(317s) Performance of Homogeneous and Heterogeneous Ion-Exchange Membranes during Donnan Membrane Process Based Alum Recovery

Donnan Membrane Process (DMP) was applied to recover alum from water treatment residuals using ion-exchange membranes and the coupled transport of aluminum and hydrogen ions was studied. The study was undertaken for two families of ion-exchange membranes; the homogeneous membranes represented by Nafion 117 and the heterogeneous membranes represented by Ionac 3470 membrane. Homogeneous membranes are coherent ion exchanger gels while heterogeneous membranes consist of colloidal ion exchanger particles embedded in an inert binder. In the process, the recovered Al3+ could be concentrated to a high value of over 4500 mg/L (80% recovery) with Nafion 117, but the recovery was relatively low (25% recovery) with Ionac 3470. Since the Donnan Membrane process is driven by the electrochemical potential gradient, the presence of high turbidity and natural organic matters in the residuals did not influence the transport process and no noticeable immediate fouling was observed in either of the membranes. The Al3+ recovery profile led to identification of three zones of mass transport; the kinetically driven linear zone, equilibrium driven saturation zone and osmosis driven dilution zone. All the three zones were observed in Nafion 117 during a 24-hour experimental run. For Ionac 3470, only the linear zone was observed during the experimental period. Osmosis effect was not noted for Ionac 3470. The interdiffusion coefficient value of was found to be one order of magnitude greater for Nafion 117. Within an ion exchange membrane, a diffusing ion hops from one charged site to the next and that constitutes the primary intramembrane ion transport mechanism. Scanning electron microphotograph and x-ray fluorescence showed clusters of non-conducting inert phases within the Ionac 3470 membrane containing no ionogenic groups. The lower interdiffusion coefficient for Ionac 3470 was attributed to its larger fraction of the non-conducting phase compared to Nafion 117.