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Abstract
Kinetics of Zn(II)-extraction from SO4
2− medium by purified Cyanex 272 (BTMPPA, H2A2) and of Zn(II)-stripping from Zn(II)-BTMPPA complex by H2SO4 solution have been investigated using the single falling drop technique and F-method of data treatment. At 303 K, the empirical extraction and stripping flux equations are: Ff (kmol/m2s) = 10−8.4
[Zn(II)] [H+
]
(int)
−1 ([H2A2
]
(o)
0.5 + 1.07[H2A2
]
(o)) (1+0.708 [SO4
2−
])−1 and Fb (kmol/m2s) = 10−5.25
[Zn-BTMPPA complex]
(o)
[
H+
] [H2A2
]
(o)
−0.5 (1 + 1.5 [HSO4
−]), respectively. In both cases, values of Ea, ΔH±, and ΔS± have been estimated. With the help of flux equations and Ea-values, the mechanisms of extraction and stripping are proposed. The extraction is diffusion controlled in all parametric conditions at high temperature region (htr). At lower temperature region (ltr), it may be under diffusion, intermediate, or chemical control depending on the experimental parameters. The chemical control step is: . High negative ΔS± values suggest that this step occurs via an SN2 mechanism.
On the other hand, the stripping is intermediate control at htr and chemical control at ltr. The chemical controlling step is either: ZnA+ → Zn2+ + A−; or, ZnA+ + HSO4 −→ ZnHSO4 + + A−; depending on [HSO4 −] in the medium. The slow reaction step in stripping occurs via an SN2 mechanism. The equilibrium constant derived from the rate study matches well with that obtained from the equilibrium study.