Mechanism of Permeation of Rare Earth Metal Ions through Supported Liquid Membrane with Addition of Chelating Reagent to Stripping Phase

The effect of the addition of diethylenetriaminepentaacetic acid(DTPA) to the stripping phase on the kinetics of permeation of rare earth metal ions, La³⁺, Pr³⁺ and Nd³⁺, through a supported liquid membrane (SLM) containing 2-ethylhexyl hydrogen 2-ethylhexylphosphona-te (PC-88 A) was investigated. The flux of Ln³⁺ steeply decreased with increasing pH in the stripping phase (pH₈) and was not detectable above pH 3. 0 in the absence of DTPA. On the other hand, in the presence of DTPA, it is worth noting that Ln³⁺ permeated through the SLM even in the range of pH_s where the permeation of Ln³⁺ was not detractable without DTPA, and that flux remained almost constant between pH_g of 1.4 and 3.3. Experimental results on the flux under the various conditions of stirring speeds, pH of feed phase, DTPA concentrations and SLM thickness, led to the assumption that the permeation rate was controlled by the diffusion of Ln complexes in SLM with the concentrations of equilibrium at the extraction interface and zero at the stripping interface. The calculated rates and the average concentrations of Ln in SLM under the above assumption were well coincided with the experimental results, which supports the proposed mechanism. It is also suggested that increasing concentrations of dissociated DTPA species should cause to form complexes with Ln³⁺ to compensate the decrease in proton activity in the stripping phase with increasing pH.