kaolin magnetic nanocomposite and its application in wastewater treatment

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JIEC 3527 1–13 Journal of Industrial and Engineering Chemistry xxx (2017) xxx–xxx

Contents lists available at ScienceDirect

Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec

Synthesis and characterization of Fe3O4/kaolin magnetic nanocomposite and its application in wastewater treatment

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A. Magdya , Y.O. Fouada , M.H. Abdel-Aziza,b,* , A.H. Konsowaa

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a b

Chemical Engineering Department, Faculty of Engineering, Alexandria University, Alexandria, Egypt Chemical and Materials Engineering Department, Faculty of Engineering, King Abdulaziz University, Rabigh, 21911, Saudi Arabia

A R T I C L E I N F O

Article history: Received 26 March 2017 Received in revised form 29 June 2017 Accepted 18 July 2017 Available online xxx

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Keywords: Adsorption Chemical co-precipitation Dye Kaolin Magnetic nanocomposite

A B S T R A C T

Fe3O4/kaolin magnetic nanocomposites were prepared by chemical co-precipitation method. The nanocomposites were characterized using various instrumental techniques. The prepared nanocomposites were tested and evaluated as an adsorbent in the removal of the anionic C.I. Direct Red 23 dye. The effects of contact time, initial dye concentration, adsorbent dose, temperature and initial pH of the solution on the adsorption process were studied. The removal efficiency reached 100%. The results showed that linearized Langmuir isotherm with a maximum adsorbent capacity of 22.88 mg g
1. Adsorption process is spontaneous, endothermic, and follows the pseudo-second order model within the range of temperatures studied. © 2017 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

6

Introduction

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Green approaches are increasingly in need in the industrial processes to reach environmental protection. More severe regulations are being imposed to minimize harmful industrial effects on the air, soil and water [1]. Different researches have been carried out on different physical and chemical processes as flocculation, adsorption, membrane filtration, coagulation, precipitation, ozonation, electrochemical techniques, and fungal decolonization for removing different pollutants from aquatic bodies [2–7]. Adsorption proved its superiority among the different techniques for removal of dyes, oily wastes, heavy metals and organic pollutants from water; this arises from its high efficiency, simple design and facile operation. However, expensive adsorbents represent the main limitation of adsorption use [8]. The most common adsorbent in commercial adsorption units is the activated carbon, yet it suffers from high cost of synthesis and regeneration [1]. Several researches investigated the use of other adsorbents as zeolites, clay, bagasse, fly ash and saw dust. Moreover, some untraditional adsorbents were studied for dye removal such as deoiled soya, activated rice husks, wheat husk, red mud and carbon slurry [9]. Luckily, late progress in

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* Corresponding author at: Chemical Engineering Department, Faculty of Engineering, Alexandria University, Alexandria, Egypt. E-mail addresses: [email protected], [email protected], [email protected] (M.H. Abdel-Aziz).

nanotechnology has revealed an insight into this field. Nanoparticles, generally distinguished by a high surface area, have been drawing in much intrigue for their distinctive properties and possible applications. It has been reported that magnetic nanoparticles exhibit a high surface-to-volume ratio leading to an augmented adsorption capacity [10–12]. Magnetic nanoparticles also satisfy the ease of separation requirement; the removal of the magnetic nanoparticles from the adsorption solution can be easily attained by the use of an external magnetic field. Accordingly, magnetic iron oxide nanoparticles represent an effective, economic and non-harmful candidate for adsorption researches [13,14]. Nevertheless, the high surface energy of the magnetic nanoparticles may cause aggregation during catalytic reactions. This drawback revealed the need to use stabilizers or supports with the nanoparticles. The main function of these supports is to control the particle size, morphology and dispersion of the nanoparticles [15–17]. Owing to their low cost, availability as well as structural and thermal stability, clay minerals represent a good option to incorporate with the nanomaterials to overcome the difficulties of aggregation and high zero point of charge of the iron oxide nanoparticles. To synthesis these magnetic composites numerous methods have been used as impregnation, ball milling, one-pot method [18,19] and chemical co-precipitation [20–22]. Owing to the fact that there is no special chemicals or procedures needed and due to its simplicity chemical co-precipitation is the most frequently used method. The resulting magnetic composites could be used to remove different varieties of pollutants [23,24].

http://dx.doi.org/10.1016/j.jiec.2017.07.023 1226-086X/© 2017 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

Please cite this article in press as: A. Magdy, et al., Synthesis and characterization of Fe3O4/kaolin magnetic nanocomposite and its application in wastewater treatment, J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2017.07.023

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Nomenclature

List of symbols AT Temkin isotherm equilibrium binding constant BT Constant related to heat of sorption bT Temkin isotherm constant C Constant Ce Final (equilibrium) dye concentration Co Initial dye concentration d Interplanar spacing KC Equilibrium constant KF Freundlich equilibrium constant KL Langmuir equilibrium constant k1 Pseudo first order rate constant k2 Pseudo second order rate constant kid Rate constant of intra-particle diffusion kinetic model m Mass of adsorbent qm Maximum monolayer adsorption capacity qt Amount of dye adsorbed per unit mass of adsorbent RL Separation factor T Temperature V Solution volume DG Free energy change DH Enthalpy change DS Entropy change

