Activated carbon based oil palm shells were prepared by physical activation using steam which was further grafted with silver. The Response Surface Methodology (Doehlert design) was used to optimize both the impregnation of silver and the atrazine removal. The effects of three variables of preparation conditions of the composite activated Carbon-Silver (AC-Ag): Concentration of silver, impregnation temperature and impregnation time on the atrazine removal (Y 1) was investigated on one hand. In another hand, three variables of atrazine removal from liquid phase: Temperature, pH and ratio of Atrazine/CaCl 2 (r(Atraz/CaCl 2)) on the adsorption capacity of atrazine (Y 2) were also investigated. Based on the Doehlert designs, the quadratic models were developed to correlate the preparation variables and the adsorption variables to the response. The optimum conditions of preparation of AC-Ag were found to be: Concentration of silver of 0.063 mol/L, impregnation temperature of 223 ℃, impregnation time of 1.3 hand atrazine removal of 384.62 mg/g. The optimum conditions of atrazine adsorption were found to be: Temperature of 25.0 ℃, pH of 7.7 and r(Atraz/CaCl 2) of 0.37 which gave 209 mg/g of atrazine adsorption capacity. These results demonstrated that the preparation and adsorption conditions have a significant influence on the removal of atrazine.
Atrazine (2-chloro-4-ethylamine-6 isopropylamino-s-triazine) is a selective triazine herbicide frequently used in agricultural sector. Contamination of water and soil by atrazine has a negative impact on aquatic ecosystems and induce severe hormonal disturbances in amphibians [
One effective alternative to eliminate this recalcitrant compound could be the adsorption process. Adsorption is a process whereby a contaminant adheres to the surface of an adsorbent, such as activated carbon, due to hydrophobic and electrostatic interactions between the adsorbate and the adsorbent [
Activated carbons are widely used to adsorb organic micropollutant from liquids or gas [
The Oil palm shells were collected locality from Bafangin the West region of Cameroon. The precursor was cleaned several times with ionized water and sun dried. Then, they were crushed and sieved to particle sizes ranging from 2.0 - 2.5 mm. The dried residues were carbonized in a furnace tube (Carbolite1200C UK) at 400˚C for 2 h under a flow of N2 gas, at a heating rate of 10˚C/min. Before, it was activated at 850˚C for 6 h (heating rate of 10˚C/min) under steam (0.1 mL/min) and cooled to room temperature. After activation, the samples were washed in distilled water, dried, ground, and sift to obtain powder with particle size less than 50 µm. To functionalize the surface of activated carbon, the activated carbons were treated with HNO3 (1 mol/L).
Activated carbon-silver composites were prepared from the suspension of 0.6g of activated carbon in 1.6 mL of water at desired concentration (0.05 - 0.1 mol/L) of silver (AgNO3). The mixture was introduced in the furnace tube (at hydrothermal carbonization) in the dark. After 1 h, the temperature was increased in the range (188˚C - 292˚C) at the desire time in the domain of (1.3 - 3.7 h). Then, the composite was washed with distilled water and dried at 105˚C until constant weight and kept in a hermetic bottle for further test.
The composites were characterized using powder EDX for the chemical composition of the AC-AgFT-IR spectroscopy was applied in order to identify the functional groups and chemical bonding on the adsorbents. For this purpose, spectra were determined between 4000 and 400 cm−1 using an FT-IR spectroscope (Spectrum Vertex 70 DTGS). The morphological analysis of the activated carbon was performed by Scanning Electron Microscopy (SEM) (JEOL JSM-5400, Japan).
