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Saturday 15 October 2016
Kinetics studies of uranium sorption by powdered corn cob in batch and fixed bed system
Published Date January 2016, Vol.7(1):79–87,doi:10.1016/j.jare.2015.02.004 Open Access, Creative Commons license,
Funding information Author Mohamed A. Mahmoud a,b,, aNuclear Material Authority, Kattamiya Road, Maddi, P.O. Box 530, Cairo, Egypt bChemical Engineering Department, Jazan University, Saudi Arabia Received 13 November 2014. Revised 24 January 2015. Accepted 18 February 2015. Available online 26 February 2015. Abstract Sorption of uranium (VI) from aqueous solution onto powdered corn cob has been carried out using batch and fixed-bed technique. The experimental results in batch technique were fitted well with pseudo second-order kinetics model. In the fixed bed technique, Thomas and Bohart–Adams models were evaluated by linear regression analysis for U(VI) uptake in different flow rates, bed heights and initial concentrations. The column experimental data were fitted well with Thomas mode (r2= 0.999), but the Bohart–Adams model (r2= 0.911), predicted poor performance of fixed-bed column. Keywords
adsorption capacity at equilibrium, (mg of U(VI)/g adsorbent)
qt
adsorption capacity at time t (mg of U(VI)/g adsorbent)
r2
correlation coefficient
t
time (min)
τ
the time required for 50% adsorbate breakthrough (min)
V
volume of the solution (l)
x
mass of adsorbent in the column (g)
Z
bed depth of column (cm)
Introduction
The growth of technology in nuclear industries has led to the emergence of many of environmental pollution problems, it is so important to develop number of methods for removing hazardous elements from industrial liquid wastes. Uranium is the important element in nuclear applications. Nuclear power is derived from uranium, which has no significant commercial use other than as a fuel for electricity generation. For this reason, the recovery, concentration and purification of uranium are of great importance. Because of the expected shortage of uranium in near future, researches are to be directed to the recovery of uranium from nonconventional resources such as sea water, industrial waste waters, mine waste water, and other waste sources in relation to the pollution of the natural environment [1] and [2]. The most commonly used methods for the removal of heavy metals from wastewater are chemical precipitation; membrane processes, ion exchange, solvent extraction, photocatalysis and adsorption [3]. Adsorption process has long been used in the removal of heavy metals and other hazardous materials such as, color, odor and organic pollution.
Although activated carbon is widely applied for pollutant removal, natural materials which are relatively cheaper and eco-friendly have also been successfully employed as adsorbents for heavy metal removal from aqueous solutions and wastewaters due to their availability, low-cost, unique chemical composition and renewability. The reduced running cost has been the focal point for research on application of natural materials. Cost is a very important factor when considering material for use as adsorbents. The recent attention in this field is evident in the number of research currently being done on the use of low cost agricultural wastes for metal removal from aqueous solution. Among the numerous adsorbents, Agriculture material is one of the most widely used and economic adsorbent in the adsorption process such as coir pith [4], orange peels [5], palm-shell [6], rice straw [7], cellulose beads [8] sunflower [9], has been investigated. The objective of this study was to investigate the adsorption potential of uranium (VI) onto powdered Corn cob (PCC) in batch and fixed-bed technique. In batch process kinetics of uranium removal onto PCC at different parameters (temperature, pH, initial concentration, and adsorbent dose) are investigated. The performance of fixed-bed column was evaluated by Thomas and Bohart–Adams models at different flow rates, bed heights and initial concentrations.
Material and methods
Preparation of adsorbent material
Corn cob is an agricultural by-product generated in Middle East. Corn cobs were washed with distilled water several times to remove dirt and particulate materials. The washed Corn cobs were dried at 80 °C. The dried corncobs were ground and sieved to obtain powdered Corn cob (PCC), of a particle size 300–425 μm and stored in dissector for further use.
