КОЛЛОИДНЫЙ ЖУРНАЛ, 2007, том 69, № 3, с. 385-389
УДК 541.183
ADSORPTION OF PHENOL AND METHYLENE BLUE BY ACTIVATED
CARBON FROM PECAN SHELLS © 2007 R. A. Shawabkeh*, E. S. M. Abu-Nameh**
*Department of Chemical Engineering, Mutah University, Jordan, 61710 Al-Karak E-mail: rshawabk@mutah.edu.jo **Department of Basic Science, Prince Abdullah Bin Ghazi Faculty of Science and Information Technology,
Al-Balqa Applied University, Jordan, 19117 Al-Salt, P.O. Box 7272 E-mail: eyad@lycos.com Поступила в редакцию 16.12.2005 г.
Activated carbon was produced from pecan shells by chemical activation using phosphoric acid. This activation was followed by treatment with sodium dodecylsulfate to enhance its surface for removal of phenol and methylene blue from aqueous solution. The results showed a great ability for methylene blue removal with sorption capacity of 410 mg/g at pH 9 and solution concentration of 35 mg/L, while moderate adsorption was obtained for phenol with a capacity of 18 mg/g at pH 11 and the same solution concentration. The increase or decrease in solution pH has a favorable effect on sorption of both adsorbates. Langmuir and Freundlich models were used to fit the experimental data.
INTRODUCTION
Activated carbon is known to be an effective adsorbent for removal of solutes from fluids. The choice of this carbon in environmental remediation depends on its surface area, adsorption capacity and non-reactivity with the target solute. Previously, a new active carbon from pecan shell was prepared as an adsorbent for removal of metal ions, radioactive isotopes and organics from aqueous solutions [1]. This work involved activation of the pecan shell with phosphoric acid while air is injected during the activation process [2]. Then several attempts were performed on this carbon by either study its active surface functional groups, enhance the surface area or utilize it for removal of different solutes from aqueous solutions.
Toles et al. [3] studied the effect of functional groups on the surface capacity of this carbon. They proved that the wet activation process with air gives the greatest quantity of surface functional groups and the highest metal uptake. Ahmedna et al. [4] compared the properties of activated carbon from pecan shells with those of carbons from rice straw, rice hulls, and sugarcane bagasse and showed that the types of shells and binder and activation method determine the properties of the carbon. Dastgheib and Rockstraw [5] studied the functional groups of the activated carbon from pecan shells and proposed that the acidic groups detected using the Boehm titration method not only be considered as oxygen-containing acidic groups, but also as oxygen/phosphorus groups. Ng et al. [6] provided that steam activation to pecan shells produced carbon which has a potential to replace Filtrasorb 400 in applications involving removal of geosmin from aqueous media. They developed process flow diagrams for the
7 КОЛЛОИДНЫЙ ЖУРНАЛ том 69 < 3 2007
large-scale production of this carbon, derived from steam or phosphoric acid activation, and carried out an economic evaluation to estimate the cost to manufacture these carbons [7].
Although this carbon was extensive studied, there was not any attempt to utilize different activation techniques to enhance its capacity and selectivity for target solute(s).
This study is attempted to improve the surface of the activated carbon synthesized from pecan shells throughout modification with sodium dodecylsulfate surfactant. The obtained activated carbon will be tested for removal of phenol and methylene blue (MB) from aqueous solutions.
EXPERIMENTAL
Pecan shells were brought from Las Cruces area, New Mexico-USA. Sodium dodecylsulfate (SDS) was provided from Mallinckrodt Co. Methylene blue and phenol were analytical grade supplied by Merck. Hydrochloric acid, nitric acid and o-phosphoric acid were analytical grade supplied by Sigma Chemical Company.
The activation procedure was performed by mixing of 30 g of powdered pecan shells (the particle size is less than 45 |m) with excess hot o-phosphoric acid and boiled over Bunsen burner. Meanwhile air was injected through out the paddles of glass stirrer into the solution at flow rate of 1.5 L/min until the mixture get solidify as shown in Fig. 1. At this moment, the temperature of the mixture was recorded at 270°C. Then the activated shells were directly placed into cold 0.5 M SDS solution (4 ± 1°C). The mixture was kept stirred at 500 rpm for 5 h. After that the modified carbon was filtered, washed 5 times with 1 L each
Air Inlet
Motor
Air Inlet
Motor
Stirrer
Stirrer
AC AC without
Test type Standard treated treatment
with SDS with SDS
Ash content, wt. % D 2866-94 13.2 4
Moisture content, wt. % D 2867-95 17.8 2
Apparent density, g/L D 2854-96 490 495
pH D 3838-80 7.5 6.5
at equilibrium. The most popular model is the Langmuir one, which relates the rates of adsorption and desorption of a solute at equilibrium by equation [8]
kadsCeN( 1- 0e) = kdesN0e
(1)
where kads and kdes are the rate constants of adsorption and desorption at the surface of the adsorbent, respectively. N is the maximum number of the sites occupied by the solute of interest at the equilibrium solute concentration Ce and 0e is the ratio of adsorption capacity at specific solute concentration to its limiting value. The above equation is rearranged to
qe
KL Q Ce
1 + KT C e
(2)
Fig. 1. Schematic diagrams for the reaction apparatus.
of deionized water, dried at 105°C, and stored in a des -iccator for further uses.
