475
Adsorption of Organic Molecule (acetic acid) on Activated Carbon in Aqueous
Y. Morad*, M. Hilali, L. Bazzi, A.Chaouay
Laboratory of materials and environment, Faculty of Science Ibn Zohr University, Agadir, Morocco
*Corresponding author. E-mail : yassine.morad@gmail.com Received 13 Sept 2014, Revised 01 Oct 2014, Accepted 24 Oct 2014
Abstract
The main applications of activated carbon are among other purification, decolorization, deodorization and general detoxification of drinking water and the purification of air and chemicals, food ... etc. The research focused on the adsorption of acetic acid on activated carbon. Studies in "batch methods" were used to determine the contact time (4 hour to 16,5% in removal efficiency and a concentration of 0,2 mol/l of substrate). However, this contact time may vary with the concentration of organic molecule. The influence of parameters such as: temperature, concentration of adsorbate in solution, time, mass of adsorbent and pH, revealed a significant improvement in capacity and speed acid adsorption acetic on this support. Moreover, adsorption of this compound is well described by kinetic models of Langmuir and Freundlich.Also the comparison of activated carbon with other soils has concluded that the marl calcareous responds well to the adsorption of acetic acid with a removal efficiency of 75 % for a time of 0,5 h.
Keywords: organic molecule, temperature, pH, activated carbon, marl calcareous, yellow clay, gray clay.
1.
Introduction
The intensive use of organic molecules in everyday life has created problems in both the environment and in food [1,2]. It is important to mention that environmental pollution is due to release of effluents from textile industries and in food; the toxicity is due to the incorporation of several organic molecules [3].
To counter this, several decontamination methods have been developed, we quote as examples: adsorption [4,5], ion exchange [6,7], flocculation-coaculation [8]...etc. In this work we present the results for adsorption on powdered activated carbon of acetic acid. To do this, we determined sequentially the spectral study of the organic molecule, the contact time, the influence of parameters such as temperature, concentration of adsorbate in solution, pH and initial substrate concentration. This study is necessary to try to better understand the mechanisms governing the adsorption.
2.
Materials and methods
The coals used (Brand Aldrich), Measures the concentration of solutions of this organic molecule, at different reaction times, and were followed by the chemical method of assay. The experiments were conducted in
476
"batch method" (in a single flask of 50 ml) at room temperature (25 °C). It should be noted that the temperature control was done by simply reading the thermometer. Also to ensure good dispersion of solid particles of activated carbon, we adopted the value of 10 g/l (or 0,5 g/50 ml) for the solid / liquid ratio.
Adsorption isotherm: To investigate the adsorption capacity of our samples, we applied the models of Langmuir and Freundlich.
3.
Results and discussion
3.1. Influence of some parameters on the adsorption 3.1.1.Contact time
The study of the adsorption of acetic acid on powdered activated carbon (PAC), obviously implies the determination of contact time, time which corresponds to the adsorption equilibrium or a state of saturation of the support the substrate. In this case, the experimental procedure followed is simple "batch method" and is contacting, separately, 0.2 mol/l acetic acid with 10 g/l of powdered activated carbon. Analysis by method chemical dosing will determine the residual concentrations of this substrate, in samples taken at different reaction times. Thus the determination of equilibrium time, allowed the establishment of adsorption isotherms that are essential for calculating the maximum adsorption capacity and to identify the type of adsorption to occur in mono or multilayer. The results obtained after these experiments, figure 1 showed that the contact time of 4 hours is obtained and is not a sufficient removal of the organic molecule, since it is only 16,50%.
Moreover, the extension of time until 6 hours does not lead to an improvement in the percentage removal of this compound, which remains stable. What justifies well, taking account of this contact time for the other adsorption experiments. This result was obtained by the relation [9]:
Q: adsorption capacity of the support (mg/g). C0: initial concentration of organic molecule (mg/l) at t = 0. Ct: concentration of the organic molecule (mg/g) at time t of the adsorption process. V: volume of the solution (substrate) (l). m: mass of the support (g). Other factors such as the initial concentration, temperature, pH and the mass of adsorbent can influence the adsorption capacity [10]. Moreover, it finds its application in the various models used for adsorption.
Figure1. Influence of contact time on adsorption of acetic acid on activated carbon
477 3.1.2. Influence of initial concentration and temperature
The examination of Figure 2 shows the influence of parameters such as the initial concentration of the adsorption capacity of the organic molecule (the mass of the support being fixed). We find that it grows along with increasing the initial concentration.
Figure2. Effect of initial concentration of acetic acid and temperature
But for the temperature, the experimental results show a high retention for the 25 °C and this proves that this setting will affect negatively the process by high energy contribution, to increase the repulsive forces localized at the interfaces of environments liquid and solid[11]. So it is interesting to note that the contribution of heating plays an opposite role in the kinetics of retention of the organic molecule, regardless of their affinity for this medium. This means that the retention process can be exothermic (ΔH <0) and in those conditions lead to a physisorbtion [12].
