Counterion transport:

Counterion transport in which an acidic carrier, HC, loses a proton and creates a complex, MC, metal ion at the feed solution-SLM interface. That complex diffuses to the SLM-strip solution interface while it liberates a metal cation within the receiver phase and concurrently picks up a proton from the strip solution. The reproduced carrier, HC, diffuses back to the feed solution-SLM interface, picks up the other metal ion and the procedure continues. An acidic carrier molecule shuttles among the feed and the strip solution-SLM interfaces.

The driving force within a counter transport SLM system is the pH difference among the feed and the strip solution. For capable transport, high feed and low strip solution, K_d values are required. The K_d difference among the feed and strip solution is managed through a pH gradient. To divalent metal ion and an acidic carrier, the reaction at the feed solution-SLM interface is as follows:

M²⁺   + 2 HC →         MC_{2 ( membrane )}   +  2 H⁺ _(aq)