Nuclear Science and Technology, Vol. 3, No. 4 (2013), pp. 12-19
Adsorption of U(VI) ion by modifiled chitosan flakes: optimization of process parameters and thermodjrnamic studies
Ho Thi Yeu Ly'', Nguyen Mong Sinh^
'HCM City University of Technical Education,
^Lam Dong Union of Science & Technology Associations.
'Corresponding author: yeulyvD@gmail. com (Received 7 October 2013, accepted 25 January 2014)
Abstract The cross-linking chitosan modified by citric acid has been studied for sorption of U(VI) from aqueous solution in a bath system. The effects of parameters such as pH, initial U(VI) concentration, contact time and temperature on the sorption of U(VI) were analyzed using response surface methodology (RSM). The Minitab was used to conduct the regression analysis and the analysis of variance of the U(VI) adsorption by modified chitosan. The U(VI) concentration of 100 mg/L, pH 4.2, temperamre of 313 K and contact time of 211 min were determined as optimum conditions for U(V1) adsorption by acid citric grafted chitosan (C-GCh). The maximum efficiency of U(VI) removal by C- GCh was 97,35 % at the optimum sorption conditions. Thermodynamic parameters such as AG, AH, AS for U(VI) adsorption process in aqueous solution were also determined.
Keywords: response surtace methodology (RSM), U(VI), adsorption, cross-linked chitosan, citric acid
L INTRODUCTION
Response-surface methodology (RSM) comprises collection of mathemathical and statistical techniques for experimental design with aiming to explore the relationships between several explanatory variables by experimental methods [1, 2. 3]. Typically, this involves several experiments models, using the results of one experiment to provide direction for what to do next [4]. This next action could be to focus the experiment around a different conditions, or to collect more data in the current experimental region in order to fit a higher-order model or confirm what we seem to have found.
The application of statistical experimental design techniques in adsorption process development can result in improved product yields, reduced process variability, closer confirmation of the output response to nominal and target requirements and reduced
development time and overall costs. This methodology is widely used in chemical engineering, notably to optimize the adsorption process [1,3,4]. In this study, the combined effect of pH, initial U(VI) concentration, temperature and the contact time on U(VI) removal from aqueous solution by modified chitosan has been investigated using Box- Behnken design (BSD) in RSM. The pseudo- first-order and pseudo-second-order models were applied to study the kinetics of U(VI) adsorption onto C-GCh. Thermodynamic data were interpreted using Vant Hoff and Clausius Clapeyron equations.
n. MATERIAL AND METHODS A. Materials
The chitosan in flake form with deacetylation degree of 87% and molecular weight of 10^ - 10^ p a ) was supplied from Research and Development Center for
©2013 Viemam Atomic Energy Society and Vietnam Atomic Energy Institute
Radiation Technology (VINAGAMMA Center). Glutaraldehyde, citric acid, U(VI) and the other reagents were supplied from Merck Company. All solutions were prepared with double distilled water. The citric acid grafted chitosan (C-GCh) were prepared by the modified chitosan that is chemical crosslinked with glutaraldehyde according to the procedure reported by Nguyen Van Sue and Ho Thi Yeu Ly [5. 6]. The polymer samples were ground and screened in 0.2 - 0.45 mm [5, 6].
B. E:q)erimental design
HO THI YEU LY, NGUYEN MONG SINH
In order to clarify the effects of pH, initial concentration of U(VI), temperature and the contact time on U(VI) ) removal capacity (%), batch experiments were conducted based on the BBD. The coded values of the process parameters were determined by the following equation
X, = -
Ax (1)
where Xi- coded value of the i variable, X, - encoded value of the i"" test variable and Xo - encoded value of the i**" test variable at center point.
The range and levels of individual variables are given in Table 1. The experimental design is given in Table 2, along with experimental data and predicted responses. The regression analysis was performed to estimate the response fijnction as a second order polynomial [2, 3].
P„ -h > P- Xf -F Xt + The Box-Behnken experiment design
(BBD) was applied for determination the optimal conditions for removal of U(VI) ion from aqueous solutions by C-GCh. Briefly, 50 ml solution of U(VI) for concentrations are given in Table 1 and 0,1 g C-GCh were added in 150 ml a Erleimieyer flask for each experiment. The mixture was agitated in an incubated orbital shaker at . the desired temperature for predetermined time intervals.
