Preparation and characterization of an organo-palygorskite-Fe3O4 nanomaterial for removal of anionic dyes from wastewater

Applied Clay Science 139 (2017) 45–53

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Research paper

Preparation and characterization of an organo-palygorskite-Fe3O4 nanomaterial for removal of anionic dyes from wastewater Antonieta Middea a,b, Luciana S. Spinelli b,c,⁎, Fernando Gomes Souza Jr b,d, Reiner Neumann a,e, Thais L.A.P. Fernandes a, Otavio da F.M. Gomes a,e a

Centro de Tecnologia Mineral (CETEM), Av. Pedro Calmon, 900, Cidade Universitária, Rio de Janeiro 21941908, Brazil Universidade Federal do Rio de Janeiro, Instituto de Macromoléculas, Laboratório de Macromoléculas e Colóides na Indústria de Petróleo, Av. Horácio Macedo, 2030, Rio de Janeiro 21941598, Brazil Universidade Federal do Rio de Janeiro, Instituto Alberto Luiz Coimbra de Pós-Graduação e Pesquisa em Engenharia, Programa de Engenharia da Nanotecnologia, Av. Horácio Macedo, 2030, Bl. G, Ilha do Fundão, Rio de Janeiro 21941-598, Brazil d Universidade Federal do Rio de Janeiro, Instituto Alberto Luiz Coimbra de Pós-Graduação e Pesquisa em Engenharia, Programa de Engenharia Civil, Av. Horácio Macedo, 2030, Bl. D, Ilha do Fundão, Rio de Janeiro 21941972, Brazil e Universidade Federal do Rio de Janeiro, Museu Nacional do Brasil, Programa de Pós-graduação em Geociências, Av. Quinta da Boa Vista, S/N Bairro Imperial de SãoCristóvão, Rio de Janeiro 20940040, Brazil b c

a r t i c l e

i n f o

Article history: Received 24 June 2016 Received in revised form 4 January 2017 Accepted 13 January 2017 Available online xxxx Keywords: Nanostructures Methyl orange Indigo carmine Magnetic properties Magnetic structures Surface properties

a b s t r a c t The organophilization of clays to increase interfacial interactions is a typical practice when the objective is its encapsulation with a polymer. This work intend to synthesize and characterize an organo-palygorskite-Fe3O4, to change the hydrophilicity of the material and investigate its efficiency in the removal of two anionic dyes from aqueous solution with the aim to use this material to treat wastewater contaminated with dyes. The organophilization of modified palygorskite–Fe3O4 was done with a cationic surfactant (cetyl trimethylammonium bromide, CTAB) and the material obtained was characterized by XRF, SEM, XRD, FTIR, Nano Zetasizer, density, BET surface area, TGA/DTA and contact angle measurements. XRF and FTIR confirmed the presence of carbon in the modified palygorskite–Fe3O4 nanoparticles while XRD showed new crystalline phases from CTAB. Furthermore, a positive zeta potential value appeared and an inversion in the nanoparticles' hydrophobicity happened due to the increase of organic carbon content in the organo-palygorskite-Fe3O4. The adsorbed anionic dye on organo-palygorskite-Fe3O4 may be completely recovered by the action of a magnet, showing an excellent adsorptive property and economical alternative adsorbents to treat dye wastewater. The negative values calculated for the free energies of adsorption indicate that the adsorption of methyl orange and indigo carmine on the active sites of organo-palygorskite-Fe3O4 surface was spontaneous and that the indigo carmine had the best interaction with the magnetic organo clay. © 2017 Elsevier B.V. All rights reserved.

1. Introduction The presence of anionic dyes in aqueous environments can cause harmful to aquatic life in rivers where they are discharged as a result of their toxicity. In general, dyes are used in many industries such as the textile, leather, food, dyeing, cosmetics and paper and, the removal of dyes from water is a matter of great interest in the field of water pollution because even the presence of small dyes concentrations can affect the water color and turn it unpleasant (Hidarian and Hashemian, 2014; Shen et al., 2009; Bhatt et al., 2012; Kiransan et al., 2014; Chen et al., 2011a; Anbia and Salehi, 2012; Dalaran et al., 2011; Elemen et al., 2012). The use of available and effective adsorbents such as magnetic clay derivatives is an alternative to the various treatment technologies that ⁎ Corresponding author. E-mail address: [email protected] (L.S. Spinelli).

