22
Enhancement removal of Basic Blue 9 dye from wastewaters using natural Moroccan wood as efficient adsorbent
A. Regti, A. Tourit, R. Laamari, M. El Haddad*
Equipe de Chimie Analytique & Environnement, Faculté Poly-disciplinaire, Université Cadi Ayyad, BP 4162, 46000 Safi, Morocco
*Corresponding author. E-mail : [email protected] Recieved 21 Apr 2014, Revised 14 May 2014, Accepted 14 May 2014
Abstract
The adsorption of Basic Blue 9 as cationic dye from aqueous solutions onto Moroccan Pine Sawdust was investigated. Effects of various parameters were studied like contact time, pH, adsorbent dose and initial concentration of Basic Blue 9 dye for the removal efficiency % onto Moroccan Pine Sawdust. Removal efficiency % can be attained 96% in less than 20 min. The experimental data fitted by pseudo- second-order kinetic model to describe the adsorption of Basic Blue 9 on Moroccan Pine Sawdust. Thermodynamic results revealed that the adsorption of Basic Blue 9 onto Moroccan Pine Sawdust is an endothermic and spontaneously process. The Gibbs energy G0 decreased from - 2.57 kJ/mol to - 3.08 kj/mol with increase in temperature from 298 K to 323 K indicating a increase in feasibility of biosorption at higher temperature. The Langmuir and Freundlich isotherms for adsorption of Basic Blue 9 onto Moroccan Pine Sawdust were investigated.
Accordingly, Moroccan Pine Sawdust was shown to be a very efficient, eco-friendly and low cost adsorbent and a promising alternative for removal dyes from aqueous solutions.
Keywords: Removal dye, Adsorption, Pine Sawdust, Basic Blue 9
1. Introduction
Effluents of textile industries are highly colored and disposal of these wastes into receiving waters causes drastic damages to the environment. Indeed, they may significantly affect photosynthetic activity and also be toxic to some aquatic life due to the presence of metals, chlorides, etc. [1]. Most of dyes released during textile clothing, printing and dyeing processes are considered as hazardous and toxic to some organisms and may cause direct destruction of aquatic creatures. In addition, dye wastewaters are commonly characterized by high salts content and low biodegradation potential [2] which makes effective removal by conventional wastewater treatment processes difficult [3]. The dyes have low biodegradability. Conventional biological wastewater treatment processes are not efficient in treating dyes present in wastewater [4-6]. Therefore, dye-wastewater is usually treated by physical and chemical methods, such as sonochemical, photochemical, electrochemical, coagulation and flocculation, membrane separation, bio-degradation, photo-fenton processes, oxidation or ozonation [7-16]. However, in developing countries, these methods are still too expensive to be used widely.
Adsorption is considered to be superior to other physico-chemical or biological techniques for treatment of dye effluents. Highly efficient adsorbent material for the removal of dyes is activated carbon. However, high cost of commercially available activated carbons, high operating cost and problems with regeneration hamper the use of activated carbon for large-scale application. This has lead many researchers to search for low-cost adsorbents such as natural agro-industrial or plant waste materials for the removal of dyes and other contaminants from wastewater, as a replacement for costly commercially adsorbents [17-24]. For ongoing our research program,
23
we have already described the biosorption removal of textile dyes from aqueous solutions onto inexpensive and eco-friendly biosorbent like calcined bones [25-28] and calcined mussel shells [29,30].
This study was focused on the possible use of a Moroccan Pine Sawdust to remove Basic Blue 9 dye from aqueous solutions. The adsorption studies of the dye were carried out under various parameters such as pH, adsorbent dose, initial dye concentration, contact time and Temperature. Equilibrium adsorption data were analyzed by Langmuir and Freundlich isotherm models. The adsorption kinetic data of Basic Blue 9 onto Moroccan Pine Sawdust was tested by the pseudo-first-order and the pseudo-second-order kinetic models.
Thermodynamic study was also achieved.
2. Materials and methods
Moroccan Pine woods were collected from a forest near Safi city and were washed several times with tap water and dried in open air. The sawdust obtained from Moroccan Pine wood was sieved as a particle size range comprised between 50 µm and 250 µm. The Moroccan Pine Sawdust obtained was abbreviated as MPS. The adsorbent MPS was washed another time with distilled water and dried for 12 h at 50°C. The resulting material MPS was stored in a glass bottle for further use.
