Adsorption study of reactive dyes onto porous clay heterostructures

Adsorption study of reactive dyes onto porous clay heterostructures

Applied Clay Science 135 (2017) 35–44 Contents lists available at ScienceDirect Applied Clay Science journal homepage: www.elsevier.com/locate/clay ...

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Applied Clay Science 135 (2017) 35–44

Contents lists available at ScienceDirect

Applied Clay Science journal homepage: www.elsevier.com/locate/clay

Research paper

Adsorption study of reactive dyes onto porous clay heterostructures J.E. Aguiar a, J.A. Cecilia b, P.A.S. Tavares a, D.C.S. Azevedo a, E. Rodríguez Castellón b, S.M.P. Lucena a, I.J. Silva Junior a,⁎ a b

Departamento de Engenharia Química, Universidade Federal do Ceará, Grupo de Pesquisa em Separações por Adsorção, GPSA, Campus do Pici, Bloco 709, CEP 60455-760 Fortaleza, CE, Brazil Departamento de Química Inorgánica, Cristalografia y Mineralogía, Faculdad de Ciencias, Universidad de Málaga, 29071 Málaga, Spain

a r t i c l e

i n f o

Article history: Received 13 June 2016 Received in revised form 1 September 2016 Accepted 2 September 2016 Available online 16 September 2016 Keywords: Dye Clay Porous clay heterostructure Adsorption

a b s t r a c t The present research evaluates the adsorption of reactive dyes, Remazol Violet 5R (RV5R) and Acid Blue 25 (AB25), using Porous Clay Heterostructures (PCHs) prepared from natural bentonite. Natural bentonite, PCH with silica pillars (Si-PCH) and PCH with silica-zirconia (SiZr-PCH) were characterized by elemental analysis, XRD, N2 adsorption-desorption at −196 °C, FT-IR, TG and XPS. The adsorption experiments were carried out in a stirred tank in order to evaluate the effect of pH, contact time and initial concentration. The adsorption isotherms were well fitted by Langmuir (L), Langmuir-Freundlich (LF) and Redlich Peterson (RP) models. The equilibrium data were described using the Langmuir-Freundlich model for both dyes and both materials, obtaining a maximum adsorption capacity of 209.9 mg g−1 and 265.9 mg g−1 for AB25 using Si-PCH and SiZr-PCH, respectively. In the case of RV5R, the maximum adsorption capacity was 127.07 mg g−1 and 185.7 mg g−1 for Si-PCH and SiZr-PCH, respectively. The adsorption process takes place by electrostatic interactions between the silanol groups of the PCHs with functional groups of the dyes, such as amine or hydroxyl. From the obtained results, it can be concluded that PCHs showed to be a promising material for the adsorption of dye. © 2016 Elsevier B.V. All rights reserved.

1. Introduction In the last decades, the widespread contamination of groundwater and soils as a consequence of synthetic organic chemicals, e.g. dyes coming from the textile and related industries, has been considered an important issue worldwide (İyim and Güçlü, 2009). Dyes can be classified as a function of the charge in anionic, cationic and non-ionic compounds. Among them, anionic dyes are widely used as colorant in the textile industry in wool, silk, acrylic, leather or nylon. These dyes are formed by aromatic compounds with nitrogen or sulfur centers leading to an obvious coloration. Nonetheless, these compounds are harmful and potential human carcinogen, even at low concentrations. The use of colorants in the textural industry generates effluents highly colored which provoke numerous operational problems in municipal wastewater treatment due to high biological oxygen demand (BOD) and high chemical oxygen demand (COD), high conductivity and alkaline nature of these effluents (Ozturk et al., 2009). In addition, the presence of metal together with the dyes in the wastewater can provoke a microbial inhibition. The treatment of the wastewaters depends on the physicochemical properties of the dye as well as the treatment technology selected (Abbassi et al., 2013). Several methodologies have been proposed to diminish the content of colorant of wastewater such as membrane ⁎ Corresponding author. E-mail address: [email protected] (I.J. Silva).

http://dx.doi.org/10.1016/j.clay.2016.09.001 0169-1317/© 2016 Elsevier B.V. All rights reserved.

separation, flocculation-coagulation, ozonation, oxidation, sedimentation, reverse osmosis, flotation, precipitation, and aerobic or anaerobic treatment (Leodopoulos et al., 2014; Yagub et al., 2014). However the most of them are infeasible due to their high cost or low efficiency. The adsorption is a usual method to the treatment of wastewater for dye removal due to the availability of adsorbents, its simplicity in operation and high efficiency (Almeida et al., 2009). Actually, the dyes adsorption is focused in the use of natural solid materials, which can be able to remove pollutants from contaminated water with low-cost. In this sense, it has been proposed clay minerals, siliceous materials or zeolites as natural absorbents by physical and/or chemical interactions with the dyes. Among them, clay minerals, and specially bentonites, have shown to be interesting adsorbents with low cost materials, large availability, thermal and chemical stability in a wide range of pH and interesting properties in the adsorption field (Abbassi et al., 2013). Bentonite is formed mainly by montmorillonite, which displays a lamellar structure where Si4+ species are partially replaced by Al3+ species in the tetrahedral positions and Al3+ species are partially substituted by Mg2+ or Fe2+ species in octahedral positions. This fact gives rise to the development of negative charges in the layers, which are balanced by the presence of Na+, K+ or Mg2+ cations strongly hydrated located in the interlayered space (Brindley, 1980). Montmorillonite has been treated or modified with several methods to improve its physicochemical properties. Thus, it has been proposed the acid treatment to improve the specific surface area of this smectite (Al-Khatib et al., 2012; Pentrák et al., 2012; Toor et al., 2015) or the

