(45b) Morphology Tuning of Polyelectrolyte Multi-Layer Membranes for Same Sized Ion Separation

In this talk, I will discuss a novel membrane surface-modification approach aimed at separating same sized, monovalent ions—ammonium (NH₄⁺) and potassium (K⁺). NH₄⁺ and K⁺ are critical nutrients in high demand as raw fertilizer materials and recovering them from nutrient-rich liquid sources like anaerobic digestates could reduce the reliance on traditional processes which are highly energy intensive. Achieving selective separation between NH₄⁺ and K⁺ would provide us with the flexibility to adjust the nutrient (N:K) ratio, enabling a final product suitable for a wide range of fertilizer applications. NH₄⁺/K⁺ separation is intrinsically challenging since the hydrated radii of both NH₄⁺ and K⁺ are identical (0.33 nm) with a slight difference in hydration energy (~10 KJ/mol) between them. In our prior work^(1,2), we have demonstrated that uncrosslinked and crosslinked version of polyelectrolyte modified membranes derived from weak polyelectrolytes demonstrate a wide range of NH₄⁺/K⁺ separation performance – 10-40% for %NH₄⁺ rejection and 40-80% K⁺ rejection. The polyelectrolytes include two types of polycations (i.e. positively charged polyelectrolyte) poly allylamine hydrochloride (PAH) and polyethyleneimine (PEI) and one type of polyanion (i.e. negatively charged polyelectrolyte) polyacrylic acid (PAA). These polyelectrolyte combinations are coated on a commercial nanofiltration (NF 270) membrane using the layer-by-layer (LbL) self-assembly approach. Under optimum conditions, these membranes could provide ~2.5X higher NH₄⁺/K⁺ selectivities compared to the underlying NF 270 membrane, and as a pressure-driven separation processes, this is the only reported study of achieving same-sized ion separation with such high selectivities. A key feature identified in this work is the importance of polyelectrolyte morphology – linear vs. branched polymer chains. Specifically the presence of branching, as in the case of the membranes involving PEI, appears to selectively influence the dehydration of NH₄⁺ ion, without impacting the K⁺ ion thus providing high selectivities. In my current research, we expand on this approach to tune the membrane morphology by manipulating the ratio of linear to branched chain polymers . We also investigate the role of molecular weight of polyelectrolytes in determining the overall performance of the membranes and we based this work on the hypothesis that polycation and polyanions of similar molecular weights form tighter effective voids and a mismatch in molecular weight would lead to higher void fractions leading to higher NH₄⁺ recoveries, but lower selectivities. Such precise morphology tuning effects and their impact on membrane ion-ion separation performance has no precedence in prior literature. This approach of separating NH₄⁺ from same-sized monovalent ions offers new opportunities and design principles for developing membranes with high ion-ion selectivities, which could be extended to other ion separation applications.

References:

1) Piash et al., AIChE Journal12 (2022): e17869.

2) Piash et al., Journal of Membrane Science715 (2025): 123438.