs-CoFe2O4@PPy

PMS

CoFe2O4

Fe2O4@PPy magnetic composites.

896 X. Li et al. / Chemical Engineering Journal 316 (2017) 893–902

tion; V (L) is the volume of the solution; and m (g) is the weight ofthe adsorbent used.

The percentage removal of dye (R, %) was calculated using thefollowing equation:

Rð%Þ ¼ C0 � Ce

C0� 100 ð2Þ

2.5. Catalytic degradation experiments

Batch degradation experiments of MEB were carried out in atemperature controlled shaker at 20 �C with a shaking speed of200 rpm using 250 mL Erlenmeyer flasks. CNTs-CoFe2O4@PPy(1.0 g/L) were firstly dispersed into 100 mL of dye aqueous solu-tion. Then, a required amount of PMS in the form of Oxone wasadded into dye solution to initiate the reaction. The solution sam-ples were taken at given time intervals, filtered immediatelythrough a 0.22 lm filter film, and quenched with 1 mL anhydrousethanol. The initial concentration of MEB solution was varied from50 to 500 mg/L. The pH of dye solution was not adjusted and con-trolled during the decolorization. The residual concentration ofMEB was monitored at the maximum absorption wavelength.

To determine the radical species generated in the catalyst/PMS/MEB system, two kinds of quenching agents, tert-butyl alcohol(TBA) is widely used as a quenching agent for hydroxyl radicals,while ethanol (EtOH) is an effective scavenger of hydroxyl and sul-fate radicals. Prior to the addition of CNTs-CoFe2O4@PPy and PMS,a specified amount of EtOH or TBA was added into the MEB solu-tion with an initial dye concentration of 50 mg/L.

To test the reusability and catalytic activity, the used catalystwas collected with a magnet, washed with water, and dried at70 �C after the degradation reaction. The recycled catalyst wasre-dispersed into the fresh dye solution, and the MEB degradationwas re-initiated by adding the same amount of PMS under similarexperimental conditions. The recycling and degradation experi-ments of catalyst were repeated five times.

3. Results and discussion

3.1. Characterization of CNTs-CoFe2O4@PPy hybrid material

The size and morphology of CoFe2O4, CNTs, CNTs-CoFe2O4 andCNTs-CoFe2O4@PPy were investigated by SEM. As depicted inFig. 1a, the as-prepared CoFe2O4 appeared to be spherical in shapewith the size of about 100–300 nm. The pure CNTs appeared to betubular in shape, and the long nanotubes twisted together (Fig. 1b).It can also be seen that CoFe2O4 submicrospheres were uniformlydispersed on the network structure of CNTs (Fig. 1c). However,the SEM image of the CNTs-CoFe2O4@PPy displayed in Fig. 1dshowed that the hybrid material had the coarse surface and irreg-ular shapes consisting of a large number of clusters. These may beascribed to the fact that CoFe2O4 submicrospheres and CNTs werecoated with a polymer layer and formed a large number of aggre-gates, which were in good agreement with the morphologyobserved from the TEM image. As shown in Fig. 1e and f, the diam-eter of CNTs in CNTs-CoFe2O4@PPy composites increased, and itssurface became more rough in comparison with those of bareCNTs, indicating that a thin polymer layer with a thickness of sev-eral nanometers was coated on the surface of CNTs. Thus CNTs-CoFe2O4@PPy composites formed a three-dimensional networkstructure where a large number of pores with different sizesbetween the aggregates were clearly observed, which would facil-itate molecular diffusion and provide sufficient space for theadsorption of target pollutants from water.

