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Modification of nickel ferrite with cationic surfactant: dye removal from
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textile wastewater using magnetic separation
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Elham Narian1, Mokhtar Arami2*, Hajir Bahrami3, Elmira Pajootan4
Abstract
In this study, the modification of nickel ferrite nano-particles with cationic surfactant
(Cetyltrimethyl ammonium bromide [CTAB]) and its capability as a magnetic adsorbent for the
removal of direct dyes (C.I. Direct Red 31, C.I. Direct Red 81 and C.I. Direct Red 80) from
textile wastewater have been investigated. The FTIR analysis, XRD diffraction, SEM images and
the determination of isoelectric point (pHZPC, zero point charge) were used to analyze the
morphology to characterize the synthesized magnetic adsorbent. The effect of operating
parameters including pH, initial dye concentration, adsorbent dosage and inorganic salts on the
dye removal efficiency was studied. The results showed that varying the pH of solution did not
affect the dye removal efficiency. It was also concluded that by increasing the adsorbent dosage,
the available adsorption sites and the dye removal efficiency would increase. Adsorption
isotherms such as Langmuir, Freundlich and Tempkin were investigated and the results showed
that Direct red 31 and Direct red 81 had followed all of the mentioned isotherms and Direct red
80 had followed Langmuir isotherm. The kinetics studies showed that the dye removal was
conformed to the pseudo-second order kinetic. Finally, it was concluded that the modified nickel
ferrite (CTAB-NiFe2O4) could be used efficiently for the dye removal from textile effluents
followed by easy magnetic separation of adsorbent.
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Textile Engineering Department, Amirkabir University of Technology, 424 Hafez Ave, Tehran, Iran, 15875-4413.
Tel: +98 2164542614, Fax: +98 2166400245, Email: [email protected].
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*Corresponding author: Textile Engineering Department, Amirkabir University of Technology, 424 Hafez Ave,
Tehran, Iran, 15875-4413. Tel: +98 2164542614, Fax: +98 2166400245, Email: [email protected].
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Textile Engineering Department, Amirkabir University of Technology, 424 Hafez Ave, Tehran, Iran, 15875-4413.
Tel: +98 2164542614, Fax: +98 2166400245, Email: [email protected].
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Textile Engineering Department, Amirkabir University of Technology, 424 Hafez Ave, Tehran, Iran, 15875-4413.
Tel: +98 2164542614, Fax: +98 2166400245, Email: [email protected].
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Keywords: Adsorption, CTAB-NiFe2O4, Dye removal, Isotherm, Kinetics
INTRODUCTION
Textile industry consumes large quantities of synthetic dyes. Over 7 × 105 tons of dyes are
produced worldwide each year, and 10–15% of them are discharged into the water streams
(Elemen et al. 2012; Wang and Li 2013). Different types of dyes are used in textile industry such
as basic dyes, acid dyes, reactive dyes, direct dyes, azo dyes, etc. Azo dyes which are
characterized by one or more azo bonds as the chromophore group in their structure, constitute
60–70% of all the manufactured dyes in textile industry due to their chemical stability and
versatility (Mahmoud et al. 2012; Garcia-Segura et al. 2012; Tunç et al. 2012; Collazzo et al.
2012).
Many dyes and pigments are toxic in nature and effluents containing synthetic dyes with
complex aromatic molecular structures, recalcitrant and inhibitory nature cannot be treated by
conventional wastewater treatment methods. Therefore, the removal of dyes from wastewaters is
very important before they are released into the aquatic systems (Patel and Vashi 2012; Anbia
and Salehi 2012; Wu et al. 2011; Kousha et al. 2012; Wu et al. 2013).
There are different strategies for wastewater treatment including: physical, chemical, biological,
electrochemical, filtration and advanced oxidation processes (AOPs) (Liao et al. 2012; Kousha et
al. 2012; Wu et al., 2013). Considering the disadvantages and limitations of the mentioned
methods such as high cost, generation of by-products, etc., (Shen et al. 2011; Alver and Metin
2012; Al-Rashed and Al-Gaid 2012) many researchers have found the adsorption process to be
an effective and simple technique for the removal of various pollutants from effluents (Wang et
al. 2012; Errais et al. 2012; Shen et al. 2011; Li et al. 2012).
