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Journal of Environmental Management 156 (2015) 225e235

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Journal of Environmental Management

journal homepage: www.elsevier .com/locate/ jenvman

Hydrotalcite-TiO2 magnetic iron oxide intercalated with the anionicsurfactant dodecylsulfate in the photocatalytic degradation ofmethylene blue dye

Liany D.L. Miranda a, Carlos R. Bellato a, *, Jaderson L. Milagres a, Luciano G. Moura b,Ann H. Mounteer c, Marciano F. de Almeida a

a Departamento de Química, Universidade Federal de Viçosa, Av. PH Holfs, s/n, 36571-000 Viçosa, Minas Gerais, Brazilb Departamento de Física, Universidade Federal de Viçosa, Av. PH Holfs, s/n, 36571-000 Viçosa, Minas Gerais, Brazilc Departamento de Engenharia Civil, Universidade Federal de Viçosa, Av. PH Holfs, s/n, 36571-000 Viçosa, Minas Gerais, Brazil

a r t i c l e i n f o

Article history:Received 14 November 2014Received in revised form15 March 2015Accepted 29 March 2015Available online

Keywords:Layered double hydroxidesTiO2

AdsorptionPhotocatalysis

* Corresponding author.E-mail address: bellato@ufv.br (C.R. Bellato).

http://dx.doi.org/10.1016/j.jenvman.2015.03.0510301-4797/© 2015 Elsevier Ltd. All rights reserved.

a b s t r a c t

The new magnetic photocatalysts HT/TiO2/Fe and HT-DS/TiO2/Fe, modified with the anionic surfactantsodium dodecylsulfate (DS) were successfully synthesized in this work. Titanium dioxide (anatase) fol-lowed by iron oxide were deposited on the hydrotalcite support. Several catalyst samples were preparedwith different amounts of titanium and iron. The photocatalysts were characterized by infrared andRaman spectroscopy, X-ray diffraction, scanning electron microscopy. Photocatalytic performance wasanalyzed by UVevisible radiation (filter cutoff, l > 300 nm) of an aqueous solution (24 mg/L) ofmethylene blue (MB). The most efficient catalyst was obtained at an iron oxide:TiO2 molar ratio of 2:3.This catalyst showed high photocatalytic activity, removing 96% of the color and 61% of total organiccarbon from the MB solution after 120 min. It was easily removed from solution after use because of itsmagnetic properties. The reuse of the HT-DS/TiO2/Fe23 catalyst was viable and the catalyst was struc-turally stable for at least four consecutive photocatalytic cycles.

© 2015 Elsevier Ltd. All rights reserved.

1. Introduction

Synthetic dyes are widely used in the textile, food, leather, paperand plastics industries that often end up in industrial wastewaters.Dyes are potentially harmful to the environment when dischargedinto surface waters without adequate treatment because of theirchemical stability and toxicity (Wang et al., 2014). Therefore,several methods, including chemical oxidation (Ferrarese et al.,2008), microbial degradation (Haritash and Kaushik, 2009),adsorption (Walcarius and Mercier, 2010), flocculation (Golob et al.,2005) and photocatalytic degradation (Tong et al., 2012) have beendeveloped to eliminate these organic pollutants fromwastewaters.Photocatalytic degradation, in which decomposition or chemicaldissociation is caused by exposure of a substance to visible or ul-traviolet light, is considered an especially promising technique(Chala et al., 2014) (C.-Y. Chen et al., 2012). In general, inorganicsemiconductors are used as photocatalysts, with titanium dioxide

(TiO2) having become the most widely investigated because it isnaturally abundant, inexpensive, non-toxic and stable duringphotocatalysis (Ma et al., 2013) (Ma et al., 2014).

When TiO2 is irradiated with photons at an appropriate wave-length, this semiconductor undergoes an electronic excitationprocess and electrons in its valence band are promoted to theconduction band. Through this photoelectronic excitation process,the photocatalyzer becomes a very efficient oxidizing or reducingagent. TiO2 has demonstrated exceptional performance in thephotocatalytic degradation of various organic dyes (Chala et al.,2014) (C.-Y. Chen et al., 2012) (Gao et al., 2013) (Hsiao et al.,2014). However, nanoparticles of commercial TiO2 are difficult torecover after use and can accumulate or even plug up reactors, thuslimiting their practical application (Huang et al., 2013). Moreover,due to their high hydrophilicity, it is very difficult for hydrophobicorganic pollutants to reach the highly hydrophilic nano-TiO2 sur-face, which is necessary for their photodegradation. To solve theseproblems, supports for TiO2 nanoparticles such as montmorillonite(Ooka et al., 2003) (Miao et al., 2006) (Zhang et al., 2008), zeolites(Tayade et al., 2007) and carbon nanomaterials (Sellappan et al.,

L.D.L. Miranda et al. / Journal of Environmental Management 156 (2015) 225e235226

2011) (Sampaio et al., 2011) (Yao et al., 2010) have been effectivelyused to improve photocatalytic activity. Takeuchi et al. (2007, 2009)found that a simple mechanical mixture of zeolites and TiO2increased the photocatalytic activity of nano-TiO2 significantly. Anet al. (2008) showed that TiO2 supported on surfactant-modifiedmontmorillonite completely degraded decabromodiphenyl ether.G�omez-Solís et al. (2012) deposited TiO2 on silicon carbide (SiC)and found that the SiCeTiO2 catalyst had higher photocatalyticactivity towards organic dyes than unmodified TiO2. Huang et al.(2013) immobilized nano-TiO2 on hydrophobic layered double hy-droxides (LDH) and used it to efficiently remove dimethyl phthalatein water (Huang et al., 2013).

