Influence of magnetic field on the adsorption of organic compound by clays modified with iron

Applied Clay Science 97–98 (2014) 1–7

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Research Paper

Influence of magnetic field on the adsorption of organic compound byclays modified with iron

Aline Auxiliadora Tireli a,⁎, Francielle Candian Firmino Marcos b, Laís Ferreira Oliveira c,Iara do Rosário Guimarães a, Mário César Guerreiro a, Joaquim Paulo Silva c

a Department of Chemistry, Federal University of Lavras, Lavras, 37200-000, Brazilb Department of Chemistry, University of São Paulo, São Carlos, 13566-581, Brazilc Department of Exact Science, Federal University of Lavras, Lavras, 37200-000, Brazil

⁎ Corresponding author. Tel./fax: +55 35 38291271.E-mail address: [email protected] (A.A. Tireli).

http://dx.doi.org/10.1016/j.clay.2014.05.0140169-1317/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

Article history:Received 20 November 2013Received in revised form 19 May 2014Accepted 20 May 2014Available online xxxx

Keywords:ClayAdsorptionOrganic compoundMagnetic field

Insertion of iron into montmorillonite (Mt) resulted in two modified materials, when different treatments wereused: i) pillared clay (FePILC) and ii) magnetic clay (FeMAG). The ability of the modified clays to remove the or-ganic dye methylene blue (MB) by adsorption was tested. Additionally, we evaluated the effects of adsorptionafter exposure to a pulsedmagnetic field, the results were monitored by UV–vis spectroscopy and chemical anal-ysis of total organic carbon. Allmaterials were characterized by X-ray powder diffraction (XRD), Fourier transforminfrared spectroscopy (FTIR), temperature-programmed reduction (TPR), scanning electron microscopy coupledwith energy dispersive spectroscopy (SEM/EDS) and specific surface area measurements. The catalytic activityof the clays modified with iron was evaluated in hydrogen peroxide decomposition reaction. The XRD patternsshow the formation of crystalline iron phases on the surface of the clays, besides confirming the pillaring procedurefor FePILC with a basal spacing of 1.79 nm, an increase of 0.53 nm over the montmorillonite. The specific surfacearea of FePILC was 210.9 m2/g. FeMAG had part of this original structure broken, with an area of 177.1 m2/g andmagnetic properties demonstrated by the attraction to a permanent magnet. The TPR profiles and EDS indicatedthat the different heat treatments exerted great influence on the amount and phases of iron formed. Thematerialsshowed high capacity for removal of theMB dye, and the isotherms fit the Langmuir model. Adsorption of the dyesignificantly increased in the presence of a magnetic field, changing the Langmuir parameters and especially themaximum adsorption capacity for all materials. The best results were observed for FeMAG.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Pigments and dyes are disposed into wastewaters from variousindustries, mainly by the textile production (Fatimah et al., 2010; Haiet al., 2011). They are a serious problem when dealing with textilewaste because they are water soluble, chemically and photolyticallystable (Guimarães et al., 2012). Effluents containing dyes must not bedisposed in natural bodies, because they do not allow solar lightpenetration and decrease the dissolved oxygen amount, damagingaerobic processes (Banković et al., 2012).

Clays have been used by humanity since ancient times formanufacturing, ceramic objects, and more recently in several techno-logical applications. Clay has also become indispensable to modernlife; it is the material of many kinds of applications and they are abun-dant in nature, inexpensive and environmentally friendly (Bergayaet al., 2006; Carretero and Lagaly, 2007). Their structural propertiescan be modified by simple methods such as pillaring, to produce

materials with higher surface area, porosity, thermal stability and great-er capacity for adsorption and/or degradation of contaminants (Bergayaet al., 2006; Kurian et al., 2012; Tong et al., 2009).