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J.E. Aguiar et al. / Applied Clay Science 135 (2017) 35–44

incorporation of bulky cations in the interlayered spacing to favor the electrostatic interaction between the bulky cations and the dyes (Tong et al., 2010; Kıranşan et al., 2014; Anirudhan and Ramachandran, 2015; Grundgeiger et al., 2015). However, the acid treatment leads to the partial digestion of the clay together with the loss of the silanol groups, which causes a decrease of available active sites that should interact with the dye. With regard to the intercalation with bulky cations located in the interlayer spacing, the presence of these bulky cations provokes a drastic decline of the available specific surface area. In order to improve the adsorption capacity of the montmorillonite, several methods, such as the synthesis of pillared interlayered clays (PILCs) with the use of polyoxocations (Vaccari, 1999), have been proposed. In 1995, Galarneau et al. described the synthesis of a porous material, denoted as PCH, from cationic exchange of the cations located in the interlayer spacing by a bulky cation, which produces the expansion of the interlayer spacing, and subsequent formation of silica pillars intercalated between two adjacent layers (Galarneau et al., 1995). Recently (Cecilia et al., 2013) have proposed a new method of synthesis with the incorporation of zirconium in the pillars. This method allows the control of the pore size leading to materials with high surface area (N 600 m2 g−1) with high micro- and mesoporosity. The chemical behavior of the PCH can be modified by the insertion of heteroatoms such as Al, Zr or Ti in the pillars, which produces an increasing of the acidity and provides to the material a higher thermal and mechanical stability (Cecilia et al., 2013). In addition, these materials display higher hydrophilicity than the natural clays, which improves the performance in the adsorption and catalysis, thereby generating their extensive use in the removal of different compounds (Roca Jalil et al., 2013). The present research describes the synthesis and characterization of porous clay heterostructures, which contains silica or silica-zirconium pillars, from a natural bentonite. These materials have been evaluated as adsorbents in the removing azo dyes in aqueous solutions in batch adsorption experiments. 2. Material and methods 2.1. Dyes Dye Remazol Violet 5R (trisodium; (3Z)-5-acetamido-3-[[2-hydroxy-5-(2-sulfonatooxyethylsulfonyl) phenyl] hydrazinylidene]-4-oxonaphthalene-2,7-disulfonate - C20H16N3Na3O15S4) with molar mass of 735.6 g mol− 1, UV–visible λmax = 562 nm. Dye Acid Blue 25 (2Anthracenesulfonic acid, 1-amino-9,10-dihydro-9,10-dioxo-4(phenylamino), monosodium salt - C20H13O5N2SNa) with molar mass of 416.4 g mol−1, UV–visible λmax = 605 nm. The dyes used were obtained from Sigma-Aldrich with a high degree of purity. The structures of dyes are shows in Fig. 1. Remazol Violet 5R is formed by an aromatic framework with aryl, amide and sulfonic groups, while Acid Blue 25 is also formed by an aromatic structure with amine and sulfonic groups. 2.2. Porous clay heterostructures synthesis The synthesis of the porous clay heterostructures (PCHs) were carried out following the procedure described by (Cecilia et al., 2013) in a previous research. The starting bentonite was supplied by Minas de Gador S.A. from “Sierra de Gador” (Spain). The raw bentonite was purity to obtain the montmorillonite fraction by a sedimentation process. Later, the montmorillonite fraction was treated with a NaCl solution (1 M) for 24 h to generate the homoionic Na-montmorillonite. In a typical PCH synthesis, 2.5 g of Na-montmorillonite was treated with 9 g of hexadecyltrimethylammonium bromide (HDTMBr) (Aldrich) in 100 mL of n-propanol (99.9%, VWR). After 3 days under stirring, the solution was filtered and was washed with distilled water to remove the non-intercalated HDTMBr. The solid was redissolved in 250 mL of water for 24 h. After this time, a solution of 0.9 g of

Fig. 1. Chemical structure of the Remazol Violet 5R-RV5R (a) and Acid Blue 25-AB25 (b).

hexadecylamine (90%, Aldrich), used as co-surfactant, was dissolved in 25 mL of n-propanol solution, added to the mother solution and stirred for 24 h. The Si-pillars located between adjacent layers of montmorillonite were formed by the incorporation of a solution of 11.1 mL tetraethylorthosilicate (TEOS) (98, Aldrich), as silicon source. The formation of Si\\Zr pillars with a molar ratio Si/Zr = 5 were generated by the addition of a solution of 9.4 mL of TEOS and 2.25 mL of zirconium propoxide (70%, Aldrich) dissolved in both cases in n-propanol (50 vol.%). The obtained gel was stirred during 72 h and then was filtered and washed with water-ethanol and dried at 60 °C in air for 12 h. Finally, the surfactant was removed by the calcination at 550 °C with a rate of 1 °C min−1 during 6 h. Elemental analysis CNH were carried out to ensure the complete combustion of the surfactant.

2.3. Characterizations of porous clay heterostructures The purified montmorillonite was examined by scanning electron microscope (SEM) using a JEOL SM-6490 LV combined with X-ray energy dispersive spectroscopy (EDX). The samples were gold sputtered in order to avoid charging of the surface. Elemental composition of the bentonite was achieved by the average of EDX chemical analyses of 40 grains. X-ray powder patterns for the samples were collected on a X'Pert Pro MPD automated diffractometer (PANalytical B.V.) equipped with a Ge (111) primary monochromator (strictly monochromatic CuKα1 radiation) and an X'Celerator detector with a step size of 0.017°. The powder patterns were recorded between 10° and 70° in 2θ with a total measuring time of 30 min. Low angle measurements were obtained with the same configuration maintaining the divergence and anti-divergence aperture at 1/16° and with Soller of 0.02 rad. Measurements were carried out between 0.5–10° in 2θ with a step size of 0.017°. The textural parameters (SBET, VP and dP) were evaluated from the nitrogen adsorption-desorption isotherms at − 196 °C as determined by an automatic ASAP 2020 system from Micromeritics. Prior to the measurements, samples were outgassed at 200 °C and 10−4 mbar overnight. Surface areas were determined by using the Brunauer–Emmett– Teller (BET) equation and a nitrogen molecule cross section of 16.2 Å2.

