A method for reducing the concentration of target co-ions in a liquid feed mixture comprising target co-ions and counter-ions by adding a colloid such as a polyelectrolyte or surfactant comprising a colloidal ion and colloidal counter-ions to the feed mixture wherein the colloidal ion has the same type of charge as the target co-ion. The feed mixture containing the colloid is filtered through an ultrafiltration membrane having pores small enough to block the passage of the colloidal ion.