The pillared clays, also cited by the abbreviation PILC (PillaredInterLayered Clays), have achieved considerable interest in their use ascatalysts and adsorbents in recent years (Gil et al., 2011). Thepillarization promotes an increase in basal spacing and surface area,which provides better accessibility of the molecules to the active sitespresent inside of the clay structure (Gil et al., 2011; Yang et al., 2013).The list pertaining to the engineering and application of these materialsis extensive; there are several publications with different aspects of thetheme in the recent literature.

Materials containing iron, like iron oxide pillared clay, become veryinteresting when they display magnetic attraction in combinationwith high specific surface area. Furthermore, iron is usually employedfor adsorbents fabrication, because of the low cost (Mubarak et al.,2013). These represent an innovative and promising class of newmate-rials for removal of contaminants such as organic dyes (Banković et al.,2012; Hou et al., 2010) and contaminants in aqueous media (Mubaraket al., 2013; Rivagli et al., 2014; Zhang et al., 2010). In addition, attention

Fig. 1. Adsorption inductor device. (a) polypropylene container with cooling watersystem, (b) opening for sample introduction, (c) power supply, and (d) multimeters forcontrolling current and voltage.

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was focused on utilizing magnetic materials (such as magnetite andmagnetite silica composites) for wastewater treatment (Ferroudjet al., 2013; Tuutijärvi et al., 2009; Wu et al., 2012; Yu and Yang,2010) mainly because of simplicity and speed of separation whenthese materials are used with aqueous media.

In this work a simple and rapid synthesis was performed forobtaining modified clay with magnetic iron phases exposed. Further-more, the modified materials were tested for their ability to removethe organic dye methylene blue (MB) from an aqueous medium. Thepossibility of improving the removal process by implementing amagnetic field was also evaluated.

2. Materials and methods

The cation exchange capacity (CEC) of montmorillonite, determinedby the ammonium acetate method, is 1.75 mEq/g. Mineralogicalcomposition showed the presence of 5 wt.% of quartz and 10 wt.% offeldspar. Through chemical analysis itwas determined that theprincipalexchangeable cation is sodium and the following is the chemicalcomposition:

M1.41 (Al0.20 Si7.80)IV (Al2.69 Fe0.15 Mg1.11 Mn0.02)VI O20 (OH)4

2.1. Preparation of the modified materials

The materials were prepared by a pillaring procedure of themontmorillonite. This was done by intercalating a dilute aqueoussolution (0.4 M) of trinuclear acetato-hydroxo iron(III) nitrate[Fe3(OCOCH3)7OH]NO3 (Yamanaka et al., 1984) by slow addition at35 °C to a previously prepared aqueous suspension of the clay(1 wt.%) under constant stirring. The modified material was separatedby centrifugation, washed and dried at 60 °C for 24 hours. Two distinctroutes were employed for the thermal treatment: i) in an oven withsynthetic air flow of 50 mL/min at 500 °C for 1 hour (FePILC) and ii)in an open furnace without gas flow at 500 °C for one hour (FeMAG).

2.2. Instrumental characterization

FTIR spectra were recorded on a Digilab Excalibur spectrometer withspectral range from 400 to 4000 cm−1. Samples were ground to powderand pressed in KBr pellets. FTIR spectrawith a resolution of 4 cm−1werecollected over an average of 32 scans. XRD analyseswere performed on aRigaku (Japan) D/Max 2500 VB2+/PC X-ray powder diffractometeroperating with Kα radiation of Cu (λ = 1.54056 Å), and a generatorvoltage of 45 kV and a current of 30 mA in 2θ ranging from 2° to 60°.Nitrogen adsorption isotherms were obtained with an Autosorb1 MP,Quantachrome. The specific surface area was calculated using the BETmodel. The total pore volume was estimated from the amount of nitro-gen adsorbed at P/P0 = 0.99, the pore size distribution (PSD) was calcu-lated based on the DFT equation. All sampleswere degassed overnight at200 °C before each adsorptionmeasurement. TPR profileswere obtainedwith Chembet 3000 Quantachrome equipment. In these TPR experi-ments, the samples are heated in the presence of H2 and the reductionreactions monitored by the hydrogen consumption. Scanning electronmicroscopy (JEOL Ltd.) was coupled with an energy dispersive X-ray an-alyzer (INCA 350, Oxford Instruments). The hydrogen peroxide decom-position study was carried out with 2 mL of H2O2 (0.3 mol/L) and 30mg of each material (FePILC, FeMAG and Mt), at pH 6. The mixture wasstirred and the decomposition reaction of hydrogen peroxide (Eq. (1))was monitored by O2 gaseous formed in a volumetric glass system. Thedecompositionwith organicmoleculewas carried out in the same condi-tions, with added of 5 mL of methylene blue (50 mg/L).