Adsorption was carried out in a set of Erlenmeyer flasks (250 mL). Solutions of phenol and methylene blue with different initial concentrations were prepared from corresponding freshly prepared stock solutions. These samples were mixed with fixed mass of 0.1 g of the pre -pared activated carbon and allowed to equilibrate in an isothermal shaker (22 ± 1°C) for 48 h. Blank samples containing either activated carbon mixed with deionized wa ter or target solute solution alone with different initial concentrations were prepared in a similar manner for compar ison. After equilibration, the samples were allowed to settle, then centrifugated and analyzed using UV-1601 Shimadzu spectrophotometer.
All Pyrex glassware was soaked with 0.1 M solution containing 75 wt. % hydrochloric acid and 25 wt. % nitric acid, then washed with soap and deionized water to re move any adhere impurities.
THEORETICAL
Several isotherm models can express the variation in the target solute concentration at the surface of adsorbent
Table 1. Physical properties of activated carbon prepared from pecan shells
where Q is the maximum solute uptake by the adsorbent surface at equilibrium and KL is the Langmuir constant which represents the ratio of adsorption and desorption rate constants.
One the other hand, the Freundlich model was used (to incorporate the effect of surface energy heterogeneity) in which the energy term, KL, in the Langmuir model varies as a function of surface coverage due to heat of adsorption [9]. The Freundlich equation has the form
qe = a Ce, (3)
where a and e are constants. The index e is the adsorption intensity and the coefficient a can be related to the surface energy by the proportionality relation
AH / RT
a « RTnKT e
RESULTS AND DISCUSSION
(4)
Chemical activation of the pecan shells was carried out in two different stages. The first stage involved oxidation of the shells with the acid at elevated temperature using a newly developed glass impeller. Two different impellers were used in the activation step in order to obtain the best shear and minimal foam formation. The first one was rounded by four glass-windows to allow the slurry pass through during the rotation. Once the carbonaceous slurry gets solidified, the second impeller was used in order to avoid blockage of impeller windows. The completion of the activation experiment is identified by the transforma tion of the pasty carbon into dry char.
In the second activation step, the hot solid carbon aceous material was quenched with cold solution of aerated surfactant. Continuous mixing of the mixture was performed in a beaker while air was passed during the mixing process for 2 h.
The using of these newly impellers has the advantage that air or water can be injected in the solution continuously while they rotate at high speed.
The produced carbon was analyzed for ash content, moisture content, pH and density. Table 1 shows the results obtained using the corresponding ASTM methods. It is ob -
MB adsorbed, mg/g 400
300 -
200 -
100 -
3
Phenol adsorbed, mg/g 160
120 -
80 -
40
10 20 30 40 50
Concentration of MB in solution, mg/L
0 50 100 150 200 250
Concentration of phenol in solution, mg/L
Fig. 2. Adsorption isotherms of methylene blue onto the surface of activated carbon for solution pH 4 (7), 7 (2), and 9 (J).
Fig. 3. Adsorption isotherms of phenol onto surface of activated carbon for solution pH 4 (7), 8.5 (2), and 11 (J).
Concentration of MB in solution, mg/L 6
12 pH
Fig. 4. Effect of solution pH on the adsorption of methylene blue onto activated carbon surface. The curves correspond to 1 (7), 2 (2), 5 (3), and 9 days (4).
Concentration of MB in solution, mg/L 40
30
20
10
12 pH
0
3
2
0
0
vious that the produced carbon has high density and ash content. In comparison with the results previously obtained [10], the ash content is increased from 4 to 13.2 wt. % and the pH from 6.5 to 7.5.
Adsorption isotherms of methylene blue and phenol for the produced carbon are shown in Figs. 2 and 3, respectively. These isotherms were measured at different solution pH values ranging from 4 to 11 in order to obtain the maximum uptake for both adsorbates at specific pH values. It was shown that both at pH values lower and higher than pH 7 the adsorption of both methylene blue and phenol increase (Fig. 4). The amounts of Mb uptake obtained at pH 4 and
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