The zero point charge pH (pHZPC) of the MPS adsorbent was measured using the pH drift method [31]. In this fact, the pHZPC of the MPS was determined by adding 20 mL of 5.10-2 mol/L NaCl to several 50 mL cylindrical high-density polystyrene flasks (height 117 mm and diameter 30 mm). A range of initial pH (pHi) values of the NaCl solutions were adjusted from 2 to 12 by adding 10-1 mol/L of HCl and NaOH. The total volume of the solution in each flask was brought to exactly 30 mL by further addition of 5.10-2 mol/L NaCl solution. The pHi values of the solutions were then accurately noted and 50 mg of each MPS were added to each flask, which was securely capped immediately. The suspensions were shaken in a shaker at 298 K and allowed to equilibrate for two days. The suspensions were then centrifuged at 3600 rpm for 15 min and the final pH (pHf) values of the supernatant liquid were recorded. The value of pHZPC is the point of the curve where pHf = pHi. Measured pH was down by pH-Metre Basic 20+ model pH-meter.
Basic Blue 9 dye used in this study was purchased from Sigma-Aldrich and labelled as cationic dye.
Chemical structure and some properties of the Basic Blue 9 dye are summarized in Table 1.
Table 1: Some properties of Basic Blue 9 dye Properties of dye
Chemical structure
S N
N N
Cl- +
Name of dye Methylene Blue
C. I. Name Basic Blue 9
IUPAC Name Chloride of bis-(dimethylamino)-3,7-phenazathionium
Color index number 42015
Empirical formula C16H18ClN3S
Molecular weight 319.86 mol/L
max 665 nm
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Adsorption measurement was determined by batch experiments of known amount of the MPS adsorbent with 100 mL of aqueous Basic Blue 9 solutions of known concentration in a series of 250 mL conical flasks.
The mixture was shaken at a constant temperature using Thermo line scientific Shaker Incubator at 150 rpm at ambient temperature for 90 min. At predetermined time, the bottles were withdrawn from the shaker and the residual dye concentration in the reaction mixture was analyzed by centrifuging the reaction mixture and then measuring the absorbance by UV-Visible spectroscopy of the supernatant at the wavelength that correspond to the maximum absorbance of the sample at 665 nm. Dye concentration in the reaction mixture was calculated from the calibration curve. Adsorption experiments were conducted by varying initial solution pH, contact time, adsorbent dose, initial dye concentration and temperature under the aspect of adsorption kinetics, adsorption isotherm and thermodynamic study.
The MPS adsorbent was characterized using chemical composition, FT-IR and SEM micrograph. Chemical composition shows as follows tannins (5%), lignins (20 – 25%), polysaccharides (30 – 35%), and cellulose (1%).
FT-IR spectra were obtained using ATI Mattson Genesis series FTIRTM UNICAM instrument. The FTIR spectrum is generally used to identify some of the characteristic functional groups capable of absorbing organic.
The broad band 3200 – 3600 cm-1 indicates the presence of both free and hydrogen bonded OH groups on the MPS surface. The band 1400 cm-1 is attributed to CH3 bending vibration and some interaction between MPS stretching and in plane C–O–H and C–H bending. The bands 2917 and 611 cm-1 are assigned to C=C Stretching frequencies [32]. Figure 1 shows the SEM micrograph of MPS sample, the SEM micrograph show that the MPS has fibrous structure, rough surface morphology with some pores.
Figure 1. SEM of MPS adsorbent
The specific surface area of MPS was determined by BET method from adsorption-desorption isotherm of nitrogen at its liquid temperature 77 K and was found to be Sp = 96 m2/g.
The amount of equilibrium adsorption qe (mg/g) was calculated using the formula:
W V C qe C e
0 (1)
25
Where Ce (mg/L) is the liquid concentration of Basic Blue 9 at equilibrium, C0 (mg/L) is the initial concentration of the Basic Blue 9 in solution. V is the volume of the solution (L) and W is the mass of the PMS adsorbent (g).
The Basic Blue 9 removal percentage can be calculated as follows:
100 .%
Re
0
0 x
C C Efficiency C
moval e
(2)
Where C0 is the initial Basic Blue 9 concentration and Ce (mg/L) is the concentration of Basic Blue 9 at equilibrium.