The most important class of LDH anionic clays are the hydro-talcites (HT) that have the general formula[Mg2þ1�xAlx3þ(OH)2]xþ[Ax/n

n�$mH2O]x�, where x ranges from 0.17 to0.33. Hydrotalcites are good anion exchangers since their anions(An�) and interlamellar water molecules can be exchanged for an-ions in solution (Sahu et al., 2013) (Toledo et al., 2011) (Toledo et al.,2013) (Miranda et al., 2014). LDHs are porous materials with largesurface areas containing many hydroxyl groups, making themexcellent dispersants for nano-TiO2 particles. Furthermore,adsorption capacity of hydrophobic organic pollutants on LDHs canbe significantly improved by modification with surfactants such assodium dodecylsulfate (DS) (Miranda et al., 2014) (Bouraada et al.,2012) (Bruna et al., 2012). The special lamellar structure of LDHsimproves access to adsorption sites where photocatalytic reactionsoccur. The immobilization of nano-TiO2 particles on DS-modifiedLDH increases the hydrophobicity of the catalyst's surface,thereby increasing the efficiency of photocatalytic degradation ofhydrophobic contaminants (Huang et al., 2013) (Ooka et al., 2003).

The combination of iron oxide with HT to facilitate catalystremoval from aqueous solution through application of a magneticfield reduces catalyst recovery time and costs (Toledo et al., 2011)(Miranda et al., 2014). Several magnetic catalysts have beenmentioned in the literature for methylene blue (MB) dye photo-degradation (Chen et al., 2012) (Sun et al., 2014). In the presentwork, we synthesized two catalysts, HT/TiO2/Fe and HT-DS/TiO2/Fe,which, to the best of our knowledge, have not been reported in theliterature for photodegradation of aqueous organic species. Weevaluated the synergistic effect provided by adsorption of TiO2 onHT-DS (hydrotalcite intercalated with the dodecylsulfate ion), TiO2photoactivity and the magnetic properties of iron oxide in the newcatalysts. We also evaluated the quantities of TiO2 and iron oxideadded to the catalysts in MB photocatalytic degradation experi-ments carried out in a reactor under UVevisible radiation.

2. Material and methods

2.1. Material

Metal nitrates (Mg(NO3)2.6H2O and Al(NO3)3.9H2O), sodiumhydroxide, sodium carbonate, sodium dodecylsulfate, iron IIIchloride hexahydrate, ferrous sulfate heptahydrate and titaniumdioxide nanoparticles (particle size 25 nm, 99.7%) in the anataseform used as photocatalyst were obtained from SigmaeAldrich.Methylene blue dye was purchased from Vetec (Rio de Janeiro,Brazil). All solutions were prepared with analytical grade reagentsand high purity deionized water produced by a Milli-Q® system(Millipore, Bedford, MA, USA).

2.1.1. Preparation of HT/TiO2

HT/TiO2 were prepared by fixing the Al:(Al þMg) molar ratio at0.25 and varying themolar amount of TiO2 (0.5,1,1.5, 2 and 3mol ofTi). A 100 mL solution of Mg(NO3)2$6H2O (0.075 mol andAl(NO3)3$9H2O (0.025 mol) was added dropwise to a 100 mL

solution of NaOH (0.1805 mol), Na2CO3 (0.084 mol) and TiO2 (0.5, 1,1.5, 2 or 3 mol) and the mixtures stirred for 24 h. The catalysts werehydrothermally treated at 80 �C for 24 h and the precipitates rinsedwith distilledwater and dried at 60 �C. The catalysts were labeled asHT/0.5TiO2, HT/1TiO2; HT/1.5TiO2, HT/2TiO2 and HT/3TiO2.

For comparison purposes, a hydrotalcite was prepared withinterlayered carbonate ions (HT-CO3), [Mg3Al(OH)8]2CO3$nH2O, bythe method of co-precipitation at variable pH, as described in theliterature (Bruna et al., 2012).

2.1.2. Preparation of HT/TiO2/FeA 100 mL solution of Mg(NO3)2$6H2O (0.075 mol) and

Al(NO3)3$9H2O (0.025 mol) was added dropwise to a 100 mL so-lution of NaOH (0.1805mol), Na2CO3 (0.084 mol) and TiO2 (2 mol inrelation to Ti) and the mixtures stirred for 24 h. The suspensionswere heated to 70 �C and 20 mL of different amounts of FeCl3$6H2Oand FeSO4$7H2O (3.7$10�3 and 8.6$10�3; 7.4$10�3 and 1.7$10�2;1.1$10�2 and 2.6$10�2; 1.5$10�2 and 3.5$10�2 mol) solutions wereadded, at a fixed Fe3þ: Fe2þ molar ratio of 0.5. Iron oxide wasprecipitated by addition of NaOH (5 mol/L) to pH10. The catalystswere hydrothermally treated at 80 �C for 24 h and the precipitatesrinsed with distilled water and dried at 60 �C. The magnetic cata-lysts were labeled as HT/TiO2/Fe14, HT/TiO2/Fe24, HT/TiO2/Fe34and HT/TiO2/Fe44, based on the molar ratios of Fe:Ti used.