Common adsorbents such as activated carbon and resin are too expensive and the application of
low cost adsorbents such as industrial byproducts or agricultural wastes requires the addition of
higher amounts of adsorbent rather than the activated carbon (Wang 2012; Yu et al. 2012). In
recent years, magnetic nano-particles (MNPs) with large surface area have been introduced as an
effective and efficient nano-adsorbent for the removal of arsenic, heavy metals and dyes from
wastewaters (Wu et al. 2011; Madrakian et al. 2011; Giri et al. 2011). Another advantage for
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these nano-adsorbents is the economical magnetic separation from solutions which is a simple
and time saving procedure that produces no contaminants like flocculants, and they easily
recover in magnetic field (Zhu et al. 2012; Hao et al. 2012; Tang et al. 2012; Hritcu et al. 2012;
Safarik et al. 2011).
Literature reviews indicated that various magnetic adsorbents such as cellulose/Fe3O4/activated
carbon composites, Fe3O4-activated carbon nano-composite, Fe3O4/ ZnCr- layered double
hydroxide, N-benzyl-O-carboxy methyl chitosan, multi-wall carbon nanotube, alginate beads and
activated maize cob powder have been synthesized and used for the removal of dyes from
aqueous solutions (Endo et al. 2001; Vettorazzi et al. 1998; Gómez et al. 2009; Yao et al. 2012;
Rocher et al. 2008). On the other hand, different surfactants including cetyltrimethyl ammonium
bromide (CTAB) have been extensively used for the modification of different adsorbents to
improve their sorption properties and to prevent the aggregation of nano-particles (Fazeli et al. ;
Jin et al. 2012; Bayram and Ayranci 2012).
Ferrite nano-particles are soft magnetic materials, in which a divalent metal ion could exist.
These MNPs have interesting properties such as high specific heating, low melting point, large
expansion coefficient, etc; and are widely used in various applications such as sensors, catalysis,
nano devices and magnetic pigments (Nejati and Zabihi 2012; Xu et al. 2012; Saha et al. 2011;
Xian et al. 2011). On the other hand, nickel ferrite is one of the versatile and technologically
important, well-known soft ferrite materials with low conductivity, high electrochemical stability,
high magnetocrystalline anisotropy, catalytic behavior, etc. This ferrite has an inverse spinel
structure, in which eight units of NiFe2O4 go into a unit cell of the spinel structure. The
ferrimagnetic property of NiFe2O4 is due to the magnetic moments of anti-parallel spins between
Fe3+ ions at tetrahedral sites and Ni2+ and Fe3+ ions at octahedral sites (Mathew and Juang 2007;
Liu and Gao 2011). However, complete and reliable separation of nano-particles by magnetic
force should be ensured at the end of the process (Mandel and Hutter 2012; Kong et al. 2012).
According to the above mentioned facts, in this study NiFe2O4 MNPs were synthesized and
modified with cationic surfactant (CTAB). XRD, SEM and FTIR analysis were used for the
characterization of NiFe2O4 MNPs before and after the modification. The adsorption of three
direct dyes (DR31, DR80 and DR81) by the synthesized magnetic NiFe2O4-CTAB was
investigated. The effect of important parameters including adsorbent dosage, initial dye
concentration, pH and inorganic salts on the adsorption process was studied to determine the
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optimum conditions for the removal of dyes. Also, adsorption capacity, kinetics and isotherms of
the sorption of direct dyes on NiFe2O4-CTAB were investigated.
In this regard, the Langmuir, Freundlich and Tempkin isotherms were investigated at different
initial dye concentrations. The Linear Langmuir, Freundlich and Tempkin equations can be
represented as follows (equations 1-3):
Ce
qe
=
1
KL Q0
+
Ce
(1)
Q0
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logq e = logK f + n logCe
(2)
q e = BlnK T + Bln Ce
(3)
where qe, Ce, KL and Q0 are the amount of adsorbed dye on NiFe2O4-CTAB at equilibrium
(mg/g), the equilibrium concentration of dye solution (mg/L), Langmuir constant (L/g) and the
maximum adsorption capacity (mg/g), respectively. KF and 1/n are adsorption capacity at unit
concentration and adsorption intensity, respectively. Also KT is the binding constant at
equilibrium (L/mg) which is related to the maximum binding energy and B is a constant
corresponding to the heat of adsorption.
