Effect of electrodialysis on colloidal geometry and dynamics: Why my membrane stack was clogged even after a fine pretreatment?

As a long-standing problem, electrodialysis (ED) clogging is believed a consequence of colloids. However, its blocking causation and clogging mechanism have not been verified. In this study, electrodialysis was used to treat a colloidal saline solution, aiming to answer the question from the "nature" of ED by investigating the influence of ED parameters such as laminar flow, salt concentration, current density and pH on colloid geometry and dynamics during the desalting process. The results revealed that: (i) laminar and membrane electrostatic repulsion and adsorption could not significantly increase the particle size (maximum 2.28 times), while the applied electric field elevated the particle size by 54.52 times (119.9 ± 13.66 to 6537.5 ± 64.35 nm); (ii) when the initial feed concentration elevated 10 times (0.1 to 1 mol/L NaCl), the particle size upsurged 149-fold (5.99 ± 0.57 to >150 µm), and flocs were generated. This enhancement was mainly attributed to the compressive electric double layer effect, and the Debye length was trimmed from 0.96 to 0.30 nm; (iii) The low current density (25 A/m2) had a profound aggregation effect on small BSA particles (roughly 10 nm); (iv) The change of pH causes the conformational transition of BSA. In the strong acidic (pH = 3.0) environment, the colloidal particle size expanded by 13 times. This study confirmed that the aggregation of colloids was the culprit of spacer clogging during electrodialysis at higher salt concentrations (>1 mol/L). Furthermore, experimental data were substituted into the simulation formula to summarise the geometry and dynamic variation of BSA in ED.