http://dx.doi.org/10.1016/j.clay.2017.01.017 0169-1317/© 2017 Elsevier B.V. All rights reserved.

have been used for removing dyes from aqueous solutions (Jiang et al., 2012; Obeid et al., 2013; Deligeer et al., 2011). Palygorskite is an important clay mineral with micro fibrous morphology that has industrial, catalytic and environmental applications due to its excellent physical and chemical properties (Middea et al., 2013; Wang et al., 2010). Superparamagnetic nanoparticles such as Fe3O4 adsorbed on palygorskite surfaces makes them particularly suitable for pollutant removal because no centrifugation or filtration of the sample is needed (Faraji et al., 2010; Middea et al., 2015; Chen and Zhao, 2009; Hu et al., 2009). The application of magnetic adsorption technology to solve environmental problems has received considerable attention nowadays because after magnetic adsorbents adsorbing contaminants from water they can be easily separated and collected from the medium by a simple magnet system (Obeid et al., 2013). The use of palygorskite as a nano-sized filler in organic nanocomposites has been investigated extensively in the literature (Shen et al., 2005;

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Table 1 Structure and characteristics of anionic dyes. Molecular formula

Mw (g/mol)

λmax (nm)

Methyl orange

C14H14N3NaO3S

327.33

505

Indigo carmine

C16H8N2Na2O8S2

466.35

608

Anionic Dye

Chemical structure

Pan et al., 2009; Lai et al., 2005; Li et al., 2010; Chen et al., 2011b; Lei et al., 2009; Lu et al., 2005; Xue et al., 2006). Surface modification of inorganic nanofillers is necessary for the preparation of nanocomposites due to the high degrees of dispersion of the filler particles, and this requires adequate organic-filler interactions (Tjong, 2006; Wang et al., 2009; Ma et al., 2004). Quaternary ammonium salts or silane coupling agents are commonly used as palygorskite modifiers (Zhaoa et al., 2012). These quaternary ammonium salts are responsible for the transformation of ordinary clays into organophilic ones. By adding these salts to aqueous dispersions of clay, the organic cations replace the cations that are easily exchangeable. Thus, the quaternary ammonium cations with free long hydrocarbon chains are accommodated between the layers 2:1 of the clay mineral, making it organophilic (Paria and Khilar, 2004; Gurses et al., 2012). Cetyl trimethylammonium bromide (CTAB) is a cationic surfactant with a carbon-16 hydrocarbon tail. Palygorskite nanoparticles' surfaces in aqueous environments exhibited negative values over the entire range of pH, which makes ionic bonding with a cationic surfactant possible (Zhang et al., 2012; Jin et al., 2012; Wu et al., 2013; Wang et al., 2013). The pretreatment of palygorskite – Fe3O4 with CTAB is essential for successful encapsulation and to create strong interfacial interaction with a polymer (Lee et al., 2004; Karaca et al., 2013; Mehta et al., 2009). The studies using organo-modification pretreatment of palygorskite, such as surface cation exchange with a surfactant or surface-silylation with organosilane (Shen et al., 2005; Xue et al., 2006), have aimed to enhance affinities between the strongly hydrophilic palygorskite and the hydrophobic organic matrices. In this work, palygorskite–Fe3O4 nanoparticles, with an excellent magnetic property, after modification by a surfactant were used as a cationic adsorbent to remove methyl orange and indigo carmine from aqueous solutions. These anionic dyes were select as a model pollutant to examine the adsorption capacity due to its great importance from water treatment.

