461
Fixation by adsorption from aqueous solution of the methylene blue on the Powder graphite
M. Abbaz, R. Aba aaki, S. Et-taleb, M. Ez-zahery, R. El haouti, S. Lhanafi and N. El alem*
Materials and Environment Laboratory (MEL), University of Ibn Zohr, Faculty of Sciences, Department of Chemistry, city DAKHLA B.P 8106, Agadir, Morocco
*Corresponding author. E-mail: nelalem@gmail.com; Tel: (+212661916179) Received 13 Sept 2014, Revised 01 Oct 2014, Accepted 24 Oct 2014
Abstract
The powder graphite (PG) was used as a support adsorbent for the removal of the cationic dye, methylene blue (MB), from aqueous solution. The effects of major variables governing the efficiency of the process such as, temperature, initial dye concentration, mass of the adsorbent, the contact time and solution pH were investigated. The equilibrium time was found to be 10 min for 10 mg/L dye concentrations of 25mL of volume. The point of zero charge value pHZero determined by titration method and the effect of pH study, explain that the electrostatic attractive interactionspH pHZero, promote the fixing of dye on the PG with increasing pH. Two simplified kinetic models including pseudo-first-order and pseudo-second-order equation were selected to follow the adsorption processes. The kinetics studies indicated that the pseudo-second-order model yielded the best fit for the kinetic data. The equilibrium data were best represented by the Dubinin isotherm model, by which we determined the adsorption energy.
The thermodynamic study, says that the adsorption reaction is endothermic, spontaneous and more favorable at high temperatures and the adsorption of chemical type. This study is also used for measuring specific surface areas Ssp31.79m2/g of the powder graphite.
Keywords: powder graphite, methylene blue, adsorption, Dubinin model, specific surface areas.
1. Introduction
The dyes are used in many industrial sectors such as textile, paper, leather, food [1]. In fact, the dye industry is an important economic market, but also a source of pollution that threatens all aspects of life in our environment, water, air and soil. The water pollution has taken much of the environmental concerns. This is due to the fact that water resources are limited and the volume of wastewater generated by the various sectors is becoming increasingly important. The environmental concern of these untreated dyes wastewaters has drawn the awareness of many research studies. Accordingly, various treatment processes have been employed for the removal of dyes from wastewater, such as coagulation/ flocculation process, cation exchange membranes, electrochemical degradation, advanced oxidative process, Fenton-biological treatment, and adsorption process [2,3]. Adsorption is widely used as effective physical method of separation in order to eliminate or lower the concentration of a wide range of dissolved pollutants (organics
462 or inorganics) in the effluent [3]. Activated carbon is usually used as adsorbent due to its high adsorption capacity, high surface area, microspores structure and high degree of surface reactivity [4]. 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. The main objective of this research is to evaluate the adsorption aptitude of graphite powder, for the removal of methylene blue as a model compound for basic dyes and to optimize the process by examining the various parameters (pH, temperature, contact time etc.).
2. Materials and methods
2.1. Adsorbent and adsorbate
The absorbent material used in this study is the powder graphite (GP), which is the black carbon, obtained from Fluka (the purity 99%), with the sizes of micrometric particles. The dye cationic or Methylene Blue (MB), as a reference adsorbate, being employed in several industrial fields, was chosen for the adsorption experiments.
2.2. Experimental protocol
The adsorption experiments were carried out under static conditions in a stirred reactor (Batch method [5]), at ambient temperature (23 ± 2 ° C). The Stock solutions of 100 mg/L dye were prepared with distilled water and the required concentrations were obtained by diluting. The concentration of MB remaining in the supernatant after adsorption was determined using UV /Visible spectrophotometer at the maximum absorption wavelength of max662nm. The amounts of dyes adsorbed were calculated from the concentrations in solutions before and after adsorption according to the equation (1):
m V C
Q(C0 e)* (1)
Where: Q is the equilibrium dye concentration on adsorbent (mg of dye / g of sorbent).C0 and Ce are respectively the initial and the equilibrium concentration of dye in solution (mg/L). m is the mass of dry adsorbent used (g) and V is the volume of dye solution (mL).
