Sodium Dodecyl Sulfate-Modified Fe 2 O 3 / Molecular Sieves for Removal of Rhodamine B Dyes

Studying the removal of rhodamine B (RB) dye by using zeolite 13X molecular sieves supported by Fe2O3 nanoparticles (denoted as Fe2O3-13X) is the main objective of this study. Fe2O3-13X was synthesized and modified by the addition of sodium dodecyl sulfate (SDS). ,e prepared Fe2O3-13X was characterized by XRD, TEM, SEM, and zeta potential. ,e effects of the solution pH, SDS amount, contact time, initial dye concentration, and adsorbent dosage on the removal efficiency of RB were studied. A maximum removal efficiency of 99.3% was achieved. ,e adsorption equilibrium data of RB were fitted using the Freundlich model, yielding the maximum adsorption capacity of 89.3mg/g. ,e findings revealed that the RB adsorption onto Fe2O3-13X modified with SDS (Fe2O3-13X-Ms) was described by a pseudo-second-order kinetic equation. ,e results reported in this paper indicate that a high RB removal percentage was attained by adding SDS to Fe2O3-13X.


Introduction
Recently application of nanoparticles has increased in wastewater treatment.ese materials have been established due to their properties such as high potential for removal and recovery of the effluent from wastewater, large surface area to volume ratio, reuse, and low cost.A number of researchers have used these materials as adsorbent [1,2] or photo catalyst [3] for removal of dyes and heavy metals.
Synthetic organic dyes and rhodamine B (RB), in particular, are commonly used in different industries-including food, textile, and leather-as a coloring agent.ese dyes are usually water contaminants and are frequently found in industrial wastewater.It is difficult to remove these dyes because of their complex structure.us, they have a tendency to persist in the environment, making serious issues with the quality of water, leading to some public health problems [4,5].
In the pertinent literature, a wide range of chemical, physical, and biological methods for removal of RB from wastewater have been investigated.ese methods include chemical degradation [6], physical adsorption [7,8], photo degradation [9], and biological degradation [10].e most commonly used dye removal method is based on adsorption, due to its high potential for the removal of dyes from wastewater and the simplicity and flexibility of the design.In addition, it does not produce dangerous by-products.Magnetic adsorbents have been extensively used in environmental applications.ese adsorbents combine the adsorption process with magnetic separation, and thus, they do not require common separation processes, like centrifuge, to separate the solid phase from the solution.Among its other advantages is its potential for processing a large volume of wastewater in a short time without producing dangerous contaminants like flocculants.Magnetization of activated carbon is an example of these adsorbents [11].
Zeolites are microporous and crystalline-hydrated aluminosilicates.Both synthetic and natural zeolites are commonly used in different fields, such as ion exchange, adsorption, and heterogeneous catalysis.Zeolites are a subgroup of a large class of molecular sieves.e framework of pure silica is neutral.A negative charge of the zeolite framework stems from the presence of AlO 4 , which is stabilized by cations.ese cations can be replaced by other ions using ion exchange methods.
Magnetic zeolites can be prepared via di erent methods, such as precipitation of iron oxides over zeolite [12].It is used for the removal of dyes, heavy metals, and as a catalyst [13][14][15].Unmodi ed magnetic zeolite has low dye removal e ciency.However, when magnetic zeolite is modi ed with surfactant, the removal e ciency as well as its selectivity will increase.Modi cation of zeolite and magnetic zeolite with surfactant is preferred to other magnetic zeolite modi cation processes due to its high potential and easy and cost-e ective processes.
Surfactants are surface-active substances comprising of a hydrophobic nonpolar tail and a polar hydrophilic head.According to the charge on their polar head, they can be classi ed as anionic, nonionic, cationic, and amphoteric.Authors of a signi cant number of extant studies have focused on using cationic surfactants for modi cation of zeolite and magnetic zeolite for removal of di erent types of e uents [16,17].Research e orts to modify magnetic zeolites using anionic surfactants to remove dyes have been limited.e goal of this research was to study the e ect of modi ed Fe 2 O 3 -13X using SDS for removal of RB and understand the properties and mechanisms of the removal process.To meet this objective, the e ects of the solution pH, amount of SDS, initial dye concentration, and adsorbent dose were investigated using the batch experiment method.