3.1.3. Influence of pH
From figure 3, we find that the adsorption is highly dependent on pH. For activated carbon, where we note:
An increase of the amount adsorbed with increasing pH.
The amount adsorbed increases to a maximum value at pH = 6,5.
Adsorption and significantly better for both low acid pH< 5.
Figure3. Influence of pH on the adsorption of acetic acid on activated carbon.
478 3.1.4.Effect of adsorbent mass
The figure 4 illustrates the variation of the amount adsorbed in the masses of adsorbent used at an ambient temperature of 25 °C. Indeed, for activated carbon, we observe that the adsorbed amount of acetic acid decreases with increasing mass of activated carbon. It explains that this variation is due to the decrease in the accessible area of activated carbon particles by aggregation effect. Therefore, the mass corresponding to the optimal activated carbon is 0,5 g.
Figure4. Effect of adsorbent mass on adsorption of acetic acid on activated carbon
3.2. Study of adsorption isotherms
The adsorptions isotherms play an important role in determining the maximum capacity and in identifying the type of adsorption to occur. They are obtained primarily through the knowledge of the contact time and then, by the graphical representation of Qe=f(Ce) where Qe and Ce are respectively the amount of the organic molecular adsorbed per g of adsorbent and the concentration at the equilibrium of the organic molecular respectively. The experimental results show that the isotherm is type L, which corresponds to the classification of Gilles [13]. This indicates a growth of adsorption when the concentration of the adsorbate increases, and these curves can be mathematically described by the equation of Langmuir or Freundlich (see below). As the number of sites occupied by the solute molecules increases over the adsorption of new molecules become difficult. The molecules are arranged in a monolayer on the solid surface. The gait obtained shows that the adsorption process of the organic molecule could occur in a monolayer. Because of their simplicity, kinetic models, the most commonly used are the Langmuir and Freundlich.
3.2.1.Langmuir isotherm
Langmuir isotherm is represented by the following equation :
Qe: Quantity adsorbed at equilibrium per unit weight of the adsorbent (mg/g). Q0: adsorption capacity at saturation (mg/g) and correspond to the formation of a monolayer. a: adsorption coefficient. The linearization of the Langmuir equation determines the parametersa and Q0. Representing:
Representing:
479 Wecanthusdeduce:
Q0from the intercept.
afrom the slope.
3.2.2.Freundlich isotherm
it can be described by the following equation:
Kf and 1/n are respectively the adsorption capacity in mg/g and a constant indicating the intensity of the adsorption:
The linear form of this equation allows the determination of Kf and 1/n Representing:
The results showed in Figures 5 and 6 that the adsorption of acetic acid on activated carbon follows two linear models of Langmuir and Freundlich.
Figure 5. Adsorption isotherm of Langmuir at (25 °C; 30 °C and 35 °C)
Figure 6. Adsorption isotherm ofFreundlichat (25 °C; 30 °C and 35 °C)
480 The following table 1 resumes the different correlation coefficients. Who we can notice that the adsorption of acetic acid on activated carbon follows the Langmuir model with 0,99 of correlation coefficientat 25 °C.
Table 1. Correlation coefficient for isotherm of Langmuir and Freundlich at different temperature.
Isotherm of Langmuir Isotherm of Freundlich
Temperature 25°C 30°C 35°C 25°C 30°C 35°C
R2 0,991 0,966 0,988 0,973 0,837 0,863
3.3.The specific surface area
The specific surface area of an adsorbent is based on measurements of the adsorption capacity for a given solute. The adsorbate must have an acceptable surface. Just donations determine the value of the adsorption capacity of the monolayer from the adsorption isotherm. Knowledge of the maximum amount of adsorption (mg/g or mol/g) following the equation Langmuir, leading to the determination of the surface σ by the equation:
σ = Q0*s*N
Where N is Avogadro's number, s = 21 Ų is the area occupied by a molecule of acetic acid. The following table 2 summarizes the parameters of acetic acid adsorption on activated carbon:
Table 2.Value of adsorption parameters Q0, a, kfand σ acetic acid on activated carbon.
Activated carbon
25°C
Q0(mol/g) 9,47 E-03
Kf 1,40E-02
1/n 0,7423
σ (m2/g) 1196,7
30°C
Q0(mol/g) 6,67E-03
Kf 7,40E-03
1/n 0,649
σ (m2/g) 843,75
35°C
Q0(mol/g) 4,13E-03
Kf 6,78E-03
1/n 0,4098
σ (m2/g) 522,26
3.4. Study of acidacetic adsorption on other substrates
In the first part, we evaluated the performance of activated carbon on adsorption of acetic acid,now this subsection evaluates the influence of other substrates (soil) on the adsorption of acetic acid.