3. Results and Discussions
3.1. Analytical study of graphite powder and methylene blue 3.1.1 Graphite powder characterization
The morphology of graphite powder was analyzed by Scanning Electron Microscope (SEM), to get an idea about shape and size of the particles.
Figure 1: SEM images showing the surface of GP: (a) 50X, (b) 2000X and (c) 30000X Magnification
(a) (b) (c)
463 The Very fine micro particles can be observed with diameter 0.2-80μm, the flake shape and lamellar structure of graphite particles, where they have sharp edges and display several sheets. Figure.1 shows the SEM micrograph of graphite powder
3.1.2 Methylene blue
Methylene blue (MB) is a cationic thiazine dye that is deep blue in the oxidized state while it is colorless in its reduced form (leucomethylene blue) [19]. The molecular structure and same characteristics of the methylene blue are represented in figure.2 and table.1 respectively.
N
S+ N
C H3
CH3
N CH3
CH3
Figure 2. The méthylène blue Structure Table 1. The methylene blue Datasheet [6].
Name and indexing
Name IUPAC :
3,7-bis-(dimethylamino)- phenothiazin-5-ium Chloide Color index : Basic bleu 9
EINCS No : 200-515-2
Physical and chemical properties
molecular formula : C16H18N3SCl molar mass: 319,86g/mol Melting point: 1800C
Solubility 200C : 50g/L (water) et 10g/L (éthanol)
3.2. Parameters influencing the adsorption process 3.2.1. Effect of adsorbent mass
In order to study the effect of adsorbent mass on the adsorption of methylene blue, a series of adsorption experiments were carried out with different adsorbent dosages at initial dye concentration of 10 mg/L and the mixture was stirred for 60 min as contact time.
Figure3. Effect of adsorbent masse on MB adsorption on GP ) 25 ,
/ 10 ,
1 24 ,
5 , 4
(pH T 0C CBM mg L V mL
0 2 4 6 8 10 12
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35
residuel Concentration of BM (mg/L)
graphite powder mass (g)
464 Figure.3 shows the effect of adsorbent dose on the removal of methylene blue. It was noted that the amount adsorbed of dye increases with the increase of the mass of the GP and The optimum adsorbent mass was found to be 0.07 g of GP per 25 mL of BM solution. Such a behavior is related to the corresponding increase in the number of sites available for dye adsorption.
3.2.2. Effect of contact time
For kinetic study, 25 ml of dye solutions (10 mg/L) was agitated with 0.07g of GP at different times, keeping constant temperature and stirring rate. At each time interval the adsorption capacity was calculated.
Figure.4 shows the effect of contact time on the adsorption capacity of MB by the GP.
Figure 4. Adsorption kinetics of BM on the Graphite Powder
) 25 , / 10 ,
1 24 5 , 4
(pH T 0C CBM mg L V mL
As can be seen, a rapid adsorption of Methylene blue dye on GP was observed at the initial stages of the adsorption and equilibrium is attained within about 10 min. Such uptake indicates high degree of affinity towards dye molecules via chemisorptions.
3.2.3. Effect of pH
The pH is the most important factor affecting the adsorption capacity in wastewater treatment. To study the effect of pH on MB adsorption, 0.07g of graphite powder was added to solutions containing 10 mg/L of methylene blue dye at different pH values (2-11) at ambient temperature 25°C. The initial pH values were adjusted using HCl and NaOH. Figure.5 shows the effect of the initial pH of MB solution on the adsorption capacity of the graphite powder.
As seen from Figure.5, dye removal efficiency increased with the increasing pH. To explain the results of the figure.5 we have determined the point of zero charge pHZero8.62 of GP, by the titration method [17].
At high pH values(pH pHZero), negatively charged adsorbent sites increase, which favor the adsorption of dye cations as a methylene blue due to attractive electrostatic interactions.