3. Results and Discussions
3.1. Effect of adsorbent dose on the removal efficiency % of Basic Blue 9
Figure 2 depicts the variation of removal efficiency % of Basic Blue 9 versus MPS adsorbent dose ranging from 50 to 500 mg using Basic Blue 9 at 20 mg/L as concentration and the mixture was stirred for 60 min as contact time. It can be seen that the removal efficiency % of Basic Blue 9 increases from 80 % to 96 % with increasing MPS adsorbent amount from 60 mg to 200 mg and the response becomes constant from 200 mg to 500 mg. Such a behavior, which is related to the corresponding increase in the number of sites available for dye adsorption, is in agreement with our previous observations on safranin removal by calcined bones biosorbent [26]. The optimum adsorbent dose was found to be 200 mg of MPS per 50 mL of Basic Blue 9 solution for the following studies.
Figure 2. Effect of adsorbent dose on the removal efficiency % of Basic Blue 9.
Concentration dye 20 mg/L, solution volume 50 mL, contact time 60 min, initial pH 6.7, ambient temperature
3.2. Effect of pH on the removal efficiency % of Basic Blue 9
The pH is the most important factors affecting the adsorption capacity in wastewater treatment. In this fact, the process behavior of adsorption may become different by varying pH. Figure 3 shows the effect of pH on the removal efficiency % of Basic Blue 9 onto MPS. It was observed that the removal efficiency % of Basic Blue 9 increases from 4% to 94% by increasing pH between 2 to 14, indicated that the adsorption was strongly pH- dependent. To explain the possible pH-mechanism, the determination of pHZPC played an important role.
0 10 20 30 40 50 60 70 80 90 100
0 100 200 300 400 500
Removal Efficiency %
Adsorbent dose (mg)
26
Figure 3: Effect of pH on the removal efficiency % of Basic Blue 9 onto MPS.
Concentration dye 20 mg/L, adsorbent dose 100 mg, solution volume 100 mL, contact time 60 min, ambient temperature
Figure 4 depicts the experimental determination of pHZPC. The pHZPC value for MPS adsorbent was determined and it is above 6.1. This value confirms the ranges of optimal pH values for dye removal from aqueous solutions. The pHZPC of MPS indicated that the surface of the adsorbent positively charged at pH less than 6.1 and negatively charge at pH values above 6.1. At this pH, a significantly high electrostatic attraction phenomenon exists between positively charged cationic dye and the negatively charged surface of the MPS adsorbent because of the ionization of functional groups of MPS. As the pH of the system increases, the number of negatively charged sites increased due to the presence of carboxylate groups, which can adversely affect the adsorption of cationic dye. In this case, the adsorption mechanism occurs partly by ion exchange via releasing exchangeable proton in the interlayer and basal plane surfaces and partly via non-columbic interactions between adsorbed cation and a neutralized site [33].
Figure 4: Experimental determination of pHZPC
3.3. Effect of Basic Blue 9 concentration on its removal efficiency
Adsorption of Basic Blue 9 dye at different initial concentrations (10 – 40 mg/L) was studied as a function of contact time was shown in Figure 5. It was noticed that the adsorption curves were smooth and continuous leading to saturation at various concentrations of Basic Blue 9 on the outer interface of the adsorbent. This
0 20 40 60 80 100
2 4 6 8 10 12 14
Removal Efficiency %
pH
0 2 4 6 8 10 12
2 4 6 8 10 12 14
pHf
pHi
27
shows the possibility of mono-layer coverage of Basic Blue 9 on the outer interface of the MPS adsorbent. The process was found to be initially very rapid within approximately 10 min; thereafter the dye uptake process tended to proceed at a slow rate. The dye removal then increased with time, until equilibrium was attained after nearly 20 min from the increase of the initial dye concentration. Therefore, the equilibrium time was set conservatively at 20 min for further experiments. The initial rapid uptake of the dye indicates that the adsorption process could be ionic in nature where the basic (cationic) dye molecules bind to the various negatively charged organic functional groups present on the surface of the MPS adsorbent.
Figure 5. Effect of dye concentration on the adsorption capacity of MPS
The adsorption capacity of Basic Blue 9 dye removal increases with increase in initial dye concentration. At lower dye concentrations the ratio of adsorbate concentration to adsorbent sites is high, which causes an increase in color removal; subsequently the fractional adsorption becomes independent of initial concentration.
At higher dye concentration the removal decreases due to the saturation of the adsorption sites on the adsorbent.
However, the actual amounts of dye adsorbed (mg/g) increase with increasing of the initial dye concentration (Figure 5). This may be due to the increase in the driving force of the concentration gradient for mass transfer with the increase in initial dye concentration. Similar observations were reported by other investigators.
3.4. Kinetics study
In order to examine the diffusion mechanism involved during the adsorption process, various kinetic models were tested i.e. pseudo-first order [34] and pseudo-second-order [35]. The applicability of these kinetic models was determined by measuring the correlation coefficients (r2) as well as closeness of values between experimental and calculated adsorption capacity values.
