Kinetics of backward extraction of Mn(II) from Mn-D2EHP complex in kerosene to hydrochloric acid medium using single drop technique

The extraction equilibrium constant, K-ex for the extraction of Mn(II) from hydrochloric acid medium by di-2-ethylhexylphosphoric acid (D2EHPA or H(2)A(2)) in kerosene (mostly aliphatic) has been determined to be 10(-2.16). The rate of backward extraction of Mn(II) from Mn-D2EHP complex dissolved in kerosene by hydrochloric acid has been measured by the single rising drop technique and it is found that the flux equation at 30 +/- 1 degrees C is: J (kmol/m(2)s) = 10(-4.08)[Mn-D2EHP]((o))(1 + 0.011[H+](-1))(-1)(1 + [H(2)A(2)]((o))(0.5))(-1). Analysis of this rate equation suggests that the process is almost chemically controlled at lower aqueous acidity and higher free D2EHPA concentration regions; whereas it is almost diffusion controlled at higher aqueous acidity and lower free D2EHPA concentration regions. In the investigated D2EHPA concentration (0.04-0.50 kmol/m(3)) and aqueous acidity (pH = 1.0-3.0) regions, the process is intermediate controlled. The activation energy depends on the back-extraction parameters and is of the order of similar to 20-similar to 40 kJ/mol. This low value of activation energy supports the conclusion that the process is intermediate controlled. Temperature dependence data give Delta H+/- and Delta S+/- values of 20-40 kJ/mol and (-180)-(-250) J/K mol, respectively. The high negative value of Delta S+/- suggests that the chemical controlling step occurs via S(N)2 mechanism. (C) 1997 Elsevier Science B.V.