Kinetics of the adsorption process was also studied and the characteristic constants of adsorption
were determined using pseudo-first order, pseudo-second order and intraparticle diffusion
models, which are represented by equations 4, 5 and 6, respectively:
k
1
log(q e − q t ) = logq e − 2.303
t
(4)
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1
(qe
1
= q + k2 t
−q )
t
e
(5)
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q t = k i t 1⁄2 + C
(6)
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Where qt, k1, k2 and ki are the amounts of dye (mg/g) adsorbed on the NiFe2O4-CTAB at time t,
pseudo-first order model (1/min), pseudo-second order (g/(min.mg)) and intraparticle diffusion
rate constant (mg/(g.min0.5)); and C values are related to the thickness of the boundary layer.
EXPERIMENTAL
Materials and Methods
All the reagents were of analytical grade and used without further purifications. Nickel nitrate
[Ni(NO3)2.6H2O], ferric nitrate [Fe(NO3)3.9H2O], sodium hydroxide (NaOH), Ethylene diamine,
ethanol (96% m/m) and Cethyltrimethyl ammonium bromide (CTAB) were purchased from
Merck. C.I. Direct Red 31 (DR31), C.I. Direct Red 80 (DR80) and C.I. Direct Red 81 (DR81)
were obtained from Ciba Ltd. Structures and characteristics of each dye are shown in Fig. 1 and
Table 1, respectively. Structure of the employed cationic surfactant is also presented in Fig. 1.
The stability of dyes at different pH is presented in Fig. 2, which indicates that the maximum
wavelength of dyes does not change by varying the pH of the solution, and the dye structures are
not affected.
[Figure 1]
[Table 1]
[Figure 2]
Adsorbent Preparation and Modification
Nickel ferrite nano-particles were synthesized by co-precipitation method. In this regard, ferric
nitrate and nickel nitrate were dissolved in 20 mL of dionized water (molar ratio 2:1 [Fe:Ni]) and
mixed together using magnetic stirrer. The mixture were added dropwise to the sodium
hydroxide aqueous (150 mL, 1.5 M) containing Ethylene diamine with the molar ratio of 1:1 (Fe,
Ni:Ethylene diamine) at 80 oC. The solution were washed 3 times with 20 mL ethanol (99%), 4
times with 20 mL dionized water and then dried at 80 oC for 12 hr. It should be mentioned that
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the nano-particles were separated from washing solutions by centrifugation at 5000 rpm for 5
min. The mixed sample was calcined on alumina crucible at 500 oC for 1 hr in air, with a heating
rate of 10 °C/min. Finally the synthesized sample was milled in a planetary mono mill at the
speed of 150 rpm for 15 min (model pulversette 6 Fritsch Gmbh) with zirconium mortal and
passed through a 170 mesh sieve. The synthesized NiFe2O4 MNPs are formed according to the
suggested reaction in equation (7) .
Ni(NO3 ). 6H2 O + 2Fe(NO3 ). 9H2 O → NiFe2 O4
(7)
The produced magnetic nano-particles did not show any adsorption properties toward the
aforementioned anionic direct dyes. Therefore, a cationic surfactant (CTAB) was employed for
the modification of synthesized NiFe2O4 MNPs. For this purpose, CTAB (0.7 mM) was added to
the 1% aqueous NiFe2O4 suspension. The solution was stirred with mechanical stirrer for 5 hr at
200 rpm and then washed with distilled water to remove the extra CTAB which did not react
with NiFe2O4. The modified NiFe2O4 MNPs (NiFe2O4- CTAB) was separated using a magnet
and then finally dried at room temperature for 24 hr.