Several batch experiments were carried out in order to obtain information about the effect the pH of medium, contact time and initial dye concentration. All adsorption experiments were carried out at 22 °C. After prepare dye stock solutions, adsorption experiments were performed in a rotatory shaker (Tecnal TE-165, Brazil). For this aim, 20 mL of dye solution was added to 50 mL conical tubes containing 0.02 g of PCH. At the end of each experiment, the supernatant was collected and centrifuged for 10 min at 10.000 rpm (refrigerated microcentrifuge Cientec CT – 15000R). The dye concentration in the liquid phase (supernatant) was determined by UV–Vis light absorbance at λ = 562 nm for AB25 and λmax = 605 nm for RV5R (UV–Vis spectrophotometer Biomate 3, ThermoScientific, USA). The adsorption of AB25 and RV5R dyes in single system were studied in pH range 2 to 12 taking account the equilibrium conditions, i.e. after 60 min. The pH adjustment of the stock solution was carried out using HCl or NaOH solutions. For the kinetic adsorption, the samples were collected in the experimental tubes at pre-determined time intervals (5 to 120 min). Adsorption isotherms were determined by the analysis of the residual dye concentration from aqueous solution at increasing initial concentrations. For this aim, solutions of different dye initial concentration (25–300 ppm) were evaluated. Dye amount adsorbed in the solid phase (adsorption capacity – q) was calculated according to Eq. (1): Vsol C0 −Ceq q¼ mads

Cristobalite Montmorillonite

Calcite Quartz Plagioclase Montmorillonite

10

20

30

40

50

60

2θ (°) Fig. 2. XRD pattern of the raw bentonite.

Adsorption isotherms were fitted by Langmuir, LangmuirFreundlich and Redlich Peterson models, as described by Eqs. (2)–(4), respectively:



qmax K L C eq 1 þ K L C eq

ð2Þ

(a)

Intensity (c.p.s.)

2.4. Adsorption experiments

37

Bentonite

Si-PCH SiZr-PCH

2

3

4

5

6

7

8

2θ (°) (b) (020) (110)

Intensity (c.p.s.)

The total pore volume was calculated from the adsorption isotherm at P/ P0 = 0.996. Elemental chemical analysis, using a LECO CHNS 932 analyzer, were performed to determinate the carbon content present after the calcination of the template of the PCHs and to evaluate the dyes adsorption capacity of the adsorbents. The determination of carbon, nitrogen and sulfur takes place after the combustion of the samples at 1000 °C in pure oxygen to form CO2, NO2 and SO2. DRIFT spectra were collected on a Harrick HVC-DRP cell fitted to a Varian 3100 FT-IR spectrophotometer. The interferograms consisted of 200 scans, and the spectra were collected using a KBr spectrum as a background. About 30 mg of finely ground clay based materials were placed in the sample holder. The sample was heated under air flow of 60 mL min− 1 at 200 °C (30 min) to remove the physisorbed water and thereafter the sample was cooled at room temperature and the spectra was collected. Differential Thermal Analysis (DTA) and thermogravimetry (TGA) data were taken on a Mettler-Toledo (TGA/DSC 1) analyzer from RT instruments (New Castle, DE). The temperature was varied from RT to 1000 °C at a heating rate of 5 °C min−1. Measurements were carried out on samples in open platinum crucibles under air flow. X-ray photoelectron spectra were collected using a Physical Electronics PHI 5700 spectrometer with non-monochromatic Mg Kα radiation (300 W, 15 kV, and 1253.6 eV) with a multi-channel detector. Spectra of pelletized samples were recorded in the constant pass energy mode at 29.35 eV, using a 720 μm diameter analysis area. Charge referencing was measured against adventitious carbon (C 1s at 284.8 eV). A PHI ACCESS ESCA-V6.0 F software package was used for acquisition and data analysis. A Shirley-type background was subtracted from the signals. Recorded spectra were always fitted using Gaussian– Lorentzian curves in order to determine the binding energies of the different element core levels more accurately. The samples was introduced into the analysis chamber, and directly analyzed without previous treatment.

Intensity (c.p.s.)

J.E. Aguiar et al. / Applied Clay Science 135 (2017) 35–44

(130) (200)

(060)

SiZr-PCH



Si-PCH

ð1Þ

where Vsol is the volume solution (mL), C0 is the initial liquid concentration (mg/mL), Ceq is the equilibrium liquid concentration (mg/mL) and mads is the mass adsorbent (g).

10

20

30

40

50

60

70

2θ (°) Fig. 3. Low-angle XRD pattern of raw bentonite, Si-PCH and SiZr-PCH (a) and high-angle XRD pattern of Si-PCH and SiZr-PCH (b).

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J.E. Aguiar et al. / Applied Clay Science 135 (2017) 35–44

(a)

Samples

SBET [m2 g−1]

t-plotMicrop [m2 g−1]

VP [cm3 g−1]

VP(Microp) [cm3 g−1]

BG Si-PCH SiZr-PCH

49 644 608

18 460 382

0.11 0.77 0.82

0.01 0.28 0.21



qmax K LF C eq b 1 þ K LF C eq

ð3Þ

Absorbance (a.u.)

Table 1 Textural properties of natural montmorillonite, Si-PCH and SiZr-PCH.