55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77

Kaolin is a clay mineral having uses in many fields such as paper industry, ceramics, paints and rubber manufactures [25]. Moreover, it has been appreciably regarded as a low cost green adsorbent. The main constituent mineral in kaolin is kaolinite; kaolin may usually contain also quartz, mica and other less frequent minerals. Yet, natural kaolinite showed much lower adsorption capacity than that of conventional adsorbents. Therefore enhancement of kaolin adsorption properties are required to be able to use it on commercial scale; several methods were developed to activate the kaolin such as intercalation, plasma treatment, mechanochemical activation, thermal activation, chemical activation, thermal chemical activation and milling. Chemical activation (including acid and alkali treatment) has been reported as the most efficient method to activate kaolinite. In chemical activation, alkaline-treated kaolin had a very low surface area, on the contrary, acid-treated kaolin showed a high surface area. The acid treatment modifies the chemical composition of the kaolin forming a porous solid. Thus depending on the acid type and treatment conditions, acid activation increases the specific surface area and the pore volume of kaolinite [26]. Usual acids used in the clay acid activation are hydrochloric acid, sulfuric acid and nitric acid [27]. The dissolution rate of kaolinite in H2SO4 is three times greater than that in HCl of the

same concentration [28] which makes H2SO4 the most common acid used in the clay activation. It has been reported that 7  105 tons of dyes are produced annually from over than 100,000 available types; an appreciable fraction is discharged directly in aquatic bodies [9]. A very small concentration of dye in water (<1 mg L
1) is quite visible which makes the color the easiest contaminant to be recognized in water. Color has different negative effects when present in water, it alters the transparency and gas solubility of water effluents, it interferes with the growth of living substances and it impeded photosynthesis. Several investigations have been carried out to evaluate the magnetic composites performance in water treatment by adsorption such as adsorption of Cu(II) on magnetic starch-g-polyamidoxime/montmorillonite/Fe3O4 nanocomposites [29], removal of crystal violet from aqueous solutions by adsorption on magnetic nanocomposite hydrogels and laponite RD [30,31], removal of congo red dye by Fe3O4@MgAl-LDH composite [32], removal of heavy metals by magnetic nano-Fe3O4-dioctyl phthalate [33], adsorption of methylene blue on magnetic polyvinyl alcohol/ laponite RD nanocomposites [34], etc. The current work basically aims to enhance the physicochemical characteristics of kaolin clay as adsorbent and to combine this with the extra ordinary properties of nano iron oxide to produce a locally abundant and low cost adsorbent with high adsorption capacity and readily separable for wastewater treatment. To this end activated kaolin/Fe3O4 nanocomposite were synthesized by chemical co-precipitation method to test its feasibility as an adsorbent in the removal of direct red dye from aqueous solutions. The adsorbent was characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), scanning electron microscopy (SEM), vibrating sample magnetometer (VSM), and the BET-N2 technique. The effects of different operating parameters, namely, contact time, initial dye concentration, temperature, adsorbent dosage and initial pH of the solution on the percentage dye removal were studied. Spectrophotometry was used to determine the dye concentration in the different aqueous solutions.

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Experimental part

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Materials

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The kaolin clay used in the present research was obtained from Alexandria Company for Refractories, Alexandria, Egypt; with a particle size 0.075 mm (200-mesh). The sample was used as such without modification for the acid activation. The chemical composition of the sample was: SiO2 46%, Al2O3 35.9%, Fe2O3 1%, MgO 0.08%, CaO 0.28%, Na2O 0.1%, K2O 0.08%, TiO2 1.9% and loss on ignition 12%. Ferric chloride (FeCl36H2O) and (FeSO47H2O) were purchased from Adwic Company, NaOH procured from Universal Laboratories and H2SO4 from CDH Company Ltd. The adsorbate in the adsorption experiments is Direct Red Dye 23 (Fig. 1), it was

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Fig. 1. The chemical structure of C.I. Direct Red 23 used in the adsorption experiments.

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obtained from Dyestuffs & Chemicals Co. (Ismadye). All chemicals were from analytical grade and were used without further treatment.

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Clay activation

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The acid clay activation was carried out by adding 50 g of the clay to 500 mL of H2SO4 solution of different concentrations (2 M, 4 M and 6 M). The suspension was heated at 110  C under atmospheric pressure in a neck flask equipped with a reflux condenser for 4 h with stirring using a magnetic stirrer with a hot plate. The resulting clay suspension was then quenched by adding 500 mL ice cold distilled water. The content was then filtered, repeatedly washed with distilled water till neutralization, and dried in an oven at 100  C till constant weight was obtained. The obtained amount of activated clay was 38 g (76% yield). The resulting activated clay was grinded using a mortar pestle to a powder form [25]. To test the effect of thermal activation the raw clay was heated at 500  C for 1 h.

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Activated clay evaluation

133 134 135 136 137 138 139 140 141 142 143

146 147 148 149 150 151 152 153 154 155 156

To identify the best conditions of activation, to use them while preparing the magnetic clay composite, the adsorption efficiency of the previously prepared clay samples (activated by different acid concentrations and by heat) was tested. 0.3 g of different clay samples, activated by 2 M H2SO4, 4 M H2SO4, 6 M H2SO4 and by heat at 500  C, were added to 100 mL of 20 ppm methylene blue dye solution in a shaker for 150 min and the percentage removal of the dye was recorded in each case. The percentage removal was 98.8, 98.88, 97.3 and 80, respectively. These results showed that the best clay activation to use is the acid activation using 2 M H2SO4.