Hence we carried out further studies of adsorption on different substrates. Our choice is made for three grades of soil 1, 2 and 3.
3.4.1. Determining the type of soil
481 X-ray fluorescence analysis and characterization by X-rays show:
Table 3. Results of soil analysis by fluorescence X
Compound Soil 1 Soil 2 Soil 3
SiO2 12,68 30,24 34,03
Al2O3 3,73 10,22 12,13
Fe2O3 2,10 4,06 4,60
CaO 41,95 25,42 20,12
MgO 0,79 1,70 2,77
SO3 0,03 0,04 1,17
K2O 0,64 1,25 1,57
Na2O 0,14 0,16 0,08
P2O5 0,11 0,10 0,11
Cl 0,00 0,00 0,00
loss- of-fire 36,09 24,81 22,26
total 98,27 98,00 98,84
Figure 7. X-ray diffraction of soil 1.
Figure8. X-ray diffraction of soil 2
From Table 3 and X-ray diffraction, we can conclude that soil 1 represent high percentage of CaO41,95%.
This rate is linked to the presence of calcite. We also note the high rate of organic matter 36,09%. Therefore, we have low oxides Fe2O3, Al2O3, MgO and K2O, also rich in organic matter [14; 15].Figure 7 also shows that the soil 1 is a rich calcite. For soils 2 and 3: SiO2rich due to the presence of quartz. We also note the presence of CaO and loss on ignition 24,81% respectively and 22,26%. Also, we have significant rates of oxides Fe2O3, Al2O3, MgO and K2O. Also Figures 8and 9 shows that soils 2 and 3 are rich in quartz, calcite and clay minerals chlinochlore characteristic.
482 Figure9. X-ray diffraction of soil 3
To identify our soil, we compared the percentages from Table 3 with data from international standards. We therefore conclude that: The soil 1 is themarl calcareous, soil 2 and 3 are clays. But to better differentiate them, we made a mineralogical analysis for both soils, and we found. From table 4, we conclude that Sol 2 is yellow clay and Sol 3 is gray clay.
Table 4. Mineralogical analysis of soils 2 and 3
Mineral % Soil 2 Soil 3
Dolomite 0,00 26,00
Quartz 26,00 30,00
Limestone 56,00 32,00
Clay 18,00 12,00
3.4.2. Contact time
The figure 10 showedthe impact of time on the adsorption of acetic acid:
Figure10. Influence of contact time the adsorption of acetic acid on (marl calcareous, yellow clay and gray clay)
483 From these curves, we find that the minimum time for an adsorption maximum is that of the marl calcareous:
75% for a time of 0,5 h.Gray clay and clay yellow, they have the maximum adsorption for a minimum contact time of 1,5 hours.
3.4.3. Influence of initial concentration and temperature
The graph below (11, 12 and 13) showed the influence of the initial concentration of acetic acid and the temperature of the adsorption. For the adsorption on the yellow and gray clay, the exothermic (ΔH <0) and under these conditions would lead to a physisorbtion [12].But for the marl calcareous, it is endothermic (ΔH>0) and under these conditions would lead to a chemisorption [12].
Figure 11. Influence of temperature and initial concentration of acetic acid on marl calcareous
Figure12. Influence of temperature and initial concentration of acetic acid on the yellow clay
Figure 13.Influence of temperature and initial concentration of acetic acid on gray clay
484 3.4.4. Influence of pH
Figure 14 show the impact of pH on the adsorption of acetic acid. The adsorption of acetic acid on three soils is low for an acid pH and becomes important for high pH values, probably in the form of acetic acid is stable to acidic and basic media for the anionic form of acetic acid (acetate) becomes stable and subsequently promotes adsorption.
Figure14. Influence of pH on the adsorption of acetic acid on (marl calcareous, yellow clay and gray clay)
3.4.5. Effect of adsorbent mass
From figure 15, we can concludethat adsorption of marl calcareous requires a low mass compared to other soils for better absorption [14,15].
Figure15. Influence of the mass (marl calcareous, yellow clay and gray clay) on the adsorption of acetic acid
4. Conclusion
The results obtained in this work have shown that acetic acid (0,2mol/l) is relatively less well adsorbed on activated carbon with a contact time of 4 hours which corresponds to 16,5 % adsorption, where The Langmuir kinetic model correctly describes the process for activated carbon adsorption. However Marl calcareous present a maximum of adsorption 75 % for a minimum time 0,5 h. The effect of initial concentration had a positive influence on the kinetics and capacity retention of acetic acid, against temperature has a reverse effect for the activated carbon, yellow and gray clay, and a positive effect on the Marl calcareous. As much as a pH has a positive impact on adsorption kinetics of the organic molecule.
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