2.1.3. Preparation of HT-DS/TiO2/FeThe photocatalysts were obtained by the co-precipitation

method under an atmosphere of N2. A 150 mL aqueous solutionofMg(NO3)2$6H2O (0.075mol) and Al(NO3)3$9H2O (0.025mol) wasadded dropwise to a 750 mL solution of NaOH (0.20 mol) and so-dium dodecylsulfate (0.025 mol) and the suspensions stirred for24 h TiO2 was then added (2 mol relative to Ti) and the suspensionsstirred for another 24 h, after which they were heated to 70 �C.Twenty mL of different amounts of FeCl3.6H2O and FeSO4.7H2O(9.3$10�3 and 2.2$10�3; 7.4$10�2 and 1.7$10�2; 1.1$10�2 and6.5$10�2; 3.8$10�2 and 8.8$10�2 mol) were added to the suspen-sions. Iron oxide was precipitated by adding NaOH (5 mol/L) to pH10. The suspensions were hydrothermally treated at 60 �C for 24 hand the precipitates rinsed with distilled water and dried at 60 �C(Huang et al., 2013) (Toledo et al., 2013) (Bruna et al., 2012). Themagnetic catalysts were labeled as HT-DS/TiO2/Fe23, HT-DS/TiO2/Fe43, HT-DS/TiO2/Fe63 and HT-DS-TiO2/Fe73, based on the molarratios of Fe:Ti used. After the photodegradation experiments, thecatalysts continued to present magnetic properties and all wereremoved from solution by a 0.3 T magnet. It was necessary to add2.5-fold more iron to the HT-DS/TiO2/Fe catalysts than the HT-TiO2/Fe catalyst to ensure their magnetic properties, which is explainedby the exchange of CO3

2� for the dodecylsulfate ion.For comparison purposes, prepared hydrotalcite-TiO2 interca-

lated with anionic surfactant dodecyl, HT-DS/2TiO2.

2.1.4. Characterization of catalystsInfrared spectroscopy (IR) analysis was carried out directly on

the sample in a VARIAN 660-IR infrared spectrophotometerequipped with a PIKE GladiATR attenuated reflectance accessoryover the region of 400e4000 cm�1. X-ray diffraction analysis wasperformed on an X'Pert PRO (PANalytical) X-ray diffraction systemusing a Ni filter and Cu-ka radiation (l ¼ 1.54 Å) at an angularvariation of 0e70 (2q). UVevisible spectra were obtained on anAgilent 8453 spectrophotometer. Diffuse reflectance spectra wereacquired on a dual-beam 20 GBC, Cintra model spectrophotometer,in the region of 350e700 nm. Calcium carbonatewas used as a non-absorbing standard. The spectrum of each sample was obtained byscanning the established range at a speed of 100 nm/min, for about4 min. Measurements were acquired at a 0.5 nm resolution and

L.D.L. Miranda et al. / Journal of Environmental Management 156 (2015) 225e235 227

2 nm slit thickness. Total organic carbon was measured using aShimadzu-5000A TOC analyzer. Catalyst particle morphology wasanalyzed by scanning electron microscopy (SEM) using a JEOLmodel JSM-6010LA electron microscope with a tungsten filament,operated at an accelerating voltage up to 20 kV. In order to detectthe presence of TiO2 and iron oxide in the catalysts, analyses werealso made by Raman scattering, using a Renishaw Raman Inviamicro spectrometer equippedwith an argon laser (514.5 nm)with a50� objective (NA¼ 0.75, corresponding to a spot of approximately1 mm in diameter). To avoid heating effects, power used in theRaman scattering did not exceed 1 mW.

2.2. Adsorption and photocatalytic tests

2.2.1. Adsorption testsMB adsorption on the catalysts was evaluated using Langmuir

and Freundlich isotherm models. Values of the Langmuir constants(qmax and KL) were obtained by regression using the linearized formof the Langmuir equation [Eq. (1)]:

Ceqe

¼ 1qmaxKL

þ Ceqmax

(1)

where qmax (mg/g) and KL (L/mg) are the Langmuir constantsassociated with the capacity and the adsorption energy, qe is theamount of substance adsorbed (mg/g) and Ce is the equilibriumconcentration (mg/L).

Values of the Freundlich constants (n and KF) were obtained byregression using the linearized form of the Freundlich equation[Eq. (2)]:

lnqe ¼ 1nlnCe þ lnKF (2)

where qe is the amount adsorbed (mg/g), Ce is the equilibriumconcentration of the adsorbate (mg/L); and KF and n are theFreundlich constants related to the capacity and intensity ofadsorption, respectively.

2.2.2. Photocatalytic testsThe adsorption-photodegradation experiments were performed

in a photo-reactor, schematically represented in Fig. 1 The systemconsisted of a 125 W mercury vapor lamp without the protective

Fig. 1. Schematic of the photoreactor employed in the photocatalytic tests.

bulb, encased in a 70.0 cm tall, 4 cm diameter glass cylinder (filtercutoff for l > 300 nm). This was placed at the center of anotherglass cylinder (60 cm tall, 7 cm diameter, with a total capacity of1000mL) containing 300mL of a 24mg/L MB solution and 90mg ofcatalyst. The reaction mixture was first stirred in the dark for30 min to reach the adsorption equilibrium and then exposed toUVevisible radiation provided by the mercury lamp.

To avoid overheating caused by the mercury vapor lamp, thereactor was encased in a water sleeve that maintained the tem-perature at 30 ± 2 �C. At 10 min intervals, 3 mL samples wereremoved from the reactor using a syringe. The catalyst wasmagnetically separated from the solution before measuring ab-sorption at 665 nm. A blank reaction was carried out under thesame conditions but without catalyst addition.