2. Experimental

2.2. Methods 2.2.1. Organo-palygorskite – Fe3O4 preparation To make the magnetic palygorskite organophilic, we used a cationic surfactant that has a long apolar chain (CTAB). For this purpose, we chose a concentration above the critical micelle concentration (CMC) of the surfactant needed to stabilize ferrofluids (Deligeer et al., 2011). The palygorskite – Fe3O4 was performed using co-precipitation of ferric and ferrous chloride in an alkaline medium (Liu et al., 2008; Jeong et al., 2006, Middea et al., 2015), then mixed with 500 mL of distilled water (1.0% w/v of palygorskite – Fe3O4) and sonicated for 30 min in an ultrasonic bath. After that, a concentration above the CMC, about 1.7% w/v of CTAB, was used. The pH was adjusted to 8.0 and the ultrasonic bath was maintained for three more hours. A curve of surface tension versus CTAB concentration was previously plotted to identify the surfactant micelle formation concentration, using the du Noüy ring with a K10 tensiometer by Kruss. The CTAB's critical micelle concentration was also investigated by conductivity analysis using a MS Tecnopon mCA-150 conductivity meter. 2.2.2. Dyes adsorption experiments Dye adsorption behavior was studied by a batch method which permits convenient evaluation of parameters that influence the adsorption process such contact time and initial concentration. These studies were carried out in 15 mL conical flasks containing 10 mL of the adsorption solution with desired dye concentration and 0.1 g organopalygorskite-Fe3O4 powder. To increase the adsorption capacity as possible and keep natural pH value of anionic dyes solutions, pH below 7.0 was selected (Hidarian and Hashemian, 2014; Chen et al., 2011a; Deligeer et al., 2011). The flasks were stirred in a thermostatic shaker bath at 130 rpm and room temperature. At given time intervals, the suspension containing adsorbents was extracted and collected using a magnet (7500 G) for 60 s. Then, the decrease of dye concentration in the supernatant liquid after contact with the magnetic organoclay was analyzed using a LaMotte Smart Spectro UV–VIS spectrophotometer at 505 nm (Chen et al., 2011a; Deligeer et al., 2011) for methyl orange and 608 nm (Hidarian and Hashemian, 2014; Dalaran et al., 2011) for indigo carmine.

2.1. Materials The palygorskite sample used in this study came from ColorminasColorífico e Mineração S.A., a company located in Piauí state, northeast Brazil. The sample used was gently ground to obtain particle size under 20 μm. Its magnetized form (palygorskite – Fe3O4) was previously investigated and characterized, mainly, its magnetic power (Middea et al., 2013, 2015). The reagents ferric chloride hexahydrate (FeCl3·6H2O), ferrous chloride tetrahydrate (FeCl2·4H2O) and CTAB were analytical grade, obtained from Merck Co., used as received. Distilled water was used throughout. The anionic dyes methyl orange and indigo carmine were from Vetec and were used without further purification. Chemical structures and other characteristics are given in Table 1.

Table 2 Chemical composition and LOI (loss on ignition) of the organo-palygorskite–Fe3O4 nanoparticles (mass %). Element

Palygorskite-Fe3O4

Organo-palygorskite-Fe3O4

SiO2 MgO Al2O3 Fe2O3 TiO2 MnO K2O LOI Total

21.8 1.3 6.8 48.8 0.4 0.1 0.6 20.2 100.0

15.5 0.8 5.5 31.6 0.4 0.1 0.3 45.8 100.0

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Fig. 1. X-ray diffraction patterns of organo-palygorskite- Fe3O4.

The data were used to calculate the adsorbed amount (in mg) of the adsorbent (in g) by following Eq. (1):

Γ¼

ðCi−Cf ÞV W

ð1Þ

where Γ is the adsorption of methyl orange or indigo carmine onto the magnetic organoclay sample (mg/g), Ci and Cf are the initial and final

dyes concentrations in solution, V is the volume of solution used and W the weight of the adsorbent (Chen et al., 2011b; Middea et al., 2013). 2.2.3. XRF spectrometry and carbon content measurements The chemical analysis of the samples was performed with a Panalytical Magix PW 2403 XRF spectrometer on molten pellets with a lithium metaborate:tetraborate mixture, after LOI (loss on ignition) determination (LECO TA 701). Additionally, their carbon content was measured with a LECO SC632 analyzer.

Fig. 2. SEM images for natural palygorskite (a), palygorskite-Fe3O4 (b) and organo-palygorskite-Fe3O4 (c).

Fig. 3. Zeta potential for organo-palygorskite-Fe3O4 versus pH (a) and formation mechanism of the organo-palygorskite–Fe3O4 nanoparticles (b).