Materials and Methods
2.1.Materials.Among the materials used in this work, zeolite 13X was purchased from Sigma-Aldrich, USA; ferric nitrate nonahydrate (Fe(NO 3 ) 3 •9H 2 O) was supplied by J.T. Baker, USA; SDS was sourced from Sigma-Aldrich, USA; NaOH was purchased from Alphchem, Canada, ON; and rhodamine B from Acros, USA.e molecular formula of RB is C 28 H 31 N 2 O 3 Cl.It has a molecular mass of 479 g/mol, and the chemical structure shown in Scheme 1.

Preparation of Fe 2 O 3 -13X.
Initially, zeolite 13X granules were dried at 70 °C and crushed and sieved to obtain powder of 53 µ particle size or less.Next, 5 g of Fe(NO 3 ) 3 •9H 2 O was dissolved in 100 mL of distilled water.After that, 10 g of Z13X was added to the ferric nitrate solution under stirring for 48 hours, and the resulting iron metal exchange zeolite 13X (Fe-13X) was washed with distilled water.e resultant solid was separated and dried at 70 °C for 2 hours.
e Fe-13X was treated with 100 mL of 0.5 M NaOH.e obtained Fe 2 O 3 -13X was neutralized using distilled water, dried at 70 °C for 2 hours, and then calcinated at 550 °C for 2 hours.e color of prepared Fe 2 O 3 -13X was red to reddish-brown.

Characterization Methods.
e prepared Fe 2 O 3 -13X was characterized by TEM, XRD, and Zeta potential meter.A Philips CM10 TEM was used with an accelerating voltage of 100 kV.XRP was performed using Rigaku MiniFlex XRD, Cu Kα radiation (λ 0.1524 nm) was at angles ranging from 5 °to 80 °(2θ), the surface areas of 13X and Fe 2 O 3 -13X were calculated from N2 adsorption isotherms by using the Brunauer-Emmett-Teller (BET) method, and a Zetasizer Nano ZS 3000 HAS (Malvern, Worcestershire, UK) was employed when measuring the zeta potential.

Adsorption Experiments
2.4.1.Batch Adsorption.RB adsorption experiments were conducted using a batch technique.First, 100 mg of adsorbent was shaken in 100 mL of RB with an initial concentration of 25 mg/L.Solution pH was adjusted to 3, and SDS was added to the dye solution at di erent amounts.e mixture was stirred until equilibrium was reached, the solid phase was removed by centrifugation, and the remaining dye concentration in the solution was measured using the UV-Vis spectrophotometer (Cary 60, Agilent Technology, Germany).e adsorbent amount was in the 250-1000 mg/L range, and the pH was varied from 3 to 9.
e removal e ciency (%R) and the dye adsorption capacity (q) (mg/g) were calculated using the expressions below: where C i and C f are the initial and nial dye concentration (mg/L), respectively; V is the dye solution volume (L); and m is the adsorbent mass (g).Table 1 shows the batch removal experiments that were carried out in the di erent conditions.

Adsorption Isotherm.
Adsorption isotherm, contact time, and kinetics study were conducted by using 100 mL of RB solution at an initial concentration (25 to 100 mg/L) with 1000 mg adsorbent, 100 mg SDS, and 3 pH in a 250 mL ask at 25 °C.Aliquots were withdrawn at 10-minute intervals for investigation after centrifugation.

