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

Abstract

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.