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Preparation of kaolin/Fe3O4 nanocomposite

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19.5 g of FeCl36H2O and 17.832 g of FeSO47H2O were dissolved in 1000 mL of deionized water, the solution was heated to 70  C while stirring with a magnetic stirrer, kaolin was mixed with the solution. 250 mL of 5 M NaOH were added drop wise to precipitate the iron oxide. The mixture was filtered using a vacuum pump (Deep vacuum pump, J/B Industries Inc., USA). The precipitate was washed with distilled water several times to remove any traces of NaOH and the washing was continued until the filtrate become almost neutral (pH 7), then the precipitate was dried at 105  C. A separate experiment to assess the weight of magnetite was carried out following the same aforementioned procedure and the resultant magnetite was separated by filtration and then dried.

159 160 161 162 163 164 165 166 167 168 169

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Table 2 List of operating parameters and their ranges. Parameter

Studied range

Contact time, min Initial dye concentration, mg/L Adsorbent dose, g Initial pH of the solution

0–80 20–60 0.25–0.75 g 4–11

The weight of the resultant magnetite was 14.75 g. The amount of kaolin added was adjusted in order to obtain kaolin/iron oxide weight ratios of 1:1 and 2:1 [35].

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Adsorbent characterization

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The different techniques used to characterize the obtained magnetic composite and operating conditions are listed in Table 1.

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Adsorption experiments

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In order to study the effects of different parameters on the adsorption process (time, initial dye concentration, amount of adsorbent, temperature and pH), a set of experiments were carried out by mixing a known amount of the adsorbent with 250 mL of direct red dye solution in a rotary shaker at a constant speed of 250 rpm, samples were withdrawn at constant time intervals and the residual concentration of the dye in the solution was determined using the UV–vis spectrophotometer at a wavelength of 505 nm (l corresponding to the maximum dye absorbance). The experiments were repeated three times and the average results were reported. Table 2 lists the operating parameters studied and their range. The pH of the solution was adjusted by adding NaOH or H2SO4 and measured by pH meter (JENWAY 370 pH/mV METER). Q5 The percentage removal was calculated from Eq. (1):   Co
Ce  100 ð1Þ %Removal ¼ Co

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where Co and Ce are the initial and final concentrations (mg L
1) of the dye in solution, respectively.

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Results and discussion

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Fe3O4 and kaolin/Fe3O4 nanocomposite characterization

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Fig. 2 displays the XRD profiles of the raw kaolin, activated kaolin, and Fe3O4–kaolin composite respectively. The raw clay shows well defined reflections at 2u value of 12 , 25 , and 27

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Table 1 List of characterization techniques.

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Characterization technique

Conditions The X-ray diffraction data were collected using a diffractometer (Shimadzu XRD-6100) with CuKa at 40 kV and 40 mA; the XRD patterns were recorded in the range of 10–70 with a scanning rate of 2 /min. FTIR spectra were recorded on (Bruker Vertex 70) infrared spectrophotometer as KBr pellets with resolution of 4 cm
1, in the range of 400–4000 cm
1. The sample and analytical grade KBr were dried at 100  C over-night prior to the FTIR analysis. The analysis was performed using a scanning electron microscope (JEOL 5300) at accelerating voltage 20 kV. A transmission electron microscope (JEOL 100 CX II TEM 100 kV) was used to characterize the kaolin/Fe3O4 composite with respect to their particle size and shape. Susceptibility was measured using vibrating sample magnetometer (VSM lackshor-7410 USA). The specific surface areas, the total pore volume and the mean pore diameter of the samples were determined from nitrogen adsorption isotherms at liquid nitrogen temperature (77 K), using a BET apparatus (Belsorbminill, BEL Japan Inc.), after outgassing the samples overnight at 120  C; samples used weighed 0.28 g each. The specific surface area values were determined by applying the Brunauer–Emmet–Teller (BET) equation in the range of relative pressures from 0.05 to 0.5. The total pore volume was estimated at the adsorption P/Po value of 0.99.

Please cite this article in press as: A. Magdy, et al., Synthesis and characterization of Fe3O4/kaolin magnetic nanocomposite and its application in wastewater treatment, J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2017.07.023

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X-Ray diffraction (XRD) Fourier transform infrared spectroscopy (FTIR) Scanning electron microscopy (SEM) Transmission electron microscope (TEM) Vibrating sample magnetometer (VSM) BET isotherm studies

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Fig. 2. XRD analysis for raw kaolin, activated kaolin, and Fe3O4–kaolin composite. 200 201 202 203 204 205 206 207 208 209 210 211 212 213

corresponding to the d values of 7.365, 3.557, and 2.715 Å respectively. These peaks match to the reflections from plane [001], which are typical representative peaks of kaolinite. After treatment of the clay with acid, the peak intensity of the clay was found to slightly decrease. This is attributed to the structural disorder that happened due to the acid leaching, which affects the crystalline character of the clay. The XRD results for Fe3O4/kaolin composite show three peaks at 2u of 24, 26 and 35 and corresponding to d values of 3.617, 3.33 and 2.516 Å respectively which may relate to the magnetite phase. The d spacing showed contraction than the original clay. This indicates that most of Fe3O4 is attached to the clay surface. The FTIR spectrums of kaolin, activated kaolin, and Fe3O4/kaolin composite are displayed in Fig. 3 at a frequency range of 4000–

400 cm
1. The corresponding band assignments are presented in Table 3. The overall spectrum of FTIR is divided into two general regions 4000–1100 cm
1 (the functional group region) and 1100– 400 cm
1 (the fingerprint region). In the O

H stretching region, the raw and acid treated clay shows three noticeable bands at 3619, 3649 and 3690 cm
1 corresponds to Al

OH stretching. Inner hydroxyl groups, lying between the tetrahedral and octahedral sheets give the absorption at 3619 cm
1. A strong band at 3690 cm
1 is related to the in phase symmetric stretching and a weak absorption at 3649 cm
1 is assigned to out-of-plane stretching vibrations. The band observed at 3690 cm
1, assigned to the amount of water physisorbed on the surface of the clay [36]. There was not much variation in the peak pattern for acid treated kaolin. However, with acid treatment the peak intensity was found

Fig. 3. FT-IR spectra for raw kaolin, activated kaolin, and activated kaolin/Fe3O4 composite.