The C, H and N compositions of CNTs-CoFe2O4@PPy obtainedfrom elemental analysis was 43.28%, 1.05% and 4.84%, respectively.This result confirmed the component of the synthesized CNTs-CoFe2O4@PPy and therefore the successful preparation of CNTs-CoFe2O4@PPy magnetic composites. The specific surface area, porevolume and pore size of CoFe2O4, CNTs-CoFe2O4 and CNTs-CoFe2O4@PPy were determined. The surface area calculated fromthe BET equation was 83.81 m2/g for CoFe2O4 particles, whichwas much lower than those of CNTs-CoFe2O4 and CNTs-CoFe2O4@PPy (119.82 and 150.41 m2/g, respectively). Pore size dis-tribution curves were estimated from the adsorption isothermsusing BJH analysis. The results showed that CoFe2O4 had a totalpore volume of 0.20 cm3/g and a pore size of 8.99 nm. The corre-sponding values increased to 0.42 cm3/g and 12.75 nm for CNTs-CoFe2O4, and 0.43 cm3/g and 12.71 nm for CNTs-CoFe2O4@PPy.Such a significant increase of textural properties may be con-tributed to the introduction of CNTs and PPy, which made the sur-face of the composites more loose and rough. Hence, CNTs-CoFe2O4@PPy composites with high surface area and mesoporousstructure would provide more available surface sites for the hybridmaterial-dyes, reduce the mass transfer resistance, consequentlyleading to enhanced adsorption performance and catalytic activity.

The XRD patterns of CNTs, CoFe2O4, CNTs-CoFe2O4 and CNTs-CoFe2O4@PPy were shown in Fig. 2a. The distinct peak at2h = 26.1�was a characteristic diffraction peak of CNTs. The diffrac-tion peaks at 2h values of 30.2�, 35.4�, 43.3�, 57.0� and 62.7� couldbe assigned to the (220), (311), (400), (511) and (440) planes ofthe cubic CoFe2O4 with spinel structures, respectively [33]. Thesediffraction peaks of CoFe2O4 were also observed in the CNTs-CoFe2O4@PPy composites, suggesting that the CoFe2O4 submicro-spheres were successfully immobilized in the CNTs-CoFe2O4@PPynanocomposites. However, no diffraction peak attributed to PPypolymer was observed due to the amorphous structure of PPypolymer.

FTIR was employed to examine the surface functional groups ofCNTs-CoFe2O4@PPy hybrid material. As shown in Fig. 2b, distinc-tive adsorption peak at 600 cm�1 corresponded to metal-oxygen(M-O) stretching vibration. The adsorption peaks centered at1551 cm�1 and 1471 cm�1 arose from the C@C stretching vibrationof the pyrrole ring, the peak around 1302 cm�1 was related to C-Nstretching vibration [21,34,35]. The other peaks located at1182 cm�1, 1092 cm�1, 1040 cm�1, 914 cm�1 and 784 cm�1 wereattributed to in-plane and out-of-plane C–H and N–H bendingvibrations, indicating the formation of PPy by the monomer [34].Thus, the enhanced adsorption capability of the CNTs-CoFe2O4@PPy would be expected because of the introduction ofnumerous functional groups on the adsorbent surface.

The magnetic hysteresis loops were measured with a VSM toinvestigate the magnetic properties of CoFe2O4, CNTs-CoFe2O4

and CNTs-CoFe2O4@PPy and the results were shown in Fig. 3. Themaximal magnetization saturation value of CNTs-CoFe2O4@PPycomposites reached to 25.93 emu/g, which was lower than thoseof CoFe2O4 and CNTs-CoFe2O4 composites (80.35 and 38.99 emu/g, respectively). The decrease in the magnetization saturationmay be due to the introduction of non-magnetic PPy polymerand CNTs in the nanocomposites. CNTs-CoFe2O4@PPy compositeswith a relatively strong magnetic response to the magnetic fieldcould be easily and fleetly separated from the aqueous solutionwithin a short time by applying a permanent magnet near the glassbottle (inset in Fig. 3). CNTs-CoFe2O4@PPy composites with highmagnetization saturation would provide a fast and cost-effectiveway for the separation and recovery of CNTs-CoFe2O4@PPy com-posites and make the application of CNTs-CoFe2O4@PPy compos-ites in water pollution remediation simpler and more efficient.

Fig. 1. SEM images of CoFe2O4 (a), CNTs (b), CNTs-CoFe2O4 (c) and CNTs-CoFe2O4@PPy (d); TEM images of CNTs (e) and CNTs-CoFe2O4@PPy (f).

0 20 40 60 800

600

1200

1800

Inten

sity (

a.u.)