The adsorption processes were conducted by the addition of various amounts of NiFe2O4- CTAB
(0.45, 0.6, 0.7 and 0.8 g/L) into the jars containing 200 ml of dye solution (50 mg/L). The
synthetic solutions were prepared by dissolving dyes in the distilled water. All experiments were
performed at 25 oC for 60 min with the agitation speed of 200 rpm. H2SO4 and NaOH (Merck)
were used to adjust the pH of the solution. The samples were collected at certain time intervals
(0, 2.5, 5, 7.5, 10, 15, 20, 30, 40, 50 and 60 min) and the adsorbent was separated by the
magnetic force. The absorbance of each sample was measured at the maximum wavelength
(λmax) by UV-Visible spectrophotometer (Cecil 2021). Dye removal efficiency was calculated as
follows (equation 8):
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(C0 −Ct )×100
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E(%) =
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where E(%), C0 and Ct are the dye removal efficiency, initial dye concentration (mg/L) and dye
C0
concentration after time t, respectively.
(8)
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Characterization of adsorbent
Characterization of NiFe2O4 MNPs before and after modification was studied using XRD, SEM
and FTIR analysis. The size and surface morphology of the synthesized NiFe2O4 and CTABNiFe2O4 MNPs were investigated by Scanning Electron Microscopy (SEM). X-ray diffraction
pattern was obtained by X-ray diffractometer (XRD Philips pw1800) using Cu-Kα radiation.
Fourier transform infrared (FTIR) analysis (perkin, Elmer Spectrophotometer Spectrum One)
was used to identify the functional groups of the synthesized NiFe2O4 MNPs before and after the
modification in the range of 450-4000 cm-1. Also the isoelectric point (pHZPC) of NiFe2O4 and
NiFe2O4- CTAB was determined to investigate the surface charge of the adsorbent at various pH
values.
RESULTS AND DISCUSSIONS
Characteristics of NiFe2O4 and NiFe2O4-CTAB
Fig. 3 shows the XRD patterns of NiFe2O4 and NiFe2O4-CTAB. The results showed that spinel
was formed as the most intense (311) peak and the Miller indices (111), (220), (400), (422),
(511) and (440) confirmed the reflections of the nickel ferrite reported in the previous
published paper (Wang et al. 2010). The XRD pattern of NiFe2O4-CTAB shows no different
characteristics diffraction peaks in the spectrum, which implies that CTAB with the amorphous
feature exists in NiFe2O4-CTAB.
[Figure 3]
The surface morphology of NiFe2O4 MNPs is illustrated in Fig. 4(a). The SEM image shows the
agglomeration of NiFe2O4 nano-particles during the co-precipitation method which reduces the
surface area of the adsorbent; therefore, before modification process; the synthesized nanoparticles did not show any adsorption behavior. It is obvious from Fig. 4(b) that the modification
of NiFe2O4 with CTAB has enhanced the porosity and surface area of the synthesized adsorbent
significantly.
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[Figure 4]
The FTIR analysis of NiFe2O4 and NiFe2O4-CTAB are shown in Fig. 5. The peak at 600 cm-1
(Fe–O) is a typical spinel ferrite (Hou et al. 2011). The peaks appearing at 2921.24 and 2851.69
cm-1 are corresponding to the symmetrical and unsymmetrical stretching –CH2 groups and the
peaks at 1488.61 and 1384.18 cm-1 are corresponding to the bending –CH2 and –CH3. It is
evident that NiFe2O4 MNPs were successfully modified with CTAB.
[Figure 5]
0.25 g of the synthesized adsorbent was added to 50 mL solution for the determination of the
isoelectric pH (pHZPC). The initial pH of solutions was adjusted from 2 to 11 using HNO3 and
NaOH. The solutions were stirred for 24 hr and the final pH was recorded. The ∆pH versus
initial pH is plotted in Fig. 6. The pH in which the value of ∆pH equals zero is known as the
pHZPC which in this study is equal to 7.5 and 11 for NiFe2O4 and NiFe2O4-CTAB, respectively.
This means that the surface of the NiFe2O4 before and after modification is positively charged in
pH < 7.5 and pH < 11, respectively. Therefore, the anionic dyes can be adsorbed on the surface
of the adsorbent in these pH ranges. However, the adsorption deficiency of dye molecules on the
unmodified NiFe2O4 at pH < 7.5 could be due to the high agglomeration of nano-particles, low
surface area, and the unavailability of the sorption sites according to SEM image (Fig. 4(a)).
