Pd(II) Speciation Characteristics of Synthetic ELP Solutions and Adsorptive Efficacy of Raw Chitosan
3.4 Pd(II) adsorption characteristics of chitosan-ELP solution system
3.4.3 Equilibrium, kinetic and thermodynamic model parameters
The fitness plots for alternate kinetic models have been presented in Figs. 3.5a−3.5c. Relevant parameters obtained from the fitness plots are summarized in Table 3.1. As presented, only for the pseudo-second-order model, the regression coefficient value was higher than 0.95. Further, the evaluated adsorption capacity based on the pseudo-second-order model was in agreement with the measured adsorption capacity. Therefore, pseudo-second-order model has been inferred to be the best fit to represent the kinetics of Pd(II) adsorption from synthetic ELP solutions onto chitosan adsorbent, which is based on the assumption that the rate limiting step could be the chemisorption involving valency forces through sharing or exchange of electrons with the N atom of adsorbent.
The fitness of the pseudo-second-order model is also justified with the FTIR analysis of Pd(II) adsorbed chitosan that inferred strong Pd(II) chemical interaction with the prevalent functional groups of the chitosan. Also, Fig. 3.5c conveys that the intra-particle diffusion plot indicates the multi-linear plot in three distinct phases, and the fit plot did not meet the graph at its origin. Hence, intra-particle diffusion does not exist as a rate-limiting step Pd(II) adsorption process. Similar model fitness has been indicated for the Pd(II) adsorption kinetics of chitosan, chitosan-coated activated carbon, glutaraldehyde cross-linked chitosan, L-lycine modified cross-linked chitosan, glycine modified cross-linked chitosan and thiourea modified chitosan microspheres (Ruiz et al.
2000, Fujiwara et al. 2007, Ramesh et al. 2008, Zhou et al. 2009).
t (min)
0 100 200 300 400 500
ln (Qe-Qt)
-3 -2 -1 0 1 2 3
Experimental data (298 K) Linear fitting of 298 K (R2=0.715)
(a)
t (min)
0 200 400 600 800
t/Qt
0 10 20 30 40
Experimental data (298 K) Linear fitting of 298 K (R2=0.997)
(b)
t0.5
5 10 15 20 25 30
Qt
16 18 20 22 24 26
Experimental data (298 K) Linear fitting of 298 K (R2=0.887)
(c)
Fig. 3.5: Fitness of alternate Pd(II) adsorption kinetic models: (a) Pseudo-first order kinetic model, (b) Pseudo-second order kinetic model, and (c) Intra-particle Diffusion model.
Table 3.1: Regressed kinetic parameters to represent Pd(II) adsorption kinetics of the chitosan- ELP system.
Type of model
Experimental capacity (Qexp, mg g-1)
Qe
(mg g-1)
K1
(min-1)
K2
(g mg-1 min-1)
Kid
(min mg g-1) C R2
Pseudo-first-order model
24.56
9.34 -7×10-3 - - - 0.715
Pseudo-second-
order model 25.64 - 0.019 - - 0.997
Intra-particle
diffusion model - - - 0.304 16.82 0.887
Figs. 3.6a and 3.6b respectively present the fitness plots of Langmuir and Freundlich isotherm models to represent measured Pd(II) equilibrium adsorption data. Relevant model parameters and the coefficient of correlation (R2) have been presented in Table 3.2. The figure and Table confirm that the Langmuir isotherm model best fits with the measured batch equilibrium data. Table 3.2 confirms upon a RL value of 0-1 to affirm upon favourable adsorption of Pd(II) from synthetic ELP solutions using chitosan adsorbent.