H2O2 lð Þ→H2O lð Þ þ½ O2 gð Þ ð1Þ

2.3. Adsorption experiments

The adsorption isotherms were obtained in a batch equilibriumexperiment using 20.0 mg of the materials dissolved in 20.0 mL of 25,50, 100, 250, 500, and 1000 mg/L standard solutions of the organicdye methylene blue (MB); these solutions were kept stirring for24 hours at room temperature (25 °C) and at a pH of 6.0, adjustedwith dilute HCl. The solid was separated by magnetic attraction(Fig. 8) and MB concentration in the supernatant was monitored byUV–vis spectroscopy at 665 nm (Biosystems SP-2000).

2.4. Influence of magnetic field on the dye adsorption

The adsorption test in the presence of the magnetic field was per-formed with a device developed by the Federal University of Lavras(Batista et al., 2007), designated: Device Inductor of Adsorption(Fig. 1) which generates magnetic field lines uniformly from an elec-tromagnet. The generated field was continuously measured by aTeslameter PHYWE apparatus, with an observed value of 50.0 μT, thevoltage was kept at 6.0 V and the electric current at 2.20 A. The testswere conducted under the same conditions and concentrations citedfor the adsorption tests without a magnetic field. After adsorptionexperiments in the presence of the magnetic field were completed,analysis of the total organic carbon content (TOC) was performed witha Shimadzu 5000 A, in order to obtain data on the amount of remainingcarbon.

3. Results and discussion

3.1. Structural characterization

The FTIR spectra of the clays are presented in Fig. 2. A strongabsorption in the region around 3600 cm−1 for the montmorillonitecorresponds to the stretching of the hydroxyl groups and cations inthe octahedral sheet. In these modified materials, this absorption ap-pears overlapped with another in the region of 3400 cm−1 that is relat-ed to the presence of adsorbed water on the surface of the clays. Also in

Fig. 2. Infrared spectra of montmorillonite, FeMAG and FePILC. Fig. 5. Pore size distribution for FePILC, FeMAG and Montmorillonite.

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1645 cm−1 the band can be ascribed to the vibration of watermolecules(Zhao et al., 2012). A strong reduction of this bandwas observed for themodified materials after calcination. The last absorption region around

Fig. 3.XRD for themontmorillonite, FePILC and FeMAG. (M=Maghemite;H= Hematite;Mt = Montmorillonite; Q = Quartz; F = Feldspar).

Fig. 4. Adsorption/desorption of nitrogen at 77 K of montmorillonite, FePILC and FeMAG.

1050 cm−1 indicates the stretching of the Si\O bond (Kurian andBabu, 2013; Tyagi et al., 2006).

The XRD patterns of the three samples are displayed in Fig. 3. Formontmorillonite an intense reflection appears at 2θ = 7.58 related tod001 value (1.23 nm), other reflections related to montmorillonite(2θ = 19.94°, 29.12° and 35.10°) and associated minerals like quartz(2θ = 20.90° and 26.75°), and feldspar (2θ = 26.60°) were also ob-served (Ayodele et al., 2012; Dana et al., 2004). The XRD profile ofFePILC indicated that the pillaring procedure was successful, con-sidering the presence of diffraction at 2θ = 4.95 attributing to 001reflection = 1.79 nm. After the calcination process in the open furnacewithout gas flow, the FeMAG presented magnetic properties and acollapse of a part the original structure indicated by the reflection2θ = 9.04 (d001 = 0.98 nm); this partial collapse has already been ob-served in previous works (Belaroui et al., 2004; Rightor et al., 1991).The temperature, duration and rate of heating, as well as the presenceor absence of gas flow directly influence the resulting material(Bergaya et al., 2006). Diffractions at 2θ = 29.85, 35.67, 43.60, 54.14and 57.40 are those found in the maghemite standard, γ-Fe2O3 (Huet al., 2005; Tuutijärvi et al., 2009), and explain themagnetism acquiredby the material.