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Band (cm
1)

Peak assignments

3690 3649 3619 1642 1001, 1027 & 1115 910 791 750 683 541 461, 424

Free OH stretching Inner layer OH (Al

O

H) stretching H

O

H stretching H

O

H bending (physisorbed) Si

O stretching Al

Al

OH stretching Al

Mg

OH stretching Si

O

Al stretching Si-Ostr, Si

O

Al stretching Si-Ostr, Si

O

Al stretching Si

O stretching

to decrease indicating penetration of protons into the clay mineral layers and attack to the structural hydroxyl groups resulted in the dehydroxylation and leaching of the Al ions from the octahedral layer [25]. The FTIR data for activated kaolinite/Fe3O4 composite shows that the peaks at 3619, 3649 & 3690 cm
1 becomes very weak or almost disappeared indicating transformation in the structure. These results can be interpreted as being the consequence of Fe3+ exchanging the Al3+ groups of the crystal structure of the kaolin. The structural morphology of the iron oxide and kaolin/Fe3O4 composite revealed by the SEM images in Fig. 4 show that the synthesized composite is identified by a rough texture, because of the Fe3O4 particles covering the surface, with particles diameter ranging from 16.6 to 33.3 nm; the formation of an infinite number of pores on the irregular composite surface increases the surface area and facilitates the adsorption [37]. This morphology is different than that of the iron oxide dispersed nanoparticles with diameter ranging from 17.05 to 34.09 nm. Fig. 5 shows the TEM images of the iron oxide particles (Fig. 5a) and the clay (kaolin)/nanomagnetic iron composite (Fig. 5b). The images show that Fe3O4 particles were uniform and dispersed with a diameter ranging from 5.76 to 18.2 nm; after the formation of the composite the magnetite nanoparticles are finally divided and well distributed within the composite. Fig. 5b shows that Fe3O4 after binding with kaolin remained discrete and the composite had a mean diameter of 7.9 nm. This reveals that the binding process of the composite materials did not significantly result in the agglomeration or change in particle size. This may be attributed to the fact that the interaction occurring only on the particle surface.

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The magnetic properties of the Fe3O4 particles and the clay/ Fe3O4 composite particles were inspected using a vibrating sample magnetometer. Fig. 6 displays the magnetic hysteresis loop of the prepared composite; its saturation magnetization (Ms) value is 12.32 emu g
1, this magnetization value designates that the composite particles can be easily removed by the application of an external magnetic field after being used. Different magnetic clay composites have a magnetization value ranging from 0.29 to 34.55 emu g
1, depending on the magnetic component and the preparation conditions [38]. The Ms value of the Fe3O4 nanoparticles in the composite was reduced to about 30% of the bulk value (41.1 emu g
1); this reduction may be attributed to the fact that that the energy of a magnetic particle in an external field is directly proportional to its size, hence the decrease in particle size followed by the increase in surface area resulting from the composite formation led to a diminishing saturation magnetization value [21]. Moreover, the non-magnetic property of kaolin and the disordered structure in the amorphous materials has been found to decrease the effective magnetization [39]. The specific surface area of raw kaolin, acid-treated kaolin and Fe3O4/kaolin composite were found to be 13, 30 and 31.56 m2 g
1, respectively. Presence of Fe3O4 on the kaolin increased the surface area of composite. Increase in the surface area of other types of clay by acid activation is reported in the literature [40–42], while researches discussing the effect of acid activation on the kaolin clay are scant. However, the specific surface area of raw kaolin has been reported to range from 5 to 25 m2 g
1 [43].

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Effect of different operation parameters on adsorption of Direct Red Dye 23

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Effect of contact time and initial dye concentration The effect of contact time on the adsorption capacity was studied in the range 0–90 min at room temperature using 0.5 g of the adsorbent and 250 mL dye solution at concentrations of 20, 30, 40, 50 and 60 mg L
1. The results shown in Fig. 7 imply that the adsorption of the dye from the solution increases by increasing the contact time following a decreasing rate behavior. The adsorption goes through two distinct steps; the first lasts not more than five minutes with a removal ranging from 46 to 93% of the total dye removal; the second one with a much slower removal rate ending by achieving equilibrium after about 70 min of operation. This behavior can be attributed to the fact that in the beginning of the adsorption process adsorbate concentration gradient was high and more adsorbent active sites are available; as these sites become

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Fig. 4. SEM images (a) iron oxide nanoparticles (b) kaolin/iron oxide nanocomposite.

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Fig. 5. TEM images, (a) nano iron oxide particles and (b) clay/nanomagnetic iron oxide.

