Modeling of the Donnan dialysis process for arsenate removal

Abstract

Donnan dialysis is an ion exchange membrane process driven by an electrochemical potential gradient that is capable of removing ionic contaminants from water. To better understand and simulate arsenate (As(V)) removal by Donnan dialysis, a simple model focusing on intermembrane ionic diffusion at steady state was developed based on the Nernst–Planck equation in this study. Using experimental data from independent ion exchange reaction experiments and dialysis experiments, the self-diffusion coefficient of As(V) in the anion exchange membrane was calculated. This value was on the order of 10 −8 m 2 h −1 and related to the system pH and membrane type. The As(V) removal by Donnan dialysis (feed solution: 1 L, 1.3 × 10 −2 mol As m −3 (1000 μg As L −1) with 10 mol m −3 NaCl; stripping solution: 1 L, 100 mol m −3 NaCl) was conducted with different types of membrane at different system pH. At the end of the 12-h dialysis, the removal efficiency was lowest (35%) for the heterogeneous membrane at pH 4.5 and highest (95%) for the homogeneouse membrane at pH 9.2. Higher ion-membrane affinity, higher intermembrane ionic mobility, and thinner membrane thickness facilitated the As(V) transfer according to the model results. Using the calculated membrane phase self-diffusion coefficients and the corresponding distribution coefficients of As(V) between the membrane and the solution, the present model successfully predicted the As(V) removal profiles of different dialytic conditions.