Study of kinetics of forward extraction of Fe(III) from chloride medium by di-2-ethylhexylphosphoric acid in kerosene using the single drop technique

Abstract

The kinetics of the title system have been investigated as functions of Fe 3+, H + and Cl − concentrations in aqueous phase, D2EHPA (di-2-ethylhexylphosphoric acid, H 2A 2) concentration in organic phase and temperature using the single rising drop technique. The reaction order wrt [Fe 3+] is always unity. The logarithm of flux vs. −log[HCl] plots pass through a minimum at 0.5 M HCl with a limiting slope of −2.4 in the high concentration region (hcr) of HCl and +1 in the low concentration region (lcr) of HCl. This effect is due to the combined effect of H + and Cl −. At lcr of Cl −, the rate is inversely proportional to [H +], whereas at hcr of Cl − the rate is directly proportional to [H +]. The exponent of the [Cl −] term in the rate equation is found to vary within 0–1.2 on varying [H +] from lower to higher values. When the aqueous acidity is kept low, the rate is directly proportional to [D2EHPA] and at higher acidities of aqueous phase, the rate is independent of its concentration. A rate equation involving the extractions of Fe 3+, FeCl 2+, FeCl 2 + and FeCl 3 has been derived empirically to describe the system. The treatment of the temperature dependence data on rate gives the activation energies ( E a) of 76, 63, 15 and 10 kJ/mol for the extraction of species Fe 3+, FeCl 2+, FeCl 2 + and FeCl 3 respectively. Similarly, the enthalpy (Δ H ±) and entropy (Δ S ±) of activation have been calculated to be 76 kJ/mol and −130 J/K mol, 55 kJ/mol and −140 J/K mol, 20 kJ/mol and −392 J/K mol, and 6 kJ/mol and −280 J/K mol, respectively, for the four species. The mechanisms of extraction particularly of the rate-determining steps have been suggested.