Reverse potential phenomena in the Membrane-Electrode (M-E) process

Conventional water treatment techniques typically suffer from severe limita tions in selective removal and recovery of heavy metals from dilute solutio ns. For instance, treatment by pH control, chemical reduction, and chemical oxidation result in precipitation of heavy metals, which are subsequently landfilled. Ion exchange suffers from lack of selectivity in cation removal from multi-component mixtures, while electrochemical reduction has severe limitations in dilute solutions. In order to address the problem of selecti ve heavy metal removal and recovery, development of the Membrane-Electrode (M-E) process was undertaken. The M-E process is a hybrid of electrochemica l reduction and ion-exchange technologies and permits selective ion-exchang e from dilute aqueous solutions. High cation selectivities in the M-E process is due to controlled rate of i on-exchange. It has been discovered that cation exchange rates can be contr olled by application of an electrical potential difference (pd), and that a n inverse relationship exists between pd and the rate of ion-exchange. This behavior is termed as the "Reverse-Potential Phenomena". Typically, the ra te of ion-exchange in conventional ion-exchange processes is dependent upon the cation concentration in solution, and cation loading on the ion-exchan ge material; this rate cannot be controlled by external means. Thus far, the M-E process has been effectively demonstrated for selective r ecovery of Pb2+ and Cu2+ ions from dilute aqueous binary and ternary cation solutions [1,2]. This paper focuses on the effect of Reverse-Potential phe nomena on selectivity in the Cu2+/Ni2+ binary solution mixture.