[Figure 6]
Adsorption of DR31, DR80 and DR81
Effect of pH
pH is an important parameter in adsorption processes. pH changes could affect the surface
charge of the adsorbent and influence the dye removal efficiency (Ramakrishna 1997). The
effect of pH (3, 5, 8 and 10) on the removal of DR31, DR80 and DR81 by NiFe2O4-CTAB are
shown in Fig. 7. The results show that changing the pH does not affect the removal efficiencies
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due to the presence of N+ (quaternary amine) in CTAB molecules. Even at higher pH values (8
and 10), first OH¯ ions are adsorbed onto the mentioned positive sites, but later they are replaced
by dye molecules which have higher molecular weight and affinity to the adsorbent surface. Also
the results obtained by determination of pHZPC indicate that the surface of the NiFe2O4-CTAB is
positively charged at pH<11, so there is a strong attraction force between adsorbent and
negatively charged dye molecules. Therefore, the optimal adsorption pH was chosen 8 because
there was no need to add H2SO4 or NaOH to the dye solution.
[Figure 7]
Effect of adsorbent dosage
An optimum adsorbent concentration is essentially required to enhance the interactions between
dye molecules and adsorption sites (Y.T. Zhou 2009). The effect of adsorbent dosage on the
amount of adsorbed dye was investigated by various dosage of NiFe2O4-CTAB (0.45, 0.6, 0.7
and 0.8 g/L). Fig. 8 indicated that the removal efficiency has increased by increasing the
adsorbent dosage up to a certain limit and then it has reached a constant value. Optimum
adsorbent dosage for DR31 and DR81 was 0.7 g/L and for DR80 was 0.8 g/L. This can be
attributed to the increasing of the adsorbent surface and availability of more adsorption sites for
the removal of dyes from solution (G. Crini 2008).
[Figure 8]
Effect of initial dye concentration
The effect of initial dye concentration on the dye removal efficiency has been investigated by
varying this parameter from 50 to 125 mg/L. Fig. 9 demonstrates that the dye removal efficiency
decreases from 94%, 99% and 83% to 66%, 31% and 46% for DR31, DR80 and DR81,
respectively. At a constant adsorbent dosage, the dye removal efficiencies decreased with the
increasing of the dye concentration due to the decreasing of available adsorbent sites for the
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removal of dye molecules from the solution and also the repulsion between the dye molecules in
the bulk solution and the adsorbed ones on the surface of adsorbent (Etim et al.).
[Figure 9]
Effect of inorganic salt
To investigate the effect of inorganic anions on the dye removal efficiency, 0.02 M of Na2SO4,
Na2CO3, and NaHCO3 were added to the dye solutions. Fig. 10 illustrates that the dye removal
efficiency of NiFe2O4-CTAB does not decrease in the presence of inorganic salts because these
anions are small and they compete with dye molecules over the adsorption sites on adsorbent
surface. After the adsorption of small anions, they will switch place with dye molecules due to
their higher affinity and molecular weight.
[Figure 10]
Adsorption isotherms
The adsorption isotherm describes the relation between the amount of adsorbed dye and the dye
concentration in bulk solution at a particular temperature in equilibrium. It also indicates the
distribution of dye molecules between the liquid and solid phases at various equilibrium
concentrations (Mahmoodi et al. 2012; Mahmoodi et al. 2011). The Langmuir theory assumes
that adsorption occurs at specific homogenous sites within the adsorbent. The basic assumption
in Freundlich model is a heterogeneous surface with a non-uniform energy distribution and
reversible adsorption. Freundlich equation can be used to express the characterization of a
multilayer adsorption process.
There are two basic assumptions in Tempkin isotherm; the first is that the heat of adsorption
varies linearly as a function of surface coverage, and the second is that the adsorption is
characterized by a uniform distribution of binding energies, up to a maximum binding energy.
The calculated isotherm constants at different dye concentration values (25, 50, 75 and 100
mg/L) according to equations (1-3) are given in Table 2.