Fig. 6. Magnetic hysteresis loop of clay/nanomagnetic iron oxide. 301 302 303 304 305 306 307 308 309 310 311 312 313

gradually occupied by dye molecules the adsorption becomes slower and less effective. Also on increasing contact time the driving force between the bulk of the solution and the liquidadsorbent interface decreases with a consequent decrease in the rate of removal. Studies of different dyes adsorption on clays reported an equilibrium time ranging from 20 to 200 min depending on the operating conditions. Based on this result the adsorption experiments were run for 70 min [44,45]. The initial dye concentration represents the driving force necessary to overcome the mass transfer resistance between the composite surface and the bulk solution. Using high initial dye concentration has two adverse effects, it increases the amount of   Þ dye adsorbed but decreases the removal efficiency ðCoC
C  100 o

as shown in Fig. 7; this can be explained as follows: when the initial dye concentration is low the ratio of free surface active sites on the composite to the dye molecules is high and the adsorption is easy and efficient; when the initial concentration is high it enhances the driving force dragging more dye molecules to the composite surface, meanwhile, the number of free active sites is low compared to the number of dye molecules. As the concentration of dye increases, rapid saturation of the adsorbent and more surface sites are covered at a constant adsorbent dose consequently the adsorbent capacity gets exhausted due to nonavailability of the surface sites. This behavior ends up with an increase in the adsorbed amount of dye per unit mass of composite but a decreasing removal efficiency. The dye removal increased from 69 to 92.3% when the initial concentration decreased from

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Fig. 7. Effect of operation time and initial dye concentration on the dye removal (adsorption mass = 0.5 g; pH 7; T = 25  C). 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349

60 to 20 mg L
1 while the amount adsorbed increased from 18.46 to 41.4 mg dye/g adsorbent with the same initial concentration change. This same trend was found by previous researches studying the adsorption of dyes on clay surfaces [40,44]. Effect of adsorbent dosage To investigate the adsorption capacity of the prepared Fe3O4/ kaolin composite, the composite dose was varied using 0.25, 0.375, 0.5, 0.625 and 0.75 g; while keeping the initial dye concentration, pH and temperature constant at 40 mg g
1, 7 and 25  C, respectively; the experiment was run for 70 min. Once again two adverse effects appear with the increase in the adsorbent dose. The removal efficiency increases while the amount adsorbed of the dye per unit mass of adsorbent (adsorption density) decreases. The removal efficiency incremented from 78.8 to 100% when the weight of the adsorbent increased from 0.25 to 0.75 g while the total amount adsorbed decreased from 15.76 to 6.67 mg g
1 for the same adsorbent weight change. The behavior illustrated in Fig. 8 can be attributed to the following: the increase in the removal efficiency accompanying the increase in the adsorbent amount is due to the increment in the available free active sites on the composite surface; while the decrease in the amount of adsorbed dye is caused by: (i)

unsaturation of adsorption sites during the adsorption process generated by the higher adsorbent amount in a fixed solution volume; (ii) inter-particle interaction, such as aggregation, resulting from high adsorbent dose, leading to the diminution of the total surface area available for adsorption and the increasing of the diffusion path length travelled by the dye molecules. The mathematical relationship between the amount adsorbed ðqt Þ and the adsorbent mass (m) can be explained by Eq. (2): qt ¼

%Removal  Co V 100m

350 351 352 353 354 355 356 357

ð2Þ

where, qt: is the amount of the dye adsorbed (mg g
1), Co: is the initial dye concentration (mg/L), V: is the dye solution volume in L, and m: is the mass of the composite (g). Eq. (2) implies that qt and m are inversely proportional for fixed values of %Removal, Co and V. These findings are in agreement with previous studies [46–50]. Based on this behavior, to run the most cost-effective system it is advised to choose the adsorbent mass from the intersection of the %R curve with the qt curve. Therefore, in the following adsorption experiments the mass of the adsorbent used was 0.5 g based on the intersection of the two curves in Fig. 9.

Fig. 8. Effect of the adsorbent dose on the dye removal percentage and the amount adsorbed (initial dye concentration = 40 mg/L; pH 7; T = 25  C).

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Fig. 9. Effect of temperature on the percentage dye removal (adsorption mass = 0.5 g; initial dye concentration = 40 mg/L; pH 7). 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390

Effect of temperature The study of the temperature effect is crucial because temperature plays a major role in adsorption by having two different effects: (i) increasing the temperature reduces the solution viscosity and allows an easier diffusion of the dye molecules across the external boundary layer and inside the pores of the adsorbent [51,52], (ii) modifying the temperature may alter the adsorption capacity of the adsorbent [51–53]. For instance, increasing the temperature will increase the adsorption capacity if it is an endothermic process and will decrease it if it is an exothermic one. Six values of temperature were tested (25, 35, 40, 45 and 50  C), using 0.5 g of magnetic composite, 40 mg L
1 initial dye concentration and pH 7 for 70 min. The percentage removal of the C.I. Direct Red 23 increased from 77 to 86.5% with increasing the temperature from 25 to 55  C, as shown in Fig. 9, this increase is an indication of a better mobility of the dye molecules allowing a higher number of molecules to interact with the free active sites on the surface of the composite. Also this may be attributed to increased penetration of dyes inside adsorbent pores at higher temperatures or the creation of new active sites. Moreover, it indicates that the adsorption of the C.I.

Direct Red 23 dye is an endothermic process. Previous adsorption studies found the same outcomes [50,54].

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Effect of pH The pH of the solution is one of the main variables affecting the adsorption process; especially in the case of anionic and cationic dyes where the adsorption relies on the electrostatic attraction between the anions and cations produced and the charged surface of the adsorbent. To investigate the pH effect in the present work, five values of pH were tested (3, 5, 7, 9 and 11) at a fixed adsorbent mass, initial dye concentration and temperature. Fig. 10 shows that the maximum removal efficiency (90.23%) was obtained in the acidic medium (pH 3). Kaolinite is mainly constituted of Al and Si oxides which form hydroxide complexes in solution; the acidic or basic dissociation of these complexes at the solid–liquid interface causes the formation of either positive or negative charge on the surface depending on the solution pH surrounding the oxide particle [44]. In acidic media, surfaces are probably positively charged due to the abundant presence of H+; this leads to electrostatic attraction between the anions of the Direct Red 23 dye and the adsorbent surface. The same dye behavior was observed by

393

Fig. 10. Effect of pH on the percentage removal of the dye (adsorption mass = 0.5 g; initial dye concentration = 40 mg/L; T = 25  C).