3740

3635

qmax K RP C eq

ð4Þ

1 þ K RP C eq β

3200

3000

2800

(b) 1300-1000 946 800

1630

466 522 1030 916 1116

623 800

2000 1800 1600 1400 1200 1000

500

800

600

-1

Wavenumber (cm )

400

Fig. 5. FT-IR spectra of raw bentonite (black), Si-PCH (red) and SiZr-PCH (green) in the region 4000–2800 cm−1 (a) and 2000–450 cm−1 (b).

300 200

3. Results and discussion

100

3.1. Structural and textural characterization

0 0.0

-1

3

3400

(a)

0.2

0.4

0.6

0.8

1.0

The elemental analysis of the homoionic montmorillonite (Namont) was estimated using EDX chemical analysis by the evaluation of the average of 40 grains, obtaining the following structural formula:

Relative Pressure (P/P0) Differential Pore Volume (cm g )

3600

Wavenumber (cm )

3

-1

3800

-1

where Ceq (mg/mL) is the dye concentration of the aqueous phase in equilibrium with adsorbed dye concentration in solid phase – q (mg g−1), qmax (mg g−1) is the maximum amount adsorbed dye per gram of adsorbent, KL, KLF and KRP are the binding constants of Langmuir, Langmuir-Freundlich and Redlich-Peterson models. b and β are the exponents of the Langmuir-Freundlich and Redlich-Peterson that define the heterogeneity of the system so in the case of b and β = 1, the system is homogeneous and the model is reduced to the Langmuir equation.

Quantity Adsorbed (cm g STP)

4000

Absorbance (a.u.)



1.5

(b)

VI

IV

ðNa0:8 ÞðAl3:3 Mg0:5 Fe0:2 Þ ðSi7:7 Al0:3 Þ O20 ðOHÞ4 1.2 0.9 0.6 0.3 0.0 10

100

1000

Pore Width (Å) Fig. 4. N2 isotherms at −196 °C: (a) raw bentonite (black), Si-PCH (red) and SiZr-PCH (green). (b) Pore size distribution of raw bentonite, Si-PCH and SiZr-PCH.

where the substitution of silicon by aluminum in the octahedral positions (VI) and the substitution of aluminum by iron and magnesium in the tetrahedral positions (IV) are balanced with the presence of sodium in the interlayer spacing. According to the Schultz classification (Schultz, 1969), the obtaining formula represents a Wyoming-type bentonite, where the layer charge is lower than 0.85 electron charges per unit cell (e−/u.c.). As previous reported, the XRD pattern of the raw bentonite reveals that montmorillonite is the main mineralogical phase. In addition, it is noticeable the presence of minor amounts of plagioclase, calcite, cristobalite and quartz (Zent et al., 2001; Cecilia et al., 2013) (Fig. 2). Low-angle diffractograms of the Si-PCH and SiZr-PCH (Fig. 3a) reveal the existence of a broad peak located about 2θ = 2°, assigned to the presence of d001 reflexion line, confirming the generation of the

J.E. Aguiar et al. / Applied Clay Science 135 (2017) 35–44

30

Mass Loss (%)

25 20 15 10 5 0 0

100 200 300 400 500 600 700 800 900 1000

Temperature (˚C) Fig. 6. Thermogravimetric analysis of the raw bentonite (black), Si-PCH (red) and SiZrPCH (green).

expanded porous clay structure where the silica or silica-zirconia pillars separate two adjacent sheets (Galarneau et al., 1995) in comparison with the starting bentonite. In addition, the XRD patterns reflect that the incorporation of zirconium in the pillars produces a decrease of the intensity of the diffraction peak located at low-angle, suggesting a slight delamination of the PCH framework and leading to a more disordered structure denoted as “house of cards” (Occelli, 1988). With regard to the high-angle diffractograms (Fig. 3b), it is noteworthy that the basal diffraction lines of both Si-PCH and SiZr-PCH disappear. The non-basal diffraction peaks (020, 110, 200, 060) are detected, although their intensities decrease in comparison with the raw bentonite, which confirms the structural modification along c-axis and a random displacement in the a- and b-axis, corroborating the formation of delaminated framework in all cases. N2 adsorption-desorption isotherms at −196 °C were carried out to evaluate the textural properties of the raw bentonite and PCHs (Table 1 and Fig. 4a). Taking account that the specific surface area of the raw bentonite is only of 49 m2 g−1, both Si-PCH and SiZr-PCH display a higher SBET, 644 m2 g−1 for the Si-PCH and 608 m2 g−1 for the SiZr-PCH, respectively. This increase confirms the incorporation of pillars between the 2:1 layers of bentonite leading to a well-defined porous framework which displays micro- meso- and macroporosity in its structure, as reveals the isotherms of both PCHs. According to the IUPAC classification (Sing, 1985), and refined by Rouquerol et al. (2014), both PCHs show a combination of type I and type IIb isotherms which are associated to the presence of microporosity and the latter with monolayer-multilayer adsorption on an open and stable external surface of a powder with macroporosity. These isotherms are typical of aggregates of plate-like particles, which possess non-rigid slit-shaped pores. In addition, both PCHs display a narrow H3-type hysteresis loops with no indication of a plateau at high P/P0, which is attributed to the presence of agglomerates of particles forming slit shaped pores (plates or edged particles like cubes) (Robens, 1999; Rouquerol et al., 2014). The incorporation of a