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[Table 2]
According to Table 2, the adsorption isotherms in dye removal process by NiFe2O4-CTAB for
DR31 and DR81 follows all the mentioned isotherms and for the DR80 follows Langmuir model
with high correlation coefficients (r2 > 0.98). The results show that the Q0 value for DR31 is
higher than DR80 and DR81; this can be attributed to the smaller molecular structure of DR31
which can ease their accessibility to the adsorption sites. On the other hand, containing more
anionic functional groups leads to a strong interaction between DR80 molecules and adsorbent’s
surface which results in the higher KL value for this dye.
Adsorption kinetics
Adsorption kinetics study was conducted at different initial dye concentrations: 50, 75, 100 and
125 mg/L. Characteristic constants of sorption were determined using pseudo-first order,
pseudo-second order and intraparticle diffusion models (equations 4-6) (Table 3).
The high correlation coefficients (r42 > 0.95, obtained from the plot of t/qt versus t) in Table 3
illustrate that the dye adsorption process for DR31, DR80 and DR81 follows second-order
kinetic model.
[Table 3]
CONCLUSIONS
This research has studied the synthesis and modification of NiFe2O4 MNPs with cationic
surfactant (CTAB). The XRD patterns showed the reflections of nickel ferrite and confirmed that
the amorphous feature exists in NiFe2O4-CTAB. The SEM images indicated that the
modification of NiFe2O4 with CTAB has decreased the agglomeration of NiFe2O4 nano-particles
significantly and improved the porosity of the synthesized adsorbent. The FTIR analysis also
illustrated that NiFe2O4 MNPs were successfully modified with CTAB. The effect of operating
factors such as pH, initial dye concentration, adsorbent dosage and inorganic salts on the dye
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removal efficiency was investigated. The results demonstrated that increasing the adsorbent
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adsorption behavior of Rhodamine B dye on Duolite C-20 resin". J. Saudi Chem. Soc.,
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16 (2), 209-215.
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dosage up to 0.7 g/L for DR31 and DR81 and up to 0.8 g/L for DR80 increased the removal
efficiency. Dye removal efficiencies decreased with increasing the initial dye concentration at a
constant adsorbent dosage due to the decreasing of available adsorbent sites for the removal of
dye molecules from the solution. Varying pH values and the addition of 0.02 M of Na2SO4,
Na2CO3, and NaHCO3 to the dye solutions did not affect the dye removal efficiencies. The
adsorption isotherms studies in dye removal process by NiFe2O4-CTAB showed that DR31 and
DR81 followed Langmuir, Freundlich and Tempkin isotherms and DR80 followed Langmuir
model with high correlation coefficients (r2 > 0.98). The kinetics investigation at different dye
concentrations showed that the dye adsorption process for DR31, DR80 and DR81 conformed to
the second-order kinetic model (r42 > 0.95). This study concluded that NiFe2O4-CTAB can be
used as an effective and efficient adsorbent for the removal of anionic dyes from textile
wastewaters with the advantage of simple magnetic separation procedure.
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510
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Tabel 1. Characteristics of dyes.
Dye
Molecular weight (g/mol)
Molecular formula
λmax (nm)
C.I. Direct Red 31
713.64
C32H21N5Na2O8S2
530
C.I. Direct Red 80
1373.07
C45H26N10Na6O21S6
528.5
C.I. Direct Red 81
675.6
C29H19N5Na2O8S2
510
548
549
550
551
552
553
554
555
556
557
Tabel 2. Isotherms constants for dye removal at different initial dye concentrations (200 mL
solution, initial dye concentration: 50 mg/L, adsorbent: 0.7 g/L for DR31, DR81 and 0.8 g/L for
DR80, pH: 8, 25 oC, 200 rpm and 60 min).