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394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410

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JIEC 3527 1–13 A. Magdy et al. / Journal of Industrial and Engineering Chemistry xxx (2017) xxx–xxx 411

419

Liu et al. [55] in the adsorption of Direct Red Dye 23 on powdered tourmaline. The reduction in dye adsorption at highly alkaline conditions may be attributed to electrostatic repulsion between the negatively charged adsorbent and the deprotonated dye molecules probably the sulphonate group SO
3 . The continuous decrease in the dye removal with increasing pH of the solution indicates that electrostatic attraction is the main mechanism for Direct Red 23 dye adsorption by kaolin/Fe3O4 nanocomposite adsorbent.

420

Adsorption isotherms

421

Qualitative information concerning the interactive behavior between the adsorbate and adsorbent is provided by adsorption isotherms. The mathematical correlations obtained by the modeling analysis of isotherms are important for operational design, applicable practice of the adsorption systems, and offer the necessary data on the degree of accumulation of the adsorbate on the adsorbent at fixed temperature as well as the capacity of the adsorbent under the specified conditions. In this study the experimental data were analyzed using three isotherm models:

412 413 414 415 416 417 418

422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440

Langmuir isotherm The main assumption of Langmuir isotherm is the monolayer adsorption on the homogeneous surface of the adsorbent. This assumption implies that the adsorption occurs on a structurally homogeneous surface where all adsorption sites are similar and equivalent in energy, with each of these sites holding the same number of molecules. There is no interaction between the adsorbate molecules and if a molecule occupies an adsorption site, no further adsorption will occur at the same site. The model in its linearized form is given by the following equation; Ce 1 1 ¼ þ Ce qe K L qm qm

442 441 443 444 445 446 447 448 449 450

452 451 453 454 455 456 457 458 459

where Ce (mg L
1) is the equilibrium concentration of the dye, qe (mg g
1) is the equilibrium adsorption capacity, KL (L mg
1) is the Langmuir equilibrium constant, and qm (mg g
1) is the theoretical maximum monolayer adsorption capacity. An important characteristic of the Langmuir isotherm is the possibility to determine whether the adsorption process is favorable; this can be done by calculating a dimensionless constant separation factor, also called the equilibrium parameter [56], RL, which is defined in relation to aL and Co by: 1 ð4Þ RL ¼ 1 þ aL Co where aL = KL/qm. If (RL > 1) the adsorption is unfavorable, linear if (RL = 1), favorable if (0 < RL < 1) and irreversible if (RL = 0) [57]. Freundlich isotherm Freundlich isotherm is an empirical equation applicable to adsorption on heterogeneous surfaces and involving the formation of multi layers; it assumes that the adsorption sites have different levels of energy, it is given by the equation: qe ¼

461 460

K F C 1=n e

ð5Þ

giving upon linearization: lnqe ¼ lnK F þ 1=nlnC e

463 462 464 465 466

ð3Þ


1

9

The Freundlich constant KF is related to the adsorption capacity of the adsorbent: the higher the value, the greater the affinity for the adsorbate. The empirical parameter 1/n is related to the strength of adsorption, which varies with the heterogeneity of the material. When the values of 1/n are between 0.1 and 1.0, the adsorption process is considered favorable [58].

467

Temkin isotherm This isotherm takes into consideration the adsorbent–adsorbate interactions. By ignoring the extremely low and large value of concentrations, the model assumes that heat of adsorption (function of temperature) of all molecules in the layer would decrease linearly rather than logarithmic, as sated by Freundlich isotherm, with coverage. As implied in the equation, its derivation is characterized by a uniform distribution of binding energies (up to some maximum binding energy). The model is given by the following equation [59]:

473

qe ¼

RT RT lnAT þ lnCe bT bT

469 470 471 472

474 475 476 477 478 479 480 481 482

ð7Þ

let BT ¼ RT bT qe ¼ BT lnAT þ BT lnC e

468

483 484

ð8Þ
1

where AT = Temkin isotherm equilibrium binding constant (L g ), bT = Temkin isotherm constant, R = universal gas constant (8.314 J mol
1 K
1), T = temperature (K). BT = constant related to heat of sorption (J mol
1). The three adsorption isotherms are plotted in Fig. 11 and Table 4 shows the Langmuir, Freundlich and Temkin parameters. The correlation coefficients (R2) determine the most related model in the adsorption process. The values of R2 and the values of the equilibrium constants imply that the Langmuir isotherm is the suitable equation to describe the adsorption equilibrium of the Direct Red Dye 23 on the Fe3O4/kaolin nanocomposite, indicating a monolayer adsorption with an adsorption capacity of the composite of 22.88 mg g1; the range of RL values residing between 0 and 1 indicates that the adsorption is favorable. In solution-solid systems, with the hydration forces, mass transport effects etc. the system is much more dynamic and complicated, and obeying the isotherm does not necessarily reflect the validity of the aforementioned assumptions. In such systems the isotherm adequacy can be seriously affected by the experimental conditions, in particular, the range of concentration of the solute/adsorbate. Previous studies reported that both of Langmuir and Freundlich isothems might adequately describe the same set of liquid-solid adsorption data at certain concentration ranges, in particular if the concentration is small and the adsorption capacity of the solid is large enough to make both isotherm equations approach a linear form. Only one of the two isotherms correlates better with the data at high concentration.

