(158b) Separation of Similar Sized Monovalent Ions through Controlled Crosslinking of Polyelectrolytes

In this talk, I will discuss a novel membrane surface-modification approach aimed at separating ammonium (NH₄⁺) from same-sized monovalent ion potassium (K⁺). NH₄⁺ and K⁺ are critical nutrients which could be used as fertilizer raw materials and recovering them from nutrient-rich liquid sources could provide a way to reduce the reliance on traditional synthetic processes which are highly energy intensive. In our prior work, we have utilized a surface-modified NF membrane to separate nutrients (NH₄⁺, K⁺) and organic pollutant. However, 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 fertilizing 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 this study, we applied polyelectrolyte surface modification approach to existing commercial membranes, followed by controlled crosslinking to evaluate the separation performance of NH₄⁺ over K⁺. The crosslinking density is reliant on various factors, including the type of polyelectrolytes, crosslinking agents used and duration of crosslinking. We speculate that the presence of confined domains within the crosslinked polyelectrolyte layers facilitates a unique desolvation-induced separation which leads to higher NH₄⁺/K⁺ selectivities vs. unmodified commercial NF membranes. In fact, in certain polyelectrolyte systems, the selectivity of NH₄⁺/K⁺ was ~3X higher compared to commercial NF membranes (selectivity ~1). This approach of separating NH₄⁺ from same-sized monovalent ions thereby offers new opportunities and design principles for developing membranes with high ion-ion selectivities.