Response Surface Methodology (RSM) is a collection of mathematical and statistical techniques that are useful for modeling and analysis of problems in which a response of interest is influenced by several variables [
In the present study, the composite AC-Ag was prepared using hydrothermal carbonization by varying the preparation variables using Doehlert experimental design. The variables studies were: concentration of AgNO3(X1); impregnation temperature (X2) and impregnation time (X3). For the adsorption of atrazine, three independent tests were chosen for the statistical experimental design as follows: temperature (˚C) (X1), pH (X2) and ionic strength (atrazine/CaCl2) (X3). The range and levels of the factors which were varied according to the experimental design are given in
Exp | [AgNO3] (mol/L) | Impregnation temperature (˚C) | Impregnation times (h) | Y1 (mg/g) |
---|---|---|---|---|
1 | 0.100 | 240 | 2.5 | 344.83 |
2 | 0.050 | 240 | 2.5 | 357.14 |
3 | 0.087 | 292 | 2.5 | 294.12 |
4 | 0.063 | 188 | 2.5 | 344.83 |
5 | 0.087 | 188 | 2.5 | 333.33 |
6 | 0.063 | 292 | 2.5 | 285.71 |
7 | 0.087 | 257 | 3.7 | 277.80 |
8 | 0.063 | 223 | 1.3 | 384.62 |
9 | 0.087 | 223 | 1.3 | 333.30 |
10 | 0.075 | 275 | 1.3 | 370.40 |
11 | 0.063 | 257 | 3.7 | 270.00 |
12 | 0.075 | 205 | 3.7 | 333.00 |
13 | 0.075 | 240 | 2.5 | 294.11 |
14 | 0.075 | 240 | 2.5 | 294.12 |
15 | 0.075 | 240 | 2.5 | 297.00 |
16 | 0.075 | 240 | 2.5 | 303.03 |
17 | 0.075 | 240 | 2.5 | 299.00 |
Y1: Experimental response; Exp: experiment.
Name | Coefficient | Standard déviation | t.exp | Signif. % |
---|---|---|---|---|
b0 | 297.452 | 2.884 | 103.14 | <0.01*** |
b1 | −8.904 | 3.224 | −2.76 | 2.74* |
b2 | −28.168 | 3.224 | −8.74 | <0.01*** |
b3 | −42.360 | 3.224 | −13.14 | <0.01*** |
b11 | 53.533 | 5.395 | 9.92 | <0.01*** |
b22 | 4.883 | 5.396 | 0.91 | 39.9 |
b33 | 31.497 | 5.064 | 6.22 | 0.0550*** |
b12 | 11.495 | 7.447 | 1.54 | 16.4 |
b13 | 32.139 | 8.325 | 3.86 | 0.633** |
b23 | −67.073 | 8.325 | −8.06 | 0.0141*** |
The experimental design matrix of 17 experiments and the results are given in
where, Y is the predicted response, b0 a constant coefficient, bi a linear coefficient, bii a quadratic coefficient, bij an interaction coefficient, X1 coded variables of concentration of AgNO3 and adsorption temperature. X2 coded variables of Impregnation temperature and pH and X3 coded variables of impregnation time andratio atrazine/CaCl2) of the impregnated activated carbon variables and adsorption of atrazine variables respectively. The experimental data were analysed using software named NEMROD (New Efficient Methodology of research using optimal design); for regression analysis, to fit the equations developed and also to evaluate the statistical significance of the equations obtains [
The batch experiments for the adsorption studies were carried out at room temperature in conical flask of 150 mL. For each run, 10 mg of the adsorbent was introduced into the flask containing 100 mL of the atrazine solution at initial concentrations of 20 mg/L. The shaker was set at a desired temperature (15˚C - 35˚C) at a speed of 250 rpm and the solution at a desired pH (3.4 - 8.6) and at a desired ratio atrazine/CaCl2 (0.37 - 1.83). After reaching the equilibrium, AC-Ag were separated from the aqueous solution using filtration method with 0.45 µm what man cellulose nitrate menbrane. Then, the analysis of the residual solution was performed by UV-visible spectrophotometer Secomam at 225 nm. The quantities adsorbed at equilibrium; Qe (mg∙g−1) were calculated according to:
where, C0 and Ce (mg/L) are the initial and equilibrium concentrations of atrazine in solution, respectively, V (L) is the total volume of the solution, and m (g) is the adsorbent mass.
