Influence of Membrane Ion Sorption on Ammonium Transport in Donnan Dialysis with Cation Exchange Membranes

Donnan dialysis (DD) is a promising approach for selectively recovering ammonium ions from wastewater, owing to its simplicity and low energy consumption. However, the role of ion sorption and desorption in cation exchange membranes (CEMs), particularly interactions between ammonium ions (NH₄ ⁺) and competing ions (e.g., sodium Na⁺), has often been overlooked. Our experimental results revealed a shift in the Donnan equilibrium caused by the preoccupied counterions in the CEM. For example, when the feed and draw solutions were in a 1:1 concentration ratio, the expected ammonium recovery efficiency was 50%. However, the NH₄Cl-presoaked membrane resulted in an increase of 19.1 ± 0.5% in the solution NH₄ ⁺ concentration and a decrease of 18.8 ± 0.6% in the Na⁺ concentration. Conversely, the NaCl-soaked membrane showed an 18.9 ± 1.6% reduction in NH₄ ⁺ and a 23.0 ± 1.3% increase in Na⁺. The difference indicated that the ion exchange capacity of the membrane and counterion uptake could shift the equilibrium of the DD process. We further analyzed the process kinetics and developed a nonsteady-state model incorporating ion sorption capacity to describe the behavior. Our results confirmed that presoaked ions shifted the final DD equilibrium, potentially due to differences in their affinity and geometry. To summarize, this study provides new insights into the mechanisms of Donnan dialysis by accounting for ion sorption and offers insights for the design of more efficient and effective separation processes for ammonium recovery.