Pseudo-first-order kinetic model is based on the fact that the change in Basic Blue 9 dye concentration with respect to time is proportional to the power one. The following linear form of the pseudo-first-order model was used to study Basic Blue 9 adsorption onto MPS adsorbents:
qe qt
qe k t303 . log 2
log 1 (3)
Where, qe is the amount of dye adsorbed at equilibrium (mg/g), qt is the amount of dye adsorbed at time t (mg/g), k1 is the first-order rate constant (min-1) and t is time (min).
0 1 2 3 4 5 6 7 8
0 10 20 30 40 50 60
Adsorption capacity q (mg/g)
Time (min)
10 mg/L 15 mg/L 20 mg/L 30 mg/L 40 mg/L
28
The equation adsorption mechanism over the range of contact time by the pseudo-second-order kinetic model is shown below:
q t q q k
t
e e t
1 1
2 2
(4)
Hence, a linear trace is expected between the two parameters, log (qe – qt) and t, provided the adsorption follows first kinetics. The values of k1 and qe can be determined from the slope and intercept. The results of a plot log (qe – qt) versus t, at different dye concentration (10 – 40 mg/L) are given in Table 2. The calculated equilibrium biosorption capacity qe(cal) and qe(exp) values have shown few deviation. The pseudo-first-order rate law failed to explain the adsorption of Basic Blue 9 onto MPS with poor fit.
Table 2: Kinetic parameters for the biosorption of Basic Blue 9 onto MPS adsorbent
Basic Blue 9 concentration 10 mg/L 15 mg/L 20 mg/L 30 mg/L 40 mg/L Adsorption capacity
qe, exp (mg/g)
1.86 2.84 3.83 5.62 7.11
Pseudo-first-order model
qe, cal (mg/g) 0.01 0.13 0.20 0.23 0.33
k1 (min-1) 0.04 0.10 0.13 0.14 0.08
r2 0.717 0.688 0.976 0.947 0.746
Pseudo-second-order model
qe, cal (mg/g) 1.88 2.85 3.81 5.63 6.97
k2.10-4 (g/mg min) 1.59 1.69 1.99 1.75 2.57
r2 0.999 0 .999 0.999 0.999 0.999
A plot of t/qt versus t as shown in Figure 6 should give a linear relationship if the adsorption follows pseudo-second-order model. The qe and k2 can be calculated from the slope and intercept of the plot. The results of pseudo-second-order plot are given in Table 2. The equilibrium adsorption capacity qe(cal) and qe(exp) values are in close agreement for Basic Blue 9. The adsorption of Basic Blue 9 onto MPS is well explained by the pseudo-second-order model kinetics with very high correlation coefficient. The rapid uptake of Basic Blue 9 onto MPS indicates that the rate determining step could be chemisorption in nature [36,37].
Figure 6. Plot of pseudo-second-order kinetic model 0
5 10 15 20 25 30 35
0 10 20 30 40 50 60
t/q
Time (min)
10 mg/L 15 mg/L 20 mg/L 30 mg/L 40 mg/L
29 3.5. Thermodynamic study
Thermodynamic considerations of an adsorption process are necessary to evaluate whether the process is spontaneous or not. The Gibb’s free energy change ΔG0 is an indication of spontaneity of a chemical reaction.
For significant adsorption to occur, the Gibb’s free energy change of adsorption, ΔG0 must be negative. Both enthalpy ΔH0 and entropy ΔS0 factors must be considered in order to determine the Gibb’s free energy of the process. In the present investigations, thermodynamic studies were performed and the parameters, namely, ΔG0, ΔH0, and ΔS0, were determined at 298, 303, 313 and 323 K, respectively. The thermodynamic parameters were calculated using the following equations [38]:
liquid solid
C
K0 C (5)
0 0 RTLnK G
(6)
RT H R
LnK S
303 . 2 303 . 2
0 0
0
(7)
Where K0 is the equilibrium constant, Csolid is the solid phase concentration at equilibrium (mg/L), Cliquid is the liquid phase concentration at equilibrium (mg/L), T is the temperature expressed in ° Kelvin and R (8.314 J mol/K) is the universal gas constant.
The values of these parameters have been given in Table 3. Positive values of entropy change ΔS0 and enthalpy change ΔH0 also indicate the endothermic nature of adsorption of onto MPS. Thermodynamic studies revealed that a greater adsorption can be obtained at higher temperatures. The results of the thermodynamic calculations were shown in Table 3.