0 0.5 1 1.5 2 2.5 3 3.5
0 10 20 30 40
adsorbed amount of BM (mg/g)
Time (min)
465 Figure 5. Influence de pH sur l’adsorption de BM par le GP
) 25 , / 10 ,
1 25
(T 0C CBM mg L V mL
3.3. Adsorption isotherms and kinetic models 3.3.1. Adsorption kinetic
The adsorption kinetics shows the evolution of the adsorption capacity through time and it is necessary to identify the types of adsorption mechanism in a given system. The following models are used to describe the adsorption kinetics behavior [7]:
a. the Pseudo-first-order model
The linear form of pseudo first-order kinetic model is expressed as Equation (2):
t K Q t
Q
Qe ()) ln e 1
ln( (2)
Where: K1 Is the rate Constant of pseudo first-order adsorption (min1). Qe The concentration of MB adsorbed after the equilibrium, Q(t) The concentration of MB adsorbed in time t. The plot of ln(QeQ(t)) versus t should give a linear relationship from which K1 and Qe can be determined from the slope and intercept of the plot, respectively.
b. the Pseudo-second-order model
The pseudo second order kinetic model is represented by the following linear equation:
Q t Q K t Q
t
e e
1 1 )
( 2 2 (3)
K1 Is the rate Constant of pseudo first-order adsorption (g/mg.min) determined by drawing t/Q = f(t). Table 2 lists the adsorption kinetic constants calculated by the pseudo first-order and pseudo second-order models.
Table 2. Kinetic parameter constants
Adsorbent Pseudo-first-order model
Graphite Powder
R2 K1(min1) Qe(mg/g)
0,946 0,122 0,57
Pseudo-second-order model
R2 K2(g/mgmin) Qe(mg/g)
1 0,503 3,225
0 1 2 3 4
1 2 3 4 5 6
2,9434 3,1219 3,2111 3,3157 3,6819 3,7527
adsorbed amount of BM (mg/g)
pH
466 The correlation coefficient R2 for the pseudo second-order adsorption model has an extremely high value (greater than 0.999), and the calculated Qe values also agree very well with the experimental data. These results suggest that the overall rate of the dye adsorption process appears to be controlled by the chemical sorption or chemisorptions process [8] and the rapid fixation of solutes on the most reactive sites [9].
3.3.2. Adsorption isotherm
The adsorption isotherm was obtained from the data deduced from effect of initial dye concentration at ambient temperature (2510C). The amount of dye adsorbed Q, plotted against the equilibrium concentration Ce for MB, is given in Figure 6.
Figure 6. Equilibrium isotherms for MB adsorption onto GP
) 25 , 07 , 0 ) ( , 5 , 4
(pH mGP g V mL
The graphite powder has a particularity of being ineffective when it comes to remove the dye in low concentrations. The adsorption isotherms of dye molecules show a steeply rising part (at high concentration), suggesting a strong affinity of the dye molecules with the surface sites on GP. Also, we can see that the adsorption of Methylene Blue onto GP forms a typical Langmuir-type isotherm, which indicates that dye molecules outcompete water molecules for the sites available on the surface of GP [10, 11]. Then the amount of adsorption reaches a limiting value of around 3.25mg/g.
b.1 Langmuir isotherm
The Langmuir model assumes monolayer adsorption onto a surface containing a finite number of adsorption sites of uniform energies of adsorption with no transmigration of adsorbate in the surface plane [12].
The linear form of Langmuir isotherm equation is given as (4):
m m
L Q
C Q K Q
C 1 (4)
Where, Qm is the maximum adsorption capacity for monolayer coverage, KL is a coefficient related to the affinity between the sorbent and the sorbate. The constants Qm and KL were obtained by plotting
Q
C versus C
b.2 Freundlich isotherm
The empirical Freundlich model, which is known to be satisfactory for low concentrations and based on adsorption on a heterogeneous surface. The logarithmic form Freundlich equation is given by the following (5):
) ln(
* ) ln(
)
ln(Q KF n C (5)
0 0.5 1 1.5 2 2.5 3 3.5
0 1 2 3
adsorbed amount of MB (mg/g)
Ceq(mg/L)
467 Where, KF is a constant describing the adsorption capacity and n is an empirical parameter related to the adsorption intensity (0n1). The two parameters are obtained by plotting ln(Q) versus ln(C).
b.3 Dubinin isotherm
The Dubinin isotherm model was applied to the data in order to deduce the heterogeneity of the surface energies of adsorption and the characteristic porosity of the adsorbent [13, 18].