The examination of the given results in
Nevertheless, for the removal of atrazine in function of medium conditions,
The polynomial model equation in terms of coded factors is given as:
N˚ Exp | Temperature ˚C | pH | Ratio Atrazine-CaCl2 | Y2 mg/g |
---|---|---|---|---|
1 | 35.0 | 6.0 | 1.1 | 176.51 |
2 | 15.0 | 6.0 | 1.1 | 169.64 |
3 | 30.0 | 8.6 | 1.1 | 157.69 |
4 | 20.0 | 3.4 | 1.1 | 179.25 |
5 | 30.0 | 3.4 | 1.1 | 162.50 |
6 | 20.0 | 8.6 | 1.1 | 176.00 |
7 | 30.0 | 6.9 | 1.83 | 179.9 |
8 | 20.0 | 5.1 | 0.37 | 180.52 |
9 | 30.0 | 5.1 | 0.37 | 183.82 |
10 | 25.0 | 7.7 | 0.37 | 209.45 |
11 | 20.0 | 6.9 | 1.83 | 195.53 |
12 | 25.0 | 4.3 | 1.83 | 196.02 |
13 | 25.0 | 6.0 | 1.1 | 193.88 |
14 | 25.0 | 6.0 | 1.1 | 195.23 |
15 | 25.0 | 6.0 | 1.1 | 176.32 |
16 | 25.0 | 6.0 | 1.1 | 195.5 |
17 | 25.0 | 6.0 | 1.1 | 187.14 |
The quality of the developed model was evaluated based on the correlation coefficient, R2 and the adjusted R2 indicating that the variability in the response could be explained by the mathematical model [
The Figures shows that, atrazine adsorption increases when the [AgNO3] in-
crease. It is found that, atrazine exists almost exclusively as neutral molecules, and the weak forces such as van der waals forces, hydrogen bonds and hydrophobic interaction would involve in the reciprocity of atrazine with activated carbon [
From
This response is described by the following equation:
With a significant correlation coefficient (R2 = 0.929 and R2 adjusted = 0.893). The coefficients estimated from the results are displayed in
This Figures 2-4 showed the variation of atrazine adsorption as a function of different factors.
Name | Coefficient | F.Inflation | Standard déviation | t.exp | Signif.% |
---|---|---|---|---|---|
b0 | 191.547 | 1.282 | 149.40 | ˂0.01*** | |
b1 | 2.556 | 1.14 | 1.441 | 1.77 | 9.0 |
b2 | 8.352 | 1.31 | 1.797 | 4.65 | 0.0234*** |
b3 | −4.040 | 1.04 | 1.369 | −2.95 | 0.834** |
b11 | −18.472 | 1.10 | 2.221 | −8.32 | ˂0.01*** |
b22 | −14.066 | 1.79 | 3.306 | −4.25 | 0.0529*** |
b33 | 3.144 | 1.13 | 2.234 | 1.41 | 17.3 |
b12 | 16.468 | 2.12 | 5.174 | 3.18 | 0.510** |
b13 | −7.948 | 1.34 | 3.635 | −2.19 | 4.03* |
b23 | −29.624 | 1.18 | 3.744 | −7.91 | ˂0.01*** |
sorbed atrazine, indicating that, the process was exothermic [
at high pH, atrazine can be converted to a negative charge from the protonated base in basic solution. The number of negatively charged adsorbent sites increased at pH > 6.93, limiting the adsorption of atrazine. But the increase ofr (atraz/CaCl2) at high pH enhances ion pair formation between Ca2+ and AC/Ag who is charge negatively. Consequently, electric repulsion between the negatively charged AC/Ag surface and atrazine might have occurred at high pH [
The preparation of composite AC/Ag was done under the experimental conditions given in
Ag at 44.3% and Carbon at 55.7%. The SEM images of AC and AC-Ag are shown in
The Fourier Transform Infrared (FTIR) spectra of AC and AC-Ag (
was positive when pH is below pHPzc and negative when pH is above pHPzc.
The composite AC-Ag has been prepared for adsorption of atrazine. The impregnation condition of AC-Ag and the removal of atrazine have been optimized using Response Surface Methodology (Doehlert design). The second polynomial equation has been found to fit most satisfactorily the model predicted according to the correlation coefficients obtained. The obtained adsorption capacities were 384 mg/g for optimum condition of impregnation and 209 mg/g for a medium condition for adsorption. It is clear that the medium condition has the less effect on the adsorption of atrazine. Finally, it can be concluded that AC-Ag has an excellent potential of adsorption of atrazine. Therefore, it can be efficiently used for treatment of industrial effluent containing these pollutants.
Dr. BEAS of the University of Technology of Johannesburg is highly acknowledged for his assistance in analysis and interpretation, remarks and suggestions in the write up of this paper. The authors thank the Applied Organic Chemistry Laboratory of the Chemistry Department, Faculty of Science Semlalia, Cadi Ayyad University of Morocco, for the materials and logistics support. And finally thank all the members of the Research Unit “Adsorption and Surface” of the Applied Physical and Analytical Chemistry Laboratory of the University of Yaoundé I.
Rachel, N.Y., Abdelaziz, B., Daouda, K., Julius, N.N., Gaelle, D.D.E., Abdelrani, Y., Mehdi, L. and Joseph, K.M. (2017) Optimization Study of the Removal of Atrazine from Aqueous Solution on to Composite Activated Carbon-Silver Using Response Surface Methodology. Materials Sciences and Applications, 8, 258-272. https://doi.org/10.4236/msa.2017.83018