3. Results and discussion

3.1. Characterization

3.1.1. Infrared spectroscopyThe HT-CO3 IR spectrum (Fig. 2A), shows an absorption band at

3394 cm�1 due to stretching of the interlayer water molecule bonds(nOeH). The strong absorption peak at 1359 cm�1 is assigned to thevibration of carbonate species. The bands in the range of450e780 cm�1 are attributed to stretching of AleO and MgeObonds (Toledo et al., 2013). For HT/TiO2/Fe (Fig. 2D), these bandswere superimposed on the FeeO band (Fig. 2B) and the shoulder ofthe TieOeO bond in the region of 450e780 cm�1 (Fig. 2C).

The modification of HT-CO3 by intercalation of the DS surfactantis apparent in its IR spectrum (Fig. 2E). The triplet at 2958, 2914 and

Fig. 2. IR spectra of: (A) HT-CO3; (B) magnetic iron oxide; (C) nano-TiO2; (D) HT/TiO2/Fe34 and (E) HT-DS/TiO2/Fe23.

L.D.L. Miranda et al. / Journal of Environmental Management 156 (2015) 225e235228

2844 cm�1, characteristic of stretching of the CeH bonds (n CeH) inCH3 and CH2 moieties, is evidence of the presence of dodecylsulfateions in the interlayer space. Sulfate bond stretching bands at1210 cm�1 (ns S]O) and 1057 cm�1 (nass S]O) are also visible(Miranda et al., 2014) (Bouraada et al., 2008). The absence of a peakaround 1359 cm�1 (Fig. 2E) associated with stretching of the CO3

2�

group is also evidence of the intercalation of the surfactant in theinterlayer space (Miranda et al., 2014). The characteristic iron oxideband (Fig. 2B) occurs at around 602 cm�1 due to the FeeO bond(Toledo et al., 2013) (Miranda et al., 2014). The shoulder observed at650 cm�1 (Fig. 2C), is attributed to TieOeTi vibration (Lin et al.,2011) (Low and Boonamnuayvitaya, 2013). The characteristicsbands of the FeeO and TieO bonds are better visualized using theRaman scattering technique.

3.1.2. Raman spectroscopyFig. 3 shows the Raman spectra between 100 and 1100 cm�1 for

catalysts (E and F) and their precursors, HT-CO3 (A), HT-DS (B),nano-TiO2 (C) and iron oxide (D). The spectra were normalized bymeasurement parameters to permit comparison of Raman peakintensities. Thus, the spectrum in Fig. 3C is about 350-fold and thespectrum in Fig. 3F about 0.3-fold as intense as the spectra inFig. 3A, B, D and E.

The HT-CO3 Raman spectrum (Fig. 3A) presents the strongestbands at 148, 558 and 1060 cm�1, attributed to the interlayer watermolecules, the stretching vibration of the AleOeMg linked to theoctahedral layers and CeO stretching in CO3

2� bonded to the Al3þ

bound OH groups, respectively. The Raman band at 482 cm�1 areassigned to AleOeAl bond vibrations (Burrueco et al., 2013).

Fig. 3. Raman spectra of the catalysts and its precursors. (A) HT-CO3; (B) HT-DS; (C)magnetic iron oxide; (D) TiO2; (E) HT/TiO2/Fe34 and (F) HT-DS/TiO2/Fe23.

After dodecylsulfate anion intercalation the spectrum changed,as expected (Burrueco et al., 2013), to the profile in Fig. 3B, inwhichthe 148, 482 and 558 cm�1 Raman bands are not present, while theband at 1060 cm�1 shifted to 1074 cm�1. The others Raman featuresare assigned to the intercalated dodecylsulfate.

The Raman lines at 142, 397, 516 and 636 cm�1 in Fig. 3C can beassigned to the Eg, B1g, A1g þ B1g, and Eg modes of the TiO2anatase form (Sellappan et al., 2011). The broad band at 670 cm�1 inFig. 3D is a characteristic Raman band of magnetic iron oxide andcan be associated to the totally symmetric mode (A1g) of magne-tite, Fe3O4(s) (Li et al., 2012).

Fig. 3E and F presents the spectra of HT-CO3 and HT-DS afterincorporation of TiO2 and deposition of magnetic iron oxide. Thesespectra are dominated by the TiO2 and iron oxide bands, with the1060 and 1074 cm�1 hydrotalcite bands only weakly visible. Theseresults indicate a strong incorporation/deposition of TiO2 nano-particles on the hydrotalcite surface, totally covering the structure.The stronger intensity of TiO2 Raman peaks in Fig. 3E suggests thatTiO2 nanoparticles are better incorporated onto HT-CO3 than ontoHT-DS.

3.1.3. X-ray diffractionThe X-ray diffraction patterns of the samples are presented in

Fig. 4.Hydrotalcite (HT-CO3) presented peaks of interplanar distance

(d) and magnitude (hkl) similar to the crystallographic standardsdescribed in the literature (Toledo et al., 2011) (Toledo et al., 2013)(Bouraada et al., 2008) (Zhao et al., 2008). The 2q (0 0 3) basaldiffraction spacing of HT-CO3 was estimated to be 7.6 Å, indicativeof highly crystalline hydrotalcite layers. After addition of DS, theshift in the d(0 0 3) peak to a 2q value of 3.29 and interplanar

Fig. 4. X-ray diffraction spectra. (A) HT-CO3; (B) iron oxide; (C) TiO2; (D) HT/TiO2/Fe34and (E) HT-DS/TiO2/Fe23.