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Table 3 Carbon content, density, BET surface area and d(0.5) of the nanoparticles. Nanoparticle

Carbon content (%)

Density (g/cm3)

BET surface area (m2/g)

d(0.5) (nm)

Palygorskite Palygorskite–Fe3O4 Organo-palygorskite–Fe3O4

0.37 0.18 17.60

2.43 2.68 1.98

108.86 102.40 85.41

250 700 1050

2.2.4. SEM The morphology of the samples was investigated by secondary electrons in a FEI Quanta 400 scanning electron microscope operating at 25 kV and small spot size on samples coated with gold. 2.2.5. XRD The X-ray diffraction analysis was conducted on back-loaded samples (to avoid preferred orientation) with a Bruker-D4 Endeavour diffractometer, Co Kα radiation (40 kV/40 mA), in the 2θ angle range from 5° to 80°, with a 0.02° step, λ = 1.78897 Å and a position-sensitive LynxEye detector. 2.2.6. FTIR The FTIR analysis was performed with a PerkinElmer 1720X instrument in transmission mode on samples pressed with 1:10 KBr, in the wave number range from 4000 to 400 cm−1. 2.2.7. Nano Zetasizer measurements Size and surface charge of the palygorskite samples were evaluated by a Malvern Nano Zetasizer Instrument. For surface charge

measurements, a suspension of the palygorskite nanoparticles in molar KCl · 10−3 (indifferent electrolyte) was prepared. The pH was adjusted using NaOH and HCl in the range between 2.0 and 11.0.

2.2.8. Density The nanoparticles' densities were determined with an AccuPyc 1330 (He) pycnometer from Micrometics Instrument Corporation.

2.2.9. BET surface area The nanoparticles' surface areas were measured with an ASAP 2010 analyzer from Micrometics Instrument Corporation. The samples were outgassed under vacuum at 200 °C for 24 h. After pretreatment the surface areas were calculated by Brunauer-Emmett-Teller (BET) method.

2.2.10. TGA The TGA/DTA analysis of the nanoparticles was performed with a Mettler Toledo TGA Star system, at a heating rate of 10 °C/min, under air atmosphere with a flow rate of 50 mL/min.

2.2.11. Evaluation of the nanoparticles' hydrophobicity The degree of hydrophobicity of the nanoparticles was determined by measuring the contact angle between the solid surface and water. The nanopalygorskite samples (natural and modified) were prepared as pressed pellets made from the dry material. The left and right contact angles were measured.

Fig. 4. TGA/DTG curves of the (a) natural palygorskite, (b) palygorskite-Fe3O4, (c) organo-palygorskite-Fe3O4 nanoparticles.

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Fig. 5. FTIR spectra and markings of (a) natural palygorskite, (b) palygorskite-Fe3O4, (c) organo-palygorskite-Fe3O4.

3. Results and discussion

2008; Li et al., 2012; Lee and Tiwari, 2012; Liu et al., 2013). Therefore, a concentration of 1% w/v was used in this work to be sure that micelles were formed.

3.1. Characterization of CTAB The concentration of a surfactant above which micelles are spontaneously formed is called the critical micelle concentration (CMC). In this work, surface tension and conductivity measurements were used to determine the CMC of CTAB. The values were about 0.07% w/v and 0.09% w/v, respectively. The CMC values of CTAB cited in the literature are about 0.08% w/v (Zhang et al., 2012; Bai et al.,

3.2. Characterization of nanoparticles 3.2.1. XRF According to XRF previous data the iron content in the natural palygorskite increased from 8.2% to 48.8% after the use of magnetite

Fig. 6. Contact angle for (a) natural palygorskite, (b) palygorskite-Fe3O4, (c) organo-palygorskite-Fe3O4.

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Fig. 7. Palygorskite-Fe3O4 (a) and organo-palygorskite-Fe3O4 (b) dispersed in methyl orange solutions submitted to a magnetic field and palygorskite-Fe3O4 (c) and organo-palygorskiteFe3O4 (d) dispersed in indigo carmine solutions submitted to a magnetic field.

co-precipitation technique and the loss on ignition remain constant around 20% (Middea et al., 2015). The XRF results for organo-palygorskite- Fe3O4 nanoparticles are presented in Table 2. The loss on ignition increased to 45.8%, more than twice, confirming the carbon presence in the nanoparticles. On the other hand, Fe2O3 content decreases from 48.8% to 31.6% due to the increase of organic material.