The linear form of the Dubinin equation is represented by equation (6):
E
Q A Q) ln m
ln( (6)
A plot of ln(Q) against
A yielding a straight line was made to confirm the model where 2With GK RT
A ln( e) (7)
Where ,Qm Adsorption capacity (mg/g), E Energie caractérise l’adsorption (J/mol), ∆G Free energy (J/mol), Ke Constant equilibrium of adsorption process, T Absolute Temperature (K), R Gaz constant (8,314 J/mol.K), Exposent relate to the distribution of the pore size, the solid surface is more homogeneous as is elevated . Table.1 shows the values of the parameters of the three isotherms and correlated correlation coefficients at ambient temperature.
Table 3. Langmuir, Freundlich and Dubinin Isotherms parameters.
Adsorbent Langmuir isotherm
Graphite en powder
) / (mg g
Qm KL R2
12,987 0,184 0,657
Freundlich Isotherm
N KF R2
0,592 1,860 0,621
Dubinin Isotherm
Α E(KJ/mol) R2
4 408,2 0,946
In comparing the values of the correlation coefficient (R2) for the three tested isotherms, we can be see remark that the Dubinin model provides a better fit for the adsorption isotherm. As such, the high values of the parameter α and the activation energy E, confirm respectively, the homogeneous particle size characteristic of GP and the process of adsorption is chemical type [14].
3.4. Specific surface area
The surface area of a solid is more important for understanding and interpreting the particles sorption properties on the solid. There are two categories of methods: the physical determination of the size and the morphology of solid particles and measurement of the adsorption of gas or solute molecules which have known dimensions and interpretation of the resulting data with a particular adsorption model [15].
The value of Qm obtained from Langmuir equation and the hypothesized area covered by dye molecule can be used to estimate the specific area of solid by using the following relation (7):
468 M
A N
SspQm A (7)
Where Ssp is specific surface area of solid, NA is Avogadro number, A is the area of the adsorbed molecule and M is molecular weight. The specific surface area of graphite powder Ssp31.79m2/g was estimated experimentally from the maximum amount Qm12.987mg/g of methylene blue adsorbed as a monolayer where A130Å2the area of the methylene blue molecule [16]. It appears that the graphite powder has an exceptionally high specific surface area value, which explains the highly porous surface of GP, this is evidence that proves the adsorption capacity of GP.
3.5. Thermodynamics study of adsorption
We studied the effect of temperature on the adsorption of methylene blue in contact with the support studied, at different temperatures (25, 35, 50, 60C). The thermodynamic parameters, such that the adsorption energy (Table.4) ; free energy change (G) , enthalpy change (H) and entropy change (S), [10] which have been calculated using following equations (8):
) ln(Ke RT G
(8)
Where R is gas constant and K is the equilibrium constant and T is the temperature in Kelvin.
According to van’t Hoff equation (9):
RT H R Ke)S
ln( (9)
and the free energy change can be obtained as (10):
S T H G
(10)
H and S can be obtained from the slope and intercept of a van’t Hoff plot of ln(Ke) versus 1/T.
Table 4. Thermodynamic parameters (pH4,5, CBM 10mg/L,V25mLetm(GP)0,07g)
∆H (KJ/mol)
∆S
(KJ/mol.K)
∆G(KJ/mol)
25°C 35°C 50°C 60°C
49,41 0,22 -1,77 -2,00 -2,34 -2,55
As can be seen from Table.4, the adsorption process of MB is more favorable at highest temperatures. The thermodynamic data demonstrate a spontaneous and favorable adsorption process in all the temperature range(G0). Positive values of entropy change ΔS and enthalpy change ΔH indicate the endothermic nature of adsorption process and chemical type(H 40KJ/mol).
4. Conclusion
In this study, graphite powder (GP) was tested and evaluated as a possible adsorbent for removal of Methylene blue dye from aqueous solution. The adsorption kinetics of BM is well described by the model of the second order, which explains better affinity fast dye retention. It was shown that the Dubinin isotherm could best describe the adsorption isotherms. The electrostatic attractive interactions, promote fixation of the dye onto the GP when pH value increases. The thermodynamic study shows that the elimination of methylene blue from aqueous solution is favorable at high temperature and the adsorption process is endothermic and chemical type. Finally, the study shows that the graphite powder has a high specific surface area which consequently shows the porous nature and greater adsorption capacity.
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