Preparation of uranium stock solution
All chemicals and reagents used in this work were analytical grade. Stock solution of uranium (VI) was prepared by dissolving appropriate amounts of UO2(NO3)2·6H2O, Aldrich, USA, in distilled water. For experiments the required concentration was prepared by dilution. The concentrations of U(VI) in solution were determined spectrophotometrically employing Shimadzu UV–VIS-1601 spectrophotometer using arsenazo (III) as complexing reagent [10].
Sorption experiments in batch technique
Batch experiments were first carried out to determine the potential of PCC to adsorb U(VI)) from aqueous solution and to investigate the optimum parameters of adsorption (adsorbent dose, contact time, pH, temperature, and initial concentration). 50 mL of different concentrations (25–100 mg/L) of U(VI) solutions with a range of pH values from 3 to 10 was transferred in a conical flask with 0.3 g of PCC. The solution was agitated at 200 rpm in a thermostatic shaker water bath for different time (10–180 min) at different temperature (303, 313, 323 and 333 K). The samples were withdrawn and centrifuged at 5000 rpm for 5 min and the supernatant solutions were analyzed. The pH of the solutions was adjusted with 0.1 M Na2CO3 or 0.1 N HCl.
Sorption capacity and removal efficiency
Sorption capacity (q) of U(VI) was defined as:
equation1
In addition, the removal efficiency (Re) is calculated according to the following equation:
equation2
Sorption kinetics in batch technique
Kinetics of sorption of U(VI) onto PCC was analyzed using two kinetic models (pseudo first-order and pseudo second-order models). The comparing between data of experiments and models was analyzed by the correlation coefficients (r2).
Pseudo-first-order model
Lagergren’s equation of pseudo first-order model describes the sorption capacity of solids in solid–liquid systems [11] and [12]. It is supposed that one adsorbate is adsorbed onto one sorption site on adsorbent surface.
The linear form of pseudo first order model was given by equation:
equation3
Values of k1 and qe were calculated from the slope and intercept values of the straight line of plotting log (qe − qt) versus t, respectively.
Pseudo-second-order model
The pseudo second-order model has been applied for the analysis of kinetics of chemisorption from liquid solutions. The linear form of pseudo-second order model [13] and [14], given by the equation:
equation4
The plot of t/qt versus t should give a straight line and the K2 and qe were calculated from the values of intercept and slope, respectively.
Sorption experiments in fixed-bed technique
Glass column of 2 cm internal diameter and 30 cm height was used in fixed bed experiments. PCC was packed with different bed heights (2.5, 5 and 7.5 cm) in the column with a layer of glass wool at the bottom. Three flow rates (1, 2 and 3 mL/min) were pumped to the top of the packed column by using peristaltic pump with different initial ion concentrations (25, 50, 75 mg/L) at 303 K. The effluent samples were collected at regular intervals and analyzed. Fixed bed studies were terminated when the column reached exhaustion.
Kinetic models of break through curves in fixed-bed column
For good design of fixed bed system, it is important to predict the breakthrough curve for effluent parameters. Thomas [16], Bohart–Adams [17] kinetic models were used to predict the dynamic behavior of the column.
Thomas model
Thomas model is one of the most widely used models in column performance studies. Thomas model is given in linear form by the following expression:
equation5
The parameters of Thomas model (kTh and qe) can be determined from a plot of Ln[(C0/Ct) − 1]against time (t) at a given flow rate.
Bohart–Adams model
Bohart–Adams model is used for the description of the initial part of the breakthrough curve. The linear form of Adam-Bohart model is given by the following expression:
equation6
The parameters kAB and N0 were determined from the intercept and slope of linear plot of ln (Ct/C0) against time (t), respectively.
Results and discussion
Characteristics of adsorbent
Fig. 1(a) and (b), represents the SEM photographs of adsorbent before and after sorption with 500× magnification. Fig. 1(a), shows that the adsorbent surface is rough, porous and irregular shapes allowing for good sorption between U(VI) ions and PCC. After sorption, Fig. 1(b), shows the loss of porosity and roughness of the adsorbent surface.