Isotherm and Kinetic Models
3.1.Isotherm Models.Equilibrium isotherms show the interaction between the adsorbates and the adsorbents and are thus important to optimize the application of the adsorbents.Langmuir and Freundlich models were tted to the equilibrium experimental adsorption data.e Freundlich isotherm model assumes that distinctive locales are contained within a few adsorption energies, so it can be associated with nonideal adsorption occurring on heterogeneous surfaces [18].e nonlinear form of this model is as follows: 2 Advances in Materials Science and Engineering where q e is the dye adsorption capacity at equilibrium (mg/g), C e is the dye concentration at equilibrium (mg/L), 1/n is the factor of heterogeneity, and K f is the Freundlich constant.e above equation is linearized to evaluate the parameters of linear regression: e main assumption of the Langmuir isotherm model is that the adsorbent surface is covered with a limited number of active sites, distributed in a homogeneous manner over the surface.It is given by the following nonlinear equation [19,20]: where q m is the maximum theoretical adsorption capacity (mg/g) and K L is the Langmuir constant (L/mg).is model can be linearized in different forms and one of them is e basic physical characteristics of Langmuir adsorption isotherm could be expressed as the dimensionless constant separation equilibrium parameter (R L ) that is determined by [21] where C i is the initial RB concentration (mg/g) and R L factor varies depending on the isotherm data.e calculated R L values point to the isotherm type [22].
Commonly the tool used to compare between the models is R 2 (the coefficient of determination).Due to linearization (transformations), the present extreme points may perhaps disappear and be created by new points.For this reason, the best fitness should not depend only on R 2 [23,24].One of the proposed solutions to solve this problem is to minimize the sum squared error (SSE) between the experimental and predicted values using the Origin software: where Q c and Q e are the calculated and experimental values, respectively.

Kinetic Model.
A study about the kinetic of RB adsorption on Fe 2 O 3 -13X is valuable because it provides information about the adsorption mechanism, which is necessary for selecting the optimum conditions for largescale batch processes.Generally, dye adsorption mechanism comprises of the following steps, whereby the slowest step or a combination of several steps determines the control rate of the sorption process: (1) External mass transfer of molecules from the bulk solution to the boundary layer film surrounding the exterior surface of the adsorbent solid particles.(2) Diffusion of the molecules through the boundary layer to the sites (external or internal) on the surface of the adsorbent.In this process, binding may be chemically or physically dependent on the energy.is step is usually assumed to be rapid.(3) Adsorption of the molecules onto the adsorption site, after which they diffuse into the interior of the solid particles (intraparticle diffusion).
e adsorption mechanism of RB onto the Fe 2 O 3 -13X surface was investigated by the pseudo-first-order, pseudosecond-order, and intraparticle diffusion kinetics models.
In the pseudo-first-order model, the linear form is given by [25] ln q e − q t  � ln q e − K 1 t, (8) where q t is the actual dye concentration at time t (mg/g), q e is the equilibrium RB concentration (mg/g), and K 1 is the pseudofirst-order rate constant, which is obtained from the linear plot ln(q e − q t ) versus time.e main assumption of a pseudo-second-order model is that the chemisorption of the adsorbate on the surface of adsorbents is the limiting step.e following equation can represent this model [26]: where K 2 is the pseudo-second-order rate constant (g/mg/min), which was calculated by plotting t/q t versus time (t).e probability of the effect of adsorption of intraparticle diffusion was studied using the following model [27]: where K 3 is the intraparticle diffusion rate constant (mg/g/min 1/2 ).[28] and the dispersion of amorphous particles of Fe 2 O 3 within the structure of Z13X [15].e average crystallite size (D) was computed from the peak of high intensity at 2θ 35.7 °using the Scherrer equation:

Results and Discussion
where D is the average crystallite size (nm), K is the Scherrer constant (0.9), λ is the wavelength of X-ray radiation applied (0.1540 nm), β is the full width at half maximum (FWHM) of di raction (radians), and 2θ is the Bragg angle.e average crystallite size computed was 30 nm.
Figure 2 shows the micrograph of Fe 2 O 3 -13X, investigated by TEM.Approximately uniform black spherical particles with the average size of 9-49 nm could be observed, con rming the existence and precipitation of Fe 2 O 3 particles.Due to the nanosize, as well as the high-speed movement of Fe 2 O 3 nanoparticles during precipitation, the particles seem to collide with the surface and settle deeply through the zeolite softened material.
e SEM micrographs of 13X and Fe 2 O 3 -13X are shown in Figures 3(a) and 3(b), respectively.Figure 3(a) shows that the zeolite 13X particles form agglomerates consisting of nearly cubic-shaped grains of di erent sizes.Figure 3(b) shows the changes in the zeolite 13X particle morphology due to precipitation of magnetic particles.e image reveals that the zeolite 13X particles are covered by Fe-oxide clusters.
Table 2 shows the BET surface areas and pore volumes of 13X and Fe 2 O 3 -13X, and the results show that the surface area and pore volume of 13X were 573 m 2 /g and 0.36 cm 3 /g, while for Fe 2 O 3 -13X, 541 m 2 /g and 0.21 cm 3 /g.e decrease of the surface area of Fe 2 O 3 -13X can explain blocking of the microspore of 13X by the magnetic particles which decrease the pore volume of Fe 2 O 3 -13X compare to 13X.

E ect of the Solution pH.
e dye removal process is a ected by the solution pH. e zeta potential of Fe 2 O 3 -13X and modi ed Fe 2 O 3 -13X-Ms at varying pH is shown in Figure 4. e positive values of the Fe 2 O 3 -13X zeta potential at pH values 3 and 4 changed to negative values after adding SDS.
e in uence of the solution pH on the RB removal efciency by Fe 2 O 3 -13X was studied at six pH values, ranging from 3 to 9, for the dye concentration of 25 mg/L in 1 L solution, with 1000 mg adsorbent, and using 100 mg SDS.As shown in Figure 5, the % removal e ciency changed signicantly as the pH was increased from 3 to 9, having a maximum of 99.3% at pH 3, declining rapidly to 17% at pH 9.
ese results can be explained by the possible RB binding mechanism to the modi ed surface as shown in Figure 6.
At pH below 4.0, electrostatic interactions between the negatively modi ed surface of Fe 2 O 3 -13X and RBH + molecules will occur, contributing to the high dye removal percentage [31].On the contrary, at pH above 4.0, the removal mechanism changes because of the formation of the zwitterion, and the combination e ects of positive charge and negative charge of the RB molecules will a ect the removal process [32].
erefore, the binding of the dye molecules and the modi ed surface of Fe 2 O 3 -13X is reduced because of the repulsion force between the molecules of RhB ± and the negative surface of the adsorbent.Consequently, the dye removal e ciency will decrease as the pH of the solution increase above 4. Similar results were reported by Jain et al. [4].

E ect of the Surfactant Amount.
In micellization phenomena, surfactant molecules arrange themselves so that the nonpolar hydrophobic portions are shielded, forming the core, while the polar heads are located at the watermicelle interface touching the water molecules.Micellization occurs when the surfactant concentration is equal or less than its critical micelle concentration (CMC) at which micelles form.CMC of SDS was 230 mg/L [33].In this work, sodium dodecyl sulfate (SDS) at concentrations below and above its CMC (i.e., 20, 30, 40, 50, 80, 100, 200, and 300 mg) was added to 1 L (25 mg/L initial concentration) dye solution, which was adjusted to pH 3 to investigate the e ect of the surfactant amount on the % removal e ciency.
As shown in Figure 7, the % removal e ciency of RB by unmodi ed Fe 2 O 3 -13X at SDS 0 was low (34%).However, when Fe 2 O 3 -13X was modi ed with anionic surfactant (SDS), the removal e ciency increased to 99.3%. is behavior might be explained as follows.Before adding the SDS to the solution, the repulsion force between the positive surface charge of Fe 2 O 3 -13X and the positively charged RB will decrease the % dye removal, while adding the SDS increased the dye removal e ciency in-line with the surfactant amount.
e increase in %R can be explained as follows.At a solution pH of 3, the adsorbent surface had negative charge, like the charge of micelles.However, under these conditions, hydrophobic adsorption surface served as an appropriate substrate for adsorption of hydrophobic tail of the surfactant, creating a bilayer or monolayer with the negative sulfonate head in the direction of a solution [34].   is led to sorbate-sorbate associations, which contributed to increased adsorption [35].A maximum %R was obtained at the SDS concentration of 100 mg/1 L solution.At higher SDS concentrations, RB adsorption decreased because of the aggregates of SDS in the solution, which can hinder the micelle formation on the adsorbent surface.Similar ndings were reported by Shariati et al., who modi ed Fe 3 O 4 nanoparticles with SDS and utilized it to remove safranin O [36].

