Literature Review
4.6 Adsorption Isotherms The Langmuir isotherm equation
with small deviations for both dyes. It is therefore more likely that the adsorption of Reactive yellow and Reactive red dyes on MLP might take place through a second-order mechanism.
- Earlier also it has been shown by Allen ci al.8' that the adsorption kinetics of the acid dyes are described by a pseudo second-order chemical reaction and that this reaction is significant in the rate-controlling step.
Table 4-4 Diffusion Rate Coefficients for Adsorption of Reactive Red MSB on MLP for Different Concentrations of the Dye Solutions
Dye mg/L
k, min i
R !2
g!mg/min
R
32 5.2x10' 0.981 3.2x10 3 0.974
65 5.8x10' 0.984 7.710 0.980
85 5.9 x10' 0.983 6.5x10 0.985
101 5.7 x10l 0.990 5.8x10 0.987
117 5.8 x10 0.984 4.9 x10' 0.983
4.6 Adsorption Isotherms
5 10 15 20 25 30
Ce mg/L
coefficients can be found. Another coefficient, RL, known as the separation factor such that 0
<R1 < 1 for favorable adsorption, is found from the expression,Ri. = 1/(I+bCe)
3.5 3.0 2.5 2.0 1.5
0.5
0.0
40 Ce (mg/L)
Figure 4-12 Langmuir isotherm for reactive yellow C8G adsorption onto MLP
20
10 /
5-
o
0
Figure 4-13 Langmuir isotherm for reactive red adsorption onto MLP.
35
Table 4-5 Langmuirlsotherm for Reactive Yellow
qrn(mg/g) 67.11 51.81 22.57 13.66 11.93
b(L/mg) 0.034 0.031 0.0283 0.028 0.028
R 0.943 0.946 0.934 0.948 0.934
RL 0.908 0.840 0.708 0.670 0.525
Table 4-6 Langmuirlsotherm for Reactive Red
qn (mg/g) 12.39 9.85 8.25 4.37 2.09
b(L/mg) 16.10 11.59 7.64 3.92 3.62
R 0.990 0.984 0.990 0.990 0.990
R1, 0.089 0.021 0.014 0.05 0.013
The empirical isotherm equation given by Freundlich and useful in describing nonspecific adsorption is
logq = log Kf + fl 1ogC ---(4)
whereq and C have similar meaning as earlier with Kf and n being the Freundlich coefficients. When eq.4 is obeyed, the plot of log qevs log C yields a straight line and both the Freundlichcoefficients could be obtained from the slope and the intercept of the plot. Reactive yellow and reactive red adsorption on MLP followed both Langmuir isotherm (Figure 4-12 and Figure 4-13) and Freundlich isotherm (Figure 4-14 and Figure 4-15) respectively. The Langmuir plots for Reactive yellowhave good linearity, R 0.93 to 0.95, and the Langmuirmonolayer adsorption capacity (qm) decreased from 67.11 toll .93 mg/g for MLP
36
amount varying from 0.5 to 1.0 gIL, whereas for Reactive red have also have good linearity, R-0.99 , and the Langmuir
1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7
log Ce (mglL)
Figure 4-14 Freundlichisotherm for reactive yellow adsorption onto MLP.
2.0 - 1.8 1.6 1.4 1.2
0.8 0.6 0.4 0.2 0.0
00 0.5 LO 1.5 2.0
log Ce (mg/L)
Figure 4-15Freundlichisotherm for reactive red adsorption onto MLP.
2.0 1.8 1.6 1.4 1.2 1.0
'-' 08 O 0.6 0
0.4 0.2 0.0
37
monolayer adsorption capacity (qn) decreased from 12.39 to2.09 mglg for MLP amount varying from 0.5 to 1.0 gIL . The adsorption equilibrium parameter, for Reactive yellow.b.