The presence of typical palygorskite elements, such as magnesium and silica, and its impurities like Al, Fe, K and Ti were also noted (Baltar et al., 2009). 3.2.2. XRD The organophilization of palygorskite–Fe3O4 was also analyzed by Xray powder diffraction and the powder patterns of organo-palygorskite-

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Fig. 8. Influence of contact time on the adsorption dyes, indigo carmine (λ = 608 nm) and methyl orange (λ = 505 nm) onto organo-palygorskite-Fe3O4 (a). Adsorption of dyes, indigo carmine (λ = 608 nm) and methyl orange (λ = 505 nm) onto organo-palygorskite-Fe3O4 during 60 min (b).

Fe3O4 are presented in Fig. 1. Diffraction peaks of natural palygorskite and palygorskite-Fe3O4 nanoparticles were studied before and showed some impurities, such as kaolinite and quartz for natural palygorskite and the magnetite's crystalline phase after the palygoskite magnetization (Middea et al., 2013, 2015). In this study, after organophilization process, CTAB adsorbed only on the surface of palygorskite-Fe3O4 during the process, without destroying the crystalline structure, while new crystalline phases of CTAB appeared.

3.2.3. SEM The SEM images of natural palygorskite, palygorskite-Fe3O4 and organo-palygorskite-Fe3O4 are presented in Fig. 2. The fibrous morphology of natural palygorskite is revealed by Fig. 2a, where needle-like fibers can be seen forming a randomly oriented network. These needles seem to be aggregate in a presence of magnetites (Fig. 2b). However, a magnetically oriented structure (Fig. 2c) occurred with the presence of the hydrophobic tails of CTAB. This may be due to the strong agitation and ultrasound treatment of the nanoparticles' surfaces during the organophilization process.

Fig. 9. Adsorption isotherms fitted with Langmuir model of dyes, indigo carmine (λ = 608 nm) and methyl orange (λ = 505 nm) onto organo-palygorskite-Fe3O4 during 60 min.

3.2.4. Zeta potential The surface charge results of the nanoparticles used in this work are shown in Fig. 3. According to the literature the natural palygorskite and palygorskite-Fe3O4 nanoparticles exhibited highly negative zeta potential values over the entire pH range studied (Alkan et al., 2005; Liu et al., 2008; Middea et al., 2015). Therefore, the positive zeta potential values shown in Fig. 3 appeared due to the increase of organic carbon content in organo-palygorskite-Fe3O4. This can be attributed to the increased hydrophobic interactions between CTAB cations, which added positive charge within the shear plane (Sarkar et al., 2012). It can be also concluded that the palygorskite-Fe3O4 nanoparticles showed an effective organophilization. Fig. 3 also showed a formation mechanism proposes for the organo - palygorskite - Fe3O4 nanoparticles. 3.2.5. Carbon Content – Density – BET – Nanosizer In Table 3 four different characterizations results of organopalygorskite-Fe3O4: Carbon Content – Density – BET surface area – Nanosizer, were compared with results of natural palygorskite and palygorskite-Fe3O4. An increase in the carbon grade occurred after adding CTAB to the palygorskite-Fe3O4 nanoparticles at around 17% w/w. On the other hand, the densities varied from 2.43 to 1.98 g/cm3, showing that density changed due to the increase of carbon content. The BET surface area values obtained for natural palygorskite, palygorskite-Fe3O4 and organo-palygorskite-Fe3O4 show a more significant decrease when carbon was added to the nanoparticles. These results suggest that a change occurs in the nanoparticles' surfaces. Finally, the value of the particle diameter at 50% in the cumulative distribution, d(0.5), of the nanoparticles increased as carbon was added to the structure, from 250 nm for the natural palygorskite to 1050 nm for the organo-palygorskite-Fe3O4 showing a good linear correlation with BET surface area (−0.947). 3.2.6. TGA/DTA TGA/DTA characterization results for the organo-palygorskite-Fe3O4 nanoparticles used in this work are shown in Fig. 4 and the results compared with the TGA/DTA results of natural palygorskite and palygorskite-Fe3O4 nanoparticles performed in a previous work (Middea et al., 2015). A typical three-stage mass loss during heating was found in all nanoparticles. The first stage, at around 72.4 °C (natural palygorskite), 70.0 °C (palygorskite–Fe3O4) and 60.8 °C (organopalygorskite–Fe3O4), can be attributed to evaporation of the free surface water present on the nanoparticles; the second one, at about 221.3 °C (natural palygorskite), 195.5 °C (palygorskite–Fe3O4) and 236.1 °C (organo-palygorskite–Fe3O4), was related to zeolitic water loss (Petcharoen and Sirivat, 2012; Middea et al., 2015) and the third, at