E ect of the Adsorbent Amount.
e adsorbent amount has an important e ect on RB removal.e adsorbent amount was varied from 200 to 1000 mg/1 L solution, while the solution pH was xed at 3, along with the initial dye concentration (25 mg/L), SDS amount (100 mg/L), and mixing time (1 hr).It can be seen from Figure 8 that the adsorbent amount increased the %R, likely due to the increased Fe 2 O 3 -13X surface area, or existence of a larger number of adsorption-active sites [37].

E ect of the Initial Concentration and Contact Time.
As shown in Figure 9, the %R decreased from 99.3% to 87.5% as the initial RB concentration increased from 25 to 100 mg/L. is indicated that the dye adsorption onto the adsorbent depends on the initial RB concentration.At lower initial dye concentrations, higher surface area was available to the smaller number of RB molecules.Conversely, at higher dye concentrations, a large number of dye molecules interacted with the accessible adsorption sites.
Figure 9 shows the in uence of initial concentration on the %R of RB.As seen from the graph, for all initial dye concentrations, removal was rapid during the rst 10 minutes and attained equilibrium in 1 hour.is behavior may be attributed to the second-order type adsorption process, as will be discussed in the next section.

E ects of Ionic Strength.
Wastewater from textile industries and dying process commonly contain other types of impurities such as alkali, salts, and acids, and the existence of these ions may compete with RB molecules on the adsorbent active sites.e e ect of ionic strength on the RB removal onto Fe 2 O 3 -13X was investigated in the NaCl concentration in the range of 0.001 to 0.1 mol/L.
Table 3 shows the e ect of NaCl concentration on the RB removal.
e results illustrated that increasing salt concentration decrease the removal e ciency.
is could be ascribed to the rivalry of Na+ ion and the positive charge of the dye (RB+) on the active sites on the adsorbent surface.Similar results were found by Shariati et al. [36].

Adsorption Kinetics.
e isotherm experimental data for RB adsorption on Fe 2 O 3 -13X were tested with the Langmuir and Freundlich models.Table 4 shows the    Figure 10 shows a plot of di erent RB equilibrium concentrations (C e ) versus adsorption capacity (q e ) by using a linear form of the Freundlich model ( 3). e high R 2 value and low SSE value indicated that the linear form of the Freundlich model represents the experimental data.e value of n > 1 was satisfactory to direct toward favorable adsorption of RB by Fe 2 O 3 -13X material under test conditions [7].
Figure 11 shows the plots of the RB adsorption amount on the adsorbent (Fe 2 O 3 -13X-Ms) at di erent RB equilibrium concentrations (C e ) under the optimum conditions versus C e /q e using a linear form of the Langmuir isotherm model (5).
According to Table 4, the high value of SSE and low R 2 value indicate that the linear Langmuir model was not the suitable model to depict the adsorption phenomenon.
Our ndings indicate that q m at 25 °C in the concentration range examined in this study was 89.3 mg/g.e R L values for 25-100 mg/L RB dye concentrations varied from 0.059 to 0.0156, implying favorable adsorption.
Figure 12 shows the nonlinear isotherm of the Freundlich and Langmuir models for adsorption of RB on the adsorbent.e related parameters are shown in Table 4 for both models.
e Freundlich model shows high R2 coe cient value and low SSE value compare to the Langmuir model.
erefore, again the results indicated that the nonlinear Freundlich isotherm model ts the experimental data, also the isotherm constants of nonlinear and linear form of the Freundlich model found close to each other.
So depending on the revealed results, it can thus be concluded that the adsorption isotherms of RB on the adsorbent can be tted well by the nonlinear and linear Freundlich model, demonstrating the (multilayer) heterogeneous adsorption characteristic.
A comparison of the results of the removal of RB by Fe 2 O 3 -13X-Ms with other reported adsorbents is given in Table 5.As shown in Table 5, the adsorbent employed in the present study exhibited good performance for the uptake of RB from wastewater.

