Elucidating organic and nutrient transport mechanisms in polyelectrolyte modified membranes for selective nutrient recovery

This work focuses on understanding and analyzing the transport mechanisms of organics and nutrient ions (NH₄⁺, K⁺) through polyelectrolyte-based membranes, aimed at the selective recovery of nutrients from nutrient-rich resources such as anaerobic digestate. In this study, commercial nanofiltration (NF) membranes were modified by the layer-by-layer (LbL) deposition of oppositely charged polyelectrolytes, utilizing a wide range of parameters such as polyelectrolyte type, deposition pH, salt (NaCl) concentration, polymer cross-linking, etc. Such modifications resulted in membranes exhibiting characteristics that indicate a trade-off relationship between nutrient passage and nutrients/organics selectivity. Our findings suggest that nutrient passage is primarily facilitated by surface charge, while size-exclusion plays a vital role in retaining the organics. Ionically crosslinked polyelectrolyte multilayer (PEM) membranes exhibit superior nutrient passage (up to ∼30 % higher) compared to commercial NF membranes, while covalently crosslinked PEM membranes achieve higher (∼12 % higher) organics rejection. In addition, membranes with such covalent crosslinking exhibit intra-nutrient (NH₄⁺/K⁺) selectivity of ∼1.8, when tested with binary NH₄⁺/K⁺ mixtures – a rather surprising membrane property, since both ions have similar hydrated radii. This study provides a fundamental framework of membrane design for selectively recovering nutrients from a variety of nutrient-rich sources.