39

small amount of zirconium into the pillars provokes a slight shrinkage in the hysteresis loop due to silicon and zirconium alcoxides have different hydrolysis rate, leading to the formation of blind cylindrical, coneshaped and wedge-shaped pores (Sing, 1985) which have been described in a “house of cards” structures (Occelli, 1988). In the present study, the pore width distribution of Si-PCH and SiZr-PCH (Fig. 4b) displayed similar pattern for both adsorbents. In both case, it is noteworthy the coexistence of micro- and mesoporosity by the insertion of pillars between adjacent layers and macroporosity due to the interparticle voids. It has been reported in the literature that the incorporation of Zr species in the pillars causes a delamination of the material (Cecilia et al., 2013); however the presence of pillars with a molar ratio Si/Zr = 5 seems maintain the same pore size distribution. Fig. 5a shows the \\OH stretching region of FTIR spectra between 4000 and 2800 cm−1. The raw bentonite displays a set of overlapped bands between 3760 and 3525 cm− 1, with a maximum about 3635 cm− 1, which is assigned to the stretching modes of the \\OH groups located inside the 2:1 layers (Zviagina et al., 2004). The formation of a pillared framework, using silica or silica-zirconia pillars, leads to the arising of a new band located about 3740 cm−1, which is attributed to the stretching vibration of the silanol groups (Si\\OH) located between two adjacent layers (Vilarrasa et al., 2014). The band with a maximum at 3635 cm−1 is attributed to the montmorillonite that remains unaltered. According to previous assignation bands (Fig. 5b) (Farmer, 1998; Madejová, 2003) for the OH-groups for dioctahedral smectites, the raw bentonite exhibits a complex band with a maximum located about 1030 cm−1 and a shoulder at 1116 cm−1 assigned to the symmetric stretching vibration mode of Si\\O\\Si groups. The band located about 916 cm−1 is attributed to the bending vibration mode of Al\\Al\\OH. The band located about 800 cm−1 is assigned to symmetric stretching mode νs (Si\\O\\Si). The band located about 623 cm−1 is ascribed to the Al\\O and Si\\O out-of-plane vibrations, while the bands located at 466 and 522 cm− 1 is assigned to Al\\O\\Si and Si\\O\\Si bending vibration. With regard to the region between 2000 and 500 cm−1 of the PCHs, both Si-PCH and SiZr-PCH exhibit a broader adsorption band 1300–1000 cm−1 in comparison to the raw bentonite. This fact is attributed to the formation of SiO2 or SiO2\\ZrO2 species as pillars between adjancent layers of montmorillonite (Pinto et al., 2014). The band located between 975–930 cm− 1 is attributed to the dangling Si-Od due to Si\\OH and Si\\O (Chmel et al., 1990) and the band located at 800 cm−1 (Vilarrasa et al., 2014), is assigned to symmetric stretching mode, being more evident for both PCHs and mainly for Si-PCH due to this adsorbent has a higher silica content. Finally, the band located about 1630 cm−1 is attributed to bending mode of water ν2 (H\\O\\H) (Vilarrasa et al., 2014). The thermogravimetric analysis of the starting bentonite and both PCH are shown in Fig. 6. The TG analysis reveals two mass losses. This first one, between 30 and 150 °C, is associated to the progressive loss of water molecules adsorbed on the surface of the adsorbent. This process is more evident for the starting material, while both PCHs contain a lower amount of water on the surface of the material. This fact can

Table 2 Binding energies of starting montmorillonite, Si-PCH and SiZr-PCH before and after the adsorption process. Sample

Si 2p

O 1s

Al 2p

Fe 2p

Ca 2p

Mg 2p

Na 1s

Zr 3d

S 2p

N 1s

BG BG-AA25 BG-RV5R Si-PCH Si-PCH-AA25 Si_PCH-RV5R SiZr-PCH SiZr-PCH-AA25 SiZr-PCH-RV5R

103.7 102.7 102.8 103.6 103.4 103.3 103.1 103.5 103.3

531.9 531.9 532.0 532.8 532.8 532.7 532.8 532.4 532.3

74.6 74.6 74.7 75.1 74.9 74.6 75.2 74.8 74.6

712.6 712.6 712.4 712.3 712.3 712.3 712.4 712.1 712.2

347.6 347.6 347.4 – – – – – –

49.9 49.6 48.9 49.8 49.8 49.7 49.6 49.7 49.7

1072.4 1072.3 1072.7 – – – – – –

– – – – –

– 168.3 168.4 – 168.5 168.3 – 168.4 168.3

– 400.3 401.0 – 400.9 401.0 – 400.8 401.3

183.1 182.7 182.8

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J.E. Aguiar et al. / Applied Clay Science 135 (2017) 35–44

Table 3 Atomic concentrations of starting montmorillonite, Si-PCH and SiZr-PCH before and after the adsorption process. Sample

C 1s

Si 2p

O 1s

Al 2p

Fe 2p

Ca 2p

Mg 2p

Na 1s

Zr 3d

S 2p

N 1s

BG BG-AA25 BG-RV5R Si-PCH Si-PCH-AA25 Si_PCH-RV5R SiZr-PCH SiZr-PCH-AA25 SiZr-PCH-RV5R

11.26 10.99 10.22 6.68 6.60 6.27 10.67 10.27 9.89

19.53 19.60 20.20 30.31 30.25 30.09 24.47 24.06 24.44

57.22 56.52 56.20 60.74 60.30 60.63 58.11 58.12 58.64

8.74 8.93 9.53 1.83 1.63 1.55 2.27 2.16 1.96

0.51 0.47 0.39 0.21 0.16 0.18 0.22 0.21 0.16

0.35 0.19 0.20 – – – – – –

1.38 1.81 1.71 0.23 0.17 0.38 0.58 0.54 0.39

1.01 0.50 0.40 – – – – – –

– – – – – – 3.68 3.37 3.59

– 0.46 0.43 – 0.58 0.46 – 0.56 0.38

– 0.59 0.41 – 0.71 0.58 – 0.81 0.60

be attributed to the higher amount of\\OH available of the starting material, as was shown in the FT-IR spectra (Fig. 5a), which interacts with water molecules. The second mass loss, located between 400 and 750 ° C, corresponds to the dehydroxylation of structural\\OH groups located between the tetrahedral sheets of 2:1 bentonite layers. This band is only detectable for the raw bentonite due to the calcination process, required to remove the organic cation in the synthesis of PCH, is a exothermic process and removes a higher proportion of the\\OH groups. Nevertheless, the FTIR spectra of both PCHs reveal the presence of silanol groups. In order to analyze the chemical composition of the raw bentonite, Si-PCH and SiZr-PCH samples on their surface, XPS measurements