Kinetic Model.
e experimental kinetic data of adsorption of RB onto the modi ed Fe 2 O 3 -13X were tted with  Advances in Materials Science and Engineering the pseudo-rst-order, pseudo-second-order, and intraparticle models.e results are shown in Table 6.In this work, for pseudo-rst-order, the R2 value is low. is shows that the kinetics of adsorption of (RB) on Fe 2 O 3 -13X-Ms does not t to the pseudo-rst-order model.Figure 13 shows the tted linear plots of the pseudosecond-order model.From Table 6, the values of R2 are more than 0.998 for all initial dye concentrations.is approves that the adsorption kinetics of (RB) on Fe 2 O 3 -13X-Ms ts well to a pseudo-second-order kinetic model.Also, the values of adsorption capacity calculated by this model were close to the experimental values of adsorption capacity. is result conrms that the rate of RB adsorption over Fe 2 O 3 -13X may be controlled mainly by a chemisorption process and agrees with an earlier suggested mechanism obtained from the breakthrough curve of the RB removal [41].
Figure 14 shows the plots of the intraparticle di usion model.It can be seen from this gure for all initial dye concentrations, there are two segments, each segment shows di erent mechanism of adsorption.In the rst linear segment, the adsorption capacity changes fast with time until it reaches equilibrium; this indicates that the boundary layer di usion may be the control step.While in the second segment, the experimental data points were the horizontal line which indicates that the equilibrium had been reached which indicate happening on the intraparticle di usion.e calculated values of C, K 3 , regression, and correlation constants are presented in Table 6.e nonappearance of such feature points to that the experimental data do not t the intraparticle di usion model because of low values of R 2 ≤ 93, in comparison with the high values of R 2 estimated from the pseudo-second-order model and the intraparticle di usion was not only the limiting control step.Moreover, the plot does not pass through the origin, indicating that the rst stage of this plot involved boundary layer adsorption [42].

Figure 8 :
Figure 8: Removal e ciency versus adsorbent amount.Initial dye concentration 25 mg/L, pH of the solution 3, time 60 min, and SDS amount 100 mg/1 L solution.

Figure 9 :
Figure 9: Removal e ciency versus contact time.Solution pH 3, adsorbent dosage 1000 mg/L solution, and SDS amount 100 mg/L solution.

Table 1
15.5 °, 23.3 °, 26.6 °, and 31 °corresponding to (111), (331), (533), (542), and (751) planes, respectively, are clearly observed in synthesized sample but with less intensity due to the overlapping of Fe 2 O 3 re ection lines.is proves that the crystalline structure of Z13X is not damaged during the preparation steps.No peak due to Fe 2 O 3 has been detected due to the lower crystallinity of Fe 2 O 3 -13X 4.1.Characterization of Synthesized Samples.Figure 1 shows the XRD patterns of Z13X and synthesized Fe 2 O 3 -13X.e diffraction maxima for Z13X at 2θ equal to 6.26 °,

Table 3 :
E ect of ionic strength on the %RB removal.

Table 4 :
Nonlinear and linear Freundlich and Langmuir constants for the adsorption isotherm of RB on Fe 2 O 3 -13X-Ms.Figure 10: Freundlich isotherm of RB on Fe 2 O 3 -13X-Ms.Time 60 min, pH of the solution 3, and surfactant amount 100 mg/L.

Table 5 :
Comparison of maximum adsorption capacity reported in extant studies with that of Fe 2 O 3 -13X-Ms.