486 485 487

Adsorption kinetics

514

To determine the mechanism of adsorption, the rate limiting step and the time required for completion of adsorption reaction, kinetic models are used. Three kinetic models were applied to the current experimental data for a better understanding of the adsorption process:

515

Pseudo-first-order rate model A first-order rate equation to describe the kinetic process of liquid-solid phase adsorption was introduced by Lagergren, it is considered to be the earliest model pertaining to the adsorption

520

488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513

516 517 518 519

ð6Þ
1

where qe (mg g ) is the equilibrium adsorption capacity, KF (L g ) is the Freundlich adsorption equilibrium constant, n is the Freundlich intensity factor, Ce (mg L
1) is the equilibrium concentration of the dye.

Please cite this article in press as: A. Magdy, et al., Synthesis and characterization of Fe3O4/kaolin magnetic nanocomposite and its application in wastewater treatment, J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2017.07.023

521 522 523

G Model

JIEC 3527 1–13 10

A. Magdy et al. / Journal of Industrial and Engineering Chemistry xxx (2017) xxx–xxx

Fig. 11. Plot of adsorption isotherms (a) Langmuir, (b) Freundlich, (c) Temkin. 524

526 525 527 528 529 530

532 531

rate based on the adsorption capacity. It is deduced as follows: dqt ¼ k1 ðqe
qt Þ ð9Þ dt where qe and qt (mg g
1) are the adsorption capacities at equilibrium and time t (min), respectively; k1 (min
1) is the pseudo-first-order rate constant for the kinetic model. Integrating the above equation with the boundary conditions of qt = 0 at t = 0 and qt = qt at t = t, yields qe ¼ k1 t ð10Þ ln qe
qt which can be rearranged to: lnðqe
qt Þ ¼ lnqe
k1 t

ð11Þ

Table 4 Isotherm constants for Direct Red Dye 23 adsorption on Fe3O4/kaolin composite. Isotherm

Constants

Langmuir

R2 = 0.9965 qm = 22.88 mg g
1 KL = 0.57 L mg
1 RL = 0.4–0.67 R2 = 0.826 1/n = 0.305 KF = 9.518 L g
1 R2 = 0.8834 AT = 7.32 L g
1 BT = 4.505 J mol
1

Freundlich

Temkin

To distinguish kinetic equations based on the adsorption capacity from solution concentration, Lagergren’s first order rate equation has been called pseudo first order [60]. The value of the adsorption at equilibrium and the rate constant are obtained by plotting lnðqe
qt Þ vs t (Fig. 12a).

533

Pseudo-second order rate model The basic assumptions of this model are the adsorption may be second-ordered, and the rate limiting step may be chemical adsorption involving valent forces through sharing or the interchange of electrons between the adsorbent and adsorbate [61]. The linear form of the pseudo-second-order model is given in Eq. (12):

538

t 1 1 ¼ þ t qt k2 q2e qe

534 535 536 537

539 540 541 542 543 544

ð12Þ

where k2 is the pseudo-second-order rate constant (g mg
1 min
1). Similar to the pseudo-first-order model, the k2 and qe parameters are determined from the slope and intercept of a plot of qt vs t (Fig. 12b).

546 545 547

Intra-particle diffusion model This model proposed by Weber and Morris [62] is an empirical model assuming that the adsorption capacity varies with the square root of time and is expressed as:

550

548 549

t

qt ¼ Kid t1=2 þ C

ð13Þ

Please cite this article in press as: A. Magdy, et al., Synthesis and characterization of Fe3O4/kaolin magnetic nanocomposite and its application in wastewater treatment, J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2017.07.023

551 552 553

G Model

JIEC 3527 1–13 A. Magdy et al. / Journal of Industrial and Engineering Chemistry xxx (2017) xxx–xxx

11

Fig. 12. Linearized adsorption kinetics models (adsorption mass = 0.5 g; pH 7; T = 25  C), (a) pseudo-first order, (b) pseudo-second-order, (c) intra-particle diffusion. 555 554 556 557 558 559 560 561 562 563 564

where Kid is the rate constant of intra-particle diffusion (mg g
1 t
1/2), C is a constant related to the boundary layer thickness (mg g
1). If the model is valid a plot of qt vs t1/2 should yield a straight line where Kid and C can be calculated from the slope and intercept (Fig. 12c). Table 5 summarizes the values of the equilibrium constants and correlation coefficients (R2) of the three models. Checking the correlation coefficient (R2) values and comparing the experimental capacities (qe,exp) with the calculated capacities (qe,cal) for each of the three models indicate that the kinetic data of

Table 5 Kinetic parameters for the adsorption of the Direct Red 23 dye on the Fe3O4/kaolin nanocomposite. Co, mg L
1 qe,exp (mg g
1) Pseudo-first order qe,cal (mg g
1) k1 (min
1) R2

20 9.14

1.82668 0.0482 0.6383

30 13.53

5.36985 0.0583 0.86

40 17.6569

50 18.92259

60 20.04184

8.354497 10.99016 0.059 0.0419 0.9076 0.9051

12.47716 0.0518 0.9507

Pseudo-second order qe,cal (mg g
1) 9.090909 13.5318 k2 (g mg
1 min
1) 0.257996 0.063062 2 R 0.9996 0.9986