L.D.L. Miranda et al. / Journal of Environmental Management 156 (2015) 225e235 229

spacing of 26.8 Å indicates intercalation of dodecyl ions betweenthe hydrotalcite lamella (Huang et al., 2013). A proposed structurefor HT-DS/TiO2/Fe and a scheme for hydroxyl radical generation onits surface are shown in Fig. 5.

The incorporation of nano-TiO2 particles together with thedeposition of iron oxide on the surface of the catalysts led to ahybrid and disordered compound, leading to broad and weakdiffraction peaks d(0 0 3) for HT/TiO2/Fe34 and HT-DS/TiO2/Fe23(Miranda et al., 2014) (You et al., 2002) (Barbosa et al., 2005) (Lvet al., 2008). Magnetite and maghemite (Fig. 4B) can be identifiedby the 2q diffraction peaks at 30.1, 35.4, 43, 53.4, 57 and 62.5 Å,indicating their surface deposition which is responsible for thecatalysts' magnetic properties (Quinones et al., 2014) (He et al.,2012). The magnetic properties are advantageous since they makeit possible to easily remove the catalysts from solution after use by asimple magnetic process (Toledo et al., 2013) (Miranda et al., 2014).The characteristic diffraction of the anatase phase d(1 0 1) at a 2q ofabout 25 Å can be seen in the spectra of HT/TiO2/Fe34 (Fig. 4D) andHT-DS/TiO2/Fe23 (Fig. 4E) (C.-Y. Chen et al., 2012). Fig. 4D and Eshows that deposition of iron oxides and immobilization of TiO2 didnot change the lamellar structure characteristic of HT-CO3 and HTintercalatedwith DS since the diffraction characteristics of HT/TiO2/Fe34 did not change from those of its precursors, HT-CO3, ironoxides and nano-TiO2. There was only an overlapping of the peaks,demonstrating that the combination of iron oxide, TiO2 and HT-CO3

is likely to be a physical process. The samewas observed for HT-DS/TiO2/Fe23 (Fig. 4E).

3.1.4. Scanning electron microscopyHT-CO3 (Fig. 6A) presented a more regular, smoother surface

than the modified hydrotalcites (Fig. 6B and C) (Huang et al., 2013)(Costa et al., 2008). The deposition of iron oxide and the random

Fig. 5. Schematic illustration of the HT-DS/TiO2/Fe cataly

distribution of TiO2 particles on the surface of the catalysts formedagglomerations with a spongy appearance andwere responsible forthe increased heterogeneity of the catalysts' surfaces (Fig. 6C)(Huang et al., 2013) (Toledo et al., 2013).

3.1.5. Diffuse reflectanceMeasurement of absorption by diffuse reflectance in the

UVevisible region is a convenient and effective method to inves-tigate the semiconductor band gap since electronsmigrate from thevalence band to the conduction band in photocatalytic semi-conductors by absorbing light (Lu et al., 2012). The intensity of theenergy reflected by the catalyst can be obtained using the Kulbelka-Munk function, F(R) (Eq. (3)).

FðRÞ ¼ ð1� RÞ22R

(3)

where R is the absolute diffuse reflectance of the beam, that is theratio between the reflected intensity of the sample and of a non-absorbent standard, calcium carbonate. The relationship betweendiffuse reflectance and band gap energy is given in Eq. (4):

ðFðRÞhlÞ2 ¼ Cðhl� EgÞ (4)

where hl is the energy of a photon and C is the constant of pro-portionality. The band gap energy (Eg) was obtained by plotting(F(R)hl)2 versus hl, as described in the literature (Lv et al., 2008)(Song et al., 2012).

The diffuse reflectance spectra for the UVevisible absorption ofHT-DS, nano-TiO2, iron oxide and HT-DS/TiO2/Fe23 samples areshown in Fig. 7. There was no absorption in the UVevisible regionfor HT-DS, in agreement with the literature (Huang et al., 2013).However, nano-TiO2, iron oxides and HT-DS/TiO2/Fe23 presented

st and formation of hydroxyl radicals on its surface.

Fig. 6. SEM of the catalysts: (A) HT-CO3; (B) HT-TiO2/Fe34 and (C) HT-DS/TiO2/Fe23.

L.D.L. Miranda et al. / Journal of Environmental Management 156 (2015) 225e235230

absorption bands. Absorption shifted towards a longer wavelength,or lower band gap energy, in HT-DS/TiO2/Fe23 compared to nano-TiO2. The decreased energy suggests that less energy would berequired for photocatalytic activity (Lv et al., 2008) (Song et al.,2012). The value of the band gap determined for TiO2 (3.3 eV) isin agreement with the values reported in the literature (between3.2 and 3.3 eV) (Sellappan et al., 2011) (Rashad et al., 2013) (Songet al., 2012).

Light absorption in the HT-DS/TiO2/Fe23 catalyst shifted to-wards longer wavelengths in the visible range. This shift is pur-portedly due to electronic interaction between the molecularorbitals of iron oxides (which possess narrow band openings, 0.1 eVfor magnetite and 2.2 eV for maghemite) and TiO2 (3.2e3.3 eV)forming a new molecular orbital and reducing the band difference.