The higher crystallinity of FeMAG compared to FePILC is veryevident. Because the collapse of the FeMAG structure the iron phasesbecomesmore exposed which is observed in the diffractogram and ver-ified by EDS analysis, similar resultswere also reported inNogueira et al.

Fig. 6. TPR profiles of Montmorillonite, FePILC and FeMAG.

Fig. 7. SEM and EDS analysis of montmorillonite (a

Table 1Specific surface area (SSA), pore diameter and volume of the montmorillonite, pillared(FePILC) and magnetic (FeMAG) clays.

Sample SSA (m2/g) Pore diameter (Å) Vtotal (cm3/g)

Montmorillonite 39.1 32.0 0.066FePILC 210.9 21.9 0.223FeMAG 177.1 27.2 0.191

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(2011). In FePILC, iron phases arewithin thematerial, making it difficultto identify and quantify the iron. The largest amount of iron exposed onthe surface of FeMAG was also observed in the results obtained by EDSand TPR.

The results obtained by isotherms of adsorption/desorption of N2

(Fig. 4) indicated a significant increase in the specific surface area ofthe modified clays, from 39.1 m2/g for the Mt to 210.9 and 177.1 m2/gfor FePILC and FeMAG respectively. The pore size distribution centeredat 22 Å for FePILC, 26.7 Å for FeMAG and 32 Å for montmorillonite(Fig. 5, Table 1). The increase of specific surface area for both materialswas relevant, once another authors have reported lower values for

and b), FeMAG (c and d) and FePILC (e and f).

Fig. 8. The H2O2 decomposition in water and methylene blue for the montmorillonite,FePILC and FeMAG.

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mineral clays modified with iron, like Daud and Hameed (2010) whoobtained 185.4 m2/g and Tireli et al. (in press), 139.2 m2/g.

The isotherms (Fig. 4) were categorized as type II and, for all mate-rials, hysteresis loop 3 at higher relative pressure typical of aggregatedpowders such as clays (Letaief et al., 2003; Rouquerol et al., 1999), andthe result corroborates the average pore size (Fig. 5, Table 1).

The temperature-programmed reduction profiles (Fig. 6) indicatedthe pronounced presence of iron in the materials that were modified.The Mt did not show a significant consumption of hydrogen whichindicates the absence of reducible species.

A peak centered on 430 °C was observed for the modified clays andis also evident for pure hematite (Jung et al., 2012) this can be attributedthe transformation shown in Eq. (2).

3Fe2O3 þH2→2Fe3O4 þH2O ð2Þ

This region of consumption of hydrogen around400 °C has also beenlinked to the conversion of γ-Fe2O3 in magnetite via thermal reductionin the presence H2 (Jung et al., 2012). FeMAG showedmagnetic proper-ties and confirmed the presence of iron phase maghemite by XRD; thismay have resulted in the two regions of hydrogen consumption over-lapping and the observed peak broadening.

Analysis by EDS provides a semiquantitative surface elemental anal-ysis. For montmorillonite, the presence of the elements Si, Al, O andMgwas observed as constituents of the structure of the montmorillonite,with Si, Al and O as the major elements. After the pillaring process forFeMAG and FePILC, a significant decrease in the amount ofMg is evidentwhich is indicative that the exchange of these ions for the oligomer wasperformed satisfactorily during the expansion of the clay layers (Fig. 7b,d and f).