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Table 4 Isotherm parameters and the free energy for adsorption of anionic dyes onto organo-palygorskite-Fe3O4. Anionic dye

1/Γm (g/mg)

Γm (mg/g)

1/K.Γm (g/L)

K (L/g)

Mw (g/mol)

K (L/mol)

ΔG (KJ/mol)

Methyl orange Indigo carmine

0.0124 0.0074

80.645 135.135

0.2566 0.0455

48.32 162.64

327.33 466.35

15,816.58 75,846.01

-23.96 -27.84

approximately 478.9 °C, for natural palygorskite, can be attributed to the palygorskite's dehydroxylation (Calvo et al., 2012; Middea et al., 2015). The mass loss occurred around 308.1 °C and 488.9 °C for palygorskite-Fe3O4 revealed a structural change of the magnetite to maghemite and the dehydroxylation of the mineral, respectively (Hao et al., 2010; Middea et al., 2015). Finally, we can attribute the mass loss at about 439.6 °C of organo-palygorskite-Fe3O4 to the CTAB molecules' decomposition (Lee and Lee, 2004), as showed in Fig. 4. Thus, the success of the organophilization is established by the TGA/DTA results. 3.2.7. FTIR FTIR spectra of the natural and modified palygorskite nanoparticles are shown in Fig. 5. In addition, Fig. 5a shows the spectrum of natural palygorskite and the wavenumber of each characteristic bands. The same occurred in Fig. 5b for palygorskite-Fe3O4. Fig. 5c revealed the FTIR spectrum and markings of the organopalygorskite-Fe3O4 product obtained. This spectrum is quite different from its precursors. The asymmetric stretching vibration of CH3 appeared at 3016 cm−1 and 2940 cm−1 and the asymmetric and symmetric CH2 stretching vibration frequencies are located at 2916–2848 cm−1 (Shen et al., 2005), 1462 cm−1, 731–719 cm−1 (Lei et al., 2008) and 1473–1382 cm− 1, proving that the palygorskite-Fe3O4 nanoparticles were organophilized. Besides this, the band at 1487 cm− 1 can be assigned to the asymmetric angular deformation of the groups (CH3)4 N+, while the others at 912 cm−1 and 960 cm−1 can be attributed to the group RN(CH3)3+ (Wang et al., 2010; Viana et al., 2012), indicating the organic cation present in the CTAB structure (Lee and Lee, 2004). Vibrations around 1140–981 cm−1 are attributed to the Si-O-Si group and in the 829–759 cm−1 region are associated with Si, like in natural palygorskite and palygorskite-Fe3O4 (Alkan et al., 2005; Madejova, 2003; Grzeta et al., 2002; Liu et al., 2007). 3.2.8. Contact angle Contact angle measurements are often used as an empirical indicator of wettability and are useful for evaluation of compatibilizer species. In this study sessile drop method was used to evaluate the hydrophobic nature of the pressed surfaces. The measured contact angles images are shown in Fig. 6. The results show that the contact angle for natural palygorskite (θ = 0.0°), Fig. 6a, refers to hydrophilic surfaces, having large wettability by water. This result is consistent with values found in the literature, which mention hydrophilicity for silicon oxides (Subrahmanyam et al., 1999). In presence of magnetite, the contact angle increases (θ = 45.6°) as shown in Fig.6b due to its hydrophobic nature (Shang et al., 2008) and became hydrophilic again for organopalygorskite-Fe3O4 (Fig. 6c) above the CTAB critical micelle concentration, when an inversion in the nanoparticles' hydrophobicity occurred due to the formation of micelles. 3.2.9. Dyes adsorption studies As shown in Fig. 7, the organo-palygorskite-Fe3O4/methyl orange (Fig. 7b) or indigo carmine (Fig. 7d) were easily separated from treated aqueous solution within 60 s by placing a permanent magnet (7500 G) near the conical flasks and as it was expected the same does not occur with palygorskite-Fe3O4 nanoparticles before their organophilization (Fig. 7a and 7c). Fig. 8a present the effect of mixing time on the adsorption of methyl orange and indigo carmine onto the organo-palygorskite-Fe3O4. Mixing