Fig. 7. The diffuse reflectance UVevisible spectra of HT-DS, nano-TiO2, magnetic ironoxide and HT-DS/TiO2/Fe23.

Similar phenomena were observed between graphene and TiO2(Liu et al., 2013), carbon nanotubes and TiO2 (Sampaio et al., 2011)and graphene, TiO2 and magnetite (Tang et al., 2013).

3.2. Adsorption and photocatalytic activity of the synthesizedcatalysts

Adsorption isotherms of MB on HT-DS/TiO2/Fe are shown inFig. 8 and the values of the adsorption constants (KL, Q max, KF, n andR2) are shown in Table 1. The Langmuir model (R2 > 0.99) describedthe adsorption process better than the Freundlich (R2 > 0.94)model, as previously found for adsorption of pollutants on hydro-phobic organic LDHs (Huang et al., 2013). According to the Lang-muir model, when one molecule of MB is adsorbed onto a givenlocation of the adsorbent, this location becomes unavailable toother molecules and a single layer of dye molecules is formed(Miranda et al., 2014). MB adsorption capacities based on Qmaxvalues decreased in the order: HT-DS/TiO2/Fe23 > HT-DS/TiO2/Fe43 > HT-DS/TiO2/Fe63 > HT-DS/TiO2/Fe73. The catalyst with an

Fig. 8. Langmuir adsorption isotherm for MB on the HT-DS/TiO2/Fe catalysts.

Table 1Isotherm model parameters for the adsorption of MB on HT-DS/TiO2/Fe23.

Catalyst Langmuir constants Freundlich constants

KL (L/mmol) Qmax (mg/g) R2 KF(min) n R2

HT-DS/TiO2/Fe23 0.473 133.37 0.990 71.636 4.926 0.945HT-DS/TiO2/Fe43 0.532 119.30 0.990 66.007 5.208 0.951HT-DS/TiO2/Fe63 0.669 98.62 0.996 57.538 5.747 0.947HT-DS/TiO2/Fe73 0.821 43.41 0.998 26.888 7.092 0.941

L.D.L. Miranda et al. / Journal of Environmental Management 156 (2015) 225e235 231

iron oxide:TiO2 molar ratio of 2:3 exhibited the highest MBadsorption (53.4%).

The photocatalytic activity of hydrotalcite can be significantlyaffected by the amount of TiO2 incorporated on its surface, thus theHT:TiO2 ratio should be optimized. As shown in Fig. 9A, after120 min reaction, the HT-2TiO2 catalyst exhibited a higher

Fig. 9. Photocatalytic activity of the HT catalysts: (A) with different amounts of TiO2;(B) with TiO2 and different amounts of iron oxide and (C) interspersed with DS sur-factant, with TiO2 and different amounts of iron oxide in the removal of MB. Initialconcentration of MB 24 mg/L; catalyst dose 0.3 g/L; reaction temperature 30 ± 2 �C.

photocatalytic activity than HT-CO3 and catalysts prepared withdifferent amounts of TiO2 (HT/0,5TiO2; HT/1TiO2; HT/1,5TiO2 andHT/3TiO2). Increasing the quantity of TiO2 incorporated into the HT-CO3 up to 2mol Ti led to increased generation of e�/Hþ pairs and anincreased photodegradation rate. However, above this optimumlevel, TiO2 blocked the passage of light and increased light scat-tering, resulting in a lower rate of photodegradation for the HT-3TiO2 catalyst (Wang et al., 2014) (Huang et al., 2013). Approxi-mately 13% of the MB was degraded by direct photolysis in theabsence of catalyst, indicating that radiation alone had a weak ef-fect on MB degradation (Fig. 9A).

The magnetic characteristic of the catalysts arises from the ironoxide deposited on their surface, while photocatalytic activity ismainly located in the TiO2 layer. It is thus necessary to define asuitable iron oxide:TiO2 molar ratio to achieve efficient photo-degradation while ensuring the magnetic characteristics of thecatalysts (He et al., 2012). The photocatalytic activity of the HT/2TiO2 catalyst with different amounts of iron oxide was thereforestudied (Fig. 9B) and it was observed that the HT/TiO2/Fe34 catalystshowed the greatest photo-activity. A small reduction (10%) in MBdegradation was observed using magnetic HT/TiO2/Fe34 comparedto nonmagnetic HT/2TiO2 catalysts (Fig. 9B). However, themagneticcatalysts have the advantage of being easily removed from solutionby applying a magnetic field, thus reducing time and costs of theirseparation/recovery (Miranda et al., 2014).

Photodegradation of organic pollutants by TiO2 photocatalysisoccurs mainly on or near the surface of the catalyst and thusadsorption is a critical factor in the efficiency of the process (Huanget al., 2013) (Ooka et al., 2003) (An et al., 2008). Previous in-vestigations have shown that hydrotalcite-iron oxide intercalatedwith surfactants are good adsorbents for MB (Miranda et al., 2014).Therefore, we evaluated the synergistic effects of combiningadsorption on HT-DS, photo-activity of TiO2 and the magneticcharacteristic provided by iron oxide of the new magnetic catalystsfor MB photodegradation.

Photocatalytic activity of the HT-DS/TiO2 catalyst with differentquantities of iron oxide was studied and it was observed that theHT-DS/TiO2/Fe23 catalyst showed the highest degree of photo-activity (Fig. 9C). Magnetic HT-DS/TiO2/Fe23 photoactivity wasslightly lower than that of nonmagnetic HT-DS/2TiO2, resulting in a6% reduction in MB degradation, but this decrease is outweighed bythe advantage of ready removal of the magnetic catalyst from so-lution by applying a magnetic field (Miranda et al., 2014).