2 Theta (degree)

CNTs CoFe2O4

CNTs-CoFe2O4

CNTs-CoFe2O4@PPy

4000 3500 3000 2500 2000 1500 1000 500

Tran

smitta

nce(

%)

Wavenumber(cm-1)

600

784

914

10401182

1302

1471

1551

3379

a b

Fig. 2. XRD patterns of CNTs, CoFe2O4, CNTs-CoFe2O4 and CNTs-CoFe2O4@PPy (a); FTIR spectrum of CNTs-CoFe2O4@PPy (b).

X. Li et al. / Chemical Engineering Journal 316 (2017) 893–902 897

Fig. 3. Magnetization curves of CoFe2O4, CNTs-CoFe2O4 and CNTs-CoFe2O4@PPy.

898 X. Li et al. / Chemical Engineering Journal 316 (2017) 893–902

3.2. Adsorption of the dyes

3.2.1. Effect of initial dye concentration on dye adsorptionThe adsorption capacities of MB, MO and AR increased with the

increase in initial dye concentration from 50 to 500 mg/L (Fig. 4),and the maximum adsorption capacities were determined as137.00, 116.06 and 132.15 mg/g, respectively. This can beexplained by the fact that the higher initial dye concentration,the greater driving force to overcome the mass transfer resistanceof dye molecules between the aqueous and solid phases, so higheradsorption capacities were obtained at higher initial dye concen-tration. However, the percentage removal of three dyes decreasedwith increasing of initial dye concentration. At a given adsorbentdosage, the limited number of available adsorption sites allowedthat only part of dye molecules could be adsorbed. Therefore, thepercentage of three dyes removal was higher at lower initial dyeconcentration but lower at higher initial dye concentration.

Different types of dyes including positively charged MethyleneBlue (MEB), Crystal Violet (CV) and Malachite Green (MG) wereadopted as adsorbates to assess the adsorption ability of the as-prepared CNTs-CoFe2O4@PPy. Based on the removal rate (Fig. 4),it was found that CNTs-CoFe2O4@PPy exhibited relatively lowaffinity for positively charged MEB, MG and CV, and the removalefficiencies of 8.5%, 7.9% and 6.5% were achieved at initial dye con-centration of 200 mg/L, respectively. CNTs-CoFe2O4@PPy showedmore specific adsorption ability toward anionic dyes than cationicdyes, which may be attributed to the difference in molecular struc-

0 100 200 300 400 5000

20

40

60

80

100

120

140

MB MO AR MEB MG CV0

10

20

30

40

50

60

Remo

val(%

)

Cationic and anionic dyes

Qe(m

g/g)

Initial concentration of dye (mg/L)

MB MO AF

Fig. 4. Effects of initial dye concentration on dye adsorption onto CNTs-CoFe2O4@-PPy. The inset showed the adsorption performances of anionic and cationic dyesonto CNTs-CoFe2O4@PPy.

ture of organic dyes, leading to the different adsorption mecha-nisms between dye molecules and adsorbent. MEB, MG and CVpossess positive groups like -N+, resulting in stronger electrostaticrepulsion between positively charged dyes and positively chargednitrogen atoms in PPy chains, thus the removal rates for MEB, MGand CV were very low. So CNTs-CoFe2O4@PPy could adsorbstrongly the negatively charge organic dyes than positivelycharged dyes, indicating a selective adsorption behavior.

3.2.2. Effect of initial pH on dye adsorptionThe effect of initial pH value on the adsorption of MB, MO and

AR onto CNTs-CoFe2O4@PPy was investigated in the pH range of2.0–9.0. As can be seen in Fig. 5, the solution pH in the range of2.0–9.0 had little impact on the adsorption of MB and AR. It illus-trated that CNTs-CoFe2O4@PPy exhibited higher adsorption perfor-mances for MB and AR, more than 98% of MB and AR were removedunder the pH studied. While the adsorption of MO was highlydependent on solution pH. The adsorption capacity of MO ontoCNTs-CoFe2O4@PPy increased with the increase of solution pHfrom 2.0 to 3.0, and changed slightly when solution pH was largerthan 3.0. These results showed that CNTs-CoFe2O4@PPy could beused for dye adsorption with high removal rate in a wide pH rangeof 3.0–9.0, where more than 97% of MB, MO and AR was removed.This can be explained by the following aspects: (i) CNTs-CoFe2O4