Fig. 1. SEM image of unloaded PCC (a) U(VI) loaded PCC(b).
The FTIR spectrum of PCC before and after sorption (Fig. 2) displays a number of sorption peaks, indicating the complex nature of the adsorbent material. The band at 3417 cm−1 was assigned to the OH group in free alcohols. The band at 2920 cm−1was assigned to the CH stretching. The band at 1615 cm−1 was assigned to the asymmetric stretching of COO in ionic carboxylic group. The band at 1388 cm−1was assigned to the symmetric COO stretching in pectin. The band at 1012 cm−1 was assigned to the COH stretching in alcohols. After metal loading, the CO deformation band (1384 cm−1) in pectin remained constant while shifts occurred in the wave numbers 3417, 2920 and 1615 cm−1 indicating an interaction of these functional groups with sorbed U(VI) and also the appearance of wave number 1738 cm−1 in the U(VI) loaded spectra may indicate the interaction of this group with U(VI) ion.
Table 1 shows that the sorption of U(VI) by PCC was found to be increased with increasing the time and attained a maximum value at 60 min (Fig. 3). The U(VI) uptake increased with changing pH of U(VI) solution from 3 to 10. The decreasing of sorption capacity at lower pH is due to the competition between H+ and U(VI) ions. However, with increasing pH the sorption capacity increased probably due to the decreased H+ concentration that provided more sorption sites for U(VI) ions. The optimum pH for U(VI) uptake by PCC was at pH 5 (Fig. 3). The decreasing in the uptake of U(VI) after pH 5 is due to the formation of stable complexes UO2CO3, [UO2CO3]2−[15]. On changing the initial concentration of U(VI) solution from 25 to 100 mg/L, the sorption capacity of U(VI) increased from 7.22 mg/g to 14.21 mg/g. The uptake of U(VI) was studied using different doses of PCC (0.3, 0.6, 0.9 and 1.2 g). The results indicated that the percent of sorption increased with increase PCC dose due to the increasing of sorption sites. The effect of temperature on the sorption of was studied from 301 to 333 K. The results indicate that increasing the temperature of the solution decreasing the removal of U(VI) indicating that the process is exothermic in nature. The values of correlation coefficients, (r2) in the results of kinetics data (Table 2), showed good compliance with the pseudo second-order kinetic model than pseudo first-order kinetic model (Fig. 4).
Table 1. Parameters of batch sorption of U(VI) onto PCC.
Parameter
Removal efficiency (Re %)
q (mg/g)
pH:
3
85.55
3.56
(Condition: 25 mg L−1, 0.3 g, 3 h, 303 K)
4
93.20
3.88
5
98.26
7.22
6
95.14
4.07
7
90.21
4.03
8
70.03
3.13
10
42.56
1.77
Initial concentration (mg/L):
25
98.26
7.220
(Condition: pH = 5, 0.3 g, 60 min, 303 K)
50
98.39
8.199
75
98.50
12.31
100
85.32
14.21
Adsorbent dose (g):
0.1
30.16
11.31
(Condition: 75 mg L−1, 60 min, pH = 5, 303 K)
0.3
98.50
12.31
0.6
98.50
6.156
0.9
98.50
4.104
1.2
98.50
3.07
Temperature (K):
303
98.50
12.31
(Condition: 75 mg L−1, 60 min, 0.3 g, pH = 5)
313
91.58
11.44
323
80.38
10.04
333
55.29
6.911
Fig. 3. Effect of pH on the sorption of U(VI) onto PCC at different times.
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http://www.sciencedirect.com/science/article/pii/S1876610215021840
H stretching. The band at 1615 cm−1 was assigned to the asymmetric stretching of 
O deformation band (1384 cm−1) in pectin remained constant while shifts occurred in the wave numbers 3417, 2920 and 1615 cm−1 indicating an interaction of these functional groups with sorbed U(VI) and also the appearance of wave number 1738 cm−1 in the U(VI) loaded spectra may indicate the interaction of this group with U(VI) ion.
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