After adsorption process, the solution was filtered. The U(VI) concentration in solution was determined by UV-Vis spectrophotometer at 652 nm. Vml U(VI) solution sample, 5ml chloroacetic acid (ClCH2COOH)-sodium acetic (CH3C00Na) buffer solution (pH 2.5) and 1.0ml 0,1 % Arsenazo-III aqueous solution were added to glass flask, respectively, the final solution volume was filled up to 25 ml by adding deionized water.
Table I: Levels ofdifferent process variables in coded and un-coded form for adsorption of U(VI)
1 = 1 ft-1 k
Z ZP,;X,)^ (2)
Where Y is the predicted response, p,, Pj Py are coefficients estimated from regression.
They represent the linear, quadratic and cross products of X|, xi, xson response.
Variable pH Concentration, mg/L
Temperature, K Contact time, min
Code A B C D
Levels -1
3 100 293 120
0 4 120 303 180
+1 5 140 313 240
ADSORPTION OF U(VI) ION BY MODIFILED CHITOSAN FLAKES:
A statistical program Minitab 16 was used for regression analysis of the obtained data and estimation coefficient of the regression equation. The equations were validated by the statistical tests based on the analysis of variation (ANOVA) method.
Response surfaces were drawn to determine the individual and interactive effect of the test variable on the % removal of U(VI). The optimal values of the test variables were fist obtained in code units and then converted to the uncoded units.
m . RESULTS AND DISCUSSION A. Experimental design and fitting of quadratic model
To examine the synergistic effect of four different independent variables, on U(VI) adsorption by C-GCh, 27 experiments were performed and predicted values of removal of U(VI) were given in Table 2. Regression analysis was performed to fit the response fiinctions, i.e. percentage adsorption of U(Vl).
The regression models developed to represent for response functions of pH (A), concentration (B), temperature (C) and contact time (D). An empirical relationship between the response and input variables has revealed the second order polynomial coefficient for each term of the equation determined through multiple regression analysis (in coded terms).
Hun 1 I i i 5 S 7 i
)
10 U 12 13 14
A 0 1 0 0 0 -1 -1 1 0 0 1 0 -1 -1
Table H: Experimental conditions and observed response
B 0 0 1 -1 0 0 1 0 -1 0 1 -1 0 0
c
1 1 1 -1 1 1 0 0 1 0 0 0 0 -1
J
-1 0 0 0 1 0 0 1 0 0 0 -1 -1 0
U(V1) removal (%)
82,84 81,46 88,35 78,83 93,86 71,96 67,85 76,67 94,30 86,45 70,08 85,29 60,37 58,55
82,17 81,23 88,60 79,84 92,76 72,05 66,51 77,53 95,97 86,45 69,57 84,19 60,76 58,41
15 16 17 18 19 20 21 22 23 24 25 26 27
A 0 0 0 1 -1 -1 0 0 0 0 1 0 1
B -1 0 0 0 0 -1 1 1 1 0 -1 0 0
values of BBD C
0 -1 -1 0 0 0 0 0 -1 0 0 0 -1
D 1 -1 1 -1 1 0 -1 1 0 0 0 0 0
U(V1) removal (%) Exp.
93,76 62,51 76,32 63,11 68,81 74,45 69,54 86,170 69,06 86,69 80.12 86,22 59,24
Theo.