were encapsulated into PPy matrix. Thus, the surface of CNTs-CoFe2O4@PPy was positively charged due to incorporating by FeCl3as oxidant during the polymerization process, and these positivecharges could still remain present even at basic pH values[21,36]. While MB, MO and AR are three kinds of anionic dyesand possess negative -SO3

� groups, which could be easily adsorbedonto CNTs-CoFe2O4@PPy surface through electrostatic attraction;(ii) MB, MO and AR with more than one benzene ring in their struc-ture having p electrons and therefore p-p stacking interactioncould be formed between the aromatic backbone of the dye andthe hexagonal skeleton of CNTs. These results indicated thatCNTs-CoFe2O4@PPy was quite effective in removing anionic dyesin aqueous solution over a wide pH range, which was an importantadvantage for the practical application in anionic dyes adsorptionby CNTs-CoFe2O4@PPy.

3.2.3. Effect of contact time and ionic strength on dye adsorptionFrom an economical point of view, the contact time required to

reach adsorption equilibrium is an important parameter to evalu-ate the adsorption ability of CNTs-CoFe2O4@PPy. Fig. 6 showedthe effect of contact time on the adsorption capacities of MB, MOand AR onto CNTs-CoFe2O4@PPy. It was found that the adsorptioncapacities of MB, MO and AR onto CNTs-CoFe2O4@PPy significantly

1 2 3 4 5 6 7 8 9 10

36

38

40

42

44

46

48

50

Qe(m

g/g)

Solution pH

MB MO AR

Fig. 5. Effect of initial pH on dye adsorption onto CNTs-CoFe2O4@PPy.

0 100 200 300 400 5000

102030405060708090

100Q

t(mg/

g)

t(min)

MB MO AR

Fig. 6. Effect of contact time on dye adsorption onto CNTs-CoFe2O4@PPy.

X. Li et al. / Chemical Engineering Journal 316 (2017) 893–902 899

increased at the initial stage and then increased at a slow speedwith the increase in contact time. This phenomenon could beexplained by the fact that a large number of vacant active siteson the surface of CNTs-CoFe2O4@PPy were available for dye mole-cules at the initial stage, which was favorable for more dye mole-cules to be adsorbed by CNTs-CoFe2O4@PPy. As adsorptionprocess proceeded, gradual occupancy of these surface sites sloweddown the adsorption rate and the adsorption became less efficient.

It can be seen that an increase in ionic strength from 0.2 to2.0 mol/L did not significantly affect the adsorption of MB ontoCNTs-CoFe2O4@PPy (Fig. 7). However, the increase of MO and ARremoval from 62.0% to 86.5% and 70.0% to 81.7% was respectivelyobserved. Achievement of a high removal rate at a relatively highionic strength implied that CNTs-CoFe2O4@PPy exhibited highaffinity and suitability for the removal of anionic dyes from water.

3.2.4. Adsorption isothermsAdsorption isotherms are commonly used for the description of

how the target pollutants interact with active sites on the adsor-bent surface. Langmuir isotherm is based on the assumption thata monolayer coverage occurs on the homogeneous adsorbent sur-face where all the adsorption sites have equal affinity. While theFreundlich isotherm is often applicable to a multilayer adsorptionthat occurs on the energetically heterogeneous adsorbent surface.Langmuir and Freundlich isotherm models can be represented inthe linear form as follows [37,38]:

0.0 0.4 0.8 1.2 1.6 2.00

20

40

60

80

100

Qe(m

g/g)

Ionic strength(mol/L)

MB MO AR

Fig. 7. Effect of ionic strength on dye adsorption onto CNTs-CoFe2O4@PPy.

