Stepwise membrane modification for nutrient recovery: Linking exclusion mechanisms to performance

Niche applications like nutrient recovery require membranes with specific ion retention for separation like for example NH 4 + and K + . Membrane modification is a promising strategy for developing application-specific separation properties. In this study, a three-step modification approach was applied to an ultrafiltration polyacrylonitrile membrane support for ion-specific retention. First, alkaline hydrolysis of the membrane support introduced negatively charged carboxylate groups enabling Donnan exclusion with a significant decrease in water flux due to pore swelling based on permeance and salt retention measurements. Second, the deposition of polyelectrolyte multilayers adds additional size exclusion to the Donnan exclusion by partially covering membrane pores, thereby increasing the retention of monovalent and multivalent ions. Third, the retention of salts is further enhanced by crosslinking forming a rigid polymer network improving exclusion. Moreover, the crosslinking provides a difference in the retention of NH 4 + and K + due to the difference in their dehydration energy resulting in nutrient separation. The selectivity can be further improved by increasing the pH of the manure digestate to permeate neutral NH 3 while retaining K + providing separation based on charge difference. Overall, the stepwise membrane modification shows a versatile pathway to tailor ion-specific retention, offering a strong potential for nutrient recovery and other niche applications.