Kinetics and mechanism of stripping of Mn(II)–D2EHPA complex by sulphuric acid solution

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The kinetics of stripping of Mn(II)–D2EHPA chelate (MnA 2.HA.H 2A 2) existing in kerosene phase by aqueous sulphuric acid solutions at constant sulphate ion concentration of 0.50 M have been investigated by the constant interfacial area stirred (Lewis) and non-stirred (Hahn) cells. Both pseudo-rate constant ( q) or (rate / area) and flux ( F) methods of the rate data treatment have been applied. The empirical rate equations have been derived. Results have been compared among themselves and other published works on Mn(II)–D2EHPA chelate stripping kinetics. Rate constants obtained from ( q) or (rate / area) and ( F) methods differ in magnitude and units and an explanation of this has been given. An analysis of rate equations obtained from the Lewis cell experimentation ( F b = 10 − 4.1 [MnA 2.HA.H 2A 2] (o)(1 + 0.005[H +] (i) − 1 ) − 1 (1 + 7.94[H 2A 2] (o) 0.5) − 1 ) suggests that the process is chemically controlled at lower aqueous acidity and higher free D2EHPA concentration regions; whereas, it is diffusion-controlled at higher aqueous acidity and lower free D2EHPA concentration regions. In the investigated D2EHPA concentration and aqueous acidity regions, majority of data fall in the intermediate controlled region. The suggested mechanisms are supported by the values of activation energy ( E a). In the kinetic regime, the reaction step: MnA + → Mn 2+ + A − occurring in the bulk aqueous phase is the slowest step which proceeds through S N2 mechanism as indicated by the comparable higher negative value of Δ S ± in the stated condition. In Hahn cell technique, the empirical rate equation is: F b = 10 − 5.9 [MnA 2.HA.H 2A 2] (o)(1 + 0.0025[H +] (i) − 1 ) − 1 (1 + 2.8[H 2A 2] (o) 0.5) − 1 . Analysis of this equation together with the E a value of 12–42 kJ/mol indicate that the stripping process is completely diffusion controlled in the low pH and extractant concentration region and in other parametric condition, that is intermediate controlled. For both cells, the ratio of ( k f) to ( k b) equals almost to K ex of 10 − 2.44 obtained from the distribution study.