Pd(II) Adsorption and Desorption Characteristics of Nitrogen Functionalized Chitosan Derivatives
5.3 Pd(II) adsorption characteristics of CH-ME and CH-TETA derivatives
5.3.1 Effect of adsorption parameters on batch adsorption characteristics
Figs. 5.1a and 5.1b illustrate the effect of pH on the Pd(II) adsorptive performance of CH-ME and CH-TETA derivatives, respectively. For both cases, the experiments were conducted at 720 min contact time, 1 g L−1 adsorbent dosage, and 50 mg L−1 initial Pd(II) concentration, which were set based on few trails and prior experience. For a variation in initial solution pH from 0.5 to 10, the adsorption capacity of CH-ME enhanced from 28.07 mg g-1 (pH 0.5) to 36.38 mg g-1 (2 pH)
followed by a steady decline to 28.32 mg g-1 (pH 10). Corresponding metal removal varied from 56.14–72.76 % (pH 0.5−2) and thereafter declined to 56.64 % (pH 10). For the CH-TETA, the adsorption capacity values varied from 31.85−41.25 and 41.25−32.88 mg g-1 for corresponding pH variations from pH 0.5−2 and pH 2−10, respectively. Therefore, the figures illustrate that the optimal removal efficiency and adsorption capacity were obtained at pH 2. There are 72.76% and 36.36 mg g-1 for CH-ME and 82.50% and 41.25 mg g−1 for CH-TETA derivatives.
0 2 4 6 8 10
27 30 33 36 39
Initial solution pH Adsorption capacity (mg g-1 )
(a) 50
55 60 65 70 75
Removal (%)
60 65 70 75 80 85
(b)
Removal (%)
0 2 4 6 8 10
32 34 36 38 40 42 44
Initial solution pH Adsorption capacity (mg g-1 )
Fig. 5.1: Effect of pH on Pd (II) adsorption characteristics of (a) CH-ME and (b) CH-TETA derivatives.
For low pH range (1−6) and high chloride concentration (33−500 g L-1), Pd(II) mostly exists in PdEDTA2- form and protonation effect will be dominant, i.e., significantly higher influence of H+ will be prevalent on –NH2 groups (Fujiwara et al. 2007, Ramesh et al. 2008, Zhou et al. 2009).
Therefore, –NH3+ groups exist significantly on the CH-ME and CH-TETA derivatives. These abundant groups will have a stronger affinity to attract Pd(EDTA)-2 and therefore facilitate significant adsorption at a pH of 2. Based on this hypothesis, it can be inferred that Pd(II) adsorption on CH-ME and CH-TETA derivatives is very likely to occur due to electrostatic attraction and ion-exchange in the ELP solutions, where in the electrostatic interaction between
protonated amine groups on the CH-ME and CH-TETA derivatives and noble metal ions is according to the following reactions (Eqs. 5.1 and 5.2):
As shown, for both cases of CH-ME and CH-TETA, for an optimal pH of 2, the PdEDTA-2 gets functionally bonded to protonated -NH2 groups prevalent in the resin structure.
Figs. 5.2a and 5.2b illustrate the variation in % removal and adsorption capacity (mg g−1) with adsorbent dosage for both CH-AZ and CH-TETA derivatives. These data were obtained at pH 2 for CH-ME and CH-TETA, 720 min contact time, and 50 mg L−1 Pd(II) initial concentration. As shown, for CH-ME derivative, the adsorption capacity reduced from 141.06 to 19.71 mg g-1and removal % increased from 56.43 to 78.88 % for a variation in adsorbent dosage from 10 to 100 mg. Similarly, for CH-TETA derivative, the corresponding adsorption capacity reduced from 162.26 to 20.92 mg g-1, and removal % increased from 64.90 to 83.70 %. Based on the trends, the
maximum removal % of 78.79 and 82.49 and relevant adsorptive capacities of 21.89 and 41.25 mg g−1 were obtained at an optimal equilibrium dosage of 1.8 and 1 g L−1 for the CH-ME and CH- TETA derivatives, respectively. The enhancement in Pd(II) removal efficiency with adsorbent dosage was due to the enhancement in the number of the active sites available for Pd(II) adsorption.
Simultaneously, the metal uptake reduced due to the reduction of metal ions available per unit gram of adsorbent.