Ce

Qe¼ 1

KLQmaxþ Ce

Qmaxð3Þ

lnQe ¼ lnKF þ 1nlnCe ð4Þ

where Qmax is the maximum adsorption capacity of the adsorbentcorresponding to complete monolayer coverage of dye on the sur-face (mg/g). KL is the Langmuir isotherm constant (L/mg), which isrelated to the free energy of adsorption. KF and 1/n are characteristicconstants of Freundlich isotherm, respectively, which represent theadsorption capacity and adsorption intensity.

The equilibrium adsorption data of three dyes onto CNTs-CoFe2O4@PPy were analyzed using the Langmuir and Freundlichisotherm models, and the results were shown in Table 1. The R2

values calculated from Langmuir isotherm model for the adsorp-tion of MB, MO and AR onto CNTs-CoFe2O4@PPy were 0.9981,0.9990 and 0.9833, respectively, indicating that the Langmuir iso-therm model was more suitable for describing the adsorption pro-cess of three dyes onto CNTs-CoFe2O4@PPy. Based on the Langmuirisotherm model, the maximum adsorption capacities for MB, MOand AR, were found to be 136.99, 116.69 and 135.14 mg/g, respec-tively, which were very close to those obtained from the experi-mental data. This indicated the formation of monolayer coverageof MB, MO and AR on the surface of CNTs-CoFe2O4@PPy and theadsorption took place at specific homogeneous binding sites.

3.2.5. Adsorption kineticsIn order to further understand the mechanism of adsorption

process, two kinetic models including the pseudo-first order andpseudo-second order kinetic models were applied to describe theexperimental data. The pseudo-first order and pseudo-secondorder kinetic models can be expressed in the linear form as follows[39,40]:

lnðQe � QtÞ ¼ lnQe �k1

2:303t ð5Þ

tQt

¼ 1k2Q

2e

þ tQe ð6Þ

where Qe and Qt (mg/g) are the amounts of dye adsorbed at equilib-rium and at time t (min), respectively. k1 (min�1) and k2 (g mg�1 -min�1) are rate constants of the pseudo-first order and pseudo-second order, respectively.

The kinetic parameters and the correlation coefficients (R2) forthe adsorption of three dyes onto CNTs-CoFe2O4@PPy were calcu-lated by linear regression (Fig. 8) and the results were summarizedin Table 2. As shown in Table 2, the correlation coefficients (R2)were very low, and the equilibrium adsorption capacities calcu-lated from the pseudo first-order model were not fit well withthe experimental data. In contrast, the correlation coefficients(R2) of the pseudo-second order kinetic model for all the pollutantswere over 0.99, and the Qe,cal values determined from pseudo-second order kinetic model were very close to the experimentalvalues (Qe,exp). Therefore, it can be concluded that pseudo-second

Table 1Langmuir and Freundlich isotherm parameters for organic dyes adsorption ontoCNTs-CoFe2O4@PPy.

Adsorption isotherms MB MO AR

Langmuir Qmax 136.99 116.69 135.14KL 0.14 0.21 0.04R2 0.9981 0.9990 0.9833Freundlich Kf 56.38 57.52 51.761/n 0.16 0.12 0.15R2 0.9435 0.9619 0.9399

0 100 200 300 400 500

0

1

2

3

4

5

6

7

MB MO AF

t/Qt

t0 100 200 300 400 500

-3

-2

-1

0

1

2

3

4

MB MO AF

ln(Q

e-Qt)

t

a b

Fig. 8. The pseudo-first-order (a) and pseudo-second-order (b) kinetic models for organic dyes adsorption onto CNTs-CoFe2O4@PPy.

Table 2Pseudo-first-order and pseudo-second-order model parameters for organic dyesadsorption onto CNTs-CoFe2O4@PPy.

Adsorption kinetic models MB MO AR

Pseudo-first order modelQe,cal 34.21 11.14 20.79k1 7.69 � 10�3 8.65 � 10�3 8.38 � 10�3

R2 0.9236 0.8598 0.9492

Pseudo-second order modelQe,cal 96.06 72.72 74.79k2 0.72 � 10�3 2.91 � 10�3 1.28 � 10�3

h 6.61 15.39 7.13R2 0.9992 0.9998 0.9994

900 X. Li et al. / Chemical Engineering Journal 316 (2017) 893–902

order kinetic model was better in describing the adsorption kinet-ics of three dyes onto CNTs-CoFe2O4@PPy hybrid composites.