The ability of a material to break the H2O2 molecule provides a goodindication of its catalytic ability, since the radical intermediates generat-ed from H2O2 are of great interest for oxidation of various organic com-pounds (Gulkaya et al., 2006).

Table 2Langmuir and Freundlich parameters, calculated for adsorption of methylene blue (MB)by montmorillonite, FeMAG and FePILC.

Materials Langmuir Freundlich

qmax kL R2 Kf 1/n R2

Mt 434.25 0.006 0.979 11.66 0.53 0.930FePILC 191.54 0.004 0.993 7.16 0.46 0.977FeMAG 258.42 0.003 0.988 5.41 0.53 0.982

The Mt had low capacity to decompose H2O2; this increased about10 times when using FeMAG (Fig. 8). The iron phases identified in thismaterial were maghemite and hematite, where the oxidation state ofiron is three (Fe III). However, it has been reported that the decomposi-tion of H2O2 in the presence of Fe III was favored by the reduction of FeIII to Fe II in the presence of H2O2 (Olasehinde et al., 2008; Tireli et al., inpress) giving the classical Fenton reaction sequence that occurs veryquickly with the Fe II/H2O2 system. A portion of the FeMAG structurehad collapsed but the iron species were still present and possiblymore accessible; it was deposited under the surface of the materialand confirmed byXRD (Fig. 3) and EDS (Fig. 7) and therefore explainingthe higher activity of FeMAG. FePILCwas less able to decompose hydro-gen peroxide than FeMAG, producing approximately 4.0mL of O2, prob-ably due to the more difficult access for active phases within thelamellar clay.

In the presence of the organic molecule MB, which is a radical scav-enger, there was no significant decrease in the formation of O2 (Fig. 8)suggesting that themechanismof degradation occurs via oxygen vacan-cy for the Mt and FePILC. In the FeMAG case this difference was greaterand is indicative that the formation of radicals OH and OOH occursthrough Fenton's process (Oliveira et al., 2008).

The leaching test was performed to observe the likely output of theactive phase for the solution; the results were positive indicating thatall iron stays in the material and the decomposition of H2O2 occursonly in the presence of the modified materials, being characterized asa heterogeneous phase process.

3.2. Removal of methylene blue

After 24 hours of contact of theMB solutionwith the clays, the equi-librium concentrations, Ceq (mg/L), and the amount of adsorbedmateri-al per unit mass of adsorbent, qeq (mg/g), were calculated. Theisothermswere fitted according to the Langmuir and Freundlichmodels(Table 2),with nonlinearfitting.Withoutmagnetic field themontmoril-lonite was more efficient in the removal of MB (qeq = 434.25 mg/g),followed by FeMAG (qeq = 258.42 mg/g) and FePILC (191.31 mg/g).The removal of MB by activated carbon had a maximum capacity of102.0 mg/g (Ramos et al., 2009); considering that activated carbon isknown for its high adsorption capacity, the results obtained for theclays are very satisfactory. A iron pillared clay was able to remove 40%of methylene blue (Tireli et al., in press). Hou et al. (2010) found forFe-pillared clay values of 98.62 mg/g in the adsorption process usingrhodamine B. Clays are widely employed in the removal of various clas-ses of contaminants in aqueous media by adsorption processes (Daudand Hameed, 2010; Fatimah et al., 2010; Hou et al., 2010; Rivagli et al.,2014) because of high surface area and structures.

3.3. Influence of magnetic field on the dye adsorption

It has been reported that hydrogen bonds between water moleculesare reduced in the presence of a magnetic field, permitting the forma-tion of water molecule monomers (Toledo et al., 2008). Thus, it isclear that the magnetic field exerts great influence on the physical andchemical processes that occur in aqueous solutions.

Measurements of zeta potential in solutions under the influence ofmagnetic fields showed that increase of the field greatly influenced

Table 3Langmuir and Freundlich parameters for adsorption of methylene blue (MB) in thepresence of a magnetic field.