0 20 40 60 80 100
0 30 60 90 120 150
Adsorbent dosage (mg) Adsorption capacity (mg g-1 )
(a) (b)
55 60 65 70 75 80
Removal (%)
0 20 40 60 80 100
0 30 60 90 120 150 180
Adsorbent dosage (mg) Adsorption capacity (mg g-1 )
65 70 75 80 85
Removal (%)
Fig. 5.2: Effect of adsorbent dosage on Pd(II) adsorption characteristics of (a) CH-ME and (b) CH- TETA derivatives.
Figs. 5.3a and 5.3b depict Pd(II) adsorptive performance of CH-ME and CH- TETA derivatives for a variation in contact time. These experiments were conducted at pH 2, 1.8, and 1 g L−1 dosage of CH-ME and CH- TETA derivatives, respectively along with 50 mg L−1 initial Pd(II) concentration. As shown, for CH-ME derivative, the adsorption process is rapid from 5 to 840 min and reaches equilibrium at 840 min. Similarly, for CH-TETA derivative, the adsorption process is rapid from 5 to 300 min and reaches saturation at 300 min. Based on the profiles, the maximum adsorption capacity was obtained at 840 (22.22 mg g−1) and 300 min (40.76 mg g−1) with corresponding metal removal % of 79.98 and 81.52 for CH-ME and CH- TETA derivatives, respectively.
0 200 400 600 800 1000 1200 8
10 12 14 16 18 20 22
Contact time (min) Adsorption capacity (mg g-1 )
42 49 56 63 70 77 84
Removal (%)
48 54 60 66 72 78 84 90 96
(b)
Removal (%)
(a)
0 200 400 600 800 1000
20 25 30 35 40 45
Contact time (min) Adsorption capacity (mg g-1 )
Fig. 5.3: Effect of contact time on Pd(II) adsorption characteristics of (a) CH-ME and (b) CH- TETA derivatives.
Figs. 5.4a−5.4d present the effect of initial Pd(II) concentration and temperature on the adsorptive performance of CH-ME and CH- TETA derivatives with synthetic ELP solution. These experimental investigations were conducted for 50−300 mg L−1Pd(II) solution concentration and 298−333 K temperature and for fixed choice of pH 2, 1.8 g L−1 (CH-ME derivative) and 1 g L−1 (CH- TETA derivative) adsorbent dosage, and 840 min (CH-ME derivative) and 300 min (CH- TETA derivative) contact time. At 298 K, the Pd(II) capacity and removal % varied from 22.64−85.58, 40.89−117.08 mg g−1 and 81.50−51.35, 87.05−39.03 %, for CH-ME and CH- TETA derivatives, respectively. Thus, higher concentrations detrimentally influenced metal removal %.
This is in agreement with theoretical insights. The temperature effect on noble metal adsorptive characteristics for both resins is in agreement with the theoretical insight i.e., lower % removal values have been obtained at higher temperature (70.04-45.14 % and 81.78–29.23 % at 333 K for a variation in initial solution concentration from 50−300 mg L-1) than those obtained at lower temperature (81.50-51.35 % and 87.05–39.03 % at 298 K for a variation in initial solution concentration from 50−300 mg L-1) for CH-ME and CH- TETA derivatives, respectively.
50 100 150 200 250 300 40
50 60 70 80 90 100 110 120
50 100 150 200 250 300
20 30 40 50 60 70 80 90
50 100 150 200 250 300
10 20 30 40 50 60 70 80 90
50 100 150 200 250 300
45 50 55 60 65 70 75 80 85
-1 Adsorption capacity (mg g) (d)
Initial Pd(II) concentration (mg L-1)
298 K 313 K 333 K
(c)
Removal (%)
Initial Pd(II) concentration (mg L-1)
298 K 313 K 333 K
Adsorption capacity (mg g-1 )
Initial Pd(II) concentration (mg L-1)
298 K 313 K 333 K
(a) (b)
Removal (%)
Initial Pd(II) concentration (mg L-1)
298 K 313 K 333 K
Fig. 5.4: Effect of temperature and initial Pd(II) concentration on adsorption capacity (a, c) and removal % (b, d) for CH-ME (a, b) and CH-TETA (c, d) derivatives.