3.3. Catalytic degradation studies

To examine the degradation kinetics and decolorization effi-ciency of MEB under different experimental conditions, 100 mg/LMEB was treated for 60 min by PMS alone, catalyst alone and cat-alyst/PMS process, respectively. As shown in Fig. 9a, in the absenceof CNTs-CoFe2O4@PPy, the addition of PMS (4.0 mmol/L) broughtabout 38% removal of MEB after 60 min. After coupling CNTs-CoFe2O4@PPy with PMS, the MEB removal was accelerated andenhanced significantly, yielding a higher degradation rate of 95%in 60 min. However, only 17% of MEB was adsorbed using CNTs-CoFe2O4@PPy as the adsorbent under the same conditions. Theseresults indicated that CNTs-CoFe2O4@PPy presented a higher activ-ity in activating PMS to generate reactive radicals for the oxidationof MEB. Based on the results above, decoloration rate of MEB wassignificantly enhanced in the system of CNTs-CoFe2O4@PPy andPMS, which could be attributed to the following processes: the firstwas the adsorption of MEB on the surface of CNTs-CoFe2O4@PPy,but adsorption capacity was very low. The second was the degra-dation of MEB by CNTs-CoFe2O4@PPy as the catalyst, which playeda prominent role in promoting the degradation efficiency oforganic dye from water. Once PMS was added into the MEB solu-tion in the presence of CNTs-CoFe2O4@PPy, reactive radicals suchas SO4

�� and �OH were generated from PMS being activated by theCoFe2O4 on the surface of catalyst. And then the generated SO4

��

and �OH reacted with MEB both adsorbed on the surface of CNTs-CoFe2O4@PPy and dissolved in aqueous solution, resulting in a highdecolorization rate. The enhanced catalytic performance of CNTs-

CoFe2O4@PPy indicated its great potential application in theremoval of organic dyes from wastewater.

The effect of initial dye concentration in the range of 50–500 mg/L on the catalytic activity of CNTs-CoFe2O4@PPy was inves-tigated in the presence of 1.0 g/L catalyst and 4.0 mmol/L PMS(Fig. 9b). It was observed that the decolorization rate of MEBdecreased with increasing initial dye concentration from 50 to500 mg/L. The decolorization rates were higher at lower initialdye concentrations (50 and 100 mg/L), among which nearly 100%and 95% of MEB were removed after 30 and 60 min, respectively.Then the decolorization rate of MEB decreased from 72% to 58%after 60 min reaction with the increase of initial dye concentrationfrom 200 to 300 mg/L, and further decreased to 43% when initialdye concentration increased up to 500 mg/L. Under similar exper-imental conditions except for initial dye concentrations, theamount of generated reactive radicals were not sufficient enoughto oxidize MEB completely, thus the decolorization efficiencydecreased with the increasing of initial dye concentration. Despiteof this, color could be effectively removed with initial dye concen-tration lower than 100 mg/L, indicating that the as-prepared CNTs-CoFe2O4@PPy was a high-effective catalyst for the removal of MEBfrom aqueous solution.

To identify the major reactive oxygen species that were respon-sible for catalytic oxidation reactions, tert-butyl alcohol (TBA)without a-hydrogen as a strong scavenger for �OH and ethanol(EtOH) with a-hydrogen as a quenching agent for �OH and SO4