17.79359 0.036138 0.9983

19.30502 20.70393 0.020498 0.016499 0.9964 0.9952

Intra-particle diffusion Kid (mg g
1 t
1/2) 0.0768 C (mg g
1) 8.5489 R2 0.9093

0.5623 13.725 0.974

0.9715 12.487 0.9607

0.3113 11.277 0.9961

1.0725 12.862 0.9491

the adsorption of the direct red dye on the magnetic nanocomposite are better statistically described with the pseudosecond order than with the pseudo-first order or intra-particle models. Besides, the equilibrium uptake capacities calculated from the pseudo-second order model plot are also closer to the experimental values. The pseudo-second order model was found to be the best model to describe the adsorption of numerous dyes on different adsorbents, e.g., the adsorption of acid red 57 onto calcined alunite [63], the adsorption of reactive black 5 onto activated carbon [64], the adsorption of C.I. Basic Red 46 dye on bentonitic clay [1], the adsorption of Safranin dye using magnetic mesoporous clay [8], the adsorption of methylene blue on montmorillonite clay [9], on activated kaolinite [26] and on Fe3O4/bentonite nanocomposite [65] and the adsorption of methyl violet 2B dye using a magnetic composite [66].

565

Thermodynamic studies

581

The values of the thermodynamic parameters associated with the adsorption process are the actual indicators for practical application of the process. The negative value of the change in Gibbs free energy (DGo) indicates a spontaneous process; the sign of the enthalpy change (DHo) reveals the effect of the temperature on the process and the sign of the entropy change (DSo) shows whether the disorder increases or decreases after the process. These parameters can be calculated from the equations:

582

lnKC ¼

D So R




DHo R



1 T

ð14Þ

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566 567 568 569 570 571 572 573 574 575 576 577 578 579 580

583 584 585 586 587 588 589

G Model

JIEC 3527 1–13 12

A. Magdy et al. / Journal of Industrial and Engineering Chemistry xxx (2017) xxx–xxx

Fig. 13. Eestimation of thermodynamic parameters for the Direct Red 23 dye adsorption on the nanocomposite.

KC ¼

qe Ce

ð15Þ

DGo ¼ DHo
TDSo 591 590 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614

ð16Þ

where KC is the equilibrium constant (L g ), DS is the entropy change (J mol
1 K
1), R is the universal gas constant (8.314 J mol
1 K
1), DH is the enthalpy change (J mol
1), T is the absolute temperature (K), qe is the amount adsorbed per unit mass of adsorbent at equilibrium (mg g
1), Ce is the equilibrium concentration (mg L
1), DG is the Gibbs free energy change (J mol
1). DH and DS are calculated from the slope and intercept of the plot of ln KC vs 1/T illustrated in Fig. 13, and then DG calculated from Eq. (15). The values of the thermodynamic parameters for different temperatures are summarized in Table 6. The positive value of the DH confirms that the adsorption process is endothermic as was, previously, observed from the effect of the temperature on the removal efficiency. The value of enthalpy change is also helpful to determine the type of adsorption whether physical or chemical. Typically, the enthalpy change for a physisorption is below 84 kJ mol
1 and for chemisorption it ranges from 84 to 420 kJ mol
1 [66], this fact points out that the adsorption of the Direct Red 23 dye on the prepared nanocomposite is physical in nature. It was expected that adsorption process is exothermic due to the heat released after formation of the bond between dye molecules and adsorbent surface. The endothermic adsorption of Direct Red 23 dye on kaolin/Fe3O4 nanocomposite appears to be unusual behavior, however, several
1



Table 6 Thermodynamic parameters for the adsorption of Direct Red 23 dye on Fe3O4/kaolin nanocomposite. Temperature (K) Thermodynamic parameters

R

DG (kJ mol ) DH (kJ mol ) DS (kJ mol o

298 308 313 318 323 328


1.21467
1.79665
2.08764
2.37863
2.66962
2.96061


1

o

16.128


1

o

58.198


1

K


1

authors have reported endothermic adsorption of reactive dyes on different types of adsorbents [50,54]. The positive DS reveals the increase in disorder and randomness at the composite/solution interface during the adsorption. The negative DG values, indicate that the adsorption of the red dye is a thermodynamically spontaneous process within the range of temperatures studied. The decrease in negative DG values accompanying the increase in temperatures shows an increase in feasibility of sorption at higher temperatures, observed already by the positive value of DH . Once again, the physical nature of the adsorption is confirmed by the value of DG since the physical adsorption has a free energy change in the range
20 to 0 kJ mol
1, while the chemisorption has a range of
80 to
400 kJ mol
1 [67].

615

Conclusions

628

The prepared magnetic clay nanocomposite proved efficacy in the adsorption of the anionic Direct Red 23 dye from aqueous solutions. The parameters controlling the uptake of the dye on the adsorbent are the contact time, initial dye concentration, adsorption dose, temperature and pH. The removal efficiency reached 100% under a certain set of operating conditions (t = 70 min, initial dye concentration = 20 mg L
1, adsorbent mass = 0.75 g, T = 25  C and pH 7). The adsorption follows Langmuir isotherm with a maximum adsorbent capacity of 22.88 mg g
1. The pseudo-second order kinetic model was found to be the best model describing the process. Thermodynamic studies revealed a spontaneous and endothermic adsorption process accompanied by an increase in entropy in the solution bulk. In order to complete the assessment of the prepared magnetic clay nanocomposite adsorbent the regeneration performance and change in adsorption capacity with adsorption–desorption cycles need to be considered.

629

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Please cite this article in press as: A. Magdy, et al., Synthesis and characterization of Fe3O4/kaolin magnetic nanocomposite and its application in wastewater treatment, J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2017.07.023