Materials Langmuir Freundlich

qmax kL R2 Kf 1/n R2

Montmorillonite 482.31 0.007 0.987 12.52 0.53 0.941FePILC 223.19 0.004 0.989 6.85 0.46 0.968FeMAG 834.65 0.001 0.994 4.58 0.823 0.961

Fig. 9. The H2O2 decomposition in water and methylene blue for the montmorillonite, FePILC and FeMAG.

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the adsorption of dyes and metals; larger field strengths allowed forlarger adsorptions (Peng et al., 2006;Wu et al., 2012; Zhang et al., 2005).

The magnetic field exerted great influence on the adsorption of MBfor allmaterials, and the Langmuir and Freundlich parameters increased(Table 3). In the cases of montmorillonite and FeMAG, the magneticfield worked directly in the solution containing the dye, resulting in in-creased mobility of the MB molecules in the aqueous medium. With alarger organization, the dye molecules (MB) could penetrate more eas-ily into adsorption sites of the clays. Similar behavior was observedwhen employing an organo-bentonite for methyl blue adsorption(Hao et al., 2012).

Another very important factor was reported by Bel'chinskaya et al.(2009) and refers to the changing morphology of clays exposed to amagnetic field; the surfaces of materials were observed by scanningelectron microscopy (SEM) to become more heterogeneous, with con-sequent increase in surface area and adsorption of formaldehyde. AlsoWu et al. (2012) reported that an increase in the zeta potential of solu-tions in contactwith themagnetic field caused a decrease in the electro-static repulsion between the particles of magnetic adsorbent and theadsorption increased.

This observation became even more evident when the magneticfield was applied to magnetic clay. The Langmuir parameters showeda large general increase and the maximum adsorption capacity (qmax)nearly tripled (Table 3) under these conditions indicating that themate-rial is much more efficient at removing the dye (Fig. 9). In this case thegreater influence of themagnetic field was on the particles of themate-rial and not on the MBmolecules in solution; this occurred because thephases of iron formed under the surface of thismaterial asγ-Fe2O3 havegreat magnetic attraction (Bel'chinskaya et al., 2009) and favored parti-cle orientation. The other iron containing clay (FePILC) did not show animprovement as meaningful as FeMAG because the iron phases in thismaterial are inside the clay mineral layers and not exposed like in

Fig. 10.Removal percentages ofmethyleneblue byUV-Vis at 665nmand total organic car-bon (TOC).

FeMAG; this property did not help with the organization of the particlesin themagnetic field. In this case thefield acted only on the dye solution.

The determination of total organic carbon (TOC) provided importantdata on the carbon content present in the solution after the adsorptionprocess using a magnetic field. Another authors employed the TOC de-termination for to evaluate the efficiency of adsorption and degradationof organic dyes (Gao et al., 2013).

Initial measurements were based on the concentrations of totalcarbon supplied to the MB standards (25, 50, 100, 250, 500, and1000 mg/L). The data obtained by TOC were correlated with removalof coloration observed by UV–vis absorption at 665 nm (Fig. 10). Thequantification of carbon showed that the removal of staining was byadsorption, because the initial amount of carbon decreased.

4. Conclusions

In this work, a montmorillonite was modified with iron; differentheat treatments were responsible for the formation of materials withdifferent characteristics and different iron phases. The magnetic claywas obtained by heat treatment in open furnace, and XRD indicatedthat the phase of iron present in this material is maghemite.

All clays showed high adsorption capacity of methylene blue, andthe magnetic field positively influenced adsorption. For those whichhad nomagnetism, thefield acted in the solution causing themethylenebluemolecules to becomemoremobile and easily adsorbed. For FeMAGthis influence was coupled with the organization of adsorbent particleswith phases of magnetic iron; the higher efficiency on the removal ofmethylene blue was verified by the coloration decrease and confirmedfor TOC analysis.

Acknowledgements

Wegreatly appreciate the Chemistry and Exact Science Departmentsof the Federal University of Lavras (UFLA), also the funding organiza-tions for this project, National Counsel of Technological and ScientificDevelopment (CNPQ) and Higher Education Co-ordination Agency(CAPES).

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