��

were added into system before the beginning of experiments[30,41]. As shown in Fig. 9c, without the addition of quenchingagents, CNTs-CoFe2O4@PPy exhibited high catalytic activity inMEB removal, a fast and efficient decolorization of MEB wasobserved, and nearly 100% MEB was removed after 30 min at theconditions of 50 mg/L MEB solution, 1.0 g/L catalyst, and4.0 mmol/L PMS. However, the more the quenching agents were,the lower the decolorization speed and efficiency were. With theamount of TBA increased from 5 to 10 mL, the prolonged reactiontime of 50 and 60 min was needed with removal efficiency ofMEB approximate 100%, respectively. As the TBA dose furtherincreased to 20 mL, the decolorization rate decreased to 88% at60 min for MEB. While in the presence of EtOH, the catalytic activ-ity was strongly inhibited compared with that of TBA addition, andthe increase in EtOH dose decreased dramatically the degradationspeed and decolorization rate. For example, the MEB decolorizationrate decreased from 92% to 60% in the first 30 min with theincrease of EtOH from 5 to 20 mL. The much more decrease ofMEB removal by EtOH than by TBA implied that SO4

�� radicals were

0 10 20 30 40 50 60

0.0

0.2

0.4

0.6

0.8

1.0

C t/C0

t(min)

50 mg/L 100 mg/L 200 mg/L 300 mg/L 400 mg/L 500 mg/L

0 10 20 30 40 50 600.0

0.2

0.4

0.6

0.8

1.0

C t/C0

t(min)

CNTs-CoFe2O4@PPy PMS CNTs-CoFe2O4@PPy+PMS

0 10 20 30 40 50 60

0.0

0.2

0.4

0.6

0.8

1.0

C t/C0

t(min)

1 cycle 2 cycle 3 cycle 4 cycle 5 cycle

0 10 20 30 40 50 60

0.0

0.2

0.4

0.6

0.8

1.0

C t/C0

t(min)

None 5 mL EtOH 10 mL EtOH 20 mL EtOH 5 mL TBA 10 mL TBA 20 mL TBA

a b

c d

Fig. 9. MEB removal under different conditions (a), effect of initial dye concentration on MEB degradation by the PMS activation with CNTs-CoFe2O4@PPy (b); effect of TBAand EtOH on MEB degradation (c) and repeated recycling of CNTs-CoFe2O4@PPy with PMS for the degradation of MEB (d).

X. Li et al. / Chemical Engineering Journal 316 (2017) 893–902 901

the main radical species controlling the oxidation reaction in thecatalyst/PMS/MEB system.

The stability and reusability of as-prepared catalyst are alsoimportant for their practical application in the removal of organicpollutants from wastewater. Five recycling runs were conducted toevaluate the catalytic activity and reusability of the CNTs-CoFe2O4@PPy catalyst. As shown in Fig. 9d, it was found that theoxidation activity of the recycled catalyst dropped slightly in com-parison with the fresh catalyst, the decolorization efficiency ofMEB was still as high as 87% within 60 min in the fifth run. There-fore, the CNTs-CoFe2O4@PPy exhibited good catalytic ability andcould be reused.

4. Conclusions

The meso-CNTs-CoFe2O4@PPy composites were synthesizedthrough a chemical oxidative polymerization of pyrrole in the pres-ence of CNTs-CoFe2O4. This material was used as adsorbent for theremoval of organic dyes. CNTs-CoFe2O4@PPy exhibited betteradsorption ability toward anionic dyes than cationic dyes. CNTs-CoFe2O4@PPy showed high adsorption capacity and quick adsorp-tion rate in a wide pH range for anionic dyes including MB, MOand AF. Studies on the adsorption isotherms and kinetics ofCNTs-CoFe2O4@PPy for three dyes showed that the equilibriumadsorption and kinetic process could be well described by Lang-muir isotherm and pseudo-second-order kinetic model, respec-tively. Moreover, CNTs-CoFe2O4@PPy could be used as anefficient heterogeneous catalyst to activate PMS to generate reac-tive radicals such as SO4

�� and �OH for the degradation of MEB.Nearly 100% of MEB could be removed after 30 min at the condi-tions of 1.0 g/L catalyst, 4.0 mM PMS and 50 mg/L MEB solution.These results indicated that the synthesized CNTs-CoFe2O4@PPy

had great potential to be used as an effective adsorbent and cata-lyst for the treatment of wastewater containing cationic and anio-nic dyes.

Acknowledgements

The authors gratefully acknowledge the financial supports fromthe National Natural Science Foundation of China (No. 21407071),and the Fundamental Research Funds for the Central Universities(NO. lzujbky-2014-123).

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