(a)

(a)

180

140

80

6

60

4

-1

8

q (mg g )

120

pHfinal

10

120 8 100 80

6

60

4

40

40 20 0 0

2

4

6

8

10

2

20

0

0

2 0 0

12

2

4

(b) 12

160 140

6

80 60

-1

4

40 20 0 8

10

q (mg g )

8 100

pHfinal

-1

q (mg g )

120

6

12

12

140

10

4

10

(b)

160

180

2

8

pHinitial

pHinitial

0

6

120

10

100

8

80

4 40

2

20

0

0

12

pHinitial Fig. 7. Effect of pH [ο] and different between pHINITIAL and pHFINAL [*] in adsorption of RV5R (a) and AB25 (b) on Si-PCH.

6

60

pHfinal

-1

140

10

100

12

160

12

160

pHfinal

180

q (mg g )

were carried out. Tables 2 and 3 compile the binding energies and atomic concentration of the adsorbents. XPS spectra of the raw bentonite reveals the presence of a band about 531.9 eV in the O 1s region, a band about 102.8 eV in the Si 2p region and another band about 74.6 eV in the Al 2p region which are attributed to the presence of silicate species (Barr, 1990). In addition, it is noticeable the presence of a band located at 49.9 eV in the Mg 2p region assigned the Mg2+ species in the form of MgO (Briggs and Eds, 1992), which can be located in the interlayer spacing or probably in the tetrahedral positions. Finally, a band located at 347.6 eV in the Ca 2p region and another one at 1072.4 eV in the Na 1s region have been

2 0 0

2

4

6

8

10

12

pHinitial Fig. 8. Effect of pH [ο] and different between pHINITIAL and pHFINAL [*] in adsorption of RV5R (a) and AB25 (b) on SiZr-PCH.

J.E. Aguiar et al. / Applied Clay Science 135 (2017) 35–44

identify for the starting material which have been assigned to Ca2+ and Na+, respectively (Moulder et al., 1992) which are located in the interlayer spacing. The incorporation of the silica or silica-zirconia pillars to obtain the PCH leads to changes in the atomic concentrations of adsorbents. Firstly, the bands located in the O 1s region and the Si 2p region suffer a shift at higher binding energy about 532.8 eV and 103.2 eV, respectively, which is attributed to the silicon and oxygen species in the form of silica due to the formation of a pillared structure. For the SiZr-PCH adsorbent, it is noticeable a new band about 183.1 eV in the Zr 3d region, which is assigned to ZrO2 species. In the case Al 2p and Mg 2p regions, these bands maintain their binding energies, which indicates that montmorillonite framework is not affected. Finally, the bands located in the Ca 2p region and Na 1s region disappear. This fact indicates a total cationic exchange, HDTM+ by Na+ and Ca2+ and later removal of the organic matter in the calcination step. With regard to the atomic concentration (Table 3), it can be observed how the magnesium, iron and aluminum content decreases and calcium and sodium disappear of the surface of the adsorbent when the PCHs are formed, which confirms the modification of the starting montmorillonite. In addition, it is noticeable a decrease of iron, magnesium and mainly aluminum content in comparison to the raw bentonite. This decrease is higher for the SiZr-PCH in comparison with the Si-PCH, which can be in accordance with partial delamination of the SiZr-PCH sample by the presence of zirconium giving rise to the montmorillonite layers more accessible on the surface of the adsorbent.

3.2. Adsorption experiments Figs. 7 and 8 show the effect of pH in adsorption of RV5R and AB25 on Si-PCH and SiZr-PCH, respectively. In all cases, the maximum amount adsorbed takes place at lower pH (pH = 2) for AB25 and RV5R dyes. Previous research has established that the higher adsorption capacity takes place at pH b 3.0 due to the surface of the clay becomes positively charged leading to an interaction with the anionic dye (Tabak et al., 2009). In this case, it can be observed how pHs maintain unaltered after the adsorption process discarding the interaction of OH+ 2 species with the anionic species of AB25 and RV5V at lower pH. Thus, the interaction between the adsorbent and dyes should be attributed to the interaction by hydrogen bond between the silanol groups of the PCHs and the electron pairs of the sulfonic groups of the dyes, which are protonated at low pH as well as amine, hydroxyl and azo groups (Yariv, 1996). The increase of the pH provokes a deprotonation of the \\OH both the adsorbent and the dyes, as indicates the decreasing the pH after the adsorption process, leading to the generation of adsorbents negatively charged which causes a electrostatic repulsion between species for adsorbents and dyes. The kinetic profiles of RV5R and AB25 on Si-PCH and SiZr-PCH are shown in Figs. 9 and 10, respectively. Both figures display that a fast decrease of the dye concentration in the first minutes, reaching the equilibrium conditions between 40 and 60 min. In addition, both figures show how the amount adsorbed is slightly higher for SiZr-PCH in comparison to Si-PCH for RV5R and AB25 dyes.