Fig. 10A compares MB degradation by HT-CO3, HT/TiO2/Fe34,HT-DS/TiO2/Fe23, TiO2 nano-particles and photolysis. The HT-DS/TiO2/Fe23 catalyst showed the highest rate of MBphotodegradation.

The photodegradation process and the possible MB degradationpathways are illustrated in Fig. 5. When a photon (hn) with energyequal to or greater than the band gap energy hits the HT-DS/TiO2/Fecatalyzer, an electron in the valence band (VB) is promoted to theconduction band (CB), leading to simultaneous generation of a gapin the valence band (hþ) and an excess of electrons in the con-duction band (e�) (Eq. (5)) (Huang et al., 2013).

HT� DS=TiO2=Feþ hv/HT� DS=TiO2

�e� þ hþ�=Fe (5)

Although various reactionmechanisms for MB degradation havebeen reported in the literature, it is accepted that the stronglyoxidizing band gaps can directly attack the MB adsorbed on thesurface (Eqs. (6) and (7)) or can indirectly oxidize MB through theformation of hydroxyl radicals (�OH) (Eq. (8)). The gaps (hþ)generated possess sufficiently positive potentials to generate hy-droxyl radicals from water molecules adsorbed on the surface, andthese radicals can in turn oxidize MB (Eq. (9) and Fig. 5) (Huang

L.D.L. Miranda et al. / Journal of Environmental Management 156 (2015) 225e235232

et al., 2013) (Konstantinou and Albanis, 2004) (Jing et al., 2011).

hþ þMB/oxidation products (6)

e� þMB/reduction products (7)

Fig. 10. (A) Comparison of the photocatalytic activity of the catalysts for the removal ofMB; (B) Variation in the total organic carbon content (TOC) of the solution during theMB photodegradation; (C) Absorption spectra for MB degradation using the HT-DS/TiO2/Fe23 and (D) Photograph the MB catalyst solution before (left) and after (right)photodegradation, and HT-DS/TiO2/Fe23 being attracted by a magnet (right).

hþ þH2Oads/�OHþ Hþ (8)

�OHþMB/CO2 þ H2O (9)

When oxygen is present it can act as a CB electron acceptor,initiating a series of free radical chain reactions that also result in�OH formation (Eqs. (10)e(14)) (Nogueira and Jardim, 1998) (Ziolliand Jardim, 1998).

O2 þ e�/O�2

� (10)

O�2

� þHþ/HO2� (11)

HO2� þ HO2

�/H2O2 þ O2 (12)

H2O2 þ e�/OH� þ OH� (13)

H2O2 þ O�2

�/OH� þ OH� þ O2 (14)

LDHs are hydroxyl-rich materials and their hydroxyl groups canreact with valence band gaps to produce �OH (Eq. (15) and Fig. 5)(Huang et al., 2013).

hþ þ OH�/�OH (15)

These �OH produced on the surface are considered to be moreeffective in attacking the adsorbed MB because of their mutualproximity (Huang et al., 2013). Therefore, greater adsorptionprobably caused the higher percentage ofMB degradation observedfor the HT-DS/TiO2/Fe23 catalyst compared to the HT/TiO2/Fe34catalyst. After 120 min of reaction, MB removal efficiency (Fig. 10A)was 41, 72 and 96% and TOC removal efficiency (Fig. 10B) was 15, 42and 61% for HT-CO3, HT/TiO2Fe34 and HT-DS/TiO2/Fe23, respec-tively. It should be noted that all tests were performed using thesame amount of catalyst. However, the magnetic HT-DS/TiO2/Fe23catalyst exhibited photocatalytic performance similar to TiO2 alone,although a lesser amount of TiO2 was present in the new catalyst.

Typical evolution of MB absorption during radiation is pre-sented in Fig. 10C. A sharp decrease (53.4%) in MB absorption peakintensity was found after adsorption in the dark for 30min, becauseof the relatively high MB adsorption capacity (133.37 mg/g) of theHT-DS/TiO2/Fe23 catalyst. Upon radiation, the characteristic MBabsorption peak decreased gradually and almost no color wasobserved after 120min, indicating thatMBwas degraded byHT-DS/TiO2/Fe23. Fig. 10D shows the HT-DS/TiO2/Fe23 catalyst attracted toa 0.3 T magnet after use in MB photodegradation, illustrating itscomplete removal from solution.

The similarity in photocatalytic activity of HT-DS/TiO2/Fe23 andTiO2 can be attributed to synergistic effects of four factors: (1) HT-DS/TiO2/Fe23 is an excellent dispersant and transporter for nano-TiO2 particles; (2) HT-DS/TiO2/Fe23 has a high adsorption capacityfor MB (133.37 mg/g), which assists in photocatalytic performance;(3) the HT surface is rich in hydroxyl groups that favor production of�OH by reaction with gaps; and (4) HT-DS/TiO2/Fe23 shifts lightabsorption to a longer wavelength, and less energy is needed forphotocatalytic activity.

The novel magnetic HT-DS/TiO2/Fe23 catalyst showed satisfac-tory performance in MB degradation when compared to other TiO2supported catalysts reported in the literature (Table 2).

3.3. Analysis of the material after reaction

Four cycles of MB photodegradation were performed with thesame HT-DS/TiO2/Fe23 catalyst in order to evaluate its stability.There was a small reduction in MB degradation upon reuse of HT-

Table 2MB dye degradation by different TiO2 supported catalysts.