- - varied from 0.028 to 0.034 Llmg with the increase in MLP amount and similar trend observed for reactive red. The results are summarized in Table 5 and Table 6. In order to predictwhether the adsorption process by MLP is favorable or unfavorable for the Langmuir type adsorption process, the isotherm shape can be classified by a term "R1 ", a dimensionless constant separation factor.284142 The dimensionless parameter, RL, had values in the range of 0.53 - 0.90 for reactive yellow and 0.08 -0.013 for reactive red, consistent with therequirement for a favorable adsorption process defined by 0 <R< 1.
The Freundlich plots have slightly better linearity, R 0.99 for Reactive yellow. The adsorption capacity, K f, showed a decrease from 143.52 to 19.76 LIg with increase in MLP amount from 0.5 to 1.0 gIL. Similar results found for Reactive red dye. The adsorption affinity, n, lies between 0.21 and 0.27 satisf'ing the condition n < I for favorable adsorption.
The valuesof the coefficients are shown in Table 4-7 and Table 4-8.
Table4-717reundlich Isotherm for Reactive Yellow
ii 2.15 2.05 2.04 2.12 1.84
Kt (LIg) 143.52 63.04 55.11 59.18 19.76
R 0.998 0.995 0.998 0.994 0.99
Table4-8Freundlich Isotherm for Reactive Red
n 0.21 0.27 0.23 0.24 0.22
Kf (L/g) 3.23 7.48 13.88 16.6 20.8
R 0.958 0.963 0.96 0.944 0.971
CM
It is to be noted that the applicability of Langmuir equationis limited to the formation of a chemisorbed monolayer on ahomogeneous surface of identical sites that are equally
- availableand energetically equivalent such that each site carries equal number of molecules, which do not interact with one another82 (no adsorbate-adsorbate interactions). On the other hand, the Freundlich equation describes adsorption (possibly multilayer in nature) on a highly heterogeneous surface consisting of non-identical and energetically nonuniform sites. In the present work, although both the equations are obeyed, the Freundlich isotherms have a slightly better correlation coefficient indicating that the MLP surface is heterogeneous in the long- range, but may have short-range uniformity.
39 /
CHAPTER V
Conclusion
It is found from this study that the powder made from mature, dried leaves of the tree Mahogani(SwiteniaMahogani)could be a usefulbiosorbent for removal of dyes from aqueous
medium. The results indicate the following:
(i) Biosorption of the dye was favored in acidic pH range. Still, removal of up to 60 % of the dye could be achieved of the aqueous dye solution.
-- (ii) Adsorption of the dye reached equilibrium at 450 mm. Second-order kinetics was found to be most suitable in describing the biosorption. Intraparticle diffusion and liquid film diffusion might have some influence in controlling the biosorption process.
iii) The second-order rate coefficients had widely different values but the experimental and theoretical q values now match each other with small deviations for both dyes. It is therefore more likely that the adsorption of Reactive yellow and Reactive red dyes on MLP might take place through a second-order mechanism.
(iv) Reactive Yellow and Reactive Red dyes adsorption on MLP agreed with both Langmuir and Freundlich isotherms. The isotherm plots showed that the Freundlich equation gave slightly better linearity than the Langmuir equation (R ) -M.94 for Langmuir plots; 0.99 for Freundlich plots) indicating the MLP surface to be heterogeneous in the long range. but having some amount of uniformity locally. Langmuir monolayer adsorption capacity (qrn) decreased from 67.11 toll.93 mglg for reactive yellow and from 12.39 to2.09 mg/g for reactive red, MLP amount varying from 0.5 to 1.0 g/L.T'he adsorption equilibrium parameter, for reactive yellow, h, varied from 0.028 to 0.034 L/mg with the increase in MLP amount and similar trend observed for reactive red.The adsorption capacity. K1, showed a decrease from 143.52 to 19.76 L/g with increase in MLP amount from 0.5 to 1.0 gIL. Similar results found
40
for reactive red dye. The adsorption affinity, n, lies between 0.21 and 0.27 satisfying the condition n < 1 for favorable adsorption.
41
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