Abstract: The kinetics and mechanism of extraction of Mg(Ⅱ), Ca(Ⅱ), Zn(Ⅱ), Ni(Ⅱ), Cu (Ⅱ) and Pb(Ⅱ) from nitrate medium with 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester (HEHEHP) in octane by means of the single drop method were studied at 298 K.The reaction rate constants (k_f), the activation energies (E_a) and the activation entropies (ΔS≠) were calculated.The experimental results indicate that the extraction process may be controlled by the following interfacial reactions: M²⁺ +HA_(i)=MA_(i)+H⁺(fast)MA_(i)+ 2H₂A_2(O)=MA₂ · 2HA_(O)+HA_(i)+H⁺(slow) and their k_f and E_a vary regularly with the change of hardness of Lewis acid.

Key words: 2-Ethylhexyl phosphonic acid mono-2-ethylhexyl ester, Extraction kinetics, Rule of hard and soft acids and bases(HSAB), Divalent metal ions