92,10 62,73 76,11 62,98 70,19 74,08 70,83 86,90 68,64 86,45 80,58 86,45 58,78
Table lU: Analysis of variant (ANOVA) for response surface quadratic model Source
Constant A B
Coef 86,4533 2,3908 -4.6417
T 123,880 6,852 -13.302
P 0,000 0,000 0,000
HO THI YEU LY. NGUYEN MONG SINH
c
D A « A B x B C x C D x D A x B A - C A x D B x C B x D C x D
9,0217 5.9942 -14,7054 0,9358 -4,1292 -3,8829
•0.8600 2.2025
1.2800 0,9550 2,0400 -0.6975
25.854 17.178 -28.095 1,788 -7.889 -7,419 -1,423 3,644 2,118 1.580 3.375 1,154
0.000 0.000 0.000 0,099 0.000 0.000 0,180 0,003 0,056 0,140 0,006 0,271
Results of ANOVA for the removal of U(VI) ion (%) were shown in Table 3. The larger of T-va!ue and the smaller the magnitude of P-value, the more significant the corresponding coefficient is. Values of P less than 0.05 indicates that the model term is statistically significant. From the P value it has been found that among the test variables used in the study, A, B, C. D, A", C". D", AC. BD are significant model among the test variables of this study. Therefore, we have an equation as follow:
U(VI) removal (%) = 86.45 + 2.39A - 4.64B + 9.02C + 5.99D - 14.7IA-- 4 . 1 3 0 " - 3.88D- +
2.20AxC + 2.04BxD (3) The predictive regresion coefficient R"-
96.82 % is suitable with the adjusted R-- 98.80
%. The fit of the model was also expressed by the coefficient of regression R" - 99.45 %, indicating that 99.45 % of the variability in the response, could be explained by the quadratic modell. This implies that the prediction of experimental data Is rather accurate.
B. Response sur&ce estimation for the maximum removal capacity of U(VI)
To investigate the interactive effect of two factors on the removal of U(VI). The plots of the main interactions which effect the responses, i.e. % removal of U(VI) significantly were presented in Fig. 1. A surface plot is a two dimensional representation of the response for selected factors. Optimum conditions for maximum percentage removal of U(VI) using C-GCh were obtained by using "The Solver Add-in of Microsoft Excel".
The optimum values with targeting the process parameters within the range defmed in Tab.l obtained by substituting the respective coded values of variables are: pH - 4.2, initial U(VI) concentration - 100 mg/L, temperature - 313 K. contact time - 210 min. At this condition the maximum percentage U(VI) removal was 97.31 %, which was in close agreement with those obtained by optimizing the regression model (97.35 %) Eq. (3).
ADSORPTION OF U(VI) ION BY MODIFILED CHITOSAN FLAKES: .
Fig. 1: shows the BBD countour profiler of pH (A) and concentration (B) by (a); concentration (B) and temperature (C) by (b); concentration (B) and contact time (D) by (c); temperature (C) and contact time (D).
C. Thermodynamic parameter
In this work, the effect of temperature on adsorption capacity of U(VI) ions onto C-GCh was studied in the range of 293 - 313 K. The initial U(VI) concentrations of 80 - 300 mg/L were studied, while the pH, amount of adsorbent (C-GCh) and contact time were fixed at 4.2; 0.05 g and 600 min, respectively. Fig 2.
Shows that removal efficiency of U(VI) increases with the increase of temperature from 293 - 313 K is attributed to the existence of chemical adsorption during the adsorption process. Thermodynamic parameter such as in the standard free energy (AG"), enthalpy (AH°) and entropy (AS") were calculated using following equations [7, 8]:
AG"=-RTlnKc InK, = ^ - ^
(4)
(5)
Where KL is the Langmuir constant related to the energy of biosorption, R is the gas constant (8.314 J/mol K), and T is the solution temperature (K). The values of AH"
and A S " can be calculated, respectively, from the slope and intercept of the van't Hoff plot of InKL versus 1/T. The values of AH", AS° and AG parameters were summarized in the Table IV. The positive value of A H " confirms biosorption process of U(VI) is endothermic.
The negative values of AG° at various temperatures indicate the feasibility and spontaneity of the biosorption process.
The increase of AG" with temperature indicates that the biosorption is more favorable at high temperatures. The positive value of AS"
shows the affinity of C-GCh for U(VI) and it also confirms an increase in the randomness at the solid-solution interface during the biosorption process.