Tempkin isotherm
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
Freundlich isotherm
Langmuir isotherm
Dye
r32
KT
LnKT
B
r22
n
KF
r 12
KL
Q0
DR31
0.996
1.46
0.378
28.3
0.999
3.12
35.08
0.999
0.113
142.86
DR80
0.195
7.36×10-21
-48.36
-1.36
0.206
41.67
62.95
0.984
3.85
50
DR81
0.999
32.38
3.48
10.7
0.999
6.58
43.45
0.998
0.208
90.909
574
575
576
577
Tabel 3. Kinetic constants for dye removal at different initial dye concentrations (200 mL
solution, adsorbent dosage: 0.7 g/L for DR31, DR81 and 0.8 g/L for DR80, pH: 8, 25 oC, 200
rpm and 60 min).
Intraparticle diffusion
Pseudo-second order
model
r52
I
Pseudo-first order
Dye (mg/L)
Ki
r42
K2
(qe)Cal
r32
K1
(qe)Cal
(qe)exp
DR31
0.46
33.3
5.68
0.99
-3.6
66.66
0.99
0.385
60.53
62.98
50
0.71
31.91
8.51
0.99
4.03×10-3
90.91
0.94
0.092
49.09
84.85
75
0.8
31.27
11.2
0.99
2.38×10-3
111.1
0.97
0.076
65.46
103.55
100
0.73
41.65
11.6
0.99
2.37×10-3
125
0.85
0.069
69.82
118.84
125
DR80
0.89
13.11
7.45
0.99
2.305×10-3
71.42
0.95
0.101
58.88
61.95
50
0.9
13.55
7.51
0.99
2.178×10-3
71.42
0.94
0.053
46.77
60.25
75
0.89
12.95
6.5
0.98
2.228×10-3
66.66
0.91
0.04
39.71
103.55
100
0.81
14.27
5.13
0.95
4.298×10-3
52.63
0.93
0.055
29.3
118.84
125
DR81
-3
66.667
0.95
0. 064
48.64
59.8
50
0.89
13.6
6.8
0.99
2.89×10
0.79
21.68
7.35
0.99
3.31×10-3
76.923
0.942
0.067
42.76
71.2
75
0.73
28.21
7.28
0.99
4×10-3
83.33
0.95
0.076
43.25
78
100
0.7
31.71
8
0.99
3.9×10-3
90.909
0.9
0.057
36.89
82.5
125
578
579
580
581
582
583
584
585
586
587
Figures Captions
588
589

590

Figure 2. pH stability of dyes.
591

Figure 3. XRD of NiFe2O4 and modified NiFe2O4.
592

Figure 4. SEM images of (a) NiFe2O4 and (b) NiFe2O4- CTAB.
593

Figure 5. FTIR of NiFe2O4 and NiFe2O4-CTAB.
594

Figure 6. pHZPC plot of NiFe2O4 and NiFe2O4-CTAB.
595
596
597

Figure 7. Effect of pH on dye removal by NiFe2O4-CTAB (a) DR31, (b) DR80 and (c)
598
599
600

601
602
603

604
605
606
607

Figure 1. Structures of (a) Direct red 31, (b) Direct red 80, (c) Direct red 81 and (d)
CTAB.
DR81 (200 mL solution, initial dye concentration: 50 mg/L, adsorbent dosage: 0.7 g/L for
DR31, DR81 and 0.8 g/L for DR80 , 25 oC, 200 rpm and 60 min).
Figure 8. Effect of adsorbent dosage on dye removal by NiFe2O4-CTAB (a) DR31, (b)
DR80 and (c) DR81 (200 mL solution, initial dye concentration: 50 mg/L, pH: 8, 25 oC,
200 rpm and 60 min).
Figure 9. Effect of initial dye concentration on dye removal by NiFe2O4-CTAB (a) DR31,
(b) DR80 and (c) DR81 (200 mL solution, adsorbent dosage: 0.7 g/L for DR31, DR81
and 0.8 g/L for DR80, pH: 8, 25 oC, 200 rpm and 60 min).
Figure 10. Effect of inorganic salts on dye removal by NiFe2O4-CTAB (a) DR31, (b)
DR80 and (c) DR81 (200 mL solution, initial dye concentration: 50 mg/L, adsorbent
dosage: 0.7 g/L for DR31, DR81 and 0.8 g/L for DR80 , pH; 8, 25 oC, 200 rpm and 60
min).