Table 3: Thermodynamic parameters for the removal of Basic Blue 9 onto MPS
Temperature (K) Ln K0 ΔG0 (Kj/mole) ΔH0 (Kj/mole) ΔS0 (j/mole K)
Ea (Kj/mole)
298 1.03 - 2.57
1.84 14.87 67.49
303 1.05 - 2.64
313 1.08 - 2.74
323 1.12 - 3.08
The negative value for the Gibbs free energy showed that the adsorption process was spontaneous in nature.
It could also be concluded from the activation energy value that the adsorption typically proceed after exceeding 67.49 kJ/mol energy barriers. The activation energy value also gives us information on whether the adsorption is mainly physical or chemical. Nollet et al. [39] suggested that the physisorption process normally had activation energy ranging from 5 – 40 kJ/ mole, while chemisorption had higher activation energy comprised between 40 – 800 kJ/ mole. The overall process was determined as endothermic from as shown in Table 3 (ΔH0 = 1.84 kJ/mole). Table 3 also shows that the ΔS0 value was positive; that is, entropy (randomness) increased as a result of the adsorption.
3.6. Isotherms study
The search for the best fit equation using linear regression analysis is the most commonly used technique to determine the most suitable isotherm to explain the mechanism for adsorption. The equilibrium data obtained from the Basic Blue 9 concentration on adsorption capacity experiment was interpreted by different isotherms
30
models. The Langmuir model [40] assumes that the removal of dyes occurs on an energetically homogenous surface by monolayer sorption and there are no interactions between the adsorbate on adjacent sites.
e L
e
e C
q K q q C
max max
1
1
(8)
Where Ce (mg/L) is the equilibrium concentration of the Basic Blue 9, qe (mg/g) is the amount of dye adsorbed par unit mass of adsorbent. qmax (mg/g) and KL (L/mg) are Langmuir constants related to adsorption capacity and rate of adsorption, respectively.
The important features of the Langmuir isotherm model can be defined by the dimensionless constant separation factor RL which is expressed by:
1 0
1 C R K
L
L (9)
where C0 is the initial metal ion concentration (mg/L) and KL is the Langmuir constant (L/mg). RL shows the nature of the adsorption mechanism. There are four possibilities for RL value:
Unfavorable RL > 1, Linear RL = 1, Favorable 0 < RL < 1, Irreversible RL = 0
The Freundlich isotherm [41] is based on the assumption that the adsorption process takes place by interaction of metal ions on a heterogeneous surface. There is a logarithmic decline in the energy of adsorption with the increase in the occupied binding sites. The linear form of the Freundlich isotherm equation is:
e
f
CeK n
q 1log
log
log (10)
Where qe (mg/g) is the equilibrium dye concentration onto the adsorbent, Ce (mg/L) is the equilibrium dye concentration in solution, KF (mg/g) (L/g)1/n is the Freundlich constant related to sorption capacity and n is the heterogeneity factor. The Freundlich constants KF and 1/n are obtained from the intercept and slope of straight line of the plot of log qe versus log Ce. The value of n indicates whether the adsorption process is favorable (n >
1 – 10) or not.
These types of isotherms are usually associated with ionic solute adsorption (e.g., metal cations and ionic dyes) with weak competition with the solvent molecules [42]. The correlation coefficients r2 0.999 for Langmuir isotherm and r2 = 0.837 for Freundlich isotherm. A comparison of the r2 values for the two models suggested that the Langmuir isotherm provide a better fit for the experimental data compared to the Freundlich isotherm.
The dimensionless constant separation factor RL was 0.2, showing the favorable adsorption mechanism closeness of the model to the Langmuir isotherm. This result suggests that the dye was homogeneously adsorbed on a monolayer surface of the adsorbent. The maximum monolayer adsorption capacity was 22.12 mg/g.
4. Conclusion
This paper presents the results of a detailed adsorption study for removing Basic Blue 9 dye from aqueous solutions using a low cost, eco-friendly and abundantly available Moroccan Pine sawdust material. Operational parameters such as the adsorbent dose, pH, contact time and initial dye concentrations clearly affect the removal efficiency %. The Langmuir and Freundlich isotherms could all be used to model isothermal adsorption of Basic Blue 9 on MPS, and maximum adsorption capacity was found as 22.12 mg/g at pH 9 and 298 K. The adsorption kinetics of Basic Blue 9 onto MPS was analyzed and follows the pseudo second-order model. Thermodynamic parameters indicate that the adsorption process is spontaneous and endothermic.
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