Modeling of the cadmium transport through a bulk liquid membrane

The model of metal ion permeation through a bulk liquid membrane (BLM), based on the Nernst–Planck equation, was developed. It was satisfactorily tested using the experimental data on the cadmium permeation from Cd(NO3)2 solution through BLM (D2EHPA in kerosene) to 1 M H2SO4 solution. According to the model simulations, the cadmium permeation was controlled by Keq on the feed side; the knowledge of the exact Keq value on the strip side under the condition cH2SO4≫cCdcH2SO4≫cCd was not required. None of the following: an assumed Cd-D2EHPA complex (CdP2(HP)2, CdP2HP), presence of undimerized D2EHPA in BLM, or ion pair formation Cd2+·SO42- in the strip solution, influence the goodness of fit. Assuming the model based on the Fick’s law with the constant cadmium mass transfer coefficient, a large dispersion of its optimal fitting values which depends on the model assumption was found. Thus, that coefficient can only be treated as a fitting parameter.

► The Cd permeation model through a BLM using the Nernst–Planck equation is formulated.
► The Cd permeation model through a BLM based on the Fick’s law is formulated.
► There is no influence of the kind of Cd complex, undimerized D2EHPA on the model fit.
► Cd permeation is controlled by the extraction equilibrium constant on the feed side.
► Cadmium mass transfer coefficient in the Fick law model is only a fitting parameter.