(a)

1.0

(a)

1.0

0.8

0.8

0.6

0.6

C/C0

C/C0

41

0.4 0.2

0.4 0.2

0.0 0

20

40

60

80

100

0.0

120

0

Time (min)

20

40

(b) 1.0

1.0

0.8

0.8

0.6

0.6

C/C0

C/C0

60

80

100

120

100

120

Time (min)

0.4

(b)

0.4 0.2

0.2

0.0

0.0 0

20

40

60

80

100

120

Time (min) Fig. 9. Kinetic profile of adsorption for RV5R on Si-PCH at pH = 2 and T = 22 °C: (a) RV5R; (b) AB25: (□) 100 ppm (○) 200 ppm and (Δ) 300 ppm.

0

20

40

60

80

Time (min) Fig. 10. Kinetic profile of adsorption for RV5R on SiZr-PCH at pH = 2 and T = 22 °C: (a) RV5R; (b) AB25: (□) 100 ppm (○) 200 ppm and (Δ) 300 ppm.

42

J.E. Aguiar et al. / Applied Clay Science 135 (2017) 35–44

(a)

240

Table 4 Equilibrium adsorption parameters according to Langmuir (L), Langmuir-Freundlich (LF) and Redlich Peterson (RP) models to dyes adsorption. qmax (mg g−1)

R2

χ2

– 0.1095 –

48.642 61.022 58.798

0.9825 0.9971 0.996

5.86 1.13 1.61

– – 1.116

– 0.0002 –

25.2 23.6 20.7

0.989 0.991 0.997

0.823 0.035 0.264

– 0.806 –

– – 0.933

– 0.0334 –

193.1 209.9 219.6

0.995 0.998 0.997

22.8 8.75 17.8

Reactive Violet + Si-PCH L 13,125.2 105.3 – LF – – 0.924 RP 12,533.2 103.2 –

– – 1.014

– 0.0139 –

124.5 127.0 121.3

0.993 0.994 0.994

13.6 14.8 15.7

Acid blue 25 + SiZr-PCH L 18,553.1 76.6 – LF – – 0.799 RP 22,717.4 82.4 –

– – 0.932

– 0.038 –

242.1 265.9 275.3

0.992 0.996 0.994

57.3 35.3 55.0

Reactive Violet + SiZr-PCH L 15,124.4 88.0 – LF – – 0.807 RP 16,707.1 91.2 –

– – 0.967

– 0.031 –

171.7 185.7 183.0

0.990 0.994 0.991

37.1 30.0 41.8

K (mL/mg)

a

b

β

14,674.0 – 35,306.3

301.6 – 600.4

– 0.428 –

– – 0.907

Reactive Violet + BG L 2758.3 109.1 LF – – RP 1829.6 88.2

– 1.870 –

Acid blue 25 + Si-PCH L 16,371.4 84.7 LF – – RP 20,162.1 91.8

160 120

L LF RP

80 40 0 0.00

0.05

0.10

0.15

0.20

0.25

-1

Ceq (mg mL )

(b)

240 200

-1

q (mg g )

160 120 80 40 0 0.00

0.05

0.10

0.15

0.20

0.25

0.20

0.25

-1

Ceq (mg mL )

(c)

240 200

-1

q (mg g )

160 120 80 40 0 0.00

KLF (mL/mg)

Acid blue 25 + BG

-1

q (mg g )

200

0.05

0.10

0.15 -1

Ceq (mg mL ) Fig. 11. Adsorption isotherms of RV5R and AB25 on raw bentonite using an initial dye concentration (25–300 ppm) at pH = 2 and T = 22 °C for (a), Si-PCH (b) and SiZr-PCH (c): (◄) BG-RV5R; BG-AB25 (►); (■) Si-PCH-RV5R; (●) Si-PCH-AB25; (▲) SiZr-PCHRV5R; and (♦) SiZr-PCH-AB25. Theoretical profile: The lines correspond to fitting (nonlinear regression) of experimental data according to Langmuir model (red line), Langmuir-Freundlich (green line) and Redlich Peterson (blue line).

The Fig. 11a represents the adsorption isotherm obtained from the starting material, Si-PCH and SiZr-PCH, respectively. The starting bentonite reaches lower adsorption capacities than those obtained for both PCHs. With regard to the PCHs, it can observe how RV5R dye exhibits a lower adsorption capacity for both PCHs possibly by steric effects due to RV5R dye is bulkier than AB25 dye. Fig. 11b–c also reveal that SiZr-PCH shows a higher adsorption capacity than Si-PCH. This fact could be attributed to the presence of zirconium species that can

modify the electronic density of the adsorbent as well as a slight delamination of the porous material, as was shown in XRD, leading to a higher amount of silanol groups available to the adsorption, especially for the smaller molecule. In addition, it can be observed how all adsorption isotherms display a similar pattern. The adsorption takes place mainly at lower Ceq. The non-linearity of the isotherms indicates the presence of chemical interactions between the silanol groups of the porous material and the functional groups of the dyes, discarding a physical adsorption by the use of adsorbents as molecular sieve. Langmuir, Langmuir-Freundlich and Redlich Peterson models (Table 4) were used in the modeling of experimental adsorption data. The equilibrium data were well described using the Langmuir-Freundlich model for both dyes and both materials, with maximum adsorption capacity of 209.9 mg g−1 and 265.9 mg g−1 for AB25 in the Si-PCH and SiZr-PCH, respectively. For RV5R, the maximum adsorption capacity was 127.07 mg g−1 and 185.7 mg g−1 in the case of Si-PCH and SiZrPCH, respectively. The raw bentonite displays the lowest adsorption capacities, due to the low specific surface area and porosity in comparison to both PCHs. The use of PCHs as adsorbent for the dye adsorption improves the adsorption capacity in comparison to other clay minerals or other natural sources. Thus, it has been reported q values of 75 mg g− 1 (Karaca et al., 2013) and 39 mg g− 1 (Kuleyin and Aydin, 2011), using modified clays and natural zeolite, respectively, for Remazol adsorption. In the case of the adsorption of acid blue with clay minerals, it is reported a q values between 100–150 mg g−1 for bentonite modified with acid treatment (Juang et al., 1997; Ullah et al., 2016), 67 mg g−1 for Na-bentonite (Ozcan et al., 2004). These data indicate that the insertion of pillars improves the dye adsorption capacity. With regard to the KL constant of the models, which defines the interaction between adsorbate and adsorbent, it is noticeable that the dye lesser bulky, i.e. AB25, displays higher adsorption values than RVR5 dye. According to those data shown previously, SiZr-PCH adsorbent shows a higher affinity by the adsorbate due to the incorporation of zirconium species modifies the electronic density and generates acid sites that must favor the interactions with both dyes.