time is an important parameter because this factor can reflect the adsorption kinetics of an adsorbent for a given initial concentration of the adsorbate showing the time required for complete adsorption. The curves showed that rapid adsorption of dyes during the initial period of 20 min occurred as well as a higher adsorption from indigo carmine. Then, for kinetic study the time of 60 min was chosen and used to allow complete adsorption and to obtain the adsorption isotherm of indigo carmine and methyl orange onto organo-palygorskite-Fe3O4 to understand the mechanism of the adsorption systems (Fig. 8b). Fig. 8b showed that the adsorption capacity of dyes increase with increase of initial anionic dyes concentration. In addition, a higher increase in the indigo carmine adsorption is related to the structure of this molecule, that present two [SO3]− groups (Table 1), i.e., with greater availability of adsorption sites and surface area for contact than methyl orange (Hidarian and Hashemian, 2014). The best model that describes the adsorption process of the two systems was the Langmuir isotherms (Anbia and Salehi, 2012). The Langmuir sorption model assumes that adsorption may only occur at a fixed number of localized sites of the sorbent with all adsorption sites. Therefore, once a dye molecule occupies a site, no further adsorption  can take place at that site (Dalaran et al., 2011). Thus, the plot C i Γ versus Ci in Fig. 9 should give a straight line with 1/Γm as the slope of the line and 1 / (K·Γm) representing the intercept (Eq. (2)) (Middea et al., 2013). Ci 1 1 ¼ þ  Ci Γ K  Γm Γm

ð2Þ

From this plot, the isotherm parameters for adsorption of anionic dyes and the free energy of adsorption at T = 298 K can be calculated according to Eq. (3). −ΔG ¼ RT ln K

ð3Þ

where R is the gas constant (8.31451 J/Kmol) and T is the temperature in Kelvin. Table 4 summarizes the isotherm parameters for adsorption of anionic dyes on organo-palygorskite-Fe3O4. This table shows negative values of ΔG for both systems, indicating the spontaneous nature of the adsorption process. For indigo carmine the value of ΔG, in module, is higher than for methyl orange, which is in agreement with adsorption results. 4. Conclusion The production of palygorskite-Fe3O4 nanoparticles organophilized with cetyl trimethylammonium bromide (CTAB) was proven by different characterizations. The main characteristic introduced after organophilization of palygorskite-Fe3O4 nanoparticles were a charge reversal capacity, from negative to positive, generated by the CTAB positive charges and a change of the hydrophobicity of the nanoparticles, from partially hydrophobic to hydrophilic. Besides that, the organo-palygorskite-Fe3O4 show an excellent adsorption capacity for the removal of methyl orange and indigo carmine from wastewater that was confirmed by observation of negative values of free energies of adsorption, indicating spontaneous processes.

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Acknowledgments The authors acknowledge the financial support of the Foundation of Support of the State of Rio de Janeiro (FAPERJ E-26/201.498/2014) besides Colorminas-Colorifício e Mineração S.A. for supplying the palygorskite samples. References Alkan, M., Tekin, G., Namli, H., 2005. FTIR and zeta potential measurements of sepiolite treated with some organosilanes. Microporous Mesoporous Mater. 84, 74–83. Anbia, M., Salehi, S., 2012. Removal of acid dyes from aqueous media by adsorption onto amino-functionalized nanoporous silica SBA-3. Dyes Pigments 94, 1–9. Bai, Y., Xu, G.Y., Xin, X., Sun, H.Y., Zhang, H.X., Hao, A.Y., Yang, X.D., Yao, L., 2008. Interaction between cetyltrimethylammonium bromide and beta-cyclodextrin: surface tension and interfacial dilational viscoelasticity studies. Colloid Polym. Sci. 286, 1475–1484. Baltar, C.A.M., Luz, A.B., Baltar, L.M., Oliveira, C.H., Bezerra, F.J., 2009. 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