Monovalent/Divalent Membranes

Anyone know of membranes that have selectivity for monovalent AND divalent ions. We have a client that would like to implement such membrane in an electrolytical system to extract cadmium and nickel from a cadmium/nickel/chromium rich lixiviate. They wish to recover nickel and cadmium as a solution and dispose of chromium as a precipitate. The separation of these metals is achieved through a pH gradient through the electrolytical system, and the requirement of a membrane would be to ensure that no chromium contaminates the product, which in this case would be a rich cadmium/nickel solution.