Equilibrium concentration (mg L-1)
0 50 100 150 200
Ceq/Qe
0.5 1.0 1.5 2.0 2.5 3.0 3.5
Experimental data (298 K) Experimental data (313 K) Experimental data (333 K) Linear fit of 333 K (R2
=0.997) Linear fit of 313 K (R2
=0.999) Linear fit of 298 K (R2
=0.999)
(a)
log Ceq
1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4
log Qe
1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0
Experimental data (298 K) Experimental data (313 K) Experimental data (333 K) Linear fitting of 298 K (R2=0.966) Linear fitting of 313 K (R2=0.977) Linear fitting of 333 K (R2=0.981)
(b)
0.0030 0.0031 0.0032 0.0033 0.0034 0.8
1.0 1.2 1.4 1.6
Von't Hoff plot
Linear fitting (R2=0.974)
ln K c
1/T (K-1) (c)
Fig. 3.6: Fitness of alternate models to represent Pd(II) equilibrium and adsorption thermodynamics: (a) Langmuir model (b) Freundlich model and (c) Van’t Hoff model.
Table 3.2: Regressed equilibrium parameters to represent Pd(II) adsorption equilibrium of chitosan-ELP system.
Temperature (K)
Langmuir parameters Freundlich parameters Qo
(mg g-1)
b
(L mg-1) R2 RL Kf n R2
298 90.91 0.033 0.999 0.102-0.446 10.14 2.506 0.966 313 83.33 0.029 0.999 0.102-0.405 8.71 2.427 0.977 333 76.92 0.026 0.997 0.116-0.439 7.94 2.513 0.981 Fig. 3.6c depicts the thermodynamic behaviour of the adsorption process, and Table 3.3 summarizes relevant parameters obtained for the Vant Hoff’s model. The table affirms negative
G values to indicate upon spontaneous Pd(II) adsorption and negative H values to affirm upon exothermic adsorption process.
Table 3.3: Regressed thermodynamic parameters for Pd(II) adsorption equilibrium of the chitosan-ELP system.
Temperature
(K) Kc ΔHº
(kJ mol-1)
ΔSº (J K-1 mol-1)
ΔGº
(kJ mol-1) R2
298 2.40
-15.61 -39.39
-3.875
0.974
313 2.99 -3.284
333 3.59 -2.496
Comparative assessments of obtained results with those presented in the relevant literature are summarized in Table 3.4. As summarized, only one literature refers to adsorption characteristics of Pd(II) using chitosan, and this literature refers to aqueous solutions (Sharififard et al. 2013). All other materials refer to chelating resins, which are more complex materials in comparison to chitosan in the context of their natural abundance. While the removal% for the literature reported chitosan (Sharififard et al. 2013) was 93% for an adsorbent dosage of 10 g L−1, the removal
efficiency for the synthetic ELP–chitosan system in this work has been 78.68% for an adsorbent dosage of 1.6 g L−1. Thus, it is apparent that the obtained results in this work are comparable with those reported in the literature and, the solution complexity did not have a strong influence on the Pd(II) adsorption characteristics of the chitosan-ELP system.
Table 3.4: Efficacy of Pd(II) adsorption and desorption efficiencies of chitosan with respect to the best available data in the literature.
Adsorbent name Contaminants
Adsorption Desorption
Source Adsorbent
dose (g L-1)
pH
Adsorption capacity (mg g-1)
Eluent Desorption (%)
L-lysine modified cross-linked chitosan
Pd(II) Au(III)
Pt(VI)
3.33 2 109.47
0.7 M Thiourea–
2 M HCl
99.98 Fujiwara et al. 2007
Thiourea modified chitosan microspheres
Pd(II)
Pt(VI) 3.33 2 112.4
0.5 M EDTA–
0.5 M H2SO4
98.38 Zhou et al.
2009
Glutaraldehyde cross-
linked chitosan Pd(II) 0.15 2 180 - - Ruiz et al.
2000 Glycine modified
cross-linked chitosan
Pd(II) Au(III)
Pt(VI)
3.33 2 120.39
0.7 M Thiourea–
2 M HCl
98.27 Ramesh et al. 2008 Chitosan
Pd(II)
Pt(VI) 10 2
62.5 - -
Sharififard et al. 2013 Activated carbon
coated with chitosan 43.48 -
Ethylenediamine modified magnetic chitosan nanoparticles
Pd(II)
Pt(VI) 0.5 2 138
0.4 M HNO3–1.0
M Thiourea
97.3 Zhou et al.
2010
chitosan
Pd(II) Na2EDTA
NH4OH
1.6 6 90.91 2 N
NaOH
6.31 Present work