Catalyst Experimental conditions

Dose (g/L) % degradation MB solution (mg/L) Radiation time (min) Ref.

SICeTiO2 1 52 20 180 G�omez-Solís et al., 2012TiO2eMn doped 1 ~100 5 60 Rashad et al., 2013TiO2eCo doped 1 ~100 5 60 Rashad et al., 2013BiVO4eFe 1 81 5 30 Chala et al., 2014ZnAlTi-HDL 1 ~100 10 120 Wang et al., 2014Fe3O4@HDL@Ag/Ag3PO4 1 100 10 60 Sun et al., 2014ZnAlTi-HDL 2 100 35 250 Sahu et al., 2013Fe3O4/ZnCr-HDL 1 ~95 5 180 Chen et al., 2013HT-DS/TiO2/Fe23 0,3 ~100 24 120 This study

L.D.L. Miranda et al. / Journal of Environmental Management 156 (2015) 225e235 233

DS/TiO2/Fe23 (Fig. 11), with photocatalytic activities of 94, 92, 89and 83% in the first to fourth cycles, suggesting a good potential forcatalyst recycling. Wang et al. (2014) reported that MB degradationwas reduced to 76% after four cycles using ZnAlTi-LDH as catalyst.

After use in MB treatment, the HT-DS/TiO2/Fe23 catalyst wasremoved from solution using a magnet and then dried at 60 �Cwithout washing. Recovered catalyst was analyzed before and afterboth adsorption and irradiation in order to check the stability of HT-DS/TiO2/Fe23 after one cycle (2 h) and four cycles (8 h) of MBdegradation using IR spectroscopy (Fig. 12C) and X-ray (Fig. 12D)analyses. Small changes were observed in the IR absorption bandsafter MB adsorption. The 1600 cm�1 band became sharper due tooverlapping of the n C]N band and the 1485 and 1339 cm�1 bandintensities increased because of overlapping of the MB n C]C andCAr-N bands, respectively (Yu and Chuang, 2007). After UVevisibleirradiation, these changes disappeared (Fig. 12D), indicating thatMB that had adsorbed on the catalyst was degraded. The similaritybetween catalyst spectra before and after photodegradation in-dicates that the dodecylsulfate ion remained in the interlayer space.A similar phenomenon was observed for TiO2 supported on LDHintercalated with DS after photodegradation of dimethyl phthalate(Huang et al., 2013). The recycled catalyst's IR spectrum (Fig. 12D)contained characteristic DS bands: a CH2 band between 2844 and2958 cm�1, a symmetrical stretching vibration at ~1210 cm�1 andan asymmetrical stretching vibration at ~1057 cm�1 arising fromthe sulfonyl (eS]O) vibration (Miranda et al., 2014), evidence thatDS still existed in HT-DS/TiO2/Fe23 after 8 h of irradiation (Fig.12D).

To investigate further the possible degradation of DS in the

Fig. 11. Effect of HT-DS/TiO2/Fe23 catalyst reuse in MB photodegradation. 1-First cycle,2-Second cycle, 3-Third cycle and 4-Fourth cycle. Initial concentration of MB 24 mg/L;catalyst dose 0.3 g/L; reaction temperature 30 ± 2 �C; reaction time 2 h.

interlamellar space of HT, the X-ray diffraction patterns of thesamples were obtained (Fig. 13). After repeated use, the catalyst'scrystallinity did not significantly change, with basal spacing, d(00 3), remaining at 25e26 Å, once again indicating that the DS ionsremained in the HT interlalamellar space, even after 8 h of irradi-ation. The characteristic TiO2 and the iron oxide peaks were alsopreserved (Fig. 13).

4. Conclusions

Novel magnetic catalysts, HT/TiO2/Fe and HT-DS/TiO2/Fe, weresynthesized in this work and their activity evaluated in photo-degradation of MB dye. Synergistic effects of adsorption on HT-DS,photoactivity of TiO2 and magnetic properties provided by ironoxide were observed. The photocatalytic activity of the catalystswas significantly affected by the amount of TiO2 and iron oxide

Fig. 12. IR spectrum of materials: (A) MB; (B) HT-DS/TiO2/Fe23; (C) HT-DS/TiO2/Fe23after adsorption and (D) HT-DS/TiO2Fe23 after irradiation.

Fig. 13. (A) The X-ray of the HT-DS/TiO2/Fe23 catalyst; (B) after 2 h of UVeVisibleradiation and (C) after 8 h of UVeVisible radiation.

L.D.L. Miranda et al. / Journal of Environmental Management 156 (2015) 225e235234

incorporated into HT and DS-modified HT. The highest MB (96%)and TOC (61%) removals after 120 min of reaction were obtainedwith the HT-DS/TiO2/Fe catalyst at an iron oxide:TiO2 molar ratio of2:3. The HT-DS/TiO2/Fe23 catalyst shifted light absorption to alonger wavelength, and less energy was needed for photocatalyticactivity. Moreover, the catalyst could easily be separated from thetreated solution for reuse by simply applying an external magneticfield.

Acknowledgments

The authors acknowledge the financial support of the Fundaç~aode Amparo �a Pesquisa do Estado de Minas Gerais (FAPEMIG, Uni-versal Demand, process number: APQ-00416-11) and the ConselhoNacional de Desenvolvimento Científico e Tecnol�ogico.

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