Modeling Al3+/H+ ion transport in Donnan membrane process for coagulant recovery

Abstract

AbstractA single‐step Donnan Membrane Process (DMP) has recently been investigated for selective recovery of Al3+ from water treatment plant residuals using cation exchange membranes. The DMP works on an electrochemical potential gradient and is not susceptible to fouling even in the presence of high concentration of particulates and natural organic matter (NOM). A step‐wise algorithm is presented to predict the kinetics of Al3+  H+ coupled transport using the Nernst‐Planck equation. The model includes the effect of solvent osmosis. Two characteristically different cation exchange membranes, namely the homogeneous Nafion 117 and the heterogeneous Ionac MC 3470, were included in the study. Both the membranes have hydrophilic properties when compared with recognized hydrophobic material, such as the polytetrafluoroethlylene (PTFE) surface. The average Al3+H+ interdiffusion coefficient (¯DAlH) value determined under nearly identical experimental conditions was an order of magnitude greater for the Nafion 117 membrane (10−10 m2/s) in comparison with Ionac MC 3470 membrane (10−11 m2/s). The lower diffusion coefficient values for Ionac MC 3470 membrane was attributed to the presence of nonionogenic pockets in the membrane phase that offer greater diffusional resistance to ion transport. A step‐wise protocol for the coupled transport of Al3+ and H+ during DMP successfully predicted the Al3+ recovery profile for both homogeneous and heterogeneous membranes. The Nafion 117 membrane could selectively recover Al3+ from water treatment plant residuals and could exclude dissolved organic carbon (DOC) in the recovered solution through Donnan co‐ion exclusion. © 2004 American Institute of Chemical Engineers AIChE J, 51: 333–344, 2005