J.E. Aguiar et al. / Applied Clay Science 135 (2017) 35–44

3.3. Evaluation of materials after the adsorption process In order to confirm the adsorption of the dyes on the raw bentonite and both PCHs, elemental analysis (CNHS), FTIR and XPS were carried out. FTIR spectra of the adsorbents after the adsorption process are shown in Fig. 12. In all cases, it has not been detected the typical bands of the dyes due to the most intense bands of the dyes overlap with the Si\\O bands. However, in the region of the silanol groups,

(a)

Absorbance (a.u.)

BG BG-AB25 BG-RV5R

4000

3800

3600

3400

3200

3000

-1

Wavenumber (cm )

Absorbance (a.u.)

(b)

3740

Si-PCH Si-PCH-AB25 Si-PCH-RV5R

4000

3800

3600

3400

3200

3000

-1

Wavenumber (cm )

Absorbance (a.u.)

(c)

3740

SiZr-PCH SiZr-PCH-AB25 SiZr-PCH-RV5R 4000

3800

3600

3400

3200

3000

-1

Wavenumber (cm ) Fig. 12. FTIR spectra of the materials after the adsorption process.

43

Table 5 Elemental analysis (CHNS) of the starting material, Si-PCH and SiZr-PCH before and after the adsorption process. Sample

C (wt.%)

H (wt.%)

N (wt.%)

S (wt.%)

BG BG-AB25 BG-RV5R

0.017 0.641 0.451

0.046 2.268 2.073

– 0.058 0.030

– 0.128 0.064

Si-PCH Si-PCH-AB25 Si-PCH-RV5R

0.021 1.031 0.936

0.041 2.449 2.456

– 0.143 0.114

– 0.287 0.224

SiZr-PCH SiZr-PCH-AB25 SiZr-PCH-RV5R

0.014 1.315 1.217

0.042 2.917 2.718

– 0.164 0.131

– 0.301 0.259

about 3740 cm− 1, it is noticeable how the silanol band disappears after the adsorption process, confirming that the adsorption process takes place between the silanol groups of the PCHs and the electron pair of functional groups of the dyes. The elemental analysis (CHNS) was carried out to corroborate the presence of dyes on the adsorbent after the adsorption process (Table 5). CHNS data reveal the existence of nitrogen atoms attributed to the azo- and amine-groups, and sulfur atoms which are assigned to the presence of sulfonic groups. The elemental analysis shows how both Si-PCH and SiZr-PCH have higher amount of nitrogen and sulfur after the adsorption process than the raw material, confirming those shown in the adsorption data. In addition, both nitrogen and sulfur content is directly related with adsorbed dye, being higher for AB25 dye, which is also in accordance with the adsorption data. The XPS spectra of the PCHs after the adsorption process (Tables 2 and 3) show the arising of new bands located about 401.0 eV, in N 1s region, attributed to amine groups (Moulder et al., 1992), and another band in the S 2p region located about 168.4 eV, assigned to the presence of sulfur in the form of sulfonic groups (Moulder et al., 1992). According to those shown previously, the atomic concentrations of N and S for SiPCH and SiZr-PCH adsorbent are higher than that shown for the starting material. In addition, the atomic concentrations also show higher concentration values of N and S when is adsorbed AB25 due to this compound displays lower volume than RV5R dye. 4. Conclusion The natural clays are effective precursors to synthesize porous clay heterostructures for the removal of reactive dyes, concluding that these adsorbents can remove the remaining dye from aqueous solutions in the treatment of textile wastewater. The study also concludes that SiZr-PCH adsorbent has a better time performance in the adsorption of the dyes than Si-PCH adsorbent. The adsorption process takes place by an interaction between the silanol groups of the montmorillonite and/or the PCH adsorbent with functional groups such as amine, hydroxyl or sulfonic groups of the dyes. This adsorption is influenced by the pH of the system. The optimum contact time for equilibrium was reached in 40 min for RV5R and AB25 adsorbents. It can be concluded that dosage increase of anionic dyes causes a decreases of the efficiency of the adsorption process mainly in the case of RV5R dye. The dye AB25 is better adsorbed in comparison with the dye RV5R due to bulky of each compound. References Abbassi, R., Yadav, A.K., Kumar, N., Huang, S., Jaffe, P.R., 2013. Modeling and optimization of dye removal using “green” clay supported iron nano-particles. Ecol. Eng. 61, 366–370. Al-Khatib, L., Fraige, F., Al-Hwaiti, M., Al-Khashman, O., 2012. Adsorption from aqueous solution onto natural and acid activated bentonite. Am. J. Environ. Sci. 8, 510–522. Almeida, C.A.P., Debacher, N.A., Downs, A.J., Cottet, L., Mello, C.A.D., 2009. Removal of methylene blue from colored effluents by adsorption on montmorillonite clay. J. Colloid Interface Sci. 332, 46–53.

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