HO THI YEU LY, NGUYEN MONG SINH 120
100
| 8 0
1 60
£ 4 0
^ 2 0 0
ttlte^^
- » - 2 9 3 K ^ ^ | ^ ^ ; ; ^ _ _ _ _ ^ - • - 3 0 3 K ^ ^ * = = a a - * - 3 1 3 K
' ''^m^.md'^
,
6 0 0 Fig. 2: Effect of temperature on U(VI) removal Table IV: Change of thermodynamic with temperature
Temperature (K) 293 303 313
q™
(mg/g) 208.68 201.53 208.61
KL (L/mg)
0.12 0.83 10.06
X2 8.43 14.44 48.64
AG (kcal/
mol) -2.73 -4.19 -5.65
AH (kcal/
mol)
40.05
AS (cal/ mol.K)
146.01
Results in Table IV also showed that the dependence of maximum capacities (q,n) of C- GCh for U(VI) were 208.68, 201.53, 208.61 mg/g at 293, 303 and 313K, respectively. The values of KL were increased with increasing the temperature of solution. High KL values suggested high adsorption affinity [7, 8].
D. AdsOTption kinetic
Kinetic models have been used to investigate the sorption mechanism in order to determine the stage which plays a decisive role to the adsorption process. In this research, the adsorption kinetic data obtained from different concentrations of U(VI). The pH. temperature and adsorbent amount were fixed at pH 4.2, 302 ± 1 K and 0.05g. Kinetic models used for fitting experimental data are followed pseudofirst- order (6) and pseudo-second order models (7) [9, 10].
Log(q. - q.) = logq, - ^ t (6)
- = f^ + -t (7) where, k,, ki are the pseudo-first-order
(min"') and pseudo-second-order constants (g/mg.min), q, , qe are amount of U(VI) adsorbed at time t and equilibrium (mg/g).
The plots of log(qe-q,) from Eq.(6) and t/qt from Eq.(7) against t (min) revealed a linear relationship. From the plots, one can calculate ki, ki and predict q^. The linear plots for the pseudo-first-odder and the pseudo-second-order models at different initial concentration of U(VI) are presented in Fig.3 and 4, respectively. The values of the parameters and correlation coefficients are also presented in Table 5.
ADSORPTION OF U(VI) ION BY MODIFILED CHITOSAN FLAKES:
Table V: Adsorption kinetic model rate constants at different initial concentrations of U(VI) adsorbate 4 I
Co (mg/L) qe. exp Pseudo first
order
Pseudo second order k, R- qe. cal k.
R- qe. cal
80 79.95 0.0097 0.9227 7.94 0.00060
0.9995 84.03
100 99.80 0.0056 0.9673 6.74 0.00040
0.9993 104.17
150 144.62 0.0114 0.9424 118.72 0.00004 0.9994 151.52
400 Fig. 3. Plots of pseudo-first-order equation for U(VI) adsorption
p
~Sb
t/qt (min
10 - 8 - 6 - 4 - 2
(
• U{VI)80mg/L
• U(VI) 100 mg/L
*U(VI) 150 mg/L ^ ^
) 200 400
t ,min 600 800 Fig. 4. Plots of pseudo-second-order equation for U(VI) adsorption As one can see from the Fig. 3, the
plots of U(VI) adsorption onto the C-GCh did not followed the pseudo-first-order model with large discrepancies, whereas it followed the pseudo second-order model as indicated in the Fig.4. The obtained parameters from that model were rather uniform for all investigated concentrations (80 to 150 mg/L), indicating
that the experimental data were good fitted with the pseudo-second-order model. Thus, the U(VI) adsorption of C-GCh obeyed the psedo-second-order better than the pseudo- first-order model. These results are very useful for removal of U(VI) from polluted water by using C-GCh.
18
HO TH! YEU LY. NGUYEN MONG SINH IV. CONCLUSIONS
This study is aiming to determine the optimum conditions for removal of U(VI) ion simultaneously from aqueous solutions by C- GCh through thermodynamic and kinetic parameters of the adsorption process. The results showed that the pH 4.2, U(VI) initial concentration of 100 mg/L, temperature of 313K and contact time of 211 min as the optimum conditions for removal of U(VI) from aqueous solutions, and at these conditions the maximum removal capacity of U(VT) was 97.35 %. Thermodynamic calculations indicate that the adsorption of U(VT) onto C-GCh was spontaneous and endothermic. The maximum adsorption capacities were 208.68, 201.53, 208.61 mg/g for 293, 303 and 313 K, respectively. Results of the adsorption kinetics study revealed that pseudo-second- order equation provided the best correlation to the data.
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